Are LTD Stirlings Suitable for power generation?

Discussion on Stirling or "hot air" engines (all types)
Wellington
Posts: 110
Joined: Mon Apr 18, 2016 4:02 am

Are LTD Stirlings Suitable for power generation?

Postby Wellington » Sat Apr 23, 2016 5:26 am

I've noticed a few large LTD projects one of which claims to have a 300W output (but whats the input?). Would i be right in saying that the whole point of a large LTD stirling design is that it requires only a very low level of heat to generate power with the downside being that it is as big as a tank? In other words can you use a LTD stirling to generate hundreds of watts with just a candle or a mildly sunny day? If so what kind of input output power ratio is achievable?
.Wellington.

Alfista
Posts: 60
Joined: Thu Feb 18, 2016 9:14 pm

Re: Are LTD Stirlings Suitable for power generation?

Postby Alfista » Mon Apr 25, 2016 5:56 am

Since I do not have advanced math skills, I must use some crude calculations to arrive at input and output figures for such machines but if you are interested in more sophisticated analysis, I would recommend "Stirling Maschinen" by Werdich and Kübler. It is available on Amazon. The size must be quite large even for 100 watts. Figuring a MEP of 2.5 to 3.5 psi, a displacer bore of 60", a power piston of 12" and 12" stroke, you might see 1 i.h.p. at 90 rpm after loses, you may harvest a third of that in electrical wattage. You could feed it with solar heated water below boiling temperature, picture 400 feet of black, 4" pipe in a warm, sunny location. It is an intriguing project. Hot water can be stored and the machine can run 24 / 7 but it is a project for the truly dedicated.

Wellington
Posts: 110
Joined: Mon Apr 18, 2016 4:02 am

Re: Are LTD Stirlings Suitable for power generation?

Postby Wellington » Mon Apr 25, 2016 11:30 am

thanks for the example Alfista. So If I follow you correctly: 1 horsepower = 745 watts divided by three = aprox 248 net watts output per hr for a machine 60 inches in diameter with an MEP (mean effective pressure) of 3 psi (pounds per square inch) @ 90 rpm (revolutions per minute). 248Watts x 24hrs = 5,952Watt net hrs per day. Nice If you have the dedication to build such a thing as you say and don't mind strange looks from your neighbours. Thanks again
Wellington.

burnit0017
Posts: 76
Joined: Sat Jun 26, 2010 9:54 am

Re: Are LTD Stirlings Suitable for power generation?

Postby burnit0017 » Sat May 14, 2016 5:47 pm


Wellington
Posts: 110
Joined: Mon Apr 18, 2016 4:02 am

Re: Are LTD Stirlings Suitable for power generation?

Postby Wellington » Wed May 25, 2016 10:30 am

Alfista wrote: You could feed it with solar heated water below boiling temperature, picture 400 feet of black, 4" pipe in a warm, sunny location. It is an intriguing project. Hot water can be stored and the machine can run 24 / 7 but it is a project for the truly dedicated.


ok, so we've established that hundreds of watts is possible but 400ft of 4" pipe is an awful lot of pipe. I'm curious to know why people build such large machines rather than go for a 100w compact andy ross alpha machine if the goal is power output. Is there some kind of advantage to the ltd other than a large solar dish collector? ie does it have a high power output to heat input ratio compared to alpha gamma and beta? my initial assumption was that the large displacement gives more power but the larger the displacement the more heat input that is required. can anyone clarify for me what the main advantages are to building a large LTD vs a compact machine like the andy ross 100watt if the goal is power generation. Thanks
Wellington

Ian S C
Posts: 1970
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: Are LTD Stirlings Suitable for power generation?

Postby Ian S C » Thu May 26, 2016 2:00 am

For power generation a small high temperature engine directly heated by the sun, charging a battery bank would probably be cheaper than the large LTD motor. For a reliable power supply it could be complimented by a wind turbine.
Ian S C

Wellington
Posts: 110
Joined: Mon Apr 18, 2016 4:02 am

Re: Are LTD Stirlings Suitable for power generation?

Postby Wellington » Thu May 26, 2016 2:17 am

Ian S C wrote:For power generation a small high temperature engine directly heated by the sun, charging a battery bank would probably be cheaper than the large LTD motor. For a reliable power supply it could be complimented by a wind turbine.
Ian S C


but whats the motivation of building such a large machine? is it because you can heat them with very little heat and get a larger output power ratio compared to a high temp engine and whats the advantage of such a massive displacer?
Wellington.

Alfista
Posts: 60
Joined: Thu Feb 18, 2016 9:14 pm

Re: Are LTD Stirlings Suitable for power generation?

Postby Alfista » Thu May 26, 2016 6:50 am

ok, so we've established that hundreds of watts is possible but 400ft of 4" pipe is an awful lot of pipe. I'm curious to know why people build such large machines rather than go for a 100w compact andy ross alpha machine if the goal is power output. Is there some kind of advantage to the ltd other than a large solar dish collector? ie does it have a high power output to heat input ratio compared to alpha gamma and beta? my initial assumption was that the large displacement gives more power but the larger the displacement the more heat input that is required. can anyone clarify for me what the main advantages are to building a large LTD vs a compact machine like the andy ross 100watt if the goal is power generation. Thanks
Wellington


Wellington, you are asking why a person would choose a more difficult path when an easier one appears to be available. I have gathered from some information by looking at various large LTD Stirling projects, these seem to only make sense if 1.) there can be some sort of efficiency inherent in the plans, 2.) there is an intentional choice in the use of a low tech system, ostensibly for the sake of maintenance or lack of industry. Here is one good example: The Tamera solar village. There, the fluid is pumped (by the engine) through translucent tubes on the greenhouse roof. Solving the primary problem of most greenhouses, too much heat ! That fluid is then the heat reservoir for the engine. It is this synergistic efficiency that makes good sense in this application. Also, as anyone who has tried to produce solar powered steam knows, it is far, far easier to get water to 200 degrees F. than it is to get it to 250 F for useable steam pressure.

If a person has ready access to very inexpensive, burnable fuel then steam or a high temperature Stirling engine makes sense. If the goal is to derive the energy from solar, the LTD Stirling has some virtue in that solar water heating is about 5 times more efficient than photovoltaics and about one-sixth as expensive, also such a system requires no heliostat when compared to the "average" solar powered Stirling or steam system.

Wellington
Posts: 110
Joined: Mon Apr 18, 2016 4:02 am

Re: Are LTD Stirlings Suitable for power generation?

Postby Wellington » Thu May 26, 2016 9:09 am

Alfista wrote:
If a person has ready access to very inexpensive, burnable fuel then steam or a high temperature Stirling engine makes sense. If the goal is to derive the energy from solar, the LTD Stirling has some virtue in that solar water heating is about 5 times more efficient than photovoltaics and about one-sixth as expensive, also such a system requires no heliostat when compared to the "average" solar powered Stirling or steam system.
[/size]



Thanks Alfista, I'm starting to fill in the gaps. So If I have understood you correctly people opt for such a large system because if your going 100% solar it needs to be super large to come anywhere close to matching the power outputs of a much smaller sized high temp engine and like in the case you mentioned 400ft of pipe is a lot of pipe. Thanks again
.Wellington.

Alfista
Posts: 60
Joined: Thu Feb 18, 2016 9:14 pm

Comparing heat input for LTD Stirlings vs solar steam

Postby Alfista » Fri May 27, 2016 9:40 am

Here is some more data for comparison. First, my hat is off to anyone who is able to make either one of these systems work to produce useable power. I have made some successful solar steam powered models but building to larger sizes increases the challenge exponentially.

I am presenting this to explain the relative attractiveness of the solar powered LTD Stirling engine : To bring one pound of water from 62 degrees fahrenheit to 212 degrees fahrenheit at sea level and at atmospheric pressure requires the absorption of 150 btus. To then evaporate that same pound of water requires another 967 btus, 150 plus 967 = 1,117 btus and still no useable steam pressure ! To bring one pound of water from 62 degrees fahrenheit to 338 degrees fahrenheit (100 psi) and evaporate the entire pound, we need 150 + 876 (accounting for the latent heat of evaporation) plus another 126 btus or 1,152 btus. Now if we can apply about 450 degrees F to a surface area of about 7 square feet (applied to one pound of water), then we have the task well in hand and soon we will be making steam power. The difficulty is self evident.

By comparison, our solar heat source may be applied to a very large collector, we may find that it is a relatively easy task to bring 10 or 20 gallons of water to a temperature of 180 or 200 degrees F. If we use oil, under favorable circumstances, we may get temperatures of 350 degrees F., not yet boiling. In other words, we have the advantage of using very large, relatively inexpensive collectors, which may still reach high temperatures. We may be talking about hundreds of pounds of water absorbing thousands of btus or 1 kw of solar energy per square meter * a maximum absorption of 90% or 900w or about 3,000 btus of heat at maximum absorption under ideal conditions per square meter of collector. The efficiency of converting this power in an LTD Stirling is generally taken as High Temperature - Low Temperature / High Temperature. In a larger engine you might get something close to a 25% efficiency. I am aware that 90% absorption is very optimistic but even a solar trough collector may absorb 70% - 87%, a flat plate collector may yield around 60% to 70%.


Wellington
Posts: 110
Joined: Mon Apr 18, 2016 4:02 am

Re: Comparing heat input for LTD Stirlings vs solar steam

Postby Wellington » Fri May 27, 2016 10:42 am

Alfista wrote: Here is some more data for comparison. First, my hat is off to anyone who..............



you make some very good points. I agree with you that solar steam is very impracticle on a large scale not to mention the additional danger of boilers. Solar thermal is very simplistic and low tech reliable and when aiming to provide living needs like warm shower water there is no need for water to reach boiling point. I think I will eventually go for a simple hand pump on a stainless garden sprayer container for a heated 3min shower as ive seen people doing this with reliable results (electric pumps just complicate everything). My goal is to have a solar thermal system on the deck of a 30ft sailboat with an engine that can be detached from the collector and taken below deck and attached to a stove in the winter mths to charge a boats 12v electrical system or in the worst case scenario put in a campfire to charge 12v batteries. I'm looking for an alternative to a glass vaccum tube collector that can withstand the vibrations of a boat. Maybe adding vacuum insulation to a steel or copper black painted vacuum heat pipe may not be possible as the materials ive seen will degrade fast from UV exposure. Any ideas on this goal would be appreciated. All the best, your a ton of good advice.
.Wellington.

joweizbrot
Posts: 3
Joined: Tue May 17, 2016 2:25 am

Re: Are LTD Stirlings Suitable for power generation?

Postby joweizbrot » Sun Oct 29, 2017 9:17 am

Alfista, thanks for the interesting insights.
I think you erroneously used the same heat capacity of liquid water also for steam in your calculations but this is not the main point I wanted to make.
I don't understand well the deep thermodynamic reason for preferring steam to Stirling or vice versa, one should probably look at scaling laws and how different application requirements vary. Incidentally I am very interested if somebody has any hints on this subject.
A comment I feel like making about your " The difficulty is self evident" is that a fair comparison should keep in mind also that steam engines exist which take advantage of the energy which goes in the latent heat, for example the Watt-Bolton engine with condenser and the modern steam turbines, used for example on boats and the large electrical generator ones.
For a modern interpretation of the Watt Bolton engine see for example https://eprints.soton.ac.uk/381695/1/Re ... 6_2014.pdf

Ian S C
Posts: 1970
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: Are LTD Stirlings Suitable for power generation?

Postby Ian S C » Mon Oct 30, 2017 1:33 am

Here's what you get in old style Robinson hot air engine, not sure of the weight, must be getting on toward a ton, it's got a 10 inch bore, and produces about 1/3 hp.
Ian S C
[img]
DSC00729%20(640x480).jpg
[/img]
Attachments
DSC00729 (640x480).jpg
DSC00729 (640x480).jpg (254.96 KiB) Viewed 245 times


Return to “Stirling and "Hot Air" Engine Forum”

Who is online

Users browsing this forum: Baidu [Spider], Bing [Bot] and 42 guests