The simplest system is one that just heats air that is drawn directly into rooms by natural ventilation (the stack effect), or by forced ventilation (using fans). These systems are really only appropriate for climates with long, cold winters that have many sunny days. These options may solve specific problems of ventilation or seasonal coldness in a room, perhaps in a commercial or industrial context – a warehouse, storage room or hangar, where it is necessary
building, its orientation and the local climate. Large collector areas are required since there is less sunshine in the winter. This form of space heating represents only 0.8 per cent (1.2GW) of solar thermal applications worldwide. Collectors can be glazed or unglazed. Japan and the US have most of the glazed systems and Switzerland and Canada the most unglazed.
For the maximum comfort of building occupants, the solar heating air should enter the occupied space slowly, continuously and from several different points so that it does not feel draughty. The most efficient operating temperature is around 32°C (90°F) as this is good for comfort and any higher temperature would have a greater heat loss.
Window box collectors
This simple system involves having an air-based solar collector beneath a window pointed in the direction of the sun. The collector is a flat, multilayered panel, sealed on three of its four edges and fitted to the outside of the building. Sunlight passes through the glass and strikes the black metal plate, which absorbs the heat. It warms the air flowing around it which rises, and cool air is drawn in at the bottom. The warm air rises into the building beneath the window. The collector’s layers are, from front to back: glazing, an upper airspace, a black metal plate, a lower air space, insulation and base. The inlets and outlets are airtight, vented and can be controlled by a thermostat. If the stack effect is not strong enough to draw sufficient air into the bottom from inside, a fan may be used. Thermostatic control of the fan is possible, perhaps powered by a small PV module. The heated air can be directed through ductwork further into the building.
Roof- or wall-mounted collectors
These take external air, warm it and use fans to route it into a property. The solar collectors are placed on the roof or equator-facing wall. There are two types of design. One contains material with thousands of tiny capillaries, which absorb the sun’s heat. The heated incoming air is then drawn down through ducts into the building where it is discharged around the floor level of the lower floor. Another design is glazed and contains a black plate that heats the air drawn upwards, between it and the glazing. The latter usually draws air from within the building at floor level, as this requires less heating, and releases it at ceiling level.
Rooftop collectors may be connected into a mechanical ventilation system with heat recovery. This ventilation Figure 4.1 The window-box collector.
Source: Kishore, 2009
Figure 4.2 An air-based roof-mounted solar
heating system: the collector on the roof draws air from within the building, heats it, and returns it to the building. Ideally the warm air is returned at ground level, shown here at ceiling level for simplicity.
PV module for pump pump warmed air out air intake controller solar air collector
shines, these systems also help to dehumidify the internal climate, displacing internal moist air with warmed dry fresh air, ideal for ‘wet’ rooms, such as kitchens and bathrooms.
The Trombe Wall
Another Frenchman is the inventor of this design – Felix Trombe. Here, the entire face of a building becomes the solar collector and consists of an equator-facing (south in the northern hemisphere; north in the southern hemisphere), glazed, thermally massive, dark coloured wall. The wall is positioned a short distance behind a large area of glazing. At the top and bottom of this wall are ventilation slots through to the building’s interior. Cooler air at floor level is drawn by air-pressure differences (the stack effect) into the gap. It rises in the gap between the glazing and the wall, heating up on the way. At the top, it flows back into the room behind it. The design of the airflows within the building can conduct this warmed air into other parts. Additionally, the thermal mass of the wall can also conduct heat from one side to the other and radiate it into the room. The disadvantage of the Trombe Wall is that it removes the view on the equator-facing side of the building. Solar walls
A development of the Trombe Wall, perhaps more suited to industrial buildings, is the solar wall, also called a transpired air collector. An equator-facing wall
is clad in aluminium or stainless steel coated with solar absorbing paint. The cladding is a few inches from the wall, and is perforated with thousands of tiny holes. The sun heats up the metal, and a fan at the top of the wall draws up the heated air into a HVAC system. Ideally, this also has built-in heat recovery as above. Ducting transmits the heated air around the building. This is useful for large industrial and commercial buildings, both new and refurbishments. Various companies supply kit solutions.
Figure 4.4 A Trombe Wall; the whole of the wall
becomes a solar collector.
Source: © Wikimedia Commons
Figure 4.3 An air-based wall-mounted solar heating
system: the cooler air is taken from ground level and returned, heated, higher up.
solar air
collector pump
controller cool air intake
Figure 4.5 The transpired air collector or solar wall; (a) cutaway diagram and (b) industrial application. Source: Corus