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Solar Heating Systems

A roof does not necessarily have to be orientated exactly to the south in order to serve as a mounting surface for solar collectors. Variations from southern orientation of up to 30° lead to only low losses. Even absolute east or west orientation can be offset through the use of a corresponding larger collector surface. A roof's slope can even be between 20° and 60°, whereby a solar heating system with less slope has a higher energy yield in summer, and one with more slope has a higher energy yield in winter. Special stands are recommended for flat roofs.

Smart Dimensions a must

Properly dimensioned solar heating systems offer the best guarantee for satisfactory operation. Precisely knowing a household's hot water consumption is required when finding the proper dimensions but one should also take into consideration the possibilities of lower consumption. For these questions, the advice of a specialist is recommended. A tip for deciding the dimensions for a small solar heating system: daily hot water consumption of 50 liters per person (at 45° C) yields a collector surface of 1.2 m² by 1.5 m² per person.

Choose a Suitable System

Two circuit, indirect system with controlled circulation

Two circuit indirect systems with controlled circulation are most predominant in Germany. These use heat-transfer fluid that is transported by pumps to the hot water storage tank. Once there, the solar heat is transmitted from the heat transfer fluid to the potable water through a heat exchanger. In order to protect solar heating systems from freezing damage, there is a water-antifreeze mixture in the circulation pipes, and, due to separate circuits, the heat-transfer fluid and the domestic water do not mix. The heated potable water can then flow to the hot-water faucets. In comparison, one circuit systems heat water directly in the collector (usually in countries without danger of freezing).

In thermosiphon systems the regulator and the solar circulation pumps are not necessary because of convection: The solar radiation heats the heat-transfer fluid, its density then decreases as its temperature increases. The fluid becomes lighter and rises inside the circulation pipes. Therefore, a pump is not necessary. In order for such a system to function, however, the water tank must be installed above the collector.

Hot Water Storage Tank and Heat Exchanger

The purpose of the hot water storage tank is to stockpile energy for days with poor solar radiation. Its volume capacity should be 1.5 to 2 times more than the daily hot water consumption - that means 80 to 100 liters per person.

Enameled steel tanks are normally used, such as those known from conventional heating technology. They need a magnesium-or an external current-anode for corrosion protection. Stainless steel storage tanks have a longer life expectancy, but are more expensive.

Good solar storage tanks have a slim, cylindrical form in order to develop a layering of temperature in the tank. This allows for optimal usage of the heated potable water in the upper storage region, thus the entire contents of the tank don't need to be heated to the desired temperature. Undesired mixing of the tank contents through incoming cold water is prevented through a special pipe construction or a baffle plate. The arrangement of the solar circuit heat exchanger in the lower, colder tank area causes the solar panel to work at a more economical level of efficiency due to the low incoming water temperature.

Warm Water Storage Tanks with two Heat Exchangers

In order that the conventional heater does not have to reheat an unnecessarily large volume, its heat exchanger is located in the upper part of the tank.

The entire surface of the tank should be tightly fit with a layer of insulation at least 10 centimeters thick without any gaps. To further lower heat loss, the connections in the cold lower level are led out only from one area.

The Solar Heat Circulation

Within the solar heat circulation, heat is transported from the collector to the hot water storage tank. In order to minimize heat loss, the distance from the collector to the tank should be as short as possible. For systems in one or two family homes, copper pipes with a circumference of 15 mm to 18 mm are enough to guarantee an optimal transportation of heat. The pipes are sufficiently insulated with 30 mm of insulation. For pipes with a circumference of 30 mm, the insulation should have at least the same thickness as the pipe. The insulation must be able to withstand high temperatures, and the outdoor section has to be UV- and weather-resistant. The following materials are used as insulation: mineral wool, polyurethane pipe wrappers, and foam rubber.

Mountings and safety equipment in the solar circuit

The prevalent flow rate in small solar heating systems amounts to 30 to 50 liters per hour per square meter of collector surface. The solar circulation pump has to be able to guarantee this rate of flow. As a general rule, conventional pumps with an electric input between 40 W and 80 W suffice. Also, the pump should always be installed in the colder reflux of the solar circulation system. In this way the pump will not be exposed to high temperatures during operation. Finally, stop valves are mounted in front of and behind the pump, so that the entire system does not have to be emptied when replacing a defective pump.

The typical operating pressure of solar heating systems, which can be controlled by a manometer, lies at approximately 4 bar. The safety valve should open at an approximately 0.3 bar triggering pressure. With a recuperation tank, the heat-transfer fluid can be captured and then fed back into the solar circuit through one of the refilling taps. Thermometers fitted in forerun and reflux are used to check the system's operation. To prevent heat loss out of the tank because of insufficient solar radiation or at night due to convection (the heat-transfer fluid cools in the cold collector, and through the force of gravity, then circulates towards the storage tank) a rebound valve is mounted in the outward flow. The expansion tank keeps the pressure in the system stable and takes up the volume difference of the heat-transfer fluid that is caused by the temperature difference. For safety reasons, the volume of the expansion tank has to be sufficiently large. It should be able to take up the entire volume of heat-transfer fluid. The vent valve serves to ventilate the solar circuit after it has been filled with heat-transfer fluid. It is to be mounted on the highest part of the solar circuit.

The Regulation

Often a simple controller for temperature differences is enough to regulate a small solar heating system for water heating. Through the use of two temperature sensors, the regulator ascertains when the temperature in the collector discharge is higher than the temperature of the solar circuit heat exchanger in the tank, and then it activates the circulation pump. To start the pump, the solar regulator is usually calibrated so that the necessary temperature difference between the collector and tank is between 5° C and 8° C. If this temperature difference sinks to 2° C to 3° C, then the solar regulator will shut off the solar circuit circulation pump.

Anticipatory Planning for Building Construction

If you are building or renovating a house, but still cannot decide on a solar heating system, just remember that preparations for future installation of a solar heating system can be made during construction (ducts for two copper pipes 18' and a quintuple-core cable from the boiler room to the roof). This will save you a lot of work and money later.

Text and images used with permission from the
German Section of the International Solar Energy Society (ISES)

Concise and comprehensible explanations of the basic concepts in solar heating and photovoltaics can be found in our Solar-Lexicon.

Reports on technology, business and politics, as well as presentations on innovative systems and products can be found in the Solar Magazine