How can we use heat pumps to generate electrical energy efficiently?

[amongiello]

Ok I remember watching a video when I was twelve of this guy who used basically a refrigerator in reverse to create electrical energy from heat energy in outside air to power a house. Freon was run through black heat collecting panels, then through a compressor to make it hot. The heat energy was extracted efficiently because of the temperature difference and then the freon was decompressed therefore cooling it and then it was run through the panels and the cycle was repeated. The initial energy used to run the compressor was less then the electrical energy being produced from the heat.

Would anyone be able to discredit this idea? What technology is missing to make this work? Can you point me to the equations that are involed in making this system whether or not the net electrical output is negative?

Anyone out there addicted to the thermodynamics enough to answer this one? :)

Quick note: I've seen threads about this before and the discussion quickly went to "perpetual motion," and "free energy." This is nothing of the sort in theory. Obviously the energy is simply the conversion of heat energy from the air being converted to electrical energy. The energy is by no means "free".

Hope to hear from you soon.

 

[ZapperZ]

Er... I don't get it.

In intro physics, don't we all learn that a refrigerator is simply a heat pump run in reverse? What am I missing here that is making this so special?

Zz.

 

[cesiumfrog]

Yes, you can run a refridgerator in reverse to generate electricity. The catch is that you need one end of the heat pump to be somewhere hot, and the other end needs to be somewhere cold. As you generate electricity, the hot end cools down and the cold end warms up. Once both ends are the same temperature, you can't generate any more electricity.

This is why it isn't so useful in the situation you describe.. the house would just warm up to the same temperature as outside, and then you can't generate any more power (eg. for air conditioning).

 

[russ_watters]

Someone started a thread like this in engineering a week or so ago, and as I said then, I don't see how it can work. There isn't anywhere to extract mechanical/electrical energy from. The compressor isn't a turbine and even if it was, there wouldn't be anything moving the refrigerant around.

A steam engine has a turbine and a pump - the turbine extracts mechanical energy and the pump puts some back in. An air conditioner or heat pump only has one mechanical device: the pump. No turbine. Instead, it has a throttling valve or expansion valve.

Now the OP isn't very specific, but it is possible to to use the waste heat of a refrigeration or heat pump cycle to run another energy producing thermodynamic cycle. But if you calculate your efficiencies, you will find that it isn't possible to generate excess electricity that way.

 

[russ_watters]

Er... I don't get it.

In intro physics, don't we all learn that a refrigerator is simply a heat pump run in reverse?

Well, not quite. It is often said that way (and I typed-it into my post first...), but if that were literally true, you'd have your refrigerant running backwards through the compressor, which isn't possible. In reality, a heat pump has valves that just reverse the flow through the coils changing which one gets hot and which gest cold.

An a/c unit or heat pump is a carnot cycle and running one way moves heat and the other generates mechanical energy. But making each actually run requires different physical parts.

If a Carnot machine cycles around the path clockwise, a high-temperature isothermal expansion from A to B, an adiabatic expansion cooling down from B to C, a low-temperature isothermal compression from C to D, and finally an adiabatic compression warming up from D to A, it functions as a heat engine, removing energy from the high-temperature reservoir as heat, transforming a portion of that energy to useful mechanical work (the enclosed area) done on the external world, and ejecting the remainder of the energy as waste heat to the low-temperature reservoir.

If a Carnot machine is driven (by an external agency, such as a motor) around the cycle counter clockwise, an adiabatic expansion cooling down from A to D, a low-temperature isothermal expansion from D to C, an adiabatic compression warming up from C to B, and finally a high temperature isothermal compression from B to A, then it functions as either a refrigerator or a heat pump, depending on whether removing heat from the low-temperature reservoir or adding heat to the high-temperature reservoir is of primary interest. The mechanical energy required to force the machine around the cycle is the work done on the machine, the area enclosed.

http://oak.cats.ohiou.edu/~piccard/phys202/carnot/carnot.html

 

[cesiumfrog]

Russ, it's a good point that a fridge would need some modification to work anything like a steam engine. But you shouldn't need a second "mechanical device".

If the compressor can work like a turbine, and if the cold reservoir is higher than the hot heat-exchanger, then it should work using gravity (same principle as a hydroelectric dam).

 

[amongiello]

I don't think it is impossible to extract electrical energy from heat energy, I do not believe there are any thermodynamic laws that go against that. None of the ones stated up there have discredited this idea, if you think they are I've probably not described it well enough. The real question is, and I was hoping for someone involved in heat pump design to help me get started on, is the problem of the efficiencies. At what temperatures would this actually work? Maybe better is someone who works on reclaiming heat energy by turning it into electrical. What kind of efficiencies for heat energy in a fluid being turned into electrical energy are being reached today?

To discredit the idea you would need to show somehow that the work used to gather the heat energy would always be more than the heat energy gathered. I'm not sure this is the case. Or to disprove the idea you'd have to do some sort of proof showing that compressing a fluid to make the temperature difference will always require more energy than the amount that can be gained from extracting energy from it. But I think the COP's of heat pumps shows this isn't the case and obviously why they are used. Now if you aren't trying to heat a house but rather you insulate the energy gathering system so that as much of it as possible turns into electrical energy, what do we get? Something that almost works or something that isn't even close?

 

[amongiello]

And Cesiumfrog, heat pumps use colder than inside air to make the warmer inside house air even warmer. The temperature could be serveral hundred degrees different before the heat energy you got out was simply the energy put in by the electricity. Right? How else do you explain COP's?

 

[amongiello]

And Russ, your post was exactly what i was looking for, but if that is the case, are geothermal powerplants a myth? If they arn't, what is to say that with enough research and engineering skill, you couldn't theoretically extract the huge amounts of energy that exists in the ambient air around us. I mean geez how much energy does it take to change the temperature of 30000 cubic feet of air by 10 degrees? I haven't been in that class for a long time but I'm pretty sure the same amount of energy would move a car pretty far ;).

 

[russ_watters]

Russ, it's a good point that a fridge would need some modification to work anything like a steam engine. But you shouldn't need a second "mechanical device".

If the compressor can work like a turbine, and if the cold reservoir is higher than the hot heat-exchanger, then it should work using gravity (same principle as a hydroelectric dam).

That's just not how heat engines work, cesiumfrog. You can't power a turbine with gravity unless you have a large difference in height to create the pressure needed to drive it. Heat engines don't do that because it isn't practical: they use pumps to create the pressure needed to push the working fluid through the cycle.

The only devices that I'm aware of that use passive circulation of refrigerant are heat pipe heat exchangers. They circulate slowly because the gravitational potential energy is so low - there certainly isn't enough of it to put a turbine into the loop.

 

[russ_watters]

And Russ, your post was exactly what i was looking for, but if that is the case, are geothermal powerplants a myth? If they arn't, what is to say that with enough research and engineering skill, you couldn't theoretically extract the huge amounts of energy that exists in the ambient air around us.

You really need to start at the beginning and learn just what thermodynamics is, because everything in your posts is just misunderstandings of high-school level basics.

Thermodynamics utilizes a temperature difference to provide energy to drive a turbine (or vice versa). It is like the hydro dam analogy: the height difference is what gives you the pressure needed to drive the turbine. You can't put a hydro dam in the middle of the ocean, because there is nothing to drive the flow. Similarly, you can't extract heat energy from the air because it needs somewhere to go. Geothermal plants take heat energy from the ground and discharge it into the air - and the temperature difference is what gives you the energy transfer.

Also in one of your other posts is a pretty concisely stated direct violation of the first law of thermodynamics. You can't get something for nothing. You can't get more total energy output than input.

 

[russ_watters]

For some of the more specific questions...

I don't think it is impossible to extract electrical energy from heat energy, I do not believe there are any thermodynamic laws that go against that.

No one said you can't extract mechanical/electrical energy from heat. That's the whole point of thermodynamics. What you can't do is create heat energy with electrical energy and then use that heat energy to create electrical energy. That's what a solar-powered flashlight pointed at a mirror would do - and it should be obvious why it doesn't work.

The real question is, and I was hoping for someone involved in heat pump design to help me get started on, is the problem of the efficiencies. At what temperatures would this actually work?

Heat engines in theory work at any temperature and the efficiency is a function of temperature. Here's the equation and a calculator: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/carnot.html

Heat pump efficiency is the inverse of the carnot efficiency, so if you use a heat pump to drive another thermodynamic cycle, your efficiencies are the inverses of each other and always cancel out to a maximum of 1.0 (100%). For example, if you use a carnot engine with temperatures of 300 and 450K (fairly typical), your efficiency is .333. If you use a heat pump to create the 300-450K delta-T, the COP is 3.0. and 3*.3333 = 1.0.

Maybe better is someone who works on reclaiming heat energy by turning it into electrical. What kind of efficiencies for heat energy in a fluid being turned into electrical energy are being reached today?

That's a vague question because of the host of possible ways to do it, but a good single-cycle steam engine gives you somewhere around 40% efficiency.

To discredit the idea you would need to show somehow that the work used to gather the heat energy would always be more than the heat energy gathered.

That's the first law of thermodynamics and can be seen easily enough in the Carnot efficiency equation by putting two cycles back to back as I said above Heck, it calculates the efficiencies for you. Let's use

Or to disprove the idea you'd have to do some sort of proof showing that compressing a fluid to make the temperature difference will always require more energy than the amount that can be gained from extracting energy from it.

That is a direct contradiction of the 1st law of thermodynamics.

But I think the COP's of heat pumps shows this isn't the case and obviously why they are used.

Oooooooooooooooooohhhhhhhhhhhhhhhhhhhhhhhhhhhh... That's what this is about. You don't understand the concept of COP. A COP is not free energy. It is not >100% efficiency (which is why they don't call it efficiency). Heat pumps transport heat, they don't generate it. Just like a pump pumping water uphill against gravity, a heat pump transports heat against a temperature difference. And just like the higher you pump the water, the more energy is required, the bigger you make the temperature difference in a heat pump, the more energy is required. Occasionally, people don't realize that you can't pump water uphill, then have it fall back down to drive a turbine, but a lot of people don't realize that heat pumps do exactly the same thing and can't be used to create free energy for the same reason.

 

[Andrew Mason]

You appear to be saying that heat can be collected from solar collectors on the roof and used to drive a heat engine. Since the heat in the panels would be greater than the outside ambient heat, this would provide a steady temperature difference that would drive a heat engine (it would make no sense to output the heat engine to the inside of the house). The maximum efficiency, as has been pointed out, would be the efficiency of a Carnot engine operating between those two temperatures. That efficiency increases as the higher temperature increases.

But, you also suggest that if you use a compressor to compress the freon gas to increase its temperature you will get a greater increase in energy from the heat engine than you put into the compressor. This is not correct. That would be a violation of the second law of thermodynamics.

AM

 

[zevoenli]

So, if the liquid Freon was gasified by the sun in a solar panel, within the Carnot loop, and allowed to expand past turbine blades to a liquid form, the work available at the output shaft would be the same amount of energy required to chill those plates to the temperature of an identical system with compressor, not turbine and the panels in the shade.
This would express as equall, yes or no?

 

[russ_watters]

You still need a compressor to move the fluid. The pressure at the front of the turbine is higher than the pressure at the back. You need a pump to overcome that pressure difference: the heat addition in the solar collector is at constant pressure. Look at the process diagrams!

Also, this part doesn't make any sense to me:

the work available at the output shaft would be the same amount of energy required to chill those plates to the temperature of an identical system with compressor

What plates?

 

[zevoenli]

Sorry, it was a bit late in the day to be attemting this, so here goes again.
As was said earlier, a reversable process is needed to achieve efficiency of 100%
A reversable Carnot engine.

IF : The solar collectors raise the pressure and temperature such that the low temperature liquid (?Freon?) entering the panels experiences a phase change as a result of the increased energy. This resultant pressure increase is then allowed to expand past a turbine.
The whole system is prevented from undergoing the same temperature increase by the placement of heat radiation coils on the return piping in the same manner that conventional air conditioning systems do to prevent backflows occurring.
The gas then experiences a reverse in the phase change after the turbine and the energy can be extracted as mechanical force at the turbine shaft, and the exhausted subsequant liquid is collected in such a manner as to be fed by gravity and temparature reduction to the input of the solar collectors to repeat the cycle.
This process is the mirror of the closed loop cooling cycle whereby power input is used to compress a gas to a liquid and absorb heat to move it elswhere, (the back of the fridge, outside, etc.).
Therefore, just as the energy required to cause a liquid to lower in temperature and gasify is used in the compressor, so can it be extracted from a high tenperature gas through a turbine which allows it to cool to a liquid after exhausting it from the fan, (perhaps in a chamber with sloping walls?).

 

[russ_watters]

IF : The solar collectors raise the pressure...

Again, the solar collector will not raise the pressure.

Solar collectors are not compressors(/pumps). They operate at constant pressure. Think about it: fluids flow from areas of high pressure to low pressure, so if the pressure was higher in the solar collector than in the area upstream of the collector (away from the turbine), the fluid would flow away from the turbine, not toward it. The reality is that since there is no obstruction to the flow of the fluid, the pressure is constant.

There is no getting around this: you need a compressor. Every thermodynamic cycle has one.

The whole system is prevented from undergoing the same temperature increase by the placement of heat radiation coils on the return piping in the same manner that conventional air conditioning systems do to prevent backflows occurring.

None of that makes any sense. There is no backflow issue: you have a pump that makes the fluid flow in only onen direction. And the temperature doesn't equalize throughout the cycle because each step in the cycle changes the temperature.

It really sounds like you don't have any idea what happens in a refrigeration cycle. Look at the diagram!

exhausted subsequant liquid is collected in such a manner as to be fed by gravity and temparature reduction to the input of the solar collectors to repeat the cycle.

More word salad - if the fluid is recirculated, it isn't exhausted. Gravity? Gravity can't circulate the fluid unless there is a decent change in elevation and even then it doesn't provide you with anywhere near enough pressure to run a thermodynamic engine. Most run at hundreds of PSI. You could build a tower to the moon and still not be able to use gravity to replace the compressor of a decent heat engine.

This process is the mirror of the closed loop cooling cycle whereby power input is used to compress a gas to a liquid and absorb heat to move it elswhere, (the back of the fridge, outside, etc.).

That would have to be in a compressor...

Therefore, just as the energy required to cause a liquid to lower in temperature and gasify is used in the compressor, so can it be extracted from a high tenperature gas through a turbine which allows it to cool to a liquid after exhausting it from the fan, (perhaps in a chamber with sloping walls?).

No. You have it backwards at best. In a steam engine, the pressure and temperature drop through the turbine - in a refrigeration cycle, the pressure and temperature drop through an expansion valve. You could replace the expansion valve with a turbine, but it sounds like you want to do that twice.

Regardless of all this minutae, what it sounds like you want to do here is pretty mundane. You aren't doing it right, but you could very easily replace the boiler of a steam cycle with a solar collector (people are working on that very idea). You wouldn't use a refrigerant as the working fluid in a solar driven heat engine because the temperature is far too high. For a large solar plant, the temperature is too high even for water - researchers are looking at using molten sodium, iirc.

 

[zevoenli]

Sorry, you lost the idea in the first line. I thought I had made it clear that the liquid in the solar collector would be heated to such a point that it underwent a phase change after exiting a built in nozzel and expanding into steam, after absorbing enough energy.

This energy could then be removed by the turbine which would indeed be above the solar collector as it is a gas turbine, the exhausted steam would then be allowed to give up it's heat (whether by air transmision or by heat transfer to a waterjacket) which would lower the temperature to the point where the opposite phase change occurs and the liquid state reforms.

This liquid would fall down to the bottom of the steam receptor attached to the turbine exhaust and circulate, with the aid of gravity as it cools to the base of the collectors to be reheated and replace the moved fluid at the high TEMPERATURE side of the nozzel that is placed within the panels below the point where the liquid reforms in the heat exchanger.

If the volumes are right and the placement of the atomising nozzel is correct, I can see no violation of the thermodynamic principles and convection current properties where pressure is a function of heat and volume.

I HAVE loked at the diagrams, made the best sense of the math as I could, I have seen cold fall and hot rise and felt the temperature increase as the velocity slows in those toilet room hand driers.

I accept that to initiate the working state of equalibrium within the system a pump may be used for a short time, however there could be engineering solutions that would shorten or lengthen passageways to enable this system to operate over a wide temperature range.

I appologise for the words without math but I never did learn the language!

 

[russ_watters]

Sorry, you lost the idea in the first line.

[shrug] Just saying it doesn't make it so. The solar collector will not raise the pressure of the working fluid and convection is simply not a powerful enough force to drive a turbine.

If you don't believe me, pick some conditions and calculate it! It isn't hard. For example:

Potential energy is mgh and and power is mgh/t. If, for example, your device raises 1kg of water per second 10m via convection (boiling it at the bottom, condensing it at the top), you'll get 1*9.8*10=98 n-m/s (98 watts) of power from it. But boiling 1kg of water requires 2,260,000 joules (per second...ie watts). So your device would take an input of 2.2 megawatts to produce an output of 98 watts (without considering if you need a condenser fan...). That's an efficiency of 0.004%. All the rest of the energy is lost to the environment at the condenser.

Much better to use a regular steam cycle with a pump/compressor.

...I have seen cold fall and hot rise and felt the temperature increase as the velocity slows in those toilet room hand driers.

I'm not sure what you are talking about there - toilet room hand dryers have electric heating coils to heat the air. Not much of anything changes as the air goes out the nozzle.

I can see no violation of the thermodynamic principles and convection current properties where pressure is a function of heat and volume.

I'm not quite sure what you are saying here, but it sounds like you think that by putting a nozzle in the cycle, you increase the pressure. That isn't the case: pressure drops through a nozzle. It is the same sort of device as a turbine, just without the extraction of mechanical work. What you'll end up with in your device is a big circuit of roughly equal and very high pressure, with water dripping down one side and steam slowly rising up the other. With convection the only means of circulation, you'll input an enormous amount of energy and release 99.996% of it to the atmosphere at the condenser.

I think you may be best served not just by looking at the diagrams of the cycle, but reading and learning the descriptions of what happens in each piece of it. You'll learn things like the fact that a boiler raises the temperature/energy of the working fluid, but does not add pressure.

 

[zevoenli]

You seem to be contradicting youself here,
"The solar collector will not raise the pressure of the working fluid"
"What you'll end up with in your device is a big circuit of roughly equal and very high pressure"
What makes the pressure rise?
"convection is simply not a powerful enough force to drive a turbine."
Where did i say i intended to use convection?
What I said was that as the fluid changed from liquid to gas by an energy input at the nozzel there is also a change in velocity relative to the increase in energy or am i wrong there also.
"If you don't believe me, pick some conditions and calculate it!"

"??Potential energy is ?mgh and and power is ?mgh/t." What is this, mass, gravity and height? if so, do the calculation for 1ml of water/s from 98 degrees C and see what you get because the volume of the turbine is all I need to fill.
Why would I want to boil 1kg of water to spin a turbine blade for a millisecond??
"All the rest of the energy is lost to the environment at the condenser."
Not all the energy held within the fluid is allowed to escape, only the energy to allow it to condense and return to the nozzle through the panel, heating up as it goes.
"toilet room hand dryers have electric heating coils to heat the air. Not much of anything changes as the air goes out the nozzle." Except with changing atmospheric conditions such as humidity and temperature of the incoming air!
"by putting a nozzle in the cycle, you increase the pressure." No, the volume increases with the temperature change as the steam is allowed to expand past the turbine blades before reforming into a liuid (low pressure relative to the gas.
"What you'll end up with in your device is a big circuit of roughly equal and very high pressure, with water dripping down one side and steam slowly rising up the other. With convection the only means of circulation, you'll input an enormous amount of energy and release 99.996% of it to the atmosphere at the condenser."
I am not trying to boil the water at the bottom, and the convection occurs more vigorously toward the nozzle as the fluid is heated back up to steam.
"You'll learn things like the fact that a boiler raises the temperature/energy of the working fluid, but does not add pressure."
This is only the case if the other side of the circuit is open to the atmosphere isn't it, ie; the engine exhaust. Or else pressure would continue to build as more and more heat is absorbed by the liquid within the boiler at the contained volume leading to an increse of pressure or is that the wrong interpritation of the p/v graph?

 

[Karl007]

Hi,

Effectively this is a solar powered system. Whereby the 'vibrating' energy of the air molecules, as caused by the sun, is condensed. Think of it like a magnifying glass focusing the sun rays. This is just an indirect, mechanical way of doing the same thing.

There is no free energy here and no laws of thermodynamics have been broken (the energy comes from the great big nuclear fusion reactor in the sky); its just an unconventional and on the cosmic scale of things, a very inefficient method of extracting useful energy form solar power. However, this is more than enough for our worldly needs and is an otherwise unused, reliable and readily available energy source.

Hope that helps

Kind Regards

 

[russ_watters]

Welcome to PF.

This is a three year old thread. No need to dig it up.

Locked.