250822 Exercises solutions

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Department of Energy Politecnico di Milano Author Emanuela Colombo Pag.1 of 16 Date 29/08/2022 Milan, 5th August 2022 Exam – Advanced Thermodynamic and Thermoeconomics 25-08-2022 Total score: 21 points (exercise 1 roughly 2/3 of the total) Exercise 1. Consider the refrigerating cycle illustrated in Figure 1. The working fluid adopted is a mixture of water and ammonia (5% and 95% on a molar basis respectively). The plant is aimed at refrigerating the warehouse of a supermarket. Stream 6, being the heat absorbed by the refrigerator, can be considered as the useful effect of the plant. The electricity consumed by the compressor is taken from the national grid at the cost of 0.15€/kWh. The heat released by the condenser (Stream 3) is discharged to the environment with no additional cost. The fluid undergoes an isenthalpic transformation though the lamination valve, where a pressure drop occurs to restart the cycle. Notice that this refri gerating cycle works in between the temperatures of 45°C (Tcond) at the condenser and 5°C (Teva) at the evaporator. PLEASE NOTE THAT Figure 1 represents the direction of the ENERGY flows. Consider T0 and P0 to be respectively 293.15 K and 1 atm and the following table as additional data. Condenser Evaporator Lamina�on valveCompressor 1 2 3 4 56 7 Table 1 State Absolute value of Exergy [kW] 1 Electric power ? 2 Condenser inlet 13.2 3 Heat discharged 0.9 4 Lamination valve inlet 10.4 5 Evaporator inlet 8.0 6 Heat absorbed 0.4 7 Compressor inlet 7.3 Table 2 Total Cost of Investments [€] 16800 Total cost of Operation & maintenance [€/y] 1680 Capacity Factor [%] 85 Compressor [%] 30 Condenser [%] 25 Lamination valve [%] 5 Evaporator [%] 40 Interest rate on capital [%] 2.5 Plant operative lifetime [years] 15 FIGURE 1. PLANT CONFIGURATION…