In this thesis I investigated the energy saving potential of a low temperature lift heat pump (LTLHP) such as water source heat pump (WSHP), and studied the utilization of a plate heat exchanger (PHE) as the LTLHP evaporator. Due to the facility limitation, I only tested and studied the evaporator for LTLHP. Since the LTLHP requires a large water flow rate, its heat source liquid-to-refrigerant PHE is operated at lower refrigerant mass flux than typical applications. I varied the vapor quality, heat flux, evaporation pressure, and refrigerant mass flux to provide unique heat transfer characteristics, and I studied their effects on evaporation heat transfer. Based on the collected data, I concluded that at a low mass flux range, evaporation heat transfer is dominated by nucleate boiling, and convective boiling has mall influence. In addition, I carried out a simulation to compare the performance of WSHP with air source heat pump (ASHP).