Enhancing geothermal heating and cooling systems by utilizing phase change materials: A systematic review

Over the past decade, the research community has demonstrated increasing interest in advancing geothermal heating and cooling (GHC) systems through the integration of phase change materials (PCMs), tripling the number of scientific publications between 2021 and 2024. Within this context, this review evaluates two main application forms: PCM-based thermal energy storage (PCM-TES) units and PCM in ground heat exchangers (PCM-GHEs), the latter used with PCM-enhanced heat transfer fluids, PCM in heat transfer pipes, PCM in backfills, or PCMs deployed in ground vicinity. This review yields that on average, PCM integration improves heat transfer between the ground and the thermal load by 27 %, increases the coefficient of performance of heat pumps by 15 %, stabilizes heat transfer fluid temperature by 1–3 °C, and reduces GHE length by 10 - 90 %. For PCM-TES units, cost savings of up to 55 % are achieved with tariff-based operation. However, system performance is sensitive to both ground and phase change temperatures. Especially for PCM-GHEs, which require favorable operating conditions in both short- and long-term, and thermal conductivity enhancement of PCMs is often a necessity. Current technologies demonstrate a technology readiness level of 4–8, yet require validation through long-term, full-scale testing and comprehensive evaluations of cost effectiveness, as under-investigated aspects today. • A review of phase change material (PCM) use in ground heating and cooling (GHC) • PCM leads to system size reduction, cost savings, and efficiency improvements • Solar/air sources show synergy in GHC systems with PCM integration • Long-term performance depends on multiple technical and design conditions • Long-term full-scale testing and cost analyses remain under-investigated