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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.
Building regulations on solar gain
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.
Passivhaus design
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 sun space.
Trombe walls
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.
Sun screens
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 -
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
These 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.
Decrement delay
Decrement delay is a fairly new subject which is about how insulation behaves in a dynamic situation (dynamic in the sense of fluctuating temperatures). It is quite a simple subject in theory but there are immense numbers of variables in terms of materials and their positioning and also with regard to climate. It still does not figure in the building regulations although it can have a very marked effect on how insulation behaves.
Because it is a new and complex subject you will need expert advice if you are thinking about using the building’s fabric in this way.
The traditional way to evaluate insulation in a wall or roof is to assume a steady state where the temperature on one side of the insulation is fixed at one level and on the other side it is fixed at a different level and then see (or predict) how quickly heat flows through the wall. For instance a 225mm thick solid brick wall (which has very little insulation) with a winter air temperature on the outside of 0°c and a temperature on the inside of 20°c will have a certain number of watts per square metre escaping out through the wall (about 53 watts/sq.m.).
Similarly in summer the same wall might be at 40°c with the sun beating down on it and if it is 20°c inside then heat will transfer towards the inside.
It gets more interesting when you have a situation where there is the same summer temperature but you start with a cold wall (cold from the night before) rather than one which is already warm. The heat starts moving in through the wall but before it gets through it has to heat up the bricks, so it gets delayed and only seriously starts to show up on the inside after a good few hours.
Similarly, when it starts cooling down at night it takes considerable time for the hot bricks to cool down so much of their heat goes into the room before it starts to travel to the outside. In this way a time lag has been introduced. It’s a bit like the time lag with the earth’s seasons which causes the coldest months to be around January/February rather than in December and the hottest ones to be July/August rather than June when the sun is highest.
The trick is to design walls and roofs so that there is about a 12 hour time lag so that it is always exactly out of phase with the sun. In this way you can cancel out the effects of overheating and overcooling.
It is important to notice that this only works when there is an outside temperature range which swings above and below the indoor desired temperature. Ideally it swings equal amounts both directions. None of this works in a very long, even, hot or cold spell. Only insulation will work then because once the bricks have heated up (or cooled down) after (say) 12 hours they cannot soak up (or give out) any more heat.
The really interesting bit comes when you have a wall which includes mass and insulation either combined (such as insulating clay blocks) or in sandwich layers using dense materials such as woodfibre board. It gets more complicated if you vary the position and size of layers because you can play around with the decrement factor to create different decrement delays. The diagram below shows heat coming in quickly through the insulation (but much less of it) and then slowing down when it hits the thermal mass of the solid wall.
Later on during the night the trapped heat has trouble getting out through the insulation so it goes into the room.
The obvious application for this is in desert areas where it is regularly very hot during the day and very cold at night but in the UK climate there are also seasonal periods when this effect can be utilized, especially in the spring and autumn months.
There is an interesting study from the Netherlands which shows two identical houses, except one has a high decrement time and the other not. The graph on the right in this study has a blue trace showing the lower swing of internal temperature.
Various suppliers are publishing information on decrement times -
Natural Building Technologies NBT show decrement delays of up to 12 hours hours in their literature for pitched roofs.
do you have any information on construction/design of thermal rock store, or were i may find information.
many thanks