Investigation On The MnCl₂/CaCl₂-NH₃ Two-stage Solid Sorption Combined Cooling And Power System Driven By Exhaust Heat

The exhaust gas emission causes a lot of energy waste and it also has a serious impact on the environment.Consequently,how to efficiently recover the exhaust heat is extremely urgent.Considering that the exhaust gas has a relatively high temperature and a large amount of heat,the solid sorption refrigeration cycle can be used to recover the exhaust heat and provide refrigerating capacity.Simultaneously,both the evaporator and the condenser are equipped with fans,so some electricity is consumed.In this paper,a novel solid sorption combined cooling and power system driven by the exhaust waste heat is proposed.The main target of the system is to output the cooling capacity,and the electricity is used for driving system itself.Detailed research contents are as follows:?1?By analyzing the cool supply and power demand of the sorption refrigeration system and the operating environment,a novel MnCl₂/CaCl₂-NH₃two-stage solid sorption combined cooling and power system driven by exhaust waste heat was established.The heat transfer environment is extremely critical:the temperature of the exhaust gas fluctuates greatly,and is between 200^? and500^?;simultaneously,the ambient temperature in summer is relatively high,generally above 30^?.Consequently,the single-stage solid sorption refrigeration cycle cannot meet the requirements.The two-stage solid sorption refrigeration cycle is employed.According to the critical operating environment,the MnCl₂/CaCl₂-NH₃ working pair was selected,and the expanded natural graphite treated with sulfuric acid?ENG-TSA?was selected as the matrix.?2?The three-dimensional model of the two-stage solid sorption combined cooling and power system driven by the exhaust waste heat was established,which consisted of a MnCl₂ sorption bed,a CaCl₂ sorption bed,an evaporator,a condenser and an expander.Considering that the system is maybe in a vibrating state at the running time when the system is installed on a truck or a boat,the traditional structure of sorption beds is not suitable,so two novel sorption beds have been developed.The sorption bed consisted of a number of unit tubes,and the compound sorbent was filled in the unit tube.?3?Considering that the performance of the scroll expander plays a decisive role in the power generation performance of the system,the static performance test system and the dynamic performance test system of the scroll expander are established.The dynamic performance test system of the expander shows that it is practicable to use the high-temperature and high-pressure refrigerant ammonia from the MnCl₂ sorption bed to drive the expander.The expander can continuously output the shaft work,but the mechanical friction loss of the expander greatly reduces the output of shaft power.?4?The theoretical model of two-stage solid sorption combined cooling and power system was established.Considering that the sorption bed consists of many unit tubes,the thermodynamics and heat transfer model of a unit tube was proposed.Then the calculation method of the sorption bed was proposed.For the unit tube,the temperature gradient in the radial direction is small,which is mainly due to that the ENG-TSA is utilized and the radius of the unit tube is small.When the packing density is 500 kg/m³ and the ratio between the MnCl₂and the ENG-TSA is 5:1,the thermal conductivity is up to 16.5 W/?m K?.For the sorption bed that was divided into several zones along the air/exhaust gas flow direction,the partitioning calculation method has been used.?5?The experimental system of the two-stage solid sorption combined cooling and power system was established.Firstly,the hot air is employed to simulate the exhaust gas.Experimental results show that higher heat source temperature can effectively shorten the resorption time,so the refrigerating capacity in a cycle can be increased.When the heat source temperature increases from 210^? to 270^?,the refrigerating capacity in a cycle increases from 1.19kW to 1.52 kW.The refrigerating capacity in a cycle decreases with the decrease of the refrigerating temperature.When the refrigerating temperature decreases from 0^? to-18^?,the refrigerating capacity in a cycle decreases from 1.42 kW to 1.04 kW.The COP and SCP are 0.13⁰.15 and 89.6 W/kg¹32 W/kg,respectively.?6?By analyzing the experimental results of the system,some problems have been found such as the long cycle time and the limited heat transfer effect of condenser and evaporator so on.So some optimization has been conducted as follows:?a?The arrangement mode of unit tubes in the sorption beds is changed from the aligned arrangement to the staggered arrangement,and the total number of unit tubes is reduced from the 200 to 121.?b?Both the evaporator and the condenser are directly selected from the mechanical compression refrigeration system.?c?The expansion valve between the evaporator and the liquid storage tank is firstly utilized,which ensures that the refrigerant ammonia is always in low-temperature and low-pressure state when it flows into the evaporator.As a result,the entire weight of the optimized system is reduced from the 260 kg to150 kg,and the initial investment is also reduced.Experimental results show that when the refrigerating temperature is-25^?-0^?,the refrigerating capacity is1.63 kW-2.46 kW.The maximum COP and maximum SCP are 0.258 and 186.4W/kg,respectively.For the earlier-established system,the cycle time is about240 min,and the resorption time is 120 min.For the optimized system,the cycle time is about 65 min,and the resorption time is 30 min.A reasonable resorption time is very beneficial for the storage of the goods,since there is no refrigerating capacity outputted during the resorption time.?7?When the cooling and power are output simultaneously by the system,experimental results show that when the resorption time is 30 min,the average shaft power in a cycle is about 109.2 W,which can provide a part of the power for the fan.Refrigerating capacity obtained from the combined cooling and power system are basically same as that obtained from the refrigeration system.Because the combined cooling and power system outputs the mechanical shaft during the resorption process,it can supply part of the power for the fans.Therefore,the COP and the exergy efficiency of the combined cooling and power system are higher than that of the refrigeration system.?8?In order to determine the performance of the two-stage solid sorption combined cooling and power system in the practical application,a 4.2 m refrigerated truck is chosen for application characteristics analysis.The original system can meet the required refrigerating capacity of refrigerated trucks at the refrigerating temperature of 0^? and-5^?,which means that it can meet requirements of fresh goods transportation.The optimized system can fully meet the required refrigerating capacity of refrigerated trucks,so the optimized system is installed on the truck to conduct the test,and sorption beds are heated by the engine exhaust gas.Two sorption beds are directly cooled by the ambient air and no electric fan is installed at the sorption bed.As a result,the cooling process of sorption beds depends on the velocity of the truck.The temperature of the engine exhaust gas during the heating process is 200-250^?,and the ambient temperature is 25^?.The experimental results show that the system driven by engine exhaust waste heat can meet the requirement for transporting not only fresh goods but also some frozen goods.The feasibility of a novel solid sorption combined cooling and power system driven by the exhaust waste heat has been verified in this paper.The main target of the system is to output the cooling capacity,and the electricity is used for driving system itself.