Study On An Innovative Multi-Effect And Double-way Thermochemical Sorption Refrigeration System

Solid-gas sorption refrigeration is one of the promising energy-saving and environmentally friendly refrigeration technologies. These heat-powered green refrigeration systems have received more and more attention with the recognition of rational utilization of energy and the concerns about ecological problem. To promote the economic and social sustainable development, solid sorption refrigeration would play an important role in the reutilization of low-grade thermal energy and renewable energy sources in the future. However, low system performance is the biggest drawback for the solid sorption refrigeration systems, which has been still hindering their extensive application. Furthermore, the low working performance experienced in the conventional sorption refrigeration cycles, renders them uncompetitive to the existing vapor compression refrigeration and liquid-gas absorption refrigeration.In an effort to overcome the drawbacks of the conventional sorption refrigeration cycle, an innovative combined double-way thermochemical sorption thermodynamic cycle based on the adsorption and resorption processes was developed. In addition, new consolidated composite sorbents made from reactive salts and porous graphite matrix, with enhanced heat and mass transfer properties, were manufactured for use in the proposed double-way sorption cycle. Based on the adsorption/desorption characteristics of the composite sorbents-ammonia working pairs, thermochemical reaction dynamics and the cycle characteristics of the proposed double-way sorption cycle, a re-heating strategy and a two-stage desorption technique with different reactants were used to optimize the combined double-way sorption refrigeration cycle. Moreover, in order to further improve the system performance and accomplish the COP higher than 1, an innovative multi-effect and double-way thermochemical sorption refrigeration cycle with internal heat recovery was developed by employing better heat management strategy. The working performance of the advanced sorption refrigeration system with high efficiency driven by low-grade thermal energy was predicted by theoretical analysis, and the main conclusions obtained were as follows:Experimental verification showed that the proposed combined double-way thermochemical sorption cycle based on the adsorption and resorption processes is feasible for refrigeration application. Both adsorption refrigeration and resorption refrigeration processes are combined to provide useful cold in one cycle. The presented double-way sorption cycle has a distinct advantage of larger cooling capacity per unit of heat input compared with the conventional adsorption or resorption refrigeration cycle. This is because two cold productions could be obtained during one cycle at the expense of only one heat input at a high temperature. The first cold production is obtained during the adsorption process when the refrigerant vaporizes in the evaporator, and the second cold production occurs during the resorption process when the low-temperature sorbent absorbs decomposition reaction heat at a low temperature. In comparison with the conventional adsorption cycle and resorption cycle, the proposed combined double-way sorption cycle could improve the COP by 167 % and 60 %, respectively.Experimental results showed that the proposed re-heating process is an effective technique for improving the global conversion of the sorbent during the resorption phase of the combined double-way sorption cycle. The increase in the COP varied between 12 % and 48 %, when compared with the combined double-way cycle without re-heating. The low pseudo-evaporation temperature and high heat sink temperature can further improve the working performance. Furthermore, the driving heat source temperature of the combined double-way sorption cycle could be lowered effectively by employing the new two-stage desorption technology with different reactants, and the temperature drop can be regulated by choosing the secondary sorbents with different equilibrium thermodynamic characteristics. Thus, the new desorption technology can provide an effective method for the application of the double-way sorption refrigeration to some locations where the available heat source temperature is relatively low.Moreover, an innovative multi-effect thermochemical sorption refrigeration cycle was proposed to further improve the system performance by employing two internal heat recovery processes among the different sorption working pairs. In this multi-effect cycle, three cold productions could be obtained at the expense of only one high-temperature heat input. The COP improvement obtained with the multi-effect cycle varied between 23 % ~ 50 % and 146 % ~200 % when compared with the conventional double-effect cycle and the single-effect cycle, respectively. In addition, the heating, cooling and heat recovery processes between two reactors could be performed effectively by using multifunction heat pipes without additional power consumption. In comparison with the conventional sorption machine, the heat pipe type sorption machine could improve the heat recovery efficiency and the working reliability of sorption machine. The corrosion of the reactor caused by the low-grade waste heat/gas and the salty seawater could also be avoided by employing the heat pipe technology. The amount of heat consumption was effectively reduced, and the SCP and COP could be improved by 46.7 % and 28.4 % respectively by utilizing heat pipe type energy recovery technique.Based on the above-mentioned studies, an advanced double-effect and double-way thermochemical sorption refrigeration cycle was developed by combining the adsorption process, resorption process and internal heat recovery strategy. The advanced system could have four cold productions during one cycle at the expense of only one high-temperature heat consumption. During every operating phase, the cold production consists of the adsorption refrigeration based on the evaporation process and the desorption refrigeration based on the resorption process. The double-effect and double-way sorption cycle had an ideal COPi as high as 2.29 when the FeCl2 was used as the high-temperature sorbent. At a condensation temperature of 30°C, evaporation temperature of 0°C and resorption refrigeration temperature of 5°C, theoretical results showed the COP employing the proposed double-effect and double-way cycle varied between 1.50 and 1.26 in the range of mass ratio R from 5 to 10.In comparison with conventional sorption refrigeration cycle, the presented innovative double-effect and double-way thermochemical sorption refrigeration cycle can significantly improve the working performance of sorption refrigeration systems, and a COP higher than 1 could be obtained in the advanced sorption refrigeration cycle. The research results establish a fundamental theory for the high-efficient sorption refrigeration driven by low-grade thermal energy.