June 06, 2005

Sun At Work 

I wrote quite a few posts in the past to point out the problems and weak spots of the renewable energy sources, and recently I realized a left out a technique for the use of solar energy that, instead, works pretty well.

I am talking about solar panels for water heating: these are feasible and rather cost-efficient. In my opinion, the reason is that hot water for domestic use doesn't need to be really hot: 60 C (140 F for the metrically-challenged persons) is enough, and even a temperature as low as 40 C is not useless. Instead, any decent steam turbine requires high-temperature superheated steam to work. Also, water is quite good for heat accumulation, because it is rather dense and has a high heat capacity (in other words, a little of water holds quite a lot of thermal energy) and a simple layer of cheap polyurethane foam constitutes a suitable insulation for accumulation tanks.

A solar panel is basically a coil of copper tube (which has a high thermal conductivity) painted black and palced inside a thermally insulated box with a glass pane on the front: the solar radiation thus collected will heat up a thermal exchange fluid (water + antifreeze, usually) circulating through the coil. This fluid is then forced to pass in another coil placed inside a tank, in order to transfer heat to the tap water. The whole system is simple, and its only moving part is a water circulation pump (plus a simple automated control system, but those are highly reliable nowadays), and it efficiency can be as high as 60%. In a place like Northern Italy, solar panels of the appropriate surface can produce all the hot water a household requires from May to October (depending from the weather), and some even during the coldest months (a gas or electric boiler is still required for support). The whole system with installation costs from 3 to 5 thousand Euros (IIRC) in Italy for a 6-room house*, but it will pay off rather quickly; if the solar panels are integrated in the house from the design stage the saving will be even higher.

But the beauty of solar water heating is that simpler solutions, to be used in the summer only, can be built by anyone with a little DIY skills and equipment: a coil of black plastic tube laid out on a rooftop can produce a considerable amount of hot water, and you can use your imagination to figure out other cases.

Solar power can be used for central heating too, but the problem with this is that central heating is most required on those short and often cloudy winter days which aren't exactly the best for collecting solar radiation. Many people experimented different solutions for solar heating: huge, thickly-insulated tanks of warm water; hot hair passed through brick structures (a design reminescent of certain accessory facilities of blast furnaces) etc, but the conclusion is that even the most elaborate (and not cheap) systems can provide at best three days (more or less) of autonomy without sun.

I'm rather skeptical towards solar central heating, especially considering the optimal performance of modern gas-fired boilers, but I am a big fan of solar panels for hot water, and I think that policies to boost their use are right-on.

* The European Union does subsidize up to 30% of the cost, but guess what: that's just enough to cover the bureocratic expenses...


I've been meaning to do this for years. Now that my water heater has bit the dust this would be a good timeto update the whole system.

That's cool. I gather you live in New Mexico, that should be a sunny place. I think you will appreciate the benefits of a solar panel.
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