# Questions about a hypothetical Air Motor **URL:** \ **Category:** Factual Questions\ **Created:** [July 2, 2015, 8:34pm UTC](https://boards.straightdope.com/t/questions-about-a-hypothetical-air-motor/724079 "2015-07-02T20:34:09Z")\ **Posts on this page:** 16\ **Page:** 2
**Author:** ![Habeed](https://avatars.discourse-cdn.com/v4/letter/h/a587f6/32.png) [@Habeed](https://boards.straightdope.com/u/Habeed)\ **Post date:** [July 3, 2015, 6:11pm UTC](https://boards.straightdope.com/t/questions-about-a-hypothetical-air-motor/724079/21 "2015-07-03T18:11:31Z")
> [@Amateur\_Barbarian](#): > > I realized from the start that this thread was about an isolated technical question and not a proposal for an alternative energy system, but… between hypercompressed air and H2, I’d go with H2. Both have tremendous “creation” losses, but H2 seems to have more flexible application and more power density. Long ago, a proponent put H2 in perspective for me: don’t think of it as a fuel, because it becomes nonsensical against other fuel choices because of the enormous inefficiency of creating and compressing it. It’s better thought of as a pipeline… much as compressed air is. Making the compressed fluid an active one compounds the energy storage, though. Some H2 designs even use the high-pressure fuel to drive an auxiliary turbine on its way to fuel-cell or combustion pressures. > > Back to the temperature/pressure efficiency discussion, though. I am curious to find out if there is any improvement to be had by raising the gas temperature. Barbarian, why not methane instead of H2? It’s basically an additional step, where you add C02 to your H2, and you get a fuel that has 1. Volumetric density competitive with gas and diesel (it’s not quite as high but it’s close enough) 2. Doesn’t leak out of tanks as easily - modern technology has no trouble keeping methane in natural gas lines 3. The liquid form is cryogenic but not nearly as cold or expensive to keep in a tank 4. There’s vast quantities of a natural source of the methane, more than there is liquid oil worldwide, to make it cheap and easy to adopt 5. Conventional engines including piston engines and turbines that have a century of engineering behind them burn it just fine with minimal modifications 6. It does work in fuel cells 7. It wouldn’t require as radical a set of changes to make a methane burning airliner, or a methane burning train, and you can get methane burning 18 wheeler trucks and cars right now. And so on. H2 is a dead end. If we were serious as a civilization about going to a fully renewable system, we’d create methane using high temperature steam electrolysis followed by high temperature Sabatier. The CO2 could be collected by burning methane or coal the first time in electric power plants. The heat would probably come most cheaply and easily from some kind of advanced nuclear design that isn’t as inherently dangerous as the nuclear cores that have melted down so many times. ---
**Author:** ![Amateur\_Barbarian](https://avatars.discourse-cdn.com/v4/letter/a/59ef9b/32.png) [@Amateur\_Barbarian](https://boards.straightdope.com/u/Amateur_Barbarian)\ **Post date:** [July 3, 2015, 6:17pm UTC](https://boards.straightdope.com/t/questions-about-a-hypothetical-air-motor/724079/22 "2015-07-03T18:17:23Z")
> [@Habeed](#): > > Barbarian, why not methane instead of H2? Methane is a hydrocarbon and emits CO2. H2 turns into water. 🙂 Most methane production can be considered carbon-neutral (organic matter-based) but I think if we’re going to pick a power technology, carbon-zero has a leg up on carbon-neutral, at least for the next century. One of the alternative-energy arguments that always seems to go off a cliff is the assumption that we’re going to try to convert everything to that technology, which has absurd consequences somewhere or another. Everything does not need to be solar-powered, or wind-powered, or electric, or fuel-cell, or H2. I think hydrocarbon fuels will be a part of the mix for a long, long time because (for example) there’s no good alternative for aircraft with any meaningful payload and range. But using every non-carbon tech we have in every place it makes sense is the right track. ---
**Author:** ![Habeed](https://avatars.discourse-cdn.com/v4/letter/h/a587f6/32.png) [@Habeed](https://boards.straightdope.com/u/Habeed)\ **Post date:** [July 3, 2015, 6:27pm UTC](https://boards.straightdope.com/t/questions-about-a-hypothetical-air-motor/724079/23 "2015-07-03T18:27:59Z")
> [@Amateur\_Barbarian](#): > > I think hydrocarbon fuels will be a part of the mix for a long, long time because (for example) there’s no good alternative for aircraft with any meaningful payload and range. But using every non-carbon tech we have in every place it makes sense is the right track. You can obviously capture the C02 from the atmosphere, it just isn’t the most efficient option. And liquid methane can be used instead of liquid kerosene - it has less volumetric density, but more energy per kilogram, so it’s probably a wash for payload and range. Heavier aircraft require more fuel mainly because they need to have more lift from their wings, which in turn means more drag (by making the wings bigger or more aggressive at developing lift you increase drag). Bigger aircraft to hold larger internal fuel tanks also consume more fuel due to more drag. Either way, I suspect it doesn’t end up mattering much. ---
**Author:** ![Habeed](https://avatars.discourse-cdn.com/v4/letter/h/a587f6/32.png) [@Habeed](https://boards.straightdope.com/u/Habeed)\ **Post date:** [July 3, 2015, 6:46pm UTC](https://boards.straightdope.com/t/questions-about-a-hypothetical-air-motor/724079/24 "2015-07-03T18:46:21Z")
A bit of quick and dirty figuring : the current 747, 15% of the internal volume is fuel tanks. This probably isn’t quite right but it’s close enough. With liquid methane, to store the same energy, you’d need fuel tanks 60% larger - so it would be 24% of the internal volume for the same range. If you went to a lifting body shape for an airliner you’d probably have more internal volume and greater fuel efficiency (albeit you obviously have to do billions of bucks in R&D). In any case, that’s close enough, and I’m sure you can make up for some of the missing volume with better use of space or clever engineering. Also, perhaps the liquid methane airplanes would be used for short cross-country hops, and there would be a line of long range, kerosene burning aircraft for the long distance journeys. The full fuel capacity of modern airliners isn’t usually used for most flights anyway. ---
**Author:** ![Amateur\_Barbarian](https://avatars.discourse-cdn.com/v4/letter/a/59ef9b/32.png) [@Amateur\_Barbarian](https://boards.straightdope.com/u/Amateur_Barbarian)\ **Post date:** [July 3, 2015, 7:00pm UTC](https://boards.straightdope.com/t/questions-about-a-hypothetical-air-motor/724079/25 "2015-07-03T19:00:06Z")
Well, the cargo option is obviously airships. I am not quite clear what keeps hindering their development except that they’re rather slow. But they could use a variety of power sources - including, for very light or slow use, solar cells - and not waste power on lift. ---
**Author:** ![Habeed](https://avatars.discourse-cdn.com/v4/letter/h/a587f6/32.png) [@Habeed](https://boards.straightdope.com/u/Habeed)\ **Post date:** [July 3, 2015, 7:45pm UTC](https://boards.straightdope.com/t/questions-about-a-hypothetical-air-motor/724079/26 "2015-07-03T19:45:56Z")
> [@Amateur\_Barbarian](#): > > Well, the cargo option is obviously airships. I am not quite clear what keeps hindering their development except that they’re rather slow. But they could use a variety of power sources - including, for very light or slow use, solar cells - and not waste power on lift. What’s “hindering” their development is that they are at the mercy of the wind. This makes them very difficult to not crash on landing. Also, helium is the only truly nonrenewable resource, and is the only substance in the world that can cool the high end superconducting magnets. It would be a terrible idea to waste it on filling gasbags. ---
**Author:** ![Amateur\_Barbarian](https://avatars.discourse-cdn.com/v4/letter/a/59ef9b/32.png) [@Amateur\_Barbarian](https://boards.straightdope.com/u/Amateur_Barbarian)\ **Post date:** [July 3, 2015, 7:51pm UTC](https://boards.straightdope.com/t/questions-about-a-hypothetical-air-motor/724079/27 "2015-07-03T19:51:59Z")
> [@Habeed](#): > > What’s “hindering” their development is that they are at the mercy of the wind. This makes them very difficult to not crash on landing. I don’t see aircraft as being a great deal different. Airships would need at least as large a landing area, to allow for ground capture in windy conditions. The idea of being able to, say, dock them to skyscrapers was always a bit idiotic. > [@](#): > > Also, helium is the only truly nonrenewable resource, and is the only substance in the world that can cool the high end superconducting magnets. It would be a terrible idea to waste it on filling gasbags. Or fifty million GET WELL balloons a year. There’s always… H2. Plenty of that to be had. :rolleyes: 😃 ---
**Author:** ![AdamF](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/adamf/32/6246_2.png) [@AdamF](https://boards.straightdope.com/u/AdamF)\ **Post date:** [July 3, 2015, 8:28pm UTC](https://boards.straightdope.com/t/questions-about-a-hypothetical-air-motor/724079/28 "2015-07-03T20:28:07Z")
> [@Amateur\_Barbarian](#): > > Well, the cargo option is obviously airships. I am not quite clear what keeps hindering their development except that they’re rather slow. Being slow is a big drawback. A jet plane can make several round-trips for each trip by an airship. ---
**Author:** ![Amateur\_Barbarian](https://avatars.discourse-cdn.com/v4/letter/a/59ef9b/32.png) [@Amateur\_Barbarian](https://boards.straightdope.com/u/Amateur_Barbarian)\ **Post date:** [July 3, 2015, 8:39pm UTC](https://boards.straightdope.com/t/questions-about-a-hypothetical-air-motor/724079/29 "2015-07-03T20:39:25Z")
> [@AdamF](#): > > Being slow is a big drawback. A jet plane can make several round-trips for each trip by an airship. Sure. But you have to look at fuel cost per ton, and compare that to jet, train and truck cargo. I’m not a fervent proponent of airships. They’re just another rotating \*Coming Next Year! \*cover for the PopSci magazines. But I am surprised that they’ve never found a place amid air cargo and transport options. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [July 3, 2015, 9:32pm UTC](https://boards.straightdope.com/t/questions-about-a-hypothetical-air-motor/724079/30 "2015-07-03T21:32:42Z")
The thumbnail sketch on airships is high costs per ton-mile like an airplane, but with slow and unreliable delivery like a train/ship. There are some quibbles hidden in my thumbnail, but that’s the big picture. The only people looking at airships seriously are folks who want to airmail ginormous loads like battle tanks and oil drilling rigs and mining equipment out into the landlocked boonies where barges and ships can’t reach. IOW, the military and the oil/mining companies. Both are niche markets whose inhabitants are relatively price insensitive. ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [July 3, 2015, 11:16pm UTC](https://boards.straightdope.com/t/questions-about-a-hypothetical-air-motor/724079/31 "2015-07-03T23:16:55Z")
> [@TriPolar](#): > > I’m thinking about an air motor driven by high pressure compressed air up to 4500 psi. This is about improving existing air motor configuration by pre-heating the inlet air, so the motor could be any kind of positive displacement motor like a piston motor or a rotary vane motor. Maybe this would apply to turbines also. I have designed and operated many machines expanding a high pressure fluid (process gases) to low pressure and extract the power. A few of the machines I have designed are for air expansion - that is for Air Separation Plants - where you make Nitrogen, Oxygen, Helium etc. I will try to answer all your questions. Some of your questions, presuppose certain things and so are not very good questions. [QUOTE=TriPolar] Highly compressed air is supposed to be inefficient in driving motors because the air cools significantly as it expands in the motor. [/QUOTE] This is one of those questions where you presuppose something. When you expand gases through an expander, they cool. Any humidity in the air - forms ice crystals at the low temperature - the velocities of expanded air are high and these ice crystals eat up the mechanical parts they contact. Same thing happens with trace CO2 in air which is around 400ppm - it forms crystals too. So to expand air from high to low pressure - you have to either make sure that the resulting temperature is above the ice formation temperature of CO2 or water OR you have to dry the air and remove CO2. Commonly in a Air separation plant water is removed to 1ppm (or less) using molecular sieves and CO2 is removed to 1 ppm (or less) using Temperature Swing Adsorption NaX adsorbents. [QUOTE=TriPolar] Question 1: What causes the inefficiency in expanding very high pressure air to drive a motor? [/QUOTE] Again - not a very good question. The efficiency losses are almost the same as a centrifugal compressor. These losses are fluid dynamic, thermodynamic and bearing bases. The first two are in combination called [Polytropic Efficiency](http://www.arc.vt.edu/ansys_help/cfx_mod/CDDCEFFD.html). A well designed expander will give you a ploytropic efficiency of 75% to 85% and a centrifugal compressor will give you 80% to 90%. [QUOTE=TriPolar] To preheat the inlet air I’m considering regeneration of heat from the exhaust through a passive heat exchanger and then additional heating by burning fuel. [/QUOTE] The passive heat exchanger is a high cost, high space taking idea with lots of fouling over time. Gas to Gas heat exchangers are notoriously big, expensive and need lots of maintenance if there is any oils/fouling. Do not understand the burning fuel part - are you going to burn and add the resulting gases to the air for expander ? [QUOTE=TriPolar] Imagine the heat exchanger as the inlet air passing through a coiled copper pipe inside the the exhaust pipe from the motor. [/QUOTE] Sounds trivial - but I assure you that you need 10 times or more the length of the exhaust pipe to get any appreciable heat transfer. Now you will get into back pressure issues on the engine exhaust. Wait I am confused - are you talking about heating the air with a car exhaust or exhaust from the expander ? You do understand that the expander exhaust is going to be cooler not hotter. [QUOTE=TriPolar] Question 2. In a such a heat exchanger is it better to maintain a high pressure in the inlet air to facilitate the transfer of heat, or to reduce the temperature of the inlet air to lower the midpoint temperature that will be reached? [/QUOTE] You want the air pressure to be as high as possible to have higher output of the expander. The temperature you can heat it upto is dependent on the dew point of the car exhaust - you do not want the car exhaust to go below dew point because it will corrode parts. Also - for a very good gas to gas heat exchanger, the approach temperature is about 20F - so if you have a gas at 200F - and want to heat another gas using this, then the best temperature the second gas will go upto is 180F. [QUOTE=TriPolar] So once the inlet air passes through the heat exchanger it needs to be heated more. My thought is to inject some propane and pass the mix over a preheated catalyst. This brings up the same questions about temperature and pressure as in the heat exchanger, but also something else. Question 3a. Would catalytic burning of fuel in pressurized air result in a combustion? [/QUOTE] Yes - with lots of caveats. Good combustion requires a specific ratio of air and fuel. If there is excess air then you get excess NOx and if there is excess fuel then you get unburnt hydrocarbons or soot. You can only pressurize air and propane so much - before it self ignites - remember diesel engine. [QUOTE=TriPolar] Question 3b. Catalytic burning require an optimal ratio fuel/air ratio? [/QUOTE] Yes [QUOTE=TriPolar] Question 3c. Is catalytic burning more efficient than a higher temperature flame as the makers of catalytic heaters claim? [/QUOTE] Depends on the application. The amount of heat released is the same. The air to fuel ratio decides the final temperature. Question 4. Does any of this make sense or sound insane (other than than why am I even thinking about this). [/QUOTE] ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [July 3, 2015, 11:26pm UTC](https://boards.straightdope.com/t/questions-about-a-hypothetical-air-motor/724079/32 "2015-07-03T23:26:03Z")
Also - the main idea in the OP is already being used commercially (experimentally) to store energy. This idea gives the biggest challenge to battery technology being developed by Tesla. In this idea - you take power from the grid when power is excess (off peak hours) and use it to liquefy air. You store the liquefied air in tanks, when the demand goes up - you pump the liquid air to high pressures - heat it up to room temperature (when it becomes a gas again) and run it through an expander to generate power that goes back to the grid. Thus it acts like a rechargeable battery. It is 25% - 50% efficient. Read more [here.](https://en.wikipedia.org/wiki/Cryogenic_energy_storage) ---
**Author:** ![ralph124c](https://avatars.discourse-cdn.com/v4/letter/r/8797f3/32.png) [@ralph124c](https://boards.straightdope.com/u/ralph124c)\ **Post date:** [July 3, 2015, 11:50pm UTC](https://boards.straightdope.com/t/questions-about-a-hypothetical-air-motor/724079/33 "2015-07-03T23:50:28Z")
> [@Habeed](#): > > What’s “hindering” their development is that they are at the mercy of the wind. This makes them very difficult to not crash on landing. Also, helium is the only truly nonrenewable resource, and is the only substance in the world that can cool the high end superconducting magnets. It would be a terrible idea to waste it on filling gasbags. Good point. I understand that the only real source of helium is natural gas wells in Texas. Question: if we do not extract it, is the helium escaping into the atmosphere anyway? ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [July 4, 2015, 12:00am UTC](https://boards.straightdope.com/t/questions-about-a-hypothetical-air-motor/724079/34 "2015-07-04T00:00:19Z")
> [@ralph124c](#): > > Good point. I understand that the only real source of helium is natural gas wells in Texas. Question: if we do not extract it, is the helium escaping into the atmosphere anyway? Predominantly Texas, but surrounding states have it too - [here](http://geology.com/articles/helium/) is a map. No - the helium is well protected in the formation - if it could escape , then it wouldn’t be there in the first place. (helium accumulates over long geographical times from radioactive decays) ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [July 4, 2015, 12:14am UTC](https://boards.straightdope.com/t/questions-about-a-hypothetical-air-motor/724079/35 "2015-07-04T00:14:53Z")
> [@Nefario](#): > > I used to be pretty good at thermo… :o > > Apparently the process from energy input to compression to mechanical energy output is horribly inefficient. I really don’t understand why one would intentionally cool and reheat the compressed air unless this is to get more air into the tank. That requires more “wasted” energy. Why not insulate the tank and store hot air? But once you have air energy stored in the tank the conversion efficiency would seem to depend on motor design. Some energy is lost to the creation of entropy in the air expansion. > > Anyway, I found the following links; the first is exactly what you’re describing but the gas state descriptions leave out steps that they assume you know. The second describes storing compressed air energy in abandoned gas wells and assumes the earth absorbs the heat and that the government will finance the system so cost and efficiency do not matter and the available storage volume is so great that there will be a net benefit. > > [The Oil Drum: Canada | Compressed Air Energy Storage - How viable is it?](http://canada.theoildrum.com/node/3473) > > [http://energystorage.org/compressed-air-energy-storage-caes](http://energystorage.org/compressed-air-energy-storage-caes) Thanks for the info. The first link is broken, I’ll see if I can figure it out later. I think one of the main problems this would face is the losses from every change to the incoming air. As someone already mentioned, with thermodynamics you can’t win, and you can’t even tie. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [July 4, 2015, 12:16am UTC](https://boards.straightdope.com/t/questions-about-a-hypothetical-air-motor/724079/36 "2015-07-04T00:16:18Z")
> [@am77494](#): > > I have designed and operated many machines expanding a high pressure fluid (process gases) to low pressure and extract the power. A few of the machines I have designed are for air expansion - that is for Air Separation Plants - where you make Nitrogen, Oxygen, Helium etc. > > I will try to answer all your questions. Some of your questions, presuppose certain things and so are not very good questions. Thanks for all of this. As you have seen I have a lot to learn just to ask the right questions. [Previous page](https://boards.straightdope.com/t/questions-about-a-hypothetical-air-motor/724079.md?page=1)