Passive Solar Design
Not to be confused with solar panels or Passivhaus (although these may both be associated with passive solar design), passive solar basically means collecting the sun’s energy with the minimum use of gadgets - simply allowing the sun to get into the house and trap it there.
Almost all houses do gain heat from the sun whether or not by design and many overheat in summer. Even on overcast days there is usually some useful radiant heat coming from the sky. However, partly because of the UK’s maritime climate and its generally short and unreliable periods of sunshine there have not been many serious attempts at passive solar housing.
The SAP calculations in the building regulations do take solar gain into consideration and the continental Passivhaus design method is partly built around carefully utilizing solar gain. The lack of experience and practical examples makes passive solar design a subject which few professional designers can handle. The calculation of how much solar energy is available on a particular building site is relatively simple, just requiring sets of tables and a survey of surrounding shading. The design of the building in terms of its shape and orientation may be a bit more difficult. It is important to maximise the south facing window area (and minimise that to the north) and this may be in conflict with other aspects of the design such as privacy, circulation, views, etc.
The real problem comes with the building technology, particularly with the storage of heat and the distributing of it round the house. The capacity that elements of the building have for storing heat and the speed at which they can store it and release it is very difficult to calculate. For instance a three storey house with an open plan ground floor and open stair well will respond very differently from a bungalow with lots of closed rooms. On top of this comes the subject of decrement delay which is still not well understood in practical terms.
If you do decide to go for passive solar design then the PHPP ( passivhaus planning package ) may be a good place to start because it incorporates the best of low energy design with robust building technology in a way which is well proven and dependable. In fact it may be the only route worth taking because of the well developed design software available.
There are a couple of other approaches which may be worth investigating especially for extensions on older houses. They are similar but both depend on special design features: the trombe wall and the conservatory or sunspace.
The Trombe wall
The principle of the trombe wall is shown below. It can be applied to a room or to a whole house. The trombe wall is heated up by sunlight being trapped by double or triple glazing. The wall then slowly releases its heat into the house. Obviously, again, it make sense to have all the larger areas of glazing facing south and limiting the size of those to the north.
This gives little control over flow of heat so it is then possible to have thermostatically managed flaps which allow heat to either come straight into the room or be forced into the trombe wall, to be released later when needed
The wall can have ducts within it to help distribute the heat.A further development of this is to add more thermal storage within the walls, floor and maybe ceiling. This may then require a fan to move the air round.
To solve the problem of overheating in summer it is possible to use screens over the glazed areas which allow the low sun of autumn, winter and spring to reach the glazing but it progressively cuts off the high summer midday sun.
Conservatories and sun spaces
Conservatories can be an excellent way of trapping the sun’s heat and they can, to some extent, act as a second skin of insulation. They can also be a very attractive feature on a house but they normally suffer from large temperature fluctuations and the addition of some kind of thermal storage can help overcome this. There are two approaches -
- rock heat store
- internal heat store
The rock heat store is a way of taking the hottest air from the top of the conservatory and forcing it down into a heat store formed of rocks and allowing it to percolate up into the space to heat it later. The size and shape of the store, and of its rocks, and the power of the fan, all these as they relate to the volume of the conservatory need calculating to get the best results. It may also be possible to duct the warm air into the rest of the house. Some reports of rock stores have come up with problems to do with smells being given off by rocks which were not completely clean or dry. Another approach to this is to build the wall between the conservatory and the house very massively to help absorb heat.
Given that the hot air rises to the top of the conservatory it can make sense to duct it into a massive floor at first floor level. Hollowcore prestressed solid concrete plank floors may work well for this. Obviously some type of control is needed to prevent overheating of floors in summer and there is the possibility of drawing in cool air through such a floor to provide nighttime cooling.
Sun spaces , which tend to be more integrated into the design of the house often span over two storeys and can incorporate massive elements in their structure and in the wall with the house. This can be used to buffer the temperature and store heat.
Growing deciduous climbers such as grape vines can provide an attractive way of cutting down on solar glare in summer while letting light in in winter. Overheating can be reduced by incorporating self opening roof lights in the same way that greenhouses do. This is much more economical and reliable than electrically driven mechanisms. (However, this may raise security problems)
Interestingly, there have been some successful large scale passive solar buildings designed for the UK climate such as this multi storey student accommodation block at Strathclyde University. This building has virtually no extra heating installed. The south facing wall is totally glazed and has transparent insulation behind the glazing which captures solar energy into the brickwork behind it from where it is slowly released into the building. Automatic blinds are incorporated in the glazing to prevent overheating in summer.
