Frost Control Using Osmotic Membrane Dehumidification

FROST CONTROL USING OSMOTIC MEMBRANE DEHUMIDIFICATION

Frost buildup is a major concern for heat pumps and refrigerator/freezers. Osmotic membrane dehumidification represents an opportunity for controlling frost simply and effectively. The unique application of this technique to heat pumps provides the advantage of independent dehumidification in the cooling mode and frost control in the heating mode.

An osmotic membrane dehumidifier can use a semi-permeable membrane to facilitate capillary condensation of water vapor and the transport of condensed water through the membrane into a salt solution by osmosis. Here a humid gas stream is brought into contact with a semi-permeable membrane, which separates the gas stream from an osmotic (e.g., salt) solution. Some of the pores of the membrane are small enough to permit capillary condensation. Liquid formed within these pores can connect with liquid formed in adjacent pores, collectively forming continuous paths of liquid. These liquid bridges extend across the thickness of the semi-permeable membrane and provide paths by which water can travel across the membrane. Because the membrane is so thin, water concentration gradients across the membrane can be large. This can provide a large driving force for water transport between the humid air and the osmotic fluid. An illustration of this two-step spontaneous process is given below in Fig. 1.

The effectiveness of the membrane, measured as the rate of water removal per unit area of membrane, is key to the compactness and longevity of the dehumidification system. Automotive air conditioning systems have significant constraints on size, while residential, commercial and industrial air conditioning systems are expected to last for generations. High performance membranes can have a major impact on all of these systems, with no significant energy requirements for dehumidification and with reject heat from the vapor compression cooling process used to re-concentrate the osmotic solution, as shown in Fig. 2.

Typically, capillary condensation occurs in the pores of hydrophilic membranes where surface tension attracts and draws condensed vapor along the sides of the pores. We have now confirmed that hydrophobic pores in a limited size range will enhance both capillary condensation and the transport of condensate out of the pores. Test results using membranes containing these pores demonstrate uniquely high fluxes of water.

Beckstein and Sansom, in an article in Proceedings of the National Academy of Science demonstrate under condensing conditions that hydrophobic pores will remain void of condensate if they are under a diameter of 0.8 mm and will fill with liquid if over 1.4 nm (Ref1); that is, capillary condensation can happen away from the surface in a large pore. The hydrophobic surface will drive water vapor away from the surface and cause a density gradient that will result in condensation and cluster formation between water molecules. That is typical of biological pores that selectively transport water, aquaporins, which are lined with hydrophobic residues. Kaneko and Iiyama show the importance of cluster-like water assemblies in hydrophobic nanopores using small angle x-ray scattering to elucidate clusters (Ref 2). Permeation through nanometer pores is rapid because of the reduced friction resulting from repulsion of water from the hydrophobic surfaces.

 

Fig. 1

Fig. 2

 

Osmotic dehumidification has been demonstrated in a laboratory scale device using hydrophobic membranes fabricated by Aquaporin Inside, with magnesium chloride used as an osmotic fluid. Water fluxes obtained were more than twice those obtained with available hydrophilic polymeric membranes.

Enhanced performance of the hydrophobic membrane system extends the potential for effective application of osmotic dehumidification to areas like frost control on surfaces containing refrigerant. A heat pump uses a vapor compression system to cool the interior space in warm weather and, by reversing refrigerant flow, to heat the interior space in cold weather. In the cooling mode, independent dehumidification leads to substantial energy savings; in the heating mode, dehumidifying some outside air can inhibit and substantially prevent the formation of frost.

Frost is generally caused by water vapor in very humid air at temperatures somewhat above the freezing point of water freezing on surfaces containing refrigerant at temperature well below the freezing point. Inhibiting the formation of frost involves reducing the humidity of the outside air in contact with cold surfaces and/or raising the temperature of the surfaces in contact with the outdoor air. An osmotic dehumidifier located outdoors in very effective in accomplishing both objectives simultaneously.

As shown in the attached schematic diagram (Nanocap Heat Exchanger.pdf), cold refrigerant exiting the expansion process to the outside is warmed in a liquid-liquid heat exchanger by the osmotic fluid used in the dehumidification process. Outside air that has been dehumidified is transported in a duct to another heat exchanger to warm the refrigerant further. The air is dry enough to eliminate the potential for frost formation, even with refrigerant temperatures below the normal freezing point. The efficiency of the process is enhanced as the temperature is reduced because capillary condensation occurs more readily at low temperature.

Fig. 3

References:

1.     Beckstein and Sansom, Liquid-Vapor Oscillation of Water in Hydrophobic Nanopores, Proceedings of the National Academy of Sciences Vol 100, No 12(2003) 7063-7068

2.     Kaneko and Iiyama, Structural Understanding of Water Confined in Hydrophobic Nanopores, WATSURF, Les Houches, France Apr. 2013