Experimental and theoretical analysis of adsorption-based desalination plant

In modern society, energy sustainability and fresh-water production are two of main challenges to be faced to ensure a good quality of life for the human species and to preserve planet Earth. The energy demand comes up against the problem of the scarcity of currently exploitable energy resources and the need to increasingly limit the consumption of fossil fuels to reduce polluting emissions into the atmosphere. A solution in this direction consists of increasing the exploitation of renewable energy sources. However, this is still hindered by several issues, such as the mismatch between energy supply and energy demand and the sub-optimal capacity to store energy from renewable sources. Another important consequence of human population growth is the increased need for fresh water. This demand contrasts with the scarcity of this resource in many parts of the world. Thermochemical heat storage (THS) represents one of technologies designed to reduce the mismatch between energy supply and energy demand. THS base principle is to store energy through a chemical or physical reaction, and then release it through the reverse reaction when required. In this work the focus is on THS technologies based on physical reactions, which are named adsorption processes. Adsorption is a process in which molecules or particles adhere to the surface of a solid or liquid material. In adsorption, the molecules or particles that adhere to the surface are typically referred to as adsorbates, and the material to which they adhere is called the adsorbent. This reaction is spontaneous and involves heat release. Adsorbate and adsorbent can be separated by the reverse physical reaction, that is named desorption process. Desorption process needs heat supply from the ambient to the system composed by adsorbate and adsorbent. Therefore, the thermal energy storage process concurs with the desorption phase. Then, adsorbent and adsorbate are kept separate. When the stored thermal energy is needed, the two materials are brought back into contact, which leads to an exothermic reaction and thus a release of energy. An adsorption-based THS system is composed by at least two chambers. One chamber containing the adsorbent material and one chamber containing the adsorbate. These chambers must be brought in contact when storage or release of thermal energy is required. Water vapor from a salt-water mass can be used as fluid phase. Adding a third chamber to the system, in which water vapor released in desorption process can condense, a dual purposes system is obtained. This system, in addition to storing and disposing thermal energy, allows also the production of fresh-water from salt water. The main idea of this thesis work developed at Politecnico di Torino is to analyze a system that can couple THS technology with the thermal desalination process. A system operating according to the principle described above has been installed at the Polytechnic. The work presented in this thesis focuses on the theoretical and experimental study of the plant, the definition of a protocol for its use and the development of an analytical model capable of simulating its behaviour. The results obtained were derived by analysing the performance of 1 kg of silica gel. Fresh-water production capacity and energy efficiency were evaluated. Results show that about 1 L of water per day can be produced with this plant. In perspective, the plant will be used to analyse the performance of other innovative adsorbent materials from in-house production.