TOP PLANT: Stillwater Solar-Geothermal Hybrid Plant, Churchill County, Nevada

Courtesy: Enel Green Power North America

Owner/operator: Enel Green Power North America
The Stillwater hybrid facility is the world’s first renewable energy project that pairs geothermal power’s baseload generation capacity with solar power’s peak capacity. Inaugurated in May, the 26-MW solar plant is integrated with the adjacent 33-MW geothermal plant, which began operations in 2009, and provides energy to run the geothermal plant’s auxiliary loads.

Combining the best of two renewable energy technologies, the Stillwater hybrid facility balances the continuous generation capacity of geothermal energy with the peak capacity of solar energy. The new solar plant’s photovoltaic (PV) panels cover 240 acres next to a geothermal plant in Churchill County, Nevada.

The Stillwater geothermal project, which received $40 million in tax support under the American Recovery and Reinvestment Act of 2009, harnessed innovative technologies to add solar energy to the facility and now provides 59 MW of combined capacity to power about 45,000 local homes. NV Energy has a contract to buy all the power generated by the plant.

“As the first of its kind in the world, this project demonstrates how we can tap renewable energy sources to provide clean power for American families and businesses and deploy every available source of American energy,” Steven Chu, secretary of the U.S. Department of Energy, said in May. “Supported in part by the Recovery Act, the Fallon facility is expanding domestic renewable energy sources and helping to build the infrastructure we need to stay competitive in the global race for clean energy technologies.”

How Geothermal Power Works

Geothermal energy is the only nonhydro renewable energy source able to provide baseload power because it relies on the continuous flow of heat from underground water sources rather than depending on the availability of wind or sun. In many geothermal reservoirs, however, the water temperatures are moderate (below 400F) and not hot enough to produce steam with the force needed to efficiently turn a turbine. Nonetheless, such moderate temperature reservoirs can generate electricity using a binary system.

Francesco Venturini, Enel Green Power North America’s (EGP NA) president and CEO, told POWER in October about the Stillwater geothermal plant’s medium enthalpy binary system. He explained that the facility uses the system because of the moderate temperature of the nearby geothermal reservoir. The plant’s binary system uses two fluids: hot water from underground wells heats isobutane and causes it to flash into vapor, which then turns the turbines to generate electricity.

In addition, the facility uses proprietary technology that increases efficiency by minimizing the parasitic load—energy losses that occur from operating the various pumps and fans required in the power generation cycle. The plant has a closed-loop system that continually replenishes the geothermal resource. Once the geothermal fluid has passed through the plant, it is pumped back into the ground. Therefore, there are zero intentional emissions from the process and virtually nothing is emitted to the atmosphere.

Venturini added that his company has strong expertise in the geothermal energy sector. “EGP NA is the first company in the world to produce electricity from geothermal sources dating back to 1904 in Larderello, Italy. The company has over 700 MW of geothermal capacity in full operation and uses innovative operational and drilling techniques to optimize its projects’ output,” he said.

An Electric Combination: Geothermal and Solar Technologies

Adding the solar component to the Stillwater facility was something that occurred to EGP NA management after building the geothermal component, Venturini said. He also explained his company’s goals related to developing new synergies between geothermal and solar energy.

“Having succeeded in proving the concept in a commercial-scale application, EGP NA Green Power’s Stillwater Solar project paves the way to address key drawbacks for both geothermal and solar technologies: resource risk/parasitic load and generation intermittency, respectively,” Venturini said. The geothermal plant also provides auxiliary power to the solar plant when there is no sunlight, “thus eliminating the need for backfeeding power from the utility,” according to Venturini.

Currently, a number of utilities use renewable energy sources like solar power during hours of peak consumer demand and combine it with a baseload coal or natural gas plant to ensure a steady power supply. In contrast, the Stillwater facility combines two renewable energy technologies to produce electricity at the same location and thereby increases the generation of zero-emission electricity.

Combining geothermal and solar energy at the Stillwater plant also makes it possible to use the same infrastructure, further reducing environmental impacts. That integration includes the control system, electrical protection and island mode capability, fire detection/protection schemes, electrical interconnection, and the use of a common operations and maintenance staff.

“It has to be said that since this geothermal-solar project is a first of its kind, it met the regular challenges of everything new in terms of combining two advanced technologies, as well as challenges of a regulatory and administrative nature,” Venturini said.

This innovative hybrid power plant demonstrates that the strengths of these different renewable technologies combine to create a better whole. Together, they:

  • Enhance the thermal efficiency in the geothermal unit when it is lowest, typically during the hottest and sunniest times of the day or year.
  • Stabilize production during the day, enabling a more load-following production profile.
  • Reduce investment risk due to the uncertainty of the geothermal resource and compensate for geothermal reservoir temperature depletion without reducing production.

Plant Profile

The solar power component at the Stillwater facility consists of more than 89,000 polycrystalline premium photovoltaic (PV) CNPV-295P modules on fixed mounts (Figure 1). They were manufactured by CNPV Solar Power SA, an integrated manufacturer of solar PV products. Las Vegas–based Bombard Renewable Energy was the general contractor for the solar project.

1. Some like it hot. The Stillwater facility integrates 26 MW of photovoltaic solar generating capacity with 33 MW of baseload geothermal power. In the geothermal plant’s binary system, hot water from underground wells heats isobutane and causes it to flash into vapor, which then turns the turbines to generate electricity. Courtesy: Enel Green Power North America

The benefit of adding solar to the Stillwater facility’s production capacity has been confirmed in generation measurements to date. “Average daily generation in the peak hours is significantly enhanced by the PV system, while the geothermal plant begins to reach optimal generation levels when solar generation ramps down,” Venturini explained.

“From a source point of view, there were no development difficulties, as the geothermal plant (operational since 2009) was already positioned in an area with good solar irradiation levels, so EGP NA just had to install the PV facility and connect it to the grid,” Venturini said. “It also helped that Enel Green Power was finishing construction of its pilot project in Italy integrating a solar thermal system to boost efficiency of a combined cycle natural gas power plant.”

EGP NA employs approximately 50 staff members in the state of Nevada. The company has roughly 30 employees who operate and maintain the Stillwater Solar Geothermal Hybrid Plant and EGP NA’s Salt Wells Geothermal Plant (which also became operational in Nevada in 2009).

Venturini noted that the project’s success derives from a number of factors:

  • The hard work and commitment of the engineers and staff at EGP NA.
  • The collaboration with and support of NV Energy, state and local government agencies, and the local communities—all of which were essential in completing the project.
  • Federal and state energy policies supporting renewable energy that were critical in EGP NA’s decision to commit to this successful investment.

Looking Ahead

The Stillwater hybrid plant is already being recognized as a trendsetter in the renewable energy sector that will probably encourage future hybrid projects. On June 28, 2012, the Geothermal Energy Association (GEA) recognized the Stillwater Solar-Geothermal Hybrid Project for advancing geothermal technology. The GEA singled out the Stillwater facility for being the first hybrid power plant of its kind. The association pointed out that “this technology may help to allow future projects that would otherwise have been unfeasible as stand-alone geothermal or solar projects to be more economically and technologically viable.”

In his remarks at the dedication ceremony for the Stillwater Solar Plant in May, Nevada Governor Brian Sandoval praised EGP NA for its innovation and leadership and looked forward to future growth in the Nevada renewable energy sector. “Clean energy is a key sector for Nevada. It provides energy from local sources, drives innovation, and most importantly, brings high-quality jobs and economic growth to the local communities. Enel Green Power’s first-of-a-kind solar geothermal hybrid project is a living example of these benefits and I support the further growth of this industry in Nevada,” Sandoval said.

Angela Neville, JD is POWER’s senior editor.

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