Stirling Engine - why can't we make them?

[bobdole369]

Everything I've ever read about a Stirling engine or its variants seems to point to them simply not scaling. Why not? Is there something inherent about sizing up to 1kw or more that I just am not seeing? I do know that there have been a few actual successes - such as Whispergen, and Sunpower - but these units are TERRIBLY expensive, and require a LOT of maintenance (Whispergen looks to need a rebuild every 2000 hours of use) - and the unit seems to put out about 30A at 12V - so 350W???? Prices of over $10k and fuel usage of just shy of a quart of diesel or heating oil and hour.
It doesn't seem to approach the promised efficiencies.

The Honda ultraquiet gens seem to use about half that at full output (3.8 hours per 0.6gallon)

 

[Mark R]

Stirling engines have extremely poor power density (i.e. they need to be extremely large, and move a very large amount of gas, to generate a specified amount of power).

A gas or diesel engine just sucks in air/fuel, compresses it to the required level, then ignites it at maximum compression to get high power output. A Stirling engine needs to be permanently pressurised, and in order to match the power output of a gas engine, would need to be permanently pressurised to the maximum pressure reached inside the cylinder of a gas engine. The problem with permanently pressurising an engine to that sort of pressure is that the gas will leak out of the seals on any moving part.

Operating at lower pressures mitigates the leakage problem, but destroys the power output of the engine, while friction losses stay the same; so proportionally, efficiency gets wrecked.

What this means is that the moving parts and seals need to be made to exceptional tolerances (expensive) and have a short lifetime as even tiny amounts of wear will reduce their performance to unacceptable levels.

 

[Modelworks]

Stirling engines have extremely low torque.

 

[wirednuts]

if i had a country farm house, with no neighbors, i would build a car-sized sterling engine out back that is ran by steam that i generate using sunlight focused by parabolic mirrors. have that dump into a large battery bank system, and the excess steam can be used to heat water tanks. that would be just one way i would harness energy anyway

 

[Murloc]

Thermodynamics are frustrating, all the cool stuff is completely useless.

 

[Icewind31]

Sterling engines have an use with solar concentrators as a small scale renewable source (think remote locations). Using it as a primary, and also burning fuel to generate the heat for it is highly inefficient.

 

[bryanl]

Plutonium-fueled Stirling engines for future deep space missions?

http://green.blogs.nytimes.com/2011...-nuclear-energy-moves-to-mars/?pagemode=print

"One alternative is to develop a better way to convert heat into electricity in space. The National Academy report said that the method NASA uses now is only about 6 percent efficient. A Stirling Engine system could produce five times as much electricity from each unit of heat, reducing the need for plutonium, but it has many moving parts and has not been adapted to space use."

 

[Red Squirrel]

I've always wondered the same. The concept is rather nice, there is lot of high heat available everywhere (the sun) even in winter, when you concentrate it. I have seen on TV big dishes that concentrate the sun to a sterling engine and it produces power. But I guess it's the maintenance that's a killer.

Though, why not use steam instead? It seems it's a concept we don't see at all anymore outside of nuke plants. Could one not create a steam turbine powered by concentrated sunlight?

 

[Mark R]

Though, why not use steam instead? It seems it's a concept we don't see at all anymore outside of nuke plants. Could one not create a steam turbine powered by concentrated sunlight?

You can. However, steam and water are corrosive and for industrial use require lots of expensive materials and maintenance. Additionally, higher temperatures and pressures are better, there comes a point where building the heat collectors becomes too difficult.

The typical design for solar collector power plants is pumping a special heat transfer oil through pipes at the focus of parabolic collectors (my making the collectors long troughs, with a single pipe, they are much easier to build and maintain than individual dish collectors). The oil is then circulated to a heat exchanger, where it is used to generate very high-pressure, very high temperature steam for turbines. The oil step permits the collectors to operate at much higher temperatures than water/steam alone, and avoids the difficulty of having super-pressurised pipes exposed to the elements on moving concentrators.

An alternative is to use the "solar tower" design, of a vast number of reflectors focussing light to the top of a tower, where a collector transfer the heat to a liquid, which then transports the heat to a steam generator. Liquids vary, but molten salt at about 300 C has been used effectively. In some cases, a tank of fluid can be used to provide heat storage, allowing the plant to provide power continually, even in the event of temporary cloud cover, or to continue providing energy during the evening demand peak.

 

[Tuna-Fish]

Though, why not use steam instead? It seems it's a concept we don't see at all anymore outside of nuke plants. Could one not create a steam turbine powered by concentrated sunlight?

Huh? Pretty much all coal power plants that exist use steam turbines.

There are real, funded efforts to build up solar/steam power generation in areas well suited for it -- notably, Eskom in South Africa and BrightSource in Southern California.

 

[sm625]

Using a fresnel lens you should be able to run a solar thermal steam engine setup anywhere there is sunlight. On the Green Power Science channel on youtube, some guy burns holes in cinder blocks and beer bottles using fresnel lenses from old tvs.

 

[bobdole369]

Huh? Pretty much all coal power plants that exist use steam turbines.

Yup, and they are not as simple as you think. There is a reason it costs a LOT to run one of these. So there probably isn't anything off the shelf you can buy that is meant to run at the decent pressures needed. It'll take more than the piping used for hot water radiators.

 

[StirlingMaker]

Physical Scaling is difficult. When the gas cylinder measurements are doubled,
the gas volume goes up by 8 but the heat transfer surface goes up by only 4.
Bigger is not better!
Stirlings are low power engines. Kockums make 100hp and Tessera Solar make 25Kw but these figures are very low compared to petrol and diesel.
So only low power applications make sense.
There is no economy of production scale - thus prices are high.
They are however, very reliable - enough for NASA to propose Stirling for the electrical generation for the moon lab.
So, they are suitable for low power, long running applications
In the past this was water pumping. Now it's small electricity generation - and perhaps heat pumping?

 

[Georgeisdead]

You can. However, steam and water are corrosive and for industrial use require lots of expensive materials and maintenance. Additionally, higher temperatures and pressures are better, there comes a point where building the heat collectors becomes too difficult.

The typical design for solar collector power plants is pumping a special heat transfer oil through pipes at the focus of parabolic collectors (my making the collectors long troughs, with a single pipe, they are much easier to build and maintain than individual dish collectors). The oil is then circulated to a heat exchanger, where it is used to generate very high-pressure, very high temperature steam for turbines. The oil step permits the collectors to operate at much higher temperatures than water/steam alone, and avoids the difficulty of having super-pressurised pipes exposed to the elements on moving concentrators.

An alternative is to use the "solar tower" design, of a vast number of reflectors focussing light to the top of a tower, where a collector transfer the heat to a liquid, which then transports the heat to a steam generator. Liquids vary, but molten salt at about 300 C has been used effectively. In some cases, a tank of fluid can be used to provide heat storage, allowing the plant to provide power continually, even in the event of temporary cloud cover, or to continue providing energy during the evening demand peak.

Steam and water such as you find in industrial boilers do not require exotic alloys. Simple stainless steel is a typical material of construction.

Remember, a stirling enginer requires a gas that does not condense anywhere near ambient temperatures and pressures. That's why air is part of a typical stirling engine.

 

[sao123]

I've always wondered the same. The concept is rather nice, there is lot of high heat available everywhere (the sun) even in winter, when you concentrate it. I have seen on TV big dishes that concentrate the sun to a sterling engine and it produces power. But I guess it's the maintenance that's a killer.

Though, why not use steam instead? It seems it's a concept we don't see at all anymore outside of nuke plants. Could one not create a steam turbine powered by concentrated sunlight?

would not a pressurized steam turbine system suffer as a result of reduced or no gravity?

 

[Tiamat]

Thermodynamics are frustrating, all the cool stuff is completely useless.

At least undergrad thermodynamics yeah. Graduate Thermodynamics you realize all of the cool stuff is ridiculous for real-world :/