Compressed air is like the stock market: buy low sell high

This text was originally posted at archimerged.wordpress.com. Fortunately it was mirrored at many sites, for example bloglines, because wordpress is down. Comments were left at a few sites. Archimerged's replies are included below.

Compressed air is like the stock market: Buy low, sell high.

By archimerged

Compressed air is like the stock market: Buy low, sell high.
April 10th, 2006 by archimerged

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.


 * Posted on: Mon, Apr 10 2006 3:58 AM

Replies to comments at halfbakery
The idea was also posted on halfbakery. Many comments were added, and replied to. The comments are not quoted but the replies are by archimerged who releases them under GFDL.

Links:
 * OTEC reference for OTEC mentioned in an annotation.
 * Earth's energy balance Interesting lecture notes.

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

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.

\[Crayziness\] 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

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 12 2006

Posting at krazy letters
What most people don't know about energy is about to flood the planet. Energy is not something you use up. There is no shortage of it, but it must be captured and concentrated to be useful. This requires spending money to design and build machines that will do so. Such machines need not wear out quickly, and they can capture all the energy we need every day, but they are expensive and not yet designed.

The important thing is that enough people must become aware of the possibility of building machines that move slowly and ponderously and very gently with no wear and tear, all the while producing large amounts of energy in the form of compressed air. The energy comes from the sun, and eventually all of it goes back out into space. It was merely detoured for a few days into the position of some water, or the motion of some molecules of air. I say "no wear and tear" because this means the machines last many many years. Every day they put energy out, but nothing need be put in except for routine maintenance.

Even among those who should know better, it is widely believed there is no free lunch. The sun is our free lunch. We don't need to (and shouldn't) duplicate the sun by developing fusion power on earth. We should just design and build efficient heat engines which every day produce vast quantities of compressed air. This can be converted to electricity or used directly to power all kinds of machinery, or used to extract carbon dioxide from the atmosphere.

One such machine is described at http://www.halfbakery.com/idea/buyairlowsellhigh. There are many similar possibilities.

Please help by reading the questions at halfbakery, and help me to write answers that ordinary people can understand, in order to build support for spending the money to design and build these machines. Making them work is not the problem. Getting people to believe they are possible -- and to get together to fund them -- is the problem. You can help.

Reply to lappy512
QUOTE(lappy512 @ Apr 13 2006, 08:28 PM)
 * I totally disagree. When you compress air, it creates heat. The heat energy will be dispersed, resulting in wasted energy. Also, to compress that air, you'd need a motor with lots of power (Joules/Second) (after compressing it a little) to put more air into the tank.
 * Power=Joules/Second=Watts.
 * To compress it, you'd need a motor with a huge watt consumption.

Ok, first I'll explain in technical terms. Unfortunately probably not what an average untrained person can understand. Please keep asking questions and telling me I am wrong so that I can see what you don't understand.

"When you compress air, it creates heat"

Yes, as pressure rises without input of heat (adiabatic compression), the temperature rises. Temperature is the average kinetic energy of one degree of freedom of motion of the particles of the gas. So the above reflects the fact that work done compressing the gas has increased the average velocity of the particles.

My heat engine goes to quite a lot of trouble to remove this heat as it is created, so that the working gas does not become harder to compress. The higher the temperature, the higher the average kinetic energy of the gas particles, the harder they push on the moving surface which is compressing the gas. The whole secret to a heat engine is the fact that it is easier to compress cold gas than hot gas.

"The heat energy will be dispersed, resulted in wasted energy"

Yes, a heat engine operating between 273K and 303K will disperse 90% of the energy passing through it as heat. This is absolutely unavoidable, as proved by Carnot in 1824. So one might call it "wasted", but I would call it "used."

"To compress that air, you'd need a motor with lots of power (Joules/Second) (after compressing it a little) to put more air into the tank."

One could compress air using a motor. However, that is not how I do it. I use water in silos above the compression tank. At the beginning of a cycle, the tank is completely full of water. Air at atmospheric pressure flows into the top of the tank as water drains out the bottom to the lowest reservoir. Then, those two valves are closed, and another is opened, connecting the bottom of the tank to a higher silo of water. Water flows out the bottom of the silo, down below the tank, through the valve, and up into the tank, compressing the air. There are no motors involved anywhere. This machine has no solid moving parts aside from valves.

How did the water get into the silos above the tank? After the compressed air has absorbed heat and increased in temperature from 273K to 303K (or whatever), the air is allowed to expand in the tank, pushing water out the bottom, through a valve, and up into a silo above. The air gets colder during this process, because the molecules which did work on the water lost kinetic energy and the average kinetic energy of the gas molecules decreased. The heat engine again goes to a lot of trouble to keep the temperature of the expanding gas equal to that of the surrounding air. So heat is absorbed from the air at 303K and goes into the working gas and from there into the position of the raised water.

QUOTE(lappy512 @ Apr 13 2006, 08:28 PM)
 * To sum it up, energy is energy. There's loss when it turns into heat, but it's still energy. Basically, your "heat engine" is another form of a solar panel, because it takes heat/solar energy and converts it into potential engergy.

No, energy is motion and heat is motion. Actually, energy is a scalar whose value depends on the state of motion of the observer. Energy and momentum together form a four-vector which is invariant under a Lorentz transformation. If an observer is moving with the same velocity as a particle, it appears to him that the particle has zero kinetic energy. But to an observer moving with respect to the particle, it has nonzero kinetic energy. (But I guess ordinary people, including people who make the important decisions, don't know this. The fact that they don't is a good part of why we are using fossil fuels unnecessarily.)

In the case of light, energy is equal to the frequency of the photons (converted to joules by multiplying by Planck's constant, which is in joule-seconds, or joules per hertz) times the number of photons.

Light energy = (frequency in hertz) * (joules / hertz) * (number of photons)

The connection between photon reciprocal wavelength and photon frequency is also given by a constant, the speed of light. Just as Planck's constant in joule-seconds is "really" joules per hertz, the speed of light in meters per second is "really" hertz per inverse meter. So any quantity in reciprocal meters can be converted to hertz by multiplying by the speed of light. It's still energy.

(meters/second) = (1/seconds) / (1/meters) = hertz per inverse meter.

1/(wavelength in meters) = (frequency in hertz) / (hertz per inverse meter)

To get from wavelength to frequency, multiply inverse wavelength (in inverse meters) times the speed of light (in hertz per inverse meter).

To get from wavelength to energy, multiply inverse wavelength by speed of light and by Planck's constant. (inverse meters)(hertz per inverse meter)(joules per hertz).

And finally, the connection between temperature and energy is given by a third constant, Boltzmann's constant, in joules per kelvin. The fact that it is a constant reflects the fact that temperature of a gas is 1/2 the average energy of a degree of freedom of a gas particle. In other words, a kelvin is just a funny unit of energy which can be converted to joules by multiplying by Boltzmann's constant. (Or one can express temperature in joules -- it is always a very small number because at ordinary temperatures individual molecules of gas have very small amounts of kinetic energy).

Solar panels require photons with wavelength under 600 nm. Heat engines obtain heat energy from the air, but that energy originally came from photons from the sun with much longer wavelength. There is a lot more energy available at these longer wavelengths. So a heat engine is not just another solar panel. Although it can absorb light, it mainly absorbs energy through collisions with gas molecules from the surrounding air.

QUOTE(lappy512 @ Apr 13 2006, 08:28 PM)
 * Another idea is to get Solar panels everywhere, since the average amount of sun shining on the earth is about constant. Then, during the daytime, excess energy could be converted by electrolysis into Hydrogen. During the nighttime, the hydrogen could be converted back into electricity using fuel cells.


 * Because Solar cells are proven technology, the second idea is probably better. (feel free to disagree)

Of course I disagree. Hopefully you will too after you understand the principles involved. But in the mean time, please keep trying to explain why I am wrong.

Mynck: compressed air is a bad way to store energy, in that if it gets cold, you can't get as much work out of it. But it has advantages over hydrogen (safety, and hydrogen is very hard to keep inside of a tank). Anyway, this machine naturally produces compressed air, but if you wait for the hottest part of the day and then convert the compressed air to some other form of energy like hydrogen gas, or electricity, or gasoline, it works out the same. Or if your upper reservoir happens to be located at a pumped-storage facility, you can leave excess energy stored as water in the high reservoir, and convert it to electricity very quickly on demand.

[edited to add note about Boltzmann's constant also being a conversion factor to joules, like Planck's constant, and other minor changes]

This post has been edited by Archimerged: Apr 14 2006, 10:23 AM

Reply to thisoldmage @ Apr 14 2006, 07:08 PM
post Apr 14 2006, 08:42 PM Post #6

Actually, I'm not asking for money. I'm asking for questions and sincere attempts to understand so that I can develop better explainations (and fix any problems that get exposed). If I can communicate effectively enough, the project will get developed. Also, you don't have money now, but you will in 10 years. Ten years is not that long a time.

Seriously, you guys have at least as much science knowledge as most of the general public and unfortunately, probably more than the power elite (who have mostly forgotten what they knew). So if I can't explain it to you, how can I explain it to them?

Maybe someone needs a science fair project? (Or for next year...) A model of this machine built from hoses and plastic soda bottles ought to work just fine, although it would produce only tiny amounts of power. (Discuss the design before trying it).

Drawings and animations would be very useful, if you put them on the renewable energy wikia site. One way to get the project built is to attract the attention of someone who can build it. This is done by attracting the attention of lots of people, until someone who knows the right person draws their attention to it. When I have a good article with illustrations and maybe even a working model I could use some shills to give it diggs... (Which is where I ran accross this site in the first place, fearless leader lappy512 submitted his algebra problem to digg.com).

Regarding solar panels, current costs are several thousand dollars per killwatt of peak output. Large coal-fired power plants cost only $500 to $1000 per kilowatt. Also, you have to store the energy produced. And you can't run a car off it. Cars can be modified to run off compressed air... I don't know how much this machine might cost to build and need to figure it out. Considering that power plants need fuel but this machine (like solar panels) doesn't, it can cost somewhat more per kilowatt.

Reply to thisoldmage @ Apr 15 2006, 06:53 PM
post Apr 15 2006, 09:39 PM Post #13 QUOTE(thisoldmage @ Apr 15 2006, 06:53 PM)
 * So, Archimerged, how would I produce this on a small scale? If I can use it to even power a light or make a buzzer sound using hoses and plastic soda bottles I would like to try it out. It would be a nice science project[...]

I'm working on a new version my blog but am still editing the post there, i.e., what you read now will change -- I don't have the connections between the tanks right yet. (as of 16 April 14:20 wordpress is down and the blog is AWOL. See the page at the Renewable Energy Design wikia where I am rewriting what wordpress lost). The hope is to find a system that doesn't need a whole series of reservoirs at closely spaced heights. This one would have two upper water reservoirs (one slightly higher than the other) and many expansion/compression tanks at different heights below the reservoirs. It also has a lower reservoir, and the point is to pump water up from the lower reservoir to the two upper reservoirs.

The fact you need to know about efficient heat engines is they must be nearly reversible. That means when you open a valve, the pressures should be almost the same and the flow will run for a while and then stop. In the system I described on halfbakery, I had one compression/expansion tank and very many reservoirs at different heights. I'm sure that works, but there are advantages to having many tanks and just a few reservoirs. But I don't have it working quite yet and the system for moving air from one tank to the next doesn't stop flowing by itself.

A model probably wouldn't need the heat pipes, but that means the temperature inside the tanks might get too high during compression or low during expansion unless the process moves so slowly it doesn't finish in time. (It is possible to make heat pipes out of copper tubing and propane or some other refrigerant but you would need a lot of them). If you build it with computer controlled valves and sensors and program the machine to watch the weather forecast so it knows when the low and high temperatures are going to be, it ought to pump more water upward than it lets fall down. It would be enough to measure how much water it pumps to demonstrate feasibility, but for the literal minded, you could rig up a water powered generator.

With too many losses (for instance if the pressure tanks leak or the temperature changes inside the tanks too much), the machine would end up lowering more water at low temperature than it raises at high temperature. For machines with solid moving parts, it always turns out that way unless the temperature difference is at lot more than 30K. Because it has always turned out that way, and because until very recently, fossil fuels have been working fine, people have just not tried hard enough to think of a way to eliminate the losses.

QUOTE(thisoldmage @ Apr 15 2006, 06:53 PM)
 * My other question is, how long would it take to earn back an investment in an energy creating source?

A solar panel will return you your investmen in >10 years, right? I can see people wanting it if they return their investment in 10 or even 20 years, its like a grant. But if it takes 40 or 50, I can't see an individual taking that kind of leap. If the machine works like I hope (I suppose there might be a gotcha I haven't noticed yet)... a large version costing millions or billions of dollars will pay itself off well under 30 years, and might last a lot longer than 30 years. But I'm afraid to actually do the calculations, because probably it won't turn out that way... Besides you can't really do the calculations without doing some experiments first to see how it actually performs.

[...] This post has been edited by Archimerged: Apr 16 2006, 01:25 PM

Replies to comments at thinkcycle.org
thinkcycle.org Re: don't overvalue

Thanks for thinking about this.

1. Compressed air isn't like a bank. It's like the stock market. You store energy in pumped storage or in a spring or in chemical bonds. You invest in compressed air because it soaks up extra energy when the temperature rises.

2. Using compressed air as fuel is convenient because it is nontoxic, storable, pipeable, etc. My major use is as working gas in a heat engine, where it stays. A minor use is as the form of output. I prefer compressed air output because I can convert pumped storage to compressed air with arbitrarily high efficiency, depending on how long I take to do it, or if I want fast conversion, on how many different huge pipes and ball valves I have with very slightly different pressures.

3. Re efficiency ("you need to keep an eye on whether or not this causes a net gain in efficiency before you can declare that it will solve global warming"), I describe a reversible heat engine which can achieve high efficiency if you spend enough on the infrastructure. The problem is in obtaining the resources ahead of time to build something that pays off over years.

To cast the problem in these terms, the energy capital cost (expressed in joules, not dollars) should be converted to watts by dividing by the expected useful life in seconds and subtracted from the power output to see if the machine has positive or negative power output. I think these machines have postive output, but it's the investors who must be convinced. Unfortunately, they care about the output expressed in dollars per quarter after subtracting capital cost in dollars per quarter of useful life. This result does not necessarily have the same sign as the net power output.

Posting at digg.com
[http://www.digg.com/science/Compressed_air_is_like_the_stock_market:_Buy_low,_sell_high. digg.com] Potential energy is like money in the bank: its value is not likely to change. Gas pressure is like money invested in the stock market. The value varies with temperature. The inefficiency converting pressure to potential energy is like the commission. Once enough people understand this, our energy problem is solved and we can fix global warming.

Mirrors:      

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