Stirlingengineforum.com

Over the years, I've often heard of Tesla's ambient heat engine (cold hole) and blown it off as just another one of his wacky schemes (he had many). However, a recent post made me reconsider the possibility.

Consider a regenerated Stirling cycle with 300k source input (aka ambient) and 100k sink. Since internal energy (U) is linear temperature (Kelvin) let's say 300k=300U and 100k=100U. Now, knowing that for a given volume ratio (i.e. same compression and expansion volume ratio) with input (Q) equal work (W) we find that 300k Q=300k W, 100k Q=100k W, while 300k Q and W are 3x 100k Q and W, due deltaT=deltaU=deltaQ=deltaW. So, for expansion at 300k, we need 3Qin for 3Wout while for compression at 100k we need 1Win for 1Qout. In this manner, eff=.66 where (300-100)/300 is inline Carnot. Unfortunately, the compression process taxes the system twice: (1) via mechanical backwork=1Win (2) via heat sink=1Qout, thus doubling losses whereby ideal eff=.33 and Carnot laughs.

I've mentioned several times in this forum that any 600k expansion is only meager 2x output of same 300k expansion (room temperature freebie) and nobody noticed. 100k sink is indeed frosty (-280F) but a free lunch is worth exploring.

Ambient powered Stirling spinning a cryo-mode Stirling to freeze the cold hole to -280F. Cryo-mode Stirling Warm-side-rejected-heat fed back to ambient powered Stirling warm side.
Free lunch plus ice-cream dessert with a cherry on top.

A well insulated box with a view of the night sky through a couple of sheets of thin clear plastic with a cavity between will get cold inside. Around 4F lower than ambient air temperature on a humid night, and cold enough to freeze ice cubes on a clear-sky, low humidity night.

Matt, two identical Stirling Engines running with the same temperature differential, the one at a colder overall temperature will be more efficient. Carnot rule. However if you factor in the refrigerator efficiency used to make an artificial cold temperature, you will end up worse off.

Now if you live on Pluto??? LOL

Stroller wrote: ↑Mon Apr 29, 2024 2:30 am Ambient powered Stirling spinning a cryo-mode Stirling to freeze the cold hole to -280F. Cryo-mode Stirling Warm-side-rejected-heat fed back to ambient powered Stirling warm side.
Free lunch plus ice-cream dessert with a cherry on top.

Trying to run a heat engine on "Cryo-mode Stirling Warm-side-rejected-heat" continuously, would, of course be a loosing battle. The cryo-cooler would have to struggle like mad trying to pull heat up out of a "cold hole".

Once a "cold hole" is established it just needs to be preserved. Insulated. Trying to continue to draw heat from a cryogenic "cold hole" would be the equivalent of running a pump 24/7 to pump water from a dry well. A complete waste of energy.

If you want to concentrate heat to elevate the temperature of the hot side with a heat pump, it would probably be better to get the heat from somewhere where a lot of heat actually exists. The relatively hot surrounding ambient environment, or hotter if possible. Cheap solar heated hot air panels or a greenhouse..

Once established the "cold hole" has scarcely any heat left in it to give.

Apologies Tom, I was just making an 'over unity' joke.

But you can't preserve a cold hole by pumping ambient temperature air into it, which is what the proposal amounts to.

I think the goal of a refrigerator in the kitchen is to not run continuously and to maintain a constant cold hole. And it still is one of the top users of electrical power in the home. That, hot water tank, AC, and electric heat, or heat pump.

Stroller wrote: ↑Mon Apr 29, 2024 1:05 pm Apologies Tom, I was just making an 'over unity' joke.

But you can't preserve a cold hole by pumping ambient temperature air into it, which is what the proposal amounts to.

How so? No air is pumped into the ice or whatever "cold hole".

There are many ways for ambient heat to get to a colder place. Pumping air on it is only one. You seem to be implying no others exist. We are not fools here. Doh!

In an arrangement as shown previously;
First the displacer moves down the cold air is pushed up to be heated.
As the gas is heated and expands, the ice is covered and protected from heat, insulated from the heat by the displacer.

As the hot gas expands and drives the piston, with 100% efficiency. All the heat is converted to work. As a result the gas cools back down to the same cold temperature it started at.

Before TDC all the heat introduced is used up but the piston continues to TDC by momentum, cooling the gas slightly colder than the ice cold temperature it started at.

At around TDC the very cold gas is shifted back down cooling the ice.

At this point atmospheric pressure drives the piston inward and the gas temperature begins to rise.

Before the gas temperature rises too much the gas is shifted back to the hot side to introduce more heat as the ice is again covered and protected.

So rather than pumping hot (ambient or whatever, potentially hotter) air down to the bottom cold side, the cold side, if anything, loses some heat to the COLDER working fluid.

A Stirling engine acts similarly to a heat pump, drawing heat from BOTH the hot and the LESS HOT side, converting BOTH into work.

Here is a description of the first two stages of a Carnot cycle.

The Carnot Cycle
The Carnot cycle consists of the following four processes:

A reversible isothermal gas expansion process.

In this process, the ideal gas in the system absorbs qin amount heat from a heat source at a high temperature Thigh , expands and does work on surroundings.

A reversible adiabatic gas expansion process.

In this process, the system is thermally insulated. The gas continues to expand and do work on surroundings, which causes the system to cool to a lower temperature, Tlow

In an "ideal" Carnot cycle the working fluid temperature has already cooled to the temperature of the cold side by BDC.

But why stop there?

As the gas nears BDC and cools below Tlow there is additional heat available from the cold side to continue expanding the gas, simultaneously "refrigerating" the ice.

.

This isn't some proposal for some new "perpetual motion" engine or some such thing. It's just how Stirling engines actually work.

But a "real" Stirling engine is supposed to have a regenerator, which helps keep the hot and cold sides separated.

So in this experiment I added a regenerator and also extended the piston's throw just a bit to extend the piston travel and produce a bit more cooling.

This apparently worked.

After an extended run of about three hours, with the engine running on boiling hot water poured into a vacuum insulated flask to retain as much heat as possible to power the engine, the cold (ambient) upper side of the engine, apparently grew slightly cooler than the ambient surroundings.

https://youtu.be/P11q-BAhvqk?si=0iX6FTunotYXa8wk

There is nothing extraordinary about this.

Heat can be used to power a refrigeration system. The Vuilleumier cryocooler runs on heat, using heat to produce cryogenic cold and is hardly distinguishable from a Stirling engine in its basic operating principles.

Tom Booth wrote: ↑Mon Apr 29, 2024 3:07 pm

Stroller wrote: ↑Mon Apr 29, 2024 1:05 pm But you can't preserve a cold hole by pumping ambient temperature air into it, which is what the proposal amounts to.

How so? No air is pumped into the ice or whatever "cold hole".
....
As the hot gas expands and drives the piston, with 100% efficiency. All the heat is converted to work. As a result the gas cools back down to the same cold temperature it started at.

Engineering realities never work out so neatly. The outside of the upper plate is constantly being warmed by the atmosphere towards ambient T. Some of that energy is going to be leaking into the working fluid and getting transferred by the displacer back to the cold hole.

Stroller wrote: ↑Mon Apr 29, 2024 10:42 pm

Tom Booth wrote: ↑Mon Apr 29, 2024 3:07 pm

Stroller wrote: ↑Mon Apr 29, 2024 1:05 pm But you can't preserve a cold hole by pumping ambient temperature air into it, which is what the proposal amounts to.

How so? No air is pumped into the ice or whatever "cold hole".
....
As the hot gas expands and drives the piston, with 100% efficiency. All the heat is converted to work. As a result the gas cools back down to the same cold temperature it started at.

Engineering realities never work out so neatly. The outside of the upper plate is constantly being warmed by the atmosphere towards ambient T. Some of that energy is going to be leaking into the working fluid and getting transferred by the displacer back to the cold hole.

That's your opinion based on what?

Some heat will leak into the ice box in your refrigerator, due to the fact that there is no perfect insulation.

We are talking, however, essentially, about the refrigeration system itself.

How much of the heat leaking into your icebox is leaking through the evaporator coils that surround it?

The expanding gas in a Stirling engine cools, potentially below Tcold absorbing some heat from Tcold

It is expanding the same way the refrigerant in the evaporator coils in your refrigerator expand cool and draw heat OUT of your ice box.

The displacer in a Stirling engine is the functional equivalent of a refrigeration systems expansion valve.

Carnot believed heat runs through a heat engine like water through a water wheel.

Carnot was wrong. Carnot never took a measurement of a Stirling engine. His conclusions were based on fantasy.

That fantasy has persisted for the past 200 years with everybody apparently just assuming it applies to Stirling engines, because Carnot thought his fantasy applied to all engines.

Have you measured this heat you think gets transfered to the "cold hole" in a running Stirling engine?

Here is your basic Stirling engine refrigeration cycle:
Fairly simple and straightforward.

While expanded and cold near BDC the working fluid is moved down.

While compressed and hot near TDC the working fluid is moved up.