As global warming becomes more obvious, the need for space heating in most developed countries will decrease, even as the need for space cooling increases (see Chapter 8). Nevertheless, for many years to come the energy required to keep buildings comfortable for their occupants during the depths of winter will still be a sizeable fraction of the world's energy needs. In Europe currently this fraction is estimated at more than 12%. Even with enhanced standards for insulation and increasingly sophisticated management of passive solar gain, new buildings will still need space heating. Demographic changes are also coming into play. Housing occupancy rates are dropping: there are more and more single-person dwellings, which take a proportionally greater energy share to heat due to the reduced contribution of personal body heat. Low and zero carbon energy sources will have to be utilised, of which the most obvious is the sun.
In theory, hot water from a passive solar water heating system can be used for space heating as well. This would normally be an indirect system (see Chapter 2) with a primary loop transferring solar energy via a heat transfer fluid from the solar collectors to a heat exchanger inside a thermal store containing the space heating water. A true passive solar water space heating system would use pump-free thermosyphonic action to circulate the heated water through a series of wall-mounted radiators or underfloor heating pipes. Underfloor heating (see Chapter 12) is often recommended for all types of solar water space heating as it operates efficiently at lower temperatures than conventional radiators, typically 30-45°C versus 60-90°C.
The problem in practice is that there is a significant risk of air bubbles forming in the underfloor heating pipes and blocking circulation, although the inclusion of a small electric 'purge pump' - that cuts in when the flow stagnates and circulates the water fast enough to flush out any bubbles - can reduce this risk to acceptable levels. On most buildings there would be an additional need for DHW, so the collectors would have to be proportionally larger, as would the thermal store if high stagnation temperatures are to be avoided. Back-up heaters would not just be larger, but would be likely to be needed more often. In practice, therefore, the most effective way of using a passive solar water heating system to reduce space heating energy demand might be to use it to preheat the feed
water into some form of low carbon water heater, such as a biomass boiler (see Chapter 7). This could provide both DHW and space heating even on the coldest, most overcast days. An intermediary thermal store would also be needed.
Passive space heating systems using solar heated air also have the virtues of simplicity, reliability and low capital cost. Managing solar gain to minimise the energy needed for space heating and cooling is an important part of any building's basic design concept. Tools and techniques include appropriate orientation, the use of shading and high performance glazing, and the utilisation of the building's thermal mass (see Chapter 1). Further contributions to internal comfort during cold weather can be made by utilising dedicated solar collectors to heat internal air. These normally yield temperatures well below 60°C, which can feel subjectively cool to building occupants, so large flows of low velocity air through a number of inlets are to be preferred. This is easily achieved with passive solar air heating systems.
For new build, one of the simplest options is the thermal wall . This is a south-facing double- or triple-glazed window that allows sunlight to fall on a massive wall, constructed of masonry, insitu concrete or some form of water container - this latter is often termed a water wall. The south facing side of the wall is usually painted black, although a low emis-sivity, high absorbency material such as metal foil can boost performance significantly. Heat absorbed by the wall during the day continues to be released during the evening and overnight; in fact, the wall becomes a giant storage radiator. Experience suggests that solar heat will take around 8-10 hours to reach the interior of the building through a 200 mm thick concrete or block wall.
Some form of shading, either in the form of an overhang above the wall, fixed louvres on the window or internal blinds, restricts solar gain when the sun is high in summer and maximises it in the colder months. Overhangs and fixed louvres may not restrict solar gain enough during summer mornings and evenings, so blinds or shutters will be the obvious alternative if increased cooling load is to be minimised. Orientation is quite critical. Unlike solar collectors and PV modules, which are relatively intolerant of quite major deviations from true south (see Chapter 9), windows forming part of a passive solar air heating system should be no more than 15° east or west away from true south.
The Trombe wall, named after its French developer, adds the refinement of air vents at top and bottom of the thermal wall. This allows air to circulate by natural convection from the floor level of the interior space through the space between the wall and the window, where temperatures are higher, and up and out at the top of the wall, where it may be ducted up to floors above. The same principle can be applied to the water wall, made up normally of black painted metal or plastic tanks, which usually warm up quicker thanks to internal convection currents and can store up to five times more heat per unit mass than conventional walls. In some applications, architectural considerations might dictate the choice of a translucent water wall, made up of glass fibre reinforced plastic tanks, which would sacrifice some potential efficiency in return for the extra natural daylight penetrating the interior space.
Both thermal and Trombe walls - and the Barra System below - have the problem that a large conventional thermal mass is heavy and so has obvious structural implications. 'Virtual thermal mass', however, can be added to an otherwise conventional building by the judicious use of phase change materials (see Chapter 15).
In the Barra System, developed in Italy, the air flows from the thermal wall into ducts cast into a heavy reinforced concrete floor slab, converting it into a thermal store that would radiate its heat during evening and overnight. This is said to give a more uniform south-north heat distribution and hence greater comfort for the occupants. A similar effect can be achieved by directing the air along channels in the soffit of a massive long span concrete floor. Refinements like these could supplement thermal or Trombe walls, or partially replace them when such features are inappropriate or impractical. In such cases, the necessary hot air could be generated in externally mounted thermosyphoning air panels (TAP), also sometimes known as solar chimneys (although the term solar chimney is best reserved for the vertical shafts used in many pioneering low energy buildings to extract warm air from interiors during summer) (see Chapter 8).
In principle, TAPs bear a superficial resemblance to the flat plate collectors used for solar water heating. An absorber, usually metallic, sits inside a glazed, insulated box. The absorber heats the air, and the hot air flows out and into the building through vents at the top of the box, to be replaced by colder internal air from vents at the bottom. A backdraught valve prevents cold air flowing in the reverse direction on cold nights. The relatively low temperatures at which TAPs operate mean that thermal losses to the environment are correspondingly low, increasing their efficiency. Various forms of shading or venting to the atmosphere, usually manually operated, can be used to modulate peak heat gain. TAPs are often covered up for the summer months, but they are much less likely to be affected by either high or freezing temperatures than water-based collectors. Some TAPs draw in outside air until its temperature drops below the point at which the TAP is capable of heating it to the internal temperature - this helps ventilation.
One common application of TAPs is to mount them directly below windows, venting straight into the room inside. A practical benefit of all TAPs compared to thermal and Trombe walls is that they free up the interior wall for shelving, etc. TAPs are also widely used on the commercial scale for grain drying and similar activities, which usually take place in late summer while the sun is still relatively high in the sky. These normally heat outside air. In some cases, the TAP is effectively the entire southern elevation of the building. Invariably there will be some form of back-up air heating, and hot air from the TAPs will normally be forced through a network of ducts by some form of fan.
Manually operated damper
Manually operated damper
Non-return Back pass - membrane absorber
Non-return Back pass - membrane absorber
Manually operated damper
Thermosyphon air panel Thermosyphon air panel in heating mode in ventilation mode
TAPs can aid summertime ventilation as well as heat rooms
The aesthetics of external TAPs can be enhanced by the use of textured glazing or the use of a different dark or black colour for the absorber. Specifying double wall polycarbonate for the external glazing can reduce vandalism risks. Unglazed TAPs are also possible.
These basically utilise conventional metal cladding as the absorber and circulate air in a cavity between it and the main wall insulation. More sophisticated versions are described in Chapter 10.
The low capital and running costs of passive solar air heating systems, and their inherent reliability, can make them a cost-effective option for many applications. Their main drawbacks are that without a fan to circulate the heated air, it is difficult to ensure even heat distribution throughout the building or to install a cost-effective thermal store in a convenient location. Details of active solar air heating systems can be found in Chapter 10.
Was this article helpful?