Long Term Thermal Energy Storage in Adsorbent Beds for Solar Heating Applications

Abstract

Long term thermal energy storage in adsorbent beds is investigated as means to store excess solar heat from the summer and release it during the winter for residential heating applications. Adsorption is a reversible exothermic process which releases heat when mesoporous materials, called adsorbents, are exposed to gaseous species, called adsorbates. The adsorbate is adsorbed into the crystal lattice and energy is released. Since the process is reversible, heat can be used to regenerate saturated adsorbent, release the adsorbate and store energy. Adsorption thermal energy storage has many advantages over conventional thermal storage methods and can be combined with all types of solar panels. When incorporated into a solar heating system, the adsorption thermal energy storage system could provide solar heat year round and reduce the home's energy cost by up to 80%. In this study, an activated alumina and zeolite 13X hybrid from Rio Tinto Alcan was identified as the best adsorbent when combined with water as the adsorbate, through an extensive screening procedure. The adsorption and regeneration cycles of the adsorbent were characterized using the prototype system. A maximum performance of 200 kWh/m3 was recorded. A model for predicting adsorbent performance based on the regeneration temperature was created and then tested using a different adsorbent. The efficiency of the system was also calculated in order to identify critical design parameters. The adsorbent was modified by impregnating inorganic salt hydrates into the crystal lattice. The resulting modified adsorbents have a 25% increase in performance, with some adsorbents reaching 300 kWh/m3. Using the performance and efficiency results of the prototype, the practical and economic feasibility of a large scale adsorption thermal energy storage system was investigated in various scenarios. Although adsorption shows several practical improvements over other forms of thermal energy storage, the cost of the systems remains a bottle neck to commercialization.