Solar air heat
Solar air heating is a solar thermal technology in which the energy from the sun, solar 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 commonly 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)
- 1 Unglazed air collectors and transpired solar collectors
- 2 Glazed air systems
- 3 See also
- 4 References
Unglazed air collectors and transpired solar collectors
The term "unglazed air collector" refers to a solar air heating system that consists of a metal 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 as SolarWall by 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.
Since that time, 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.
The SolarWall transpired collector technology and inventor John Hollick were honoured in 2014 by the American Society of Mechanical Engineers (ASME). They featured the 80 best inventions, inventors and engineering feats of the past two centuries, including Edison, Ford, Westinghouse, Carrier, the steam engine and the Panama Canal in an exhibit entitled Engineering the Everyday and the Extraordinary”. ASME focused on nine categories of engineering: Environment, Food, Safety, Manufacturing, Energy & Power, Transportation, Health, Exploration and Communication. The SolarWall technology and John Hollick were featured in the Energy & Power category.
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. For lower flow rates and higher delivered temperatures, a 2-stage solar air heating system is recommended.
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 heated ventilation air is drawn under negative pressure into the building’s ventilation system where it is then distributed 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. In these cases the heat from the system is ducted along the roof to the nearest air handling unit.
Transpired collectors can be configured to heat the air twice to increase the delivered air temperature making it suitable for space heating applications as well as ventilation air heating. 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.
Transpired solar collectors can also be combined with photovoltaic (PV) panels to create a hybrid solar system. The heat from the back of the PV modules (which is often four times more than the electrical energy produced by the PV module) is removed by the solar air system (which doubles as the PV racking system) and is used for building heating purposes. 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.
Conserval Engineering has recently developed and patented a transpired solar collector for nighttime cooling. Radiation cooling to the night sky is based on the principle of heat loss by long-wave radiation from a warm surface (roof) to another body at a lower temperature (sky). On a clear night, a typical sky-facing surface can cool at a rate of about 75 W/m2 (25 BTU/hr/ft2) This means that a metal roof facing the sky will be colder than the surrounding air temperature. A modified transpired collector can take advantage of this previously ignored cooling phenomena. As warm night air touches the cooler surface of the transpired collector, the heat is transferred to the metal, radiated to the sky and the cooled air is then drawn in through the perforated surface. The cool air is then drawn into the economizer cycle typically found on newer HVAC units. 
Glazed air systems
Functioning in a similar manner as a conventional forced air furnace, systems provide heat by recirculating conditioned building air through solar collectors. Through the use of an energy collecting surface to absorb the sun’s thermal energy, and ducting air to come in contact with it, a simple and effective collector can be made for a variety of air conditioning and process applications.
A simple solar air collector consists of an absorber material, sometimes having a selective surface, to capture radiation from the sun and transfers this thermal energy to air via conduction heat transfer. This heated air is then ducted to the building space or to the process area where the heated air is used for space heating or process heating needs.
Due to varying air-ducting methods, collectors are commonly classified as one of three types:
- a) through-pass collectors,
- b) front-pass,
- c) back pass,
- d) combination front and back pass collectors.
Through-pass air collector
In the through-pass configuration, air ducted onto one side of the absorber passes through a perforated or fibrous type material and is heated from the conductive properties of the material and the convective properties of the moving air. Through-pass absorbers have the most surface area which enables relatively high conductive heat transfer rates, but significant pressure drop can require greater fan power, and deterioration of certain absorber material after many years of solar radiation exposure can additionally create problems with air quality and performance.
Back, front, and combination passage air collector
In back-pass, front-pass, and combination type configurations the air is directed on either the back, the front, or on both sides of the absorber to be heated from the return to the supply ducting headers. Although passing the air on both sides of the absorber will provide a greater surface area for conductive heat transfer, issues with dust (fouling) can arise from passing air on the front side of the absorber which reduces absorber efficiency by limiting the amount of sunlight received. In cold climates, air passing next to the glazing will additionally cause greater heat loss, resulting in lower overall performance of the collector.
Air heat applications
A variety of applications can utilize solar air heat technologies to reduce the carbon footprint from use of conventional heat sources, such as fossil fuels, to create a sustainable means to produce thermal energy. Applications such as space heating, greenhouse season extension, pre-heating ventilation makeup air, or process heat can be addressed by solar air heat devices. In the field of ‘solar co-generation’ solar thermal technologies are paired with photovoltaics (PV) to increase the efficiency of the system by cooling the PV panels to improve their electrical performance while simultaneously warming air for space heating.
Space heating applications
Space heating for residential and commercial applications can be done through the use of solar air heating panels. This configuration operates by drawing air from the building envelope or from the outdoor environment and passing it through the collector where the air warms via conduction from the absorber and is then supplied to the living or working space by either passive means or with the assistance of a fan.
Ventilation, fresh air or makeup air is required in most commercial, industrial and institutional buildings to meet code requirements. By drawing air through a properly designed unglazed transpired air collector or an air heater the solar heated fresh air can reduce the heating load during daytime operation. Many applications are now being installed where the transpired collector preheats the fresh air entering a heat recovery ventilator to reduce the defrost time of HRV's. The higher your ventilation and temperature the better your payback time will be.
Process heat applications
Solar air heat can also be used in process applications such as drying laundry, crops (i.e. tea, corn, coffee) and other drying applications. Air heated through a solar collector and then passed over a medium to be dried can provide an efficient means by which to reduce the moisture content of the material.
- Active solar
- Passive solar building design
- Heat recovery ventilation
- Passive house
- Low-energy house
- Zero-energy building
- List of low-energy building techniques
- Sustainable architecture
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