Experimental and Numerical Evaluation of Solar Water Heaters with Vertical Mantle Heat Exchangers

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Experimental and Numerical Evaluation of Solar
Water Heaters with Vertical Mantle Heat
Exchangers
School of Mechanical and Manufacturing
Engineering University of New South Wales

• Yen Chean Soo Too
• Graham L. Morrison
• Masud Behnia

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Vertical Mantle Heat Exchanger
Mantle heat exchanger
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Vertical Mantle Heat Exchanger

• wider gap mantle
• (Shah, 1999 Knudsen, 2004)
• 33.5 mm gap
• natural convection

• narrow gap mantle
• (in Australia)
• approx. 3.5 mm gap
• forced convection

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Project Objective

• Overall research
• - evaluating the performance of
  pumped-circulation solar water heaters with
  vertical mantle heat exchangers
• - investigating a range of mantle designs

• Preliminary stage narrow gap mantle
• - outdoor and laboratory experiments
• - TRNSYS modelling
• Narrow gap mantle analysis will be presented.

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Experimental Approach
Collectors
Outdoor test
Controlled indoor test
Temperature controlled water source
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Measurements

• in charging mode

Outdoor
1.8 L/min
Tm,i 65C
Tm,i 25C
Tm,i 59C
2 L/min
Indoor
Tm,i 52C
Tm,i 60C
Tm,i 23C
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Overall Heat Transfer Coefficient, U
based on log-mean temperature difference, ?TLM
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Heat Transfer Correlation (mantle side)
mean Num correlation for 3.5 mm gap mantle is
based on - hydraulic diameter, Dh 2w
Num 4.86 0.00054 ReDh 1.32 Pr 0.27
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Heat Transfer Correlation (mantle side)

• measured mean Num data lies within 10 of the
  predicted values

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Heat Transfer Correlation (tank side)
mean Nut correlation for the tank side is
based on - tank height, H
Nut 0.0041 RaH 0.453
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Heat Transfer Correlation (tank side)

• measured mean Nut data lies within 10 of the
  predicted values

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One-dimensional Heat Transfer Analysis
Modified Type 60 stratified tank routine in
TRNSYS model

• Mantle side
• Num 4.86 0.00054 ReDh1.32 Pr0.27
• hm Numkm / Dh

• Tank side
• Nut 0.0041 RaH0.453
• ht Nutkt / H

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Modified TYPE 60
TRNSYS Type 60 - stratified tank with a heat
exchanger coil
Modified Type 60 - mantle heat exchanger model
using experimental correlations
Modified Type 60 code
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TRNSYS Modelling
Mantle heat exchanger system or direct-coupled
tank system
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Predicted System Performance

• Annual solar contribution (in Sydney)
  Australian Standard AS4234
• 79 for direct system
• 72 for system with mantle heat exchanger (MHE)
• loss of performance is outweighed by the benefit
  of freeze protection

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Further Investigations
Forced convection
Natural convection
Transition
Approx. 3.5 mm narrow gap mantle (in this study)
33.5 mm wide gap mantle (Shah, 1999 Knudsen,
2004)
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Further Investigations
narrow gap mantle
wider gap mantle