Is it efficient to compress biogas?

Biogas applications for permaculture and off-grid energy solutions.

[@bigfatpauly3609]

"Efficient" might be one of those words that summarizes too much. A hydroplant converting flowing water into electricity, that is then used to turn a motor's shaft, isn't actually very efficient. Using the water's power more directly would be more efficient. But we convert that power into electricity because electric power is easier to transmit to distant locations. I think you should probably be thinking about compressing the biogas in a similar fashion. It isn't just about pure efficiency, but also what problems you're trying to solve for. Which really makes it more of an engineering problem than a straightforward measure of efficiency. From some quick searches, biogas seems to have approximately 6kWh of heat energy per cubic meter. Of course we can't convert that to electrical energy with 100% efficiency. But a decent approximation might be 2kWh if that same cubic meter of biogas was used to generate electricity? Not sure what the power draw of your compressor is, but I'd be shocked if it is pulling 2kW for an entire hour to compress a single cubic meter of biogas. So I doubt very much that you are at a net energy loss via the compression. I might be wrong, but I think the biggest thing you could do to improve the overall efficiency, is getting as much carbon dioxide out as possible. If your biogas is 30% CO2, then your compressor is wasting 30% of its used power to compress a gas that doesn't help improve the power density of your biogas. Just want to add, I only discovered your videos a few hours after you had posted this. I was looking for good info on scrubbing and you've definitely given me some ideas. You've done some great work here, and I've really been enjoying going through your backlog of videos. Thanks for sharing your experiments with the rest of us!

[@BiogasUSA]

This is an outstanding comment. Thank you for sharing your thoughts here, very insightful. It looks like my compressor requires 1.8 kw

[@bigfatpauly3609]

Looks like your compressor will compress 50 liters per minute, based on the Amazon listing info. Assuming a constant rate of compression, it should take 20 minutes (1/3rd of an hour) to compress 1000 liters (1 cubic meter) of biogas. If the compressor is pulling 1.8kW for those 20 mins, that would be about 0.6kWh (1.8kWh/3) to compress 1 cubic meter. If we use the 2kWh estimate for generated electricity from the cubic meter of biogas, then 0.6kWh is only 30% of the energy in the compressed biogas. So I would estimate a decent rule of thumb is that you'll be using about a third of your biogas's potential electrical power to compress it. That is all based on some quick searches and back of the napkin kind of math, so I encourage double checking my estimates. But if I had to guess, the overall efficiency of compression is probably better than my estimate. Since that compressor is capable of very high pressures, you probably aren't pulling a full 1.8kW when doing lower pressures like your propane tank.

[@BiogasUSA]

Those are at least some real numbers to talk about. Thank you for taking the time. Really appreciate, I like the word “energy deficit” too. Wishing I would have used that in the video.

[@atlasboucher543]

I love to see you do another compressing video

[@BiogasUSA]

I should. I have improved the way I do it considerably since my last one. I should do an update video

[@brightmal]

Some factors to consider. If you're getting electricity from solar and or wind, the smart move would be to note when you have more electricity than the house needs. In other words, use energy that would be thrown away or exported to the grid. Another option is to set the system up so that it only uses biogas to run the compressor when your gas storage bladder gets full. Again, using energy that would otherwise go to waste. Another interesting point is that if you're compressing biogas up to between 100 and 150 psi, that is enough for water to differentially absorb carbon dioxide but not methane. If you can do that, you're storing pure methane in those LPG tanks rather than biogas. Much more energy in that.

[@BiogasUSA]

Interesting points. The gas I had tested was compressed over 100psi. know solubility of gas in water is higher at lower temps and higher pressures. To clarify, are you suggesting running my gas through a filter system at 100+psi to more effectively filter out co2? I have never considered that before. Maybe a series of filters and a 150psi back pressure valve between the compressor and the tank. What a clever idea, I wonder how much more effective that would be.

[@brightmal]

@@BiogasUSA My initial idea for a DIY approach would be to start with a larger LPG tank. You fill it to about three-quarters full with water, then pump in your biogas, through the water, and once it reaches over 100 psi inside the tank, the gas at the top should be about 98% methane. The complicating issue is that you also need a way to remove a portion of the water with some regularity to release the carbon dioxide stored in it, without decreasing the pressure in the bottle below that 100 psi pressure, and also put new fresh water in. And when you release the pressure on that saturated water, be prepared, as it may resemble something like dropping Mentos in Coke!

[@briancohen-doherty4392]

Since it's beyond me, has anyone done the math regarding co2 saturation levels to get an idea of how much water would be needed to do a full tank of methane?

[@brightmal]

@@briancohen-doherty4392 you would have to have a way to let some of the saturated water out regularly, and push fresh water in.

[@brightmal]

In addition, different compositions of water would a different capacities for absorbing CO2. Some experimentation would be required.

[@arbienacecruz5171]

The problem we're facing with uncompressed biogas is the space it occupies. We are currently using water displacement in large tanks to capture the biogas. And it does take up space. So, I guess considering those things, compressing biogas for ease of use and transport can be considered when talking about "efficiency" of compressing biogas. Thanks for this!

[@michawojtaszek2052]

I think you can compress the gas using solar energy. After all, the compressor doesn't have to run 24/7, you actually need it for several minutes a day. Unless you have a really large biogas installation, but even then you will still run the compressor for several hours a day. That is, when the sun is shining. When it comes to comparing biogas with hydrogen, there are two completely different things. Water electrolysis consumes a lot of energy itself, plus the energy needed to compress/cool hydrogen... Therefore, hydrogen production is unprofitable. With biogas you only have the energy needed to compress/purify it, so much less of it is needed. In fact, in the case of a small installation, you need several dozen watts of energy to drive a small compressor. Theoretically, you can fill such a cylinder using a hand pump, such as one used to manually fill air gun tanks. They allow you to achieve up to 200PSI.

[@kawaiisenshi2401]

Id say the 1st step is determine how much energy in electricity your equipment utalizes Specifically it's peak power output in watts vs it's continuous power needs to be utilized, the next variable to consider how long you would be using that power load for. Once you have those details Determine what ur end goal is and sort them by priority Consider sorting by the ABC method Or the getting things done Quadrant framework of *Urgent & important *Important ,but non urgent *Non important, but urgent *Non important, & not urgent What's most important for u? Energy independence? Saving money you don't have to spend on the grid? Having fun ( awesome! ) follow ur fancy For example If the last goal of having fun was your primary goal then cost be darned and do what seems the most interesting next! If it's saving money, consider which power generation method Whether that be on grid, solar off grid, biogas energy is going to financially give you the best return on investment etc etc I

[@jblue3163]

Is it efficient? Maybe not. Is it worth it? Probably so for other reasons than efficiency. I believe you could make it more efficient right now and it’ll be “more free” lol. But energy in the states is a way to volatile speculative thing and that’s why at times in the future it could be very efficient to compress your own gas. Like if it costs you $1 to compress free gas today and the grid is down or just really expensive one year from now kinda thing. So imo it will always be worth it for peace of mind, availability, and security reasons. I’m just getting into this world of “free” bio gas. I really appreciate all your videos. I learn from them

[@BiogasUSA]

That’s a great point, stability and independence is a factor we should consider also. Thanks for the comment, and thanks for watching.

[@nicolascharlery8323]

Software and telecom background, so not entitled to give the right answer, but interested in the topic nonetheless. My thoughts are: - in a perfect system, energy in = energy out. Since you loose energy somewhere in the process (heat released by the compressor for example), then compressing has an energy cost that will never be compensated - Efficiency would be measured comparing two systems. Transferring electrical energy from A to B using a wire is pretty efficient comparing any solution involving storage of energy in the middle - What type of efficiency? (electrical, volume, ..) - the raw energy comes from the sun, so I would compare its a biogas system to another system that stores solar energy (from the sun to its final storage form)

[@BiogasUSA]

Great thoughts. I suppose “efficient” is the wrong word and I’m wishing I was a whole lot more articulate right now. The question should have been “is there more energy in the bottle than it took to get it there?” Since all biogas is waste energy to me, I just want to make sure I’m not running an energy deficit.

[@ignasanchezl]

I calculated energy consumption for filling a high pressure 4500PSI tank, it would take about 8Kwh to fill a 33L tank. That tank would contain about 100Kwh of energy if it were 90% methane, if I were to use that methane on a 40% efficient generator (very optimistic) and then use the energy from that generator to power the compressor, I would require 20Kwh of my gas to do so. Meaning to store 100Kwh of 90% methane gas at 4500psi I waste 20% of the energy of the gas. It's an ok number, the problem comes when you realize biogas is not 90% methane, but much less.

[@ignasanchezl]

easy option is to lower the pressure, 2000 psi, less than half the capacity, but would take less than a quarter of the energy to reach that pressure.

[@atlasboucher543]

I think so for saving it for winter time

[@medina3420]

You can run a Generator on Biogas with an Aquarium pump and gas kit. Start the Generator with tiny bit of gas, To get the Pump running, then switch to biogas. works. Then Free Electricity. So then compressed biogas become closer to Free.

[@neilclarke4338]

If you have a cheap source of biogas then you could store the gas at high pressure. Use a small compressor to compress the biogas to increase the energy density and store in pressure vessels. Decant through an expansion station as required to drive a gas engine to drive the compressor.

[@BiogasUSA]

Yes. This is the idea. I’d like to know how much energy it costs to compress it, vs. how much energy is now compressed.

[@mt.sinairefuge5360]

The reason to compress would be to use for automotive purposes. Otherwise I dont see how it could pay off.

[@robertmawanda]

Being on 100% solar, pumping biogas or splitting H20 with solar electricity does me no loss.

[@fervi8124]

The way commercial biogas plants do it is first to cool it, this requires a good size chiller, for amateur applications you could use a cooling device, like an a/c, or a freezer, the point is to lower the temperature of the gas as cold as possible (I am not giving you operation points of the plant) but if you are able to do this thermal exchange thru a heat exchanger, the gas (methane) will condense enough so you can compress it. The only major problem with compressing the gas is that you have to have a safe means to keep the gas cooled and compressed and that is expensive, commercial plants DO NOT store it (you dont want to have a potential hazard). One other thing that you have to be careful with is H2S, which is poisonous, you have to watch for it, seriously, not to mention that methane, since it does not smell. makes it difficult to detect if you have an explosive atmosphere.

[@BiogasUSA]

Hello thanks for commenting. I am familiar with the operations of commercial cryo plants, and the hazards of h2s and LEL. On this channel I do my best to apply that knowledge to biogas at a hobby/permaculture level. I have talked about liquifying methane on here before. I’d like to build a cryo plant and try it one day, but I have so many goals to hit between here and there. I invite you to watch a few more of my videos, and to share knowledge and expertise where you can and want to.

[@DannyD3343]

How does the btu of your gas compare to like propane at same measurement?

[@BiogasUSA]

The unit of measure typed out is BTU/SCF and pipe gas (commercially processed methane) is 1000 btu/scf. A propane comparison is less relevant, but I looked it up anyway out of curiosity. It’s 2500 btu/scf

[@brightmal]

@@BiogasUSA Yeah, propane definitely has more energy than methane as it has more carbon-carbon bonds. The longer the chain, the more energy, but the fuel burns less cleanly if we go right up to bigger chains. In many ways, propane is a sweet spot, but it's much easier to make methane from waste organic material than propane.

[@paulishism]

You can pressurise your gas bag by putting weight on it. The advantage of compressing gas into canisters is that it makes it a mobile source of dense energy, which is what is needed to power a car. Think of it like putting your electrical energy into a chemical battery. In this case the chemical energy store is gas instead of poles of a battery. I love that you came back after months, fair play. Have you looked into how to convert your car to biogas yet? Also how good are you with the metric system? I feel like a lot of the science is done with that these days.

[@BiogasUSA]

Thank you for the welcome hahaha. I do have a few more ideas for videos I can make, but I’m running out. I want to see if I can get someone to sponsor the car conversion idea. It’s still on the horizon. Also availability of an extra car I don’t mind blowing up is a factor.

[@paulishism]

@@BiogasUSA There are companies doing the conversions, but they are not very well known. Perhaps one of them would do a sponsored video for the increased fame. Are you planning to convert a diesel car or a petrol car. I hear petrol is a bit more straight forward because diesel also works as a lubricant in your engine. Your video about filtering H2S with water made me wonder if the water coming out of the digester is also saturated with H2S. People tell me that the fertiliser doesn't smell, but is that because the H2S is too concentrated? Should we be worried about the dangers of digester liquid off gassing and killing people?

[@BiogasUSA]

@@paulishism I’m planning on converting a gas car with fuel injection. I think converting a carbureted engine would be too easy. That’s a great point about the h2s in the effluent. I don’t know much about the effluent, I have been thinking there’s probably a lot that we can learn from it. Like are we feeding the system too much/not enough. Maybe the ph of the effluent could tell us something. I don’t believe there’s high concentrations of h2s in it because it’s so good for plants. Plants like a little h2s, but too much will kill them like my filter water does. Still something to be cognizant of, maybe people shouldn’t be having buckets of it in enclosed areas just in case. Another interesting thing I have discovered since I made that video, the black dust that settles out of my Lye water is probably pyrophoric.

[@paulishism]

@@BiogasUSA Would you be able to do one of your H2S tests on a container of your effluent? I hear if you agitate the liquid then the H2S of gasses more. When people poor it onto the plants it's probably agitated enough to not effect the plants and just disappears into the atmosphere. Do you know what chemical the black dust is? Did it burn when you tried drying it?

[@BiogasUSA]

@@paulishism that’s an excellent idea. I will give the effluent an h2s test. I’m not completely sure, but I suspect the black dust is iron sulfide (FeS). I haven’t had any of my powder burn, but I’ve encountered similar black powder elsewhere that spontaneously ignited with exposure to oxygen.

[@abab-kl2gt]

a 2kw generator needs about 3 gallons of porpane an hour..... ??? "1 m3 biogas is equivalent to 0.46 kg LPG" so 12.5m³ of biogas for 2kw. A ⅓hp 3gl air compressor needs 0.5kw for 1 hour, it runs 1/10 of that so 0.05kw. How much biogas is in a 3gl air compressor? You need 0.32m³ to git that.

[@chrislinda3623]

Your question seems to show a big misunderstanding as far as what compressing the gas is doing. Compressing gas gets not more useful energy out of burning gas so it is 100% inefficient to compress it unless you are using the decompression cooling effect. Assuming bio gas compresses as well as air a 2 cubic foot tank at 100 psi is going to have about 13.6 cubic feet of unpressurised gas when released. As far as how much "power" is in your gas 1 W is equal to 3.41 BTU/hr or other way around 1 BTU/hr is 0.293 Watt. You said your gas is about 700 BTU per cubic foot so 13.6 cubic feet will have 9520 BTU if you compress that into a 2 cubic feet tank it will still produce 9520 BTU when burnt so all your compressing did was take it from 13.6 cubic feet of gas and make it 2 cubic feet. 1 hour of 1 KW is about 5 (or 4.87) cubic feet of your gas. Compressing that gas gets you no more energy out unless your gas is spinning a turbine as it depressurizes before burning or you are burning it under pressure, it just makes it smaller and portable so if you take that same 13.6 cubic feet and compress it into a canister your going to get the same heating when you burn it just 13.6 cubic foot gas bag is a pain to handle where a pressurized canister is much more manageable. Is it more efficient to burn gas or use gas in a generator to produce electricity to do the task you want it depends on the task and the devices you are using to do the task. Firstly a petrol driven electric generator is normally about 50-60% efficient assuming your generator is not 100% designed to run on bio gas its probably going to run less than that could be anywhere from 30-50% at a guess. Simple test you can do is get a KW meter and run a 1 KW device for an hour (or a 500w device for 1 hours for half the numbers) on your generator running on bio gas see how much you used based on above calculations 9.7 cubic feet of gas producing 1KW would be your generator running close to 50% efficiency. Heating burning gas is going to be basically 100% efficient similar to electricity in a heating coil. Introduce an electric heat pump in normal operating conditions suddenly the heat pump can be around 240% efficient at heating because its moving heat not making heat even at 50% efficiency to produce the electricity you getting more heating on a heat pump. Not to mention for heating you can use the waste heat of the generator producing electricity to heat as well improving your wasted 50% of energy making electricity. Then you have a very complex system that is very efficient. For cooking burning gas is probably going to yield better results because electric cooking devices take longer to heat up so your using them for longer if your paying a 50% penalty to make the electricity from gas then your paying two costs. If you want to use a washing machine use the electricity no one wants to heat water manually churn the water to wash etc. Refrigeration lets just say that gas fridges went out of style a long long time ago but it is a way to use the energy you put into compressing the gas. Thoughts on Hydrogen As far as hydrogen gas generated from electricity its about 20-30% efficient to make the hydrogen so only do it if you are using "waste" electricity. Most hydrogen gas for industry is produced using energy intense steam reforming on methane so if your burning the gas just burn the methane bio gas. Side note handling hydrogen is NOT like methane, hydrogen can escape every container its ever put in over time it works its way between the molecular structure of its container weakening its structure. It also has no smell and fills a room top down so by the time it finds an ignition point its likely taking the building with it. It is way way more explosive than methane. I would STRONGLY caution people with a can do attitude from working with hydrogen it really is something you want a good understanding of before you play with it. If you are going to "play" with it do it in tiny amounts until you have respect for it we are talking cubic inches of gas not feet. Hydrogen fuel cells for electricity are cool tech but not practical for much other than devices that have very interesting design requirements like space based tech.

[@BiogasUSA]

I got a lot out of this comment. Thank you so much for taking the time. You mentioned “waste energy”. to clarify, I see all uncompressed biogas as waste energy. I know I’m not creating more energy by compressing it. I was wondering if it was “worth it” I suppose. “Efficient” is definitely the wrong word. I wish I was as articulate as you. Also appreciate the disclaimer on hydrogen. I did not know it was that dangerous.

[@livingecology]

good question, I'm not a numbers person so will wait to hear what they have to say, but taking it ruff I've seen a dude compress biogas with a frige compressor and had a full tank in a few minutes, the electricity demand of friges is at top 400w wich is not so much especially compared to your cars energy demand so rufly if you can ride for more then an hour and a half on free gas that took you a few minutes of 400w to make I think it's good math for hydrogen the thing is safety , it's not flammable but explosive. hens hydrogen bombs , so probably not so good for your car

[@livingecology]

and iven if you use a stronger compressor say 2000w the tank fills up faster and still the energy equation is by far in your favor if you get a full tank of gas

[@BiogasUSA]

I’m hearing unsubstantiated claims that it takes 4 cf of biogas to compress 1cf of biogas. I’m trying to figure out how to substantiate/debunk this claim.

[@livingecology]

sounds counterintuitive but I'll wait to see what you find.. I wonder what is the energy density of propane gas comered to methane, i guess its not hard to look up , it can give a clue to theas claims

[@BiogasUSA]

This conversation is taking place on another platform, but it sounds like the claim is based on internal combustion engines being only 20% efficient and the fact that any energy conversion is going to cost. I think that biogas being free is a wonky element in that equation though. As long as there is more energy in the bottle than it took to get it there I’m going to call compression efficient. I’d like to figure out how to get some real numbers.

[@livingecology]

en.m.wikipedia.org/wiki/Energy_density was looking up energy density chart but not sure if I understood correctly as it seems they compare things in different densitys

[@vasilispants8241]

If you use solar panels to power the compressor then I guess it's fine, otherwise you would have to change your car with a new electric one.