Theoretical And Experimental Studies On The Diffusion Absorption Heat Transformer With H₂O-CaCl₂-R134a-TEGDME

Diffusion absorption heat transformer（diffusion AHT）is a thermally-driven technology to lift low-grade heat with the advantages of no moving parts and no need of electricy,which is of significance for the recovery and utilization of low-grade thermal energy.In this paper,theoretical and experimental studies have been carried out on the performance of the diffusion AHT with H₂O as refrigerant,Ca Cl₂ as absorbent for refrigerant,R134a as diffusion gas and TEGDME as absorbent for diffusion gas.The main contents and corresponding conclusions are as follows:（1）Based on reasonable assumptions,a complete theoretical model of the diffusion AHT conbining two bubble pump transport models and thermodynamic model is established for the first time.The bubble pump transport model is a semi-empirical model,which is used to calculate the vapor generating and solution transporting performance of bubble pump under different geometry and operation parameters.Attribute to the combination of bubble pump transport model,system performance under different operating parameters of system and bubble pump can be calculated by the complete theoretical model,which lays a foundation for futher studies.（2）Based on the bubble pump transport model,the influences of five main parameters on the performance of the key component-refrigerant generating bubble pump are obtained by control variate method:（a）the increases of diffusion gas flow rate,high temperature reservoir liquid level and refrigerant generating temperature,or the decreases of system pressure and mass fraction of Ca Cl₂ are beneficial to the generation of refrigerant;（b）with the increase of diffusion gas,concentrated solution flow rate and pumping ratio increase first and then decrease;（c）with the increase of high temperature reservoir liquid level,concentrated solution flow rate and pumping ratio increase;（d）the influences of refrigerant generating temperature,system pressure and mass fraction of Ca Cl₂ on concentrated solution flow rate and pumping ratio are not obvious.（3）Based on the complete theoretical model,the influences of eleven main parameters on the performance of diffusion AHT are obtained by control variate method:（a）with the increase of diffusion gas flow rate or decreases of high temperature reservoir liquid level,system pressure and mass fraction of Ca Cl₂,the gross temperature lift and total heat input increase,so system performance and energy consumption should be weighed;（b）when refrigerant generating temperature increases,gross temperature lift increases while total heat input decreases,so it can be increased moderately to reduce total heat input;（c）the total heat input is only affected by the low temperature reservoir liquid level,diffusion gas generating temperature,low temperature absorber temperature,lifting tube number of diffusion gas generator,pressure difference between diffusion gas separator and low temperature reservoir and condensen temperature,which means the total heat input can be reduced by optimizing values of the six parameters.（4）Through the improvement of experimental setup such as the arrangement of valves and components,three kinds of experiments are conductded:refrigerant generating bubble pump experiment,thermally-driven diffusion AHT experiment and gas-pump-driven refrigerant generating/absorbing loop experiment.（5）The influences of diffusion gas flow rate,high temperature reservoir liquid level,refrigerant generating temperature,system pressure and mass fraction of Ca Cl₂ on the refrigerant flow rate,concentrated solution flow rate and pumping ratio are verified by the refrigerant generating bubble pump experiment.The accuracy of the bubble pump transport model is acceptable:the average relative errors of refrigerant flow rate,concentrated solution flow rate and pumping ratio are 18.29%,5.46%and 8.55%,separately.（6）The influences of evaporator heat input,high temperature reservoir liquid level,diffusion gas flow rate and mass fraction of Ca Cl₂ on the absorber temperature lift,the maximum absorber temperature lift,gross temperature lift,the maximum gross temperature lift and refrigerant generator heat input are verified by the thermally-driven diffusion AHT experiment.The average relative errors of gross temperature lift,the maximum gross temperature lift and refrigerant generator heat input are 30.94%,10.34%and 4.36%,separately.In theoretical model,lumped parameter method is used to calculate state parameters of each component,so the temperature distribution in each component can not be calculated,which results in large average relative errors of gross temperature lift.It means that the theoretical model only can be used for steady-state operation calculation.（7）The influences of diffusion gas flow rate and system pressure on absorber temperature lift,the maximum absorber temperature lift,gross temperature lift,the maximum gross temperature lift and refrigerant generator heat input are verified by the gas-pump-driven refrigerant generating/absorbing loop experiment.The average relative errors of gross temperature lift,the maximum gross temperature lift and refrigerant generator heat input are24.00%,11.88%and 7.80%separately,in which the accuracies of the maximum gross temperature lift and refrigerant generator heat input are acceptable.（8）In this paper,Ca Cl₂ with low cost is used as the absorbent for refrigerant,which possesses higher thermal stability than HCOOK and lower corrosiveness than Li Br.By using Ca Cl₂,a nice temperature lift is obtained in the experimental setup and the decomposition of HCOOK in high temperature is solved,so the upper limit of driven heat source temperature is broadened.