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Thermal Energy Storage Included in California Power Purchase Agreements

December 20, 2011

More and more, the energy industry is recognizing the value of thermal energy storage (TES) for concentrating solar power plants to provide cost-competitive, reliable, and dispatchable power that meets peak demand. TES is so valuable, in fact, that BrightSource Energy, Inc. and Southern California Edison (SCE) recently stipulated the use of TES in several revised power purchase agreements (PPAs). The agreements involve at least three power tower solar thermal plants that are slated to be built and operating between 2015 and 2017.

In late November, BrightSource announced the new PPAs, which must still be approved by the California Public Utilities Commission (PUC). The company stated in a press release that its own version of TES, called SolarPLUS, helps reduce the total cost of energy by increasing a power tower plant's capacity factor, that is, how much power a plant produces during a year. Furthermore, the increased efficiencies and capacity gained by using TES will allow BrightSource to build one less plant than it originally planned, while generating the same amount of energy.

A power tower solar thermal system features a field of mirrors, or heliostats, that reflect solar energy to a boiler atop a tower. In the case of BrightSource, the company uses its patented software program to adjust the position of the heliostats to optimal angles throughout the day, so that the maximum amount of the sun’s energy is reflected onto the boiler. The boiler produces high-temperature, high-pressure steam, which in turn powers a steam turbine to produce electricity. When TES is added to the system, steam can be directed to a heat exchanger, where hot molten salts are further heated to an even higher temperature, thus storing the heat energy for future use. When energy is needed (for example at night or when the sun is covered by clouds) the heat in the molten salts is used to generate steam to run the turbine.

In its press release, BrightSource cited a recent study by the National Renewable Energy Laboratory and Ohio State University that points to the high value of concentrating solar power technologies with TES. (See “The Value of Concentrating Solar Power and Thermal Energy Storage", published by NREL.)

BrightSource says the added value of SolarPLUS is derived from:

  • Shifting electricity production to periods of highest demand
  • Providing firm capacity to the power system; replacing the need for conventional power plants as opposed to just supplementing their output
  • Providing ancillary services, such as spinning reserves to help support a reliable grid
  • Avoiding the variability and integration costs that other renewable resources like photovoltaics (PV) and wind create for utilities and grid operators, reducing the need for additional fossil fuel units required to back up intermittent renewables that put a hidden financial burden on ratepayers.

(Note that another NREL report, however, makes the case that TES in CSP plants could actually boost penetration of PV and wind power by buffering the intermittency of those renewable generating technologies. See the December 6 article on NREL's Concentrating Solar Power news website.)

Under the original power purchase agreements with SCE, BrightSource would provide approximately 4 million megawatt-hours of electricity annually from seven power plants. Because of the higher efficiencies and capacity factors associated with TES, the new set of agreements will provide approximately the same amount of energy annually, but with one less plant. Not building the additional plant reduces the amount of land needed and related environmental impacts, reduces the cost of delivering the energy, and avoids transactional costs that could be borne by California’s ratepayers.

The new set of contracts, if approved by the PUC, will encompass two solar thermal plants scheduled to deliver electricity in 2015 and three solar tower power plants with TES scheduled to deliver electricity in 2016 and 2017.

For more information, see the November 28 announcement published by Business Wire.

—Karen Atkison