[Greenbuilding] Dessiccant Potential

nick pine nick at early.com
Mon Sep 24 16:31:01 CDT 2012


"Eli Talking" <elitalking at rockbridge.net> wrote:

>I have been intrigued by the idea of using heat to recharge (dry out) 
>desiccant to be used to adsorb humidity within a space.

Me too.

> I have been incrementally upgrading my home thermal envelope to be tight.

That's good, with a blower door test.

> I night flush the house during the summer and generally achieve 70F with a 
> high relative humidity of 80% in the morning.

That's bad, if your house contains hygroscopic materials like wood, fabric, 
or concrete.

>Since I live in a wooded setting, the active solar opportunities are 
>limited for recharging the desiccant.

Dry air can be transported farther than warm or cool air...

I just posted this on the yahoo solar heat list, regarding a new 1800 ft^2 
house in Florida:

>... r32.5 of foam under roof. Walls are roughly 1700 sqft... about an R14.
Windows and door along exterior walls is roughly 325 sqft. I think an R-1.5 
can
be assumed to be minimum for those areas.

That's 1800ft^2/R32.5 = 55 Btu/h-F + 1700/14 = 121 + 325/1.5 = 217, totaling
393. Or less, with lower window and wall conductances :-)

>Being mostly masonry house not much leakage. Not leakage in walls, only 
>windows
and doors. If I pay attention to their install be very tight house.

Maybe 0.1 ACH, ie 0.1x1800ft^2x8'/60 = 24 cfm, making the total thermal
conductance be 417 Btu/h-F.

>>NREL says 1730 Btu/ft^2 of sun falls on the ground on an average 82.1 F 
>>August
day with a 74.5 low and a 0.0175 humidity ratio in August in Tampa. The deep
ground temp is 72.3. ASHRAE says an 80 F house with a 0.120 humidity ratio 
can
be comfortable.

With 600 kWh/mo of indoor electrical use, ie 2843 Btu/h, the house only 
needs
(82.1-80)417 + 2843 = 3719 Btu/h of cooling on an average August day.

> We are going to have radiant ceiling.

The cooling could come from 3719/(80-72.3)/8.33/60 = 1 gpm of 72.3 F water
moving through the ceiling, or less, with a smaller house conductance.

If occupants evaporate 2 gallons per day of water and air leaks add
24hx24cfmx60m/hx0.075lb/ft^3(0.0175-0.0120) = 14 lb, the house needs 31 
lb/day
of dehumidification. FSEC is exploring solar-dried bentonite clay clumping 
cat litter,
which can absorb about 20% of its weight in a daily cycle, so the house 
could have
150 lb of clay in a glazed box on the roof.

Who needs AC? :-)

Nick 





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