Thermal performance of a double-pass solar air heater (SAH) with ribbed absorber surface – an experimental study

Abstract

Purpose Use of packed beds, enhanced tubes, nano-fluids and artificial ribs are few passive techniques to increase heat transfer in solar air heaters (SAHs). Artificial ribs attached to the absorber plate of the SAH will enhance the turbulence near the plate. Experimental analyses are conducted to find the thermal performance of SAH with ribs of regular geometries including rectangular, semi-circular and triangular in cross section. This paper aims to present the improvement in thermal performance of SAH with modified-arc. Design/methodology/approach Absorber plates are designed with ribs of rectangular, triangular, semi-circular and modified-arc in cross-section using existing data in literature. Physical dimensions of the ribs are designed by adapting procedure from literature. Absorber plates are manufactured with ribs and coated with blackboard paint and fixed to the existing SAH. Experiments are conducted with a variable-speed blower fixed to the inlet section of the SAH, which is used to supply air at different mass flow rates in a range between 0.495 and 0.557 kg/min. Findings Efficiency is found to be a strong function of mass flow rate of air through the SAH from the present experimental investigations. It was found that use of modified-arc ribs enhanced the efficiency of SAH by 105.35 per cent compared to SAH with plane absorber plate. Efficiency of SAH with modified-arc ribs is found to be higher by 24.43, 45.61 and 63.21 per cent, respectively, for SAH with semi-circular, rectangular and triangular arc ribs on its absorber plate. Research limitations/implications Experiments on SAH are conducted during daytime from 9:00 am to 5:00 pm in open atmospheric conditions. Solar intensity is continuously changing during the experimentation from morning to evening. Calculations are made based on the observations with average values of solar intensity and temperature readings. More accurate values of SAH efficiency can be obtained with constant heat supply to the absorber plate by simulating the experimental setup in indoor conditions. Temperature and flow rate observations could be more accurate with sophisticated instrumentation rather than using simple thermocouples and orifice meters. Social implications SAHs are basically used to supply hot air for both rural and industrial applications. These are used for crop drying, preheating of air, removal of moisture from leather, chemicals, etc. Conventionally, formers in India are using open sun drying to remove moisture from agricultural products. In this method, the moisture can be removed up to a level of 20 to 25 per cent. Use of SAH can remove moisture up to below 5 per cent and process is clean without reducing the quality of agricultural products. Enhancing the efficiency of SAHs will surely increase its usage by formers for crop drying. Originality/value Use of artificial ribs on absorber plate of SAH is most economical among many of the active and passive techniques. Numerical and experimental investigations are found in literature with regular cross-sectional ribs, including rectangular, triangular and semi-circular. The present work proposed new shape of the ribs named as modified-arc, which was not presented in the literature. Experimental analysis proved that the use of modified-arc makes the SAH more efficient in heat transfer.

Experimental study on solar air heater with encapsulated phase change material on its absorber plate

“…Ribs on absorber plate creates more turbulence near the plate and hence local heat transfer coefficients increases with simultaneous increase in efficiency of SAH. Gupta and Garg, 10 Esen, 11 El-Sebaii et al, 12,13 and Nandkishor and Mohan 14 verify this fact experimentally.…”

A comprehensive review of experimental investigation procedures and thermal performance enhancement techniques of solar air heaters

“…Artificial roughness of various forms and geometrical shapes investigated by different researchers is reported in the literature. The use of rib roughness in numerous shapes and geometries, such as square, 88,90,[103][104][105] triangular, 92,95,106,107 rectangular, 106,107 inclined, 93 V-shaped, 89,108 W-shaped, 94 discrete, 93,94,109 chamfered, 88 wedge shaped, 107 arc shaped, 97,106,110,111 L shaped, 96 U shaped, 91 porous rib type roughness, 112 and pentagonal ribs 98 result a better performance compared to conventional-type solar air collector. Many of the investigators reported considerable enhancement in heat transfer with less penalty of friction by the application of…”

“…Geometry of different rib profiles 106 medium. 121,124,126 Therefore, it is concluded from most of the studies that incorporating paraffin wax as heat storage material reduced the discrepancy between the energy supply and demand and enhancement in energy efficiency by providing thermal stability.…”

The thermal impact of stored water as supplementary energy source for a solar air heater

“…It has been determined that surface roughness plays a significant role in heat transfer and friction behaviors in solar air heater (SAH) ducts. Various approaches have explored the use of surface roughness created by repeating ribs in the same pattern on the SAH duct's surface as an effective means of increasing system efficiency 15,21 . Implementing fins or ribs on the surface disrupts the formation of a laminar sub‐layer, which contributes to overall thermal losses and reduces efficiency 22–28 …”

“…Various approaches have explored the use of surface roughness created by repeating ribs in the same pattern on the SAH duct's surface as an effective means of increasing system efficiency. 15,21 Implementing fins or ribs on the surface disrupts the formation of a laminar sub-layer, which contributes to overall thermal losses and reduces efficiency. [22][23][24][25][26][27][28] Although SAHs suffer from poor thermal efficiency due to heat loss from various parts and low heat transfer coefficient, they remain the most cost-effective system of converting solar energy into heat.…”