Development of a heat transfer model for waste-water heat recovery system

Condensers are an essential part of a vapor-compression heat pump cycle, which is commonly used for residential waste-water heat recovery systems. Heat exchangers with the multi-phase flow, such as the condenser, are one of the most complicated elements of a heat pump cycle. This study is mainly focused on condensers and their behavior in the case of simultaneous heat rejection and condensation. Rejected heat from the condenser has a significant potential for energy recovery and can be later used to supply heat to another source such as waste-water of which heat will be recovered or tap water. For the storage of this recovered energy, phase change materials are used, and they are aimed to store the rejected heat from the condensing refrigerant. In the mathematical model, the energy equation for the double pipe condenser with R134a refrigerant is resolved in the enthalpy form. The upwind scheme is used to discretize the governing equations. Different empirical equations for the heat transfer coefficient from literature are compared. After verification of the condenser model with similar work from literature, paraffin as phase change material (PCM) is added to the system. The thermal energy that is released during the condensation is stored by PCM. Later on, this recovered and stored energy is aimed to be used in another process. For the modeled condenser, temperature distribution and vapor quality are given with time. For the PCM results, temperature variations with time are evaluated. According to the results, the condenser model with and without PCM can be used to predict the thermal behavior of a condenser, and the developed model shows good consistency with the literature.