School of Engineering, Sustainable Energy Centre, University of South Australia, Mawson Lakes Boulevard, Mawson Lakes, South Australia, 5095, Australia, ph: +61 (0)8 8302 3497, fx: +61 (0)8 8302 3380, email: martin.belusko@unisa.edu.au

Solar heating and natural cooling systems have often proved unacceptable to the domestic market. High initial cost, long payback periods and their aesthetics have all hindered the commercial success of such systems. Domestic energy demand for heating and cooling in Australia is approximately 40% of total energy consumption. Previous systems have focused on meeting most of this demand irrespective of cost. By developing a system which performs less effectively but is significantly less expensive, it is possible to have a system which is economical and significantly reduces the dependence on conventional heating and cooling systems.

The paper examines current steel roof constructions and ways for integrating them into a solar heating and natural cooling system. A roof integrated unglazed solar collector operating in conjunction with a phase change thermal storage device, can function as a solar heating and natural cooling system. These features lend themselves to a system which not only can be economical but which does not alter the appearance of a residential home. Not meeting the total demand, the system operates together with conventional heating and cooling systems.

Typical steel roofs consist of corrugated colour bonded steel attached to a moisture barrier with insulation on the roof cavity floor. The resulting arrangement is a collector of large area with a reasonable amount of back insulation (see Figure). In winter, although less efficient than a typical glazed air collector, using the entire roof area enables large quantities of low grade heat to be collected and stored in the thermal storage unit.

Summer operation is based on reducing the cooling load by venting the roof cavity during the day and storing the coolness from radiatively cooled air in the thermal storage unit during the night. Venting the roof can reduce the cooling load by up to 15 %. The current natural ventilation methods have been shown not to be effective. Consequently forced venting is the best alternative.

Investigations into the various options for economically improving the efficiency of the collector have revealed that the current design is the most promising. Research has shown that a transpired collector can significantly improve the performance of a solar air collector. When applied, it can reduce wind losses as well as improve local heat transfer coefficients. However due to the geometry of the roof integrated collector, transpiration was found not to be of benefit.

A performance analysis of the system based on experimentation highlighted that the roof integrated heating and cooling system has the potential to meet approximately 50% of domestic demand. The paper also discusses the economic analysis carried out for the system.

Keywords: solar collector, roof integrated, transpiration, heating, cooling