The Reversible engine

[Tom Booth]

Playing the conspiracy card is fruitless in this case. I am merely here as a friend and am only attempting to help.

"Apparent vested interest" does not equal "conspiracy theory'.

Your so-called "help" is almost invariably negative, contentious, insulting, derogatory and more often than not a diversion from the subject at hand relating to some trivial nonsense, and a complete waste of time.

You virtually never participate in a conversation constructively in any way. Your basically a "Troll" and a heckler who apparently has nothing better to do but make a display of his pompous know it all, egoistic attitude.

Do me a favor and please find someone else to "help".

[Tom Booth]

(...)

Phillips has a video demonstrating that a single machine can run as a motor, generator, Stirling Heat Engine, Stirling Cold Engine, Stirling Heat Pump, and, Stirling Cryo-Cooler. I appreciate that the fact is amazing. It also proves that there's no difference between a heat pump or engine and that the direction of rotation changes it to an AC/Cryo-Cooler and or Cold Engine. It doesn't matter what it is called, Stirling Machines are capable of all four modes.

The colloquial term "heat pump" describes a home unit used to bring in heat from a colder outside and release it to a hotter inside. Units are also available for both that and air conditioning. Same unit, just has extra hardware to reverse the direction of heat travel. Typically those units use the irreversible Linde Joule-Thompson process, so can't run as an engine.

Using a Stirling Engine to heat a building or cool computers is about as stupid as it sounds. Engines require high delta-T. Heat pumps require low delta-T. Trying to run an engine from computer heat while trying to cool the computer, would lead to the conundrum of the engine cooling it's own hot plate making it less efficient. It also makes it less powerful, but nobody talks about that. People have a fixation on efficiency.

If the temperature rises to help the engine the computers get fried. It would be an evil balance at best. The same is true for heating a space with the cold side. It's the cold side, dummies.

Now using a Stirling Machine as a cryo-Cooler to cool computers would work great. Throw out the irreversible Linde AC. Yes. Stirling Cryo-Coolers are already a profitable business. A Stirling redesigned to work for residential heat pumping and cooling at a much more efficient smaller delta-T would potentially be viable. They could possibly be more efficient and robust, last longer. They probably would be much more expensive. Especially the first ones.

There are several, to put it gently, misleading statements or assertions in your above essay.

A Stirling engine itself does not "run as a motor, generator".

Also a Stirling heat engine cannot run as a "Stirling cold engine". If you run it "on cold" (below ambient) it is really just still running on heat in the same way that it always runs on heat.

Furthermore there is nothing particularly significant about the direction of rotation, forward or reverse. What matters is the phase angle which can be changed.

I'm sure you already know a Stirling engine will run with the heat on top or the heat in the bottom. That doesn't really change anything about the way it operates.

So you really only have two modes of operation.

It is either driving itself on a heat source as a heat engine or it is being driven by a motor externally as a heat pump.

You confuse things by making two functions into four then throw in motor and generator but in reality the single machine either drives itself using a heat source or it is driven by an external motor.

And no, your attempt at confusing matters does not prove "that there's no difference between a heat pump or engine and that the direction of rotation changes".

A Stirling engine consumes heat and outputs work, regardless of its direction of rotation, or to which side heat is applied.

A Stirling cooler driven by a motor does not need to consume heat in order to operate, the external motor takes over that function. So it functions better as a heat pump and moves heat and does not need to drive itself or do any external work. And it does not matter which way it is rotating when driven as a heat pump except for which way the heat is moved.

So you have just an engine running forward or backward or just a heat pump running forward or backward.

The heat pump, however, is not "a heat engine running in reverse". That's pure baloney. Delusional Carnot "reversible engine" nonsense.

[Tom Booth]

I might make a few additional comments about this somewhat confusing video.

https://youtu.be/GFfMruoRMGo?si=-38CyhGy_w-RzbOb

There are four "ways of operation" mentioned in all.

Two are driven by a motor

1) driven "forward" by the electric motor, the narrator calls the engine a "refrigerator"
The temperature of the head eventually drops to cryogenic liquid air temperature (-320°F) and liquid air can be seen trickling down off the head and boiling off. The heat is being moved down to the cooling jacket and carried away. Work from the electric motor is being used to move heat from the head to the cooling water jacket.

2) driven in "reverse" by the electric motor, the narrator calls the engine a "heat pump".
The temperature of the head reaches a glowing red hot 1290°F (700°C). Note that the heat is being moved OUT from the water circulating through the water jacket.

The engine is also twice shown running with the electric motor switched off.

1) Running as a "Cold gas ENGINE" on the residual COLD (-320°F but warming quickly) retained in the head (after running as a "refrigerator"/cryocooler) Actually the heat to run the engine is coming from the relatively hot (ambient temperature) water in the lower water jacket. The engine is consuming this heat from the circulating water jacket to produce work output.

2) Running as a "Hot gas ENGINE on the residual HEAT (1290°F but cooling rapidly) retained in the heater head (after running as a "heat pump". The engine is again consuming this residual heat to produce work output.

Driven by the electric motor the Stirling engine moves heat as either a "refrigerator" or as a "heat pump" one way or the other according to the direction of rotation.

Running on a temperature differential without any work input from the electric motor, the Stirling engine runs by consuming heat and converting the heat to work. The heat either comes from heat applied to the heater head or from residual heat in the heater head from previously being driven by the electric motor as a heat pump. OR it can run on heat from the relatively hot water when the head is still cryogenically cold from previously being driven by the electric motor as a "refrigerator"/cryocooler.

Driven forward or backward by the MOTOR the head gets cryogenically cold, or intensely hot respectively.

Running as a heat engine in the "forward" direction, the engine is consuming heat from the heater head in the usual way that a heat engine operates as a "hot gas engine".

Running as a heat engine in the "reverse" direction heat is consumed from the relatively warm water jacket.

Hopefully this makes the video less confusing.

As a point of observation, I would say that the engine would have run much longer as a "cold gas engine" if all that liquid air had been preserved in some way and the cold head insulated. As it was, in the video, the liquid air, along with the residual cold in the metal heater head just boiled away and the head was quickly heated up by the surrounding relatively HOT ambient air

It might have also run longer as a "Hot gas engine" on the residual heat in the heater head after being driven as a "heat pump" if the heater head were insulated to retain heat (and the circulating water in the cooling jacket turned off or removed) rather than allowing most of the heat to just quickly dissipate into the relatively cold surrounding ambient air.

It would also make things more clear if the narrator had used some neutral terms such as "clockwise" and "counterclockwise".

[Fool]

The demonstration appears to show that to run as a heat pump or refrigerator requires work input, and that forces the ∆T to increase.

It also appears to show when running as an engine, with power output, ∆T decreases.

[Tom Booth]

The demonstration appears to show that to run as a heat pump or refrigerator requires work input, and that forces the ∆T to increase.

It also appears to show when running as an engine, with power output, ∆T decreases.

I agree with that.

I mean, as worded, the statements are accurate.

Well, almost.

"to run as a heat pump or refrigerator requires work input, and that forces the ∆T to increase"

Before the motor is switched on, there is no ∆T, or does not need to be any.

If you buy a new refrigerator, before plugging it in, there is not any temperature difference.

What happens is the refrigerant in gaseous form is compressed. The compressed gas condenses to a liquid releasing heat and the heat is driven off with a fan or by convection or both. The liquid is then allowed to expand through a valve into a low pressure area created on the vacuum side of the compressor and reverts back into a gas. As the liquid/gas expands it needs to take back the energy it lost while being compressed and cooled. That is, the expanding gas now gets VERY cold.

The point is, all this CREATES a ∆T it does not "increase" a temperature difference.

Once started the cold gas in the evaporator takes in heat which is then MOVED to the condenser.where the heat is driven off by a fan.

......

"...when running as an engine, with power output, ∆T decreases."

Yes, because as the heat is converted to work it is "used up" and the temperature of the hot side decreases as the heat is CONVERTED to work.

The heat/energy is not being moved, it is being consumed, or converted until it is gone and there is no longer any ∆T.

Now, I'm talking "ideally".

Actually the compressor in a refrigerator does some work on the gas which adds to the temperature to some degree, and that bit of extra heat needs to be driven off as well.

Also in the engine, some of the heat driving the engine may to one degree or another or in one way or another "leak through" to the cold or ambient side without doing any work, resulting in some rise in temperature at the cold side, but if possible that is something to be avoided or minimized. It is not the function of the engine to move heat.

[Tom Booth]

I forgot we are talking about a Stirling cooler, but the cooling process in a Stirling cryocooler is essentially the same as in a refrigerator or heat pump.

Compress the gas and drive off the heat and then expand the gas to absorb heat. The same compression with heat out followed by expansion where heat is taken in, just without phase change, but again, the heat is only MOVED, not converted.

It is kind of like the difference between the gasoline in your cars gas tank being CARRIED in the tank as you drive down the road.

It takes very little energy to move or carry gasoline in a gas can.

As opposed to the gasoline actually being CONSUMED by the engine to produce power or work output.

Moving heat is not the same thing as converting heat

Yes your car does SOME work carrying gasoline around in the tank. But a fuel pump is not an engine.

[Fool]

Tom,

"Now, I'm talking "ideally"."

Okay.

"Compress the gas and drive off the heat and then expand the gas to absorb heat. The same compression with heat out followed by expansion where heat is taken in, "

Please allow me to describe this in a little more detail:

The gas is compressed by work being input. That results in a temperature and pressure increase, above ambient.

Drive off heat. Heat transfers from the hotter gas to outside cooler ambient. Pressure drops but is still above ambient.

The gas is expanded. That results in a temperature decrease to below the cold space. Pressure below ambient.

Absorb heat. Heat transfers from the cold refrigerated space onto the colder gas until the temperature and pressure return to the starting values/ambient. Where the cycle begins again.

The above full cycle is depicted on a PV diagram by traveling counter clockwise around the loop. The area inside the loop is the amount of work needed per cycle to accomplish the cooling. That work equals the cooling.

It takes more work to compress a gas volume at the higher temperature than the work output during lower temperature expansion. That describes why refrigeration cycles require work input. If the gas is cooled during compression, it takes less work, more efficient but still requires work input.

[Tom Booth]

(...)

The above full cycle is depicted on a PV diagram by traveling counter clockwise around the loop. The area inside the loop is the amount of work needed per cycle to accomplish the cooling. That work equals the cooling.

(...)

I suppose that may be what a PV diagram depicts, but just like the PV diagram for an engine it does not represent reality.

A refrigeration system can have a COP much greater than 1 so quite obviously the work input does not "equal" the cooling.

Take a look at any textbook example.

Your initial assumption:

The gas is compressed by work being input. That results in a temperature and pressure increase, above ambient.

Leaves out that the uncompressed gas already contains heat which is also driven off. The gas temperature does not just increase because of work input, but because of the heat already contained in the gas being made to occupy less space.

You then say:

Absorb heat. Heat transfers from the cold refrigerated space onto the colder gas until the temperature and pressure return to the starting values/ambient. Where the cycle begins again.

The above full cycle...

To have a full cycle you need to go back to where you started, "The gas is compressed by work being input. That results in a temperature and pressure increase," but I assume that is what you meant.

Your purpose however seems to be to prove or illustrate that a heat pump can only move the same quantity of heat in joules as is put in as work.

Everyone knows that is not true.

The math is very simple.

If the COP of the heat pump is 3.0 and 1000 joules of heat are MOVED out the work required is

1000/3=333 joules

So in a heat pump with a COP of 3.0 it takes about 333 joules of work to move about 1000 joules of heat each cycle

That is not "equal".

[Fool]

You are correct. Thanks for pointing it out. The work almost never equals the heat, Qh or Qc. Unless of course there is 100% efficiency. Or zero work out, zero heat in.

"That work equals the cooling." <<< Bad Bad Bad

What I wish I'd said is, the area enclosed in the loop is the heat expelled to the hot plate Qh, minus, the heat absorbed into the cold plate Qc. Plate being the word for heat exchanger.

I figure you know the following equations. I supplying them for reference and for other students.

COP for a heat pump, heating:

COPh = Th/(Th-Tc)

COP for cooling:

COPc = Tc/(Th-Tc)

Just some quick numbers:

300 600 COPh 2 COPc 1 Engine 0 .5
300 320 COPh 16 COPc 15 Engine 0.0625
40 300 COPh 1.15 COPc 0.15 Engine 0.86

Ideally.

You started with ambient. 1 Compress, 2 reject heat Qh, 3 expand, 4 absorb heat Qc back to ambient.

Saying gas contains heat is like saying a flywheel contains work. Contains Internal energy? Yes. Heat or work? No.

"Absorbs heat back to ambient" is including the initial internal energy contained by the volume of gas.

[Tom Booth]

"other students"

?