Solar thermal electricity generation is often touted as one of the most promising renewable energy solutions due to its lower cost and higher efficiency on a large scale compared with photovoltaics.

While promising, building such a plant is still expensive. I have recently gained interest in a hybrid power plant concept - using solar heat to aid in the generation of steam at existing fossil-fuel power plants. It combines the political popularity of the word hybrid with the generators and turbines already in place at existing coal-fired power plants to make a solar system which has the capacity to be cheaper and more effective (e.g. more watt-hours of solar pumped out to the grid) than a standalone plant.

It is obvious that hybrid power plants are not a long-term solution to the problem (just as hybrid cars are not), but looking at our current electrical generation mix (50% coal, 20% nuclear, 20% gas, 7% hydro, 3% everything else) it is clear that more renewable energy is necessary and has not been delivered. Anything that can make use of existing equipment and integrate renewable energy into the system is worth pursuing, especially if it can be done in a way that is cost-competitive with fossil fuels (which have most of their real costs externalized).

Such a plant would produce a constant electrical output. The amount of solar power will vary throughout the day and based upon weather conditions. Coal burn in the boiler would be modulated based upon solar radiation, keeping the amount of steam flowing to the turbine more or less constant.

SEPARATE WATER CIRCUIT IS USED FOR THE ARRAY TO AVOID HAVING TO WITHSTAND THE FEEDWATER PRESSURE. A BYPASS SYSTEM IS ALSO INCLUDED TO ALLOW COAL-ONLY OPERATION.

Advantages:

• Reduced coal burn in the boilers during sunlight hours.
• Cost of solar implementation is lower, using already-existing coal-fired plants with turbines and generators already in place.
• Utilization of equipment (turbines and generators) is higher than a solar-only plant due to constant power generation, reducing the payback period.
• Steam inlet temperatures to the turbine are higher than standalone solar, meaning lower heat rates and better electricity sales and utilization of equipment.
• Pollutant reduction from adding solar to a coal-fired plant is much greater than adding solar to a natural gas plant, as the carbon/joule ratio in coal is higher than gas.
• The absence of turbine/generator expenses means more money could be spent on collectors to capture more energy and burn less coal.
• A cheaper array may be built if it is only being used as a feedwater heater; temperatures and pressures may be much lower than in a standalone array working at the turbine inlet temperature. Large coal-fired plants require more power for feedwater heating than most commercial-scale solar plants produce in total.

Disadvantages:

• Siting of collectors arrays may be a challenge at existing power plants; transmission of the heat over long distances becomes impractical.
• May create a mentality that "coal is okay so long as it is supplemented with solar"

Links

• Cutting Coal Use with Sunshine