7 ms·
You might get away with pissing on Carnot's grave once - but twice - never. By that I mean the energy of compression is lost. A compressor is an analog to an in
by aurizon 7y ago
You might get away with pissing on Carnot's grave once - but twice - never. By that I mean the energy of compression is lost. A compressor is an analog to an internal combustion engine - in reverse. You put in energy via amotor and compress the air. Unless you can use that heat of compression it is lost. Then you run another analog to an internal combustion engine - in reverse.
https://www.google.com/search?ei=jFgWXoCkI474-gTJspPoDQ&q=energy+efficiency+of+an+air+compressor&oq=energy+efficiency+of+an+air+compressor&gs_l=psy-ab.12...4691.7128..9727...0.2..0.100.661.5j2......0....1..gws-wiz.......0i71j0i13j0i13i5i30.PGxHy-B_CRo&ved=0ahUKEwiAmpzFh_XmAhUOvJ4KHUnZBN0Q4dUDCAs https://www.google.com/search?ei=jFgWXoCkI474-gTJspPoDQ&q=en...
Extract:-
• Is compressed air free?
No, compressed air is not free. Although "it's only air," compressed air is actually very expensive because only 10 to 20 percent of the electric energy input reaches the point of end-use. The remaining input energy converts to wasted heat or is lost through leakage. For example, to generate 5 CFM it takes 1 HP.
So 10-20% for the compressor.
Then the expansion:-
https://en.wikipedia.org/wiki/Pneumatic_motor https://en.wikipedia.org/wiki/Pneumatic_motor
sir motors are in the 60-65% efficiency range.
Then the electric motor to drive the generator is about 93% efficient (some as high as 95%)
This string of 10-20% plus 60-65% plus 93.95%
Gives a net range of 5.58% efficient all at low end of range to about 12.35% at the high end.
At best you lose 87% and at worst you lose 94.5%
But this is wasted energy anyway, but it has to pay for manitanance and staff etc, so in the worst case it is terrible and the best case is sub-marinal.
Looks like a free energy racket to me?
- p1necone 7y agoAre there many downsides to just lifting weights/pumping water up hills? What if you connected a lot of very large weights and electric motors to the grid and intelligently wound them up/let them down based on demand? Maybe a two tiered approach combined with capacitors for fine tuning?
- petschge 7y agoThe main downside is that you need a place to store the water at the top. Favorable geography is most likely already in use. And building new lakes on hill tops alters the ecosystem quite a bit.
- pfdietz 7y ago> Favorable geography is most likely already in use. Favorable geography ON RIVERS is limited. Off rivers, the possibilities are enormous and largely unexploited.
- PeterisP 7y agoOff rivers it's hard to get a sustainable supply of the huge amounts of water required to make a reasonably large operation to work.
- pfdietz 7y agoNo, not really. The evaporative losses are significantly less than a water-cooled thermal power plant of similar average capacity, even in desert conditions. And compared to water extraction for irrigation the water needs even to charge the reservoirs would be modest.
- SailingSperm 7y agoThis startup is stacking concrete blocks as a 'battery' https://qz.com/1355672/stacking-concrete-blocks-is-a-surprisingly-efficient-way-to-store-energy/ https://qz.com/1355672/stacking-concrete-blocks-is-a-surpris...
- asymptotically2 7y agoIt's bullshit. https://www.youtube.com/watch?v=NIhCuzxNvv0 https://www.youtube.com/watch?v=NIhCuzxNvv0
- qqqwerty 7y agoThat is a poor quality source. A lot of "hand wavy" arguments with little to no citations.
- dahart 7y agoThat looks interesting, I hope the company succeeds and we see more of this. Side note, the article has a comment about peak summer power usage that made me laugh out loud: > For example, Mumbai hits peak consumption in the summer when air conditioners are on full blast, whereas London peaks in winters because of household heating. Ideally, energy captured in one season could be stored for months during low-use seasons, and then deployed later in the high-use seasons. It seems hilarious to talk about storing power to run AC in order to solve an excess of heat problem. I honestly feel bad for people who die in heat waves in large cities, but it seems pretty absurd that we’re not already running AC cooling on solar like everywhere.
- opencl 7y agoPumped hydro is by far the most commonly used utility-scale energy storage in use today. Efficiency is in the range of 70-80% so it's a lot better than compressed air in that regard, the main downside is that you just need a really large amount of water and a place to put all of it. Lifting weights for energy storage is not really very conceptually different from pumped hydro (both store gravitational potential energy), water just happens to be vastly cheaper per ton than pretty much anything else. Concrete is something like $50 per ton and water is more like $0.50.
- oh-4-fucks-sake 7y agoI wonder what it'd cost to use a waste material like slag. Slag weighs like 90% of the weight of concrete and is 10% of the price--because we basically throw it away. I guess the biggest cost would be containers to hold it. Even if the (cost container to hold slag + cost of slag) = (cost of equal weight concrete)--we're not using up concrete that could be used for things that actually need concrete. Here we just need something heavy that won't collapse when lifted. Plus, concrete releases co2 when poured. I guess co2 would have to be released to manufacture the containers--but even then...interesting thought.
- twic 7y agoHow about instead of pushing water up into the air, you pulled air down into the water. Build a big floating object at sea and drag it down into the water with a cable attached to a motor/generator.
- kortex 7y agoThat's really clever. Only real downside is underwater maintenance is a PITA. Although I suspect for the quantities involved, it still may be impractical.
- nkurz 7y agoSeveral similar approaches have been tested, although they use a turbine rather a moving chain to generate energy: http://euanmearns.com/a-review-of-underwater-compressed-air-storage/ http://euanmearns.com/a-review-of-underwater-compressed-air-....
- kerkeslager 7y agoVast size.
- wahern 7y agoThey're using a different process: https://en.wikipedia.org/wiki/Cryogenic_energy_storage https://en.wikipedia.org/wiki/Cryogenic_energy_storage Not particularly efficient, but doesn't seem to be as fundamentally broken as compressed air storage.
- robocat 7y agoText from link: "In isolation the process is only 25% efficient, but this is greatly increased (to around 50%) when used with a low-grade cold store, such as a large gravel bed, to capture the cold generated by evaporating the cryogen. The cold is re-used during the next refrigeration cycle.[8] Efficiency is further increased when used in conjunction with a power plant or other source of low-grade heat that would otherwise be lost to the atmosphere. Highview Power claims an AC to AC round-trip efficiency of 70%, by using an otherwise waste heat source at 115 °C."
- wahern 7y agoI was being pessimistic about their claims. Theoretically you could likewise capture the heat from compressed storage, too. Plus, it didn't seem fair in the comparison to compressed storage to include efficiency gains from using residual heat from elsewhere during expansion as technically that heat is input energy.
- aurizon 7y agoThis is suspect. It looks like optimism to me. I agree you can assist it, but with the intrinsic Carnot limitations added, I dount they can beat 25-30%. It still may be economic to save even 20% of energy that would otherwise be wasted,. That added waste at 115C can make the difference
- rklaehn 7y agoThere are lots of ways to get better efficiency for this process. One is to store the heat of compression in a gravel bed for use when evaporating the liquid air again. Another way is to use waste heat from an industrial process to evaporate the liquid air. It is also possible to use the cold to increase the efficiency of a combustion carnot process such as a natural gas turbine.
- kerkeslager 7y agoYou might try reading the article before responding to it.
- deleted 7y ago[deleted]
- jp57 7y agoIt's not compressing the air, though, it's liquifying it (presumably only the nitrogen), and then using the force of the expanding gas as it boils later to drive a turbine. (I don't know why the author uses the strange phrase "freezing to liquid"). There's not a compressor. Well, there might be one in the refrigerator that "freezes" the air, but the air that is being used for energy storage is not, itself compressed. In any case, as with any other energy storage, I don't think they're claiming it's lossless, but rather that the liquid nitrogen has a much higher energy density than compressed air. It sounds plausible, given that the gaseous to liquid volume ratio for nitrogen is something like 600:1, but I don't actually know. Might still have very high losses, I guess.
- hinkley 7y agoI was under the impression that liquid nitrogen was typically created using a compressor. Is it more efficient to produce it cryogenically?
- pfdietz 7y agoThe cycles for air liquefaction and separation are highly optimized, but the basic idea is to compress air, cool the compressed air back toward room temperature, then send it through an expander (a turbine, say), converting much of its remaining thermal energy to work. This leaves the expanded gas colder than when you started.
- ramses0 7y agoHow would you power a weed-whacker via non-traditional methods? AFAIK there are three main technologies in use today: gas, battery, and plug-in electric. Assuming that gas is on the way out, chemical batteries have contamination/recycling issues, and plug-in electric isn't appropriate for all use cases (definitely less convenient). I can't think of anything other than those technologies which comes remotely close to power a small engine for ~30 minutes at a time, once a week. Will we ever get to "nuclear-powered" weed-whackers? Wouldn't it be Great(tm) if there were the equivalent of AA/AAA rechargeable batteries but for larger devices? Maybe the 6-cell laptop format is the winner (but look at the variation in that use-case)...
- kortex 7y agoI have an electric weed wacker, and for my purposes it works great. But even for more industrious weed wacking demands, I don't think small gasoline engines are contributing that much to global carbon. Pollution and smog, otoh, two strokes are notorious for polluting.
- Mirioron 7y ago>Will we ever get to "nuclear-powered" weed-whackers? I wanted to say "probably not", but I looked some stuff up and now I'm unsure. Essentially all small scale "nuclear power" uses a Radioisotope Thermal Generator. You essentially have a radioactive rock that generates heat and the heat difference is used to generate electricity through the Seebeck effect. The fuel that's typically used for this is Plutonium-238, because it almost exclusively decays as alpha particles (helium-4 nuclei), it has a half life of 87 years while being the most (of the long half life ones) energy dense at around 0.57 W/g. This is what's used in space flight missions and has actually been used to power pacemakers in the past with essentially no ill effect on the patient. Pu-238 is still too heavy for the amount of power it generates for a weed-whacker. What changed my mind though is that apparently Polonium-210 also almost exclusively decays as alpha particles, but it has an incredible 140 W/g power density. The problem is that it has a half life that's only about a third of the year. It still could technically be used for a weed whacker though. From a practical standpoint though, small scale "nuclear power", through RTGs, is way too inefficient for it to be used by regular people. Even if you eliminated all the risk of contamination, it would still only make sense for things that need to be running 24/7 that you couldn't swap out the battery on. Fission and fusion, so far, require a far larger plant to generate power.
- kragen 7y agoAs I understand it, they're refrigerating air, not compressing it. If so, your calculations are entirely inapplicable. Carnot efficiency approaches 100% as your cold reservoir approaches 0 K, so it's plausible for such a cryogenic system to get very high efficiencies from an air motor, much higher than normal.
- oh-4-fucks-sake 7y agoSeems like you're pretty knowledgeable on the topic! I'd be super interested to see an efficiency comparison between the frozen air approach and the "concrete (or any other cheap, heavy thing) battery approach" For the uninitiated: the process of using excess energy to drive motors to lift/stack heavy objects tethered to a pulley. Consuming that energy is by slowly lowering them down as they drive an engine/turbine. As a rider, I'd be equally interested in the realistic energy density between the two methods: how many MWH can you store per acre. Thirdly, I would guess the concrete battery would require less maintenance--but I'd love to see a comparison of that too. Thank you for your comment and getting me thinking about this.
- nine_k 7y agoMotors efficiency, an generators efficiency, can easily be 0.95, and even 0.98. So overall the lift approach must be pretty efficient, even when we subtract losses for friction in the rope and of the air. The energy density isn't great, though, you need really much mass, and large elevations, to store anything interesting. Pumping large amounts of water uphill is more feasible, even if less efficient, and is actually done.
- aurizon 7y agoYes, pump motors and generators. so 96% motor and 96% pump up = are in the 95-7% efficiency, pumps about the same up or down. So .96 x .96(up) and .96 x .96 (down) = 84.9% and friction drops that a little. It is quite good if you hacve a handy high dam pond to use as many places do. To build a high dsm pong may not be economic unless you have a topographic valley you can dam and fill (and murder all the locals and flying snapping turtles) Dams have many objectors, so new ones are hard to build, except for China.
- natmaka 7y agoThe 'concrete'-based approach (gravity storage) is pursued, for example by a firm named Heindl Energy https://heindl-energy.com/ https://heindl-energy.com/ There are offerings and deployed energy-storage systems based upon flywheels https://en.wikipedia.org/wiki/Flywheel_energy_storage https://en.wikipedia.org/wiki/Flywheel_energy_storage
- Taniwha 7y agoRemember that peak power is much more valuable than non-peak power (in some places non-peak renewable power has a negative value) so buying it at a discount, wasting most of it in the process, and selling it at peak time can still be economical.
- Mirioron 7y agoIf the numbers in the parent post are true, then you'd probably still be better off just pumping the water high up with that energy and letting it fall down when the energy is needed. On the other hand nabla9 quoted numbers that look very good.
- Taniwha 7y agoyeah, or the concrete block thing - but water requires valleys you're allowed to flood. BTW one solution is to use existing hydro dams as partners to wind/solar, add extra generation capacity to the dams, share the transmission infrastructure between the wind/solar/hydro and simply let the dams fill when the wind blows/sun shines. That's more efficient than pumped storage
- frandroid 7y agoThe province of Québec stops or reduces the flow of many of its hydroelectric dams at night due to the much market lower prices spot prices for electricity, especially since nuke plants flood the markets with unused capacity.
- Taniwha 7y agoThat's great - but really the dams need to be able to help handle peak grid capacity, and to do that they need to be able to generate more than they were originally designed for so that they can trade off against those other sources that weren't around when the dams were built
- 7y ago
- Bendingo 7y ago> A compressor is an analog to an internal combustion engine - in reverse A compressor is not the reverse of a IC engine. IC engine is a heat engine, and the reverse of that is a heat pump (not a compressor). Furthermore, Carnot efficiency refers to heat engine efficiency.
- extrapickles 7y agoThey are referring to an internal combustion engine as a device that coverts heat/expanding gas->work, where the reverse is work->heat/compressed air.
- aurizon 7y agoI beg your pardon. A Carnot engine is indeed a heat engine. The heat of combustion creates hot compressed gas - on which it runs by expanding that gas. A compressor is also a Carnot engine, you compress the gas and you must extract the heat from the hot gas or your air tank will fill with hot gas. As this cools, unless you can use that heat for other tasks, it is wasted. A carnot engine is not reversible. If you run it backwards you must submit to the equation and take your losses. They have been hard at work for almost 200 years trying to beat Carnot - he won them all, no survivors, but he gained immortality... https://en.wikipedia.org/wiki/Carnot_cycle https://en.wikipedia.org/wiki/Carnot_cycle The Carnot cycle is a theoretical ideal thermodynamic cycle proposed by French physicist Sadi Carnot in 1824 and expanded upon by others in the 1830s and 1840s. read the wiki for more
- lutorm 7y agoAn internal combustion engine runs on an Otto (or maybe Atkinson) cycle, though, not a Carnot cycle. All thermodynamic cycles are heat engines, but their efficiencies aren't the same. How do you mean a Carnot cycle isn't reversible? You can run the cycle in either direction. That's not the same as saying is has 100% efficiency.
- aurizon 7y agoThe Otto cycle means the Otto mechanical sequence. Reversible means run it backwards and it returns the burned gasses etc to fresh fuel, heat etc. etc with 100% recovery
- nabla9 7y agohttps://en.wikipedia.org/wiki/Cryogenic_energy_storage#Efficiency https://en.wikipedia.org/wiki/Cryogenic_energy_storage#Effic... >In isolation the process is only 25% efficient, but this is greatly increased (to around 50%) when used with a low-grade cold store, such as a large gravel bed, to capture the cold generated by evaporating the cryogen. The cold is re-used during the next refrigeration cycle.[8] >Efficiency is further increased when used in conjunction with a power plant or other source of low-grade heat that would otherwise be lost to the atmosphere. Highview Power claims an AC to AC round-trip efficiency of 70%, by using an otherwise waste heat source at 115 °C.