Combined harvester

The article 'Double take' (

) highlighted the benefits of CHP — potentially the most efficient way to obtain energy from fossil fuels. But crucially, unlike renewables, CHP is always available at critical times.

The article excited the reader by teasingly omitting vital information yet, naturally, promoting perceived benefits. The innovation is extending CHP technology to appliances suitable for small business markets. But payback and efficient use is only realised if both electricity and heat are required most of the time.

This is simply not the case for domestic use, where 3kW of electricity is only required occasionally, such as for ironing or running an electric cooker, and significant heat loads will vary widely and spasmodically from 0kW to 3kW and do not necessarily coincide with electrical load. Critically, no mention is made of how the proportioning modulation works, maximum/minimum heat loads that are obtainable, nor a price target.

The product is likely to be a sales person's dream so several cruel pitfalls await the unwary. Detailed tests by BEAMA (The British Electrotechnical and Allied Manufacturers' Association) for over two years indicates utilisation factors for all domestic renewables, including micro-CHP, vary widely — around 10 per cent (in other words, actual usage compared with full operation 24 hours a day, seven days a week).

This intolerable low usage would make the technology financially inefficient and, domestically, unmarketable. Hence the government burdens other energy users to pay higher prices, and, main electricity generators to suffer reduced utilisation factors. These burdens are the Renewables Obligation and future Feed-in tariffs. Taken to the extreme, many users of micro CHPs will run them, dumping heat if necessary, to get a juicy 40p/kWh.

Regrettably, the more electricity or gas saved by domestic users in any green technology, the higher the cost per kWh in their reduced quarterly bills due to the two-tier pricing system. Wastefully running CHP for long periods to sell electricity also reduces the domestic gas bill per kWh.

The sensible installations will be in such areas as hospitals, hotels, nursing homes and small SME processes. The last thing we all want is higher electricity prices to subsidise unsuitable domestic micro-generation.