In April, researchers from the U.S. Department of Energy's (DOE) National Renewable Energy Laboratory (NREL) announced they had quantified the value that concentrating solar power (CSP) plants can add to an electric grid. NREL's computer model evaluated the operational impacts of CSP systems with thermal energy storage (TES) within the California electric grid managed by the California Independent System Operator.
The TES aspect is important, says NREL senior analyst Paul Denholm. ‘The primary purpose of the analysis was to identify the additional value that thermal storage brings to CSP,’ he says. ‘It is a surprisingly under-analyzed piece. When we started looking at the literature, there was basically nothing out there on how much more CSP is worth when you added thermal energy storage.’
California has a renewable portfolio standard (RPS) of 33% by 2020. According to NREL's paper, ‘An Analysis of Concentrating Solar Power with Thermal Energy Storage in a California 33% Renewable Scenario,’ the California Public Utilities Commission (CPUC) developed scenarios associated with the state meeting this RPS. The scenarios included CSP but not significant amounts of TES.
CSP with TES is a dispatchable source of renewable energy, making it more valuable than other variable-generation sources, such as photovoltaics and wind without TES. CSP with TES has the flexibility to provide power during periods of high demand – and value – and stay offline during periods of low demand, the paper noted.
The CPUC scenarios depicted various mixes of generation and assumptions about changes in operating requirements. The scenarios showed the different regions in California, as well as various combinations of biomass, geothermal, small hydro, solar PV, distributed PV, CSP and wind. For the NREL paper, the researchers chose the ‘Environmentally Constrained’ scenario, which assumed a mix that included a higher contribution from solar than the other scenarios, which had names such as ‘Cost Constrained’ and ‘Time Constrained.’
NREL researchers took the Environmentally Constrained scenario and created a baseline scenario, then added four types of generators: a baseload generator with constant output, a photovoltaic system, a CSP plant providing dispatchable energy and another CSP plant providing both energy and operating reserves.
That, as it turns out, was an almost overwhelming amount of data. The model simulated one year's worth of generation, hour by hour, from thousands of power plants. There were millions of combinations of these generators.
‘The modeling was challenging,’ Denholm says. ‘It took several days to get results. We ran four computers at once, and it took two days to spit out results. And these are new state-of-the-art computers.’
The results, in the form of ‘operational value per unit of delivered energy,’ measured in dollars per megawatt-hour, indicated that when CSP can provide reserves, that adds about $17/MWh to the total value, compared to CSP without reserves. CSP with TES adds $30/MWh to $51/MWh incremental value compared to a baseload resource. The added value was $32/MWh to $40/MWh compared to PV. These numbers are denoted in a range of low capacity to high capacity, which depends on the ability of CSP to provide operating reserves and the expected cost of new capacity.
Those were the results the researchers expected. ‘We knew storage would be worth more, but I didn't have a good feel for how the numbers would work out,’ Denholm says.
The team will use the model for additional research, such as creating scenarios with different technology options and generation mixes. Denholm says the DOE has tasked the team with another study.
‘One really big, glaring thing we didn't do was look at power towers,’ he says. ‘We looked at trough technology. They are building CSP towers in California, and we didn't do those.’
Nora Caley is a Denver-based freelance writer.