Solar Power 101

Concentrated solar-thermal power

Just as humans first focused sunlight with glass lenses more than 2,700 years ago, concentrated solar-thermal power (CSP) creates electricity using the same basic mechanism. CSP utilizes mirrors to point sunlight into a receiver to heat up a fluid and generate steam. That steam spins the turbines that then create electricity.

CPS systems are utility-scale solar plants that are typically built on large swaths of land and can power tens of thousands of homes and businesses.

One of the advantages of a CPS system is that the fluid can retain the heat for a long time, which is especially handy on cloudy days when CPS systems aren't able to harness as much sunlight. However, the technology is still in limited use and the industry is not expected to grow as much as utility-scale PV solar.

Gas- and coal-powered plantsalso use steam to spin turbines but with CSP, there's no nasty pollution or waste-or climate-warming emissions-like there are from burning fossil fuels.

Passive solar design

Most of us have seen at least one example of passive solar heating: a greenhouse, which is designed to take maximum advantage of sunlight.

A passive solar system isn't so much about equipment as it is about strategy. This method of controlling indoor temperatures doesn't even require electricity-just the right building materials and conditions to take advantage of the sun's warmth.

Whether it's primarily for heating or cooling, there are a few key requirements for a passive solar system in a building:

• Aperture: large windows that face within 30 degrees of true south, with no shade between 9 a.m. and 3 p.m. (In North America, this is where the sun sits the highest during the day.)
• Thermal mass: materials like concrete, brick, or stone that can hold onto heat
• Distribution: Thermal energy is distributed around the building via: conduction, like when you touch the thermal mass; convection, moving thermal energy through the air, as through vents and fans; or radiation, moving through electromagnetic waves.
• Controls: things like thermostats, awnings, or blinds that can help prevent overheating or under-heating the building throughout all kinds of weather

Solar water heaters

The sun's thermal power can also be used to heat tap water in a home or business. Similar to the PV systems, solar water heaters utilize collector panels to absorb solar radiation. Water will heat up as it's circulated through the panels and sent to a storage tank. These panels are typically installed on rooftops to capture the most direct sunlight, and some systems include pumps and valves to move the water through the system.

For a typical U.S. home, heating water accounts for a good chunk of its total energy use: about 20 percent. The cost of a solar water heater varies, based on factors such as the size of the storage tank, how many collector panels are needed, and installation fees. And while it can be pricier up-front compared with a conventional water heater, a solar water heater can cut your water heating bills by 50 to 80 percent. That's because sunlight is free!

(If you're not in such a sunny location, heat pump water heatersare another efficient and eco-friendly option.)

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