The Stillwater power station in Churchill County, about 80 miles east of Reno, will add a 24-MW array of solar photovoltaic panels (peak capacity) to an existing 27-MW geothermal power plant, making it the first of its kind in the U.S.
The newly combined facility is being built by Enel Green Power North America (EGP-NA), which is a division of Enel Green Power—the world’s largest renewable energy company.
EGP-NA owns and operates more than 70 plants with an installed capacity of around 800 MW in 20 states that are powered by renewable hydropower, wind, geothermal, solar, and biomass energy, and has some 500 MW of green power projects under construction in Nevada, Utah, Minnesota, Kansas, and Oklahoma.
The novel green-power project—which was announced in late August by Nevada Senator Harry Reid, Department of Energy Secretary Steven Chu, and EGP-NA President Francesco Venturini—will produce electricity that will be sold to local Nevada utility NV Energy.
Joining green energy sources
When the solar array is completed, the 240-acre site will feature 7,000 poles supporting some 81,000 fixed polycrystalline-silicon photovoltaic panels that will convert the sun’s rays into power, said Bill Price, Vice President of Geothermal Construction and Engineering at EGP-NA. Meanwhile, Stillwater’s nearby hydrothermal wells will also continue to generate electricity.
The concept behind this variety of combined facility is to augment the current power source with solar energy during periods of high demand, such as during hot Nevada summers when air conditioning use spikes. Together, the complimentary systems can reliably supply the surrounding region with electricity 24 hours a day, seven days a week, Price said.
This groundbreaking installation would provide the nation with an opportunity to lead the world in this market.
At the introductory press presentation, Venturini reportedly said that he believes this kind of solar-geothermal plant will be popular among American utilities with access to geothermal resources.
“This is an incredible new technology that can be used across the country,” stated Secretary Chu, who added, “All these things are going to be necessary as we move into renewables.” He noted that the groundbreaking installation would provide the nation with an “opportunity to lead the world in this market.”
Binary geothermal plant
As it stands now, the Stillwater geothermal facility uses binary geothermal technology—the most common type of geothermal electricity plant being constructed today.
In its binary system, he explained, moderately hot (“medium enthalpy”) water from geothermal wells feeds into a heat exchanger where its heat passes to a secondary working fluid of isobutane, which has a much lower boiling point than water. Because the isobutane working fluid is heated in a closed-loop pipe system, the fluid flash vaporizes into pressurized gas. The high-pressure gas then drives a turbine-generator set, which produces electricity. The isobutane vapor is next condensed back into a fluid state by an air-cooled system.
The company is building Stillwater’s fixed solar array in-house using local expertise and labor.
EGP-NA’s Stillwater and nearby Salt Wells facility are the only geothermal plants in the world that use large-scale electric submersible pumps in the wells to pressurize and thus extract geothermal water. The company is building Stillwater’s fixed solar array in-house using local expertise and labor.
Combined vs. hybrid
The use of both solar and geothermal power-generation systems, Price noted, not only improves the power plant’s electrical production profile, but reduces costs and the facility’s parasitic load by allowing the combined system to make use of the same plant infrastructure—inverters, transformers, reactive power equipment, switchgear—that prepares the DC power feed for the AC electrical grid.
Although the combined PV solar-geothermal plant benefits from the co-location of the two renewable energy sources, it would be too much to describe it as a hybrid system, a term that implies a greater degree of integration.
Such a description would, however, apply to another type of solar-geothermal plant installation: a hybrid concentrating solar power (CSP)-geothermal facility. A true hybrid solar-geothermal power plant would be one in which a geothermal system and a CSP system would both heat the same working fluid that flows into the power-generation turbines. A spokesperson for EGP-NA said that it is in the early stages of developing such a “test plant” in the same location in Nevada.
Illustration by Leslie Wood