2019 RMH / Stirling Cycle Generator CHP Thread - Inresol AB teases 10 KW generator coming soon.

Hi again.

I've looked through past posts, the most recent from early 2018, and have not seen anything posted about this (or a working implementation, please correct me if I'm wrong about either).

Discovering the existence of RMHs immediately made me think about Stirling Engines, not only because they could in theory work very efficiently together, but also because they both represent technology that was all but abandoned by science, despite having a sort of beautiful simplicity and elegance to them that seemed to beg for a second look.  Traditional wood stoves have only relatively recently made incremental improvement, and Stirling Engines are used only in niche situations with no commercial availability.

I've heard this story before, but here goes:

Here's a teaser video from 2/2018, which just postdates the most recent post I found, wherein a company seems to be showing large scale production of 10KW stirling engines.

Here's the companies products page.  The security certificate is not valid, and it appears still incomplete.  Note the power bank and passive nitrogen harvester.  More importantly, there's also an existing pellet stove accessory, which suggests that would can get plenty hot (but you guys already knew that), which is good, because it requires a 1000 C input heat to get 10KW.

Check out the specs! Overall efficiency of 88 - 94%, with 28 - 34% of that going to electric, and the rest going to hot water.

They have resellers lined up, and one claims that Inresol will take the world by storm, though they may be biased.

Cursory searches didn't yield anything recent regarding the status of this endeavor, so I contacted the American reseller to see if he had any info.  I may try the company tomorrow.

It would seem that a good solution for integration would be to run the heat to the engine, rather than to the cob sink, and then routing the waste hot water output to the sink, and presumably then to a water heater and/or traditional boiler and/or pellet boiler in basement.

At first I thought that removing all that energy in the form of electricity would lower the combustion temperature, cause flue gas condensation, and the potential for fire.  Then I realized that the cob heat sinks don't cause this, so it has to be possible.  The difference is that to achieve the 1000 C you need, then you'll have to put the hot end of the engine right in the gas combustion chamber (right?), which means that close to a third of the heat energy will be taken out.  I'm not sure if that works out to lowering the temperature in that chamber by close to the same amount, but it must lower them to a significant extent.  That said, I've heard that the secondary chamber actually gets much hotter, depending on the design.  That's to say nothing about the cold water that is now running over the cold side of your engine, and further drawing out heat.  I'm assuming that doubles as  a coolant, though perhaps it can  be regulated, and the engine can use air for cooling, to at least some extent   That may be a deal breaker for an easy out of the box installation with current stove designs.  Anyone have thoughts on this?

I wonder what other pitfalls there are that I'm not seeing.  I'm sure there are many, but I haven't seen many mentioned in existing threads.

I'm in the process of finding land to build a house on, and plan on using RMHs to supplement central heat.  If by then the product actually becomes available, actually functions anywhere close to advertised, and actually can be purchased and no one has tried it, I'm willing to take one for the team.  A final condition is that the cost is only within an order of magnitude of a 10 KW gas generator.

Update:

The US distributor of any future Inresol products responded to my message.

He said:

Inresol is not shipping engines at this time due to technical and financial issues, Qnergy in Utah is only company I know selling quality Sterling engines at this time.

This first part of his response is not surprising.  The second part is to me, because around once a year I look in nto the state of the Stirling Engine market, such as it is, and consistently found nothing but toys and plans.

I've reached out to them about pricing.  I'll also reach out to Inresol to see if there's any ETA.  Pardon me if this is old news.

From the images, the Qnergy model appears larger, and a bit more challenging to decouple from the housing and fuel source.

Update:

The Melvin became available about 3 weeks ago.

I wrote the owner asking if he had heard of RMHs, and, if so, what his thoughts might be.

One advantage to the latter two engines is actually the lower output.  10KW is a lot for residential use, even in the summer.  The others will require lower temps and less fuel.

Update:

There is a "rocket style" wood fuel accessory made out of a 55 gallon drum.  Per the promo video, this is an operation that just got out of the garage.

I saw a kickstartereque video of Sefton Motors and it looked like fun.  I'd love to have a way to turn woody biomass into electricity.

Brian, thank you for this topic. Please keep us in the loop as you learn more. I  stopped watching Stirling engines a few years ago, but I agree that coupling one with a RMH is interesting.

If they want  1000C, I'm curious about their materials on the hot end, and especially how much higher they could handle without damage.

If the form factor works, I'd love to mount the hot end directly above the  J tube, right in the center of the barrel. It poses some interesting fabrication challenges sealing the joint, but placing it there has two big advantages...

Peak temperature input for the Stirling engine

Positioning the cooling effect there should help the barrel's down draft.

If this starts to look viable, I'll enjoy calculating the amount of firewood necessary to produce  10kw for an hour.

Mike Jay wrote:I saw a kickstartereque video of Sefton Motors and it looked like fun.  I'd love to have a way to turn woody biomass into electricity.

The guy is pretty friendly, if you wish to speak to him.  Relative to solar, the price per watt is outstanding.  I just checked, and if accurate, it outputs more watts to price than best case solar in the US, and that's not taking into account that it you can use it 4 - 6 times more (and since the topic is RMHs, I'm guessing not many of us are in optimal solar zones).  Google gave me an estimate of 2.67 to 3.43 USD/watt for solar.

I don't want that to sound like an endorsement.  Who can say how well this thing really works.... ?   Seriously, who?

Ashley Reyson wrote:Brian, thank you for this topic. Please keep us in the loop as you learn more. I  stopped watching Stirling engines a few years ago, but I agree that coupling one with a RMH is interesting.

If they want  1000C, I'm curious about their materials on the hot end, and especially how much higher they could handle without damage.

If the form factor works, I'd love to mount the hot end directly above the  J tube, right in the center of the barrel. It poses some interesting fabrication challenges sealing the joint, but placing it there has two big advantages...

Peak temperature input for the Stirling engine

Positioning the cooling effect there should help the barrel's down draft.

If this starts to look viable, I'll enjoy calculating the amount of firewood necessary to produce  10kw for an hour.  I'm not sure what that implies for the scaling of the oven.

I estimated it at 3 KG dry wood per hour per KW @ 6.66% efficiency, but that was just napkin math.  Supposedly, if you use something like pressurized helium for the fluid (hydrogen is too expensive and will leak through metal), you can get something like 25% to 35% efficiency in converting the heat to electricity in a good Stirling Engine.  That's what Inresol is promising.  If you put the waste heat into water, then your total efficiency can be as high as 95%.

The guy who makes the Melvin and I had a brief conversation via e-mail.  He told me the components for the hot side are made with steel, but that upon request he could upgrade that to a different metal, which would allow more heat and thus RPM/output.  I told him I was concerned about overheating the thing, and he said, in the context of RMHs, that he didn't think that would be a problem, as you can always compensate for too much heat.  As I mentioned above, he sells a burn tank for wood that is not insulated, but is a RMH.  One could insulate it, and presumably make more heat.  I pointed him to the an example of a commercial RMH (the Liberator), and he said it looks like it would work as an input.  He mentioned that it could tolerate around 640 C (1200 F), "maybe hotter" with the steel components.

I still have concerns, even with the lower heat requirement, about how difficult it will be to find a sweet spot.  Too far from the flue gas combustion chamber and you're not running efficiently. Too close, and if you don't overheat the engine, maybe you pull out too much heat from the stove, leading to incomplete combustion (creosote build up?  fire?).  It may be prudent to hire a professional when I get to that point.

I have an update on Qnergy too.  I'd filled out a web inquiry asking the price, and if they have ever considered an RMH as an input.  The latter was probably a mistake if I wanted to be taken more seriously.  Someone responded by thanking me for the inquiry, told me the cost of the SE is 20K US, and said nothing else.  This caught me by surprise, since I know they have two models -- one outputs 5.7 KW and the other 1.5 or so -- and I didn't specify which.  I wrote back asking for which that was, and the guy responded by telling me that is the cost of the engine.  That struck me as odd, because it assumes that I can enumerate the rest of the components, and put a price on them.  How should I know that?  Anyway, I guess that means the difference in the two models is in the other components, i.e. one makes more heat.  So I asked the guy to ballpark a cost installed, for a system generating 4 - 6 KW/HR per day.  I'll let everyone know if he writes back, but no matter what he says, the price is such that you'd need to run it all the time to ever approach making your money back.  Granted, it's not all about the money, but that doesn't mean I want to throw it away.

Qnergy wrote back to me.  The whole kit and caboodle for their 1.2 KW generator is 31,000 USD.  Probably too much to justify in all but the most niche applications.

I picked up my correspondence with Mr. Sefton of the Melvin Engine.  I asked him specifically if he had any concerns about placing the engine too close to the bell, and sucking out so much heat that combustion efficiency is reduced, and condensation occurs.    His design has a hose coming off of the top of the stove.  It seems common sense would suggest that the length of that hose can be used to find the sweet spot, now that I think about it.

I think you're right that Qnergy isn't in the game at that level. As fun as stirling engines are conceptually, if the goal is electric power generation from wood, one can purchase a turnkey wood gassifier with generator that has much more output for much less money.

The Melvin engine may be a start, but now that you're getting me interested in this topic again... I think a serious focus on wood powered electricity requires some adjustments to that approach.

If I recall correctly, an ideal stirling engine's efficiency is carnot efficiency, meaning that it's efficiency is a function of the hot/cold temperature differential.
That implies that we'd be nuts to not put the hot side of the stirling engine in the highest temperature place possible. A first cut at that would be right on top of the J tube, certainly not heat transfer via a hose.

The limiting factor in this is of course the materials in the engine. My recollection from Paul's 8 DVD set is that the innovator's event RMH's of the last few years are routinely running around 2500F and some are getting above 3000F.

Ashley Reyson wrote:I think you're right that Qnergy isn't in the game at that level. As fun as stirling engines are conceptually, if the goal is electric power generation from wood, one can purchase a turnkey wood gassifier with generator that has much more output for much less money.

The Melvin engine may be a start, but now that you're getting me interested in this topic again... I think a serious focus on wood powered electricity requires some adjustments to that approach.

Great point.  I'd also like a wood gasifier.  The only problem I see with them is that if you're making only 3 KW hours a day, that adds up to about a ton fertilizer component every year, which may be more than you need and can give away.  That equates to probably not much more than a single large window AC unit consumes in a summer.  I'm not sure what else it's good for, or how easy it is to get rid of, or how toxic it is.  You've got me interested though.  I don't know how efficient the Melvin is, but if we assume 15%, then it won't use much more wood, and produce less than 5% of the waste, perhaps?  That's a guesstimation.

I find gasifiers interesting, but on the other hand I'm fascinated by SEs, and so I will not deny that I am biased in this regard.  As I've said before, anything that works and is within an order of magnitude of price per watt to a gas generator, and I'm in (The Qnergy System definitely does not meet that).  Speaking of my bias, when contemplating the efficiency, another way of thinking about it is that the overall system of a RS with integrated SE will actually be very high, because it outputs two usable forms of heat (hot water given a water jacket and hot gasses) along with the electricity, and of course it will radiate heat.  Also, the stove alone will get more energy out of the wood than the gasifier.

Ashley Reyson wrote:
If I recall correctly, an ideal stirling engine's efficiency is carnot efficiency, meaning that it's efficiency is a function of the hot/cold temperature differential.
That implies that we'd be nuts to not put the hot side of the stirling engine in the highest temperature place possible. A first cut at that would be right on top of the J tube, certainly not heat transfer via a hose.

The limiting factor in this is of course the materials in the engine. My recollection from Paul's 8 DVD set is that the innovator's event RMH's of the last few years are routinely running around 2500F and some are getting above 3000F.

It's a shame you guys were not around in the Bronze Age.

I'm not an expert, but I presume it would be very difficult and expensive to find a material that can withstand that sort of heat, and work well for an engine.  Maybe at an old Soviet rocket scrapyard.  I assume those furnaces are made of stone.  I'm pretty sure you're right re: carnot efficiency.