Presentation on theme: "TECH 57210 Sustainable Energy I Energy Sources and Systems 8 Dr. Darwin L. Boyd Fall 2011."— Presentation transcript:
TECH Sustainable Energy I Energy Sources and Systems 8 Dr. Darwin L. Boyd Fall 2011
Fuel Cells Fuel Cell – Electrochemical energy conversion device in which fuel and oxidant react to generate electricity without any consumption, physically or chemically, of its electrodes or electrolyte. Storage cell: reactants self contained and electrodes consumed Lead-Acid Battery Reaction Pb + PbO 2 + H 2 SO 4 2 PbSO H 2 O + _ H 2 SO 4 Pb Storage Cell Fuel cell: reactants supplied continuously and electrodes invariant Overall Fuel Cell Reactions: H 2 + O 2 H 2 O + heat + electrons Fuel Cell _ + Air (O 2 ) H2H2 H2OH2O
4 Photographs from FC History US Army MCFC, 1966 Allis-Chambers PAFC engine, 1965 William Grove's drawing of an experimental “gas battery“, 1843 William Jacques' carbon battery, 1896
5 Bipolar Plate Cathode + Anode - Electrolyte H+ HYDROGEN (H 2 ) OXYGEN (O 2 ) Bipolar Plate O- e - H+ O- e - WATER (H 2 O) + HEAT H 2 2H + + 2e - ½O 2 + 2H + + 2e - H 2 O H+ PEMFC: Protons formed at the anode diffuse through the electrolyte and react with electrons and oxygen at the cathode to form water and heat.
6 Single cells are arranged into “stacks” to increase total voltage and power output Cathode: O 2 + 4H + + 4e - 2H 2 O 1.2 V Anode: 2H 2 4H + + 4e V Total Cell: 2H 2 + O 2 2H 2 O 1.2 V per cell Power = Volts X Amps Ballard PEFC Stack
7 Fuel Cell System Fuel Processor Fuel Cell Stack Spent-Gas Burner Thermal & Water Management Air Fuel H2H2 Exhaust Electric Power Conditioner
8 Fuel Processor Power Fuel Processor BARRIERS Fuel processor start-up/ transient operation Durability Cost Emissions and environmental issues H 2 purification/CO cleanup Fuel processor system integration and efficiency On-Board Fuel Processing
9 Five major types of fuel cells Fuel Cell Type Temperature Applications Electrolyte / Ion Polymer Electrolyte Membrane (PEM) ° C Electric utility Portable power Transportation Perfluorosulfonic acid / H+ Alkaline (AFC) 90 – 100° C Military Space KOH / OH- Phosphoric Acid (PAFC) 175 – 200° C Electric utility Distributed power Transportation H 3 PO 4 / H+ Molten Carbonate (MCFC) 600 – 1000° C Electric utility Distributed power (Li,K,Na) 2 CO 3 / CO 2 - Solid Oxide (SOFC) 600 – 1000° C Electric utility Distributed power APUs (Zr,Y) O 2 / O-
10 Alkaline Fuel Cell (AFC) Applications Space Transportation Features High performance Very sensitive to CO 2 Expensive Pt electrodes Status “Commercially” available AFCs from Apollo & Spaceshuttle Spacecrafts-- NASA Equations Cathode: ½O 2 + H 2 O + 2e¯ → 2OH¯ Anode: H 2 + 2OH¯ → 2H 2 O + 2e¯
11 Phosphoric Acid Fuel Cell Equations Cathode: ½O 2 + 2H + + 2e¯ → H 2 O Anode: H 2 → 2H + + 2e¯ Applications Distributed power plants Combined heat and power Some buses Features Some fuel flexibility High efficiency in cogeneration (85%) Established service record Platinum catalyst Status Commercially available but expensive Excellent reliability and availability Millions of hours logged UTC Fuel Cells 200-kW
12 Equations Cathode: ½O 2 + CO 2 + 2e¯ → CO 3 = Anode: H 2 + CO 3 = → 2H 2 O + CO 2 + 2e¯ Fuel Cell Energy MCFC stack Molten Carbonate Fuel Cells Applications Distributed power plants Combined heat and power Features Fuel flexibility (internal reforming) High efficiency High temperature good for cogeneration Base materials (nickel electrodes) Corrosive electrolyte Status Pre-Commercially available but expensive
13 Equations Cathode:O 2 + 2e¯ → 2O = Anode: H 2 + O = → H 2 O + 2e¯ Solid Oxide Fuel Cells Applications Truck APUs Distributed power plants Combined heat and power Features Slow start – subject to thermal shock High temperature High power density (watts/liter) Can use CO and light hydrocarbons directly “Cheap” components, solid electrolyte Low-yield manufacture Status Vehicle APUs
14 Equations Cathode: ½O 2 + 2H + + 2e¯ → H 2 O Anode: H 2 → 2H + + 2e¯ Polymer Electrolyte Fuel Cells Applications Transportation, Forklifts, etc. Power backup systems Consumer electronics with methanol fuel Features Quick start Low temperature Expensive Pt electrodes Easy manufacture Operating window limits 53-67% thermal efficiency Status Vehicle demonstrations underway Stationary/backup power “commercially” available Toyota Fuel Cell Forklift
15 Direct Methanol Polymer Electrolyte FC (DMFC) Applications Miniature applications Consumer electronics Battlefield Features A subset of Polymer Electrolyte Modified polymer electrolyte fuel cell components Methanol crossover lowers efficiency Status Pre-Alpha to Beta testing Equations Cathode: 1.5 O 2 + 6H + + 6e¯ → 3H 2 O Anode: CH 3 OH + H 2 O → CO 2 + 6H + + 6e¯
Fuel Cells Advantages Emissions Efficiency Some have fuel flexibility Disadvantages Cost – (Pt catalyst) Some are high temp – may be slow to start Some sensitive to fuel impurities (need pure H 2 )
Hydrogen Hydrogen is a secondary energy source Very clean energy both in ICE and fuel cells Poor energy density by volume Hydrogen Storage
Physical storage of H2 Chemical storage of hydrogen New emerging methods Hydrogen Storage Overview Metal Hydride (“sponge”) Carbon nanofibers Compressed Cryogenically liquified Methanol Alkali metal hydrides Sodium borohydride Ammonia Amminex tablets DADB (predicted) Solar Zinc production Alkali metal hydride slurry
Compressed Volumetrically and Gravimetrically inefficient, but the technology is simple, so by far the most common in small to medium sized applications. 3500, 5000, 10,000 psi variants.
Liquid (Cryogenic) Compressed, chilled, filtered, condensed Boils at 22K (-251 C). Slow “waste” evaporation Kept at 1 atm or just slightly over. Gravimetrically and volumetrically efficient but very costly to compress
Metal Hydrides (sponge) Sold by “Interpower” in Germany Filled with “HYDRALLOY” E60/0 (TiFeH 2 ) Technically a chemical reaction, but acts like a physical storage method Hydrogen is absorbed like in a sponge. Operates at 3-30 atm, much lower than for compressed gas tanks Comparatively very heavy, but with good volumetric efficiency, good for small storage, or where weight doesn’t matter
Carbon Nanofibers Complex structure presents a large surface area for hydrogen to “dissolve” into Early claim set the standard of 65 kgH 2 /m 2 and 6.5 % by weight as a “goal to beat” The claim turned out not to be repeatable Research continues…
Methanol CH 3 OH Broken down by reformer, yields CO, CO 2, and H 2 gas. Very common hydrogen transport method Distribution infrastructure exists – same as gasoline
Ammonia Slightly higher volumetric efficiency than methanol Must be catalyzed at deg. C for hydrogen release Toxic Usually transported as a liquid, at 8 atm. Some Ammonia remains in the catalyzed hydrogen stream, forming salts in PEM cells that destroy the cells Many drawbacks, thus Methanol considered to be a better solution
Alkali Metal Hydrides “Powerball” company, makes small (3 mm) coated NaH spheres. “Spheres cut and exposed to water as needed” H 2 gas released Produces hydroxide solution waste
Sodium Borohydrate Sodium Borohydrate is the most popular of many hydrate solutions Solution passed through a catalyst to release H 2 Commonly a one-way process (sodium metaborate must be returned if recycling is desired.) Some alternative hydrates are too expensive or toxic The “Millennium Cell” company uses Sodium Borohydrate technology
Amminex Essentially an Ammonia storage method Ammonia stored in a salt matrix, very stable Ammonia separated & catalyzed for use Likely to have non-catalyzed ammonia in hydrogen stream Ammonia poisoning contraindicates use with PEM fuel cells, but compatible with alkaline fuel cells.
Amminex High density, but relies on ammonia production for fuel. Represents an improvement on ammonia storage, which still must be catalyzed. Ammonia process still problematic.
Diammoniate of Diborane (DADB) So far, just a computer simulation. Compound discovered via exploration of Nitrogen/Boron/Hydrogen compounds (i.e. similar to Ammonia Borane) Thermodynamic properties point towards spontaneous hydrogen re-uptake – would make DADB reusable (vs. other borohydrates)
Solar Zinc production Isreli research effort utilizes solar furnace to produce pure Zinc Zinc powder can be easily transported Zinc can be combined with water to produce H2 Alternatively could be made into Zinc-Air batteries (at higher energy efficiency)
Alkaline metal hydride slurry SafeHydrogen, LLC Concept proven with Lithium Hydride, now working on magnesium hydride slurry Like a “PowerBall” slurry Hydroxide slurry to be re- collected to be “recycled” Competitive efficiency to Liquid H2
Storage Method Comparison Sodium Hydride slurry.91.0Must reclaim used slurry DADB (numbers for plain “diborane”and sodium borohydride, should be similar) Amminex Zinc powderunsure US DOE goal