Stirling heat motors

I see how the current Stirling heat motors work. This post isn't advocating a major change. I've got a question about volume and I can't seem to find an answer. Stirling engine forums seem resistant to any discussion that deviates from the current standard setups, shutting down questions about alternative approaches to heating the air that would require a change in configuration. So, I'm going to ask here to see why I'm wrong in my thinking and approach. Also, I'm going to leave out the cooling cycle of a true Stirling in the interest of getting this question answered.

If I weld a lid onto a pot and it's air tight, then put that pot over a fire, it will eventually explode. The bigger the pot and fire, the greater the volume of hot air and pressure, the bigger the explosion. This suggests a greater volume of heated air has a greater amount of energy. First, am I correct in this assumption? Does a greater volume of air have more potential energy than a smaller volume? This seems like a common sense question, but I want an answer anyway. Maybe I'm wrong and a sealed thimble explodes with the same force as a sealed cooking pot.

If I took that same pot and welded a pipe on it that lead to the small cylinder and piston of the size we currently see in most heat motors, would the expanding air from the pot transfer pressure to the cylinder and drive the piston, assuming the cylinder is not too far from the pot? If the answer is yes, would the hot air from the pot over the fire drive the piston harder than a candle beneath the little sealed cylinder we usually see?

If I used a larger pot over a bigger fire with that exact same cylinder, would more horse power result? Yes or no. Save the possibility of blowing the piston for later, if there is a later with this discussion.

I see current heat motors using a very small area to heat. Yes, I'm aware this is for efficiency. I'm wondering if the HP can be upped my heating a greater volume of air, basically. I've been told it will not and that just goes against the way it seems things should work. I would like to understand why it wouldn't, if that's the case. I'm honestly not going to argue, just trying to understand this question about increased volume as pertains to HP.

To increase surface area, lets say I fill the pot with metal shavings, which will heat up inside the pot. This should, in theory, heat a greater volume of air inside the pot, increasing pressure and driving the piston with greater force, or HP. In theory.

For the sake of a clear answer to this question of volume vs HP, lets not get into efficiency or fuel usage just yet. If I'm wrong in my thinking, those things will not matter, anyway. I just want an answer to the question about a greater volume of heated air equalling greater horse power. Does it?

Also, I don't understand where the continued pressure would come from in the pot. The piston would relieve/exhaust some of the pressure with each stroke. It seems like the air inside the pot is already expanded. So, where would the pressure for another stroke come from? Wouldn't cooler air be required in there so it can be expanded for more pressure? Or, does the air inside the pot constantly cool and expand as it comes in contact with the surface area, leading to constant pressure as the fire burns? I mean, with steam engines, water has to be added to the boiler because the level lowers as the water is turned to steam to drive the steam engine. What about the air in a sealed cylinder or pot? Doesn't it need to be replentished if the piston relieves some of the pressure with each stroke? I realize this is probably a stupid question, or extremely basic to some of you. Good news is if I can understand the answer, I won't ask again.

Again, I'm not saying anything needs to change or that anyone is doing anything wrong with the current methods or that this is a better idea or that this hot sealed pot idea is even possible. I'm just trying to better understand something about the Stirling that's been nagging me.

If you find this question annoying, wait till I post a question about flash steam engines. That'll be loads of fun.

The amount of energy in a large mass of hot gas is larger than the energy in a small mass of hot gas. Actually, it true for all of the phases.

Bill Bianchi wrote: If I weld a lid onto a pot and it's air tight, then put that pot over a fire, it will eventually explode. The bigger the pot and fire, the greater the volume of hot air and pressure, the bigger the explosion. This suggests a greater volume of heated air has a greater amount of energy. First, am I correct in this assumption? Does a greater volume of air have more potential energy than a smaller volume? This seems like a common sense question, but I want an answer anyway. Maybe I'm wrong and a sealed thimble explodes with the same force as a sealed cooking pot.

It's not the volume of gas that matters, it's the mass (all else equal). The working fluid (i.e. gas) carries the heat. Therefore, the greater the mass of working fluid, the greater the heat, and the greater the potential work from the engine. A small engine with a very high pressure may have more mass of working fluid than a large engine with very low pressure. Again, it's the mass that matters. When you're talking about a working fluid with no phase changes (such as a stirling engine), then you see that the amount of heat added is proportional to the temperature of the working fluid (all else equal). This is one reason why you want the highest possible temperatures to achieve the most work from an engine (all else equal).

Bill Bianchi wrote: If I took that same pot and welded a pipe on it that lead to the small cylinder and piston of the size we currently see in most heat motors, would the expanding air from the pot transfer pressure to the cylinder and drive the piston, assuming the cylinder is not too far from the pot? If the answer is yes, would the hot air from the pot over the fire drive the piston harder than a candle beneath the little sealed cylinder we usually see?

Careful to not merely copy other engines, but make sure to understand the principles involved. This is what you're investigating with the inquiry, so I salute you. Yes, most stirling engines use a small power piston relative to a much larger displacer. However, this is primarily because these engines operate on a low temperature differential. What's important for engine power is the RATE at which the heat enters the system along with the rate at which it can be cooled, and the peak temperatures/pressures, etc. A large really hot fire iS necessary to generate serious power, but there may be other bottlenecks in the system. For example, it doesn't matter how big or hot your fire if the area where heat enters your engine is very small and with poor heat transfer characteristics... it can't generate serious power unless the working fluid is actually heated at the required rate.

Bill Bianchi wrote: If I used a larger pot over a bigger fire with that exact same cylinder, would more horse power result? Yes or no. Save the possibility of blowing the piston for later, if there is a later with this discussion.

Not a simple yes or no question. See the previous answer. However, in general, the answer is yes, but the temperature of the fire is also important and not merely the size. Also, if the fire is too large or hot then there will be more thermal losses, so you might get more power but lower efficiency.

Bill Bianchi wrote: I see current heat motors using a very small area to heat. Yes, I'm aware this is for efficiency. I'm wondering if the HP can be upped my heating a greater volume of air, basically. I've been told it will not and that just goes against the way it seems things should work. I would like to understand why it wouldn't, if that's the case. I'm honestly not going to argue, just trying to understand this question about increased volume as pertains to HP.

A small area to heat is not necessarily good for efficiency. This is done mainly to simplify construction. If you want more HP, then it's necessary to heat a larger mass of air per cycle and/or achieve a higher engine speed. This becomes problematic due to friction losses (both in seals and in having to shunt the gas back and forth), and more important efficiency tends to suffer in a stirling with high speeds because the heat transfer rates are capped and this lowers the temperature differential across the engine (when I write "engine", I don't mean the actual metal but the working fluid itself).

Bill Bianchi wrote:To increase surface area, lets say I fill the pot with metal shavings, which will heat up inside the pot. This should, in theory, heat a greater volume of air inside the pot, increasing pressure and driving the piston with greater force, or HP. In theory.

It will increase surface area, but it might decrease heat transfer rates... this will lower hp. I'm not saying it won't work, just that it's more complicated than simply noting that the surface area goes up.

Bill Bianchi wrote:For the sake of a clear answer to this question of volume vs HP, lets not get into efficiency or fuel usage just yet. If I'm wrong in my thinking, those things will not matter, anyway. I just want an answer to the question about a greater volume of heated air equalling greater horse power. Does it?

Again, not volume but mass that matters. All else equal (same pressure, etc.) the larger volume has more mass, so in that sense you're thinking properly. Feel free to get into the other issues because a stirling is interesting in that the higher the power the lower the efficiency tends to be.

Bill Bianchi wrote:Also, I don't understand where the continued pressure would come from in the pot. The piston would relieve/exhaust some of the pressure with each stroke. It seems like the air inside the pot is already expanded. So, where would the pressure for another stroke come from? Wouldn't cooler air be required in there so it can be expanded for more pressure? Or, does the air inside the pot constantly cool and expand as it comes in contact with the surface area, leading to constant pressure as the fire burns? I mean, with steam engines, water has to be added to the boiler because the level lowers as the water is turned to steam to drive the steam engine. What about the air in a sealed cylinder or pot? Doesn't it need to be replentished if the piston relieves some of the pressure with each stroke? I realize this is probably a stupid question, or extremely basic to some of you. Good news is if I can understand the answer, I won't ask again.

I'm glad you asked this as it shows you may not yet understand the stirling. Nothing wrong with that! Consider the following thought experiment. You have your sealed pot with a pressurized gas inside. You also have a sealed piston in a cylinder that is perhaps half the volume of your pot. Now, assume the piston cycles at a very low speed on the order of one rotation every 20 seconds or so. Now, connect the top of the cylinder to the pot via a flexible tube (let's assume it can handle the heat and pressure). Now, let's put a massive flywheel on the crank that allows the piston to keep cycling. Now, shortly before the piston reaches the top of the cylinder you put your pot in a roaring fire. It heats up the gas inside and pressure starts to rise. The piston reaches top dead center and then it starts a power stroke. Well, since the pressure in the system is building (or not falling nearly so fast as it would otherwise), then you will get some torque on the flywheel. Now, since the gas is expanding out of the pot and into the cylinder, the temperature may not be rising... it might be rising, might be isothermal, or might be dropping... but you're adding heat so you're adding force that would otherwise not be there. Now, just before the piston reaches the end of the stroke you take your pot out of the fire and onto a platform where dozens of water spray nozzles spray cold water onto the vessel. The pressure of the air in the pot drops rapidly just in time for the compression stroke. So, the piston can return to the top of the cylinder with a lot less pressure in the system. During recompression the pressure will rise, but it won't rise nearly so much as would be the case without the water cooling. This allows the work of the power stroke to exceed the work of the compression stroke, and net work out results. Note that you have to keep cooling the pot during the compression stroke because work is converted to heat as the compression takes place, and you want to remove this heat to minimize back pressure during the compression stroke. Rinse and repeat. NOTE: In actual stirling engines what happens is that the heat entering the system lowers the rate at which the pressure drops due to expansion, and the heat leaving the system lowers the rate at which the pressure rises due to compression. The net result is a mean effective pressure, force, torque, and work. A REGENERATOR may be added to pick up some of the excess heat added during the power stroke. To understand how this works think about keeping the pot in the fire only during the first part of the power stroke, then removing it and letting the gas expand down to where the temperature of the gas approaches ambient. Now, when the compression stroke starts there is less heat to remove... the only heat that must be removed is the heat of compression. Now, keeping the pot in the fire longer and adding more heat will increase the power of the engine, but without the regenerator it will not increase the efficiency much. What the regenerator does is allow for adding the extra heat for more engine work, but then recycling the heat back into the system... now that increases efficiency a great deal while also getting the extra power. Steam engine systems with compounding can do this by reheating the steam that expanded from one stage to the next, and the heat is recycled by using the high temperature superheated steam finally exhausted from the engine to preheat combustion air and/or preheating the boiler feed water.

Bill Bianchi wrote:Again, I'm not saying anything needs to change or that anyone is doing anything wrong with the current methods or that this is a better idea or that this hot sealed pot idea is even possible. I'm just trying to better understand something about the Stirling that's been nagging me.

Good deal, keep thinking along these lines... it has helped me tremendously to make thought experiments like these. I now understand engines very well and got to the point where I see all heat engines alike with only trivial differences between them.

Bill Bianchi wrote:If you find this question annoying, wait till I post a question about flash steam engines. That'll be loads of fun.

Actually, a steam engine has been called a "practical Carnot engine". We often consider the stirling as most closely matching Carnot, but a steam engine in a certain configuration does it best - at just about as good. This is why modern steam power plants can approach 50% net thermal efficiency without exceeding 1000F steam temperatures... because they are so good at following a Carnot cycle. The way this is done is through compounding, steam reheat, and heat regeneration. The stirling engine does this in a much simpler configuration, but it's still a real bitch to get high efficiency, and even harder to get both high efficiency and high power. A steam engine gets both (but not without sophistication). A piston steam engine can approach 40% net thermal efficiency under similar conditions (compounding, steam reheat, heat regeneration).

That was one heck of an answer. Thank you very much. Seriously informative.

All right, so not all my thoughts on this were completely off base. That's nice to know. And, thanks for explaining the parts I flat out didn't understand.

Seems like a valve on the pipe leading to the cylinder would help, if it cuts the pressure on the return stroke and opens to push the piston again after its all the way back in the cylinder. That would be a lot of opening and closing, though. Don't know if a reliable valve like that exists. It would at least address the flywheel working against the pressure on the return stroke. If the valve also opened the cool chamber at the same time only on the return stroke, maybe it would help "suck" the piston back into the cylinder as the hot gas cools and contracts?

The cool chamber, where the hot air exhausts into, looks like another challenge. How the heck do you cool that without expending more energy or overwhelming the cooling the cooling agent used to cool the working fluid?

This may be another stupid question, but has anyone tried using ammonia as the working fluid in a Stirling, a mixture like that found in an absorbsion refrigerator? To a newb like me, the thinking goes like this. The ammonia liquid gets heated, expands, drives piston forward, exhausts into an unheated chamber where it condenses. That second chamber should get ice cold as it condenses, providing a wide temperature difference, which should help the engine run more efficiently.

That seems like a good idea, but there's probably a ton of legistics that would have to be overcome, if its even workable. Maybe the ammonia corrodes the crap out of the piston and cylinder.

On that note, could the same ammonia mixture be used on a thermal electric generator to heat one side, then condense on the other plate and cool that side?
I'm thinking 2 connected containers, one on each plate of the TEG. Heat one tank, which also heats that TEG plate on the hot ide, route the ammonia gas to the second tank on the other plate, where it condenses back to liquid and turns ice cold before going back over to the hot tank to repeat the cycle.
Again, sounds nice, but lots to figure out for a demonstration model. I like it, though, because the heat applied would, in theory, also power the cold side, enhancing energy output through extreme differences in temp without having to add more input energy than normal. In theory, which is all any of this is.

We can get into possible heat sources next, if I'm not being too boring with this thought experiment. Just trying to look at possibilities. I need to know what's possible for a fiction book I'm writing.. I'm not going to try to build one, but I need to understand this if I'm to present it as a viable method.

Bill Bianchi wrote:Seems like a valve on the pipe leading to the cylinder would help, if it cuts the pressure on the return stroke and opens to push the piston again after its all the way back in the cylinder. That would be a lot of opening and closing, though. Don't know if a reliable valve like that exists. It would at least address the flywheel working against the pressure on the return stroke. If the valve also opened the cool chamber at the same time only on the return stroke, maybe it would help "suck" the piston back into the cylinder as the hot gas cools and contracts?

Removing heat from the gas during the compression stroke takes care of things. That's all it needs. A valve like this will not help. All piston engines have to store some energy in a flywheel to overcome recompression unless there are multiple cylinders. Three or four power pistons could be devised to generate a positive torque profile over the cycle so that a flywheel is not required, and each of these essentially independent stirling engines could share the same furnace and cooling heat sink. This would probably be necessary to make a truly efficient and high power stirling engine with limited funding. In fact, I designed just such an engine a few years back that used four separate large power cylinders each with a displacer and a single biomass furnace that heated all four engines. The system was very slow moving on the order of one cycle per second, but highly pressurized for very high torque. No flywheel was required, but it did call for a gear box to increase speed for driving an alternator. That approach can be highly efficient and see enough power to be useful. Imagine such an engine at 30% efficient and 1 KW electrical output and fueled by biomass, and with heat recovery for all space heating and water heating. At such a low speed the thing would last a lifetime.

Bill Bianchi wrote:The cool chamber, where the hot air exhausts into, looks like another challenge. How the heck do you cool that without expending more energy or overwhelming the cooling the cooling agent used to cool the working fluid?

I'd say you use water cooling at a fairly high rate. It won't consume very much energy. The main problem is designing the system so that it can cool at the required rate even with a good heat sink in the form of cool water. You need high surface area and good thermal conductivity without excessive internal volume. I've considered a good way to do it.

Bill Bianchi wrote:This may be another stupid question, but has anyone tried using ammonia as the working fluid in a Stirling, a mixture like that found in an absorbsion refrigerator? To a newb like me, the thinking goes like this. The ammonia liquid gets heated, expands, drives piston forward, exhausts into an unheated chamber where it condenses. That second chamber should get ice cold as it condenses, providing a wide temperature difference, which should help the engine run more efficiently.

Well, no, that's not right. The ammonia vapor releases heat as it condenses and will be at a higher temperature than ambient when this occurs (i.e. not ice cold). Think of a steam engine with condenser... everything is well above ambient including the condenser.

Bill Bianchi wrote:That seems like a good idea, but there's probably a ton of legistics that would have to be overcome, if its even workable. Maybe the ammonia corrodes the crap out of the piston and cylinder.

Physics is the main obstacle. Ammonia does well with most steel alloys, but eats copper and most copper alloys, and aluminum doesn't like it either.

Bill Bianchi wrote:On that note, could the same ammonia mixture be used on a thermal electric generator to heat one side, then condense on the other plate and cool that side?
I'm thinking 2 connected containers, one on each plate of the TEG. Heat one tank, which also heats that TEG plate on the hot ide, route the ammonia gas to the second tank on the other plate, where it condenses back to liquid and turns ice cold before going back over to the hot tank to repeat the cycle.
Again, sounds nice, but lots to figure out for a demonstration model. I like it, though, because the heat applied would, in theory, also power the cold side, enhancing energy output through extreme differences in temp without having to add more input energy than normal. In theory, which is all any of this is.

Again, you need evaporation and not condensation for cooling. This system you describe would require a dedicated vapor compression cycle. Imagine a highly efficient heat engine driving a refrigerant compressor. The refrigerant is used to cool the engine. The heat gets dumped at the condenser of the vapor compression system. The heat engine is no longer highly efficient since much or most of the work from the engine is used to power the cooling system.

Bill Bianchi wrote:We can get into possible heat sources next, if I'm not being too boring with this thought experiment. Just trying to look at possibilities. I need to know what's possible for a fiction book I'm writing.. I'm not going to try to build one, but I need to understand this if I'm to present it as a viable method.

What do you mean by "friction book"... are you referring to friction heaters by chance?

Fiction, as in a fictional novella. I'd like to show characters using alternative energy technologies that a lot of the reading public may not be knowledgable about yet. But, I don't want to show something generating energy in the story that won't work in the real world.
Gasification is one I intend to show the characters using. It's fairly unknown to a lot of people, so it'll be new or exotic to them. I'm looking for more technologies like that.

I thought absorption refrigerators removed heat from the inside, which keeps food cold, by heating the liquid, which turns to gas and travels up the tube, then condenses back to liquid, absorbing heat from the box, then drains back down the tube where it will be reheated to a gas to repeat the cycle.
Are you telling me that the condensing of the gas will not remove heat from the area where it condenses? How do those icy balls get cold after heating?
My thought was that if the heat can be removed from the other side of a TEG, there would be a wide temperature difference between the heated side and the refrigerated side, increasing electric output.
You're telling me it's not possible to use the absorption cycle to heat one side of a TEG and cool the other side?
Or, that it's not possible to use the absorption cycle to refrigerate the cool chamber of a Stirling?

How does an absorption refrigerator work? Looks like I've got it all wrong. Damn wiki explanations.

Looking forward to learning more.

Bill Bianchi wrote:Fiction, as in a fictional novella. I'd like to show characters using alternative energy technologies that a lot of the reading public may not be knowledgable about yet. But, I don't want to show something generating energy in the story that won't work in the real world.
Gasification is one I intend to show the characters using. It's fairly unknown to a lot of people, so it'll be new or exotic to them. I'm looking for more technologies like that.

Yep, you did write "fiction"... guess I was in engineering mode (funny). Thank you for doing research as I HATE reading a fiction book that totally dicks things up in the engineering/physics department. It's really distracting to a person who knows better. BTW, I've been helping a friend along the same lines... he's also writing a fiction book and I often correct his work. Since his story has an apocalyptic setting, he has a particular need to understand the kind of technologies that I like to study (gasification, steam power, absorption, etc.). I've been trying to convince him (with success) that a post apocalyptic world has a need to understand and implement such technologies... especially one where millions of zombies are roaming the landscape as this tends to put a damper on infrastructure development.

Bill Bianchi wrote:I thought absorption refrigerators removed heat from the inside, which keeps food cold, by heating the liquid, which turns to gas and travels up the tube, then condenses back to liquid, absorbing heat from the box, then drains back down the tube where it will be reheated to a gas to repeat the cycle.

If you examine the process you just described, then you'll see that you provided no provision for removing heat from the refrigerant. You show the liquid being heated, vaporizing, then absorbing heat by condensation (sic), then the liquid draining down to be reheated again. What actually occurs is that liquid refrigerant evaporates to provide the cooling effect, and the absorbent takes in the refrigerant vapor to drive the process (it absorbs refrigerant vapor to keep the pressure so low that the liquid refrigerant evaporates). The absorbent must be cooled during this process. Eventually the absorbent will take in too much refrigerant, so it's necessary to heat the solution and drive out the refrigerant (essentially recharging it). The refrigerant vapor driven out by heat is cooled and condensed so that a continual supply of liquid refrigerant is available.

Bill Bianchi wrote:Are you telling me that the condensing of the gas will not remove heat from the area where it condenses? How do those icy balls get cold after heating?

That's right. Condensation releases heat. The icy ball is heated to drive the ammonia refrigerant out of the water absorbent. The ammonia vapor cools, condenses, and collects in the ammonia side of the unit. Once the absorbent is reconstituted, then it can absorb refrigerant vapor again and provide cooling.

EVAPORATION: If you could zoom in on the surface of a glass of water and see the individual water molecules, then you would see them jiggling and bumping and banging around against each other. Temperature measures only the AVERAGE kinetic energy of these molecules. However, STATISTICALLY it's clear that some of these molecules have more energy than others. Energy is constantly being transferred back and forth between the molecules. Every once in a while a particular set of collisions between the molecules result in a molecule getting banged so hard that it literally flies out of the pile. It becomes WATER VAPOR. This is evaporation. Since that particular water molecule clearly has a lot more energy than most of the others, then the AVERAGE kinetic energy of the remaining water molecules must go down. This is seen as a REDUCTION IN TEMPERATURE. CONDENSATION IS THE REVERSE PROCESS.

Bill Bianchi wrote:My thought was that if the heat can be removed from the other side of a TEG, there would be a wide temperature difference between the heated side and the refrigerated side, increasing electric output.
You're telling me it's not possible to use the absorption cycle to heat one side of a TEG and cool the other side?
Or, that it's not possible to use the absorption cycle to refrigerate the cool chamber of a Stirling?

You'll have to explain more precisely what you're considering (i.e. what you mean by using an absorption cycle to heat and cool a TEG... in what configuration is this done?). What is the benefit?

Back to the topic of stirling engines. I actually have some admiration for the stirling cycle, but I do not like the way stirling engines are generally designed. I've come up with many designs that I believe to be superior... well, at least for certain applications. I am interested in a system that is designed primarily for simplicity, longevity, reliability, quiet operation, biomass fuel, heat recovery for useful purposes, and high thermal efficiency. This has led me to a very particular design that is unlike anything I've ever seen before. No need to discuss specifics because describing it in words is damn near impossible. I will say only that it's designed for a very low speed on the order of 1-2 hertz, and a low power (less than 2 hp). Thermal losses are lessened by using compact heat exchangers, so the system is highly pressurized to get the volume of the engine down. Pressure is on the order of 1000 psig. Piston expanders are used, and there are sealed piston rods used to contain the working fluid in the system which is dry air. Most interesting is the use of external heat exchangers along with check valves to shunt the working fluid through the system in a particular flow path to optimize heat transfer. There is a regenerator. While I realize this is all a pipe dream at the moment, it's still interesting to consider the implications of having a highly efficient heat engine fueled by biomass that just loafs along at a super low speed for months on end. One of the main advantages is NOT being nearly so reliant on a battery system. A good battery of modest size that is properly maintained and never sees significant discharge will last a lifetime. Furthermore, battery losses can be almost eliminated if the electrical loads in the home are closely matched to the alternator driven by the engine. In this case the battery serves primarily as a voltage regulator rather than as an energy storage device. Air conditioning and space heating is particularly interesting under these conditions. A simple piston refrigerant compressor can be driven at low speeds to provide a highly efficient vapor compression cycle for both air conditioning and space heating purposes. Of course, the heat provided at the water cooled part of the engine (i.e. providing heated water) is the primary source for heating applications, if the vapor compression cycle is provided for air conditioning during the summer months, then might as well make use of it for space heating during the winter months. Actually, it turns out that electricity usage can be so low in the off grid setting that a highly efficient heat engine will not provide enough heat under those conditions to provide sufficient space heating or water heating. Therefore, the engine power can be increased and the vapor compression system should be configured to heat pump mode (simple valve line up) to provide both more heated water from the engine cooling system along with bringing in outside heat with the heat pump configuration.

I suppose I'm just pointing out what's possible under these conditions. Actually building such a system would be truly daunting, but I do know it's possible. It would be nice if someone with serious money would build something like this.

Bill, I'm discussing the compounded piston steam engine with reheat and heat regeneration. In my opinion, this is the most practical way to get both high efficiency and decent power in an external combustion engine.

A great discussion is the engine designed by Dr. Robert Bourke. See www.newsteamengine.com. His is optimized for vehicle use, but a stationary system would be even more efficient, and biomass fuel can be used. His design is extremely sophisticated, and there really is no way around this sophistication if one desires to achieve those kinds of efficiency figures. However, I am convinced that the same basic approach (compounding, reheat, heat regeneration) can be used to achieve 20% net thermal efficiency without exceeding a steam temperature of about 700F and staying well below 1000 psig. Something like this would make biomass gasification obsolete for many stationary applications. Think about it: You have equal or superior net efficiency, there is less fuel processing required, the system is quieter, all the heat from the system is available at the condenser and steam is an excellent heat transfer medium (much better for cogeneration than trying to harvest heat from a gas engine), and the system can operate at very low speeds for long periods (steam engines excel at this).