Thermoelectric Membrane Distillation System Engineering ...

49th International Conference on Environmental Systems ICES-2019-117 7-11 July 2019, Boston, Massachusetts

Thermoelectric Membrane Distillation System

Engineering Design Improvement Concepts

Jeffrey M. Lee1 and Lance Delzeit2

NASA Ames Research Center, Moffett Field, CA, 94035, USA

Jurek Parodi3

Bionetics Corporation, Yorktown, VA, 23693, USA

Gregory Pace4

KBRwyle, Houston, TX, 77002, USA

and

Serena Trieu5

Logyx LLC, Mountain View, CA, 94043, USA

All of the membrane distillation technologies that National Aeronatics and Space

Administration (NASA) has examined to date require external heating and cooling subsystems

to drive the distillation and condensation processes. Since energy is added to the system to

change liquid water into vapor, and energy is rejected from the system to convert vapor back

into a liquid, a higher efficiency is achieved when the enthalpy of liquefaction is recaptured

for use in supplementing the enthalpy of vaporization. The Thermoelectric Membrane

Distillation (TMD) system embeds thermoelectric devices acting as heat pumps directly at the

membrane surface into a self-contained device, thereby heating the retentate while

simultaneously cooling the permeate. A flexible testing apparatus has been developed to

quickly validate the TMD concept and to characterize different key performance parameters,

which have been utilized to develop models for the design of engineering prototypes. This

paper describes the validation of our proof-of-concept work and the design improvements

implemented to improve performances, and the degradation of performances observed during

long-duration testing.

Nomenclature

ARC = Ames Research Center

BEB = Brine Evaporation Bag

CAD = computer-aided design

CFD = computational fluid dynamics

CPVC = chlorinated polyvinyl chloride

DAQ = data acquisition

DCMD = Direct Contact Membrane Distillation

DOC = Direct Osmotic Concentration

ESM = equivalent system mass

ID = internal diameter

1 Chemical Engineer, Bioengineering Branch, M/S 239-15. NASA Ames Research Center. 2 Chemist, Bioengineering Branch, M/S 239-15. NASA Ames Research Center. 3 Aerospace Research Engineer, Bioengineering Branch, M/S 239-15, NASA Ames Research Center. 4 Senior Research Engineer, Bioengineering Branch, M/S 239-15, NASA Ames Research Center. 5 Engineer, Bioengineering Branch, M/S 239-15, NASA Ames Research Center.

International Conference on Environmental Systems

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IWP = Ionomer-membrane Water Processor

K = kelvin

MD = membrane distillation

NASA = National Aeronautics and Space Administration

NI = National Instruments

PP = polypropylene

PVC = polyvinyl chloride

SGMD = Sweeping Gas Membrane Distillation

T = temperature

TEC = thermoelectric cooler

Tf = final temperature

Ti = initial temperature

TIME = Thermoelectric Integrated Membrane Evaporative Subsystem

TMD = Thermoelectric Membrane Distilliation

TRL = technology readiness level

V = Volts

VMD = Vacuum Membrane Distillation

I. Introduction

URRENT state-of-the-art membrane distillation technologies use bulky external heating and cooling subsystems

to provide the heating and cooling necessary to drive the distillation process, which adds to launch mass, create

heat losses, and are a source of viscous flow losses1. The Thermoelectric Membrane Distillation (TMD) system

eliminates these external subsystems by embedding thermoelectric devices directly at the surface of the membrane.

Membrane Distillation (MD) is driven by water vapor pressure gradients across the two sides of a hydrophobic

membrane, which defines the physical location of the phase interface and mass transport. The membrane is permeable

to water vapor but impermeable to the liquid water phase and to most contaminants present in urine and other

wastewater sources. The use of membranes for phase separation is particularly well suited in microgravity

environments. The innovation behind TMD is to incorporate thermoelectric devices at the membrane surface, which

act as heat pumps by heating the retentate while simultaneously cooling the permeate. Thus, mass transport, energy

transport, and phase separation are achieved into a single, self-contained system with no moving parts that reduces

equivalent system mass (ESM) and increases reliability when compared to state of the art distillation systems.

II. TMD Background

MD uses a hydrophobic membrane that is permeable to water vapor to separate waste water (retentate) from

distilled water (permeate). Heating the retentate and cooling the permeate is usually used to create a water vapor

pressure differential across the membrane, which drives the water vapor flux through the pores of the membrane itself.

Adding heat to the warm retentate side vaporizes liquid water that diffuses through the membrane pores to the cold

permeate side, where it then recondenses. Higher temperature differentials have the advantage of creating higher vapor

pressure driving forces. However, higher retentate temperatures increase the permeation of volatile contaminant gases.

NASA has investigated the following types of MD to recover water from urine, urine brine, and hygiene water for

space missions. The Direct Osmostic Concentration (DOC)2 project and the Thermoelectric Integrated Membrane

Evaporative Subsystem3 (TIME) projects both examined the use of Direct Contact Membrane Distillation (DCMD),

which uses a membrane to separate the liquid phases of the permeate and retentate with water vapor transport within

the pores of the membrane. The Brine Evaporation Bag4 (BEB) project investigated the use of Vacuum Membrane

Distillation (VMD), employs a vacuum on the permeate side of the membrane. The Ionomer-membrane Water

Processor5,6 (IWP) investigated Sweeping Gas Membrane Distillation (SGMD), which uses a flowing gas such as dry

air over the permeate side of the membrane. All these MD methods require external subsystems such as pumps,

blowers, chillers, condensers, and heaters to provide the latent heat energy for bringing water from its initial liquid

phase to a gas phase and then back to a final liquid phase. Moreover, recuperative heat exchanges and/or a heat pumps

are needed in order to recover the heat of condensation and increase energy efficiency.

C


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The vapor compression cycle used

in MD is shown in Figure 1. In order

to minimize the amount of work

necessary to drive this process and

thus to use the power available

onboard the spacecraft more

efficiently, the enthalpy of

liquefaction should be recaptured and

used to supply part of the enthalpy of

vaporization.

In TMD, the thermoelectric device

embedded at the surface of the

membrane is used as a heat pump that

simultaneously removes heat of

condensation from the permeate side

and rejects it to the retentate side, thus

incorporating both cooling and

heating in a single, self-contained

system. The heat taken from the cold

side is ideally the enthalpy released by the recondensing water vapor. The heat rejected to the warm side is used to

vaporize the water. The transport of heat from the permeate to the retentate requires some work (electrical in this

case) according to the 2nd law of

thermodynamics. Thus, additional heat is

rejected at the permeate due to this added

electrical work. As a result, the total heat

rejected to the permeate is higher than the

required enthalpy of vaporization and the

additional heat must be rejected to the

environment. Figure 2 shows a diagram of the

main components constituting TMD and the

related mass and energy transport process. In

this configuration, the thermoelectric device

is sandwiched between two membranes, one

separating the hot plate of the thermoelectric

from the retentate, and the other separating

the cold plate of the thermoelectric from the

permeate.

III. TMD Testing

The approach used to validate the TMD technology concept consisted of several iterative rapid prototyping, testing,

and simulation phases that addressed issues observed during each new phase and to gather the information needed to

develop the requirements for a higher technology readiness level (TRL) system. Several test apparatus were

manufactured to test the performance of the TMD technology in different operational scenarios and to quickly generate

critical data. High-fidelity computer aided design (CAD) models for the each test apparatus have been developed and

the outcomes of the respective flow simulation analysis have been compared to the experimental results, which

allowed tuning of the parameters used in the computational fluid dynamics (CFD) tool. A set of flow simulation

analyses were performed to develop future test apparatus with the objective of increasing the performances of the

system. This paper summarizes the experimental results obtained during the proof-of-concept testing of the initial

TMD prototypes and the engineering designs of improvement concepts of the system.

Figure 1. T-s diagram for vapor compression cycle.

Figure 2. Mass and energy transport diagram of TMD with

thermoelectrics sandwiched between membranes.


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Figure 3 shows the diagram of the

first generation of the testing

apparatus, used for a set of batch mode

experiments. This configuration

consisted in a thermoelectric cooler

(TEC) sandwiched between two

coupons of hydrophobic membrane,

all kept in place by two symmetric

assemblies made of plastic fittings.

This resulted in a gap between the two

membrane coupons, requiring water

vapor transport within this gap from

the hot membrane coupon to the cold

one. The main advantage of this

configuration was the easiness to

accommodate the thickness of a

commercially available TEC (4mm)

by using off-the-shelf silicone flange

gaskets and the creation, through the

gap between the two membranes, of a

barrier to heat flow from the hot

retentate to the cold permeate. Each of

the two symmetric assemblies

consisted in a flange connected to an elbow with a hose barb fitting to which is attached a transparent and graduated

column. A CAD cutout of the of one of the assembly with a gasket and the TEC is shown in Figure 4.

The membrane used in all TMD experiments is a

high-porosity microporous membrane from Celgard,

named EZ2090. It is composed of a single layer of

polypropylene (PP) material and it is characterized by

a thickness of 20 micron, a porosity of 67%, and an

average pore size of 97 nanometers. Other properties

include stability over a pH range between 2 and 12, a

highly uniform pore structure, and a moisture vapor

transmission rate of 100,000 g/m2/day. It is designed

specifically for applications that require a

hydrophobic but highly breathable barrier such as

membrane distillation. It has been down-selected

among other membrane candidates because of its

higher porosity and pore size and comparable

chemical resistance and mechanical strength.

The TEC used during this first set of experiments

is a commercially available module from TE

Technology, part number CH-109-1.4-1.5. The

dimensions of the module are 40mm x 40mm x 4mm.

It has a hole with a diameter of 13mm in the center.

The minimum and maximum operational

temperatures are respectively 40ºC and 80ºC. At 50ºC

on the hot side, the maximum voltage input is 15.2 V

and the maximum amperage is 6.1 A. The maximum coefficient of performance is function of the input voltage and

of the temperature differential between the cold and the hot side. The module is potted on all sides between the two

ceramic plates to provide waterproofness.

A series of two commercially available high-temperature, silicone flange-gaskets with a third, custom made gasket

of the same material located in the middle are used to secure the TEC and the membranes between the flanges of the

two assemblies, preventing the system from leaking, protecting the ceramic plates of the TEC from excessive

compression (and cracking) when the flanges are connected and tightened together, and allowing the wires of the TEC

out of the system.

Figure 3. Diagram of TMD static testing apparatus.

Figure 4. Cutout drawing of one of the two symmetric

assemblies of the testing apparatus.


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Each side of the system integrates an internal thermocouple that is

inserted inside the assembly through the column so that its probe is in

direct contact with the surface of the membrane. Moreover, each side of

the system integrates an external thermocouple, the probe of which is stick

to the surface of the plastic elbow. A power supply with adjustable current

and voltage output provides the desired power to the TEC during the

experiment.

Initial tests were conducted with deionized water and recording

temperature and static pressure head (water level in the graduated

columns) across the membranes as a function of power input to the TEC.

The general operating procedure was to fill the permeate and retentate

sides until the column levels reached the same initial and equal reference

level. Power to the TEC was then turned on and held constant, generally

until steady state was achieved. At steady state, the power was turned off

and the system allowed to passively dissipate heat until achieving room

temperatures.

Figure 5 shows the first version, consisting in chlorinated polyvinyl

chloride (CPVC) flanges and ¾” CPVC fittings, Buna-N gaskets, and

flexible hoses with ¼” internal diameter (ID) as columns. The results of

the preliminary tests done using the CPVC flanges and fittings showed a

level increase of the water columns on both sides of the TEC, which goes

against the law of conservation of mass. In particular, the trends of the

water column on the cold side of the TEC are consistent with expectations,

growing in function of time, and growing faster at a higher input power,

which corresponds to a higher differential temperature between the two

sides. On the hot side, instead, the water column increased during the

transient phase of the process, independently of the power input,

maintaining the level for a certain time into the steady state phase, only to

start decresing afterwards. This observations indicated that there were

phenomena affecting the behavior of the system. In particular, the

difficulty to fill the two sides of the system suggested that some air

remained trapped within the chambers between the flanges and the hose

barb.

Thus, the testing apparatus was modified to implement transparent flanges and fittings in order to identify any

presence of air bubbles in the chambers at the end of the filling process. The flanges also have capped ports used for

removing excess air bubbles present in the system.

Figure 6 shows the third version of the testing apparatus, which was used for most of the experiments conducted

to characterize the flow rates

across the system in function of

the TEC power. This version is

characterized by clear, custom-

made acrylic flanges, 1 ½” clear

PVC elbows, and clear acrylic

tubes with 3/8” ID as graduated

columns. This configuration

allowed the observation and the

removal of any air bubbles

trapped during the filling of the

system. Moreover, it verified the

position of the thermocouple

probes and reduced the thermal

conductivity between the two

sides of the system. A CAD

section view of the of this

version is shown in Figure 7.

Figure 5. First generation testing

apparatus.

Figure 6. Third generation testing apparatus.


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Several experiments conducted

with this version of the apparatus

and with initial conditions

characterized by the absence of

any visible air bubbles in the

system still showed a rapid

increase of the water column on the

hot side of the TEC during the

transient phase of the experiment.

This observation led to the

conclusion that the main physical

phenomenon responsible for this

behavior is the volumetric

expansion of water related to the

rapid growth of temperature during

the transient phase. Above 4ºC, the

density of water decreases and its

volumetric expansion coefficient

increases with rising temperatures.

The variation in volume was calculated as:

𝛥𝑉 = 𝑉 ∫ 𝛼𝑉(𝑇)𝑑𝑇𝑇𝑓𝑇𝑖

(1)

where 𝑉 is the initial volume of the water, 𝑇 its temperature, 𝛼𝑉(𝑇) is the volumetric expansion coefficient of water

as a function of temperature 𝑇, and 𝑇𝑖 & 𝑇𝑓 as the initial and final temperatures, respectively. To validate these

adjustments, after the system was cooled down at the end of the experiment and the the water columns reached room

temperature, the corrected and uncorrected values converged. A subsequent reversal of the TEC current to create the

temperature conditions for reversing the permeation was also conducted, and showed that water did flow back as

would be expected.

The graph in Figure 8 summarizes the results obtained during a set of experiments conducted using the system

configuration described above. The graph shows the relationship between the volume of water produced, the

temperature differential between the two sides of the system, the temperature of the permeate, and the flow rate across

the membranes in function of

power. As expected, all four

variables increased with

power. The volumes and flow

rates are corrected for thermal

expansion. Each datapoint in

the graph represents the

average of all the datapoints

collected during the steady

state of each run. In fact, each

run can be divided into two

phases. The first phase is the

transient, during which there is

a rapid increase of temperature

on the hot side of the TEC and

a rapid creation of a

temperature differential

between the two sides of the

system. During this phase, the

level of the permeate does not

show any apparent change

while the level of the retentate

Figure 7. Cutout drawing of the the third version of the testing apparatus.

Figure 8. Summary of experimental results obtained using third version TMD

with 40x40 TEC.


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increases dramatically. In correspondence with the stabilization of the temperatures and temperature differential,

which signals the beginning of the steady state, where the level of the permeate starts to increase while the level of

the retentate decreases linearly. Each run lasts 8 hours; after the power supply is turned off and the system cools down,

both the permeate and retentate reaches the initial temperature and the values of the uncorrected and corrected volumes

converges.

Figure 9 is an example of a flow simulation analysis conducted to investigate the behavior of different parameters

of the system during a run with initial and boundary conditions identical to the values measured during an actual

experiment. The objective

was to determine the

effects of conductivity and

convection on the

temperature profiles

internal to the system.

Figure 9 in particular

shows the results at the

beginning of the steady

state. A comparison with

the actual experimental

data determined that the

flow simulation tool can be

used with a good degree of

confidence to analyze new

configurations and new

engineering concepts. The

main limitation of this tool

were found to be the

impossibility to select

multiple solid materials

with different material

properties and the impossibility to simulate phase changes and mass transport of the water vapor.

A second set of batch mode experiments using the same testing apparatus was conducted using a TEC with a

smaller heating/cooling surface area and a single membrane coupon. The TEC used during this set of experiments is

a commercially available module from TE Technology, part number TE-65-0.6-0.8. The dimensions of the module

are 12mm x 12mm x 2.6mm. The minimum and maximum operational temperatures are respectively 40ºC and 80ºC.

At 50ºC on the hot side, the maximum voltage input is 8.9 V and the maximum amperage is 2.1 A. The module is

potted both on all sides

between the two ceramic

plates to provide

waterproofness. The TEC

is kept in place against the

membrane coupon by a

custom made silicon

gasket flange. In this

configuration, the water

vapor is transported

directly from the retentate

to the permeate across the

membrane pores.

A summary of the

results obtained during this

set of experiments is

shown in the graph in

Figure 10. The graph

compares the volume of

water produced, the

temperature differential

Figure 9. Simulation results at the begininning of the steady state in third version

TMD.

Figure 10. Summary of experimental results obtained using third version TMD

with 12x12 TEC.


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between the two sides of the

system, the temperature of the

permeate, and the flow rate across

the membranes with respect to the

orientation of the TEC and its

power. The first subset of

experiments was conducted with

the cold plate of the TEC facing

the membrane, while the second

subset was conducted with the hot

plate of the TEC facing the

membrane. All four variables

increase with power and the

highest flow rate is achieved in

the former case. Figure 11 shows

the results of the flow simulation

analysis conducted to compare the

temperature profiles inside the

retentate chamber and at the

membrane surface during the two

subset of experiments described

above in a 1g or “normal gravity”

environment. The temperature

gradient at the membrane surface

is higher when the cold plate of

the TEC faces the membrane and this corresponds to a higher flow rate according to experimental data. The membrane

constitutes a barrier to convective flows and to thermal conduction between the retentate and the permeate. Thus, a

higher temperature gradiend and lower average temperature along the surface of the membrane also means a lower

temperature of the permeate and a higher temperature gradient across the membrane itself, which explains the higher

flow rates.

A fourth-generation system was developed to automate the system control and data collection process by

integrating low-pressure transducers on both sides of the apparatus, using clear 1 ½” PVC tees instead of elbows to

integrate the transducers and to achieve a direct view of the surface of the membrane, not subject to diffraction. This

new configuration allowed

a continuous, more precise,

and automated reading of

the static pressure of the

liquid columns using

National Instruments (NI)

data acquisition (DAQ)

hardware with LabVIEW

software. The data from the

LabVIEW program is used

to analyze the pressure and

its correlation from the

temperature readings. Any

pressure variation also

corresponded to a variation

in volume across the TEC,

which was calculated in the

LabVIEW program.

Moreover, the input voltage

and amperage provided to

the TEC is automatically

adjusted to maintain the

desired temperature or

Figure 11. Simulation results of temperature profiles inside the retentate

chamber ans a the membrane surface.

Figure 12. Diagram of last version of static TMD testing apparatus.


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temperature differential between the two sides of the

system. The diagram of this new configuration is

shown in Figure 12 and the system is shown in Figure

13. However, this design had the disadvantage of a

much longer transient phase due to the bigger volume

of the retentate and permeate fluid reservoirs, which

is almost four times higher compared to the ones in

the third-generation TMD.

IV. TMD Engineering Design Improvement Concepts

The observations and experimental results collected with the prototype testing apparatus described above and the

flow simulation analysis conducted to investigate the behavior of different parameters of the system were used to

develop new system configurations and new engineering concepts that would maximise the performances of TMD. A

design that places heating and cooling elements in the vicinity of

the membrane within the diffusion layer reduces thermal losses

when compared to larger system and reduces the need for

turbulent flow to supply heat of vaporization and liquifaction.

Smaller retentate and permeate fluid reservoirs decrease the

transient time and heat losses.

Figure 14 shows the design of system derived from the third-

generation TMD testing apparatus characterized by thinner fluid

reservoirs that integrates the TEC into the membrane itself.

Conductive meshes are attached to the ceramic plates of the TEC

on both the retentate and permeate sides to distribute heat transfer

from the plates to the liquid with more homogeneous temperature

within the reservoirs. This configuration would be more

representative of the conditions found in microgravity. The heat

flux provided by the TEC must be limited by the transport rate

across the membrane since the water cannot reach the point of

boiling. The maximum temperature differential achieved between

the permeate and retentate in this configuration is 63 kelvins (K),

higher compared to values from both experimental and simulation

data of the existing prototypes, while the average temperature

differential across the membrane is 1.56 K with a standard

deviation of 1.88 K. The temperature profile along the vertical

axis of the membrane, centered with respect to the TEC, is shown

in the graph in Figure 15. For comparison, the graph in Figure 16

shows the values for the third generation testing apparatus.

Figure 14. Engineering concept design

improvement.

Figure 13. Fourth generation TMD testing apparatus.


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Figure 16. Temperature profiles along the vertical centerlines on the two sides of the membrane

for the concept design.

Figure 15. Temperature profiles along the vertical centerlines on the two sides of the membrane for the

third generation TMD.


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V. Conclusion

The initial proof-of-concept tests have generated promise for TMD as a new means for membrane distillation.

TMD uses water selective hydrophobic membranes to maintain phase separation, and embeds thermoelectric heat

pumps at the membrane surface. This arrangement allows for a complete, self-contained wastewater distillation system

that has the advantage of eliminating bulky external support systems such as heating and cooling systems, pumps,

blowers, and recuperative heat exchangers. For these reasons, TMD is especially applicable for space missions where

volume and mass are at a premium and where insufficient gravity exists to adequately manage liquid-vapor interfaces.

Proof-of-concept has been validated through experimental demonstration and flow simulation analysis. New

engineering design concepts have been developed and investigated to increase system performances. Future steps are

experimental testing with a representative waste solution, testing of a continuous flow system and engineering designs

that include electrical circuitry printed on membranes and different TEC distribution and size relative to membrane

open area.

References

1 Shaw, H., Flynn, M., Wisniewski, R., “Evaluation of Brine Processing Technologies for Spacecraft Wastewater,” 45th

International Conference on Environmental Systems 12-16 July 2015, Bellevue, WA, ICES-2015-202.

2 Flynn, M., Gormly, S., et al., “Development of the Direct Osmotic Concentration System”, Proceedings of the 40th

International Conference on Environmental Systems, AIAA 2010-6098, 2010.

3 Thibaud-Erkey, C., Fort, J. H., and Edeen, M. A., “A New Membrane for the Thermoelectric Integrated Membrane

Evaporative Subsystem (TIMES),” 2000-01-2385, 30th International Conference on Environmental Systems, 2000.

4 Delzeit, L., Fisher, J. W., Pace, G., and Shaw, H., “Brine Evaporation Bag Design Concept and Initial Test Results,” AIAA

2012-3525, 42nd International Conference on Environmental Systems, 19 July 2012, San Diego, California.

5 Kelsey, L. K., Straus, J., and MacCallum, T., “Employing ionomer-based membrane pair technology to extract water from

brine,” AIAA 2012-3527, 42nd International Conference on Environmental Systems, 15 - 19 July 2012, San Diego, California.

6 Kelsey, L. K., Padsadilla, P., and MacCallum, T., “Development of Ionomer-membrane Water Processor (IWP) Technology

for Water Recovery from Urine,” 44th International Conference on Environmental Systems 13-17 July 2014, Tucson, AZ, ICES-

2014-269.

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