Research On Performance Of Large Compression Ratio Positive Pressure Dual Source Heat Pump Water Heater

Under the influence of global technological innovation and environmental protection requirements,the use of renewable energy has become an important research topic for countries to achieve energy conservation,pollution control and energy efficiency.Among renewable energy sources,solar energy and air energy have received great attention in recent years because of their advantages of easy access and environmental friendliness.Solar energy has the ability to be clean,renewable and easy to use,and it is free.The air source heat pump is regarded as a kind of heat energy regeneration system,and it has been vigorously promoted by China in recent years under the favorable policy of "coal to power".But a single energy source is relatively backward,and the combination of multiple energy sources is the way of future development.The combination of solar collectors and air-source heat pumps is a breakthrough attempt in multiple energy combinations.A solar-coupled air-source heat pump system is proposed.The heat-collecting evaporator is a conventional multi-row air-cooled fin-tube evaporator coated with a solar film.Early research shows that when used to make domestic hot water,the system COP is significantly improved compared to conventional air source heat pump hot water systems.However,the small solar absorption area of the heat collection evaporator leads to the main source of heat produced by the unit still being air energy.Based on this,the structure of the heat collection evaporator was modified,and a new positive pressure dual source heat collection evaporator was proposed.Introduce radiative heat transfer correction(solar energy absorption)into the mathematical model of a conventional air source heat pump evaporator.Especially in the mathematical model of the conventional air source heat pump evaporator,a new mathematical model suitable for the positive pressure dual source heat pump water heater is established by considering the non-uniformity of the wind speed distribution and the flow self-organization theory.The simulation program written in MATLAB is used to simulate the performance of the system.Under the premise that the air volume of the fan,the distance between the fins and the tube length of the heat collection evaporator are unchanged,the simulation model is used to optimizethe aspect ratio of the evaporator.As a result,there is an error of about 5%between the calculated value and the experimental value,and it has high accuracy.Solar radiation has a better effect on improving the heating performance of the system and improving the stability of the unit operation.The optimal aspect ratio of the evaporator is 1.1,optimized the annual average COP of the rear unit has increased by 10.4% before optimization.The monthly average COP of the unit after optimization has also significantly improved,especially in July,11.4% higher than before optimization,and 32.8% higher than the conventional air source heat pump,and the energy saving effect is good.Since the positive pressure dual source heat pump water heater needs to be operated all year round,frosting is inevitable.However,the research on the positive pressure dual source heat pump water heater under frost conditions has not been involved.The positive pressure dual source heat pump water heater uses solar energy + reverse cycle defrost.This article conducts experiments on the basis of the established positive pressure dual source heat pump water heater prototype,quoting the anti-frosting perfection theory,and comparing and analyzing the characteristics of the two with conventional air source heat pump water heaters during operation to quantify the delayed frosting / defrosting performance.The results show that during operation,the conventional unit performs a total of 6 defrosts,while the positive pressure dual source unit only has 2 times.The defrost cycle of the positive pressure dual source unit is 4.14 times that of the conventional unit.The heating performance coefficients of the positive-pressure dual-source unit and the conventional air-source heat pump unit during operation were 2.83 and 2.75,respectively.The positive-pressure dual-source unit increased the heating performance coefficient by 2.9% over the conventional unit.The defrost performance coefficients during operation are1.66 and 1.5,respectively.The positive pressure dual source unit has a 10.7%higher defrost performance coefficient than the conventional air source heat pump unit.The anti-frosting perfection during operation is 0.587 and 0.1545,respectively.The positive-pressure dual-source unit has a 7.51% higher anti-frosting perfection than the conventional air source heat pump unit.Single-stage positive-pressure dual-source units suffer from problems such as reduced air intake,excessive compression ratio,and high exhaust temperature during winter operation,resulting in shutdown of the unit’s high-temperatureprotection and insufficient heating capacity.In view of this situation,a compression cycle of supplemental gas and enthalpy was introduced.In order to study the system performance,experimental investigations were performed on the characteristics of the positive pressure dual-source heat pump water heater with a large compression ratio to start the supplementary gas and increase enthalpy.Compare the characteristics of heat pump water heaters,and find a suitable control strategy for starting the supplementary gas and increasing enthalpy.The results show that when the outdoor dry-bulb temperature is 0 °C,when the exhaust temperature is close to the compressor’s high-temperature protection temperature(110 °C),the water temperature of the water tank of the conventional single-stage positive-pressure dual-source heat pump water heater is about 55 °C,and the make-up gas is started.The enthalpy-increasing unit can heat the water to 66.2 °C,which is 11.2 °C higher than the single-stage positive-pressure dual-source heat pump water heater.Heating water to 55 °C,the conventional single-stage system takes longer to heat up than the start-up gas supplement enthalpy system at the same ambient temperature.At 0 °C,7 °C,14 °C,and 20 °C,the single-stage system takes longer than the start-up gas supplement.The heating time of the system is 28.6 min,21.8 min,18.7 min and15.3 min.The single-stage system can only heat water to 45.7 °C at-7 °C.The supplemental gas enthalpy system can produce hot water at 55 °C stably,and the system COP is still 2.42.According to the change of COP,the operation control strategy of the positive pressure dual-source heat pump water heater is proposed as follows: when the outdoor ambient temperature is greater than or equal to14 °C,the system is only allowed to run in single-stage mode.When the outdoor ambient temperature is lower than 14 ℃,the system is controlled by the exhaust temperature whether the supplemental air increases enthalpy.When the exhaust temperature is lower than 60 ℃,the system operates in single-stage mode,otherwise the supplemental air increases the enthalpy.