Various Designs of Solar Collectors for Enhancement of Thermal Performance

Two Pass Air Flow Type Collector

A two-pass solar air heater consist of two glass covers, an absorber plate, and a rear plate. The air is first made to pass through the channel formed by two glass covers and then through the channel formed by the rear plate and the absorber plate.

This type of collector is not suitable when the incoming air is at temperature well above the atmospheric temperature because air flowing on the top side of absorber plate may cause substantial loss from the inlet air to ambient by way of convective heat transfer to the inner glass cover and subsequently out to ambient. The dust in the air, if any, will build up on the inside surface of the temperature cover thus reducing the transmittance of the cover.

Absorber Plate with Selective Surfaces

It is desirable to obtain absorber plate surface with the combination of high absorbance for solar radiations and low emittance for long wave radiations. It is possible to achieve this combination of properties because there is very little overlap in wavelength ranges of the solar radiations and long wave radiations. Coatings having high absorption for solar radiations and low emittance for long wave radiations can be applied to substrates. The properties of surface coatings used in the flat plate collectors. These are “nickel black” on galvanized iron, “copper black” on copper, ebanol copper on copper and “black chrome” plating on nickel plated steel. The experimental coatings reported are copper oxide on anodized aluminium or nickel, interference layers of aluminium-molybdenum compounds on molybdenum and lead sulphide crystals on aluminium. Absorber plate with highly selective surfaces requires only one transparent cover even at fairly high operating temperatures.

Tank Type Collector

In an Integral storage collector unit, the hot water storage tank is the solar absorber. The tank or tanks are mounted on the insulation box with glazing on one side and are painted with black or coated with a selective surface. The sun shines through the glazing and hits the black tank, warming the water inside the tank. The single tanks are typically made of steel, while the tubes are typically made of copper. Achievable temperature with such collectors is a little bit less then in flat-plate collectors.

Pool Collector

The single largest application of active solar heating systems is in heating swimming pools. Special collectors have been developed for heating seasonal swimming pools: they are unglazed and made of special copolymer plastic. These collectors cannot 25 With-stand freezing conditions. Approximate maximum operating temperature of such type of solar collector is 10-20 °C above ambience.

Figure 2: Different types of liquid and air heating solar collectors

Click on image to enlarge

Evacuated Tube Collectors

Evacuated tube collectors are mostly used to heat water in residential applications that require higher temperatures. Sunlight enters through the outer glass tube and strikes the absorber tube(s) and changes to heat. The heat is transferred to the liquid flowing through the absorber tube. The collector consists of rows of parallel transparent glass. The tubes are designed in such a way that air is evacuated from the space between the two tubes forming a vacuum. Conductive and convective heat losses are eliminated because there is no air to conduct heat nor to circulate and cause convective losses.

Performance of solar collectors

The investigation performance of double pass-finned plate solar air heater has been done by The results of A.A. El-Sebaii et al.⁴ showed that the double pass v-corrugated plate solar air heater is 9.3–11.9% more efficient compared to the double pass-finned plate solar air heater. It was also indicated that the peak values of the thermo hydraulic efficiencies of the double pass-finned and v-corrugated plate solar air heaters were obtained when the mass flow rates of the flowing air equal 0.0125 and 0.0225 kg/s, respectively.

Evaluation of thermal performance of wire mesh solar air heater by P. Velmurugan et al.,⁵ employing a low carbon steel wire mesh observed 5% increase in overall efficiency is observed when compared with conventional system. Mass flow rate influences the thermal efficiency of the air heater. Solar intensity has no effect on thermal efficiency.

Optimization of bed parameters for packed bed solar energy collection system has been carried out by B. Paul et al.,.⁶ In this work two types of packed bed collectors, one with wire mesh screen matrix bed and other with pebble bed, were optimized on the basis of minimum cost per unit energy delivered.

Five different shapes of elements of storage material have been investigated by Ranjit Singh et al.,.⁷ Effect of system and operating parameters on heat transfer and pressure drop characteristics of packed bed solar energy storage system with large sized elements of storage material has been done Correlations have been developed for Nusselt number and friction factor as function of Reynolds number, spheriocity and void fraction

Thermo hydraulic investigations on a packed bed solar air heater having its duct packed with blackened wire screen matrices of different geometrical parameters (wire diameter and pitch) has been done by M.K. Mittal et al.⁸ Based on energy transfer mechanism in the bed, a mathematical model is developed to compute effective efficiency. A design criterion is also suggested to select a matrix for packing the air flow duct of a solar air heater which results in the best thermal efficiency with minimum pumping power penalty.

In an experimental investigation carried out by L. Varshney et al.,⁹ on heat transfer and fluid flow characteristics of a solar air heater having its duct packed with wire mesh screen matrices. The investigations covered a wide range of geometrical parameters of wire mesh screen matrix (wire diameter, pitch and number of layers). Correlations developed for Colburn j-factor and friction factor indicate that heat transfer coefficient and friction factor are strong functions of geometrical parameters of matrices.

N.S. Thakur et al.,¹⁰ done experimental investigation on a low porosity packed bed solar air heater covering a wide range of geometrical parameters of wire screen matrix, i.e. wire diameter 0.795 to 1.40 mm, pitch 2.50 to 3.19 mm and number of layers from 5 to 12. It is observed that both the heat transfer coefficient and the friction factor are strong functions of geometrical parameters of the porous packed bed. A decrease in porosity increases the volumetric heat transfer coefficient.

The packed bed type solar air heater can be successfully used to enhance the heat transfer rate. The high heat transfer area to volume ratio promotes heat transfer capability and the turbulence producing airflow path through the bed provide for a rapid increase of heat exchange. Porosity strongly influences the heat transfer coefficient and hence thermal performance of packed bed solar air heater. It can also be concluded that a considerable enhancement of heat transfer can be obtained from the packed bed solar air heater by lowering the porosity of the bed. The porosity of the bed governs the performance of packed bed solar air heater at a given Reynolds number, the porosity and area to volume ratio are the key parameters for heat transfer. The Reynolds number also has been found to be a strong parameters affecting heat transfer and friction factor for packed bed solar heater.

References