Solar water heating is a technology that every homeowner can use to save dollars on utility bills. Questions most frequently addressed to the Florida Solar Energy Center (FSEC) about solar water heating are answered below. Please note that many replies are general and more explanation may be required for those with individual questions. For additional information, contact the FSEC Public Information Office, call a solar dealer, or consult the references listed.
What is a Solar Water Heater?
A solar water heater uses the sun's energy rather than electricity or gas to heat water, thus reducing your monthly utility bill. When installed properly, solar water heaters are more economical over the life of the system than heating water with electricity, dedicated heat pumps, heat recovery units or propane.
In Florida, three types of solar systems are used: pumped, integral collector storage (ICS), and thermo-siphon. The direct circulation system (see Figure 1, page 2) circulates potable water from the water storage tank through one or more collectors and back into the tank. The solar collector is the main component of the solar system. It is usually a metal box with insulation and a black absorber plate that collects solar radiation and heats the water. The circulating pump is regulated by either an electronic controller, a common appliance timer, or a photovoltaic (PV) panel.
In integral collector storage systems, the solar water storage system is built into the collector. The potable water in the collector unit is heated by the sun and delivered by city or well water pressure to an auxiliary tank (which contains non-solar back-up heating) or directly to the point of use.
A thermosiphon solar water heating system has a tank mounted above the collector (normally on the roof) to provide a natural gravity flow of water. Hot water rises through piping in the collector, which is mounted below the tank; heavier cold water sinks to the lowest point in the system (the collector), displacing the lighter hot water which rises to the tank.
The ICS and thermosiphon systems are simple since they use no pumps or controllers and water always flows through the collector.
How do solar water heating system work?
As sunshine strikes the collector, the water inside it is heated. If the circulating pump is regulated by a PV panel, the pump starts turning as the PV panel is activated by the same sunshine. This direct current (DC) motor pump moves water from the tank through the collector and back to the tank. As the sun's intensity changes throughout the day, the circulating pump also changes its speed accordingly. By the end of the day, the water in the tank has been circulated many times through the collector and has been heated to usable hot water temperatures.
If the circulating pump is regulated by an electronic differential controller, a sensor at the outlet of the collector and a sensor at the bottom of the tank (Figure 1) activate the circulating pump when the water in the collector is about 15-200 F warmer than the water at the bottom of the tank. The pump then circulates water from the collector and the tank. This process continues as long as the water temperature at the collector outlet is about 50 F higher than that in the bottom of the tank. If the temperature difference decreases further, the controller automatically shuts off the pump.
Common appliance timers also may control system operation. The timer is set to operate during a period of the day when solar radiation is available to heat the potable water. It is important that the timers used in these systems incorporate battery back-up in the event of power failures. In order to avoid loss of energy from the tank during overcast days, the collector feed and return lines are both connected at the bottom of the storage tank. During normal operation, natural stratification allows the warmer water to rise to the top part of the tank.
During periods of insufficient sunshine or high hot water demand, a backup electrical element in the storage tank heats the water. The check valve prevents heat loss when the circulating pump is off. The circulating pump consumes only a small amount of electricity — around $5 to $10 worth per year, or in the case of PV — none.
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