Energies

Heat pumps are increasingly used for low-temperature heating applications. But their operation can create electricity demand peaks during periods of high thermal load. This paper presents a concept and calculation study for integrating phase change materials (PCMs) with air-to-water heat pump (AWHP) systems as a condenser-side thermal buffer for residential buildings. The analyzed retrofit-oriented storage unit is a 200 L tank containing 30% PCM by volume, corresponding to 60 L of paraffin PCM and 140 L of water. The sizing calculation gives 3.08 kWh of PCM thermal storage, 0.81 kWh of sensible storage in the remaining water over the 55–60 °C interval, and almost 3.89 kWh of total short-term thermal reserve for the PCM-plus-water tank. A 200 L water tank over the same 5 °C interval would store approximately 1.16 kWh. At the nominal 14 kW heat pump output, the PCM buffer corresponds to about 13.2 min of operation. At representative part load it corresponds to about 53 min at 25% load and 26 min at 50% load. The PCM sizing model predicts storage capacity, buffer time and possible cycling relevance, but it does not by itself predict a seasonal COP increase. Therefore, annual electricity and economic effects are evaluated only as COP-uplift sensitivity cases, including 0%, 3%, 4%, 5%, 8%, 15% and 28.6%. Given the assumptions in this study, the PCM add-on reaches positive net present value at approximately 11% COP uplift when the initial marginal investment and discounted year 10 replacement cost are considered. Approximately 14.1% is reached if the replacement cost is added upfront as a simplified lifecycle basis. The results indicate that the condenser-side PCM buffer may be technically useful for short-term thermal buffering and possible on-off cycle reduction, but its annual energy and economic benefit requires dynamic simulation or experimental validation. Moreover, peak-demand reduction, demand shifting and renewable electricity utilization are discussed as potential applications of the buffer concept.

To address the temperature non-uniformity of heating platens in small-scale photovoltaic (PV) module laminators, this study develops a wire-wound hot platen with a Square loop layout and arithmetic-progression wire spacing. The heating wires are represented by an equivalent surface-heat-flux boundary, and CFD simulations are used to evaluate the effects of wiring layout, wire length, and spacing on the temperature field. The optimized Square loop layout reduces the full-field working-surface-temperature standard deviation from 1.887 °C to 0.552 °C, a reduction of 70.8%. A FLUENT-UDF incremental PID model is then developed using the top-surface area-weighted average temperature as feedback. At the 4800 s endpoint, the simulated controlled average is 150.55 °C, with a steady-state error of +0.55 °C and a maximum positive deviation of approximately 0.96 °C at approximately 4480 s; the holding-stage response shows a small limit-cycle oscillation of approximately ±1 °C. The local center-point temperature T₁ is 153.0 °C, indicating a center-to-edge spatial non-uniformity that is distinct from the controller overshoot. A small-scale laminator platform with 13 thermocouples was used to benchmark the simulated platen-level temperature field. The measured 13-point average temperature is 150.55 °C, the maximum temperature difference is 3.0 °C, and the 13-point sample standard deviation is 1.05 °C; the maximum absolute error and mean absolute error are 1.30 °C and 0.913 °C, respectively. These results support the proposed surface-heat-flux model and incremental PID design for small laminator hot platens under the tested platen-level conditions.

Temporal mismatches between geothermal output and user demand limit resource utilization, whereas thermal energy storage (TES) can decouple heat production from consumption. This review organizes geothermal–TES coupling by resource depth and answers four engineering questions. First, temperature, duration, geology, and end-use requirements determine compatibility: water tanks and low-temperature phase-change materials are preferred for short-term regulation; BTES and ATES provide seasonal capacity where geology permits; and pressurized-water, molten-salt, or thermochemical systems are reserved for higher-temperature resources. Second, short-term TES primarily suppresses cycling and peak demand, whereas seasonal TES restores annual subsurface balance and raises geothermal-well utilization. Third, selection must jointly consider recovery efficiency, usable discharge temperature, auxiliary electricity, land and drilling requirements, technology readiness level, and cost rather than storage density alone. Fourth, coupling introduces constraints that are absent from stand-alone components, including groundwater advection, thermal breakthrough, corrosion and scaling, well-flow limits, and temperature matching. Nordic experience demonstrates the value of integrating underground TES with district heating and cooling, but economic feasibility remains site specific. Indicative storage-capacity costs are approximately 0.1–1.3 EUR/kWhth for ATES, 0.6–3 EUR/kWhth for BTES, and 0.5–1.7 EUR/kWhth for PTES, with strong scale and boundary dependence. A compatibility matrix, maturity assessment, and staged development roadmap are provided to support system selection and research prioritization.