| The industry of Refrigeration & Air-conditioning continuously adopts the environmentally friendly policy with the growing crisis of chemical and fuel energy, depletion of ozone layer and increasing global warming potential. Until recently, low-grade energy, such as geothermal energy, industrial waste and solar power, are abundant around our life, so it has become increasingly significant about the importance of how to use the existed low-grade energy. This paper discusses about the innovative adsorption refrigeration technology, combining the physical adsorption cycle with the chemical adsorption cycle. The combined adsorption refrigeration cycle could utilize the low-grade energy to produce freezing power and refrigeration power simultaneously, resulting in the utilization of gradient thermal energy.The low-grade energy always exists in the environment with the temperature ranging from 60 to 100 oC. The adsorption refrigeration technology, using the environmentally friendly refrigerants and being powered by low-grade energy, will bring tremendous economic and social benefits. The adsorption refrigeration will become a competitive method to effectively utilize thermal energy such as solar energy or waste heat as primary driving energy. A gradient thermal driven adsorption refrigeration cycle is constructed to utilize low-grade energy. Theoretical and experimental performance analysis of the cascading cycle is presented.A two-stage adsorption freezing cycle is adopted as the first stage and the silica gel/water adsorption air-conditioning cycle is used as the second stage. In the gradient thermal driven cycle, the two-stage chemical adsorption refrigeration cycle is used as the first stage. In order to get the adsorption freezing effect for the heat source with the temperature lower than 100 oC,a novel two-stage adsorption freezing machine is developed. In the system the composite adsorbents of CaCl2 and BaCl2 developed by the matrix of expanded natural graphite are chosen as high temperature adsorbent and low temperature adsorbent, respectively. The adsorption and resorption processes are coupled together for constructing a two-stage desorption process to generate the freezing effect. The experimental results show that the system can generate the refrigerating power at-15 oC when the cooling temperature is 25 oC and the heat source temperature is as low as 70 oC.When the hot water inlet temperature ranges from 70 to 90 oC, cooling water is at 25 oC, cycle time ranges from 13 to 43 min and the evaporation temperature ranges from-15 to 0oC, COP ranges from 0.08 to 0.20 and SCP ranges from 110 to 226W/kg, the exergy efficiency ranges from 0.08 to 0.31.To extensively utilize low-grade energy, the silica-gel/water adsorption refrigeration is adopted as the second stage. Heat and mass recovery techniques are used to improve the COP of the system. In the experiments the heat source temperature ranges from 70 to 90 oC, condensing temperature is controlled at 25 oC. The results show that thermal coefficient of performance(thermal COP) of gradient thermal driven cycle ranges from 0.13 to 0.28, which improves 87% compared with the adsorption freezing cycle under the heat source of 90 oC and evaporating temperature of-15 oC. The refrigerating capacity is 4.56 kW under driving heat source at 70 oC and the improvement is 76.5% if the heat source temperature increases to 90 oC. The exergy efficiency of the cycle are calculated to be 0.11-0.13 under the above conditions, and the exergy efficiency for heat utilization varies from 0.31 to 0.42, which is improved 93% and 78% compared with the adsorption freezing cycle and adsorption cooling cycle, respectively when the heat source temperature is 90 oC.A new type of consolidated composite adsorbent for CaCl2 is developed by the matrix of ENG-TSA(Expanded natural graphite treated by the sulfuric acid). Adsorption performance of consolidated composite CaCl2 was tested, and the results showed that for the sample with the density of 400kg/m3 and salt mass ratio of 80%, adsorption quantity ranges from 0.4015kg/kg to 0.4585kg/kg while the cooling temperature ranges from 25 to 35 oC and evaporating temperature ranges from-10 to 15 oC,respectively. The cycle adsorption quantity of consolidated CaCl2 with ENG-TSA as matrix is similar with that of consolidated CaCl2 with ENG as matrix, whereas the addition of ENG-TSA can greatly reduce cycle time as well as improve the specific cooling performance per kilogram adsorbent(SCP).The cycle time for the composite adsorbent of CaCl2/ENG-TSA is around 3300 seconds, which is 33% reduced if compared with that for the adsorbent of CaCl2/ENG. When the condensing temperature is 30 oC, the maximum SCP for the adsorbent of CaCl2/ENG-TSA is 65.75W/kg which is increased by 48% if compared with the adsorbent of CaCl2/ENG.In this paper, the virtual adsorbent heat exchanger was constructed. The theory of entransy dissipation was introduced to analyze the heat transfer performance of the adsorption system, and then the entransy dissipation rate of the adsorber was investigated by a modeling approach. The relations between the entransy dissipation rate and the cycle time under the conditions of different heat source temperature and different heat transfer coefficient was presented, respectively. The entransy temperature difference was introduced as a criterion of the heat transfer performance of the adsorbent system. The relations between the entransy dissipation and heat transfer coefficient, as well as the relations between entransy temperature difference and heat transfer coefficient under different heat source temperature were analyzed. The research showed that at the switch time for the alteration of heating and cooling processes for adsorption system, the entransy dissipation occupies about over than 80% of the total value of the entransy dissipation, and the heat recovery could decrease this value effectively. Under the condition of high heat source temperature and low heat transfer coefficient the study also indicated that the heat recovery could significantly decrease the entransy dissipation and the average entransy temperature difference, thus improving the system performance greatly. |
PDF Full Text Request