Solar heating is the use of heat from the sun to provide direct heat for home heating. In this article, we narrow the definition down to that of using the Sun's heat for water heating.

In South Africa, the solar constant can almost reach 2000Watts/sq.m, with an average value over 24 hours of 220 Watts/sq.m. This is approximately double the values for Northern Europe. With such comparatively high values of insolation, even relatively crude solar panels can provide excellent results.

Solar Heating System

The simplest solar water heating system consists of a collector which takes cold water from the hot water tank, circulates it through pipes iwhich warms it and returns the warm water to the top of the tank. The solar panel can consist of a simple flat plate collector. This is an example of a passive system. Its efficiency can be improved by adding a circulating pump and some electronic controls to maintain the temperature. Some care is needed in areas prone to frost, as “icing up” in cold weather could cause a problem.

More complex systems are possible, where the collector heats a working fluid, which circulates between the collector and the tank. Inside the tank, a heat exchanger transfers heat from the working fluid to water in the tank. The components are:

• Solar collector(s),

• Pump,

• Electronic controller,

• (Optional) solar PV electrical supply.

In most modern systems, the temperature is controlled a by a microprocessor based control unit that the user can control from a small keypad. In some instances the entire electrical system is also powered by a solar photovoltaic panel. The advantage of this (especially for South Africa at present) is that the pump can still operate during power outages.

Solar Collectors

Whilst the simplest collectors can simply consist of variations placing the heating tank ('”geyser”) in a sunny location, the amount of heat that can be absorbed is limited. This is because the temperature can never rise above a level where heat is transferred back to the environment, in accordance with the second law of thermodynamics.

Flat Plate collectors consist of a network of thin copper pipes behind a glass plate. The pipes are attached to a blackened metal plate that absorbs heat energy from the sun. The whole assembly is mounted behind a thick glass panel. Flat panel collectors can also be made using pressed metal plates. The water inside the collector plates can become very hot, leading to heat losses back to the environment. Flat plate collectors are perfectly adequate for South African conditions.

Vacuum tube collectors consist of two concentric glass tubes with a vacuum between them. The inner tube contains the piping, or more often a copper rod containing a working fluid (heat pipe) that transfers heat to one end of the tube. The vacuum system prevents heat losses back to the environment, and the use of a heat pipe effectively removes problems of icing. A vacuum system works well, even in cloudy weather. In South Africa, these collectors can become extremely hot, even to the point of boiling the water. Heat pipe collectors should theoretically be placed at an angle of more than 25 degrees to the horizontal to allow the working fluid to circulate.

Because these units are mass-produced in countries like China for the Northern Hemisphere market (for which they are best suited), they rival the flat plate collector in price. Some care needs to be exercised in choosing a collector that is hail-resistant.

Economics of Solar Water Heating

ESKOM has an aggressive solar water heating program with the aim of offsetting the provision of further power stations by implementing solar water heating in as many homes as possible. Many homeowners could have an effective system for around R10 000, which would certainly repay itself within 2-3 years. There are a number of conditions that have to be met and these are all set out on ESKOM's web-site. Installing solar water heating may be the best energy-saving tip.

Home DIY projects

Whilst ESKOMs rebate system for all practical purposes rules out DIY for water heating, there are still areas where solar water heating can be used, such as pool heating, underfloor heating, etc where experimenters can use home made flat plate collectors in their own designs.