One critical element of a successful transformation of the grid with a high proportion of solar and wind power generation is the maintenance of power system flexibility. A flexible power system reliably supplies all customer energy demand at different time scales, from the very short to the long term — while managing the variability and uncertainty introduced by both the load and the VRE (variable renewable energy) delivered to the system. In addition to conventional power plants, system flexibility can be provided by grid infrastructure, demand-side response, and electricity storage. Even market, policy, and regulatory frameworks play an important role in unlocking flexibility. Operating solar generation in a flexible manner, when needed, is yet another option to increase overall system flexibility.
Investigating the Use of Solar in Grid-balancing Requirements
The value of this flexibility was illustrated in a breakthrough study carried out by First Solar for an actual utility system — Tampa Electric Company (TECO). We simulated operations of TECO and its generation portfolio to investigate the use of solar as an active participant in balancing requirements rather than being operated solely to deliver the maximum amount of electricity in real time.
For this relatively small utility system, the study found that under high solar penetration conditions, the operating reserves that were needed to accommodate solar uncertainty became a significant cost driver, leading to more conservative thermal plant operations: the need for additional reserves led to less efficient thermal plant operation at a reduced output. Consequently, curtailment of a large amount of solar became necessary to ensure that there was sufficient operating margin available from thermal generation.
On the other hand, this study found that leveraging the full flexibility of solar generation reduced uncertainty, enabled leaner operations, and even (surprisingly) reduced overall solar curtailment, especially as penetration levels exceeded 20% on the TECO system. This resulted in solar bringing significant additional value due to reduced fuel costs, reduced operations and maintenance costs, and reduced emissions. The key is to have both thermal and solar contribute to providing the balancing required (footroom and headroom) to manage the uncertainty associated with the load and solar generation.
Unleashing Solar’s Flexibility
What does it take to unleash this flexibility in solar generation? Many modern utility-scale solar plants have the technical capabilities to be flexible (dispatchable) and contribute to regulation and balancing requirements through precise and very responsive output. The entire suite of solar dispatch capabilities is made available to the system operator in determining economic dispatch. The system operator can elect to use the solar resources to provide either energy or regulation reserves, and this choice may vary throughout the day.
Provision of these grid services requires downward dispatch of solar, and some services require the plant operator to maintain headroom to enable upward dispatch. Since this results in lost solar energy production, the cost of solar providing these services is an opportunity cost that can be estimated in the context of economic dispatch. Obtaining grid services from solar plants can, in some instances, have the positive effect of enabling system operators to reduce fuel costs by reducing thermal generator commitments and increasing the efficiency at which they operate, as illustrated in the TECO study.
Taking Advantage of Solar’s Flexibility in Organized Markets
For an integrated utility that has a single ownership of all of the resources, performing this dispatch optimization is somewhat straightforward. However, in the context of the organized markets, there are several challenges that need to be addressed to fully realize the full potential of flexible solar. How would VREs bid their flexibility into energy and ancillary service markets? Existing methods of calculating opportunity cost for ancillary services are largely based on thermal opportunity cost of producing less energy and dispatching at less-efficient set points. Compared to thermal generators, VREs have more uncertainty surrounding day-ahead or hour-ahead maximum production levels. On the other hand, VREs may have no marginal cost of providing ancillary services if they are already curtailed due to system-wide conditions.
Most existing solar plant contracts rely on the full energy output of the plant and do not envision using the plant for grid balancing. Also, renewable energy certificates (RECs) are a primary revenue source in many cases, and a solar power plant would not want to forgo REC revenue by offering to be held in reserve unless doing so provided the generator with positive net revenue. However, as shown in the TECO study, flexible solar can add significant value to the overall system performance and cost. The Wind Solar Alliance report “Customer-focused and Clean: Power Markets for the Future” offers in-depth recommendations on some of the market and policy reforms that will support the transition to clean and affordable electricity.
Recent cost declines in energy storage technologies enable solar to further extend its contributions to grid flexibility by providing firm dispatchable capabilities, which in turn enables even higher solar penetrations. Adding storage to the grid can shift energy to when it is most needed, even if the sun has already set, combining the flexibility of solar with the firm capacity and energy-shifting capabilities of storage. This does require significant capital investment in storage resources. The economics of these options will likely vary by circumstance, for example, a recent study finds that curtailing solar may be a cheaper option than adding a significant amount of storage.
Dr. Mahesh Morjaria
Vice President – PV Systems
Dr. Morjaria leads an R&D effort addressing key challenges associated with integrating utility-scale solar plants into the power grid. Prior to joining First Solar in 2010, Dr. Morjaria worked at GE for over twenty years where he held various leadership positions. His academic credits include a B.Tech from IIT Bombay and a Ph.D. from Cornell University.
 E3, Tampa Electric Company, and First Solar, “Investigating the Economic Value of Flexible Solar Power Plant Operation.” 2018. https://www.ethree.com/wp-content/uploads/2018/10/Investigating-the-Economic-Value-of-Flexible-Solar-Power-Plant-Operation.pdf.
 California ISO, National Renewable Energy Laboratory, and First Solar, “Using Renewables to Operate a Low-Carbon Grid: Demonstration of Advanced Reliability Services from a Utility-scale Solar PV Plant.” 2017. https://www.caiso.com/documents/usingrenewablestooperatelow-carbongrid.pdf.