Fred R. Hart),, Jr. zs a senior prznczpal engzneer at Stone &

Webster Engzneerzng Corporatzon. He has provided extenszve deszgn,

engmeerzng and consulting servzces on more than 50 hydroelectrzc projects, zncluding 16 pumped

storage projects. Fred E. Depenbrock is an assistant

vzce president at Stone & Webster Management Consultants, Inc., with expertzse zn electrzc utzlity planning

and operatzons, engzneerzng analyszs, and economzc and regulatory studzes

Partrick W. Ward zs a senzor mechanzcal engzneer at Stone &

Webster Engzneerzng Corporatzon He has prowded deszgn and

engzneermg serwces on hydroelectrzc and other power-produczng faczhtzes.

Daniel L. Shectman zs a power engineer wlth Stone & Webster

Engzneering Corporatzon. He has authored numerous generatzon

planning studies and electmclty generatzon and storage technology

evaluations for varzous electrzc utzlztles

Options in Energy Storage Technologies Existing and emerging technologies for energy storage have the potential to vastly increase power system flexibility and improve system response characteristics, while minimizing environmental damage.

Fred R. Harty, Jr., Fred Depenbrock, Patrick W. Ward and Darnel L. Shectman

T he use of energy storage tech- nologies provides two major

benefits to an electric power trans- mission system: more effective use of existing systemwide plant investment, flexibility in system- wide operations and better re- sponse to price changes. Stored electricity is made available when needed to meet immediate changes in demand, allowing more effective operation of baseload units at higher, more-effi- cient and essentially constant lev- els of power. Also, an energy stor- age facility that can respond quickly and efficiently to generate or store energy over a wide range of loads provides substantial dy- namic value to the grid by displac- ing less-efficient and more-expen-

sive facilities that must otherwise be dedicated to this dynamic type of service. Energy storage facili- ties provide additional benefits by using off-peak power for pump- ing and/or charging, thus allow- ~ng more-efficient operation of the baseload units and choice of tim- ing of energy buying and selling transactions, whether on-peak or off-peak.

The most commonly used en- ergy storage technology is hydro pumped storage. This technology has evolved into advanced pumped storage (APS), which provides greatly enhanced opera- tional flexibility relative to its bet- ter understood traditional parent. Other technologies that are being pioneered as options for energy

]uly/August 1994 21

storage are compressed air energy storage (CAES), battery energy storage systems (BESS), and su- perconducting magnetic energy storage (SMES).

"Energy Storage" IS more than just energy storage. Energy stor- age technologies such as APS are being used to enhance system reh- abihty while helping the rest of the system to be more cost com- petitive and environmentally friendly. As an example, the Ad- vanced Pumped Storage fadlity at Dinorwig, located rn North Wales, provides 1800 MW of dynamic ca- pacity to meet a wide array of sys- tem operating needs, including energy transfer and correction of short-term system imbalance

T he ultrafast response of some energy storage tech-

nologies used m the management of transmission capacity may be the most valuable benefit of this technology. APS facilities have prowded and are being designed for capacities on the order of 2000 MW with reaction times in sec- onds for major changes m output. BESS and SMES can prowde even faster response (milliseconds) should a critical transmission line fail. Thus, system stability is maintained by BESS and SMES until the APS reacts in the next 10 to 15 seconds, thereby providing time until any other corrective ac- tion or follow-up is initiated. The balance among the use of these technologies will ultimately de- pend upon the relative costs and needs for increasingly fast but possibly short-term response of technologies such as BESS and SMES versus other technologies

such as APS which can react in seconds and be sustained for hours.

The energy storage technologies described in this paper can pro- vide major benefits to an electric power system, as well as air pollu- tion control. Examples of the ap- phcations and benefits of the en- ergy storage technologies are shown m Table 1.

Besides the traditional elements of energy storage, the major ele- ments of dynamic energy savings are spmning and quick start re- serve, with an estimated value of $100/kW to $600/kW for displac- ing thermal units on spinning re- serve; load regulation by fast ramping systems, with an esti- mated value range of $20/kW to $200/kW, system frequency regu- lation, with an estimated value range of $10/kW to $500/kW, and black start capability (units capable of start-up during a black- out) having an estimated value of up to $5/kW. The range of values reflects the complexity of electri- cal generation and &stribution

grids and the high variability of key parameters among utilities.

I. Types Of Energy Storage Technologies

A. Hydro Pumped Storage Conventional pumped storage

(CPS) uses an upper reservoir and a lower reservoir which typically consist of existing lakes or rivers or reservoirs constructed with dams Energy is stored in the up- per reservoir in the form of water which is pumped using off-peak electricity from the grid During the generation cycle the water is discharged through reversible pump-turbines to produce power. CPS responds to system changes in terms of mLnutes and provides the operator with peak shaving operations once or twice dail3a

Advance pumped storage, cur- rently proposed for several U. S. projects, is similar to CPS, except that the reservoirs are configured at much higher heads and the plants are designed hydraulically and mechanically for ultrafast

Table 1: Energy Storage Technologies Apphcat~ons and Benefits-- Examples

Application/Viewpoint

Type of Benefit Generation Transmission Corporate Customer

Strategic Em,ss,ons Outages R~sk of fuel price Blackouts (fewer (reduce/shift) (reduced effect) shocks or and shorter)

shortages (lower)

Dynamic System Power Quality Frequency (Improved) (Improved control)

Load-levehng Baseload Grid Network (Improved (Improved utilization) utilization)

Power Quality (Improved)

Energy Market DSM (Reduced (Buy/Sell need for choices) temporary

disconnection)

22 The Electnczty Journal

loading and ramping and fre- quent changes among the pump- ing, generating and spinning-in- air modes. The high heads and frequent mode change capabilities of APS require modem equip- ment designs for the pump-tur- bine units to sustain the high duty demands. Existing CPS stations have been sized as large as 2100 MW with five to 12 hours of stor- age. Proposed APS applications have similar storage times and power capacities. Capacity fac- tors a for CPS and APS range from 10 to 35 percent, with round trip electrical effioencies (generating to pumping energy ratio) ranging from 72 to 75 percent and higher

CPS s~te locations are llrruted to terrain that affords the gross head required to make best use of the water storage volume. APS appli- cations wxth the lower reservoir above ground are limited to essen- tially the same locations as CPS, with the requirement of nearby high terrain to achieve the high gross head The use of under- ground lower reservoirs removes the restriction of available topog- raphy such that the lower reser- voir elevation can be selected al- most at will to achieve the desired gross head. Underground reser- voirs are, however, limited to geo- logical conditions favorable to such development.

B. Compressed Air Energy Storage

A compressed air energy stor- age (CAES) plant can be de- scribed as a simple combustion turbine that has been modified to operate in separate compression

and generation cycles A CAES plant has additional components including the storage chamber and a combination motor/gener- ator that does double dut~ power- ing the compressor during the charging cycle and producing electricity during the generating cycle. Air storage options include caverns in salt or rock formations, depleted natural gas fields, and salt-water bearing aquifers.

CAES is not a pure storage tech- nolog~ To operate most effi-

Existing conventional pumped storage stations have been

sized as large as 21 O0 M W with five to 12 hours of storage.

cientl~ CAES utilizes fuel (oil or gas) which is burned with the stored air that is recovered from the storage cavern. Pure storage technology would not use addi- tional outside energy Economic advantages come from the use of off-peak electric power, rather than premium fuel, for the com- pression cycle

CAES is similar to other ther- mal plants, which, unlike APS, lack the ability to respond with ul- trafast ramping and frequent mode changes. CAES systems can be sized to match the utility's particular load curve. Two dem-

onstration projects have been de- veloped, with capacities of 35 MW with six hour storage and 110 MW with 26- hour storage.

C. Battery Energy Storage Systems

Battery energy storage systems (BESS) constitute a pure storage technology which stores electric- ity as electrochemical potential en- erg~ Conventional storage batter- ies use lead acid batteries. The batteries are charged during off- peak periods and provide power to the grid during periods of peak demand. BESS plants typically provide 10 percent to 15 percent capacity factors with round trip electrical efficxencies ranging from 74 to 76 percent.

The lead acid energy storage battery is typically used for one hour storage for light- and heavy- duty applications. Light applica- tions include spinning reserve, peak shaving, and shallow cy- cling. Heavy-duty applications in- clude load leveling, frequency regulation and deep discharge. Battery energy storage facilities have extremely fast response times and can achieve full load charge or discharge rates in less than about five milliseconds. Commeroal applications usmg lead acid batteries have ranged from 200 kW with two hours of storage to 20 MW with one-half hour storage.

Advanced energy battery stor- age systems are in the planning stage for initial field tests and are expected to be sized for applica- tions of one MW with four to eight hour storage. Advanced sys-

July~August 1994 23

terns will use sodium/sulfur or

zinc/bromine designs. The ad- vanced energy battery storage sys- tem is environmentally attractive

because the emissions are virtu- ally zero. As with lead-acid bat- tery storage, the advanced energy

battery storage system will pro- vide extremely fast response time, on the order of five milliseconds, to achieve full load charge or dis- charge rates.

BESS technologies are typically

modular and can be located essen- tially anywhere. The size of the fa-

cility would be determined by the energy needs for the particular ap- plication Modularization and ap- plication sizing are great advan- tages for distributed generation applications.

D. Superconducting Magnetic Energy Storage

Superconducting magnetic en- ergy storage (SMES) is at an early stage of technical development. A

500-kW micro-SMES was success- fully demonstrated recentl3a The

energy is stored in a magnetic field that is produced during off- peak periods. SMES uses a circu- lar coil built in a trench, some- what like a cyclotron atomic particle accelerator. The energy is

stored as direct current orculating in a superconducting coil whose coil conductor is made from a low- temperature nonconducting allo~ such as niobium titanium al- loy. The low temperature is achieved through the use of a re- frigeration system. Postulated

SMES locations are limited to sparsely populated areas due to

questions of potential health ef-

fects of the magnetic fields. SMES has rapid response times

to provide for system stability and can provide pulsing bursts to

serve short-term needs. SMES is similar to BESS in that the time needed to achieve full load dis- charge rate, or full charge rate, is less than five milliseconds. Simi- larl~ major benefits of this tech- nology are the dynamic, strategic, and load-leveling capabdltxes of

the energy storage facility

%

The amounts of energy stored in the micro-SMES project are rela- tively small when compared to CAES, CPS, and APS; however, SMES systems are being pro- posed to provide storage ttmes of

up to four hours with capacities ranging up to 2500 MW with

round- trip electrical efficiencies of up to 95 percent.

II. Current State of Development

A. Hydro Pumped Storage

Conventional pumped storage facilities have been in commercial

operation since the 1890s and are located above ground using existing lakes and rivers or con- structed reservoirs. Recently the trend has been to locate CPS and

APS closer to the load. Since fa- vorable sites using the CPS sta- tions are not always available near the load, APS stations using underground reservoirs are being planned for some projects. An ex-

ample is the Mt. Hope Water- power Project, located in New Jer- sey near East Coast load centers. This project has an upper reser-

voir located at grade and the pow- erhouse and lower reservoir exca- vated about 2800 ft and 2500 ft below grade, respectivel:~ This APS station will be able to achieve full load in about 15 seconds from spinning in air and will be capa- ble of frequent mode changes.

The use of pumped storage de- signed as a closed-loop system, such as at Mt. Hope, is ecologi- cally attractive because the inter-

action with the ecosystem is mini- mal. A closed-loop system uses a

captive water volume which is transferred back and forth be- tween the two reservoirs.

B. Compressed Air Energy Storage

CAES technology may now be considered to be commercially available. The Alabama Electric Cooperative's 110-MW salt dome CAES facility became the first commercial CAES plant to oper- ate in the U.S. Operating experi- ence has been relatively success-

ful and xs typical of the operation of a gas-turbine-based peaking

station. Due to AEC's successes,

24 The Electnczty Journal

interest in CAES has steadily in- creased. Westinghouse Electric Corp. has developed stand- ardized designs and has an- nounced the commercial availabil- ity of 100-MW and 300-MW CAES plants. EPRI has solicited interest from member utilities in several planned demonstration projects. Locations for such com- mercial applications are limited by the storage requirements for the compressed air, because the cavern openings must be airtight and the surrounding medium must be capable of withstanding the pressure forces.

B. Battery Energy Storage Systems The BESS use of lead acid batter-

ies has been researched since the early 1900s, with commercializa- tion for large-scale electricity stor- age having occurred in the late 1980s. Research on advanced bat- tery energy storage technologies using sodium/sulfur or zinc-bro- mine began in the 1960s; commer- cialization is expected within the first quarter of the next century. These advanced technologies are expected to have longer life, lower costs and increased reliabil- ity compared to lead acid battery facilities.

C. Superconducting Magnetic Energy Storage SMES is currently experiencing

a high level of R&D effort and mi- cro scale systems have been suc- cessfully operated. Commercial systems are expected to be avail- able by the late 1990s. Issues which need to be resolved include

high costs, potential impacts of magnetic fields, and the verifica- tion of emergency energy dump systems.

III. Environmental Impacts Environmental impacts are a

major consideration for develop- ment of any power-generation fa- cility. Minimizing these impacts obviously helps facilitate positive interaction with local communi- ties and expedites facility licens- ing. Each of the above energy storage technologies minimizes impacts on the environment by the nature of the technology itself and by the improvements that take place to baseload unit opera- tions. It should be noted, how- ever, that energy-storage technolo- gies do increase the total amount of energy generated by baseload plants (by the ratio of roundtrlp efficiency). In spite of this, they may reduce total environmental impacts because they improve

overall system efficienq5 allow displacement of fossil-fired peak- ing units in adverse environ- mental locations and address other factors related to the redistri- bution of generation.

Operational environmental im- pacts of CPS and APS are limited primarily to units connected to rivers, streams or existing lakes where fish could be impacted and the effects of water level fluctua- tion must be considered. But APS closed-loop systems using spe- claUy constructed reservoirs virtu- ally eliminate operational environ- mental impacts.

BESS has essentially zero im- pact on the environment during normal operation but the poten- tial for leaking cells and the need for recycling of old cell materials associated with the BESS batteries are issues for development of in- dustry standards.

CAES flue gas emissions would require licensing and permit con-

July~August 1994 25

siderations during the develop- ment and operation phases as would any other combustion tur- bine application.

SMES and high-voltage trans- mission and distribution systems have recently been the subject of queshons with respect to the im- pact of the magnetic field devel- oped by the coils of SMES and by the high-voltage transmission lines. SMES as well as BESS, CPS, and APS have no operational emissions.

IV. Roles Of Energy Storage Technology

Dynamic benefits of energy stor- age technologies vary with their ability to respond to changes in load and ability to sustain their as- sistance in controlling voltage, fre- quency and system stability The mterachon times range from milli- seconds (for BESS and SMES) to seconds (for APS) to rmnutes (for CPS and CAES). The ultrafast ramping rates of BESS, SMES and APS have the potenhal to provide valuable services: system stabfl- lt3~ peak shaving, load following, spinning reserve, pov~er factor correction, frequency regulation and momentary carryover capa- bilities. These capabilities im- prove overall system operation, improve efficiencies on other gen- erating units, reduce wear and tear on conventional units and im- prove transmission system usage. "Black start" capability is another major benefit of stored energy sys- tems, in which the energy storage plant can be designed to initiate restoration of the system after a blackout

CPS and APS are the most ma- ture of the technologies described above, having been proven by commercial apphcation over many years

Compressed air energy storage has had recent limited commer- cial application and is in a transi- tion toward increased commercial acceptance.

Battery energy storage systems have recently provided large-scale electricity storage applications

Advanced battery energy stor- age systems are technically achiev-

able and are being considered for possible future commercial appli- cahons on a smaller scale. Super- conducting magnehc energy stor- age remains in the R&D stage.

Advancements in stored energy technologies should continue to improve transmission system reh- ability by providing improved system response and improved operating efficiency As noted above, response hmes of five milli- seconds for full load may be achieved using BESS and SMES, while fast starting and ramping rates are available from APS These response characteristics pro- vide the utilities and system op-

erators with a variety of reserve and operating regimes that were not envisioned in the past, but which may be required by the de- velopment of open-access trans- rmssion and other externally dic- tated requirements.

The various energy storage tech- nologies may also be combined to provide a broader range of capa- bilities. For example, CPS, APS, or CAES could be combined with BESS or SMES for providing en- hanced system reliability and dy- namic operation. System capacity factors of 10 to 35 percent and large plant sizes (50 MW - 2000 MW) could be supplied by APS, CPS or CAES. Transient operat- ing needs of relahvely low capac- ity factor (under five to 10 per- cent) and generally smaller plant sizes (200 kW - 20 MW) could be provxded by BESS or SMES en- ergy storage technologies for com- bination with APS dynamic capa- bilities for frequent mode changes and rapid and sustained loading.

C ombining system capacity and dynamic system opera-

tion capabilities at a common site could enhance the overall system efficiency and provide the opera- tors with a wide variety of reserve and operating regimes. Wide- spread incorporation of certain re- newable technologies which can- not be readily dispatched to follow load will also be enhanced through the use of large-scale en- ergy storage systems. •

Endnotes

1. Capacxty factor is defined as an- nual MWhr output divided by net rated output m MW times 8760 hours

26 The Electricity Journal