Topics 1. A Very Brief History 2. Electrolysis 3. Fuel Cell Basics - Electrolysis in Reverse - Thermodynamics - Components - Putting It Together 4. Types of Fuel Cells - Alkali - Molten Carbonate - Phosphoric Acid - Proton Exchange Membrane - Solid Oxide 5. Benefits 6. Current Initiatives - Automotive Industry - Stationary Power Supply Units - Residential Power Units 7. Future
A Very Brief History Considered a curiosity in the 1800’s. The first fuel cell was built in 1839 by Sir William Grove, a lawyer and gentleman scientist. Serious interest in the fuel cell as a practical generator did not begin until the 1960's, when the U.S. space program chose fuel cells over riskier nuclear power and more expensive solar energy. Fuel cells furnished power for the Gemini and Apollo spacecraft, and still provide electricity and water for the space shuttle. (1)
Electrolysis “What does this have to do with fuel cells?” By providing energy from a battery, water (H 2 O) can be dissociated into the diatomic molecules of hydrogen (H 2 ) and oxygen (O 2 ). Figure 1
Fuel Cell Basics “Put electrolysis in reverse.” fuel cell H2OH2O O2O2 H2H2 heat work The familiar process of electrolysis requires work to proceed, if the process is put in reverse, it should be able to do work for us spontaneously. The most basic “black box” representation of a fuel cell in action is shown below: Figure 2
Fuel Cell Basics Thermodynamics H 2 (g) + ½O 2 (g)H 2 O(l) Other gases in the fuel and air inputs (such as N 2 and CO 2 ) may be present, but as they are not involved in the electrochemical reaction, they do not need to be considered in the energy calculations J / mol·K J / mol·K J / mol·K Entropy (S) kJ / mol 00Enthalpy (H) H 2 O (l)O2O2 H2H2 Table 1 Thermodynamic properties at 1Atm and 298K Enthalpy is defined as the energy of a system plus the work needed to make room for it in an environment with constant pressure. Entropy can be considered as the measure of disorganization of a system, or as a measure of the amount of energy that is unavailable to do work.
Fuel Cell Basics Thermodynamics Enthalpy of the chemical reaction using Hess’ Law: Δ H = ΔH reaction = ΣH products –Σ H reactants = (1mol)( kJ / mol ) – (0) = kJ Entropy of chemical reaction: ΔS = ΔS reaction = ΣS products – ΣS reactants = [(1mol)(69.91 J / mol·K ) ] – [ (1mol)( J / mol·K ) + (½mol)( J / mol·K ) ] = J / K Heat gained by the system: ΔQ= TΔS = (298K)( J / K ) = kJ
Fuel Cell Basics Thermodynamics The Gibbs free energy is then calculated by: ΔG = ΔH – TΔS = ( kJ) – (-48.7 kJ) = -237 kJ The external work done on the reaction, assuming reversibility and constant temp. W = ΔG The work done on the reaction by the environment is: The heat transferred to the reaction by the environment is: W = ΔG = -237 kJ ΔQ = TΔS = kJ More simply stated: The chemical reaction can do 237 kJ of work and produces 48.7 kJ of heat to the environment.
Fuel Cell Basics Components Anode: Where the fuel reacts or "oxidizes", and releases electrons. Cathode: Where oxygen (usually from the air) "reduction" occurs. Electrolyte: A chemical compound that conducts ions from one electrode to the other inside a fuel cell. Catalyst: A substance that causes or speeds a chemical reaction without itself being affected. Cogeneration: The use of waste heat to generate electricity. Harnessing otherwise wasted heat boosts the efficiency of power- generating systems. Reformer: A device that extracts pure hydrogen from hydrocarbons. Direct Fuel Cell: A type of fuel cell in which a hydrocarbon fuel is fed directly to the fuel cell stack, without requiring an external "reformer" to generate hydrogen.
Fuel Cell Basics Putting it together. Figure 3
Types of Fuel Cells The five most common types: Alkali Molten Carbonate Phosphoric Acid Proton Exchange Membrane Solid Oxide
Molten Carbonate Fuel Cell (MCFC) carbonate salt electrolyte 60 – 80% efficiency ~650˚C operating temp. cheap nickel electrode catylist up to 2 MW constructed, up to 100 MW designs exist Figure 5 The operating temperature is too hot for many applications. carbonate ions are consumed in the reaction → inject CO 2 to compensate
Phosphoric Acid Fuel Cell (PAFC) phosphoric acid electrolyte 40 – 80% efficiency 150˚C - 200˚C operating temp 11 MW units have been tested sulphur free gasoline can be used as a fuel Figure 6 The electrolyte is very corrosive Platinum catalyst is very expensive
Proton Exchange Membrane (PEM) thin permeable polymer sheet electrolyte 40 – 50% efficiency 50 – 250 kW 80˚C operating temperature electrolyte will not leak or crack temperature good for home or vehicle use platinum catalyst on both sides of membrane → $$ Figure 7
Solid Oxide Fuel Cell (SOFC) hard ceramic oxide electrolyte ~60% efficient ~1000˚C operating temperature cells output up to 100 kW high temp / catalyst can extract the hydrogen from the fuel at the electrode high temp allows for power generation using the heat, but limits use SOFC units are very large solid electrolyte won’t leak, but can crack Figure 8
Benefits Efficient:in theory and in practice Portable:modular units Reliable:few moving parts to wear out or break Fuel Flexible:With the a fuel reformer fuels such as natural gas, ethanol, methanol, propane, gasoline, diesel, landfill gas, wastewater treatment digester gas, or even ammonia can be used Environmental: produces heat and water (less than combustion in both cases) near zero emission of CO and NO x reduced emission of CO 2 (zero emission if pure H 2 fuel)
Current Initiatives Automotive Industry Most of the major auto manufacturers have fuel cell vehicle (FCV) projects currently under way, which involve all sorts of fuel cells and hybrid combinations of conventional combustion, fuel reformers and battery power. Considered to be the first gasoline powered fuel cell vehicle is the H 2 0 by GM: GMC S-10 (2001) fuel cell battery hybrid low sulfur gasoline fuel 25 kW PEM 40 mpg 112 km/h top speed Figure 9
Fords Adavanced Focus FCV (2002) fuel cell battery hybrid 85 kW PEM ~50 mpg (equivalent) 4 kg of compressed H 5000 psi Approximately 40 fleet vehicles are planned as a market introduction for Germany, Vancouver and California for Current Initiatives Automotive Industry Figure 10 Figure 11
Daimler-Chrysler NECAR 5 (introduced in 2000) 85 kW PEM fuel cell methanol fuel reformer required 150 km/h top speed version 5.2 of this model completed a California to Washington DC drive awarded road permit for Japanese roads Current Initiatives Automotive Industry Figure 12
Mitsubishi Grandis FCV minivan fuel cell / battery hybrid 68 kW PEM compressed hydrogen fuel 140 km/h top speed Plans are to launch as a production vehicle for Europe in Current Initiatives Automotive Industry Figure 13
Current Initiatives Stationary Power Supply Units A fuel cell installed at McDonald’s restaurant, Long Island Power Authority to install 45 more fuel cells across Long Island, including homes. (2) Feb 26, 2003 More than 2500 stationary fuel cell systems have been installed all over the world - in hospitals, nursing homes, hotels, office buildings, schools, utility power plants, and an airport terminal, providing primary power or backup. In large-scale building systems, fuel cells can reduce facility energy service costs by 20% to 40% over conventional energy service. Figure 14
Current Initiatives Residential Power Units There are few residential fuel cell power units on the market but many designs are undergoing testing and should be available within the next few years. The major technical difficulty in producing residential fuel cells is that they must be safe to install in a home, and be easily maintained by the average homeowner. Residential fuel cells are typically the size of a large deep freezer or furnace, such as the Plug Power 7000 unit shown here, and cost $ $ If a power company was to install a residential fuel cell power unit in a home, it would have to charge the homeowner at least 40 ¢/kWh to be economically profitable. (3) They will have to remain a backup power supply for the near future. Figure 15
Future “...projections made by car companies themselves and energy and automotive experts concur that around 2010, and perhaps earlier, car manufacturers will have mass production capabilities for fuel cell vehicles, signifying the time they would be economically available to the average consumer.” Auto Companies on Fuel Cells, Brian Walsh and Peter Moores, posted on Technical and engineering innovations are continually lowering the capital cost of a fuel cell unit as well as the operating costs, but it is expected that mass production will be of the greatest impact to affordability. A commercially available fuel cell power plant would cost about $3000/kW, but would have to drop below $1500/kW to achieve widespread market penetration.
References (1) FAQ section, fuelcells.org (2) Long Island Power Authority press release: Plug Power Fuel Cell Installed at McDonald’s Restaurant, LIPA to Install 45 More Fuel Cells Across Long Island, Including Homes, (3) Proceedings of the 2000 DOE Hydrogen Program Review: Analysis of Residential Fuel Cell Systems & PNGV Fuel Cell Vehicles, Figures 1, 3 4 – Table 1 Fuel cell data from: Types of Fuel Cells, fuelcells.org Fuel Cell Vehicle data primarily from: Fuel Cell Vehicles (From Auto Manufacturers) table, fuelcells.org