Energy II: Hydrogen, pie-in-the-sky-drogen
Since writing the first installment of this series, I came across this guy, whose website I have read with great interest. He concentrates primarily on energy production, so he has some great information on different forms of that as well as a set of predictions that I found instructive in making my own. This series, however, concentrates primarily on fuels for energy, and their development and use as part of a system of energy usage which argues against some sources as basically energy sinks.
Case in point: hydrogen.
Not too long ago, I was imagining my own private power plant, one with which I could generate income by selling electricity. The power plant would have solar panels and wind turbines to generate the electricity to electrolyze water, generate oxygen, which I would sell, and hydrogen gas, which I would use to run an array of fuel cells. It was at that point that I made the rather obvious realization that the amount of energy released by running hydrogen through a fuel cell (238 kiloJoules per mole [kJ/mol]) is exactly the same amount of energy required to liberate hydrogen gas from water. If I was already generating that much electricity, why recycle it into hydrogen?
This is the root of my impression that the hydrogen economy is a pipedream, at best: we can get no more energy out of hydrogen than we put into producing it. Electrolysis is the most efficient mechanism; the alternative is liberating hydrogen from other compounds, at greater energy expense. The most likely source is hydrocarbons, and breaking two carbon-hydrogen bonds (hydrogen being a diatomic molecule) takes no less than 712 kJ/mol (bond energy varies somewhat by position of the hydrogen atom in question on a complex hydrocarbon molecule). If one were to, as has been proposed, burn hydrocarbons to produce hydrogen to pass through a fuel cell, one would essentially waste 474 kJ/mol of hydrogen used, and would be better off just burning the hydrocarbon itself.
Add to this the fact that hydrogen must be compressed into usable density, or even liquid form, which compression also requires energy. This useful discussion, notably published by a strong hydrogen proponent, sets the amount at 9-12% of the energy value of the hydrogen produced -- that means that you get only the 88-91% of the energy from the hydrogen by the time you've compressed it to a usable density. Liquifying hydrogen takes 33% of the hydrogen's energy, but makes hydrogen much easier to handle -- and much more efficient by volume than compressed hydrogen (though still less than gasoline, for those interested: 1 gallon of gasoline has as much energy as 1.6 gallons of liquid hydrogen). Storing compressed or liquid hydrogen presents considerable hazards: temperature changes transmitted to the hydrogen by conditions outside the tank increases the pressure on the tank, resulting in explosion if the tank isn't vented -- resulting in more energy loss. There are some solutions being investigated (see this article, for example), but consider, for example, how variable are the levels at which individual car owners maintain their cars. You can't even be sure that the guy tailgaiting you has checked his brake pads in the last ten years. How confident would you be that the driver of the car driving or parked next to you was following the manufacturer's recommendations for maintenance of his hydrogen tank or mechanisms for temperature control thereof? Further, the tiny hydrogen atom, less than 1/20 the size of an octane molecule, can actually slip between atoms in a molecular lattice, so leakage is an issue that must be overcome when one talks about long-term storage. All of this means that we're effectively burning energy before we get one smidge of it from the hydrogen fuel itself.
Bottom line? We can't start up a fuel-cell, and use it to liberate hydrogen to both sustain the fuel-cell's operation and store for use in other motors. We have to therefore use something else to "create" hydrogen for fuel. The Rocky Mountain Institute, cited above, envisions thermonuclear plants to do that, but I alluded to the inherent inefficiency and danger of nuclear fission in the previous post (and I will explore it in a little more detail in the fourth installment of this series). I would prefer the wind/solar combination, given its much lower tangible and intangible cost to the energy system.
But that brings me back to the question I asked above with regard to my own imaginary power plant. The answer is this (at least based on these numbers): if I send electricity out into the power lines directly from wind and solar, the loss of energy from production to use at the other end of the line is about 8% -- or 92% efficient. If I put a bank of lithium-ion batteries in my plant to stabilize what is, admittedly, irregular power generation from wind and solar, I increase the loss to about 23%, for an overall 77% efficiency. With hydrogen as an intermediary, the loss is about 28% from inefficiencies in electrolysis, another 14% from storage, 20% from inefficiencies in the fuel cell, and 8% from power line transmission -- a total loss of 54%, or 46% efficiency. If we're talking about cars, charging a battery-electric vehicle (BEV) from the socket represents 77% efficiency from production to use in the car, while filling up a fuel-cell vehicle with hydrogen represents only 54% efficiency by the time the energy reaches the wheels.
As I was researching this, I was impressed by how cool hydrogen as a fuel could be. I even read that hydrogen subjected to extremely high pressure exhibits superconductive properties (the pressure I'm talking about is about 1 million atmospheres [atm], which has been reproduced for only short periods in laboratories). But it is not the replacement for gasoline: too much energy is lost in production, storage, and transportation of hydrogen to make it truly effecient system-wide.
Case in point: hydrogen.
Not too long ago, I was imagining my own private power plant, one with which I could generate income by selling electricity. The power plant would have solar panels and wind turbines to generate the electricity to electrolyze water, generate oxygen, which I would sell, and hydrogen gas, which I would use to run an array of fuel cells. It was at that point that I made the rather obvious realization that the amount of energy released by running hydrogen through a fuel cell (238 kiloJoules per mole [kJ/mol]) is exactly the same amount of energy required to liberate hydrogen gas from water. If I was already generating that much electricity, why recycle it into hydrogen?
This is the root of my impression that the hydrogen economy is a pipedream, at best: we can get no more energy out of hydrogen than we put into producing it. Electrolysis is the most efficient mechanism; the alternative is liberating hydrogen from other compounds, at greater energy expense. The most likely source is hydrocarbons, and breaking two carbon-hydrogen bonds (hydrogen being a diatomic molecule) takes no less than 712 kJ/mol (bond energy varies somewhat by position of the hydrogen atom in question on a complex hydrocarbon molecule). If one were to, as has been proposed, burn hydrocarbons to produce hydrogen to pass through a fuel cell, one would essentially waste 474 kJ/mol of hydrogen used, and would be better off just burning the hydrocarbon itself.
Add to this the fact that hydrogen must be compressed into usable density, or even liquid form, which compression also requires energy. This useful discussion, notably published by a strong hydrogen proponent, sets the amount at 9-12% of the energy value of the hydrogen produced -- that means that you get only the 88-91% of the energy from the hydrogen by the time you've compressed it to a usable density. Liquifying hydrogen takes 33% of the hydrogen's energy, but makes hydrogen much easier to handle -- and much more efficient by volume than compressed hydrogen (though still less than gasoline, for those interested: 1 gallon of gasoline has as much energy as 1.6 gallons of liquid hydrogen). Storing compressed or liquid hydrogen presents considerable hazards: temperature changes transmitted to the hydrogen by conditions outside the tank increases the pressure on the tank, resulting in explosion if the tank isn't vented -- resulting in more energy loss. There are some solutions being investigated (see this article, for example), but consider, for example, how variable are the levels at which individual car owners maintain their cars. You can't even be sure that the guy tailgaiting you has checked his brake pads in the last ten years. How confident would you be that the driver of the car driving or parked next to you was following the manufacturer's recommendations for maintenance of his hydrogen tank or mechanisms for temperature control thereof? Further, the tiny hydrogen atom, less than 1/20 the size of an octane molecule, can actually slip between atoms in a molecular lattice, so leakage is an issue that must be overcome when one talks about long-term storage. All of this means that we're effectively burning energy before we get one smidge of it from the hydrogen fuel itself.
Bottom line? We can't start up a fuel-cell, and use it to liberate hydrogen to both sustain the fuel-cell's operation and store for use in other motors. We have to therefore use something else to "create" hydrogen for fuel. The Rocky Mountain Institute, cited above, envisions thermonuclear plants to do that, but I alluded to the inherent inefficiency and danger of nuclear fission in the previous post (and I will explore it in a little more detail in the fourth installment of this series). I would prefer the wind/solar combination, given its much lower tangible and intangible cost to the energy system.
But that brings me back to the question I asked above with regard to my own imaginary power plant. The answer is this (at least based on these numbers): if I send electricity out into the power lines directly from wind and solar, the loss of energy from production to use at the other end of the line is about 8% -- or 92% efficient. If I put a bank of lithium-ion batteries in my plant to stabilize what is, admittedly, irregular power generation from wind and solar, I increase the loss to about 23%, for an overall 77% efficiency. With hydrogen as an intermediary, the loss is about 28% from inefficiencies in electrolysis, another 14% from storage, 20% from inefficiencies in the fuel cell, and 8% from power line transmission -- a total loss of 54%, or 46% efficiency. If we're talking about cars, charging a battery-electric vehicle (BEV) from the socket represents 77% efficiency from production to use in the car, while filling up a fuel-cell vehicle with hydrogen represents only 54% efficiency by the time the energy reaches the wheels.
As I was researching this, I was impressed by how cool hydrogen as a fuel could be. I even read that hydrogen subjected to extremely high pressure exhibits superconductive properties (the pressure I'm talking about is about 1 million atmospheres [atm], which has been reproduced for only short periods in laboratories). But it is not the replacement for gasoline: too much energy is lost in production, storage, and transportation of hydrogen to make it truly effecient system-wide.
posted by heavynettle at 21:08
2 Comments:
Wow. This is fascinating. I used to live in a house that was "off the grid." It was a cabin in hills outside Pecos (west of the Pecos), N.M. My water was pumped by a solo powered well that filled a tank good for several days (when the sun didn't shine). The solar power was also stored in 12 regular car batteries tucked under the stairwell and which provided enough power to watch TV. Alas, the solar power wasn't great enough for everything, so it was supplemented with propane, which was needed to run the refrigerator. But the tank only needed to be filled about twice a year.
One thing that became clear by producing my own energy was the benefits of intelligent conservation. The cabin worked because it was incredibly well made and INSULATED. It ran on very little energy, so a single solar panel was sufficient to keep me comfortable. So when you build your urban power station, I'd recommend your first step is to stuff and chaulk.
One question about hydrogen. I've heard hydrogen proponents say that another benefit of hydrogen is that it is essesntially non-poluting. We may loose energy creating it, but fossil fuels or other energy sources used to make hydrogen fuel can have their emissions more effectively trapped at a large plant (say, with scrubber stacks) than their emissions can be trapped by millions of individual cars with, as you note, varying states of maintenance. Is this true? I guess wind/solar is even less poluting, but I wonder if hydrogen cars woudn't still be a step forward from gasoline power?
BTW, electric cars were developed very early and stayed popular through the 40s. Their speed and range was limited, so they were often marketed as ladies' cars for short-range shopping trips. It seems the continuing irrational aversion to conservation & lowered emissions is still with us, based partially on this girlie image.
Use of hydrogen in vehicles would represent a decrease in air pollution, not just because centralized plants are more easily made clean, but because the hydrocarbon combustion reaction itself, when used to liberate hydrogens in a car that uses hydrocarbon-hydrogen fuel rather than pure hydrogen, is also cleaner. Bear in mind, however, that the centralized plant argument applies to BEVs, too -- and that brings into play all the comparisons I made about energy loss via hydrogen v. electricity. If we couldn't run the whole tamal on wind and solar, then a natural gas power plant sending out electricity would still be cleaner than internal combustion engines; and BEVs would be more efficient in that model than hydrogen vehicles. Natural gas, of course, is one of those "free energy" sources I talked about in part I: it already has energy in it, and we have only to separate out the most useful parts. The fact that it burns much more cleanly than gasoline -- and much, much, much more cleanly than coal -- makes the push by some for natural gas power plants a very reasonable thing.
Unfortunately, natural gas, as with the other fossil fuels it could replace, is also finite, and cannot be part of a sustainable system at the energy usage levels expected by mid-century. The very limited availability of any fossil fuel makes any energy loss in the combustion of it that much more costly. Again, too much energy is lost to create hydrogen, relative to the still-improving efficiencies of batteries and battery chargers.
I should acknowledge, by the way, that hydrogen cars also are exploiting, and thereby furthering development of, better chargers and batteries, so hydrogen car development is not a bad thing. It's just that we can't expect to run our transportation economy on them.
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