Sun + Fossil Fuel = Efficiency

I read an article (http://www.technologyreview.com/energy/22080/page1/) about how about using the heat of the sun can boost the efficiency of existing fossil fuel power plants. I thought this was an interesting concept to explore.

One type of power plants that is prevalent is called a combined cycle plant. It uses two different types of engines (cycles) to create electricity. One cycle is a combustion engine called a gas turbine or combustion turbine (Brayton Cycle) and the other is a steam engine or steam (Rankine) cycle. It is represented by the graphic (Source: Texas Utilities).

Let’s explain what’s going on here and how adding solar energy can improve the engine’s efficiency. Think of the Gas Turbine Cycle above as if it were an aircraft engine, which are also gas turbines. As you can see this engine drives an electric generator to create electricity. The exhaust gases from this engine are very hot, and if allowed to escape to the atmosphere, will dissipate and be of no use.

However, power plant designers have devised a way to capture and reuse this waste heat by running it through a boiler and powering a steam turbine cycle, represented at the bottom portion of the graphic above. Combined Cycle Power Plants are currently the most efficient way to create electricity.

So, how do we introduce the sun into this mix? In some combined cycle power plants, the boiler in the steam cycle is capable of being fired with additional fossil fuel, usually natural gas. This boosts the output of the plant during the peak times of the day when electricity use is at its highest. However, because it is firing additional fuel to obtain the increased output, it will usually be less efficient to do so. So why do it? The price of power is higher at the height of electric use or peak time of the day, so it is worthwhile to have reduced efficiency for additional plant output. Using heat provide by a solar collecting array eliminates the need for the additional fossil fuel.

Although sunlight may be free, the technology that turns sunlight it into steam is not. The power plant will have to build and pay for the solar array, but, as a result, over time the plant will produce more energy when it is most needed at a more predictable cost rather than the volatile and unknown expense of the displaced fossil fuel long into the future.