Large LTD Stirling for CHP

Discussion on Stirling or "hot air" engines (all types)
Post Reply
MisterQED
Posts: 10
Joined: Wed Jan 29, 2014 11:18 am

Large LTD Stirling for CHP

Post by MisterQED » Thu May 21, 2015 11:36 am

Most of the work I have seen on Stirling Engines is directed toward small high pressure HTD Stirlings.
I understand because HTD means more efficiency due to increased possible Carnot efficiency, but unless the heat source is self contained, HTD leads to inefficiencies as any heat below the chosen temperature is wasted. i.e. How much energy is lost as hot exhaust? If you tried to power a Stirling using a propane torch, then even in ideal conditions, how much of the energy of that torch is absorbed by the engine and how much is lost to the air? This inefficiency increases with the target temperature as any heat below the target temperature must be discarded unless it can be recycled.
The high pressure part also makes sense as it increases thermal conductivity, but couldn't the same gains be made with scaling the whole system up? i.e. double the volume and half the pressure or quadruple the volume and quarter the pressure. This seems to be the path taken by Sunventions in a video I just can't stop thinking about.
Their design uses the sun to heat oil to power a single large cylinder LTD beta configuration to generate 1.5kw of nearly silent power. A review of the setup is here http://www.solarheatengines.com/2012/01 ... se-engine/.
Has anyone else followed this path?
The reason I ask is that this path would seem to avoid the need for tight tolerance machining and since the heating and cooling are piped in, with correct design, most of the structure could be made from wood/fiberglass or best fiberglass reinforced wood with steel pieces only needed for key components.
Also if the design were modified into an opposing cylinder configuration, then higher pressures could be used and some scaling down would be possible. So double the cylinders, double the pressure and possibly quarter the piston area. So drop the diameter in half.
Am I missing something?

Bumpkin
Posts: 131
Joined: Fri Mar 02, 2012 1:42 pm

Re: Large LTD Stirling for CHP

Post by Bumpkin » Sun May 24, 2015 11:31 am

Hi MisterQED. I think sometimes people follow technology away from their goals. I can't see much point in Stirling engines trying to compete with modern photovoltaics or internal combustion, but when burning alternative fuels for heat in cool climates, a CHP Stirling makes a lot of sense. Unfortunately most power-producing Stirlings are designed for relatively high temperature differences that make alternate fuels problematic. I tend to think that a high displacement low tech engine might be the best answer for residential CHP. So if you're missing something, I am too. :confused:
http://spragueriver.com/index.php/bumpkin-tech

Bumpkin
Last edited by Bumpkin on Wed Nov 25, 2015 12:29 pm, edited 1 time in total.

MisterQED
Posts: 10
Joined: Wed Jan 29, 2014 11:18 am

Re: Large LTD Stirling for CHP

Post by MisterQED » Mon May 25, 2015 9:03 pm

"I can't see much point in Stirling engines trying to compete with modern photovoltaics". This is a strange statement as modern Stirlings far exceed modern photovoltaics. PVs range up to 15-20%, but Stirlings reach 40% and hold the record for most efficient power conversion. http://www.theguardian.com/environment/ ... ity-system. Add in that a PV systems only last 20 years and Stirlings can last nearly indefinitely with service and the scales tip away from PV till cost is examined. Stirlings are currently too expensive. So we are of the same mind.
I looked at your site and get the direction you are trying to go, but from my understanding you are substituting on e problem for another. Yes, the thermal transfer from a gas to a metal mesh is great, but the temperature transfer from a solid to a metal mesh is poor. So yes you can get the heat out, but how do you put the heat in? The solution is to do exactly what all high performance Stirlings do and move the heating and cooling outside of the cylinders and I think I know how to do that with off-the-shelf parts. The key is liquid heating and cooling with lots of surface area supplied directly with heating or cooling by the liquid. At least that is what I think, though till I build one, I certainly could be horribly wrong.
Best of luck and sorry to hear about your setbacks,

Bumpkin
Posts: 131
Joined: Fri Mar 02, 2012 1:42 pm

Re: Large LTD Stirling for CHP

Post by Bumpkin » Wed May 27, 2015 5:48 am

I have far less money than sunshine, and my p.v.s are certainly more than 20 years old, but if we have different priorities for efficiency that's fine. Thanks for looking at my thoughts on heat transfer, I can see I'll have to work on being more clear.

Bumpkin

MisterQED
Posts: 10
Joined: Wed Jan 29, 2014 11:18 am

Re: Large LTD Stirling for CHP

Post by MisterQED » Wed May 27, 2015 8:53 am

"I have far less money than sunshine", well that is both a good and a bad thing. Lots of money would be nice, but constraints lead to originality. I am constrained by both time and money. I'm sorry if I was off-putting with my comments, but I was hoping to start a dialog. Chances are we are both in a way wrong and also right. Reality and experience are the only judges that matter. If your way works, then you are right, if not then less right, though not necessarily wrong. I'm not a big fan of Edison, and even this quote he probably stole from someone else, but maybe we all have to find a thousand ways to not make a Stirling. I believe in my path and you believe in yours or at least I hope you do. Naysayers used to be a dime a dozen, but the internet turned them into drops in an ocean of negativity. I have no interest in being part of that wave.

That being said not all ideas are completely good and criticism, if constructive, should be welcomed by any on the quest for real knowledge. My criticism, of what I thought was your idea, was meant to be constructive. You are certainly right about the issues with heat transfer and the advantages of transferring heat from a hot mesh to a gas, that is how the regenerator works, but what I missed was the method for making the mesh hot. The regenerator should be made of iron because of its thermal mass, and formed in such a way to minimize linear conductivity, but your mesh should be optimized for conductivity, so something like copper and formed in such a way to maximize linear conductivity if you are heating the end of the piston or horizontal of you are heating the sides. After some thought, it seems that you could create multiple circular meshes mounted on a conductive spring. In this way the piston would compress the arrangement into the piston crown and heat it then as the piston retracts the spring would expand to allow the gas to come in a be heated. The challenge would be to find a material that is thermally conductive but would also be thin yet survive in constant compression and expansion or at the very least constant cyclical motion. Copper or aluminum would work harden, but a carbon mesh might be perfect.You might be able to use the copper mesh sheets if you can make springy conductive separators or springy tabs that come off the copper screens. The good news is you could make the copper meshes like they make circuit boards with photo etching, so the first one would be tough to make but making the next 100 would be easy. Again just a suggestion, if that is your idea.

On a similar vein, I had thought, and still occasionally think, about solving the heating and cooling issue with sprayers. i.e. Take an alpha Stirling arrangement have an timed injector that would spray hot oil into the hot cylinder and a cool water spray in the cold cylinder, then have a drain that would allow the products to be recovered and recirculated. The systems would be complicated and probably would have a lot of problems with fluid mixing and too much complexity, but like your idea, it does solve the heat transfer issue. A million tiny droplets would have lots of surface area.

Best of luck and keep going.
I will try to keep going too, this is important work.
The first person who can find the best way to turn wood into electricity will change the world and that is worth all the naysayers,

Rich

Bumpkin
Posts: 131
Joined: Fri Mar 02, 2012 1:42 pm

Re: Large LTD Stirling for CHP

Post by Bumpkin » Thu May 28, 2015 8:40 am

No worries of being off-putting. If we can't be genuinely thankful for genuine criticism, we can't evolve. I've already changed part of the mentioned page, so again thanks. We're of like mind about getting power from wood with large displacement and off-the-shelf parts, but we need to consider "who's shelf?" Some thoughts on radiant effects, and also mention of a spray system, were in this thread. viewtopic.php?f=1&t=1116 I found the part about "black gas" particularly thought provoking.

Bumpkin

Ian S C
Posts: 2221
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: Large LTD Stirling for CHP

Post by Ian S C » Fri May 29, 2015 3:14 am

A good many years ago there was an artical in "Model Engineering" about how one bloke solved a couple of problems in one go. First his gas tank froze, second the cooling water boiled, so he put the gas tank in the water supply, at first it got a bit carried away, too much gas pressure, but by adjusting the amount of gas tank in the water, found a balance.
Just a thought, Ian S C

MisterQED
Posts: 10
Joined: Wed Jan 29, 2014 11:18 am

Re: Large LTD Stirling for CHP

Post by MisterQED » Fri May 29, 2015 2:25 pm

My theory is that a power producing alpha style Stirling engine can be made from parts found at any decent home supply store: Home Depot, Loews, etc. The one missing piece for a usable engine is a high temp oil pump as without it you are force to use gravity and some rather scary hot liquid transfer maneuvers. Also some small bearings would be nice. The parts are as follows, 4" PVC pipe, fiberglass packing tape, assorted copper tubing of various sizes, a lot of J-B Weld, fine mesh steel wool, wood, RTV silicone to make 4 rolling sock seals, Paracord (which will be disassembled to create fiber reinforcement for the seals), six sections of black pipe to house the pairs of hot of cold ends and the regenerators, Glycerine (or other oil/antifreeze with high smoke point), several gallon of water, an air pump, a Schroeder valve, four metal buckets (for fluid pumping, a cheap DC drill, a wheelbarrow wheel (flywheel and bearings) and a blow torch to heat the fluid.
The system will be a pressurized T arrangement of alphas so that the pressure in one opposes the pressure in the other. The downside is it definitely will not be balanced at all and I will have to coat the PVC with J-B Weld or something else to keep it below the 170.F operating temp.
At least that is what I will try to prove in the next month.

Post Reply