Stirlingengineforum.com

Hello all,

I am currently working on a project for a generator system and was hoping people on this board might be able to help me out hunting down some information. I'm looking for a 5kWe to 10kWe stirling generator with roughly 500C to 600C hot side and < 50C cold side (air cooled basically)

I'm looking for general information on what might be out there as far as commercially available products or R&D projects. What kind of efficiencies can I expect from this range? I know ~30% is typical but is upwards of 40% possible? I'm familiar with some of Sunpower's stuff as well as the engines developed for KRUSTY but that stuff is mostly focused on space rated low mass optimization. I'd like to find a more affordable alternative.

Thanks!

I don't know of anything being produced in that power range being marketed to the general public. Some utility scale stuff, maybe.

As far as R&D, there doesn't seem to be a lot going on any more these days. Not entirely sure why.

Sadly, I guess what I'm doing is about it, but only just getting started.

I have gotten together about a half dozen gasoline powered generators of various power ranges I will soon be attempting to convert to an experimental Ringbom Stirling type power drive.

It seems this sort of conversion has been successful in the past, but apparently never got much further than some proof of concept prototypes.

As far as the scant information on the subject goes, it appears as though such conversions can result in a Stirling with a power output comparable with that of the original engine.

If successful, such a conversion would be considerably less expensive than building an engine from scratch.

The main difficulties so far reported there seem to be in the areas of pressurization and lubrication.

As far as lubrication, I've been kicking around a couple ideas.

The goal, it would seem would be to keep engine lubricating oil out of the heated displacer chamber.

Making the displacer itself oil free is not an issue, as it is loose fitting and could be made entirely frictionless.

The IC engine though, typically requires oil.

With the Ringbom arangement, possibly the two functional sections could be separated by a physical barrier. Two options come to mind.

On the left is a simple fluidyne "trap". This is used in plumbing under nearly everybody's kitchen sink, to keep the household breathing air separated from the potentially odoriferous air in the sewage line.

Theoretically, any engine blowby, working it's way towards the displacer would be caught in the trap, which could be filled with water or some other neutralizing, fire preventative liquid.

The trap might also be filled with some very porous oil absorbent dry medium that could soak up any oil, but still allow the free flow of air

Finally on the right is a diaphragm of rather large diameter.

Inevitably, this would introduce some "dead,air space" and possibly some timing issues due to pressure transfer delays or something.

The diaphragm idea, though, could possibly allow some partial pressurization, where the more valuable gas, such as helium could be used on the displacer side while that could be balanced by pressurized air on the engine side of the diaphragm.

Probably finding a diaphragm material that was impermeable to helium would be difficult. But if some suitable material could be found (possibly graphene based) the "leaky" IC engine could be kept charged with "free" air, while the more valuable gas could be retained on the displacer side, if there proved to be any advantage in that.

Some kind of intermediate pneumatic plunger/piston or hydraulic separation might also be an option.

At any rate, such a physical barrier would possibly allow the IC engine to continue operating with ordinary engine lubricant. Some papers/research suggests using water.

Somehow water doesn't seem like a viable solution to me. Aside from rust and corrosion, I would think water could be as explosive as oil. Thinking about steam boiler explosions. The lubricating properties of water also don't strike me as being particularly effective.

Tom,

Thanks for the reply. I contacted several manufacturers and so far it seems like only Qnergy is producing a commercial product. Sunpower's generator was a 1-off for kilopower. I did manage to get a quote from Qnergy for their PCK80 engine, a 7.8kWe 30% eff engine that is pricey and dependent on volume but it's something. No data on efficiency as a function of THot and TCold.

I also know of a 25kWe 30% eff @400C THot development in Boulder but holy crap that thing is like 8,000lbs.

For lubrication most that I've seen use He gas bearings but that limits them to basically 50% operating power and higher. I've thought about developing one in house but that adds a lot more time, $$$, and uncertainty to an already ambitious project.

looking4stirling wrote: ↑Wed Sep 28, 2022 11:11 am ....I did manage to get a quote from Qnergy for their PCK80 engine, a 7.8kWe 30% eff engine that is pricey and dependent on volume but it's something. ...

That's more than I've been able to get, still waiting for a return call that never came (as far as I know). Do you mind putting a number on "pricy"?

Just curious.

looking4stirling wrote: ↑Wed Sep 28, 2022 11:11 am .... I've thought about developing one in house but that adds a lot more time, $$$, and uncertainty to an already ambitious project.

Even more curious. "In house", so, not just a personal pursuit ? "Project" - care to share any additional details ?