Researchers from Catholic University of Leuven (Belgium), Gabès National Engineering School (Tunisia) and the North-Western University (South Africa) created a new type of solar air heater, which is about three times cheaper than the existing analogues. Such installations may be used for dehumidification of agricultural products, green-houses heating, in-door ventilation (e.g., in shops) – everywhere when there is a need for inexpensive and stable heating.
The design of this new appliance is pretty simple: a metallic plated heated by sunlight is covered by glass on the top, and a channel runs under it, through which a fan blows the air. However, these are the details that make this installation unique. The scientists welded big V-shaped crossbars on the absorber’s surface – steel plates simple in form positioned angularly to each other. They blow the air, create vortices and increase the baffling, due to which the air stays longer inside the channel and is heated more efficiently.
Similar are used in other types of solar collectors, but usually their manufacturing is complicated and they increase flow resistance significantly. In the new design such crossbars are bigger and are positioned more rarely (from 4 to 8 crossbars in total), hence, the pressure losses remain minimal, and the heat exchange becomes better significantly. All the elements of the installation may be manufactured from easily accessible materials – steel, glass and timber, and a small electrical fan similar to those used for cooling the desktops will work.
The tests were performed in the lab in Gabès given the air velocity of 2.7 m/s and the solar radiation of about 1,100 W/m². In the base version (without the crossbars) the output air was heated from 22 °C to 38 °C. When four bars were added, the temperature grew up to 45 °C, when there were six crossbars – up to 52 °C, and when there were eight of them – up to 55 °C, i.e., 33 °C higher than the input temperature. At the same time, the pressure losses in the channel grew only by fractions of pascal, which did not affect the fan operation.
The calculations performed by ANSYS Fluent software with account of the turbulence and solar radiation fully confirmed the test results – the deviation did not exceed 3%.
The configuration with eight crossbars slightly off-center demonstrated the best results. Such asymmetry created soft baffling of the air flow and provided for a more even heating inside the installation. In this option, the fuel efficiency reached 73%, i.e., almost double vs the standard solar air heaters without crossbars.
The full assembly of elements cost approximately USD 75, i.e., USD 118 per one square meter of active area. Compare with USD 300 and higher per one square meter in commercial solar air heaters of similar capacity.
Hence, the Belgian and African researchers demonstrated that you can improve the efficiency of energy systems not only by applying expensive nanocoating or complex technologies. Well-thought-through geometry and accurate engineering calculations will be enough.
