(96b) 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