If every south-facing roof in Britain had a solar panel, we would make a huge cut in our national carbon emissions. If you have such a roof, make the most of it: Government grants are available (www.clear-skies.org), the panels are not difficult to install and you will get plenty of hot water even on cloudy days. Unlike photovoltaic (solar electric) panels, solar thermal panels are a simple and cheap technology, a radiator in reverse on your roof. Your solar panel will provide 60-70% of your hot water over a year. Unfortunately, they are no good for central heating as the panel rarely gets hot in winter. A solar panel can't help you if the temperature of the panel falls below the temperature of the water in your hot water tank, usually around 50C (much less when you empty it). So there is quite a lot of the year when the panel is warm, even quite hot, but unable to do anything useful.
That is, until now. Last week I turned my back on the sun-baked south and ventured as far north as I possibly could in one day without flying: the Orkney Isles. The following morning I was greeted in the Orcadian drizzle by Alton Copland, the director of Ice Energy Scotland and a man on a mission to transform his business - selling heat pumps - with some innovative integrated technology.
There are three deep boreholes under Alton's house containing pipes carrying a refrigerant, which extracts warmth from the ground to heat the house above. Ordinarily, the fluid comes out of the ground at around 60C, which his heat pump converts to a 50C hot-water output using simple fridge technology. The heat in the ground may be there for the taking but a lot of electrical energy is needed to shift it and compress it, typically one unit of electricity for every three-to-four units of heat moved. An efficiency of 350 per cent is pretty good but it would be much better - around 600 per cent - if the ground temperature was just a few degrees warmer. If only there was a free source of energy available to stick down the pipes and raise the ground temperature, ideally a warm liquid between 12C and 50C, as anything hotter might damage the pipes...
Sure enough, sitting proudly on Alton's rain-spattered roof is a big black solar panel. When the panel is hot, it heats his water tank directly. When it cools, it gets redirected into the ground, providing that extra lift to enable the heat pump to reach the magical 600-per-cent efficiency. Because the solar panel operates at such low temperatures, its output rockets, contributing not only to hot water but also to central heating (via the heat pump) across the year. And, needless to say, Alton's energy bills tumble.
The idea is brilliant but it's taken a lot of trial and error to develop a robust commercial product as Vera, Alton's wife, will, somewhat wearily confirm. Happily, that moment has now come and Britain's third Ice Energy Solar kit (patent pending) will shortly be installed in Tree House, connecting the ground pipes we sank last November with our big, bright, solar panel. As we will be literally saving the sun for a rainy day, we will surely have the sanest rooftop in the whole of our crazy capital.