Overall efficiency cogeneration plants

Fig. 9.2 shows how a simple open circuit gas turbine can be used as a cogeneration plant (a) with a waste heat recuperator (WHR) and (b) with a waste heat boiler (WHB). Since the products from combustion have excess air, supplementary fuel may be burnt downstream of the turbine in the second case. In these illustrations, the overall efficiency of the gas turbine is taken to be quite low ((tjo)cg = ccJf ca 0.25), where the subscript CG indicates that the gas turbine is used as a recuperative cogeneration plant. 

At 1-10 W (watts), fuel cells could be used as battery replacements at 100 W to 1 kW, fuel cells could find military applications which require lightweight portable power sources for communications and weapon power at 1 - 10 kW, fuel cells could supply power to residential buildings and serve as auxiliary power units in vehicles and trucks. At higher power levels, the solid oxide fuel cell (SOFC) could be an effective approach for the distributed power generation and the cogeneration (i.e., combined heat and power). Above 1 MW, the SOFC could be integrated with a turbine power plant to improve the overall efficiency of power generation and reduce emissions. ... 

Another approach to increasing power plant efficiencies is to use a nonther-mal conversion method for power production, such as fuel cells. Fuel cells rely on electrochemical conversion of the chemical energy in the fuel to electric power. In the cogeneration mode, these systems have been reported to be operable at overall efficiencies as high as 85% (Schora, 1991). Large-scale power plants based on fuel cells have not been developed yet and are not expected to be available for generating central station power until well into the twenty-first century. 

MC Power The world s largest molten carbonate fuel cell cogeneration plant, a 250-kW facility built by MC Power, began producing power at the US Navy s Miramar Naval Air Station in the spring of 1997. With cogeneration factored in, the company said, the plant has an overall efficiency of close to 80 percent. Unfortunately, the company folded in early 2000 after... 

In some European cities, waste heat from fossil fuel electric power plants is used for district heating with an overall energy efficiency of 85%. These plants were not originally constructed as cogenerating units. Waste heat from industrial process plants can also be used. Geothermal sources are used to provide heat for district heating systems in Iceland and Boise, Idaho. 

The efficiency of the total fuel cell system can be calculated by accounting for the generation efficiency of the fuel cell stack, the efficiency of power conversion devices, and the accessory load of the related subsystems. In order to obtain the best overall fuel cell power system efficiency and better economics, a system optimization is required. The optimization involves minimizing the cost of electricity or heat and electric products as in a cogeneration system all the component processes of the system should be integrated into an efficient plant with low capital cost. Often, these objectives are conflicting, so compromises, or design decisions, have to be made. In addition, project-specific objectives, such as desired fuel, environmental emission levels, potential uses of rejected heat, desired output levels, volume (volume/kW)... 

Cogeneration may improve the plant economics because systems and operations are shared between multiple production activities or because overall thermal efficiency is usually increased from when power is produced alone. 

In a cogeneration power plant, the effective overall thermodynamic efficiency of the plant is increased by utilizing a portion of the waste heat generated by the power generation... 

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