Solar air heating is a solar thermal technology in which the energy from the sun, insolation, is captured by an absorbing medium and used to heat air. Solar air heating is a renewable energy heating technology used to heat or condition air for buildings or process heat applications. It is typically the most cost-effective out of all the solar technologies, especially in commercial and industrial applications, and it addresses the largest usage of building energy in heating climates, which is space heating and industrial process heating. Solar air collectors can be divided into two categories:
Unglazed Air Collectors or Transpired Solar Collector (used primarily to heat ambient air in commercial, industrial, agriculture and process applications) Glazed Solar Collectors (recirculating types that are usually used for space heating)
Collector types Solar collectors for air heat may be classified by their air distribution paths or by their materials, such as glazed or unglazed. For example:
through-pass collectors front-pass back pass combination front and back pass collectors unglazed glazed
Unglazed air collectors and transpired solar collectors
Background The term "unglazed air collector" refers to a solar air heating system that consists of an absorber without any glass or glazing over top. The most common type of unglazed collector on the market is the transpired solar collector. This technology was invented and patented by Canadian engineer John Hollick of Conserval Engineering Inc. in the 1990s, who worked with the U.S. Department of Energy (NREL) and Natural Resources Canada on the commercialization of the technology around the world. The technology has been extensively monitored by these government agencies, and Natural Resources Canada developed the feasibility tool RETScreen to model the energy savings from transpired solar collectors. John Hollick and the transpired solar collector were honored by the American Society of Mechanical Engineers (ASME) in 2014 as being one of the best inventions of the industrialized age, alongside Thomas Edison, Henry Ford, the steam engine and the Panama Canal – in a New York exhibition recognizing the best inventions, inventors and engineering feats of the past two centuries. Several thousand transpired solar collector systems have been installed in a variety of commercial, industrial, institutional, agricultural, and process applications in over 35 countries around the world. The technology was originally used primarily in industrial applications such as manufacturing and assembly plants where there were high ventilation requirements, stratified ceiling heat, and often negative pressure in the building. 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 2-stage system, the first stage is the typical unglazed transpired collector and the second stage has glazing covering the transpired collector. The glazing allows all of that heated air from the first stage to be directed through a second set of transpired collectors for a second stage of solar heating. Another innovation is to recover heat from the photovoltaic (PV) modules (which is often four times more than the electrical energy produced by the PV module) by mounting the PV modules onto the solar air system. In cases where there is a heating requirement, incorporating a solar air component into the PV system provides two technical advantages; it removes the PV heat and allows the PV system to operate closer to its rated efficiency (which is 25 C); and it decreases the total energy payback period associated with the combined system because the heat energy is captured and used to offset conventional heating.
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