SEE 611A- Energy Systems: Modeling and Analysis 2022-23 2nd Semester Lalit M. Pant Department of Sustainable Energy Engineering Disclaimer: The instructor owns the copyright of all the distributed course materials unless otherwise mentioned. Contents of this lecture/file are only intended for the students of the course and should not be distributed in print or through electronic media without the consent of the instructor. Students can make their own copies of the course materials for their use. What is a fuel cell? • An energy conversion device • Not an energy storage device • Opposite of electrolyzer • π»2 + 12 π2 → π»2 π • First fuel cell was invented in 1838 Source: O’Hayre, Cha, Colella, & Prinz, Fuel Cell Fundamentals, 3rd ed., Wiley, 2016. Electrolyzer Fuel Cell 2 Why study hydrogen fuel cells? • Fuel cells are a promising alternative to conventional energy conversion devices β’ High efficiency, high run time, zero emissions, low noise • Increased interest Source: https://spectrum.ieee.org/ β’ Automotive, backup power, portable power Source: http://www.ultracell-llc.com/rugged.php Source: auto.hindustantimes.com 3 Fuel cell Overview • • • • • Clean/green, no-noise, no emissions, portable Fuel and the conversion mechanism are separate Continuous operation as no depletion of the device itself Easy recharging. Just need to refill fuel bottles Can be scaled easily. Energy can be scaled by putting extra fuel. Power can be scaled by adding extra fuel cells. Source: O’Hayre, Cha, Colella, & Prinz, Fuel Cell Fundamentals, 3rd ed., Wiley, 2016. 4 Basics of Fuel Cell Operation • π»2 + 12 π2 → π»2 π • Same reaction for all hydrogen fuel cells, no matter the type/electrolyte Source: O’Hayre, Cha, Colella, & Prinz, Fuel Cell Fundamentals, 3rd ed., Wiley, 2016. 5 Fuel cell vs other green energy systems Source: O’Hayre, Cha, Colella, & Prinz, Fuel Cell Fundamentals, 3rd ed., Wiley, 2016. • High current: β’ Internal resistance losses become more important β’ Electrolyte is crucial 6 Types of Fuel Cell • Two major types: Low temperature and high temperature PEMFC PAFC AFC AEMFC MCFC SOFC Polymer membrane Liquid H3PO4 (immobilized) Liquid KOH (Immobilized) Polymer membrane Molten carbonate Ceramic Charge carrier H+ H+ OH − OH − CO2− 3 O2- Temperature 80 200 80-200 80 650 600-1000 Catalyst Pt Pt Pt Ni/Fe/C Ni Ceramic Cell components Carbon based Carbon based Carbon based Carbon based Steel Ceramic Compatible fuel H2, CH3OH H2 H2 H2 H2, CH4 H2, CH4, CO Electrolyte Table adapted from O’Hayre, Cha, Colella, & Prinz, Fuel Cell Fundamentals, 3rd ed., Wiley, 2016. 7 Cell Performance • At STP: β’ πΈππππ = 1.23 π • Real Performance β’ Different losses crucial at different currents • Simple cell potential equation: • π = πΈππΆπ − π log π π0 − ππ + π log 1 − π π liπ 8 Cell Performance • π = πΈππΆπ − π log π+ππ π0 − ππ + π log 1 − π π πππ • Typical values for low temperature fuel cell like PEMFC: β’ πΈππΆπ = 1.23 π β’ π = 0.05 π β’ π0 = 0.05ππ΄/ππ2 β’ ππ depends on the membrane thickness and hydration β 0.1 − 3 ππ΄/ππ2 β’ π β 0.02 − 0.25 Ω ⋅ ππ2 β’ ππππ depends on fuel cell design and operating characteristics β 2 − 3 π΄/ππ2 β’ π = 0.05 π • What will be cell voltage at 0 current and internal crossover of 1 mA/cm2 ? 9 Heat Balance • How much heat is produced? • How much heat/energy in? β’ Enthalpy of fuel: πΔβ αΆ ππ₯π • How much energy out/lost? β’ Electric work/power: ππ β’ Heat loss β The difference between OCV and real potential is the loss β Additional entropic loss • παΆ = πΔβ αΆ ππ₯π − ππ = ππ • παΆ = ππΉ πΔβππ₯π ππΉ − π π = ππ ( 1.25 S − π 1) 10 Mass Balance • How much reactants/products are flowing in/out? • How much reactants flow in? β’ Ideal: β’ Real: π ππΉ ππ ππΉ • How much consumed/produced? β’ Consumed: π β’ Produced: π 1 π π (H2: 2πΉ, O2: 4πΉ) πΉ π π2 πΉ (H2 O: π ) 2πΉ • παΆ ππ’π‘ = παΆ ππ ± ππΉπ • Why is stoichiometry >1? 11 Low Temperature Fuel Cell: PEMFC Overview • Reactions: β’ Anode: π»2 → 2π»+ + 2π − or Hydrogen Oxidation Reaction (HOR) 1 β’ Cathode: 2π»+ + 2 π2 + 2π − → π»2 π or Oxygen Reduction Reaction (ORR) • Overall: π―π + ππ πΆπ → π―π πΆ • Advantages: β’ Highest power, efficiency β’ Rapid start-up β’ Low temp -> safe, suitable for portable applications • Disadvantages: β’ Platinum as catalyst -> high cost Source: O’Hayre, Cha, Colella, & Prinz, Fuel Cell Fundamentals, 3rd ed., Wiley, 2016. β’ Expensive polymer electrolyte β’ Water management issues β’ Poor impurity tolerance (CO, S) 12 Parts of a PEM Fuel Cell • • • • • Electrolyte: Separate anode and cathode. Conduct protons. Do not allow electrons and gases to cross Catalyst layer (CL): Facilitate the reactions on top of a catalyst surface with a high catalyst area Micro-porous layer (MPL): Control the water in fuel cell by using PTFE in the layer Gas diffusion layer (GDL): Allow gases to diffuse under the land for uniform reaction Current collector plate: Collect electrons in a highly conductive plate and provide path for reactants and products 13 Parts of a PEM Fuel Cell stack & system Source: Larmine & Dicks. Fuel Cell Systems Explained. Wiley, 2003 14 Parts of a PEM Fuel Cell stack & system Forced cooling system Source: Larmine & Dicks. Fuel Cell Systems Explained. Wiley, 2003 15 High-Temperature Fuel Cell: SOFC Overview • Reactions: 1 β’ Cathode: 2 π2 + 2π − → π2− or Oxygen Reduction Reaction (ORR) β’ Anode: π»2 + π2− → π»2 π + 2π − or Hydrogen Oxidation Reaction (HOR) • Overall: π―π + ππ πΆπ → π―π πΆ • Advantages: β’ High combined efficiency β’ No need for precious catalysts at high temperatures β’ Can do internal reforming • Disadvantages: β’ Mechanical issues due to high temperatures β’ Safety issues at high temperature β’ Low thermodynamic voltage Source: O’Hayre, Cha, Colella, & Prinz, Fuel Cell Fundamentals, 3rd ed., Wiley, 2016. 16 Why CHP or Co-Generation? • Fuel cell ΔπΊ reduces with temperature β’ Efficiency reduces • Heat engine efficiency increases with temperature • Best of both: β’ Heat engines cannot operate at very high temps (>1500) due to material issues. Limited efficiency β’ SOFCs cannot operate at low temperatures Source: O’Hayre, Cha, Colella, & Prinz, Fuel Cell Fundamentals, 3rd ed., Wiley, 2016. 17 Co-Generation Schemes Source: O’Hayre, Cha, Colella, & Prinz, Fuel Cell Fundamentals, 3rd ed., Wiley, 2016. 18 19
