anybody here know anything about heat pumps or A/C?

[eternalfrost]

i am a physics grad student currently and i have been kicking around an idea in my head a bit for the past few months. ive been interested in stirling engines. they are basically a piston engine that run off heat directly rather then combustion.

i recently stumbled across some info on home heating systems that use underground pipes to pull thermal energy out of the ground. basically acting like an A/C or refrigerator in reverse. basically, i would like to link such a heat pump to a stirling engine and use it to generate power.

so i am wondering if anyone here knows what it would take to convert an old A/C window unit into a heater. ideally it would take energy from a source about room temperature and boost it up to as hot as possible (higher temps=higher effiency for stirlings).


anyways, this isnt a super serious project more just tinkering in the basement.

or else anyone know any good sources of info for this? i know that at least a few of you are plumbers/construction types

[Butch50]

Basically, just turn it around. As you know a heat pump is an AC unit that faces inward during the winter and outward during the summer, simply speaking. You pull the heat out of the house in the summer and dump it outside. You feel the hot exahust at the condenser.

[eternalfrost]

yes i know this. i guess i didnt word it so great above...

i standard A/C is built to move lots of thermal energy, but does so with a fairly low temperature difference between the inside and outside heat exchangers. you can imagine it as a foot wide pipe with a slow rate of flow through it. sure it moves alot of water (heat) but thats not what i want for this application.

what i want is to maximize the hot heat exchangers temperature. absorb thermal energy from the ground etc at room temperature, run it through the compressor and get it as HOT as i can, at least a few hundred degrees.

a stirling engine basically has a heat input and a heat output. you want to create as big a difference in temperature between them as you can.

so while i am aiming to be moving heat, it is a bit different goals then a plain A/C

imagine using the hot side of your A/C as a stove to cook on. thats the type of temps that would be ideal. im not sure if this is even possible with A/C stype heat pumps but thats what im looking into.

[Dnderhead]

Just build the condenser in the pot but a window air conditioner whould not be big enough to do the job.as most between 5000-8000 BTU some 12000 BTU maybe a hole house air?

[Butch50]

I may not be following you, correct me if I am not. You want to remove heat from a source, then boost that heat up higher. You can certainly do that, but you can not boost it up higher without a commensurate amount of energy being inserted into the loop. Using an AC to do that, if I understand your purpose, will be an inefficient heat exchanger/booster due to the energy load required to run the compressor.

The amount of energy you put into your stirling engine is going to be more than you recover from it, in other words. If you are after a more energy efficient system, you might consider thinking of taking the, I assume warm, air from your geothermal source and boosting it up with the least costly form of fuel you have available. It could be electric heat strips, or gas burners, or what have you.

Ultimately I believe that stirling engines will always run on an energy deficit, simply because the laws of physics don't allow us to create new energy, only convert energy. There is always a loss of energy in the conversion, and you have to manufacture the energy for the stirlin somehow.

The exception would be if you can harness a stirling engine to a natural energy differential that already exists - the conversion loss is still there but you don't have to pay for it, so to speak. One energy differential that exists is a thermocline in a body of water. There can be several degrees difference in only a matter of inches when the conditions are right. Another if you live in a cold climate is winter ice. Store it and use it all year with insulation.

[eternalfrost]

the laws of physics don't allow us to create new energy, only convert energy. There is always a loss of energy in the conversion, and you have to manufacture the energy for the stirlin somehow.

you are right in saying this. let me try to explain the design a bit clearer. i am sure it will work, but i am not sure how much usuable power could be produced, it will depend on the efficiencys of the key parts.

we start with some heat source. the thermal mass of the earth works as a basically unlimited source. cold, low pressure vapor refrigerant is pumped through a heat exchanger in the earth. the now warm(absorbed energy) vapor heads back up and is compressed. this turns it into a liquid and boosts its temperature according to ideal gas laws. at this point we must put in energy to the compressor. howaver an important point to see is this power input is NOT what is raising the temp of the refrigerant. the refrigerant contains a constant ammount of thermal energy but is 'compressed' into a smaller volume which raises the temperature( again see ideal gas law). this HOT refrigerant is run through a heat exchanger, energy spontaneously flows from the hot liquid to the stirling engine. the now cool liquid flows through the de-pressurizing valve, which turns it back to a vapor and drops the temperature(gas law) ready to go back into the ground and absorb more energy from the warmer ground

ok so far the above simply describes how any normal refrigerator or a/c or heat pump works...

i wont go into the specifics of how stirling engines work here but put simply. you heat the chamber which raises the temperature and pressure and pushes the piston out. you then cool the chamber which lowers the pressure. you then INPUT WORK to compress the (cold) piston back to starting point. the important point is that the work you must put in is less the the work coming out because of the temperature differences during the parts of the cycle. this is how a normal engine works in you car. each cycle you must INPUT WORK to compress the piston and reset it.


now if all goes well, the same should apply to the whole system. yes, you have to input work to pump the heat from the cool (room temp) ground to the hot stirling engine input. but the energy you are pulling up is more then what it takes to pull it there. the energy produced by the stirling generator neglecting effiency losses will be greater then the power to run the generator.

systems like this are an estblished technology but are currently only used to heat air in houses. these systems have effiencies of 300-500% meaning if you put say 1000 joules into your heat pump compressor, you get 3000-5000 joules of heat coming out of your radiator.
==============================================

its like saying burning coal wont work because you first have to put work in to dig it out of the ground.

no energy is being created, if you monitored the ground temperature, it would cool down slightly. but because of its sheet size it wont run out. just like you cant make it cold outside by turning your a/c around backwards

[carlos castenada]

hi,

i'm in commercial / industrial air con/ heating. you wouldn't be able to use anykind of air con unit for this purpose, as the temps at the condensor are not in the range that you want.
the liquid that flows from the condensor needs to be sub cooled for the unit to cool, (below its saturation temp for its pressure).

most units have safety controls for excessive pressure and or temp, which aren't designed for any higher. also if the discharge pressure and temp are raised
this would also have an effect on the suction pressure. higher suction pressures on a unit that it isn't designed for mean less effiency and compressor failure.

the heat given off at the condensor is mostly the heat given to the system at the evaporator and the heat of compression. in theory if it could be used you would just be adding heat to the evaporator then harvesting this heat from the condensor. i personally couldn't see the point as you could just add heat to your unit
and miss out the middle man.

cheers

[decoy]

perhaps you need to look into the basics of refrigiration and gases.

an Airconditioner or refrigerator will do exactly what you want, you may need to alter the gas used to achive the desired result you are looking for.
airconditioners are already used to heat cold water in cold enviroments, admitedly there more efficent were the outside temperature is warmer.

the use of a liquid or gas with a suitable boiling point is the key, then subject it to vacume or pressure.. to lower or increase its boiling point to suit your needs.

here is a link on a thread that i replied to that you might find intresting on how to make a simple heatpipe.
http://homedistiller.org/forum/viewtopi ... =17&t=8185

anyway this is the principle used in early refrigeration utelising amonia as the heat transfer gas, and today heat pipes are used in laptops, computers water heating and all sorts of other heat transfer applications.





here are a few couple of links.
http://www.answers.com/topic/heat-pipe

http://www.cotsjournalonline.com/home/a ... ?id=100367

you might also want to look into peltier transistors

google is your friend.
cheers.

[eternalfrost]

thanks for the quick reply decoy. im confident that the theory behind it should work, ive talked it over with a few professors. the problems with making it actually work is finding a heat pump that could actually run at those temps and optimizing a stirling engine and generator to work with that temp.

a stirling engine can run off very low temperature differences. they have demo models that will run off the difference between your hand and the room temp when you hold it. however the power output on these is verrryyyyy low, like 1/4 W. the output power goes way up as the temperture difference between the heat sink and source increases. to get anything even remotely useful you want at least a few hundred degree difference

[Hack]

Here's my wild idea. Maybe instead of a sterling engine, try a steam engine or turbine, but instead of water use something with a lower boiling point that you can recapture and reuse. A steam engine has tremendous torque and is perfect for generating power. Using something with a lower boiling makes it easier to use the temperature range of your ground loop heat pump.

I also wonder whether the amount of power generated will be enough to run the heat pump let alone produce excess power.

[HookLine]

Using an AC to do that, if I understand your purpose, will be an inefficient heat exchanger/booster due to the energy load required to run the compressor.

IIRC, reverse-cycle air-cons used in the heating mode (the reverse-cycle part) are actually probably the most efficient heat pumps available. They will give you back 2-3 times the energy (as heat) that is required to run them (as electricity). It seems impossible, but it is due to the fact that they are moving existing heat, not making it.

[eternalfrost]

IIRC, reverse-cycle air-cons used in the heating mode (the reverse-cycle part) are actually probably the most efficient heat pumps available. They will give you back 2-3 times the energy (as heat) that is required to run them (as electricity). It seems impossible, but it is due to the fact that they are moving existing heat, not making it.

exactly hook, did some more researching tonight and it seems there are working fluids that can put out up to 150C output temps in a closed-cycle comression loop like this. only problem is it looks like they are now banned or being phased out in the US due to the CFC's/ozone depletion.

[eternalfrost]

IIRC, reverse-cycle air-cons used in the heating mode (the reverse-cycle part) are actually probably the most efficient heat pumps available. They will give you back 2-3 times the energy (as heat) that is required to run them (as electricity). It seems impossible, but it is due to the fact that they are moving existing heat, not making it.

exactly hook. in the world of pure theory, the energy emitted by the pump will put out the amount of energy absorbed from the exchanger plus the amount of energy inputted by the compressor. stirling engines are also interesting because they are the only physical engine that can theoretically reach the Carnot cycle (100% efficiency) and are much more efficient then standard electric or internal combustion engines.

in the real world, heat pumps can give you 2-10 times the ammount of energy back depending on the type and design. stirling engines typically run around 25-40% efficiency

did some more researching tonight and it seems there are working fluids that can put out up to 150C output temps in a closed-cycle comression loop like this. only problem is it looks like they are now banned or being phased out in the US due to the CFC's/ozone depletion.

[Puma]

Humm, I don't see using a heat pump or A/C unit for this. The energy you make will be less than what you use to make it. I know what you are saying about the geo-thermic system. The temps under the ground depend on were you live. For me that's 52*f year round. That's good for heating (using a heat pump) when it is cooler than 52* outside. Anyway, there is a much easier and more effective way to do this.

Look into evacuated solar tubes. They are mucho efficiant (96%) and can produce very high temp (350*f+) regardless of outside temps. Just a thought. Or for a large scale, parabolic miror or troff and you have all the heat you will ever need.

The solar tubes work even on clowdy days.

Sun=free energy

[Hawke]

I worked in a store that had a groundwater system. Was hot in summer and cool in winter. Really don't think it would produce enough excess heat to run a sterling.

[eternalfrost]

ive been doing alot more studying all day and i think ive found why this system, while feasible theoretically wouldnt work as well as i thought in real life.

the heart of the system is the idea that geo-heat pumps can produce many times more thermal heat then the electrical heat you put into them, then capturing a portion of this heat through a stirling generator to produce an excess.

the problem is that the two halves of the system need exact opposite conditions to run at top efficiency. heat pumps work amazingly well, but only really produce over 100% when the temperature differential is very small and become more and more inefficient as the differential increases. stirling on the other hand love huge differentials.

both systems will run in the others environment, but very inefficiently. running at high temperatures will make the generator happy and very efficient but will create huge losses in the pump. running it cool will pull up huge ammounts of energy from the ground, but its not in a state that will run the stirling very well.

so thats the reason why it simply isnt a 'free energy' save the world idea i guess.

i havent really gone through and crunched the numbers so im still not sure if you could actually create usuable power with this but even if you could my guess is it wouldnt be much.

im stil not giving up on it quite yet tho, if you could make one that could simply power your home, or even half of your needs it would be amazing. the beauty about it is it would run 24/7 exactly the same year round. it would be totally renewable without the headaches of solar or wind conditions and just sit in your basement like a hot water heater

[Butch50]

Another way to look at it is to reverse the process. Decide what apparatus you are going to drive with the stirling engine, then figure out how many ftlbs of torque you will need to drive it. Now take that information and determine the size of the stirling engine that can provide that kind of torque. Thermal differential is a relatively "gentle" power source compared to steam or internal combustion. To get the kind of torque that say a 10 hp steam engine can deliver, you would probably need a stirling engine the size of a house? I am just guessing there, but intuitively it seems that the stirling engine is going to have to be enormous, or you are going to have to provide a heat differential that is a couple of orders of magnitude.

One possible way around that would be to couple a moderately large stirling engine to a large heavy precisely balanced fly wheel. If you could get it up to speed and then have the stirling take over the drive to deliver enough torque to overcome the resistance to the wheel from the load and friction, etc....you might get a smaller engine to work. You would either need a booster engine to start it and maybe occasionally bump it up a bit, or you would have to have a long gradual speed build up. Flywheels are interesting energy delivery systems in and of themselves, that aren't used nearly as much as you would think.