Rock bed thermal energy storage for solar cooking application (Potential for solar cooking in Uganda)

Abstract

This thesis focus on the development of a rock bed heat storage for storing solar energy for cooking and other applications requiring heat in the range of 1000C to 3500C. The potential of using such a system in Uganda is evaluated and it is observed that the northern part of the country has a promising potential for small domestic systems as well as for other large scale solar industrial applications. The thermal performance of an air-rock based system is investigated experimentally and theoretically using the modified Schumann's one-dimensional energy balance equation. The effects of inlet temperature, air flow rate, particle size on the charging behaviour of the rock storage were investigated. Increasing the bed inlet temperature increases the energy content of the bed but the temperature profile along the bed is similar. Result of varying air flow rate indicates faster bed charging with increased air flow. More stratification occurs with smaller sized particles. The degradation of higher temperature at the bed top with storage time was investigated and it was observed that faster loss in stratification occurred in a highly stratified bed compared to a bed with is almost fully charged. The variation of pressure drop for different rocks sizes with air flow rates is investigated and the experimental results were compared with different pressure drop correlations reported in literature. A strong dependence of pressure drop on air mass flow rates and particle sizes was observed and comparison with other pressure drop results showed the Dunkle's correlation predicting slightly higher values compared to experimental results in all experimental runs. The discharging of a heat bed of rocks was performed by reversing the air flow direction through the bed. The test results showed that faster cooking rate can be achieved using a rock stove and this rate can be control by simply adjusting the air flow speed through the bed. And finally, the energy content of the bed can be enhanced by incorporating phase change materials in a bed of rocks.