Performance And Application Of CaCl₂/Expanded Graphite Adsorbent For Double Htat Pipes Type Refrigeration System

With the development of the world economy and the addition of the energy depletion, the energy and environment issues are the current focus of global attention. Solid adsorption refrigeration get more and more global attention for it can ustilize efficiently the low grade thermal energy and does no harm to the environment. For the application of adsorption refrigeration, the chemical adsorption happened in metal chloride-ammonia is the best working pair at the freezing situation where the temperature is lower than 0℃. CaCl₂-ammonia working pair has the advantage of low evaporating temperature and it can be used to make ice comparision with the physical adsorbents using water as refrigerant. In recent researches, the chemical adsorption icemaker which ustilize the CaCl₂-NH₃ as working pair have been reported. But there are three problems in the application of metal chloride. Firstly, the thermal conductivity of the metal chloride is very low (thermal conductivity of CaCl₂ is only 0.1-0.3W·m^-1K^-1), can not release the adsorption/desorption heat out of adsorber, this causes longer cycle time and decreases the cooling power of the adsorption refrigeration system. Secondly, there is serious agglomeration phenomenon happened in repeated desorption/adsorption process, this decreases the adsorption capacity of adsorbent. Thirdly, in the designing of the adsorption system which use metal chloride-ammonia as the working pair. The exhaust gas and cooling water are switched by the four-port valve, although this system is very simple but it would cause the corrosion problem between exhaust gas and steel tube wall. For solving all of above problems, a new kind of compound adsorbent which is composed of expanded graphite and CaCl₂ is studied. Furthermore, a novel double heat pipes adsorption chiller is presented in thesis. Based on this work, the main contents in this paper include as follows:1. A new kind of compound adsorbent which is composed of CaCl₂-expanded graphite adsorbent is studied in this paper. The expanded graphite will enhance the mass transfer of CaCl₂ by plenty of porous structure inside expanded graphite and solve the adsorption capacity deterioration happened in long term adsorption/desorption process. The consolidated compound adsorbent has a higher thermal conductivity because of consolidated technology is used in compound adsorber. For the pellet CaCl₂ thermal conductivity is 0.31 W·m^-1·K^-1, the consolidated compound adsorbent with 20% expanded graphite thermal conductivity is 7.2 W·m^-1·K^-1. The thermal conductivity increased more than 23 times. So it is possible to decrease the cycle time and volume of adsorber using consolidated compound adsorbent.2. A novel double heat pipes adsorber is established in this paper, which structure is compact and excellent heat transfer performance. The double heat pipes type adsorber integrated with two kinds of heat pipes, namely, heating heat pipe and cooling heat pipe. The heating heat pipe is separated heat pipe and cooling heat pipe is two phase close thermosyphon heat pipe. The heating heat pipe is to heat the adsorber and the cooling heat pipe is to cool the adsorber. The adsorber is heated or cooled by switching these two kinds of heat pipe and the corrosion problem between exhaust gas and steel tube wall is solved. The heat transfer performance is enhanced obviously because the heat transfer in phase change process happened in heat pipes is quite higher than convection heat transfer in traditional adsorber heat exchanger.3. In the base of the compound adsorbent and double heat pipes adsorber, a novel chemical adsorption chiller driven by waste heat from exhaust gas is established. This machine has two adsorber integrated double heat pipes, two condensers and two flooded evaporators. The chemical adsorption chiller is composed of two parts, namely, ammonia compressor and ammonia refrigerator. Ammonia compressor is composed of two condensers installed in parallel, two adsorbers and exhaust gas-water heat exchanger. Ammonia refrigerator is composed of ammonia condensers installed parallel and two flooded evaporators. The adsorption and desorption process is switched by the cooling and heating heat pipe and there are total five valves in whole system. It improves the operating reliability and cuts down the cost of system. The system is designed in equipment style with compact dimension. It will satisfy the commercialized requirement. With the optimum designing of machine, the heat exchangers are not the pressure vessel and it will decrease the check fee for the users every year.4. The simulation model is established for the adsorption refrigeration system. The simulation model is validated by the experiments. The simulation model can show the system dynamic operating performance very well. It is a reliable method to study the double heat pipes adsorption refrigeration system and optimum of the system.5. The test rig for testing the adsorption chiller machine performance is constructed. The experimental results prove that double heat pipe integrated into adsorber will enhance the heat transfer process of adsorber. The speed of desorption and adsorption process is fastened. The cycle time for heating and cooling process is shortened. When adsorber temperature change from 35℃-120℃, the heating time or cooling time is twelve minutes which is decreased almost 100% compared with compound heat pipes type adsorber using same heat pipe working medium. The averaged cooling power is 2.64kW, SCP is 200W/kg. It has proved that the double heat pipes adsorption chiller is efficient and feasible.