[Greenbuilding] Dessiccant Potential

[nick pine]

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

>What I propose is a simpler technology using desiccant in a box with a 
>humidity switch to a fan blows indoor air through between above 50%RH.

Sounds doable, with sun-dried desiccant, but we don't need a switch or a 
fan. This can work passively.

>>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...

> What do you mean by that?

A remote elevated solar desiccant bed with a long concentric tube air-air 
heat exchangercan supply dry air to a house basement. If the heat exchanger 
makes dry box air and moist house air close to the same temperature, less 
dense moist house air can rise up into the elevated bed and denser bed air 
can fall back into the house to replace it. Unlike warm or cool air, moist 
and dry air can be transported in long airtight tubes with no insulation.

> 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.
>
>>Being mostly masonry house not much leakage, 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 417 Btu/h-F.
>
> Where does 417 btu/hrF come from?

C cfm of air leakage adds about C Btu/h-F to the house conductance, eg 
393+24 = 417.

> Is .1ACH enough.  Using the ASHRAE formula Requred ventialtion/min 
> =(.01cfm/sf Area(sf)) + (7.5cfm/occupant x #occupants)

No. I'd provide a mere 7.5 cfm per full time occupant in a house with no 
smokers and minimal VOCs and minimal kitchen and bathroom exhaust fan use.

>>>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. (Average?)

Yes.

> ASHRAE says an 80 F house with a 0.120 humidity ratio can be comfortable.
>
> What is your source of information for average humidity ratio for August?

NREL's Blue Book... http://rredc.nrel.gov/solar/pubs/bluebook/

> Where in ASHRAE is reference to humidity and comfort?

The ASHRAE comfort standard. You can buy one and use the BASIC program 
therein.

> 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.
> I see you are subtracting the ground temp from 80F comfort conditions. 
> How do you propose to do this?

Move 1 gpm of 72.3 F water through the radiant ceiling.

> 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.
>
> What is your source of infomration on 2gal/day evaporation from occupants?

The Andersen windowfolk.

> 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.
>
> http://www.grace.com/engineeredmaterials/MaterialSciences/SilicaGel/SilicaGelTypes/SilicaGelTypeAdsorption.aspx
> This chart shows the Silica Gel can adsorb 40% humidity of weight at 80%RH 
> air and 26%weight at 50%RH

Silica gel requires higher drying temperatures. See FSEC's bentonite clay 
experiments.

> Sensible cooling can be achieved with daily temp swing averaging.

Not in Tampa.

Nick