Тексты по специальности ТЭС / Combined Heat and Power

Combined Heat and Power

Introduction

Combined Heat and Power (CHP) is a very efficient way to generate heat and power simultaneously. The common acceptance is that a heating source (such as a gas, coal or oil 'boiler') performs its primary function and then discharges the hot exhaust into the atmosphere. CHP reclaims some of the 'lost' power. In the US alternative names are "Cogeneration" and "Total Energy". For what its worth we think that Cogeneration best describes the principle, it is unlikely that the woolly phrase 'Combined Heat and Power' was coined by a scientist or engineer.

In essence, the Combined Heat and Power technique is usually implemented by the addition of a heat exchanger which absorbs the exhaust heat (normally wasted) from an existing generator and uses that to drive an electrical generator. The main generator would typically be providing the space heating and hot water for a home, industry or community and the recovered energy would be used to supply electricity for lighting, cookers, refrigerators and other kitchen machines, for example. The ratio of demand would therefore be reasonable for the relative amounts of energy required in many normal domestic, community or industrial units.

CHP can reduce the carbon emissions, from about 10% to more than 30%, depending on the amount emitted by the primary generator. The primary generator may not be a low carbon-emitter but the associated reduction in emissions is significant. The costs are relatively low, consequently the government (Labour, 2000) has identified CHP as one of the most important ways of achieving the UK's Kyoto commitments.

[We think that Fuel cells are also worth mentioning as mirrors of conventional CHP. Their prime output is electricity, not heat, but they operate at high temperatures (depending on type) and that heat can either be wasted or put to good use using Cogeneration. See the item on Fuel Cells on the main page.]

How is CHP Progressing So Far?

During the decade to 2000, CHP in the UK had grown at a rate of about 7% per year, which means it had about doubled, nevertheless the current saving in power is still at a low level. Most of the installations have been concentrated in schemes where the electrical capacity is relatively small (eg commercial, public and community residential), however, the absolute savings are naturally greater when the capacity is larger (industrial).

And for the Near Future?

The UK target for installations set for the end of the year 2000 was 5,000 MWe* and by 2010 a rather vague, further target of 12,000 to 19,000 MWe.

[* Note: the 'e' in MWe refers to the fact that power is electrical]

James Meek, the science correspondent writing in The Guardian on 2 September 2000 reported that by 2025 well over half of us in the UK will have our own generator attached to our domestic boiler. Currently these boilers waste about 30% of the heat in the exhaust. The expected system has been named "Micro CHP" (systems with up to 5 kW of potential electrical output).

A device which could enable this is the Stirling engine invented at the beginning of the 19th century although there are more recent alternatives. The article claims that the first devices should be available by 2002 [however, see our own brief article on Micro-CHP under the CHP subsection on the 'Alternative Energy' page] and would add about £500 to the price of a boiler but the saving might be in the region of £150 to £300 per annum.

The advantage of Micro CHP (as well as other forms of CHP) is that the demand for electrical power rises in the cold of winter and this is the time when most could be made available because the heating generator has its heaviest workload. EA Technology has estimated that when over half the households are using Micro CHP (by 2025?), the combined output would be at least equal to the country's nuclear power stations.

Our Comments

At envocare, we feel that any improvement is better than none but in this case the approach does not tackle the fundamental problem of avoiding the use of fossil fuels. Combined Heat and Power undoubtedly has its value, but as an amelioration rather than a radical solution. Nevertheless, since it is a relatively simple technique with some merit, we have to ask, why is it going to take so long to introduce it on a large scale and realise its potential?

Footnote: in Denmark which has an exemplary record for energy efficiency we read that all new thermal power generation, for the 20 years to 2003, has been built as Combined Heat and Power (CHP) [Ref: Roger Dettmar, All at Sea, IEE Review, May 2003]. Ironically this has resulted, on at least one occasion, in a forced reduction of electrical power from the Dane's wind turbines during an extreme cold weather spell to avoid excessive generation.