Theoretical And Experimental Studies On The Diffusion Absorption Heat Transformer

As an important way to upgrading the temperature of heat sources,absorption heat transformers(AHTs)can effectively utilize and recycle the low grade thermal energy.AHTs are very potential for the energy saving and emission reduction in industries.However,electric mechanical pumps are still necessary to produce and maintain the pressure differences in almost all AHTs.Namely,the low grade thermal energy can be utilized and recycled only if the extremely high grade electric energy is supplied,which is contrary to the aim of the low grade energy driving.In this dissertation,the overview and development of AHT was reviewed.On the base of the previous research,a diffusion absorption heat transformer(diffusion AHT)was proposed by introducing the principle of diffusion absorption refrigeration.The diffusion AHT could be completely driven by heat without any electricity.Thermosyphonic bubble pumps instead of electric pumps were employed in a diffusion AHT.In this way,the low grade thermal energy could be utilized and recycled with no moving parts,high stability and reliability.In order to solve the corrosion problem caused by aqueous LiBr in conventional AHTs,aqueous HCOOK was innovatively employed as absorbent for refrigerant in this dissertation.Aqueous HCOOK of low cost,low corrosivity and good environmental compatibility was promising in practical applications.Theoretical and experimental studies on the diffusion AHT with R134 a as the diffusion gas,TEGDME as the absorbent for diffusion gas,H2 O as the refrigerant,LiBr or HCOOK as the absorbent for refrigerant were conducted in this dissertation.The main contents were as follows:(1)An integrated thermodynamic model of the diffusion AHT was first built.Ideal gas model,modified Patel-Teja equation of state and electrolyte NRTL model were used to calculate the physical properties and phase equilibrium of working fluids according to characteristics of different substances.The model of bubble pumps were also built to design the experimental setup of the diffusion AHT.(2)In the detailed analyses of the cycle performance of diffusion AHT,influences of different paramters like temperatures,flow rates and concentrations of working fluids on the system performance were investigated.In the constant temperature working condition(90°C of temperature of generator for diffusion gas,90°C of temperature of generator for refrigerant,90°C of evaporator temperature,30°C of condenser temperature,30°C of temperature of low temperature absorber,120°C of temperature of high temperature absorber),it was found that the highest COPs could be realized as 0.1701 and 0.1693 by suitable flow rates and concentrations of working fluids in the diffusion AHT with LiBr and HCOOK as the absorbents for refrigerant,respectively.The performance of the diffusion AHT with HCOOK was a bit worse than that with LiBr in the comparison.The results showed that lower condenser temperature,lower temperature of low temperature absorber,lower temperature of high temperature absorber,higher temperature of generator for diffusion gas,higher temperature of generator for refrigerant,higher evaporator temperature were all beneficial to the improvement of COP when the other parameters were specified.(3)According to the cycle performance calculation results of diffusion AHT,the experimental setup of diffusion AHT with HCOOK as the absorbent for refrigerant was designed and built.The experimental setup could ran steadily and effectively after much debugging and improvement.Different kinds of experiments have been studied on the performance of the diffusion AHT in this experimental setup such as the generation and absorption performance of diffusion gas,the temperature lift performance of diffusion AHT with electric gas pump,the temperature lift performance of diffusion AHT with thermally driven pump.(4)In the experiment of the generation and absorption performance of diffusion gas,the flow rates of diffusion gas(0.085~0.428g/s)could be adjusted steplessly by the variations of heat input to generator for diffusion gas and numbers of bubble pumps on work.Larger heat input to the generator for diffusion gas could lead to larger flow rate of diffusion gas,larger flow rate of pumped TEGDME solutions,higher temperatures of generator for diffusion gas,higher pressures,but smaller pumping ratios of bubble pumps.For higher efficiency of the diffusion gas generation,a fewer number of bubble pumps were suggested in the diffusion gas supply with required flow rates.(5)In the experiment of temperature lift performance of diffusion AHT with electric gas pump,the gross temperature lift was realized as high as 17.6°C when the evaporator temperature was 99.5~111.0°C.In the experiment of temperature lift performance of diffusion AHT with thermally driven pump,the gross temperature lift was realized as high as 11.8°C when the evaporator temperature was 115.0~125.4°C.It was found similar impacts on temperature lifts in both experiments.Larger evaporator heat input,smaller flow rate of aqueous HCOOK,more concentrated aqueous HCOOK,lower evaporator temperature,lower system pressure,were all beneficial to the temperature lift performance.(6)On the base of the built thermodynamic model of diffusion AHT,80 groups of highest temperatures in the high temperature absorber could be predicted with some experimental results.92.5% of the differences between the predicted results and the experimentally measured results were smaller than 4°C,while 63.75% of the differences were smaller than 2°C.It was indicated that the built theoretical model of high accuracy and reliability in this dissertation was remarkably instructive to the design calculation and performance prediction of diffusion AHT.