Enel's Stillwater plant uses solar mirrors to reheat geothermal brine in hot afternoons

Ars Technica reported in 2016 that Enel Green Power's Stillwater plant in Fallon, Nevada combines geothermal with a 26MW solar PV array and a concentrated solar power system, which the Department of Energy called the only triple-hybrid renewable plant in the US. Soon after the geothermal portion opened in 2009, Enel noticed the desert's ambient heat slowed the air-cooled system's heat shedding and cut midday output.

Since it could not make the afternoon cooler, Enel made the brine hotter. Its 16-foot parabolic mirrors heat the pipe so brine goes from 300 to 390 degrees Fahrenheit. The CSP system came online in 2015, and the DOE said that from March to December 2015 it increased overall output by 3.6 percent on average.

Together, PV produces by day when geothermal output sags, CSP boosts geothermal in the heat, and at night the cool air means geothermal runs well without CSP. In October 2016 the DOE and Enel announced studies to optimize the three sources.

Air cooling is least effective exactly in the hottest hours.

Heating brine raises the temperature difference the plant works with.

Solar PV fills the daytime dip and geothermal covers the night.

At night CSP is not needed because the cool air restores geothermal performance.

If an environmental condition hurts one energy source at a given time, pair it with a source that peaks at the same time.

The DOE measured a 3.6 percent average output gain in 2015, and the plant produces 24/7. The DOE and Enel started joint studies in October 2016 on optimizing Stillwater's three energy sources. Enel could not say whether more such projects were planned.