Study On The Performance Of The Open Absorption Heat Pump System With Stable Output Driven By Unstable Heat Source

Efficiently harnessing of low-grade energy sources,such as widespread solar energy and industrial waste heat,is crucial for improving energy efficiency and reducing carbon emissions.The traditional electric-driven mechanical compression heat pump realizes the heat transfer at the expense of high-grade electric energy.In contrast,absorption heat pumps recover heat with thermal energy and minimal pump power,as they leverage the principle of state parameter variation such as vapor pressure difference between solution and refrigerant.Open absorption heat pumps offer several advantages over closed absorption heat pumps,including the ability to directly absorb the latent heat from humid gas without limitations related to the dew point,fewer sealing components,a simpler overall structure,etc.While most existing research on open absorption heat pumps has focused on steady-state models under stable operating conditions,it is essential to explore their performance when driven by unstable and intermittent heat sources like on-site solar energy and industrial waste heat.To effectively utilize such low-grade unstable heat energy sources,this study establishes three different cycle configurations based on the open absorption heat pump.These configurations are designed to address the temperature instability of heat source and the discontinuity of thermal energy.A theoretical calculation model is simultaneously developed to identify key influence parameters and uncover their effects on the stable output of the system.Meanwhile,comprehensive experimental testing is conducted to validate the reliability of proposed theoretical models.In this paper,a comparative study of the operating characteristics of a direct-flow open absorption heat pump system and a mixed-flow open absorption heat pump system is carried out.The results show that for the same latent heat recovery rate,the direct-flow open absorption heat pump system has lower absorption and generation temperatures,and the final output temperature is also lower.It was found and demonstrated in detail that the mixed-flow open absorption heat pump system decouples the absorption process from the generation process by mixing the concentrated solution from the generator with the dilute end-ofabsorption solution as the beginning of the absorption,which ensures that the state of the beginning of absorption solution is not affected by the state of the end-of-generation solution.In the case of unstable heat source,the heat output and performance of the system can be stabilized only by changing the flow rate of dilute solution into the generator.Based on the theoretical findings,this paper presents an experimental study of a mixed-flow open absorption heat pump system driven by a variable-temperature heat source.The system output instability due to the occurrence of heat source temperature fluctuations is eliminated by adjusting the dilute solution flow rate into the generator during the experiment.The results show that when the unstable heat source varies in a small temperature range,the mixed-flow open absorption heat pump system can still achieve a more stable output by regulating the system solution flow.In this work,the theoretical model of the open absorption heat transformer and heat pump is established.The theoretical results show that the open absorption heat transformer driven by a variable heat source exhibiting minor temperature fluctuations,can also maintain a consistent output by adjusting the flow parameters within the system.With identical design parameters,the generation temperature and output hot water temperature ranges fall between72.93℃-79.41℃,and 59.94℃-60.27℃ for the open absorption heat pump,while these ranges are 50.73 ℃-56.23 ℃ and 60.12 ℃-61.28 ℃ for heat transformer,respectively.In the hybrid system comprising both the open absorption heat pump and the open absorption heat transformer,the generation temperature variation range is notably broader at 28.24℃,comparing to the narrower ranges of 4.64℃ for the heat pump and 6.48℃ for the heating transformer in their individual models.In this scenario,the fluctuation range in the output hot water temperature remains within a narrow margin of 1.34℃,and the output load change rate does not exceed 1.55%.Consequently,even under wide temperature range heat source driving conditions,the hybrid system with open absorption heat pump and heat transformer can provides technical means to achieve narrow temperature range thermal output.Meanwhile,this study establishes a theoretical dynamic model for an open absorption heat pump energy storage system,based on which both partial energy storage and pure energy storage modes are analyzed.The charging and discharging processes of the open absorption heat pump energy storage system are both unsteady.With the same design parameters,the theoretical relative deviations of COP and energy storage density of the two modes are 3.68 %and 4.33 %,respectively.To validate the theoretical model,this paper also conducts experimental research on the partial energy-charging and pure energy-releasing processes.The results indicate that the proposed theoretical model coincides well with the experimental findings.The relative deviations between the theoretical and experimental coefficient of performance(COP),energy storage density(ESD),and charging time per unit energy are 3.6%,3.5%,and 2.7%,respectively.The research findings of this dissertation hold significant theoretical and practical significance to achieve stable output of open absorption heat pump systems driven by unstable heat sources.