Mechanical Engineering : Solar Air Heater Classification and Advantages

This post provides the description and analysis of different types of solar air heaters used in space heating and drying purposes. The Solar air heaters have the following advantages over other solar heat collectors.

Solar Air Heater Advantages

The need to transfer heat from working fluids to another fluid is eliminated as air is being used directly as the working substance. The system is compact and less complicated.
Corrosion is a great problem in solar water heater. And this problem is not experienced in solar air heaters.
Leakage of air from the duct does not create any problem.
Freezing of working fluid virtually does not exist.
The pressure inside the collector does not become very high.
Thus air heater can be designed using cheaper as well as lesser amount of material and it is simpler to use than the solar water heaters.

Solar Air Heater Disadvantages

Air heaters have certain disadvantages also the first and foremost is the poor heat transfer properties of air. Special care is required to improve the heat transfer.
Another disadvantage is the need for handling large volume of air due to its low density.
Air cannot be used as a storage fluid because of its low thermal capacity.
In the absence of proper design the cost of solar air heaters can be very high.

Solar Air Heater Applications

The applicability of the solar air heater depends on various factors like high efficiency, low fabrication cost, low installation and operational cost and some other specific factors regarding specific uses. Extensive work in solar air heaters has been done. Various geometries have been proposed and their theoretical investigation is carried out. But it needs commercial exploitation.

Solar Air Heater Classification

A conventional solar air heater is essentially a flat plate collector with absorber plate, a transparent cover system at the top and insulation at the bottom and on the sides. The whole assembly is encased in a sheet metal container. The working fluid is air, though the passage for its flow varies according to the type of air heater.
Material for construction of air heaters are similar to those of liquid flat plate collectors. The transmission of solar radiation through the cover system and its subsequent absorption in the absorber plate can be given by expressions identical to that of liquid flat plate collectors. Selective coating on the absorber plate can be used to improve the collection efficiency but cost effectiveness criterion should be kept in mind.

Non-porous Type solar air heater

In non-porous type, air stream does not flow through below the absorber plate but air may flow above and/or behind the plate.

In first type, no separate passage is required and the air can flows between the transparent cover system and the absorber plate. ( see the figure). In this heater as the hot air flows above the absorber, the cover receives much of the heat and in turn, loses it to the ambient. Thus a substantial amount of heat is lost to the ambient and hence this air heater is not recommended.
The non-porous type with air passage below the absorber is most commonly used. A plate parallel to the absorber plate is provided in between the absorber and the insulation, thus forming a passage of high aspect ratio.
In another variety of non-porous type air heater, the absorber plate is cooled by air stream flowing on both sides of the plate.

Depending on the type of the absorber plate, the air heater can be non-porous and porous. Figure below shows the non-porous absorber type air heaters.

It may be noted that the heat transfer between the absorber plate and the flowing air being low, the efficiency of air heaters is less. The performance, however, can be improved by roughening the absorber surface or by using a vee-corrugated plate as the absorber plate. Turbulence induced to the air flow helps increase the convective heat transfer.
The radiative loss from the absorber plate are significant, unless selective coatings are used, decreasing the collector efficiency. Also, the uses of fin may result in a prohibitive pressure drop, thus limiting the applicability of non-porous type.

Porous type solar air heater

The second type of air heaters has porous absorber which may include slit and expanded metal, overlapped glass plat absorber and transpired honeycomb.

The sir heater with porous type of absorber has the following advantages:

Advantages of porous solar air heater

Solar radiation penetrates to a great depth and is absorbed along its path. Thus the radiation loss decreases. Air stream heats up as it passes through the matrix.

The pressure drop is usually lower than the non-porous type.

It may be noted however, that an improper choice of matrix porosity and thickness may cause reduction in efficiencies as beyond an optimum thickness, matrix may not be hot enough to transfer the heat to air stream.

Wire mesh porous bed formed by broken bottles and overlapped glass plate are some examples of porous type absorbers used in Solar air heaters.