[Greenbuilding] Dessiccant Potential

[Eli Talking]

My comments in blue.  

Christian Corson writes:
Buy a single ASHP and call it good. 3/4 ton will get it done. Install it yourself and save a G. $1600.00 bucks and a lifetime of heating/cooling and de-humidification. 
Done and done.

An air sourced heat pump is the off the shelf predictable performance way to cool and dehumidify a house. I do speck these mini split heat pumps on houses for my clients. However, they still have the problem of using refrigerants that are never supposed to leak. With the vast amount of refrigerant cooling done, incrementally there is steady leaking going on. Also, depending on a proprietary machine such as the Mitsubishi Mr Slim Mini Split System depends on an international supply chain and dependence on a single company for support. If either of these conditions is interrupted the system suffers or fails. 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. 


It took some concentration to work through Nick’s numbers. They are certainly thought provoking.   

-----Original Message----- 
From: nick pine 
Sent: Monday, September 24, 2012 5:31 PM 
To: greenbuilding at lists.bioenergylists.org 
Subject: Re: [Greenbuilding] Dessiccant Potential 

"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.


>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?

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.

Where does 417 btu/hrF come from?

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


>>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?) The deep
ground temp is 72.3. 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?

Where in ASHRAE is reference to humidity and comfort?

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?

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?

.0120 is the humidity ratio for 80F at 55%RH according to psychrometric chart.  


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

Who needs AC? :-)

Sensible cooling can be achieved with daily temp swing averaging.  However, removing the humidity is the key.  Desiccants can do that where any kind of heat is used to recharge (dry) the desiccant. 

Eli 

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