White Papers

The Stillwater Triple Hybrid Power Plant: Integrating Geothermal, Solar Photovoltaic and Solar Thermal Power Generation

By Giuseppe DiMarzio, Lorenzo Angelini, William Price, Chun Chin and Steve Harris


The Stillwater geothermal project is located in Nevada, USA, and is owned and operated by Enel Green Power North America, Inc. (EGP-NA). The first phase of the project began with a geothermal plant, a 47 MW gross binary plant which was commissioned in 2009. A desire to increase output led EGP to add 26 MW of solar photovoltaic (PV) power to the project in 2011. The solar PV project size was tailored to complement the geothermal plant output degradation during hot summer temperatures. In 2013, design began on an additional solar project using Concentrated Solar Power (CSP) thermal technology. The augmentation project relies on linear parabolic trough systems to add energy to the incoming geothermal fluid, which allows the binary plant to increase output. The project consists of collectors, a heat exchanger, a circulating pump and a control system integrated with the geothermal plant. The solar field adds about 17 megawatts of thermal energy, and is estimated to add an equivalent of up to 2 megawatts of boost in power generation to the geothermal power plant. The following paper presents the Stillwater resource park as a prime case study for the benefits of collecting multiple renewable energy generation types, which harness multiple resources, such as existing infrastructure, transmission capability and human capital. It serves as a template for similar projects blessed with comparable resources. Performance contributions of the geothermal, solar PV and solar thermal aspects are presented, with commentary on operations and lessons learned for the integrated unit. Stillwater was the first geothermal power plant to be paired with solar photovoltaic generation and is also the first to be coupled with concentrating solar power field on a major utility scale. When completed, it will be the first power plant to incorporate the three renewable technologies.

To request a copy of this paper, send your name, company name and e-mail address to generation@powereng.com. Please include the title of the paper you are requesting.