Optimal waste heat temperature in semiconductor cleanroom HVAC: Configuration-dependent cost structure and operating strategy

Heating, ventilation, and air-conditioning (HVAC) systems account for a large share of the energy use of advanced semiconductor fabrication facilities (fabs). Recovering chiller condenser waste heat to outdoor air conditioner (OAC) heating coils in place of LNG boiler heating has emerged as an efficiency measure. The waste heat temperature is the condenser water return temperature (CWRT), which simultaneously governs chiller coefficient of performance, cooling tower fan power, recoverable heat, and auxiliary boiler use. Prior work, however, remains limited to individual demonstrations, leaving the optimal waste heat temperature across configurations and outdoor conditions unaddressed. This study develops a system simulation of a fab OAC and thermal utility system. It analyzes the cost of displacing LNG heating with waste heat for two configurations differing in backup heating scope: a steam-backup hybrid and a boiler-free variant. The cost divides into a high-gain boiler displacement region, a steam humidification plateau, and a loss region with nothing left to displace. This structure is robust to LNG price variation. Both optima are closely approximated by tracking the lowest CWRT sustaining the displacement. Annual scenarios of different realization levels evaluate its feasibility and operating window: in central Korea, the strategy cuts the operating cost of one recovery chiller and its matched outdoor-air units by up to about 44% with steam backup and about 42% boiler-free against a boiler-only baseline, most of it realizable without added sensors. Finally, the unit analysis extends to the fab level, relating the chiller conversion ratio to total operating cost to guide operation and design.