Division of Energy
Using Solar Energy - Passive Solar Water Heating
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Figures in this page provided courtesy of the Florida Solar Energy Center.
All solar water-heating systems employ a collector area and a storage tank. Usually collectors consist of an insulated box containing a winding array of water pipes attached to metal sheeting or fins that have been painted flat black to absorb solar radiation. Most collectors have a cover of either tempered glass or plastic to better contain the solar energy. The main use of a solar water heater without a cover glass is to heat water for swimming pools. These low-temperature collectors usually do not heat the water above 90 degrees Fahrenheit, which is sufficient for swimming pools.
MID-TEMPERATURE SOLAR COLLECTOR (up to 1600 F)
Passive systems do not use a pump to circulate water from the collector to storage or other locations. They employ three means: gravity, the tendency of hot water to rise above cold water and water pressure.
Passive solar water-heating systems can be categorized as either direct or indirect. A direct passive solar water-heating system is the simplest. The crudest form of direct passive solar water heating is to paint a water storage tank black to absorb heat into the water. Such systems were employed in the past at summer campgrounds where facilities were mostly used during warm, sunny periods of the year. These systems must be drained dry at the end of the season to avoid damage from freezing.
Care must be taken with such a system to avoid scalding injuries since the water in the tank can reach very high temperatures on sunny days. In addition, these systems can hold only as much water as the tank holds. That means that if everyone wants to take a shower at the same time at the end of the day, some folks may end up taking cold showers. Referred to as Integral Collector Storage systems, modern versions of these systems often enclose the storage tank(s) in an insulated box with one or more layers of glazing to let the sun in without letting the heat out as readily.
Next in complexity are passive direct systems that store water in a tank that is separate from the collector area. The term ‘direct’ means the water to be used by building occupants is run directly through the solar collector. One type is called a Thermosiphon System. A box-type collector heats the water, and the storage tank is positioned higher than the collector so that convection draws the heated water up into the storage tank without using a pump.
For more information in solar collectors and heating applications, please visit the U.S. Department of Energy's Solar Heating Basics page.
Active Solar Water Heating
In active solar water-heating systems, pumps circulate water from the storage tank to the collector. Passive systems do not use a pump. The diagram below shows the essential elements of an active system, but there are many variations available. In Missouri, the most important variation is the difference between direct and indirect systems.
In a direct system, the water to be used by the building occupants runs directly through the solar collectors. In an indirect system, either water or another heat-conducting liquid runs through the collectors and then passes through a heat exchanger to heat the water used by building occupants. A heat exchanger requires more piping than shown in the diagram above. Depending on the operating conditions, the cold water delivery pipe would be positioned to allow water to pass either through the heat exchanger loop or directly to the storage tank.
In Missouri, any solar water-heating system designed for year-round use should be an indirect system using a heat-transfer fluid that contains an anti-freeze agent such as non-toxic propylene glycol so that damage from freezing can be avoided. Heat-transfer fluid needs to be changed every three to five years to avoid system failure.
Excessive heat buildup also can harm a system. Controllers are usually solid-state devices that direct the pumps in a solar water-heating system to operate in a manner that optimizes the transfer of heat from the collector to storage to avoid dangerous levels of heat buildup in the collector. Operation of the heating element in backup systems is sometimes also tied into the system's controller.
Heat exchangers in solar water-heating systems resemble a pipe or pipes within a larger pipe. The pipe containing the water to be heated passes through a larger pipe that is flowing the fluid from the collector panel, allowing the heat of the sun to transfer into the water. Most collectors used for water heating are of the mid-temperature type as shown in the box-shaped collector diagram above. Certain industrial and institutional applications of solar water heating use ‘evacuated tube’ collectors. Welding a heat-collecting metal plate to a water pipe and placing it inside a vacuum tube allows the equipment to heat water to a temperature of 3500F. Cost trade-offs must be considered when choosing one type of system over the other.
Pumps used in these systems are small, between 1/100 to 1/12 horsepower, because they move a relatively small amount of liquid through a closed loop. Some manufacturers now provide photovoltaic panels that attach to their solar water-heater collectors to power these pumps. This practice works well because little power is needed to operate the pumps, and the pumps should not need to run when the sun is not shining.