Thursday, April 23, 2009

Solar Hot Water


Let the sun do the work

Solar water-heating systems are different from the solar systems that make electricity. Collectors on your roof still grab energy from the sun, but in this case the energy is used to heat water. Of course not everyone has the right roof or enough sun to do the job. But the average U.S. household spends 11% of its energy budget heating water. So why not investigate your home's potential to provide you with a hot shower or warm pool from this abundant, free source?
Solar Hot Water

Solar water heaters for indoor water use save the most money if your household uses a lot of hot water--and if you use it from late morning through early afternoon when the solar collectors have maximum exposure to the sun. Smaller households that use hot water mostly early in the morning and in the evening can benefit, too, but the savings will be smaller.

Once the solar collectors heat the water, it's stored in an insulated tank until needed. When the sun isn't shining, the water in the tank slowly cools down, and a back-up water heater kicks in to boost the temperature.

By the way, if you had a solar water heater in the 1970s that didn't hold up, don't hold it against today's products. There's been a dramatic increase in reliability and efficiency since then.

See the very end of the article for more details on pools and hot tubs.

Top Tips

At home

  • How low can you go? Using less energy is always less expensive than producing energy--even if that energy comes from the sun. So before you make a move to solar heating, look for ways to reduce your hot water use. Take shorter showers, and showers instead of baths. Install water-saving faucets and showerheads. If you're buying a new dishwasher or clothes washer, choose a water-efficient model. Set your water heater temperature to 120°F, and turn it to "low" or "vacation" mode if you're going to be out of town for more than a few days.

  • Blanket your tank. Cut hot-water energy use by wrapping the water heater with an insulating blanket. They're inexpensive and readily available at home improvement stores.

  • Back it up. For regular household needs (kitchens, bathrooms and laundry), solar water heaters provide water from 110°F to 180°F when the sun is shining. To ensure plenty of hot water first thing in the morning and during cloudy stretches, however, most solar homes have a back-up fossil-fuel-fired water heater. The back-up heater can either be a conventional water heater or a tankless heater.

When shopping, look for

  • Certification. Choose a solar water heater system that's certified by Solar Rating and Certification Corporation (SRCC) an independent organization that verifies the performance of solar equipment.

  • An Energy Star. The government's labeling program for the most energy-efficient appliances and equipment, Energy Star, will begin covering solar water heaters in 2009. To earn the Energy Star label, a solar water heater must have certification from the SRCC and a "solar fraction" of at least .50. A solar fraction is the portion of the water-heating needs served by the solar system rather than the back-up water heater. A solar fraction of .50 means that 50% of the hot water is supplied by the solar heater and 50% by the back-up equipment.

Other Considerations

  • If you are adding solar water heating to an existing home, you'll need to consider whether the roof can take the added weight, whether you have adequate unshaded south-facing space, and whether there's room near your existing water heater for an additional storage tank and pipes. A solar designer or installer can evaluate these considerations for you.

  • Solar water heaters are most cost effective in sunny climates. In cloudy climates, you may need a larger, more expensive system.

  • For ideal performance, the solar collector should be located in an unshaded area that faces south. It should be tilted at an angle close to the latitude (a 37° angle for 37° latitude). But any orientation within 45° of south and any tilt from 15° to 60° will work well enough. If you have a pitched roof, you can mount the collector flush with the roof rather than having it jut out. Most solar water heaters today are streamlined and look like skylights on the roof from the exterior.

  • If your home is heated with hot water circulating in tubes under the floors, look into going solar. You need hot water anyway, so why not solar? In such radiant-floor heating systems, a solar water heater is backed up by a conventional one.

  • Check the Database of State Incentives for Renewables and Efficiency (DSIRE) for information about incentives and rebates for solar water heating.

Benefits... your wallet
Solar water heating is much less expensive than solar electric and usually pays for itself in four to nine years. Most systems cost from $2,000 to $4,500. A typical household with an existing electric water-heating system could save up to $500 a year by going solar. If you have gas water heating, the savings are lower and the payback longer. But as natural gas supplies decline and prices rise, solar water heating will become a hedge against rising energy costs. the Earth
Solar water heaters reduce greenhouse gas emissions, air pollution, and demand for fossil fuels. According to the U.S. Department of Energy, replacing an electric water heater with a solar water heater offsets the equivalent of 40% to 100% of a typical passenger car's CO2 output.

Common Mistakes

Doing it yourself. Some very experienced DIYers buy solar water heating components and do the installation themselves, but it's not a project for the faint of heart. Most people have the systems professionally installed.

Getting Started

Here are some basic terms you'll encounter when you dip your toes into the world of solar water heating.

Most solar water heater systems have two basic parts: a collector and a storage tank. The collector sits on the roof (or another sunny location), and the storage tank usually sits next to the home's conventional water heater. Although most residential systems have a separate storage tank just for the solar heated water, some systems have tanks where the solar-heated and conventionally heated water mixes.

  • Flat plate collectors are the most common. A pump circulates liquid through the collector, which typically looks like a shallow glass-covered box. When the sun is shining, it heats up the liquid in the collector. When the liquid is hot enough, the pump comes on and moves the hot water to a storage tank. In some systems, the liquid is potable water that's stored in a tank that feeds into the home's hot water pipes. In other systems, the liquid is an antifreeze solution that flows through a heat exchanger to transfer heat (but not liquid) to the home's potable water.

  • An evacuated tube collector also uses a pump to circulate water. It's more efficient than a flat plate collector, but a lot more expensive. Water is heated inside a vacuum, so there's less heat loss than with flat plate collectors. Rather than looking like a glass box, the collector has rows of metal fins. As with a flat plate collector, the heated water is stored in a tank, usually next to the conventional water heater.

  • Batch collectors are considered passive water heaters. They don't use pumps, which reduces electricity use and maintenance. But they require the storage tank to be placed higher than the collector, so weight can be an issue if the collector and tank are on the roof. When water in the collector gets hot enough, it naturally rises and is replaced by cooler water from the storage tank. Batch collectors are best in mild climates where freezes are rare (in cold climates, they need to be drained for the winter months).

  • Before hiring a contractor, ask the following questions:

    • Does the solar professional have experience designing and installing the type of system you want?
      If you are in the market for a solar pool heater, for example, don't hire a contractor who has only installed photovoltaic systems.

    • How many years has the contractor has been in the solar business and how many installations have they done? Solar is booming in many parts of the country; be cautious about hiring a newbie contractor.

    • Is the contractor licensed? Some states require solar contractors to have special licenses; check with the contractors' license board in your state about requirements.

    • What specific services will the solar contractor provide? Most offer a "turnkey" service: they'll analyze your site and energy needs, design an appropriate system, procure the equipment and materials, handle the utility company and rebate paperwork, obtain any necessary permits, and install the system.

    • Are the bids you received based on comparable information? When evaluating bids, make sure they are for the same type and size system. The bids should include all costs associated with buying and installing the system, including hardware, installation, permits, and grid connection (for PV systems). For PV systems, the bids should state the expected energy output in kilowatt-hours. Bids for solar hot water systems should include an estimate of how much energy will be saved in kilowatt-hours or "therms" (which each contain 100,000 Btu).

    • For general advice on what questions to ask contractors and other tradespeople, see our "What to Ask Your Contractor" article.

Pools and hot tubs

Solar pool heaters represent the majority of solar water heating systems installed in the United States. Solar pool heaters are simple technologies that require little maintenance and are cost effective. A typical solar pool heaters costs $3,000 and $4,000, and will pay for itself within two to seven years.

There's no need for a storage tank since the pool provides storage. The pool's existing pump circulates the water between the pool and the collector. The collector size needs to be 50% to 100% of the pool's surface area (the larger the collector, the longer you'll be able to extend the swimming season), but the collectors are more streamlined and unobtrusive than the solar collectors used for household hot water. In cold climates, the collector is usually drained for the winter so that water in it doesn't freeze.

You can also get a solar hot water system designed for use with a hot tub. As with a solar pool heater, the hot tub's pump circulates water between the hot tub and the collector. When the water in the tub reaches the desired temperature, the pump turns off. Solar hot tub heaters can be ground-mounted or roof-mounted.

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