[Greenbuilding] composting (in extremely cold climates)

[Nick Pine]

Dry compost has a heating value of about 10K Btu/lb, like dry wood. In a 
big box under ideal conditions (50% moisture, 15% O2, 130 F, 25:1 C/N 
ratio), it can decompose at 15%/day, with lots of leftover heat in the 
form of warm moist air. The box might have a humidity and oxygen sensors 
and controls and a condensing air-air heat exchanger to remove heat from 
the compost air.

Nick

20 VCOMP=4*4*8'compost box volume (ft^3)
30 DCOMP=20'density of dry compost (lb/ft^3)
40 HCOMP=10000'heating value of dry compost (Btu/lb)
50 RCOMP=2'compost decay rate (%/day)
60 DHEAT=RCOMP/100*VCOMP*DCOMP*HCOMP'max daily heat output (Btu/day)
70 DMASS=DHEAT/HCOMP'compost consumption (lb/day)
80 PRINT"box volume (ft^3):";VCOMP,"max daily output (Btu):";DHEAT
90 DFILL=DMASS/DCOMP'max daily fill (ft^3)
100 MTIME=VCOMP/DFILL'min fill time (days)
110 PRINT"daily fill (ft^3):";DFILL,"min fill time (days):";MTIME
120 TC=130'compost temp (F)
130 PC=EXP(17.863-9621/(460+TC))'compost vapor pressure ("Hg)
140 WC=.62198/(29.921/PC-1)'compost humidity ratio
150 L=2'heat exchange cube size (ft)
160 DP=.5'heat exchanger plate spacing (inches)
170 A=L^2*12*L/DP'heat exchange area (ft^2)
180 TI=70'room temp (F)
190 ICFM=2470'room air flow (cfm)
200 VI=ICFM/(L^2/2)'room airflow velocity (fpm)
210 VIM=VI/88'room air velocity (mph)
220 UA=2+VIM/2'airfilm conductance (Btu/h-F-ft^2)
230 RA=1/(UA*A)+1/ICFM'room air thermal resistance
240 CCFM=200'compost airflow (cfm)
250 PRINT "room airflow (cfm):";ICFM,"compost airflow (cfm):";CCFM
260 TR=80'return air temp estimate (F)
270 TA=(TC+TR)/2'average heat exchanger temp (F)
280 PHEAT=(TA-TI)/RA'peak heat output (Btu/h)
290 SHEAT=(TC-TR)*CCFM'sensible heat (Btu/h)
300 WR=WC-(PHEAT-SHEAT)/(4500*CCFM)'return air humidity ratio
310 PR=29.921/(.62198/WR+1)'return vapor pressure ("Hg)
320 TR=9621/(17.863-LOG(PR))-460'return air temp (F)
330 IF ABS(TR-TRL)>.1 THEN TRL=TR:GOTO 270
350 PRINT "return air (F):";TR,"peak heatflow (Btu/h):";PHEAT

box volume (ft^3): 128      max daily output (Btu): 512000
daily fill (ft^3): 2.56           min fill time (days): 50
room airflow (cfm): 2470   compost airflow (cfm): 200
return air (F): 111.3823    peak heatflow (Btu/h): 51617.86