Heat Engine Invention - hydraulics questions

Hello tinkerers, I have an idea for an engine that runs off a temperature difference (like a steam engine or Stirling engine).  Unlike those it only needs a 20-40 degree temperature difference and it operates at convenient temperatures like in the 60-150F range.  I won't disclose the core details of the engine since I have little dreams of it becoming a permies.com kickstarter some day.  Kickstarter output would be a patent for permies.com, plans, video of a build, etc.

This magical engine would output power in the form of hydraulic pressure.  By my crappy math, I think it could make .56 kw of power when running.  

My thought is that this engine could pressurize a large pressure tank and then that pressure could run a hydraulic motor/generator to make electricity.  The pressure tank would then be the battery in the system.  

So instead of buying lots of solar panels and batteries, you could use some of these engines running off a heat difference generated by solar or RMH and store it in a big tank.

My first issue is that I'm not very familiar with hydraulics and pressure tanks.  I'm having trouble figuring out if they actually make large pressure tanks, what they would cost and how much energy they can store.

My second issue is that I'm assuming they make hydraulic motors that are coupled to an electric generator.  Is that actually a thing or would we have to cobble those two pieces together.

Anyone have some hydraulic experience to lend to this problem?  Let's invent something cool!

Most small hydraulic motors have a simple keyed shaft, so could easily be hooked up to a generator head. I don't understand how you are storing hydraulic pressure. Like a water tank, with air compressing as it's filled with fluid?

A large pressure tank is certainly possible, though one strong enough to handle a few thousand PSI would be quite pricey.

Hi Mike, I like the project!

Hydraulic motors certainly do exist so that part of the build is certainly possible.  Also, hydraulic motors typically have a pretty high power density, meaning you can get a good amount of power from a small volume.

I guess the issue I see is pressurizing the fluid in a static state.  Do you intend to compress air to move the fluid?  All the hydraulic motors I am familiar with are only pressurized while in operation.  The moment power is cut the hydraulic pressure is lost as well aside from a trivial amount left in hydraulic lines and that is eliminated the instant any valve is opened.

Certainly you can get a hydraulic reservoir To hold fluid but I am thinking that stored energy would involve compression, especially if batteries are ruled out.

Did I get this about right or did I miss something?  I love the idea of a little heat engine, so maybe some type of kinetic energy storage will work.  Just off hand I don’t know how hydraulic fluid will store energy, but perhaps I have missed something?

I am very curious about this project.

Eric

Thanks gents!  Yes, the hydraulic pressure would be stored in a pressure tank (like a well pressure tank with a gas bladder in it).  Here's a 5 gallon one but I can't seem to find anything bigger: https://www.mcmaster.com/59595K46/

Back in college there was a small team working on a hydraulic hybrid car.  I was very impressed but they didn't have the funding to get far enough with it before I graduated.  It had a small engine pressurizing a fairly large tank (maybe 50 gallons).  That pressure then ran 4 motors on the wheels.  It regenerated when braking to repressurize the tank.  They calculated that if they were going at 60 mph and had no pressure in the tank, they could brake to zero and then accelerate back up to 55 with the regenerated energy.  Electrical regen is limited by the size of the motor but hydraulic motors are so small you can have a 100 hp motor on each wheel.  They hadn't realized that their little car would go zero to sixty really fast with motors that big on it ;)

So at that time, they had a pressure tank that would hold enough pressure to drive the car 3-4 miles before the engine needed to kick back on.  Much more than that little tank in the above link.

So Eric, to more specifically answer your question, the engine would pressurize the hydraulic fluid and send it into the tank.  In the tank it compresses a nitrogen balloon to something like 1000 to 3000 psi (I think).   If the tank is large, say 500 gallons, that represents a lot of stored static pressure.  Connected to the tank and engine would be the hydraulic motor that runs a generator.  Any time we need electricity, the motor spins and makes juice.  That "used" hydraulic fluid would go to a reservoir at low pressure while it waits to be repressurized by the engine.

Hey, I was using the wrong search words.  I was looking for hydraulic pressure vessels.  Apparently they're called "accumulators".  Here are a few:
https://www.boschrexroth.com/en/xc/products/product-groups/industrial-hydraulics/topics/cylinders/large-hydraulic-cylinders/products-and-features/hydraulic-piston-accumulators/index
Looks like they come up to 0.5m diameter and 5.5m long.  I gotta think that would hold a bunch of energy but it might be prohibitively expensive...

https://www.grainger.com/category/hydraulics/hydraulic-accumulators/piston-accumulators-accessories?categoryIndex=5
Their biggest one is 2,310 cubic inches with a max pressure of 3,000psi for $3700.  

So, how much energy is represented by 2,310 cubic inches of fluid at 3,000 psi?

Mike, I get it now.  The hydraulics is the power source, not the energy storage device.  I bet that 500 gallons pressurized to over 1000psi is going to store a whole lot of energy!  

Just as a thought--since you are pressurizing with compressed air, would it be simpler, more efficient, and overall fewer parts if the whole device was pneumatic?  I was just thinking that the fewer devices involved the better it would be.

But just a thought.

There are 231 cubic inches per gallon. I would say it could run a typical motor under load for a few minutes, maybe. Of course, the larger the motor you want to run, the shorter it will run.

I'm not sure how to do the math for a tank whose pressure is slowly bleeding down as it runs, especially with some kind of regulator to keep the pressure or rpm's steady.

Eric Hanson wrote:Mike, I get it now.  The hydraulics is the power source, not the energy storage device.  I bet that 500 gallons pressurized to over 1000psi is going to store a whole lot of energy!  

Just as a thought--since you are pressurizing with compressed air, would it be simpler, more efficient, and overall fewer parts if the whole device was pneumatic?  I was just thinking that the fewer devices involved the better it would be.

But just a thought.

Not quite.  The hydraulic fluid is the working material.  It's being pushed by the engine into the accumulator/tank.  As it goes into the tank, it compresses a balloon of nitrogen in the tank which raises the pressure on the hydraulic fluid.  The gas never leaves the balloon, the only thing going in or out of the tank is hydraulic fluid.  So you're right that the energy storage portion is pneumatic but it never leaves the tank.  I believe hydraulic fluid is a better way to transfer energy than pneumatics, primarily because it doesn't compress.

I agree Jordan, I'm not quite sure how to do that math either.  That first company's biggest cylinder (.5m dia by 5.5m long) would store a metric shit ton of energy.  I think that works out to tad over 1 cubic meter, or 265 gallons of capacity.

Don't really have any pertinent experience, but.. thoughts anyhow!

First gotcha on the tanks is that a nominal xxx gallon tank wonn't hold that much pressurized fluid, because either a bladder or piston plus associated air uses some of the volume.

Secondly, there will be a lower pressure limit where the motor won't spin..

Thirdly, I am guessing that it will be most practical to have the motor/generator speed regulated so that you get nice smoothish power... so, that will raise the minimum pressure limit.

Here is some math for calculating available flow from an accumulator:

https://www.qualityhydraulics.com/blog/accumulators/hydraulic-accumulator-sizing


300in3 at min 1500psi needs a 3.9 gallon tank per the example in an isothermic application. This is basically 1.3 gallons. So, for these particular pressure needs the ratio seems to be usable volune about 1/3rd nominal tank size.

So a 285 gallon nominal cylinder like the one found above might store something like 95 gallons usable fluid.


Here is some math for determining hydraulic motor flow rate based on size and rpm; very simple. Also for determining torque.

 https://www.womackmachine.com/engineering-toolbox/formulas-and-calculations/hydraulic-motor-calculations/


So, if you had 95 gallons of fluid and wanted 5 mins of runtime, at 1800rpm, you can allow flow of 19gpm, giving a displacement of 2.439ci.


Take the min allowable psi and the cubic inches, and you get a torque of 582 in-lbs, which is on the scale of a small cordless drill.


Ok, next person can take that torque and rpm and figure out what sort of alternator we can spin with it.. assuming my uninformed math isn't leading us astray...

D Nikolls wrote:
So, if you had 95 gallons of fluid and wanted 5 mins of runtime, at 1800rpm, you can allow flow of 19gpm, giving a displacement of 2.439ci.

Take the min allowable psi and the cubic inches, and you get a torque of 582 in-lbs, which is on the scale of a small cordless drill.

Thanks D!  How did you get from 2.439 in3 and 1000psi and end up with 582 in-lbs?  Or were you using a lower minimum psi?

I'm also not experienced with hydraulics but it sure seems like 95 gallons of pressurized fluid would run the equivalent of a cordless drill for a lot longer than 5 minutes.  I don't know if there's a boo boo with your math but that seems quite low...

So why is Nitrogen used as a working gas and not air, or CO2 or some other gas?  I know I can pay extra to get my tractor tires filled with Nitrogen to stop leaks, I just don’t know why it works or if this is related.

Nothing you have said so far seems to violate any laws of physics that I can remember, but my intuition tells me that something is not going to pencil out here.

Harvesting heat energy to do mechanical work is generally not a very efficient process. The lower the temperature, the larger the area I suspect your contraption will need to produce meaningful quantities of power. Also, if the end result is going to be to convert the mechanical energy back into electricity, then you have to take into account those efficiency losses as well.

What I think you are going to wind up realizing is that you will need a giant garage-sized contraption to run a single LED lightbulb, or something along those lines :) It could be done, but it would probably not make sense from a financial standpoint. I just bought used solar panels for 33 cents a watt. I could have picked up 11kw of panels for the price of that big accumulator tank. All the plumbing, hydraulic motors and generator head would probably cover a pretty decent battery bank.

You could also look into Thermoelectric generators - they take a heat difference and turn it directly into electricity (at I want to say a couple of % efficiency). You can even make a generator just out of twisted pairs of dissimilar metal - I think aluminum and iron will work. You need thousands of pairs to generate even a few volts, but you then have no moving parts or giant scary tanks of 3000 psi hydraulic fluid. If your house caught on fire, can you imagine what that tank would do when it decided to release its stored metric shit ton of energy, plus several hundreds of gallons of flammable hydraulic fluid?

Thanks Carl, the engine I'm imagining would be of a reasonable size and, if my math is correct, puts out around 560 watts.  Generating the heat is a definite consideration but there are several ways of doing that.  On my property we have too many trees for solar.  No flowing water for hydro and too many trees for wind.  Thus we have plenty of biomass.  So that's why my mind has been working on ways to turn wood into electricity.  Wood gas operating a generator is another approach but it seems a bit to touchy to me.

I should check into TEGs again as well as steam engines and Stirling engines.  Last time I investigated them there wasn't anything big enough on the market for home energy production.

I do have quite a bit of experience with hydraulics everything seems to work out so far, I have direct experience with a hydraulic motor running a 240 Amp alternator it takes 5 gal per min at 1500 psi.

The first design we ran was belt driven with a tensioner, but the current design is direct drive with a 'love joy' coupler.

Sweet, that's great to hear Marc!  Do you know how many watts (or volts) that alternator is putting out?  Trying to get a feel for how much power that amount of hydraulic storage would meaningfully produce.

It a 12 volt system. I think it would still produce with less pressure it needs the volume for the pump though.

I can play with the system tomorrow to see at what the minimum pressure of the system is. I don't know if there is still a flow meter around if there is I can take a volume measurement too.

Mike Haasl wrote:

My first issue is that I'm not very familiar with hydraulics and pressure tanks.  I'm having trouble figuring out if they actually make large pressure tanks, what they would cost and how much energy they can store.

My second issue is that I'm assuming they make hydraulic motors that are coupled to an electric generator.  Is that actually a thing or would we have to cobble those two pieces together.

Anyone have some hydraulic experience to lend to this problem?  Let's invent something cool!

They make large pressure tanks.  Think rural LPG tanks.  They cost about $1/gallon (size) for used tanks.  I believe they are rated fairly high for pressure, say above 500psi You might be better served by a fas cylinder like they use for welding or a scuba tank.  

Thanks Marc!  So if it is 240A and 12V, I think that is 2,880 watts.  

Mike Haasl wrote:
So, how much energy is represented by 2,310 cubic inches of fluid at 3,000 psi?

Assuming isothermal expansion (i.e. it's allowed to warm up as it expands by absorbing heat from the surroundings), and that's just compressed air (which can hold far more energy than an incompressible fluid), expanding from 3000 psi to atmospheric pressure, .42 kWh

Soooo not a ton...

Eric Hanson wrote:So why is Nitrogen used as a working gas and not air, or CO2 or some other gas?  I know I can pay extra to get my tractor tires filled with Nitrogen to stop leaks, I just don’t know why it works or if this is related.

Lack of rust, lack of oxygen destroying seals and lack of moisture are the main reasons.   The other major is that nitrogen is the most plentiful gas and very easy to separate from the others.  which means it is cheap too.   It is used in racing tires because as the tire warms up it changes pressure less than air filled tires.  Same for shocks plus oxygen in a pressurized system is going corrode the metal container and by sticking to nitrogen that problem is eliminated.  CO2 can break down into some oxygen and cause corrosion under the right conditions plus has some wierd temperature/pressure properties


Now on other fronts accumulators are commonly used on many closed center systems and on some open center systems.  Early case magnum tractors for example used to accumulators to smooth shifting out.  Those sell for a bit over $400 a piece new after market.  Have also seem them in some track hoes and So I would check with someone who does hydraulic repair looking for sources of them in the salvage that might get you to what you want.

Wood gas operating a generator is another approach but it seems a bit to touchy to me.

If you make the wood into charcoal, the process becomes dead simple. Do a search for Gary Gilmores "simple fire" on youtube. There is also lots of information at https://forum.driveonwood.com/

I ran a little 2000 watt generator on charcoal which had enough juice to run an angle grinder - I want to say it could consistently produce about 600 watts. Charcoal is easy to make, but it is messy to process to the correct size for an engine. Once you get all the kinks worked out, it can be quite simple and reliable - I just never actually needed the generator, so it never quite floated to the top of my priority list. I have a little propane inverter generator now that I probably run for like 3 hours a year or something like that.

Jack Edmondson wrote:

Mike Haasl wrote:

My first issue is that I'm not very familiar with hydraulics and pressure tanks.  I'm having trouble figuring out if they actually make large pressure tanks, what they would cost and how much energy they can store.

My second issue is that I'm assuming they make hydraulic motors that are coupled to an electric generator.  Is that actually a thing or would we have to cobble those two pieces together.

Anyone have some hydraulic experience to lend to this problem?  Let's invent something cool!

They make large pressure tanks.  Think rural LPG tanks.  They cost about $1/gallon (size) for used tanks.  I believe they are rated fairly high for pressure, say above 500psi You might be better served by a fas cylinder like they use for welding or a scuba tank.  

Unfortunately typical pressures for an LPG tank and a hydraulic system are about an order of magnitude apart... and you need a working gas to be compressed, hence the piston or bladder arrangements..

Although I do not know the details of why you can't just have half the tank or so full of gas and half fluid... I feel like it would commonly be done that way if there was no issue..

Mike Haasl wrote:

D Nikolls wrote:
So, if you had 95 gallons of fluid and wanted 5 mins of runtime, at 1800rpm, you can allow flow of 19gpm, giving a displacement of 2.439ci.

Take the min allowable psi and the cubic inches, and you get a torque of 582 in-lbs, which is on the scale of a small cordless drill.

Thanks D!  How did you get from 2.439 in3 and 1000psi and end up with 582 in-lbs?  Or were you using a lower minimum psi?

I'm also not experienced with hydraulics but it sure seems like 95 gallons of pressurized fluid would run the equivalent of a cordless drill for a lot longer than 5 minutes.  I don't know if there's a boo boo with your math but that seems quite low...

I used 1500psi as a lower bound, and used the third calculator in my 2nd link..

The torque is equivalent, if the math is right. This doesn't say anything much about the horsepower/wattage...

Marc Dube wrote:It a 12 volt system. I think it would still produce with less pressure it needs the volume for the pump though.

I can play with the system tomorrow to see at what the minimum pressure of the system is. I don't know if there is still a flow meter around if there is I can take a volume measurement too.

Do you know the displacement of the hydraulic motor? It would be interesting to see if the math in my links is matching up..

Have you played with loads on the alternator?

So..

1) Does it need to be hydraulic pressure, or could your idea work with air instead? Stored air is more useful on a homestead, and less polluty.. still dangerous and cumbersome though.


2) It seems like converting to electricity asap is key.. so, an accumulator that is just big enough for a very modest generator/alternator, and aim to let it keep spinning all the time that the gizmo is gizmoing?


3) I agree with the stated worry that inefficiences will make this a non-starter.. but a bit of thought can't hurt...

Just remember that while hydraulic has some great advantages it comes with some big disadvantages too.  In this case I am going to guess that lost efficiency will eat up most of your gains.  Now its big advantage is lossless storage just sitting there.  But high speed hydraulic motors are less efficient so most include a transmission that gears the speed way up that also cost efficiency.   And changing forms of energy will cost too.  So making electric out of the stored energy will also cost  Say it heat energy was X.  You lost 60% converting it to rotatory motion via a stirling cycle.  Getting hydraulic pressure costs you another 20%.  Getting that hydraulic back to rotary costs you say another 30%.  Getting that rotary back to electric costs another 25%.   Lets keep the math showing.  100 watts of heat energy -->to 40 watts of rotary-->to 32 watts of stored hydraulic pressure-->22.4 watts of rotary motion high speed-->16.8 watts of electrical energy.  Now if you went straight to electrical storage such as a battery or super capacitor you would have 50% to 60% of that left coming back out of the battery.  So by going the hydraulic route you are basically tripling your loss rate while adding a very expensive battery in the middle.  If you can make the economics work more power to you.

I'm also guessing that to get a large enough accumulator to store enough power to get through 12-24 hours of use may cost more than an equivalent battery bank.  

So then using the heat engine to pump fluid into a small accumulator in order to meter it evenly to a motor/generator and then to the battery bank may be best...

I am not sure why it would really need an accumulator. The invention outputs high pressure fluid at some flow rate. Route that fluid to a hydraulic motor that runs at roughly the rated pressure and flow rate. Couple an alternator or whatever to the hydraulic motor. If there are variations in the flow, the alternator will simply produce more or less power. An accumulator obviously does not produce any power - it only stores it. So unless you need the generating system to meet some surge of demand, I dont see how it would really help.

So how much flow would the device produce exactly? You mentioned that it should output 560 watts. How much energy has to go into it to get that output?

Sorry Carl, I neglected to mention that the engine is a sort of slow piston compression device.  I'm thinking it would compress a hydraulic piston 4" on a heating stroke, then retract on the cooling cycle.  Those cycles would not be fast, maybe a cycle every 4-10 seconds.  So that's why I think it would need a small accumulator to average out those pulses for a downstream hydraulic motor.

I don't have a good feel for the cycle rate until we build one.  Or we find someone who's really good at calculating thermal effects on metals...   If it's 4 seconds per cycle, I calculate 1 liter/min at 338 bar (4900 psi).  I have no idea how much energy needs to go into it to get that power out but I'm assuming it's 30-60% more.  The key here is that this engine would take low grade heat differences and convert them into usable power.

Mike Haasl wrote:Sorry Carl, I neglected to mention that the engine is a sort of slow piston compression device.  I'm thinking it would compress a hydraulic piston 4" on a heating stroke, then retract on the cooling cycle.  Those cycles would not be fast, maybe a cycle every 4-10 seconds.  So that's why I think it would need a small accumulator to average out those pulses for a downstream hydraulic motor.

I don't have a good feel for the cycle rate until we build one.  Or we find someone who's really good at calculating thermal effects on metals...   If it's 4 seconds per cycle, I calculate 1 liter/min at 338 bar (4900 psi).  I have no idea how much energy needs to go into it to get that power out but I'm assuming it's 30-60% more.  The key here is that this engine would take low grade heat differences and convert them into usable power.

If you want to share any more details, I can probably do a fair amount of the engineering (I'm a materials engineer, and have spent the last decade working with pressure vessels, boilers, steam turbines, etc.)


That said, I suspect you'll be terribly disappointed by the reality of thermodynamics. There's some really excellent reasons why we use high temperature differentials in heat engines. You're limited by the carnot efficiency (i.e. the absolute MAXIMUM amount of energy that can be extracted from a temperature differential), which for 80 F to 60 F (for instance) is only 3.7%

To put that in perspective, if you transfer 1,000,000 btus from your hot reservoir to your cold reservoir, the ABSOLUTE MOST energy you could extract from that 20 F temperature differential is just over 10 kWh, meaning you need a really big cold reservoir (that's enough energy slipping through to heat 24,000 gallons of water 5 degrees). For a number of reasons, you won't be approaching carnot efficiency, so the situation is even more bleak than that...

The other big reason we use large temperature differences, is that the differential in temperature is what drives the transfer of heat. All else being equal, conductive heat flux is directly proportional to the difference in temperature (i.e. a 200 degree differential will conduct 10x the heat energy that a 20 degree differential will). This means that to produce a meaningful amount of work, your engine will likely have to be a LOT, a LOT bigger than you're thinking, just for heat transfer surfaces.


All that said, I love weird heat engines, and would love to learn more. Happy to run some numbers for you if you need...

Mike Haasl wrote:
So, how much energy is represented by 2,310 cubic inches of fluid at 3,000 psi?

231ci is equal to 1 gal.  
2310ci/231ci=10 gallon.

psi x gpm / 1714=hp

If we use all 10 gallons in one minute that is 10 gpm

3000 psi x 10 gpm / 1714 = 17.5 hp for one minute.

kw = 17.5 hp/ 1.341 = 13.05 kw for 1 minute.

13.05 kw / 60 minutes = .21 kw for 1 hour.

The most efficient hydraulic motors I have used are about 90% oae.

On the 4" bore piston pump.

edit  Sorry about the mistake, I am getting rusty.

4" bore x 4" stroke = 12.57 square inches x 4 in = 50.27 ci

6 strokes per minute = 301.62 ci per minute / 231 = 1.3 gpm.

That 5-8% efficient TEG is looking better and better :)!

Here is an article about a 1000W unit that runs on the waste heat from diesel exhaust.
https://www.greencarcongress.com/2014/12/20141203-gmz.html

So from Amazon: https://www.amazon.com/Comimark-40x40mm-Thermoelectric-Generator-SP1848-27145/dp/B07W4PY9CZ/ref=sr_1_6?crid=3JEXL0RYFDW7Q&keywords=Thermoelectric+generator&qid=1642477969&sprefix=thermoelectric+generator%2Caps%2C255&sr=8-6

They claim you can get about 3 watts per unit with a 100 degree (presumably C) temperature difference (and obviously some way to dump the heat on the cold side. For $4.50 each, that is more expensive than solar panels, but really not bad at $1.50 per watt.

An 8" chimney pipe would fit pretty much exactly 16 units around its circumference, and at 10 units tall (around 16") you would be able to generate 500 watts from chimney heat. You would likely need a water jacket to help dump the heat, and a boost converter or two to allow you to run the units in parallel to keep the resistance down.

If I had more free time, and $720 laying around, I might be inclinded to try something like that. I would even kick in a few dollars for a kickstarter that made a drop-in chimney generator. Just saying.

Mike Haasl wrote:Sorry Carl, I neglected to mention that the engine is a sort of slow piston compression device.  I'm thinking it would compress a hydraulic piston 4" on a heating stroke, then retract on the cooling cycle.  Those cycles would not be fast, maybe a cycle every 4-10 seconds.  So that's why I think it would need a small accumulator to average out those pulses for a downstream hydraulic motor.

I don't have a good feel for the cycle rate until we build one.  Or we find someone who's really good at calculating thermal effects on metals...   If it's 4 seconds per cycle, I calculate 1 liter/min at 338 bar (4900 psi).  I have no idea how much energy needs to go into it to get that power out but I'm assuming it's 30-60% more.  The key here is that this engine would take low grade heat differences and convert them into usable power.

If a single piston's cycle is 50% heating, 50% cooling, couldn't you add a second or third piston at 180* or 120* phasing to get full use of the heat and cold sources? I'm assuming that the "low-grade heat" (waste heat?) is constant... and maybe you are pumping cooling water?

No Kenneth, it's a fairly odd "engine".  It's not rotary...

Thanks for the offer Nick, you're just the kind of engineer I'm after :)  I'll shoot you a pm.

I don't want to post too much about the actual engine for fear of losing the ability to patent it for permies.com....

Mike Haasl wrote:No Kenneth, it's a fairly odd "engine".  It's not rotary...

Thanks for the offer Nick, you're just the kind of engineer I'm after :)  I'll shoot you a pm.

I don't want to post too much about the actual engine for fear of losing the ability to patent it for permies.com....

Easy to misunderstand... I really meant "timing" in regards to phase, similar to a piston ICE where multiple pistons are in various "phases" of the cycle at any given time. Therefore, there is always one in the power stroke, or in your case possibly always one in the "heating phase" of the cycle. So, two would increase the frequency of the pulsing, possibly making the pulsing less severe and at a higher amount of power? three, four, etc... would smooth it further.
Or possibly like a double-acting pump, which is doing work in both directions, is a better example.

I like "idea" of your idea (since you are being mysterious with the details) and hope it goes well. I'm really interested in building a large scale LTD Stirling engine, similar to what's been done at Tamera.
I think the naysayers will quote all sorts of maths and inefficiencies, and alternate "better" ways, but if the input is "free" or otherwise "waste" then the numbers change. Further, if the functions are stacked, the numbers change again. If the "lack of costs" is internalized rather than costs being externalized, you could see a win-win-win scenario.

Ahh, great point.  Having multiple "piston engines" would average out the pulses more.

I'd love to see a nice sized Stirling or steam engine that people could play with to run a homestead.  You should definitely build one!

Here's a nice Stirling engine I saw the other day:
https://old.reddit.com/r/Damnthatsinteresting/comments/s2rqis/huge_stirling_engine_powered_by_6_tea_light/

So I did some playing around with the system I have access too (enough parts finally came in to run it)

It is a small hydraulic motor attached to a 240 Amp alternator that charges a 12v battery. The hydraulics go through an overly complicated system with a flow control, pressure valve, and a pilot operated check valve because it is tied in with other operations.

It is settup up to run at 1500psi and 5gpm I was playing with these settings and found that 1200psi and 3gpm worked for just charging the battery but could not keep up to a load of 30amps. My next setting was 1350psi and 3.75 gmp and this was sufficient to keep up with the 30 Amp load.

Thanks for checking into that Marc!  Do you know how many amps it was taking to charge the battery?

My two cents' worth of tinkering and experimentation has shown me if you want any decent power, steam is really the best solution.

I fiddled with various Stirling setups, and I truly admire Robert Stirling's invention, but it really is pretty wasteful with regards to the fuel consumption. Ford produced a Stirling motor in the 1970s which achieved 240 HP, but was (and still is) a technological nightmare. Robert Stirling's second patent in 1847 was for a dual-action model which produced 45 HP, but it also weighed several tons and had "hot" pistons which were 16" in diameter. The "cool" pistons were 4 feet in diameter.

There are many steam solutions which don't require a giant dangerous boiler. In fact, the power companies don't use big boilers anymore either. They use tube designs which minimize the dangers.

Hobbyists have been using various "water tube" designs to not only minimize danger, but to also reduce weight (especially for boats and airplanes).

I tried to stay away from steam for years, but kept coming back to it. I guess there's a reason the power companies still use it.

Edit: a huge issue in generating CLEAN electricity is driving a generator at the typical 3,600 RPM. There are units out there which accept PTO inputs of 540 and 1,000, but they're simply geared-up to output 3,600. Therefore, if the RPM input deviates even a litte, the output of a standard generator will be seriously distorted. At best, it reduces efficiency, at worst, it damages equipment.