1. 25 Fuels are similar for any system.doc.lnk
Fuels are similar for any system. Solving the fuel problem is a separate issue.
(This relates to material in Chapter 3)
The misconception that getting people out of cars will solve our transportation energy problems distorts much of our thinking. Roughly put, the facts are that:
Public transit today is no more energy-efficient than single-occupant cars
Electric energy produced from fossil fuels generates just as much of the principle greenhouse gas, carbon dioxide, as energy produced by internal combustion vehicle motors.
Solving the problems of global warming and the shortage of fossil fuels is a matter of finding alternative fuels. Period. Shifting to public transportation systems will address the problems of congestion and can improve energy efficiency per passenger mile. However, we are fighting a rear-guard action as long as we continue to use fossil fuels. Just as we have seen with passenger cars over the past three decades, every improvement in efficiency will be offset by an increase in usage. In the short term we must fight mount that rear-guard fight. As we do so we cannot lose sight of the fact that in the long run world needs to find an alternate, preferably a renewable energy source to replace fossil fuels.
THE REAR-GUARD ACTION: MAKING THINGS MORE EFFICIENT
1.1. What is energy efficiency?
We are accustomed to measuring energy efficiency as miles per gallon (or liters per 100 Km outside the U.S.). Give ourselves some credit for improving according to this metric. Figure 1 derived from the U.S. Department of Energy figures shows automobile efficiency improving by about 50% between 1978 and 2000. On the downside it shows that truck efficiency did not increase nearly as much. The figure also shows that actual mileage decreased as light truck registrations roared ahead over the same period. As American industry became more efficient in energy consumption per ton of vehicle, American consumers developed a lust for more vehicle weight. We have met the enemy and it is us.
Our ultimate objective of course is to move people and goods around, not metal. By this measure cars are still a dismal failure. If we assume that the payload of the average car is a 200-pound commuter, then the efficiency is still only about 1% of that of a highway truck or simply walking:[1]
Payload (lb.) / Payload (tons) / Miles/gal / gal/mile / Gallons/ton milePrivate Car / 200 / 0.1 / 20.7 / 0.048309 / 4.83E-01
Semi Truck / 60000 / 30 / 5 / 0.2 / 6.67E-03
Walking/biking (10 mi./day) / 200 / 0.1 / 1000 / 0.001 / 1.00E-02
The
1.1.1. Energy expended per net ton mile (excluding packaging, vehicle itself)
The law of conservation of energy, known as the first law of thermodynamics, ensures that whenever energy is converted in form its total quantity remains unchanged.
Entropy is the measure of disordered, or unusable energy in the universe. “Entropy increaseth” is a shorthand summary of the second law of thermodynamics. Something is lost in any conversion of energy from one form to another. That is why a perpetual motion machine is impossible.
In automobiles this means that
Energy is lost in converting crude oil into gasoline
Energy is lost in using an engine to convert gasoline to power
Energy is lost converting power into motion
These are only a few of the losses. It takes a lot of energy to get crude out of the ground and to a refinery and more energy to get the gasoline to market. There is spillage, seepage and evaporation to consider.
The frightening statistic is how much energy is lost in each conversion. Figure 2 shows that car engines are only about 30% efficient in converting gasoline into rotary motion. Two thirds of the energy potential is lost to heat and friction in the process of vaporizing the fuel, expanding the fuel by burning it in a cylinder to push a piston, converting the linear motion of the piston into rotary motion in the crankshaft and getting the power to the back wheels.
After gasoline is converted into rotary motion the question remains how efficient the motion of tires on pavement is in getting us to our destination. A car loses energy in:
Overcoming wind and rolling resistance. It takes a certain amount of energy to keep a car traveling at a constant speed. Modern automobiles are most efficient operating on a straight flat road at about 55 miles per hour with fully inflated tires and no cross winds. As we know from the window stickers on new cars, even this ideal highway mileage, which few drivers ever achieve, is not very good if you consider that all the energy is spent merely fighting resistance.
Braking. First you spend energy getting the car in motion. Then you convert that kinetic energy back into heat which the brakes dissipate into the environment. One of the efficiencies of electric trains and cars, and hybrids such as the Honda Insight, is that they convert kinetic energy back into electricity as they brake instead of simply throwing it away.
As Figure 3 shows, electric motors of the power required in transportation approach 95% efficiency. That figure is misleading, however, because it is only the last in a series of other, less efficient conversions required to bring electricity to the motor in the first place:
Energy to get the fuel to an electric generator
Conversion of fossil fuel (mostly) into electricity, with about 40% efficiency. GE[2] is working on bringing that up to 55 or 60% in their next generation of power generation equipment.
Transmission of electricity to the point of use. Here the efficiency is quite high, about 95%[3]
At the end of the day, the cumulative efficiency of electric motors for vehicle propulsion is no better than equivalent to gasoline or diesel. It is, however, cleaner and quieter at the point of its ultimate use.
There are several initiatives, including one by Princeton University and British Petroleum, to find a way to immobilize part of the 3 to 4 billion tons of CO2 we generate each year in salt aquifers or the deep ocean. Whether or not this is feasible in the first place, it will certainly be easier if that CO2 can be captured at generation plants instead of at individual automobile tailpipes.
Electric power offers a major potential advantage in that it does not need to be generated from fossil fuels. Whatever its other drawbacks, nuclear power, which still accounts for about 20% of our power needs, does not contribute to global warming by producing greenhouse gases. It is much easier to generate electricity from renewable sources such as wind, geothermal energy and solar cells in fixed sites instead of on board a vehicle. Given the advantages of generating electric power elsewhere, the question of getting it into the vehicle becomes of high interest.
CAPTURING ELECTRIC POWER USING FUEL CELLS
Up until now electric cars have run on batteries. They are expensive, heavy, and don’t offer much range. Battery technology only improves incrementally. Relatively few people believe they offer a long-term solution. There has to be a lighter, more concentrated way to carry electric energy aboard a vehicle.
Hydrogen offers an alternative. Fuel cells convert hydrogen gas into electricity. The controlled chemical reaction they use to oxidize hydrogen gas into water is a much more efficient means of converting it into energy than exploding it or burning it with an open flame. Also, the electric energy that fuel cells produce is more useful for driving a machine than the heat that would be created by combustion. Using fuel cells and hydrogen, then is an alternative to using batteries in an electric vehicle.
Hydrogen is an unwieldy fuel to carry around. As the Hindenberg disaster demonstrated, it has a tendency to explode. Keeping it liquid requires ultra-low temperatures, but carrying it in gaseous form requires bigger tanks than are practical to design into cars. For these reasons most fuel cell technologies involve a two-step process. First there is a chemical process to generate hydrogen; then a second reaction using the hydrogen to generate electricity. The major variation among fuel cell systems involves techniques of reforming some kind of base fuel into hydrogen. Big ones can run as hot as 1800° F. Smaller systems, suitable for use in vehicles, operate at lower temperatures and are generally less efficient.
The fuel cells that are closest to being commercialized use conventional carbon-based fuels gaseous and liquid fuels. The process of reforming them to release hydrogen gas perforce creates some kind of carbon compounds, usually CO2, the most benign to release into the atmosphere.
On a gross scale, then today’s fuel cells have the same drawback as internal combustion engines. They add to the CO2 in the air. They do offer a few incremental improvements. They make more efficient use of the fuel, which means they release less CO2 per unit of energy. On top of that, the catalytic processes they use are inherently cleaner than combustion. Fuel cells generate significantly less of the really noxious gases like carbon monoxide and nitrous oxide.
Carbon is not essential to the operation of a fuel cell the way it is to internal combustion engines. It is only along for the ride, the most convenient means of freighting hydrogen atoms to their point of use. Carbon can be removed from the system when the technologies are developed to conveniently store pure hydrogen or when a better, preferably a reusable carrier is found.
The characteristics of an ideal hydrogen carrier are:
Physically, it needs to be rather light. If a vehicle is going to carry it around, it needs to be able to carry a fair amount of hydrogen per unit weight of carrier. It also needs to be in a form that is easy to work with.
Chemically, it would take little energy to bind it to and separate it from hydrogen gas. As an analogy, consider carbon dioxide and water. It takes little energy to dissolve CO2 in water (H2O) to form carbonic acid, and little energy is released getting it back into gaseous form. It is the fizz in your soft drink. It is the theory behind burying our excess CO2 in the ocean. Researchers need to find a compound that will accept and release H2 as readily as water accepts and releases CO2. That way the system can recharge the spent carrier with hydrogen instead of releasing it into the environment.
Chemically, it should not easily get clogged up or “poisoned” by elements other than hydrogen that ruin its ability to accept and hold hydrogen.
Environmentally, it cannot be too corrosive, toxic or otherwise unfriendly to humanity if it escapes due to a car crash or whatever.
Cost has to be within reason.
Experimenters are working today with metal hydrides, simple molecules made up of only metal and hydrogen. They satisfy the first criterion in that they generally carry a sufficient amount of hydrogen per unit of weight. The most commonly used of these, a magnesium-nickel alloy, is able to carry about 3% of its weight in hydrogen. [4]
The other criteria are more of an issue. Hydrides can be “poisoned” by a number of contaminants. They can be explosive and corrosive. However, with many good minds working on the problem it is likely that they will find new compounds or work around the problems with existing ones at some point in the future.
As is often the case in technology, there is a leading-edge alternative that appears too good to be true -–and may be. Graphite Nanofibers are microscopic platelets of carbon that can be spaced so closely that there is room for hydrogen molecules but not oxygen or anything bigger between them. More than that, the platelets have something of an electrical charge that helps them hang onto the hydrogen. Once it is forced into the platelets, it will stay there without a great deal of pressure. One researcher projects that a one cubic foot fuel tank at 1000 pounds/square inch pressure (not that much; eight times the pressure in a bicycle tire) could hold enough fuel for 4000 miles. They claim this is about six times the amount that can be stored by a similar weight of metal hydrides.[5] This is a roundabout endorsement of metal hydrides; by this math, they should be able to fuel a car up to 600 miles, which would be wonderful in itself.
Fuel cell systems are a natural complement to our electric grid. While electric generators are capable of putting out more or less the same amount of power around the clock, demand for electricity is highly cyclical, usually peaking in the afternoon. This fact is reflected in our electric bills; rates are set higher during peak hours to encourage homeowners to schedule.
Various schemes in the past have attempted to store the excess energy available at night for reuse during the day. One of the most obvious devices was to use cheap nighttime electricity to pump water uphill to a reservoir, then let it flow downhill during the day to generate more valuable peakload electricity.
If fuel cells ever become a major part of our energy system, the process of splitting water into oxygen to release into the air and hydrogen for storage as a fuel will demand a great deal of energy. It would make sense to draw that energy from the grid at night when there are fewer competing uses are low. The scheme adds more value than pumping water for two reasons. First, the process of reconverting hydrogen into electricity is very efficient, and secondly, hydrogen is transportable; it can be used to fuel vehicles.