Solar Building Design

Passive solar building design involves the modeling, selection and use of appropriate passive solar technologies to maintain the building environment at a desired temperature range usually based around human thermal comfort throughout the sun's daily and annual cycles. As a result it generally minimizes the use of active solar, renewable energy and especially fossil fuel technologies.

Passive solar building design is only one part of thermally efficient building design, which in turn is only one part of sustainable design, although the terms are often used erroneously as synonyms. Passive solar design does not relate to factors such as ventilation, evaporative cooling, or life cycle analysis unless these operate solely by the sun.

Direct Solar Gain

Elements of passive solar design, shown in a direct gain applicationDirect gain involves using the positioning of windows, skylights and shutters to control the amount of direct solar radiation reaching the interior spaces themselves, and to warm the air and surfaces within the building. The use of sun-facing windows and a high-mass floor is a short-cycle example of this. John Hait's "Passive Annual Heat Storage" method is an example of an annualized solar approach primarily using this path.

Direct solar gain systems suffer because historically there were no reasonably priced transparent thermally insulating materials with R-values comparable to standard wall insulation. This is now changing in Europe, where superinsulated windows have been developed and are widely used to help meet the German Passive House standard.

In the northern hemisphere, a design that uses too much south-facing glass can result in excessive heating and an uncomfortably bright living space at certain times of the year. Conversely, in the southern hemisphere the same is true for north-facing glass.

Indirect Solar Gain

Passive solar design using a trombe wallIndirect gain, in which solar radiation is captured by a part of the building envelope designed with an appropriate thermal mass such as a water tank or a solid concrete or masonry wall behind glass. The heat is then transmitted indirectly to the building through conduction and convection. Examples of this are Trombe walls, water walls and roof ponds. The Australian deep-cover earthed-roof, innovated by the Baggs family of architects, is an annualized example of this path.

In practice indirect solar gain systems have suffered from being difficult to control, and from the lack of reasonably priced transparent thermally insulating materials.

Isolated Solar Gain

Isolated gain, involves passively capturing solar heat and then moving it passively into or out of the building using a liquid (for example using a thermosiphon solar space heating system) or air (perhaps using a solar chimney), either directly or using a thermal store.

Sun-spaces, greenhouses, and "solar closets" are alternative ways of capturing isolated heat gain from which warmed air can be taken. In practice it has been found that some owners use these structures as living spaces, heating them with conventional fuels and therefore significantly increasing, rather than reducing, the environmental impact of the building.

Don Stephens' "Annualized Geo-Solar" (AGS) heating is an annualized example of this option, which offers the advantages of preventing over-heating when living spaces are already deemed warm enough, and of extending time-delays until such heat will be desired.