Theoretical And Experimental Investigation On A Solar Driven Hybrid Absorption And Vapor Compression Coupled Cooling Cycle

Due to the good compatibility between the cooling output from the solar air conditioning system and the cooling load in buildings,the large-scale application of solar cooling system can help reduce the load of power grid in summer.However,as the intensity of solar radiation is low and unstable,the solar air conditioning system usually faces the problems of intermittent and instability.On the other hand,in many cases,the installation area for solar collectors is limited,which has restricted development of solar air conditioning system.Improving the cycle efficiency,reducing the solar collector area and realizing stability by developing hybrid system are the key points of the solar air conditioning application.Focusing on these issues,this paper proposed the solar driven air-cooled absorption and vapor compression hybrid cycle to improve the continuity,stability and economy competitiveness of the system.The efficiency of vapor compression system can be improved by coupling the solar cooling technology.Besides,the stability can be realized by the combination of absorption and vapor compression cycles.The limitation of air-cooled absorption chiller was firstly analyzed.Due to the high cooling temperature,the driving temperature of air-cooled absorption chiller must be raised,which will lead to a low solar conversion efficiency and high crystallization risk.Two hybrid cooling cycles（sub cooling and cascade coupled cycles）between solar driven air-cooled absorption cycle and vapor compression cycle were proposed.The mechanism of the upgrade of energy level and the reduction of electricity consumption was revealed.The hybrid cycles can utilize the low grade solar thermal energy to reduce the electricity consumption and improve the solar cooling efficiency.In order to improve the continuity and stability of the solar air conditioning system in a long term running,a new heat pump driven cooling storage cycle based on LiBr-H₂O concentration difference was proposed by the simultaneously coupling of the heating and cooling energy between absorption and vapor compression cycle.The cooling storage cycle is irrelevant to the environment conditions,since the only energy input is the electricity to the heat pump cycle.The ESD（energy storage density）is high and the heat loss is low.In this way,the efficient multi energy complementation of solar air conditioning system can be realized.The theoretical models of the proposed cycles were established.An air-cooled absorption chiller was developed and tested to validate the models.Based on that,the air-cooled absorption cycle was improved by introducing the adiabatic flash process.The secondary heat exchanging between refrigerant water and chilled water is avoided in the new cycle,therefore the conventional chilled water circulation pump and the heat transfer pipes in evaporator are removed.In this way,it is capable of improving the COP and cooling capacity with a lower size and cost of material,as well as electricity consumption.During the experiments,the evaporation temperature of 8.6℃can be achieved and the COP_th can reach 0.76.During the 9 hours outdoor experiment,this unit was able to maintain the room temperature with an average COP_th at 0.65.Further,the system performance of hybrid cycles was investigated experimently.The results showed that the COP_ele,HP can be improved greatly with the supply of hot water between 70℃and 90℃in the cascade coupled mode.The experimental results of the heat pump driven cooling storage system（rated cooling capacity/power input are40kW/10kW）showed that the ESD can reach 77.8 kWh/m³ and the ESE is about 2.81with a lower heat loss.These results furtherly proved the feasibility to improve the system efficiency,the operation stability and continuity of solar air conditioning system by coupling the absorption and vapor compression cycle.Finally,the optimization of solar driven air-cooled absorption cooling system was conducted.The influence of different parameters on the performance of hybrid system was analyzed.Although the sub cooling coupled system can achieve a higher exergy efficiency with a low heat consumption,the improvement of COP_ele,sys was only15.9%²9.8%,while the cascade coupled system can reach an increase of63.9%¹66.7%.In the cascade coupled system,the absorption sub-system can work independently or work in a cascade mode to reduce the power consumption when the hot water temperature is low.In this way,the efficient temperature range of solar system was extended with a higher utilization ratio.The compatibility of the energy balance between absorption cycle and heat pump cycle in the cooling storage system was analyzed.It was found that the COP_ele,HPle,HP and ESE will be increased with a lower initial concentration of LiBr-H₂O solution and evaporation temperature of heat pump.When the initial concentration was increased from 52%to 57%,the ESE will be decreased by about 34.4%.The average ESD will be increased to 140.2kWh/m³ with a concentration difference of 8%.