Types of Solar Water Heaters
There are many types of solar water heaters used in the world. Most of these are designed for northern climates where freezing weather is a major concern and sunny days are not as common as in Florida. Others are used in extremely hot climates or where electricity is extremely expensive and not always available, such as Hawaii, Caribbean Islands and Central America. Fortunately, on the Suncoast we do not have these issues.
All solar systems have three parts in common:
- A solar collector (or panel) which collects the suns energy and heats the water
- A special storage container (the solar storage tank) to save the heated water for periods when solar energy is not available such as night time or rainy days
- A delivery system which usually consists of, an electronic control, insulated pipes and a circulating pump.
There are two major types of solar systems installed locallye: Direct Systems and Drainback Systems. We will discuss the merits of each.
Direct systems, also called an “active or “open loop system”, is most often installed in the central and southern areas of Florida where freezing conditions are rare. In a direct system your potable water is circulated directly thru the solar collector. An electronic control uses a set of temperature sensors to compare the temperature of the solar collector with the temperature of the bottom of the tank where the coldest water is located. When the solar collector is warmer than the bottom of the tank, the electronic control turns on the circulating pump that draws cold water from the bottom of the solar storage tank and circulates it through the solar collector. The solar heated water is then returned to the solar storage tank. When the bottom of the tank has reached a predetermined temperature or insufficient solar energy is available, the control turns the circulating pump off.
The circulating pump is very small and uses less energy than a 20 watt light bulb. Another version of this system uses a small photovoltaic (solar electric) panel to operate a direct current (DC) circulating pump.
Advantages: A direct system typically offers the highest operating efficiency because there is no efficiency lost through a heat exchanger, since the potable water from the solar storage tank is circulated directly through the collector.
Disadvantages: Direct solar systems require a set of safety valves:
- A pressure relief valve to release super-heated water when the solar pump is turned off when the solar tank is filled with hot water and the remaining water in the solar collector is allowed to boil
- A freeze protection valve which protects the solar collector from freezing during a power outage. The valve then opens drawing water from the solar storage tank and allowing the water to pass through the collector keeping it from freezing. This water is then released onto the roof. However a freeze protection valve requires a properly functioning check valve on the solar return line to prevent water from going through the return line and not passing through the solar collector, resulting in a frozen collector
- A vacuum relief valve to allow air into the collector to allow it to be drained.
- An air vent to allow any air to escape from the solar collector
All of these valves are mechanical and require routine maintenance to check for proper operation, leaking, scaling or corrosion. During periods of freezing weather the control circulates and sacrifices the hot water in the solar storage tank running it through the solar collector to keep it warm. In addition since potable water is used throughout the entire system, scaling and corrosion of the copper plumbing reduces the lifetime of the solar collector.
A drainback system consists of an unpressurized drainback reservoir, which typically holds ten gallons of water and air. The reservoir is connected directly to the solar collector with a separate high head circulating pump and has a heat exchanger located in the bottom of the reservoir.
When the electronic control calls for solar heat the drainback pump pushes and circulates the water through the solar collector heating the water in the reservoir. As the water is first drawn out of the reservoir, the air in the empty solar collector replaces the water in the reservoir.
A separate circulating pump, controlled by the same electronic control, is connected to the heat exchanger and the solar storage tank allowing the solar heated water in the reservoir to heat the storage tank. When the tank is heated, solar energy is no longer available, or freezing conditions exist, the electronic control turns off both pumps and gravity allows the water in the solar collector to drain back into the reservoir filling the collector once again with air. An empty solar collector cannot freeze or overheat. Once the power is interrupted to the pumps, the solar system simply drains and fills the solar collector with air.
Advantages: The system is designed to be fail-safe. All mechanical valves are eliminated. The solar collectors drain whenever the power to the pumps is interrupted, eliminating damage to the solar collector from freezing in the winter and overheating in the summer. Going on vacation, simply unplug the solar controller and the system drains. When you return just plug it back in.
In addition since the same water is passing through the plumbing and solar collector over and over, scaling and corrosion are eliminated. The life expectancy of a drainback system is twice that of a direct system. Removing all of the mechanical valves eliminates annual maintenance. The reservoir and heat exchanger both have a ten year warranty. For the Suncoast a drainback system is far superior to a direct system.
Disadvantages The heat exchanger makes the system slightly less efficient than a direct system. And as you might expect, the drainback reservoir, second pump and heat exchanger make this system a bit more expensive than a direct system. However the advantages of a drainback system far outweigh the disadvantages.