Report Utility Scale Grid Energy Storage Greater Use of Renewable Energy and Reduced Greenhouse...

8/2/2019 Report Utility Scale Grid Energy Storage Greater Use of Renewable Energy and Reduced Greenhouse Emissions

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Utility Scale Grid Energy Storage: DistributingRenewable Energy and Reducing Greenhouse Gas

EmissionsThien Do

The University of AdelaideSA, Australia

[email protected]

AbstractEnergy storage plays a pivotal role in distributing

renewable energy into the power grid, and yet it is often

overlooked. Renewable energy is intermittent and unpredictable

due to their reliance on natural events. Due to these factors,

renewable energy is not suitable to be connected directly into the

grid. Energy storage is the link connecting renewable sources to

the power-grid. This paper discusses effective methods of storingelectrical energy, and techniques involved with implementing

storage units into the grid including load shifting, peak shaving,

frequency regulation, spinning reserve, and uninterruptable

power supply applications. These techniques increase the use of

renewable sources in the grid and reduce greenhouse gas

emissions. This paper also examines the types of utility scale grid

energy storage systems currently available, identifying the

advantages and disadvantages of each. It will then summarise

and compare all the storage types, discussing the suitability of

each for various circumstances.

I. INTRODUCTION

The energy demand from consumers fluctuates at variouspoints within any given day. To accommodate for thesechanges, power plants are required to continuously alter theirenergy production levels throughout the day, which results indecreased operational efficiency. Grid energy storage helps tosolve this problem by allowing power plants to produceenergy at a constant level. To implement this technologyeffectively, techniques of load shifting and peak shaving areemployed.

Load shifting involves the storage of energy during off-peak demand periods, and the release of this stored energy

during peak periods. Basically, some energy from the plant issaved during low demand periods, and then used later when agreater demand is required. This effectively levels out theelectricity demand during the day so that power plants do nothave to alter their output load frequently. As a result, base-load power plants are able operate at a constant levelcorresponding to a maximum efficiency. Greenhouse gasemissions are significantly minimised this way by allowingfossil fuel-powered plants to operate constantly at maximumefficiency. This process is illustrated in figure 1.

Figure 1: Graph illustrating the method of load shifting. The horizontal linerepresents constant power production while the curve represents powerdemand.

Peak shaving is very similar to load shifting. The maindifference is that it requires second source of energy, whichhas traditionally been a fossil-fueled generator. This secondsource is used to provide the extra energy required during

peak hours, while the base load plant continues to producepower at a constant level. To reduce greenhouse gas emissions

and to employ more use of renewable energy, grid operatorshave coupled a renewable energy source together with astorage unit for peak shaving services, instead of using afossil-fueled generator which emits greenhouse gases. Duringoff-peak periods, renewable sources can be used to rechargethe storage unit; then during peak periods, the storage unit willdischarge power back into the grid.

Grid energy storage works very effectively withrenewable sources as it solves the problem of intermittency.Photovoltaic cells (PV) and wind turbines are inherentlyflawed in that they are intermittent; the amount of electricalenergy they produce is highly dependent upon uncontrollablevariables such as the weather. Intermittency in the grid can

damage load-equipment due to fluctuations in voltage andpower. Energy storage systems can eliminate thisintermittency and enable reliable renewable energy to bedischarged into the grid.

PV and wind turbine systems are also flawed in that theycannot control when energy is released into the grid, ie.sunlight or wind may not be available when it is required, orconversely, large amounts sunlight or wind may be presentwhen their energy is not required. This is obviously noteffective. Grid storage coupled with the renewable source can


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control when energy is released into the grid, effectivelyeliminating this flaw.

The ability to control the time of energy release into thegrid also has strong financial advantages. Energy is expensiveduring peak periods, and is cheap during off-peak periods.Low-cost energy can be used to recharge the storage unitduring off-peak periods, and then during peak periods, theenergy can be sold at higher prices.

Frequency regulation is another important serviceprovided by some energy storage systems, and is one that isrequired by all grid operators. Frequency regulation is aservice which provides stability in the grid by balancingsupply and demand: when there is excess electricity in thegrid, the frequency regulating system will absorb energy fromthe grid; when there is an energy deficit, the system willdeliver energy back into the grid. Traditionally, grid operatorshave used generators powered by fossil fuels to providefrequency regulation- which use fuel inefficiently and producea large amount of greenhouse gas emissions. By incorporatinga storage unit into the system instead, the level of greenhousegases emitted may be significantly reduced. With theincreased injection of intermittent renewable energy in thegrid, frequency regulation has become increasingly important.

There are various technologies suitable for energy storageincluding: batteries, compressed air energy storage (CAES),

pumped hydro, flywheels, super-capacitors, hydrogen storageand superconductive magnets. Utility scale systems generallyrequire energy storage technologies with capacities greaterthan 1MW [1]. From the figure 2, we can see that some

batteries, CAES, pumped hydro, high power fly wheels, andsupercapacitors have energy capacities in this required range.

Note from the graph that the available discharge time of highpower fly wheels and supercapacitors is in the measure of

seconds. These systems are more suitable to providefrequency regulation services or backup power inuninterruptible power supply (UPS) applications rather thanload shifting and peak shaving.

Figure 2: A graph comparing energy storage technologies in terms of dischargetime and system capacity [2].

The capacity of hydrogen storage is not shown on thisdiagram. This information is not readily available asdevelopment of hydrogen storage for grid-connectedapplications is still in its early stages [3]. Current resourcessuggest that, until relatively recently, hydrogen storage hasmainly focused on hybrid vehicles and off-grid applications.Although hydrogen is yet to be used for grid-connectedstorage applications, it holds great potential for future energy

storage due to its high energy density (39.4kWh/kg threetimes greater than any other chemical fuel), and abundance(hydrogen is the most abundant element on earth) [4].

II. BATTERIES

Rechargeable batteries are one of the best options for thestorage of PV energy as PV generation systems often requireenergy storage of a few MW for a few hours [2]. There aremany types of rechargeable batteries used for storingrenewable energy; leadacid, nickelcadmium, sodium-sulfur(NaS), lithiumion, metalair, and flow batteries. From figure2, we can see four types of batteries which are suitable forutility scale systems; namely lead-acid, nickel-cadmium,sodium-sulfur, and flow batteries.

Lead acid batteries provide a default option for storage asthey are well-developed, low in cost, and are readily available[1]. But these have low energy densities, typically resultingwarehouse-size buildings [5]. They also have low life spans(approximately five years) due to their acidic nature whichcorrode battery components [5]. Over time, nickel-cadmium

batteries have replaced lead-acid batteries, but the industry isalready moving away from this technology as more advanced

batteries are developed [1]. These advanced batteries come inthe form of sodium-sulfur and flow batteries as they provide abetter alternative to grid storage applications.

A. Sodium-Sulfur Batteries

1) Operation Principles

The Sodium-sulfur (NaS) battery is a type of molten metalbattery. It consists of a molten sulfur positive electrode, amolten sodium negative electrode, and a sodium beta-aluminaceramic electrolyte to separate the electrodes as shown infigure 3.

Figure 3: Diagram illustrating the discharge process of a NaS battery.


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During the discharge process, the sodium electrode forceselectrons through the external load and into the sulfurelectrode. To balance out the charges of this process, positivesodium ions pass through the electrolyte and into the positiveelectrode. The ions react with the sulfur to produce sodium

polysulfide (Na2S4). This process is explained graphically infigure 3.

The beta-alumina ceramic electrolyte is important to the

NaS battery structure due to its ability to conduct sodium ionswell, while preventing electrons to pass through freely [2].This minimises self-discharge of the battery, thus increasingthe efficiency.

2) Advantages

All the performance characteristics of the NaS battery aresuperior to those of the conventional lead acid battery. Byusing the lead acid battery as the standard battery, we cancompare the characteristics of both batteries to get a betterunderstanding of the NaS batterys advantages. The mainadvantages of the NaS battery are listed below:

High efficiency (87-90 percent); compared to 74-78 percent for that of the lead acid battery [2]

Long life cycle ; triple that of the lead acidbattery [2]

High energy density; three to five times greaterthan that of the lead-acid battery [2]

Compact (room-size), as opposed to warehousesizes. [2]

Rapid response rate; suitable for UPS systems [2]

The compactness is credited to the high energy density.This enables the battery to be installed in densely populated

areas where other large storage options are not feasible.Construction of new transmission lines, substations and power

plants can also be avoided [5].A major advantage which is characteristic of most

batteries, including the lead acid battery, is rapid responserate. In particular, the NaS battery is able to completelydischarge in 1ms [2]. Rapid response rates enable storagedevices to be used for UPS systems and backup powerapplications. UPS systems are used to provide load equipmentwith continuous power, particularly in the case of a poweroutage.

3) DisadvantagesThe greatest disadvantage of the NaS battery is its cost. It

costs about $2,500/kW, approximately twenty times that of alead acid battery [2]. However, prices are expected to ease asthe batteries are mass produced [2,5].

The operating temperature of the NaS battery is alsodisadvantageous. The battery must be kept at approximately300C during operation for optimum performance; if theelectrodes are allowed to solidify, the damage would beirreparable [2].

4) Current Installments

NaS batteries have been installed in Japan since the1990s. In 2001, more than 20 batteries were installed in Japan,including two 6 MW batteries. Altogether, there were enough

NaS batteries to light the equivalent of about 155,000 homes[5]. In 2006, the American Electric Power(AEP) utilitycompany installed a 1.2 megawatt NaS battery in North

Charleston [5]. Another US utility company, PG&E, isplanning to install a 4 megawatt NaS battery in Silicon Valley,California, by end of 2010 [6].

B. Vanadium Redox Flow Batteries

The other main type of battery suitable for utility scaleenergy storage comes in the form of flow batteries, alsoknown as regenerative fuel cells. Flow batteries are a type ofrechargeable battery where the electrolytes are pumpedthrough an electrochemical cell from external tanks. There arethree main types of flow batteries; polysulfide-bromide (PSB),

zincbromine (ZnBr) and vanadium-redox battery (VRB). Thefollowing section will focus on the VRB.

1) Operation Principles

The VRB is a unique battery in that it has just oneelctroactive element in its two electrolytes; that is, there isonly one type of element which transfers the charges duringthe chemical reactions. This element is vanadium, which hasfour different possible oxidation states: V2+, V3+, V4+,V5+.

Figure 4 illustrates the structure of a VRB system. Thereare two half cells, two tanks and two pumps. The half cellsconsist of the negative and positive electrodes; this is where

the chemical redox reactions occur to produce the electriccurrent. The two tanks hold the electrolytes (consisting ofvanadium solution in sulfuric acid), which are pumpedthrough the half cells during operation to continuouslyreplenish the chemical fuel. It is necessary to store theelectrolytes in large external tanks as the energy density of theelectrolytes is very low, thus large volumes of electrolytes arerequired to make up for the low density and to increase thecapacity of the system.

Figure 4: A diagram depicting the structure of a basic VRB system [6].
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2) Advantages

The VRB is inherently unique compared to other batteriesdue its structure and chemical make up. Thus, it has uniqueadvantages to other batteries, making it a good batteryalternative. Again, it is useful to compare its performancecharacteristics to lead acid batteries which are the standard

battery. The main advantages are listed below:

Relatively high efficiency (72-84 percent) [2] High life cycle; two to three times that of the leadacid battery [2]

Large capacity; double that of the lead acid battery[2].

Rapid response rate; suitable for UPS applications[7,8,9]

Mechanical rechargeability; replenished solutioncan be pumped in to replace depleted solution in thecase that there is no electricity recharge the battery[9]

The large capacity of the VRB is due to the large amount

electrolyte solution stored in the external tanks; to increase thecapacity of the system, simply store more solution in theexternal tanks. The tanks also allow for mechanicalrechargeability. When there is no available electricity torecharge the battery, more solution can be poured in to replacethe depleted solution [9]. This essentially gives the battery theability recharge to rapidly, rather than having to wait forelectrical power to slowly recharge it.

The VRB also has a low operating cost per kilo-watt-hour. As the storage capacity increases, the cost decreases aslow as US$150 /kWh [9,10], which is even lower than that ofthe lead acid battery [10].

3) Disadvantages and Further Developments

The main disadvantage the VRB is its low energy density(approximately 25Wh/kg) [8], which is even less than that oflead acid batteries. Low densities mean that a larger area isrequired for storage. To improve this current density, effortshave been made to increase the concentration of vanadiumions in the electrolyte; increasing the concentration willincrease energy density. At present, the concentration ofvanadium ions in sulfuric acid is limited to 2M becausegreater concentrations will render the solution unstable.Studies are now focused on improving the stability of 3-4Msupersaturated vanadium solutions in sulfuric acid. In 2001,

the University of New South Wales (UNSW) achieved higherconcentrations of vanadium in a new Generation 2 vanadium

bromide redox cell [8].

4) Current Installments

The VRB was developed at the UNSW in 1985 and isnow installed in Australia, Thailand, USA and Japan. In 1997,an 800kWh battery was installed in Japan by the Kashima-Kita Electric Power station. A second battery rated at 1MWhwas installed in Japan by the Sumitomo Electric and Kansai

Electric Power Co., Inc. Both were installed for load shiftingsevices. More recently, a 6MWh VRB was installed for thestorage of wind energy in Hokkaido, Japan [8].

III . COMPRESSED AIRENERGY STORAGE

Compresses air energy storage (CAES) stores energy inthe form of compressed air beneath the ground in reservoirssuch as salt mines and aquifers. When power is required, thisair is heated and expanded through gas turbines, which thengenerate electricity into the grid. These plants have very largecapacities compared to other storage options other than hydro-

pumped storage; it is considered to be a form of bulk energystorage.

A. Operation principles

Figure 5 below displays the main components of a typicalCAES system. In periods of low demand, energy from the gridis used to force ambient air into the system where it iscompressed to approximately 11 atm by the compressor [11].The air is then cooled and further compressed in a boostercompressor system. This final storage pressure ranges from 12to 100 atm, depending on the type of storage reservoir andrequirements of the CAES system [11]. If high pressures aredesired, it is necessary to divide the booster compressor

process up in stages and add intercooling. The air is thenready to be stored in the underground reservoir. Due to thecompression stages, the air gets heated. Storage temperaturesare typically lowered to around 40C to minimise thermalstresses in the reservoir walls [11].

During the power generation cycle, the high-pressured airis expanded by igniting it with gas. This increases the velocityof the air as it passes through and drives a gas turbine, whichgenerates power back into the grid.


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Figure 5: A labelled flow diagram displaying the main components of a typicalCAES system.

B. Advantages

CAES is a form of bulk energy storage; it has a very largecapacity, larger than most types of storage. Existing plantshave capacities over 100MW, which is much larger than thecapacity of batteries, flywheels and supercapacitors. The only

other type of bulk energy storage is hydro-pumped storage.Thus, CAES is advantageous for situations requiring largestorage capacities.

Another major advantage of CAES is its high ramp rate,meaning that the system is capable of increasing its outputrapidly in response to a major load change or outage. Thischaracteristic enables CAES plants to act as a spinningreserve. Hirst and Kirby define spinning reserve as:generators online, synchronized to the grid, that can increaseoutput immediately in response to a major outage and can

reach full capacity within 10 minutes [12]. For example, inthe case of a power outage, the CAES system is able to comeon line quickly and provide emergency backup power until

normal systems come back online.The main advantages of CAES are summarised below:

Large system capacity; greater than 100MW[2]

High ramp rate; suitable for spinningreserve [10]

The storage medium is air, which isabundant, free, and readily available

The environmental impact is minimal [10];the storage reservoir is an underground site whichwas already present (such as an abandoned mine orempty aquifer), thus the plant has no further impacton the environment.

C. Disadvantages

There are various disadvantages regarding the CAESsystem, which would explain why very few CAES systemshave been built. Firstly, it requires an underground reservoirsuch as an unused empty salt dome, an aquifer, or anabandoned mine. If its service is required in a region where nosuitable underground site is available, another storagealternative may have to be employed. Secondly, the system isnot self-contained; instead it depends on a pipeline to supplynatural gas for the combustion chamber. Finally, it has arelatively low efficiency (70-78 percent), which is the lowestefficiency of all the storage types discussed in this paper.

D. Further Developments

To counter the above disadvantages, a number ofdevelopments have been under way.

A possible solution to the first and second problems is tocombine the CAES with a thermal plant; this is called athermal and compressed-air storage (TACAS) system [13].Here, the compressed air is forced into conventional gas tanksor pressure vessels for storage instead of an underground

reservoir. When the compressed air needs to be expanded, it ispassed through a thermal plant which heats it, instead ofigniting it with gas. The system does not require anunderground storage reservoir and is self-contained. TACASsystems have a life expectancy of 20 years [13].

Further developments have also been made to achievegreater efficiencies in CAES. During the cooling process, asignificant amount of energy is discarded and as a result, a lot

of energy is wasted [14]. Efforts have been made to developadiabatic systems where the ambient heat is stored and usedlater to preheat the air before it enters the gas turbine.

The heat can be stored in pebble beds in direct contactwith the air as it passes through. The difficulty with this is thatthe pebbles may break into dust particles due to the thermalstresses and damage rotating machinery. Another option is touse heat exchangers with concentrated brine as the coolingliquid. Brine is proposed in this idea as it is a low-boilingliquid, is cheap, and readily available. Adiabatic plants arecurrently under development and can potentially achievegreater efficiencies and zero direct CO2-emissions[14,15].

D. Current Installments

There are currently only two CAES plants built inthe world. The first plant is rated at 290 MW andwas built in Huntorf, Germany, in 1918 [15]. Thesecond was built with a capacity of 110 MW inAlabama, USA in 1991.

IV. PUMPED HYDRO STORAGE

Traditional hydroelectric dams are one of the oldest formsof power generation, yet it provides by far the greatest amountof renewable energy in the world [16]. Hydropower producesover 16 times the amount of solar and wind power combined[16], and in the US, it accounts for 77 percent of renewableenergy capacity. Part of this 77 percent includes a 16 percentcontribution by pumped storage alone [17].

A. Operation Principles

Pumped hydro storage is a form of hydroelectric power-generation where energy is stored by pumping water from a


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lower reservoir to a higher reservoir. During hours of

Figure 6: Diagram depicting the operation of a typical pumped storage system.Note that the water flows through the same pathway in both generating andpumping modes of operation. This is possible due to the reversible pump-turbine

peak demand, this stored water is allowed to flow thoughturbines which generate electricity in the same method asconventional hydro stations. When energy production is inexcess, water is pumped from the lower to upper reservoir to

store this excess energy. Reversible pump-turbine andgenerator-motor equipment serve as both a pump and agenerator, allowing the system to operate more effectively asshown below in figure 6.

B. Advantages

Pumped storage is the major type of bulk energy storagedue to its massive capacity and well developed technology.The main advantages of pumped storage are listed below:

Relatively high efficiency for bulk storage(73-85 percent) [2]

High ramp rate (can discharge 800MW

within seconds); suitable for spinning reserves [16]

Largest system capacity compared to anyother grid energy storage currently available [17,18];the largest existing pumped storage plant has acapacity of 2400 MW [18]

No fuel is required, only the volume andmotion of water [16].

Operates on robust, well developed, andreliable technology; pumped storage is one of theoldest energy storage technologies and is widelyaccepted.

Besides its energy storage and peak shaving capabilities,pumped storage also has significant benefits in managing theenergy grid; pumped storage stations can provide networkfrequency control and spinning reserve. These services are

possible due to the systems very high ramp rate [16, 17]. Forexample, the Grand Coulee Dam in Washington State can gofrom operating at low load to full load (around 800 MW) inmatter of seconds [16].

Pumped storage is now used to resolve the intermittencyof renewable sources such as solar and wind energy where the

energy from PV cells and wind turbines can be used to pumpthe water to the upper reservoir [17].

C. Disadvantages and Further Developments

The disadvantages of pumped hydro stem from landrequirements and its environmental impact. Pumped hydrorequires two large water reservoirs, including one at an

elevated level; many regions do not have the landscaperequirements for such a system [10]. Furthermore, even if anarea has the required landscape, there is strong opposition tosuch facilities due to the significant environmental impact

brought upon by the size and dynamic behaviour of the system[10].

Underground pumped storage is a solution to theseproblems. An underground pumped storage system is onewhere the upper reservoir is on the surface of the ground,while the lower reservoir is located in mined cavernsunderground [19]. In regions where conventional pumpedstorage is desired, but where no appropriate sites are present,underground pumped storage may be the solution. Undercertain underground conditions with suitable rock formations,the lower reservoir can be placed underground [19].Underground pumped storage is especially desirable becauseit has minimal impact on the environment [10]. There are nounderground pumped storage plants built yet, as it is stillunder development. However, the idea of undergroundhydroelectric power-generation is not a new one; undergroundhydroelectric plants have been operating in Germany since1951 [19].

D. Current Installments

Many pumped storage systems have been installedworldwide for their grid services and benefits in balancing theintermittency of wind and solar energy. In 2009, more than127,000 MW of pumped storage capacity was in operationworldwide. This amount is expected to grow rapidly over thenext four years, totalling more than 203,000MW by 2014 [20].Most plants have capacities ranging from approximately 200MW to 500MW, although the largest plant in the world has acapacity of 2,400 MW located in Guangzhou II, China [18].

V. HIGH POWER FLYWHEELS

The types of storage systems discussed above have aremainly used for peak shaving and load shifting services. Withregards to high power flywheels, the focus is based onfrequency control and UPS applications due to their rapidresponse rates, high life cycles and high efficiencies. They arenot suitable for peak shaving and load shifting due to their lowdischarge durations.

Frequency regulation generally accounts for 1 to 2percent of the power generated daily [21]. Employing a fossilfuelled generator to provide this service would emit


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greenhouse gases, by installing a high power flywheel instead,up to 90 percent of the greenhouse gas emissions will bereduced [21].

A. Operation Principles

A flywheel is a device which stores electrical energy askinetic energy in a spinning wheel. The mechanical section of

the system is comprised of a motor to spin the flywheel, aflywheel which stores the kinetic energy, and a generatorwhich extracts energy from the spinning flywheel. Figure 7illustrates how these components are connected. The amountof energy stored is proportional to the flywheels mass and thesquare of its rotational speed. Hence, the most efficient way tostore energy in a flywheel is to increase its speed. Themaximum speed of the flywheel is limited by the stress on theflywheel due to the inertial loads, thus limiting the energycapacity of the flywheel.

Figure 7: An illustration depicting the main components of a traditionalflywheel system[26].

High power flywheels are an advanced form of thetraditional flywheel, such as the one depicted in figure 7. Theadvanced flywheel is placed in a vacuum sealed container andis suspended by a magnetic bearing to minimise frictionallosses from the air and bearings. These conditions are ideal formaximum rotating speeds. Recent technology advances incarbon fibre composite materials have allowed theseflywheels to withstand the increased speeds by strengtheningthe material of the flywheel itself. All of these developmentstogether have resulted in todays high power flywheels, whichhave much higher capacities.

B. Advantages

The characteristics of high power flywheels are especiallywell suited frequency regulation services. Their mainadvantages are listed below:

High efficiency (87-94 percent efficient)[2]; higher than that of batteries

Rapid response rate; in the order ofmilliseconds [21]

Short recharge time; can fully charge within15 minutes [22]

High life cycle (70,000 cycles); 35,000times more cycles than the lead acid battery [2]

High life expectancy (typically 20 years)[23]

No capacity degradation; flywheel lifetimeis almost independent of discharge/charge cycledepth [24]

Can operate equally well at a low or highlevel of charge [24]

Frequency regulation devices must be able to constantlyand rapidly absorb and release energy into the grid. All theadvantages described above help flywheels regulatefrequency. Their rapid response and short recharge timesallow flywheels to rapidly absorb and release energy into thegrid, while their high life cycles allow them to continuouslyabsorb and release energy into the grid. Also, the flywheelslack of capacity degradation and ability to operate well at lowor high levels of charge allow it to continuously operate athigh performance without fear of damage due to overuse. Thisis important, as frequency regulation is a service whichrequires constant operation throughout the day, every day.

C. Disadvantages and Current Installments

Currently there are no utility scale installments offlywheels in the grid; they have typically been installed insmaller scales in local systems for UPS applications and back-up power. However, this will soon change when the firstutility scale, 20 MW flywheel plant, is installed in New York,US. This plant has a much larger capacity than previousflywheel systems as it is made up of a matrix of 200flywheels. This comes at a massive cost of US$70-million,however future costs of such plants are estimated to be lessthan US$50million. Construction has already begun and is

expected to be completed by the end of the first quarter of2011. Two more 20 MW flywheel-based plants are expectedto be installed in the US in the near future [21,25].

VI. SUPERCAPACITORS

Supercapacitors, also known as Ultracapacitors, storeenergy in the form of static electricity, just as ordinarycapacitors do. However, supercapacitors are able to storemuch greater amounts of energy due nano-technologydevelopments which have allowed increased surface areas on

the electrodes. Supercapacitors are power devices; they cansupply short bursts of power short durations, typically in therange of seconds [2]. They are suitable for grid managementapplications such as frequency regulation and UPS systems,

but are also used in storage applications for renewable energysuch as solar, wind, and hydro storage [26,27]. With regardsto grid management applications, they are very similar to high

power flywheels.

C. Operation Principles


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A supercapacitor is a double layer capacitor. Figure 8below displays a diagram of a supercapacitor; there are twooppositely charged plates (the current collectors), two porouselectrodes (consisting of the electrolyte), and a separator. To

build up static charge, the positive plate attracts negative ionsfrom the electrolyte to the plate, while the negative plateattracts positive ions. As charged ions move to the plate edges,

Figure 8: This figure looks at the ultracapacitor at different levels of

abstraction; leftmost is the ultracapacitor module, followed by a schematic, andrightmost is a detailed diagram of a single cell [27].

oppositely charged ions will be repelled from the edges andgather to the middle, next to the separator. Ultimately, thereforms two layers of capacitance [28].

The electrodes only store energy on their surfaces;therefore to increase the storage capacity, the surface areamust be increased. This is the reason for the porous electrodes,as it provides a high surface area in a given volume [29].Furthermore, the separation of charges in the double layers isabout 0.3 to 0.5 nm, as opposed to 10 to 100 nm in electrolyticcapacitors, and 1000 nm in polystyrene capacitors [28]. Thisallows the layers to be structured more compactly together,

further increasing the surface area per unit volume. As aresult, the capacitance per square centimetre ofsupercapacitors is approximately 10,000 times larger thanthose of ordinary dielectric capacitors [28].

B. Advantages

The advantages of supercapacitors are very similar tothose of high power flywheels; we can see this through themain advantages of supercapacitors listed below:

Very high efficiency (> 95 percent efficient)[2,27,29]

Rapid response rate; in the order of

milliseconds [26] High life cycle (100,000 cycles); 50,000

times more cycles than the lead acid battery [2]

High life expectancy of up to 15 years [13];the life expectancy does not degrade with the numberof cycles, rather it depends on the temperature andvoltage [30]

Very high power densities compared tobatteries (double that of a lithium ion battery) [28];this allows them to rapidly discharge power into thegrid

By comparing the advantages of supercapacitors againsthigh power flywheels, we see that the former has a greaterefficiency range, and a higher number of life cycles, but thelife expectancy is lower. However, these differences are notvery significant, and their response rates are also very similar.Thus, the supercapacitor is well suited to the same services asthose performed by the high power flywheel, ie. frequency

regulation and UPS applications.

C. Disadvantages and Current Installments

There are currently two main challenges facingsupercapacitor manufacturers; cost, and the fact that thetechnology is relatively new. Due to these factors, utility

providers have been reluctant to employ this new technologyand so there have been no utility scale installations to date[13,27]. However, it is evident that there is increasedawareness of this technology among companies, and that costsare decreasing [27]. Despite the lack of utility scaleinstalments so far, supercapacitors have found employment inother applications related to the grid including UPSapplications and storage of solar, wind, and hydro energy[13,28,31]. Their rapid response rates, high life cycles, andhigh efficiencies make them suitable for frequency regulationon the grid, but this service requires a large scale of units to be

produced which has been unfeasible due high costs.

VII. SUMMARYAND COMPARISONOF STORAGE TYPES

The table below summarises the characteristics of thevarious types of energy storage systems discussed in this

paper. As each have their own advantages and disadvantages,thus some are more suited to different applications thanothers; there is no single storage system which suits allapplications. Buy studying the different advantages of eachstorage type, we can determine the suitability of each fordifferent types of applications.

With regards to load shifting and peak shavingapplications, those storage systems with large systemcapacities and long discharge times in the range of hours aresuitable. Batteries, CAES and pumped storage are suchsystems used for such applications. Batteries are most suitablein densely populated areas as they are smaller and do notrequire large on-site features such as reservoirs at high

altitudes or underground mines, but these have lower energycapacities than CAES and pumped storage. CAES andpumped hydro are more suitable in situations in which bulkstorage is required, but where the necessary sites available.

Regarding to the distribution of renewable energy,suitable storage capacities for PV and small scale wind energystorage lie the range of a few mega-watts [2]. Batteries havethe capacities within this range. It also happens that PV andsmall scale wind farms are typically located in densely

populated areas, which is where batteries are well suited. Forlarge scale wind farms, CAES and pumped storage are most


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suitable. Large scale wind farms require much larger storagecapacities and are typically found in less densely populatedareas suitable for CAES and pumped storage.

Concerning frequency regulation, a range ofcharacteristics are considered to determine whether a systemis suitable. Those with rapid response rates, high powerdensities, high life cycles and high efficiencies are required.Rapid response rates and high power densities allow units to

rapidly absorb and release energy into the grid, whichbalances out the fluctuations. High life cycles allow units tocontinuously charge and discharge into the grid throughout theday. Finally, high efficiencies significantly reduce losses andgreenhouse gas emissions in the grid. Flywheels and

supercapacitors the most suitable storage units used forfrequency regulation services.

For UPS and spinning reserve applications, the suitabilityof a system depends on its response rate and capacity. Thosewith rapid response rates (typically in the range ofmilliseconds) are suitable for UPS applications, while thosewith slower response rates and larger capacities are suitablefor spinning reserves. Batteries, flywheels, and

supercapacitors are suitable for UPS applications, whileCAES and pumped storage are compatible for spinningreserves.

TABLE ISUMMARYOF STORAGE TYPE CHARACTERISTICS

Storage

Type

Energy

Density

[32]

Power

Density

[32]

Capacity

[2]Efficiency

(%)

[2]

Plant

Capital

Cost

US$/kW

[10]

Storage

Capital

Cost

US$/kWh

[10]

Response

Rate

Discharge

Time

[2]

Life Cycles

(1000 Cycles)

with DoD+

80% [2]

Lead-Acid

low moderate 1kW -10MW

74-78 120 170 UPScompatible

hours 0.2-2

NaS high high 40kW-10MW

87-90 2,500 n/a UPScompatible

hours 3-6

VRB low high 40kW-20MW

72-84 1,500 150 UPScompatible

hours 2-5

CAES very low low 100MW-1GW

70-78 390 1 spinningreserve

compatible

hours 9-30

HydroPumpedStorage

low low 100MW-2.4GW

73-85 1100 10 spinningreserve

compatible

hours 30-60

Flywheel very low high 10 kW -1MW ^

87-94 150 300 UPScompatible

seconds 40-70

Super-capacitor

very low high 10 kW -1MW

> 95 n/a n/a UPScompatible

seconds 10-100

^ For single flywheels, ie not a matrix of flywheels+ DoD Depth of Discharge

VIII. CONCLUSION

The various types of energy storage systems discussed inthis paper can be divided into two main categories: one forlong discharges in load management, the other for shortdischarges in frequency regulation. Batteries, CAES and

pumped storage are suitable for load management servicesthrough peak shaving and load shifting, while high powerflywheels and supercapacitors are suitable for frequencyregulation.

With the increased distribution of renewable energy intothe grid, both load management and frequency regulationservices have become increasingly important. It is imperative

for utilities to be aware of all the types of storage systemsavailable, especially the new technologies, so that they may

have a greater flexibility in choosing the most suitablesystems for their various circumstances. Many utilities arereluctant to adopt the newer technologies such as underground

pumped storage and supercapacitors for fear of the problemsattached to being one of the first to employ such alternatives;these technologies harbour great potentials and increasingtheir use in the industry will allow the energy storage industryfurther develop and grow.

With the increased development of storage technologies,there will be more effective management of the grid, more


10/10

effective distribution of renewable energy, and lessgreenhouse gas emissions from the grid.

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