Energy flow control of nanofluid-based direct absorption solar collectors with functional optical coatings for efficient solar harvesting

Abstract

The nanofluid-based direct absorption solar collector (NDASC) ensures that solar radiation passing through the tube wall is directly absorbed by the nanofluid, reducing thermal resistance in the energy transfer process. However, further exploration is required to suppress the outward thermal losses from the nanofluid at high temperatures. Herein, this paper proposes a novel NDASC in which the outer surface of the collector tube is covered with functional coatings and a three-dimensional computational fluid dynamics model is established to study the energy flow distributions on the collector within the temperature range of 400−600 K. When the nanofluid’s absorption coefficient reaches 80 m−1, the NDASC shows the optimal thermal performance, and the NDASC with local Sn-In2O3 coating achieves a 7.8% improvement in thermal efficiency at 400 K compared to the original NDASC. Furthermore, hybrid coatings with Sn-In2O3/WTi-Al2O3 are explored, and the optimal coverage angles are determined. The NDASC with such coatings shows a 10.22%−17.9% increase in thermal efficiency compared to the original NDASC and a 7.6%−19.5% increase compared to the traditional surface-type solar collectors, demonstrating the effectiveness of the proposed energy flow control strategy for DASCs.

1. Introduction

Solar energy, which is renewable and abundant, offers a promising solution to clean and sustainable energy systems [1−3]. Various solar energy harvesting techniques, such as concentrated solar power (CSP) [4] and photovoltaic (PV) [5], have been explored extensively in recent years. Solar collectors have been widely used to harness solar thermal energy [6]. In traditional surface-type solar collectors [7], solar radiation is initially absorbed by the collector surface. After the surface temperature of the collector tube increases, the heat is then transferred to the working fluid inside the collector. Due to the thermal resistance between the collector surface and the working fluid, solar energy cannot be fully absorbed by the working fluid. However, direct absorption solar collectors (DASCs) enable solar radiation to pass through the collector wall and be directly absorbed by the working fluid, reducing the hindrance of heat conduction by the collector wall [8]. Therefore, DASCs are considered the next generation of high-performance collectors.

DASCs were introduced in the 1970s as a simplified design compared to traditional collectors, aiming to improve efficiency by directly absorbing solar radiation with the fluid [9]. MINARDI et al [10] first proposed a direct absorption solar collector that utilized a suspension of micro-sized carbon particles in “Indian ink” as the volumetric receiver. Over the past few decades, there has been a renewed interest in nanofluid-based direct absorption solar collectors due to the advancements in nanoparticle production and nanofluid preparation techniques [11, 12]. KHULLAR et al [13] first applied the aluminum nanofluid with a volume fraction of 0.05% in direct absorption parabolic trough solar collectors (DAPTSCs), reporting a 5%−10% higher thermal efficiency than traditional trough solar collectors. OTANICAR et al [14] numerically evaluated the thermal performance of nanofluids with various nanoparticles in low-temperature DASCs. They observed a 5% improvement in thermal efficiency by utilizing nanofluids as the absorption medium in solar collectors. In another study [15], the DAPTSC was constructed using Al2O3/H2O nanofluids. The results showed that the DAPTSC achieved the highest thermal efficiency at the minimum incidence angle, and the thermal efficiency increased with the volume fraction of nanofluids. In conclusion, DASCs with nanofluids as the solar absorption medium enable them to capture solar radiation better and prevent its escape through the working fluid, thus exhibiting higher thermal efficiency than traditional surface-type solar collectors.