August 13th, 2020
This analysis is a preliminary assessment of the potential benefits of CSP in providing grid flexibility using reduced form simulations with limited geographical scope and many simplifying assumptions. Gaining a more thorough understanding of how CSP can enable greater PV and wind penetration will require detailed production simulations using security – constrained unit commitment and economic dispatch models currently used by utilities and system operators. These simulations should consider the operation of the entire power plant fleet including individual generator characteristics and constraints, and the operation of the transmission system. The geographical footprint should cover the entire Western interconnect including possible transmission expansion to take advantage of greater spatial diversity of the wind and solar resources as well conventional generators.
To date, production simulations have not considered CSP operations in detail. Both the WWSIS and the first phase of the California 33% Renewable Portfolio Standard integration studies (CAISO 2011) included CSP, but assumed fixed schedules for CSP dispatch. This assumption limits CSP’s ability to shift generation to when needed most and to provide grid flexibility to enable PV and wind. Future and ongoing studies, including the second phase of both the California study and the WWSIS will evaluate the benefits of TES in more detail. To perform these simulations, production cost models will need to include the ability of CSP to optimally dispatch the solar energy resource, and not rely on heuristics or schedules often used to estimate the operation of conventional storage plants such as pumped hydro. However, the ability to optimize CSP, including scheduling both its energy and ability to provide operating reserves, is limited by lack of certain data sets needed for a more detailed simulation. A greater understanding of the predictability and variability of the solar resource, including the sub-hourly variation and the effects of spatial diversity in mitigating variability, is needed. This data will also be needed to determine any required increase in operating reserves over various time scales as a function of solar penetration. In addition, more data is needed on the actual characteristics of CSP plants—those now being deployed and under development—including ramp rates, turn-down ratio, part-load efficiency, and start times under various conditions.
While it will be some time until solar technologies achieve very high penetrations in the U. S. grid, international experience in wind deployment demonstrates the importance of increasing overall grid flexibility. Key factors in improving grid flexibility include increasing the ramp range and rate of all generation sources and the ability to better match the supply of renewable resources with demand via increased spatial diversity, shiftable load, or energy storage. The use of thermal energy storage in concentrating solar power plants provides one option for increased grid flexibility in two primary ways. First, TES allows shifting of the solar resource to periods of reduced solar output with relatively high efficiency. Second is the inherent flexibility of CSP/TES plants, which offer higher ramp rates and ranges than large thermal plants currently used to meet a large fraction of electric demand. Given the high capacity value of CSP/TES, this technology could potentially replace a fraction of the conventional generator fleet and provide a more flexible generation mix. This could result in greater use of non-dispatchable solar PV and wind meaning CSP and PV may actually be complementary technologies, especially at higher penetrations.
The preliminary analysis performed in this work requires advanced grid simulations to verify the actual ability of CSP to act as an enabling technology for other variable generation sources. Complete production simulations using utility-grade software, considering the realistic performance of the generation fleet, transmission constraints, and actual CSP operation will be an important next step in evaluating the benefits of multiple solar generation technologies.