2 KWh LTD Stirling Engine

[Hutch687]

I am currently working on building an LTD Stirling Engine (or 2) to provide electricity for my home. I am using Evacuated Tube Collectors to heat vegetable oil to 180-200 Centigrade in a closed system. I have not decided on my coolant yet, but it will either be water being pumped in copper pipes in a closed system through the bottom of my pond, or perhaps another liquid that has a very low boiling point, such as Alcohol or Acetone without pumping, but instead put inside a large, hollow heat sink.

I have not decided between a beta or gamma type engine yet. I am not concerned with size, but each part of the engine must be small enough for me to be able to lift/maneuver on my own or with the help of a friend. I am getting a professor from my alma mater to help me with some of the more complex calculations. Currently I have three current engine designs in mind as possibilities.

The First, which seems the most practical is the Sunpulse Electric:

https://www.youtube.com/watch?v=v_e0981CLDI

The village at Tamera has a larger, 1.5 KWh version. This design looks very efficient, but it also looks overly complicated and I have been unable to find the internal design of it anywhere other than a few poor diagrams.


The Second option would be based on this Stirling Engine built by Saitama university:

http://www.bekkoame.ne.jp/~khirata/acad ... index.html
http://www.bekkoame.ne.jp/~khirata/acad ... strct.html

This has the advantage of being sealed for pressurization of the air inside, but would likely be quite large.


My Third option is a scaled up version of the coffee cup stirling:

http://www.grand-illusions.com/acatalog ... gine_1.jpg

I really like this one due to the simplicity of the design and ease of maintenance, even though it would likely have to be larger due to being more inefficient.


So, my question to all of you is which do you think I should build, and why? Also, any help on finding specific design plans for the first two would be appreciated.

I will post pictures and videos of my progress and the final product over the course of this project. My plan is two have a 1/10th scale version finished within 1 year, and the final product complete within 18 months.

So, any and all help would be appreciated!

[Aviator168]

Actually, the Sunpulse is simpler than the scale up LTD. The big thing you see moving up and down is actually the power piston. Assuming you can get 20% mechanical power, @ 180rpm, you are going to need to heat at least 10 grams of air up by 100 degree and cool it down again in every cycle. As to how that gets down, it is up to your imagination. If you are going to design it yourself, I suggest you to take a class in heat exchangers.

my 2 c

[Ian S C]

And win a Lottery.
There is a site on Google from the University of Canterbury (NZ) on the development of a large, low temperature Stirling Engine.
Ian S C

[Aviator168]

Hey Ian. This is by no mean, is not impossible. The biggest challenge is the displacer. If the stroke is 5 cm, you are talking about a displacer with a diameter of greater than half a meter. Making that thing light enough is a challenge, not impossible.

[Hutch687]

Hey Aviator,

I would prefer to use a design that has been built and tested by someone else if possible. That would just be simpler in my opinion (im a big fan of simplicity).

Perhaps if someone can help me decipher the exact internal setup of the Sunpulse I will build it since the dimensions for it have been released.

Here is a link to the patent: https://patentscope.wipo.int/search/en/ ... eryString=

If you go to the documents page and open the 2nd PDF (48 pages) it is fairly extensive and explains most of it, but I still havent been able to figure out the design and location of the heat exchanger, the heat sink, and exactly where the working fluid (air) moves.

If the community here thinks the Sunpulse is my best bet and/or want to see it built then I'm okay with it I think, since I know it works reliably and would have a lot less trial and error.

[Aviator168]

If you look at Fig.1 12 is the heater, 11 is the regenerator, and 13 is cooler. The displacer being such smaller volume; it is indeed a very smart design.

BTW. You can get rid of the water/oil pumping mechanics; instead, use electric pumps with electronic control. You can also, change design a bit and use off the shelf radiators for the heat exchangers; it will save you lots of work.

Correction: I need to correct my earlier post "heat at least 10 grams of air up by 100 degree and cool it down again in every cycle"
shoud be "heat at least 40 grams of air up by 100 degree and cool it down again in every cycle"

[Alfista]

Here are a few calculations on heat collection and power production in a solar heated, LTD Stirling engine system. I have been looking at the feasibility of such a system. Without putting too fine a point on it, it is easy to see why such a system must be deployed within a larger, synergistic project. For instance, in the case of Tamera Solar village, the larger of the two engines is also part of the system which supplies hot water, including heat for cooking in the kitchen and serves to cool the greenhouses. These products are probably more valuable than the electricity produced by the LTD engine.

The heat collection is no small matter. If you figure a collector efficiency of 33% (over 8 hours) and a thermal efficiency of the engine at 10% (whereas several large engines in use actually operate at about 5%), if you collect 25k btus per day, an average of about 3k btus per hour over 8 hours, the best case i.h.p. output is 88W. Using these figures, if you collect 100,000 btus per day, an average of 12,500 btus per hour, the best case i.h.p. output may be 352W. I realize that this does not actually take TΔ into account but this is based on raising temperature above the ambient temperature. Also, it does not take into account losses related to heat storage or transfer. By contrast, imagine four pv panels, each 4 feet by 8 feet. Of course they are more expensive than the solar thermal collectors per square foot but the advantages are quite obvious.

I am currently building a model LTD engine, with two aluminum plates which are each 4 square feet, aiming for about 8w of indicated output, maybe six watts of shaft power and three or four watts of electrical power.

[Aviator168]

Yeah. You need to account for all the pumping lost. Force oil and water through those small tubes takes a lot of energy. However, they can still claim high efficiency since the heat rejected by the engine is being used to heat the swimming pool.

I am currently building a model LTD engine, with two aluminum plates which are each 4 square feet, aiming for about 8w of indicated output, maybe six watts of shaft power and three or four watts of electrical power.

Assuming 4 square feet is the heating/cooling area, you have to be very lucky to get that power.

[Hutch687]

I know Jurgen claims 12% efficiency for electricity generation on the Sunpulse, and 90%+ when he includes the heat. I have plenty of uses for heat, and I have no qualms about having an always-ready BBQ for some smoked pork.

Hey Alfista, I am very aware of the advantages of PV panels as my folks have them on their house. I am wanting to use a stirling engine partly for the fun of it, and especially because I will be able to do all necessary maintenance myself. So thank you for informing me about the easier method of PV cells, but I really just want to do the Stirling engine. So any help or input on that would be greatly appreciated.

[Aviator168]

When you lower the temperature, everything has to get bigger in size and it costs a lot of energy to move them back and forth.

[Alfista]

Hey Alfista, I am very aware of the advantages of PV panels as my folks have them on their house. I am wanting to use a stirling engine partly for the fun of it, and especially because I will be able to do all necessary maintenance myself. So thank you for informing me about the easier method of PV cells, but I really just want to do the Stirling engine. So any help or input on that would be greatly appreciated.

Hutch,
my reference to the PV panels was not meant to discourage you from building a large LTD Stirling. On the contrary, I admire your ambition. I suppose that I was reacting to the notion of building such a large engine, 2kW is an enormous output from a LTD Stirling. Having said that, perhaps it would be best to begin with either the size of engine that you would like to build or what resources you have to devote to harvesting heat. However, if you truly wish to begin with the output as a goal, then in my opinion, it is too great a task to set out upon without developing such a system in stages, by first building some models and scaling up.

I will recommend a few books which may be helpful. Dr. Senft's book "Low Temperature Differential Stirling Engines" is terrific. It provides both theory and an excellent set of drawings and instructions for building the N-92. One may not find better crafted plans than these. The book also has a good bibliography.

An excellent, cursory overview of "high powered" Stirling Engines, including LTD engines, can be found in "Stirling-Maschinen: Grundlagen - Technik - Anwendungen" by Werdlich and Kübler.

There are a couple of other books that are worth mentioning, these are written for folks who do not have access to machine tools such as a lathe and mill. Larsen has two books on LTD engines, ideal for beginners. Roussel's "10 Moteurs Stirling LTD de type Gamma: plans, schémas, réalisations" is so easy to follow. He does an excellent, easy to understand analysis of power output and at least a couple of the models involve small generators. This book, although perhaps not as essential as the Senft, is nevertheless so delightful, well written and entertaining that I would highly recommend it.

Just two more books, a little further down on the list : "Der Stirlingmotor, einfach erklärt und leicht gebaut" by Dieter Viebach. I found this book a little denser and more difficult but the models are well thought out, the plans and building instructions are thorough and they include many excellent suggestions on building without machine tools.

Lastly, I would suggest anything by Ivo Kolin, the inventor of the LTD engine. Sadly, the only book that I have been able to find by Kolin is "The Evolution of the Heat Engine". He is so brilliant. The few articles on the Stirling Engine found in this book would in themselves justify the price of the book. The book is illuminating.
I hope that you may find these suggestions helpful.

[Aviator168]

Alfista, while you are at it. Do you have an estimate the amount of work can be extracted from a heat engine in cycle? For example, if you manage to inject 100 J of heat into the working fluid, how much work can be extracted with a cylinder and a piston?

[Alfista]

I am sure that someone here on the forum can give you a better answer than I can.

In relation to my previous post. Kolin was a physicist and spent a lot of time on this question in his book "The Evolution of the Heat Engine." Although I aspire to improve my understanding of these things, I am confined to very basic conversations about heat and power.

I will offer one formula which I came across in the Viebach book "Der Stirlingmotor: einfach erklärt und leicht gebaut" He writes about determining the pressure or vacuum created by a change in temperature. Δp = (T2-T1)/T1 I realize that this is very similar to the simple version of the formula for determining Carnot efficiency. This reads that the Δ (Druck) Pressure is equal to the high temperature minus the low temperature, the result is then divided by the low temperature. Temperature is measured in Kelvin, Pressure is measured in bars. Eg. Let's say T2 is 423K (301.7F) - T1 348K (or 166F) divided by 348 = .2155, let's say .2 bars. Now divide this figure in half because not all of the gas is efficiently heated and we have .1 bar or 1.47 psi. It seems quite plausible.

Also, the Carnot efficiency number is often given in relation to this question because, in effect, it sets a theoretical limit on the maximum amount of work that can be extracted from a change of temperature or difference in temperatures in a fluid. I confine myself to working with the simple version of the formula.
Th-Tl / Th = C% Subtract the low temperature from the high temperature, then divide the result by the low temperature. Using the temperatures in Kelvin given above, (423 - 348) / 423 = .177. So this engine will have a Carnot efficiency of 17%. The Carnot efficiency is the efficiency with which heat is transformed into mechanical power. This figure however represents a maximum, ideal efficiency. It tells us that our engine will not approach an efficiency as high as 17% in this case. It also tells us that as we increase the temperature difference, we also increase the theoretical efficiency of the engine. Similar to the formula above, quoted by Viebach, Senft suggests dividing the number in half in order to bring it close to a number which may reflect a real world efficiency of an engine, suggesting an efficiency of about 8% and the true number may be still lower ...

[Aviator168]

The whispergen has an over all electrical efficiency of 9% with a tiny regenerator and no economizer.

[Ian S C]

Whisper Gen 1kW electrical output, 5kW hot water(for domestic HW supply, central heating).
Ian S C