Solar Power isn't Feasible!

Solar Power isn't Feasible!
This cartoon was on the cover of the book "SolarGas" by David Hoye. It echoes the Sharp Solar slogan "Last time I checked nobody owned the sun!"

Saturday, June 23, 2012

The Solar CITIES IBC Tank Biodigestor just got simpler!

 Actually it has been this simple for over a year now, but we just got around to learning enough Blender to make these 3D images.

 Gosh it's easy to build a family scale biodigestor!  All you have to do is buy a used IBC Tote Tank (those 1000 liter/275 gallon practically indestructible HDPE tanks that they ship liquids in all over the world; on Craig's List they can be had for about $125!) and put a 2 inch pipe (usually in two sections) running from about a half meter above the tank to about 15 cm above the bottom for introducing the feedstock (ground up food waste in our case),  a 1 inch pipe running from the middle of the tank to a valved elbow for letting out the effluent/fertilizer, and a 1/2 tank fitting with elbowed valve to let out the biogas that results.

You don't have to alter the tank in any way; all the inputs and outputs can be made through the cover.

You may find it a bit difficult to find the bulkhead tank fittings/adaptors in some hardware shops, but check out aquarium, pool and boating shops, or look online and you should find them.

To get the gas and the effluent out you can use garden hoses or clear plastic tubing. No big deal there; this stuff isn't under much pressure at all.

The gas comes right out the top so you don't need any pipe there.  The fertilizer comes out of the "dead zone" in the middle of the tank (fats and oils, which contain lots of energy, tend to float, and proteins and carbs tend to sink, so the top and bottom of the tank are biologically acive) so you want that one inch pipe going down to the halfway mark.  The feeding pipe should reach to about 10 or 15 cm above the bottom of the tank to leave room for the rocks and active sludge without the food getting blocked on its way down and into the tank.

But that's about it.  You'll want the feeding pipe up top to be about a half meter above the tank so that enough of a head can build up to force the gas out and so that you can pour all your food waste in without overflowing since the effluent outlet is half the diameter of the input pipe.  As you add the ground up food waste slurry (we use an Insinkerator brand garbage disposal to make it) fertilizer fluid will be coming out of the 1 inch pipe, but it will come out slower. You could use a larger pipe, but sometimes they don't fit the lid easily.  We've found with the proper feeding regime the dimensions depicted here work fine!

 Make sure the lid has an intact rubber O-ring gasket; one of ours didn't and while it eventually worked out using silcone, you can experience troublesome leaks. Some used IBC vendors sell you lids without the rubber ring inside; check and see just to be sure!
Then just fill your tank with about 50 to 100 kg animal manure, top off with water, screw the lid on and wait for a couple of weeks. When you get your first flammable gas you are ready to start feeding ground up food scraps -- about a 25 liter buckt of mixed food waste and water each day!


Nick Palmer said...

This looks so easy, I just might try to make one up. How much energy per day (therms/kwh) would one get from the 25 litres food waste feedstock?

T.H. Culhane said...

Hi Nick, great to hear from you and thanks for the links and referrals! I just sent my facebook groups links to your excellent site which I've enjoyed perusing. To answer your question in a pedestrial way (without talking therms) the 25 liters feedstock slurry gives approximately 1000 liters of uncompressed gas per day which allows one to cook on a medium flame on a single burner for about 2 hours or generate electricity from a 2kW engine for about 45 minutes. This is from observation and use. The official figures are 1 cubic meter of biogas (or 1000 liters) equals 2.5 kg of firewood, or One cubic meter of biogas is equivalent to 1.613 liter of kerosene or 1 cubic meter of biogas is equivalent to 4.698 kwh of electricity. These are theoretical figures though. In a home system performance varies. Not a problem though -- you should definitely build one. Even in the colder temps here in Germany I get about 1/2 hr of cooking a day from the kitchen waste my wife and I produce. Try it, you'll never regret it! Also, feel free to join us in our active facebook group, "solar CITIES biogas innoventors and practitioners" and share your results. We look forward to getting to know you!

Nick Palmer said...

That's the sort of thing I needed to hear! I live in Jersey so I'm a little South of you - maybe the reaction might be even faster.

I just found out last night that a friend of mine has *3* unused IBC tanks in his shed... Obviously now is the time to have a go! I'll be over to those facebook links as soon as I post this comment.

I think what you're doing in the Third world is hugely significant.

Anonymous said...


I really like this design, since you need only one IBC tank, but I wonder how much of the gas can be stored in the tank. I expect, that the valve on the pipe for letting out must be closed if not introducing the feedstock. Then the maximal pressure of the gas is equivalent to the heigth of the water in the feeding pipe above the tank. This can't 1000l of uncompressed gas. Also when the tank is full of gas and you open the valve for letting out, then it goes too much of the fertilizer out before you can introduce the feedstock. So my questins are, am I wrong? and wouldn't be better to add two IBC tanks for storing the gas (using preassure of the water as in the other design on your pages).

Nick Palmer said...

Anonymous. The IBC tank is not used to store gas, just to generate it. Th gas is stored in another system. Look at facebook page for more - this website is not updated very frequently but the FB is almost daily