Stirling Engine & Heat Pump

[GuernseyPete]

If a heat pump has a COP of >3, could a Stirling Engine use the heat energy to both power the heat pump and generate additional power?

EG Heat pump requires 1KW to salvage a further 2Kw from the environment, resulting in >=3Kw output. Therefore 1Kw input to heat pump and 2Kw available surplus. Even at 50% efficiency that should result in 1Kw of usable energy, shouldn’t it? Is my maths wrong or what am I missing?

[Tom Booth]

This is from an old newspaper advertisement:

A heat pump doesn't make heat-it simply moves the heat that's supplied by nature. And since it takes less energy to move beat than it does to make it, a heat pump is a highly energy-efficient system. So. efficient, in fact, that it can deliver more than two units of heat for each unit of electricity needed to move· it.

From The News-Herald, 22 July 1982

I think heat pump technology has only improved since then.

To my mind heat is fuel for a heat engine, so a heat pump would basically be just a fuel pump to move the available heat in the environment to a location where it could be utilized by a heat engine.

Most engines have no problem whatsoever using some of their own output to operate a fuel pump.

But I've seen this idea debated a dozen times over the years, so why hasn't someone just gone ahead and tried it?

You have two systems not many people actually understand. Heat pumps are a rather specialized field, likewise heat engines are a bit on the obscure side. How to actually combine the two to work harmoniously together is something of an engineering challenge or I would have tried it myself a long time ago. But until I see someone actually do so, I wouldn't say it's impossible.

[GuernseyPete]

If nobody can suggest a reason why this wouldn’t work, I’m going to go ahead and try it. The implications are too far reaching to ignore and I can’t think of a reason why it wouldn’t work. I also asked the same question on the “Just have a think” YouTube channel and, so far, nobody has responded. It will take me a while to build it with my limited engineering skills and equipment!

[Fool]

Your question gets asked on several different forums, and answered better than I can do here. Here is one link:

https://www.physicsforums.com/threads/h ... to%20heat).

I would look for a good off the shelf Stirling Engine that gets your requested 50% efficiency at your requested 1000 Watts power requirement first. Then look at it's specifications very carefully. Or you could, if you know how, build one to those parameters. Remember a heat pump's COP is dependent on it's temperature difference. Here is a link on that:

https://clade-es.com/blog/what-is-the-c ... heat-pump/

Here is another:

https://en.m.wikipedia.org/wiki/Coeffic ... erformance

And finally:

https://www.livescience.com/55944-perpe ... hines.html

Good luck with your ventures.

[GuernseyPete]

Thank you for these links, I will give it a try. The physics forum link wasn’t helpful as they seemed to equate the configuration to a ‘free lunch’ or perpetual motion machine! There was also a claim about it breaking the 2nd law of thermodynamics, all of which demonstrated a lack of understanding about how these technologies work. They seem to have missed the point that heat pumps harvest energy from the environment that already exists. And heat pumps are used widely these days with COPs in excess of 3, I was being conservative.

I’m less knowledgeable about the efficiency of Stirling engines so I’ll focus on this first. Thanks again.

[Tom Booth]

Thank you for these links, I will give it a try. The physics forum link wasn’t helpful as they seemed to equate the configuration to a ‘free lunch’ or perpetual motion machine! There was also a claim about it breaking the 2nd law of thermodynamics, all of which demonstrated a lack of understanding about how these technologies work. They seem to have missed the point that heat pumps harvest energy from the environment that already exists. And heat pumps are used widely these days with COPs in excess of 3, I was being conservative.

I’m less knowledgeable about the efficiency of Stirling engines so I’ll focus on this first. Thanks again.

If you are seriously going to try such a thing, IMO it is unlikely to work if the focus is on transporting heat only

If you compress a gas it gives off heat which heat can be used to expand a gas in a heat engine to produce work

Pretty obviously this is a loosing battle as you can't get any more work out from expanding the gas than you put into compressing it. On top of that there will be loses

However an air conditioner with a COP of say 3.5 is throwing away heat. A heat pump with a COP of 3.5 is using heat but is also acting as an air conditioner at the point where the heat is being removed.

A Stirling engine can run on either heat or, in effect, cold, so, since a heat pump and air conditioner are essentially the same device, if there is any possibility of such a thing working, I would think it would depend on utilizing both the heat and the cold produced by the heat pump as it actually does both simultaneously.

Heat is taken away from one area and put into another.

Overall I would say the focus should be more on the production of cold. Infact, arguably, we already have at least one working example of a simple air conditioner and heat pump operating "perpetually" on ambient heat:

https://youtu.be/Rq3K6Ma0wIU?si=8aGDEZ-0kbeFNHaZ

[Tom Booth]

Above I meant to say "air conditioner and heat ENGINE".

Also the COP of a cooler can be effectively doubled by using water cooling on the condenser side.

That is, the compressor pumps/compresses the refrigerant into the condenser tubing. The condenser tubes, of course get hot, but if these are water cooled to remove the heat, because of the thermal capacity of water the heat can be removed more efficiently, which also means the compressor has less work, but the cooling capacity is increased. So instead of a COP of 2.5 or whatever, that could be increased to 5.0

So you can significantly increase the COP of a cooling system by dumping heat.

I think you can also increase the COP of a heat pump by heating the expansion/cooling tubes, say with solar, but it is more difficult to compress hot refrigerant than it is to compress cold refrigerant, so there seems to be a greater advantage to lowering the temperature to produce cold to run the Stirling engine rather than raising it.

There is a vast supply of ambient heat if you can use cold to create a temperature difference. The cold is also, theoretically easier to maintain, since the Stirling engine itself runs on (or consumes) heat and "rejects" heat at a lower temperature.

[GuernseyPete]

I agree, the temperature difference is key. Looking into the efficiency ratings for Stirling Engines I can see my target of 50% efficiency is currently unrealistic. My goal won’t be to use the configuration to produce anything more than a self-sustaining heat pump with a useful heat output . With a thermal efficiency of, say, 33% it would be necessary to use a closed system to utilise the heat energy lost in the Stirling Engine or somehow transfer the pump and compressor directly from the heat pump. Still lots to think about!