Thermochemical storage (TCS), and more generally high-density energy storage (HDES) and phase change storage (PCS), is a very promising technology for energy demand management, especially with a view to harnessing renewable energies having non-programmable and variable trends in both the short and long term (seasons).
During the previous LA.4.01 and LA.4.02, an experimental plant was constructed for steam (and thus heat) storage/release tests up to 6 kg of material. An initial experimental campaign was carried out on that facility using 13X zeolites as the sorbent, which are one of the most widely studied materials for seasonal heat storage applications due to their good activity and high stability. Parallel to the work on zeolites, the development of high-density energy materials based on magnesium sulfate supported on alumina was carried out. The porosity of the support was optimized to accommodate, on the one hand, a high amount of active phase, so as to have high energy densities, and to ensure, on the other hand, good vapor diffusion within the pores. The best material obtained has an energy density of 188 kWh/m3, which is higher than that characteristic of zeolites (120 kWh/m3), and has macropore diameters of 1 micron. Comparison in terms of the performance of such a macroporous material with a microporous material having the same salt load, and thus the same energy density, showed that the macroporous sorbent exhibits a higher maximum temperature rise due to the reaction. This result thus highlighted that for application purposes, in addition to salt load and material porosity, which determine the energy density one has available, pore diameter is also a parameter of considerable importance, as it influences the way heat is released: larger pores allow a greater rise in reaction temperature, for the same amount of time, and thus higher powers and faster heat release.
Although obtaining materials with high-density energy is a key aspect in the development of a thermochemical system for meeting residential heat loads, it alone cannot guarantee efficient performance from an application point of view. Indeed, in parallel with material development, the reactor design should also be considered, which must ensure low losses of the released heat, good material and heat transfer properties, and the delivery of the required thermal power. In this regard, two types of reactors used for thermochemical storage are reported in the literature: integrated reactors and separate reactors. In integrated reactors, hydration and dehydration reactions take place in the same vessel, which contains the entire amount needed to meet the seasonal heat load, while in separate reactors, absorption and desorption reactions take place in the same vessel in a cyclic manner as follows: absorption (winter) and regeneration (summer) reactions are conducted in a reactor of small size while the entire amount of fresh anhydrous material, needed to cover up to the entire heat demand, is stored in a specific tank. All spent and humidified material is also stored in a second specific tank. The exact amount of material needed by the reactor for the absorption or desorption reactions must be taken from one of the two tanks (dried material tank during winter and humidified material tank during summer) and piped into the reactor. From there, once the reaction is completed, it must be discharged and conveyed to the other tank. The process is repeated cyclically until the tank from which it is drawn is empty. This results in the need to continuously operate sorbent handling between reactor and storage tanks.
Although the plant is more complex than the integrated reactor system, the separate reactor system represents a very promising solution for the application of thermochemical storage on a large scale; having a smaller reactor actually brings advantages in terms of better process control and lower bed load losses, and, moreover, the compact size of the reactor allows it to be placed where there is space. With this in mind, a pellet handling system for thermochemical storage was designed and built during LA.4.03, and then installed to complement the test system built in previous LAs. This system is equipped with two tanks (storage of anhydrous and hydrated material) and a single reactor in which the material hydration and dehydration reactions occur. Movement of the material between the reactor and storage is carried out by pneumatic transportation. In parallel, modeling of the thermochemical storage reactor on the plant was completed during LA4.03. The obtained model was validated by absorption/regeneration cycles performed on the test plant. Finally, a plant diagram was proposed that could include all the aspects covered during the three-year period and represent how the proposed system could be interfaced with a residential-type building to meet the relevant heat loads. The operation of the proposed system was simulated in the Trnsys environment during an entire winter season, with the aim to highlight the satisfaction of the comfort levels of the dwelling, to calculate the consumption of process auxiliaries, and to identify operating conditions that could reduce such consumption.