How much power possible from a LTD Stirling generator

[Tom Booth]

There is this problem.

In order to get heat out of a heat pump, the working fluid (gas, refrigerant or air) needs to be compressed.

Compressing the gas (with or without a phase change to liquid) generates heat, or rather, the large quantity of gas forced to occupy a much smaller space is forced to give off heat.

Now how does a Stirling engine get power? By using heat to expand a working fluid.

So, compressing air releases heat, if we use the heat to run a Stirling engine to compress more air, the best we could possibly hope for would be to break even, but with all the loses, no chance of that happening.

But suppose as we compress the gas to generate heat, the heat is removed immediately ?

Removing the heat while the gas is in the process of being compressed makes compression easier, because taking the heat away quickly causes the compressed gas to cool and contract.

But it is still very unlikely enough "high grade" heat, that a Stirling engine needs to run could be retained in some heat battery, or used directly by the engine. You are still just running in circles. Compressing the working fluid to get heat, then using the heat to expand the same or some other working fluid to operate a heat engine by expanding the working fluid, then using the resulting work output to compress more air to get more heat.

If we take the heat away quickly to make compression easier, the temperature will never amount to anything.

Like cooling the compressor with a water jacket. We need to use cold water. That water will have to stay cold. It can absorb a lot of heat because of it's high heat capacity, but the water will not get hot enough to operate a heat engine. Maybe a very sensitive, but very low power output ltd. But the LTD could never power an air compressor.

But suppose we just throw away the warm water?

The working fluid is now compressed and coed and this was accomplished fairly easily due to removing the heat quickly.

Now if we release the compressed cooled gas into an evaporator, (the cold side of a heat pump) we get a very cold temperature because the compressed gas had a lot of heat removed, so that when it expands again, it is very cold. That is how a household freezer works. The gas in the lines is compressed and the heat removed immediately, usually with a fan, then re-expanded in an insulated space (the freezer box) to produce freezing cold, but all that heat of compression is wasted, and because it is removed so quickly, it never really gets extremely hot so is not very useful.

But there is the cold as the by product, and there is SOME elevation in temperature produced by the heat of compression.

If there is some "free" source of cold running water the working fluid could be compressed and cooled very easily because we could just dump all the heat of compression into a river or wherever the cold water came from.

Then we might be able to run the Stirling engine on the cold produced when the working fluid is allowed to expand and cool.

Remarkably the gas can be cooled much more if it is expanded through an engine or turbine, because that makes the gas do some work as it expands, which removes "internal energy" causing it to get much colder than it would ordinarily.

That work can be put to use.

We now, perhaps, have enough cold to operate the Stirling engine with a fairly high temperature difference. Not by using the heat produced by the heat pump, so much as the cold.

That is essentially how the "dippy bird" toy operates. It runs between the temperature difference of the cold part of the "heat pump" the evaporative cooler is wasting/removing all the heat. The engine is running on cold.

But not really cold, it is running on heat in the air. Heat that is freely available.

So that is why I suggested early on, utilizing the cold side of your heat pump.

Is there perhaps some refrigerant that is easy to compress and condense to generate heat, which heat can then be used to expand some other working gas with a greater ratio of expansion?

There are all kinds of possibilities to explore, but the only accounts of actual success in such an endeavor I've been able to find has been where the heat of compression from the compressor was dumped directly into cold water from a river. Cold air can then be expanded using the surrounding ambient heat to do the actual work of compressing more air and dumping all the heat somewhere.

This seems to have been successfully accomplished by Charles Tripler using his liquid air machine.

https://books.google.com/books?id=hF0DA ... &q&f=false

Tripler apparently was able to produce 10 gallons of liquid air and then use just 3 gallons expanded in a steam engine to run his air compressor to produce 10 gallons more.

But he was incorporating cold water from a nearby stream to remove the heat of compression.

Basically a round about way if using evaporative cooling to run a heat engine.

[Tom Booth]

Another point to consider in favor of utilizing the cold side of the heat pump is this:

If a heat engine converts heat into some other form of energy, in effect leaving behind cold as a by product, then when running on the hot side of the heat pump and using ambient air as the cold side, the heat will need to be continually generated. That is, the heat pump will need to run all the time.

If however the cold is used and ambient air is the hot side heat source, the heat in the air is ever present. It does not have to get generated by the heat pump.

So the Stirling engine takes in a lot of ambient heat, converts the heat into work. So think about that. If the ambient heat energy is converted into work output with cold as the byproduct, the heat pump should not have to work as hard or as continuously to produce cold to run the Stirling engine.

So if the heat pump is used to maintain a "cold battery" with most of the heat of compression from the heat pump removed via a cold water jacket around the condenser tubing to ease up the work load on the compressor, the Stirling engine running on ambient heat would not really be passing much, if any heat through to the cold battery, so the heat pump might only need to run intermittently, while the Stirling engine could be running all the time.

The cooling water could be taken from and returned to a pool where it could cool down naturally, or put through radiators. Some of the wasted heat of compression could possibly be used for low grade heating or even to supply a little extra heat to the Stirling engine, but IMO the best chance for any success would be to run the Stirling on the cold side of the heat pump primarily relying on ambient heat as your primary heat source, the heat pump only maintaining your "cold battery".

[Tom Booth]

Another thought though:

If the condenser (hot) tubing of the heat pump could be passed direct through the hot side of a large LTD type Stirling, coiled tightly around inside the displacer chamber, the Stirling engine itself might serve the purpose of removing the heat from the compressed gas in the heat pump condenser coils - circulating air around and through the coils/tubing similar to how a fan would cool the condenser coils on the back (or under) a refrigerator/freezer or air conditioner/heat pump.

The condensed gas (liquefied refrigerant) might then also be expanded through a small turbo-generator and the cold evaporator coils could be routed directly through the other side of the displacer chamber.

The Stirling engine itself would then serve as the compressor for the heat pump. The Stirling engine and heat pump would essentially be the same machine very tightly integrated.

I had a thread on the forum years ago:

This was an early iteration of the idea:

Air_Stirling_Gen.jpg (26.99 KiB) Viewed 1774 times

Which evolved into this:

Resize_20221113_022020_0391.jpg (93.5 KiB) Viewed 1774 times

I also had an animated version.

[Tom Booth]

This required copy pasting and editing about 100 gif images, I had misspelled the word insulation but didn't notice until after animating the gif, and I wasn't about to bother unpacking it and editing 100 images. So I'm a bad speller.

ezgif.com-gif-maker.gif (80.18 KiB) Viewed 1771 times

This was intended for using air as the working fluid/refrigerant but could be adapted to a more conventional heat pump.

The main idea is just to put the expansion and condenser tubes right inside the Stirling engine.

I was calling this a Stirling Turbine at the time (2009). A lot of the links on the thread are broken now, but possibly can be retrieved using the Wayback internet archive.

viewtopic.php?f=1&t=461

I've never had the time or resources to actually make an attempt at building any of these Stirling/heat pump contraptions and kind of doubt any of these ideas would actually work as crudely illustrated, if at all, but part of the reason I'm setting up a big workshop for building Stirling engines is to fool around with this kind of project.

I did get as far as building what I called an "ice pump" to test the concept, but I was using very leaky check valves made out of some warn ball bearings balanced on some small springs from ball point pens and they didn't seal very well. This is basically a Stirling engine displacer in a tin can manually operated, so I am not "pumping" air with the handle, just raising and lowering the displacer.


https://youtu.be/S8MBj6_sdSw


It proved to me that it was possible to compress air using a temperature differential. It didn't work great, mostly because of the leaky check valves I think, but the inside valve was soldered in and I never tried making another, but I thought it proved the concept anyway.

Maybe. I'd have to build a better one with some real check valves to know for sure, but it did blow up the balloon a little!

That is real pressure/compression right? But it is heat doing all, or most of the work.

[Tom Booth]

Aside from the leaky check valves, the empty coffee can also conducts a lot of heat, so there was probably a relatively small temperature difference. A chamber made of a section of plastic tubing pvc pipe with metal end caps might work better.

If the construction of this "ice pump" isn't clear, it is basically just a Stirling engine displacer chamber and displacer, but with check valves instead of a piston.

The idea being, if it could pump air, then it could drive a turbine.

Resize_20221113_034231_1679.jpg (77.41 KiB) Viewed 1766 times

Anyway, that old thread is here: viewtopic.php?f=1&t=461

The "displacer" I used in that can was just a block of wood. Now I'm wondering if a regenerator type displacer would have worked better.

If it could compress air and drive a turbine, then heat might be generated from the compressed air and cold generated by expansion through the turbine, so it would act as a heat pump.

[Tom Booth]

Anyway, aside from the heat pump, Personally I think it is possible to generate as much power from an LTD style engine, and by that, I just mean the engine design, having the relatively large flat plates/heat exchangers, as any other Stirling engine. It is just a mater of how much heat is put into it. Theoretically, IMO it could probably generate more power due to the very large heat input surface area.

I'm guessing you may be trying to overcome the objections raised that a heat pump has greater efficiency with a small temperature difference, whereas a Stirling engine requires a high temperature difference.

Really we are not far away from a flaming ball radiating plenty of heat, so the Earth's atmosphere is a giant hot house. We are living in an oven.

The real problem is getting rid of some of the heat, or isolating the engine from all that heat to have a temperature differential. So part of the engine can be unheated.

[dlaliberte]

I'm guessing you may be trying to overcome the objections raised that a heat pump has greater efficiency with a small temperature difference, whereas a Stirling engine requires a high temperature difference.

Yes, I am concerned about those objections. But I'm also not so concerned given the fact that the heat pump can "provide" 5 times more energy, in the form of heat, than the electric energy required to provide it. And we can keep storing the heat when the heat pump is very efficient so we can use it later when the heat pump is less efficient.

I am encouraged by your experiments that appear to show a Stirling engine can convert a portion of the input heat to mechanical energy while not necessarily losing much of the remaining heat to the colder output. It seems that much greater efficiency is possible than is admitted by normal science, but I think the focus on "efficiency" is effectively confusing the multiple variables involved (temperature, pressure, and moreover, flow rate of energy) and some re-conceptualization will be required to understand what is going on while maintaining the conservation of energy (which is generally unquestioned by everyone).

Another thing to consider is that the greater "efficiency" of Stirling engines with greater difference of temperature might be a boon if we can integrate the features of a Stirling engine design with that of a heat pump, since the latter is least efficient with greater temperature differences. The two technologies are thus complimentary. And maybe it will be easier to start with a Stirling engine design and morph it into a heat pump rather than starting with a heat pump system, with separate components for the compressor, expansion valve, and two heat exchangers, and figure out where to insert one or more Stirling engines. This is exactly what you have been doing with your alternative designs, I suspect.

[Tom Booth]

...And maybe it will be easier to start with a Stirling engine design and morph it into a heat pump rather than starting with a heat pump system,... This is exactly what you have been doing with your alternative designs, I suspect.

Yes, I started out just trying to design a Stirling engine for a friend (long story). He specified it should have a cooling system.

After some research I thought an air-cycle heat pump would be the deal cooling system for a hot air engine.

After about maybe a month of improving my design, I ended up with a combination Stirling / heat pump that seemed in my imagination, that it could run on indirect solar heat in the air that was being made available to the engine by the air cycle.

My hunch is that my friend, being a millitary contractor, he used my idea as a proposal. The millitary was taking bids on a small tactical Stirling Solar/multi fuel generator at the time. I found out much later.

So he called and told me it was all off, because my design "violated the second law of thermodynamics", so he said he was told.

Well I had no idea what he was talking about at the time. I'm just a dumb engine mechanic.

Seemed like a perfectly viable engine to me, so I had to find out what this "LAW" was all about.

My research may be a bit biased in that this supposed "LAW" pretty much threw a monkey wrench into my life and plans, putting an end to a month of hard research and design work, and a life of luxury, I was quite proud of my design.

After more than a decade, I'm still not convinced it cannot work. The 2nd law seems contrived. There is no real science behind it IMO.

[Tom Booth]

It is also important to know why a heat pump is considered inefficient at greater temperature differences.

I don't think in necessarily applies to the way the heat pump might be used in conjunction with a heat engine.

We are not necessarily trying to bring heat in from a very cold winter outdoor temperature to heat a warm house, just move heat from point A to point B which could be at higher, lower or equal temperature.

For example, all summer the heat pump can be moving heat from the hot summer air outside to wherever the engine may be, which is unlikely to.be anywhere that it is hotter. But the heat pump will make it hotter, very efficiently. It would basically be moving heat on a level plane not up hill. Such arguments don't apply to the proposed application.

If using the heat pump to just dispose of heat to maintain a "cold battery", efficiency can be maximized by dumping the heat into cool water, then the heat pump is pumping heat down hill the way the heat would be tending to go anyway, which would make the heat pump very efficient.

[Tom Booth]

Also, "they" say, ignorantly IMO, not really knowing much about either, that a heat pump is the opposite of a heat engine, which IMO is not really true at all.

So it is not a matter of lifting heat up with the heat pump and lowering it back down with the heat engine.

The heat pump just moves heat to where it can be used by the heat engine.

The heat pump only moves the heat, the engine CONVERT$ it. Is there a difference? I think so.

My truck converts gasoline. I can use a pint of gas to carry 20 gallons of gas in gas cans in the back of my pickup.

Transporting fuel is very different from converting it. Heat is fuel for a heat engine.

Another thing as I'm sure you are already aware, not all heat engines require a great temperature difference to run efficiently, which I would assume, is the reason the thread has the title it does.

Well than why hasn't anyone done it yet ?

I think they have, actually, but every time they do they get clobbered over the head with this irrational obsession with adhearance to the all sacred second law.

If anyone happens to come up with some such device, they are immediately labled a fraud, a "pseudoscientist", shamed, mocked, excluded from investment circles, you name it, they will be ruined and forgotten.