Concentrated solar power (CSP) with thermal energy storage (TES) can address mounting problems associated with integrating renewable power into the grid, enabling renewable resources’ share of the energy supply to grow w

Growing concerns about over‐generation (projected to increase exponentially as renewables approach 50% of the energy supply) and solar and wind variability can be mitigated by CSP with storage. CSP with storage acts as a catalyst, allowing increased solar energy and wind power without adding carbon or other emissions associated with the  “peakers” that might otherwise be needed for reliability – while providing substantial quantities of clean power. 

Concentrated solar power  (CSP) uses mirrors to concentrate sunlight on a receiver, heating fluids to upwards of 1,000°F or more. The heated fluid can either produce steam immediately, driving turbines just like those in fossil fueled plants to produce electricity, or the heated fluid can be stored to generate power at a later time.

The ability to store and produce energy at any time means CSP is dispatchable—i.e., able to provide or withhold power as needed by the grid, thereby enabling the grid to absorb more renewables that are less flexible.

Of the multiple technologies that comprise CSP, two are now being commercialized: parabolic troughs and power towers. The most recent CSP+TES plant in the U.S. is the 280 MW Solana parabolic trough facility, in Gila Bend, Arizona.

Solana has six hours of storage, allowing it to convert heat stored in molten salt form to electricity at any time, day or night. On a typical summer day, for example, Arizona Public Service (APS) could dispatch Solana to generate electricity for up to six hours after the sun goes down, to supply air conditioning demand through a warm evening. On a cold winter day, APS could dispatch Solana to start operating at 4 am, provide electricity until the end of the morning peak, then  come back on line for the evening peak.

As with conventional energy resources, the market for CSP and other renewable energy resources is driven, in part, by government policies. While these policies have been successful in encouraging private sector investment and deployment, reliability and cost issues have arisen due to lack of attention to the need to 

procure a balanced mix of diverse resources as renewables’ share of the energy supply increases.

Germany, with the world’s highest renewable energy levels, is going through “the worst structural crisis in the history of energy supply," according to Peter Terium, CEO of RWE, the country’s second largest utility.

Similar concerns have echoed in Italy, Spain, and elsewhere in Europe.” Here in the U.S., California has made remarkable progress towards its 33% renewables portfolio standard (RPS) while maintaining reliability and minimizing cost increases, in part due to the grid flexibility provided by its hydropower and its mild climate.

Nonetheless, similar issues to those experienced in Europe are beginning to emerge; to maintain an affordable, reliable grid as renewables increase, their lessons must not be ignored.

Governments promote renewable energy to achieve several important objectives: energy security, improved air quality, and climate protection. To meet these objectives, while maintaining grid reliability and minimizing undue costs, a diverse portfolio of renewables that collectively support grid needs is vital.

The grid is a highly complex system. As renewable portfolio standards increase, several grid‐related concerns emerge, including overgeneration (which can destroy key grid elements), fluctuations in power, lack of predictability, and steep ramp rates (i.e., the rate at which the net power load changes).

Recent analysis by Energy + Environmental Economics (E3) concludes that over‐generation will be the largest renewable energy integration challenge, estimating that “over‐generation will increase exponentially at RPS levels approaching 50%.” Over‐generation would require generators to reduce (curtail) output, reducing their income and becoming an economic threat to them. While some suggest the “duck curve” and the E3 conclusions represent worst‐case scenarios, the seriousness of the risks they portray are widely recognized.

Generation resources called “peakers” have traditionally provided the precise, continuous balance between supply and demand needed for grid reliability. Within this decade, increased renewables would require peakers to operate at or near their reliability thresholds, and to ramp up and down at rates that may be difficult to achieve—increasing their emissions, as well as their costs. For example, the highest average ramp rate in California is presently about 30 MW per minute. By 2020, it is estimated to be three times higher.

These challenges may increase as intermittent renewable levels increase, and threaten to undermine renewable policy objectives – and erode public support for these policies. CSP+TES can mitigate these problems while decreasing emissions.


The Regulatory Assistance Project (RAP) takes a positive outlook on high renewable penetration, indicating CSP+TES, along with other clean energy resources — such as electrical storage, demand response, and energy efficiency — can cost effectively address grid reliability concerns.

CSP+TES offers essentially all the electric power products and services provided by fossil‐fueled plants, without their carbon and other emissions. CSP+TES provides capacity and operational attributes the grid needs as renewables increase, such as fast‐ramp rates that can be sustained for multiple hours.

As a synchronous, steam‐cycle resource, it can provide regulation services that “shore up” intermittent generation resources, while contributing carbon‐free energy. CSP+TES also provides other vital grid services such as voltage support, frequency response, spinning and nonspinning reserves.

CSP’s ability to store energy adds flexibility to the grid. Several studies quantify the comparative value of power from CSP+TES. Lawrence Berkeley National Laboratory estimates CSP+TES with six hours of storage would provide $0.035/kWh more value than solar without storage.

The National Renewable Energy Laboratory (NREL) came to a similar conclusion, estimating the range of additional value of CSP+TES as $0.03 to $0.04/kWh. Notably, these studies focus only on the flexible reliability benefits that are most critical as RPS targets increase; they do not attempt to calculate the value of the other grid benefits CSP+TES provides.

In a further report, NREL provided additional analyses of CSP+TES’ ability to address emerging changes to the energy load curves. Like the E3 report, NREL found that marginal curtailment increased rapidly after threshold levels of nondispatchable solar were added to the grid; however, it also found that adding CSP+TES can significantly decrease curtailment, increasing the effectiveness of the nondispatchable solar.

NREL’s shows how CSP and PV complement one another. NREL’s projection shows three curtailment rates: for PV alone, PV plus CSP+TES, and PV plus CSP+TES assuming CSP+TES is not dispatched to provide the flexibility benefits it could offer. The red “PV” curve shows significant curtailment once PV reaches ~14% of grid power (the projected PV level for California’s 33% RPS). The green “PV plus CSP” curve shows curtailment decreases when PV and CSP provide power during the day and CSP provides additional morning and evening power to reduce some curtailment. The white curve shows the impact of CSP+TES when it provides additional flexibility in the form of higher turndown.

More analysis is needed, but this study suggests high renewable portfolio standards can be implemented without loss of grid reliability when CSP+TES is added to the portfolio.


Current approaches to procuring renewable energy could lead to an unnecessarily unstable grid and increased costs, ultimately stunting renewable energy growth. CSP+TES could provide a cost effective means to increase renewables penetration while maintaining a stable grid. A revised procurement process, intentionally targeted to achieve a least‐cost, least‐emissions and reliable grid, would enable CSP+TES and other solutions to contribute to a clean energy future.

“Turndown” refers to the operation of a power plant below its rated capacity. Coal and nuclear plants have historically been designed to provide baseload power. They operate at least cost and highest efficiency when at full capacity. They can, however, operate at partial load although doing so accelerates damage.

Frank (Tex) Wilkins, Arthur Haubenstock, Kate Maracas, and Fred Morse

Concentrating Solar Power Alliance, Perkins Coie LLP, and Abengoa Solar


Frank (Tex) Wilkins is Executive Director of the Concentrating Solar Power Alliance. Tex previously led DOE’s CSP, Solar Industrial, and Solar Buildings Programs. He holds degrees in mechanical engineering from the University of Maryland.

Arthur Haubenstock is senior counsel in Perkins Coie’s Environment, Energy & Resources practice. Arthur previously held senior positions with BrightSource

Energy and PG&E. He holds a J.D. from Georgetown University and a B.A. from Wesleyan University.

Kate Maracas is an energy consultant, was Vice President of Development for Abengoa Solar, and has practiced in the energy sector for over 30 years. Kate holds

degrees from Thunderbird Graduate School and Arizona State University.

Fred Morse is Senior Advisor of US Operations for Abengoa Solar. Fred previously served as Executive Director of the White House Assessment of Solar Energy and Director of DOE’s Solar Heat program. Dr. Morse holds a B.S. from RPI, an M.S. from MIT, and a PhD from Stanford.