• Solar Water Heater (Flat Plate Collector)

SOLAR WATER HEATER (FLAT PLATE COLLECTOR)

In order to heat water using solar energy, a collector is fastened to the roof of a building, or on a wall facing the sun. In some cases, the collector may be free-standing. The working fluid is either pumped (active system) or driven by natural convection (passive system) through it.
The collector will be made of a simple glass topped insulated box with a flat solar absorber made of sheet metal attached to copper pipes and painted black. In some cases, before the solar energy is absorbed, a parabolic mirror is used to concentrate sunlight on the tube.
A simple water heating system would pump cold water out to a collector to be heated, the heated water flows back to a collection tank. This type of collector can provide enough hot water for an entire family.
Heat is stored in a hot water tank. The volume of this tank will be larger with solar heating systems in order to allow for bad weather, and because the optimum final temperature for the absorber is lower than a typical immersion or combustion heater.
The working fluid for the absorber may be the hot water from the tank, but more commonly (at least in pumped systems) is a separate loop of fluid containing anti-freeze and a corrosion inhibitor which delivers heat to the tank through a heat exchanger (commonly a coil of copper tubing within the tank.). Another lower-maintenance concept is the 'drain-back': no anti-freeze is required; instead all the piping is sloped to cause water to drain back to the tank. The tank is not pressurized and is open to atmospheric pressure. As soon as the pump shuts off, flow reverses and the pipes empty by the time when freezing could occur.
When a solar hot water and hot-water central heating system are used in conjunction, solar heat will either be concentrated in a pre-heating tank that feeds into the tank heated by the central heating, or the solar heat exchanger will be lower in the tank than the hotter one. It is important to remember, however, that the main need for central heating is at night when there is no sunlight and in winter when solar gain is lower. Therefore solar water heating for washing and bathing is often a better application than central heating because supply and demand are better matched.
The water from the collector can reach very high temperatures in good sunshine, or if the pump fails. Designs should allow for relief of pressure and excess heat through a heat dump.

Economics, Energy and System Costs

In sunny, warm locations, where freeze protection is not necessary, a batch type solar hot water heater can be extremely cost effective. In higher latitudes, there are often additional design requirements for cold weather, which add to system complexity. This has the effect of increasing the initial cost (but not the life-cycle cost) of a solar hot water system, to a level much higher than a comparable hot water heater of the conventional type. When calculating the total cost to own and operate, a proper analysis will take into consideration that solar energy is free, thus greatly reducing the operating costs, whereas other energy sources, such as gas and electricity, can be quite expensive over time. Thus, when the initial costs of a solar system are properly financed and compared with energy costs, then, in many cases the total monthly cost of solar heat can be less than other more conventional types of hot water heaters (and also in conjunction with an existing hot water heater). In addition, local incentives can be significant.