Why solar water heating? According to Solar Rating and Certification Corporation, an independent national testing lab, an average home solar water heating system for a family of four saves about 3,400 kilowatt–hours per year. That’s equal to more than eleven barrels of oil.

Which home energy uses are the most expensive? The pie chart to the left shows where the money goes in a typical Florida family’s annual electric bill. The chart is based upon an electric power consumption simulation developed by the U.S. Department of Energy’s Lawrence Berkeley National Laboratory, using these assumptions:

  • Florida home built in 1990 with 2,400 square feet of living area
  • 9.5 SEER air conditioner with thermostat set at 81°F during day and 78°F at night
  • Thermostat set during cold weather at 64°F during day and 68°F at night
  • Two adults and two children in household
  • 85 gallons of 140°F hot water used per day
  • Electric cost with fuel surcharge and utility taxes of 14 cents per kilowatt-hour

Naturally, costs (and savings) can be much greater for families with heavier than average hot water consumption.

Hot water facts

  • Water heating is the second largest energy user in the average home. Only space heating and air conditioning use more energy.
  • Most water heaters are installed by builders who are more concerned with construction costs than with future utility bills. A conventional electric water heater cost less to install than solar.
  • The average home water heater lasts 13 years, or less, so don’t wait for your inefficient electric model to go bad…
  • The savings from replacing it with solar start to pay off immediately!

Oh yes, and there is one other thing we forgot to mention: Using electric power to heat 85 gallons of water from 72°F to 140°F each day for one year sends over three tons of powerplant carbon dioxide and sulfur emissions into Earth’s atmosphere. Yes, really.

Solar energy is clean, safe and free. Naturally, switching to solar water heating means going green in a big way. but you might also be surprised to learn that solar water heating gives you benefits that improve upon conventional electric or gas water heaters:

  • More hot water. Most electric water heaters hold enough hot water for about 20 minutes of continuous use. Then the electric element reheats another tankful. You can run out of hot water if two people are showering at the same time, or if you take a shower after the clothes washer or dishwasher have been running. Solar storage tanks are sized to store an entire 24 hour’s worth of hot water because there is no sunshine at night. But this gives you the added benefit that you are less likely to run out of hot water when there are multiple uses at the same time.
  • Hotter water. The water delivered by a solar collector panel can be hotter than the thermostat setting on your electric water heater. This means you will have hotter water for the dishwasher and for showers if you you want it. (A mixing valve on the storage tank protects you against scalding.)
  • Plenty of hot water during emergencies. With a passive solar water heater, or a system with a solar–powered circulating pump, you will have your customary supply of hot water even during electric power outages. This is an important feature even if you have a standby power generator, because standby generators are not typically sized to handle the power consumption (4,500 to 9,000 watts) of a conventional electric water heater.

System Performance Because incoming solar radiation (called insolation) is often interrupted by cloudy or rainy weather and fluctuates in intensity with seasonal changes in the sun’s position in the sky, solar water heating systems are usually sized to collect and store an entire 24 hours or more worth of energy to meet hot water demand. A modern solar water heating system storage tank will usually have a backup electric heating element to meet unusual peaks in hot water use or extended periods of very cloudy or rainy weather.

While solar thermal water heating systems are viewed as ”pre-heating“ systems in many regions of North America, the sunbelt states enjoy solar insolation levels that allow solar potable water heating systems to economically meet 85 to 90 percent of the annual hot water need.

Different Types of Solar Water Heating Systems There are four major types of solar water heaters:

Direct circulation. This is the type of system typically installed in South and Central Florida and other non–freezing climates within the United States. It is the type shown above.

Direct circulation systems circulate potable water from a hot water storage tank through one or more flat–plate solar collector panels, which are typically shallow aluminum boxes lined with side and rear foam insulation board, glass cover plates and a high absorbency black–coated metal (usually copper) absorber plate. The absorber plate has tubes or other fluid channels and the simple operating principle is not unlike a water–filled garden hose laying out in the hot summer sun. Water is drawn out of the bottom of the tank (where it is the coldest) and sent to the solar collector(s) by a very small circulating pump, which in turn is regulated by a thermostatic electronic controller, a common appliance timer or a small photovoltaic panel. Protection of the absorber plate from occasional freezing weather is achieved by manually or automatically (with a freeze sensor/solenoid combination) opening a drain valve that empties the collector panel(s). Efficiency improvements have been achieved over the last 30 years or so by reducing the iron–oxide content of the cover plate glass, by incorporating closed cell insulation materials, and through the development of increasingly “selective” coatings for the absorber plate that trap more of the solar energy striking the plate.

Florida direct circulation systems are typically installed with an automatic freeze protection feature. The system’s electronic control has a sensor to detect the onset of freezing air temperatures, and warm tank water is circulated through the collector(s) during the freezing conditions. Also, the collector(s) can be isolated from the water storage tank during rare freezing nights by closing manual valves at the tank. Water will then drain out of the collector(s).

Indirect circulation systems. These systems are more common in northern climates where frequent freezing temperatures are anticipated. While the roof apparatus and control mechanisms are identical to the direct circulation system, an antifreeze solution is circulated through the solar collector and back into a heat exchanger in the hot water storage tank. The addition of a heat exchanger adds to the cost of the system and creates some degree of heat transfer energy loss, and this combined with the fact that these systems are typically installed at higher latitudes, where incoming solar radiation is reduced, makes indirect solar water heating less viable than its direct circulation cousin.

Integral collector storage. Also called “batch” solar heaters, integral collector storage (ICS) systems combine the hot water storage tank and the solar collector surface into a single component, eliminating the need for circulating pumps or automatic control systems. In its most simple implementation, a water storage tank painted black and sitting out in the sunlight is a rudimentary ICS system. Hot water is drawn from the high point of the tank, where the hottest water is found, for end–use. Unheated tap water refills into the bottom of the tank under mains pressure as the heated water is used. ICS systems are the least efficient of all solar water heating types because stored heat is lost through the solar–exposed surface of the storage tank during nighttime, which of necessity cannot be insulated to prevent heat loss. On the other hand, an ICS system can be as simple as a black rubber bag, and thus these systems can be a very practical water heating solution that raises the standard of living in economically depressed regions with high solar insolation.

Thermosyphon. This type of system is the most common in Japan, Israel and Australia, which have enjoyed an installed base of millions of residential and commercial systems for the last 30 years. Like the ICS system, the thermosyphon system eliminates the circulation and control system by taking advantage of the thermosyphon principle, whereby circulation flow can be induced by placing the hot water storage tank above the collector panel. The colder water in the bottom of the storage tank falls by gravity through a circulation line into the bottom of the solar collector panel, where it is heated. As the water is heated in the solar collector panel, it rises and feeds through a circulation line from the collector into a high point in the water storage tank. Unlike the ICS system, which combines storage and energy collection, the thermosyphon system seeks to optimize system efficiency by fully insulating the storage tank and taking advantage of improved optimal solar collector panel design. The primary drawback to thermosyphon systems in the United States has been appearance: solar collectors are typically installed on a nominally south–facing roof, and American homeowners (and their homeowners associations) have been reluctant to accept bulky hot water storage tanks on their roofs.