As the penetration of wind and solar generation becomes a significant portion of grid power, utilities and government policy makers have begun to sponsor analyses to compare alternative renewable resource portfolios. This has resulted in calculations of net system costs in order to capture the full range of costs and benefits of different renewable technologies. This report surveys the recent research literature on the economic and reliability benefits of Concentrating Solar Power (CSP) with thermal energy storage, especially in comparison with other solar technologies. Its objective is to summarize in one report the major findings and conclusions of those analyses.
Attributes of CSP with Thermal Energy Storage
CSP plants use mirrors to focus sunlight into an intense solar beam that heats a working fluid in a solar receiver, which typically boils water to drive a conventional steam turbine that produces electricity. In many ways, it is like a fossil‐fueled steam power plant, the main difference being that its fuel supply is from the sun. The inclusion of thermal energy storage with a CSP plant removes, to a great extent, interruptions to its production that result from the intermittency of the solar resource. Storage also enables its power to be shifted to periods of highest demand and aids system flexibility, which is becoming increasingly important for grid operation.
Specifically, the combination of a steam turbine backed by stored thermal energy enables the plant to provide many of the functions necessary to support the transmission of power, short‐term energy balancing, protection against system contingencies, and resource adequacy. These include:
• ancillary services such as spinning or non‐spinning power reserves that could be brought quickly onto the grid if needed, and regulation (the plant’s ability to automatically increase or decrease its power on time‐frames of seconds to account for variability in demand or supply);
• flexibility in meeting capacity needs such that, similarly to a conventional gas‐fired plant, its energy can meet resource adequacy requirements at different times of day and in response to evolving needs;
• reduced requirements for integration into the grid, which is made easier by using storage or varying its production to lessen grid ramps (the rate of increase/decrease in grid system power) and reduce operator uncertainty due to solar forecast errors; and
• support for power quality, such as reactive power support, dynamic voltage support, and primary frequency control that is needed to prevent blackouts.
Although utilities and regulators are beginning to calculate net system costs when valuing alternative renewable resources, a number of reviewed studies show that more comprehensive methods are needed. Each renewable technology needs detailed simulations of its operations under a range of future scenarios for the grid, including comparison with the performance of alternative renewable technologies. The studies reviewed that did this analysis came to similar conclusions on the system costs and benefits of CSP when compared to alternative solar technologies. Nearly all of the referenced studies identified further analysis needed to better understand the implications to grid operation and performance due to variable solar and wind as it reaches 33% penetration. This could result in the need for additional ancillary services, increased operational flexibility, and improved forecasting of wind and solar. CSP with storage fits these forthcoming needs.
Storage generally allows CSP plants to shift electricity generation to whenever it is most needed throughout the day, overnight, or the next day as determined by the utility or system operator. At low penetrations of solar power on the grid, solar correlates well with daily peak demands. As solar penetration increases, however, analyses show that the peak demand net of renewable energy then shifts to the evening hours. CSP with storage obtains the highest capacity value of any solar resource as these grid changes take place, because its storage capability allows for shifting energy into the periods of highest capacity need.
To make procurement decisions that include a balance of both solar PV and CSP, utilities need to see reasonable estimates of quantifiable economic benefits. In simulations of the California power system, for example, recent studies by the Lawrence Berkeley National Labs (LBNL) reviewed in this report found that the comparative value of CSP with storage increases as the amount of solar on the grid increases. If CSP with 6 hours of storage and PV with no storage were each providing 5% of the grid’s power, CSP power would have an additional value of $19/MWh (1.9¢/kWh). At grid penetrations of 10% each, CSP power would be worth an additional $35/MWh (3.5¢/kWh). The added value results from a calculation of grid integration costs and market benefits. The National Renewable Energy Laboratory (NREL), in recent simulations of part of the Colorado/Wyoming power system, found similar results to LBNL: the comparative value of CSP storage increases as the penetration of wind and solar increases, and the value of CSP power increases relative to that of PV. Another NREL study shows that a renewable energy portfolio that includes CSP with storage provides operational flexibility that may enable both increased PV penetration and a reduction in investment in fossil‐fuel generation.
Renewable energy provides clean, sustainable power from abundant U.S. resources. The technologies to generate it are constantly improving and becoming less expensive. The grid into which these new technologies must integrate will also have to change to accommodate them. That change will come about with the help of analysts who model the grid to predict how it will operate under all possible conditions using the entire portfolio of generation and non‐generation resources. Work continues on improving the computer models, making them more accurate and running more scenarios. This report is meant to provide a source for the latest information on CSP integration into the grid and quantifiable benefits. As such, it is the intention of the CSP Alliance to update this report at least once a
year to incorporate the latest studies.
Frank (Tex) Wilkins Executive Director CSP Alliance