Solar panels produce electricity. Literally, when sunlight hits them, photons start moving around and generating an electric current. This is why the technical name for them is photovoltaics: light + electricity. But there is another kind of solar power that tends to get short shrift: solar thermal. Solar thermal can be applied in many different ways, so it’s not quite as easy to stick a label on as photovoltaics. All the methods use energy from the sun to heat fluid, but the next steps (and applications) can vary widely.

• Solar hot water: Collectors that look a bit like “regular” solar panels trap heat.  In “open loop” systems, water runs directly through the collector. Think garden hose in the sunlight. In “closed loop” systems, a freeze-resistant liquid (like glycol) gets superheated by the sun, then heats the water in a storage tank by means of a heat exchanger. For a residential home with average hot water use, systems can cost as little as $5,000-$8,000 to install.
• Solar air conditioning: In another type of heat exchange, the sun takes the place of an electric compressor by heating a refrigerant fluid. It changes from a cool low-pressure gas to a hot high-pressure gas. That gas goes through a condensor, becomes liquid, and then can absorb heat from your sticky August rooms.
• Solar pool heat: Operating on the basic principle of an open loop solar thermal system, pool water circulates through solar collectors, through a filter, then back into the pool. (In hot climates, pumping water through the system at night can actually cool the pool water as well.) Standard systems consist of plastic or rubber tubing–one reason they’re so cheap. In climates where freezing is an issue and you don’t want to drain the pool, there are more expensive systems whose copper tubing is glazed with glass. Unglazed systems tend to cost $3,000-4,000; glazed, a bit more.
• Concentrated solar power: On a whole other level, CSP is a thermal technology that actually produces electricity. It’s most effective in large installations and is becoming a major player on the utility-scale solar scene. Essentially, mirrors are angled to reflect light on a single collector point. The intensity of this focused energy turns water (or another fluid) inside the collector into boiling steam; the steam then powers a generator that produces electricity.
  • A plant just opened in California that uses 24,000 mirrors to concentrate sunlight onto just two collector towers, generating electricity for Southern California Edison. The news-worthy element of this particular plant, designed by eSolar, is that the electricity it produces is actually cheaper than conventional electricity in SCE’s portfolio. eSolar is partially funded by Google and already has deals on the books with PG&E, among others. Cost parity with traditional energy is the single biggest hurdle for solar electric generation to cross–many believe CSP could be the answer to solar’s cost woes.

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