Posts Tagged anerobic digestion

2% National Coutput -or- 10,000 Cows per MW

A commenter on my recent pro-transmission article questioned some statements of mine about the availability of renewable electricity from cow manure.  I had stated that 

  1. It was most likely baseload.
  2. There was not much of it in many states.

The first observation comes from the fact that the digester is sized so that
it runs as much as possible to produce the best economics.  Manure also
breaks down over time, and so is likely to yield more electricity if used as
quickly as possible after it’s produced.

Total Cow Output (Coutput?)

I thought it would be interesting to run some numbers and see how much
electricity might be available from cow manure.  First I had to come up
with some numbers for kWh/cowyear.  I found an example of a 50 kW engine running 14h/day on the produce of 300 cows.

That’s 850 kWh/cowyear, or about 100 W/cow, or 10,000 cows/MW.  A single cow produces enough manure to run one incandescent light bulb (or four CFLs) year round.

The US has about 100 million cattle, capable of producing 10 GW of electricity (if all the manure was gathered and processed in anaerobic digesters,) or 85 million MWh/year.  Total US Electricity production in 2007 was 4,208 million MWh, so 100% conversion of cow power into electricity could supply 2% of total US electricity.

100% conversion is a rather heroic assumption, especially for pastured cows, but I’m not including other types of livestock (pigs, chickens, etc.) so 2% of electricity seems like a reasonable potential estimate for total anaerobic digestion of manure. 

2% is actually a higher number than I would have expected, although that potential for electricity generation is unlikely to be reached, since much might be converted to liquid fuels for transport.

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