The first unglazed transpired collector in the world was installed by Ford Motor Company on their assembly plant in Oakville, Canada. With the increasing drive to install renewable energy systems on buildings, transpired solar collectors are now used across the entire building stock because of high energy production (up to 500-600 peak thermal Watts/square metre), high solar conversion (up to 90%) and lower capital costs when compared against solar photovoltaic and solar water heating. Method of operation Unglazed air collectors heat ambient (outside) air instead of recirculated building air. Transpired solar collectors are usually wall-mounted to capture the lower sun angle in the winter heating months as well as sun reflection off the snow and achieve their optimum performance and return on investment when operating at flow rates of between 4 and 8 CFM per square foot (72 to 144 m3/h.m2) of collector area. The exterior surface of a transpired solar collector consists of thousands of tiny micro-perforations that allow the boundary layer of heat to be captured and uniformly drawn into an air cavity behind the exterior panels. This solar heated ventilation air is drawn into the building’s ventilation system from air outlets positioned along the top of the collector and the air is then distributed in the building via conventional means or using a solar ducting system. The extensive monitoring by Natural Resources Canada and NREL has shown that transpired solar collector systems reduce between 10-50% of the conventional heating load and that RETScreen is an accurate predictor of system performance. Transpired solar collectors act as a rainscreen and they also capture heat loss escaping from the building envelope which is collected in the collector air cavity and drawn back into the ventilation system. There is no maintenance required with solar air heating systems and the expected lifespan is over 30 years. Variations of transpired solar collectors Unglazed transpired collectors can also be roof-mounted for applications in which there is not a suitable south facing wall or for other architectural considerations. A number of companies offer transpired air collectors suitable for roof mounting either mounted directly onto a sloped metal roof or as modules affixed to ducts and connected to nearby fans and HVAC units. Higher temperatures are also possible with transpired collectors which can be configured to heat the air twice to increase the temperature rise making it suitable for space heating of larger buildings. In a