Influence of airflow rates on air source heat pump operating parameters

Abstract

Air source heat pumps (ASHPs) are energy-efficient and environmentally friendly heating systems widely used to meet the heating needs of buildings. Therefore, researches that ensure the effective use of ASHP systems are crucial for improving the energy efficiency of buildings. Faults, design flaws, or user-controlled settings can cause changes in the airflow rates of the evaporator and condenser in ASHPs. This study examines the effect of changes in the airflow rates of the evaporator and condenser on the operating parameters of ASHPs in heating mode using a developed physics-based model. Although the results of experimental studies in the literature provide useful information in this regard, a model-based study would allow for the examination of different systems with their specific parameters. The model is an advanced version of the author's previous model, which now includes the effects of airflow rates in addition to outdoor air conditions, and it has been validated by comparison with experimental data correlations published in the literature. The logarithmic mean temperature difference-based solution method employed by the model allows for its potential for further development and improvement. Four different sizes of air conditioners with heating capacities ranging from 2.5 kW to 8.2 kW were investigated. Corresponding to EN-14511 test standards, 7 °C outdoor temperature and 90% relative humidity condition and 20 °C indoor temperature condition were taken account. The effects on the ASHP parameters were determined by varying the airflow rates in ratios between 0.2 and 1.4 with 0.2 intervals. The airflow ratio term is defined as the ratio of the flow rate to the maximum airflow rate provided by the fans. It is assumed that the condenser heat load is kept constant at its nominal rate to maintain a consistent indoor temperature using inverter-driven variable speed compressor technology. The results show that changes in the condenser airflow rate have a greater impact on system parameters than changes in the evaporator airflow. Reducing the condenser airflow ratio to 0.4 with the silent mode option reduces the COP value by 21% and increases energy consumption by 44%, indicating that manufacturers should inform customers about the potential decrease in performance when using lower condenser airflow rates. A devastating performance drop is observed when airflow ratios in either the condenser or evaporator drop below 0.4. The performance curve of the ASHP produced by the model can help determine the optimum airflow rates for optimal performance. Systems which can be designed close to optimum airflow rates are less affected by airflow changes. In frost-free conditions, the impact of changes in evaporator airflow on performance is less significant than that of the condenser. However, decreasing the evaporator airflow rate increases the susceptibility of the ASHP to frosting. For example, reducing the evaporator airflow to 0.6 extends the frosting condition limits from 6 °C and %65 relative humidity to 9.7 °C and %50 relative humidity. The results of this study will be valuable for researchers, manufacturers and users in terms of performance evaluation and improvement based on airflow rates. The presented model can be further improved and has the potential to facilitate the investigation of various conditions, including frosting, in future studies.