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This study proposes a novel thermal energy storage (TES) concept using two phase change materials (PCMs) (inorganic salt and metal alloy) as the storage media. The metal alloy PCM is encapsulated in a tube which is inserted in the inorganic salt PCM. Thus, the metal alloy PCM serves as the heat storage material as well as the heat transfer enhanced fin for the inorganic salt PCM. After validation, a numerical model is developed to simulate the charging and discharging processes of the presented TES unit. Furthermore, the influence of the storage material selection, the phase change temperature difference between those two PCMs, and the location of heat transfer surface on the thermal behavior of the charging and discharging process are discussed. The results show that, compared with the unit filled with only salt PCM, the proposed unit can significantly reduce the charging and discharging time by 33.2% and at least 50.3%, respectively. When selecting metal PCM, it is not recommended to use the metal PCM with higher melting temperature than the selected salt PCM. With 75 K melting temperature difference, the melting and solidification time of the unit was prolonged by 47.1% and 6.1%. In addition, if the unit has bottom heat transfer surface, the melting and solidification time of the unit are 356 and 1989 min, corresponding to 4.5 charging power and 0.8 kW discharging power.

An air solar-driven open sorption system for comfort cooling is investigated experimentally in an indoor test facility. The main components of the system are a solar dehumidifier and a cold storage with desiccant. One operation cycle is 36 h. On the first day of the operation cycle, the solar dehumidifier is charged. At night the cold storage is charged and on day two, the cold storage is discharged. A material investigation is undertaken to find the most suitable desiccant for both components, including samples of different silica gels and zeolites. The scope of this work is to determine the usability and the performance of the system. The usability of the system is addressed by an investigation of the climatic conditions of fourteen different locations. It is shown that a small porous silica gel is the best desiccant for both the solar dehumidifier and cold storage, that the system can work well in eight of the investigated climates, and that the system can supply cool, dry air for comfort cooling of buildings.

Future energy systems with a large share of fluctuating renewable energies demand thermal energy storages that are flexible and reliable. Sodium acetate trihydrate (SAT) has been investigated for many years as heat storage materials but the focus of the investigations were mostly on short-term applications. SAT has a high energy storage density and a large supercooling degree which make it an ideal flexible heat storage material. Heat storages utilizing stable supercooling of SAT can store heat almost heat loss free in both short-term and long-term, which offers great benefits for the energy system. A review of SAT heat storages is presented in this article. The investigations show that flexible heat storages using SAT have clear advantages over a traditional water based heat storage in both short-term and long-term heat storage. Significant decrease of the storage size (at least 60%) is considered a merit for applications in single family houses where there is a spatial constrain. Moreover, the SAT heat storages offers flexibility in system operation, which is considered beneficial for renewable energy such as solar heating systems. The main challenges for the heat storages, for instance, low heat exchange capacity rate, phase separation and spontaneous crystallization, are addressed. Remedies to overcome the challenges are discussed. The paper will also investigate stability of supercooling and reliability of triggering methods for controllable release of the latent heat stored in supercooled sodium acetate trihydrate. Recommendations for further investigations are proposed.