A COMPARISON ADVANTAGES AND DISADVANTAGES OF PUMPED

HYDROELECTRIC ENERGY STORAGE AND UNDERGROUND PUMPED

HYDROELECTRIC STORAGE

By (Wisterock William Stephenson)

A comparison Advantages and Disadvantages of Pumped Hydroelectric Energy Storage and

Underground Pumped Hydroelectric Storage

EXECUTIVE SUMMARY

Currently, many countries worldwide have experienced an increase in the demand for

electricity for both household and use in the industries. The need for constant generation and

supply of electricity has created the demand for energy storage facilities by the electricity

generating companies to ensure that they meet the demand and continuous flow through the grid

at all times. Among the major characteristics of electricity companies is that the quantity of

electrical energy that can potentially be generated is, in most cases, fixed for a short period while

the demand fluctuates throughout the day and night. Energy storage refers to the capturing of the

energy that is generated at a specific time and utilised at a later time. The storage devices

accumulate electrical energy and store it for use at a future time or date. There are numerous

electricity storage technologies, which occur in many forms for instance, electrical in the form of

batteries, mechanical in the form of flywheels; chemical energy in the form of hydrogen;

potential energy in the form of pumped hydro; and thermal in the form of phase change

materials. There is, therefore, the need to identify the best alternative energy storage technology

capable of storing a large quantity of electricity. This dissertation focuses on UPHES and the

PHS storage technologies as they are capable of storing larger capacity of electrical energy.

This study, therefore, sought to assess and analyse the advantages and the disadvantages

of PHS and UPHES. A comparison was made while considering the performance and features of

these technologies. This study also reviews the research done on the storage technologies and the

circumstances under which these technologies are more suitable for their applications.

Throughout the literature review, it was noted that these technologies have distinct applications,

and no storage technology can be applied in all cases. The researcher, therefore, narrowed down

to technologies suitable for large energy storages that are also less costly and have the capability

of continuously supplying electricity to meet the rising demand for electricity.

This study analysed the parameters that are of keen interest in the process of storing

electrical energy. These parameters include the storage capacity, power capacity, round-trip

efficiency, response time and the cost involved in the implementation process. A Decision Tree

diagram and the Analytical Hierarchy Process were used to analyse the performance and

outstanding features of these storage technologies. The outcomes showed that indeed, UPHES

storage technology is more suitable for large energy storage than the PHS storage technology.

The analysis also showed that UPHES, if implemented, could become more advantageous than

the contemporary PHS, because UPHES, to some extent is an improved version of the PHS

system.

Table of ContentsACKNOWLEDGMENT...........................................................................................................................2

DEDICATION...........................................................................................................................................3

EXECUTIVE SUMMARY.......................................................................................................................4

List of Tables..............................................................................................................................................8

Table of Figures.........................................................................................................................................9

CHAPTER ONE: - INTRODUCTION..................................................................................................10

1.1.Background........................................................................................................................................10

1.2. Problem Statement...........................................................................................................................13

1.3. Objectives..........................................................................................................................................17

1.4. Specific objectives.............................................................................................................................18

1.5.Research questions............................................................................................................................18

CHAPTER TWO: - LITERATURE REVIEW.....................................................................................20

2.1. Introduction......................................................................................................................................20

2.2. Overview of the storage technologies and their implications........................................................20

2.3. Present status of energy storage......................................................................................................22

Figure 1 Comparative estimation of currently installed energy capacities worldwide...............................23

2.4. Merits Of UPHES and PHS Over Other Storage Technologies...................................................25

2.5. Framework of storage facilities.......................................................................................................28

Figure 2 Classifications of storage technologies........................................................................................28

2.6. The Need for Pumped and Underground Pumped Hydroelectric Energy Storage.....................28

2.7. TYPES OF ENERGY STORAGES................................................................................................31

2.7.1. Pumped Hydroelectric Storage (PHS).........................................................................................31

2.7.2. Flywheel Energy Storage (FES)....................................................................................................32

2.7.3. Storage in form of Hydrogen........................................................................................................33

2.7.4. Geothermal energy storage...........................................................................................................33

2.8. How PHS Operates...........................................................................................................................34

Figure 3Typical design of PHS facility.....................................................................................................37

2.8.1. Underground Pumped Hydroelectric Energy Storage................................................................39

2.9. The Description and Operation of the UPHES...............................................................................42

2.9.1. The Design of the UPHES Facility................................................................................................44

CHAPTER THREE:- METHODOLOGY............................................................................................47

3.1. Power capacity..................................................................................................................................47

3.2. Storage capacity................................................................................................................................48

3.3. Round-Trip Efficiency......................................................................................................................49

3.4. AHP Analysis....................................................................................................................................49

3.4. Decision Tree Diagrams...................................................................................................................52

3.5. Analysis.............................................................................................................................................53

3.6. Classification of the storage technologies in AHP..........................................................................54

3.7. Decision Tree Diagram for selecting the best storage technology.................................................55

CHAPTER FOUR: - RESULTS AND DISCUSSION..........................................................................57

4.1. Efficiency of PHS..............................................................................................................................57

4.2. Power and Storage Capacity of PHS...............................................................................................58

4.4. Efficiency of UPHES........................................................................................................................60

4.5. Comparison of the Advantages and the Disadvantages of PHS and UPHES...............................62

4.5.1 Advantages of PHS System............................................................................................................63

4.5.2. Disadvantages of PHS Storage System.........................................................................................64

4.6. Core Advantages and Disadvantages of UPHES compared to those of PHS facilities................66

4.6.1. Disadvantages of UPHES Storage Facility...................................................................................67

4.7. Future of PHS...................................................................................................................................68

CONCLUSION........................................................................................................................................69

References.................................................................................................................................................71

List of Tables

Table 1 AHP Comparison Criteria................................................................................................48Table 2 Criterion for selecting Power Quality...............................................................................50Table 3 Storage Alternatives Subjected to Fuzzy Logic...............................................................51Table 4 Classification of Storage Technologies in AHP...............................................................51Table 5 Ranking of storage Alternatives using Fuzzy Logic and Classification Criteria.............52Table 6 Storage Capacities for Major countries in the World.......................................................56Table 7 Power Capacity of a proposed UPHES Sstorage System.................................................58

Table of Figures

Figure 1 Comparative estimation of currently installed energy capacities worldwide.................22Figure 2 Classifications of storage technologies...........................................................................27Figure 3 Typical design of PHS facility........................................................................................36Figure 4 An Estimate proposed size of a desirable UPHES System.............................................39Figure 5 Typical Design of UPHES facility..................................................................................40Figure 6 Decision Tree Diagram for Selecting Best Storage Technology....................................53

CHAPTER ONE: - INTRODUCTION

1.1. Background

Among the major features of the electrical power industries is that the quantity of

electrical energy capable of being generated, is in most cases constant over a short period,

whereas the demand fluctuates throughout the day and night (Barnes and Levine 2011 p. 5).

Thus, establishing technology for storing electrical energy to meet the demands whenever

required can be a breakthrough in the distribution of electricity (Graditi et al. 2014).

Energy storage is often a term used to refer to the capturing of energy generated at a

particular time for utilisation at a later time (Butler, Miller, and Taylor 2002). The storage device

is known as an accumulator. It is crucial to try and meet the objective of developing energy

storage so that electrical energy storage devices are capable of managing and maintaining the

supply of the quantity of power needed to meet the demands of the users during the times when it

is required most. In many instances, electrical power is in high demand during the peak season

(Butler, Miller, and Taylor 2002).

The storage technologies can aid in making the renewable energy sources whose output

of power may not be controlled by the grid operators, smoothly and in a way that can be

dispatched (Tam, Blomquist, and Kartsounes 1979 p. 330). Therefore, this paper intends to

evaluate the advantages and disadvantages of both the “Pumped hydroelectric storage (PHS) and

Underground Pumped Hydroelectric Energy Storage (UPHES)”, and determine which of the

storage technologies is more favourable.

Since electricity was discovered, there have been attempts to find an effective method of storing

energy for utilisation on demand. Over the past decades, the energy storage industry has

progressively evolved and adapted to the changing energy demands and improvements in

technology (Ribeiro et al. p. 1744). The systems of storing energy offer a variety of technological

methodologies to help in the management of power supply to establish a more robust

infrastructure for energy (Sunderkötter and Weber 2012). The result is the reduction of storage

costs and bringing utilities to the consumers (Slocum 2010). Further, it is also recommended to

comprehend the diversified approaches that are presently being implemented all over the world

to store energy (Smith, SenSr, and KroposkiSr 2008). The two commonly used utility-scale

electricity storage technologies include “Pumped hydroelectric storage (PHS) and Underground

Pumped Hydroelectric Energy Storage (UPHES)”, which are the focus of this study.

It is crucial to take note that correctly choosing the technology for storing energy will

help in the smoothening of power surges and permit the dispatch of electric power at a later time

(Poonpun and Jewell 2008). Before advancing to determine which of the two storage approaches

is more appropriate, it is important to consider their advantages and disadvantages. Energy

storage also requires a storage device that is specifically designed to allow electric energy from

the grid and converts it into an energy form suitable for storage, and conversely converts it back

into electric energy. This conversion takes into consideration the losses or energy due to

inefficiencies when it returns the energy into the grid (Smith, SenSr, and KroposkiSr 2008). The

energy storage approaches have a wide spectrum of applications due to their special storage

capacities and power that the technique is capable of receiving from a wide range of devices

(Ribeiro et al. 2001).

These energy storage approaches have their assorted features and unique properties.

However, despite these distinctions, the techniques have a general base parameter (Butler,

Miller, and Taylor 2002). The most common base is the capacity to store energy, that is, the total

electrical energy that the storage technique can accumulate (Levine et al. 2007). In this case, the

standard unit is the megawatt-hours (MWh) or the kilowatt-hours (kWh) for applications

consuming little energy (Levine et al. 2007. p. 132). The other common base parameter is the

power capacity, which refers to the greatest immediate production that the system of storing

energy can supply, and the common units of measurement are megawatts (MW) or Kilowatts

(kW) (Steffen and Weber 2013). Thus, to determine which of the methods is best, the efficiency

parameter must be considered in the calculations of a recoupment of the storage system at the

time of projection (Rehman Al-Hadhrami and Alam 2015).

The efficiency parameter indicates the total of electric power on the output within the

percentage from the device electric charges (Steffen and Weber 2013). It is also recommended to

consider the property of the quality of power, which is the capability of the storage approach to

transfer the required energy with the best quality that has no harmonics, spikes or any other

associated difficulties (Bin et al. 2014). There is also a “Round-Trip Efficiency” parameter that

was used to evaluate the effectiveness of the storage device. It indicates the efficiency in the

same way in which the efficiency property shows in the percentage of the quality of electric

power that is capable of being recovered from the electricity utilised to charge and discharge the

storage devices (Smith, SenSr, and KroposkiSr 2008). The other property is the response time

that indicates the duration which extends from the request and power storage output response

(Ribeiro et al. 2001). It is, therefore, crucial to implement energy storage using the best storage

techniques. Selecting the best alternative will be done by comparing the advantages and the

disadvantages of PHS and UPHES.

1.2. Problem Statement

The concept of energy storage is well developed but to some extent unexplored. Currently,

“Pumped hydroelectric storage (PHS) and Underground Pumped Hydroelectric Energy Storage

(UPHES)”, are considered as excellent technologies for storing energy for use in future when the

demand arises as the electricity industry is going through a series of transformations (Ramos,

Amaral, and Covas 2014 p. 1099). Identifying the advantages and disadvantages, how they work,

their applications, and future implications are essential in selecting which of the technologies of

storing electricity is the most effective (Kumar and Radhakrishna 2008). It is noted that different

countries prefer different storage technologies (Sankar et al. 2012 p. 213). Due to their

efficiency, there is a hard task of choosing the best technology for energy storage between the

two technologies.

Furthermore, these technologies in themselves are perfect for particular applications, but

again, no technology is ideal for every situation (Kumar and Radhakrishna 2008). The two

technologies have a large power range and installations, which further make it difficult to choose

between the two (Barnes and Levine 2011 p. 7). It is critical to note that the storage of energy is

capable of optimising the existing power generation and transmission infrastructures while at the

same time preventing costly upgrades (Lombardi, Vasquez, and Styczynski 2009). The

fluctuations of energy resources can alter their penetration onto the electricity networks or cause

power surges (Kumar and Radhakrishna 2008). Due to these fluctuations, it is critical to note that

the devices for storing energy have the advantage of managing the existing irregularities and,

therefore, support the implementation of energy resources (Ekman and Jensen 1146).

Additionally, considering the production of power, the energy storage devices can

enhance the overall quality of power and the reliability that is currently becoming essential for

the modern commercial applications (Smith, SenSr, and KroposkiSr 2008). The energy storage

devices can also minimise energy emissions by supporting the transitions to new and clean

storage technologies, which are environmentally friendly (Ess et al. 2012).

According to Eyer, Iannucci, and Corey (2004), for a long period, there have been many

impediments that have hindered the commercialisation of the energy storage devices. In most

instances, the benefits of energy storage are inconclusive, and the users often do not have the

knowledge about the benefits of energy storage when considering cost saving and the quality of

power (Eyer, Iannucci, and Corey 2004. p. 12). This condition is due to the higher capital costs

that are related to the energy storage technologies and the inadequate experience for the majority

of the stakeholders involved, such as the investors, the designers in the market, and transmission

system operators (Ingram 2009). To the extreme, there is also uncertainty as to who will pay for

the cost of storing energy (Crampes and Moreaux 2010 p. 325). Some of the stakeholders

consider the storage grid to be the infrastructure, particularly in markets in which energy storages

are mainly dispatched as grid assets (Denholm 2013).

Electrical energy storage technologies have indicated the ability to cope with some

essential features of electric energy, for instance, the hourly fluctuations in the demand and

pricing of electricity (Butler, Miller, and Taylor 2002). For a long time, energy storage

technologies have played three major roles (Yucekaya 2015 p. 46).

Firstly, they have been able to minimise the cost of electricity through the storage of

electricity accumulated during the off-peak seasons at the time when the prices are low, for

utilisation at times of peak seasons, when electricity would be bought at higher prices (Crampes

and Moreaux 2010 p. 326).

Secondly, in an attempt to enhance the reliability of electrical power supply, the electrical

storage systems have been able to support the user during the times when failures of power

network happen as a result of natural calamities (Levine et al. 2007).

Thirdly, they have been able to maintain and enhance the quality of power, the power

frequency, and voltages (Rehman Al-Hadhrami and Alam 2015). Thus, choosing the most

effective electricity storage technology will depend on which of the two technologies is more

efficient in terms of cost reduction, power reliability, power quality, frequency and the voltage

supplied through the system (Crampes and Moreaux 2010 p. 340).

The researcher noted that electricity is always consumed at the same time it is generated.

The implication is that a proper quantity of electricity must always be available to meet the

fluctuating demands (Poonpun and Jewell 2008). Any imbalance between the demand and

supply will in most cases, destroy the stability and the quality of the power supply, that is, the

voltage and the frequency of its transmission (Bradbury, Pratson, and Patiño-Echeverri 2014).

Again, the researcher noted that it is crucial to note that the places where electricity is produced

are often situated far away from the areas where it is consumed (Yucekaya 2015 p. 48). The

consumers and power generators are usually connected via power grids, thus forming a power

system (Denholm 2013). The researcher also found that in the emerging markets, in the on-grid

locations, the electrical energy storage technologies are anticipated to solve problems

encountered frequently, such as excess fluctuations of power and the unreliable supply of

electrical energy, associated with utilisation of great amounts of renewable energy.

The researcher, therefore, found it important to identify the most efficient storage

technology through comparing the advantages and disadvantages of PHS and UPHES. The study

focuses on the efficiencies and benefits of using the two electrical storage technologies. The

efficiencies are determined regarding the costs saved, the quality of power supplied by the

technology, the frequencies, and voltages released, and the reliability of the system to supply

electrical energy considering the fluctuating nature of electrical power supply (Poonpun and

Jewell 2008). In finding the best storage technology, the researcher investigated the efficiencies

of both the UPHES and the PHS.

While considering the two energy storage technologies, the researcher found that the

efficiency of PHES, which is a tried and tested energy storage technique, has presently reached

between 70% and 80% (Levine et al. 2007). Again, it is known that globally, PHES installation

upholds an estimate of 3% of power generation capacity (Robert 1933 p. 1). However, in most

instances, it is noted that PHES has been limited by geographic parameters (Levine et al. 2007).

The researcher needs to find the best alternative between UPHES and PHES electricity energy

storage technologies. Thus, some decision-making criteria has been deployed and evaluated to

find the best storage technology that can deal with the existing alternatives incorporated in such

decisions.

In this paper, the researcher used two methods to determine the solution to the current

problem of finding an alternative energy storage technology. These methods include Analytical

Hierarchy Process (AHP) and the Decision Tree Diagram (DTD) (Hobbs and Meier 2012). The

AHP methodology used six criteria that include output power characteristic of the technologies,

the needed space for installing the storage technology, the anticipated lifespan of the technology

during its operation, and the temperatures under which the technology operates (Zakeri and Syri

2015). These criteria were used along with the determined efficiency and the quality of power

transmitted by the two energy storage technologies.

On the other hand, the DTD used six similar criteria but expanded on them through

assigning particular ranges to them to create room for comparison (Hobbs and Meier 2012).

Assigning the ranges permits a higher level of user specificity, thus resulting into a more tailored

energy storage solution.

1.3. Objectives

The following general and specific objectives were the centres of this study. The

identified two methods for evaluating the selection criteria for energy storage technologies

helped the researcher to answer these research objectives.

General objective

The overall objective of this study was to select the best energy storage technology

between PHES and UPHES energy storage technologies through assessing their advantages and

disadvantages.

1.4. Specific objectives

1. To evaluate the available electrical energy storage techniques so as to identify the

present state of technologies and evaluate the more favourable alternative electrical energy

storage technology.

2. To determine the efficiency of UPHES and PHES so as to determine which of the

electrical energy storage technology is more preferable. .

3. To explore the current decision criteria used to select electrical energy storage

technologies.

4. To identify an appropriate model that can be utilised to arrive at the decision for

choosing electrical energy storage technology.

5. To determine the criteria against which the two technologies can be evaluated

depending on user requirements and select acceptable storage technology in forming part or

overall solution to the electrical energy storage technology.

1.5. Research questions

1. What are the available electrical energy storage techniques and present state of

alternative storage technologies that are more favourable?

2. How efficient are UPHES and PHES in their capabilities to store electrical energy for

use at a later time?

3. What are the current decision criteria used to select electrical energy storage

technologies?

4. Is there an appropriate model that can be used to arrive at the decision for selecting

electrical energy storage technologies?

5. What are the criteria against which the UPHES and PHES can be evaluated

considering the user requirements so as to select the best storage technology?

CHAPTER TWO: - LITERATURE REVIEW

2.1. Introduction

This section explores the working of UPHES and PHES storage technologies; the

research done on their application and their circumstances under which these methods are

suitable; and the current trends in their implementation.

2.2. Overview of the storage technologies and their implications

Recently, most countries around the world have experienced an increase in the need for

bulk electrical energy storage as a consequence of the increase in the adoption of renewable

energy resources (Levine et al. 2007). The adoption of the renewable energy resources has been

driven by the desire to eradicate the environmental impacts, energy shortage issues, the reduction

in the increasing costs of new power plants (Slocum 2010). There are many studies conducted to

identify and deploy the most suitable technology to help in solving these problems (Jackson et al.

1999). Consequently, there has been the need to store energy wastes from a series of industrial,

commercial or even domestic processes and reduce the energy losses to the minimum (Ku 1995).

With this regard, the storage of energy has attracted more attentions from scholars because of its

ability to minimise energy consumption, cost, and can also be utilised as an alternative energy

source (Zhou, Ang, and Poh 2006).

Notably, reliable and affordable electricity storage is a prerequisite for utilisation of

renewable energies in inaccessible localities, in the integration into the electric grid system and

the enhancement of any future decentralised supply of electricity system (Shiu and Lam 2004). It

is, therefore, noted that energy storage system has a critical role in the attempts to combine the

future, sustainable electricity supply with the standards of technical services. Thus, any delays in

the responses to electrical energy requirements and insufficient or excessive electricity levels are

not acceptable to the industrial, commercial or even private user as it may result in failures in

applications (Shiu and Lam 2004). Therefore, it is crucial to implement energy storage system

that matches energy requirements of the users.

When utility enterprises realised the essentials of the flexibility provided by the storage

technologies in electricity networks, there was the construction of the first central energy storage

station and a PHS facility that was put into usage in 1929 (Butler, Miller, and Taylor 2002).

Later, there was the subsequent advancement of the electrical energy supply industries with the

interest of economies of scale at larger central generation stations, considering their broad and

comprehensive transmissions and networks of distribution (Barnes and Levine 2011 p. 9). By

2005, over 200 PHS facilities were being used all over the world offering over 100GW of the

generation capacity (Mitchell and Mitchell Dell 2001). However, there is pressure from the

deregulation and environmental concern resulting to investing in major PHS systems falling off

(Zhou, Ang, and Poh 2006). Further, a pursuit in the practicality of applying electrical energy

storage systems is presently enjoying revitalisation. Revitalisation came as a result of worldwide

transformations in the utility regulatory environments, the ever-increasing dependency on

electricity industries, commercial and home utilisation, the quality of power matters, and the

growth of renewable new sources of electrical supply (Ginocchio, Parker, and Sewalk 2007).

Pumped storage hydro systems have become the most common type of utility-sized

electrical energy storage system (Deane, Gallachóir, and McKeogh 1293). Initially, these

systems were viable economically only because they replaced the most costly generating units

(Medina et al. 2014). It is also essential to take note that the demand for electrical power changes

throughout the day (Scieri, Miller, and Miller Richard 1984). There are some instances when

power demand is very high, in which case, if the power station fails to generate more power

immediately, there is a possibility of power cuts among the users connected to the same grid

(Rehman Al-Hadhrami and Alam 2015). Again, considering that most of the power people use is

generated by the fossil fuel power stations that take longer durations to crank to full power, it is

crucial to have a power storage facility that can store this energy whenever it is produced. It

should also keep on supplying power until the power stations can catch up (Ribeiro et al. 2001).

The implication here is that the storage facility must be one that has a faster response time.

2.3. Present status of energy storage

The motivation for producing and supplying clean energy around the world has become

considerably competitive as a result attempts to increase the efficiency, reduction in the emission

of the greenhouse gasses and promotion of clean and more sustainable energy production

(Yüksel 2010 p. 464). Many storage technologies have been used to help solve the problems of

energy storage. The commonly used storage technologies currently being employed include the

PHS and the UPHES, which have been deployed in the electricity grid (Denholm 2013). Thus, to

understand the energy storage situation of the world, it is essential to conduct a comparative

estimate of the presently installed capacities of the world’s energy storage projects presented in

the figure below:

Figure 1 Comparative estimation of currently installed energy capacities worldwide

Currently, the electricity transmission and the transport sectors are the possible fields in

which the energy storage systems can fully be implemented (Hadjipaschalis, Poullikkas, and

Efthimiou 2009 p. 1513). These energy storage systems (ESS) enhance the current power plants

while at the same time does not allow costly upgrades (Rehman Al-Hadhrami and Alam 2015).

They also act as regulators of the fluctuations of the electrical energy from the renewable energy

resources that has deterred their penetration of the market (Ingram 2009). Therefore, through the

use of the ESS, sources of renewable energy can be utilised to assist in the transition for new and

clean energy generation technologies (Mitchell and Mitchell Dell 2001). Although many factors

are hindering the commercialisation of these storage technologies, for instance, higher capital

costs and lack of experience (Scieri, Miller, and Miller Richard 1984). However, the utilisation

of the electricity storage technologies is predicted to increase shortly as renewable energy, and

the quality of power is considerably becoming critical (Slocum 2010).

Each technology has particular characteristics that make them suitable for specific

conditions (Ibrahim, Ilinca, and Perron 2008 p. 1226). The energy storage technology by

pumping is the most mature of all; it has been used since the twentieth century (Butler, Miller,

and Taylor 2002). It is based on storing energy through pumping of water from a river or lower

reservoir to an upper reservoir. The proper slope to be given between the two reservoirs so that

the technology is efficient must be at least 100 m (Olsen et al. 2015 p. 54). A series of

characteristics of different technologies relating to the energy storage technologies have allowed

them to be utilised for various applications depending on the particular parameters of the

applications (Jackson et al. 1999). These parameters include power and storage capacities,

response time, costs and the economies of scale, the lifetime monitoring and the control devices,

efficiency and the constraints involved in the operation (Poonpun and Jewell 2008). These

parameters assist in the determination of the most suitable type of storage technology to be used

(Zakeri and Syri 2015). It is noted that the ESSs have become important due to their capabilities

to increase the efficiency of the electricity used resulting from the production of an electricity

reserve (Jackson et al. 1999). In this study, the PHS and UPHES happen to be the best large

capacity energy storage technologies.

The PHS and the UPHES technologies depend on the availability of water reservoirs

(Martin 2011 p. 77). It is thus, crucial to take note that the most suitable kind of reservoir relies

on the characteristics of the site of construction, for instance, the topographical structure, the

composition of soil, and the local environmental regulation (Ginocchio, Parker, and Sewalk

2007). The commonly used reservoirs designs are in most cases, excavated, embankments, or a

joint of the two (Zhou, Ang, and Poh 2006). The excavated reservoirs are commonly used on flat

terrains while the embankments are commonly used with the sloping terrains (Jackson et al.

1999). In the aquifer UPHES storage technology, the water level in the surface reservoir usually

falls and rise frequently (Martin 2007 p. 77). The magnitude of the fall and rise depends on the

surface area and the reservoir’s volume with respect to the quantity of pumped or rejected waters

(Yüksel 2010 p. 467). However, as the surface area of the reservoir increase, the losses due to

evaporation also increase. Thus, the volume and the depth of the reservoir should always be

traded with the permitted water level changes (Jackson et al. 1999). It is also crucial to note that

while the UPHES does not consume any water during its operation, the owner of the reservoir

must possess exclusion water rights that will take into account any losses as a result of

evaporation or irrigation usages (Yüksel 2010 p. 468).

The cost of excavation of a reservoir can vary widely subject to the type of soil, size and

the rates of local labour and the economics (Medina et al. 2014). Further, a hydraulic interface,

which permits water to be pumped in and out of the reservoir is needed for an aquifer UPHES to

allow the installation of an underground or surface reservoir water piping and valves that

interface the reservoir to the well (Martin 2007 p. 77).

2.4. Merits of UPHES and PHS Over Other Storage Technologies

Flexibility

When compared to other storage technologies, the UPHES and the PHS prove to be more

flexible sources of electrical energy because the stations are capable of being ramped up and

down very quickly and efficiently to cope with the dynamic demands of energy (Tam,

Blomquist, and Kartsounes 1979 p. 330). The hydro turbines possess the start-up duration of an

order of just a few minutes. These storage facilities may take approximately 60 to 90 seconds to

develop a unit of cold start-up to full load that proves to be shorter than those of other storage

facilities such as gas turbines or even steam plants (Colarelli and Grunwald 2002 p. 7). In most

instances, the generation of power can be reduced quickly in case there is a surplus generation of

power.

To add to this, the hydroelectricity power stations have longer economic lives, as some

plants have been in service for over 50 to 100 years (Shiu and Lam 2004 p. 49). Additionally, the

costs of operation concerning labour are very low because the power projects are automated and

do not require many personnel on the site at the normal operational hours.

Suitable for Industrial Usage

In most cases, other storage technologies usually supply public electricity networks, and

may not be able to meet industrial application at the same time. However, most hydroelectric

power plants are built to serve both the public and some industrial applications. A key example is

a hydroelectric power plant constructed to offer substantial quantity of electrical energy that is

required for the electrolytic aluminium plants.

Lower costs of power

One of the greatest advantages of hydroelectricity over other technologies of electricity

storage is the eradication of the fuel costs. Once the hydro plant has been established, the

operation costs of the hydro station are unlikely to increase due to the increase in the costs of

fossil fuels like oil, coal or natural gas, as there is no need of imports. According to the Navigant

Consulting, Inc. 2010, the long lifespan implies that not only the costs are spread across the long

timeframe but it also takes into consideration that the equipment for generating power utilised at

the facility can operate for a longer duration without the need for major replacement or repairs.

Reduced Emission of CO2

Hydroelectric power projects do not burn fossil fuels, thus do not produce CO2 directly.

However, some CO2 is emitted during the construction of the power plant; the quantity released

cannot be compared to those released by other storage facilities such as those of fossil fuels that

fuel electricity generation and storage technologies.

Also, when compared to nuclear energy storage technology, hydroelectric projects do not

generate any nuclear waste and has none of the dangers related to nuclear leaks. When compared

to wind energy storage facilities, the load factor of a hydroelectricity power plant is more

predictable as it is capable of generating power whenever the need arise and can be frequently

varied and regulated to cope with the variations in the demand for power.

Other useful Purposes of the Reservoirs

During the construction of pumped hydroelectricity power plants, reservoirs are usually

created and often offer facilities for water sports and can also become tourist attraction sites,

which is an additional economic benefit to a country (Shiu and Lam 2004 p. 450). Other

reservoirs can be used to provide water for irrigation to support agricultural production as they

provide constant water supply. Additionally, larger hydroelectricity power plants reservoirs can

be used to control floods that would otherwise have a considerable effect on the people living in

the downstream of the plant.

2.5. Framework of storage facilities

The framework below shows the normal PHS system used more frequently and were

often classified as direct or indirect storage facilities. The former is noted to have few energy

changes, best energy efficiency, and few devices as compared to the latter (Olsen et al. 2015 p.

54). Overall, these storage devices must possess low discharge. The low discharge is due to the

storage in the short term and long term; higher efficiency; higher lifetime exposure to the

unforeseeable cyclic circumstances and be in a position to deliver electrical energy more or less

quickly as the demand changes. Additionally, the discharge is due to the wider operational

temperature ranges; lower maintenance costs and be regarded as non-pollutants (Poonpun and

Jewell 2008).

Figure 2 Classifications of storage technologies

2.6. The Need for Pumped and Underground Pumped Hydroelectric Energy Storage

The systems of energy storage are crucial because, in addition to allowing storage of

energy, the generation systems permit the integration of renewable energies into the electricity

grid. The major beneficiary of these storage systems is wind energy; storage solves most of the

problems associated with wind generation (Mitchell and Mitchell Dell 2001). The electrical

energy storage facilities are in high demand by the electricity-producing industries because

unlike other commodities in the commercial market, the conventional industries generating

electricity have little or no other storage facilities. Thus, with the availability of the storage

facilities, system planners would only need to construct sufficient generating capacities that

would meet the electricity demands as opposed to only the peak demands (Ku 1995). With this

brief explanation of the energy storage and with the help of the guide, extensive research can be

carried out with the need for storing energy through hydroelectric methods.

The real point of the benefit of Hydroelectricity is the disposal of the expense of fuel

(Mahlia et al. 2014 p. 532). Because of the unsteady geopolitics circumstance on the planet, the

costs of conventional fossil energises get to be vacillated (Poonpun and Jewell 2008). In the case

individuals use Hydroelectricity, no imports are required. As indicated by World Watch Institute

(2012), the conventional expense of electricity from a hydro project bigger than 10 megawatts is

$0.3 to $0.5 USD pennies for every kilowatt-hour. Again, the hydroelectric plants possess long

financial lives, with fewer projects still in administration for more than 50 to 100 years. The

work expenses of working the plant are low, as plants are mechanised and have a few staff

nearby with a specific end goal to screen the operation. Additionally, the storage facilities offer

considerable benefits such as load following, peak power, and standby reserves (Eyer, Iannucci,

and Corey 2004).

On the other hand, a crucial benefit of utilising Underground Pumped Hydroelectricity

Energy Storage is, it can decrease the outflow of carbon dioxide (CO2). A few individuals

asserted that CO2 is still created amid the assembling and development of the undertaking (Luo

et al. 2015). To add to this, a study led by European Commission (2005) expressed that

hydroelectricity delivers a minimal measure of greenhouse gasses and externality of any vitality.

In this manner, we may infer that Hydroelectricity is the cleanest vitality on the planet (Tong,

2010). There are however a few drawbacks of utilising Hydroelectricity. The primary hindrance

is that hydroelectric tasks can be problematic to encompassing amphibian biological systems

both upstream and downstream of the plant sites (Tietjen 2007). The era of hydroelectric force

changes the downstream environment (Zhou, Ang, and Poh 2006). Water leaving a turbine

contains next to no suspended silt, which can prompt scouring of informal stream lodging of

riverbanks (Yüksel 2010 p. 468).

The storage technologies occur in many forms for instance, electrical in the form of

batteries, mechanical in the form of flywheels, chemical energy in the form of hydrogen,

potential in the form of pumped hydroelectricity, and thermal in the form of phase change

materials (Farid et al. 2004). However, the economics of these storage facilities depend on their

capital costs and the price arbitrages (Eyer and Corey 2010). Thus, choosing the right electrical

energy storage facility that is more efficient and cost-effective is crucial for electricity generating

companies (Barnes and Levine 2011 p. 14).

In this dissertation, the emphasis was put on the two types of pumped-storage systems,

that is, the Conventional PHS and the UPHES systems as they are ideal for large energy storage

(Steffen 2012).

In short, the rationale for the electrical energy storage system is grounded on the ability

of the storage facility to provide a storage surplus of energy that would otherwise go to waste for

use in future (Kumar and Radhakrishna 2008). It would also help in arbitraging whenever the

storage occurs during the times of low energy prices, making energy to be discharged at times of

higher prices (Rehman Al-Hadhrami and Alam 2015). Thus, it would also provide the electricity

system services that help in the maintenance of the stability of the system such as load levelling

to assist in correcting the imbalances in the supply and demand of electricity (Connolly 2010).

Therefore, when properly designed and integrated into the electricity grid, the storage facilities

have the ability to smoothen the variability and enable the dispatch of electricity in the future

(Bradbury, Pratson, and Patiño-Echeverri 2014).

2.7. TYPES OF ENERGY STORAGES

2.7.1. Pumped Hydroelectric Storage (PHS)

Pumped hydroelectric energy storage (PHS) is known to be a more mature and the

biggest storage technology available. PHS refers to a kind of energy storage technology utilised

by the electrical power systems for balance load (Perich 2007). This technique stores electrical

energy as gravitational potential energy of pumped water from a lower raised reservoir to a

higher raised reservoir (Barnes and Levine 2011 p. 18). Pumped hydroelectric storage (PHS) is

viewed as one of the most developed storage technology for utility-scale electrical energy and

has been implemented commercially since the 1890s (Steffen 2012). PHS has been serving as a

stabiliser of the electric grid through shaving of peak, balancing load, regulation of frequency,

generation of the reserve (Perich 2007). It represents one of the oldest and large-scale energy

storage technologies (Barnes and Levine 2011 p. 29). Since the 2000s, there have been needs to

establish PHS devices all over the world (Van Dorp 2009). The need came as a result of the low-

carbon resources for electrical energy, for instance, the wind, nuclear, and solar resources are not

able to flexibly adjust their energy output to meet the demands of the fluctuating electrical

energy needs (Scieri, Miller, and Miller Richard 1984). Compared with any other storage

technology, the PHS has the largest storage capacity (Steffen and Weber 2013). According to

Inage (2009), Pumped Hydroelectric Storage is presented in the following ways, which include

the following:

Store it in hydrogen form

Geothermal

Flywheel

2.7.2. Flywheel Energy Storage (FES)

The operation of flywheels is not very complicated, but one that contains a certain

amount of mass, which rotates to hold kinetic energy (Siostrzonek, Piróg, and Baszyński 2008).

This type of storage technology has been used since the 1950s (Luo et al. 2015). The capacity of

a flywheel can be utilised as a standalone energy storage system accompanied with a distributed

generation asset or in certain instances, with a hybrid configuration of another medium of energy

storage facilities such as batteries (Mitchell and Mitchell Dell 2001). Additionally, the energy

stored through the acceleration of the rotor and the maintenance of the energy in the system at

very high speed is in the form of inertial energy (Graditi et al. 2014). Thus, considering the

speed, this storage technology can be disintegrated with high speed and low speed (Connolly

2010). The high-speed FES offers long term storage whereas the low capacities provide shorter

periods of energy storage (Siostrzonek, Piróg, and Baszyński 2008). The flywheel technology is

regarded as a perfect energy storage equipment because of its low maintenance costs, long

lifespan, and a higher efficiency that is free from discharge effects, environmentally friendly, a

wide range of operational temperature and can persist in harsh conditions (Siostrzonek, Piróg,

and Baszyński 2008). Also, the ability of the flywheels to attain rapid cyclic recharges and

discharges suit them in power quality applications (Luo et al. 2015).

2.7.3. Storage in form of Hydrogen

The energy hydrogen-based energy facilities are among the PHS promising storage

technologies that have experienced more attention (Schoenung 1999). Essentially, hydrogen is

known to be an energy vector implying that it has the potential of being used as a portable fuel at

the point of generation that can be decoupled with the point of utilisation (Mitchell and Mitchell

Dell 2001). It is important to note that hydrogen systems are distinct in comparison with other

storage facilities since they make use of two distinct processes during the course of charging and

discharge of the storage system (Schoenung 1999). The hydrogen storage and energy production

usually incorporate electrolyser units that separate water into hydrogen and oxygen by using

electricity (Yüksel 2010 p. 468).

Furthermore, the hydrogen that is compressed is stored in high-pressure tanks

(Schoenung 1999). The low costs involved in the hydrogen storage technology have the potential

of presenting the best option for the renewable energy integration (Connolly 2010).

2.7.4. Geothermal energy storage

Once again, larger storage volumes and longer storage duration can be accomplished

through storage of hot or cold water underground. In this storage technique, underground water

is usually obtained from the layer and later injected back at various temperatures at separate

localities. Under the thermal form of storage technology, even boreholes can be used to help in

storage and exchange of heat. Thermal energy storage is beneficial because of its lower costs, but

the losses are always increased considerably at higher energy densities while the materials to

maintain and tolerate repeated cycling to higher temperatures are unavailable.

Again, the thermal form of electrical energy storage systems can be utilised to raise the

flexibility within the storage system (Hasnain 1998). Unlike the hydrogen storage system, the

thermal system only joins the electricity and heat segments with each other (Hasnain 1998).

Therefore, through the introduction of a district heating into energy systems, the electrical energy

and heat may be offered from a similar storage system to systems of energy making use of

Combined Heat Power (CHP) facilities (Boyle, Everett, and Ramage 2003). Thus, bringing an

added flexibility to the system that allows large penetration of the recurrent renewable energy

resource (Barnes and Levine 2011 p. 36).

2.8. How PHS Operates

In the actual sense, the PHS facilities are not meant to generate electrical power, but

rather to store electricity to be quickly released when it is demanded (Alamri and Alamri 2009 p.

2). The operation of a PHS facility requires two reservoirs, in which one is placed at a high

altitude end and the other at a lower altitude (Luo et al. 2015). Typically, a PHS facility is

equipped with generators and pumps that connect the upper and lower reservoirs as shown in

figure 1. These pumps make minimum use of the electrical energy from the power at times of

off-peak to pump water from lower reservoirs to the upper reservoirs so as to store the electrical

energy (Lund et al. 2015). The production and pumping of electricity can be done either through

a single unit, reversible pumping turbines or separating the pumps and turbines (Scieri, Miller,

and Miller Richard 1984). The changes in mode occur between pumping and the generation of

electricity might happen within periods of minutes, and can go up to more than 40 times a day

(Mitchell and Mitchell Dell 2001).

It is crucial to note that the quantity of electrical energy that can be stored in the PHS

installations depend upon the volume capacities of upper reservoirs and the difference in the

altitude or “head” between the upper and the lower reservoirs (Robert 1933 p. 905). The lower

reservoir must be big enough to be able to fill the upper reservoir (Steffen and Weber 2013).

Thus, the energy that is in the upper reservoir is stored in the form of gravitational potential

electrical energy. The gravitational potential energy is proportionate to the altitude and the

water’s volume is represented by U=VgH; in this case, U represents the potential energy,

represents the water’s density given by 1,000 kg m-3 (Steffen and Weber 2013). V represents the

volume of water; g represents the acceleration as a result of gravity while H represents the

differences in heights between the lower and the upper reservoirs (Eyer and Corey 2010).

Also, it was noted that that the volume of the water is limited by the capacities of the

upper reservoir whereas H is assumed to be invariable (Steffen and Weber 2013). However, it

varies slightly when the upper reservoirs are emptied, while the lower reservoir is filled or vice-

versa, and to some extent, the height H is limited by geographical factors that must enable

smooth construction of the upper reservoir at considerably higher altitudes of 100m and above

(Medina et al. 2014).

On the other hand, the reservoirs must not be too far from each other to allow the

connection by a relatively shorter run of tunnels because a long tunnel run would prove to be

expensive to construct and would also be uneconomical as it would waste excessive energy via

the friction of the pipe (Yüksel 2010 p. 468). Therefore, the normal requirement of a

conventional PHS facility installation is an area where it is possible to build large reservoirs

separated by steep slopes of more than 100m in height from naturally or constructed lower body

of water (Barnes and Levine 2011 p. 38). It implies that water must be available in large

quantities, and the area should not encroach on protected lands (Yüksel 2010 pp. 469).

During the peak hours, water is allowed to flow from the upper reservoirs to help in the

generation of power at higher prices (Mitchell and Mitchell Dell 2001). At times when electricity

is required on the electricity grid to meet the increasing demand for electricity by the users, the

water that was initially pumped into the upper reservoir is allowed to flow back down via the

turbines that power the generators to establish electricity. The electricity is then fed back into the

electric grid (Barnes and Levine 2011 p. 55). However, the losses of electrical energy during the

process of pumping makes the PHS facility a net consumer of electric power in general, but the

system can increase the revenues through selling more electrical power during peak periods, at

the time when the prices are higher (Bradbury, Pratson, and Patiño-Echeverri 2014).

Notably, a PHS facility has 200-300 m of hydraulic head. Again, it is important to note

that there are two types of PHS facilities (Wilson, McGregor, and Hall 2010). These include pure

PHS facility, which is also known as off-stream, and it wholesomely depends on the water that

was initially pumped into the upper reservoir to act as the source of energy (Steffen 2012). This

type of PHS is in other instances known as closed-loop system. However, in some instances,

others may use the term closed-loop system to refer to a system that is entirely separated from

the natural ecosystem. For instance, the U.S. Federal Energy Regulatory Commission defines the

system as a project that is not constantly connected to any natural flowing waters. A model of the

PHS is shown in the figure below.

Figure 3Typical design of PHS facility

The second type of PHS facility is the combined, hybrid, which is also referred to as

pump-back, and uses both the natural stream flow water and pumped water in the generation of

power (Steffen 2012). The figure below represents the conventional PHS facility constructed

along a river.

Conventionally, PHS may use an open sea as the lower reservoir as it is applicable in

areas which are geologically appropriate, that is, areas that have high-elevation differences

(Barin et al. 2009a). In some instances, it can be expensive to construct a PHS facility because of

the absence of a natural vertical difference, and thus an artificial vertical difference must be

constructed (Uddin 2003 p. 335). There are also variations in the conventional PHS based on the

utilisation of an adjustable or a variable-speed turbine (Ku 1995). The adjustable-speed PHS

plant is usually constructed based on turbine and motor generator, which operates over the range

of a rotation speed, which is 10% efficiency beyond and below the nominal efficiency (Gulliver

and Arndt 1991). The speed variation is normally attained through the utilisation of a solid rotor

with three-phase winding, which can then be driven at variable frequencies (Luo et al. 2015).

Here, a cyclo-converter transforms the AC power into recommended frequencies through AC-

DC-AC converter (Farid et al. 2004).

Further, the stator in a normal PHS plant is connected directly to a three-phase bus that is

energised at line frequencies (Carrara and Marangoni 2014). The nominal +10% change in the

rotation frequency significantly impacts the performance of the facility (Farid et al. 2004). A

20% variation in the velocity of rotation translates into a 50% change in the output of power

since the mass flow is also associated with the third power of the speed of the rotation. The

adjustable-speed PHS plants have many benefits (Eyer, Iannucci, and Corey 2004). They do not

require pony motors to start the pumping of water; the synchronous power operations extend to a

wider range than for the normal turbines, ranging from approximately 70% to a full power

capacity (Barin et al. 2009a). Again, the changing rate of the output of power is driven by a mass

flow that is associated with the third power of the speed of rotation (Luo et al. 2015).

Additionally, the full output of power can be successfully delivered from water-variance of a

factor of two (Ess et al. 2012).

Furthermore, the speed of rotation is capable of being adjusted to avoid resonances within

the facility and cavitation modes occurring in the water flow, thus resulting in a longer lifespan

and reduced maintenance (Denholm 2013). There is also a higher level of the overall efficiency

that can go as high as 0.3 enhancements on an annual basis (Zakeri and Syri 2015). Currently,

PHS systems are the largest-capacity type of power grid energy storage technology at present

and represents more than 99% of the bulk of the energy storage capacity in the world (Steffen

and Weber 2013). This capacity represents approximately 127GW and spread all over the world

in more than 300 installations (Robert 1933 p. 910).

2.8.1. Underground Pumped Hydroelectric Energy Storage

Currently, the German Scientists and engineers are working on the feasibility of a

particular type of PHS storage facility known as Underground Pumped Hydroelectricity in which

coal or salt mines are acting as the lower reservoirs (Allen, Doherty, and Kannberg 1984). This

storage facility makes use of similar methods of operation as a normal PHS facility. The only

difference between the PHS and the UPHES comes in handy concerning the location of the

reservoirs (Connolly and MacLaughlin 2010).

Again, under PHS, correct geological formations and suitable fields are essential factors

in the construction of a PHS facility (Gulliver and Arndt 1991). However, the UPHES storage

facility can be constructed on a flat terrain depending on the accessibility and availability of the

underground water reservoir deep underneath the surface; and the availability of an upper

reservoir on the ground levels (Allen, Doherty, and Kannberg 1984).

During the construction of a UPHES, there are large capital costs involved that depend on

the local topographical and the direct environmental damages that act as the major challenge of

the UPHES plant (Perich 2007). The UPHES is intended to reduce the environmental impact

because many of the interventions would be installed underground (Oberhofer and Meisen

2012). Thus, depending on conditions, the upper reservoir might be built in the caverns levels

(Allen, Doherty, and Kannberg 1984). It can be viewed as the solution to the energy storage

crisis. However, the practicality of the design and the actual construction of bigger UPHES poses

as a challenge (Yang and Jackson 2011 p. 8340). Small UPHES might be easily built,

particularly in the case where the prevailing underground and surface waters can be utilised

(Tietjen 2007). To date, there is no large UPHES storage facility that has ever been installed and

put into use levels (Allen, Doherty, and Kannberg 1984). Thus, it is essential to take into

consideration the design of the UPHES systems, including both the small and the large ones. The

figure below shows the design of a UPHES storage facility.

Figure 4 An Estimate proposed size of a desirable UPHES System

The proposed UPHES is considered the most suitable large energy storage system that

can eradicate the problems encountered with the PHS installations (Barin et al. 2009a). The

reliance on the surface topological structure is alleviated, even though the suitable underground

geological structure is a requisite for the construction of the UPHES plant levels (Allen, Doherty,

and Kannberg 1984). The underground system possesses vertical water flow paths that

significantly decreases losses that are related to the transverse water flows (Ess et al. 2012).

Additionally, the environmental impact of the UPHES is minimal as compared to those of the

conventional pumped hydroelectricity since only a single surface reservoir is needed (Perich

2007). Again, the UPHES eradicates new river dams and the big powerhouse on the surface as

opposed to the PHS while minimising the wildlife habitat interference as well as reducing the

noise during the generation of power (Gulliver and Arndt 1991).

Additionally, considering the economic cost of constructing the facility, it is noted that

the operation of a UPHES plant relies on the excavation of the deep shaft that demands

surprisingly little investment costs (Fertig 2014). This circumstance is due to the lower

excavation per storage capacity as compared to PHS facility and is also highly automated

(Steffen and Weber 2013).

Figure 5 Typical Design of UPHES facility

2.9. The Description and Operation of the UPHES

The UPHES requires a surface water reservoir that will contain the water that will be

pumped up from the aquifer until all of it is used (Martin 2007 p. 77). This water that is pumped

and held at the surface acts as the stored potential energy concerning the aquifer (Martin 2007 p.

77). This stored energy can be converted into electricity through the turbine generators or in

some instances being partially allocated to other uses for instance in agriculture (Connolly and

MacLaughlin 2010). The UPHES being an adaptation of the conventionally surface-pumped

hydroelectricity, which utilises underground water structure as the lower reservoirs (Perich

2007). The lower reservoir is the core of the UPHES and can be excavated from geological rocks

located at different depths or sometimes it can tap the prevailing aquifer or any naturally existing

underground water structures levels (Allen, Doherty, and Kannberg 1984). The UPHES can be

classified as small with roughly 10kW to 0.5 MW and large with approximately 0.5MW to 3000

MW installations (Robert 1933 p. 921). It is important to note that the large systems are usually

the ones targeted to mitigate the changes in loads of primary urban centres to enable the variable

renewable energy to become more consistent (Slocum 2010). The UPHES have installations

ranging between 1000MW and 3000MW that are regarded the most economical scale that is

based on the varying load supplied to consumers by the conventional plants fired by coal

(Oberhofer and Meisen 2012).

The small UPHES installations usually serve single users, smaller communities, the

agricultural sector or even industrial operations levels (Allen, Doherty, and Kannberg 1984).

These installations can make use of the existing underground water structures and consume no

net water except the water loss through evaporation (Barin et al. 2009a). However, optimising

the smaller UPHES installations requires considerations or costs of electricity, the formations of

the geological structure, and the characteristics of the water table, the available infrastructures,

the load user profile, and the accessibility and availability of the renewable energy sources

(Lombardi, Vasquez, and Styczynski 2009).

The underground aquifer pumped hydroelectricity storage technology is normally

designed for the storage of electrical energy as gravitational potential energy in the separated

water from surface reservoirs and subterranean aquifers levels (Allen, Doherty, and

Kannberg1984). The energy storage is managed through the pumping of water from the

underground sources into the surface reservoirs for energy storage (Barin et al. 2009a). The

stored energy is later on recovered through the release of the surface stored water back to the

initial water source via turbines that generate electricity (Scieri, Miller, and Miller Richard

1984). The UPHES storage system is found most suitable when utilised in tandem with a

variable renewable energy source, for instance, the wind and the solar photo voltaic since energy

acts as a buffer to the variable output and can be relied upon to supply an on-demand power for

the user loads (Becker et al. 1999).

2.9.1. The Design of the UPHES Facility

Conventionally, larger systems require the excavation of lower reservoirs from the

suitable geological strata. In most cases, it is suggested that hard rock, for instance, granite can

act as lower reservoirs bed for suitable structural characteristics for installation (Olsen et al. 2015

p. 56). Also, the solution mining that establishes larger underground caverns has also been seen

as a possibility of creating the UPHES facility (Olsen et al. 2015 p. 57). About its construction, a

larger lower reservoir would be excavated in a network of extruded, narrow caverns as opposed

to a single larger cavern (Becker et al. 1999).

The accessibility, availability and the ability of the water turbines and pumps, which have

the capability of operating at extremely higher pressures, are critical as they determine the

decision of how deep the lower reservoir can be built (Gulliver and Arndt 1991). Additionally,

designing a trade-off for the depth of the reservoir must be taken into consideration since as the

depth increases; the water volume that is required to generate a similar amount of power

diminishes (Scieri, Miller, and Miller Richard 1984).

Then again, larger UPHES require that the main power stations to be situated below the

lower reservoirs to alleviate cavitation challenges, which could lower the lifespan of the UPHES

storage device (Smith, SenSr, and KroposkiSr 2008). Additionally, the underground power

stations call for a safer personnel access to considerable depths beneath the surface of the earth

(Ramos, Amaral, and Covas 2014 p. 1099). However, there are major barriers to the

implementation of a UPHES facility, which include difficulties in the excavation of a large scale

UPHES, lack of stable water reservoirs located at considerable depths below the earth’s surface,

presumably from the hard subterranean rock (Connolly and MacLaughlin 2010). The UPHES is

environment-friendly because the underground power station is not an amenable location for the

settlement of humans and thus, is remotely operated (Becker et al. 1999).

Under UPHES, an essential parameter for optimising the design includes the hydraulic

head, the flow capacity, and the electrical efficiency of the system (Lombardi, Vasquez, and

Styczynski 2009). However, the parameter of interest, in this case, is the flow capacity, that is,

the flow that can be re-injected into the aquifer, and not the flow that can be pumped out or

produced from the system (Martin 2007 p. 77). It is crucial to note that the core of the aquifer of

a UPHES system is an integrated pump turbine and motor generator unit (Connolly 2010). This

unit plays the role of pumping water utilising electrical energy and generates electricity from the

water power (Scieri, Miller, and Miller Richard 1984). This kind of integrated equipment exists

commercially for the larger UPHES installations and in most cases, deploys the Francis reaction

type of turbines combined with asynchronous AC electric device (Allen, Doherty, and Kannberg

1984).

On the other hand, a unit size and design for a conventional UPHES system is not

currently available commercially (Scott 1977 p. 21). Thus, an option designed for the aquifer

UPHES pump turbines involved the utilisation of a standard centrifugal or vertical turbines well

pumps in the forward directions for the pumping and reverse for the operation of the turbines

(Martin 2007 p. 77). In many instances, the motor centrifugal pumps are mostly utilised to pump

water. They exist in submerged or in non-submerged designs having a wide range of available

head ratings, flow rating, and the power rating for the versions available commercially

(Hadjipaschalis, Poullikkas, and Efthimiou 2009 p. 1522). The units are usually centrifugal or

vertically designed turbines that are integrated with AC induction motors (Connolly 2010).

CHAPTER THREE: - METHODOLOGY

This section explores the methods of determining the effectiveness of a pumped electricity

storage facility through determination of the parameters of energy storage devices. The methods

involve the determination of the efficiency, response time, Round-Trip efficiency, energy

capacity and power capacities.

3.1. Power capacity

The power capacity of a storage facility is the maximum instantaneous output the plant

can generate, and the measurement is kilowatts (kW) or megawatts (MW). A conventional

storage device should, at least, possess a power capacity ranging from 1kW to hundreds of

megawatts and a capacity of storage of a proximately 1 to 3 hours (Smith, SenSr, and KroposkiSr

2008). Additionally, a storage facility that is utilised with a renewable energy technology should

possess a power capacity that ranges from 10 kW to 100MW, perfect recycling abilities and a

greater lifetime of 100 to 1,000 cycles annually(Steffen and Weber 2013).

The capacity of power is also a function of the rate of flow and the hydraulic head,

whereas the storage capacity is a function of the volume of the reservoir and the hydraulic head.

Therefore, calculating the output of power of PHS storage facilities require that the following

relation is utilised:

Pc = gQHn

In which case:

PC = power capacity measured in Watts (W)

ρ = mass density of water measured in kg/m3

g = acceleration as a result of gravity in m/s2

Q = discharge through the turbines as measured in m3/s

H = effective head measured in m

n = efficiency of the facility

3.2. Storage capacity

The storage capacity is a term referring to the amount of energy that is available in the

storage devices after the completion of charging cycles. The storage capacity is measured

regarding the total energy that is kept in the storage capacity. The energy that is stored by the

UPHES must be higher than the useful energy at a given point of operation.

The following relationship provides the storage capacity of the PHS:

Whereby:

SC = represents the storage capacity in megawatt-hours (MWh)

V = the volume of water that is must be drained and filled every day in m3

ρ = mass density of water in kg/m3

g = acceleration due to gravity in m/s2

H = effective head in m

n = efficiency

3.3. Round-Trip Efficiency

From the above relationships, it is clear that the storage and power capacities depend on

the volume and the hydraulic head of the reservoirs. The round-trip efficiency is given by = c.

d, where c represents the charging efficiency that is, the efficiency with which electrical energy

that is received by the PHS facility is converted to potential energy in the upper reservoir of a

PHS, and d represents the efficiency of the discharge (Eyer and Corey 2010). In some instances,

the round-trip can be calculated as follows. ¿EoE1 , where is represented the round-trip

efficiency, Eo represents the output of electricity in kWh and E1 is the electricity input also

measured in kWh. Overall, it is also critical to take note that all the parameters except the

efficiency parameter deviate from -30% to +30% accounting for the uncertainties in resources of

data (Center 2009). The changes in efficiencies are limited to 10% because any increase beyond

this value would result in some technologies having more than 100% efficiencies, and this is not

practically possible (Sunderkötter and Weber 2012).

3.4. NEplan Simulation.

Neplan is a power systems analysus software used in by professional and engineers to

simulate a design or a network before it is been incorporate to the real life experience, it is

simply refer to as a powers system analysis, they software or program can be used to quickly to

resolution, as well as used to decide the non-linear RC prolblems

, they were used to quickly solve the non-linear load flow problem and calculate short circuit currents, but their use has been extended to many other areas such as power system stability, protection and coordination, contingency / reliability, economic modelling, etc.

This article provides a list of the most common software packages used for power systems analysis, and surveys both commercial and non-commercial software (listed in alphabetical order by vendor name).

3.4. AHP Analysis

The AHP analysis was proposed by Saaty and is based on a pair wise comparison (Saaty

1990). It makes use of a subjective assessment of the relative significance that is changed into a

series of the overall score or weight, thus making classification of the structure of the issue under

consideration in a hierarchical manner (Graditi et al. 2014). The utilisation of an AHP as the only

decision supporting tool could become a problem mainly because the technique might, in some

instances, overlook the association that exists between the values and the adjustments

(Diakoulaki and Karangelis 2007). The AHP method uses the consistency index to determine the

consistency ratio because it does not allow the acceptance of priorities in case the levels of

inconsistency are high (Zhaoyu, Shengzhu, and Hong 2010). A consistency ratio of 0.10 or

below is accepted meaning that the consistency is regarded as a b issue of concern if the

consistency ratio goes beyond 0.1, and in such a case, the pair wise comparison must be

reassessed (Diakoulaki and Karangelis 2007). However, it is important to note that before AHP

is applied, one should choose the criteria and the alternative to be included in the hierarchy

(Kintner-Meyer et al. 2012).

Additionally, it is important to take consideration of the real database for a particular case

considered in the analysis (Hobbs and Meier 2012). Again, the criteria and the alternatives may

be then utilised to build the pair wise comparison matrix (Barin et al. 2011). Thus, the weights of

the criteria must be approximated, and it is performed through the measurement of AHP, basing

on the Saaty’s theory (Diakoulaki and Karangelis 2007). In this case, the alternatives are the

various storage technologies under consideration (Saaty 1990).

According to Saaty (1990), the criteria for comparison are listed in the table below.

Table 1 AHP Comparison Criteria

Weight Comparisons Explanation1 Equal Two activities contribute equally to the objective to be achieved.

3 Moderate (weak)

Experience and judgment slightly favour one activity over another activity.

5 Essential (strong)

Experience and judgment strongly favour one activity over another

7 Very strong An activity is favoured very strongly over another; its dominance demonstrated in practice.

9 Absolute (extreme)

The evidence favouring one activity over another is of the highest possible order of affirmation.

The AHP method is always used to determine the flexibility of the storage facility for the

smart grid operation (Denholm 2013). The normal reserve sources have different characteristics

and distinct natures (Lund et al. 2015). It therefore means that, determining the best alternative

storage system requires that, the flexibility index of the system must be measured to assess the

appropriateness of the system in converting CPV power shortages (Kintner-Meyer et al. 2012).

Notably, the flexibility of the technologies for storage vary concerning the unlike aspects. Some

storage systems may be used to follow the changes in power in CPV power (Zhaoyu, Shengzhu,

and Hong 2010). Other systems may be used to smoothen power fluctuations in small durations

while enhancing the quality of power of the storage system. Therefore, the AHP diagram will be

used to design the flexibility metric tool of the storage systems (Lund et al. 2015). It should be

noted that each criteria source of flexibility has weights, which share the flexibility index that is

proposed (Barin et al. 2011). The proposed flexibility tool ranks all the alternatives from the

most flexible to the least flexible system for the real-time operations (Zhaoyu, Shengzhu, and

Hong 2010).

The AHP in measuring the flexibility of the storage facility is constructed while

considering criteria, which have their own flexibility measurements (Hobbs and Meier 2012).

While making the decision of determining the criteria according to their impact on flexibility,

Saaty (1990) states the choices must be decomposed into the steps below.

• Defining the problem and finding the available specification that is known and

building the hierarchy's structure from the objective through the main criteria so as to

lower the alternates’ levels.

• Constructing a set pair wise comparison matrices while considering each criterion as

well as considering each criterion to the main goal.

• Using the priorities to weigh each of the alternatives and each criterion.

• Ranking the alternatives concerning the goal to assess up on the existing alternatives

and their weights.

3.4. Decision Tree Diagrams

The Decision Tree Diagrams (DTD) have played the role of addressing the differences

between energy-related needs in the field while making sure no single energy technology is

recommendable for use in all humanitarian settings (Diakoulaki and Karangelis 2007).

Therefore, these diagrams represent a vivid means of assessing which factors influence the

choice of energy storage strategy for individual contexts, based on a simple reaction to a series of

questions about the local priorities, accessibility, availability and many other related factors

(Fertig 2014). This method recognizes that many contexts might demand more than one type of

energy storage technology, particularly over the long term period. Thus, recognising that the

short-term and the long-term energy strategies could, by necessity, distinguish that the diagrams

cover two phases include acute emergency and the protracted settings (Farid et al. 2004). The

protracted settings Diagram is meant for every field-based stakeholder having the role of

determining long-term energy strategies, and because of that, offers guidance on the inter-

linkages between various considerations and the cross-sectoral of every strategy.

3.5. Analysis

Table 2 Criterion for selecting Power Quality

Criterion Criterion of Power Quality—C.R. = 0.0399

UPHES PHS H2 FLY

UPHES 1.00 1.00 0.14 0.20 0.20PHS 1.00 1.00 0.14 0.14 0.20H2 7.00 7.00 1.00 1.00 3.00FLY 5.00 7.00 1.00 1.00 3.0

The table above shows the consistency ratios determined through the simulation of the

AHP techniques as considered for each and every power capacity measurements. As suggested

by Saaty, the outcomes of consistency ratio must be below 0.1 (Saaty 1990). The relative weight

is calculated by summing the data from each column. Every individual data of the capacity of

power measurement is further dived by the summation of the column where the data is inserted.

The storage alternatives above are subjected to the following qualitative criteria.

• Power quality (PQ)

• Storage capacity (SC)

• Round-Trip efficiency (RTE)

• Power capacity (PC)

• Response time (RT)

• Cost of constructing the technology

Table 3 Storage Alternatives Subjected to Fuzzy Logic

RTE

PC SC COST

RT PQ RW

RTE 1.00

1.00

3.00

7.00 5.00

0.33

0.20

PC 1.00

1.00

3.00

7.00 5.00

0.33

0.20

SC 0.33

0.33

1.00

3.00 3.00

0.14

0.08

COST

0.14

0.14

0.33

1.00 0.33

0.11

0.03

RT 0.20

0.20

0.33

3.00 1.00

0.14

0.05

PQ 3.00

3.00

7.00

9.00 7.00

1.00

0.44

3.6. Classification of the storage technologies in AHP

Table 4 Classification of Storage Technologies in AHP

Scenario. Classification in AHP—power qualityRTE LM TM COST PQ RW CL

UPHES 0.01 0.02 0.02 0.00 0.00 0.057 3rd

PHS 0.01 0.02 0.02 0.00 0.00 0.055 4th

H2 0.00 0.05 0.00 0.00 0.00 0.148 2nd

FLY 0.04 0.01 0.02 0.01 0.01 0.167 1st

The above table shows clearly the rankings of the storage technologies, with UPHES

ranked above PHS. This classification also made use of Fuzzy Logic. The Fuzzy logic was first

brought up by Zadeh Fuzzy and is regarded among the most effective methodologies that

incorporate many fields of appliances (Barin et al. 2009b). The testing of Fuzzy logic is done

under multi-rule based decisions that incorporate a collection of item propositions under

consideration (Diakoulaki and Karangelis 2007). There is a system known as Mamdani fuzzy

system that basically accept numbers as inputs while translating the input numbers into linguistic

terminologies, for instance, low, medium, and high (Barin et al. 2009b). The Fuzzy logic leads to

the ultimate ranking of the technologies as indicated in the table above.

Table 5 Ranking of storage Alternatives using Fuzzy Logic and Classification Criteria

Criteria

UPHES

PHS

H2 FLY

RTE 85 80 55 90Costs 450 75

012

0040

0SC 0.65 0.6

00.8

00.4

0PC 0.85 0.8

50.5

00.8

0

RT 0.55 0.75

0.80

0.90

PQ 0.40 0.40

0.85

0.80

From the table above, the AHP analysis puts the UPHES as the best storage alternative

considering its performance and characteristics. The Round-Trip efficiency of UHPES is greater

than that of the PHS system. Even though PHS has the greatest reaction time between the two,

the UPHES are still preferable because their costs are low, and the storage and power capacities

are much higher than those of PHS Systems (Center 2009). However, in case one would want the

technology with the greatest RTE, RT, and PQ, one would go for the Flywheel (FLY) followed

by the hydrogen (H2) storage technologies.

3.7. Decision Tree Diagram for selecting the best storage technology

From the decision tree diagram, the following framework was established to determine

the storage technology with the most desirable performance features.

Figure 6 Decision Tree Diagram for Selecting Best Storage Technology

The Decision Tree Diagram analyses the sustainability and acceptability of the UPHES as

compared to other storage technologies. Before selecting any technology, it is important to assess

all available technology and make a comparison. The comparison then narrows down to the two

technologies under consideration (Kintner-Meyer et al. 2012). The Tree Diagram clearly

evaluates the procedures to implement a preferable technology between the two technologies

under consideration. While constructing the storage technology, the environmental laws and

regulations in place must be respected (Zhou, Ang, and Poh 2006). The technology with the least

environmental impact will not encounter much resistance as compared to the one that greatly

affects the environment (Ginocchio, Parker, and Sewalk 2007). From the tree diagram, UPHES

is favoured because it affects the environment the least.

Furthermore, UPHES can be built easily because only the upper reservoir needs to be

built in instances where there are no upper natural reservoirs such as dams (Van Dorp 2009). The

UPHES are also found good because conducting safety and awareness is not easy.

CHAPTER FOUR: - RESULTS AND DISCUSSION

This section involves an in-depth study of the operation, benefits, merits and the demerits

of each technology and ending with a concluding remark about the reliability of the technologies

and the most favourable alternative to large energy storage.

4.1. Efficiency of PHS

PHS’s efficiency varies considerably as a result of the long technological history and

long lifespan of the facility (Zakeri and Syri 2015). The Round-Tip Efficiency (RTE) of the PHS

facility that is, the electricity that is generated divided by the electricity that is used for pumping

water, which has an older design might be below 60% whereas the state-of-the-art PHS systems

could attain beyond 80% efficiency (Scieri, Miller, and Miller Richard 1984). It is important to

note that a hydroelectric power needs a considerably larger volume of water to generate electrical

energy (Van Dorp 2009). Thus, Schoenung and Hassenzahl (2002) came up with the following

equation that relates the water’s volume V, in cubic metres; the energy stored E, in Kwh; and the

average head that drives a turbine h, in metres. It further assumes an efficiency level of 0.90

when converting energy to produce electricity. The equation is as follows:

V (m3) ≈ 400 * (E (kWh)/h (m))

Therefore, making use of the relationship, a reservoir having an approximate of 1 metre

in diameter, and an average of 200 metres, and having enough water to produce 10,00 MWh, can

then be filled up to a 25 metres depth. However, since these installations are large, they demand

a considerably intensive planning while considering the environmental and other related permits

to allow their installations (Zhou, Ang, and Poh 2006). It is essential to take note that the

efficiency of any storage facility is evaluated from the ratio between the energy released and the

energy stored.

4.2. Power and Storage Capacity of PHS

Presently, there is more than 90 GW in more than 240 PHS energy facilities worldwide

that are equivalent to approximately 3% of the generating capacity of the world. Any single PHS

facility’s capacity varies from the range of 30 MW to 4,000MW of the electrical energy that is

stored. The power capacities of PHS facilities represent the function rate of flow and the

hydraulic head, at the same time the stored energy becomes a function of the volume of the

reservoir and the hydraulic head. Notably, both the storage and power capacities depend on the

head and the volume of the reservoirs.

Additionally, the PHS facility must always be designed with the greatest possible

hydraulic head as opposed to a larger upper reservoir. The implication is that constructing the

PHS facility with a larger hydraulic head and smaller reservoirs are less expensive, considering a

facility of an equivalent capacity that has a smaller hydraulic head and larger reservoirs. This fact

relates to the nature of the smaller size of the storage device, pumps and the turbines including

the piping and the lower amount of materials that have to be eliminated to pave the way for the

reservoirs (Smith, SenSr, and KroposkiSr 2008).

Currently, Japan has installed the largest PHS facility with the highest capacity

worldwide, followed by the China and the USA. The table below shows the installations of the

PHS capacities of the major nations around the world.

Table 6 Storage Capacities for Major countries in the World

Country Installed PHS Capacity /(MW)Japan 27 438China 21 545USA 20 858Italy 7 071Korea, South 4 700United Kingdom 2 828Switzerland 2 687Taiwan 2 608Australia 2 542Poland 1 745Portugal 1 592South Africa 1 580Thailand 1 391Belgium 1 307Russia 1 246Czech Republic 1 145Luxembourg 1 096Bulgaria 1 052Iran 1 040Slovakia 1 017Argentina 974Norway 967Ukraine 905Lithuania 900Philippines 709Greece 699Serbia 614Morocco 465Ireland 292Croatia 282Slovenia 185Canada 174Romania 53Chile 31Brazil 20

4.3. Round-Trip of PHS

The efficiency of a PHS is dependent on the pump, turbine, motor and generator

efficiencies including the rates of evaporation. In most instances, the PHS have a round-trip

efficiency ranging between 65% and 75%, even though the round-trip efficiency of an improved

pump and high heads can achieve an efficiency of up to 76%. For instance, the Tianhuangping

pumped storage facility in China, located 175km from Shanghai, is one of the largest PHS

systems in Asia. The upper and the lower reservoirs are separated by a 560m, and each reservoir

can store up to 8 million cubic metres of water. The installation installed in 2001 has the

capability of producing 1.8GW while operating at 70% round-trip efficiency. There are also

other storage facilities of similar installations in other parts of the world.

4.4. Efficiency of UPHES

The operational efficiency of an aquifer UPHES storage technology is an essential

measure of how feasible the system is during its operation (Martin 2007 p. 77). A first order

efficiency of the turbine of a centrifugal pump is that it is similar to the efficiency of the pump.

Even though it was initially designed as a pump, a centrifugal pump has the capability of

operating in the reverse as a turbine at certain efficiencies ranging from 65% to 85%. This

methodology is proposed as the preferable option for the aquifer UPHES scenario since it makes

use of the existing technologies, is available commercially, and also represents a low-cost

problem-solving the solution (Schainker 2004 p. 2315). However, because of the difficulties

involved in predicting the performance of a particular centrifugal pump, testing the performance

is mandatory to characterise the capacity of the flow, the velocity of water range, and efficiency

of the turbine. The following figure shows the approximated UPHES system and component

efficiencies of a proposed UPHES facility in the UK.

Table 7 Power Capacity of a proposed UPHES Storage System

Component Low Target HighPumpingVFD pumping drive 94 95 97Power drive 96 98 99Motor 94 96 97Pump 60 70 75Pipe friction 96 97 98Total 49 61 68

GeneratingPipe friction 96 97 98Trurbine 70 80 85Generator 93 95 96Rectifier 95 97 98Inverter 94 96 97Total 56 69 76Round-trip efficiency 27 42 52

Efficiency (%)

In the above table, the turbine or the pump is seen to have the most impact on the UPHES

system’s efficiency. The electrical system components, which include the motor generators, have

considerably higher efficiencies. It is important to note that the Round-trip efficiency does not

impact the aquifer UPHES, and thus, is not a figure of consideration. However, the operational

turbine efficiency must always be considered since at the time of pumping the water, the energy

that would otherwise be unutilised is made use of for pumping the water. Thus, the cycle of

pumping can be viewed as free, and the cycle of generation can be considered as the efficiency

of merit for the aquifer UPHES system.

From the AHP, it was noted that in case the design for UPHES is followed correctly, and

then the UPHES would be a preferable option for the storage of renewable energy sources. The

UPHES technologies have potentially larger capacities and are also capable of storing excess

renewable energy for quite longer periods than the PHS facilities, thus making them the best

alternative storage technologies that can handle the challenge of intermittency of the renewable

energy sources. Concerning the load shifting reasons, the UPHES are projected to be the optimal

choice. Again, it is noted that the UPHES are the ideal storage technologies that produce quality

power, even though PHS provide the most appropriate time response when the two technologies

are compared. The UPHES have a higher power capacity and low levels of losses of power.

Further, the costs related to their construction are lower than those of the PHS, even though the

costs depend on the parameters that were subjected to this study and the type of application of

the storage system (Bin et al. 2014).

4.5. Comparison of the Advantages and the Disadvantages of PHS and UPHES

With due consideration of the operation and the performance of the two types of storage

technologies, the following advantages and disadvantages clearly emerged. The analysis of their

operation and performance helped the researcher to identify the standing features of these

technologies, and the one with the most outstanding performances features was recommended.

4.5.1 Advantages of PHS System

The PHS systems have the ability to protect the power system from the outages through

storage of electricity. It is noted that when combined with an advanced power electronic device,

the facility is capable of reducing harmonic alterations, and eradicate the sags and surges in

voltage transmitted. PHS systems offer an option to the peak power through storage of cheaper

base-load electrical energy and emitting it at peak-hours. It is considered as an alternative to

other kinds of generators such as the peak-load generators that generate electrical energy at

considerably higher cost than the load-base types.

Additionally, PHS systems are known to provide considerably larger capacities of

electrical energy using lower operation and maintenance costs, and thus, are considered more

reliable. The storage costs for electricity that are levelled using PHS systems are quite lower than

other electricity storage technologies. PHS is also known to be a net utiliser of power. It can also

assist in resolving the intermittency matters related to other renewable technologies when use in

combination with other forms of renewable technologies in pumping up the water back to the

upper reservoir (Carrara and Marangoni 2014).

PHS is important for quality power supply for the renewable and clean energy sources,

for instance, the wind, solar, biomass, wastes from municipal, wave, and tidal, which, by their

nature, are intermittent and thus are not able to generate and nameplate power capacities (Slocum

2010). PHS also helps countries to integrate the renewable energy variables for instance wind

and solar into the power grid through the provision of a dynamic reaction and to provide a

crucial back-up at times of excess power demands (Connolly 2010).

The PHS systems have large power and storage capacities and faster reaction times, and

therefore, usually identify load-levelling as the ideal applications. It is noted that the PHS facility

can have a reaction time as shorter as 10 seconds or even lesser from full reversal of operations

or shutdowns. It can also be noted that, the PHS facilities can be used as the ideal application

because of their large power capacities and their sufficient time of discharge.

4.5.2. Disadvantages of PHS Storage System

Throughout the years, there have been some worries about the natural effect of

hydroelectric plants. For one thing, a hydroelectric energy changes the stream of a waterway

both in front and behind the dam. This situation essentially changes the waterway biological

system. The dam hinders the ascent of transient fish. The turbines harm or murder of the fish

relocating downstream (Connolly and MacLaughlin 2010). Because of the change of stream,

oxygen levels in the water drop. These changes influence both plant and creature life in the

stream and along its banks (Zakeri and Syri 2015). The water level ascents and falls with vitality

use, driving amphibian and riparian plants to adapt to continuous changes in water level.

The extent of some more up to date undertakings; has raised various attentiveness toward

the general population of these locales according to Fogarty’s 2012 book on “Investing in the

renewable power market” (Ingram 2009). The dams are situated in a zone inclined to seismic

tremors, and disappointments in dams would be heart-breaking. Recent research demonstrates

that hydroelectric offices in tropical locales may deliver as much methane, a nursery gas, as

offices that create vitality from fossil powers (Fertig 2014). The reason: the deterioration of plant

matter. Some portion of the plant matter originates from the vegetation wrecked when the supply

is at first filled. Later, plants develop and disintegrate as the water level ascents and fell

regularly. This matter stays under scrutiny.

Also, the implementation of the PHS facility demands a terrain with considerable

elevation differences between the two reservoirs and the large amounts of water. Additionally,

the establishment of a PHS facility can take several years to complete, and on most occasions it

can take up to a decade. Again, even though the operation and maintenance costs of construction

are low as there is always higher upfront capital investment during the civil construction, which

may only be recovered over several years (Fertig 2014).

Furthermore, there are serious environmental concerns that have resulted into significant

cancellations of the proposed PHS facilities (Patocka 2014). Additionally, the blocking of the

natural water flows interferes with aquatic ecosystem and flood imparted on the initially dry

areas damage the terrestrial wildlife and considerably alters the landscape. The pumping of water

also increases the temperature we of water and stirs up the sediments found at the bottom of the

reservoir and damages the quality of water. An example of a PHS facility failure includes the

failure of the upper reservoir of Taum Sauk PHS energy storage plant in the USA, which acts as

a reminder of the potential damages of the facility (Eyer and Corey 2010). The Taum Sauk PHS

station was overfilled leading to its embankment being overtopped and breached. The

overflowed water is washed away to more than one square kilometre of the forest.

4.6. Core Advantages and Disadvantages of UPHES compared to those of PHS facilities

Underground Pumped Hydropower Energy Storage in general and their use, in particular,

has certain advantages over Pumped Hydroelectric Energy Storage (Wong 1996). These

advantages are clearly listed as follows:

The UPHES facilities are considered more efficient as they comply with the electrical

energy requirements of the current generation. They are built to meet the current energy

requirement specifications as opposed to the PHS facilities that were built many years ago, and

were thus, often furnished with outdated and technologies that are not efficient (Becker et al.

1999). The PHS facilities are becoming difficult to maintain because the existing PHS facilities

require upgrade and renovations to meet the energy requirements (Verma, Gambhir, and Goyal

2013 p. 2298).

Again, re-engineering of the PHS through adding lower reservoirs to help in pumping the

units back to help the upper reservoir at times of off-peak hours and then combining PHS

stations for the utilisation of intermittent energy from renewable sources included wind turbines

and solar panels (Gulliver and Arndt 1991).

Availability: the proposed UPHES facilities are considered as inexhaustible resources, as

long as the water cycle will last (Verma, Gambhir, and Goyal 2013 p. 2354). The facilities do not

pollute the environment in the same manner as in the proportion that is caused by the oil, coal,

and any other related technology that emit greenhouse gases or cause acid rain (Pickard and

Abbott 2012). Generally, it does not pollute the atmosphere, thus, there are no need to use

expensive methods to clean the emissions of greenhouse gases.

The UPHES facility preserves the ecosystem balance as the underground waters are not

inhibited and rarely pollute the waters used for human consumption and other usages such as in

irrigation. On the contrary, the PHS facilities interfere with the aquatic environment, threatening

the survival of the aquatic life resulting in an imbalance in the ecosystem (Pickard 2012 p. 473).

Work produced at room temperature: the proposed UPHES does not use cooling systems

or boilers that consume energy, which in many cases also contaminate the water. Thus, it is more

cost effective concerning the water storage for irrigation; it does not also interfere with the

recreational activities in the water resources such as rowing and swimming and avoid flooding to

regulate the flow of water.

Again, the comparison of costs of a classic PHS and conventional UPHES plants made

showed that economic realisation of the UPHES is both feasible and reasonable as compared to

PHS. The UPHES is more feasible because it has a low storage capacity cost that may allow

economic sizing to operate on a weekly cycle basis (Pickard and Abbott 2012). Additionally,

PHS facilities are more limited to topographical structure than the UPHES facilities.

4.6.1. Disadvantages of UPHES Storage Facility

The construction of dams, insurmountable obstacles: to establish a UPHES facility, a

dam must be constructed in places where there are no upper reservoirs. The construction is

another expense that would otherwise be avoided if a naturally occurring reservoir was existing

further; there are several obstacles to the implementation of the UPHES plants (Gulliver and

Arndt 1991). The obstacles include the presence of Salmon and other species that have to be

overcome along the rivers to spawn the encounter walls that cannot allow water to pass.

However, the PHS facility is more expensive because, in some instances, both the upper and

lower reservoirs have to be constructed (Gulliver and Arndt 1991).

Pollution of the water - The reservoir water has salinity conditions, dissolved gases,

temperature, nutrients, and other properties of the water flowing down the river. These pollutants

may pollute the waters in the upper reservoirs, which in some cases are often used by humans

and in agricultural production although, the pollution rates cannot be compared to PHS rates of

pollution.

Deprivation of sediment to lower reaches - Sediments accumulate in the reservoir nutrient

impoverished the rest of the river to the mouth (Fogarty, 2012). These sediments may lead to the

blockage of underground waterways, thus forcing rivers to dry.

Furthermore, the costs of excavation and lining new surface reservoirs and the problem of

maintenance of the quality of water also present major unforeseeable concerns during the

construction and operation of a UPHES facility.

4.7. Future of PHS

Presently, there are over 200 PHS installations around the world, which constitute more

that 99% of all the grid-connected storage capacities (Palmer and Cooper 2013). As from the

1970s, the global PHS power capacities have been growing steadily by approximately 3GW

every year (Kumar and Radhakrishna 2008). Currently, there are approximately 290 sites that are

in operation all over the world, with other 40 facilities still being constructed, and other 550

installations are have been planned to be built shortly. However, PHS facilities are comparatively

limited by the shortage of suitable sites for reservoirs.

Due to this limitation, the UPHES’ are viewed as a viable solution to solve the problem

of shortage of reservoirs. It offers the solution through excavating an underground reservoir that

is a hundred metres underneath the earth surface to allow the exchange of water between it and

the surface reservoir that is constructed immediately on top of it and diked, making use of spoils

from the excavated reservoir. It is thus, clear that UPHES facilities are more advantageous in

electricity storage than the PHS technologies.

CONCLUSION

Overall, it is evident that there are some advantages of underground pumped

hydroelectric energy storage that overpower those of pumped hydroelectric energy storage.

Also, the identification and selection of a storage facility are complicated tasks that require an

analysis going beyond the economic, the techno-economic factors to include many series of other

characteristics. It is also important to acknowledge that, indeed there is no perfect storage

technology for every application.

Furthermore, the construction of the PHS and the UPHES energy storage technologies

are also limited by the topographical and geological factors. Thus, identifying the most

appropriate option will help reduce cost and increase efficiency storage of electricity. There is

the great feasibility of the project to be based on the advantages of underground pumped

hydroelectric energy storage. UPHES facilities are viewed to have the potential of alleviating the

problem of large energy storage and providing a continuous supply of electrical energy well

deployed in the electricity grid.

This study has clearly shown the best electrical energy storage technologies that are

available. A comparison made between PHS and UPHES has shown that UPHES storage

technologies have the best storage capabilities and performance as compared to the PHS systems,

thus, would be more preferable.

Even though the improved PHS facilities offer a wide range of solutions to the problems

of energy generation and storage, the UPHES are projected to offer a better solution at

minimised costs and reduced environmental impact. Again, energy storage is likely seen to be

forming an essential part of the future energy storage system in the case where the variable and

the uncontrollable renewable energy generation, which significantly makes a contribution to the

supply of electrical energy. The two energy storages if well implemented, have the capability of

providing flexibility within the energy system, therefore, reducing the need for new generation

capacities while contributing to efficient usage of low-carbon electrical power. Governments

should, therefore, enact programmes that support the growth of renewable energy through the

construction of better storage facilities such as improved PHS and UPHES to ensure

continuously and availability of electricity to promote economic growth.

REFERENCE LIST

Alamri, B.R. and Alamri, A.R., 2009, April.Technical review of energy storage technologies when

integrated with intermittent renewable energy.In Sustainable Power Generation and Supply,

2009.SUPERGEN'09. International Conference on (pp. 1-5). IEEE.

Allen, R.D., Doherty, T.J. and Kannberg, L.D., 1984. Underground pumped hydroelectric storage (No.

PNL-5142).Pacific Northwest Lab., Richland, WA (USA).

Barin, A., Canha, L., Abaide, A., Magnago, K. and Wottrich, B., 2009, September.Renewable hybrid

systems using biogas fuzzy multi-sets and fuzzy multi-rules.In Energy Conversion Congress and

Exposition, 2009.ECCE 2009. IEEE (pp. 1180-1184). IEEE.

Barin, A., Canha, L.N., Abaide, A.R., Magnago, K.F., Wottrich, B. and Machado, R.Q., 2011.Multiple

criteria analysis for energy storage selection.Energy and Power Engineering, 3(04), p.557.

Barin, A., Canha, L.N., Magnago, K., Abaide, A.D.R. and Wottrich, B., 2009, May. Multicriteria

decision making for management of storage energy technologies on renewable hybrid systems-

the analytic hierarchy process and the fuzzy logic.In Energy Market, 2009.EEM 2009. 6th

International Conference on the European (pp. 1-6). IEEE.

Barnes, F.S. and Levine, J.G. eds., 2011.Large energy storage systems handbook.CRC Press.

Becker, M.F., Bruce, B.W., Pope, L.M. and Andrews, W.J., 1999.Ground-water quality in the central

High Plains aquifer.Colorado, Kansas, New Mexico, Oklahoma, and Texas, pp.2002-41.

Bin, L.I.U., Laijun, C.H.E.N., Shengwei, M.E.I., Feng, L.I.U., Junjie, W.A.N.G. and Sixian, W.A.N.G.,

2014.The impact of key parameters on the cycle efficiency of multi-stage RCAES

system.Journal of Modern Power Systems and Clean Energy, 2(4), pp.422-430.

Boyle, G., Everett, B. and Ramage, J. eds., 2003.Energy systems and sustainability (p. 347). Oxford:

Oxford University Press.

Bradbury, K., Pratson, L. and Patiño-Echeverri, D., 2014. Economic viability of energy storage systems

based on price arbitrage potential in real-time US electricity markets. Applied Energy, 114,

pp.512-519.

Butler, P., Miller, J.L. and Taylor, P.A., 2002. Energy storage opportunities analysis phase ii final report

a study for the doe energy storage systems program. Sandia Report No. SAND2002-1314, Sandia

National Laboratories, Albuquerque, NM (May 2002). BERNARD S. LEE.

Carrara, S. and Marangoni, G., 2014.Modeling the integration of Variable Renewable Energies (VRE)

into the electrical grid in the WITCH model: techno-economic impacts of different approaches.

Center, H.D., 2009.Technical Analysis of Pumped Storage and Integration with Wind Power in the

Pacific Northwest.

Colarelli, D. and Grunwald, D., 2002, November.Massive arrays of idle disks for storage archives.In

Proceedings of the 2002 ACM/IEEE conference on Supercomputing (pp. 1-11). IEEE Computer

Society Press.

Connolly, D. and MacLaughlin, S., 2010.LOCATING SITES FOR PUMPED HYDROELECTRIC

ENERGY STORAGE.

Connolly, D., 2011. The integration of fluctuating renewable energy using energy storage.

Crampes, C. and Moreaux, M., 2010. Pumped storage and cost saving. Energy economics, 32(2),

pp.325-333.

Deane, J.P., Gallachóir, B.Ó. and McKeogh, E.J., 2010. Techno-economic review of existing and new

pumped hydro energy storage plant.Renewable and Sustainable Energy Reviews, 14(4), pp.1293-

1302.

Denholm, P., Jorgenson, J., Hummon, M., Jenkin, T., Palchak, D., Kirby, B., Ma, O. and O’Malley, M.,

2013.The value of energy storage for grid applications.Contract, 303, pp.275-3000.

Diakoulaki, D. and Karangelis, F., 2007. Multi-criteria decision analysis and cost–benefit analysis of

alternative scenarios for the power generation sector in Greece. Renewable and Sustainable

Energy Reviews, 11(4), pp.716-727.

Ekman, C.K. and Jensen, S.H., 2010. Prospects for large scale electricity storage in Denmark.Energy

Conversion and Management, 51(6), pp.1140-1147.

Ess, F., Haefke, L., Hobohm, J., Peter, F. and Wünsch, M., 2012. The significance of international

hydropower storage for the energy transition.Prognos AG, Berlin.

Eyer, J. and Corey, G., 2010. Energy storage for the electricity grid: Benefits and market potential

assessment guide. Sandia National Laboratories, 20(10), p.5.

Eyer, J., Iannucci, J. and Corey, G., Dec 2004. Energy storage benefits and market analysis

handbook.Tech. Rep. SAND2004-6177, Sandia Laboratories, Sandia National Laboratories

Albuquerque, New Mexico 87185 and Livermore, California 94550.

Farid, M.M., Khudhair, A.M., Razack, S.A.K. and Al-Hallaj, S., 2004. A review on phase change

energy storage: materials and applications. Energy conversion and management, 45(9), pp.1597-

1615.

Fertig, E., Heggedal, A.M., Doorman, G. and Apt, J., 2014.Optimal investment timing and capacity

choice for pumped hydropower storage.Energy Systems, 5(2), pp.285-306.

Ginocchio, A., Parker, M.L. and Sewalk, S., 2007. Review of the Legal and Regulatory Requirements

Applicable to a Small-scale Hydro-energy Storage System in an Agricultural Setting. University

of Colorado, Boulder, School of Law.

Graditi, G., Ippolito, M.G., Rizzo, R., Telaretti, E. and Zizzo, G., 2014, September. Technical-

economical evaluations for distributed storage applications: An Italian case study for a medium-

scale public facility. In Renewable Power Generation Conference (RPG 2014), 3rd (pp. 1-

7).IET.

Grünewald, P., Cockerill, T., Contestabile, M. and Pearson, P., 2011. The role of large scale storage in a

GB low carbon energy future: Issues and policy challenges. Energy Policy, 39(9), pp.4807-4815.

Gulliver, J.S. and Arndt, R.E., 1991. Hydropower engineering handbook.McGraw-Hill.

Hadjipaschalis, I., Poullikkas, A. and Efthimiou, V., 2009.Overview of current and future energy storage

technologies for electric power applications.Renewable and sustainable energy reviews, 13(6),

pp.1513-1522.

Hasnain, S.M., 1998. Review on sustainable thermal energy storage technologies, Part I: heat storage

materials and techniques. Energy Conversion and Management, 39(11), pp.1127-1138.

Hobbs, B.F. and Meier, P., 2012. Energy decisions and the environment: a guide to the use of

multicriteria methods (Vol. 28). Springer Science & Business Media.

Ibrahim, H., Ilinca, A. and Perron, J., 2008. Energy storage systems—characteristics and

comparisons.Renewable and sustainable energy reviews, 12(5), pp.1221-1250.

Inage, S.I., 2009. Prospects for large-scale energy storage in decarbonised power grids.International

Energy Agency, IEA.

Ingram, E.A., 2009. PUMPED STORAGE-Development Activity Spotlights-A sampling of pumped-

storage new development and project rehabilitation in Europe, Asia, Africa, and North America

illustrates the breadth and depth of this market sector. Hydro review worldwide, 17(6), p.12.

Jackson, J.A., KloeberJr, J.M., Ralston, B.E. and Deckro, R.F., 1999. Selecting a portfolio of

technologies: An application of decision analysis.Decision Sciences, 30(1), p.217.

Kintner-Meyer, M., Balducci, P., Colella, W., Elizondo, M., Jin, C., Nguyen, T., Viswanathan, V. and

Zhang, Y., 2012. National assessment of energy storage for grid balancing and arbitrage: phase

1, WECC. Pacific Northwest National Lab oratory: Richland, WA.

Ku, A., 1995. Modelling uncertainty in electricity capacity planning.Unpublished doctoral

dissertation).London Business School, London, UK.

Kumar, J.A. and Radhakrishna, C., 2008, November.Sustainable Energy Future by AD2030-India Case

Study.In Energy 2030 Conference, 2008.ENERGY 2008. IEEE (pp. 1-8). IEEE.

Levine, J., Martin, G., Moutoux, R. and Barnes, F., 2007.Large scale electrical energy storage in

Colorado.CERI Research Report.

Lombardi, P., Vasquez, P. and Styczynski, Z.A., 2009, July.Optimised autonomous power system. In

Integration of Wide-Scale Renewable Resources into the Power Delivery System, 2009

CIGRE/IEEE PES Joint Symposium (pp. 1-13). IEEE.

Lund, P.D., Lindgren, J., Mikkola, J. and Salpakari, J., 2015. Review of energy system flexibility

measures to enable high levels of variable renewable electricity. Renewable and Sustainable

Energy Reviews, 45, pp.785-807.

Luo, X., Wang, J., Dooner, M. and Clarke, J., 2015.Overview of current development in electrical

energy storage technologies and the application potential in power system operation.Applied

Energy, 137, pp.511-536.

Mahlia, T.M.I., Saktisahdan, T.J., Jannifar, A., Hasan, M.H. and Matseelar, H.S.C., 2014. A review of

available methods and development on energy storage; technology update.Renewable and

Sustainable Energy Reviews, 33, pp.532-545.

Martin, G.D., 2007. Aquifer underground pumped hydroelectric energy storage. ProQuest.

Martin, G.G., 2011. Underground Pumped Hydroelectric Energy Storage.Large Energy Storage

Systems, p.77.

Medina, P., Bizuayehu, A.W., Catalao, J.P., Rodrigues, E.M. and Contreras, J., 2014, January. Electrical

Energy Storage Systems: Technologies' State-of-the-Art, Techno-economic Benefits and

Applications Analysis. In System Sciences (HICSS), 2014 47th Hawaii International Conference

on (pp. 2295-2304).IEEE.

Mitchell, D. and Mitchell Dell N., 2001.Method of producing electricity through injection of water into

a well. U.S. Patent Application 09/872,452.

Oberhofer, A. and Meisen, P., 2012. Energy storage technologies & their role in renewable

integration.Global Energy Network Institute.

Olsen, J., Paasch, K., Lassen, B. and Veje, C.T., 2015. A new principle for underground pumped

hydroelectric storage.Journal of Energy Storage, 2, pp.54-63.

Palmer, J. and Cooper, I., 2013. United Kingdom housing energy fact file 2013. Department of Energy

and Climate Change.

Patocka, F., 2014.Environmental Impacts of Pumped Storage Hydro Power Plants.

Perich, S., 2007.Hydroelectric power and desalination. U.S. Patent Application 11/780,980.

Pickard, W.F. and Abbott, D., 2012. Addressing the intermittency challenge: Massive energy storage in

a sustainable future.Proceedings of the IEEE, 100(2), p.317.

Pickard, W.F., 2012. The history, present state, and future prospects of underground pumped hydro for

massive energy storage.Proceedings of the IEEE, 100(2), pp.473-483.

Poonpun, P. and Jewell, W.T., 2008. Analysis of the cost per kilowatt hour to store electricity.IEEE

Transactions on energy conversion, 23(2), pp.529-534.

Ramos, H.M., Amaral, M.P. and Covas, D.I., 2014. Pumped-Storage Solution towards Energy

Efficiency and Sustainability: Portugal Contribution and Real Case Studies. Journal of Water

Resource and Protection, 6(12), p.1099.

Rehman, S., Al-Hadhrami, L.M. and Alam, M.M., 2015. Pumped hydro energy storage system: a

technological review. Renewable and Sustainable Energy Reviews, 44, pp.586-598.

Ribeiro, P.F., Johnson, B.K., Crow, M.L., Arsoy, A. and Liu, Y., 2001. Energy storage systems for

advanced power applications.Proceedings of the IEEE, 89(12), pp.1744-1756.

Robert, B., 1933. Hydroelectric installation.U.S. Patent 1,921,905.

Saaty, T.L., 1990. How to make a decision: the analytic hierarchy process. European journal of

operational research, 48(1), pp.9-26.

Sankar, S., Saravanakumar, S., Padmarasan, M., Manikandan, C.T. and Jayalakshmi, D.,

2012.INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING & TECHNOLOGY

(IJEET).Journal Impact Factor, 3(3), pp.211-221.

Schainker, R.B., 2004, June. Executive overview: energy storage options for a sustainable energy future.

In Power Engineering Society General Meeting, 2004. IEEE (pp. 2309-2314). IEEE.

Schoenung, S.M., 1999. Hydrogen energy storage comparison.Prepared for DOE under contract no.

PE-FC36-96-GO10140 A, 3.

Scieri, F. and Miller, R.L., Miller Richard L, 1984.Hydroelectric generating system.U.S. Patent

4,443,707.

Scott, F.M., 1977. Underground hydroelectric pumped storage-A practical option.Energy, vol. 2, fall

1977, p. 20-22,2, pp.20-22.

Shiu, A. and Lam, P.L., 2004. Electricity consumption and economic growth in China.Energy policy,

32(1), pp.47-54.

Siostrzonek, T., Piróg, S. and Baszyński, M., 2008, September.Energy storage systems the flywheel

energy storage.In Power Electronics and Motion Control Conference, 2008.EPE-PEMC 2008.

13th (pp. 1779-1783). IEEE.

Slocum, A., 2010. Concepts for Robust Renewable Energy Generation and Storage.

Smith, S.C., SenSr, P.K. and KroposkiSr, B., 2008, July.Advancement of energy storage devices and

applications in electrical power system. In Power and Energy Society General Meeting-

Conversion and Delivery of Electrical Energy in the 21st Century, 2008 IEEE (pp. 1-8). IEEE.

Steffen, B. and Weber, C., 2013. Efficient storage capacity in power systems with thermal and

renewable generation.Energy Economics, 36, pp.556-567.

Steffen, B., 2012. Prospects for pumped-hydro storage in Germany.Energy Policy, 45, pp.420-429.

Sunderkötter, M. and Weber, C., 2012.Valuing fuel diversification in power generation capacity

planning.Energy Economics, 34(5), pp.1664-1674.

Tam, S.W., Blomquist, C.A. and Kartsounes, G.T., 1979. Underground pumped hydro storage—An

overview. Energy Sources, 4(4), pp.329-351.

Tietjen, J.S., 2007. Pumped storage hydroelectricity.Encyclopaedia of Energy Engineering and

Technology. CRC, Boca Raton.

Uddin, N., 2003. Preliminary design of an underground reservoir for pumped storage.Geotechnical &

Geological Engineering, 21(4), pp.331-355.

Van Dorp, M., 2009.Dealing with energy needs in humanitarian crisis response operations.

Verma, H., Gambhir, J. and Goyal, S., 2013. Energy Storage: A Review. International Journal of

Innovative Technology and Exploring Engineering (IJITEE) ISSN, pp.2278-3075.

Wilson, I.A.G., McGregor, P.G. and Hall, P.J., 2010. Energy storage in the UK electrical network:

Estimation of the scale and review of technology options. Energy Policy, 38(8), pp.4099-4106.

Wong, I.H., 1996. An underground pumped storage scheme in the Bukit Timah granite of

Singapore.Tunnelling and Underground Space Technology, 11(4), pp.485-489.

Yang, C.J. and Jackson, R.B., 2011.Opportunities and barriers to pumped-hydro energy storage in the

United States.Renewable and Sustainable Energy Reviews, 15(1), pp.839-844.

Yucekaya, A., 2015. Energy Storage with Pumped Hydrostorage Systems Under Uncertainty. In Energy

Systems and Management (pp. 43-54). Springer International Publishing.

Yüksel, I., 2010. Hydropower for sustainable water and energy development.Renewable and Sustainable

Energy Reviews, 14(1), pp.462-469.

Zakeri, B. and Syri, S., 2015. Electrical energy storage systems: A comparative life cycle cost analysis.

Renewable and Sustainable Energy Reviews, 42, pp.569-596.

Zhaoyu, P., Shengzhu, L. and Hong, Z., 2010, October. The application of Ttriangular fuzzy AHP in the

power system load forecasting. In Computer Application and System Modeling (ICCASM), 2010

International Conference on (Vol. 15, pp. V15-494).IEEE.

Zhou, P., Ang, B.W. and Poh, K.L., 2006. Decision analysis in energy and environmental modeling: An

update. Energy, 31(14), pp.2604-2622.