Readers letters

Bottom priority

PE

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There are many far better ways to increase energy security and/or reduce emissions than PV installations

Peter Tindale wants to have solar panels on all new buildings, which would generate a “huge” amount of energy (Letters, PE September). Unfortunately this energy supply peaks at entirely the wrong time, midday in June, and costs an astonishing amount of money.

Domestic photovoltaic (PV) installations get 41.5p/kWh subsidy, about nine times that accorded to (much reviled) onshore wind (around 4.5p/kWh), all of which is paid for by electricity consumers, including some who are not exactly rich. There are many far better ways to increase energy security and/or reduce emissions. Until we are all in Passivhaus-standard buildings, solar PV at this price should stay right at the bottom of the priority list.

And anyone who wants to save the world with PV should site it where it will do the most good – in (say) Arizona, the load factor is over 20%, while it is typically 10% in the UK. No, the panels don't work flat out whenever it is light.

Brian Daniels, writing in the same issue, wonders why we are building electric cars when internal combustion engines are 45% to 50% efficient. While a big marine diesel might make 50%, car petrol engines are about 30% efficient while diesels can make 40% – but only at optimal speed and power.

Except in a series hybrid, car engines spend most of their time at fairly low power, well away from optimal conditions. Real-world efficiencies may be 20% for petrol and 30% for diesel. Pre-tax diesel and petrol prices are around $20/GJ, so even at 30% efficiency motive power costs around $67/GJ. Compare that with (say) coal costing $4/GJ burnt in a 37% efficient station, leading (pessimistically) to 33% delivered efficiency.

Assuming an 80% efficient electric powertrain yields motive power at $15/GJ, that's less than 25% of the liquid fuel figure.

Of course tax, distribution and other costs affect these numbers, very much to the benefit of the electric car user – until there are enough of them and the government wants to recoup the lost tax revenue by charging us by the mile.

Rick Jefferys, Berkhamsted, Herts

Next letter: Built-in inefficiency

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