WHY COMBINED HEAT AND POWER - THE CONCEPT
Much of the world's economy relies on abundant supplies of fossil fuels; for power generation, for transport, for heating and as feedstock for many production processes.That these fossil fuel reserves are finite cannot be denied and when they run out, many of the world's economies will collapse. Moreover, there is a general increase in demand for these resources, not least from those parts of the world which have not enjoyed the benefits of plentiful fuel supplies in the past.
Whilst there may still be considerable reserves of solid fuels, liquid and gaseous fuels, which are generally easier to transport and use, are being used at an ever increasing rate.
Alternative fuel and energy technologies may come on stream eventually, but in the interim, the only solution to an ever dwindling fuel resource is efficiency of use.
Whilst direct burning of fossil fuels is often used to generate low grade heat, (low grade heat assumed as being in the form of hot water below 100oC) relatively efficiently, (a boiler is about 80% efficient), this is a waste of the potential in the fuel which can produce much higher temperatures.
For the generation of electricity, the laws of thermodynamics dictate that upgrading of the potential energy in fossil fuel is also an inefficient process. Although modern centralised power stations are much more efficient than they used to be, they still waste half of the potential energy in the raw fuel as heat. This is evidenced by the huge cooling towers and ponds associated with power stations.
Combining the process of electricity generation whilst harnessing that heat usually wasted is the combined heat and power concept.
Overall thermal efficiencies approaching 90% can be achieved and, moreover, by generating electricity where it is to be used, transmission losses are saved and the heat is available as a by product of the electricity generation process. By combining the production of two energy streams from a single source, there are also savings in greenhouse gas emissions.
Because of the design rating of a CHP set, of which more later, such a machine is usually running in parallel with the local area mains electricity supply and generates at the voltage of that supply. This is referred to as "embedded" generation. Naturally, where a country has an established grid, all power generation stations are "embedded" because they are all connected to the same grid distribution, but here we specifically refer to privately owned generators which are run to suit the convenience of their owners rather than the demands of the electricity supply market.
An embedded CHP set is paid for by the fuel savings made in:
- Reduction in transmission losses (in wiring, transformers etc.)
- Reduction in heat wasted during the upgrading of potential chemical energy in fuel to electricity.
It can also make savings in maximum demand charges at the site where it is installed.
Where fuel is virtually free as in the case of the majority of biogases where it is released as part of another process, a generator running on that gas can pay for itself within two years.
All embedded generators have to comply with certain regulations applied by the electricity supply authorities. The regulations in the UK are referred to as the "G59" regulations (from the number of the relevant documentation). Other countries have similar requirements.
A combined heat and power system can be installed anywhere that has a demand for both electricity and heat. This applies to many locations, but typical examples include hospitals, hotels and leisure centres.
A CHP system can also be combined with a vapour absorption chiller to provide air conditioning. The vapour absorption chiller can be driven by the hot water from the CHP set. A very elegant system can be set up such that heating is provided in winter, cooling in summer and electricity all year round - a superbly efficient use of energy.
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