mece3410u report - renewable microgrid for a community in fiji

FACULTY OF ENGINEERING AND APPLIED SCIENCE

MECE3410U

Renewable Microgrid for a Community in Fiji

GROUP PROJECT REPORT

Course Instructor: Dr. Yuelei Yang

Teaching Assistant: Mohammed Alziadeh

Project Report Submitted On: April 11, 2016

# Last Name First Name ID

1 Bower Lowell 100500898

2 Karanwal Tushar 100481186

3 Owais Syed 100506689

4 Pandya Devarsh 100455628

MECE3410U - Renewable Microgrid for a Community in Fiji

1

Table of Contents List of Figures ................................................................................................................................. 2

List of Tables .................................................................................................................................. 3

1.0 Introduction ............................................................................................................................... 4

1.1 Problem Statement ................................................................................................................ 5

1.2 Project Objectives ................................................................................................................. 5

2.0 Design Details ........................................................................................................................... 6

2.1 Proposed Design ................................................................................................................... 6

2.2 Project Assumptions ............................................................................................................. 7

3.0 Analysis and Discussion ........................................................................................................... 9

3.1 Component Selection ............................................................................................................ 9

3.2 System Analysis .................................................................................................................. 16

3.3 Economic Analysis ............................................................................................................. 24

4.0 Conclusion .............................................................................................................................. 27

5.0 Nomenclature .......................................................................................................................... 28

6.0 Appendix ................................................................................................................................. 29

6.1 Figures................................................................................................................................. 29

6.2 Sample Calculations............................................................................................................ 33

6.3 EES Code and Results ........................................................................................................ 35

References ..................................................................................................................................... 37


2

List of Figures

Figure 1: Map of Fiji [21] .............................................................................................................. 4

Figure 2: Proposed system diagram ................................................................................................ 6

Figure 3: Geothermal Organic Rankine Cycle ................................................................................ 9

Figure 4: R134a Molecular Structure ............................................................................................. 9

Figure 5:40 VSI Leroy Somer alternator ...................................................................................... 10

Figure 6: T701 wind turbine from Pika Energy ............................................................................ 11

Figure 7: Vertical gin pole from ARE and Econotower from Pika Energy .................................. 12

Figure 8: B801 battery charge controller from Pika Energy......................................................... 13

Figure 9: X7601 islanding inverter from Pika Energy.................................................................. 14

Figure 10: 12 CS 11P deep cycle battery from Rolls.................................................................... 15

Figure 24: T-S diagram of ORC process ...................................................................................... 16

Figure 25: Zoomed in to see states 3 and 4 ................................................................................... 16

Figure 11: Efficiency curve of the LSA 40 VS1 operating at 50 Hz [7]. ..................................... 18

Figure 12: Short-circuit curve for start-up of LSA 40 VS1 [7]. ................................................... 18

Figure 13: Equivalent circuit (a) and phasor diagram (b) for a synchronous generator [13] ....... 19

Figure 14: Variable-speed system with full capacity converters [13] .......................................... 19

Figure 15: Active-stall control of a wind turbine [13] .................................................................. 20

Figure 16: Average wind speed in Fiji .......................................................................................... 20

Figure 17: T701 turbine power curve ........................................................................................... 21

Figure 18: Estimated turbine power output by month .................................................................. 21

Figure 19: Series/parallel connection of batteries [11] ................................................................. 22

Figure 20: Clockwise Rotation Phase Sequence (1-2-3) [14] ....................................................... 23

Figure 21: Counter Clockwise Rotation Phase Sequence (3-2-1) [14] ......................................... 23

Figure 22: Generator Frequency Lower than Grid’s [14] ............................................................. 24

Figure 23: Generator and Grid in Phase [14] ................................................................................ 24

Figure 26: Full-scale proposed system diagram ........................................................................... 29

Figure 27: Fiji power grid [3] ....................................................................................................... 30

Figure 28: Number of cycles vs. depth of discharge for 12 CS 11P battery [22] ......................... 31

Figure 29: EES Results ................................................................................................................. 36


3

List of Tables

Table 1: ORC component parameters ............................................................................................. 9

Table 2: LSA 40 VSI component parameters [7] ......................................................................... 10

Table 3: T701 wind turbine component parameters ..................................................................... 11

Table 4: Econotower and gin pole component parameters [8] [10] .............................................. 12

Table 5: B801 charge controller component parameters [8] ........................................................ 13

Table 6: X7601 inverter component parameters [8] ..................................................................... 14

Table 7: 12 CS 11P battery component parameters [12] .............................................................. 15

Table 8: Tabulated state values of ORC ....................................................................................... 17

Table 9: Cost break down for ORC .............................................................................................. 25

Table 10: Cost breakdown for wind turbine and associated components ..................................... 25

Table 11: Economic summary ...................................................................................................... 26


4

1.0 Introduction

The nation of Fiji is located in the Pacific Ocean and comprised of 332 islands; approximately 110

of which are inhabited [1]. The country has a total population of 909,389 contained within a total

land area the size of New Jersey. In 2015, 1150.5 GWh of electricity was produced with renewable

generation comprising 90.9% of total production [2]. This remarkably high percentage of

renewable generation is the result of a dedicated effort by the Fijian government to reduce the need

for costly fossil fuel imports. They have set the target of 100% renewable energy generation by

2020 and appear to be on track to realize this goal [3]. Renewable sources of energy are primarily

Figure 1: Map of Fiji [21]


5

hydroelectric with biomass, solar, and wind contributing a smaller share (see Figure 27). The

primary source of non-renewable energy is diesel which must be imported.

This report will propose and analyze a renewable energy microgrid for a small community on one

of the islands within Fiji. Since only renewable energy has been specified for the microgrid, a

primary and secondary source of energy will be used to ensure a constant supply of power. The

International Renewable Energy Agency (IRENA) has identified that an increase in non-hydro

renewables would diversify the energy mix and add resilience to the energy grid in Fiji.

Additionally, both IRENA and an analysis by McCoy-West et al. [4] have determined that the

nation has a high development potential for geothermal energy with 53 thermal areas identified

from detailed surveys. Based on this information geothermal energy has been selected as the

primary source of power for the community. An analysis of wind power prospects in Fiji was

carried out by Kumar and Prasad [5] which determined that most sites have a moderate potential

for wind generation during the months of April to October with an average power density of

approximately 160 W/m2. The sites are less suited to wind power generation during the months of

November to march with an average power density of 100 W/m2. Wind energy has therefore been

selected as the secondary source of energy since it is less reliable compared with the approximately

constant generation of a geothermal site.

1.1 Problem Statement

A small community of 100 occupants on the islands of Fiji requires an energy generation system.

The island has sufficient wind and sunshine. This community consumes an average of 6000 kWh

of electricity per month. To plan for any changes in load, this consumption will be increased by

20% to a total of 7200 kWh (~9.7 kW)

1.2 Project Objectives

Identify the primary and secondary energy sources

Create a schematic of the system

Select and size a type of generator and determine its rated parameters

Perform an economic analysis


6

2.0 Design Details

2.1 Proposed Design

Figure 2: Proposed system diagram

The proposed design shown above is the micro grid which will provide enough energy for a

community of 100 occupants in Fiji. The primary energy source of this micro grid is a supply of a

hot geothermal water. This constant supply of heat runs an organic Rankine cycle (ORC) which

has R-134a circulating as working fluid.

Referring to Figure 2, the ORC has four main components: turbine, condenser, pump, and heat

exchanger. The heat exchanger is used to transfer the heat from the well water from the geothermal

boreholes to the working fluid. This causes the working fluid to change phase to super-heated

vapor. The high quality super-heated vapor is then fed into a turbine, where it is expanded from

high pressure to low pressure. Mechanical work is produced from the turbine which is connected

to a 3 phase synchronous generator which is the primary power source for the community. The

expansion of the working fluid causes a decrease in entropy, and the resulting outlet stream from


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the turbine is a mixture of vapor and liquid. The condenser is used as a heat dump; cooling water

lines sourced from the ocean provide sufficient cooling to induce phase change to saturated liquid.

This is crucial as it is not possible to compress a mixture without experiencing cavitation damage

to the compressor. The pump is used to pressurize the saturated liquid to desired levels before the

addition of heat from the geothermal source.

The secondary power source is three T701 wind turbines which converts the kinetic energy of the

wind into a 190 V DC power output. A lightning arrestor is included at the base of the tower to

protect the system from a direct lighting strike. The junction box connects the three wind turbines

and the DC disconnect allows this portion of the system to be isolated for maintenance. The B801

charge controller manages the flow of energy from the wind turbines and 3 phase synchronous

generator to the battery bank. The controller is bi-directional and transforms power between the

190 V DC incoming from the turbines, 48 V DC battery bank voltage, and 380 V DC output. The

battery bank is comprised of eight batteries connected in series-parallel for increased voltage and

capacity. The inverter connects the DC and AC portions of the proposed system and allows for bi-

directional flow so that the primary 3 phase synchronous generator can charge the battery bank.

Both the streams of electricity from the organic Rankine cycle-generator unit and the wind turbines

are synchronized to ensure no damage to the generator or micro grid occur. After the

synchronization module, the voltage is stepped up by the use of a transformer. This is to reduce

transmission losses as the power flows through the distribution grid. To supply energy to the

occupants, the voltage is stepped down after transmission and sent to homes in the community.

The electricity standard for Fiji is 240 V at 50 Hz [3] and the system was designed to meet this

requirement. Although the power grid on the community is isolated, electronics and appliances

will likely be designed to work with this standard.

2.2 Project Assumptions

ORC:

- No leaks

- No losses in pipes and fittings

- Constant geothermal temp

- Constant mass flows of fluids

- Heat exchanger efficiencies are 100%

- 10 degree gradient for effective heat exchange

3ϕ Synchronous Generator:

- Manufactured by solid steel forging

- Rotor is 2/3 slotted for windings and 1/3 unslotted for behaviour as pole


8

- Speed of 1500 rpm with 4 poles

- Nonsalient pole alternator must be selected for high speed application

- Peripheral velocity below 175 m/s to dictate physical dimensions

- Short Circuit Ratio (SCR) between 0.7 and 1.1

- Current density between 3 A/mm and 5 A/mm

- Stator current must be less than 1500 A

- Single turn coils required for 1500 rpm and simplicity in design

- Coils assumed to be full pitched and corresponding winding factor (𝑘𝑤) is 0.955

- Synchronous generator was assumed to be in Y-connection

Wind:

- Air properties 25C and 1 atm

Power Grid

- Transmission losses are negligible

- Distribution grid is existing

- The electricity used in the community is 240 V at 50 Hz


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3.0 Analysis and Discussion

3.1 Component Selection

Geothermal Organic Rankine Cycle

Figure 3: Geothermal Organic Rankine Cycle

The primary power source for this community will be a Geothermal Organic Rankine Cycle

(GORC). The system uses 1,1,1,2 Tetrafluoroethane (R134a), as the working fluid.

Table 1: ORC component parameters

Technology Produced Power

Range

Heat Source

Temperature Range Turbines

Micro ORC 10 kW Around 100˚C Lysholm Turbine –

60% of cost

Heat Exchangers Working fluid Size Cost

Compact brazed heat

exchangers

R134a, R245fa, R22,

Other Refrigerants 0.6·1.5·1.5 (m3) $25000 - $2500/kW

Figure 4: R134a

Molecular Structure


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Synchronous Generator

3ϕ synchronous generator for the Organic Rankine Cycle (ORC) was selected based on several

design considerations to ensure optimal functionality with the proposed system. The LSA (Leroy

Somer Alternator) 40 VS1 was chosen to operate along with the ORC. This is a 3ϕ 1500 rpm, 50

Hz, 380 V, 10.5 kVA turbo alternator. The synchronous generator can be operated at 8.4 kW under

rated operating conditions while assuming a power factor of 0.8. Some of the other specifications

of this low voltage turbo alternator are that the winding pitch is approximately 2/3. The insulation

class of this alternator is H meaning the generator can operate at 155°C at a thermal life expectancy

of 100,000 working hours instead of the typical 20,000 to 25,000 working hours [6] .The maximum

working temperature of this class insulation is approximately 180° C [6]. The efficiency of the

LSA 40 VS1 is approximately 88.7% which is observed to be about 2% lower than the 60 Hz

model at the same rated conditions.

Table 2: LSA 40 VSI component parameters [7]

Rated Power Output (kVA)

Rated Voltage Output

(V) Rotational Speed

(rpm)

10.5 380 1500

Figure 5:40 VSI Leroy Somer alternator


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Wind Turbine

The wind turbines selected for the community are a T701 model manufactured by Pika Energy.

The three-blades on this horizontal-axis wind turbine (HAWT) are glass-reinforced polymer and

the nacelle has an upwind rotor with free yaw. The system includes a brushless permanent magnet

AC generator with slip ring design. The generated AC power is rectified into DC voltage for

transmission within the microgrid. This turbine can be connected to a Wi-Fi remote monitoring

system and has been has received a Limited Power Performance (LPP) certification by the Small

Wind Certification Council (SWCC) [8]. The parameters for this turbine are summarized below.

Table 3: T701 wind turbine component parameters

Peak Power Output (kW)

Rated Power Output

(kW) Output Voltage

(V DC)

1.7 @ 13.5 m/s 1.5 @ 11 m/s 190

Cut In Wind Speed (m/s)

Survival Wind Speed

(m/s) Swept Area

(m2)

3.3 66 7.1

Figure 6: T701 wind turbine from Pika Energy


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Wind Tower

The wind tower will be an Econotower from Pika Energy combined with a vertical gin pole from

American Resource & Energy (ARE). The windtower is a modular system that can be made with

locally sourced pipe sections and is designed to be used with eight supporting guy wires. The

vertical gin pole allows for easy setup by one person and would be left attached to the windtower

so that it could be quickly disassembled and secured in the event of dangerously high winds. The

wind turbine has a wind survival speed of 66 m/s (237.7 km/hr), however during cyclone Winston

gusts of up to 84.7 m/s (305 km/hr) were recorded on the island of Fiji [9]. An additional advantage

of a non-permanent installation is that the turbine can be lowered for service.

Table 4: Econotower and gin pole component parameters [8] [10]

Total Height

(m) Econotower Tube

Ginpole Weight Capacity

(kg)

12.8 Sched. 40 2.5 inch diameter 2048

Figure 7: Vertical gin pole from ARE and Econotower from Pika Energy


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Charge Controller

The B801 charge controller is used to manage the flow of DC power from the wind turbines to the

battery bank and inverter used in the microgrid. The charge controller will step up the incoming

turbine voltage of 190 V to 380 V to reduce line losses. The controller will prioritize loads and

manage battery storage to ensure the maximum number of cycles can be reached within the bank.

Protection features include sensors for over/under voltage, high temperatures within the controller

and battery bank, and high current flow.

Table 5: B801 charge controller component parameters [8]

Rated Power

(W) Rated Input Voltage

(V DC) Rated Input Current

(A)

4000 190 10

Rated Output Voltage (V DC)

Battery Voltage

(V DC) Max. Battery Current

(A)

380 24 to 48 80

Efficiency (%)

Standby Power Consumption

(W) Temperature Range

(°C)

95 7 -20 to 50

Figure 8: B801 battery charge controller from Pika Energy


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Inverter

The inverter for the system is an X7601 islanding inverter which will convert the DC power output

of the charge controller to the AC power used in the distribution system for the microgrid.

Although capable of connecting directly to lithium ion batteries, lead acid batteries have been

specified for the system requiring the additional battery charge controller. The inverter allows for

bi-directional energy flow so that the primary synchronous generator can also be used to charge

the batteries if required. Additionally, an internal autotransformer can be used to support critical

loads within the community.

Table 6: X7601 inverter component parameters [8]

Rated Continuous Power

(W) Rated Surge Power

(W) Rated Input Voltage

(V DC)

8000 12000 for 10 seconds 380

Rated Output Voltage

(V AC) Peak Efficiency

(%) Max. Temperature

(°C)

3𝜙 208 V @ 60 Hz 97 60

Figure 9: X7601 islanding inverter from Pika Energy


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Battery Bank

The battery bank for the proposed microgrid will consist of eight 12 CS 11P deep cycle lead acid

batteries connected in a series and parallel. A bank of batteries connected in series will have a total

voltage equal to the sum of each batteries voltage while maintaining the amp hour capacity of a

single battery. While a bank of batteries connected in parallel will have a total capacity equal to

the sum of each batteries capacity with a voltage of a single battery. By connecting the battery in

series and parallel both the operating voltage and amp hour capacity can be increased [11]. This

battery was selected due to the very favorable relationship between number of cycles and depth of

discharge reaching a 50% discharge after approximately 3200 cycles (see Figure 28).

Table 7: 12 CS 11P battery component parameters [12]

Single Battery Voltage

(V DC) Single Battery Amp Hour

(A) Specific Gravity

12 357 1.280

Figure 10: 12 CS 11P deep cycle battery from Rolls


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3.2 System Analysis

The following section analyzes the main components of the system. Sample calculations and the

created EES code are included in the Appendix.

Geothermal Organic Rankine Cycle

The GORC was simulated using Engineering Equation Solver with parameters outlined in

Appendix 6.3 EES Code. The following figures show the temperature-entropy plots of the process.

Figure 11: T-S diagram of ORC process

Figure 12: Zoomed in to see states 3 and 4


17

In an ideal situation, the all expansions in the process would be isentropic, and all heat additions

in the process would be isobaric. For this simulation we introduced turbine and pump efficiencies

of 90% and 80% respectively, to get an accurate estimation of mass flows and work inputs for the

proposed system. The state properties are shown below:

Table 8: Tabulated state values of ORC

State Enthalpy

(kJ/kg) Pressure

(kPa) Energy Rate

(kW) Entropy

(kJ/kg-K) Temperature

(C)

1 303.9 2500 98.14 0.9569 92.2

2 276 600 89.14 0.967 35.34

3 81.51 600 26.32 0.308 21.55

4 83.45 2500 26.95 0.3093 22.82

5 169.6 2500 - 0.5753 77.54

Setting the power demand at 9 kW, the mass flow rate of the working fluid (R134) is determined

to be 0.323 kg/s which is a reasonable flowrate considering the peak load the cycle has to produce.

Thermal efficiency is calculated by using the specific work of working fluid in the boiler divided

by the difference in enthalpy of working fluid at turbine inlet and outlet. The proposed design is

11.76% thermally efficient which is within the rage of commercially available Organic Rankine

Cycles.

Synchronous Generator

The 3ϕ synchronous generator selected for the electrical infrastructure designed was chosen based

on several design parameters. As mentioned above, an Organic Rankine Cycle feeds into the

synchronous generator. The ORC turbine behaves similarly to steam turbines commonly found

and thus require a generator that work at a high speed. The speed of the turbo alternator was chosen

to be 1500 rpm in order to meet this requirement. The LSA 40 VS1 was chosen based on the

working conditions and assumptions specified above. The synchronous speed as dictated by the

fundamental relation with 4 poles and a frequency of 50 Hz was 1500 rpm (𝑁𝑠) or 25 rps (𝑛𝑠) as

required for some design considerations that will be discussed below.

Peripheral speed is defined as: 𝑛𝑝 = 𝜋𝐷𝑟𝑛𝑠 where 𝐷𝑟 is the rotor diameter. The rotor diameter can

be chosen based on a suitable peripheral velocity or vice versa depending on the design constraints.

The design constraint was to be below 175 m/s for the peripheral velocity so a value of 100 m/s

was selected. The Short Circuit Ratio (SCR) was found to be 0.7 for the LSA 40 VS1 which was

in the required bounds of the design [6]. Furthermore, the efficiency of the generator can be

analyzed over the range of apparent powers that the generator may output. The ideal scenario with

a power factor of 1 on the 50 Hz LSA 40 VS1 yields approximately 88.7% efficiency and this


18

efficiency is optimized at 88.8% at 8 kVA [6]. The altered scenario where the system operates at

a power factor of 0.8 yields lower efficiency of 82.7% at the 10.5 kVA rating selected for the

application with the ORC.

Figure 13: Efficiency curve of the LSA 40 VS1 operating at 50 Hz [7].

Figure 14: Short-circuit curve for start-up of LSA 40 VS1 [7].

Figure 14 shown above is the short-circuit curve in the Y connection as required for the system.

The Y-connection is preferred to reduce the insulation required due to 58% reduction in the voltage

per phase (resulting from the factor of 1/√3) between the slots and increase the cross sectional area

of the conductors. The increased current corresponds to a higher power output as well which is a

typical requirement of the turbo alternators. The delta connection was avoided since it is known to

distort line voltages significantly instead of crossing them out like a Y-connection when a lagging

load is attached to the system. The figure above can be used for understanding the transient

characteristics of the generator at start up.


19

Wind Turbine

Figure 15 shows the equivalent circuit and phasor diagram for a synchronous generator. The main

difference between most synchronous generators and the selected wind turbine is that the

excitation voltage (𝐸𝑓) will be created due to the flux generated by mechanical rotation of the

permanent magnets instead of an external power source [13]. In this type of generator the stator

winding flux (𝛷𝑎) due to the current 𝐼𝑎 is linked with leakage flux (𝛷𝑎𝑙) but does not impact the

field winding. The armature reaction flux (𝛷𝑎𝑟) however does link with the field winding and acts

against the flux, reducing power production.

The turbine selected for the community is permanent magnet synchronous generator (PMSG)

combined with full capacity power converters (see Figure 16). These converters transform the AC

output of the PMSG into the 190 V DC output of the turbine.

Figure 15: Equivalent circuit (a) and phasor diagram (b) for a synchronous generator [13]

Figure 16: Variable-speed system with full capacity converters [13]


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Speed control is accomplished by electronic stall regulation which is a type of active-stall control.

To avoid damaging the turbine at high wind speeds, turbines must have some type of speed control

that limits the rotational speed while producing maximum power. For electronic stall control, the

blade angle of the turbine is increased so that turbulence develops on the back of the blade and

rotational speed is reduced (see Figure 17).

Wind Energy

The power output of the wind turbines can be estimated using average monthly wind speeds

measured in Fiji at various sites (see Figure 18) which was created by finding the midpoint through

measured data from Kumar and Prasad [5].

The performance curve for the T701 wind turbine was created using data from a Small Wind

Certification Council (SWCC) report [8] and used to create the relation between wind speed and

Figure 17: Active-stall control of a wind turbine [13]

5.0

5.5

6.0

6.5

7.0

7.5

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Ave

rage

Win

d S

pee

d (

m/s

)

Jan Feb Mar Apr May Jun Jul Aug Oct Sep Nov Dec

Figure 18: Average wind speed in Fiji


21

turbine power output seen in Figure 19. The wind speed range encountered in Fiji has a minimum

and maximum value of 5.4 m/s and 7.2 m/s respectively and a trendline was created for this portion

of the power curve. The curve at this point is approximately linear with an R2 value of 0.9863.

The monthly estimated power output for the three turbines was then calculated based on the

trendline for the region of interest and can be seen in Figure 20. The maximum output of the three

turbines is 1448 W and was reached in July and the minimum output of 534 W for February. The

average yearly output of the three turbines is approximately 1035 W.

y = 164.69x - 702.95R² = 0.9863

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 5 10 15 20

Po

wer

Ou

tpu

t (W

)

Wind Speed (m/s)

Figure 19: T701 turbine power curve

200

400

600

800

1000

1200

1400

1600

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Po

wer

Ou

tpu

t (W

)

Figure 20: Estimated turbine power output by month

Jan Feb Mar Apr May Jun Jul Aug Oct Sep Nov Dec


22

The efficiency of the microgrid (𝜂𝑀𝐺) was calculated and includes the efficiency of the charge

controller and inverter. For the proposed system 𝜂𝑀𝐺 was calculated to be approximately 92.1%.

Therefore the actual output of the turbine will be decreased. Assuming that transmission losses to

the inverter are negligible, the peak power output of one wind turbine will be reduced from 482.8

W to 444.9 W. Considering these losses the yearly output of the three wind turbines is

approximately 8379 kWh. The efficiency of a single turbine in terms of converting the kinetic

energy of the wind into electrical energy (𝜂𝑊𝑇) was also calculated considering the maximum and

minimum wind speed of 7.2 m/s and 5.2 m/s respectively. The efficiency range under these

conditions was approximately 30.9% to 27.0%.

Battery Bank Capacity

The battery bank of eight deep cycle batteries have been connected in series and parallel in order

to increase the system voltage and amp hour capacity (see Figure 21). The rating for a single 12

CS 11P battery is 12 V DC and 357 amp hours (Ah). The system voltage for the battery bank is

therefore 48 V DC with a 2856 Ah capacity. It is recommended to keep above a 50% discharge

for normal use of batteries [11], therefore the rated capacity of the battery bank well be assumed

to actually be 1428 Ah. Since the rated power for the charger controller is 4.0 kW, the community

could be run at half capacity in the event of both the primary and secondary energy sources failing.

In this scenario community could be run entirely off the battery bank at half consumption for

approximately 2.8 days.

AC Power Synchronizing

To synchronize different outputs from generators to a grid, four parameters must be met. The first

is the phase sequence (or phase rotation). This is the sequence of the phase angle shift between the

three winding pairs. An example of the two possible rotation phase sequences is shown in the

Figure 21: Series/parallel connection of batteries [11]


23

images below. When the magnet of a generator is rotating, the peak instantaneous voltage of the

windings will usually be 120° from each other.

Figure 22: Clockwise Rotation Phase Sequence (1-2-3) [14]

Figure 23: Counter Clockwise Rotation Phase Sequence (3-2-1) [14]

The voltage also needs to be matched in order to synchronize different outputs of AC power. In

order to achieve this the magnitude voltage of the incoming generator must be matched to the

sinusoidal voltage of the grid [14]. If the two voltage do not match, a voltage differential will be

produced. If the generator’s voltage is higher, the generator will be overexcited and it will put out

MVAR [14]. If the generator voltage is less than the grid’s, then the generator will absorb apparent

power.

Frequency of the generator and the power grid must be equal, or it would cause damage to the

generator. Usually syncroscopes, essentially an electronic machine which displays different wave

forms, are used to match these frequencies. If the generator’s frequency is slower than the grid’s,

as shown in the figure below, then the generator would behave like a motor. This is due to the grid

trying to match the generator to the grid [14]. This would cause slipping poles between the rotor

and stator and could possibly damage the generator. If the generator’s frequency was higher than

the grid, then the generator would input power into the grid [14]. This would be in the form of a

very high current rush.


24

Figure 24: Generator Frequency Lower than Grid’s [14]

Lastly, the phase angle between the generator and grid has to be zero. This is usually done by

comparing the peaks or zero crossings of sinusoidal waveforms.

Figure 25: Generator and Grid in Phase [14]

3.3 Economic Analysis

Diesel Mitigation

Currently, diesel is one of the methods used to generate electricity in Fiji [15]. The proposed design

will aim to mitigate the use of diesel. Typical diesel generators use 0.4 litres of diesel for every

kWh they produce [16]. To produce the annual electricity for the 100 occupant community, a total

of 28,800 litres of diesel will be required. At the moment, diesel costs $0.73 per litre. The cost to

provide for the energy needs of the community would be $21,024 annually.

The diesel consumption of the community has been estimated at 28,800 L/year and this value can

be used to estimate the kilograms of CO2 that have been removed from the atmosphere due to the

installation of this microgrid. Given an estimated emission factor of 2535 g/L for diesel fuel used

in electricity production [17], using the proposed microgrid has removed approximately 73,008 kg

of CO2 from the atmosphere each year.


25

Organic Rankine Cycle and Generator

Table 9: Cost break down for ORC

Component Number of

Components

Unit Cost

(CAD) Purchase Cost

(CAD)

Micro ORC 1 $25000 [18] $25000

Total Purchase Cost

(CAD) $25000

Installation Cost

(CAD) $250000 [18]

Total Cost

(CAD) $275000

Referring to the above table, the total principle cost for implementing a Geothermal Organic

Rankine Cycle is $275,000 [19]. The costs attributed to geothermal drilling are usually high. The

price per foot of depth is approximately $2600 [19]. Operational and maintenance costs related to

organic rankine cycles are usually lower than steam power plants, due to less moving parts. The

O&M costs to run an ORC is approximately $0.01/kWh [15]. The total O&M costs to run this

proposed system is approximately $720 annually.

Wind Energy

Table 10: Cost breakdown for wind turbine and associated components

Component Number of

Components

Unit Cost

(CAD) Purchase Cost

(CAD)

Wind Turbine 3 $5995 [20] $17985

Wind Tower/Gin Pole 3 $4500 [20] $13500

Charge Controller 1 $0 $0

Inverter 1 $2000 $2000

Battery 8 $1055 [21] $8440

Total Purchase Cost

(CAD) $41925

Installation Cost

(CAD) $25000 [22]

Total Cost

(CAD) $66925

Table 3 summarizes the initial cost for the wind turbine and associated components. There is no

cost for the charge controller because it is included in the cost of the turbine. A cost for the inverter

could not be found when the report was created and was assumed based on inverters of a similar

capacity. The installation cost was the upper limit estimated by Bergey Wind Power but is likely


26

an underestimate given the difficulties of installing on a remote Pacific island. The shipping cost

of the components was not included and could significantly increase the cost.

Economic Overview and Payback Period

The total costs of the proposed micro grid are shown in the table below.

Table 11: Economic summary

Component Capital Investment

(CAD) Operations and Management (O&M)

(CAD/year)

Micro ORC $275000.00 [18]

$720.00

Synchronous Generator $5100.00

Wind Energy Components $66925 $234

Total Capital Cost

(CAD) $347025.03

Total Annual O&M Cost

(CAD)

$954

The total capital investments are upwards of $350,000. The total operational and maintenance

costs can range up to a total of $1000 annually. The low O&M costs of the ORC is much more

appealing than implementing a steam generator. A majority of this costs is attributed to the

borehole and geotechnical services required to set up the geothermal organic rankine cycle.

It is important to know the moment when the capital investment of the proposed design would pay

for itself. The savings from diesel annually are $21,024. Factoring in the total O&M costs, a net

savings of $20,070 annually is estimated. The payback period for the total capital investment

shown in the table above would then be achieved in the 17th year of operation.


27

4.0 Conclusion

This report has proposed a micro grid to be installed on the islands of Fiji to provide electricity for

100 occupants. A primary energy source of hot geothermal water was used to heat an organic

Rankine cycle (ORC). The ORC provides 9 kW in order to provide 100% of the energy that the

community needs. The turbine of the ORC is paired with a 3 phase synchronous generator which

runs at 1500 rpm, providing 380 V output at 50 Hz rated for 10.5 kVa. The generator runs at 0.8

power factor, which provides 8.4 kW at rated speed. Estimated monthly output from the

synchronous generator is therefore a constant 6250 kWh.

Wind is used as a secondary energy source to supplement generation from the ORC. Maximum

power output from the three wind turbines occurs between the months of May to August. After

including the microgrid efficiency, the output during this time is approximately 900 kWh. Output

during the period from September to April is lower and fluctuates between 350 and 750 kWh. This

secondary source of power would satisfy peak demands and provide a backup source of power if

the primary system failed or required service. Considering both sources the maximum power

output is 7250 kWh and the minimum power output is 6580 kWh. This means that an excess of

21% to 10% of power is produced by the proposed system.

The micro grid also implements a large battery bank, which is used in the event of a large power

outage. The battery bank alone will be able to provide 50% of the power supply for the community

for a period of approximately 2.8 days.

The micro grid were also analyzed based on economic and environmental impacts. Due to Fiji

currently burning diesel to provide energy, this was used as a control case. 28,800 litres of diesel

would provide the necessary 6000 kWh for the community. A total capital investment of

approximately $350,000 was estimated, with operations and maintenance costs of $1000 annually.

The payback period attributed to this was equal to 17.3 years, factoring in the savings from not

importing diesel. When the micro grid was used, the diesel usage was eliminated and replaced with

clean and alternate energy sources. This mitigated approximately 73,000 kg of CO2 which would

have been released into the atmosphere.


28

5.0 Nomenclature

General

𝐴 − 𝐴𝑟𝑒𝑎 [𝑚2]

𝐶 − 𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑦 [𝐴ℎ]

𝐷𝑟 − 𝑅𝑜𝑡𝑜𝑟 𝑑𝑖𝑎𝑚𝑒𝑡𝑒𝑟 [𝑚]

𝐸𝑓 − 𝐸𝑥𝑐𝑖𝑡𝑎𝑡𝑖𝑜𝑛 𝑣𝑜𝑙𝑡𝑎𝑔𝑒 [𝑉]

𝐼 − 𝐶𝑢𝑟𝑟𝑒𝑛𝑡 [𝐴]

𝑘𝑤 − 𝑊𝑖𝑛𝑑𝑖𝑛𝑔 𝑓𝑎𝑐𝑡𝑜𝑟

�̇� − 𝑀𝑎𝑠𝑠 𝐹𝑙𝑜𝑤 𝑅𝑎𝑡𝑒 [𝑘𝑔

𝑠]

𝑁 − 𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑏𝑎𝑡𝑡𝑒𝑟𝑖𝑒𝑠

𝑛𝑝 − 𝑃𝑒𝑟𝑖𝑝ℎ𝑒𝑟𝑎𝑙 𝑠𝑝𝑒𝑒𝑑 [𝑚

𝑠]

𝑛𝑠 − 𝑆𝑦𝑛𝑐ℎ𝑟𝑜𝑛𝑜𝑢𝑠 𝑠𝑝𝑒𝑒𝑑 [𝑟𝑝𝑠]

𝑁𝑠 − 𝑆𝑦𝑛𝑐ℎ𝑟𝑜𝑛𝑜𝑢𝑠 𝑠𝑝𝑒𝑒𝑑 [𝑟𝑝𝑚]

𝑃 − 𝑃𝑜𝑤𝑒𝑟 [𝑊]

𝑡 − 𝑡𝑖𝑚𝑒 [𝑠 𝑜𝑟 ℎ𝑟 𝑜𝑟 𝑑𝑎𝑦𝑠]

𝑉 − 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦 [𝑚

𝑠] 𝑜𝑟 𝑉𝑜𝑙𝑡𝑎𝑔𝑒 [𝑉]

Greek Letters

𝜂 − 𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 [𝑑𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛𝑙𝑒𝑠𝑠 𝑜𝑟 %]

𝜌 − 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 [𝑘𝑔

𝑚3]

Subscripts

𝑏𝑎𝑛𝑘 − 𝐵𝑎𝑡𝑡𝑒𝑟𝑦 𝐵𝑎𝑛𝑘

𝑏𝑎𝑡𝑡 − 𝑆𝑖𝑛𝑔𝑙𝑒 𝐵𝑎𝑡𝑡𝑒𝑟𝑦

𝑐𝑐 − 𝐶ℎ𝑎𝑟𝑔𝑒 𝑐𝑜𝑛𝑡𝑟𝑜𝑙𝑙𝑒𝑟

𝑖 − 𝐼𝑛𝑣𝑒𝑟𝑡𝑒𝑟

𝑚𝑎𝑥 − 𝑀𝑎𝑥𝑖𝑚𝑢𝑚

𝑚𝑖𝑛 − 𝑀𝑖𝑛𝑖𝑚𝑢𝑚

𝑀𝐺 − 𝑀𝑖𝑐𝑟𝑜𝑔𝑟𝑖𝑑

𝑜𝑢𝑡 − 𝑂𝑢𝑡𝑝𝑢𝑡

𝑆𝑤𝑒𝑝𝑡 − 𝑆𝑤𝑒𝑝𝑡 𝑏𝑦 𝑡𝑢𝑟𝑏𝑖𝑛𝑒 𝑏𝑙𝑎𝑑𝑒𝑠

𝑊𝑇 − 𝑊𝑖𝑛𝑑 𝑇𝑢𝑟𝑏𝑖𝑛𝑒


29

6.0 Appendix

6.1 Figures

Figure 26: Full-scale proposed system diagram


30

Figure 27: Fiji power grid [3]


31

Figure 28: Number of cycles vs. depth of discharge for 12 CS 11P battery [22]

Figure 29: Average monthly wind speed distribution for sites in Fiji [5]


32


33

6.2 Sample Calculations

Wind Energy

The density of air is taken for standard conditions of a temperature of 25 C and pressure of 101

kPa and calculated using Engineering Equation Solver.

Maximum Wind Speed: 𝑉𝑚𝑎𝑥 = 7.2𝑚

𝑠

Minimum Wind Speed: 𝑉𝑚𝑖𝑛 = 5.4𝑚

𝑠

Maximum Wind Turbine Power Output: 𝑃𝑊𝑇,𝑚𝑎𝑥 = 482.8 𝑊

Minimum Wind Turbine Power Output: 𝑃𝑊𝑇,𝑚𝑖𝑛 = 178.1 𝑊

Swept Area: 𝐴𝑠𝑤𝑒𝑝𝑡 = 7.1 𝑚2

Density of Air: 𝜌𝑎𝑖𝑟 = 1.180𝑘𝑔

𝑚3

Calculate the total power available in the wind assuming that the entire velocity is reduced to the

stagnation pressure [23] under maximum and minimum wind speeds.

𝑃𝑤𝑖𝑛𝑑 = �̇�𝐾𝐸 = (𝜌𝐴𝑉) (1

2𝑉2) =

1

2𝜌𝐴𝑠𝑤𝑒𝑝𝑡𝑉3

𝑃𝑤𝑖𝑛𝑑,𝑚𝑎𝑥 =1

2(1.180

𝑘𝑔

𝑚3) (7.1 𝑚2) (7.2𝑚

𝑠)

3

≈ 1563 𝑊

𝑃𝑤𝑖𝑛𝑑,𝑚𝑖𝑛 =1

2(1.180

𝑘𝑔

𝑚3) (7.1 𝑚2) (5.4𝑚

𝑠)

3

≈ 659.6 𝑊

Calculate the efficiency of the turbine under the maximum and minimum wind speeds observed.

𝜂𝑊𝑇,𝑚𝑎𝑥 =𝑃𝑊𝑇,𝑚𝑎𝑥

𝑃𝑤𝑖𝑛𝑑,𝑚𝑎𝑥≈

(482.8 𝑊)

(1563 𝑊)≈ 30.88%

𝜂𝑊𝑇,𝑚𝑖𝑛 =𝑃𝑊𝑇,𝑚𝑖𝑛

𝑃𝑤𝑖𝑛𝑑,𝑚𝑖𝑛≈

(178.1 𝑊)

(659.6 𝑊)≈ 27.00%

Microgrid Efficiency

Maximum Wind Turbine Power Output: 𝑃𝑊𝑇,𝑚𝑎𝑥 = 482.8 𝑊

Charger Controller Efficiency: 𝜂𝑐𝑐 = 0.95

Inverter Efficiency: 𝜂𝑖 = 0.97

The overall efficiency of the microgrid will include the charge controller and inverter efficiency

and assume that resistive losses are negligible.


34

𝜂𝑀𝐺 = 𝜂𝑐𝑐𝜂𝑖 = (0.95)(0.97) = 0.921

Calculate the power lost for one wind turbine operating at maximum wind speed.

𝑃𝑜𝑢𝑡 = 𝜂𝑜𝑃𝑊𝑇,𝑚𝑎𝑥 = (0.921)(482.8 𝑊) ≈ 444.9𝑊

Battery Bank Capacity

Battery Voltage: 𝑉𝑏𝑎𝑡𝑡 = 12 𝑉

Battery Capacity:𝐶𝑏𝑎𝑡𝑡 = 357 𝐴ℎ

Number of Batteries Connected in Series: 𝑁𝑠 = 4

Number of Batteries Connected in Parallel: 𝑁𝑝 = 4

Rated Charge Controller Power: 𝑃𝑐𝑐 = 4000 𝑊

Rated Charge Controller Voltage: 𝑉𝑐𝑐 = 380 𝑉

In series/parallel mode the battery bank voltage and capacity are increased by the number of

batteries connected in series and parallel respectively.

𝑉𝑏𝑎𝑛𝑘 = 𝑁𝑠𝑉𝑏𝑎𝑡𝑡 = 4(12 𝑉) = 48 𝑉

𝐶𝑏𝑎𝑛𝑘 = 𝑁𝑝𝐶𝑏𝑎𝑡𝑡 = 4(357 𝐴ℎ) = 1428 𝐴ℎ

It is recommended that the battery bank not be taken below 50% discharge. Therefore the actual

capacity is:

𝐶𝑏𝑎𝑛𝑘′ = 0.5𝐶𝑏𝑎𝑡𝑡 = 714.0 𝐴ℎ

Calculate the rated current for the charge controller.

𝐼𝑐𝑐 =𝑃𝑐𝑐

𝑉𝑐𝑐=

(4000 𝑊)

(380 𝑉)≈ 10.53 𝐴

Calculate the number of hours the battery bank can be run at half capacity.

𝑡 =𝐶𝑏𝑎𝑛𝑘

′

𝐼𝑐𝑐=

(714.0 𝐴ℎ)

(10.53 𝐴)= 67.83 ℎ𝑟 ≈ 2.826 𝑑𝑎𝑦𝑠


35

6.3 EES Code and Results "Set parameters" T_turb= 92.2 p_turb= 2500 pcond= 600 eta_turb= .9 eta_pump= .8 W_dot = 9 "Inlet of turbine" T[1]=T_turb p[1]=p_turb h[1]=enthalpy(R134A,T=T[1],P=p[1]) s[1]=entropy(R134A,T=T[1],P=p[1]) "Inlet of condenser" s2s=s[1] p[2]=pcond h2s=enthalpy(R134A,S=s2s,P=p[2]) h[2]=h[1]-eta_turb*(h[1]-h2s) T[2]=temperature(R134A,H=h[2],P=p[2]) s[2]=entropy(R134A,H=h[2],P=p[2]) x[2]=quality(R134A,H=h[2],P=p[2]) "Inlet of pump" p[3]=p[2] h[3]=enthalpy(R134A,P=p[3],X=0) s[3]=entropy(R134A,P=p[3],X=0) T[3]=temperature(R134A,P=p[3],X=0) Tcond=T[3] "Inlet of boiler" s4s=s[3] p[4]=p_turb h4s=enthalpy(R134A,S=s4s,P=p[4]) h[4]=h[3]+(h4s-h[3])/eta_pump T[4]=temperature(R134A,H=h[4],P=p[4]) s[4]=entropy(R134A,H=h[4],P=p[4]) s[5] = entropy(R134a, P=2500, X=0) h[5] = enthalpy(R134a, P=2500, X=0) T[5] = Temperature(R134a, P=2500, X=0) p[5] = 2500 "Work and heat calculations" wout=h[1]-h[2] qout=h[2]-h[3] win=h[4]-h[3] qin=h[1]-h[4] "Thermal efficiency" eta_thermal=(wout-win)/qin m_dot = W_dot/wout T_H = 600+273.15 T_L = 193.2+273.15


36

Q_dot[1] =m_dot*h[1] Q_dot[2] =m_dot*h[2] Q_dot[3] =m_dot*h[3] Q_dot[4] =m_dot*h[4] "Economic Analysis" Monthly_demand = 6000 "kWh" Desiel_cost = 0.73 "per litre" Annual_desiel_usage = 23000 "litre" Annual_operational_cost = Annual_desiel_usage * Desiel_cost Cost_electricity = .331 "monthly usage over 95 kWh" Cost_electricity2 = .172 "monthly usage less 95 kWh" Annual_demand = Monthly_demand *12 Revenue_annual_1 = Cost_electricity * Annual_demand Revenue_annual_2 = Cost_electricity2 * Annual_demand Cost_ORC = 25000 Cost_Installation = 250000 Total_system_cost = Cost_ORC+Cost_installation Payback_1 = Total_system_cost/Revenue_annual_1 Payback_2 = Total_system_cost/Revenue_annual_2

Figure 30: EES Results


37

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mece3410u report - renewable microgrid for a community in fiji

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