Combined Heat and Power (CHP)* is an efficient way to generate electricity and heat simultaneously.
Fuels (such as gas, coal, oil, biomass and hydrogen etc) are burnt to release energy which is then harnessed to serve some useful purpose. The most basic form of the released energy is heat (as in a domestic boiler) and this can then be distributed via a heat-exchanger and a circulating fluid to be used for water and space heating. Quite a large proportion of the heat may be wasted through the flue but by careful engineering that too can be used. For example a second heat exchanger could extract more of the heat by partly condensing the hot exhaust gases and that is the principle of a condensing boiler. No electricity is produced by this method and it is therefore not a CHP system but it has the advantage of being highly efficient at 90% or more.
There are several methods by which Combined Heat and Power can be generated and the dual forms of output power explain to a considerable degree why they too can be highly efficient. However, these techniques are inherently more complex and some of the options are more practical than others.
External combustion technology lies at the heart of a way to achieve high efficiency and produce CHP. The technique here is to use some of the heat input to drive a Stirling‡ engine which produces rotary power and can be used to drive an alternator to generate electricity. While that explanation oversimplifies the system it illustrates the principle of a form of CHP. Since the Stirling engine operates by being heated from the outside it is described as an external combustion engine (ECE). Although the general public are probably not too aware of the ECE it is a well established and developed technology which is capable of being used widely. It is characterised further by being efficient, it is quiet because the combustion is not explosive and, in principle, it can use almost any combustible fuel and any source of heat, within reason.
Internal Combustion engines (ICE) also form a practical basis for CHP. Suppose you took the engine out of your car and ran it to drive an alternator. This set up gives electricity but there is also the heat going into the cooling system and out of the exhaust pipe; now those outputs can be put to good use (eg fed into a central-heating system) instead of deliberately dissipating them into the atmosphere. In this CHP mode the ICE is much more efficient than when used in the conventional locomotive mode. The ICE technology is well known and tried and very familiar to the public. While this method is also producing CHP it differs from the previous example because it relies on internal combustion, it is very noisy and is more limited in choice of fuels (see Feedback below).
Fuel Cells are a fundamentally different way of producing CHP. The Fuel Cell involves no moving parts and serves its primary function of generating electricity, although it would need conversion (inversion is a better term) to be compatible with mains supplies. It so happens that Fuel Cells run hot and so it doesn't require much imagination to see that they also form the basis for CHP systems. Different types of cell operate at quite different temperatures but they all work at high or very high temperatures. The ideal fuel is hydrogen, not the most practical fuel because it is a gas, but commonly available hydrocarbon compounds can be used with suitable chemical processing. The technique is virtually silent, very efficient and theoretically can produce very low carbon emissions (providing the hydrogen is sourced by a low carbon process). At this time, the maximum powers are limited and there is much more research and development needed. Fuel Cell technology is full of promise, but for now that is as far as it goes. For further information on Fuel Cells click here.
The efficiencies of practical CHP systems can be of the order of 90% but there is a bonus effect with the CHP systems because part of the output power is in the form of electricity, normally an expensive utility.

How has CHP Progressed in the UK, by 2006?
During the decade to 2000, CHP in the UK had grown at a rate of about 7% per year, which means, in that period, it had about doubled, nevertheless, in absolute terms, the saving in power was still small because of the very low starting base. Most of the installations have been concentrated in schemes where the electrical capacity is relatively modest (mini CHP; for example commercial, public and residential small communities). The absolute savings are potentially greater when the capacity is larger (industrial).
The other area with promise is at the lower extreme in the 'micro CHP' domestic systems where mass penetration would be needed to reap the benefits. Unfortunately, in 2006, there seems little progress on this front. There is not much evidence of suitable micro-systems being available and not much evidence of familiarity at the 'coalface' (eg the public, plumbers, architects etc). Most replacement boilers are distressed installations (breakdown of previous boilers) and to be accepted by the customer the replacement must be understood, trusted, immediate and not over costly to purchase and maintain. Further, these days in most of the UK they must conform to new legislation in force.
Our experience of trying to get information on micro-CHP boilers from putative suppliers revealed an attitude of disinterest, nay, sometimes apathy. All this is set against a backdrop of successful campaigning, backed by knee-jerk political legislation, to promote condensing boilers and the latter are dominating the domestic market.

And for the Near Future in the UK, with emphasis on Micro CHP?
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.

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. 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.

All this sounds promising but with legislation from the ODPM focusing attention almost exclusively on condensing boilers, little vigour from the producers and with no efficiency standard equivalent to SEDBUK, micro-CHP has, to say the least, an uphill struggle. We have viewed the CHP market, from the customer point of view, for several years and noted a manãna effect, although that is calibrated in years not days.
Our own enquiries to EC Power UK Ltd, a Bristol based firm who produce Natural Gas and Diesel mini-CHP units (see the link above and on our Alternative Energy page) produced one of the few constructive items of feedback. They indicate that, in Q2, 2006, the 'Whispergen' is the only domestic micro CHP product in the UK. It is based on a four cylinder Stirling engine and can be powered by diesel, kerosene, natural gas or LPG. It is manufactured in New Zealand by WhisperTech and we believe it is marketed by Powergen (a UK company of E.ON). However, getting any meaningful information from either Powergen or WhisperTech has proved impossible so far.
Ceres Power is a company exploiting technology originally developed at Imperial College. Their speciality is Fuel Cells and they have a £2.7 million contract with British Gas (March 2006) to design, build and evaluate fuel cell CHP units. On making enquiries to BG we were told that practical CHP systems would not be available until next year (2007). Sceptics that we are, we're not holding our breath on that one
We may be proved wrong but can't see that internal combustion CHP is viable for domestic, micro CHP power because of the high noise inherent in the genre.

In view of the paucity of micro CHP models to assess we couldn't get a handle on costs and savings. The best guesstimate that arises from reading around indicates a CHP unit might cost several hundred £s extra to buy but would save one to three hundred £s pa in running costs. We've seen a payback figure of 3.3 years quoted but that is likely to depend on the technology and the maturity of the products. But then you've heard that one about statistics haven't you?

We find the subject interesting and our visitors are showing a keen interest but so far history indicates there have been many unfulfilled promises. If any of our visitors could add any down-to-earth information or correct our perceptions we would be only too pleased to receive feedback.