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Combined Heat and Power (CHP), Micro-CHP, Mini-CHP

By Gordon Shaw

Generating electricity at the point of use is very efficient plus with CHP you get heat as well

Combined Heat and Power (CHP)* is an efficient way to generate electricity and heat simultaneously.
Fuels (such as natural 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 (see our Reference page on Stirling Motors and Generators) 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 (which is an ICE) 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 petrol, diesel or occasionally gas vapour.
  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 see our Reference page on Fuel Cells.
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.
CHP systems can be large-scale, mini-CHP, or micro-CHP. The larger and mini systems, for industrial application and for use in small organisations such as hospitals, schools and community centres or grouped households, have got proven track records (especially in parts of Europe, notably in Denmark). The micro-CHP systems, which would be applicable to domestic users in individual households still need to prove their worth. Micro-CHP systems are potentially mass-market products and the scale of the market is very large in terms of units that could be installed and there is a lot of profit to be made for firms that can produce and market them successfully.
Although micro-CHP systems are small and their maximum electrical output is only around 1kW, the impact could be significant if adopted en-masse. 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. Similarly Roger Dettmer [1] estimates that in a few years, assuming a modest national uptake of 10% (about 2 million households), that accounts for 2GB, equivalent to two Sizewell Bs.
The real-life efficiency of CHP systems can be very high because one is using an intrinsically efficient system (where nearly all the energy is being used) but on top of that you are generating some electricity which is much more expensive than gas, for example (almost four times). Further, the electricity is generated at the point of use; the distribution of electricity through the national grid is notoriously inefficient. Additionally the national grid as a utility is stretched and would benefit from consequent reduction in loading.
Ideally, you should be using both the electricity and the associated heat output usefully at the same time; commonly that is likely to be the case. CHP systems produce more heat than electricity (with micro; of the order of 6:1 to 10:1) which is a likely requirement and the demand for both tends to rise and fall together depending on the seasons. Typically, in the cold months, energy consumption in the home increases from the late afternoon and continues through the evening with space and water heating being the main requirements but also lights, TV, kettles and computers etc call for concurrent electricity consumption. This timing is when the national electric supply and distribution is placed under stress.
As far as the electricity output is concerned a CHP system supplies only part of the required load and is normally dependent on the traditional electric supply being available. Even so, the potential advantages are significantly reduced costs and carbon emissions. We suppose a further advantage might be gained if there is an electricity cut; then the user of conventional gas fired boiler may be stuffed, while the CHP user could get electric power to run not only the boiler controls but other key domestic appliances as well. This is a theoretical idea of ours and would depend on a back-up battery supply to bootstrap the system while the electrical generator came on to stream (about 1 to 10 minutes).
In 2010 in the UK, we saw the introduction of feed-in tariffs which are welcome to encourage users to install the more expensive micro-CHP systems and create a mass market with resulting fall in costs expected. One can see that there is more to CHP systems than it may seem at first glance.
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.
So what micro-CHP options are available in the latter part of 2010? Having dismissed ICE engines as unacceptable in the home and consigned domestic sized fuel cells to the wish list, what we are left with are ECE engines. According to Roger Dettmer in the IET Magazine [1] there are two contenders: the Free-Piston Stirling Engine (FPSE) used in the Baxi Ecogen and the Organic Rankine Cycle (ORC) engine in the Genlec system. Both of these techniques use a compact version of their versatile types to drive an alternator (to give an alternating voltage similar to the mains supply) but there are interesting differences. Remember that low maintenance and high reliability are essential for such an application.
The Stirling engine has fascinated us for the ingenuity in its operation (for more detail see our Reference page on Stirling Motors and Generators) but this FPSE builds on this with more magic. The FPSE can be a truly sealed unit whose output is electrical only with no external moving parts which gives two advantages. Because of the sealed construction it can use helium as the working gas instead of air and this has a better flow and heat transfer properties. Secondly, because of the sealed-for-life guarantee, maintenance is minimal and reliability is very much enhanced (providing the engine is well engineered and manufactured).
The ORC engine owes its place due to the enormous quantity of similar devices already used in cars and homes. The Rankine engine is used very extensively including the worlds largest generating plants where super-heated steam is the working fluid. The Rankine engines in the micro-CHP units are, of course, miniature by comparison. As its name suggests the ORC uses organic compounds instead of steam so that it can operate at lower temperatures. Domestic 'fridges and car air-conditioning systems use the ORC (but operate in reverse as heat pumps) in the millions. The ORC engine in the Genlec boilers therefore can source standard mass-produced components taking advantage of their outstanding track record of high reliability and low cost.
Costs of new CHP units and benefits of Feed-in tariffs over the next 10-25 years. We can only give a tentative outline in these matters. Costs of domestic CHP installation will vary and we hope they will come down in real terms as the benefits of mass production kick in. It can't be predicted whether the competitive technologies will coexist or perhaps one will become dominant. Our crystal ball suggests that the extra cost of installing a new CHP boiler over condensing boiler might be several hundred £s (Genlec £650 [1]) but might save about one hundred £s pa in running costs. The payback period on that basis is likely to be several years but will be very variable depending on personal factors as well as suppliers data and service.
However, in late 2010, things are much rosier due to The Energy Saving Trust 'Feed-in Tariff Scheme'. With this it is possible to think in terms of the complete installation paying for itself in a finite time over several years. This works by crediting the householder and generator owner (who may be one and the same) with amounts based on a three part tariff. The scheme applies to five different technologies (Solar PV, Wind Turbine, Hydro, Anaerobic Digestion and Micro-CHP) and each has its own set of levels and regulations. The three parts are: a rate based on how much you generate, an amount paid for feeding surplus electricity back to the supplier and thirdly an intrinsic saving resulting from using your own electricity which reduces your electricity bill. For installations between 15th July 2009 and 31st March 2012 Micro-CHP attracts a generation tariff of 10p/kWh and the lifetime of the tariff is 10 years. Generally speaking we feel that micro-CHP is the poor relation and the other technologies are more favourably treated (especially solar PV), however, the tariffs bear some relation to the investment costs hence the micro-CHP tariff scheme is still quite worthy.
Be warned, in our opinion government schemes are bureaucratic; regulations are subject to caveats and sudden changes, and regulations are more important than logic. Remember that these initiatives are, quite rightly, pump-primers and decrease over the years until they will disappear. The advice then is to read carefully all the details and restrictions, discuss with your potential supplier and do all the checks and balances before committing to a large outlay. Having said that, the scheme appears to be sound and practical and we welcome it. The source we recommend you start with is this Energy Saving Trust web site. It not only gives the facts but there are some helpful suggestions in places (eg such as what questions to ask of the 'free' offers from installers who then collect the generation tariff).
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.
References:
[1] "The Mighty Micro", Roger Dettmer, Engineering and Technology, Vol. 5, Issue 3, February/March 2010.

Notes:
* Alternative names for CHP are "Cogeneration" and "Total Energy"; we like cogeneration.

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Originated: 30 November 2000,  Last amended: 29 October, 2013