Sizing
Let's start with demand: the level of electricity demand in a house over a day is very variable, typically with short-lived but substantial peaks. Any system sized to meet the peaks will have a very low load factor and therefore be difficult to justify on economic grounds unless electricity can be exported to other users via the grid or there is storage available.
Electricity export is, of course, technically possible but faces several practical problems.
Firstly, rather less than half the price we pay for electricity is for the generation cost: the rest is for transmission, distribution, billing etc. Demand reductions (that is, in what we buy) are worth 6 p/kWh to 7 p/kWh, but exports will usually be worth only the cost of the displaced generation, say 2 p/kWh to 3p/kWh .
Secondly, the metering and grid connection arrangements for local generation can be complex and costly.
So, until these connection and export issues are resolved, we are probably better off sizing systems so that all the electricity generated can be used within the house and export is unnecessary. We still need to balance the capacity and cost of the system against the number of hours each year that we have a demand that matches or exceeds the capacity. A consensus is emerging that an appropriate level for typical UK housing is between 500 W and 1 kW.
Generation technology
The next choice is the generation technology. Renewable energy, in the form of wind or photovoltaics is one possibility. At present, these technologies are expensive and, with a few exceptions, their output at any particular time is uncertain. This is a further disincentive to grid connection, especially under the New Electricity Trading Arrangements (NETA). While measures to reduce this unpredictability are under discussion2, the technologies are still some way from making a significant market impact.
Currently the more promising technical option is micro-chp. This is more predictable in the sense that the energy supply is always available but, like any chp plant, for efficient operation, the timing of the demands for heat and power should be (more or less) synchronised. The usual rule of thumb for chp systems is that there needs to be 4000 hours per year of simultaneous load for a system to be economical. Central heating systems usually operate for fewer hours than this – perhaps 3000-3500 hours (mostly at part-load) and the timing of demand for heat and power does not match perfectly. In houses, the evening demands for heat and power are reasonably in step, but the morning heat demand corresponds less well with electricity use. There is some scope to improve the matching by managing the timing of space or water heating provision. Provided that the additional cost of micro-chp, compared to a simple boiler, can be kept low, the usual rules of thumb do not necessarily apply though, and developers of micro-chp believe that economically viable systems can be produced.
Traditionally, small-scale chp has meant internal combustion engine systems, with the associated problems of noise and maintenance. Although these problems are reduced with the latest gas engine systems, the leading technical option at the moment is the external-combustion Stirling engine, with fuel cells as perhaps a longer-term option.
For micro-chp, the heat:power ratio of the technology is important. An efficient fuel cell producing 1 kW of electricity might produce 1 kW of heat, while a Stirling engine of similar electrical output would produce something like 7 kW of heat. This means that, all other things being equal, a Stirling engine is better suited to most existing UK houses, in which the demand for heat is considerably higher than the demand for electricity. A fuel cell would be a better match for highly-insulated housing, or could be used in existing housing if provided with additional heat generation. Currently, Stirling engine technology is significantly cheaper than fuel cells, and is the preferred technology in the short-term.
Stirling engines
Several developers are close to field trial demonstrations of Stirling engine micro-chp systems. In the UK, products have been developed by EA Technology using WhisperTech engines and Advantica with the Sunpower engine, and similar developments are under way in other countries. These typically generate electricity with an efficiency of around 15%, but this is in addition to providing a heating service equivalent to a conventional boiler. Higher generation efficiency would, in any case, result in a need to export electricity. If the target capital costs are met, these might have payback times of the order of five years or less and provide around 2000 kWh per dwelling per year.
Because of the need to maintain a workable balance between heat and power outputs, the main target market would probably be smaller, reasonably well-insulated houses – though, with supplementary heating, larger homes could also be served. A free-piston Stirling engine is a potential source of vibration and, measures to control this notwithstanding, floor-mounted installations might be preferred.
Persuading owner-occupiers to pay an extra £500 or more for a 'boiler' may not be easy, despite the running cost benefits. The development of a substantial market may require the emergence of energy services companies that can lease the equipment to householders. Social landlords may more readily see the affordable warmth benefits of a higher initial cost, lower running cost system.
Government has said that it is anxious to facilitate field trials that would demonstrate the technology and provide information that would allow the regulatory and network interface problems to be addressed. In her speech to the Green Alliance on October 24 this year, Margaret Beckett reaffirmed the government would shortly be publishing a draft chp-strategy to help ensure that the UK doubles the amount of this power-generation technology in use.
Fuel cells
Fuel cells are currently much more expensive than Stirling engines, but should be vibration-free. For application in typical houses, most fuel cells have the disadvantage of being too efficient at generating electricity: a fuel cell producing 500 W of electricity would typically produce only 500 W of heat. There are several possible solutions: use them only in highly-insulated dwellings, provide separate top-up heating or export the excess electricity. With DTI funding, Johnson Matthey and TXU (Europe) are exploring the most promising configurations. In the USA, Plugpower (also in collaboration with Johnson Matthey) has been developing a 7 kW unit, more appropriate to the US market, while in Japan the Ebara Corporation, Tokyo Gas and Ballard have produced a prototype 1 kW unit.
Closer to home, the Swiss company Sulzer Hexis has been field-testing domestic fuel cell systems since 1998 and has moved to controlled marketing through city utilities. Their system is designed to avoid the need to export electricity, having an electrical output of 1 kW and a thermal output from the fuel cell of 3 kW with supplementary gas heating of 16 kW to 22 kW according to application. Electrical efficiency is 25%. Within the UK, Birmingham University and Adelan are also developing a novel catalytic combustion plus fuel cell system with the housing market in mind.
Issues
So where does this leave home-generated electricity? Micro-chp in the form of Stirling engines initially, and perhaps fuel cells eventually, certainly have the potential to make a significant impact on carbon emissions from housing. Even 500 W per house, aggregated over the number of potential installations adds up to a considerable number. The principal barriers are capital cost, regulatory constraints and customer confidence.
Target capital costs for Stirling engines promise reasonable pay-back times, but customers may be reluctant to make the initial investment. For wide-scale deployment, innovative financing including the development of energy service companies may be necessary.
Regulatory constraints are not generally specific to domestic applications, but apply to all small-scale generation. The recent Government response to Ofgem's reports on the impact of NETA on small generators addresses some of these. Problems relating to metering and grid connection for micro-chp remain to be resolved.
Customer confidence can be built by successful field trials and, crucially, by the development of a competent and easily accessible support network providing installation, maintenance and trouble-shooting.
So, one day soon, you may need to ask not only what's the heating capacity of that boiler, but also what's its power output?.
Source
Building Sustainable Design
Reference
References
1 The European micro-chp market, Frost and Sullivan www.frost.com
2Government response to Ofgem's reports. www.dti.gov.uk/energy/consult_gen.pdf
Postscript
Roger Hitchin is principal energy consultant at BRE. For more information call 01923 664773.
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