Direct Absorption Solar Thermal Collectors Research Articles

Hybrid Plasma-Liquid Treatment of Carbon Nanotubes for Application in Direct Absorption Solar Thermal Collectors

Covalent functionalization of carbon nanotubes (CNTs) with oxygen-based or nitrogen-based functional groups is one of the most popular techniques to modify the surface energy and inherent properties of the CNTs.1 This functionalization generates interest for a broad range of applications including mechanically superior advanced composites,2 more sensitive gas sensors,3 enhanced energy storage materials4 or ultra-conductive fibers.5 As an alternative to the harsh chemicals used by traditional chemical functionalization, plasma-based surface treatments can be used with great efficacy and reduced reaction times, providing an array of application-specific benefits.Awareness of the environmental pollution caused by fossil fuels and the ever-increasing demand for energy has driven research focussed on zero-carbon renewable energy sources. Conscious of this, direct absorption solar thermal collectors are gaining prominence, both for domestic heating and steam generation. CNTs have been investigated as an additive to take advantage of the near blackbody absorption and high thermal conductivity to improve system efficiency. Critical to the success of the additive nanoparticles is the stability in the fluid, however, due to their high surface energy CNTs tend to agglomerate which can limit the efficacy of their integration into real-world products.In this work, macroscopic ribbon-like assemblies of carbon nanotubes are functionalized by plasma-induced non-equilibrium electrochemistry (PiNE) implemented through a simple direct current-based plasma-liquid system. This system utilizes the plasma-generated species in an electrolyte of 10 %vol ethanol in water, with or without a nitrogen precursor, for the oxygen and/or nitrogen functionalization of the carbon nanotube assembly. For this treatment, a ribbon-like piece of CNT was used as the anode and a helium plasma discharge triggered by applying 10 mA and 1 - 1.6 kV to act as the cathode. The plasma-generated species are then expected to migrate towards the CNTs and functionalize the sidewalls.The oxygen content is shown to be increased by between 70 % and 300 % when the treatment solution of 10 %vol ethanol is used in the ribbon electrode configuration. When ethylenediamine is added as a nitrogen precursor, the atomic concentration of nitrogen reaches 23 %, with amine groups, pyrrolic groups and graphitic nitrogen observed in the x-ray photoelectron spectra. This nitrogen content is unmatched in quantity when compared to either a simple soaking procedure or an electrolysis process with a platinum foil replacing the plasma as the counter-electrode. This demonstrates that the plasma either directly or indirectly, through plasma-generated species, facilitates and enhances the availability of nitrogen from the ethylenediamine precursor. The potential plasma-induced chemical pathways which lead to the functionalization of the CNTs are also investigated and discussed.In the application of a direct absorption solar collector system, it is found that the covalent functionalization provided by the plasma-liquid system enhances the stability of the CNT-ethylene glycol nanofluid, with the oxygen-functionalized samples producing the most stable nanofluids. This is explained through the lower value for contact angle measurements, suggesting greater hydrophobicity of the CNTs.References(1) Mallakpour, S.; Soltanian, S. Surface Functionalization of Carbon Nanotubes: Fabrication and Applications. RSC Adv. 2016, 6 (111), 109916–109935. https://doi.org/10.1039/c6ra24522f.(2) Williams, J.; Broughton, W.; Koukoulas, T.; Rahatekar, S. S. Plasma Treatment as a Method for Functionalising and Improving Dispersion of Carbon Nanotubes in Epoxy Resins. J. Mater. Sci. 2013, 48 (3), 1005–1013. https://doi.org/10.1007/s10853-012-6830-3.(3) Ham, S. W.; Hong, H. P.; Kim, J. H.; Min, S. J.; Min, N. K. Effect of Oxygen Plasma Treatment on Carbon Nanotube-Based Sensors. J. Nanosci. Nanotechnol. 2014, 14 (11), 8476–8481. https://doi.org/10.1166/jnn.2014.10007.(4) Hulicova-Jurcakova, D.; Seredych, M.; Lu, G. Q.; Bandosz, T. J. Combined Effect of Nitrogen- and Oxygen-Containing Functional Groups of Microporous Activated Carbon on Its Electrochemical Performance in Supercapacitors. Adv. Funct. Mater. 2009, 19 (3), 438–447. https://doi.org/10.1002/adfm.200801236.(5) Li, L.; Liu, E.; Shen, H.; Yang, Y.; Huang, Z.; Xiang, X.; Tian, Y. Charge Storage Performance of Doped Carbons Prepared from Polyaniline for Supercapacitors. J. Solid State Electrochem. 2011, 15 (1), 175–182. https://doi.org/10.1007/s10008-010-1087-8. Figure 1

Zirconium diboride-based nanofluids for solar energy applications

Direct absorption solar collector (DASC) are being considered of great interest in combination with colloidal systems consisting of nanoparticles dispersed in suitable liquids, also called nanofluids. Nanofluids are able to carry out, at the same time, the double function of volumetric solar absorbers and heat transfer media. In this work, novel ceramic nanoparticles of zirconium diboride (ZrB2) have been synthesized, dispersed in ethanol and characterized from points of view of microstructure, stability and optical properties. ZrB2 is representative of the family of Ultra-High Temperature Ceramics (UHTCs). Some of their peculiar characteristics, in particular ultra-refractoriness, high-temperature stability and promising bulk optical properties favourably compared bulk UHTCs to conventional solid solar absorbers used in Concentrated Solar Power to date. The present work, for the first time, shows the optical spectra of ZrB2 nanoparticles. We prove the appealing optical properties of ZrB2 nanofluids for direct-absorption solar thermal collectors. This opens a new application field for UHTCs.

Hybrid Plasma-Liquid Treatment of Carbon Nanotubes for Application in Direct Absorption Solar Thermal Collectors

Awareness of the environmental pollution caused by fossil fuels and the ever-increasing demand for energy has driven research focussed on zero-carbon renewable energy sources. Conscious of this, direct absorption solar thermal collectors are gaining prominence, both for domestic heating and steam generation. Carbon nanotubes (CNTs) have been investigated as an additive to take advantage of the near blackbody absorption and high thermal conductivity to improve system efficiency. Critical to the success of the additive nanoparticles is the stability in the fluid, however, due to their high surface energy CNTs tend to agglomerate which can limit the efficacy of their integration into real-world products. Covalent functionalization of the CNT sidewalls with oxygen-based or nitrogen-based functional groups is one of the most popular techniques to modify the surface energy of the CNTs.1 As an alternative to the harsh chemicals used by traditional chemical functionalisation, plasma-based surface treatments can be used with great efficacy and reduced reaction times.In this work, macroscopic ribbon-like assemblies of carbon nanotubes are functionalized using a simple direct current-based plasma-liquid system. This system utilizes the plasma-generated species in a fluid of 10 %vol ethanol in water, with or without a nitrogen precursor, for the oxygen and/or nitrogen functionalisation of the carbon nanotube assembly. For this treatment, a ribbon-like piece of CNT was used as the anode and a helium plasma discharge triggered by applying 10 mA and 1 - 1.6 kV to act as the cathode. The plasma-generated species are then expected to migrate towards the CNTs and functionalisation the sidewalls.The oxygen content is shown to be increased by between 50 % and 200 % when the treatment solution of 10 %vol ethanol is used in the ribbon electrode configuration. When ethylenediamine is added as a nitrogen precursor, the atomic concentration of nitrogen reaches 23 % in the ribbon electrode configuration, with amine groups, pyrrolic groups and graphitic nitrogen observed in the x-ray photoelectron spectra. This nitrogen content is unmatched in quantity when compared to either a simple soaking procedure or an electrolysis process with a platinum foil replacing the plasma as the counter-electrode. This demonstrates that the plasma either directly or indirectly, by means of plasma-generated species, facilitates and enhances the availability of nitrogen from the ethylenediamine precursor. The potential plasma-induced chemical pathways which lead to the functionalization of the CNTs are also investigated and discussed.In the application of a DASC system, it is found that the covalent functionalisation provided by the plasma-liquid system enhances the stability of the CNT-EG nanofluid, with the oxygen-functionalized samples producing the most stable nanofluids. This functionalization method generates interest for a broader range of applications including mechanically superior advanced composites,2 more sensitive gas sensors,3 enhanced energy storage materials4 or ultra-conductive fibres.5 References (1) Mallakpour, S.; Soltanian, S. Surface Functionalization of Carbon Nanotubes: Fabrication and Applications. RSC Adv. 2016, 6 (111), 109916–109935.(2) Williams, J.; Broughton, W.; Koukoulas, T.; Rahatekar, S. S. Plasma Treatment as a Method for Functionalising and Improving Dispersion of Carbon Nanotubes in Epoxy Resins. J. Mater. Sci. 2013, 48 (3), 1005–1013.(3) Ham, S. W.; Hong, H. P.; Kim, J. H.; Min, S. J.; Min, N. K. Effect of Oxygen Plasma Treatment on Carbon Nanotube-Based Sensors. J. Nanosci. Nanotechnol. 2014, 14 (11), 8476–8481.(4) Hulicova-Jurcakova, D.; Seredych, M.; Lu, G. Q.; Bandosz, T. J. Combined Effect of Nitrogen- and Oxygen-Containing Functional Groups of Microporous Activated Carbon on Its Electrochemical Performance in Supercapacitors. Adv. Funct. Mater. 2009, 19 (3), 438–447.(5) Li, L.; Liu, E.; Shen, H.; Yang, Y.; Huang, Z.; Xiang, X.; Tian, Y. Charge Storage Performance of Doped Carbons Prepared from Polyaniline for Supercapacitors. J. Solid State Electrochem. 2011, 15 (1), 175–182. Figure 1

Photothermal conversion enhancement of triangular nanosheets for solar energy harvest

Metallic plasmonic nanofluids have been adopted to enhance the photothermal conversion in direct absorption solar thermal collectors, however, their absorption band is not broad enough to realize a sufficient adsorption. Driven by the Local Surface Plasmon Resonance (LSPR) effect, this work explores the enhancement of triangular nanosheets in photothermal conversion, to facilitate the optimization of blended nanofluids. Firstly, the size effects of triangular nanoparticles on photothermal conversion are examined using the Discrete Dipole Approximation method. Then, the blended nanofluids based on silver triangular nanosheets and gold nanorods, are proposed and a high efficiency of 76.9% is achieved experimentally with a very low volume concentration (0.0001%). The enhancement mechanism is revealed furtherly by numerically investigating the excited electric field. We believe that this work can shed light on the design of blended nanofluids based on triangular nanosheets for solar energy harvest.

A review of nanofluid-based direct absorption solar collectors: Design considerations and experiments with hybrid PV/Thermal and direct steam generation collectors

Over the last 100 plus years, solar thermal energy has been used for residential heating applications, industrial process heating, electricity generation, and thermochemical reactions. Because of the vast number of applications, numerous designs have been developed to improve the efficiency of converting incoming solar energy into useful heat and to lower the cost. Conventional solar thermal collectors required a solid surface to absorb and convert incoming solar energy to useful thermal energy. Developments in materials science have enabled a new type of absorber—a volumetric absorber—which utilizes nanoparticles suspended in a fluid to absorb sunlight. Since most working fluids only weakly absorb sunlight, well-engineered ‘nanofluids’ are attractive because only a low volume fraction of nanoparticles is needed to obtain a large shift in the optical properties. This review, on the 10-year anniversary of the first appearance of nanofluid-based direct absorption solar thermal collectors, provides a forward-looking perspective on the challenges and opportunities associated with nanofluids as direct absorbers. Through a critical comparison of design considerations, as well as the most recent experimental results of less well explored areas like hybrid photovoltaic/thermal systems and direct steam generation, this review aims to provide discourse on the next steps for development.

Influence of Six Carbon-Based Nanomaterials on the Rheological Properties of Nanofluids.

Nanofluids, dispersions of nanosized solid particles in liquids, have been conceived as thermally-improved heat transfer fluids from their conception. More recently, they have also been considered as alternative working fluids to improve the performance of direct absorption solar thermal collectors, even at low nanoadditive concentrations. Carbon-based nanomaterials have been breaking ground in both applications as nanoadditives during the last decade due to their high thermal conductivities and the huge transformation of optical properties that their addition involves. In any application field, rheological behavior became a central concern because of its implications in the pumping power consumption. In this work, the rheological behavior of four different loaded dispersions (0.25, 0.50, 1.0, and 2.0 wt%) of six carbon-based nanomaterials (carbon black, two different phase content nanodiamonds, two different purity graphite/diamond mixtures, and sulfonic acid-functionalized graphene nanoplatelets) in ethylene glycol:water mixture 50:50 vol% have been analysed. For this purpose, a rotational rheometer with double cone geometry was employed, which included a special cover to avoid mass losses due to evaporation at elevated temperatures. The flow curves of the twenty-four nanofluids and the base fluid were obtained by varying the shear rate between 1 and 1000 s−1 for seven different temperatures in the range from 283.15 to 353.15 K. The shear-thinning behaviors identified, as well as their dependences on carbon-based nanomaterial, concentration, and temperature, were analyzed. In addition, oscillatory tests were performed for samples with the clearest Non-Newtonian response, varying the deformation from 0.1 to 1000% with constant frequency and temperature. The dependence of the behaviors identified on the employed carbon-based nanomaterial was described.

Solar-Coal Hybrid for Steam Boiler

Solar-coal hybrid is the use of solar and coal energy for producing steam for electricity as opposed to coal alone. The boiler feed-water is pre-heated using solar energy before introduced into the coal boiler, where the steam generating process is finalized for use in the electrical generating turbine. Reviewed is liquid-based direct absorption solar thermal collectors, in which incident sunlight is absorbed directly by the working fluid. Described is the amount of reduction in coal consumption, thus reduction in pollutant.

Carbon nanotube heat transfer fluids for solar radiant heating of buildings

Solar-based radiant heating systems represent a sustainable, and relatively low-cost, technology to raise the temperature of the interior thermal mass of our buildings. Through the use of direct absorption solar thermal collectors, the same working fluid which absorbs the solar energy can be used to transfer the energy for storage in the thermal mass of the structure using a network of pipes embedded in concrete floors. This study investigates a promising working fluid which can be used in such systems – one which is based on multi-walled carbon-nanotubes suspended in normal base fluids. A major stumbling block affecting the wide spread use of carbon-nanotube nanofluids is their low dispersion stability at elevated temperatures, which significantly reduces the absorption capabilities of the nanofluids and could lead to clogging of the pumps used to circulate the fluids. In this paper, we report on a scalable UV-ozone (UVO) treatment technique to produce highly stable dispersions for the elevated temperatures experienced by working fluids in radiant heating systems. To probe suitability of UVO treated multi-walled carbon-nanotube (MWCNTs) for solar-assisted radiant heating systems, this paper investigates the effects of exposure time and temperature on stability, optical absorbance properties, the extent of functionalisation, and the photothermal conversion performance of UVO-treated MWCNT nanofluids. No agglomeration or degradation of the MWCNTs was observed at elevated temperatures (up to 150 °C), highlighting the stability of proposed nanofluids.

Plasmonic nanofluids based on gold nanorods/nanoellipsoids/nanosheets for solar energy harvesting

Due to the localized surface plasmon resonance (LSPR) effect excited on the surface of metallic nanoparticles, plasmonic nanofluids have been used to improve the efficiency of direct absorption solar thermal collectors (DASC) as working fluids. In this study, optical properties of plasmonic nanofluids containing gold nanoparticles with different shapes, sizes, aspect ratios and concentrations are studied numerically. The results show that the LSPR of gold nanorods and nanoellipsoids can be improved by tuning the aspect ratio. Particle size has little effect on extinction coefficient. Nanosheets show a great potential in solar thermal conversion. To achieve a broad-band absorption in both visible and near-infrared spectral region at a low particle concentration, the gold blended nanofluids made up of 20% nanoellipsoids of AR=2, 60% nanorods of AR=5, and 20% nanosheets of l/h=7 are proposed according to the plasmon resonance absorption band. An enhancement in solar energy harvesting of 104% for the gold blended nanofluids is achieved compared to the gold sphere nanoparticles even at an extremely low concentration and small particle size, which overcomes the instability of nanofluids under high concentration.

Optical property of nanofluids with particle agglomeration

Nanofluids have been used to improve the efficiency of direct absorption solar thermal collectors (DASC) as working fluids because of their excellent heat transfer and optical absorption properties. However, aggregates could be generated in nanofluids especially in the two-step preparation. A theoretical approach for calculating the extinction coefficients of nanofluids with particle agglomeration is proposed based on diffusion limited cluster aggregation (DLCA) simulation and generalized multi-particle Mie solution (GMM) method, in which the dependent scattering effect between particles is considered. The influences of particle agglomeration, particle diameter and particle volume fraction on the extinction coefficient of nanofluids with particle agglomeration are investigated. Moreover, the harvested solar energy under the real solar irradiation at sea level for the nanofluids with particle agglomeration is also studied. The results show that the particle agglomeration reduces the absorption peak in the short-wavelength as well as enhances the long-wavelength scattering. The nanofluids with particle agglomeration have a higher performance in solar energy harvest.

Evaluating the Optical Properties of TiO2 Nanofluid for a Direct Absorption Solar Collector

Recent studies specify that designated nanofluids may increase the proficiency of direct absorption solar thermal collectors. To determine the efficiency of nanofluids in solar applications, their capability to change light energy to thermal energy must be identified (i.e., the absorption spectrum of the solar material). In view of that, this study compares model predictions to spectroscopic measurements of extinction coefficients over wavelengths that are important for solar energy (200– 1100 nm). In the first decade of nanofluid research, most of the focus was on measuring and modeling the fundamental thermophysical properties of nanofluids (i.e., thermal conductivity, density, viscosity, and convection coefficients). Lately, considerable focus is given to the fundamental optical properties of nanofluids. However, the effect of particle size, shape, and volume fraction of nanoparticles as well as alternation of the base fluids, which can significantly affect scattering and absorption, have not been addressed to date in the literature. In this study, the effects of size and concentration of TiO2 nanoparticles on the extinction coefficient were analyzed using the Rayleigh approach. The results show that smaller particle size (<20 nm) has a nominal effect on the optical properties of nanofluids. Volume fraction is linearly proportionate to the extinction coefficient. Considering a nanoparticle size of 20 nm, almost 0% transmissivity is obtained for wavelengths ranging from 200 to 300 nm. However, a sudden increase of 71% in transmissivity is noted from 400 nm, gradually increasing to 88% and becoming similar to that of water at 900 nm. Promising results are observed for volume fractions below 0.1%.

High temperature and long-term stability of carbon nanotube nanofluids for direct absorption solar thermal collectors

Stable dispersions (nanofluids) are produced using plasma functionalized multi-walled carbon nanotubes (MWCNTs). To our knowledge, this study presents a first quantitative demonstration of nanofluid stability over extended periods of time (currently tested up to 8months) and after intense heating. No agglomeration is found to occur in the water and glycol-based nanofluids after heating at 85 and 170°C, respectively. Significant agglomeration does occur for suspensions produced using the non-polar Therminol® VP-1 heat transfer fluid. Optical characterization of the nanofluids demonstrates that the MWCNTs are highly absorbing over the majority of the solar spectrum, allowing for close to 100% solar energy absorption, even at low concentrations and small collection volumes. These absorption properties coupled with the stability of the nanofluids make them ideal candidates as direct solar thermal collectors.

Trends and Opportunities in Direct-Absorption Solar Thermal Collectors

Efficient conversion of sunlight into useful heat or work is of increasing global interest. Solar-to-thermal energy conversion, as opposed to solar-to-electricity, is enabled by solar thermal collectors that convert sunlight into heat at some useful temperature. We review here recent developments in solar thermal energy conversion. Our emphasis is on “direct-absorption” solar thermal collectors, in which incident sunlight is absorbed directly by a working fluid. This contrasts with conventional solar thermal collectors where the sunlight strikes and is absorbed by a solid receiver, which then transfers heat to the working fluid. Both liquid-based and gas-based direct-absorption collectors are described, although liquid-based systems are emphasized. We propose that if “direct-absorption” technologies could be developed further, it would open up a number of emerging opportunities, including applications exploiting thermochemical and photocatalytic reactions and direct absorption of a binary fluid for absorption refrigeration.

Spatially Varying Extinction Coefficient for Direct Absorption Solar Thermal Collector Optimization

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.

Nanofluid optical property characterization: towards efficient direct absorption solar collectors

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.