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Solar Thermal - How it Works?
Water heating can account for as much as 25 percent of a typical familys energy costs and up to 40 percent of the energy used in a typical dairy operation. A properly sized solar water-heating system could cut those costs in half. Hot water is also needed for pen and equipment cleaning and a host of other agricultural uses.
A solar water heater is a long-term investment that will save you money and energy for many years.
Solar water heaters can operate in any climate. Performance varies depending, in part, on how much solar energy is available at the site, but also on the temperature of incoming water. The colder the water, the more efficiently the system operates. In almost all climates, you will need a conventional backup system. In fact, many building codes require you to have a conventional water heater as the backup.
Solar Water Heater Basics
Solar water heating systems use the sun to heat either water or a heat-transfer fluid, such as a water-glycol antifreeze mixture, in collectors most commonly mounted on a roof. The heated water is then stored in a tank similar to a conventional gas or electric water tank. Then, when water is drawn from the water heater, it is replaced with the solar-heated water from that tank. Some systems use an electric pump to circulate the fluid through the collectors.
Types of Solar Water Heaters
There are four basic types of solar water-heating systems available. These systems share three similarities: a glazing (typically glass) over a dark surface to gather solar heat; one or two tanks to store hot water; and associated plumbing with or without pumps to circulate the heat-transfer fluid from the tank to the collectors and back again.
Draindown systems pump water from the hot water tank through the solar collectors, where it is heated by the sun and returned to the tank. Valves automatically drain the system when sensors detect freezing temperatures.
Drainback systems use a separate plumbing line, filled with fluid, to gather the suns heat. These systems operate strictly on gravity. When the temperature is near freezing, the pump shuts off and the transfer fluid drains back into the solar storage tank. Antifreeze closed-loop systems rely on an antifreeze solution to operate through cold winter months.
Antifreeze solutions are separated from household water by a double-walled heat exchanger.
Breadbox batch systems are passive systems in which the storage tank also functions as the collector. One or two water tanks, painted black, are placed in a well-insulated box or other enclosure that has a south wall made of clear plastic or glass and tilted at the proper angle. This allows the sun to shine directly on the tank and heat a batch of water. An insulated cover can provide freeze protection.
Sizing Your System
Just as you have to choose a 30-, 40-, or 50-gallon conventional water heater, you need to determine the right size solar water heater to install. Sizing a solar water heater involves determining the total collector area and the storage volume required to provide 100 percent of your buildings hot water during the summer. Solar equipment experts use worksheets or special computer programs to assist you in determining how large a system you need.
