Recycled concrete integration with PCM: A sustainable approach to high-efficiency latent heat thermal energy storage systems

Renewable thermal energy sources have emerged as a sustainable option to meet the heat demand for space heating and domestic hot water production in buildings. In this framework, thermal energy storage systems are crucial for optimizing the deployment of renewables. Latent Heat Thermal Energy Storage Systems (LHTESSs) using Phase Change Materials (PCMs) as a heat-storing medium offer higher energy density and reduced storage volume requirements, but face challenges due to lower heat transfer performance and cost-effectiveness. In order to enhance the storage thermal performance, metallic inserts, such as metal fins, are typically installed in these systems. However, this solution is confronted with higher fabrication and maintenance costs and corrosion.To overcome the afore-mentioned issues, this study experimentally explores a novel LHTESS that employs a mixture of Recycled Concrete (RC) aggregate and the paraffinic PCM RT-44HC as heat-storing material. Unlike previous studies, which primarily focused on building components such as roof and ceiling tiles, this work considers the RC-PCM compound as the primary heat-storing medium in thermal energy storage systems. In order to consider different RC-PCM mixtures, RC was crushed into different grain sizes, and the thermo-physical properties of the resulting compounds were assessed. Moreover, the influence of the Heat Transfer Fluid (HTF) inlet temperature on the LHTESS thermal performance was assessed. The proposed RC-PCM compound offers a significant improvement in system thermal performance during charging and discharging cycles, up to 75%, compared to a similar storage using pure PCM as heat-storing medium. Furthermore, finer particles demonstrated approximately 10% higher heat transfer rates than coarser particles due to their lower void fraction. Additionally, the proposed solution reduces PCM usage by 55%, improving sustainability by efficiently reusing waste materials. However, the storage energy density decreases by about 40% due to the lower PCM content in the mixture compared to the pure PCM solution.