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OPINION Your letters

be on a par with Watt’s separate steam condenser. It appears that, even now,

some producers of rare metals are storing rather than discarding thorium, as a waste product, in anticipation of its use as a future nuclear fuel. Support for the energy sector, and in particular energy innovation, is of course essential. But we need to distinguish between supporting innovation and subsidising commercial-scale energy production. Significant subsidies

for production consume economic resources that could help deliver future energy innovation. As the UK drives forward with an ambitious programme to deploy various forms of green energy, it is becoming clear that a combination of energy-dense, lower-carbon methane, partly from expanding reserves of shale gas in the UK and elsewhere, together with uranium, and later thorium, can be the key fuels of the future. This combination can provide the foundations of an energy policy to deliver future abundant, clean energy from compact power plants. The era of cheap energy is

over only if we choose so. If we use technical innovation to accelerate rather than supplant moves towards greater energy density, we can deliver energy that is both cheaper and more abundant. And, as a useful side- effect, we will help decarbonise our economy in the process.

l Professor Colin MCinnes FREng FRSE is Professor of Engineering Science at the University of Strathclyde and director of the Advanced Space Concepts Laboratory. A version of this article orginally appeared in Ingenia magazine, published by the Royal Academy of Engineering.

Not a limitless fuel for CHP Paul Woods’ article described the numbers behind comparing CHP and heat pump efficiency – showing that CHP can use natural gas to produce low carbon heat and electricity (‘Power relation’, December Journal, page 28). However, it would be easy

to conclude from the article that natural gas is a limitless resource. Unfortunately this is not the case. Discussions of CHP should not just compare carbon and cost with current conventional systems but should directly compare the use of natural gas as a heating fuel with other potential uses of this finite resource. Therefore, on a basic

level, CHP should never be compared to using 40% efficient grid electricity, as the natural gas consumed (forever) by the CHP system would not be used in a coal-fired power station; 50% would be a more appropriate figure for a modern gas-fired power station, even accounting for distribution losses. But the argument is more far-reaching than this. The natural gas could alternatively be used in industrial processes (perhaps as part of a large- scale CHP installation) or manufacturing. Natural gas is a finite, high-entropy resource. Space heating and hot water have relatively low entropy requirements. Is it appropriate to waste this difference in entropy when other viable (similar cost, carbon emissions, etc) options exist? Andrew MacKay

Seasonal data needed The aquifer thermal energy storage (ATES) project described in ‘Underground Solution’ in the October Journal was very interesting, but it would be highly beneficial

to see what is the predicted seasonal performance of the system. This should take into account

not only efficiency of the heat pump itself, but also all energy input to run associated plant – including submersible pumps which are to deliver warm or cold water from the underground energy stores. It would also be of great benefit to see how this technology stacks up financially against gas-fired boilers sited within the building and serving the Greenwich Museum estate and traditional chillers. With regards to the lifetime

of the technology mentioned at the end of the article, although boreholes ‘should have a life of minimum 30 years but probably longer’, the major plant will require to be changed much sooner. This would include heat pumps, plate heat exchangers

and submersible pumps. Michal Koscielniak

Wasteful transformers The simple energy-saving measures reported for the ‘Quick Wins’ case study (November Journal, page 40) are fine, but the estimate of savings from disconnecting maintenance transformers is somewhat optimistic: 100 transformers each drawing 0.6 amps at 240 volts on no load does not equal 14 kW, but 14 kVA. Those reporting the figures have forgotten about power factor which, on no load, will be quite low. Transformers do waste energy. There must be an enormous standing loss in buildings all over the US due to their socket outlets (‘receptacles’) being 120 volts instead of 240. Stuart Bridgman Wellington, New Zealand

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