Assessment of phase change material within hollow steel balls mixed with graphite powder-enhanced concrete for underground energy structures

Abstract

Thermal energy storage using phase change materials (PCMs) offers great potential for improving energy efficiency and conservation. This paper explores a novel phase change energy storage concrete by encapsulating capric acid-myristyl alcohol (CA-MA) within hollow steel balls (HSB) to create PCM-HSB aggregates. Graphite powder (GP) was added as an admixture to enhance thermal conductivity. The thermal performance and mechanical properties of GP/PCM-HSB concrete were tested at various replacement ratios: PCM-HSB (10.0 %, 12.5 %, 15.0 %) and GP (2.5 %, 5.0 %, 7.5 %, 10.0 %). Results indicated that as the PCM-HSB replacement ratio increases to 15.0 %, the specific heat capacity of the concrete increases by 61.9 %, enhancing its heat storage capacity and enabling the concrete to better maintain a stable internal temperature as temperature changes. The addition of GP has a positive impact on the thermal conductivity. When the GP content increases to 5.0 %, the thermal conductivity of the concrete increases by 60.4 %, effectively solving the problem of the decrease in thermal conductivity caused by the addition of PCM-HSB. The advantage of GP/PCM-HSB is that it encapsulates the phase change material inside the hollow steel balls, which can effectively prevent the leakage of the phase change material and improve the stability and durability of the material. At the same time, it can store and release heat efficiently during the phase change process, enhancing the energy storage capacity of the concrete. The addition of graphite powder (GP) significantly increases the thermal conductivity of the concrete. The best-performing mix ratio was identified as 15.0 % PCM-HSB and 5.0 % GP. The GP/PCM-HSB composite phase change energy storage concrete demonstrated excellent thermal and mechanical properties, making it highly suitable for the future development of underground energy structures.