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Several U.S. utilities are experimenting with hybrid concentrated solar thermal technology to boost generation at gas, coal, and even geothermal power plants. These projects are aimed at reducing fossil fuel consumption and carbon emissions, and to fulfill renewable portfolio mandates. As a result, concentrated thermal solar is emerging as a viable baseload or peaking-power option—as an add-on to an existing plant, or as part of a greenfield endeavor.

Adding a solar thermal plant to an existing generating facility can be more cost effective than adding a stand-alone solar photovoltaic (PV) plant. But new technology developments with solar thermal are helping to drive down costs even further. Heat storage capability and scalable modular design are two such development factors now being tested by utilities.

Since much of the connection, control room and power plant equipment at an existing generating facility can be shared by the solar addition, the cost of installing the solar equipment needed for a hybrid facility can be half or less of what would be required for a standalone solar unit, according to Hank Price, a director of technology at Abengoa Solar.

Figure 1. A 26-MW photovoltaic array is being added to Enel’s Stillwater geothermal plant in Nevada. Source: ENEL

Emissions reductions can be another driving factor in a utility’s decision to go with a solar-fossil hybrid, which can help it meet state renewable portfolio standards. “An augmented (hybrid) system reduces carbon dioxide emissions as much as a stand-alone solar thermal plant with the same size array, but at a much lower cost,” says Craig Turchi, a senior engineer in the concentrated-solar power program at the National Renewable Energy Lab in Golden, Colo.

And simple fuel efficiency is a strong economic driver in the decision to adopt a hybrid solar booster. Solar hybrid plants show increased fuel efficiencies over a basic combined-cycle plant of anywhere from a few percentage points to as much as 25 percent, according to various system providers. While a solar plant without storage can only provide half its nameplate kWh capacity—given operation is limited to daylight hours—when storage is added, solar thermal plants remove the limitation and can provide heat ’round the clock. One source of the efficiency boost from a hybrid is startup time: “Hybrid plants don’t suffer the thermal inefficiencies associated with the daily startup and shutdown of a steam turbine,” Turchi says.

Solar Steam

The basic thermodynamics of tapping solar thermal energy are simple.

In a standard coal- or gas-fired plant, heat in the boiler creates steam to drive a turbine, which produces electricity. In a typical combined-cycle power plant, a gas-fired turbine is paired with a steam turbine; exhaust heat from the turbine engine drives a separate cycle to raise fuel efficiencies to 55 percent and above. An integrated solar combined-cycle power station can further increase efficiencies by double digits, by providing a separate line of additional steam to the steam turbine, displacing fossil fuel cost for the same amount of electricity.

The higher the temperature of the solar-derived steam, the more efficient the hybrid steam addition can