Outline. Determining Equivalence Factors II. Fuel Cell Stack. Fuel Cell Basic Principles. Overview of Different Fuel Cell Technologies

Transcription

1 Vehicle Propulsion Systems Lecture 8 Fuel Cell Vehicles Lars Eriksson Professor Vehicular Systems Linköping University May 3, 8 / 4 / 4 Deterministic Dynamic Programming Basic algorithm N J(x ) = g N (x N ) + g k (x k, u k ) k= x k+ = f k (x k, u k ) Algorithm idea: Start at the end and proceed backward in time to evaluate the optimal cost-to-go and the corresponding control signal x Deterministic Dynamic Programming Basic Algorithm Graphical illustration of the solution procedure x 3 3 k = N N t 4 JN(xN) ta tb k = N N t ta tb 3 / 4 5 / 4 Examples of Short Term Storage Systems Pneumatic Hybrid Engine System 6 / 4 7 / 4 Heuristic Control Approaches Parallel hybrid vehicle (electric assist) ECMS Equivalent Consumption Minimization Strategy µ depends on the (soft) constraint µ = q(t f ) φ(q(t f )) = /special case/ = w Different efficiencies µ = q(t f ) φ(q(t f )) = { wdis, q(t f ) > q() w chg, q(t f ) < q() Determine control output as function of some selected state variables: vehicle speed, engine speed, state of charge, power demand, motor speed, temperature, vehicle acceleration, torque demand ntroduce equivalence factor (scaling) by studying battery and fuel power s(t) = µ(t) H LHV V b Q max ECMS Equivalent Consumption Minimization Strategy 8 / 4 9 / 4

2 Determining Equivalence Factors Collecting battery and fuel energy data from test runs with constant u gives a graph Slopes determine s dis and s chg / 4 / 4 Fuel Cell Basic Principles Convert fuel directly to electrical energy Let an ion pass from an anode to a cathode Take out electrical work from the electrons Fuel Cell Stack The voltage out from one cell is just below V. Fuel cells are stacked. / 4 3 / 4 Components in a Fuel Cell Stack Overview of Different Fuel Cell Technologies 4 / 4 5 / 4 AFC Alkaline Fuel cell PEMFC Proton Exchange Membrane Fuel Cell Advantages: Relatively high power-density characteristic Operating temperature, less than o C Allows rapid start-up Good transient response, i.e. change power Top candidate for automotive applications Other advantages relate to the electrolyte being a solid material, compared to a liquid Disadvantages: Among the most efficient fuel cells 7% Low temperature 65- C Quick start, fast dynamics No co-generation Sensitive to poisoning 6 / 4 of the PEMFC for some applications operating: temperature is low The electrolyte is required to be saturated with water to operate optimally. Careful control of the moisture of the anode and cathode streams is important 7 / 4

3 The Other Types of H Fuel Cells Hydrogen Fuel Storage Other fuel cell types are 75 C 65 C C PAFC Phosphoric Acid Fuel Cell MCFC Molten Carbonate Fuel Cell SOFC Solid Oxide Fuel Cells Hydrogen storage is problematic - Challenging task. Some examples of different options. Hotter cells, slower, more difficult to control Power generation through co-generation Compressed Hydrogen storage Liquid phase Cryogenic storage, -53 C Metal hydride Sodium borohydride NaBH4 8 / 4 Comparison of H Fuel Cells US DOE 9 / 4 DMFC Direct Methanol Fuel Cell Basic operation Anode Reaction: CH3 OH + H O CO + 6H + + 6e Cathode Reaction: 3/O + 6H + + 6e => 3H O Overall Cell Reaction: CH3 OH + 3/O => CO + H O Main advantage, does not need pure Hydrogen. outside automotive battery replacements small light weight Low temperature Methanol toxicity is a problem / 4 / 4 Fuel Cell in USA US DOE Fuel cells need hydrogen Steam reforming of methanol. Generate it on-board CH3 OH + O CO + 4 H / 4 3 / 4 Quasistatic Modeling of a Fuel Cell Causality diagram Power amplifier (Current controller) Fuel amplifier (Fuel controller) Standard modeling approach 4 / 4 5 / 4

4 Fuel Cell Thermodynamics Starting point reaction equation H + O H Open system energy Enthalpy H Fuel Cell Performance Polarization curve Polarization curve of a fuel cell Relating current density i fc (t) = fc (t)/a fc, and cell voltage U fc (t) H = U + pv Available (reversible) energy Gibbs free energy G G = H TS Open circuit cell voltages U rev = G n e F, U id = H n e F, U rev = η id U id F Faradays constant (F = q N ) Heat losses under load Cooling system P l = fc (t) (U id U fc (t)) Curve for one operating condition Fundamentally different compared to combustion engine/electrical motor Excellent part load behavior When considering only the cell 6 / 4 7 / 4 Single Cell Modeling Fuel cell voltage U fc (t) = U rev (t) U act (t) U ohm (t) U conc (t) Fuel Cell System Modeling A complete fuel cell system Activation energy Get the reactions going Semi-empirical Tafel equation U act (t) = c + c ln(i fc (t)), or U act (t) =... Ohmic Resistance to flow of ions in the cell U ohm (t) = i fc (t) R fc Concentration, change in concentration of the reactants at the electrodes U conc (t) = c i fc (t) c3, or U conc (t) =... Power at the stack with N cells P st (t) = fc (t) U fc (t) N 8 / 4 9 / 4 Fuel Cell System Modeling Describe all subsystems with models P (t) = P st (t) P aux (t) P aux = P + P em (t) + P ahp (t) + p hp (t) + P cl (t) + p cf (t) em electric motor, ahp humidifier pump, hp hydrogen recirculation pump, cl coolant pump, cf cooling fan. Submodels for: Hydrogen circuit, air circuit, water circuit, and coolant circuit 3 / 4 3 / 4 Fuel Cell Vehicles Fuel Cell HEV Short Term Storage Short term storage. Recuperation. FC has long time constants 3 / 4 33 / 4

5 Fuel Cell Vehicle Components Electric Motor The Hy.Power vehicle, going over a mountain pass in Switzerland in. Technology demonstrator Lower oxygen contents, 5 m Cold weather 34 / 4 Components Fuel Supply and Fuel Cell Stack 35 / 4 Components Fuel Cell Stack and Heat Exchanger 36 / 4 Components Fuel Cell Stack, Controller and Heat exchanger 37 / 4 Components Power Electronics and Super Caps 38 / 4 39 / 4