[Greenbuilding] Dessiccant Potential

Eli Talking elitalking at rockbridge.net
Mon Sep 24 09:49:09 PDT 2012

Since becoming aware of equipment that uses heat to recharge (dehumidify) desiccant, I have been intrigued by the idea of using heat to recharge (dry out) desiccant to be used to adsorb humidity within a space.  
http://www.iaqsource.com/product.php?p=novel-aire_comfortdry-250&product=173848 shows a unit that uses hot water to recharge the desiccant that could potentially be produced from solar energy.  

I have been contemplating the possibility of removable desiccant that can be recharged by any heat source outside the thermal zone.  I have been incrementally upgrading my home thermal envelope to be tight.  I night flush the house during the summer and generally achieve 70F with a high relative humidity of 80% in the morning.  I am wanting the identify how much desiccant would be required to dry that air to 50%RH at 75F.  Since I live in a wooded setting, the active solar opportunities are limited for recharging the desiccant.  However, if the desiccant can be removed, it can be carried to an outdoor heater or drier for recharging.  In my situation, I could take it to a solar dryer located in my field with full sunlight.  One could also put it on a wood stove located outside when dehumidification is needed during rainy weather.  Once a mass of desiccant has been recharged(dried) it can be stored in a vapor tight container such as a zip lock plastic bag until it is needed.  In this way, one could stock up on dry desiccant.

Below I attempt to quantify the issues.  I am hoping some of the smart and knowledgeable people on this list might review the numbers to see if I have I understand the issues correctly.  For example, the delta volume at 50%RH and 80%RH at 75F divided by delta humidity ratio should give me the volume of a #humid air, at least at that temperature.  If I am right, 1# of vapor (1 pint in liquid volume) occupies more than 21cf.  That is a huge volume expansion which give me some intuitive understanding of vapor pressure.  

In locations with better solar access than I do, I would propose the conversion of a thermal syphon solar air heater located on the South side of the building be tasked with recharging desiccant in the Summer.  Many technical issues to be resolved to make that work. For now, I am focusing on my situation using the removable desiccant.  The question is how much desiccant is needed.  

I have become re-familiarized with the Psychometric chart to determine humidity ratio and volume, enthalpy at each combination of RH and Temperature.  From this information I attempt to determine the weight of the humid air at both 50% and 80% RH at 75F.  From these two conditions, I determine of the change in humidity ratio and volume. Dividing the cf/#da(dry air) into the zone volume to determine the #da in the zone.  I multiply that times the humidity ratio in #ha(humid air)/#da(dry air) to come up with the actual weight of the vapor in the space.  Since we only want to reduce humidity to 50% and not bone dry, the humidity to be removed is only the difference.  

I probed into a local university library to access the ASHRAE references to read their 2 main articles regarding desiccants.  I learned that the solid desiccants physically adsorb humidity through capillary action as opposed to liguid desiccants absorbing humidity with a chemical reaction.  Page F32.4 was a chart that showed the adsorption capacities of 3 different properties.  The Gel 8 which is described in comparison with other desiccants has the capacity to adsorb .21 x weight of desiccant in vapor at 70F.  I think this is a Silica Gel.  I am looking for a good source of these desiccants.  At any rate, the article also says that left in contact with the humid air, the desiccant will adsorb vapor until it matches the RH of the air.  Therefore, the size of the mass of desiccant needs to have the capacity to adsorb 100%RH vapor so that when it adsorbs 50% of that, the air and the desiccant will be 50%RH, the target condition for comfort.  I imagine a fairly simple box in the thermal zone with a fan that circulates room air through the desiccant until a humidity sensor switches off the fan when the target humidity say 50% is achieved and cut on again when the humidity goes above say 55%RH.  

The zone I show in my numbers is 24’x16’x8’=3072cf.  If my logic is correct, than I need 2x weight of vapor at 50%RH or weight of air at 
100%RH divided by adsorption capacity of dessicant of .21 to find required weight of desiccant of 34#.  I think this is somewhat oversized because we are starting out at 80%RH and not 100%RH.  

I installed a Heat Recovery Ventilator in my house instead of Energy Recovery Ventilator that allows latent heat recovery because I do not have active dehumidification such as compressor driven refrigerant systems. However, with this active desiccant removal, an ERV would be a key part of reducing the vapor removal needed by drying out the incoming fresh air when it is more humid than the outgoing stale air.  A tight building with latent heat recovery of ERV, allows for the possibility of using heat the recharge desiccant that can be used to dehumidify indoor air to achieve comfort and healthy conditions without refrigerant driven systems with their energy cost and incremental environmental degradation that is inevitable with leaking refrigerant.  

I do not have a person to review these issues with locally.  I hope this list will wade through the logic to identify my errors and add to their own experiences on using desiccants.  


       Tdb(F)              75.00  75 change   
       RH(%)                 50.0                     80.0      
       Vol(cf/#da)              13.68  13.80                     0.12    
       Vol(#da/cf)              0.073                  0.072                   0.001    
       Zone Volume(cf)              3,072                  3,072                   3,072    
       Weight Dry Air (#da)            224.56                222.61                      1.95    
       Humidity Ratio (#ha/#da)            0.0095  0.015                     0.01          21.82  cf/#ha 
       Weight Humid Air (#)              2.133                  3.339                      1.21    
       Vol Water Vapor as Liquid(cf)          0.03441              0.05386                      0.02    
       Ratio Volume Liquid/vapor       0.000011           0.000018      
       Adsorption Capacity Req (2x # at 50%RH)(#)                 4.27                     6.68                      2.41    
       Adsorption Capacity of Desiccant relative to desiccant weight  0.21 0.21 0.21   
       Weight of desiccant req to dehumidify to 50%RH(#s desiccant)                         34.04   
      Sensible Heat Added/latent heat removed(btu/#ha)   970   
      Latent Heat Removed-Sensible Heat Added(btu/#da)                    1,170    
      Sensible Heat Added/#da(btu/#da)                       5.21    
      Change in Temp(F)=sensible heat added(cf) *{1/zone vol (cf)}*{1/Air Specific Heat 1.08(btu/cf*F)}=                       0.35    
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