Editing Solar thermal collector (section)

==Heating air==
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 [[Industrial processes|process area]] where the heated air is used for space heating or process heating needs. Functioning in a similar manner as a conventional forced-air furnace, solar-thermal-air systems provide heat by circulating air over an energy collecting surface, absorbing the sun's thermal energy, and ducting air coming in contact with it. Simple and effective collectors can be made for a variety of air conditioning and process applications.{{citation needed|date=January 2021}}

Many applications can utilize solar air heat technologies to reduce the carbon footprint from the 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 cogeneration|solar co-generation]]', solar thermal technologies are paired with photovoltaics (PV) to increase the efficiency of the system by taking heat away from the PV collectors, cooling the PV panels to improve their electrical performance while simultaneously warming air for space heating.{{citation needed|date=January 2021}}

===Space heating and ventilating===
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. A pioneering figure of this type of system was George Löf, who built a solar-heated air system in 1945 for a house in Boulder, Colorado. He later included a gravel bed for heat storage.{{citation needed|date=January 2021}}

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.{{citation needed|date=January 2021}}

===Process heating===
Solar air heat is also 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.{{citation needed|date=January 2021}}

High temperature process heat can be produced by a [[solar furnace]].

===Solar air heating collector types===
Collectors are commonly classified by their air-ducting methods as one of three types:
* through-pass collectors
* front-pass
* back pass
* combination front and back pass collectors

Collectors can also be classified by their outer surface:
* glazed
* unglazed

===Through-pass air collector===
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Offering the highest efficiency of any solar technology the through-pass configuration, air ducted onto one side of the absorber passes through a perforated 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.

====Glazed systems====
Glazed systems usually have a transparent top sheet and insulated side and back panels to minimize heat loss to ambient air. The absorber plates in modern panels can have [[Absorbance|absorptivity]] of more than 93%. Glazed Solar Collectors (recirculating types that are usually used for space heating). Air typically passes along the front or back of the absorber plate while scrubbing heat directly from it. Heated air can then be distributed directly for applications such as space heating and drying or may be stored for later use. Payback for glazed solar air heating panels can be less than 9–15 years depending on the fuel being replaced.

====Unglazed systems====
Unglazed systems, or transpired air systems have been used to heat make-up or ventilation air in commercial, industrial, agriculture and process applications. They consist of an absorber plate which air passes across or through as it scrubs heat from the absorber. Non-transparent glazing materials are less expensive and decrease expected payback periods. Transpired collectors are considered "unglazed" because their collector surfaces are exposed to the elements, are often not transparent and not hermetically sealed.

===Unglazed transpired solar collectors===
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====Background====
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. 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 countries around the world. This 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. 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 750 peak thermal Watts/square metre), high solar conversion (up to 90%) and lower capital costs when compared against solar photovoltaic and solar water heating.

[[Image:Transpired Air Collector.PNG|thumb|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 of all the solar technologies, especially in large scale applications, and it addresses the largest usage of building energy in heating climates, which is space heating and industrial process heating. They are either glazed or unglazed.

====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 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.

Hot air that may enter an HVAC system connected to a transpired collector that has air outlets positioned along the top of the collector, particularly if the collector is west facing. To counter this problem, Matrix Energy has patented a transpired collector with a lower air outlet position and perforated cavity framing to perpetrate increased air turbulence behind the perforated absorber for increased performance.

This cutaway view shows the SolarWall transpired solar collector components and air flow. The lower air inlet mitigates the intake of heated air to the HVAC system during summer operation.

The extensive monitoring by Natural Resources Canada and NREL has shown that transpired solar collector systems reduce between 10 and 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 no suitable south-facing wall or for other architectural considerations. 

Each ten-foot (3.05 m) module will deliver 250 CFM (425 m3/h)of preheated fresh air typically providing annual energy savings of 1100 kWh (4 GJ) annually. This unique two-stage, modular roof-mounted transpired collector operating a nearly 90% efficiency each module delivering over 118 L/s of preheated air per two square meter collector. Up to seven collectors may be connected in series in one row, with no limit to the number of rows connected in parallel along one central duct typically yielding 4 CFM of preheated air per square foot of available roof area.

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.