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This is, or may be, somewhat of a departure from a "pure" Stirling heat pump, but could certainly be driven by a Stirling engine.

In the mid 1800's John Gorrie invented a relatively simple mechanical ice making machine (a kind of heat pump).

""If the air were highly compressed, it would heat up by the energy of compression. If this compressed air were run through metal pipes cooled with water, and if this air cooled to the water temperature was expanded down to atmospheric pressure again, very low temperatures could be obtained, even low enough to freeze water in pans in a refrigerator box." The compressor could be powered by horse, water, wind driven sails, or steampower." http://www.phys.ufl.edu/~ihas/gorrie/fridge.htm

Hot compressed air "cooled with water", of course, produces hot water. And why could it not also be driven by a Stirling engine?

Unfortunately at that time, there was an established northern natural ice industry which saw Gorrie's machine, which could manufacture ice "out of thin air" in the hot summer in Florida a threat.

https://www.smithsonianmag.com/history/ ... -66099329/

There is no doubt, however, the machine actually worked. At the time this seemed truly miraculous, so much so that Gorrie's invention met with much ridicule and disbelief.

In general, refrigeration or heat pump systems using ordinary atmospheric air as the working fluid (refrigerant) still aren't used much, which I suspect is simply because air cannot be patented. Nobody can make money selling and servicing a machine of that kind simply because anyone could build it out of ordinary plumbing pipe. Also there is not much need for the extremely cold temperatures potentially produced on the cold side (-160° F) flash freeze drying type applications.

But it seems to be making a comeback, maybe, primarily do to environmental concerns.

I think, with a bit of imagination, such a simple low tech "air cycle" heat pump system could be adapted to the purpose of transferring heat from a compost to a hot water storage tank, or directly to a living space. Essentially just a Stirling engine powered air compressor.

In general, refrigeration or heat pump systems using ordinary atmospheric air as the working fluid (refrigerant) still aren't used much, which I suspect is simply because air cannot be patented

Could also be that the performance is terrible compared to other refrigerants. Just to get a feel for the potential, I used a 1.5mm nozzle on my shop compressor which hasn't run in weeks but still had 140psi in the tank. So the air in the tank was room temperature (22C as measured w/thermocouple in contact with tank).

I released it through the nozzle into a small PVC pipe sealed at the nozzle so that I wasn't pulling room air with it. I tried a few pressures like 10, 20, and 30psi with the same thermocouple in front of the nozzle at varying distances within the tube.

Best drop I could get was 3C under ambient. I'm curious to know what you might see with your own experiment.

skyofcolorado wrote: ↑Thu Jun 23, 2022 10:46 am ...

Best drop I could get was 3C under ambient. I'm curious to know what you might see with your own experiment.

I haven't had the opportunity to experiment with air-cycle refrigeration just yet.

The throttling used in an Air-Cycle system however is different from standard vapor compression throttling because there is no phase change involved, Just releasing the compressed air through a nozzle results in a minimal Joule-Thmpson type cooling.

Here though, we are discussing the utilization of the HEAT of compression, which in your experiment has already dissipated from the compressor and tank if as you say, it had already returned to room temperature. The heat of compression was already lost.

In a heating application, any cold produced, unless used for refrigeration of some sort, serves no beneficial purpose. The resulting compressed air though can be used.

In other words, the idea would be to draw air into a compressor, from pipes that pass through a compost to pre-heat the air being compressed. Ideally the compressor should be directly water cooled immediately and the the directly heated hot water pipes into a water storage tank in the process removing much of the heat from the compressed air in the pipes.

The cooled compressed air can then be used in a bootstrap manner to reduce the load on the air compressor by being discharged through an air motor.

In an air-cycle system, the bootstrapped air motor helping to drive the compressor is the throttling device. This takes heat out of the air in the form of "work'.

The resulting cold air leaving the air motor could be used to cool the cold side of the Stirling engine. Returning cold air to the compost would actually be very detrimental and serve no good purpose.

The "excess" or unneeded cold air could be discharged to atmosphere in an open air cycle, or reheated with an ambient air heat exchanger before being recycled through the compost heap.

This PDF is about the best brief description of an air-cycle refrigeration system I've come across:

https://grimsby.ac.uk/documents/frperc/ ... search.pdf

Probably the best arangements would be to have the Stirling engine, air compressor and air motor all coupled together on the same shaft.

Hi, I did my experiment on Peltier plate and got mixed outcome, please see the following:

Experiment settings:
I changed the structure I originally designed in the first post.
Instead of pumping heat from the steam, I placed a sealed stainless steel pot inside the reactor.
The pot is filled with 5 liters of water and installed with two silicon tubes and a small sink pump.
To obtain the heat energy, water in the pot will be pumped through one of the tubes to an aluminum heat exchange block.
And then the Peltier plate will pump the heat into a steel tray containing 500ml of water.

Here's the general design:
https://drive.google.com/file/d/1XDkla6 ... sp=sharing

And a picture of it(shot at top of the reactor prototype)
https://drive.google.com/file/d/1ngpm-e ... sp=sharing

The Peltier plate I used is a CL-C067(70Watt), which can freeze water by the support of air cooling.

Initial hot water(in the steel pot) temperature is 38 degrees Celsius.

The target I set for this experiment is to make 500ml of hot water(85 degrees Celsius) to cook a cup of coffee.

---------------------------------
Result:
The starting water temperature in tray is 29.6 degrees Celsius.
After 40 minutes, it was heated to 39.1.

From the beginning to the end hot Side stayed around 80 degrees Celsius:
here's the footage:
https://drive.google.com/file/d/1o9l7Ux ... sp=sharing

After the experiment, I checked the hot water in the steel pot and it remained at 38 degrees Celsius.
https://drive.google.com/file/d/1raU4CG ... sp=sharing

The heat energy pumped into the target tray of water:
500 g * 4.2 (specific heat capacity) * (39.1 - 29.6) = 19950 Joule
The work is done in 40 * 60 = 2400 seconds
So the actual out put power to the water is only 19950 / 2400 = 8.3 Watt (Joule / second)
Compared to the power consumption of that Peltier plate, which is 70 Watt, it is absolutely less efficient than a regular electrical resistance heating device.

-------------------------------------------------------------
Conclusion:
It's obvious that this(Peltier heater plus compost heat) will not be a viable cooking heat source.
The Peltier heater did go near the temperature I desired(85 degrees Celsius), but it failed to heat up the water efficiently.

However, the compost reactor seems to be a reliable heat source, which remained at the same temperature after 40 minutes of draining.
Now I'm more convinced that this reactor can help heat up a house.

---------------------------------------------
Additional result: Compost piles can be revived!

I did a experiment on this batch of compost.
Every time when the core temperature of the compost falls below 50 degrees Celsius, I add and mix new materials into the pile.
By doing so, I successfully pushed the compost heap back into thermophilic stage.
here's the record done in 23 days:
https://drive.google.com/file/d/1BjA3ck ... sp=sharing
A stands for steam temperature.
B is the core temperature of the heap.

This result means that even with one reactor, it's still capable of providing stable heat output!