Predicting Service Life of Heat Pump Refrigerant Cycles Based on Corrosion- and Fatigue-Induced Degradation
This study develops a specific degradation model for the refrigerant cycle in heat pumps. The model allows a failure time prediction of the refrigerant cycle with respect to the degradation processes corrosion and fatigue. To enhance the model’s accuracy, specific stressors (mechanical stress amplitude, mechanical stress cycles, relative humidity and temperature) associated with these degradation processes are derived from measurements and simulations and used to estimate the service life. In the modeling approach, the impact of a defect growing on the surface of a pipe onto the lifetime of the refrigerant cycle is investigated. A literature model was adapted, such that the equation describing corrosion incorporates temperature and humidity. Additionally, an approach to describe the crack growth influenced by both, corrosion and fatigue, is introduced. The study utilizes the simulation result of the mechanical stresses induced by compressor operation at three different frequencies (i.e. 30 Hz, 60 Hz, and 90 Hz), as well as the calculated results of mechanical stresses resulting from thermal expansion. To each frequency belongs a specific location in the refrigerant cycle with maximum mechanical stresses. At each location of maximum mechanical stresses, its unique stressors are determined. Both, mean and maximum values of the stressors are determined to predict the worst-case and mean failure times at the specific location. The results show that at the location of maximum mechanical stresses due to compressor operation at 90 Hz, the failure time calculated using mean stressor values is 19 years; while using maximum stressor values, it is approximately 12 years. For the location of maximum mechanical stresses caused by thermal expansion, the failure time calculated with maximum stressor values is 17 years. The simulation results are integrated into a more comprehensive analysis from simulation and experimental aging of six heat pumps over more than 2 years in the HP Resilience project.