Stirling

randall gabriell wrote:I will keep this thread posted on our progress but we are aiming at a sterling that could be made for 500$ give or take and produces 1kW.

If you come up with that, then I'll buy it. What is the projected net thermal efficiency, working pressure, and working fluid?

This thought is most likely unworkable, but here it goes for your consideration.

Since Stirlings seem to be good for high speed operation, but not so great for horse power, would it make more sense to drive a static electric generator with it?

Yeahbut time! Yeahbut, static electricity is useless. You can't power anything with it. Yeahbut, it wouldn't make enough electricity to do anything, based on--well--based on opinion off the top of my head, calculations not really necessary to just intuitively know this.

Okay, I can't seem to find information disputing that the long wire method of collecting and using static electricity to pulse-charge a battery is invalid or impossible. (Long wire strung 10' off the ground collects static electricity. Spark gap at end discharges static electricity into an automotive coil when charge builds high enough to jump the spark gap. Coil lowers voltage and raises amperage. That electricity, which now has amperage, is pulsed into a deepcycle battery. Battery charges from ambient energy--static electricity)

Instead of a long wire to collect electricity, generate it with a static electric generator, then convert it and pulse it into a deep cycle battery. Stirling could power the static electric generator.

The advantage I see is Lenz's Law should not apply to a static electric generator, reducing the need for horse power.

If I'm not all wet with the long wire method, adding a Stirling-powered static electric generator to the long wire should add an active power system to a passive power system to achieve a more all around consistent source of energy.

A wind turbine might also be used to turn a static electric generator, but that is a different discussion for low-wind turbines.

Maybe there is too much loss in a system like this due to loss at the spark, but I don't see it discussed much, so I wouldn't know. I've never read of anyone actually trying it, either.

Even if the method would work, a real-world attempt would require circuits to protect the battery from over charging and maybe to smooth out the pulses . It would also pose risks of electric shock and exploding batteries, so maybe we should wait until TPTB develop it for us in a controlled lab setting and release it to the permies.

I'd give my right arm for a stirling engine. Anyone bought the hyperex Tech Inc super stirling out? I'd love to buy one for testing. ANy one want to go in with me??

Has any one seen this Stirling? The ST05G is a basic 500 Watt Stirling Engine. They sell full drawings that one could have machined at emachineshop.com or any other CNC enabled machine shop.

Here is the link. It looks promising.

http://www.ve-ingenieure.de/projekt_st05g_cnc_engl.html

Also here is a guy who is really making usable engines. So it CAN be done!

http://www.starspin.com/stirlings/jimd6.html

Stirlings seem to be the silver bullet for us off girders! The stirling solution seems so obvious, so right there and yet so out of reach!

Might it be that using modern materials and machining technology, might we not be able to build these engines in the 1-2hp range for an affordable price?

Am I just off my rocker.

It seams to me that a 1.5hp stirling coupled with a Vulcan Gasifier, using a belt driven fan to produce the draw necessary to keep the gasifier going and storing the extra sun gas in a weather balloon for later use could provide a situation where you could run your little engine most o the day, keep a battery bank charged, rain or shine, using readily available biomass as fuel. The problem with an ICE is its short lifespan. It seems to me that the very nature of the stirling would mean a longer life, less hassle or concern with really clean gas, meaning you could use less quality fuel and gasifier etc. It all seems to make sense IF... we could get that engine!!

The Robertson/ Erricson engines were one of the most popular domestic engine in there time for many of the same reasons we want something like it.

Comments, input??

Bill Bianchi wrote:This thought is most likely unworkable, but here it goes for your consideration.

Since Stirlings seem to be good for high speed operation, but not so great for horse power, would it make more sense to drive a static electric generator with it?

I would not say unworkable.... but there are some physical laws that may get in the way. First, power out is always less than power in. Second, every conversion is less than 100% efficient. Generating low voltage/high current power is generally pretty good from rotation. True most available generators (include alternators in there too) are made to fit with an IC motor that runs at speeds where an IC motor runs best. Running them outside of that range gives poor performance. However, they can all be rewound to work well at a different speed. Converting rotational power to static and then back down as current is extra conversions and would probably require some (lossy filtering) so as not to hurt the battery. I am not sure how efficient static generators can be made, I would guess not just because the only use for static seems to be for experimentation so the drive to make them efficient would not be there... In fact most static electric is generated from low voltage high current instead. I would suggest that in general generating power as close to final use levels would be best.

In the end power in, minus losses, is power out.

Justus Walker wrote:Stirlings seem to be the silver bullet for us off girders! The stirling solution seems so obvious, so right there and yet so out of reach!

Might it be that using modern materials and machining technology, might we not be able to build these engines in the 1-2hp range for an affordable price?

Am I just off my rocker.

It seams to me that a 1.5hp stirling coupled with a Vulcan Gasifier, using a belt driven fan to produce the draw necessary to keep the gasifier going and storing the extra sun gas in a weather balloon for later use could provide a situation where you could run your little engine most o the day, keep a battery bank charged, rain or shine, using readily available biomass as fuel. The problem with an ICE is its short lifespan. It seems to me that the very nature of the stirling would mean a longer life, less hassle or concern with really clean gas, meaning you could use less quality fuel and gasifier etc. It all seems to make sense IF... we could get that engine!!

The Robertson/ Erricson engines were one of the most popular domestic engine in there time for many of the same reasons we want something like it.

Comments, input??

A few thoughts - there is no need to use a small gasifier designed to generate engine grade gas (i.e. an Imbert) for an external combustion application. A simpler furnace could work and would allow for using wood with a lot less processing. Also...

I think many external combustion engine configurations with biomass fuel can be configured to be more versatile and more efficient overall than biomass gasification with internal combustion. Unfortunately, the development of these systems is not there as you know. In particular, I think Rankine or Brayton systems are more promising than the Stirling. I've already mentioned the prospect for Rankine (i.e. steam) systems. A micro scale Brayton cycle might be configured from an existing air compressor, and using an appropriate expander. Something like a steam engine running on compressed air might work well if the expansion ratio were sufficiently high (compounding would help). The two features that I believe can make this option more promising than a Stirling engine include (1) the heater is not restricted with respect to size and can be configured for counterflow heat exchange (both will permit higher efficiency in heat exchange than a Stirling), and (2) a lot of heat can be regenerated easily by using the heated air exhausted from the expander to support all combustion in the furnace. NOTE: It would be very difficult to get high efficiency in this kind of set up. The losses in the air compressor is a major loss. High temperatures would be required. I still think the Rankine cycle is the way to go for micro scale (i.e. a good steam engine).

Marcos Buenijo wrote:

A few thoughts - there is no need to use a small gasifier designed to generate engine grade gas (i.e. an Imbert) for an external combustion application. A simpler furnace could work and would allow for using wood with a lot less processing. Also...

I think many external combustion engine configurations with biomass fuel can be configured to be more versatile and more efficient overall than biomass gasification with internal combustion. Unfortunately, the development of these systems is not there as you know. In particular, I think Rankine or Brayton systems are more promising than the Stirling. I've already mentioned the prospect for Rankine (i.e. steam) systems. A micro scale Brayton cycle might be configured from an existing air compressor, and using an appropriate expander. Something like a steam engine running on compressed air might work well if the expansion ratio were sufficiently high (compounding would help). The two features that I believe can make this option more promising than a Stirling engine include (1) the heater is not restricted with respect to size and can be configured for counterflow heat exchange (both will permit higher efficiency in heat exchange than a Stirling), and (2) a lot of heat can be regenerated easily by using the heated air exhausted from the expander to support all combustion in the furnace.

So, then what if one was to use both? Use the waste heat from the steamer to run the stirling.... Or use the stirling as a chiller. The nice thing about a stirling is that heat does not have to be well controlled. So it would be possible to use more than one source of heat. I don't know if it would be as efficient as just PV cells to use solar heat though. The solar would need to be tracked and in the end sqft of solar collection is what it is. It ends up being wood heat to be practical. Be nice to try both.

Len Ovens wrote:
So, then what if one was to use both? Use the waste heat from the steamer to run the stirling.... Or use the stirling as a chiller. The nice thing about a stirling is that heat does not have to be well controlled. So it would be possible to use more than one source of heat. I don't know if it would be as efficient as just PV cells to use solar heat though. The solar would need to be tracked and in the end sqft of solar collection is what it is. It ends up being wood heat to be practical. Be nice to try both.

The temperature from the steamer is too low to achieve worthwhile efficiency for a Stirling engine. The Stirling as a chiller requires a lot of mechanical energy. It's definitely preferable to use a traditional vapor compression cycle for air conditioning if the mechanical energy is available. The steam exhaust from a steam engine can power an absorption/adsorption chiller very efficiently, especially under some positive pressure to boost the temperature. Steam heat is highly controllable with respect to temperature by simply adjusting the pressure or the temperature of the heat sink for the condenser.

On controlling the heat, if one is using a biomass furnace, then the furnace output can be highly controlled. See the silver fire wood gasifier stoves for one example (their forced draft gasifier unit in particular). It is possible to place a hopper on a highly insulated hearth with a grate. Once combustion has started and the hearth temperature is up, then the biomass will be consumed at a rate proportional to the rate at which air is supplied to the ash pit under the grate, and this can be done with a very small blower fan. The pyrolysis gases can move to an insulated combustion chamber right next to the hearth and mixed with preheated air for efficient combustion. This can then be used to power a micro heat engine if desired. Note that the hopper must be sealed at the top.

On solar thermal power generation, it won't compete with PV - at least not on the micro scale. Even in that case I say a Rankine cycle would be the way to go, but an organic working fluid would probably be better without access to temperatures higher than about 400F. Still, I don't think it's worth it either way in the micro scale without access to a very good heat engine suitable for the project.

Marcos Buenijo wrote:

Len Ovens wrote:
So, then what if one was to use both? Use the waste heat from the steamer to run the stirling....

The temperature from the steamer is too low to achieve worthwhile efficiency for a Stirling engine.

Ya, 900 degree steam on the output side of a steam engine would require even hotter input. I should have said the other way around, but I think things start to get too complicated. The ST5 site (for a 5HP Stirling) compares the stirling to both gas and diesel for genset operation. They do cost per kwh output and the stirling looks good (of course). They list the overhaul cost of the gas genset at $0 I am guessing because it gets replaced instead. It would be interesting to see the same kind of comparison with steam included.

The attraction of the stirling over steam for most people would be:
- no boiler means safer (in most people's mind)
- simplicity, stick heat on the end and run.
- low maintenance

I have purposely over simplified though. The stirling requires a hotter furnace meaning the safety may be over stated, and at higher temperature wear may be increased as well. The stirling looks to me to be similar in complexity. They both have a furnace, not just a fire and the that furnace needs to produce steady heat. The steamers needs a steam generator, but the stirling also needs a high pressure generator to keep the gas in the unit at the rated pressure and to make sure it is dry. Both units also require cooling (technically the steam engine doesn't as it could just exhaust the used steam) In the stirling case, the cooling needs it's own circulating power to move the coolant. I am not sure water can be used with out additives but in any case, the cooling system must be very efficient to get good power out as stirling requires temperature difference. So the steam engine is actually dealing with lower temperatures and pressures and the condensing side of things is less critical. I do not know how the efficiencies compare or the ease (includes cost) of home building. The stirling pretty much needs to be built from scratch, every piece machined to spec. And the machining looks more complex too.... though once you have the mill and have gone through the learning curve, it may not make that much difference. It appears a steam engine can be built by modifying something already built (2stroke IC engine, air compressor, etc.). Even buying the the stirling engine part (at over 3k) one has to realize there is a lot of supporting system to add to it and the person thinking of buying one needs to look at the whole system and the costs and decide if they can build all of it. So far as I know, there are no "complete" packages.

This has been an interesting study I hope to see pictures and results from someone who goes through such a setup (steam too).

I think many people when considering external combustion engines tend to favor the Stirling for reasons other than sound engineering considerations. The Stirling has the advantage of external combustion (meaning wide fuel capacity and potentially clean combustion), but this applies equally to a modern steam engine. When people consider the "steam engine", then they often imagine the inefficiencies and pollution associated with antiquated steam technology and practices. The Stirling has no such emotional and historical baggage. However, objectively speaking, a modern steam engine will achieve similar efficiencies and better performance than a modern Stirling engine, and the higher power/weight of a steam system would be favorable from a cost perspective. Unfortunately, in both cases these systems have not been developed. This is so quite simply because low cost internal combustion systems are available along with low cost petroleum based refined fuels.

I am attracted to physics and other intellectual pursuits (like economics and philosophy) partly because I've experienced on several occasions that close inspection and analytical thinking often fosters conclusions that are counterintuitive. I believe modern steam power is an example of this experience. On close inspection, the potential for modern steam engines is a lot more favorable than people generally realize. Of course, few take the time to make this close inspection.

Marcos Buenijo wrote: This is so quite simply because low cost internal combustion systems are available along with low cost petroleum based refined fuels.

IC engines that last as long as stirling or steam, cost as much or more. The average 3.5kw genset available at the big box with a b&s motor will not last a year doing 24/7... maybe not even half that. Even replacing the motor with something from a car will not last that much longer. Diesels start at $3500 with 2 year warranty... means not designed for continuous run, and go up from there. The diesels they use for backup in nuclear plants have a MTTF of under 10000 hours... about a year of continuous running. Basically, to get an IC that will last long enough for 24/7 use (not counting fuel costs which would keep you from running over night for sure) one is looking at a Lister(oid). If you can find a Lister, it will need TLC before it runs or cost a lot. A Listeroid from India will still cost $3500 without shipping... and generally still need some TLC (even brand new) before it runs. It will still need a generator, cooling, fuel system, shed, etc. At least it will be able to run on waste oil if properly treated... but waste veg oil has become a commodity with a price in most places. I don't know if a Lister(oid) can be run on wood gas reliably. or what it would take to do so, or if it would have even half the efficiency. The general rule of thumb for IC engines is gas gives 10 HP for 1 hour per gallon and diesel gives 20 hp for 1 hour per gallon (from the boat design world). I don't know where wood gas fits. That is the one thing about wood gas ICs is can they be made to last? They are all conversions from gasoline ICs and there are not many of those out there that last. There are some boat motors that are pretty good, but they will be older and need rebuilding before use as they all seem to have gone diesel. There are also some old stationary gas engines from days gone by that could last. Some of them even have intermittent firing.

All of the sudden, externally fired engines that can last the life of the owner don't look so bad.

Len Ovens wrote:
IC engines that last as long as stirling or steam, cost as much or more. The average 3.5kw genset available at the big box with a b&s motor will not last a year doing 24/7... maybe not even half that. Even replacing the motor with something from a car will not last that much longer. Diesels start at $3500 with 2 year warranty... means not designed for continuous run, and go up from there. The diesels they use for backup in nuclear plants have a MTTF of under 10000 hours... about a year of continuous running. Basically, to get an IC that will last long enough for 24/7 use (not counting fuel costs which would keep you from running over night for sure) one is looking at a Lister(oid). If you can find a Lister, it will need TLC before it runs or cost a lot. A Listeroid from India will still cost $3500 without shipping... and generally still need some TLC (even brand new) before it runs. It will still need a generator, cooling, fuel system, shed, etc. At least it will be able to run on waste oil if properly treated... but waste veg oil has become a commodity with a price in most places. I don't know if a Lister(oid) can be run on wood gas reliably. or what it would take to do so, or if it would have even half the efficiency. The general rule of thumb for IC engines is gas gives 10 HP for 1 hour per gallon and diesel gives 20 hp for 1 hour per gallon (from the boat design world). I don't know where wood gas fits. That is the one thing about wood gas ICs is can they be made to last? They are all conversions from gasoline ICs and there are not many of those out there that last. There are some boat motors that are pretty good, but they will be older and need rebuilding before use as they all seem to have gone diesel. There are also some old stationary gas engines from days gone by that could last. Some of them even have intermittent firing.

All of the sudden, externally fired engines that can last the life of the owner don't look so bad.

I've come to similar conclusions. A micro wood gas engine system used to operate a small Honda engine at low speed for battery charging at a constant rate could be a reliable and cost effective system. However, it doesn't seem suitable for anything beyond a backup power unit. I realized that when restricted to use as a backup power system (to back up a PV array), then it makes more sense to use a refined fuel like propane or Diesel. In my opinion, the costs and labor involved in processing wood fuel to make it suitable for a small wood gas engine system is not worth the trouble. However, I do believe a large wood gas engine system to provide stationary power for a small community makes a lot more sense. I've seen descriptions of large wood gas engine systems used to supply power in remote "third" world regions that were highly efficient, and they required minimal processing of the fuel (like large wood chunks and/or coconut husks). Also, if one desires to fuel an automobile with biomass, then a wood gas engine system makes the most sense by far.

A wood gas engine system at the 10 hp and under range can see good efficiency. At optimal output which tends to be at the highest rated load for a genset, the net thermal efficiency can be anywhere from 15-20% (assuming a good system - and this is NET). Generally, the larger the engine and higher the compression, then the higher the efficiency. The efficiency varies dramatically over the power range. A problem with this is that a modest (read: realistic) off grid home does not consume electricity at a high rate. Even if one desires to operate a wood gas engine system at a low output to more closely match the output to the demands, then it will be discovered that the gasifier must be operated at a certain minimum air flow rate to maintain proper temperatures. The only way to operate at a lower rate is to build a smaller gasifier, and this exacerbates the fuel processing requirements. It's a catch 22. The only setting I've considered the wood gas engine system to make sense is to serve as a backup unit for bulk charging a large battery system during inclement weather (when the solar panels can't meet demands). However, again, if this is done infrequently, then one might as well save a ton of money and labor by using Diesel fuel and a highly efficient Diesel genset.

Now, if one truly desires complete energy self-sufficiency, then I think biomass (wood in particular) has untapped potential. An external combustion heat engine might be the ideal solution IF it can make use of wood with minimal processing (like wood splits, stick wood, baseball sized wood chunks) while operating efficiently (and quietly) for long periods at a lot output that matches well the demands of the home. Most important (in my opinion, at least) is to make full use of the heat from the system. It's a sad waste of limited resource to not do this. When the engine is operated at a low output for long periods, then this can also be optimized.

So far, I think the lowly piston steam engine is the best candidate to fill this niche.

Marcos - You keep coming back to the steam engine as a good fix, but then you keep saying that there is not one available on the market. So it is not a good fix!

Justus Walker wrote:Marcos - You keep coming back to the steam engine as a good fix, but then you keep saying that there is not one available on the market. So it is not a good fix!

The problem is, there isn't really anything on the market to take biomass and make rotary power. There are steam engines available, there is a stirling available, gasifiers are make you own. I suppose a listeroid run on waste oil might be considered as well. The biggest problem with them all is both cost and size. There are smaller steam engines around (1 - 2 HP) but they are old technology. The new technology steam engines just starting to be produced in beta form, are 10 HP plus. The ST5 is 5HP as is the standard lister. For a continuous run engine, 1 to 2 HP is about right for most homes to provide both power and heat. Anything more provides too much of both. The sizing of these things should come as no surprise because the average IC genset one buys for the farm is 3.5KW... the right size for running once a week and so that is the target market for new technology. The smaller ones will come in time I think.

Between the steam and the stirling, the small steamer is easier to DIY and easier in a DIY setting, to get good results with.

Justus Walker wrote:Marcos - You keep coming back to the steam engine as a good fix, but then you keep saying that there is not one available on the market. So it is not a good fix!

I see your point. I mean only that a good micro steam engine system would fill this niche (micro scale combined heat and power with biomass fuel) better than other options. Also, while external combustion alternatives such as steam, Stirling, and Bratyon engines require development (i.e. there is not one available on the market as you note), I believe developing a steam system will be a lot easier than doing so with a Stirling cycle of equal performance.

I was surfing the web the other day looking for info on Stirling engines and came across the website of a fellow in the UK who builds Stirlings, and also sells plans and castings for them if you are fortunate to have a metal lathe and other essential tools. He has an excellent selection of youtube videos showing how to do aluminum and brass sandcasting, along with some how-to type videos on Stirlings. One design in particular caught my eye, a stovetop fan with about a 7" fan blade that sits atop a wood stove and helps to circulate the stove's heat. You can see that here: http://myfordboy.blogspot.co.uk/p/blog-page_27.html
His youtube channel is here: https://www.youtube.com/user/myfordboy/videos

There's something very empowering about being able to cast and machine equipment yourself. It's a secure feeling knowing that you will be able to manage in the face of catastrophic failure of society if it happens. I did a bunch of aluminum casting a few years ago, built my own electric furnace, crucible, tongs and mixed my own sand. Still have the equipment, and planning to resurrect the skill. I never did try anything as challenging as a Stirling engine though (yet).

Hi,
I'm new here but have been researching biogas and digesters and using it to produce electricity.
What I have not found out is where small "biogas generators" are used. Some are less than 1 kw in power costing around $500

They are sold by many Chinese firms and some are sold in Africa but I can find nothing about their performance in the field.
They seem a much more sensible answer to Stirling engines which will always be expensive.

There have been long-term problems using biogas as a fuel. Have they been overcome?

GrahamK