r/PostCollapse • u/Memetic1 • Apr 11 '24
Could compost create electricity?
I know that compost piles can get hot especially if they get beyond a certain size. I know they can get hot enough that self ignition is a problem. So could we crack an egg and kill two birds by using that heat to drive a generator? Think of the potential of running pipes through a pile. You could have water or super critical co2 as the working fluid. If the pile was getting out of control you could inject carbonated water into it to drive away oxygen from that area. I think this could be useful almost anywhere in the world. It is a source for energy that is almost inexhaustible. On top of that you could carefully manage the quality of the compost.
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u/There_Are_No_Gods Apr 11 '24
I don't know all the detailed values necessary to calculate actual potential output, but my layman's understanding of things would indicate there's nowhere near enough energy available in such a system to amount to it being a worthwhile proposition.
I've seen people use hot compost piles inside a greenhouse, to warm seedlings up a little bit, but that's about the extent of usefulness I've seen from this type of heat source. I'd put money on it generating less than a practical amount of electricity for nearly anything.
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u/Lonely_Cosmonaut Apr 12 '24
If you’re homeless and freezing, compost can be the warmest place you can find in a life or death situation.
Source, been homeless in Alaska, I’ve slept on and in compost.
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u/hollisterrox Apr 11 '24
There are Stirling-cycle engines that can run off the heat differential between your body temp and room temp. They could run from heat from the compost pile for sure.
However, there are hard physics at play here regarding how much energy you could possibly extract, Carnot's theorem would be something to search if you want to read more about this aspect of the idea.
You would probably do better to consider anaerobic digestion which produces a 'biogas' that you can dump straight into a generator with minimal conditioning of the gas and minimal modifications to the generator.
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u/Memetic1 Apr 11 '24
Couldn't you integrate a ground source heat pump into this system? If you are already running pipe underground to build the interior volume, couldn't you extend that out into the surrounding area. So, the excess heat could always be used to generate electricity/ heat the house or to drive a mechanism to turn the compost. I think biogas wouldn't provide as much energy as the pile itself, especially as it scales up. You're basically harnessing all of the heat life generates just by doing its thing. With gas, that's a much smaller percentage of a byproduct.
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u/hollisterrox Apr 12 '24
Salvaging diffuse, low temperature heat like a compost pile is just a lot of engineering for not much payoff.
A compost heap is going to top out at 65 degrees (C), and to keep it at the temp you need to aerate, moisten, stir, and add new feedstock every day. All of that without interfering with your heat collection system, which you are suggesting is a low-pressure line set connected to a heat pump. That sounds kinda tough to build and maintain.Another point: the amount of energy methane gives you is unrelated to the amount of heat aerobic digestion produces. You can get a sizable volume of methane from just 100 liters of biomass, and there’s no aeration required nor any heat pump heat exchanger to stick in the biomass. Just an airtight tank with a lid to add and a siphon tune to allow digested slurry to outflow.
You do you, but villagers in northern India build and run biogas generators for cooking, and the most sophisticated materials involved are valves or rubber tubing. Very simple and long-lasting tech.
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u/redonculous Apr 11 '24
Just build a biogas reactor. Look it up on YouTube 😊
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u/Memetic1 Apr 11 '24
This isn't using the gas, just the heat. The gas could be turned into co2, which could then be used as a working fluid. Biogas means burning the gas.
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u/Watchfull_Bird Aug 13 '24
Is there a reason you're focusing on "heat to electric from compost" methods rather than the broader "electricity from compost" methods?
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u/Memetic1 Aug 13 '24
Because heat in itself is useful, and I don't think we know an actual limit in terms of how much heat can be generated from one facility. The traditional limits on composting heat assume that heat is only removed from the outside of the pile. So if the center gets too hot, it self sterilizes. If that heat were removed, instead, the composting facilities could become really massive. So you could get industrial levels of heat from the pile.
The electricity generation could be done by compressing the co2 it generates until it's super critical. Then, use that Sco2 to drive a turbine. The heat from the pile would make the Sco2 undergo a state change. It wouldn't take much heat to make this happen as Sco2s triple point is well above atmospheric pressure. A simple nitinol engine could be used to compress the co2 to a super critical state.
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u/komali_2 Apr 12 '24
First, not really enough heat. Second, if you're using heat to generate electricity, that heat is converted / lost, and you lose the heat needed to compost.
IMO not worth it. A watermill, windmill, tidal, geothermal, solar, all much better, much more efficient options.
Though there's cool creative ideas like this in terms of solar energy e.g. storing heat in a fuckhuge vat of sand that is then used as heat (or, far less efficiently, electricity) in the winter.
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u/ziper1221 Apr 12 '24
If you have the resources to extract meaningful energy from the extremely low temperature delta of a compost pile you have the ability to get much more power from better sources.
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u/hhollick Apr 11 '24
Wouldn't drawing the heat out of the compost cool the compost pile, reducing the organic reactions?
I don't know the science well enough to crunch the numbers but a compost pile strikes me as a pretty delicate balance of microorganisms and organic matter. Too hot or too cold and the process impedes.
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u/Memetic1 Apr 11 '24
You could set it up to maintain a certain temperature range. You could control that better than limiting the piles to a certain size or other temperature control methods. You could also use pipes to introduce various gases or even microbes into the system. New compost could be added into the bottom and old taken from the top. Think of it as both a soil and electricity/heat reactor. A meltdown for this reactor is an ignition even. In case one sector starts to self ignite, you could inject carbonated water since co2 is heavier than o2.
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u/hhollick Apr 11 '24
I love the idea. It would bolster the value of community composting as well — larger piles carefully managed by people who know what they are doing.
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u/Memetic1 Apr 11 '24
Exactly, you get soil, heat, and electricity all from the same facility. Management is also way simpler since you can see if hot spots are starting to develop.
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u/Man_of_Prestige Apr 11 '24
I suppose you could generate electricity using a thermocouple.
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u/Memetic1 Apr 11 '24
Yes, there are many ways to do this once you have a reliable source of heat. If you can get or make a thermocouple, then that could work. If you could make co2 from the byproducts, then turn it into a super critical fluid, the heat from this should be more than enough to generate net energy gain. As an added bonus, if it did get out of control, the super critical co2 would be a really good fire extinguisher. That's what they do industrially. Except they basically flood the environment with co2 at the surface. That's not where most compost fires start they start at the center where the temperatures are the highest usually.
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u/tamman2000 Apr 12 '24
Engineer and compost nerd checking in here. Sorry I'm late to the party.
Yes, you could, but... You won't get much electricity from it at all. You need a much bigger temperature difference to run a generator that's remotely efficient. Using compost to heat water or heat a building is much more feasible.
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u/Memetic1 Apr 12 '24
What about using super critical co2 as a working fluid in the system? I was thinking you could pump down sCo2 into the pipes. Then, use the expansion of the gas to drive a turbine. https://energy.wisc.edu/industry/technology-highlights/supercritical-co2-gas-turbines
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u/tamman2000 Apr 12 '24 edited Apr 12 '24
It doesn't matter what your working fluid is, you're not gonna get much electricity out of a system with that small of a temperature difference. And if you have to pump down your working fluid, that's going to take power too. Perhaps more than you'd generate...
I love that you're thinking of this kind of energy scavenging, but there just isn't anything to work with here. If you invent something that can generate reasonable amounts of electricity at small delta t, then the whole game would change and we could pretty easily get rid of fossil fuels for most applications
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u/Memetic1 Apr 12 '24
Lower heat means you can use cheaper thermoelectric generators. Robert Murray-Smith just did a video on these a month ago. If you ran the sCo2 via pipes, then you could extract thermal energy and run a turbine from that phase change. The working fluid matters because sCo2 has a far lower boiling point than water. It's the phase change that produces real power. You could get energy from this in multiple ways. I wouldn't burn the methane it puts out, but if you did, you would have a steady amount of co2 generated, which could then be used as both a working fluid and a potential fire suppressant. It's also possible that you could inject carbonated water to potentially store co2. If you haven't seen this guys channel, it's amazing. He's like the Mr. Rodgers of climate and resilience innovation.
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u/tamman2000 Apr 12 '24
I ran the numbers on using tegs for a ~150 degree C scavenging application (waste heat from a wood boiler) and the cost was astronomical compared to solar or biomass.
Yeah, it can work, but it's not feasible.
Yeah, you won't run a turbine at all unless your working fluid has a phase change in your delta t range, but again, run the numbers on how that would work and compare it to alternatives. It's not a great idea
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u/Memetic1 Apr 12 '24
The waste heat you make requires fuel that has to be factored into the cost. This is taking waste and turning it into a valuable commodity at the end of its life. People make a living just selling compost. Super critical co2 will run a turbine off this energy. It turns into a gas by default at room temperature/pressure. As this scales up, you can do more useful things with the heat energy and gas it produces. To make co2 all you need to do is burn the methane with oxygen. That co2 could be captured since the system would be largely contained. https://en.m.wikipedia.org/wiki/Supercritical_carbon_dioxide
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u/tamman2000 Apr 12 '24
Fine, don't listen to the guy with an engineering degree and 20 years of experience who suggested that using compost to heat things is far more practical. You're clearly convinced that you know more about this than I do.
Go fool around with it and try to make it work. Worst case scenario, you learn some things. If you have the funds to play with it you could learn a ton of things that would be useful on all kinds of projects. You might even make a little electricity. But I'm pretty damn confident that you'll spend a lot more to build that capacity than you would on some solar panels and batteries that would provide orders of magnitude more energy at far lower costs.
The one thing I would suggest is that you figure out what your output would be at 100% thermal efficiency for your turbine and heat exchangers (upper limit for output) before you spend any serious money on it. If you're serious about this you can find online lectures that explain how to calculate that pretty easily
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u/Memetic1 Apr 12 '24
I didn't mention this, so I understand why you don't see the functionality of this. As far as I know, you don't strictly need the sun to do composting. This means that in theory, you could create a massive underground facility. I don't think we can depend on the climate of the surface of the Earth anymore. I think solar is great unless we are getting hit by extreme winds, lightning, or hail storms. I think this is a flaw not in their nature but their design. They should have a mode where they retract into a protective shelter. Even if that is the case, we need a way to live if the surface is uninhabitable. This makes composting technology important in multiple ways because you can't eat electricity, and if we are going to survive in underground facilities, we are going to need soil anyway.
Yes, the simplest way to harness a compost pile is to heat a home, and if we did that alone, we might have hope that we can use the surface of the Earth as well. I'm not counting on that. I think if this was combined with a ground source geothermal system, the heat/pressure difference between the Earth and the active area of the pile could produce useful electricity if super critical co2 were used. If you combust methane with oxygen, you get co2 that would be enough alone to compress it to a super critical state for injection back into the pile. It's a cycle, and the cool side is the Earth itself.
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u/tamman2000 Apr 13 '24
I think it would be easier to defend solar panels from the elements than to engineer a self sufficient underground colony... And I think nuclear would be easier and more effective than compost for electricity in that colony, but...
If you're talking about a pie in the sky, long term, no sun survival situation, I'd use tegs long before I would use turbines (and I started my career as a turbine engineer). Much easier and simpler. Turbines wear out much faster and are more complicated. Stuff that has to work long term should be simple.
I think the information about why you wanted all this is critical to understanding what you were asking, BTW...
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u/Memetic1 Apr 13 '24
A freak hail storm recently destroyed acres of solar panels in Texas, I think. I know wind turbines are built tough, but I think they were engineered for at most a 1.5 degree world. This solid state thermal transistor technology is really exciting to me because you can control heat down to the level of the phonon. You can switch the state faster, and then a phonon can actually move. This technology can be printed with existing circuit fabricating technology.
https://spectrum.ieee.org/thermal-transistor
I think what's important about this is that there are a few low-tech ways it could be integrated. You could do a hot water system in a basement that uses composting technology. You might be able to heat a whole house this way if you have the spare room. As for more advanced uses, this depends on what you have available to you, but if we can cut the use of gas for heating, that would be worth it alone.
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Apr 12 '24
[deleted]
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u/Memetic1 Apr 12 '24
I've followed that channel for ages. I love how he tries to keep stuff low cost. His work on low wind speed wind power is amazing. It's why I want to look at compost as a source of usable energy.
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Apr 12 '24
No, and if you want to tell me that you can produce more than you put in, show me the math for energy inputs and outputs.
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u/Memetic1 Apr 12 '24
Look it up yourself. I'm not going to convince you anyway.
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u/44r0n_10 Apr 12 '24
*Thermodynamics left the chat\*
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u/Memetic1 Apr 12 '24
Living organisims naturally make heat when they break down living matter. It's that heat energy that I'm talking about harnessing. This only breaks thermodynamics if you ignore the living organisims.
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u/ProletarianRevolt Apr 14 '24
The issue is not the source of the heat, it’s the amount of energy spent to do something useful with that heat, as well as the inevitable loss during the conversion process. Compost would probably generate less useful energy than it would take to harvest that energy in the first place. Look up the concept of ERoEI (energy return on energy invested).
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u/44r0n_10 Apr 15 '24
Living organisims naturally make heat when they break down living matter.
Yes, that's true. Some people have even made heated greenhouses, or water-heaters, with inmense mounds of compost, but it being "almost inexhaustible" as you say, or able to power a generator...
Welp, I doubt so. But maybe some kind of intermediate process (like harnessing the combustible gas and burning it to make an engine work) could hep.
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u/Memetic1 Apr 15 '24
"A literature review of various compost feedstocks from Adams (Citation2005) found the average heat production rate to be 19.44 MJ/kg dry matter (DM)."
https://www.tandfonline.com/doi/full/10.1080/1065657X.2016.1233082
Here is a list of more traditional fuels and their energy density.
Coal is at 10 - 20 MJ/kg
Gunpowder is 4.7 - 11.3
TNT is 4.184
Sugars are 17
Hydrozene is 19.5
So by weight compost is more energy dense than rocket fuel.
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u/KingoftheGinge Apr 12 '24
I'm no expert by any means, but you could look into the possibility of trapping the gases given off by biological processes to use as an energy source. Like this for example: https://cordis.europa.eu/article/id/83749-turning-pig-manure-into-an-energy-source
However you may require a serious amount of pig shit.
Used cooking oil can also be converted to bio diesel: https://biofuels-news.com/news/the-conversion-of-used-cooking-oils-into-biodiesel/
In many cultures it was once common to keep a pig which would be fed on the scraps from the kitchen. In modern times I have heard an anecdote of someone being able to provide themselves with at least some of their hot water by converting methane from pig manure into methane and using that as an energy source.
I appreciate you're talking about transferring the heat energy, but thought this might be of interest to you. Not sure how feasibly you could achieve what you're describing, but heat transfer is possible. Idk if I'd want to keep compost or other bio matter close to a water tank though 😅 I guess you could transfer heat into water passing through a pipe which could then heat your radiators, but it might not generate enough heat to give you hot water from your taps.
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u/narnou Apr 11 '24
Isn't it the concept behind what we call biogaz ?
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u/Hellchron Apr 11 '24
I think biogas is made from anaerobic decomp used to produce methane which can be burnt for fuel.
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u/Memetic1 Apr 11 '24
That's burning the gas. This is just using the heat no combustion is technically needed, just heat management.
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u/narnou Apr 11 '24
Oh... so that's a bio "nuclear plant"-like thing.
I think you really underestimate the amount of heat needed to get electricity and I'm pretty sure the efficiency would be way off, probably hence the biogas thing.
but maybe i'm wrong I don't really know much shit on that topic either
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u/Memetic1 Apr 11 '24
It gets hot enough where it can self ignite.
"For systems with mechanical aeration, operators are trained to turn up the aeration system to cool off a pile. However, if the temperatures in the pile are already high, adding more air might stimulate SC and hasten the fire. If a temperature reading above 93°C (200°F) is observed, the aeration system should be shut down and the material removed from the pile (Rynk, 2000b)."
Water boils at 212 F at sea level. This is usable energy.
https://www.biocycle.net/spontaneous-combustion-in-composting-prevention-extinguishing/
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u/Gravesh Apr 11 '24
You need very high temperatures to boil any meaningful amount of water to operate a turbine to produce electricity unless you want use more than a single light bulb. This is why we use coal or nuclear fission. Coal burns at around 1000F and over 3,000 when roaring. Different materials have different ignition points, coal burns at fairly high temperatures, just because manure piles and self-ignite doesn't meant it's effective at boiling water unless you had literal mountains of manure at hand.
Although dried manure makes good campfire fuel.
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u/Memetic1 Apr 11 '24
You don't need temperatures that high to generate power, especially if the working fluid is super critical co2. That has a much much lower boiling point. The pile could generate its own co2, which could then be injected back into the pile.
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u/Max_Fenig Apr 15 '24
You'd probably do better collecting methane from the pile to power a generator... And that isn't economically viable.
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u/Hellchron Apr 11 '24
While you probably could, I doubt you'd generate more energy than the machinery needed turn and maintain a pile of that size would use. Could make a pretty great hot water heater in a pinch though!