Cogeneration is the simultaneous production of power and useful heat from fuel. It is essentially a power plant with heat recovery and thereby derives a potentially higher efficiency because of that. this intrinsic potential efficiency advantage was the primary reason it was promoted so heavily during the 1980s. However, theoretical advantages are often lost to the harsh reality of operational and maintenance nuisances associated with running a small power plant. That, coupled with the impending deregulation of the electric utility industry and the uncertainty over electric rates has almost eliminated it from consideration for most customers.
Never-the-less, cogeneration can be a prudent strategic investment for customers who understand the rigors and requirements of the power generation business (such as pulp and paper and petrochemical plants) and where the right combination of fuel and electric costs and the energy user's use of waste heat is in balance. The following material has been included to help you to decide.
While some cogeneration systems are large (several hundred megawatts) and even burn coal and garbage, most aren't. Most systems are relatively small (30-3,000 kW) and burn natural gas. reciprocating and gas turbine engines are most often used to drive an electric generator at your site and run in parallel with the local electric utility. Fuel is converted to electricity at an efficiency typically ranging from 25% to 30%. However, unlike the central power plant, the engine's heat rejection is captured and used to satisfy some, or even all, of the site's thermal requirements. A well designed cogeneration system can have a total system efficiency of 70% or more twice as efficient as the typical electrical utility's central stations.
However, the selection and design of the system must consider the variations in both power and thermal requirements . . . not just the average requirements. Remember that the engine will probably have to be throttled back to less than full power during periods of low electrical energy demand and that the waste heat available from the engine at that time is probably insufficient to provide the site's peak heating load. Also remember that even the best cogeneration systems have to be maintained off-line and that the site will need supplemental heating to accommodate this. Where the system designer factors these and other related issues into the feasibility study you are likely to derive economic benefits from cogeneration. However, where the customer is approached by some "wheeler dealer" promising guaranteed energy savings using this concept, take a close look before signing on the dotted line.
