The performance of solar air heater with high density polyethylene sheet as top cover and sand layer as an absorber

Abstract

Agricultural products especially vegetables and tobacco loose nutrients due to direct sun drying method Aflatoxins infect cereals including maize due to high moisture content during harvesting and storing in most maize growing parts in the country. A Solar Air Heater (SAH) mainly consists of a panel, insulated hot air ducts and air blowers for an active system. The panel consists of an absorber plate themtally insulated from the bottom and sides, and a glass or plastic cover is fixed above the absorber plate to form a passage for air flow. The main applications of Solar Air Heaters (SAHs) are space heating, drying and paint spraying operations. Factors that affect SAH efficiency include collector length, collector depth, type of absorber plate, top cover plate and wind speed. The main objectives of this research were to construct and determine the efficiency of a SAH with HDPE sheet as the top cover and brown sand layer as an absorber plate with an aim of increasing the efficiency from the current 52% to a value above this. The SAH was constructed and placed outdoors for data collection and tests were conducted in an open field (to avoid shading from trees and buildings) between 0900 and 1500 Hrs. and data was recorded after every 20 minutes for 100 hours between July and August, 2011. Results show that, among the three samples investigated, (HDPE, LDPE and glass), clear HDPE sheet has the highest transmittance value of 0. 84 at wavelength 791.90 nm followed by LDPE which had 0.72 at 820.89 nrn and glass which had 0.72 at 735.02 nm. The average absorptances of brown and grey sand samples were found to be 59% and 56% respectively with a significant value of P less than 0.05 which indicates that there is significant difference between the two means. The average daily solar radiation was obtained as 841 W/ml with a maximum of 9S6Wlmz and a minimum of 529 W/ml which increases in the morning to a peak at 1300 Hrs. and starts to decrease afterwards. The average ambient temperature obtained was 23°C with 36°C as the highest The average temperature increase (AT) through the heater was found to be 17°C with a nnstirnurn of 31°C at 6_.83xl0’6 Kg/s mass flow which occurred at 1240 Hrs. The elficiency of the SAH was found to be 54% which occurred 20 minutes after start of experiment The efficiency increased with increasing air mass flow rate and decreased as the temperature |mameter increased The values of Collector heat removal factor (FR) and Collector overall heat Ins eoetiicient (UL) were found to be 0.84 and 7.98 W/m2K respectively. Results show that the heat loss coefiicient decreases with decreasing air mass flow rate.