Advancing geothermal power generation with concentrated solar power technologies

Stillwater hybrid geothermal-solar plant, Nevada (source: Enel, video screenshot)

Alexander Richter 14 Sep 2018

With a background in thermodynamics, heat transfer and fluid mechanics, research Guandong Zhu at NREL is looking at hybridization of geothermal power generation with concentrated solar power to increase efficiency and temperatures of fluids for power generation.

An article published earlier this year, discusses research done at the National Renewable Energy Laboratory (NREL) in the U.S., focusing on the work done by Guangdong Zhu.

Guangdong Zhu, called GD, has a bachelor in electrical engineering and a master in thermal engineering, and did his PhD in mechanical engineering at the University of New Mexico. He now works for NREL working on Concentrated Solar Power, looking into combining renewable energy technologies. As part of hat he started looking into bringing together geothermal and CSP technologies.

Working with colleague Craig Turchi, GD tackled an occasional problem in geothermal energy: a small portion of geothermal installations experience a decline in reservoir production—and therefore power generation—over time.

One solution for such geothermal plants is to add heat in some way to get the geothermal cycle back to its design-point efficiency. “Solar hybridization of existing suboptimal geothermal plants can give the geothermal industry some ‘insurance.’ Operators and investors can realize that they can tolerate higher uncertainty in geothermal reservoir assessments. In essence, hybridization can improve the bankability of the plant,” GD explains.

GD points to multiple recent projects as proof that hybridization works in the field. In Stillwater, Nevada, a CSP solar field is installed with the geothermal plant. Turchi and GD used a modeling tool to assess the hybrid plant’s performance and develop a strategy for best operations by working with engineers from the plant and researchers from Idaho National Laboratory.

Another project is at Coso, California, where 30 years of operational data have documented declining performance. NREL has been designing the best hybridization scenario to boost power performance and have found that geothermal-plus-CSP with thermal energy storage is more economic than using photovoltaics-plus-batteries.

An ongoing project at Raft River, Idaho, is determining the feasibility of a new configuration for solar and geothermal—using a solar “topping” cycle for high-temperature CSP, with the exhaust steam feeding into a geothermal “bottoming” cycle.

As a natural extension, another project considers designing a hybrid plant right from the beginning—as a ‘greenfield’ project—to fully explore the potential of two complementary renewable technologies.

“To do so, we need to deal with mismatch of the resource—solar is intermittent, geothermal is consistent night and day,” says GD. “But thermal energy storage stores higher-temperature solar heat, so hybridization can provide more efficient dispatchability, allowing geothermal to be more competitive in the future grid.”

Source: NREL