Experimental Investigations on Thermal Storage in a Solar Dryer

Abstract –

Thermal energy storage for solar dryer has recently attracted increasing interest as the availability of solar energy is intermittent. The effectiveness of thermal energy storage using Phase Change Material (PCM) in solar dryer application was investigated in this work. A solar cabinet dryer having 3 evenly spaced trays that provide maximum heat flow with drying area of 0.5 m2 (each) that caters to a family was constructed and its performance evaluated. The experiments were done on the dryer with and without thermal storage for 3 days each during the month of April when the sunshine remains considerable in Annamalai Nagar located at 11.39° N Latitude and 79.69° E Longitude. The temperature at different vital locations was recorded from 10 a.m. onwards until the cabinet temperature fell to that of the atmosphere. The mass flow rate of air during experimentation varied between 0.001-0.003 kg/s. A paraffin (n-docosane)-kerosene composite of 12.5 kg and 2:1 mass ratio was used for thermal storage in the dryer. The PCM was filled in 20 numbers of 20 mm diameter aluminum tubes having 500 mm length and 100 aluminum cans of 150 ml capacity (excluding space for phase change). The results were analysed for the variation of thermal absorption and storage based on air velocity, temperature gradient, and solar intensity. The cabinet temperature remained 25-30°C higher than that of atmosphere during mid-day. The peak heat gain fell by 18% on the days of experimentation with thermal storage due to the heat conducted by PCM pipes. On an average, the thermal efficiency of the system with thermal storage improves by 50 % of its efficiency without thermal storage unit. It was found that the dryer with thermal storage can sustain the required temperature for 3 hours after sunshine hours and then it falls gradually.

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INTRODUCTION

In a tropical country like India, the main source of economy and livelihood is agriculture. Even though farmers are trying to increase the crop productivity, it is very difficult to preserve them due to lack of cheaper and effective technology. Thermal energy storage has been an active area of research at present. Efficient alternative energy utilization must involve energy storage to cater to the fluctuations in demand and to obtain a higher performance. Energy storage system assumes greater significance in situations dealing with intermittent supplies of input energy. The selection of thermal storage system for an application depends on factors such as storage duration, economics, temperature supply and utilization requirements, storage capacity, heat losses and space availability. Thus this experimental work has the objectives of finding the supply temperature and thermal storage duration in accordance with the dryer requirements and PCM storage space availability.

PHASE CHANGE MATERIAL (PCM)
A good thermal energy storage medium must remain stable with high heat capacity. The technology of solar thermal storage using sensible heating of a media called as first generation storage systems has developed well and so being utilized commonly. The technology of latent heat based solar thermal storage has good applicability and is being developed by considerable research right now. Latent heat storage systems using phase change materials have high thermal storage capacity when compared with that of sensible heat storage materials. The PCM behavior depends on the environmental conditions of the solar system which involves thermal cycling utilizing the latent heat. Thus one of the best approaches has been to use phase change materials (PCM) for the thermal storage. Thermal energy storage system can accumulate energy as sensible heat or as the heat of fusion or a combination of both. Latent heat storage is better than sensible heat storage as it has high storage density and lesser temperature swings. PCMs can store 2-3 times more heat or cold per volume or per mass, as can be stored as sensible heat in water in a temperature interval of 20◦C.

CONCLUSIONS

The following conclusions have been arrived based on the analysis of experimental results: The paraffin n-docosane is suitable for thermal storage in a solar cabinet dryer. The PCM released its sensible heat very rapidly, and longer time was needed to transfer the latent heat due to frozen layer formation. As the solidification extended, the amount of latent heat transferred to heat transfer fluid became smaller. The melting and solidification of PCM occurred in a phased manner and so heat transfer cycle extended for longer duration. The PCM mixture near the can and pipe walls solidifies sooner than the mixture at the centre of the can. This is due to the thermal resistance offered by the PCM. During solidification process, the solidification front is moving towards outer surface whereas heat transfer occurs in the opposite direction inwards through the already solidified portion. Thus the thermal resistance of wax increases. The composite PCM mix ratio of 2:1 suited well for the application since it melts at a temperature much suitable for solar dryer application in this geographical area. During the phase change the wax melts over a temperature range and it is difficult to find the fraction of wax that melts at a particular temperature. Thus the phase change occurs in fractions which have been proven by the variations in temperature gradient during the thermal cycle. The PCM cans arranged at the base store the heat convected to the bottom side from the absorber thus reducing the bottom side heat loss. The variation of atmospheric air flow because of velocity changes created mass flow rate variations marginally and due to it, uneven temperature rise occurred inside the solar dryer.

Reference And Download report :

http://www.rericjournal.ait.ac.th/index.php/reric/article/download/1420/637