Retrofit Pathways for HVAC Decarbonization: A sustainable solution for harnessing ultra-low-grade waste heat for energy efficiency and net-zero buildings

Buildings are responsible for nearly one quarter of global energy consumption, with 30-50% of this demand arising from Heating, Ventilation and Air-Conditioning (HVAC) systems. While essential for ensuring indoor comfort, conventional vapor compression cycle (VCC) units reject a substantial share of ultra-low-grade heat (<80°C) to the urban environment. This not only represents a significant loss of useful energy but also contributes to local temperature rise and intensifies urban heat island effects, thereby driving further cooling demand. In this work, a retrofit-based strategy is proposed to integrate a single-stage organic Rankine cycle (SS-ORC) with the condenser of a conventional VCC system. A novel desuperheating-driven waste heat recovery approach, combined with an Adaptive Pinch Point (APP) method, is used to maximize the recoverable work potential under varying operating conditions. Multiple working fluid (WF) combinations are assessed, including low-GWP fluids (R1233zd (E), R1234ze (E), R1336mzz (Z)) for the VCC and zeotropic mixtures (R600/R600a and R365mfc/R152a) for the ORC. A comprehensive energy, exergy, economic and environmental (4E) analysis is conducted to evaluate the performance and sustainability of the proposed system. Among the tested configurations, coupling R1234ze (E) in the VCC with R365mfc/R152a (0.1/0.9) in the ORC delivered the most favourable outcomes. This pair achieved a 3.76% enhancement in overall COP, and ORC exergy efficiency of 52.5%. From the economic standpoint, the system yielded the lowest levelized cost of electricity (0.061 US$/kWh) with a discounted payback period of only 3.8 years. Environmentally, the retrofit achieved an annual emission reduction of 70×106 kg of CO2 / yr. Beyond technical and economic viability, the integration reduces heat rejection into the urban canopy, indirectly lowering surrounding cooling loads and improving the sustainability of indoor environments. The findings establish the proposed system as an effective and economically feasible retrofit method for enhancing energy efficiency and minimizing the carbon footprint of HVAC systems to support the energy conservation, net-zero transition and the sustainable built environment.