reliable exploring photovoltaic (pv) and thermoelectric remote · a more recent driver is the...

RELIABLEREMOTEPOWER

Exploring photovoltaic (PV) and thermoelectric generator (TEG) based low power off-grid systems

© Gentherm Global Power Technologies, [email protected] | www.genthermglobalpower.com

Originally published by Gentherm Global Power Technologies, at the Rio Pipeline Conference & Exhibition, October 2017.


AbstractPipelines traverse a wide variety of terrain which often has limited access to reliable grid power. Grid reliability and environmental factors like forest fires and storms can affect power availability and geographic isolation can limit access to the site. In these cases, on-site power generation should be considered. Reliable power is required for various critical loads including SCADA (Supervisory Control And Data Acquisition), instrumentation, valve operation and cathodic protection. Difficult site conditions experienced at remote locations can provide operational challenges for maintenance and refueling of remote power systems. Using traditional power solutions like gensets in low power applications can result in reliability issues and can pose a risk to both safety and production. Photovoltaics (PV) and Thermoelectric Generators (TEGs) are the most common low power off-grid generation technologies. PV generation requires no fuel as it captures energy from the sun. However, it is weather and location dependent, requiring batteries for off-grid operation. With a rugged design and no moving parts, TEGs are ideal for unattended, continuous operation in remote areas. However, TEGs require fuel; when natural gas is not available tanked fuel needs to be considered. Careful consideration of remote site conditions is essential in the design of a remote power system. This paper will discuss the advantages of both PV and TEG systems at low power remote sites. Considering both capital and operating costs while optimizing system loads can result in a robust and cost-effective power system. In some cases, a hybrid power system incorporating the advantages of multiple generation technologies may be the ideal solution.

IntroductionThere are several factors that lead to power requirements across a pipeline. Perhaps the most obvious is the sheer distance and variety of terrain that a pipeline travels. Sections of the pipeline are geographically isolated which makes site access challenging, time-consuming and costly. For this reason, automation is selected to safely monitor and control processes, and the automation equipment requires power.

A more recent driver is the negative environmental impact of venting methane from pneumatic devices. To eliminate methane venting, pneumatic devices are being replaced by electrically powered devices.

Reliable power is required for various critical loads including SCADA, instrumentation, leak detection, valve operation and cathodic protection. Unreliable power systems can pose a risk to both safety and production volumes. In more remote locations, difficult conditions pose challenges to both commercial and onsite power generation.

Even if commercial utility power can be extended to the site, the cost of backup power systems and recurring power charges need to be considered. Grid reliability and environmental factors like forest fires and storms can affect power availability and geographic isolation can limit access to the site. When evaluating these risks, on-site power generation options should be understood and considered.

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Utility PowerElectrical utility companies supply commercial power to most oil and gas facilities. The reliability of the electrical grid is quite high in most urban areas. This translates into a high level of power availability. For critical systems, a backup power source is usually supplied by a Uninterruptible Power Supply (UPS). A simplified diagram of a UPS is shown in Figure 1 below.

Electrical power is most efficiently distributed as high voltage alternating current (AC). As such, a transformer is required to convert the power to a usable level determined by the system load. Next the power is rectified to direct current (DC) so that it can be stored in a battery. An inverter is then required for any AC loads. The battery is selected based on the required system autonomy for desired duration of power in the event of a grid power failure. Normally, the system would use grid power to supply loads and float charge the battery. When grid power is interrupted, the battery power would be distributed to loads.

Each component of the UPS adds to the cost and complexity of the system. For critical applications, redundant electrical components may also need to be considered to provide continued operation in the event of failure of any one component. In summary, the cost of the UPS is driven by complexity, redundancy and autonomy.

Electrical power companies charge consumers a rate dependent on their power usage. The system is designed with reserve power to supply additional demand instantaneously. Grid infrastructure peak power generation needs to be available and offered at all times; In remote areas, utility companies may charge a demand factor for peak power usage even if the power isn’t used. In applications like block valve sites, peak power demand is relatively high, but the frequency of operation is low. This can lead to higher power charges even when valves are not operated.

The example in Figure 2 shows a 5HP block valve which equates to 4971W power demand at 75% full-rated load efficiency. However, if we estimate once a month actuation with 3-minute duration, this works out to 249WH or 0.35W of average power. A UPS can be designed to reduce the peak demand of the system by load shaving, or using the UPS battery to supply peak energy demand of intermittent loads. In this case, if we consider the low average power demand of the site, then onsite power generation options can also be considered.


Figure 1. Simplified UPS Example

Figure 2. Valve Actuation Example

Utility Grid

Step DownTransformer

Battery Charger

Battery

Bypass

Inverter

Load

Use VFD to handle inrush demandValve nominal power: 5HP * 745.7/75% =

Energy for 3 min activation4971W * 3 min/60 min/h =

Average power for once a month activation249WH * 1/30 days/24H =

4971W

249WH

0.35W

Utility Power AC Panel 5HP Block Valve

Onsite Generation Battery 5HP Block Valve

~

Utility Power cont...Other considerations for grid power in remote locations are power accessibility and reliability, both of which have cost implications. The cost to extend the grid varies depending on the distance from existing utility infrastructure. The maintenance of this extension may be passed on to the consumer in their recurring electrical power rate. In many cases, the reliability of the power at a remote location will be lower than in urban areas and the UPS design and cost will reflect this. For example, in the event of natural disasters like storms and fires, utility power will fail. Remote sites at the fringes of the grid are likely to be the last to have power restored, which drives the autonomy of the UPS to be higher. In these cases, it would be prudent to consider a backup power generator or other on-site power generation.

Remote Power GenerationWhere commercial power is unavailable, or not cost-effective, onsite power generation can be considered. Depending on the selected power generation method, difficult site conditions can provide operational challenges for maintenance and refueling of remote power systems. Furthermore, many low power solutions have reliability issues and can pose a risk to both safety and production.


System Design OversightsThe most common oversight when designing a remote power system is treating it like a grid powered site. The grid has excess capacity and systems can be oversized with minimal impact on system cost. Grid connected loads are overestimated by using peak load ratings and adding safety factors. However, using this methodology on a remote power site will result in a high cost, oversized system that could potentially be less reliable due to increased complexity.

Another common oversight in remote system selection is overlooking operating expenses. Often the lowest cost solution can have high operating expenses, like frequent maintenance which is exacerbated by remote site geographic location. As well, lower cost systems may not prioritize reliability, which can increase system downtime. Furthermore, the cost of lost production due to power failure should also be considered when evaluating system costs.

The opposite mistake can also be true when trying to lower system capital costs by under-sizing or selecting the wrong type of batteries. This can result in more frequent power failure and accelerated aging of the batteries, especially in cycling applications. Valve regulated lead acid batteries are commonly used for remote power. However, using low cost commercial batteries like 12V lead acid mono-blocs can result in frequent replacement intervals. Longer life industrial 2V cells may be a better option. In high temperature applications, robust flooded NiCd batteries may need to be considered. Finally, the weight of large battery banks and the costs to replace them in remote locations should not be underestimated.

Lastly, critical applications, like leak detection, cannot afford to have low power reliability. Even a small pipeline leak with relatively low environmental impact can have significant negative impact on a company’s brand and image.

Load OptimizationThe first step in selecting a remote power system is to analyze the electrical loads and optimize them to reduce cost, size and fuel consumption. Most grid connected sites use AC powered equipment, while many remote power generation technologies and UPS systems are DC based. Selecting DC equipment for remote locations simplifies electronics and eliminates conversion losses. Low power DC equipment like LED lighting should be considered when possible. Better yet, for unmanned sites lighting can often be placed on a timer switch so it only consumes power during maintenance visits.

Figure 3 (next page) illustrates how a load’s duty cycle can be considered to calculate the average power consumption of the load. In off-grid systems the battery can be designed to handle the peak loads with the generator required to supply only average power and a small reserve for battery recharge.




Load Optimization cont...Average power is an important consideration for low power off-grid sites. Radios are commonly used for communication and have a relatively high peak power consumption during transmit bursts. However, data volumes are generally quite low in remote locations and as such the transmit duty cycle is correspondingly low while the standby duty cycle of the radio will be high. Considering these factors can help size a system built around the average power consumption of the radio rather than oversizing it for an unrealistic continuous transmit scenario.

HVAC (Heating Ventilation and Air Conditioning) loads can also require a lot of power. It is best to eliminate these loads as much as possible. Where heat is required, consider using gas fueled heaters (i.e. catalytic heater) or capturing waste heat from generators in a combined heat and power system. To avoid cooling loads, opt for industrial rated equipment and use fans sparingly.

Remote Site ConsiderationsBy nature, remote sites are geographically isolated making them difficult to access. The length and difficulty of travel on poor roads, or by helicopter, boat or foot, make reducing site visits a priority. Weather conditions and extreme temperature can affect the performance of power generation systems or affect seasonal access. These and other site characteristics should be considered when evaluating a remote power system.

Space restrictions at site should also be considered since there is increasing pressure to reduce the environmental footprint of sites to lower costs. This is most apparent on offshore platforms where there is limited space to install equipment and unit weight needs to be taken into consideration. In addition, there may be requirements to install equipment in hazardous areas restricting options and increasing costs.

Challenging site conditions can also affect onsite fuel options. Does the existing fuel source need to be conditioned or alternately what fuel can be transported and stored at site? Are there any security considerations or environmental restrictions such as spill containment to consider?

The ability for Operations resources to manage the site should also be considered. How frequently do they visit and what skills and training will they require to operate the system?

The combination of all these factors can affect operating costs and drive the need for reliability in a remote power system.

GensetsA genset is a combination of an Internal Combustion Engine (ICE) and an electrical generator. Gensets are used extensively in standby and temporary power applications. ICEs are a mature and reliable source of energy designed for intermittent operation. However, running them continuously as the prime power source requires a high level of maintenance and results in a short operating life. In cases like intermittent valve operation, the genset must be sized for peak load while primarily operating at low load, reducing efficiency and reliability. In some cases, dummy loads are added increasing fuel consumption.

Figure 3. Lighting Example

Utility Power AC Panel 100W Load

Onsite Generation Battery LED Light

Grid capacity for 15A circuit * 120VAC =

Peak demand for single light bulb (continuous)

1800W

100W

Light on photocell, 50% duty cycle: 100W * 0.5 = 50W

17W LED light with 50% duty cycle: 17W * 0.5 = 8.5W

~

Photovoltaic PanelsPhotovoltaics (PV) are a growing renewable energy technology capturing power from the sun. Advances in technology and manufacturing have increased the efficiency and lowered the cost of these systems. Major panel manufacturers include Trina Solar, Canadian Solar, Jinko Solar, JA Solar and Hanwha. When coupled with battery storage, arrays of PV panels are increasingly used in off-grid applications. However, in remote locations and under adverse weather conditions these systems can have operational challenges.



PV panel power production is specified using peak sun hours. Peak sun hours refers to the solar insolation which a location would receive if the sun were shining at its maximum value for a certain number of hours. Peak Solar Radiation is 1kW/m2. Figure 5 below displays the average global horizontal irradiance for Sao Paulo, based on NASA monthly averaged 22-year data (NASA, 2017).

Most remote power systems require a steady amount of power year-round. Excess power will be produced in the peak summer months but cannot be consumed. Thus, panels are often angled towards the horizon to optimize power production during the winter months which conversely decreases power production during summer months.

Due to continuous year-round power requirements, off-grid PV systems need to be designed to operate during the worst solar conditions. A rough estimation of power production can be made by multiplying the rating of the solar panel by the number of peak hours. For example, if a location has minimum 3 hours of peak sun and a 200W PV array, how much average power can it supply?

Figure 4. Off-Grid PV System

Figure 5. Sao Paulo GHI

Average Power = 3H x 200W/24H = 25W~

PV Array Charge Controller Battery Load

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5

4

3

2

1

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Jan

uar

y

Feb

ruar

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Mar

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Ap

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Dec

emb

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tem

ber

Au

gu

st

July

Jun

e

May

1.00.90.80.70.60.50.40.30.20.10.0

Daily Radiation (kWh/m2/day)

Clearness Index

Solar Global Horizontal Irradiance



Photovoltaic Panels cont...In this case, the panels will produce rated power for 3 hours to supply load and charge a battery bank. For the remaining 21 hours of the day the batteries supply the load. Roughly speaking, with 3 hours of peak sun you need a solar array sized at least 8x the average power.

The preceding estimate is a gross simplification. There are many deratings that need to be considered including: soiling, shading, snow accumulation, panel orientation (azimuth and slope), mismatch as well as temperature and aging deratings. A 25% cumulative derating factor is common. IEEE publishes standard 1562-2007 which is a good guide for array and battery sizing in stand-alone photovoltaic (PV) systems. Furthermore, there are software simulation packages available that can aid in PV system modelling. In addition to PV panel power deratings there are several additional system losses that should be considered including solar charge controller efficiency, cable losses and battery storage efficiency. Battery round trip efficiency is a significant factor with efficiencies around 85% for lead acid chemistries.

For off-grid systems, the battery bank needs to be sized for worst case conditions. For Sao Paulo, NASA data indicates the maximum number of no-sun days to be 10 in September. The battery size will affect both the reliability of the power system and the life of the batteries. IEEE standard 1562-2007 states the minimum battery autonomy should be 5-7 days for non-critical loads and areas with high solar insolation, and 7-14 days for critical loads or areas with low solar insolation (IEEE, 2008). In addition, a PV system cycles batteries daily, the deeper they are cycled the faster they age, requiring more frequent and costly replacement.

Thermoelectric GeneratorsThermoelectric Generators (TEGs) have been used extensively in remote off-grid pipeline applications for decades. Gentherm Global Power Technologies (GPT) is the world’s largest supplier of thermoelectric generators. The company was established in 1975 to commercialize the unique lead telluride technology developed by 3M Corporation for the Apollo space program. Individual generators range in size from 30W to 550W and can be combined in parallel for up to 5kW of continuous DC power. Figure 6 below illustrates the operation of a GPT TEG.

Figure 6. TEG Cross Section



Thermoelectric Generators cont...TEGs burn gas or propane to create heat which is directly converted into DC electricity. As heat moves from a gas burner through a thermoelectric module, it causes an electrical current to flow. At the heart of every TEG is a hermetically sealed thermoelectric module, called a thermopile. This thermopile contains an array of semiconductor elements. The durable module provides a chemically stable environment for the thermoelectric material and ensures a long service life. A burner maintains a high temperature on one side while heat sink fins cool the other. The temperature difference across the thermopile creates steady DC electricity using no moving parts. This rugged design makes TEGs ideal for long-term unattended operation where site visits are difficult.

For cathodic protection applications, the generator can impress current directly into the ground bed without the use of a rectifier or battery bank. When equipped with a battery bank the TEG is sized to supply the average load of the system and float charge a battery for peak loads and backup autonomy. Float charging extends the life of the battery bank and eliminates the need for costly deep cycle batteries. In most cases, TEG systems can use significantly smaller battery banks compared to PV systems.

Ordinarily, TEGs operate directly off pipeline gas. However, on liquids pipelines TEGs can operate on propane in which case remote site refueling can pose a challenge. A hybrid power system blends the advantages of multiple generation technologies. Hybrid PV-TEG systems combine the reliability of TEGs while reducing fuel consumption by using PV panels to capture the sun’s energy. In the Gulf of Thailand PV-TEG hybrids are the preferred method for remote wellhead platforms, optimizing CAPEX while offering high reliability and a reduced system footprint (Rongsopa, 2012).

PV/TEG ComparisonsModern pipeline facilities have an increasing number of electrically powered devices. Traditionally, small electrical loads like instrumentation and SCADA might have their own independent PV power system. For loads less than 30W, PV power systems are relatively cost-effective. However, the operational burden of multiple power systems can be alleviated by combining smaller loads with larger ones like block valves and cathodic protection into one single distributed power system. A centralized system equipped with remote communications offers the added benefit of increased control and monitoring of all site equipment.

Following best engineering practices, a remote power system should be sized using manual calculations. This sizing can be verified using simulation software. In the following example, HOMER Pro micro-grid software was used to validate system size and estimate fuel consumption (HOMER Energy, 2017). Table 1 compares TEG and PV systems modelled for a 90W power system in Sao Paulo. The battery replacement interval has been estimated based on temperature derating and depth of discharge curves of the battery. In the TEG Hybrid system the TEG only operates when the battery level is low due to poor solar resource. This reduces system fuel consumption and extends the life of the battery.

Table 1. 90W Load – Sao Paulo

OptionPower

AvailabilityPV Array

(150W Qty.)

Annual FuelConsumption

(Propane)

24VDCBattery Bank

Min 10ºC(12VDC Qty.)

Battery Life(Replacement

Interval)Battery Type

TEG5120

99+% NA 5000 L100 AH

(2 batteries)5 years

12VDC 100AH5 year nom life

PV95 95%1200 W

(8 panels)NA

600 AH(6 batteries)

2 years12VDC 200AH

Deep Cycle5 year nom life

TEGHybrid

99.5%900 W

(6 panels)200–300 L

400 AH(4 batteries)

3 years



PV/TEG Comparisons cont...The following Figure compares the footprint of the PV and TEG 90W systems. Pure TEG system is on the left, followed by the hybrid system in the middle and pure PV system is on the right.

At low power levels, oversizing a PV array and battery bank to increase system reliability is feasible. However, on larger loads the footprint and cost of the system increase rapidly. For PV systems the operational expense of maintenance and battery replacement becomes a significant factor in the total cost of ownership. If natural gas is available, a pure TEG solution is often the most attractive solution as presented in the estimated TCO (Total Cost of Ownership) comparison below. An added benefit of the high reliability TEG solution is less system downtime; resulting in decreased production losses and lower maintenance costs.

Figure 7. PV/TEG Comparison

Figure 8. TEG vs. PV – Natural Gas TCO

120,000

100,000

80,000

60,000

40,000

20,000

01

Solar (PV)

TEG w/Battery

TCO – Sao Paulo, Brazil 90w (Natural Gas)

140,000

2 20191817161514131211109876543

YEARS

DO

LLA

RS

(US

D)

39.24 [997]

29.

87

[75

9]

80

.50

[20

45

]17

9.9

4 [4

571

]

98

.08

[24

91]

110.60 [2809]

98

.86

[25

11]

Pure 5120 TEG Solution

Inches [mm]

5120 TEG Hybrid Solution

Pure PV Solution

88

.20

[22

40

]

81.00 [2057]



PV/TEG Comparisons cont...When operating on tanked fuel, the higher capital cost of a hybrid system may be worth the reduced operational fuel costs. Refer to the estimated TCO comparison in the following Figure. An added benefit of the hybrid solution is the full load redundancy offered by the TEG.

ConclusionsTechnological advances are increasing the number of electrically powered devices on pipeline systems. However, grid power may not be a viable option in remote areas. Onsite power generation is an alternative requiring careful consideration. Photovoltaics (PV) and Thermoelectric Generators (TEGs) are the most common low power off-grid generation technologies. Utilizing these technologies with the right type and size of battery can deliver a robust off-grid power system.

The specific characteristics of the remote site play a large factor in selecting a power solution. The availability of fuel and the environmental conditions, like sun exposure and temperature range, are important factors to be considered when designing the system.

Power options should be evaluated with careful consideration of both capital and operating expenses, in addition to the cost of system downtime. Optimization of system loads and definition of critical power are crucial to correct sizing of the power system. Oversizing the system will add complexity reducing reliability while increasing costs.

PV generation requires no fuel as it captures energy from the sun. However, it is weather and location dependent, has a relatively large footprint and requires batteries for off-grid operation. For critical loads a large battery bank or backup power source is recommended.

A TEG’s rugged design has no moving parts making it ideal for unattended operation in remote areas. They are an ideal option for natural gas wellheads and pipelines. However, when natural gas is not available tanked fuel needs to be considered.

A hybrid power system is an attractive option which blends the advantages of multiple generation technologies. Hybrid PV-TEG systems combine the compact footprint and reliability of TEGs with the reduced fuel consumption of a PV system to harness energy from the sun.

Taking careful consideration of system requirements into the design stage will help in the selection of a remote power system built to provide reliable power while reducing the cost of ownership.

REFERENCES:

HOMER ENERGY. HOMER Pro. Retrieved from http://www.homerenergy.com/HOMER_pro.html, 2017.

IEEE. IEEE Guide for Array and Battery Sizing in Stand-Alone Photovoltaic (PV) Systems (1562-2007). Retrieved from https://standards.ieee.org, Section 6, 2008.

NASA. Surface meteorology and Solar Energy. Retrieved from https://eosweb.larc.nasa.gov/sse/, 2017.

RONGSOPA, R. Hybrid power generation for offshore wellhead platform: a starting point for offshore green energy. In: International Petroleum Technology Conference #14563, Bangkok, Thailand, 2012.

Originally published by Dominic Pituch, Manager, Sales Engineering, Gentherm Global Power Technologies, at the Rio Pipeline Conference & Exhibition, October 2017.

Figure 9. TEG vs. PV vs. Hybrid – Propane TCO

160,000

140,000

120,000

100,000

80,000

60,000

40,000

1

Solar (PV)

TEG w/Battery

TCO – Sao Paulo, Brazil 90w (Propane)

180,000

2 20191817161514131211109876543

YEARS

DO

LLA

RS

(US

D)

20,000

0TEG Hybrid

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