Energy Engineering - Electrochemical Energy Conversion and Storage

Full exam

EECS – Prof. A. Casalegno – 14 th January 202 2 1 Basic Part (4.5 points per questions, minimum score 12) 1. Write the Nernst -Plank equation for mass transport in a generic electrolyte 2. Define the efficiency for a battery following a complex operation profile 3. Write Faraday’s law applied to products in a n alkaline electrolyser 4. Exercise A PEM electrolyser stack consumes at full load 400 A at 36 0 V with an efficiency equal to 67% (LHV=120 MJ/kg). Assuming that 3% of H 2 is wasted and its performance follows the reported polarization curve, estimate the number of cell, cell area and efficiency at 20 % current load. Interm ediate Part (3 points per theory questions and 3.5 for the exercise) 1. Is the overpotential negative when O2 reduction occurs? Demonstrate analyzing �̃ 2. Show how voltage limits in batteries could reduce the available capacity during discharge at high C -rate 3. Exercise A PEM fuel cell operates at 80 °C with pure H 2 and air at 2.5 atm pressure and supplies at 0. 65 V a current density equal to 1.8 A/cm 2 and related impedance is reported in the figure . Assuming that: 1. anode behaves as an ideal H 2 electrode (�−+ ������−=0) 2. ion and mass transport effects in catalyst layers are negligible 3. the average O 2 chemical activity in the cathode channel is equal to 0.2 5 4. diffusion la yer diffusivity is equal to 0.0 3 cm 2/s (at 2.5 atm) and its thickness is 200 μm 5. cost of the fuel cell stack is 80 €/kW if operated at 1.8 A/cm 2. Design a 90 kW stack with a maximum current density of 1. 5 A/cm 2, assuming a cell area of 200 cm 2 and a fuel utilization of 9 8%, providing: minimum efficiency, number of cells and cost per kW . ������� (�) −������������ (�) 0.1 ������ ������������2 −0.0165 ������ ������������ 2 0.14 ������ ������������ 2 ∆������ 0 � 1.5 V 1.8 V 0.4 A cm -2 2 A cm -2 EECS – Prof. A. Casalegno – 14 th January 202 2 2 Difficult Part (1.5 point s) Exercise A Li -ion battery with nominal capacity of 60 Ah operates at ambient pressure and temperature, th e discharge curves at C -rate 0.1 and 2 are reported in the figure. Assuming that: 1. BV kinetics is valid for both electrodes 2. transfer coefficients of both electrodes during discharge are equal to 0.5 3. ion and mass transport effects in electrodes are negligible 4. �������������� = ������������������� 2 = [0.75 ∗������������� + 10 −6]2 5. �������� = ������������� 5 = [0.87 ∗������������� + 0.05 ]5 6. � ∗�+= 4000 ������ 7. electrolyte resistance is equal to 1 mΩ Calculate at Crate= 2 and SoC=2 5% the overpotential attributable to the negative electrode. To upload https://forms.office.com/r/1hkXfrW8n6 25% 0% 100% 75 %