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Compressed air is like the stock market: Buy low, sell high.
  (+15, -2)(+15, -2)
(+15, -2)
  [vote for,

Potential energy stored in a spring or in the position of a heavy object is like money in the bank -- its value is not likely to change. Energy stored as gas pressure is like money invested in the stock market. The value varies with temperature. The inefficiency of the device used to convert pressure to potential energy is like the stock-broker's commission. Buy low, sell high, but be sure the commissions don't wipe out the gains.

Our whole energy problem arises from a general ignorance of the above facts. Everyone knows we are just 20 years away from getting limitless energy from fusion. That's been true for the past 50 years. The energy available from fusion is like money in Uncle Bob's bank account -- he might leave it to us, but then, he might not. In the mean time, the sun is a working fusion reactor, and the neutrons it produces stay far away from us. Air in a sealed tank gains and loses energy as it warms and cools. We can play that market while waiting for Uncle Bob to die.

One approach is day-trading. Convert your potential energy to compressed air just before dawn, when the most molecules of gas can be squeezed into a container by a given amount of potential energy. At the daily high temperature in the afternoon, sell some of the high-pressure air to customers. (They might use it to power their cars). Then convert the rest back to the same potential energy you started with, and wait for the next low temperature.

Another approach is to sell on the uptick and buy on the down-tick. Extremely low commissions and high volume are necessary, but think of the profits! The volume is there -- every day much more energy arrives from the sun and is re-radiated back to space than humans use in a year. Just capture a tiny fraction of it using properly designed machines spread out over large areas of land, and there is no need to burn fossil fuel.

As you might have noticed, I have been thinking a long time about various ways of capturing energy from ambient temperatures. Very recently I thought of an approach that seems much more promising than anything I have encountered before. A machine using this approach does not operate between two heat reservoirs. Instead, it accepts heat from and rejects heat to its environment, either at daily lows and highs, or whenever the environment's temperature changes.

A day-trading setup might involve some concrete cylinders the size of grain silos for storing potential energy in the form of pumped water, and some high-pressure gas tanks. To convert stored water to high-pressure air, start with a large chamber of atmospheric pressure air connected to the water silos. Open a valve to the lowest silo, and let the water flow in until it stops. Then close that valve and open one to the next higher silo. Continue until the air in the chamber is all squeezed into the high-pressure tanks connected to the top of the chamber.

In the afternoon, the tanks are warmer and the pressure is higher. Transfer some of the air to storage tanks for later sale to customers, but you have to save some to "pay the broker". You need a little more pressure than you started with in the morning to get the water back to where it was. Reverse the morning process: open the valve to the highest silo, and let the water in the pressure chamber flow up into the silo until it stops. Then close that valve and move to the next. When you get to the last silo, you should have enough pressure left to empty the pressure chamber of water, leaving the water levels where they were and the chamber full of air at atmospheric pressure. It will help to have a lot of copper heat-transfer vanes inside and outside the compression chamber, and probably heat pipes too, so that the gas temperature stays at ambient. The process should be done slowly, over an hour or two, the water pipes have to be big so there is no friction losses, and you need enough different level silos so that even with big wide-open water pipes, the water moves without turbulence.

Obviously this scheme works. The thermodynamics is sound. The question is, do you make a profit after paying for the equipment? I don't know. But if we don't find some way to replace fossil fuels, a lot of people are going to suffer.

The other scheme (frequent "buying and selling") avoids working with high-pressure air and massive equipment, but doesn't get a daily harvest. I imagine a cheap device mass produced and distributed far and wide over thousands of acres of open fields, or deserts. After a week or two, a harvester moves slowly over the fields, collecting stored potential energy from the devices but leaving them in place to collect more. Each device includes a pressure chamber with a good thermal connection to ambient temperature, and some means for storing potential energy.

To be vivid and a little cute, imagine that the fields are filled with posts several meters tall, and the devices store potential energy by climbing the posts. The harvester lowers each machine back to the ground while capturing the energy. So, how can a heavy machine lift itself up a pole using nothing but ambient temperature variations? When the price (temperature) is low, the machine buys gas pressure in exchange for potential energy, lowering itself a little down the pole. When the temperature rises enough, the machine sells the gas pressure for a boost up the pole, ending up a little higher than it started even after paying the broker.

There are endless possible variations on the theme, and local conditions would dictate adjustments. If there is a lot of sunshine, the machines might warm up their working gas with sunlight, store the energy, and then cool the gas by sending the heat into the ground. This is more like an ordinary heat engine, but during the night, the machine can still operate whenever a big enough temperature variation happens.

A vast quantity of energy comes and goes every day. We have been using fossil fuels because they were there, and we didn't see anything wrong with it, and we didn't have anything better immediately available. Now we understand that we can't keep using fossil fuels without big unintended effects. All we need to do is capture a tiny fraction of the solar energy the earth blocks, and delay its return to space by a day or two.

Interested parties are invited to join the Renewable Energy Design Wikia (formerly Wikicity) at renewableenergy.wikia.com and work out the details.

Archimerged, Apr 10 2006

OTEC http://en.wikipedia...l_energy_conversion
reference for OTEC mentioned in an annotation. [Archimerged, Apr 12 2006]

Earth's energy balance http://apollo.lsc.v...chapter3/ebal0.html
Interesting lecture notes [Archimerged, Apr 13 2006]


       Interesting reading, I enjoyed that.
skinflaps, Apr 10 2006

       I couldn't be bothered to read until the end, I'm afraid. However, in the bit that I did read, you say that //we are just 20 years away from getting limitless energy from fusion. That's been true for the past 50 years//. Do you mean that we have been 20 years away from nuclear fusion for 50 years? If so, that doesn't sound promising.
dbmag9, Apr 10 2006

       [dmag9], I think that's the point.   

       [Archimerged], I assume it's a solar-powered harvesting machine? I like this idea, why, it's practically practical! [+]
imaginality, Apr 10 2006

       Sunrise economies that are good for scuba divers = obvious bun.
coprocephalous, Apr 10 2006

       Ah, cunning.
dbmag9, Apr 10 2006

       It doesn't approach the energy debsity of fossil fuels, but you knew that. Interesting discussion of the subject. [+]
david_scothern, Apr 10 2006

       Day trading in energy - brings back warm memories of happy days at Enron...   

       Seems to me that this is more of an advert for a wiki site, than an original half-baked idea. Not that this isn't entirely half-baked.
DrCurry, Apr 10 2006

       You should probably sell the P.E. let out by lowering the machine just before dawn. You could do this by compressing a cylinder of air, sure, and selling that. If you go and start pulling compressed air out of your heating and cooling cylinder, well that would be like taking blades off your windmill.
daseva, Apr 10 2006

       No doubt about it, this is interesting, workable and sound (so it gets my +) once it is installed; but, energy-wise, will the facility even pay for itself? That is, how does the energy used to build and operate it over its economic life compare to the output?
methinksnot, Apr 10 2006

       Dr. Curry, I use wikia because I like the availbility of LaTeX formulas and easy group editing and automatic revision history and free hosting. I have no connection to wikia corporation and make no profit from the Google ads on the site. Perhaps this site will do a better job of developing the idea than wikia.com. Lets see what happens. I would put the idea on Wikipedia but they don't take original research.   

       Gum Bob,   

       In contrast to the water silo machine, the little machine doesn't ever develop much pressure in its compressed air cylinder. You cover square miles of land with these machines and harvest the energy after weeks of collection. The machine goes up (or stores energy) whenever its pressure rises enough to trip the ratchet. It never goes down except when harvested by an external agent. (I think I take back the idea of having it give up a little potential each time to get started, I just meant to indicate it can't convert all of the added energy to potential). I don't have an exact mechanism for one of these. It has to be cheap and durable. It has to move upward (i.e. convert pressure to potential energy) whenever enough excess energy is present to complete the move. I think all of the pressure escapes during that move, and it has to start over. Whenever the temperature goes down, a check valve lets air enter the chamber, and whenever temperature rises, the expanding gas pushes the machine upward. (But probably a rising liquid would achieve better efficiency). The whole trick is to have a ratchet that lets the machine go up but not down, and doesn't stick much. And it has to be very cheap so you can put millions of them out all over the land. Note that plants harvest high quality energy in the form of visible radiation from the sun. They don't do much at all with heat. This machine works even in the dark, so long as the temperature is changing. It doesn't need water. It works all year round.   


       Yes, the total energy used in building the machine must be replaced well before the machine is worn out. If the potential energy is in water level, there is very little wear and tear. Otherwise the answers have to come from experiment. Suggestions welcome, here or on any other site I'm reading.
Archimerged, Apr 10 2006

       [Archimerged], the moveable part of these energy harvesters could/should be made light and of such an exterior shape that wind can also be used to help drive them upwards (so that less gas pressure is needed).
methinksnot, Apr 10 2006

       But big ones, or lots of little ones (which is just as good).
methinksnot, Apr 11 2006

       Imagine a big glass cylinder with closed ends.
At the top end is a check valve that lets air out, and at the bottom is a check valve that lets water in.
Now imagine this cylinder set with the lower end in the sea.

       Two things:
Any action of tide or waves will cause the check valve at the bottom to let water in periodically.
Any change in air temperature inside the cylinder will either
a) expel air out of the top check valve or
b) draw water in through the bottom check valve.

       Some comments:
It's only going to work over a few metres.
The cylinder ought to be efficient at heating the air inside.
There's plenty of sea.
Ling, Apr 11 2006

       PV/T[time one] = PV/T[time two] -- ideal gas law.   

       Just throwing that out there.   

       So, we can see that a 30 deg temp. difference (generous) from 323K(50C) to 293K(20C) will result in the volume of the cylinder increasing by a factor of 1.1024 (assuming constant P).   

       So, imagine your silo.
Dia = 5m
Height = 20m
Volume = 392.7m^3

       After heating...
Volume = 432.91
Height = 22.05m

       Alright, we've gained 2.05m of height with perfect heating, assuming adiabatic and isobaric expansion in the cylinder, a 30def temp difference, and negligible resistance from the machine. Still these figures are slightly promising. I think we should now lower the machine before dawn, harvesting the P.E. in some sort of external system, and start over at the initial temp. and height.   

       Let's just say one of these things costs 1 mil US dollars to make. And 2m is the height in our P.E. calcs. Let's use a weight of 250kg atop the cylinder to harvest the energy with, and we end up with about 5000 joules/day or .0579 watts. (This calc. assumes that a weight is dropped the 2.05m to harvest energy, different from the author's thoughts on harvesting energy, but good for rough calcs.)   

       Electricity in America costs about .00009 dollars per hour for one watt, or about .000005211 dollars per hour for your output. OK. So, how long to payoff this thing?   

       1,000,000 dollars per unit / .000005211 dollars per hour return = 1.92*10^11 hrs, or just under 22 million years. :)   

       Factors that may need rearranging:
height of column
mass of weight (too much mass, and the air in the column will deviate from ideal behavior. bad.)
Capital costs
daseva, Apr 11 2006

       [DrCurry] Did you used to work at the crooked E? Just being nosey. I've met a few people unhappily displaced from there.
Zimmy, Apr 11 2006

       [Gumbob], the 250kg weight will compress the gas, so the volume at start and finish will be different. Otherwise, why not use a 1 million ton weight? And speaking of 1 million: 1 million dollars for a silo...?   

       But anyway, the message is clear - these things aren't so efficient.
Ling, Apr 11 2006

       1 mil includes the energy sequestration assemblies and such.   

       I know, the weight approximation is an issue. A heavier weight will affect the ideal behavior. I don't know how else to model the power calcs.
daseva, Apr 11 2006

       [gumBob], no, the pressure at start and finish are the same. So PV/T always has constant P.
So ignore what I said.
A heavier weight just needs a silo that can take more working pressure. Of course, higher working pressure means more energy is required to heat the gas (more dense), and more energy can be stored.
Ling, Apr 11 2006

       [Ling], right. What I'm afraid of is that the gas will not expand as well undeer these greater working pressures. It will deviate from the ideal law and in a bad way.
daseva, Apr 11 2006

       What if you fill the silo with Hg and harvest the energy using a heavy-yet-buoyant-in-mercury weight?
methinksnot, Apr 11 2006

       Wow, lots of comments. This article was written for my blog at wordpress and posted on various other mirror sites. This site has yielded the most informed interest. Here, it ought to be divided into two separate ideas. This idea has title buyairlowsellhigh, so the discussion here ought to center on the silo idea, not on the field of little climbing machines. I'll start up a separate discussion on that.   

       [rcarty] Not barometers, thermometers. Gas (and liquid, but much smaller effect) absorbs heat energy and makes the Carnot fraction of it available as work (depending on the temperature difference achieved). The silos that look like barometers (manometers, actually) are active only while actually pressurizing or de-pressurizing the air. The valves are closed and they are just tanks of water the rest of the time.   

       [GumBob, Ling, methinksnot] Thanks for taking time to think about this. Please take the time to reread the section starting "A day-trading setup might involve some concrete cylinders the size of grain silos..." This is a very efficient machine. The volume of gas involved is much larger than one silo, and the pressure changes. We are talking 10,000 m^3 of 1 atm air repeated 300 times to get 10,000 m^3 of newly-compressed 300 atm air every day. A practical machine would be installed at a pumped-storage reservoir and would involve moving a substantial fraction of the water in the reservoir every day, downward in the early morning and upward in the afternoon. The total energy involved equals the heat absorbed at high temperature minus the heat rejected at low temperature. The efficiency can be very high. The heat and cooling is free, coming from and going to ambient temperature air. The heat is moved by gravity-feed heat pipes (no operating cost) and by fans powered by (what else?) compressed air.   

       Most of the expense of the silo design is involved in achieving nearly perfect efficiency in conversion of potential energy to compressed gas, and back. There isn't much point to that unless the goal is to produce high-pressure compressed gas for sale. (Although by using high-pressure air, we can store more of it in a reasonable volume so there are other reasons for going to the expense). I'm thinking either of establishing a compressed air utility with distribution pipes like municipal water, or else compressed air stations widely distributed. Cars would do tank exchange, maybe accomplished every day in the parking lot while the owners are at work.   

       State-of-the-art tanks hold around 300 atm (4400 psi), so I'll use that value, but scuba tank pressures of around 200 atm (3000 psi) could be used too. Unfortunately, 300 atm is about 10,000 feet of water so there are some problems. To keep the discussion centered on one mechanism, let's suppose we have water pipelines extending to the top of nearby mountains. It doesn't matter much if they are open to the atmosphere at the top, or sealed with a vacuum forming there, so long as there is a big wide tank of water so the level doesn't change much. If mountains aren't available, other techniques can be devised, or we can just use lower pressure air. The actual pressure doesn't matter much, and pretty high efficiency machines can be devised to convert medium pressure air to high pressure air for use as high enough density energy storage for city cars. With pipelines and quick change tanks at pit stops, it would work for long distance as well.   

       The important thing for efficiency is the machine must be nearly reversible. That's why I asked for many different silos with different water heights. A full-sized installation involves lots of lakes with different surface heights above sea level, including smaller and smaller ponds at higher and higher elevations, and small water tanks at intervals up the mountain-side, each with its own relatively small diameter pipeline.   

       At time of minimum temperature, we use the raised water to compress a large volume of gas, at constant temperature. The mechanism consists of many silos and one very large isothermal compression chamber ("tank"). Below all of this are water-main sized pipes and valves connecting the silos and the chamber. Beside each valve is a pressure gauge. Say 50 silos and one tank with heat pipes extending from heat source below to heat sink above, with fans to warm the heat source from hot afternoon ambient air and to cool the heat sink from cold early morning ambient air. The tank starts dry, full of air. Because we have to replace the air which was sold for money the day before, and because we are day-traders (no gas inventory at end of day), the starts at atmospheric pressure. Lets say a tank is a 26.73 m inside diameter sphere, 10,000 m^3.   

       The water-pressure gauge on the valve at the bottom of the tank reads 1 atm (relative to vacuum), so we look around at all of the silo valves and find the one with water pressure at the valve closest to (and above) 1 atm, say 2 atm. The surface of water must be about 34 feet above the valve. We open that valve, and let the water flow until it stops. The pressure gauges now read the same, say 1.5 atm, and the water level has dropped to about 17 feet. The tank is half-full of water. (I guess we overdid that, probably shouldn't fill the tank more than 1/10 at a time so the first reservoir better be closer to 1.1 atm). The temperature inside the tank increased a little, so we wait for the heat to be carried upward by the gravity-feed heat pipes to the large high-surface area heat-sink which is cooled by compressed-air-powered fans blowing ambient temperature air.   

       Then we close that valve and open the next higher pressure value. Maybe it reads 4 atm (3 atm relative, 102 feet of water above the gauge). The water flows until it stops. The temperature of the tank increases, but the heat pipes keep carrying heat up to the heat-sink: liquid propane inside the pipes evaporates, giving pressure controlled by vapor pressure of propane at the given temperature. Propane gas condenses on the coldest exposed surface, which is in the heat-sink. The cold liquid runs downward into the tank, where it evaporates again. Pretty quickly, the air inside the tank is restored to the heat-sink temperature. The fan speed is controlled to keep the heat-sink close to ambient air temperature.   

       The process continues: compare pressure readings on the water-pressure gauges and open the silo valve which shows a slightly higher pressure than the tank valve. Water will flow into the tank until the pressures equalize. Heat will flow out of the tank. The tank temperature will stay constant. If it doesn't, we need more silos with intermediate pressures.   

       Suppose the temperature is 273K (0C) and the forecast is for a high of 303K (30C). We plan ahead, and close the tank valve when the tank pressure is such that we will get 300 atm at 303K, i.e. 273/303*300 atm. When the temperature rises that high, we will have 10,000/300 = 33 m^3 of air at 300 atm.   

       Not enough. But there is a valve on the top of the gas tank leading to other tanks. We go ahead and run the pressure up to 300 atm, and transfer all 33 m^3 of air into a large bank of storage tanks, displacing water. Ambient temperature is still about 273, and we drain 10,000 m^3 of water into the lowest silo and start over. We repeat this so long as the temperature is low. We are limited only by the rate of heat transfer from the tank to ambient.   

       When the temperature rises to 303K, reverse the process. Use pressure from the stored 300 atm air in many 10,000 m^3 spherical tanks to efficiently raise water from the low silos back up into the high silos. In particular, fill the tank with 10,000 m^3 of water at 303K. Then transfer 33 m^3 of 300 atm air into the tank. Examine the water pressure gauges beside the valves, and pick the valve with a pressure slightly lower than the water pressure at the tank. Open that valve and water flows out of the tank up into the water tank at the top of the mountain until the pressure is equalized. Continue picking the closest lower pressure and opening the valve. Of course, the air temperature inside the tank drops, but the other set of gravity-feed heat pipes extending down to a heat source below the tank will carry heat rapidly upward in the form of propane vapor which condenses on the cool metal with cold air on the other side. A compressed-air powered fan keeps the heat source temperature close to ambient air temperature.
Archimerged, Apr 12 2006

       Gaaaaaaaaaaaaaaaaaaaaar. You cannot get something for nothing!!!!!!!!!!!!! You cannot possibly ever ever ever ever get more energy from the compressed air than it took to run the compressor! It doesn't matter how hot it is or anything. Go read the laws of thermodynamics.Do it!
craziness, Apr 12 2006

       oh, and about the silo compression thing. The energy it must take to get the water in the top silo in the first place will be equal to, if not more than the amount of energy you can get from the compressed air. Ideas like this have been thought of before, and they will never ever work.
craziness, Apr 12 2006

       This concept is not getting something for nothing. The changes in pressure and temperature are the main energy inputs into the system. The science behind it seems sound, if the economics are just crazy.
methinksnot, Apr 12 2006

       [Crayziness] Go and read the laws of thermodynamics. Think about all of that heat flowing from the sun into the hot afternoon air into the high surface area metal heat "source" through the heat-pipe into the expanding air. Then think about all of the heat flowing from the compressing air into the heat-pipe into the heat-sink and into the cold early morning air and out into space.   

       [methinksnot] The crazy economics around here are those of fossil fuels. People who earn salaries from corporations which look at stock prices next quarter don't care if the people of Bangladesh get flooded out a few years hence. The machine I describe will last a very long time once built. Why would it wear out? It doesn't have rapidly rotating turbines and thousand degree furnaces. It should last centuries. Why not?
Archimerged, Apr 12 2006

       The question is not: Why not? but Why not now?   

       The crazy economics comment stems from the basic fact that for you to be able to fund this project, the benefit must be greater than the cost. It is a sad reality that when I say cost I do not mean the full cost (to the environment, to society, etc) but just capital outlay.
As non-renewable energy sources become scarce, their cost will increase and the B/C ratio of your project will look better. Alternatives that were risible at US$25/barrell are now very attractive and well into implementation stage.
methinksnot, Apr 12 2006

       Well, let's just hope the price of oil stays high. And maybe we can work out the actual price per killowatt of this thing. I really suspect it would be lower than the ocean thermal energy extraction (OTEC) scheme which has been getting design money.
Archimerged, Apr 12 2006

       [rcarty and crazyness] I don't mean to be insulting and I'm sorry I repeated the taunt about studying thermodynamics. But I do know a little about that subject. What I don't know is how much things cost to build, and how long they will keep working.   

       The energy used to store energy comes from the sun, every day. Most of that energy is then radiated out to space the next night. A little of it gets stored. The energy used to store the energy gets converted to heat and radiated out to space. The total amount of energy possibly available for permanant storage is (heat_in - heat_out) (T_hot - T_cold) / T_hot, less losses to inefficiency. For 273K and 303K, about 10% of the heat can be permantly stored (or used for operating cars etc.) and the rest has to be radiated.   

       The engine I describe is simply a nearly reversible heat engine operating between a hot reservoir which exists in the afternoon and a cold reservoir which exists at night. It does one cycle per day. The heat absorbed in the afternoon is nearly all converted to work and used to raise water. However, in order to continue this, we have to let the water back down again in early next morning. At that time, we capture and compress more moles of air than we used the previous afternoon to raise the water. This is because it is simply easier to compress cold air than warm air. PV = nRT.   

       The engine is highly efficient because there is little friction involved in laminar flow of water, and because there is little opportunity for heat which gets absorbed by the heat source to avoid passing through the gas and doing work on the water before reaching the heat sink the following morning.   

       There is inefficiency: some energy is used to operate the fans, some is used to heat the water, there is friction and a little turbulence in the water flow, and there is only a finite number of different pressures available to match the tank pressure.   

       This machine will definitely work. A model of it will work. The calculations I haven't done yet are more along the lines of how much heat can be moved through a spherical tank 27m in diameter in a few hours? Can one be built with heat pipes running through it? How much water must be raised how far to store all (not just 10%) of that heat until early morning? I'll be working them out shortly if no-one beats me to it.   

       And of course, how much will it cost to build? I have no idea on that one.
Archimerged, Apr 13 2006

       Archy: I suggest you take your passion and apply it to redo GumBob's calculations, showing some ballpark approximation of the wattage per day per silo. Your thermodynamics is sound. Forget the naysayers who ignore ambient changes.   

       Nice halfbaking, though [+]. You might want to be a tad more concise to get more readers. "Every word you add dilutes the words you have".
sophocles, Apr 25 2006


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