Most types of power generation technology are capable of being integrated into a CHP system. There are obvious exceptions such as hydropower, wind power and solar photovoltaic. But solar thermal power plants can produce excess heat and geothermal energy is exploited for CHP applications. Fuel cells are probably one of the best CHP sources while conventional technologies such as steam turbines, gas turbines and piston engine plants can all be easily adapted.
The type of heat required in a CHP application will often narrow the choice. If high-quality steam is demanded then a source of high-temperature waste heat will be needed. This can be taken from a steam-turbine-based power plant, it can be generated using the exhaust of a gas turbine and it can be found in a high-temperature fuel cell. Other generating systems such as piston engines are only capable of generating low-quality steam or hot water.
The way in which a CHP plant is to operate is another important consideration. Is it going to be required to provide base-load electricity generation or will it follow the load of the user who is installing it? If the plant will be required to load follow, then a power generating unit suited to that type of operation will be needed. The best for this purpose is either a fuel cell or a piston engine power plant. However if base-load electricity generation is intended, then a gas turbine or perhaps a steam turbine will offer the best solution.
These two can also provide steam supply flexibility. With a gas-turbine-based CHP plant, excess steam can be used to generate extra electricity. A steam-turbine-based system, meanwhile, will allow steam and electricity generation to be balanced to meet site demands.
The quantity of heat that will be available will also vary from technology to technology. Table 5.1 gives typical energy conversion efficiency ranges for modern fossil-fuel-burning power plants. Most of the energy not converted into electricity will be available as heat. Where more flexibility is required, it is possible to design a plant to produce less electricity and more heat that the efficiency figures in Table 5.1 suggest. Some technologies are amenable to this strategy. Others are not.
Table 5.1 Power plant energy conversion efficiencies
Conventional coal fired 38-47
Pressurised fluidised bed 45
Integrated-gasification combined cycle 45
Heavy gas turbine 30-39
Aeroderivative gas turbine 38-42
Fuel cell 36-60
Lean-burn gas engine 28-42
Most of the technologies employed in CHP plants have their own chapters in this book where detailed accounts of their operation can be found. In discussing these technologies here, consideration will only be given to factors of specific relevance to CHP. Please refer to other chapters for fuller accounts of each technology.
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