Ashmore Mawire

Thermal insulating materials are very essential in thermal energy storage. There are both imported and locally/homemade thermal insulation materials. However, the imported thermal insulation materials are expensive compared to the locally made ones due to the high importation cost and taxes imposed on them. This paper aims to compare two thermal insulators for domestic solar heating applications. The two insulators are a wonderbag (imported from South Africa) and a hay basket (locally made in Uganda). Two identical cooking pots with a capacity of 1 litre each are separately placed on identical Photovoltaic Direct Current (PV DC) cookers inside a wonderbag and hay basket, respectively. Water heating experiments are performed in the morning and noon. Results show that when a 0.5 kg water load is used, a higher maximum temperature of 96.3 oC is achieved by the water in a wonderbag insulator compared to 94.8 oC achieved for a hay basket, within 24 min of the experiments during solar noon. A slightly higher water heating efficiency of 3.9% is shown when in the wonderbag compared to that of 3.7% attained when the hay basket is used. No significant difference in the maximum water load temperatures and heating efficiencies are observed when using a wonderbag and hay basket as thermal insulating materials for domestic solar cooking applications during solar noon. Longer heating times are taken during morning experiments (32 – 34 min) than during noon experiment (26 – 30 min). The work demonstrates that a cheaply available and locally made heat retention vessel (hay basket) can substitute the imported and expensive heat retention vessel (wonderbag). The experimental setup can be used to introduce the ideas of heat retention, solar cooking, and photovoltaics to senior undergraduate students pursuing courses in thermal physics and semiconductor physics, respectively.

In this article, a comparison is presented between a pure latent heat storage system (erythritol) and a mixed storage system consisting of equal mass ratios of erythritol and granite pebbles (5 – 10 mm) for a solar cooking application. Two small black stainless cooking pots with a capacity of 1 L were placed inside two larger 5 L cooking pots to form simple storage cooking pots. The space between the pots was filled with thermal energy storage (TES) material. In the first configuration, the space between the pots was filled with 2 kg of erythritol. In the second configuration, the storage system consisted of 1 kg of erythritol and 1 kg of granite pebbles in the same space. The first experimental tests involved charging the storage cooking pots without any load for 4 h, followed by discharging them using heating loads in insulated wonder bags to evaluate off-sunshine cooking performance for another 4 h. The second experimental test involved simultaneous cooking and heat storage alternating between charging and discharging cycles. Experimental results showed that the mixed storage system achieved higher temperatures than the erythritol storage system during charging without cooking. During discharging cycles, the heat utilization rate was faster for the mixed storage system than for the erythritol storage system. Both storage systems enabled the cooking of multiple meals within an 8-h cooking period. However, at the end of the experiments, the erythritol storage system retained higher temperatures than the mixed storage system. Future work will focus on characterizing the thermophysical properties of the mixed storage system, optimizing the erythritol-to-granite mixing ratio for improved thermal performance, and investigating alternative, locally available TES materials – such as sandstone, marble, limestone, and xylitol – for potential use in mixed storage systems.