Hi everyone, long time lurker, haven't built an rmh, but bought the DVD / book and I had random thoughts I wanted to run by everyone on decentralized power production using an rmh. I saw a few mentions of power generation on the forum, but I wanted to brainstorm on the possibilities from a different angle.
From an ancient memory, I seem to recall that the most inefficient part of power generation itself (ignoring power distribution), is the boiling of water by coal/nuclear/natural gas, etc. to generate the steam that moves the turbine blades with efficiencies from a brief internet search at very roughly 40-60%. So, my question is if a rmh results in an ultra efficient almost complete combustion of organic matter, couldn't you scale up an rmh and very drastically undercut the pricing structure of a heavily centralized business structure like power generation while providing power to a single house or possibly even an entire neighborhood (on a large enough scale) from coppiced wood?
If one of the fundamental ways of producing electricity essentially the simple boiling of water, can't we all do that ourselves on a local level, adding in heat exchangers, oxygen sensors, conveyors, PLC controls, etc. to add even MORE drastic efficiency to the process of heating and boiling water? For another example of scaling up an rmh, having worked in foundries, there is also refractory material that can withstand extreme high temperatures that could expand the capabilities of a rmh well beyond a smaller system.
I have a lot of issues with very large scale systems due to their fragile and inefficient nature and think that there is some serious opportunity here - anyone have thoughts on getting into business with power generation using multiple very large rmhs?
Well... Yes and no. Can someone generate steam with an RMH? yes. Can someone produce enough steam to generate a household's worth of power? maybe, but that is a lot of energy; and it needs to be dry steam or it will quickly and severely damage turbine blades or steam engine parts. Dry steam requires a boiler and all of the safety equipment that that implies. There are synthetic oils that go to steam at lower temps but now you are talking having to have a condenser to recycle the steam and return it to its liquid state. Lastly, steam power generation on the tiny scale required for a single house or small neighborhood is extremely rare and therefore extremely expensive. They market and sell such units in Europe, they are called "Combined heating, cooling, and power generation" , or CHCP, some are just CHP (combined heating and power). Last time I looked they started at about $15K, and most use natural gas or propane because of the extreme energy needs to produce the dry steam.
So yes it can be done. No, it is not cost effective with current technology. Most commonly they are used in conjunction with grid connected homes where they produce enough energy at low usage times to sell some back to the power company to offset the peak use power that must be purchased. The residual heat is then a "Free" by-product for heating the house in winter, or through a heat pump for cooling in the summer.
Location: Southern alps, on the French side of the french /italian border 5000ft high Southern alpine climate.
posted 1 year ago
Well, they manage to burn petrol at stoechiometric mixture For fulle efficiency, and only manage a yiels of 30/40%
How in the life, can you immagine to do better than engineers?
Let me explain.
You burn wood, and may be you can manage 90% perfect combustion. On petrol they might manage 98% since it's all hydrocarbons.
If ever you"re lucky enough, you can transfer 80% of the energy released to water.
That's already only 72% of the original energy.
Then, you might transform only 70% of that heat energy into rotational energy in the turbine. 0.504% of original energy input. Ok, you can use the heat to heat homes or else. Still need some to create the draft.
Then, you have the electricity production side of it. You can't reach a cos phi, of more that 0.89 or 89% of the energy inputed on the shaft, into electricity, so that's 44.85% of original energy input. And i have used very optimistic figures.
I don't think it's a lucrative business proposition, but we all know what happens to elements incorporated into a larger permacultural design; the inefficiencies and "waste" products can be channelled to other uses.
Let's say that I buy a factory building in an area of town up for gentrification. I convert it, according to permacultural principles, to a hard-loft-conversion co-op (easier to deliver central heating to many households). One of the many permacultural elements would be this RMH-based co-generation plant.
I live in Toronto, where the continental climate gives us cold winters and hot, humid summers. The heat and power station would probably only operate in the cool seasons, or we'd need a building-sized refrigerant loop setup, using the excess heat from the generation of electricity to power it, with the remainder going into a ground-source heat pump.
It seems needlessly complicated. It requires too much specialised equipment, and as this is supposed to be on the household or small neighbourhood scale, if it's not as cheap and straightforward to build as the RMH itself (which are, for any of you who haven't participated in a build, more expensive and complicated than you could imagine, balancing use of reclaimed materials with ease of assembly, as in, the more strange bits you want to fit in to the build because they're free, the more complicated the build) it's not likely to be very accessible to the normal person.
I think to make decentralised power production work with an RMH, you have to look at the parts of what you're trying to use to make power. The RMH is composed of a rocket heater, either j-tube or batch box, the riser and additional refractory and insulation that gives the rocket enough rocketyness to push that exhaust through a longer-than-typical exhaust system that features an air-to-mass heat exchanger.
I think the RMH is designed to be a really efficient tool for converting biomass into heat energy, and then trapping the bulk of that energy in a heat battery consisting of mass. I think that, while perhaps the rocket part of the RMH might provide the energy you need for efficient co-generation from coppiced wood, the Mass part and the parts of the design that make benches work aren't beneficial for power generation.
One exception I can see to this is if you have a heat transfer mechanism around the top part of the barrel, where you usually see people put their kettles or whatever. Even so, steam is too complicated for household use, in my opinion, and too bulky. Trying to miniaturise a system designed to work on a giant scale might not work out.
I think thermovoltaic paint, or a thermovoltaic plate that generated electricity from the difference in temperatures between the top of the riser and the cold air outside, with no moving parts, is an idea with more merit for RMH experimentation on the homestead scale. You could even do both, the plate, designed to benefit from the extreme temperature difference between the burn tunnel and the outside air, and the paint coating the chimney outside, where the temperature difference between the exhaust and the outside air would be much less, but the surface area of the chimney would be greater, and the paint cheaper.
The thermovoltaic systems I am describing, incidentally, aren't linked with any product that I know of to exist yet, at least commercially. I read articles regularly about how thermovoltaics is getting more efficient, that a lab-scale experiment produced thermovoltaic paint, and that it is benefitting from all the R & D being done in photovoltaics, but I have yet to see it implemented in any great way. This might be that way.
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein
I think you are onto the right idea, just maybe not the right heating source.
My first thought was in trying to build a simple Sterling Engine and use the heat/cool differential it operates on to produce power. The heat side would come from a Jean Pain type of compost heating system since the sheep produce more then enough compost, and sheep farming is a type of farming that is very light on electrical consumption. The cooling side would come from geothermal cooling. I saw a simple Sterling Engine that ran on solar...but the sun only shines so much in a day. Compost heat is 24/7/365. In this case, while I would only produce 1-2 KW's, I would not being using many KW's anyway, and the rest can go onto the grid. In this situation, building the Sterling Engine would be the challenge...
My second thought was in going with biogas. My sheep could produce some, but the affluent coming off the corn and silage has even higher amounts. I happen to have a duel fueled, 3 KW liquid cooled generator kicking around. If I plumbed the silage pit, and put the manure into the biogas process, it might produce some electricity that could do the same thing. The liquid cooled part of the engine is the key; it would help to heat the biogas to the right temperature to get the biogas processing. Again this is something that would produce electricity 24/7/365. The challenge her would be in building a working biogas plant.
In my case, I think I would steer clear of my home because the draw is just too much for what I could expect to produce, but I think current laws force them to buy my green power off my farm, but I could be wrong.
Any thoughts on this Adam or anyone else?
A sincere thank you to all of Permies Forums for making Christmas special to Katie and I, and our four daughters. Thank you!
Location: Mountain View, United States
posted 1 year ago
Wow, fantastic responses everyone! This is exactly the kind of dialog I was hoping to create. I will admit to being out of my league on many aspects of combustion and power generation, but I hopefully can still inspire ideas.
@Duane, excellent summary, thank you for the details on the logistics of creating steam, that's very fascinating and I currently only knew about the existence of steam tables, but that's about it, so this helps me understand more about the requirements to physically move the turbines. Are there other ways besides the Sterling engine to utilize heat to generate power besides steam generation?
@Satamax, I'm mostly thinking about the overall efficiency of the system I'm proposing from the standpoint of scale besides efficient burning of the wood. For example, at one previous job, we had 1/2 million pound melters that wasted an incredible amount of energy to keep that aluminum molten 24 hours a day. The melter was consuming this energy regardless of whether or not the rolling mill was actually running, so not exactly an efficient use of energy to simply keep that metal molten at all times versus if there were several much smaller and more flexible systems with the same capacity that could operate without that constraint, saving huge amounts of energy (and cost) in the process. Those are the kinds of efficiencies I'm wondering if you could achieve by having tens or hundreds of thousands of decentralized power plants instead of the current system of several thousand centralized power plants (in the US).
@Chris, I'm still pretty rudimentary in my full understanding of the rmh system, but yes that makes sense now that I am probably thinking more along the lines of the "rocket" portion of the RMH for massive amounts of heat generation. Are there ways to utilize just the rocket portion of an rmh for heat generation instead of heat storage or does the system then just become some kind of cupola? The air to mass heat exchanger does sound promising to somehow use to harvest the high amount of heat somehow for power generation.
Also, I'm not limiting the discussion to just ultra small scale on how to power a house, by all means if you could harvest the power of the special geometry of a "massless rocket" on a large enough scale to power a city, that would be really awesome!
@Travis, the Sterling engine idea would be great - can you use just the "rocket" part of the rmh as mentioned above while still having efficient burning of wood to power a Sterling enginer? Would construction of the Sterling engine itself still be the complicated part? The ideas I'm looking for wouldn't even necessarily have to power an entire modern house, maybe even just enough electricity to charge up a bank of marine batteries to help reduce dependence on the power grid and build resiliency. Any other ways that ocmpost heat / biogas / photovoltaics, etc would play into power generation would be neat as well.
The RMH is a highly efficient way of burning wood specifically, to heat one space. (A few people have adapted it for other fuels.) For use at much larger scales, a simple scaling of geometry might or might not work best.
The air-to-mass heat exchange works well precisely because it can operate at low temperature differentials; power generation works best at high differentials. The storage mass would essentially use the waste heat from a primary power generator. I saw a comment that someone in Europe lives in a neighborhood with a local generator with the waste heat being used for district heating. This is cogeneration, and is relatively widespread if not common.
I figure the easiest, most appropriate way here to make a power-producing Stirling engine is to make it large and low-pressure, to add to the category of 55 gallon-drum engineering. Maybe a piston with around 10 or 20 liters of swept volume, pressurized with nitrogen to say 10 psig or so. It would run at around 120-150 rpm, and this could be geared up to run a generator. The efficiency would be comparable to the TEG, or a bit better. Still economical to run in the winter with CHP. The parts might be less expensive than TEG but could easily take much longer and be more complicated to build. The dimensions have to be well designed for it to work right but I could probably point you in the right direction on that.
I clicked on this thread thinking it might talk about small 4 inch batch box rockets in each room,rather than a single large 8-10" in a central location😁
That being said, I favor wood gasification for homescale biomass to electricity systems.
They could be a relatively simple way to run a generator.
A charcoal gasifier is simpler still.
I would like to build a rocket powered "Iwasaki-type high-speed charcoal kiln" and capture the otherwise wasted heat for my home,using the resulting charcoal to power all of my IC engines.
Of course the charcoal could be used/sold for grilling or art or biochar.
I agree, converting wood to char and then gasifying the char to fuel a generator is among one of the best ways.
Woodgas presents more of a challenge with reducing the tar content below ~0.1 g/m^3 or so to not gum-up the engine.
How does the charcoal maker work that you mentioned? Any good links?
One way to do it might be with an "updraft gasifier". Unlike a "downdraft", the tar gas (smoke) isn't passed through the char, so it seems easier to separate the gas streams. There's probably a way to make a hybrid gasifier-RMH design with a producer gas pipeline coming out of it.
If that isn't ideal, another way is to make a simple hot air engine. Use a compressor to run compressed air into a steel or stainless pipe, heat the pipe to a moderately high temperature, say around 400-500F (200-250C), run the pipe to a conventional engine/generator to expand the air. Ideally the camshaft would have to be modified to get better valve timing but a 2 or 4 cycle gasoline/petrol engine would probably run (crudely) even without any modifications.
Put the whole thing in an insulated box to reduce noise. The main problem with this is the maintenance required, including engine oil changes every 50 hours or so. An industrial-grade engine run at optimal speed with synthetic oil might go for longer, I don't know, 100 hours? Maybe longer with a better oil filtration system. Normally that sort of genset isn't economical to use though except in emergencies, or camping, or on job-sites, on islands, or for transport in cars, trucks, and boats.
Turbines are probably less maintenance but tend to be less efficient. For example it seems challenging to use an automotive turbo to make a gas power turbine. Even if the turbine and compressor is run at 70% efficiency, 50% of the turbine power is needed to run the compressor. All this does is move a lot of air (which is what it's designed to do in a car). You'd probably have to pull out all the stops to get any use out of it.
I also looked at thermoacoustic engines which could be fairly simple and cheap to build, low maintenance and reliable even if they're not so efficient. But they probably wouldn't produce much power, maybe 100 watts.
I kind of like the idea of the Stirling engine that is big, slow and low pressure. It shouldbe nice and quiet, low maintenance, reliable, long-lived. As long as you don't need to move it, it seems okay if it's bulky. And it's fairly safe and efficient for what it does. And it runs in all weather and at night. It's not easy to meet all those requirements. Stirling is one of the very few ways to do that. They are extremely tricky to design, but basically there are spreadsheets now that do all of that number crunching for you. ... That's my 2 cents anyway, having looked at it for quite awhile now. A gasifier and Otto engine is good if you don't want to mess with Stirling engine design. A Stirling engine is good if you don't want to mess with gasifiers, filters and oil changes.
The way you talk about the Stirling reminds me of what people say about Lister style diesels. Simple to the point of unkillable.
As I understand it,an "Iwasaki-type high-speed charcoal kiln" consists of two 55 gallon drums. Half of one is turned onto the firebox, the other 1 1/2 barrels are essentially a bell that is filled with biomass.
The gasses from the firebox enter at the top of the bell and leave via a chimney opening low on the other end.
The chamber is insulated with soil, perlite, etc.
Wood vinegar can be condensed from the smoke early on, followed by tar,if you choose.
I would chose to return the tar to the burn chamber.
The wood vinegar has homestead uses, so I'd like to keep that.
Upgrading the firebox to a batch box should make the process cleaner and more efficient,while requiring less tending.
There is a change in the tempature of the exhaust when it becomes flammable, so monitering can provide a cue as to when to redirect the gasses from the condensing apparatus to the batch box.
An old conventional wall oven,single or double could be used,essentially acting as black oven where wood is baked into charcoal.
A lot of speculation, but here are some links to working systems:
Adam Chadek wrote:I seem to recall that the most inefficient part of power generation itself (ignoring power distribution), is the boiling of water by coal/nuclear/natural gas, etc.
Modern fossil fuel grid power generation is using gas turbines or diesel engines, neither of which use the boiling of water to drive turbines. The other advantage these systems have over coal and nuclear is that they can be throttled, so 1) you are not wasting fuel by running at full capacity during off peak times and 2) they can be throttled down when unpredictable renewables come online eg when the sun shines and wind blows, and can be ramped back up when the sun goes in/down or the wind drops.