Virginia, like several other states, is experiencing growth in the number of new power plant permit applications or proposals for planned facilities. Some of the proposed plants will be "peaking units," which means they will operate and produce electricity only during periods of high electricity demand. These peak demand periods generally occur during the extreme hot spells of summer and extreme cold spells, usually in December and January. Other proposed plants will be "baseload units," which means they will generally operate full time.
To date, the applications received for new power plants are for facilities that will be fueled with natural gas which is less polluting than existing coal-fired units. Most of these facilities will have fuel oil as a backup fuel should the gas supply be interrupted. Peaking units are usually simple cycle turbines and baseload units are usually combined cycle turbines.
Simple Cycle Turbine: A simple cycle turbine is an internal combustion engine with three major parts; an air compressor, burner(s), and power turbine. In the air compressor, a series of bladed rotors compresses the incoming air from the atmosphere. A portion of this compressed air is then diverted through the burners (also called combustors), where fuel is burned raising the temperature of the compressed air. This very hot gas is mixed with the rest of the compressed air and directed to the power turbine at temperatures up to 2350oF. In the power turbines, the force of the hot compressed air as it expands pushes another series of blades, rotating a shaft. Greater than 50 percent of the mechanical energy produced by the power turbine is consumed to drive the air compressor. The balanced of the mechanical energy turns a generator and makes electricity. The cycle efficiency, defined as a percentage of useful shaft energy output to fuel energy input, is typically in the 30 to 35 percent range.
Combined Cycle Turbine: The difference between simple cycle turbines and combined cycle turbines is that in a combined cycle turbine, the hot exhaust gases from the turbine do not directly go to the atmosphere. Instead, the hot exhaust gases from the turbine, which are typically at 1000oF, are ducted through a waste heat boiler to generate steam. This steam is then used to drive a steam turbine generator to make additional electricity. The recovery of the heat energy in the exhaust of a gas turbine in this manner can increase the cycle efficiency of a combined cycle plant to 50 percent or more. The additional electricity that can be produced by a combined cycle turbine is accompanied by additional capital costs for a waste heat boiler, steam turbine, and cooling system. However, the operating cost per unit of electricity produced is lower compared to that of simple cycle turbines due to the energy recovery.
The main natural resources used by these facilities include fuel and water.