Fuel Cell
Discussion of Fuel Cell technology - please comment!
Posted for Tom Pfotzer who wrote the below:
Fri, 28 Jul 2006 05:30:26 -0700 (PDT)
Subject:[solaroof] More About the So-called "Fool Cells"
I think the condemnation of fuel cells might be premature.
The concept of a fuel cell is valuable - e.g. the conversion of chemical energy efficiently into electricity. Rather than focus on the villain of the hydrogen-oxygen fuel cell (e.g. "where does the hydrogen come from?"), let's examine the case of an ethanol fuel cell, which "burns" alcohol to produce water and CO 2.
The basic notion I wish to advance is that there are ways to obtain high-energy-content, safe, transportable, storable compounds such as ethanol whose chemical bonds are readily broken (just use a catalyst, which causes the reaction to occur, but is not consumed in the reaction, and is the key function of a fuel cell) to efficiently produce electricity which can then be used to operate (again, rather efficiently) electric motors. The take-away here is that the chemical-to-mechanical conversion is efficient.
Then the question naturally turns to the source of the fuel. We've spent a lot of time extolling the virtues of bio-diesel, because the conversion of oil seeds, or even oil-containing algae, into high-energy-density hydrocarbons seems somehow simpler than making, for example, hydrogen. Based on my reading of the literature, making pure, stable oil is no slam-dunk, either. And the combustion products aren't all that superior in quality (pollutants) than the existing diesel fuel we get from "fossil" oil. Additionally, bio-diesel is used to run internal combustion engines which have the unfortunate trait of wasting the majority of the input-energy in the form of heat; only about a third of the input energy gets converted into mechanical energy. Two-thirds of the input power is wasted. and that's just way too much.
As of this moment, the main problem with fuel cells is that the fuel that goes into them has to be very pure, or the cell gets "poisoned" (e.g. the catalyst gets chemically altered) and it soon loses effectiveness. The necessary breakthrough for fuel cells is mainly in the area of devising a cell that can either manage or is immune to fuel-quality problems. Think of a fire - within limits, you can put almost any kind of wood on it, with a little or a lot of moisture in it, and it will still burn, and if it gets hot enough, it will burn almost anything.
The key concepts to bear in mind are:
a. The inputs to a fuel cell are chemical compounds that have a lot of energy contained in their bonds (so you need just a little fuel to produce a lot of power) and that b. That energy can be released via catalytic action (cheap and efficient conversion from chemical-to-electical energy) and c. The efficiently-converted electical power is then used in a device (a motor) that also efficiently converts electical power to mechanical power, with much lower heat losses, and finally, that d. The resulting outputs, in addition to the energy produced, are benign. There are several fuel cells which produce harmless outputs (e.g. water). Even the ones that produce CO 2 in addition to water (ethanol cells) are benign, if the CO 2 came from a plant, and not from carbon that's been sequested below the surface of the earth for the past several hundred million years (as with fossil fuels)
Now that the plusses have been laid out, let's talk about the negatives:
a. Which types of fuel cells meet the above criteria, and what is their stage of development? To cite one example, alcohol-driven fuel cells are relatively well-developed. Their key deficits are, as set out above, their requirement for pure fuel. It's relatively cheap to make ethanol, and it's harder to make it so it contains nothing but alcohol (or alcohol and water, that combination is OK). What's needed are manufacturing process advances that reduce the cost of manufacturing pure alcohol, or to advance the design of the fuel cell itself or its attendant fuel pre-processor (e.g. filters) to enable the system to handle the typical impurities that are introduced by the fuel generation process.
b. To get generate that fuel, however, we need fundamentally different processes. Instead of digging out of the ground fossil fuels which contain chemical energy from photons that hit earth several hundred million years ago (limited resource, side effect of releasing carbon sequestered underground), we need to convert the photons arriving today into chemical (storable) energy.
What can do this job of converting photons into chemical energy? Plants, having evolved over the past 3 or 4 billion years to do exactly this, are quite good at it. The amount of solar power lavished on the earth every day, most of which bounces right back into space, provides all the power we need, now and for the forseeeable future, to power human affairs. I suggest that whatever process we use to convert photons, or the effects of those photons (e.g. wind power), use the most efficient conversion process we know of, and that points directly at plants.
Fuel cells, combined with electric motors, are an efficient conversion (chemical-to-mechanical) method. In theory, they have enormous, very valuable benefits (high weight-to-power factor for the fuel, long-term storage capacity (harvest in the summmer, use all year, high-input-to-output conversion efficiency). Fuel cells as a concept are terrific. It's the fuel generation process that is not very well developed, and let's put the blame for that squarely where it belongs: we have a huge investment in fossil-fuel physical plant, and we have (as a society) not yet made the decision to abandon it and invest in the successor process. Never mind all the ghouls that run the oil companies, etc.; the main problem is that the public doesn't know enough about the alternatives, and doesn't have a well-defined collective vision of how to go from where we sit now to where we need to be in 30 years.
Bashing fuel cells, in my opinion, isn't the most direct path from where we are to where we need to be. We need to focus on the part of the process that's the most broken (fuel generation), and fix it.
That gets us squarely into the "how do we allocate limited surface area, nutrients, water between food production and fuel production?.
I'll leave that issue for another day. This is already too long as it is.
Tom
Comment - I realize your primary objective was to discuss fuel cells, but you were a little too negative about plant oils. There is a significant difference in pollutants between SVO and bio-diesel compared to petro-diesel. With the exception of N Ox?, every pollutant is dramatically lower. Sulphur, which causes acid ran, is not present. To some people, the most significant difference is that SVO and bio-diesel are carbon neutral. You are correct that diesels reject a lot of heat. If you are unable to use that heat, it does lower the efficiency. But even with that loss, a diesel engine fueled with either SVO or bio-diesel is usually more cost effiecient than a fuel cell for any load over 3 HP (or 3 KW). - Bobby