Influence of air temperature, flow rate and sand layer thickness on the rate of charging sand

Abstract

Solar thermal energy storage technology has made solar energy a potentially viable supplement for fossil fuels in much of the developing world. However one of the challenges in adopting the technology is the low efficiency of the storage media. Many thermal energy storage materials have difficulties and limitations such as handling, containment, storage and cost. Sand which is abundant, cheap, easy to handle and contain, can be used to alleviate the difficulties. Research carried out on sand as thermal energy storage material has been concentrated on the amount that can be stored. Literature on factors that influence the rate of charging sand thermal energy storage media could not be accessed. There is no existing information on the rate of charging various types of sand. The objective of this research was to determine the influence of air temperature and flow rate on the rate of charging different types of sand of different layer thicknesses. Sieve analysis was done on the sands to grade them in terms of particle size distribution. Air at temperatures of 40, 50, 60 and 70oC and flow rates of 0.0004, 0.0006, 0.0008 and 0.001m3/s was passed through a thermal energy storage tank containing four types of sand which were collected from Machakos, Mombasa, Kisumu and Nakuru. However electric heater was used to heat the air instead of a solar heating system. The Taguchi experimental design approach was used. The temperature rise for sand at intervals of 2.5 minutes was recorded. Data analysis was carried out by use of Excel Statistical Analysis Software (SAS). The result of the study showed that the rate of charging sand increases as temperature of air increases. The result also showed that the rate of charging sand increases slightly as flow rate of charging air increases. The rate of charging was inversely proportional to the sand layer thickness. The four types of sand showed different rate of charging with sand from Mombasa having the greatest charging rate at 1.14oC/min. This was followed by sand from Nakuru at a rate of 1.12oC/min, sand from Kisumu at 1.01oC/min and finally sand from Machakos at 0.9oC/min. The most influential factors on the rate of charging were sand layer thickness and charging air temperature and contributed 86.93% and 10.00% of the variation in the charging rate respectively. The air flow rate and type of sand had little influence on charging rate and contributed 2.23% and 0.49% of charging rate variation respectively. The optimum factors for the charging process were observed to be charging air temperature of 70oC, sand type from Mombasa, sand layer thickness of 0.01cm and flow rate of 0.001m3/s. A confirmation experiment proved the result to be correct by registering a charging rate of 3.60oC/min.