Solar water heating: Modern solar heating systems can keep swimming pools warm, heat your home’s water and heat your home’s interior space. Their popularity is increasing for several reasons. Solar heating systems are cost saving, reliable, adaptable, and pollution-free because they use renewable energy from the sun. Modern systems include sleek, attractive, low-relief collectors that blends in with the look of the modern house. Did you know that modern solar heating systems work well even in winter?

A solar heating system is a rewarding investment. It can cover a high percentage of your monthly heating bill, ensure hot water during power failures and increase your property value. When you purchase a solar heating system you are making a conscious, responsible decision to help reduce harmful emissions from fossil fuels, while maintaining your quality of life.

What is solar water heating? A solar heating system collects the sun’s energy to heat water or another heat transfer fluid. The water or other fluid then transfers solar heat directly or indirectly to your home, water, pool or industrial process. Solar water heaters are a very good investment. Although solar water heaters cost more initially than conventional water heaters, the fuel they use—sunshine—is free. To take advantage of solar energy you need to have an un-shaded area, such as a roof, that faces north, northeast or northwest.

What Are The Basic Components Of A Solar Thermal System?

Solar water heaters and solar space heaters are made up of solar collectors, and all systems except pool heaters have some kind of storage. In pool systems, the swimming pool itself is the storage, and the pool’s filtration pump circulates the pool water through the collectors. Active systems also have circulating pumps and controls, passive systems work without this added equipment. Two types of solar collectors are used for residential applications: flat-plate and evacuated-tube collectors.

Flat-plate Collectors

Flat Plate collectors are traditionally the most common type because they are easy to manufacture. Glazed flat-plate collectors essentially are weatherproofed boxes that contain a dark absorber plate under a glass cover.

Evacuated-tube Solar Collectors

Evacuated tubes can be seen as the newer of solar collectors. They are made of a copper “manifold” where the water flows through and a number of evacuated tube heat pipes connected to it. Figure 1 shows multiple heat-pipe Evacuated-tube solar collectors in series.

House with Evacuated-tube Solar Collectors

Evacuated tube solar collectors convert direct and diffused solar radiation into heat. Infra-red rays, which can pass through clouds, are also absorbed and converted into usable heat.

The collectors efficiently collect and transfer this energy through a special collector plate and a rapid heat transfer channel ‘the heat-pipe’, situated in an evacuated glass tube, to a highly insulated manifold heat exchanger –see Fig 1 and Fig 2. The collector plate has a special wavelength ‘selective’ coating using a semi-conductor layer. This special absorber plate converts the maximum amount of solar radiation into heat whilst having very low radiation losses.

Solar Tube Heat Transfer Comparisons

The heat-pipe has a very low heat capacity but an exceptionally rapid conductivity and therefore is a very efficient and speedy heat conductor.

It also provides the system with a diode function i.e. heat transfer is always in one direction – from the absorber to the water and never the reverse. The special fluid inside the ‘heat-pipe’ evaporates when heated, transferring heat energy to its top. The vapour condenses inside a special heat exchanger (which is located within a water manifold) and the fluid then returns to its original position, at the bottom of the ‘heat-pipe’, due to gravity and the cycle is repeated continuously.

The vacuum in the glass tube, being the best possible insulation for a solar collector, suppresses heat losses and also protects the absorber plate and the ‘heat-pipe’ from external adverse conditions. This results in exceptional performance. Due to the huge growth in the solar heating market over the last number of years, this cutting-edge technology has come down drastically in price.

Solar Storage Tanks

Solar storage tanks are basically geysers with an additional outlet and inlet that gets connected to and from the solar collector. Most types also contain a backup electrical heater element. Another difference between solar storage tanks and a standard geyser is that they are usually much better insulated. This ensures that the water temperature is kept even through the coldest night. Most of the new geysers with B-rated energy efficiency comes standard with solar ports. A standard electrical geyser can be retro-fitted with a solar collector by connecting a special valve to it that provides you with the extra inlet and outlet ports.

The collectors can be connected to the storage tank in various ways. The main systems on the market are Closed-Coupled systems and Split systems.

Close-Coupled Systems

Close coupled systems consist of roof-mounted solar collectors, combined with a storage tank located immediately above the collectors. The main disadvantage of this system is out of an aesthetic point of view…you now have a big tank mounted on top of your roof.

Split Systems

Split systems consist of the solar collector on the roof and the storage tank inside the roof. Water circulation between the storage tank and collector can be done in two ways:

Thermosiphon circulation relies on the natural rising of warm water. This however requires that the base of the storage tank be at least 300mm above the collector’s top which is not always possible in modern roofs. Forced circulation uses an electronically controlled electric pump to circulate the hot water from the collectors to the storage tank. The pump used very little power and can even be run from a photovoltaic panel. Forced systems allow for maximum efficiency and flexibility as the storage tank can now be placed wherever. It also provides freezing and overheating protection.

All of the above systems can be configured to be direct or indirect circulation systems. Direct circulation systems directly heat the water in the system. Indirect systems are used in areas where the water is hard and corrosive or in extreme cold areas. Indirect systems usually use non-freezing heat transfer fluid that is circulated through the collectors and a heat exchanger inside the storage tank. Thus, the water in the storage tanks is indirectly heated.

How Much Money Will My Solar Thermal System Cost and How Much Will It Save?

Depending on the size of the solar collectors and the quality a typical residential system costs between R10k and R30k installed. As an example a 2.4m2 panel retro-fit system on an existing 200L geyser will cost around R15k and can save the client up to R32500 in electricity over the next 5 year (assuming R2/kWh electricity costs). It is easy to see the brilliant return on investment that a solar water heating system can provide.

How Much Will Your Solar Heating System Help The Environment?

On average for every 1kWh of energy produced by a coal power station, 0.966kg of carbon dioxide is produced. Now the Solar water heater that we used in the cost saving calculation will provide you with a yearly average of over 2667 kWh. Therefore, you will personally be responsible for saving our environment of another 2576 kg of carbon dioxide per year. Carbon dioxide traps heat in our atmosphere, contributing to the greenhouse effect, which alters our planet’s climate and ecological systems. Using solar energy in place of nonrenewable fuels may also reduce nitrous oxides and sulfur dioxides, which are components of smog.

What Maintenance Does Solar Thermal Systems Require?

Modern solar systems are designed to be maintenance free but due to poor water quality in some areas and dust cleaning is needed. ITS recommends an annual checkup of the system to ensure that your system is providing you with the biggest possible saving. During this checkup the collector surface can then also be cleaned and de-scaling can be done.