N. Tziris
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Modelling and Optimisation of CO₂ Refrigeration Cycles with Advanced Subcooling Configurations
A technoeconomic investigation of waste heat integration
Transcritical CO2 refrigeration cycles offer meaningful environmental benefits as natural refrigerants, yet their efficiency degrades severely under elevated ambient temperatures—COP declining from 3.78 at 25°C to 1.83 at 40°C (51% reduction). This performance limitation necessitates advanced subcooling configurations to maintain competitiveness with conventional refrigeration technologies. While mechanical subcooling cycles (MSC) and ejector-based systems have been investigated separately, limited research has explored the integration of externally supplied low-grade waste heat to drive ejector refrigeration cycles for CO2 subcooling applications.
This thesis develops and optimizes thermodynamic models for waste-heat-driven ejector refrigeration cycles (EJRC) serving as mechanical subcooling units for transcritical CO2 vapor-compression systems. Three progressive configurations are analyzed: a baseline transcritical CO2 cycle establishing reference performance, an MSC system employing ammonia as auxiliary refrigerant, and an EJRC with both simplified Köhler efficiency models and detailed multi-ejector binary valve representations. The models are validated against established literature correlations and subjected to comprehensive parametric optimization across evaporating temperatures (−20°C to 20°C), ambient conditions (30°C to 40°C), and waste heat ratios (f = 0.8–20).
Results demonstrate that MSC achieves 56% COP improvement over baseline (COP = 2.91 vs. 1.83 at reference conditions: Tair = 40°C, Tevap = 0°C), while EJRC configurations deliver superior performance ranging from 45% to 75.7% improvement depending on waste heat availability. The optimal EJRC configuration—generator-off mode at waste heat temperature T9e = 90°C and ratio f = 10—achieves COP = 4.44, representing 73% improvement over MSC. The simplified model, calibrated with entrainment ratio ϕ = 0.4–0.7 and ejector efficiency ηej = 0.10, provides 200× computational speedup with deviations of 5.7–16.5% from multi-ejector simulations.
Techno-economic assessment reveals fundamental divergence between thermodynamic and economic optima. Generator-on configurations achieve superior COP (3.00–3.72) but exhibit negative net present value (NPV = −€117,220 to −€518,190) due to prohibitive capital expenditure and maintenance burdens. On the other hand, the generator-off configuration at f = 10 delivers marginal economic viability (NPV = €3,485, simple payback period = 12.24 years, LCOC = €0.165/kWh) compared to MSC (NPV = €206,400, LCOC = €0.1655/kWh). The analysis establishes that ejector technology becomes economically competitive only when high-grade waste heat (T9e ≥ 90°C) exists at sufficient ratios (f ≥ 10), generator operation is deactivated, and electricity pricing exceeds €0.30/kWh.
This research provides integrated thermodynamic-economic optimization framework for waste heat-driven refrigeration improvement, demonstrating that technology choice must be dictated by waste heat availability, quality, and economic boundary conditions rather than thermodynamic performance maximisation alone. ...
This thesis develops and optimizes thermodynamic models for waste-heat-driven ejector refrigeration cycles (EJRC) serving as mechanical subcooling units for transcritical CO2 vapor-compression systems. Three progressive configurations are analyzed: a baseline transcritical CO2 cycle establishing reference performance, an MSC system employing ammonia as auxiliary refrigerant, and an EJRC with both simplified Köhler efficiency models and detailed multi-ejector binary valve representations. The models are validated against established literature correlations and subjected to comprehensive parametric optimization across evaporating temperatures (−20°C to 20°C), ambient conditions (30°C to 40°C), and waste heat ratios (f = 0.8–20).
Results demonstrate that MSC achieves 56% COP improvement over baseline (COP = 2.91 vs. 1.83 at reference conditions: Tair = 40°C, Tevap = 0°C), while EJRC configurations deliver superior performance ranging from 45% to 75.7% improvement depending on waste heat availability. The optimal EJRC configuration—generator-off mode at waste heat temperature T9e = 90°C and ratio f = 10—achieves COP = 4.44, representing 73% improvement over MSC. The simplified model, calibrated with entrainment ratio ϕ = 0.4–0.7 and ejector efficiency ηej = 0.10, provides 200× computational speedup with deviations of 5.7–16.5% from multi-ejector simulations.
Techno-economic assessment reveals fundamental divergence between thermodynamic and economic optima. Generator-on configurations achieve superior COP (3.00–3.72) but exhibit negative net present value (NPV = −€117,220 to −€518,190) due to prohibitive capital expenditure and maintenance burdens. On the other hand, the generator-off configuration at f = 10 delivers marginal economic viability (NPV = €3,485, simple payback period = 12.24 years, LCOC = €0.165/kWh) compared to MSC (NPV = €206,400, LCOC = €0.1655/kWh). The analysis establishes that ejector technology becomes economically competitive only when high-grade waste heat (T9e ≥ 90°C) exists at sufficient ratios (f ≥ 10), generator operation is deactivated, and electricity pricing exceeds €0.30/kWh.
This research provides integrated thermodynamic-economic optimization framework for waste heat-driven refrigeration improvement, demonstrating that technology choice must be dictated by waste heat availability, quality, and economic boundary conditions rather than thermodynamic performance maximisation alone. ...
Transcritical CO2 refrigeration cycles offer meaningful environmental benefits as natural refrigerants, yet their efficiency degrades severely under elevated ambient temperatures—COP declining from 3.78 at 25°C to 1.83 at 40°C (51% reduction). This performance limitation necessitates advanced subcooling configurations to maintain competitiveness with conventional refrigeration technologies. While mechanical subcooling cycles (MSC) and ejector-based systems have been investigated separately, limited research has explored the integration of externally supplied low-grade waste heat to drive ejector refrigeration cycles for CO2 subcooling applications.
This thesis develops and optimizes thermodynamic models for waste-heat-driven ejector refrigeration cycles (EJRC) serving as mechanical subcooling units for transcritical CO2 vapor-compression systems. Three progressive configurations are analyzed: a baseline transcritical CO2 cycle establishing reference performance, an MSC system employing ammonia as auxiliary refrigerant, and an EJRC with both simplified Köhler efficiency models and detailed multi-ejector binary valve representations. The models are validated against established literature correlations and subjected to comprehensive parametric optimization across evaporating temperatures (−20°C to 20°C), ambient conditions (30°C to 40°C), and waste heat ratios (f = 0.8–20).
Results demonstrate that MSC achieves 56% COP improvement over baseline (COP = 2.91 vs. 1.83 at reference conditions: Tair = 40°C, Tevap = 0°C), while EJRC configurations deliver superior performance ranging from 45% to 75.7% improvement depending on waste heat availability. The optimal EJRC configuration—generator-off mode at waste heat temperature T9e = 90°C and ratio f = 10—achieves COP = 4.44, representing 73% improvement over MSC. The simplified model, calibrated with entrainment ratio ϕ = 0.4–0.7 and ejector efficiency ηej = 0.10, provides 200× computational speedup with deviations of 5.7–16.5% from multi-ejector simulations.
Techno-economic assessment reveals fundamental divergence between thermodynamic and economic optima. Generator-on configurations achieve superior COP (3.00–3.72) but exhibit negative net present value (NPV = −€117,220 to −€518,190) due to prohibitive capital expenditure and maintenance burdens. On the other hand, the generator-off configuration at f = 10 delivers marginal economic viability (NPV = €3,485, simple payback period = 12.24 years, LCOC = €0.165/kWh) compared to MSC (NPV = €206,400, LCOC = €0.1655/kWh). The analysis establishes that ejector technology becomes economically competitive only when high-grade waste heat (T9e ≥ 90°C) exists at sufficient ratios (f ≥ 10), generator operation is deactivated, and electricity pricing exceeds €0.30/kWh.
This research provides integrated thermodynamic-economic optimization framework for waste heat-driven refrigeration improvement, demonstrating that technology choice must be dictated by waste heat availability, quality, and economic boundary conditions rather than thermodynamic performance maximisation alone.
This thesis develops and optimizes thermodynamic models for waste-heat-driven ejector refrigeration cycles (EJRC) serving as mechanical subcooling units for transcritical CO2 vapor-compression systems. Three progressive configurations are analyzed: a baseline transcritical CO2 cycle establishing reference performance, an MSC system employing ammonia as auxiliary refrigerant, and an EJRC with both simplified Köhler efficiency models and detailed multi-ejector binary valve representations. The models are validated against established literature correlations and subjected to comprehensive parametric optimization across evaporating temperatures (−20°C to 20°C), ambient conditions (30°C to 40°C), and waste heat ratios (f = 0.8–20).
Results demonstrate that MSC achieves 56% COP improvement over baseline (COP = 2.91 vs. 1.83 at reference conditions: Tair = 40°C, Tevap = 0°C), while EJRC configurations deliver superior performance ranging from 45% to 75.7% improvement depending on waste heat availability. The optimal EJRC configuration—generator-off mode at waste heat temperature T9e = 90°C and ratio f = 10—achieves COP = 4.44, representing 73% improvement over MSC. The simplified model, calibrated with entrainment ratio ϕ = 0.4–0.7 and ejector efficiency ηej = 0.10, provides 200× computational speedup with deviations of 5.7–16.5% from multi-ejector simulations.
Techno-economic assessment reveals fundamental divergence between thermodynamic and economic optima. Generator-on configurations achieve superior COP (3.00–3.72) but exhibit negative net present value (NPV = −€117,220 to −€518,190) due to prohibitive capital expenditure and maintenance burdens. On the other hand, the generator-off configuration at f = 10 delivers marginal economic viability (NPV = €3,485, simple payback period = 12.24 years, LCOC = €0.165/kWh) compared to MSC (NPV = €206,400, LCOC = €0.1655/kWh). The analysis establishes that ejector technology becomes economically competitive only when high-grade waste heat (T9e ≥ 90°C) exists at sufficient ratios (f ≥ 10), generator operation is deactivated, and electricity pricing exceeds €0.30/kWh.
This research provides integrated thermodynamic-economic optimization framework for waste heat-driven refrigeration improvement, demonstrating that technology choice must be dictated by waste heat availability, quality, and economic boundary conditions rather than thermodynamic performance maximisation alone.