The use of latent heat thermal energy storage is an effective way to increase the efficiency of energy systems due to its high energy density compared with sensible heat storage systems. The design of the storage material encapsulation is one of the key parameters that critically affect the heat transfer in charging/discharging of the storage system. To fill the gap found in the literature, this paper experimentally investigates the effect of the macro-encapsulation design on the performance of a lab-scale thermal energy storage tank. Two rectangular slabs with the same length and width but different thickness (35 mm and 17 mm) filled with commercial phase change material were used. The results show that using thinner slabs achieved a higher power, leading to a reduction in the charging and discharging time of 14% and 30%, respectively, compared with the thicker slabs. Moreover, the variation of the heat transfer fluid flow rate has a deeper impact on the temperature distribution and the energy charged/released when thicker slabs were used. The macro-encapsulation design did not have a significant impact on the discharging efficiency of the tank, which was around 85% for the operating thresholds considered in this study.

Thermal energy storage (TES) allows the existing mismatch between supply and demand in energy systems to be overcome. Considering temperatures above 150 °C, there are major potential benefits for applications, such as process heat and electricity production, where TES coupled with concentrating solar power (CSP) plants can increase the penetration of renewable energies. To this end, this paper performs a critical analysis of the literature on the current and most promising concrete energy storage technologies, identifying five challenges that must be overcome for the successful exploitation of this technology. With these five challenges in mind, this paper proposes an approach that uses a new modular design of concrete-based TES. A preliminary study of the feasibility of the proposed system was performed using computational fluid dynamics (CFD) techniques, showing promising results.

Deep Reinforcement Learning (DRL) proved to be successful for solving complex control problems and has become a hot topic in the field of energy systems control, but for the particular case of thermal energy storage (TES) systems, only a few studies have been reported, all of them with a complexity degree of the TES system far below the one of this study. In this paper, we step forward through a DRL architecture able to deal with the complexity of an innovative hybrid energy storage system, devising appropriate high-level control operations (or policies) over its subsystems that result optimal from an energy or monetary point of view. The results show that a DRL policy in the system control can reduce the system operating costs by more than 50%, as compared to a rule-based control (RBC) policy, for cooling supply to a reference residential building in Mediterranean climate during a period of 18 days. Moreover, a robustness analysis was carried out, which showed that, even for large errors in the parameters of the system simulation models corresponding to an error multiplying factors up to 2, the average cost obtained with the original model deviates from the optimum value by less than 3%, demonstrating the robustness of the solution over a wide range of model errors.

This paper experimentally investigates the direct integration of 3.15 kg of phase change materials (PCM) into a standard vapour compression system of variable cooling capacity, through an innovative lab-scale refrigerant-PCM-water heat exchanger (RPW-HEX), replacing the conventional evaporator. Its performance was studied in three operating modes: charging, discharging, and direct heat transfer between the three fluids. In the charging mode, a maximum energy of 300 kJ can be stored in the PCM for the cooling capacity at 30% of the maximum value. By doubling the cooling power, the duration of charging is reduced by 50%, while the energy stored is only reduced by 13%. In the discharging mode, the process duration is reduced from 25 min to 9 min by increasing the heat transfer fluid (HTF) flow rate from 50 L·h⁻¹ to 150 L·h⁻¹. In the direct heat transfer mode, the energy stored in the PCM depends on both the cooling power and the HTF flow rate, and can vary from 220 kJ for a cooling power at 30% and HTF flow rate of 50 L·h⁻¹ to 4 kJ for a compressor power at 15% and a HTF flow rate of 150 L·h⁻¹. The novel heat exchanger is a feasible solution to implement latent energy storage in vapour compression systems resulting to a compact and less complex system.

The need for innovative heating and cooling systems to decarbonize the building sector is widely recognized. It is especially important to increase the share of renewables at building level by maximizing self-consumption and reducing the primary energy demand. Accordingly, in the present paper, the results on a wide experimental campaign on a hybrid system are discussed. The system included a sorption module working as the topping cycle in a cascade configuration with a DC-driven vapor compression heat pump. A three-fluids heat exchanger with a phase change material (PCM), i.e., RT4 with nominal melting temperature of 4 °C, was installed on the evaporator side of the heat pump, for simultaneous operation as thermal storage and heat pumping purposes. The heat pump was connected to a DC-bus that included PV connection and electricity storage (batteries). Results showed that the energy efficiency of the heat pump in cascade operation was double compared to compression-only configuration and that, when simultaneously charging and discharging the latent storage in cascade configuration, no penalization in terms of efficiency compared to the compression-only configuration was measured. The self-sufficiency of the system was evaluated for three reference weeks in summer conditions of Athens climate and it was found that up to 100% of the electricity needed to drive the system could be self-produced for a modest cooling demand and up to 67% for the warmer conditions with high cooling demand.

Thermal energy storage (TES) systems play a very important part in addressing the energy crisis. Therefore, numerous researchers are striving to improve the efficiency of TES tanks. The TES technology has the potential to reach new heights when the biological behavior of nature is incorporated into the design of TES tanks. By mimicking the branched vein pattern observed in plants and animals, the heat transfer fluid (HTF) tube of a TES tank can enhance the heat transfer surface area, hence improving its thermal efficiency without the need to add other enhancements of heat transfer methods. Accordingly, in this study, a unique additive-manufacturing-based bio-inspired TES tank was designed, developed, and tested. A customized testing setup was used to assess the bio-inspired TES tank’s thermal performance. A comparison was made between the bio-inspired TES tank and a conventional shell-and-tube TES tank. The latent TES system’s thermal performance was significantly enhanced by the biomimetic approach for the design of a TES tank, even before the optimization of its design. The results showed that, compared to the shell-and-tube TES tank, the bio-inspired TES tank had a higher discharging rate and needed 52% less time to release the stored heat.