Traditionally, the generation of electricity is carried out in a power station designed for that sole purpose, and more often that not fuelled by fossil fuels. A great deal of thermal heat is required to fuel the process of generating electricity, and in the past a significant proportion of that heat has simply been released back into the environment, through cooling towers. By contrast, Combined Heat and Power plants (CHPs) are designed to make use of virtually all the heat produced in the electricity generation process.
What is a CHP?
Combined Heat and Power plants produce both electrical energy and thermal energy as part of a single process. This process is sometimes called co-generation. Generally, CHPs produce around twice as much thermal energy as electrical energy, but this ratio can be varied according to need.
A CHP has a heat recovery system in place to collect the heat left over from the generation process and deliver it to another application that requires energy. Normally this will be another heating application, but the thermal energy can also be transformed and used for cooling purposes. Plants which incorporate an absorption cooling cycle are called Combined Cooling Heat and Power (CCHP), or trigeneration plants.
Do CHPs Offer Very Significant Advantages?
Conventional power stations are not energy-efficient because they utilise only a small part of the heat that they have to produce in order to drive the turbines. Globally, electricity generation represents a massive proportion of our total energy usage, and in some conventional power stations only around one-third of the total thermal energy that is released from the fuel, is actually doing useful work.
CHPs burn the same amount of fuel to generate electricity, but use up to 90% of the energy it releases, by capturing the thermal energy that conventional power stations waste. This obviously has economic benefits, but it also has important environmental advantages. Using fuel more efficiently means that overall consumption is reduced, thus cutting greenhouse emissions, and also slowing down the rate at which we are using up our reserves of fossil fuels.
Another advantage is that CHPs can be powered by virtually any kind of fuel that can produce steam. Those that have been designed to run on coal can equally well be fuelled by solid biomass. Many CHPs use natural gas, but could also operate on biogas. The recent interest in using biomass as an energy source has led to a number of new CHPs being planned and built.
So Why Aren’t There More CHPs?
Compared to conventional power stations, many CHP plants operate on a relatively small scale. The location of a CHP is very important, since there should ideally be a local need for the thermal energy that is recovered, as well as for the electricity that is produced. The simplest way of using the recovered energy is to heat a supply of water that can be used for domestic or industrial heating purposes; however, this is only effective if the end users are located within a close geographical radius of the CHP plant.
Electricity supplies can travel over long distances once a supply network has been installed, so the electricity can be supplied over a wider area; but if water is piped over long distances, thermal energy is likely to be lost into the environment. Ideal locations for CHPs include industrial sites that need power to drive machinery and to heat or cool different parts of the premises, business parks, hospitals, leisure centres, university campuses, and residential developments such as sheltered housing.
For industry, CHPs can represent an excellent long-term investment when offset against future savings in fuel costs, while community CHP schemes that provide a continuous supply of hot water and domestic heating can offer significant savings for residents. However, the initial installation does inevitably involve significantly financial outlay.
CHPs can also work on a larger scale. A number of UK towns have medium-sized CHPs in the town centre. One very large CHP installation in the UK is the plant at Immingham, in the Humber, which was built to supply two oil refineries with heat and power, and this plant produces more electricity than many conventional power stations.
CHPs: The Way Forward
A recent study by DEFRA (Department for the Environment and Rural Affairs) found that only around 8% of all the energy that the UK uses for space and process heating comes from CHP systems. The UK government has an obligation to promote renewable energy usage and, recognising that more widespread use of CHPs could help it achieve its targets, it has introduced a number of incentives to encourage the construction of more CHPs, though still with the emphasis on small-scale schemes.