You might say that solar power, one likely guide to our energy independence, is a two-headed creature. Solar thermal and solar PV both spend their fair share of time in the spotlight. But which one is better? It would seem, based on the numbers, that solar thermal would be the easy bread winner.
For one, efficiency is much higher for solar thermal. You can use up to 70% of the sun’s energy with a thermal solar collector. Whereas, using a PV collector, sunlight-to-electricity conversion rates average about 12% only. You can also look at it in terms of area. It is a rule of thumb that, at any given (sunny) time, the energy available from the sun is about 1kW per square meter. This is equivalent to 3400 BTU/hr/square meter. Therefore, if you can get 70% efficiency using a solar thermal collector, you would only need approximately 42 square meters to generate 100,000 BTU/hr, or the amount of heat generated by a typical gas furnace.
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Thus, your solar thermal system would require much less space (and many fewer collectors) than a solar PV system. Not only that, but solar thermal collectors are about 5 times cheaper than solar PV panels. So you’ve got a scenario in which you have a system that is 6 times more efficient at 20% of the cost of its rival. How can solar thermal not be dominating the solar industry? The choice is obvious, right?
Well, it’s not quite that simple. There are a few reasons why your kids aren’t finding Solar Thermal Man action figures in their cereal boxes. For one, heat is difficult to store without a thermal mass. Passive solar homes will likely incorporate thermal mass walls (i.e., concrete or other masonry) to store collected heat. Yet this can only be achieved via a passive solar design, starting at the home’s conception. As you can imagine, it is difficult to add thermal walls and other passive solar collectors to an existing home. Therefore, when retrofitting a solar thermal system, you will have to use an active system, which uses a transfer fluid and a large water storage tank.
Furthermore, you will need to calculate the intensity of the sun at different times of the year. In colder climates there is often as little as a few hours of intense sunlight on a winter’s day. To make up for the lack of sunshine, keeping in mind the difficulty of heat storage, you will have to increase the size of your system. In addition, because of clouding, you may need to have a backup system also. All these complications and distributed yet fast-accumulating costs may be why solar thermal space heating is still more possibility than actuality.
Another big advantage for solar PV is that it is usable year round. A solar thermal space heating system will sit idly by during the long summer months while a solar PV system will provide usable electricity all year. The system would be at its most efficient just when you need it most.
The thermal versus PV story reminds me somewhat of that old chestnut, The Tortoise and the Hare. Solar thermal, our speedy hare, is inherently faster and dramatically cheaper–the numbers jump off the page. But the slower, more determined solar PV, our valiant tortoise, doesn’t nap in the summer and, slowly and steadily, wins in the end. So far, that is.
However, I don’t want to disregard solar thermal. In fact, I could argue that solar thermal represents the future of solar energy and a ‘green’ electrical grid. There have been new innovations in thermal storage techniques (if only practical on a commercial scale). And, for the residential homeowner, a solar thermal hot water system can result in fast, worthwhile savings. After all, hot water is the second most consumptive energy user in the home.