Performance improvement of a nanofluid solar collector based on direct absorption collection (DAC) concepts
Introduction
Nanofluids are engineered colloidal suspensions of nanoparticles in a base fluid [1], [2]. The convective transport and effectively thermophysical properties of nanomaterials and nanofluids have been extensively studied [1], [3], [4], [5], [6]. Phelan et al. reported that the thermal conductivity was increased by ∼160% through adding 1% carbon nanotubes into the base fluid [7]. In terms of radiative properties, nanofluids can substantially absorb or spectrally selectively absorb solar radiation [8], [9], [10]. They can be potentially used in automotive applications, disease treatment, cooling of electrical devices, and solar energy [11].
Direct solar radiation absorption collection (DAC) concepts were presented in the 1980s [12]. In contrast to commercial evacuated tube collectors that absorb solar radiation through spectrally selective coatings, DAC systems absorb solar radiation directly through working fluids whose heat resistance can be reduced [13]. Volume trap solar collectors, black liquid collectors, small particle collectors, and other types of collectors based on DAC have been proposed for solar thermal application [14], [15], [16], [17], [18]. With the development of nanotechnologies, the absorption of solar radiation can be improved with low particle loadings, and the clogging of pumps and pipes can be avoided through the extremely small size of nanoparticles [19], [20]. Otanicar and colleagues [21], [22] investigated numerically the effects of sizes and scattering modes in DAC solar collectors on optimal solar absorption properties and found that the efficiency of the nanofluid system was improved by 5% than that of the base-fluid system. Taylor et al. extended the application of this concept to concentrated solar power systems [13]. Saidur et al.[23] presented that the particle size has little influence on the optical properties of nanofluids, while the loadings of nanoparticles is linearly related to the extinction coefficient of the nanofluids. Lenert and Wang presented that idealized DAC receiver-side efficiencies could exceed 35% [24].
The radiative properties of nanofluids are considered significant in predicting the photothermal efficiency of DAC collectors. In the models mentioned above, the extinction coefficients of nanofluids do not account for the absorption of the matrix, such as water and oil, which selectively absorb solar radiation. In our previous paper [25], it was shown that the absorption of the matrix played an important role in predicting the effective absorption coefficients. There was almost no absorption in the QCA model, while there was a noticeable absorption in the QCA’ and FV models for both dilute and dense nanofluids in the entire spectral range. It indicated the QCA’ model was more reasonable than the QCA model in considering the absorption of matrix. In order to improve the model, the radiative transfer of the nanofluid layer was considered carefully in this study, particularly in solving the radiative transfer equation. Extinction coefficients were predicted via the Fedorov and Viskanta model [25], which considered matrix absorption and particle scattering and absorption. The temperature distribution and performance of the DAC collector with nanofluids was modeled through coupling of conduction and convection in the DAC collector. The radiative and conductive properties of nanofluids were experimentally investigated. Based on the findings, the efficiencies of a direct absorption receiver were tested and used to validate the model, which is expected to optimize the design of solar nanofluid collectors.
Section snippets
Simulation model
Fig. 1 presents a schematic of the solar collector based on DAC. The nanofluid flows horizontally from the right to the left in a solar collector covered with a glass plate. A 2D model was built to analyze the radiation and conduction in the collector. The nanofluid layer is assumed to be a particulate suspension colloid filled with single spherical particles. The bottom of the collector and the left and right sides of the glass cover are assumed to be insulated.
Preparation of nanofluids
In this study, nanofluids were prepared through a two-step method in which nanoparticles such as TiO2, Al2O3, Ag, Cu, and SiO2, as well as graphite and carbon nanotubes, were added directly into Texatherm oil to prepare stable suspension colloids. Sizes of the nanoparticles are shown in Table 2. Cetyltrimethylammonium bromide was added as a dispersing agent at an SN ratio of 0.3.
Radiative properties
The radiative properties of the nanofluids were measured with a Shimadzu UV3150 ultraviolet and visible
Radiative properties
Fig. 4 illustrates the transmittance of the nanofluids. The nanoparticles were scattered, and they were tested with the integral sphere. Fig. 4(a) shows that the transmittance of the nanofluids decreased when the depth increased. Fig. 4(b) presents the results of the nanofluids of 0.01 vol.% tested in 2 mm cuvettes. A low loading can promote the efficiency of radiation absorption from 200 nm to 2000 nm. Fig. 4 shows that the transmittance of oil was about 90% and the transmittance of most
Conclusions
This study investigated the performance improvements of a DAC solar collector with nanofluids. A simulation model was proposed by combining the radiative heat transfer in particulate media with conduction and convection heat transfer in the DAC collector to predict the photothermal efficiency. TiO2, Al2O3, Ag, Cu, SiO2, graphite nanoparticles, and carbon nanotubes were applied in nanofluids, whose transmittances and testing performances were reported. These results are used to validate the
Conflict of interest statement
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Performance improvement of a nanofluid solar collector based on direct absorption collection (DAC) concepts”.
Acknowledgment
This study was supported by the Important Science and Technology Specific Projects of Zhejiang Province (No. 2012C01022-1), the National Natural Science Foundation of China (No. 51276167), and the China International Cooperation Project (No. 2011DFR60190).
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