• Energy Research

At this moment, the global energy consumption in buildings is around 40% of the total energy consumption in developed countries. Thermal energy storage (TES) is presented as one way to address this energy-related problem proposing an alternative to reduce the gap between energy supply and energy demand. One way to store energy is using thermochemical materials (TCM). These types of materials allow accumulating energy through a chemical process at low temperature, almost without heat losses. In addition, it is a stable way to perform the heat storage and TCM can be implemented for seasonal storage or/and long term storage. This study compares the cyclability, from the thermophysical point of view, CaCl2 which follows a chemical reaction and zeolite which follows a sorption process to be used as TCM for seasonal/long term storage. The main results show that the chemical reaction TCM is more energy-efficient than the sorption TCM. The CaCl2 calculated energy density is 1.47GJ/m3, being the best option to be considered to be used as TCM, even though the dehydration process of the zeolite is simpler and it occurs at higher temperatures its calculated energy density is only 0.2GJ/m3.

In our laboratories, a seasonal thermochemical storage system for dwellings and offices is being designed and developed. Based on a thermochemical sorption reaction, space heating, cooling and generation of domestic hot water will be achieved with up to 100% renewable energy, by using solar energy and waste heat. Development of the reactor and its components (adsorber/desorber, evaporator/condenser) as well as on the active material are described, and indications for further improvement are given. Simulations and experiments yield promising results for further development in demonstrators and field tests of the system set-up, and our newly developed enhanced active material yields promising results exhibiting high storage capacity, good reversibility and ease of use.

The IEA joint Task 42 / Annex 29 is aimed at developing compact thermal energy storage materials and systems. In Working Group B, experts are working on the development of compact thermal energy storage applications, in the areas cooling, domestic heating and hot water and industry. The majority of application projects were in the field of room heating and domestic hot water. In this article, an overview is given of a large number of applications. The storage technologies used in the applications are latent heat storage, open and closed solid sorption, liquid sorption and salt hydrates and composites thereof. On a broad front, a lot of progress was made in the development of components and systems, providing knowledge and experience regarding the design, numerical modeling, building, testing and economical assessing of components and storage systems. Most important findings are that the interaction of storage materials with the materials of components can be deciding for the technical feasibility, that a number of components, like reactor, heat exchangers and evaporators are less understood than initially thought and need more development, that the inclusion of storage materials in systems generate new challenges like the occurrence of non-condensable gases and thermo-mechanical effects and that standardized and simplified system approaches are needed.

Thermochemical materials (TCM) are proposed for thermal energy storage as one of the future options to achieve lower energy consumption in buildings and other industrial applications, as well as to store energy from solar energy. In this study, the thermophysical properties of two TCM, CaCl2 and zeolite, are determined with TGA and DSC and samples are cycled 4 times with TGA. Results show that the material with the highest energy density is the salt, CaCl2. Moreover, both materials under study present noble cyclability.

Bottlenecks for realizing a commercial system for thermochemical heat storage (TCS) with hygroscopic salts are the chemical, physical and mechanical stability of the salt under operation conditions. Hence, improved knowledge of thermochemical materials (TCMs) is critical to spur progress in TCS system development. Sodium sulfide hydrates (Na2S.nH2O, n=0-9) are highly interesting as TCMs because they exhibit a high energy density under operation conditions and are potentially readily available and affordable. Preparation methods for well-defined nonahydrate and pentahydrate crystals of Na2S were developed and the resulting samples were subjected to cycling experiments under conditions representative for TCS. Before and after cycling, crystal samples were taken and characterized using techniques like SEM/TEM, XRD. Mechanical strength was evaluated using a salt bed stability test. Based on the extensive characterization of sodium sulfide hydrate salts, a method has been proposed to improve the stability of the salt by blending it with cellulose. First trials on these composites yielded promising results with respect to improved material stability.