Solar heat exchangers could be mounted on the roof of the building. When conditions are favorable, these could be used to heat domestic hot water at an energy savings compared to conventional systems. A back-up system for cloudy days and evenings would be required.
In temperate climates solar water heaters are usually employed to support a conventional (i.e. gas or electric) sanitary hot water system by pre-heating the water coming off the mains; due to variations in the availability of solar energy (night / day, summer / winter, clear sky / cloudy) it is normally not advisable to rely completely on solar energy for water heating. The collected solar energy would be used to heat an additional coil in the hot water storage of the conventional system; the calorifier might want to be increased in size to allow the use of the solar energy (peak at lunchtime) with a time lag (user peak morning or afternoon).
The solar energy is collected by so-called solar collectors, which heat up water which is circulating through them, and then in turn heats the above mentioned coil in the calorifier; the collector circuit normally consists of the collector itself, a circulation pump (which can be solar powered, too), and pipework that connects the collector and the calorifier.
The main factors for the collector's efficiency are its heat loss and its absorbtion. The absorbtion capacities of the collector - in simple terms actually just a pipe with water - is increased by either welding the pipe to a plate (increasing the collector area) or focussing the solar irradiation of a larger area onto the pipe; in both cases the pipe would want to be black for increased absorbtion. The heat loss is taken care of by effective insulation; the better the insulation, the more expensive the collector usually becomes. Insulation types range from a simple glass pane on the sun side and some cork on the back side of the collector, over double glazing with low-e coating and polyethylene foam, to vacuum insulation (mostly in form of tubes) and reflector systems.
The peak efficiency of solar water heaters then varies from 40% (normal, good quality flat collector) to over 80% (vacuum tubes), giving a peak harvest of up to 500 W/m²; a "normal" household would require about 3 500 W. The choice between high efficiency and normal efficiency (equivalent to high and normal costs) depends mostly on the area available, since the achieved energy cost is roughly the same; if sufficient space is available, a normal efficiency system should be chosen, while a high efficiency system should be used with limited space.
As long as the collectors are installed at an angle of more than 20° from the horizontal, they are self-cleaning and almost maintenance free; the yearly solar harvest only varies little between a southeast, south or southwest orientation of the collectors. Some construction costs can be saved if the collectors are integrated into the roofing system, rather than supported on an independent system above the roof.
Contents of this site are ©1999-2002 Christopher R. Schaffner Last Update February 23, 2002.