A PROTOYPE OF A SMALL-SCALE ROCK THERMAL ENERGY STORAGE. A study evaluating diabase as a storage medium for the purpose of direct thermal applications.

Abstract

Access to sustainable and affordable energy for direct thermal applications, such as cooking and heating, remains a challenge in many regions. Sub-Saharan Africa is a region where reliance on traditional biomass fuels is prevalent, giving rise to severe health and environmental consequences. This study aimed to investigate the thermal performance of diabase as a storage medium within a prototype of a small-scale rock thermal energy storage system, constructed from readily available components, to evaluate its feasibility for potential low-energy applications. The experimental setup consisted of a 60 dm³ steel barrel containing 79.5 kg of 11-16 mm diabase gravel, heated by three 1.5 kW heating forks, and insulated with rock wool. Air was introduced into the system by an external compressor for heat extraction. Results from four charging and extraction cycles demonstrated that a single thermocouple in the diabase bed recorded localized temperatures up to 783.4°C. More representative average diabase temperatures, estimated from the total energy input, likely reached a range between 340.14 – 572.22°C depending on the cycle. The system successfully extracted heated outlet air, reaching up to 321.3°C with an airflow of 2.1 dm³/s and 64.9°C with a reduced airflow of 0.4 dm³/s, demonstrating the significance of flow rate on extraction efficiency. Energy extraction per cycle peaked at 3.76 MJ. Temporal heat retention analysis showed rapid energy dissipation within the first four hours with approximately 64-69% lost over 16 hours. Despite these losses, the system retained heat above ambient for several days, indicating potential for overnight storage. The study concludes that the small-scale prototype, despite limitations such as localized temperature measurements and the need for improvements regarding its system design, shows potential for direct thermal applications using readily available materials.