Using molten salt to store electricity offsets the high cost of solar thermal power, says report
By Gerard Wynn
The ability of solar thermal power to store electricity confers huge grid management benefits which help offset its high up-front cost, a US government report calculated this week.
Solar photovoltaic (PV) power dominates the generation of electricity from sunlight, with concentrated solar power (CSP), or solar thermal, coming a distant second.
That is because solar PV is cheaper to install, especially at lower levels of sunlight.
But the United States is poised to quadruple its installed CSP capacity this year, with five new utility-scale plants, to 1.3 gigawatts.
The ability of CSP to store electricity adds an important grid benefit, according to a report by the National Renewable Energy Laboratory (NREL), a research arm of the US Department of Energy.
That will help states meet both renewable energy targets and still balance the grid.
Solar PV is variable according to the strength of the sun, and that creates a headache for grid operators because of its inability to store electricity.
By contrast, CSP can store electricity, and so can be used to back up variable solar PV instead of using gas or coal-fired power, found the NREL report, “Estimating the value of utility-scale solar technologies in California”.
The savings were generated by the avoided cost of both building and running back-up fossil fuel capacity.
The report focused on the state of California, because of its abundant sunshine and ambitious climate legislation: California’s renewable portfolio standard (RPS) requires the state to increase electricity generation from eligible renewable energy resources to reach at least 33% of total retail sales of electricity per year by 2020.
Beyond 2020, California targets a further reduction in greenhouse gas emissions to 80% of 1990 emission levels by the year 2050, which may require a greater roll out of renewable power in the longer term.
CSP works by using sunlight to heat a liquid oil, which can then either be stored or used to generate steam and drive a turbine. The heat can used to melt salt in insulated tanks; the molten salt releases heat as required when it re-freezes.
Such energy storage adds a vital edge over solar PV, which dispatches power to the grid at the instant it generates electricity. Solar PV also provides grid management value, by replacing fossil fuel capacity and avoiding burning fossil fuels.
But CSP provides at least twice the benefit, because of its ability to store power, the NREL calculated.
CSP provided total grid management value of $94.6 to $107 per MWh, compared with $47.1 to $58.2 for solar PV, under a scenario of 33% renewable power in the grid.
The NREL report did not compare such benefits with the lifetime operating costs of solar power, dominated by the upfront cost of installing the equipment given zero fuel costs.
However, the Department of Energy recently updated its estimates for that levelised cost of energy (LCOE), in its Annual Energy Outlook 2014.
The AEO 2014 calculated the LCOE of CSP and solar PV at $243 and $130 per MWh for new projects respectively.
A crude comparison with the grid management benefits suggests that solar PV still has the value advantage, but CSP adds important flexibility.