Nanofluids for Direct-Absorption Solar Collectors-DASCs: A Review on Recent Progress and Future Perspectives

Abstract

Owing to their superior optical and thermal properties over conventional fluids, nanofluids represent an innovative approach for use as working fluids in direct-absorption solar collectors for efficient solar-to-thermal energy conversion. The application of nanofluids in direct-absorption solar collectors demands high-performance solar thermal nanofluids that exhibit exceptional physical and chemical stability over long periods and under a variety of operating, fluid dynamics, and temperature conditions. In this review, we discuss recent developments in the field of nanofluids utilized in direct-absorption solar collectors in terms of their preparation techniques, optical behaviours, solar thermal energy conversion performance, as well as their physical and thermal stability, along with the experimental setups and calculation approaches used. We also highlight the challenges associated with the practical implementation of nanofluid-based direct-absorption solar collectors and offer suggestions and an outlook for the future.

Keywords: direct-absorption solar collectors; nanofluids; solar thermal energy conversion.

Figure 5

Schematic shows methods of direct measurements of (A) transmittance using the transmission port and (B) transmittance and scattering combined in the integrating sphere compartment [106].

Figure 1

(A) Surface-based absorption solar collector with (B) the flow of both heat and solar light in a typical liquid flat plate collector. (C) Direct (volume-based) absorption solar collector (DASC). Reproduced with permission from [19].

Figure 2

Three different methods of Au NP synthesis with and without surface modifications. (A) Atmospheric-pressure microplasma setup, adapted from [61]. (B) Seed-mediated synthesis method, adapted from [58]. (C) Polymer coating procedure of Au NPs prepared by chemical reduction, adapted from [60].

Figure 3

Energy flow in solar thermal nanofluids (nanoparticles and base fluid).

Figure 4

(a) Spectral solar irradiance at Earth’s level (air mass of 1.5, AM1.5) [102] and (b) transmittance of common base fluids used in DASCs for a one cm sample thickness (transmittance for Terminol VP-1 is taken from [29]).

Figure 6

(a) Extinction, (b) absorption, and (c) scattering coefficients for gold (red) and copper oxide (blue) NPs in an ethylene glycol base fluid. Adapted with permission from [61].

Figure 7

(a) Comparison between the transmittance of several NFs as a function of wavelength. (b) The extinction distance for investigated NFs as a function of NP concentration, in the case of wavelength equal to 1300 nm. Adapted from [73].

Figure 8

Coefficients of (a) extinction, (b) absorption, and (c) scattering of three different types of CuO at different sizes dispersed in ethylene glycol (mean diameter: ~3.5 nm QDs, ~50 nm NPs, ~3–10 µm MPs).

Figure 9

UV–Vis spectra of Ag NPs and SiO2@Ag NPs in water: (a) after exposure to stimulated solar light, (b) after exposure to natural solar light for two weeks, and (c) after storage in the dark for three weeks (adapted from [62]).

Figure 10

Transmittance variation in (a) CuO MPs, (b) CuO NPs, and (c) CuO QDs in addition to (d) ZnO NPs in EG across a wavelength range of 300–1500 nm. The transmittance measurements were performed in a static mode, i.e., without shaking, and were then manually shaken before the last measurement “After shaking”.

Figure 11

DASC schematics; (a) simplified closed-loop system for heat transfer. Adapted from [24], (b) irradiation and heat loss sources. Adapted from [118], (c) nanoparticle and solar irradiation interaction.

Figure 12

Photo-thermal and collector efficiencies as well as the heat loss of different NFs in (A) cube-shaped and (B) flat DASCs (adapted from [58]).

Figure 13

Optical properties of different Au nanostructures; (A) absorption section, and (B) scattering section. Adapted from [120].