Combined heat and power

The production of electricity from fossil, biomass or nuclear fuels is an inefficient process. While some modern plants can achieve nearly 60% energy conversion efficiency, most operate closer to 30% and smaller or older units may reach only 20%. The USA, which has a typical mix of fossil-fuel-based combustion plants, achieves an average power plant efficiency of 33%. Other countries would probably struggle to reach even this level of efficiency.

Putting this another way, between 40% and 80% of all the energy burnt in power plants is wasted. The wasted energy emerges as heat which is dumped in one way or another. Sometimes it ends up in cooling water, but most often it is dissipated into the atmosphere. This heat can be considered as a form of pollution.

Some loss of energy is inevitable. Neither thermodynamic nor electrochemical energy conversion processes can operate even theoretically at 100% efficiency and practical conversion efficiencies are always below the theoretical limit. Hence, while technological advances may improve conversion efficiencies, a considerable amount of energy will always be wasted.

This energy cannot be utilised to generate electricity but it can still be employed. Low-grade heat can be used to produce hot water or for space heating1 while higher-grade heat will generate steam which can be exploited by some industrial processes. In this way the waste heat from power generation can replace heat or steam produced from a high-grade energy source such as gas, oil or even electricity. This represents a significant improvement in overall energy efficiency.

Systems which utilise waste heat in this way are called combined heat and power (CHP) systems (the term co-generation is often used too). Such systems can operate with an energy efficiency of up to 90%. This represents a major saving in fuel cost and in environmental degradation. Yet while the benefits are widely recognised, the implementation of CHP remains low.

Part of the problem lies in the widespread preference for large central power stations to generate electricity. Such plants are sited to suit the demands of power network and of the power generation companies which own them. Rarely will there be a local use for the waste-heat energy such a plant produces. Only if generating capacity is broken down into smaller units, each located close to the source of demand, does it become possible to plan to use both electricity and heat.

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