Performance improvement of a nanofluid solar collector based on direct absorption collection (DAC) concepts

Abstract

Nanofluids are engineered colloidal suspensions of nanoparticles in base fluids, which have good properties of radiation absorption and heat transfer and are a kind of potential working fluids for solar collector based on direct absorption collection (DAC) concepts. A simulation model of nanofluid solar collector was built based on DAC concepts by solving the radiative transfer equations of particulate media and combining conduction and convection heat transfer equations. The system efficiency and temperature distributions are analyzed by considering the absorption and scattering of nanoparticles and the absorption of the matrix. The simulation results were in accordance with the experiments’. The nanofluids improved the outlet temperature and the efficiency by 30–100 K and by 2–25% than the base fluid. The photothermal efficiency of a 0.01% graphite nanofluid is 122.7% of that of a coating absorbing collector. The study indicated that nanofluids, even of low-content, have good absorption of solar radiation, and can improve the outlet temperatures and system efficiencies.

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.