How much power possible from a LTD Stirling generator

[dlaliberte]

I am trying to determine how much electric power I might be able to generate using a Stirling engine attached to a generator. This system would resemble a Combined Heat and Power system, but with the following constraints.

The input heat to the Stirling engine is in the form of the high temperature, high pressure vapor from a 4 ton heat pump system. I did a fair amount of searching for info on what that temperature and pressure might be, and what flow rate would provide the 4 tons of heat, but it looks like I'd have to dig through service manuals at best, so I'm going someone here could help come up with a range of numbers.

The cold side of the Stirling engine will be the room temperature. And any wasted heat after generating the electric power would be blown into the rest of the house to maintain the room temperature.

I realize this temperature differential between the hot and cold sides is relatively low compared to what is required for typical CHP systems, so the power output might be comparably low.

The returning coolant should have about the same temperature and pressure as in the original heat pump system, after passing through the heat exchanger. So the net effect is that not quite as much heat will be provided to the house, but some amount of electric power will be produced instead from the fraction of heat that is subtracted from the total heat. So the question is, how much electric power could that be?

It might be simpler to think of this as a replacement for the heat exchanger, which has both a heat output and power output.

Best case estimate, proven similar system, close analogy, or whatever you've got, I'm interested in hearing about it. Thanks.

[Tom Booth]

Are you intending to, in some way, re-route the vapor from the heat pump directly through, or into the Stirling engine, or just utilize the hot air blowing out of the heat pump or what exactly?

It seems the hot side of a typical heat pump, generally, is, or should not be much more than about 200° F to prevent damage to the unit.

If tearing apart a heat pump, why not utilize the cold on the evaporator side?

[dlaliberte]

I wanted to use the greatest heat difference available but entirely inside the house. The heated air coming blowing out of the heat exchanger would not be as hot as the hot vapor from the heat pump, so it seems better to use the hot vapor directly for the hot side of the Stirling engine.

And for the greatest temperature difference available inside the house, the room temperature would be better to use for the cold side.

I wanted to not change anything else about how the heat pump cycle operates, and using the cold from the evaporator would raise its temperature in the process. For the same reason, I wanted to ensure that the coolant returning back outside is about the same temperature, though it is still a high temperature, high pressure liquid rather than a vapor.

I was thinking that the cold side of the Stirling engine would actually heat up, as theory predicts, but looking into that question is how I found this forum.

If the process of extracting some amount of heat energy from the heat pump to convert it to an equivalent amount of energy in the form of electric power "wastes" some energy in the form of heat, there is no loss of energy actually since it would simple continue on to be used to heat the house.

So if the difference in temperature between the hot and cold sides of the Stirling engine is 200° F - 70° F = 130° F, how much electrical energy could we extract. It seems the total heat energy of 4 tons, or 48kbtus, should enter into the equation as well, but it is not clear to me how. Would the Stirling engine operate at 20% efficiency and thus we should be able to extract 20% of the heat energy? Better or worse than that?

[Tom Booth]

..., so it seems better to use the hot vapor directly for the hot side of the Stirling engine.
(...)

How would you access the "hot vapor". That would, I believe, be some type of refrigerant, pressurized within the tubing, inside the unit.

The refrigerant travels in a closed loop, through the compressor, to the "hot side" tubing/coil, then through some kind of expansion valve and then back to the cold side to pick up heat, then back into the compressor.

Resize_20221110_091119_9031.jpg (101.01 KiB) Viewed 4521 times

Something along those lines usually.

The cooling actually takes place as the refrigerant passes through the expansion valve. After that is is very cold, generally, or probably, depending on the type of refrigerant, well below freezing.

The coil of tubing, outside after the expansion valve is for absorbing heat from the ambient air.

I"m just thinking if you are going to be removing the refrigerant and cutting and reworking tubing, it might be possible to locate the expansion valve so as to increase the temperature difference at the engine,

I don't think picking up additional heat from the engine would interfere with the functioning of the heat pump. Probably it would work better, by reducing the need for a defrost cycle. The heat would just be recirculated back into the house, but the cold, applied to the engine could possibly double the power output from the Stirling engine.

Most heat pumps heat the cold evaporator coils one way or another periodically anyway, to keep them from getting iced up, which ice buildup would block air flow reducing efficiency.

Anyway, sounds like an interesting project.

[Tom Booth]

In other words, it might be possible to relocate the expansion valve inside, so the cold "exhaust" from the valve could be utilized. That might actually be a relatively minor, inconsequential modification.

Resize_20221110_100452_2658.jpg (143.94 KiB) Viewed 4517 times

[dlaliberte]

I think I lost a reply I wrote, or failed to submit my reply, probably due to the confusing ui.

Anyway, I would keep the hot vapor in the tubing, but wrap the exposed tubing around the hot side of the Stirling engine. Integrating this with the heat exchanger would seem to make sense.

If it works to move the expansion valve device into the house, and then use the support cold coolant on the cold side of the Stirling engine, cool! But if the effect is that the coolant would warm up enough to affect the heat pump efficiency, that might not be so great.

[Tom Booth]

I've done that. Especially when using "preview", then forgetting it was just a preview.

Having had similar plans, I unfortunately found the existing tubing in heat pumps and such virtually impossible to work with or modify, usually soldered into some odd shape with fins attached as well as being rather thin walled and easy to break or kink, so, after several attempts gave up on the idea and decided that if I were ever to put together some such heat pump / engine it would have to be built from scratch. Simply wrapping the existing heat pump tubing around an engine cylinder without breaking it is pretty much impossible in my experience, the heat exchanger tubes are generally put together like a car radiator. Not that I want to be discouraging by any means.

Given your probable setup I had thought it might be a good idea to keep the engine itself outside, assuming that is where the compressor unit is located.

The hottest point in the system would be the compressor and the point in the tubing where the hot compressed refrigerant leaves the compressor.

The coldest, the point just after the expansion valve, so if those two points are outside anyway, it might be easier to tap into them there.

[dlaliberte]

Those points to tap into the coolant tubing make sense, except for being outside. I'm hoping to avoid wasting any heat to the outside air, hence my focus on keeping it all inside.

Since the existing tubing is difficult to change, maybe it would be better to plan on replacing the whole heat exchanger with the Stirling engine generator. Assuming there are connection points in to and out from the heat exchanger, that sounds feasible.

Another variable in all this is alternative heat pump systems that use coolant with a wider range of temperatures, if that would help.

[Tom Booth]

(...)
Another variable in all this is alternative heat pump systems that use coolant with a wider range of temperatures, if that would help.

Some food for thought:

https://youtu.be/f1FQjfyOifI


One thing to note is that to be effective at cooling with such an air cycle, the turbine needs a load. It works by converting heat into work, so without a load it doesn't really get very cold (as in the video).

The usual method of providing a load for the turbine would be to have it help drive the compressor.

But this is probably getting pretty far afield from your typical household heat pump.

Nevertheless, an interesting pdf on the subject:

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

[dlaliberte]

Compressed air seems to have some advantages, though I was thinking more of CO2. https://blog.isa.org/why-co2-is-the-mos ... g-industry

[dlaliberte]

When the heat pump is being used to air condition during hotter months, then we would want to move the Stirling engine generator outside, and perhaps integrate it with the outside heat exchanger. We don't care so much about wasted heat at this time since the problem is that we have too much.

So if we replace both the inside and outside heat exchangers with Stirling engine generators, we might get some power from both all the time, especially if the operate in reverse mode, but will likely get more power from one or the other depending on the season.

Having two Stirling engine generators would be an extra expense however, and maybe instead we could make do with just one, with auto switching lines similar to how the compressor does it to change the direction of coolant flow through the system.

[dlaliberte]

One more thought, which is actually the imputus for why I was thinking about using a Stirling engine in the first place. It would be better to drive the Stirling engine with a consistent heat source, and a heat pump system is not as consistent as possible, although they do tend to be run more or less continuously. But in cold months, there is a defrost cycle that switches the system to cooling mode, so that hot coolant is run through the outside heat exchanger to melt the accumulation of ice.

Another problem with heat pumps is that they can become very inefficient during the coldest and hottest extreme temperatures.

So a solution to both these problems is to use a thermal storage system that we continuously heat up, or cool down, using the heat pump system, and then the inside heat exchanger that heats or cools the inside would be isolated from the heat pump system, by using its own closed loop with the thermal storage unit.

During the coldest and hottest outside temperatures, the heat pump system would just shut down, and then the inside system would continue to use only the thermal storage to provide the "backup" heating or cooling.

Now if there is a problem coming up with this thermal storage system, it might be easier to just store electric power in a battery. That's good for using electric resistance heating as a backup heater, but there is nothing comparable for backup cooling.

[Tom Booth]

I'm not entirely sure in what way, or what you actually mean by: "replace (one or) both the inside and outside heat exchangers with Stirling engine generators". Or how that could be accomplished and still have some kind of functional apparatus.

I can only picture one "Stirling engine generator" sitting out in the cold, outside, and the other warm inside.

Neither doing anything particularly fascinating.

Edit: you added another post while I was writing, which I read, but still don't have any clear idea.

I'll reread your post again when I have time to contemplate it more thoroughly.

[dlaliberte]

Ok I'll spill the beans.

Originally I was just thinking about generating electric power to store for later use when the heat pump system is less efficient, as explained in my previous post. But then, thinking about how much power I could expect to gain by doing that, and comparing to the cost of just storing power from the grid, rather that using some amount of power to move heat with the heat pump system, I realized that it seemed possible to generate enough power from this extra heat to power the heat pump system itself.

Immediately recognizing the perpetual motion device red flags, I went searching for answers as to why this might or might not be possible.

So I posed the question here as independently from the heat pump system as I could, to avoid the immediate knee jerk response that, of course, it must be impossible to generate enough power to run the heat pump system.

But imagine if a self powered heat pump system is no more magical than a siphon, which, once it is started, continues to operate until the water levels are equalized.

A heat pump is already kind of magical by being able to move more heat energy into a house than was required in electric energy to move it, by a factor of 3 to 5, and even 10. So why can't we use just a fraction of that heat energy to generate enough power to run the heat pump system?

And the answer I got elsewhere, and that I expect to get here as well, is that the temperature difference determines the efficiency of heat engines. Basically, it comes down to the fact that you can't get a higher temperature difference without using some amount of power, and then the most power you can expect to get back with a heat engine is no more than that power you put in to create that difference. That does seem to make some sense.

And yet, imagine you have a super insulated tank of hot water that represents some amount of stored energy. As long as the temperature of that water is hotter than the room temperature, why can't we convert some fraction of that stored energy to electric power, while wasting the rest to heat the house? We can, but supposedly, the closer that temperature is to room temperature, the more inefficient the conversion must become. That's the normally accepted answer.

But why, and what exactly does it mean to be more or less efficient? Maybe, rather than necessarily wasting the energy that is not converted, we can convert more of the energy if we do it more slowly, so the inefficiency involves time. Haste makes waste, so go slow and save. And if so, maybe we can compensate for the slower conversion time by scaling up the heat engine, or parallelizing the process with many more smaller heat engines.

I'm refraining from accepting the idea that all the energy that is not converted to some other form must be exhausted as waste heat out the cold side of a heat engine. But I'd still like to see proof that it can be avoided.

[Tom Booth]

On the way back from a trip to Florida to visit relatives, when I was, maybe three or four years old, my father bought me and my brother's a toy.

Once home, on the fireplace mantle, I became fascinated with it, making sure to keep it's glass of water full.

Basically a type of heat engine.

The glass of water serves as a kind of heat pump or evaporative cooler.

The evaporative cooler does not work entirely by itself using just passive evaporation, the heat engine/bird powers the mechanical operation of the heat pump constantly re-wetting the felt on the birds head, the swinging back and forth speeds up the evaporation/cooling process which creates a temperature difference that allows the heat engine to continue running on the pre-existing heat in the surrounding air, entering into the bulb at the bottom which contains a fluid in a partial vacuum which keeps boiling and condensing

https://youtu.be/Rq3K6Ma0wIU

A very sensitive LTD type Stirling engine will also run on evaporative cooling, such as a wet sponge or wet piece of paper. Kept wet by a wick from a glass of water one ran for two weeks, and only stoped because it had to be moved.

Evaporation is a natural process, but a Stirling engine running with evaporative cooling can assist/speed up the process. The flywheel acts a bit like a fan to keep air moving across the wet sponge.

Similarly a heat pump has air being blown through the condenser unit, a kind of porous metal "sponge" of sorts, to rapidly remove heat.

Until I see someone actually try it, for the above, and various other reasons, I wouldn't entirely rule out the possibility.

I think, to have a chance of working though, the heat engine and heat pump would have to be tightly integrated, and I wouldn't count on any tremendous power output.