MECH5275 - Renewable Energy Assignment 3 Thermodynamics of Chemically Reacting Systems Due Friday 15th October (This assignment should take a typical student around 8 -10 hours to complete.) Notes: •...

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MECH5275 - Renewable Energy Assignment 3 Thermodynamics of Chemically Reacting Systems Due Friday 15th October (This assignment should take a typical student around 8 -10 hours to complete.) Notes: • Submit this assignment as a professionally presented report. Present an outline of your analysis and your results as you would in an engineering design report. Note that there is no need to show every tiny detail of your calculations. There should be sufficient detail so that an engineer who has expertise in the relevant area can understand what you did and could repeat your calculations. Be sure to comment on and explain interesting features of your results. Consider using LaTeX as this will make presentation of equations easier. • Submit your report as a PDF document generated directly from your Word or LaTeX file, not scanned. • For calculations of enthalpy and Gibbs free energy, if you may do calculations as a table in a spreadsheet program such as Excel then import the table into your document. If you choose to do this you must also upload your spreadsheet along with your report. • Submit your report electronically through Turnitin on the Canvas site. You are part of a design team developing a new hydrogen fuel cell powered car using a proton exchange membrane type fuel cells. Part A Start by considering an ideal hydrogen fuel cell operating at 25C (including all inlet and outlet streams). The oxidiser is pure oxygen gas. The hydrogen and oxygen are supplied at a rate to give a stoichiometric oxygen/fuel ratio. The oxygen enters the fuel cell at an absolute pressure of 220kPa, the hydrogen at 240kPa, and the products exhausted at 200kPa. Hydrogen is supplied to the cell at a rate of 0.15g/min. Assume that all the water produced during the process leaves the fuel cell in liquid form. In all calculations, assume standard environmental conditions, 25C (298K) / 101kPa. 1) Determine the chemical reaction and the partial pressures of each of the reactants and products. 2) Determine the mass flow rate of oxygen and water [1.2g/min, 1.35g/min] 3) Determine the enthalpy change associated with the reaction. [-285.8MJ/kmolfuel] 4) Determine the Gibbs free energy change associated with the reaction. [-240.3MJ/ kmolfuel] 5) Hence determine: a) the electric power output of the cell, Wrev [300.4W] b) the rate of heat rejection [56.9W] c) the efficiency of the ideal fuel cell [84.07%] d) the reversible open circuit voltage Vrev. [1.245 V] Part B Carrying pure oxygen is impractical. Consider the situation in which the oxygen stream is replaced with air. Assume the fuel cell operates at 25C with a stoichiometric air/fuel ratio, with the same hydrogen flow rate and inlet and outlet pressures as Part A (air at the same pressure as the oxygen in Part A), and that all the water produced during the process leaves the fuel cell in liquid form. 1) Determine the chemical reaction and the partial pressures of each of the reactants and products. 2) Determine the required air/fuel ratio on a mass basis. [34.32] 3) Determine the enthalpy change associated with the reaction. [-285.8MJ/ kmolfuel] 4) Determine the Gibbs free energy change associated with the reaction. [-237.7MJ/ kmolfuel] 5) Hence determine: a) the electric power output of the cell, Wrev [297.1W] b) the rate of heat rejection [60.2W] c) the efficiency of the ideal fuel cell [83.16%] d) the reversible open-circuit voltage Vrev. [1.232V] 6) Explain why the open-circuit voltage and efficiency are different to those in Part A. Part C Fuel cells generally operate at higher temperatures in order to improve the chemical kinetics and hence produce a higher electric current and more power. Consider the same ideal fuel cell as in Part B this time operating at 75C including all inlet and outlet streams. Assume constant specific heats for all substances in these calculations. (Use the values of Cp at 300K for each substance.) Use the same hydrogen flow rate, inlet and outlet pressures as before. Again assume that all water in the products is in liquid form. 1) Determine the chemical reaction and the partial pressures of each of the reactants and products. 2) Determine the enthalpy change associated with the reaction. [-284.2MJ/ kmolfuel] 3) Determine the Gibbs free energy change associated with the reaction. [-229.75MJ/ kmolfuel] 4) Hence determine: a) the electric power output of the cell, Wrev [287.2W] b) the rate of heat rejection [68.1W] c) the efficiency of the ideal fuel cell [80.83%] d) the reversible open-circuit voltage Vrev. [1.191V] 5) Explain why these are different to the values calculated in Part B. Part D At these operating conditions it is likely that some of the water in the products will evaporate within the fuel cell. The upper limit for this process is that the gaseous product stream is fully saturated with water vapo ur. Consider the same conditions as in Part C, but this time with the product stream fully saturated with water vapour – ie it will be a saturated mixture of liquid and vapour. 1) Determine the chemical reaction equation and the partial pressures of each of the reactants and products. 2) Determine the proportion of the water that leaves as a liquid [55.04%] 3) Determine the enthalpy change associated with the reaction. [-265.4MJ/ kmolfuel] 4) Determine the Gibbs free energy change associated with the reaction. [-230.9MJ/ kmolfuel] 5) Hence determine: a) the electric power output of the cell, Wrev [288.6W] b) the rate of heat rejection [43.1W] c) the efficiency of the ideal fuel cell [87.01%] d) the reversible open-circuit voltage Vrev. [1.197V] 6) Explain why these are different to the values calculated in Part C. Part E In the car, a stack of fuel cells operating at the conditions given in Part D is to provide up to 110kW of electric power at 360V to a DC motor. In reality irreversibilities will reduce the electrical energy and voltage that the real fuel cells can supply. Assume the real fuel cells can provide an open circuit voltage Voc that is 90% of the reversible voltage calculated in Part D and have an internal resistance of 0.32m and an activation voltage of 0.11V at the operating load current. 1) Determine the load current when the motor is operating at 100kW. [305.6A] 2) Determine the load voltage across each fuel cell. How many cells would need to be stacked in series to provide this? [0.869V, 415] 3) Determine the efficiency of the real fuel cells. How does this compare with the thermal efficiency of a typical internal combustion engine? [63.2%] 4) Determine the mass flow rate of hydrogen needed to provide the required electric power. [1.31g/s] 5) Assume that on average the car travels at a speed of 80km/h and requires 55kW of power (at the wheels). Also assume that hydrogen consumption is directly proportional to the power output from the fuel cells and that the electric motor and drive-chain have a combined power loss of 15%. What mass of hydrogen would be required to give the car a range of 300km? [10.4kg] 6) If the hydrogen is to be stored in a pressurized tank at 450bar and 25C, estimate the volume of the tank required to give the car a range of 300km. [400L] (Hint: There is a high pressure Nelson Obert chart on the Canvas site.) Just for fun (This will not be marked) Use your spreadsheet or program to investigate how changes to the operating parameters of your fuel cell change its performance. For example… • In reality, fuel cells will operate somewhere between fully liquid and fully saturated. Use your spreadsheet or program to investigate how the fuel cell performance changes between these two limits. • In reality, fuel cells usually require water vapour to be added to the fuel and air streams to ensure that the membranes don’t dry up. Investigate the effect of adding water vapour to your reactants.