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.
Our main focus is on domestic applications, known as micro-CHP because their large scale adoption could lead to hundreds of thousands being installed each year in the UK alone.
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 motor such as a Stirling Engine (for info click ) or Rankine Engine which gives a mechanical output in rotary or linear reciprocating form that 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, eg like petrol or diesel.
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 the technical 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 a very expensive utility.

What is the history of CHP and micro-CHP in the UK up to 2010? In 2000, the UK government (Labour) identified CHP as one of the important ways of achieving our Kyoto commitments. 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 had 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 'micro-CHP' domestic systems where mass penetration would be needed to reap the benefits. One might be tempted to think that the development costs for domestic gas CHP systems (micro-CHP) should be relatively low because the larger systems have been in use for decades. However, it seems that engineering a unit with satisfactory power levels which is small and quiet enough to hang on the kitchen wall and with a believable reliability at an acceptable price is not at all easy. Nevertheless in Q3 of 2010 there is hard evidence that actual ECE gas-fired boilers are available for installation.
The ICE option is virtually ruled out (who would welcome such a noisy smelly thing in the kitchen?). As for fuel cells, in 2006 when we first wrote about micro-CHP it was wishful thinking. Our enquiries to British Gas about fuel cell CHP suggested that units might be available in 2007 as a result of a contract with Ceres Power (a company exploiting technology originally developed at Imperial College): we were sceptical. As far as we know fuel cells are barely in sight even now.
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 Comments
We feel that an improvement in efficiency is better than nothing but if the approach does not tackle the fundamental problem of avoiding the use of fossil fuels, that is not a radical solution we would hope for. Nevertheless, Combined Heat and Power undoubtedly has its value in the real world. Large and mini systems already have some proven track record and micro-CHP seems to be on its way to a successful future.
Because the technology is likely to be used in very large quantities the contribution can be significant. Indeed if it can replace some of the nuclear power stations, that is all to the good.
As far as our own experience is concerned with a domestic installation from scratch, in Q3 of 2006, we opted for a gas condensing boiler system and the choice was clear for a variety of reasons. Now the choice would not be so clear and we fancy that we would have seriously considered micro-CHP instead.
The weakness of the two systems mentioned above is that they are described as being suitable for gas only. What we would like to see is a move to renewable (non fossil) fuels. During our research in Q2 2006 we came across the 'Whispergen' which was then the only domestic micro CHP product in the UK (manufactured in New Zealand by WhisperTech). It was based on a four cylinder Stirling engine and powered by diesel, kerosene, natural gas or LPG.