electrolysis energy return
- Thread starter lefenzy
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No, in a perfect ideal case the electrity would generate an exactly equal amount of hydrogen in terms of energy. In reality eletrolysis is VERY inefficient and the amount of hydrogen produced is only a fraction of the amount of energy put in from the electricity. This is a consequence of the laws of thermodynamics making it impossible to every create energy from nothing.
PerpetualMotionMachines--
PerpetualMotionMachines--
In good, modern fuel cells, electrolysis can be used with around 90% efficiency. That is, the electricity you get out will be about 90% of what you put in. It can never be more than you put in unless some laws of thermodynamics turn out to be wrong, which seems pretty unlikely at this point.
Yeah, the problem isn't getting the electricity from the hydrogen its getting the hydrogen from the electricty, yo lose energy on both of the conversions, but last I saw normal electrolysis was horribly inefficient and even with alot of tricks you still aren't gonna get a reasonable efficiency. There are some good ways to get the hydrogen out, but some of them involve the water being at 500 degrees C in order to force some chemical reaction that would otherwise never be favored near rooom temperature. They talk about using nuclear reactors for the process but current nuclear reactors don't produce water hot enough for the process.
It was just a blip on the big-media radar and hyped for a few years. It was quickly realized that hydrogen is nothing but an energy storage medium with more problems than solutions. There may be hope, still, though: http://www.dailytech.com/article.aspx?newsid=10805
It's been a while since I did any electrolysis, but I believe we still got around 60% of the input energy back even though we were using an el cheapo small-scale fuel cell. This implies about a 67% efficiency of the electrolysis (assuming 90% efficiency in the other direction). Like I said, I could be forgetting the numbers, but that's still pretty good compared to gas engines.
its not all that great when you consider you still haven't done anything with it, using your own number + the standard numbers I would get 30% for the nuke plant + 60% for electrolysis + 90% for feul cell (good luck with that) + 90% for an electrical moto, thats 14.6% efficienct compared to what like 25% for a gasoline engine? Not to mention the fact that going through all those steps requires lots of expensive equipment whereas after you get the oil out of the ground you only need 1 step to trun it into gasoline. Also, your statement that fuel cells get 90% efficiency is turning hydrogen to electricty is not what I have ever seen, wiki uses the number of 36% for an average car load and states that 83% is the maximum theoretical efficiency at room temperature. Using wikis numbers that would put the total efficiency at 8.8%
silverpig
Lifer
- Jul 29, 2001
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How does it have more steps?
Gasoline: Pump oil from ground, transport to refinery, refine, transport to gas station, put in car.
Hydrogen: Suck water from the ocean, electrolyze (build the plant right by the water... it's not going anywhere and the ocean won't dry up), transport to hydrogen station, put in car.
Apparently your missing the steps behind "electrolyize", I'll give them for my example of a nuclear reactor:
mine uranium>convert to UF6>enrich>convert to UO2>mill to feul pellets>put in nuclear reactor>get electricty>move electicity to hydrogen plant
or for coal:
mine coal>transport coal>pulverise coal>burn coal>get electricty>move electricity to hydorgen plant
You have to follow the ENERGY through all its conversions not just the water, its inefficiencies in transporting and converting your energy source (gasoline for an ICE, coal or nuclear for an electric car) that effect overall efficiency, losing some water in the hydrogen plant is no big deal, losing 70% of your energy converting it to electricty is.
silverpig
Lifer
- Jul 29, 2001
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And if your refinery is powered by a nuclear reactor, which was assembled by diesel burning trucks which were manufactured in a plant operated by an oil-burning powerplant?
We're comparing the efficiency of an electrolysis based hydrogen fuel system versus one running on gasoline. Both gasoline and electrolysis require some initial energy investment. It doesn't matter how this is produced as it can be the same in both instances, all that matters is how much is used per unit of energy output in your car.
Oh come on people.
Learn from nature, it is hard to be smarter than billions of years of evolution.
Every green plant on the planet has a photosynthetic center that has 98% quantum efficiency at taking water in, one photon of irradiating light, and spitting out an electron and excited hydrogen atom.
Yes, they take that energy and do other things with it (synthesize carbohydrates, et. al.) in their own internal "fuel cells", but, still, the photosynthetic process is well understood and distinct from subsequent chemical reactions within the cell.
People are working on either genetically modifying photosynthetic algae to produce free hydrogen in addition to being self-replicating, self-supporting, et. al.
I believe they're also working on creating artificial or cyborg type photosynthetic centers independent of organisms to couple with engineered structures to generate either free hydrogen or electricity.
Also the synthetic photovoltaic structures in either semiconductors or organic types of systems can be pretty efficient at a quantum efficiency scale, e.g. even your common CCD chip in a camera. The bits that lose efficiency are more related to costs of manufacturing, ability to convert high current low voltage power efficiently, transporting energy on nano/micro scales from the reaction area to more traditional collector wires, et. al.
These are all not terribly difficult things to overcome and I'm pretty sure they will be within the next couple of years.
Learn from nature, it is hard to be smarter than billions of years of evolution.
Every green plant on the planet has a photosynthetic center that has 98% quantum efficiency at taking water in, one photon of irradiating light, and spitting out an electron and excited hydrogen atom.
Yes, they take that energy and do other things with it (synthesize carbohydrates, et. al.) in their own internal "fuel cells", but, still, the photosynthetic process is well understood and distinct from subsequent chemical reactions within the cell.
People are working on either genetically modifying photosynthetic algae to produce free hydrogen in addition to being self-replicating, self-supporting, et. al.
I believe they're also working on creating artificial or cyborg type photosynthetic centers independent of organisms to couple with engineered structures to generate either free hydrogen or electricity.
Also the synthetic photovoltaic structures in either semiconductors or organic types of systems can be pretty efficient at a quantum efficiency scale, e.g. even your common CCD chip in a camera. The bits that lose efficiency are more related to costs of manufacturing, ability to convert high current low voltage power efficiently, transporting energy on nano/micro scales from the reaction area to more traditional collector wires, et. al.
These are all not terribly difficult things to overcome and I'm pretty sure they will be within the next couple of years.
Born2bwire
Diamond Member
- Oct 28, 2005
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They are also designing nanoscale photovoltaic cells around the quantum process of photosynthesis. I've seen a few papers in regard to these processes and nanoparticles (despite the trendiness of the word) seems to be poised to greatly benefit the efficiency of solar cells.
First, wtf does this have to do with this thread? Second, it's obviously not as easy as you think.
Well, we ARE talking about hydrogen...
Imagine a new class of solar panels that consisted of a vat of algae: air and light int -> hydrogen and oxygen out -> (burn for heat->power generation) or, on a smaller scale -> fuel cell-> electricity
It might me a more efficient use of land space than photovoltaic solar panels, who knows? Algae and things can live in less light than it takes to (usefully) power a solar panel, so maybe the useful operating areas of the world for solar power could be expanded.
Jeff7
Lifer
- Jan 4, 2001
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Hydrogen was never about energy production any more than gasoline is. They're about energy transport. Energy is easy to get in power plants - big, centralized locations where it can be burned. But the problem then is that it's not always where you want it. Getting it to homes, that's easy enough, as they don't move. But cars present a problem, as they're moving.
So, we need a way of sending a pocket of usable, efficient, safe energy along with them. For now, gasoline and diesel fuel do the trick, with some alternatives in use.
Hydrogen could be one way of doing that, though there are some technical problems associated with it, both in terms of storage and production.
Another option would be some kind of battery, complete with a regenerative braking system.
So it's not really a matter of getting a positive energy return on anything, it's about improved efficiency and energy transport.
Hydrogen is a terrible idea. 1) it's gaseous at room temperature 2) therefore it has to be stored at less than -259.14 C 3) even when it is stored at those temperatures it has lousy density 4) ahh I love the smell of exploding hydrogen in the morning (Oh wait, it wouldn't smell... just water).
Hydrocarbons are an exceptionally convenient package for energy, and the only "pollution" produced by them when properly used is CO2, which, granted, is a real problem. However, hydrogen is not a reasonable replacement as an energy carrier. I personally think that if the question of where to get the energy to replace fossil fuels from is solved, it is more likely that we'll start synthesizing hydrocarbons out of atmospheric CO2 than that we'll start using hydrogen. In fact, this is already being done. It is called ethanol.
Hydrocarbons are an exceptionally convenient package for energy, and the only "pollution" produced by them when properly used is CO2, which, granted, is a real problem. However, hydrogen is not a reasonable replacement as an energy carrier. I personally think that if the question of where to get the energy to replace fossil fuels from is solved, it is more likely that we'll start synthesizing hydrocarbons out of atmospheric CO2 than that we'll start using hydrogen. In fact, this is already being done. It is called ethanol.
Yes, I thought so too. His post didn't mention it, at least not as far as I could tell.
This is being worked on, as are bacterial (AKA microbial) fuel cells. My department has one of the leading guys in the area right now, though he's moving to Cornell shortly because they're giving his wife a job there too. The consensus on this stuff is that it's simply not practical for large-scale energy production because of the rather complex infrastructure needed to generate electricity directly. Burning them for heat to be used to generate electricity is feasible, but only a small fraction of the energy stored would be recovered in this way. It would be more beneficial to burn grass in this sense because it grows faster and thicker than photosynthetic algae.
It doesn't have to be cryogenically stored. Increasing the pressure will liquefy it at much higher temperatures. Very safe pressure vessels that can store liquid hydrogen at room temperature and fit in the spare tire chamber of a current car have been around since about 2001 or so, at least in labs. The ones that I read about used a carbon fiber-based composite material
Hydrogen is the most energy-dense (in a mass-density sense) material, period, which is why it's being considered as a fuel. It is not necessarily so efficient in a volume-density sense, depending on the storage pressure and temperature. It is also beneficial because whether it is combusted in an engine or oxidized via a fuel cell, only water is produced. Ethanol is not being produced from atmospheric CO2 in any process that I've ever heard about. CO2 is a very thermodynamically stable molecule. It seems to me that a highly specialized catalyst would be required to convert CO2 to ethanol - so specialized that I'm not even sure if it would be feasible since it would require precise alignments of multiple molecules at extremely short length scales.
Both electrolysis and fuel cells are practical technologies, although both extremely expensive. The problem is that both are relatively inefficient.
Electrolysis is generally an efficient process - typical commercial electrolysers can achieve efficiencies around the 80% region, although laboratory grade devices can achieve around 90%. The problem is that electrolysers are very expensive, as they require large area platinum electrodes. Both current flow (and therefore hydrogen generation rate) and efficiency are highly sensitive to cell voltage, so if you want good efficiency you have to have very low voltage and therefore large electrodes, if you want the hydrogen at a useful rate.
Fuel cells are less efficient, and various fuel cell technologies have widely variable efficiencies. Proton exchnage membrane cells (the technolgoy aimed for cars) have a typical efficiency of around 40% - with the possibility of 50% in the future. Other technologies, such as solid oxide fuel cells, can achieve better efficiency (e.g. SOFC can achieve 70%), but have other problems, such as an operating temperature of 1000 C. At any rate, fuel cells have a thermodynamic efficiency limit of about 86% - you simply can't go higher because the reaction is inherently exothermic.
Conversely, this means electrolysis is inherently endothermic, so it is potentially possible to supply some energy as heat. Electrolysers have been developed that use superheated steam at 600 C, instead of water. These devices produce more hydrogen per unit electricity because some of the energy comes from the heat - as generating electricity from heat is inefficient, this is of interest for future industrial hydrogen generation plants.
There have also been direct thermo-chemical hydrogen generation schemes put forward (e.g. sulfur-iodine cycle). This uses a series of reactions that split water into hydrogen and oxygen, that take place at different temperatures (the hot side at about 800 C) and a cold side (at about 100 C) and can achieve direct heat to hydrogen conversion of about 70%. Some large laboratory devices were built in the 60s, with the intention of using direct nuclear heat - but the projects were shelved.
Umm.... you make ethanol out of plants? Helloooo? Plants? Take in CO2 and spit out oxygen?
As to the viability of Hydrogen... Stockholm (where I come from, although I'm currently living in Illinois) has 20 some hydrogen-fueled buses. It's an interesting concept and all, but despite the (very) high cost of making (and running) those buses, they are still far inferior to regular vehicles. Why? In order to bring enough hydrogen to match a normal fuel tank, they had to cover the entire roof of the bus with a ~2 foot thick tank. According to the publicly available information, this huge volume requirement was because they couldn't make a crash-survivable hydrogen tank with a sufficiently high internal pressure to make it smaller. This might be possible to remedy with increased research, granted, but still, carrying a high-pressure tank of explosives around in a car at a high speed seems like a bad idea to me. Add in the complexity of storing it at "gas" stations (do you want to try to design an enormous pressure tank that'll be relatively cheap and, when buried in the ground for 50+ years, is guaranteed not to break?), and transporting it in bulk... I doubt that'll it be feasible, especially not anytime soon. But it's a hypothetical discussion at any rate, since at the moment we'd have to use fossil fuels to make it.
As to emissions, combustion engines are very clean these days, at least those with catalysts (aka all of them). Granted, water is better, but as far as car exhaust goes, emissions (other than CO2) are not that big of a deal.
Umm, that's not what you said. "it is more likely that we'll start synthesizing hydrocarbons out of atmospheric CO2 than that we'll start using hydrogen." Ethanol isn't even a hydrocarbon - it's an alcohol. And trying to produce ethanol from plants requires energy input, so it's still a net energy loser such that there is still a net CO2 production from these processes.
The people who designed the tank in your anecdotal example are idiots. The design of pressure vessels has been understood in insane detail for well over 50 years. Modern materials easily allow storage of hydrogen as a liquid in very compact form at densities only slightly higher than regular gasoline. Studies have even shown that these tanks are less likely to detonate than a conventional gas tank. And gas stations don't have to store hydrogen - that's what electrolysis is all about. I can plug my car in and get hydrogen back from my "waste" product of water. Unless, of course, I'm combusting the hydrogen, in which case I'm essentially throwing energy down the drain to save in capital costs, which is almost always a money-loser.
This isn't true at all. All cars still emit nitrous oxides (NO_x) and other species that are much more environmentally harmful than CO_2. Catalysts in catalytic converters get poisoned with use just like any other catalyst with the result that the emissions profile of a car changes with time, very much for the worse. "Very clean" is very relative, but extremely misleading in this sense.
You obviously know more about hydrogen tanks than I do, so I'll take your word for it.
That's exactly what I said. "Synthesize" doesn't specify a production process. You can use plants, fancy catalysts, or your grandma's cooking range; when you make a chemical compound out of other compounds, you are synthesizing it.
You are right, I misused the term "hydrocarbon". In Swedish, "kolvate" (lit. hydrocarbon) can be used to denote any organic compound. The term "organic" is in some contexts seen as implying an endorsement of the old idea that organic compounds could only be produced by living things (aka. were not possible to synthesize in the lab), and is usually avoided by chemists. My mistake.
0roo0roo
No Lifer
- Sep 21, 2002
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its flawed, but on the other hand you can generate your own hydrogen from renewables from home in theory.
whereas installing an oil rig is impossible in most areas
but no, its not viable until gas gets really really expensive. and well when it gets that expensive such requirements as 300 miles on a tank and such while go out the door.
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