Confronto tra prima e seconda legge della termodinamica testo

Pagina 1

Exercise: 1st law vs. 2nd law analysys Consider the process shown in the figure of heat recovery from hot air to produce electric power by means of a Carnot cycle. Air is at constant atmospheric pressure, enters the system at 600°C and can be considered as a perfect gas with specific heat cp = 1 kJ/kg K. Consider ideal heat transfer with the Carnot cycle, allowing minimum ΔT of 0°C with the heat source and with the ambient. The electric generator, converting the mechanical power produced by the Carnot cycle into electricity, has an efficiency of 99%. It is required to: 1. perform a the energy (1st law) and exergy (2nd law) analysis of the process, i.e. to calculate the efficiency losses Δηi associated to: (i) heat exchange between air and the Carnot cycle, (ii) heat rejection from the Carnot cycle to the ambient, (iii) heat released to the ambient by the air after the Carnot cycle heat exchanger, (iv) electrical-mechanical losses. 2. Calculate the 1st law and 2nd law efficiencies of the process 3. Find the optimal temperature Th that maximizes the efficiency of the process 600° Heat released to ambient from the Carnot cycle Hot air: 1 kg/s, 1 atm, 600°C Air released to ambient at temperature Th Heat to Carnot cycle Ambient at T0=25°C Mechanical power CARNOT CYCLE (Tmax=Th, Tmin=T0) Electric generator Electric power Mechanical- electric loss T s T0 Carnot cycle Th