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re: birds as heaters...
24 mar 1999
wombat  wrote:
>louis boyd  wrote:
>> wrote: 
>>>dkh  wrote

>>> >birds put out a lot of heat on their own...
>>> about 16 btu/h of sensible and 18 of latent heat (chicken sweat :-)
>>> for an ashrae-standard 2-pound broiler raised at 60 f on litter.

>>that's useful info. i hadn't seen numbers on the thermal output of birds

the ashrae handbook of fundamentals (hof) has many cheerful facts about
animal heat production, eg sheep heat as a function of air temperature
and fleece length.

>...wonder, though, which btu the writer is using, the btu [international]
>or btu [therm]?

looks like it hardly matters.

>they don't differ a great deal, though.  16 btu international would be
>equivalent to 4.689 watts and 16 therms 4.686 watts.  

whoa, those are apples and oranges! btu's are energy, and watts are power,
the time rate of change of energy, like miles vs miles per hour. in other
words, energy = power x time. a watt-hour is 3.41 btu, so 16 btu is 4.69
watt-hours or 0.00469 kilowatt hours (kwh.) also, a "therm" is 100k btu,
roughly equivalent to a gallon of oil, in natural gas circles. 

>anyone want to bother to calculate practical considerations such as
>how many chickens it would take to produce enough chicken manure to
>produce enough methane to produce 36 kilowatts of usable power?

the smallest creature listed in the nraes on-farm biogas production book
is a 130 pound feeder pig who produces 0.16 cubic feet (a 6.5" cube :-)
of manure per day which makes 5.6 ft^3/day of biogas which can make
0.2 kwh/day of electrical energy, ie 8.3 watts, on a continuous basis
(vs. 4.83x130^0.75 = 186 watts of heat from the pig, according to the
hof), so producing 36 kw on a continuous basis requires 4,320 pigs,
which could make you unpopular with the neighbors.

aerobically composting poop or raising animals for heat production seems
like a much better deal, energy-wise, and requires simpler equipment.


expired us patent no. 3,933,628 (us patents are available for $3 each
from the superintendent of patents and trademarks, washington, dc 20231)
describes a "method and apparatus for the anaerobic digestion of
decomposable organic materials," issued to inventor frederick t. varani
of golden, co on jan 20, 1976, and assigned to bio-gas of colorado.

he describes a way to make methane in conjunction with a 100,000-cow
feedlot, using 2 epdm-rubber-lined trenches, each 700 feet long x 80 feet
wide x 40 feet deep. the trenches have self-inflated translucent "solar
covers" and cost $0.02 per gallon, including excavation. the feedlot
generates 3.3 million pounds of manure each day, along with 6 million
pounds of water and 200,000 pounds of carbon, which the digesters turn
into about 7 million cubic feet of methane per day with a heating value
of 277 million btu per hour, along with 2 1/2 million cubic feet of co2.

the digesters contain heat exchangers for temperature control... 

  the fermentation reaction will proceed satisfactorily at any temperature
  between approximately 90 f and 115 f, however, between these limits many
  different species of bacteria become active, each in its own particular
  temperature zone carved out of this broader range. in other words, the
  digestion process is basically an equilibrium between many species of
  bacteria that live upon various substrates (food) and on one another.
  changes in temperature cause this equilibrium to shift and some of the
  more temperature-sensitive species die off or become less active while
  others assume a more active role... ideally, methanogenic bacteria should
  be kept at about 95 f and the temperature range should not be allowed to
  vary more than +/- 2 f per day from this base temperature if temperature
  shock is to be avoided.

this could be a municipal sewage treatment system, without the cows,
or an efficient way to combine sewage treatment and long term passive
solar thermal storage, for a single house, on a smaller scale.

pages 825-826 of metcalf and eddy's 1991 _wastewater engineering_ say

  typical values [of gas production] vary from 12-18 ft^3/lb of volatile solids
  destroyed... gas production can also be crudely estimated on a per capita
  basis. the normal yield is 0.6 to 0.8 ft^3/person/day (15 to 22 m^3/1000
  persons/day) in primary plants treating normal domestic wastewater. in
  secondary plants, the gas production is increased to 1.0 ft^3/person/day...
  because digester gas is typically about 65% methane, the low heating value
  of digester gas is approximately 600 btu/ft^3 (22,400 kj/m^3)... in large
  plants digester gas may be used as fuel for boiler and internal combustion
  engines, which are in turn used for pumping wastewater, operating blowers,
  and generating electricity... because digester gas contains hydrogen sulfide,
  particulates and water vapor, the gas frequently has to be cleaned in dry
  or wet scrubbers before it is used in internal combustion engines.

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