16-8 The Advantages and Disadvantages of Hydrogen as an Energy Source • Concept 16-8 Hydrogen fuel holds great promise for powering cars and generating electricity, but to be environmentally beneficial, it would have to be produced without the use of fossil fuels.
Hydrogen Is a Promising Fuel but There Are Challenges (1) • Hydrogen as a fuel • Eliminate most of the air pollution problems • Reduce threats of global warming • Some challenges • Chemically locked in water and organic compounds • Fuel cells are the best way to use hydrogen • CO2 levels dependent on method of hydrogen production
Hydrogen Is a Promising Fuel but There Are Challenges (2) • Production and storage of H2 • Hydrogen-powered vehicles: prototypes available • Can we produce hydrogen on demand? • Larger fuel cells
A Fuel Cell Separates the Hydrogen Atoms’ Electrons from Their Protons
HYDROGEN FUEL CELLS CURRENT AND FUTURE DESIGNS USE AS AN ENERGY STORAGE DEVICE RENEWABLE VS. CO2 GENERATING DISTRIBUTION METHODS AND COSTS SAFETY
Molecular Hydrogen Facts • Three times energy content of gasoline (120 Mj/kg vs. 44Mj/kg) • Cost of liquefying it is 30 to 40% of its energy content • Pipelines are 50% greater diameter than for gas (for equivalent energy transmission rate), so more $. • Distribution doubles cost of production ($1.03/kg). • Flammable concentration has a wide spread from 4% to 75%.
Types of Fuel Cells • Phosphoric Acid fuel cells: only commercially available. Since 1991, 250 units sold (until 2004). $4500/kW • Proton Exchange Membrane fuel cells. Vehicle transportation method because 25kW to 150kw size. Beta testing at $1500/kW by UTC Fuel Cells. • Solid Oxide fuel cells. Currently operate using natural gas, but could use H. Unlike those above, no noble metals. Can operate up to 1000 c.
Molecular Hydrogen Generation • Three different scales of generation: Central Station, Midsize, and Distributed. • Central Station: 1,080,000 kg/day would support 2M cars. Distributed by pipeline. Generated by fossil fuel or nuclear energy. • Midsize: 21,600 kg/day would support 40k cars. Distributed by cryogenic truck. Generated by natural gas or biomass • Distributed: 480 kg/day would support 800 cars. No distribution system needed. Renewable fuels used.
Hydrogen Generation • Electrolysis: from fossil fuels or renewable energy sources • Fossil Fuels requires carbon sequestering (next slide) • Hydroelectric, Nuclear Energy, Photovoltaic, grid based energy, wind power, have either periodic generation, which may not match usage, or have constant generation, which does not match usage. • Energy storage at peak times is a problem for these energy sources that hydrogen generation could solve. • Cost for all distributed (renewable) sources is two to five times cost of gasoline (2004)
Carbon Dioxide Sequestering • No advantage to Hydrogen production from fossil fuels as far as reducing Carbon Dioxide is concerned. Yet this is the cheapest method to generate Hydogen. • Currently, only 4% of Hydrogen is from electrolysis; the rest is from coal, oil and natural gas, which contains about 10% CO2 as well as the Hydrogen. • Norway injects CO2 into saline aquifer. • Los Alamos study of CO2 injection into magnesium formations (Peridotites, serpentinite, Gabbro, and Basalt) • Costs about $8 per ton of CO2 disposed. Permanent and no legacy problems for future generations.
CO2 and Hydrogen from fossil fuels • Small increase in cost to capture CO2 when hydrogen produced by fossil fuels. • Using coal as feedstock, sulfur captured along with CO2. • CO2 sequestering and storage costs about $4.00/barrel of oil, $24/ton of coal, $0.54/cubic feet of natural gas. • New infrastructure required to transfer CO2 from H2 production to storage. • Deep ocean already stores CO2, may give it up with warming. • World’s biological storage now 700 billion tons. To double this storage would require doubling vegetation.
Hydrogen production technologies • From natural gas. Already import natural gas. H conversion from methane by steam reforming, produces CO too. If natural gas costs $6.50/Btu, H costs $3.51/kg • From Coal. Enough to last 200yrs. Produces the most CO2 for the same H. Gasification produces syngas. Research may get costs to $0.90/kg. • From nuclear energy. Water splitting by electrolysis or thermochemical process. Need temps of 700 to 1000 c. No CO2. No cost estimates. • Electrolysis. Most expensive. Use proton exchange membrane( PEM) or liquid electrolyte (KOH).
Hydrogen production--Renewable fuels • From wind energy. Electrolyze water. Wind is the most cost effective renewable energy source: $0.04 to $0.07/kWh costs about $6.64/kg per H2 if grid back up used. • From Biomass. Only 0.2 to 0.4% of solar energy converted to H2. Costs $7.05/kg by gasification, not including fertilizers and land degradation. • From Solar energy. Either by electrolysis (Photo voltaic) or using photoelectrochemical cell (in a early stage of development). Cost now is $28.19/kg and solar energy is only available 20% of the time.
Hydrogen safety • Small leak more flammable than for gasoline, but more likely to disperse, so ignition less likely. • Static spark can ignite, so ground during transfer. • Detonation more likely than with gasoline because of wider flammable concentration and higher flame speed. • Need high pressure to transfer efficiently: 5-10k psi. • Odorless, burns with a blue flame. Small molecule precludes adding scent molecule.