The performance of nanofluidbased volumetric solar receivers: Computational and experimental analyses

  • Luqmaan Habib

Student thesis: Master's Thesis

Abstract

Conventional tubular solar receivers, such as those found in parabolic trough concentrated solar power plants, rely on a selective-surface absorbing solar radiation, thereby heating the tube. This thermal energy is transferred to a heat transfer fluid (HTF) contained within. Conceptual volumetric receivers would permit the entry of solar radiation through a transparent enclosure enabling absorption to occur directly within the working fluid. Certain nanofluids, suspensions of nanoparticles in a fluid medium, are possible candidates for the working fluid in volumetric receivers having displayed significant light attenuation capabilities. Despite this, the use of these systems in real-world applications is not well known. This study focuses on the performance of nanofluid-based volumetric solar receivers, with particular attention on direct comparisons between volumetric and selective-surface absorbing systems. Initial models revealed parameters (thermally dependent thermophysical properties, fluid refractive index, and thermal emission at elevated temperatures) that are important in theoretical analyses. Direct comparisons with selective-surface absorbing systems over a realistic receiver length of 450m and HTF flow rate of 8 kg/s were performed using ANSYSR Fluent. At steadystate, the temperature gain of the volumetric receiver exceeds that of a conventional selectivesurface receiver by no more than 3K. The lower surface temperatures inherent in volumetric receivers reduce convective losses. On the other hand, the glass tube emissivity being approximately ten fold larger than that of a selective-surface promotes radiative losses. Hypothetically, low-emissivity glass coatings would improve the performance, with radiative losses being reduced by approximately 75%. However, the reduced solar transmissivity presented by these coatings results in lower fluid temperatures, as compared to a volumetric receiver with uncoated glass. Interestingly, the transient analyses indicate that volumetric receivers reach steady-state conditions significantly faster than selective-surface receivers. Experimental configurations of stationary as well as flowing media were developed, both for surface and volumetric receiver systems. Besides confirming the transient numerical result mentioned previously, both types of volumetric receiver configurations indicated promise, with a pure water-based system matching the performance of its selective-surface counterpart. One may safely assume that an appropriately prepared light-absorbing nanofluid would improve this performance. Additionally, the numerical model of the stationary setups agreed with the experimental results, within the bounds of experimental uncertainty. The quantitative results presented in this study, in addition to clarifying the performance of nanofluid-based volumetric solar receivers, would assist in determining the most efficient and economical method of solar energy harvesting.
Date of AwardMay 2015
Original languageAmerican English
SupervisorYoussef Shatilla (Supervisor)

Keywords

  • Nanofluid Receivers
  • Solar Receivers
  • Thermal Energy
  • Heat Transfer
  • Solar Radiation
  • Nanoparticles
  • Nanofluids
  • Thermosphysical properties
  • Thermal Emission
  • Volumetric Receivers
  • Solar Energy Harvest.

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