Why do Stirling engines have low power/weight?

[Hercuflea]

I have heard that Stirling engines are not in wide use because they have a low power-weight ratio. Why is this?

It would seem to me that the Beta type stirling engine (with 1 cylinder) would be very efficient because it only has like 3 major moving parts...flywheel crankshaft and piston and only one cylinder to wear out. I can't seem to find info about the total power output of the original stirling engine designed in the 1800s...anyone know this?

 

[russ_watters]

You are mixing a few different concepts. Stirling engines are efficient, but power to weight ratio isn't efficiency. Efficiency is power out divided by power in.

The reason they have a low power to weight ratio is that they use heat exchangers to move heat in and out rather than burning fuel, which carries energy more densely and releases it more quickly.

 

[Hercuflea]

You are mixing a few different concepts. Stirling engines are efficient, but power to weight ratio isn't efficiency. Efficiency is power out divided by power in.

The reason they have a low power to weight ratio is that they use heat exchangers to move heat in and out rather than burning fuel, which carries energy more densely and releases it more quickly.

So why are Stirling engines still not in wide use in stationary/steady state power applications?

 

[russ_watters]

So why are Stirling engines still not in wide use in stationary/steady state power applications?

Low power to weight ratio --- really low --- means they are expensive to make and cumbersome to operate.

 

[Nidum]

Low power to weight ratio --- really low --- means they are expensive to make and cumbersome to operate.

+1 on that . Also :

They do not scale up to large sizes very well . Multiple reasons but main one is that the gasses that can be used in practical versions are poor conductors of heat and it becomes more difficult to get effective heat exchange as the cylinder volumes become larger .

There are in any case better engine cycles for practical power generation on larger scales .

When deciding which engine cycle to use for an application theoretical efficiency is only one of many factors to be considered .

 

[Nidum]

Just for interest : the Stirling cycle is used very effectively to produce liquid gasses : Stirling Cryogenics

 

[Hercuflea]

So what would be the most practical external combustion engine?

 

[russ_watters]

So what would be the most practical external combustion engine?

I would think that the typical definition of "practical" requires choosing as an answer, the type(s) that are used in real life. Cars, for example, primarily use the Otto cycle since it is most practical for that application.

 

[Hercuflea]

I guess to clarify...what would be the most practical external combustion engine for energy production? The Otto cycle is internal combustion.

Also, what do you think about the added benefits of Stirling and other external combustion engines in comparison to ICEs? I.e. Maintenance costs would be extremely low since none of the combustion products make contact with any lubricants.

 

[russ_watters]

Sorry, I missed the "external" part.

The most common for power applications is the Rankine* cycle.
*Autocowrecked fail corrected. Thanks @Merlin3189

 

[OmCheeto]

I guess to clarify...what would be the most practical external combustion engine for energy production? The Otto cycle is internal combustion.

Based on the Ivanpah Solar Power Facility, my guess is steam turbines.

https://en.wikipedia.org/wiki/Ivanpah_Solar_Power_Facility#Power_towers
The LPT 550 solar system produces electricity the same way as traditional power plants – by creating high temperature steam to turn a turbine.

Also, what do you think about the added benefits of Stirling and other external combustion engines in comparison to ICEs? I.e. Maintenance costs would be extremely low since none of the combustion products make contact with any lubricants.

For a land based fixed unit, it might make sense. From my research over the last hour, state of the art Stirling engines are 64 times more massive for a given output compared to automotive sized gasoline fueled engines.

 

[Hercuflea]

Based on the Ivanpah Solar Power Facility, my guess is steam turbines.

For a land based fixed unit, it might make sense. From my research over the last hour, state of the art Stirling engines are 64 times more massive for a given output compared to automotive sized gasoline fueled engines.

Where were you able to find data about power output for medium-large sized Stirling engines? Everything I see on Google is just about tin can DIY engines.

The applications I have in mind are for small to mid scale power generation in remote/underdeveloped communities. I also think they could work well in cargo ships.

Is the power to weight ratio for a rankine cycle external combustion engine significantly greater than for a Stirling engine?

 

[OmCheeto]

Where were you able to find data about power output for medium-large sized Stirling engines? Everything I see on Google is just about tin can DIY engines.

A few different places:

A Swedish company that makes them: SUNBOX FOR SOLAR PARKS

Provided me with an image that I grossly estimated the mass at 1000 kg. I may be off by +/- a factor of two or more.​

A paper written in Finnish: Utilization of Stirling engine vehicle use

Provided me with the thermal efficiencies of Stirling vs Other types of engines.​

et al, for ICE power to mass ratio​

I had to interpolate some of the data to come up with my "64 times" comment.

The applications I have in mind are for small to mid scale power generation in remote/underdeveloped communities. I also think they could work well in cargo ships.

From the paper written in Finnish, I was able to extract that Sterling engines are roughly as thermally efficient as internal combustion engines. So given their power to mass ratio disparity, I'm thinking they are going to be only good for a very niche market/problem.

Is the power to weight ratio for a rankine cycle external combustion engine significantly greater than for a Stirling engine?

I don't even know what "rankine cycle" means. I'm a retired mail room clerk. Sorry!

 

[Nidum]

@OmCheeto

The most common application of the Rankine cycle is in the steam engine .

1800 HP Rankine cycle engine

Many modern power stations till use steam technology but they use steam turbines rather than reciprocating engines .

 

[jack action]

Where were you able to find data about power output for medium-large sized Stirling engines? Everything I see on Google is just about tin can DIY engines.

​

• Popular Science, Stirling auto engine
• Phase I Results from the StirlingPowered Vehicle Project
• Automotive Stirling Engine - Mod II Design Report

 

[OmCheeto]

​

• Popular Science, Stirling auto engine
• Phase I Results from the StirlingPowered Vehicle Project
• Automotive Stirling Engine - Mod II Design Report

Bwah! Hahahaha!
I watched the video, and they mentioned "improved fuel economy". Being somewhat old, I kind of remember what typical fuel economy was back then. So I skimmed through the rest of your links. I was not disappointed in my memory.

Phase I Results from the Stirling Powered Vehicle Project
...
Fuel Economy
Fuel economv for the Stirling-powered van in
the expediter mission was reported by the Air
Force to be 6.3 mpg with all fuels used during the
10-month evaluation period. Fuel economy numbers
include fuel usage for both the gas-fired, front window
defogger and the rear-compartment heating
system. This compares with an average of 4.3 mpg
for four gasoline-powered vans and 8.1 mpg for
nine diesel-powered vans used at Langley in similar
missions during the same period.

I suppose 6.3 mpg was good compared to 4.3 mpg.