Passive solar energy : modifications of existing masonry walls to improve thermal performance.

Existing brick walls usually have a high U value, one method of improvement is by insulation, but if the wall
faces approximately south, it is possible that greater
benefits could be obtained by improving the wall's
performance as a solar collector. Three solid 225 mm walls
were constructed: one glazed, one unglazed, and (since
glazing prevents rain penetration) one waterproofed
unglazed wall. Two glazed cavity walls were also constructed one glazed with a fan to recover heat from the
cavity by forced convection, and one unglazed, with no fan.
These walls gave small heat gains.

The walls behind the glazing were then insulated on
their outer faces, and warm air collected by natural
convection. Such a collector was built using a selective
surface, a maximum efficiency of about 50% was observed. A
theoretical model was developed based on simple steady
state theory, this was found to correlate well with
experimental data. The theory could be used for any system
of low thermal storage, including forced flow collectors.

Finally these walls, and others, are compared and
their relative merits discussed.