R. Beith,     Beith & Associates, UK

Review of micro combined heat and power (CHP)

Looking first at the micro-chp scene and the Stirling cycle, this has been developed over many years aimed at the domestic market and particularly in the UK. Initial units have been trialled in houses and it is considered that most of the problems have been resolved, such as low noise and cleanliness for in-house installation, acceptable maintenance cycles, reliability, high overall efficiency (even up to 95%).

Cost is now apparently coming down to acceptable levels, but perhaps the main challenge has been how to match the typical duty of say 1 kW electricity and 5-7 kW heat output against a widely varying house demand. It seems that this electrical duty matches house ‘base load’ qusite well and peak load can be ‘mains topped’. But the heat duty on earlier units was marginal and it is now accepted sometimes to add a simple supplementary heater. Two main electricity suppliers are said to be planning marketing of Stirling cycle equipment in the near future. Some larger units may be applied in bigger establishments but the market here is said by one author to be mature but small, compared with the potentially very large domestic scene.

Domestic scale reciprocating gas/oil-fired micro-CHP units are already commercialised and being sold, mainly in Japan (say 20 000 per annum) and in limited quantities in the United States and Germany. These are sized for 2-5 kW, but the units have to be located outside because of noise, exhaust, etc.

Another technology being developed for domestic and other services is the fuel cell. One UK developer is moving towards commercialisation in terms of technical design but it is believed that the cost is still high and demonstration of product life (target say 40 000 hours) is still not fully established. The technology is attractive with the typical duty of 1:1 ratio of electricity to heat and with much higher electricity output than a typical domestic Stirling cycle unit. The package is more a ‘system’ than a ‘unit’. It will, of course, have many applications other than just domestic services.

So, for the domestic scene the technological groundwork is being well laid, but there has not yet been big market impact. There could be significant advances and perhaps a breakthrough in the near future, but two issues are important for this: (i) all domestic equipment has to be in standardised and ‘easy to install’ form for builders to understand, and (ii) householders will need to realise that these new systems operate in a different fashion and to adjust to this. For example, existing gas boilers are oversized so that they can bring temperature up quickly and then be turned off, whereas new microchp systems produce the heat more steadily but at a lower rate.

Review of small combined heat and power (CHP)

Looking next at the ‘small’-CHP range, we find a number of process and equipment routes are being exploited. Perhaps the most widely used equipment is the gas/oil reciprocating engine, typically in size ranges of the order 50-2000 kWe. They are applied in many establishments and organisations (e.g. in many universities, offices, hospitals, community centres, etc.) and reported also in many food preparation processes, such as in the dairy, meat and sugar industries. It is likely there will be other industrial applications not reported in this book. Application tends to be where energy requirement is ‘round the clock’ and the plant duty is sized as near normal full load as possible, particularly for electricity supply as this is the expensive ‘buy in’ commodity. Usually in residential installations only low temperature heat is required for radiator systems, but this is not the case for many industries. There are applications in supermarkets, also, where the excess heat is used to drive lithium bromide, absorption refrigeration systems for cold stores. This makes for good economics where the heat output is large in relation to electricity and is not all required for heating. Supermarket sizes can typically be 1200 kWe.

There are also a number of steam cycle plants typically in the 100-1000 kWe range, which mainly use biomass fuelling. Some of the smaller ones may be biogas-fuelled reciprocating engine plants. There are a range of such fuels, but typically wood (pellets or other), agricultural wastes, energy crops, or such as straw wastes are used. These plants tend to be in rural locations near biofuel sources to save transport costs. For instance, smaller plants may be fired from gas produced at a farm site anaerobic digestor (AD) or from manure processing. The pattern here is use of local available cheap fuels for local energy supply and carbon reduction is obviously one of the factors.

Another equipment combination, also mainly using biofuels, is the use of the ORC (organic Rankine cycle) to produce electricity, using low boiling point fluids to drive a turbine. Low level waste heat from another source can drive the process. It is not thought economic below, say, 10 kWe but it is reported that 130 plants have been installed over 16 countries typically in the 200 kWe range.

Another contender is the gas turbine/generator. One characteristic of these is the low electricity-to-heat ratio (can be 1:5 with only about 20% electricity efficiency) and also the physical size. They can be a good choice and competitive for larger-scale industrial/process plant situations where a big and steady process heat requirement exists and where spare electricity can be sold off; and maintenance costs are low. But at much smaller scale this route is possibly more expensive than the duty can support. However, TGs are available, mainly from US suppliers, in sizes from 30 kWe to 1000 kWh.

We should not forget district heating (DH) applications and, looking at Denmark's experience, where they have maximised this energy route, it can be noted that 60% of their population are served by DH driven by CHP. Systems often start smaller and are extended, but typically units will be 1 MW upward. They are sized to suit the housing needs in an area. They are usually gas-fired engines or at bigger scale steam turbines, sometimes using wood chip furnaces. Water heat stores are used for balancing on the heat duty side but there is no problem with electricity load not matching local needs, as this goes to the grid.

Conclusion and outlook

So it is clear there are a wide range of technologies, systems and applications for small-CHP and steady growth in a number of areas. There is still a need to custom design each application - few are identical - but the equipment components are generally readily available. On the fuel side, gas is still the main resource, but there is a growing move towards biomass fuels and a need to improve standardisation/specification of these fuels on a more national basis. The cost of these fuels is also a variable to be taken into account, but of course this is also the case for fossil fuels.

Finally, it seems clear that small-scale CHP will grow steadily and probably faster as experience develops from existing plants and as application is encouraged by energy and carbon saving benefits. But it is still essential to assess and design each application carefully from heat and electricity duty requirements to ensure a viable system, and this may limit expansion rate. Also, as was indicated for the micro-chp market, there is a need for owners/users to realise they may need to operate in a different way than they would with the old formula of plenty of cheap fuel!

Certainly this energy technology has good Government support with targets for CHP plant availability (all sizes) to almost double by 2020; and with incentives in terms of feed-in tariff (FIT) payments at the domestic scale, together with benefits in potential carbon discharge reductions. I noted, for instance, in one chapter that typically there could be a 10-20% carbon saving with the Stirling ‘system’ compared with conventional domestic gas-fired equipment. So in my view the future scenario for small- and micro-CHP is definitely encouraging.