Heat Transfer in Concentrated Solar Power Systems

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Heat Transfer in Concentrated Solar Power Systems

• By Nick Smith

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Introduction

• Concentrating Solar Power Systems (CSPs) focus a
  large area of sunlight onto a small area. Unlike
  photovoltaic systems, the sun's energy is not
  directly converted to electricity. Instead it is
  used to heat a fluid to steam, which in turn
  powers a turbine.

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Introduction (cont.)

• Parabolic Solar Trough Power Plant in Israel

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Schematic Drawings
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Objective

• Use a simplified Heat Transfer Approach to find
  the optimum fluid, and length of the parabolic
  trough.

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Assumptions

• q is evenly distributed across the tubing
• Fluid enters tube at ambient temperature, 20C
• Effects of the tubing material are negligible
• To function properly, the average fluid
  temperature at exit should be 400C

• Radiation from the sun can be approximated
  (normalizing for the earth's surface area) as 680
  W/m2
• Parabola perfectly reflects energy and can be
  approximated as a semi-circle
• Flow-rate is 0.2 kg/s

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Approach

• The heat flux on the tube is equal to the solar
  heat flux over a semi-circle of 2.5m diameter

 
 
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Calculations

• A simple energy balance equation will now give us
  the ideal length for each potential fluid

• Lengths of various materials, using average Cp
• Water 130 m
• Oil 71 m
• Mercury 3.85 m

 
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Conclusions

• While Mercury would be the ideal material, it is
  hazardous and expensive, so oil should be used,
  as it is almost twice as efficient as water. More
  precise calculations should be done using
  different types of oil to find the most ideal
  material, perhaps a mixture with mercury would
  provide the best properties.

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Appendix

• A more complete of heat transfer in CSP systems
  is available here
• http//pointfocus.com/images/pdfs/saltw-troughs.pd
  f

• Pictures and drawings are courtesy of Andrew
  Buck, modified by myself