Performance evaluation of a novel hybrid cooling system combining air source heat pump and urban underground pipe rack

Abstract

Underground pipe racks, widely constructed in urban China for laying municipal pipelines, can pre-cool hot outdoor air in summer in a manner similar to earth-to-air heat exchangers (ETAHEs) and air-tunnel heat exchangers (ATHEs). However, research into their potential for passive energy-saving is lacking in the context of global energy stress. Accordingly, this study proposes an efficient hybrid system for cooling buildings in a subtropical climate region, utilizing an underground pipe rack as the air-pipe rack heat exchanger (APRHE) to pre-cool hot outdoor air for the condenser of the air conditioner (AC) as the cold source. Given the logistical infeasibility of coupling the on-site pipe rack to an actual building as a temporary APRHE-AC hybrid cooling system, a new experimental rig is then designed, with the pre-cooled air of the pipe rack being simulated by a wind tunnel. This rig enables the quantification of the cooling capacity, power consumption, and coefficient of performance (COP) of the proposed APRHE-AC system. The results show that the APRHE-AC system can optimize the energy efficiency of cooling from 9:00 a.m. to 6:00 p.m. Notably, higher outdoor air temperatures lead to more pronounced improvements in energy efficiency. Specifically, in the experimental condition of Group A, when the COP of the AC with outdoor air as its cold source registers to its minimum value of 1.95 at 2:00 p.m., the COPs of the ACs with pre-cooled air of the pipe rack’ cable trench and comprehensive trench as their cold sources are 2.26 and 2.18, representing improvements by 15.9% and 11.8%, respectively. In Group B, such corresponding COPs are orderly 1.83, 2.25, and 2.10, meaning the latter two improvements by 23.0% and 14.8%. Such remarkable performances have collectively demonstrated the promising potential of the APRHE-AC system in reducing not only the installed capacity of AC systems but also power consumption in cooling buildings. Additionally, modest increases in initial investments for buildings in proximity to underground pipe racks yield substantial benefits during the operation of the system.