Edited by Barry Butler, Liz Merry and Diana Young
In most parts of North America, the best bang for your solar energy buck is with domestic solar water heating (DSWH). It’s a no-brainer in the desert Southwest and in semitropical Florida and Hawaii.
A complete DSWH system can be installed for $4,000 to $7,000, depending on its size, complexity and the climate. These systems are now eligible for the 30 percent federal tax credit. At today’s energy prices, over the life of the system, the cost to operate is about 20 percent lower than a conventional gas water heater and 40 percent lower than an electric one. As gas and electricity prices rise, DSWH will look like a better and better deal. The benefits are much greater since solar energy avoids 2,400 pounds of CO2 per year and provides a secure domestic source of hot water.
Solar hot water systems come in two flavors: passive and active. In warm climates, a simple passive system can provide plenty of hot water.
Passive Solar Water-Heating Systems
Passive systems are installed in areas where freeze protection is not an issue. The most common types are integral collector storage (ICS) and thermosiphon systems.
In an ICS (or breadbox) system, cold city water flows into a rooftop collector. The collector holds 30 to 50 gallons of water in a serpentine pipe with a heat-capturing coating. Hot water from the collector flows directly to a conventional water heater; in effect the sun does most of the work usually performed by the water heater’s burner. As hot water is withdrawn from the water heater, cold water is drawn into the collector, driven by pressure in the city water pipes.
This system, installed by Star Max Solar, uses a flat-plate collector and a PV-powered pump.
A thermosiphon takes advantage of the fact that water rises as it’s heated. Solar-heated water in a flat-plate collector rises through tubes and flows into the top of an insulated storage tank. Colder water at the bottom of this tank is drawn into the lower entry of the solar collector. Water thus flows in a continuous loop, continually reheating during daylight hours. When a hot water tap is opened in the house, hot water flows from the top of the storage tank, and is replaced with cold city water flowing into the bottom of the storage tank.
Although the system is simple, thermosiphons put an 800-lb storage tank high on the roof, which should be reinforced to support it. Other solar water-heating systems put the storage tank at ground level or in the basement, where it’s not a structural challenge.
Active Solar Water-Heating Systems
Active systems use an electric pump to circulate water through the collector. In warm climates, a direct (or open-loop) system is practical: City water goes into an insulated storage tank. A pump draws water out of the storage tank to pass through the solar collector and go back into the tank. Hot water for household use is drawn from the top of the storage tank, sometimes passing through a booster heater. An automatic control system starts the pump whenever the collector is warmer than the storage tank.
In freezing climates, the rooftop part of the system must be protected either by draining down when the temperature dips, or by running an antifreeze solution. These cold-weather systems require temperature sensors, electric pumps and automatic control systems, adding complexity and cost to the installation.
The most common cold-weather system today is the closed-loop antifreeze heat-exchanger system, or active indirect system. When the collector is warm, a food-safe propylene glycol antifreeze solution is pumped through the collector and on through a heat exchanger, then back to the collector. The heat exchanger heats city water for domestic use. The heat exchanger is usually located at the bottom of an insulated storage tank (sometimes the storage tank is also the home hot-water heater, with an electric or natural gas heating mechanism for use when the collector is cold). A breach in the heat exchanger would leak antifreeze into the drinking water, which is why it’s necessary to use only food-safe propylene glycol in these systems. Many local plumbing code officials require double-walled heat exchangers to permit systems in their jurisdictions.
The Energy Star program rates solar water-heating systems.
Swimming Pools and Hot Tubs
One of the most common uses for solar water-heating is to heat pool water. Pool solar collectors are lighter in weight — usually made of UV-resistant polymers — and less expensive than DSWH systems. The size should be 50 to 100 percent of the surface area of the pool. The more solar collector area, the warmer the pool will be in cool weather. The pool serves as the storage tank, and the filtration pump circulates the pool water through the collectors. A solar collector can provide all the heating necessary for a swimming pool, but hot tubs and spas need a backup or booster heater.
DSWH can also be used for space heating. The most efficient method is an in-floor radiant heating system that sends hot water through pipes embedded in the floor. Agas backup water heater is typically used because a house is coldest at night and in winter months. The solar collector area needed is generally 10 to 30 percent of the home’s floor area, depending on climate.
All hot water heaters and solar system storage tanks need to be flushed annually. The pumps and valves in an active system are electro-mechanical devices that will need periodic attention. Annual pressure testing can identify potential problems before they become major leaks. Long-term corrosion is an issue in any plumbing system, but a well-maintained system can go 35 years or more before replacement of major parts.
This article is adapted from the Solar Energy Resource Guide 2008, published by the NorCal Solar Energy Association, a chapter of the American Solar Energy Society.
Illustration by Kurt Struve.