Control of methanol fuelled HTPEM fuel cell system
Authors
Sahlin, Simon Lennart ; Sørensen, Jesper Kjær
Term
2. term
Education
Publication year
2009
Pages
198
Abstract
This project examines the electrical efficiency and control of a methanol-fueled high-temperature PEM fuel cell system. Such a system converts methanol and water into hydrogen in a reformer and generates electricity in a fuel cell stack. The setup includes an evaporator, a reformer with a burner, and a fuel cell stack, and it operates in two modes: startup and steady operation. During startup, the burner uses pure methanol and the components are heated by convection. In steady operation, the reformer is heated by burning excess hydrogen from the stack exhaust. The reformer uses the endothermic steam reforming process and is heated directly by the burner. We investigate a general feedforward control strategy that proactively sets reactant flows to the reformer and the fuel cell stack based on the desired load. To keep a constant hydrogen stoichiometry (the ratio of supplied hydrogen to what is consumed) in the stack exhaust, we propose an estimator. It shows good theoretical performance but has not been tested on hardware. The system is modeled nonlinearly for both startup and running conditions. The electrical efficiency is calculated to be approximately 18% based on methanol’s higher heating value (HHV). The nonlinear model is linearized and used to test a PI controller. We also study a fuel estimator based on current density to link electrical load to fuel demand. A mathematical search routine in MATLAB is used to select suitable controller parameters. Experiments are performed to better understand hydrogen stoichiometry. The fuel cell exhaust is emulated with a mass flow controller, and the hydrogen flow to the burner is varied to hold the reformer temperature constant. The nonlinear model’s stoichiometry is adjusted until it shows the same amount of excess hydrogen as observed experimentally.
Dette projekt undersøger den elektriske virkningsgrad og regulering af et methanoldrevet højtemperatur PEM-brændselscellesystem. Et sådan system omdanner methanol og vand til brint i en reformer og producerer strøm i en brændselscellestak. Anlægget består af en fordamper, en reformer med brænder og en brændselscellestak og er designet med to driftstilstande: opstart og normal drift. I opstart bruger brænderen ren methanol, og komponenterne opvarmes ved konvektion. I normal drift opvarmes reformeren ved at brænde overskudsbrint fra stakkens afgas. Reformeren arbejder med endoterm dampreformering og opvarmes direkte af brænderen. Vi undersøger en generel reguleringsstrategi baseret på fremadkobling (feedforward), som på forhånd doserer reaktanter til reformer og brændselscel lestak ud fra det ønskede belastningsniveau. For at holde en konstant brintstoikiometri (forholdet mellem tilført brint og det, der forbruges) i stakkens afgas præsenteres en estimator. Den viser gode teoretiske resultater, men er ikke testet på et fysisk anlæg. Systemet modelleres ikke-lineært for både opstart og normal drift. Den elektriske virkningsgrad beregnes til cirka 18 % baseret på methanols højere brændværdi (HHV). Den ikke-lineære model linearisers og bruges til at afprøve en PI-regulator. Vi undersøger også en brændstofestimator baseret på strømtæthed, som kobler den elektriske belastning til brændstofbehovet. For at finde passende regulatorparametre anvendes en matematisk søgerutine i MATLAB. For at få viden om brintstoikiometrien gennemføres eksperimenter, hvor brændselscellens afgas simuleres med en masseflowregulator. Ved at variere brintflowet til brænderen, så reformertemperaturen holdes konstant, bestemmes det nødvendige brintflow. Den ikke-lineære model justeres, indtil den viser samme mængde overskudsbrint som i forsøgene.
[This apstract has been rewritten with the help of AI based on the project's original abstract]
Keywords