A Study of Transient Behavior During Start-Up of Residential Heat Pumps

Abstract

An experimental and analytical study concerned with the transient performance of heat pumps is presented. A series of tests were performed to study and characterize the transient sensible and dehumidification performance of a heat pump in the cooling node. All the tests were conducted according to the ASHRAE Standard (1983). The effects of indoor dry-bulb temperature (72 to 80 F), indoor relative humidity (20 to 67%), outdoor dry-bulb temperature (82 to 100 F), cycling rate (0.8 to 10 cph) and percent ON-time (20 to 95%) on the transient performance (sensible and latent capacity, efficiency and sensible heat ratio) are addressed in this study. The results indicated that part load factor (PLF) can be as low as 0.65 at low percent ON-times and high cycling rates. The combination of low percent ON-times and high cycling rates produced maximum cyclic losses. The dehumidification process usually started between 60 to 150 seconds after start-up depending on the indoor ambient conditions, percent ON-time and cycling rate. The sensible and latent capacity response and part load efficiency of the heat pump increased with an increase in indoor relative humidity. There was a slight increase in PLF with indoor dry-bulb temperature. The outdoor dry-bulb temperature did not have any effect on the transient performance. Based on the functional relationship of PLF and coefficient of degradation (C_D) with the five independent variables a multiple linear regression analysis was performed on the experimental data. The analysis yielded two general equations to predict PLF and C_D. A lumped parameter heat pump transient analysis computer model was developed, which drew upon methodologies from the earlier models by Chi and Didion [1982] and Oak Ridge National Laboratory [1981]. The model was capable of simulating the transient response of a vapor compression air-to-air heat pump commonly used in residential applications. The simulated results were in good agreement with the laboratory results at high percent ON-times (> 20) and high relative humidities (> 45).