How to scale up a LTD Stirling engine?

[Rogetit]

I am new to Stirling engine. I would be very glad if anyone could help me with my questions.

I have a project that have 70-80^oC hot water as waste heat. I am trying to use the waste heat to generate electricity, target at getting ~1kW electrical power. I have looked into thermoelectric generator but the cost is too high and efficiency is too low at low temperature differential. The efficiency of LTD Stirling engine (in theory) seems much better in comparison.

For industrial application, I only found one LTD Stirling engine available, which is Sunpulse 500, but they have not responded to my request, probably because I haven't write my request in German. So, I am thinking of building my own LTD Stirling engine.

I have found a LTD Stirling engine design including dimensions from an article: http://gistworldconpro.com/uploads/4/3/ ... obitra.pdf. Their engine's efficiency looks promising. However, the article does not report the load of the LTD Stirling engine, but judging from its size I believe it needs to be scaled up for my target power output. I know that to have one large LTD Stirling engine may be impractical. I don’t mind to have 4 or 10 engines combined to give ~1kW output in total, but I don’t want to have like 50 engines either.

My questions are:
1. Is a temperature differential of 40-50^oC useful to generate electricity using? From literature and maths, it should still give 5-8% heat to electrical efficiency, which is Ok for my application.
2. How to scale up a LTD Stirling engine? Should I just scale up every dimensions?

Roger

[Wellington]

You might find the answers you seek in my post here: http://stirlingengineforum.com/viewtopic.php?f=1&t=2077
.Wellington.

[Rogetit]

Wellington, thanks for the information.

60" diameter is really large and I need 4 of them to get ~1kW. If I want to build multiple 20" engines, how would the dimensions and power scale down?

[Wellington]

I'm a newbee like yourself to stirlings and this forum and so am not the best person to answer technical questions. From what I can tell most people designing for power go down the large alpha engine route like dieter viebachs engine from the 80s. Other projects of interest to look up are derwoodvw on youtube, NASA engine in one hour on youtube , and "15w stirling" and "metronome engine" aka laminar flow aka thermal lag pulse engine. Also look into graphene heat pipes and wood gasifier stoves or rocket stoves as possible heat sources. Those examples should give you an overview of what people are currently doing and how you might proceed with your own ideas and goals. Quite a few people it seems on this forum including myself have similar/same goals to yours.

[Wellington]

Wellington, thanks for the information.

60" diameter is really large and I need 4 of them to get ~1kW. If I want to build multiple 20" engines, how would the dimensions and power scale down?

you can get 1 kw out of a small compact free piston type engine but as far as I know no company currently sells one for less than 10k and no plans are available. also watch greenpowerscience channel on youtube for lots of useful vids.
.Wellington.

[Alfista]

Rogetit,
if you have already calculated the dimensions of your engines, then you can calculate the output by the P.L.A.N. formula (mean effective) Pressure * Length of stroke * Area of piston * Number of strokes per minute, now divide all of this by 33,000 and this will give you the indicated horse power. If it were my engine, I would not count on more than a third of that as electrical output. Also, your water temperature will probably work but it would probably not result in something close to the maximum potential output for such an engine.

Also, keep in mind that the above formula has to do with actual pistons. If it is a diaphragm, I would use Pi*R2 /2. Also the Length of stroke is multiplied * 2, since the engine is double acting from both pressure and vacuum. I would suggest 2.5 psi as a possible mean effective Pressure.

To me it sounds like a massive undertaking.

[Rogetit]

Rogetit,
if you have already calculated the dimensions of your engines, then you can calculate the output by the P.L.A.N. formula (mean effective) Pressure * Length of stroke * Area of piston * Number of strokes per minute, now divide all of this by 33,000 and this will give you the indicated horse power. If it were my engine, I would not count on more than a third of that as electrical output. Also, your water temperature will probably work but it would probably not result in something close to the maximum potential output for such an engine.

Also, keep in mind that the above formula has to do with actual pistons. If it is a diaphragm, I would use Pi*R2 /2. Also the Length of stroke is multiplied * 2, since the engine is double acting from both pressure and vacuum. I would suggest 2.5 psi as a possible mean effective Pressure.

To me it sounds like a massive undertaking.

Thank you for the formula, Alfista.
How to know the Mean Effective Pressure of my engine? Is there another formula related it to temperature difference?

[Alfista]

I will give you a related calculation. The engine's Carnot efficiency is (Th-Tc) ÷ Th. Temperature measured in Kelvins. Let us say that the ambient temperature in 20 degrees C. or 293 K and the hot water is 353 K. or 80 C. Then (353-293) ÷ 353 = .1699 or 17%. Fantastic ! But unfortunately, too optimistic so now divide it in half. A really well built engine might get 8.5 %. So if you could calculate the absorbed btus, convert to watts and multiply by 8.5 to find power output in watts, convert to horse power, then using the PLAN formula, ihp × 33000 = P×L×A×N. From this, you could, in theory calculate the pressure. If it is above 5 psi, there is probably a mistake somewhere ... I would remind anyone reading this post that building large LTD engines for power production is experimental at best. I would again recommend the Senft book on LTD engines and Stirling - Maschinen by Kübler and Werdich.

[Alfista]

 A really well built engine might get 8.5 %. So if you could calculate the absorbed btus, convert to watts and multiply by 8.5 to find power output in watts, .

Multiply by .085 as it is a percentage of the heat absorbed.