Decarbonizing air-conditioning systems via ultra-low grade waste heat recovery − A comprehensive energy, exergy, economic and environmental assessment with novel desuperheating based adaptive pinch point approach

Abstract

The rising global energy demand and policy shifts due to global warming and associated climate challenges stipulate more efficient energy utilization. Air-conditioning, being a major sector of urban energy consumption, possess significant potential for reducing overall energy waste and related carbon emissions. Vapor compression cycle (VCC) is predominant in all scales of air-conditioning applications and rejects large proportions of energy as ultra-low grade waste heat, the utilization of which is largely unexplored. This study focuses on assessing the potential of this ultra-low grade waste heat (70–80 °C) through an integrated single stage Organic Rankine Cycle (ORC) by evaluating system's performance on energy, energy, economic and environmental criteria with diverse working fluid combinations (i.e. R1234ze(Z), R1243zf & R1234yf in VCC and zeotropic mixtures of R600/R600a, R601/R601a, R365mfc/R152a & R245fa/R113 in ORC). The comprehensive analysis will not only indicate the thermodynamic improvements but also demonstrate its economic viability and environment benefits. The key innovation of this work is use of zeotropic mixtures in ORC cycle coupled with desuperheating based ultra-low grade waste heat recovery approach. The performance is evaluated using a novel adaptive pinch point technique which dynamically locates the position of pinch point in ORC evaporator and ORC condenser simultaneously based on mixture fraction variations. The results demonstrated that R1243zf-R245fa/R113(0.6/0.4) yielded the most favorable performance. Thermodynamically, it achieved highest ORC thermal efficiency (6.22 %), improvement in system's coefficient of performance (4.41 %), ORC exergy efficiency (51.97 %), and system exergy efficiency (16.48 %). From the economic and environmental perspective, it also showed maximum net present value (US$ 28.50 million), emission reduction (82 × 106 kg/yr.), carbon credits benefits (∼1 million US$/yr.) and lowest discounted payback period (3.38 years). The findings highlight the potential of the proposed scheme for effective ultra-low grade waste heat utilization, demonstrating improvements in thermodynamic efficiencies along with economic viability, and significant emissions reductions.