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.