Projections on Stirling engine efficiency

[Seeker]

Hi everyone,

I’m excited about the possibility that Kamen’s Stirling engines may come into production in just over a year from now !

I would like to dedicate this thread to trying to come up with a projection on the efficiency of Kamen’s Stirling engines.

The Vanity Fair article of earlier this year described one of Kamen’s Stirlings as being the size of a shoe box and the other the size of a bread basket. I would imagine that the former engine would be used with the Segway, and the latter would be the Stirling which is part of the generator/water purifier.

It seems like Kamen is saying that the smaller Stirling would have an efficiency of 20%

This is from the Harvard Cyberposium and Kamen was talking about the Stirling which would be used on the Segway :

“Once you add the rest of the practical realities, we'll probably come out at about 20 percent efficient at turning heat into electricity. But that's about twice as good as any little engine you buy.”

So could the fact that this 20% efficient model is the size of a shoe box and the other Stirling is the size of a bread basket, be used to come up with a rough estimate of the efficiency of the Stirling used in the generator/water purifier ?

Seeker

[Casey]

I'm afraid you are way over my head on this subject. But this website gives an efficiency of 40%.

quote:High theoretical thermal efficiency
The thermal efficiency of Stirling cycle that is used for Stirling engine is equal to that of Carnot cycle which is theoretical higiest efficiency. Expression of Carnot efficiency is follows.

1-Tc/Th

Where

Th : Higher temperature in the cycle
Tc : Lower temperature in the cycle

This means as to be large for a temperature difference, theoretical heat efficiency are higher. Among Stirling engine developed, the engine to achieve a thermal efficiency 40 % exists.

http://www.bekkoame.ne.jp/~khirata/academic/kiriki/begin/general.html

[Casey]

Also, if you are talking fuel efficiency, I think that depends on the fuel. While hydrogen might achieve 40%, cancentrated sunlight would seem to me to have nearly 100% efficiency if shielding were used to avoid radiated heat from escaping the "hot" end of the engine.

[wderooy]

From 60 Minutes 2 it was my impression that the Stirling would be used
only as a separate APU and not as an on-board generator on the Segway HT. Am I wrong?

[Seeker]

quote:Originally posted by Walter deRooy

From 60 Minutes 2 it was my impression that the Stirling would be used
only as a separate APU and not as an on-board generator on the Segway HT. Am I wrong?


Hi Walter,

In a Vanity Fair article done last year, it talked about Kamen producing Stirlings of 2 different sizes, one the size of a shoe box (Segway Stirling ?) and the other the size of a bread basket (APU Stirling ?). Then, in the Harvard symposium, Kamen was referring to a Stirling which would clip on to the Segway, so this would probably be the shoebox sized Stirling.

Seeker

[don c.]

quote:Originally posted by Seeker


Hi everyone,

I#8217;m excited about the possibility that Kamen#8217;s Stirling engines may come into production in just over a year from now !

I would like to dedicate this thread to trying to come up with a projection on the efficiency of Kamen#8217;s Stirling engines.

The Vanity Fair article of earlier this year described one of Kamen#8217;s Stirlings as being the size of a shoe box and the other the size of a bread basket. I would imagine that the former engine would be used with the Segway, and the latter would be the Stirling which is part of the generator/water purifier.

It seems like Kamen is saying that the smaller Stirling would have an efficiency of 20%

This is from the Harvard Cyberposium and Kamen was talking about the Stirling which would be used on the Segway :

#8220;Once you add the rest of the practical realities, we'll probably come out at about 20 percent efficient at turning heat into electricity. But that's about twice as good as any little engine you buy.#8221;

So could the fact that this 20% efficient model is the size of a shoe box and the other Stirling is the size of a bread basket, be used to come up with a rough estimate of the efficiency of the Stirling used in the generator/water purifier ?

Seeker


If we consider the Stirling from kamen in the application of power-genration and water purification, the total effiency is the combination of electrical and heat efficiencies. This may help, from Ocean Power:

http://www.powerco.com/Power/hpu/

The HPU System

In the HPU (Home Power Unit) system, the conversion of fuel into heat and electricity is highly efficient. Around 65% of the primary energy is converted into heat and 25% into electricity. While this might appear to represent an overall efficiency of 90%, the value of electricity is substantially higher than heat. Therefore, comparison should be made with centrally generated electricity with a delivered efficiency of less than 40% (primary fuel to electricity) in addition to fuel consumption for heating with a typical efficiency of 70%. Compared to all-electric dwellings, the efficiency improvement will be 75% with slightly lower figures for units with gas or oil fired heat.

[Seeker]

Hi guys,

We know that the efficiency of a Stirling engine is somewhat related to how much heat it loses to the atmosphere & the amount of heat lost, depends largely on the surface area of the engine.

If the surface area of an engine is compared to the volume of the engine, this gives us a ‘surface area to volume ratio’. This number can be used to compare the surface areas of engines of different sizes. (Again, this surface area effects the efficiencies of these engines of different sizes)

Let’s assume an engine is shaped like a cube, and has dimensions of 1x1x1 (length,width, & height)...we can ignore units.

The surface area to volume ratio can be found by :

1 squared x 6 ( the area of the 6 faces of the cube)
------------------------------------------------------------------------
1 cubed ( the volume of the cube)

= 6/1....so the surface area to volume ratio is 6

If the surface area/volume ratios are compared for cubes of size 1,2 & 4 the surface area/volume ratios are 6---> 3---> 1.5 respectively

In other words, when you double the dimensions of an engine, it has 1/2 as much surface area, compared to its volume. (So if efficiency depended only on the surface area of an engine, an engine twice as large would be twice as efficient).

It seems that Kamen is indicating that the Stirling which is the size of a bread box is 20% efficient ( at converting heat to electricity). So if surface area ( & heat loss) were the only factor determining engine efficiency, an engine twice as large as a shoebox, might have an engine efficiency of around 40%.

But of course surface area is only 1 determinant of engine efficiency. I checked into it and these are the major determinants of engine efficiency, when you compare Stirlings of different sizes :

mach number, surface to volume ratios, temperature ratios of absolute degrees

I’m interested to know what 'mach number' is, and what ‘temperature ratios of absolute degrees’ is, as these factors may give us further insight into what the efficiency of an engine ‘twice as large as a shoebox’ might be like.

(Btw...what is a bread basket exactly ? ...is that one of those baskets you carry around at a grocery store ? )

Seeker

[Seeker]

quote:

If we consider the Stirling from kamen in the application of power-genration and water purification, the total effiency is the combination of electrical and heat efficiencies. This may help, from Ocean Power:

http://www.powerco.com/Power/hpu/

The HPU System

In the HPU (Home Power Unit) system, the conversion of fuel into heat and electricity is highly efficient. Around 65% of the primary energy is converted into heat and 25% into electricity. While this might appear to represent an overall efficiency of 90%, the value of electricity is substantially higher than heat. Therefore, comparison should be made with centrally generated electricity with a delivered efficiency of less than 40% (primary fuel to electricity) in addition to fuel consumption for heating with a typical efficiency of 70%. Compared to all-electric dwellings, the efficiency improvement will be 75% with slightly lower figures for units with gas or oil fired heat.



Hi Don,

Which fuel do you think would be the best/easiest one to use on a Stirling. Would it be propane ? That's interesting what you posted about the value of heat & of electricity. It's good that Kamen would be able to recoup the heat as well, using the water purification scheme.

Do the numbers I came up with relating to surface area and heat loss, seem to check out ?

Seeker

[don c.]

quote:

Hi Don,

Which fuel do you think would be the best/easiest one to use on a Stirling. Would it be propane ? That's interesting what you posted about the value of heat & of electricity. It's good that Kamen would be able to recoup the heat as well, using the water purification scheme.

Do the numbers I came up with relating to surface area and heat loss, seem to check out ?

Seeker


I don't think a direct relationship can be drawn between the total outside surface area/vs. engine volume of a Stirling, or any other engine, with regard to heat loss or overall efficiency. Different parts of an engine dissipate heat according to their type of material, shape, thickness, proximity to combusted fuels or coolants, etc.

The classic Stirling design is represented by a tube containing the working fluid (gas), with the 'hot-side' and 'cold-side' at either end. The higher the 'DT' , or Delta temperature (absolute temperature difference between hot and cold sides), the better the efficiency will be.

The three greatest areas of heat loss in a Stirling are: 1) The heat exchanger (the part that transfers heat energy from the external combustion source to the working fluid), 2) The regenerator (the wire mesh or other construct that alternately heats and cools the working fluid), and 3) The 'cold end', which actually may become quite hot during operation. It's the heat loss from the 'cold side' of Kamen's Stirling that he uses in purifying water, and that Ocean Power's HPU uses to provide heat and hot water for homes.

If I recall, Kamen has made a few patent claims with regard to the design and manufacture of the first two above areas.