Dear Richard,
For a week's compression you'd need something like 7:1 ;-) . The bag would be roughly 4times more material intensive, assuming that the production parameters would handle 7 or more atmosphere's (which we don't know.) This also assumes that we just make the carry-bag big enough to hold the increased volume and compress it with the ubiquitous tubing you mention. So we have a roughly 4x heavier bag + the weight of the necessarily rubber tubing + a 7 times increase in bio-gas volume, which will no longer reduce the load on the shoulders of the carrier but rather increase it c. 5 (?) kg. I think this increases the technical complexity and reduces the carriability considerably.
As an alternative, I think a strap-pull system could be conceived to double the pressure pretty easily, but would need to be carefully conceived in order not to dramatically increase price, weight and practicality.
regards,
Ron von Vordemhaarschnit/derdusche
Von: "Richard Stanley" <[email protected]>
Gesendet: Jan 13, 2012 8:23:34 AM
An: "Discussion of biomass cooking stoves" <[email protected]>
Betreff: Re: [Stoves] Biogas backpack
Not to be facetious about the bag lady in the prior post, but there has to be some practical alternative. Biogas is said to be too expensive to compress. The comparison was made to LPG for transportation. Like most one line conclusions it may or may not be though, ... In different circumstances. What amount of energy per unit atmosphere of compression is expended? Is it a linear increase with increasing pressure ? Lets have that plot then talk about practicality of transport per various volumetric requirements of the rest of the 99%, like the lady carrying the bag so, perhaps she is not left doing so, so to speak.
I have heard about an intrepid country inventor, Harold somebody, who used his refrigerator compressor to generate about 200 psi (12+ atmospheres) off from his chicken manure, to run his car around his town. What is a practical minimum compression for transport and say week long domestic use? Would an extended tank -in-tank biogas digestor, properly ballasted and suspended by ropes on a basic windlass, generating up to say 15 psi, sufficient to contain the required volume in an inner tube? (Where is Boyle when we need him! ) . Even the simple wrapping of sealed plastic bags filled with biogas, with ubiquitous inner tube tire rubber strips can generate substantial compressive forces... . It all comes down to time and effort versus cost and time of alternatives---assuming of course that the so called beneficiary wants it and can function as a community member pursuing it.
Having lived on a tank in well system for four years in Tanzania, am all for a practical solution.
Richard Stanley
Sent from my iPhone
Dear Crispin,
On 09.01.2012 19:02, Crispin Pemberton-Pigott wrote:
Dear Ron
Good to hear from you in the new year.
>I think you misunderstood the article. The bag is only for transport and temporary storage. They take the empty (and some dung) to a biogas producer, get it filled and go home to cook with it. It is supposed to be about a day's cookin' worth.
I am with David House www.completebiogas.com on this one: it is a portable digester with nothing but gas in it! J
He writes, “It's actually a gas bag, not a digester. Even so, it's an excellent innovation, and a worthy addition to the armamentarium, wherever it can serve as a means of transport in connection with a large digester near any large population.
And as well, from my point of view, it's also a bit ironic, since the very sturdy bag, selling for ~$US38, could actually be a digester if it had two additional pipes (an inlet and an outlet), and further that since it's about a cubic meter in volume, it would produce about a cubic meter of gas every day, if fed and kept warm.”
That may well be, but, not having seen the bag, it may well need some additional features apart from two pipes. You'd have to get solid materials in and out, etc. Also, I think that his proviso: "...wherever it can serve as a means of transport ... large digester ... any large population." is overwrought. When compared with walking 10 km with 20 or more kg on your head, a 3kg bag is gonna add up to 30 € pretty quick. I'm not dissing the idea of making a digester of that size, just have the impression from the relative clause that he is degrading the usefulness of the idea unnecessarily.
I think David is working on a bag digester himself. The transport of gas is interesting. If it turned out to be attractive as a cooking fuel (and delivery system) the users would perhaps be convinced to install their own systems. I am not convinced that a minibus would accept the gas ‘package’ without fear or charge.
I think they limited the size to keep it a relatively manageable size.
It is a lot lighter than a load of wood and the walking distance might be less. I wonder if a tire on a rim might be as good, and could be pumped by hand. Will a tire hold a useful volume of gas if hand pumped?
Even if the walking distance was more, one would not have the same fatigue. i think you'd have to work pretty hard to compress a whole m^3 into anything like a regular tire-- and a tractor tire with rim would be more taxing to manage than a load of firewood on one's head. ;-) If this was done in cooperation with schools, they could even get the transport with larger school children on the way home from school, delivery before school, pick up the bag and go home after school. hoping you're doing well.
regards,
ron
w
h
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