idea for solar propane tanks?

[hackberry jake]

I was thinking about sterling engines yesterday and how they work off of the power of compressed air and heat. As air is heated it expands. I started thinking about leaving a compressed air tank out in the sun how the pressure would increase. Propane tanks are cheaper per gallon than compressed air tanks. Around  here 500 gallon tanks run about $500 and 1,000 gallon tanks run about $750. Say you took 10 of the 1,000 gallon tanks and painted them flat black and plumbed them all together. Maybe use some type of hail resistant reflectors to focus more sun on them. That would be 10,000 gallons of air that expand and contract with heat and cool. You could generate power off the compressed air once the tanks reached max pressure and once the pressure fell below generating level, just let the rest of the pressure out and close the valve. At night once the temperature of the tanks is at its coldest, you might be able to generate from the vacuum caused in the tanks. Practical?

[D L Bahler]

There may be a more efficient way of using the sun's heat to power a sterling engine. Not to say your idea is not possible. But when putting forward that amount of money, there may be a way that gets better results.

However, you could conceivably use your concept in a power system that is not really a sterling cycle engine. Just a simple piston with a single pressure mechanism rather than the double-sided concept of a sterling, where temperature differential causes uneven expansion/contraction thus motion.

Just have a piston shaft or group of them where the pressure generated by the solar tanks shoves the head in one direction (thus maintaining a relatively constant pressure inside) and then the cooling off at night causes it to return. Is this what you are describing?
This would just use an isolated single pressure system

You may not want to paint the tanks black. Yes they will absorb the heat during the day better, but then they may not release it as well during the night. You need to come up with a system that has optimal temperature range within a single day period, not necessarily optimal temperature gain during the day.

[hackberry jake]

The sterling is just what got me thinking. As far as generating is concerned it would likely use a scroll expander or screw expander type generator.

[Ianab]

I think the issues will be the small amount of power you are going to have available. Air has very little mass, and you aren't going  to get a huge pressure gain simply  by warming it. It would run a TINY air motor for a while, but I don 't think it would create a useful amount of power.

Compare it to a steam engine, which can generate a lot of power, But that's running ~100 psi, and has a fire under it to continuously create more steam. Once the fire goes out it looses pressure pretty quick.

A more practical use for those tanks might be solar heat storage. Build a solar collector and use that to heat the tank full of water,. Because water is heavy you can then store a decent amount of heat. Then at night, pump the warm water though a radiator in the house. If everything is insulated the losses should be quite small. 1000 gallons of hot water is able to store a useful amount of energy, at least compared to 1,000 gallons of air.

Ian

[julio]

Hackberry I know here you can go to scrap yard and pick them up they just pull the valve out and just throw them away. I have bought them and made grills from them , I think I paid 25 or 30 bucks for them .

[hackberry jake]

The pressure difference on propane is more than on air I'm sure, but propane changes pressure substancially with heat. Here is a website with a graph showing the relationship. http://www.engineeringtoolbox.com/propane-vapor-pressure-d_1020.html It showes that at 50 degrees propane will be at 100psi. At 150 degrees it would be around 400 psi. That's a pretty big jump and 150 degrees shouldnt be too hard to achieve in the summer months if the tank is painted black. Thats why most propane tanks are either silver or white.

[Ianab]

The propane is effectively boiling at that temperature range. It's the phase change from from liquid to gas that's increasing the pressure. Like water in a steam boiler as you go over 100C. Boiling point of Propane is -42C, but it's kept liquid because of the pressure in the tank. Increase the temp, and the pressure that's keeping it liquid also increases.

Air isn't going to change in pressure near as much, unless you get cold enough to farm liquid air...

Ian

[hackberry jake]

well, it was worth a shot. Guess I don't have to go out and buy 10 massive propane tanks now.  8)

[Gary_C]

The pressure difference on propane is more than on air I'm sure, but propane changes pressure substancially with heat.

It showes that at 50 degrees propane will be at 100psi. At 150 degrees it would be around 400 psi. That's a pretty big jump and 150 degrees shouldnt be too hard to achieve in the summer months if the tank is painted black. Thats why most propane tanks are either silver or white.

There are a number of problems with your thinking. First, I hope you were not thinking of actually filling your tanks with propane and then letting it escape. That would be a loosing proposition. Then, as far as propane, you are talking about a liquid to gas phase change going on in a closed container as the temperature rises. That is why you are seeing a large pressure change as the temperature rises. And the other problem is your engine could go BOOM if the temperature gets too high. That's why propane tanks are painted with a light and reflective paint and have a relief valve to prevent the pressure from going too high.

Now with air that does not go thru any phase change at the normal temperatures you are considering, it's easy to calculate the pressure change of a fixed volume of air with a temperature change. What the ideal gas law states is with a fixed volume, the pressure divided by the temperature must equal a constant. So any increase in pressure must be accompanied by a proportional change in temperature and vice versa. And what defeats your proposal is the temperatures and pressures must be in absolute values.

So at 70 deg F. and standard atmospheric pressure of 14.7 psi and on the farenheit scale, you are starting at 500 degrees Kelvin and 14.7 psi. So if you are able to raise the temperature by 100 degrees to (170 deg F), both pressure and temperature has increased by 20 %. That results in a 2.94 psi rise in pressure.

So you could make you idea work maybe, but it would not amount to much. And that does not account for the natural heat loss you would have.

Sorry for the bad news.

[hackberry jake]

70f would be about 295k, but point taken

[hackberry jake]

I guess you could always put a bunch of mirrors on one tank and put some water on it, but then you just have a solar boiler. Which with enough $$$ and sun is feasable, but not for this small timer.

[D L Bahler]

Wait a minute,

You can present a 20% rise in temp resulting in a 20% rise in pressure.
If your tanks start at atmospheric pressure that isn't much.

But, if your tanks are pressurized suddenly your 20% gain is quite a bit higher -the question is, how much pressure can you safely put in those tanks?
Suddenly we could be talking rising for 200 PSI to 240 psi, an increase on 40 psi can actually do something, especially with the shear volume we are talking about.

The science is simple, if you have pressurized air, you have a greater mass of air in the tanks so, as the temperature increases, a greater expansion.

If you would have a pneumatic cylinder working as a pusher valve system where something moved outward to increase the storage volume as pressure inside increased and inward to reduce volume as pressure decreased, resulting in a nearly constant internal pressure, you might be able to use that to generate some form of power.

It still may not be the best use of the resources at hand, however.


What if you had tanks full of something that would naturally undergo a phase change in the target range -like ethanol (at atmospheric pressure) for example, or even nitrogen (at higher than atmospheric pressure) Then your gains could be increased dramatically. 

[Fla._Deadheader]

Google and Youtube "Organic Rankine Cycle". The Scroll compressor in automotive AC units have solved the expander problem with the ORC system. It just takes the correct compressor. The rest is not so difficult.

Go HERE
and search for Organic Rankine Cycle thread. It's died, but, excellent info, and, a couple of guys have systems running. Check out youtube for them.

[Gary_C]

The science is simple, if you have pressurized air, you have a greater mass of air in the tanks so, as the temperature increases, a greater expansion.

The first part is true, the second part, not so true.

If the mass of air in the tanks is fixed, the total mass of air in the tanks does not matter. The absolute temperatures and pressures will always increase or decrease proportionally.

The Sterling Cycle is used all the time in refrigeration systems. But those systems have never been free of costs to make them work. TINSTAFL 

[D L Bahler]

The point is, if you have a greater pressure that means there are simply more atoms jammed into the same amount of space. That is what pressure is. That equates to more mass.

The reason mass matters is, it affects the way heat is absorbed, it affects how much work an increase in pressure will do before it returns to its standard state.

But, absolute pressure is proportionate only to temperature, I understand that. But the simple fact is you have an increased pressure, you have raised that side of the equation significantly. So if you raise the temperature, your resultant pressure is still going to raise proportional to the rise in temperature -so significantly higher gains are in fact achieved with pressurized air than with atmospheric air. Anyone who has worked with pressure tanks knows this. Air pressure tanks CAN in fact reach critical pressure if left in the heat.

Why do you think they put warnings on air tanks, spray cans, etc. not to leave them sit in the sun? Because as they heat up, they expand and can (and do) explode.

[Gary_C]

The point is, if you have a greater pressure that means there are simply more atoms jammed into the same amount of space. That is what pressure is. That equates to more mass.

But, absolute pressure is proportionate only to temperature, I understand that. But the simple fact is you have an increased pressure, you have raised that side of the equation significantly.

Sorry, but I've forgotten what is the point of this exercise and I don't know what is the equation that we've raised the side of significantly.

I thought this was an idea to create energy with a sterling cycle engine, not to blow up air tanks. It takes a lot of energy to get the pressure in tanks up to 200 psi and then you could get a little back, but with efficiency losses, I doubt you could even get back what you put in.

[D L Bahler]

I think his whole idea is not to use the sterling cycle -but was inspired by it- but to use a straightforward pressure differential over a single day period to draw a ram back and forth, or some such.

He is not pumping the tanks full of air, then releasing that air. He is pumping the tanks full and trying to keep that same mass of air in the tanks, then relying on the ideal gas law to take over, as the sun heats the tanks the volume of the gas wants to expand, but can only do so by driving a motor. Then likewise when the gas cools at night it wants to reduce its volume, but again can only do so by driving the motor back in the reverse direction.

[Gary_C]

He is not pumping the tanks full of air, then releasing that air. He is pumping the tanks full and trying to keep that same mass of air in the tanks, then relying on the ideal gas law to take over, as the sun heats the tanks the volume of the gas wants to expand, but can only do so by driving a motor. Then likewise when the gas cools at night it wants to reduce its volume, but again can only do so by driving the motor back in the reverse direction.

Explaining how the tank pressure works is easy compaired to the motor part. The simple explanation is you cannot make the air you used in the motor to push the piston to go in a reverse direction. And cooling does not create a suction or negative pressure to make the air go back in the tank. Or another way of saying this is in order to do work, you have to work between a high pressure and a lower pressure. So you must always be venting your air to do work.

Or the more difficult way to explain it is with entropy and the second law of thermodynamics. What that scientific principle essentially says is that any system that is doing work is always moving towards a lower energy state, even considering the convection radiation from the sun. You just cannot get energy efficiencies greater than one.

A better way to capture the energy of the sun would just be to put some solar cells on those tanks and capture the energy as electricity and store it in batteries. You would have far less losses and a simpler system.

[hackberry jake]

I think his whole idea is not to use the sterling cycle -but was inspired by it- but to use a straightforward pressure differential over a single day period to draw a ram back and forth, or some such.

He is not pumping the tanks full of air, then releasing that air. He is pumping the tanks full and trying to keep that same mass of air in the tanks, then relying on the ideal gas law to take over, as the sun heats the tanks the volume of the gas wants to expand, but can only do so by driving a motor. Then likewise when the gas cools at night it wants to reduce its volume, but again can only do so by driving the motor back in the reverse direction.

BINGO!

[Thehardway]

Funny how some of us see energy potential in all kinds of stuff isn't it?  I have thought the same about gasoline cans that build pressure during the day and then almost collapse at night.

The stirling engine would not work real well here as the amount of torque it is capable of generating is very low.  The idea of compressing the air in the tank does increase the pressure rise exponentially but it also requires energy input to achieve.  This input energy must be recouped. In the process but would not be as the air will not "recompress" on it's own as it cools to form a suction and pull the piston back in. It will merely stay in it's new and larger uncompressed state in a larger volume container. (made bigger by piston travel)  In order to acheive a suction, the tank volume would need to remain constant.

I think what you need here is a three distinct items.

1.  A gear reduction drive on the piston so you can make the piston travel very slow over a long period of time as pressure rises.
2.  A second tank connected on the opposite side of the piston travel
3.  Valves controlled by a photo cells

Two tanks are tied to a horizontal piston.

Tank 1 would begin to expand as the sun rises and begin slowly placing pressure on the piston side. The piston would move slowly and turn the gear reduction drive to run a small generator. At the end of the day the piston will have reached the end of its excursion at the opposite end of the cylinder.  The unit sits idle over night and an exhaust valve is opened on tank 1 but closed on tank 2.  At sun rise tank 2 now begins expansion driving the piston back in the opposite direction towards tank one which is now exhausting.

The amount of energy produced is equal to the day/night temperature differential as the piston will only move the distance of the expansion produced. 

This would be a 2 day cycle.  Problem would be equalizing the days/nights and what to do if it rainedduring day or stayed warm at night.  Valves could get out of sync or piston travel be shortened.  The theory works if energy of expansion can overcome friction produced.

I believe it would be more efficient to use a solar concentrator and some water to produce a steam driven engine as steam creates a much higher pressure and volume change.  Down side is you have to have a water supply and a condensor.

Using a more volatile gas will increase energy yield but also increases risks and costs.

Interesting fact is that Nitrogen is not linear in its expansion and compression rates over a temperature range.  Nitrogen expands very rapidly at low temps but very slowly at higher temps.  (This is one of several reasons why it is now used in tires)  There has been some recent attempts at using this non-linearity as a potential generation source.

[Gary_C]

Interesting fact is that Nitrogen is not linear in its expansion and compression rates over a temperature range.  Nitrogen expands very rapidly at low temps but very slowly at higher temps.  (This is one of several reasons why it is now used in tires)  There has been some recent attempts at using this non-linearity as a potential generation source.

Not true. Nitrogen obeys the ideal gas law the same as oxygen and most other gases.

The reason for the difference in expansion rates between nitrogen filled tires and air filled tires is the nitrogen is very dry and the air is not. So the moisture in the air changes phase from liquid to water as the tires heat up and the phase change is what causes air filled tires to inflate more as you are driving.

Nitrogen Filled Tires

[Thehardway]

Gary,

I think I may have been a little obtuse in choosing the Nitrogen tire inflation as an example of non-linearity.  My bad.  Here is a better explanation that makes better sense.

Nitrogen does not expand at a linear rate across its entire temperature and pressure range.  At the temperature range and pressure at which automotive tires are utilized, its expansion rate is very minimal, controlled and predictable. (operates close to an Ideal Gas)  However, at other temperatures and pressures, it can expand very rapidly.

In a car tire, atmospheric air and the latent water it contains when compressed in a tire, can actually be less linear in expansion than nitrogen so it was a bad example.

As for Ideal Gas Law. An Ideal Gas is a hypothetical gas.  It does not truly exist.   Real world gasses only exist in ideal gas state at a specific pressure and temperature range, not over their entire range of pressures and temperatures. 

I feel like I'm talking in circles.  Not sure I helped here.  Let's just forget the whole nitrogen thing as it really doesn't apply that much to OP's original proposal.

[hackberry jake]

Another thing I didn't take into account was the expansion of the tank. If the steel in the tank expands, the overall maximum pressure would be less.