Maximizing the Sustainable Energy Generation Potential from Ultra-Low Grade Waste Heat: Thermo-4E Optimization and Decision Making Analysis for Novel Integrated Vapor Compression Cycle and the Organic Rankine Cycle with Zeotropic Working Fluids | Faculty Development Scheme (FDS), Research Grants Council (RGC) Competitive Research Funding Schemes for the Local Self-financing Degree Sector 2024/25 | Project Reference No.: UGC/FDS24/E12/24 | HK$ 636,901

In hot and humid regions, like Hong Kong, refrigeration and air-conditioning systems consume a significant amount of total energy. Consequently, a substantial portion of low-grade waste heat is discharged into the atmosphere, leading to an overall increase in carbon footprints. This research examines a methodology for recovering low-grade waste heat by introducing a novel integrated vapor compression cycle and organic Rankine cycle. The proposed system captures waste heat from air-conditioning units and utilizes it in the organic Rankine cycle for electricity generation. The study aims to enhance the thermodynamic performance of the integrated system through the use of zeotropic mixtures and optimization of the composition of the proposed working fluids. To achieve this objective, a concise methodology is adopted, employing a heat exchange network (HEN) with a linear program model. This approach allows for the customization of the organic Rankine cycle configuration, enabling evaluation based on performance indicators. The primary focus of the research is the selection of appropriate working fluid pairs for both the integrated vapor compression cycle and the organic Rankine cycle. The waste heat temperature range for the vapor compression cycle is between 50°C and 89°C.
The ORC cycle includes a single stage (SS-ORC) and dual stage (DS-ORC) configuration. Both configurations employ a desuperheating approach to recover and utilize high-quality waste heat, thereby achieving favorable thermodynamic response. To reduce irreversibilities associated with non-isothermal heat addition, zeotropic mixtures are utilized. These mixtures possess a unique characteristic known as temperature glide at constant pressure, which facilitates better thermal matching and minimizes exergy losses during heat transfer. Consequently, higher conversion efficiency (ORC thermal efficiency) is achieved compared to using pure refrigerants. Moreover, by adjusting the composition of the mixture, desired properties can be achieved.
The organic Rankine cycle in the integrated system does not rely on an external heat source. Instead, a shared heat exchanger is employed, functioning as both the condenser for the vapor compression cycle and the evaporator for the organic Rankine cycle simultaneously. To optimize system performance, a multi-objective optimization approach utilizing the non-dominated sorting genetic algorithm-II (NSGA-II) is applied. This optimization aims to maximize the thermal performance of the system while minimizing system losses. The integrated system has also been analyzed not only thermodynamically, but also thermo-economically and thermo-environmentally using 4E (energy, exergy, economic and environmental) analysis to study the comprehensive performance of the system. Subsequently, the optimal solution is selected for the chosen zeotropic mixtures and compared using decision-making methods such as Shannon entropy, LINMAP, and TOPSIS. Ultimately, the Pareto-front solution is utilized to identify the most optimized composition of the zeotropic mixture for the organic Rankine cycle.