Investigating air source heat pump cooling performance and humidity management using a physics-based model

Abstract

The rising cooling demand due to urbanization and industrialization emphasizes the need for efficient air source heat pumps (ASHPs). This study investigates how indoor and outdoor temperature variations affect the performance of three different capacity air conditioners in cooling mode, using a physics-based model that iteratively solves logarithmic mean temperature differences (LMTD) through temperature and heat balance methods. Additionally, study proposes a method for controlling relative humidity by adjusting the evaporator airflow rate in split air conditioners, and determines the necessary airflow rates based on varying outdoor and indoor temperatures. Findings reveal that cooling load reduction is the primary driver of performance improvement. For example, decreasing the outdoor temperature from 35 °C to 30 °C increases the coefficient of performance (COP) from 3.47 to 4.36 in variable cooling heat load (VH) mode, compared to 3.99 in constant cooling heat load (CH) mode. Additionally, selecting higher-capacity ASHPs enhances performance and operational range, though benefits diminish at lower cooling demands. Relative humidity (RH) control was demonstrated by adjusting airflow rates; for instance, while cooling the indoor space to 27 °C, an increase in outdoor temperature to 36.6 °C reduces RH to 32 %, whereas a drop to 29.7 °C raises RH to 73 %. To maintain 40 % RH under these conditions, the airflow rate must vary between 1.26 and 0.34. The impact of airflow rate changes on performance was also assessed. Although reducing airflow is typically expected to cause a performance loss, the increase in compressor efficiency due to a higher compressor pressure ratio mitigates this decrease, rendering it insignificant. Realistic conditions with moisture inputs were examined for two scenarios, showing that relative humidity control becomes increasingly challenging at low cooling loads, especially in crowded spaces. However, reducing the evaporator airflow rate to its minimum still enhances comfort by effectively mitigating excessive indoor humidity levels. The study provides actionable insights for optimizing ASHP performance and design, emphasizing the importance of airflow control for effective humidity regulation. The findings provide actionable insights for both users and manufacturers. For users, illustrating the impact of air conditioning selection and usage preferences on performance and comfort helps inform better decisions. For manufacturers, the study suggests developing systems that integrate temperature and humidity regulation with automatic airflow adjustment mechanisms driven by real-time relative humidity sensing.