Energy indepentend home

Digester design and construction info

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davidwillis
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Energy indepentend home

Postby davidwillis » Tue Jan 13, 2009 12:35 pm

I would like to make my house totally energy independent by using a Digester solar heat, and a generator (to be run off of the methane). I don't need it to be automatic, in fact I it would be good to have chores for the kids to do(although I don't want it dangerous).

I have a house on a two acre lot with a spring. The house is heated hydroponically with in floor heating, and very well insulated. However I live in a cold climate (Idaho).

So here is the plan:
Build a digester to produce methane (I haven't figured the size yet...I am just in the planning stages) to run generator for electricity for house (I will also have some solar panels with a bank of batteries, and the generator will only need to be run when the bank is getting low due to using more power than they can make with the solar panels). I will also use the methane to heat hot water to heat the house, and heat the digester, and hot water for the house (again, I will have some solar hot water heaters as well, so this will only be needed when there is not enough sun to produce the hot water needed). I will probably build a well insulated building to put this in to help keep it warm.

I want to grow a crop of something... this is where I need some suggestions, to fuel the digester(along with any wast food products from the house). At this point, we do not have any animals add any manure to the mix, So I would like to grow something during the summer, and store it so I can feed the digester during the winter (and summer I guess, it just won't need to be as much during the summer).

I would like to compress the methane into a tank for storage (not too much, but I need a little in case we get a cold spell with no sun).

So here are my questions...

1- What would be the best crop to grow?
2- How can I store and prepare the crop for the digester?
3- Is it best to do a batch feed digester for this?
4- after the methane is cleaned (running it through lime water and steal wool), can I compress it into my tank using a small air pump of some sort (maybe a foot pump)... I would like to do this without using any electricity. I am thinking if I just do this once a day, to pump any excess methane into a storage tank, it may work?

I am sure I will have more questions.

And thanks for the wonderful sight with all the information.

David

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Bob
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Re: Energy independent home

Postby Bob » Fri Jan 16, 2009 11:12 am

1- What would be the best crop to grow?

That's probably a location-specific question, David. Rather than grow a crop specifically for fuel, you might consider feeding your digester agricultural 'waste' from other (perhaps more valuable) crops? Are you near an urban center -- like where people rake up leaves in the fall and pay to have them hauled off? What 'waste' resources are available in your area? Brush? Yard waste? Farm waste? Sawmill waste? Horse manure?

2- How can I store and prepare the crop for the digester?

That depends a lot on the type and volume of feedstock, but it sounds like something that might lend itself to an insulated, heated pit in the ground -- a large batch digester covered with a flexible membrane for gas collection. Have you done heat loss calculations for your house, to know how to size both the heating system and the annual usage?

3- Is it best to do a batch feed digester for this?

That too depends on the answer to question 1. In general, a batch digester is best for anything that you would have difficulty pumping -- that would be likely to clog pumps or piping.

4- can I compress it into my tank using a small air pump of some sort (maybe a foot pump)... I would like to do this without using any electricity

Here is a discussion about compression and storage.

davidwillis
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Re: Energy indepentend home

Postby davidwillis » Sat Jan 17, 2009 9:12 pm

Thanks for the comments... and this wonderful sight... I am learning more each day.... here are some of my thoughts... let me know if I am off in any of this. But it looks very promising.

1- That is good advice, but I am trying to figure out a way to produce all the energy I need using only what is on my land.... I can't really explain why, but I just want to try and do it. So I have been reading around a little... and it looks like Switchgrass or corn may be good options... I think I will give corn a try, as I can eat, and sell the corn, and then use the rest to feed the digester. I was reading someplace, and I can't remember where (I have been reading a lot) That an average acre of corn would produce 18 tons of corn (including the whole plant), which if feed through a digester would produce enough methane to make 6,000 kwh of electricity in a year (if the methane was running a generator). I obviously don't need that much power, so scaling that down, 1/8 of an acre of corn would produce 2.25 ton of corn, and 750 kwh of electricity in a year (which means the digester will produce about 15000 ft^3 of methane). I won't need that much, but I will use some methane to heat water, run a range, and run a Shredder. Do my numbers seem right? I have no idea, I just read that some place.

2,3 -Could a large batch digester run on only loading once a year? That would be perfect, If I could load it once a year when the corn is ready.... Although I have a feeling it would produce a lot of gas for a short time, then be done, which would mean I would have to store a years worth of gas... not an easy thing to do. Let me know if there is a way to get the digester to go slow enough to produce on one load/year. Otherwise, I think I may have to do a hybrid, or continuous feed.

as far as preparing the corn, I was thinking of running it thought something like this.http://www.northerntool.com/webapp/wcs/stores/servlet/ProductDisplay?R=200141374&storeId=6970&productId=200141374&cm_ven=natural&cm_cat=netconcepts&cm_pla=Google&cm_ite=wood%2Bshredder, then storing it in a silo, or shed??? Still not sure how that will work... but then once a day I could get a bucket of shreeded corn, and mix it with water, and feed to the digester, and take the wast and put it back on the ground to fertilize for the next season.

I haven't done any heat loss calculations, because I am planning on doing an addition to the house this summer, as well as add solar heating, and solar photovoltaic panels... When the sun is out, I will be able to generate all the power and heat I will need with just those, but when it is cloudy, it will need some help... I think shooting for 15000 ft^3 / year is a good target for now... That would be about 41 ft^3/day).... If I produce more than I need, I will just run my generator, and feed the power to the grid and make some money... If I don't have enough, then I will have to build a bigger digester, and plant more corn, but then I will have a good idea.

4- Thanks for the link... To summarize what was there, and other places... if I compress the gas to 100 psi, I will get about 7 times as much gas in the same size container, and it will take me about 3wh to compress 1 ft^ 3 of gas...So the most I will have to compress in a day it 41 ft^3 of gas which will take 123wh of power to do... The gas stored in the container will have the capacity to produce about 2kwh of electricity... So the 123 wh is not too much compaired to the storage amount, but I don't know if it is possible to do that by hand... it is only 0.16 of a horse power for 1 hr... May be possible, as well as give me a great workout...lol
Last edited by davidwillis on Sat Jan 17, 2009 9:53 pm, edited 1 time in total.

davidwillis
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Re: Energy indepentend home

Postby davidwillis » Sat Jan 17, 2009 9:23 pm

I just ran some numbers through the Digester calculator. If I make 2.25 ton of corn, that will equate to about 15 lbs / day... Putting that in the sawdust or leaves category, it gives me 48 ft^3/day (which is about what I was figuring). However, my carbon/nitrogen ratio is 500... I know it should be 20... I guess that isn't good.

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Bob
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Re: Energy indepentend home

Postby Bob » Tue Jan 27, 2009 6:07 pm

Hi David,

No time for more than a fly-by, but here's a link that has some good info on feedstocks and gas production.

And David House's Biogas Handbook has this:
Buswell [says]… “the present estimate is that from 5 to 10 cubic feet of gas can be obtained per pound of cornstalks, and that the rate of production will be from ½ to 1 cubic foot of gas per day per cubic foot of tank volume. Taking the lower figure, a ton of cornstalks would furnish gas for 400 people for one day, allowing 25 cubic feet per capita per day. From the data given by Weber for yields from regions where 30 percent of the land is planted to corn, an area with an 8-mile radius will produce enough cornstalks to supply a city of 80,000 inhabitants continuously. In other words, the cornstalks from one acre will produce the gas for one person for a year.”

davidwillis
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Re: Energy indepentend home

Postby davidwillis » Wed Jan 28, 2009 3:54 pm

Thanks Bob, that is very good information. Right now I am trying to get some plans together, and then this summer I am going to build a digester. I am excited about it.

I am still trying to work out the gas cleaning and storage. I will post some drawings when I get the design down.

Also, do you know where you can get some monitoring equipment, to monitor temperature,ph,co2,methane,and hydrogen sulfide? I would like to measure the concintrations before and after scrubbing. Also I am going to need some type of system to circulate hot water around the tank to maintain the correct temperature(i am guessing this will not be to hard to find).

Thanks again
David

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Bob
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Re: Energy indepentend home

Postby Bob » Wed Jan 28, 2009 4:48 pm

Also, do you know where you can get some monitoring equipment, to monitor temperature,ph,co2,methane,and hydrogen sulfide?

The short answer is yes. The longer answer would depend on how much you want to spend, how frequently you want to record the data, and how much you want to do manually vs automating it -- which will have a lot to do with desired sampling rate.

Temperature is easy. You can do it with a standalone data logger that costs about $100 (a Hobo, for example). pH a bit harder (more expensive). Gas analysis much more expensive. If you are an electronics type, there are some hobbyist / d-i-y options. Otherwise lab quality stuff will cost you a lot more than the value of the gas.

davidwillis
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Re: Energy indepentend home

Postby davidwillis » Thu Jan 29, 2009 11:18 am

Thanks... It looks like I will just run it, and take periodic samples with ph paper. It would be nice to be able to monitor everything, but it probably isn't necessary, and is not worth the money. I can probably just find a system that will pump hot water around the tank when it cools down from somewhere that sells hydronic floor heating supplies. That is really all I need. As long as it is producing methane, then there is really no need to know everything.

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Bob
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Re: Energy indepentend home

Postby Bob » Thu Jan 29, 2009 11:30 am

As long as it is producing methane, then there is really no need to know everything.

True. If you can maintain the temperature and the pH you should be good. There are a number of inexpensive pH meters on the market that would be handier than pH paper.

VincenzoAI
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Re: Energy indepentend home

Postby VincenzoAI » Sun Jan 03, 2010 11:25 pm

Hi David,

How did you go with your plans? I am intending to do a similar project, although smaller to start with, for an urban environment. I would love to know how things worked out for you.

Cheers
Vincent

davidwillis
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Re: Energy indepentend home

Postby davidwillis » Wed Apr 07, 2010 9:13 am

I took a break for a bit... I still want to do this, and may get some time this summer to work on it more.

How is yours going?

davidwillis
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Re: Energy indepentend home

Postby davidwillis » Tue Jun 15, 2010 2:53 pm

Ok, it has been a cold spring, but I am trying to re-learn about digesters. And I have found out that I can get grass clipping, and cow manure and grass clippings for nothing to run my digester. So I am thinking of mixing 1/2 manure with 1/2 grass clippings. From what I understand this will produce about 4.3 ft^3 for each 1/2 lb of grass+ 1/2 lb of manure.

Also my plan is to burn the methane each day in a generator (grid tie, battery, or just to reduce electrical usage during the day). During this burn I am going to take the exhaust from the generator and pipe it through a heat exchanger to heat up a water tank. This water tank will be used to keep my digester warm, as well as hot water for my house (which will also include heating the house in the winter). I may need to also build some hot water solar panels to assist in this, but am not sure at this point. I am going to be on a time of day power meter, so I will only run this generator during the day when electricity is expensive. So for now my plan is to store methane for 1 day only.

Currently I am using between 20 and 90 kw/day of electicity (90 is peak during the winter, and 20 is about where it is in the summer). However my entire house is run on electicity, heat, range, water heater, etc. I plan to heat everything with the digester and solar panels if needed, so I should be able to bring my electricity down even lower than 20 kw/day, so for now I will try and produce 15 kw of electricity, and heat the house.

For my testing I am going to run a 12 hp 6kw generator. So that will have to run about 2.5 hours per day to generate the elecricity. At 17 ft^3 per hour per hp.... than means I need 510 ft^3 per day or 3,815 gal. Wow... is that right? Compressing it to 100 psi will still need about a 500 gal tank.

So if that is correct, I will need to feed about 60 lb of grass and 60 lb of manure/ day into the digester. And that is dry weight. So I will need almost 22,000 lbs of each (dry), for a year. I don't even know how much that is, but it sounds like a lot. So lets say I keep a months supply at my house, that is still almost 2,000 lbs of each. How large of a storage do I need to hold 2,000 lbs of manure and 2,000 lbs of grass. Also I don't know how much grass I can get in the winter, so I may have to store it all winter.

Also the calculator shows I need about a 260 gal tank for this. How many gallons of mix am I adding each day when I add water to 60 lb of dry grass, and to 60 lb of dry manure?

Anyway, I thought I would post and see if I have my numbers correct, and what your thoughts are.

Thanks
David


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