Closed-loop solar water heaters are appropriate for areas where temperatures sometimes dip below freezing.
Illustration Courtesy Energy Savers
Barb and Joe Gordon were delighted the day they had a solar hot water heater installed on their suburban Chicago home, boosting their energy independence and helping to create a cleaner world. Now when they use hot water, they take pleasure in knowing it is heated by the sun. The Gordons estimate that their 64 square feet of solar collectors heats 70 percent of the hot water the family of four uses annually, requiring their backup natural gas hot water heater to fire up infrequently.
Conventional hot water heaters are responsible for up to 25 percent of a home’s total energy use, so a solar hot water heater can significantly reduce a house’s carbon footprint and energy bills. In most residential installations, a separate solar hot water storage tank feeds a conventional natural gas, electric or propane hot water heater, which serves as a backup if needed. This ensures the home will never be without hot water—even after a few cloudy days. When quality components are used, systems can last 20 to 30 years and require little maintenance. Three main types of solar collectors are typically used for residential applications.
• Flat plate collectors are insulated, weatherproof boxes that contain a dark absorber plate covered with a sheet of glass.
• Evacuated tube panels contain rows of glass tubes connected to a header pipe. Their high efficiency makes them ideal for high-temperature or space-constrained applications.
• Integral collector-storage systems, also known as ICS or batch systems, feature one or more black tanks or tubes in an insulated box. After passing through the solar collector, preheated water continues on to a conventional backup water heater. These systems should only be installed in mild-freeze climates because outdoor pipes are prone to freezing.
Installing a Solar Water Heater
To determine if your home is a good candidate for solar water heating, also known as solar thermal, first determine your property’s solar exposure. It’s ideal to have unobstructed midday sun throughout the year, but remember that the panels don’t have to be mounted on the roof. They can be mounted as an awning, on the garage, or even in the yard, though this may add to the installation cost.
In the Northern Hemisphere, panels are typically mounted facing south at an angle 10 degrees greater than the latitude of the location. Contractors usually follow this general guideline to calculate the space required for a solar water heating system, according to the U.S. Department of Energy: For two household members, you will need 40 square feet of collector area. For every additional person, add 8 square feet if you live in a sunny area; add 12 to 14 square feet per additional person if you live in a cloudy or unpredictable climate. It is also necessary to have 2 or 3 square feet for a solar storage tank, ideally located near the backup hot water heater.
Solar water heating is a more mature technology than photovoltaic solar (solar electric), and panel efficiency exceeds 90 percent for evacuated tube collectors and around 70 percent for flat plate collectors—three or more times the efficiency of photovoltaic panels. “Solar water heaters have been around for centuries,” says Brandon Leavitt, who has installed solar systems since the 1970s and is president of Solar Service Inc. “They were one of the big sellers in the Sears Catalog in the early 1900s. After the energy crisis in the 1970s, the technology was rediscovered and improved. Today it is a reliable, fully automatic and highly efficient technology if installed correctly.”
To find a good solar installation company, Leavitt recommends using North American Board of Certified Energy Practitioners (NABCEP)-certified installers, which will ensure the company follows industry standards. It’s also wise to check references, view past installations and read customer reviews on Angie’s List, an online network where consumers review service providers.
The Cost of Solar Hot Water
The price of solar water heaters varies widely among companies because of a variety of factors such as the components used, installation costs, and warranties on parts and labor; therefore, the return on investment also varies. Thermosiphon solar systems (keep reading for “How Solar Water Heaters Work”), which are installed in warm or consistently sunny locations, have lower component and installation costs than closed-loop systems, which are necessary in colder climates.
In the United States, the renewable energy tax credit, which will cover up to 30 percent of a solar hot water heater, is currently in effect until 2016, while local incentives vary by location. The return from a solar system is highest when it displaces an electric hot water heater, because heating water with electricity is usually two or three times more expensive than using natural gas. It is also important to consider that solar savings, unlike the return on many other investments, are tax-free and add to the value of your home. “I look at energy as a commodity,” Leavitt says. “You can own it or rent it, consume it or produce it. Why not harvest the energy falling on your property and put it to work?”
How Solar Water Heaters Work
A solar water heating system may be active, meaning it uses pumps to move water or antifreeze from the solar collector to the storage tank, or passive, meaning it relies on convection to move the liquid, rather than mechanical pumps.
In active, closed-loop systems:
Price: $7,000 to $12,000
A pump circulates liquid (food-grade antifreeze in cooler climates and water in milder climates) through tubing, pulling heat out of the collectors. The household water does not move through the panels; a heat exchanger is used to transfer the heat from the liquid in the panels to the water in the solar storage tank.
In passive, thermosiphon solar hot water heaters:
Price: $4,000 to $7,000
Hot water circulates directly through panels relying on thermal convection rather than mechanical pumps. When water is heated it becomes more buoyant and thus rises into the storage tank that is mounted on the roof above the solar panel. Gravity lowers the water as needed for household use. These simple systems are lower in cost and simpler to install, but they are not recommended for colder climates. In Hawaii, thermosiphon solar water heaters are very common.
American Solar Energy Society
Database of State Incentives for Renewables & Efficiency
North American Board of Certified Energy Practitioners