Example of a Dessicant Wheel (from Engelhard/ICC Corp.)

Materials Research

The Development of Passive Humidity-Control Materials

Investigator: Dr. William Carty
Industrial Partner: NY Indoor Environmental Quality Center June 2003 to Present

Objectives

Ceramic monoliths offer a unique opportunity as a passive humidity control material provided the pore size and pore structure can be engineered. The objective of this study is to determine the feasibility of generating ceramic monoliths with controlled pore size in the range appropriate for the condensation of water at a specific humidity level. It is also necessary that the pore volume in the sample be sufficient to contain a reasonable volume of condensed water. A unique approach to porosity control in ceramic materials is proposed that exploits the phase separation observed when two or more incompatible polymeric additives are used in a ceramic particle suspension.

Experimental Approach

It is proposed to exploit polymer-polymer incompatibility (phase separation) to construct the desired pore structure. The introduction of a non-compatible polymer to a polyelectrolyte dispersed ceramic particle suspension provides an opportunity for phase separation. It is proposed that the pore size can be controlled by the molecular weight of the added polymer and that the pore connectivity can be controlled by the polymer concentration.

Expected Results

It is expected that the generation of ceramic monoliths with controlled pore size and sufficient pore volume will serve as passive humidity control materials. Passive humidity control materials offer the potential for significant savings in both heating and cooling costs, of which cooling cost reduction is more obvious.

Background and Objective

Indoor air quality is one of the primary design aspects for any building, from commercial to residential. A significant component of indoor air quality is humidity control. Maintaining humidity levels within a desired range is required for comfort, preservation of building materials, and the inhibition of mold growth. Mold growth, one cause of "sick building syndrome", can pose a significant health hazard to people sensitive to mold spores and contamination by mold can lead to millions of dollars in costs to a commercial building.

Current methods of humidity control are expensive and energy intensive. The most commonly used method is mechanical refrigeration that uses a cooling coil that is the integral part of an air cooling system. The coil is cooled below the dewpoint of the ambient air, causing condensation and dehumidification. However, in this system, humidity is of secondary concern, with the primary concern the reduction in temperature. The added benefit of air conditioning is the reduction in humidity - lower humidity makes the environment feel cooler. Passive humidity control has the potential to reduce cooling loads because a lower humidity level provides for greater comfort. (Note the common phrase that refers to the "dry heat" in Arizona in the summertime.) In addition, the collection of condensed water can lead to mold, mildew, and bacterial growth.

As an alternative or supplement to refrigerant coils, commercial and industrial building can use dessicant or enthalpy wheels, which are air-to-air heat exchangers with rotating disks embedded with a dessicant, such as silica gel or treated, hydrophilic paper. These systems help control temperature and humidity (sensible and latent heat loads). Enthalpy wheels house both a dessicant and a heat storage medium. The dessicant removes moisture from the intake air and releases the moisture (regenerating the dessicant) into the adjacent exhaust air counterflow stream. Heat wheels have a fill that transfers only sensible heat while an enthalpy wheel's fill transfers total heat.

Since the removal of water is an exothermic reaction, conventional refrigeration is then used to lower the temperature only to the desired level for distribution. Since refrigeration is done at a higher temperature than in a conventional HVAC system, a reduction in energy use is possible due to the reduced cooling capacity needs.

The advantages of a dessicant-based dehumidification system include:

• The control of latent/humidity loads in the ventilation air;
• The elimination of condensation on cooling coils and in drip pans and, consequently, the elimination of mold, mildew, and bacterial growth;
• A reduction in the mechanical cooling load, which allows the use of smaller chillers and ductwork.

The disadvantages of the systems include:

• The complicated mechanicals that require maintenance
• The increased capital costs
• Ppotential increase in cost if heat is need to regenerate the dessicant

Passive humidity control materials offer the potential for lower energy and maintenance costs and an overall lower life-cycle cost.

It is well established that as pore size decreases in a material, water vapor will condense within the pores due to a capillary condensation mechanism, with the humidity level at which condensation occurs decreasing with decreasing pore size. As the temperature increases, the humidity level for condensation increases at a certain pore size. The pore size at which condensation occurs is nearly independent of ambient temperature. This observation provides the opportunity to create materials with engineered pore structures that would passively remove humidity from the ambient air and would work at a specific humidity level - if the ambient humidity increased, water would condense in the pore structure, but if the humidity decreased, water would evaporate from the pore structure back into the environment.

Ceramics offer a unique opportunity for the creation of stable, well-controlled porosity materials and are already used for several applications, including filters, catalytic supports, and other applications. The pore size dictates the humidity cutoff level, thus when humidity is greater than the cutoff, water condenses in the pores; when less than the cutoff, water vapor is released. The passive nature of this approach provides an opportunity for reduced cooling loads in the summer, and reduced heating loads in the winter, due to decreased interior humidity in the summer and increased humidity in the winter, respectively.

Benefits

The direct benefit of this research is to produce ceramic monoliths with controlled pore size and sufficient pore volume, which serve as the passive humidity control materials. The humidity control materials do not replace the current humidity control methods, but offer the potential for significant savings in both heating and cooling costs. This successful introduction of the humidity control materials will substantially impact the use of building materials and energy conservation.

The results will also provide insights into the fundamental studies in ceramic processing. The ceramic morphology control by the interactions between polymeric additives is relatively new idea in ceramic processing. The results generated from this research will establish the relationship between pore structure and concentration and molecular weight of polymer. This extends the study on particle phase behaviors in a dilute suspension to a concentrated system, which is close to the suspensions in mass production.

The results obtained from this study can be applied to other areas that require controlled pore size and volumes with strength such as biomaterials and catalysts.

CEER is funded in large part by the United States Environmental Protection Agency.

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