PREDICTION OF POLYMER TUBE LIFETIME IN POTABLE WATER

William Camisa, Susan Mantell, Jane Davidson, Gyanender Singh

Mechanical Engineering University of Minnesota

H2O

Cool

Heat

  Water and space heating represent 10% of total U.S. energy consumption.

  Cost is perceived to be a major barrier to greater implementation

Space Htg Space Clg Water Htg Lighting Refrigeration Electronics Other

The Big 3-Thermal loads: 55% of total, 2.34 TWh

Q tot,avg ~ 200 GJ primary

 Reduce weight shipping, handling and installation costs

 Integrated manufacturing  Greater design flexibility

Polypropylene absorber, marketed through Home Depot

Why Polymeric Collectors?

Polymer Materials: Cost vs. Performance

Low cost polymers have low service temperature High performance polymers: higher operating temps, highest cost

Thin-walled polymer vessel/absorber

Cold Water Hot Water

Polymer Heat Exchanger

Glazed All Polymeric Collector

Low Cost Polymeric Collectors: Challenges and Solutions

Challenges

Polymer materials are poor conductors

Heat exchanger/absorber design guidelines

Long term performance f(T, p, env)

Characterize lifetime for low cost polymers

Suitable for all climates Glazed collectors with “smart” thermotropic coating

Solutions

Study of Polymer Degradation in Potable Water

  Demonstrate a correlation between antioxidant (AO) level with mechanical performance

  Predict level of antioxidant (AO) in PE samples: including relationship of Cl environment, flow conditions

Polymer tube Potable water (Chlorine)

Develop model for AO diffusion and Cl migration

Evaluate properties (strain at failure, molecular weight) of exposed samples

Establish relationship between AO and Cl content and property degradation

Expose samples in (1) reverse osmosis water (RO) bath and (2) Cl (5ppm) water bath

Approach includes both Theory and Experiments

Predict lifetime under typical SDHW conditions

Water Bath for Exposing Samples to Potable Water Conditions

Nominal Test Conditions Temperature

(°C) ORP (mV)

60 550

60 825

80 550

80 825

Test Duration Up to 1,100-1500 hours at 825 mV and 80°C (5 ppm Cl, 7 pH)

Materials PB, PSU, PA6,6, PP-r, PE

PE samples 0.3 or 2 mm thick with phenolic AO

AO Loss vs. Mechanical Properties

Loss of mechanical properties begins after antioxidant is depleted

l = 7.6 cm (3 in.)

h = 0.2 cm (0.08 in.)

w = 0.64 cm (0.25 in.)

w = 1.3 cm (0.5 in.)

0.3 mm thick PE with phenolic AO 2-3 days

Antioxidant Depletion will Vary through the Thickness

Con

cent

ratio

n, C

A

thickness

-H/2 +H/2

t=0

Increasing time

AO Loss in a “plate” sample exposed to potable water H

AO Depletion and Cl Migration Models

Initial condition: CA=CA0 at t=0

Boundary conditions at the surface:

Flux at boundary

Mass transfer at surface of plate to surrounding fluid

storage diffusion through thickness reaction

Model: Diffusion and chemical consumption

AO depletion

Cl migration

Unknown parameters: DA, DCl, K, kA,cl

small Equation will change to polar coordinates when considering a tube, BC’s modified as well

2mm thick dogbones

Sliced (after exposure) to generate AO concentration profiles

Evaluation of Diffusion and Reaction Constants

l = 7.6 cm (3 in.)

h = 0.2 cm (0.08 in.)

w = 0.64 cm (0.25 in.)

w = 1.3 cm (0.5 in.)

1.5 cm

0.51 cm 3.81 cm

300 µm thick Plaques

Curve fit to find DA, DCl, K, kA,cl

Validation

Data in RO and Chlorinated Water (thick samples)

RO water

Cl water

Fit to find parameters: DA, DCl, K, kA,cl

Model Validation (thin samples)

Model prediction using constants obtained from curve fit to dogbone data (previous slide)

0% 20% 40% 60% 80%

100%

0 200 400 600 800 Time (hours)

OIT

(%)

500 ppm Cl Cl model 500 ppm RO RO model

0% 20% 40% 60% 80%

100%

0 200 400 600 800 Time (hours)

OIT

(%)

1000 ppm Cl Cl model 1000 ppm RO RO model

Includes AO depletion, Cl migration and Cl reaction

Excellent agreement between model prediction and Cl data

Depletion in RO f(AOi)

Relative Importance of Loss Mechanisms are Determined from Characteristic Time Scales

DATA obtained at 80°C

Diffusion of Cl “in” is approximately 4 times faster than Diffusion of AO “out”

Use Model to Predict AO Loss in Tubes

AO

Con

cent

ratio

n, c

_a

radius

ri ro

Increasing time

t=0

Low concentration Cl or Air

ri

ro

2-3 ppm Cl

Tube Lifetime for typical Solar Applications

Reaction and Diffusion rates will be temperature dependent 50-60°C

Typical tube thickness 0.5-1 mm

“Lifetime” at 10 ppm AO remaining

Conclusions and Future Work

•  Model of Antioxidant depletion is a reasonable approach to estimating polyolefin lifetime

•  Reaction with chlorine is the dominant loss mechanism

•  Consider alternate failure criteria that combines AO loss and crack propagation

AO depletion

Pressure

Current Work

Polymer Collectors of the Future   Low cost polymers   Light weight collectors with snap together fittings   Easily replaceable, roll up glazing  Materials innovations: Smart Polymer Coatings  Available to the Do-it-yourself Market