Design and Development of a Novel Thermochemical Reactor Using Composite Sorbent for Solar Thermal Energy Storage

Thermochemical heat storage (THS) is being considered as a potential technology in order to enhance the utilization of renewable energy sources due to its high energy storage density and long term heat storage capacity. However, further advancement in such systems depends on the development of novel sorption materials and innovative solar-driven thermochemical processes. To this end, this work presents new sorption materials and a new solar-driven THS process suitable for building applications. In the first part of the study, two novel composite sorbents consisting of aerated porous concrete-calcium chloride (APC-CaCl2) and pumice-calcium chloride (P-CaCl2), respectively, were synthesized. Other than the synthesized new composite materials, vermiculite-calcium chloride (V-CaCl2) and zeolite were also selected and synthesized for a comparative performance study. A new experimental scale THS system was designed and developed for investigating the charging/discharging characteristics of all synthesized sorption materials. The experimental results show that, the values of the average energy storage density of the system operating with APC-CaCl2, V-CaCl2, zeolite, and P-CaCl2, for five hours of discharging were 186.9 kWh/m3, 174.2 kWh/m3, 182.6 kWh/m3 and 204.3 kWh/m3, respectively. For a charging temperature of 90 ºC over three hours of charging process, the mass desorption rate of APC-CaCl2 was found to be 9.84 g/min, while, it was 4.40 g/min, 13.27 g/min and 14.13 g/min for zeolite, V-CaCl2 and P-CaCl2, respectively. Considering their charging/discharging performances and their cyclic stability, P-CaCl2 and V-CaCl2 were found to be the best candidate materials for THS applications. In the second part of the study, a solar-driven THS was designed, developed and tested under real climatic conditions of North Cyprus. V-CaCl2 was selected as the working material due to its high energy density, good cyclic ability and the low cost and availability of vermiculite (host matrix) in North Cyprus. Three series of charging-discharging experiments were performed employing the newly developed solar-driven THS system. The discharging tests were performed during the night for a duration of five hours. During the discharging experiments, the temperature and relative humidity of air at the reactor inlet were in the range of 21 – 24 ºC and 80 – 90 %, respectively. Under these conditions, the average energy output over three cycles was found to be 2.1 kWh, with an average energy storage density of 156 kWh/m3. A manufactured parabolic solar concentrator was utilized to provide the required thermal energy in the charging process. Hence, the average surface temperature of the reactor was in the range of 78 ºC to 83 ºC and the average useful energy of the charging process was 3.94 kWh. The rate of moisture desorption was 6.5 g/min while the overall efficiency was 38%. Furthermore, an economic analysis was performed to compare the feasibility of the proposed solar-driven THS system and a typical heat-pump unit of the split air-conditioner under the climatic conditions of North Cyprus. The heating load of a house with an area of 90 m2 was simulated in the DesignBuilder software, and from the simulation results, it was found that a solar-driven THS system with a reactor volume of 8.75 m3 is required to meet 1366 kWh of an annual heating load of the building. The analysis showed that the net present value of THS system after ten years would be $381 with a simple payback period of 6.4 years, which illustrates the economic feasibility of the THS system. Keywords: Thermochemical Heat Storage, Solar Energy, Thermodynamic Analysis, Porous Host Matrix, Calcium Chloride, Composite Sorbent, Experimental