Thermal energy storage snapshot
Experts consider thermal energy storage as the game changer for the future of solar energy electricity generation. Heat exchange between steam produced in a concentrating solar power plant and a heat storage fluid (typically molten salt) is the first step in the process. Direct solar heating of the storage medium is an alternative. Heat energy is stored in tanks and returned to the steam turbine system to generate electricity later or after sundown. Molten salt is the most promising thermal energy storage medium at present. R&D on more advanced, less costly storage systems is ongoing.
Thermal energy storage has been successfully demonstrated in several solar power plant projects in California in the 1980s. Today, a number of smaller commercial solar plants in Spain—mostly parabolic trough type—apply energy storage to extend operation up to eight hours after sundown. In the U.S., solar power plants can be expected to use thermal energy storage in the near future. (This is online Ref. 3; see main article, CE Nov. 2012).
Molten salts (typically a 60%-40% composition by weight of sodium nitrate and potassium nitrate) offer several attributes as a storage medium. These substances have high heat capacity and relatively low cost, hold heat at very high temperatures, and remain in a fluid state without changing to steam. As an aside, molten salt is also being investigated as a heat storage medium in new adiabatic processes for compressed-air energy storage systems under development.
Expertise with molten-salt energy storage systems at BrightSource Energy Inc. stems from an engineering team that includes technology pioneers who built the nine original solar energy generating systems (SEGSs) in California noted above. Currently, the company’s solar power tower (SPT) technology is applied at one smaller facility in the state. BrightSource also owns a 6 MW solar power demonstration facility operating in Israel, which tests equipment, materials, procedures, and construction/operating methods for solar power technology.
BrightSource is also SPT technology supplier for the 370 MW Ivanpah SEGS now being built in California. The scope of supply includes solar field design as well as optimization software and a control system for positioning the heliostats that concentrate sunlight on the solar receiver, which produces superheated steam. The final step of the process feeds the steam to a turbine generator for electricity production. Ivanpah SEGS has passed the midway point of construction (see main article, CE Nov. 2012).
Two larger solar thermal plant projects with 500 MW capacity are awaiting certification from the California Energy Commission. Construction on one of the projects is expected to begin in 2013, with completion not before 2015. Two other BrightSource SPT projects with a longer time horizon, named the Siberia and Sonoran West projects, include plans to add SolarPlus molten-salt energy storage systems (photo) to the conventional SPT plant design.
SolarReserve LLC is another U.S. company active in integrating molten-salt storage with solar power generation. In this company’s technology, heliostats concentrate sunlight directly on molten salt flowing through the solar receiver (heat exchanger) atop the power tower—raising the fluid temperature from 550 F to 1,050 F (288 C to 566 C). Then, molten salt is piped down to a storage tank where thermal energy of the high-temperature fluid is stored until there is a need to generate electricity.
When electricity generation is needed, during either day or night, molten salt is directed to the steam generator where it turns water into superheated steam to drive the turbine generator. After heat exchange to produce steam, the relatively cooled molten salt is directed back to the “cold” storage tank, then pumped back up to the receiver for reheating and process repetition. Thus, liquid molten salt serves as both the energy collection and the storage mechanism.
SolarReserve is building a 110 MW solar power tower plant in Nevada, reportedly with 10-hour thermal storage capacity. The Crescent Dunes solar energy plant is scheduled for commercial operation in late 2013. Another SPT plant in planning by the company is a two-unit (2 x 100 MW) facility, which will also have molten-salt energy storage—for up to 15 hours. The Saguache County (Colo.) solar energy project passed a significant local permitting step in March 2012.
Future of thermal energy storage
While thermal energy storage systems based on heat exchange with molten salts are viable today, the importance of this technology arena has drawn the attention of further research and development. Less costly and more efficient advanced thermal energy storage methods are the objectives.
For example, the American Society of Mechanical Engineers (ASME) has formed a Thermal Energy Storage Task Force to promote development of more advanced energy storage systems. “Spotlight On: Thermal Energy Storage—Changing the Game for Solar Power,” an article by members of the ASME Task Force (presented online in ASMEnews in early 2012), is the source of the information provided below.
Current R&D indicates that thermal energy storage at temperatures around 900-1,000 C is feasible and desirable. This temperature range is substantially higher than that used with present solar power tower technology—and envisaged for thermal energy storage for many other applications in different industries. Today, thermal energy storage relies mainly on sensible heat of materials, such as molten salts, piped between a hot charge tank and a ”cold” discharge tank in a two-tank system (see photo and discussion above and in the main article, CE, Nov. 2012). Such storage systems can be expensive, noted the ASME task force article (TFA).
Substantially more thermal energy per unit mass can be stored if the storage material undergoes phase change. Latent heat of storage of the phase-change material (PCM) can often be significant compared to sensible heat, based on allowable temperature changes in the heat transfer fluid used during thermal energy storage. It can translate to lower system cost, the TFA noted. Proper design of such a system could enable near isothermal heat transfer (which is thermodynamically reversible and thus better) for a significant part of the stored energy. PCM storage can also be designed with a favorable temperature gradient (thermocline) in the storage tank. This would allow one storage tank design instead of two tanks, further reducing system costs.
In addition, using encapsulated phase-change materials offers a better way to store thermal energy. PCM capsules provide much more surface area for heat transfer and, with proper design, do not inhibit heat transfer during energy storage or retrieval. Other advantages of using thermoclines with PCM capsules were suggested in the TFA:
- Cascades of a number of PCMs with different melting temperatures can be used to obtain significant system exergy gains. (Exergy refers to the maximum useful work possible during an equilibrium process between a system and a heat reservoir.)
- With proper choice of PCM (based on operating temperature and appropriate encapsulation), indications are that storage costs could approach the U.S. Department of Energy goal of $15/kWhth (kilowatt hours thermal) in capital costs—even for thermal energy storage around 950 C.
Research is ongoing at various universities and national laboratories to make thermoclines practical. Present research is intended to reduce thermal energy storage costs by as much as 75%, which will in turn forever change the market for solar thermal power, the ASME task force article emphasized.
The Thermal Energy Storage Task Force is headed by D.Yogi Goswami, PhD, and Sudhakar Neti, PhD, and operates under the Board on Technical Knowledge Dissemination’s Strategic Planning Committee, within the Knowledge and Community Sector of ASME. Interested professionals who would like to participate in the development of solutions in this critical multidisciplinary area can contact ASME’s Brandy Smith, program manager, emerging technologies; SmithB@asme.org.
Meanwhile, the global standards organization International Electrotechnical Commission (IEC) has formed a new technical committee called Electrical Energy Storage Systems. Technical committee TC 120 intends to oversee development of international standards that address various energy-storage technologies.
Among other tasks, TC 120 will develop architectures and roadmaps to support industry in building affordable and reliable electrical energy-storage systems that can be incorporated into existing grids anywhere in the world, according to an October 2012 IEC announcement. The overall objective of TC 120 is “to accelerate integration of renewable energy and enable a more reliable and efficient supply of electrical energy,” the IEC noted.
Formation of this international technical committee further indicates the growing importance of energy storage systems in the near future—as applied to renewable energy and various other industries.
- Frank J. Bartos, PE, is a Control Engineering contributing content specialist. Reach him at email@example.com
National Renewable Energy Laboratory, Technical Report (NREL-TP6A2-45833), February 2010, “The Value of Concentrating Solar Power and Thermal Energy Storage.”
The International Electrotechnical Commission has recently published two white papers on the topic of energy storage. Copies can be downloaded at: www.iec.ch/whitepaper/energystorage
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