Typical Air Collectors or Solar Air Heaters

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Typical Air Collectors or Solar Air Heaters:

Typical Air Collectors or Solar Air Heaters

Slide 2:

Fig 3.34 shows a schematic flat-plate collector where an air stream is heated by the back side of the collector plate.Fins attached to the plate increase the contact surface.The back side of the collector is heavily insulated with mineral wool or some other material. The use of air as the heat-transport fluid eliminates both freezing and corrosion problems,and small air leaks are of less concern than water leaks. Moreover ,the heated air can be used directly for space heating.The drawback is that transfer of heat from air to water in a hot water supply system is inefficient. But solar air heater has an important place among solar heat collectors.It can be used as subsystems in many systems meant for the utilization of solar energy. Possible applications of solar air heaters are drying of agricultural products,curing of industrial products such as plastics, etc.

Slide 3:

Basically air heaters are classified in the following two categories Non-porous absorber plate type collectors. Collectors with porous absorbers. Non-porous absorber plate type collectors. In this the air stream does not flow through the absorber plate.Air may flow above and or behind the absorber plate. A non porous absorber may be cooled by the air stream flowing over both sides the plate.[fig 3.36(a)] In most common design the air flows behind the absorbing surface.Air flow above the upper surface increases the convention losses from the cover plate and therefore is not recommended if the air inlet temperature rise at the collector are large .[fig 3.36(b)] Due to low heat transfer rate, efficiencies are lower than liquid solar heaters under the same radiation intensity and temperature conditions.

Slide 4:

Performance of air heaters is improved by: Roughning the rear of the plate to promote turbulence and improve the convective heat transfer coefficeint , OR Adding fins to increase the heat transfer surface . 2) Collectors with porous absorbers. The main drawback of the non-porous absorber plate is the necessity of absorbing all incoming radiation over the projected area from a thin layer over the surface , which is in the order of few microns. These defects are eliminated in a porus absorber type collectors in two ways The solar radiation penetrates to greater depths and is absorbed gradually depending on the matrix density. Any improper selection of the matrix and the thickness may result in reduced efficiencies.

Slide 5:

The pressure drop for the matrix is usually lower than the non-porous absorber with flow behind the plate since flow per unit cross-section would be much lower. The solar air heating utilizing a transpired honey comb [ fig 3.37 (b)] is also very favourable from the pressure drop stand point since the flow cross-section is much larger. There is a method of using crushed glass layers to absorb solar radiation and heat the air. A porous bed,as shown in fig 3.37(c) made by forming layers of broken bottles ,may be readily used for agriculture drying purposes with minimal expenditure. The overlapped glass plate air heater as shown in fig 3.37(d) can be considered as a form of porous matrix,although overall flow direction is along the absorber glass plates instead of being across the matrix.

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