HEAT RECOVERY WITH OSCILLATING HEAT PIPES

Waste-heat recovery applied in HVAC air systems is of interest to increase the energy efficiency of residential, commercial and industrial buildings. In this study, the feasibility of using tubular-shaped oscillating heat pipes (OHPs), which are two-phase heat transfer devices with ultra-high thermal conductivity, for heat exchange between counter-flowing air streams (i.e., outdoor and exhaust air flows) was investigated. For a prescribed volumetric flow rate of air and duct geometry, four different OHP Heat Exchangers (OHP-HEs) were sized via the epsilon-NTU method for the task of sub-cooling intake air 5.5 degrees C (10 degrees F). The OHP-HE tubes were assumed to have a static thermal conductivity of 50,000 W/m.K and only operate upon a minimum temperature difference in order to simulate their inherent heat transport capability and start-up behavior. Using acetone as the working fluid, it was found that for a maximum temperature difference of 7 degrees C of more, the OHP-HE can operate and provide for an effectiveness of 0.36. Pressure drop analysis indicates the presented OBP-HE design configurations provide for a minimum of 5 kPa. The current work provides a necessary step for quantifying and designing the OHP for waste heat recovery in AC systems.