OT- Stirling engine calculations

[Doug_Adam]

Tom

I became interested in Stirling engines once, for about one day.
The reason I became intersted was the same as your reason, using up waste heat. The reason that I then became disinterested was after some quick sums, Stirling engines in this situation will not work. The reason is Carnot efficiency.
The maximum theoretical efficiency of a heat engine (of which a Stirling is only one type) is:
(Temp_Source - Temp_Ambient)/Temp_Source
All units in absolute temperature.
Imagine your heat source is hot enough to boil water at 373K, and your ambient temperature is room temperature at 293K. Then you will get an effeciency of 21%.
Assuming your heat source puts out 200W, at the very best (and no one has done it yet) you could get an entire 42W of power out of your system. More realistically, this will be 10W at my guess.

In other words, it is a good toy for the kiddies.

The only Stirling engines that I know that work in a realistic situation burn diesel at high temperatures. They are generally only marginally more efficient that a Diesel engine, have much lower maintenance requirements (few moving parts), take much longer to start up (external combustion engines need a long warm up period), and are much more expensive. The expense is, however, only due to their very low production volumes.

IMHO, a Stirling would be fun to build, but not anything you could make or save money on.

 

[rsdoran]

Where does the power go if not to heat? The all energy a system uses eventually gets converted to heat unless it is stored as some form of potential energy.

I do not believe you stated that Peter.

You are saying that all power used in compputers is converted to heat? In other words no devices in a computer actually consume power to operate...or the power they use can be measured by the heat they produce?

If that is what you are saying then right or wrong I do not think I will listen to you again on any subject. All energy used in a computer is not eventually converted to heat, heat is a secondary product and believe it or not ALL computers develop/use kinetic energy...think about it.

Of all people, I do not believe YOU stated that. I am disillusioned.

 

[MarkTTU]

Unless you have found a frictionless environment, kinetic energy quickly gets converted to heat.

After thinking on this one I'd like to retract my statement that you are correct. At the very least in a typical computer some of the energy consumed goes into storing data on magnetic media. Storage on magnetic media is done by placing a potential charge in a particular position on the media; all of those potential charges add up to some amount of stored energy on the magnetic media.

 

[Doug_Adam]

I do not believe you stated that Peter.

You are saying that all power used in compputers is converted to heat? In other words no devices in a computer actually consume power to operate...or the power they use can be measured by the heat they produce?

If that is what you are saying then right or wrong I do not think I will listen to you again on any subject. All energy used in a computer is not eventually converted to heat, heat is a secondary product and believe it or not ALL computers develop/use kinetic energy...think about it.

Of all people, I do not believe YOU stated that. I am disillusioned.

He is correct. All energy is destined to become low grade heat in the long term. Heat is a form of kenetic energy. For anything to do work, waste heat must be generated. There is no avoiding it. In the process of convering the energy to heat, we can do useful things with it, but it will soon become heat.

 

[Peter Nachtwey]

Did anybody repeal the first law of thermodynamics?

I do not believe you stated that Peter.

You should know me by know. I wouldn't say something like that if I could find something to back me up. This is just a variation of where does all the energy generated by a submarine's reactor go?

You are saying that all power used in compputers is converted to heat?

Yes, where else did it go? Waiting....

In other words no devices in a computer actually consume power to operate...

The devices do not really 'consume' power. They just convert energy from one form to another. Power is an energy rate. It is better to just think in terms of energy because I have never seen the term kinetic power.

or the power they use can be measured by the heat they produce?

Yes, either directly or indirctly.
http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookEner1.html
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

A laptop can store some of the energy it uses in a battery as potential energy. The energy that moves the hard disk is converted into kinetic energy. However, eventually all this energy gets converted to heat. If you turn on your computer and put it in an insolated box, what will happen?

 

[kamenges]

Originally posted by marksji:
After thinking on this one I'd like to retract my statement that you are correct. At the very least in a typical computer some of the energy consumed goes into storing data on magnetic media. Storage on magnetic media is done by placing a potential charge in a particular position on the media; all of those potential charges add up to some amount of stored energy on the magnetic media.

Technically I don't think you are 'creating' a magnetic field. You are establishing the alignment of something that is already there but is simply not coherently aligned. This does take energy but that energy is immediately converted as part of the alignment process.

Keith

 

[MarkTTU]

Technically I don't think you are 'creating' a magnetic field. You are establishing the alignment of something that is already there but is simply not coherently aligned. This does take energy but that energy is immediately converted as part of the alignment process.

Keith

In magnetic recording (audio tape, VHS, hard drive, etc) you magnetize or demagnetize small areas of ferromagnetic material. If you take an object that is not magnetized and magnetize it isn't it technically storing some small amount of potential energy?

 

[kamenges]

Originally posted by marksji:

If you take an object that is not magnetized and magnetize it isn't it technically storing some small amount of potential energy?

I don't believe so. I don't think energy storage occurs until you 'load' two fields against each other. For example, if you allow two bar magnets to align North to South and then try to pull them apart, you will store potential energy in the magnetic field as you pull them apart. However, I don't think we consider the bar magnet to contain a specific amount of potential energy simply because it is a bar magnet.

Keith

 

[Tom Jenkins]

Peter is absolutely correct. All of the power used by a PC is eventually converted to heat.

For all practical purposes there are three "kinds" of energy involved. First is kinetic energy - the air moved by the fan for example. This is converted to heat as friction slows down the air.

Second is potential energy. As long as it stays stored, say as a magnetic field in the hard disk, it is potential energy and not heat. As soon as it is converted it turns into heat.

Third is internal energy, which shows up directly as sensible (felt) heat.

All of the energy created by a power plant, nuclear or otherwise, ends up as heat.

 

[gbradley]

All of the energy created by a power plant, nuclear or otherwise, ends up as heat.

What about the energy in the power plant in my truck?
I know that it generates heat, that's why I have a radiator.
But something gets me to work each morning.
It seems like it's not just creating heat, but actually doing work by moving me from point A to point B.

 

[kamenges]

Originally posted by gbradley:

It seems like it's not just creating heat, but actually doing work by moving me from point A to point B.

You are correct. The truck is doing work. However, the energy in that work is either being converted to heat due to some type of friction (tires to road, body to air, displaced air to air, etc) OR is being converted to potential energy if you are increasing your net elevation. But in reality anything that isn't converted to potential energy or stored in the battery is burned up as heat.

It really isn't any different than sliding a stone block along the ground. That act requires that work be performed. In this case the energy is converted from kinetic energy to heat energy due to the friction with the ground.

Keith

 

[Andybr]

I absolutely agree that all of the energy will end up as heat but surely the point here is not what happens to it but where it happens. If the energy in the cooling air stream is converted to heat once it has left the computer then the end result is to cool the inside of the computer which is exactly what was intended.

Andybr

 

[gbradley]

the energy is converted from kinetic energy to heat energy due to the friction with the ground.

I know. You are right. The heat dissipates over such a large area that you hardly notice it. (unless you are a brake pad)

 

[Rod]

So my three computers are 'adding' to the final state of entrophy?

Rod

 

[Tom Jenkins]

I absolutely agree that all of the energy will end up as heat but surely the point here is not what happens to it but where it happens. If the energy in the cooling air stream is converted to heat once it has left the computer then the end result is to cool the inside of the computer which is exactly what was intended.

Andybr

You're right that the point is what happens to the heat. The fan simply moves it from where you don't want it to a place you don't care about. If you use the low grade heat to power a Stirling engine you can use it to do other work, like run another fan or an air conditioner or whatever, depending on the device connected to the engine. Then that energy eventually turns up as more heat someplace else.

That's all engineers do - they make the energy do something they want done during it's treck back to equilibrium with its environment. There are only two sources of energy for the entire earth - geo-thermal and solar. Solar energy makes the wind blow. Solar energy is converted to plant matter and extracted as bio-fuel or stored over eons as fossil fuel. Geo-thermal takes heat from the earth's core, left over from the amorphous interstellar mass that congealed into our planet, and moves it to the atmosphere or the waters, doing work we want on its trip.