The project uses an improved technology—a supercritical binary geothermal cycle—to extract energy from rock and fluids in the Earth’s crust. The technology is estimated to be more efficient than traditional geothermal binary systems, allowing lower-temperature geothermal resources to be used for power generation. Unlike coal-fired and natural gas-fired power generation plants, geothermal plants produce virtually no greenhouse gas emissions.

Binary geothermal plants typically use an Organic Rankine Cycle system, according to the Geothermal Energy Association (GEA). The geothermal fluid heats a working fluid, such as isobutane or other organic fluids such as pentafluoropropane, which boils at a lower temperature than water. The two liquids are kept completely separate through the use of a heat exchanger, which transfers the heat energy from the geothermal water to the working fluid. The secondary fluid expands into gaseous vapor. The force of the expanding vapor, like steam, turns the turbines that power the generators. All of the produced geothermal water is injected back into the reservoir.

The supercritical binary cycle—which has been explored for several decades—raises the working fluid to supercritical pressure before entering the heat exchanger, where it can absorb more heat from the geothermal fluid.

The project’s total output will be sold to Idaho Power Company under a 25-year power purchase agreement.

The Department of Energy, through the Loan Programs Office, has issued loan guarantees or offered conditional commitments for loan guarantees totaling nearly $18 billion to support 19 clean energy projects. The program’s eight generation projects alone will produce nearly 23 million megawatt-hours or enough to power almost two million homes. Additional DOE-supported projects include two of the world’s largest solar thermal projects, the world’s largest wind farm, and the nation’s first nuclear power plant in three decades.

Resources

• C.J. Bliem and G.L. Mines (1989) Supercritical Binary Geothermal Cycle Experiments with Mixed-Hydrocarbon Working Fluids and a Near-Horizontal In-Tube Condenser

• Zhaolin Gu and Haruki Sato (2002) Performance of supercritical cycles for geothermal binary design. Energy Conversion and Management Volume 43, Issue 7, Pages 961-971 doi: 10.1016/S0196-8904(01)00082-6