[Digestion] 5000 dairy cow AD system

[David]

On 1/9/2012 8:34 PM, Gould, Charles wrote:
> Mr. Harris,
>
> I like your suggestion of smaller digesters. The problem is I have 
> not been able to find a digester design robust enough yet cheap 
> enough for a 100 cow dairy. Do you know where such a design exists? 
> I have just about given up the notion that such a thing exists.

This highlights a difficulty which may be partially fundamental (and 
to that degree absolute) but which seems largely conceptual, which is 
that because situations differ so much, and the need sufficiently 
moderate that there are few if any standard "replicable" designs for 
digesters, of the sort that would be used for, say, a large block of 
tract houses, where 1,000 houses might share four or five basic floor 
plans. And no doubt specifics in this area vary country-to-country...

AgSTAR, of the US EPA, which could be said to be the primary 
government-based promoter of biogas in the US, is aimed strictly at 
manure-using digesters for farms with 500+ animals. For example, 
although it could hardly be said to be a digester at a modest scale 
(cost: $US10M+), the AgSTAR site deliberately (according to my 
conversation with a staffer) does not mention the Stahlbush Island 
Farms digester in Corvallis, OR, because it is fed plant matter 
exclusively-- therefore it is not a manure-based digester.

It becomes a kind of self-fulfilling prophecy, because the system (in 
its academic support, among its industrial implementers and so on), 
becomes adapted to larger digesters, and therefore has that much more 
difficulty accommodating a different scale of thinking. But as well, 
it seems that the variability of energy prices in the US, where 
feed-in tariffs appear to be a political impossibility, has prevented 
the establishment of an industry that might work its way down to 
"smaller" digesters, after the larger digesters on the megafarms have 
been built, similar to what seems to be happening in Germany.

I think there may well be a number of lower-cost building and process 
control technologies, perhaps along with energy crop co-digestion 
strategies, which could be put together in the proper circumstances to 
reduce the cost of digesters of a size below the AgSTAR limit in the 
US; but it would require a funder with vision to realize such an outcome.

d.
-- 
David William House
"The Complete Biogas Handbook" |www.completebiogas.com|
/Vahid Biogas/, an alternative energy consultancy |www.vahidbiogas.com

|
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"Make no search for water.       But find thirst,
And water from the very ground will burst."
(Rumi, a Persian mystic poet, quoted in /Delight of Hearts/, p. 77)

http://bahai.us/
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