Colloquia

Summary

This thesis investigates a dual-temperature phase change material (PCM) thermal energy storage system designed to enhance the performance of photovoltaic-thermal (PVT) assisted heat pumps for residential heating. The system uses two PCMs in a concentric configuration: RT25HC (melting around 25°C) as an outer buffering and insulating layer, and RT54HC (melting around 54°C) as an inner high-temperature storage layer. A laboratory prototype was developed with two stainless-steel heat exchangers and 16 thermocouples to examine charging, discharging, and heat-loss behavior, and a MATLAB model was created to evaluate system performance at residential scale. Experiments demonstrated that the outer PCM provides stable insulation and absorbs standby losses from the inner PCM, while the inner PCM reliably stores high-grade heat but exhibits stronger heat losses toward the outer PCM. When both PCMs are charged, heat propagates outward and the system achieves energy densities up to 8.2 kWh/m³. Modeling at residential scale showed that the dual-PCM system can stabilize source temperatures, shift thermal loads, and reduce annual heat-pump electricity consumption by up to 17% with a four-times larger storage volume. Seasonal effects were notable: during spring and early fall, the PCM could supply 20–40% of total heating demand, while winter contributions were limited to 5–10%. An economic analysis estimated system material and fabrication costs and compared annual energy savings across multiple storage sizes. A 4x scaled system produced annual savings of roughly €370 and achieved a return on investment (ROI) of about 10 years. Overall, the research concludes that a dual-PCM TES system can meaningfully enhance PVT-assisted heat pump performance by increasing available source temperatures, improving discharge quality, reducing electricity use, and providing compact, stable thermal storage. Although the MATLAB model contains idealized assumptions that overestimate uniformity, the trends align with experimental results. The system is technically feasible and economically promising for residential heating, with room for further refinement, long-term monitoring, and optimization.