passive solar liquid desiccant cooling SOLAR.KnowledgePublications.com

passive solar liquid desiccant cooling
28 sep 2004
>...the roof might be epdm rubber, with a passive greenhouse-type solar still
>with shallow licl lakes separated by dry epdm beds to act as water collectors
>and parasitic air heaters, like this, viewed in a fixed font like courier:

                   |           2'          |
                 carbo
             poly  s  nate 
        flat       p       clear
  clear            a            flat
                   c                 poly      nate...
                   e                     carbo
         epdm licl r lake epdm  heater  dry bed            epdm
    epdm 2x4 epdmepdmepdm 2x4 epdmepdmepdmepdmepdmepdmepdm 2x4 epdm
--------------------------------------------- top of sip ------

(what's a good lake to heater area ratio?)

a tight sip house in an average 83 f humid place like miami (with w=0.0176)
might only need 15 cfm of fresh air with 1000x15x60x24hx0.075(0.0176-0.0120)
= 9k btu/day of dehumidification and 24h(83-80)300 = 22k btu/day of cooling
to reach the 80 f/w=0.012 upper right hand corner of ashrae's comfort zone.
the cooling might come from an indoor greywater wetland or some night air,
with internal house mass. 

how many square feet of 80 f licl solution (precooked to 160 f) are needed
to remove 9 pounds of water from 80 f house air with w = 0.012 in 12 hours? 
here's one calculation, based on some crude assumptions:

1) the licl still operates at a constant temp for 12 hours per day.
2) the solar energy that enters the r1 glazing with 90% transmission
equals the sensible and latent heat energy needed for concentration.
3) the solution cools to 25 c at night. 
4) the solution gains heat like an ashrae pool loses heat. 

i'm not at all sure about 4). suggestions welcome. the next step might be
a simple tmy2 simulation. 

10 a1=12.7409'licl vapor pressure constants from the 1993 hawlader paper
20 a2=-.065536
30 a3=-8.2416e-04
40 b1=-4675.4
50 b2=+29.31
60 b3=+.66911
70 c1=372690!
80 c2=-1689.8
90 c3=-187.1
100 ta=82.8'average ambient august temperature in miami (f)
110 sg=1770'average august sun on ground in miami (btu/ft^2-day)
120 h=12'distillation day length (hours)
130 w=.0176'average ambient august humidity ratio in miami
140 pv=25.4*29.921/(1+.62198/w)'ambient vapor pressure (mmhg)
150 p=9'dehumidification load (lb h2o/day)
160 for tc=60 to 90 step 10'solution temp (c)
170 tk=273.1+tc'solution temp (k)
180 c=a1+b1/tk+c1/tk^2-log(pv)/log(10)
190 b=a2+b2/tk+c2/tk^2
200 a=a3+b3/tk+c3/tk^2
210 conc=(-b-sqr(b^2-4*a*c)/(2*a))'equilibrium soln conc (wt%)
220 tf=1.8*tc+32'solution temp (f)
230 concsurf=1000*p/(.9*sg-h*(tf-ta))'licl surf needed for conc (ft^2)
240 tk=298.1'solution temp (25 c)
250 ap=a1+a2*conc+a3*conc^2
260 bp=b1+b2*conc+b3*conc^2
270 cp=c1+c2*conc+c3*conc^2
280 pvc=10^(ap+bp/tk+cp/(tk^2))'vapor pressure at 25 c (mmhg)
290 pvi=29.921/(1+.62198/.012)'indoor vapor pressure ("hg)
300 pvl=pvc/25.4'licl vapor pressure ("hg)
310 dryrate=.1*(pvi-pvl)'lb/h/ft^2 h2o (like an ashrae pool)
320 drysurf=p/(12*dryrate)'licl surface needed to dry p lb h2o in 12 h (ft^2)
330 print tc,conc,pvc,drysurf,concsurf
340 next

still         solution      licl pv       drying        concentrating
temp (c)      conc (wt%)    (mmhg)        surf (ft^2)   surf (ft^2)

60            39.15389      5.493444      21.42437      9.927201
70            45.89019      2.75522       16.3801       13.03215
80            52.33653      1.244954      14.49746      18.96333
90            58.57794      .5091767      13.72873      34.80277

if the still temp is too low, it looks like we need lots of drying surface.
if it's too high, we need lots of concentrating surface.
a 70 or 80 c still temp seems good...

nick