By Bob Moser, Americas correspondent

The US is currently the world leader in installed geothermal electric capacity with 3,086 MW in 2010. Its neighbour Mexico ranks fourth, with 958 MW, and is considered a potential growth market due to its proximity to California and geothermal hotspots in the along the Pacific coast.

High-pressure steam at geothermal sites can deplete and falter over time, diminishing the production of energy turbines above. New studies show those turbines could at least partially run on CSP-generated steam, and a range of cost-benefits may be realized by both industries.

The problem is, it hasn't been tested yet. There are no known CSP-geothermal hybrid plants operating anywhere in the world at the moment, according to NREL.

AREVA Solar currently has geothermal sites that happen to be in valued CSP locations, near California's Imperial Valley and the Salton Sea.  Within the last 18 months, the company has had inquiries for potential hybrid sites in northern Nevada and the Mojave Desert area of California, but interest isn't widespread.

“Developers aren't looking at this really seriously right now because it's more of a retrofit option,” said Milton Venetos, AREVA Solar's vice president of systems performance. “You'd have to identify promising (geothermal) candidates and go after them, and their existing facilities.”

A natural fit

Medium-temperature geothermal spots are being explored in Nevada and Utah, throughout the west coast and Gulf of Mexico. But California through Oregon is where most development is taking place today, and that's an area attracting new solar projects as well. Karl Gawell, executive director of the Geothermal Energy Assocation, isn't surprised that growing interest in pairing technologies has accompanied geothermal expansion.

“When you look at California and ask yourself what the renewable systems are that make most sense, given the demand profile its solar and geothermal,” Gawell said. “Their output just about tracks demand load in the state. So there are obvious market benefits in those two technologies working together.”

By pairing CSP with an existing geothermal power block, a developer can realize substantial savings on the project's overall capital expenditures compared to a stand-alone solar plant. In most CSP projects, the power block represents at least 30% of total cost, Venetos said, so linking CSP and existing geothermal can save that much, if not more, over a greenfield plant.

The overall supply stream of a hybrid plant in the American southwest could be a great match for peak load times there. Geothermal plants typically offer lower performance during the afternoon, when CSP could augment supply.

Geothermal can help CSP offer renewable energy around the clock, a key selling point for negotiations with utilities that have been wary of inconsistent supply from wind and solar. CSP would also benefit by doubling up on land-use, diluting the criticism of mirror sprawl with the efficiency of geothermal, which boasts the smallest ratio of land-use to megawatt amongst renewables.

The most prominent downside is that both power sources share the handicap of water usage if they're run with a wet-cooling model, said Craig Turchi, senior engineer at NREL's CSP program. “Wet-cooling is the single biggest reason this (hybrid) could be a problem,” he said. “For most of these plants in the Southwest, water is at a premium there.”

Turkey leads on solar-geothermal

Most countries outside of the US that sport strong geothermal potential are poorly situated for solar, and vice versa. But one country that may beat everyone to solar-geothermal hybridization is Turkey, where several high-temperature geothermal sites are producing energy, and solar power potential is now being realized. Engineering group BM Holdings, which holds 140 geothermal exploration licenses throughout Turkey, is in advanced stages of pioneering a hybrid geothermal-solar plant.

The company has already drilled seven wells and filed applications for preliminary operating rights to three reservoirs of superheated geothermal steam. A hybrid plant would use geothermal steam to power an initial series of generators, after which the hot water would pass through a solar tower to bring the water's temperature up to a necessary 120 degrees Celsius.

The Turkey hybrid is on hold at the moment because of low solar energy prices offered by the government. This would be the first hybrid plant of its kind in the world, and could serve as a model for future projects elsewhere.

Back in the US, the biggest challenge in Venetos' eyes is finding the right mix of available sites that have geothermal production in place, and good solar potential. In the end, the hybrid market may simply not be as big as everyone imagines.

“Overall market size is an issue, because geothermal is still relatively small in the total energy market (less than 1% of total US energy),” Venetos said. Establishing new geothermal sites continues to be an uphill battle for developers, as capital expenditures to drill deep wells are among the highest in renewable energy.

“But people are looking at a whole array of new technologies with new eyes, including geothermal, and you have to do that,” said Gawell of the GEA. “You've got to recognize that just because something doesn't immediately leap out at you doesn't mean it isn't a good idea.”  

Image credit:Solar Heat Pump Electrical Generation System

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