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• The 1D model is in good agreement with the CFD model. • Regenerators perform effectively at high speeds. • Heat transfer mechanisms are enhanced at high speeds. • Expressions for the heat transfer coefficient vs Reynolds number were generated. • The efficiency of the designed Vuilleumier machine increases with the speed. A one-dimensional analytical model and a three-dimensional CFD numerical model were used for the investigation of the effect that the rotational speed has on several thermodynamic quantities and the performance of a Vuilleumier machine. The machine, was designed as a combination of two opposing Stirling engines. The one-dimensional program was based on the energy balance at each control volume in order to calculate the energy flow inside the machine and then losses were added. On the other hand, the CFD model utilized fundamental conservation equations at each of the numerous computational cells which resulted to accurate calculations at every dimension. It was able to provide the heat transfer coefficients inside the heat exchangers that were in turn utilized by the analytical model. The pressure drop was computed directly at each space by the numerical model and with equations for flow losses from the bibliography by the analytical model. Pressure drop increased significantly with the speed. The effectiveness of the regenerators was evaluated by an existing analytical model and resulted to reduce drastically with the drop of speed. The effectiveness plays very important role on the efficiency of the machine. Furthermore, there appears to be a discrepancy between the heat flow in the heat exchangers and the wall-gas temperature difference at high speeds which has to be examined according to the oscillatory nature of the gas flow. Heat transfer coefficients were generated in relationship with the Reynolds number for each speed investigated, yielding less thermal resistance when the speed is high. Finally, the change of heat amounts through the four heat exchangers and change of the efficiency of the Vuilleumier machine with the speed, resulted to be similar with experimental data. [ABSTRACT FROM AUTHOR]

A numerical study was carried out to investigate charging and discharging processes of different phase change materials (PCMs) used for thermal storage in an innovative solar collector, targeting domestic hot water (DHW) requirements. The aim was to study PCMs that meet all application requirements, considering their thermal performance in terms of stored and retrieved energy, outlet temperatures, and water flow rate. Work was carried out for three flat-plate solar panels of different sizes. For each panel, a PCM tank with a heat exchanger was attached on the back plate. Simulations were conducted on a 2D domain using the enthalpy–porosity technique. Three paraffin-based PCMs were studied, two (A53, P53) with phase-change temperatures of approximately 53 °C and one of approximately 58 °C (A58H). Results showed that, during charging, A58H can store the most energy and A53 the least (12.30 kWh and 10.54 kWh, respectively, for the biggest unit). However, the biggest unit, A58H, takes the most time to be fully charged, i.e., 6.43 h for the fastest feed rate, while the A53 unit charges the fastest, at 4.25 h. The behavior of P53 lies in between A53 and A58H, considering stored energy and charging time. During discharging, all PCMs could provide an adequate DHW amount, even in the worst case, that is, a small unit with a high hot water consumption rate. The A58H unit provides hot water above 40 °C for 10 min, P53 for 11 min, and A53 for 12 min. The DHW production duration increased if a bigger unit was used or if the consumption rate was lower. [ABSTRACT FROM AUTHOR]

An innovative thermal energy storage system (TESSe2b) was retrofitted in a residential building in Cyprus with a typical Mediterranean climate. The system comprises flat-plate solar collectors, thermal energy storage tanks filled with organic phase change material, a geothermal installation consisting of borehole heat exchangers with and without phase change material and a ground source heat pump, an advanced self-learning control system, backup devices and several other auxiliary components. The thermal energy storage tanks cover the building's needs at certain temperature ranges (10–17 °C for cooling, 38–45 °C for heating and 50–60 °C for domestic hot water). A performance evaluation was conducted by comparing the TESSe2b system with the existing conventional heating and cooling system. The systems were simulated using commercial software, and the performance of the systems and the building's energy needs were calculated. Based on the energy quantities, an economic analysis followed. The equivalent annual primary energy consumption with the conventional system resulted in being 43335 kWh, while for the storage system, it was only 8398 kWh. The payback period for the storage system was calculated to be equal to 9.76 years. The operation of the installed storage system provided data for calculations of the seasonal performance factor and storage performance. The seasonal performance factor values were very high during June, July and August, since the TESSe2b system works very efficiently in cooling mode due to the very high temperatures that dominate in Cyprus. The measured stored thermal energy for cooling, heating and domestic hot water resulted in being 14.5, 21.9 and 6.2 kWh, respectively. Moreover, the total volume of the phase change material thermal energy storage tanks for heating and domestic hot water was calculated to be roughly several times smaller than the volume of a tank with water as a storage medium. [ABSTRACT FROM AUTHOR]