Experimentally comparative study on two-phase natural and pump-driven loop used in HVAC systems

Two-phase natural loops (TPNLs) have been widely used in HVACs for energy conservation. Under harsh conditions, a mechanical pump is usually added in the loop (TPPLs), which is expected to enhance the performance. Unfortunately, the extent and reasons for the enhancement is unclear owing to a lack of comparative studies between TPNLs and TPPLs. In this study, an experimental setup that includes both TPNL and TPPL was established, which were measured comparatively for five typical cases. The results indicate that: (1) When a TPNL performs poorly (under-charged refrigerant, over-charged refrigerant, or insufficient height difference), a pump can enhance the performance remarkably (up to 400%), approaching the ideal cycle of TPNL, and the main contribution to the enhancement is eliminating the overheating and sub-cooling, rather than increasing heat convection. (2) For a well-designed TPNL, the pump cannot further enhance the performance, and the pump is unnecessary. (3) When the flow resistance is considerable, the pump even degrades the performance owing to a considerable pressure drop. Finally, (4) the ideal cycle of TPNL is the boundary of the enhancement, and the application principle of the pump is to approach the ideal cycle at as low a speed as possible.