Power plants fossil-fired, efficiency

Fossil Fuel-Fired Plants. In modem, fossil fuel-fired power plants, the Rankine cycle typically operates as a closed loop. In describing the steam—water cycle of a modem Rankine cycle plant, it is easiest to start with the condensate system (see Fig. 1). Condensate is the water that remains after the steam employed by the plant s steam turbines exhausts into the plant s condenser, where it is collected for reuse in the cycle. Many modem power plants employ a series of heat exchangers to boost efficiency. As a first step, the condensate is heated in a series of heat exchangers, usually sheU-and-tube heat exchangers, by steam extracted from strategic locations on the plant s steam turbines (see HeaT-EXCHANGETECHNOLOGy). 

A conventional power plant fired by fossil fuels converts the chemical energy of combustion of the fuel first to heat, which is used to raise steam, which in turn is used to drive the turbines that turn the electrical generators. Quite apart from the mechanical and thermal energy losses in this sequence, the maximum thermodynamic efficiency e for any heat engine is limited by the relative temperatures of the heat source (That) and heat sink (Tcoid) ... 

Thus, the net reaction is just Eq. 15.77. Methane-fueled MCFCs are leading candidates for local power plants in the range 1-3 MW, as their cost and efficiency are more attractive than those of PAFCs. The relatively high temperature of the exhaust gases means that they can be used effectively to drive conventional gas or steam turbines to generate additional ( topping ) electricity for increased overall efficiency. Less CO2 per kilowatt-hour is produced than by other fossil fuel-fired power generation methods, and emissions of NO , SO , and unburned hydrocarbons are minor. 

Only large gas turbines of multi-MWei size and low rpm diesel engine generators of 1 MW and more can display efficiencies above 40% [11], thus competing with modem coal-fired power plants. Nevertheless, considering the avoidance of grid losses and the dramatically increased total conversion efficiency, a clear case can be made for CHP units with respect to reductions in CO2 emissions and fossil fuel savings. 

Solar voltaic cells based on crystalline silicon have operated with a 30% efficiency for experimental cells and 15-20% for commercial units available in 2008, at a cost of around 15 cents/kWh, compared to 4-7 cents/kWh for fossil fuel-fired power plants and 6-9 cents for those fired by biomass. Costs of photovoltaic electricity have shown a continuous downward trend. Part of the high cost in the past has resulted from the fact that the silicon used in the cells must be cut as small wafers from silicon crystals for mounting on the cell surfaces. Significant advances in costs and technology are being made with thin-fllm photovoltaics, which use an amorphous silicon alloy. These cells are only about half as efficient as those made with crystalline silicon, but cost only about 25% as much. A newer approach to the design and construction of amorphous silicon film photovoltaic devices... 

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