Analysis of Nanofluid-Based Parabolic Trough Collectors for Solar Thermal Applications

Abstract

Solar-to-thermal energy conversion technologies are an important and increasingly promising segment of our renewable energy technology future. Today, concentrated solar power (CSP) plants provide a method to efficiently store and distribute solar energy. Current industrial solar-to-thermal energy technologies employ selective solar absorber coatings to collect solar radiation, which suffer from low solar-to-thermal efficiencies at high temperatures due to increased thermal emission from selective absorbers. Solar absorbing nanofluids (a heat transfer fluid (HTF) seeded with nanoparticles), which can be volumetrically heated, are one method to improve solar-to-thermal energy conversion at high temperatures. To date, radiative analyses of nanofluids via the radiative transfer equation (RTE) have been conducted for low temperature applications and for flow conditions and geometries that are not representative of the technologies used in the field. In this work, we present the first comprehensive analysis of nanofluids for CSP plants in a parabolic trough configuration. This geometry was chosen because parabolic troughs are the most prevalent CSP technologies. We demonstrate that the solar-to-thermal energy conversion efficiency can be optimized by tuning the nanoparticle volume fraction, the temperature of the nanofluid, and the incident solar concentration. Moreover, we demonstrate that direct solar absorption receivers have a unique advantage over current surface-based solar coatings at large tube diameters. This is because of a nanofluid's tunability, which allows for high solar-to-thermal efficiencies across all tube diameters enabling small pressure drops to pump the HTF at large tube diameters.

Performance analysis of a direct-absorption parabolic-trough solar collector using plasmonic nanofluids

“…In the 3-D thermal model, the fluid flow and heat transfer are coupled in the inner tube region. The thermophysical property of the nanofluids are taken the same as the base fluids, i.e., Therminol VP-1 [35], because the nanoparticle concentrations is far below 1% in volume fraction, as was also done in other related works [4,5,27,36,37]. The flow rate in this study is around 0.1 kg/s and the corresponding Reynold number is above Re D = 2300; thus, the flow is in the turbulent regime and the standard k − ε model was applied with corresponding parameters C µ = 0.09, c ε1 = 1.44, c ε2 = 1.92, σ k = 1 and σ ε = 1.3 [38].…”

Comparative analysis of direct‐absorption parabolic‐trough solar collectors considering concentric nanofluid segmentation

“…With the increase of L , the advantage of the dual‐nanofluid DAPTSC becomes more obvious due to the increasing receiver surface area and corresponding heat loss. Note that in practice, the collector length can be up to 115 m; thus, little degradation of η with increasing L makes the dual‐nanofluid DAPTSC be more attractive for the real‐world application.…”

“…For instance, at the inlet temperature of 150°C, the thermal efficiency of a SBPTSC with a surface absorptivity of 0.96 is below 50% . Moreover, the stability of the absorber coating deteriorates with increasing temperature …”

“…Thermal efficiency of 5‐10% point higher than a SBPTSC was achieved with a volume fraction of 0.05%. After that, the effects of different factors including operation temperature, particle volume fraction, and flow rate on the DAPTSC performance were thoroughly investigated, but all considered a transparent glass receiver containing the nanofluids. Recently, a new design was studied for a DAPTSC, where a highly reflective semicylindrical coating on the glass receiver was employed to increase the optical path length and thus reduce the required absorption coefficient of the nanofluids …”

“…13 Moreover, the stability of the absorber coating deteriorates with increasing temperature. 14 In order to reduce the aforementioned shortages of a SBPTSC, a direct-absorption solar collector (DASC) can be adopted as been widely used in the low temperature regime. [1][2][3]6,[15][16][17][18][19][20] In a DASC, the nanoparticles dispersed in the nanofluids directly interact and absorb the solar energy; thus, the absorber coating is no longer needed.…”