Micro machines

Conventionally, combined heat and power (chp) has been used for groups of buildings. However, this brings with it problems of individual billing, heat metering, potential breakdowns and shared maintenance. These are by no means insurmountable, but added to the British passion for individualism, particularly in connection with our homes, there has always been a resistance to being tied into any communal system.

Micro-chp overcomes the problems of system sharing by providing the power supply at a scale small enough for an individual home. Householders get cheap electricity and heat without the need to sign any communal agreements, while the environment benefits from reduced CO₂ emissions.

CHP represents one of the most significant measures in achieving the EU CO₂ reduction targets. The EC estimates that 150 MtC of a total of 800 MtC can be achieved with cogeneration measures. In order to meet the CO₂ emission reduction targets agreed at Kyoto, the EU has established the aim for chp generation as 18% of total capacity. However, it is now clear that the emerging micro-chp technologies, which were not included in this original target, may help to make up for the disappointing growth in the market for conventional chp. Micro-chp alone could save 19 MtC/y after ten years.

CHP generally represents one of the most cost-effective CO₂ abatement measures, as does micro-chp. More importantly, micro-chp can be implemented in the vast majority of existing homes, for which relatively few alternative energy efficiency measures can be implemented in a commercial manner.

Market positions

Alongside the potential environmental impact, the implications for the evolving competitive energy markets in Europe require that these technologies be taken seriously. Although some people consider generators of 3 kW (electricity) and below to be somewhat trivial, the key to micro-chp is the large numbers of units and the significant cumulative impact. EA Technology has undertaken evaluation of micro-chp technologies and concluded that the potential market for product sales alone is in excess of one million units or £2 billion annually, throughout Europe. From an electricity industry perspective, this would provide an ultimate installed capacity of a similar scale to the nuclear industry.

Micro-chp is no longer a research concept. In the UK, BG Technology has unveiled a prototype 1 kW(e) Stirling engine-based unit at an advanced state of development. New Zealand company WhisperTech is in a similar position, with a Stirling engine-based product already on sale as a dc-version for the leisure market.

A 3 kW(e) Stirling engine-based unit has been demonstrated by Sigma in Norway, and a larger 10 kW(e) unit is available from Solo in Germany. In the USA, Plug Power is field-trialling a fuel cell unit for domestic use; a similar unit is being trialled in Germany.

This has led to considerable uncertainty over which technologies and companies are likely to lead the field. Also, in what manner and at what rate the market will develop. It is evident that without the maturity of one or more of the technologies, little will be achieved, whatever the commercial and environmental drivers.

There are a number of advantages to using micro-chp in addition to the overall CO₂ savings. Not least is the potential for significant costs savings to householders. Using natural gas as a fuel, and assuming the current differential between gas and electricity prices, householders could effectively get electricity at half the current price. In addition, hot water can be thought of as a by-product of the electricity generation, and is therefore "free" for use as heating and hot water.

Demand profiles

There are of course problems associated with demand profiles – electricity may not be required at the same time as heating and hot water, nor in the same proportion. To overcome this, rather than sizing the chp engine to cover the maximum electrical load, its capacity is a proportion of the maximum required. The excess is purchased from the public supply.

Micro-chp technologies...may help to make up for the disappointing growth in the market for conventional chp

A possible set-up for a standard installation would be a chp engine of about 1 kW(e) supplying the base load electrical requirements. The heat generated would be used to supply a large (say 240 litres) hot water storage tank. This would be well insulated and act as a heat store, enabling any mismatch between electricity demand and heat requirements to be overcome. This would supply domestic hot water and heating in the same way as current heat-store systems.

An additional advantage of using a storage system is that a variety of heat sources can be used in addition to the chp engine, for example thermal solar collectors or heat pumps. It is important to note that these systems are not intended to create autonomous houses.

Exactly how such systems perform in reality is one of the questions to be answered in the Domestic Energy Optimisation (DEO) project. Energy and environmental engineering company, ECD, is a co-ordinator of this project, which is about to get underway. With funding from the European Commission, the demonstration project is looking specifically at integrated energy solutions for individual houses. The project involves several of the larger European Gas companies, including BG (UK), Gasunie (Netherlands), GDF (France) and Italgas.

Each of the partners will demonstrate in one or two test houses, a "total energy solution". Exact schemes are yet to be finalised, but examples may include photovoltaic arrays, thermal solar collectors or heat pumps. The key component in all the schemes will be micro-chp.

Technology issues

The technology involved in micro-chp is well proven, and any failings can be easily overcome with refinements in design and manufacturing as total volume increases. However, there are many questions to be answered by the DEO project and others awaiting funding, such as: Can heat demands be met? How to cope with summer/winter heat load variations? What sort of switching/control regime should be used?

There are also several non-hardware related areas that must be examined. One potential sticking point with existing electricity generators is that of grid-linking, and the selling of very small amounts of surplus electricity back to suppliers. Sizing the chp unit so that top-up from the grid is more likely may solve this problem. However, to cope with the minute-by-minute fluctuations in electricity demand, it would be wasteful to be unable to export surplus power. With a potentially huge amount of generation capacity available under the control of many organisations, how can the grid system handle this?

One possible solution is that the engines be owned and operated by existing supply companies. Householders would be offered a total energy package – a logical extension to the combined electricity and gas suppliers we now see in the market. The engine would be owned, installed and maintained by a single company, and to the householder would merely be a white box bolted to the wall. Householders would be assured of continuous supply and a known tariff for their energy.

The advantage to the energy supply company would be that they have direct control, via radio or other remote access method, switching from chp to grid supply as appropriate. In terms of supply management, this would not only solve the problem of uncontrollable exports of electricity to the grid, but could also provide particular advantages to supply companies. With the ability to switch between generation sources, suppliers will have more control over costs, which should ultimately benefit consumers.

Other European projects yet to be given the go-ahead will be looking further at market conditions for micro-chp. They will be examining how the various players – consumers, energy suppliers and regulators – will react and cope with widespread adoption of micro-chp, as well as how the existing market and regulatory mechanisms may need to be adapted.

The key aims must be to identify barriers to take-up in the current market, and to identify ways to stimulate the demand and adoption of the technology.