Climate change, national security, energy independence, jobs and energy cost are some of the factors that are merging to reinvigorate efforts in energy storage development.

Energy Storage and Variable Renewable Energy Sources

Contributed by | CALMAC

After brief but intense interest in energy storage during the 70’s and 80’s, the return of inexpensive fossil fuel dampened the sense of urgency created by insecurity about our energy future.  There have been some developments and successes. Most notably, thermal energy storage for air-conditioning applications has matured into a robust commercial product, with broad manufacturing and marketing support.     

Recent focus on energy storage is founded on more compelling and comprehensive motivations.  Climate change, national security, energy independence, jobs and energy cost are some of the factors that are merging to reinvigorate efforts in energy storage development.  And predictably, much of the effort is related to the ability of storage to support renewable energy sources, such as wind and solar, that possess some level of uncertainty or variability, or where the availability may be temporally out of phase with demand.


Renewable Energy and Electricity Generation

As reinforced throughout a recent report supported by Sandia National Laboratories1 on behalf of the U.S. Department of Energy’s (DOE) Office of Electricity Delivery and Energy Reliability and the Office of Energy Efficiency and Renewable Energy Solar Technologies Program, energy storage is a viable solution to our renewable energy needs.

Investment in energy storage is essential for keeping pace with the increasing demands for electricity arising from continued growth in U.S. productivity, shifts and continued expansion of national cultural imperatives (e.g., the distributed grid and electric vehicles), and the projected increase in renewable energy sources……”

“Perhaps the most significant trend driving the need for grid-scale energy storage is the shift to renewable energy sources, such as wind and solar……..”

“One of the most promising approaches to addressing the growing limitations of the electric grid and the increasing demand for renewable energy is to incorporate stationary energy storage technologies into the U.S. electric grid.”

Over the next 25 years, wind is expected to provide the second largest increase to US renewable generation production, eventually exceeded only by biomass fuel2.  (Fig.1)  US wind generation capacity increased by 19GW between 2003 and 2008, and is expected to grow an additional 39GW by 2035. 

Fig. 1 Forecast Renewable Energy Production (Energy Information  Administration, Annual Energy Outlook 2010)

The impact of variable generation on utility operations is remarkably complex and has been the subject of extensive analysis3,4,5,6,7.  There is general agreement that at low levels of variable generation, there will be minimal disruption to utility operations and negligible curtailment of the renewable resource.  As the renewable contribution increases, the number of variables – and goals - that must be considered expands, and the predictions become less consistent.  Additionally, there are potential benefits from energy storage other than operational stability or maximizing the kWh of renewable resources.

Because renewable sources like wind and solar may not be available on a consistent and predictable basis, the utility must construct and operate conventional power plants to supplement the renewable source when necessary.  The US has about 1 trillion watts of power production capacity and, on average, only generates slightly more than half of that value.  Introducing renewable capacity that will be available most of the time – but not always – may aggravate that condition even more since backup generation may be used even less of the time, devaluing both the investment in the renewable source and the supplemental conventional generation.  It is interesting that ERCOT, operator of the Texas electric grid, considers only about 9% of wind nameplate capacity for peak power contribution8.

A 2010 report from Sandia National Laboratories9 identifies 26 potential benefits of energy storage for the electricity grid, including several specifically related to renewable generation resources and others with a significant influence.  These include:

  • Renewables Energy Time Shift
  • Renewables Capacity Firming
  • Wind Generation Grid Integration
  • Load Following
  • Area Regulation
  • Electric Service Power Quality

There are several fundamental characteristics of energy storage technologies that define their suitability for addressing electric grid problems, not including cost.  The first is response time – how quickly the storage device can respond to a grid disturbance.  The next is the power (kW) that the storage can produce and finally the endurance, or how long the device continues to discharge at a particular level of power (kWh).    

Flywheels and capacitors have very fast response times and high-cycle tolerance and are well suited for power quality correction, voltage stabilization and accommodating short duration, abrupt differences in supply and demand. 

While batteries can also address some fast response volatility, they are often considered for intermediate and long time scale applications such as ramp up/down or bridging power as generating assets are brought on and off-line.  And there is, of course, the familiar storage duty of longer term accumulation of energy at times when supply is abundant but demand is low, for release when the opposite is true.

There is currently tremendous activity in advanced battery R&D.  Much of this development is at the ‘demonstration’ phase with several projects funded through the American Recovery and Reinvestment Act (ARRA).  Familiar consumer (albeit advanced) battery technology such as lead-acid, nickel-metal hydride, nickel-cadmium and lithium-ion  are being supplemented by an array of additional battery designs.  Perhaps the most successful currently is the high temperature sodium-sulfur battery that exhibits high power levels and high total energy capacity. Flow batteries that employ liquid electrolytes, such as the vanadium redox or zinc-bromine, provide excellent scalability because the liquid electrolytes can be stored independently of the power producing cell.

Despite the current high level of interest, the National Renewable Energy Laboratory reported in January 2009 that only four types of energy storage had total installed capacities that exceeded 100 MW – sodium-sulfur batteries, pumped hydro storage, compressed air energy storage (CAES) and thermal energy storage (TES). 

Sodium sulfur batteries are currently being applied in wind farm pilot projects and transitioning to commercial application.  Presidio, Texas has installed a 4 MW sodium-sulphur battery for emergency backup, claimed to be the largest battery of this type in the US.  They have a high energy density and charge/discharge efficiencies in the range of 90%.    

Pumped hydro storage, the technique of pumping large volumes of water to an elevated storage reservoir so that it can descend through power generating turbines at a later time, has a large installed capacity of about 20GW, with most of it installed over 30 years ago.  This capacity will undoubtedly increase in the future, but siting and environmental concerns make this a lengthy and involved process.  The ideal locations have already been exploited.

CAES is potentially capable of very high capacities but currently there is only one US installation, a 110 MW plant operating in Alabama, with a few others now proposed and in development (over 700MW total in California, Iowa and New York).  In overly simplistic terms, air is compressed and stored in underground caverns such as salt domes or aquifers when excess generating capacity is available and later released and heated with natural gas before passing through a series of turbines to generate electricity.


Thermal Storage for Cooling - End Use Application

Many studies that evaluate the potential impact of energy storage on grid operations base their conclusions on the characteristics, including costs, associated with advanced batteries, pumped hydro plants or compressed air storage, probably because these technologies convert their stored energy directly back into electrical power.  However, as noted in a NREL report, thermal energy storage isn’t always the obvious answer.

Thermal energy storage is sometimes ignored as an electricity storage technology because it typically is not used to store and then discharge electricity directly. However, in some applications, thermal storage can be functionally equivalent to electricity storage. …..  One example is storing thermal energy from the sun that is later converted into electricity in a conventional thermal generator. Another example is converting electricity into a form of thermal energy that later substitutes for electricity use such as electric cooling or heating…… Demand for electric-power cooling can be shifted by storing cold energy in the form of chilled water or ice during off-peak times and releasing that cold energy during times of peak demand. This effectively stores electricity with high round-trip efficiency.” 3

Thermal energy storage for cooling is currently being applied on a broad scale with commercial success based solely on its own economic benefits.  Some utilities offer rebates but generally, the time-of-day dependent charges (demand and energy) incorporated into virtually all commercial electrical rate structures provide the energy cost savings that justify thermal storage for cooling applications.  These are installations that building owners request and pay for now – at costs far below typical battery or utility side technologies.  In fact, many of these systems are cost competitive with conventional central plant cooling systems.

California provides an example of the possible interdependency of variable renewable energy, primarily wind, and energy storage.  As described in a recent analysis10:

“As California moves towards the goal of generating 33 percent of the state’s power from renewable sources by 2020, it will need significantly greater deployment of energy storage technologies to address the challenges posed by integration of large amounts of renewables into the grid.”

For California, as in most areas, peak renewable output does not coincide with peak demand.  (Fig. 2)11 On average, the average summer wind power returns to its higher level several hours after the utility peak has passed.  During the winter, on average, the utility load does not exhibit the same extreme variation, nor does the wind contribution.

Fig. 2   Seasonal Utility Load and Wind Generation for California (  Source data - Bai, X.; Clark, K.; Jordan, G.; Miller, N.; Piwko, R. Intermittency Analysis Project: Appendix B Impact of Intermittent Generation on Operation of California Power Grid. General Electric 2007)

Thermal energy storage is traditionally installed to address the precise electrical load responsible for the utility peak – air-conditioning.  Furthermore, hours of high wind power availability coincide with the charging hours for the thermal storage equipment.  Additionally, end use energy storage, located at the point of energy consumption, completely removes that burden from the transmission and distribution network at the most critical time, saving energy and delaying the need for additional T&D infrastructure.

Of course, utilities find more comfort in storage that they control directly.  However, that ability is beginning to develop.  A group of California utilities recently entered into an agreement to deploy 53 MW of cooling storage capacity at customer facilities that is part of the ‘smart grid’ initiative.  And of course, utilities exercise some control through manipulation of electric rate devices like real-time pricing, interruptible rates and demand charges.

There are many types of thermal energy storage systems12,13.  A common design employs a conventional chiller to circulate a cold glycol solution through a heat exchanger submerged within a tank of water.   The heat exchanger tubing is distributed throughout the volume of water and the water gradually freezes on the exterior surface of the heat exchanger tubing.  During the day, the same glycol solution now circulates through the heat exchanger where it is cooled by the melting ice before flowing through the building coils, in turn cooling the air delivered to the occupied space.  The glycol, now warmed by the air, returns to the tank heat exchanger to be cooled again.  Fig. 3 illustrates a commercially available storage tank, typically capable of avoiding 20 to 30 kW of daytime electrical power consumption.  These tanks are modular, and installations may consist of just a few, or hundreds, of these products. A basic system is depicted schematically in Fig. 4.  The system illustrated in Fig. 5 will shift about 250 to 300 kW to night time but systems using this, and similar technologies, have been installed that shift tens of MW’s to efficient and available night power generation.  

Fig. 3 Thermal storage tank for off-peak air-conditioning (Courtesy CALMAC Mfg.)

Fig. 4 Simple thermal storage system schematic


Fig. 5 Thermal storage system at educational facility (Courtesy CALMAC Mfg.)


Generation Side Solar Heat Storage

Wind energy is predicted to be the major contributor to the growth in renewable and variable electric power generation, but solar energy will also play a significant, if minor, role.  While solar power will typically be more coincident with peak utility load profiles it is often out of phase with many utilities that exhibit late afternoon peaks.   This limits the value of the renewable generating asset since conventional capacity must be available to meet the peak demand.  And of course, individual solar power plants are subject to rapid fluctuations in power production, a condition not well tolerated by Rankine cycle steam turbines commonly employed in concentrated solar power (CSP) installations.

Storage for CSP plants is most often accomplished by accumulating thermal energy at a high temperature.  Liquids such as oil or molten salts are the most common storage mediums, but porous solids like sand or rocks have also been used.

Interesting examples of large scale solar storage system can be found on the plains of Andalusia, Spain14,15.  The AndaSol power plants, two almost identical installations, each employ a solar field covering over 125 acres of parabolic trough solar collectors to produce 50 MW of electric power.  One third of the solar energy can be diverted to storage, providing up to 7 ½ hours of additional generation from about 1000 MWh of stored thermal energy.  When solar energy is available, a portion of the high temperature heat transfer fluid (synthetic oil) from the concentrating collectors is circulated through heat exchangers, heating a mixture of molten nitrate salts to about 725F from 560F as it flows from a cold tank to a hot tank.  Each tank measures 37m in diameter by 14m high and can hold 28,000 tons of salt.  When the stored energy is needed for power production, the salt from the hot tank is returned to the cold tank, passing through the heat exchanger where it returns its thermal energy to the circulating oil.   Storage increases the full load operating hours of the plant by almost 80%.


Annual Underground Thermal Energy Storage

There is no energy more fundamentally renewable than the thermal energy associated with the natural climate cycles of the earth.  Another family of storage technologies, Underground Thermal Energy Storage (UTES), more common in Europe than North America, takes advantage of this virtually free energy source. 

There are four basic types of underground systems, including a simple tank, pit, borehole and aquifer storage, each with a wide variety possible site specific design features16.  While occasionally applied to individual residences, these systems are more often devoted to multi-dwelling or district size ranges.  The storage temperatures cover a broad span and are often in the range of 80-95C for heating applications, but cooling and combined heating/cooling systems are also currently operating.

Tank storage is fairly straightforward.  Pit storage is similar conceptually, with the earth forming the sides and base of the storage volume.   The pit, fitted with an impermeable liner, can be filled entirely with water or high void porous solid like gravel or sand.  The Marstal, Denmark installation is an often cited example17.  About 14,000m3 of pit storage provides 8 million kWhth to over 1400 customers in 200,000m2 of occupied space, reducing the annual conventional heating requirement to approximately 19 million kWhth.

Borehole storage employs a matrix of piping loops extending into vertically drilled holes.  A large North American installation is located at the University of Ontario, Canada18 and consists of 400 individual boreholes drilled to a depth of 200m and encompasses a total earth volume of 1 ½ million m3

Aquifer storage is accomplished by injecting and extracting water directly into subterranean formations of saturated and permeable earth.  Systems covering a wide range of capacities have been installed, one of the largest located in Germany.  The injection and extraction wells are drilled to a depth of 1250m and are separated by 1300m or over ¾ of a mile. 

Tank and pit storage systems may be insulated, but it is obviously impossible to insulate borehole and aquifer types.  Annual losses can be substantial and large installations enjoy a significant economy of scale.  Of course, all of these systems, particularly borehole and aquifer designs, require an extensive evaluation of the site geology.

Although relatively uncommon, there are also systems that accumulate snow and/or ice produced in the winter, for air-conditioning the following summer.  Of course, some readers may still remember the seasonal ice industry that flourished into the first third of the 20th century.   Again, losses are usually quite high (20-30%), but the very low energy consumption results in efficient seasonal performance.  The Regional Hospital in Sundsvall, Sweden has been operating for over 10 years19.  It has often met 90% and higher fractions of the total cooling load for the 190,000m2 facility.  Up to 40,000m3 of snow is accumulated during the winter, either recovered from local streets and squares or artificially produced with snow guns.   Maximum capacity has been approximately 2000kW but there are plans for increases of up to 50%.



Storage is a component of virtually all processes.  Food, water supply, transportation fuels, and commerce – all include storage at some point in the distribution/consumption chain.  Up until now, power utilities have been able to take advantage of the energy stored in the chemical bonds of fossil fuel, essentially controlling the burn rate of the fuels to precisely match the consumption of electricity.  The growth of variable or intermittent renewable energy power production will require the deployment of other storage methods.  A wide selection of storage technologies are under development for the generation side of the utility grid, including advanced batteries, expanded pumped hydro and compressed air storage as well as thermal storage for concentrated solar power production.  On the customer end of the power transmission network, thermal storage for cooling applications has already built a proven track record of cost-effectively resolving the demand for electricity with its availability, directly addressing air-conditioning, the primary cause of high utility demand.  In addition, end use storage technologies, like thermal storage, provide the additional benefit of reducing congestion on the transmission and distribution network, delaying the need for additional T&D construction.


Brian Silvetti, P.E., is vice president of engineering, for CALMAC Manufacturing. More information about CALMAC can be found at



  •  Nexight Group, Sponsored by US Department of Energy, Electric Power Industry Needs for Grid-Scale Storage Applications 2010
    • U.S. Energy Information Administration. Annual Energy Outlook 2010 With Projections to 2035 2010
    • Denholm, P.; Ela, E.; Kirby, B.; Milligan, M. The Role of Energy Storage with Renewable Electricity Generation. National Renewable Energy Laboratory 2010
    • Xcel Energy, NRECA Cooperative Research Network, American Public Power Association DEED, Western Area Power Administration, Electric Power Research Institute. Characterizing the Impacts of Significant Wind Generation Facilities on Bulk Power System Operations Planning 2003
    • Milligan, M.; Parsons, B.; Zavadil, B.; Brooks, D.; Kirby, B.; Dragoon, K.; Caldwell, J. In Grid Impacts of Wind Power: A Summary of Recent Studies in the United States, Proceedings of the European Wind Energy Conference and Exhibition, Madrid, Spain, June 16-19, 2003
    • Dai, A.; Deser, C. Diurnal and semidiurnal variations in global surface wind and divergence fields. Journal of Geophysical Research 1999, 104 (D24)
    • North American Electric Reliability Corporation. Accommodating High Levels of Variable Generation 2009
    • ERCOT, ERCOT Expects Adequate Power Supply for Summer, Press Release 2008
    • Eyer, J.; Corey, G. Energy storage for the electricity grid: benefits and market potential assessment guide. Sandia National Laboratories 2010
    • Elkind, Ethan,. The Power of Energy Storage: How to Increase Deployment in California to Reduce Greenhouse Gas Emissions, Berkley Law, UCLA Law, Bank of America 2010
    • Bai, X.; Clark, K.; Jordan, G.; Miller, N.; Piwko, R. Intermittency Analysis Project: Appendix B Impact of Intermittent Generation on Operation of California Power Grid. General Electric 2007
    • Silvetti, B., Thermal Energy Storage, Encyclopedia of Energy Engineering and Technology, Taylor & Francis, 2007
    • 2007 ASHRAE Handbook, HVAC Applications, Chapter 34 Thermal Storage.  ASHRAE, Atlanta, GA 2007
    • Directorate – General for Energy and Transport. Concentrating Solar Power from Research to Implementation. European Commission 2007
    • National Renewable Energy Laboratory. Survey of Thermal Storage for Parabolic Trough Power Plants 2000
    • Mangold, D. Seasonal storage – a German success story. Sun & Wind Energy 2007, 48-58
    • EU Supports the Expansion to a 100% Renewable Energy System, Solar District Heating Newsletter, July 2010
    • Beatty, B., Dincer, I., Bright, K., Canada’s Largest BTES System 74,000m Borehole Field, Presentation Ecostock, Richard Stockton College of New Jersey, May 31-June 2,2006
    • Skogsberg, K. Seasonal Snow Storage for Cooling Applications. Lulea University of Technology 2005

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