Indirect gain

In this form of design a heat absorbing element
is inserted between the incident solar radiation
 and the space to be   heated; thus the heat is
 transferred in an indirect way.
This often consists of a wall  placed behind
glazing facing towards the sun, and this thermal
storage wall controls the flow of heat into the
building.

The main elements
 ● High thermal mass element positioned between sun and internal spaces, the heat absorbed slowly conducts across the wall and is liberated to the interior some time later.
● Materials and thickness of the wall are chosen to modify the heat flow. In homes the flow can be delayed so that it arrives in the evening matched to occupancy periods. Typical thicknesses of the thermal wall are 20–30 cm.
● Glazing on the outer side of the thermal wall is used to provide some insulation against heat loss and help retain the solar gain by making use of the greenhouse effect.
● The area of the thermal storage wall element should be aboutaway its heat, at the same time drawing in outside air to the building from cooler external spaces. Indirect gain options are often viewed as being the least aesthetically pleasing of the passive solar options, partly because of the restrictions on position and view out from remaining windows, and partly as a result of the implied dark surface finishes of the absorbing surfaces.
As a result, this category of the three prime solar design technologies is not as widely used as its efficiency and effectiveness would suggest. 15–20 per cent of the floor area of the space into which it emits heat.
● In order to derive more immediate heat benefit, air can be circulated from the building through the air gap between wall and glazing and back into the room. In this modified form this element is usually referred to as a Trombe wall. Heat reflecting blinds should be inserted between the glazing and the thermal wall to limit heat build-up in summer (Figures 1 and 2).

In countries which receive inconsistent levels of solar radiation throughout the day because of climatic factors (such as in the UK), the option to circulate air is likely to be of greater benefit than awaiting its arrival after passage through the thermal storage wall.
At times of excess heat gain the system can provide alternative benefits with the air circulation vented directly to the exterior carrying