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Solar air heating systems – how do they work?
Applications:
Advantages and limitations
Types of solar air system 
Active air solar systems

Solar air systems are capable of supplying a great part of heat demand in a building  with the roof and the facade serving as air collectors. In buildings with good heat insulation ventilation is an important factor too. Therefore, solar assisted ventilation systems with heat recovery are becoming more and more important. The design and installation of solar air systems has to take into account architectural and technological factors of the building. Active utilisation of solar energy in such systems offers greater efficiency, better heat distribution and thus greater comfort than passive systems

Solar air heating systems in modern building technology increasingly aims at reducing the space heating loads and tries to supply the required residual heating by means of renewable sources of energy as far as possible.
The central part of any active solar system consists of the collector, which absorbs radiation energy from the sun and converts it into heat. At present, solar air collectors are not yet very widespread: On a global scale, there are only a dozen manufacturers engaging in the serious production of air collector systems. While there are international and national testing standards for hot-water solar collectors, as well as uniform standards on a European level, there are no testing standards for air collectors, as yet.

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Solar air heating systems – how do they work?

The following elements may serve as "solar air collector" integrated in a building:

A heat storage can be implemented by hypocausts (concrete floor with built-in channels through which solar heated air is passing or hollow core walling) or rock-bed storage in the basement or in the core of the building. Of course, solar heated air may also be circulated directly into the space without intermediate storage, thus supplying not only heat but also fresh air and, at the same time, regulating air humidity.

Air circulation is brought about automatically as heated air is rising (natural convection); however, most solar air heating systems use fans. In well designed systems the ratio of energy demand for fan operation to total energy output (heat) should be from 1:15 to 1:25. In ideal cases, the energy needed for fan operation is generated by photovoltaic (PV) panels, the advantage being that this system is self-regulating as PV-induced fan performance increases in proportion to solar thermal output.

Heat distribution is implemented by means of conventional ventilation systems, hypo- or murocaust or directly from space to space.

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Applications:

Systems for direct air supply are not only used for halls, holiday homes, caravans, storehouses, outbuildings and places not occupied all year round but also for drying of agricultural products and heating and ventilation of factories and private property. Indirect heat supply (e.g. by hypocausts), in contrast, is preferred for space heating in residential buildings because the warm surfaces provide agreeable radiant heat to the space. Another possible application consists in ventilation air preheating combined with heat recovery.

Practice has shown that multi-function systems usually are more economical.

Typical applications include:

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Advantages and limitations

Solar air heating systems feature a number of advantages as compared to merely passive solar systems or solar heating systems using water. In comparison to passive solar systems they show the following positive characteristics:

Compared to solar heating systems using water as fluid solar air heating systems feature advantages as far as safety (leakage in the system no freeze up does not cause damage to the building) and environmental protection (no anti-freeze) are concerned.

Other positive aspects of such systems consist in their additional functions. Lower price as thermal systems, Solar air systems integrated in the building often serve as weather skin and as a barrier against street noise or as load bearing elements (snow load). The individual components of the system form an integral part of the conventional building shell and can easily be combined with mechanical ventilation, which is constantly gaining ground not only in office buildings but also in low-energy houses.

Nevertheless, solar air heating systems also have a disadvantage:

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Types of solar air system

Various types of solar air  heating / ventilation systems are available on the Irish market today.

Type 1 is a very simple construction: ambient air passes from a glazed or unglazed collector directly into the room to provide ventilation and heating. Applications include vacation cottages (dehumidification) and large industrial buildings requiring adequate ventilation.

Type 2 circulates room air to the collector. The heated air rises to a thermal storage ceiling from which it is conveyed back into the room. This system uses natural convection and is well suited for apartment buildings.

Type 3 is particularly suited for retrofitting poorly insulated buildings. Collector heated air passes through a cavity between an outer, insulated wall and an inner facade. This creates a buffer which considerably reduces heat loss via the facade of the building.

Type 4 is the classical solar air heating system and is commonly used. Collector heated air is circulated through channels in the floor or in the wall. Heat is radiated into the room with a time delay of four to six hours. The advantage of this system consists in the large radiating surfaces, which provide for a comfortable climate. Systems with forced ventilation (fans) provide the best efficiency and thermal output. They may be used in buildings with large surfaces, which serve as radiation sources.

Type 5 is an advanced version of type 4; room air is circulated through separate channels of the storage. Thus, heat can be stored for a longer period of time and released when it is needed. However, this type is rarely used as investment costs are rather high.

Type 6 combines a solar air collector and, via a heat exchanger, a conventional heating system. Thus, common radiators and floor or wall heating components may be used. This system can also provide domestic hot water and is particularly suited for retrofitting and for
buildings in which heat has to be transported over long distances.

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Active air solar systems

Active solar systems for air heating are a straightforward yet effective way of using solar energy for space heating and tempering ventilation air. They offer some unique advantages over solar water systems, can offer improved comfort and fuller use of solar gains than passive solar systems and are a natural fit with mechanically ventilated buildings.

They can be economical, with short pay-back periods and can act not only as space heating or ventilation air heating but also for water pre-heating, sun shading, electricity generation (with hybrid photovoltaic systems) and can help induce cooling.

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