shallowdeep
Posts: 343
Joined: 9/1/2006
From: California
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An attempt to split up transportation fuel alternatives based on energy source, plus some thoughts.
- The Status Quo (e.g. petroleum derivatives)
Cons:- Emissions, both particulate and carbon.
- Limited, largely foreign supply.
- Rising costs.
Pros:- High energy density.
- No infrastructure changes needed.
Thoughts: The sooner we can get off oil, the better.
- Alternative Fossil Fuels (e.g. methane, synthetic coal gases)
Cons:- Lower energy density.
- Same carbon emission problems.
Pros:- Lower particulate emissions.
- Larger domestic supply, possibly cheaper.
Thoughts: They're not solving the problem. At best a limited stop-gap.
- Crops Grown Expressly for Energy (e.g. ethanol, methanol, biodiesel, etc.)
Cons: - It may not be environmentally friendly. Trying to analyze a complex system's energy use is difficult, and it's a relatively new field with the available empirical data limited in both quantity and quality. The result is assumptions and simplifications that produce conflicting estimates and academic bickering. For instance, corn ethanol production has ranges for a net energy value from around 75% (critics like Pimentel) to 167% (USDA). Even if a biofuel has a positive energy return, that may well be lost when considering sustainability issues and the fact it's likely being burned in an internal combustion engine. Patzek, for instance, argues a gallon of corn ethanol burned in an engine returns only 1/7th of the energy that went into making it as usable work. Some biofuels may be better than others, but this really is the sort of analysis that needs to be carefully considered before making energy policy. At present there is at least some support for the idea that biofuels may be an energy sink rather than source. I found this interview with Pimentel interesting.
- Use of relatively scarce crop (or arable) land for energy production is arguably a misallocation of resources.
- There isn't enough available. Assuming there are biofuels with significant positive net energy returns, there still isn't enough land to meet current US transportation energy needs. It might be part of the solution, but it's never going to be the "one" fuel.
- The reason for the last point is that plants aren't all that efficient. They only convert about 0.1% of solar energy into potentially usable chemical energy. Even cheap photovoltaics are much more efficient, which raises the question: even if biofuels prove workable, might there be a better approach?
Pros:- Even if critics are right, and biofuels don't have positive net energy values, it still helps kick foreign oil dependence. The energy source essentially becomes coal and natural gas, with all the negatives those entail, but it's a domestic source. Some, including the USDA (see the 'Energy Balance Issue' section of this document), argue that this may make it worth pursuing regardless of environmental issues.
- A potentially lower cost, basically related to the above point. As oil prices increase, the price differential translates into profit for agriculture, and maybe even some consumer savings.
- Little infrastructure change is required to accommodate it.
Thoughts: Some biofuel systems may have some potential, but they need more careful analysis. Their current supplementary role may expand, but they'll never make a primary fuel source. Corn ethanol is probably currently a bad idea.
- Waste Materials (e.g. used frying oil, methane from manure, etc.)
Cons:- Wastes useable as fuels only exist in limited quantities and are wholly insufficient for meeting current transport energy needs.
- In some cases they may not burn as cleanly as conventional alternatives.
Pros:- Relatively cheap, as it's still an underutilized resource.
- Helps solve a waste problem.
Thoughts: If you can make it work, go for it, but it's never going to be more than a small niche.
- Rechargeable Energy Storage (e.g. hydrogen, batteries, compressed air, solid oxides, etc.)
Cons:- New infrastructure may be required.
- New power generation sources will be needed.
- At best, the method is only as environmentally friendly as the power source.
Pros:- Decouples transportation energy from any particular fuel source.
- Potentially zero harmful emissions.
- Potentially no reliance on foreign energy sources.
Thoughts: This is where we should be headed. Nothing in this category is a fuel source per se, so it really requires two further discussions:
- How efficient is each method at storing and recovering energy, how economical, how safe?
- What can be done to get a cheap, clean, and sufficient power source (probably in terms of our electrical generation infrastructure)?
- Human and Animal Power (e.g. walking, biking, dwarf hamsters, etc.)
Cons: - The obvious cons are limits on convenience, range, speed, and capacity.
- The less obvious con is the enormous energy inefficiency. Other fuels are cheap compared to food, and humans aren't particularly efficient at turning food into mechanical work. The idea that biking, for instance, is carbon neutral or even environmentally friendly (on a per energy expended basis) isn't true in our modern world. Once you factor in the energy spent to grow, harvest, transport, clean, refrigerate and cook it you were probably better off using gas. Some animal/feed combinations would be better.
Pros:- Health benefits. If we all walked or biked when it was practical, everyone, maybe excepting the medical industry, would be better off.
- The inefficiency of food as fuel gives an incentive to conserve. A 30 pound bike going 5 miles can come out ahead of a 3000 pound car going 50 miles pretty easily, even accounting for inefficient use of energy. So, in practice, it tends to be a good choice.
Thoughts: When practical, go for it but, realistically, another fuel is needed.
The last point, about the tendency to conserve with human powered transport, raises a question. How important is the choice of fuel to the transportation energy problem?
The fuel obviously matters and, in the best case, could be made essentially environmentally neutral. But that utopia is, unfortunately, still at least a few decades off in practice. However, there is quite a bit that could be done with conservation that could make that goal easier to achieve by lowering the energy required – and conservation should play into discussions about transportation energy policy.
Lighter vehicles, increased use of hybrids, more (and better) public transportation, better thought out communities that reduce required travel distances, and automated control of traffic are all things that I think could have an impact in the same time frame it will take to switch over to a truly clean fuel. Why have we all become so attached to a system that results in 6 million accidents, 3 million injuries, and over 40,000 deaths annually anyway? There's room for improvement in more than one way.
[edited to clarify one point]
< Message edited by shallowdeep -- 4/23/2008 8:57:22 PM >
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RE: Alternative Fuels Roundtable - 
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Just playing a little, mostly. Who we are socially will have a great effect on the task that will be set for the mechanical engineers, though. 
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