Showing papers by "Todd Otanicar published in 2011"

Nanofluid optical property characterization: towards efficient direct absorption solar collectors.

Abstract: Suspensions of nanoparticles (i.e., particles with diameters < 100 nm) in liquids, termed nanofluids, show remarkable thermal and optical property changes from the base liquid at low particle loadings. Recent studies also indicate that selected nanofluids may improve the efficiency of direct absorption solar thermal collectors. To determine the effectiveness of nanofluids in solar applications, their ability to convert light energy to thermal energy must be known. That is, their absorption of the solar spectrum must be established. Accordingly, this study compares model predictions to spectroscopic measurements of extinction coefficients over wavelengths that are important for solar energy (0.25 to 2.5 μm). A simple addition of the base fluid and nanoparticle extinction coefficients is applied as an approximation of the effective nanofluid extinction coefficient. Comparisons with measured extinction coefficients reveal that the approximation works well with water-based nanofluids containing graphite nanoparticles but less well with metallic nanoparticles and/or oil-based fluids. For the materials used in this study, over 95% of incoming sunlight can be absorbed (in a nanofluid thickness ≥10 cm) with extremely low nanoparticle volume fractions - less than 1 × 10-5, or 10 parts per million. Thus, nanofluids could be used to absorb sunlight with a negligible amount of viscosity and/or density (read: pumping power) increase.

Applicability of nanofluids in high flux solar collectors

Abstract: Concentrated solar energy has become the input for an increasing number of experimental and commercial thermal systems over the past 10–15 years [M. Thirugnanasambandam et al., Renewable Sustainable Energy Rev. 14 (2010)]. Recent papers have indicated that the addition of nanoparticles to conventional working fluids (i.e., nanofluids) can improve heat transfer and solar collection [H. Tyagi et al., J. Sol. Energy Eng. 131, 4 (2009); P. E. Phelan et al., Annu. Rev. Heat Transfer 14 (2005)]. This work indicates that power tower solar collectors could benefit from the potential efficiency improvements that arise from using a nanofluid working fluid. A notional design of this type of nanofluid receiver is presented. Using this design, we show a theoretical nanofluid enhancement in efficiency of up to 10% as compared to surface-based collectors when solar concentration ratios are in the range of 100–1000. Furthermore, our analysis shows that graphite nanofluids with volume fractions on the order of 0.001% or l...

Band-Gap Tuned Direct Absorption for a Hybrid Concentrating Solar Photovoltaic/Thermal System

Abstract: Two methods often proposed for harnessing renewable energy, photovoltaics and solar thermal, both utilize the power of the sun. Each of these systems independently presets unique engineering challenges but when coupled together the challenge intensifies due to competing operating requirements. Recent research has demonstrated these hybrid systems for low-temperature applications but there exists limited studies at higher concentration ratios, and thus higher temperatures. What these studies have shown is that keeping the photovoltaic (PV) cell temperature low keeps the overall system efficiency relatively high but results in low efficiencies from the thermal system. This study presents a unique design strategy for a hybrid PV/thermal system that only has mild thermal coupling which can lead to enhanced efficiency. By creating a fluid filter that absorbs energy directly in the fluid below the band-gap and a PV cell with an active cooling strategy combined effrciencies greater than 38% can be achieved.

Spatially Varying Extinction Coefficient for Direct Absorption Solar Thermal Collector Optimization

Abstract: Direct absorption solar thermal collectors have recently been shown to be a promising technology for photothermal energy conversion but many parameters affecting the overall performance of such systems have not been studied in depth, yet alone optimized. Earlier work has shown that the overall magnitude of the extinction coefficient can play a drastic role, with too high of an extinction coefficient actually reducing the efficiency. This study investigates how the extinction coefficient impacts the collector efficiency and how it can be tuned spatially to optimize the efficiency, and why this presents a unique design over conventional solar thermal collection systems. Three specific extinction profiles are investigated: uniform, linearly increasing, and exponentially increasing with the exponentially increasing profile demonstrating the largest efficiency improvement.

Characterization of a Nanofluid Volumetric Solar Absorber / Steam Generator

Abstract: Solar thermal energy has shown remarkable growth in recent years — incorporating many new technologies into new applications [1]. Nanofluids — suspensions of nanoparticles in conventional fluids — have shown promise to make efficient volumetric-absorption solar collectors [2–4]. It has also been shown that concentrated light energy can efficiently cause localized phase change in a nanofluid [5]. These findings indicate that it may be advantageous to create a ‘direct, volumetric nanofluid steam generator’. That is, a solar collector design which could minimize the number of energy transfer steps, and thus minimize losses in converting sunlight (via thermal energy) to electricity. To study this, we use a testing apparatus where concentrated laser light at 532 nm — a wavelength very near the solar spectrum peak — is incident on a highly absorbing sample. The highly absorbing samples compared in this study are black dyes, black painted surfaces, and silver nanofluids — with de-ionized water as a base fluid. Each of these samples converts light energy to heat — to varying degrees — in a localized region. This region is monitored simultaneously with a digital camera and an infrared camera. The resulting observed temperature profile and bubble dynamics are compared for these fluids. For pure water with a black backing, some very high temperatures (>300 °C) are observed with a laser input of ∼75 W/cm2 . Using a similar absorption potential, we observed higher temperatures in the nanofluids when compared to black dyes. A simplified boiling heat transfer analysis based on these results is also presented. We also noticed differences in bubble size and growth rates for the different samples. Overall, this study represents a proof-of-concept test for a novel volumetric, direct steam generator. These results of this test indicate that it may be possible to efficiently generate steam directly in a controlled, localized volume — i.e. without heating up passive system components.© 2011 ASME

Enhancing the Heat Transfer in Energy Systems From a Volumetric Approach

Abstract: Many energy systems rely on flat surfaces for energy conversion. The simplest example is the solar thermal collector which absorbs solar irradiance on a flat plate and then transfers heat via conduction, convection and radiation to both the surroundings and more importantly to the working fluid. Conversely a night-sky radiator tries to lose heat via convection and radiation to ambient and night sky respectively while being coupled either via convection or conduction to higher temperature system. The recent advent of nanoparticles, particularly liquid-nanoparticle suspensions termed nanofluids, have led to novel systems that can reduce some of these heat transfer steps by utilizing the whole fluid volume directly. This study looks at the advantages afforded by using the volumetric approach on both the radiative properties of the system and the simplification of the heat transfer networks within these systems.Copyright © 2011 by ASME

Comparative Economic Analysis of Concentrating Solar Technologies

Abstract: One of the noted benefits of concentrating photovoltaics (PV) is the reduced cell area which results in reduction of the overall system cost. A variety of studies have looked at the cost for concentrating PV systems and made comparisons to concentrating solar thermal power plants, typically resulting in concentrating solar thermal power having lower system costs. Recently a widespread design space was assessed for the potential efficiency improvements possible with a coupled hybrid PV/thermal solar energy system for electricity generation. The analysis showed that modest efficiency improvements could be made but no assessment of the economic impact was made. Although modest efficiency gains can be made such a hybrid system requires more components than one of the conventional stand alone concentrating solar power plant on its own resulting in significantly different system costs. As a result we look to compare the overall system costs of three different solar power technologies: concentrating PV, concentrating solar thermal, and the concentrating hybrid approach. Additionally we will focus on documenting the necessary hybrid efficiencies to make a hybrid system competitive as well as the feasibility and means for achieving these efficiencies.Copyright © 2011 by ASME

Band-Gap Tuned Direct Absorption for Hybrid Concentrating Solar Photovoltaic/Thermal System

Abstract: Two methods often proposed for harnessing renewable energy, photovoltaics and solar thermal, both utilize the power of the sun. Each of these systems independently present unique engineering challenges but when coupled together the challenge intensifies due to competing operating requirements. Recent research has demonstrated these hybrid systems for low-temperature applications but there exists limited studies at higher concentration ratios, and thus higher temperatures. What these studies have shown is that keeping the PV cell temperature low keeps the overall system efficiency relatively high but results in low efficiencies from the thermal system. This study presents a unique design strategy for a hybrid PV/thermal system that only has mild thermal coupling which can lead to enhanced efficiency. By creating a fluid filter that absorbs energy directly in the fluid below the band-gap and a PV cell with a passive cooling strategy combined efficiencies greater than 38% can be achieved.© 2011 ASME

Nanofluid Extinction Coefficients for Photothermal Energy Conversion

Abstract: Suspensions of nanoparticles in liquids (i.e. nanofluids) have been shown to dramatically affect thermal and optical properties of the base liquid at low particle loadings [1–3]. Recent studies by the co-authors have indicated that selected nanofluids are promising as solar energy harvesters [4,5]. In order to determine the effectiveness of nanofluids in solar applications, their ability to convert light energy to thermal energy must be known. That is, the extinction coefficient of real nanofluids must be established. Although it is relatively straight-forward to model these properties from knowledge of bulk properties, with the help of some simplifying assumptions, real spectroscopy tests do not always match these calculations. This study compares model predictions of extinction coefficients to spectroscopic measurements. Unfortunately, the models and the optical testing data do not show very good agreement. Some possible reasons for this are discussed. Also, some simple experiments are presented to investigate the extent of scattering in nanoparticle suspensions. As alluded to above, all of these tests are conducted on nanofluid compositions which are considered to be suitable for solar thermal collectors.Copyright © 2011 by ASME

Using nanofluids to enhance the operation of solar energy systems

Abstract: Nanofluids have garnered a lot of attention for their potential to modify thermal properties, particularly the thermal conductivity. While these modifications have been heavily investigated within the heat transfer and materials community, nanofluids additionally offer the potential for major modification to the radiative properties of the host fluid. Of particular interest here is how the modification of the radiative properties of fluids through nanoparticle dispersions can lead to enhanced energy conversion in systems dependent on radiative transport. By adjusting the size, shape, material, volume fraction and particle structure drastic modification can be achieved leading to enhanced efficiency within systems such as solar collectors, and night sky radiators. A summary review of the mechanisms that lead to enhanced efficiency are presented as well as new topics such as dynamic radiative property control with coreshell multifunctional nanoparticles.