| Air source heat pumps are widely used in the heating field.In cold areas,in order to provide stable and continuous heating,Low-temperature heating heat pump units mostly use multi-stage compression cycles.Among them,the cascade cycle has the characteristics of simple structure,good low-temperature cycle performance,and has good low temperature.Application prospects,but in the medium and high temperature environment,the performance advantage of the cascade cycle is reduced.How to improve the operating performance of cascade heat pumps under multiple working conditions is the research purpose of this topic.This topic firstly selects the working medium R410 A as the single working medium of the cycle,and uses the thermodynamic calculation method to find that for a single working medium,when the working conditions are stable,there is an optimal intermediate pressure,and the heating performance of the cascade cycle is better.The intermediate pressure is the control target,and the displacement of the high and low temperature compressors is the control variable.Based on the existing research results,models are established for both single-stage and cascade cycles,and the performance of single-stage cycles is studied by changing the frequency of the low-temperature stage,and the high temperature is changed.The stage frequency is changed at the same time,the low-temperature stage frequency is adjusted and the intermediate pressure is adjusted to study the cascade cycle performance.Combining simulation and experimental research,the heating performance of the cascade heat pump is studied and combined with the single-stage cycle in the exhaust temperature,COP,and heating capacity.Compare aspects.In the experiment,the high and low temperature compressors used variable-capacity compressors,the cooling end of the experiment was measured by the liquid refrigerant method,and the evaporating end was balanced by the heat balance method.The experimental conclusions are as follows:(1)When the evaporation temperature increases from-35℃ to-5℃ the temperature difference between the outlets on both sides of the condenser drops from 10.19℃ to 6.5℃.When the evaporating temperature is-35℃,when the high-temperature compressor frequency increases from 90 Hz to 210 Hz with an increase of 15 Hz,the temperature difference between the two sides gradually increases from 5.29℃ to 8.52℃.Under the experimental conditions,the maximum exhaust temperature of single-stage and cascade cycles are 98.36°C and 91.42°C,respectively,and the exhaust temperature does not exceed the safety value of 120°C.(2)The condenser inlet water temperature is 35℃,the evaporation temperature is-35℃,the maximum cascade heating reaches 8.74 k W,and the maximum reaches 19.34 k W at-5℃.When the inlet water temperature increases to 40℃,the cascade circulation heating decreases by an average of 1.10 k W,the single-stage cycle reduces by 1.63 k W on average,and the average decrease in heating capacity is 16.2%.At 35°C water inlet temperature,the heating capacity of the cascade heat pump is higher than that of the single-stage circulation by 3.55 k W,3.86 k W,3.89 k W.under the same working conditions of the two circulating.(3)Both the cascade cycle and the single-stage cycle have the best COP value.The higher the evaporation temperature,the lower the condensation temperature,and the lower the optimal compressor frequency.When the water inlet temperature is 35°C,and the evaporation temperature is-35°C,the optimal COP value of the stacked cycle is 2.25,and the maximum value of-5°C is 3.47.The maximum value of the single-stage cycle COP is 2.11 at-15°C and the optimal value is 2.48 at-5°C.In summary,both cascade and single-stage heat pumps have optimal working conditions.By adjusting the capacity of the compressor,the COP value and heating capacity of the heat pump system can be effectively improved,and the intermediate pressure can be controlled to optimize the cascade under variable operating conditions.The circulating heating capacity and COP value not only enable the cascade cycle to efficiently and stably supply heat in a low temperature environment,but also have certain advantages compared to the single-stage cycle in a medium temperature environment,which effectively improves the comprehensive heating performance of the cascade heat pump... |
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