Evaluation of the Potential of Nanofluids Containing Different Ag Nanoparticle size Distributions for Enhanced Solar Energy Conversion in Hybrid Photovoltaic-Thermal (PVT) Applications

Hybridising photovoltaic and photothermal technologies into a single system that can simultaneously deliver heat and power represents one of the leading strategies for generating clean energy at more affordable prices. In a hybrid photovoltaic-thermal (PVT) system, the capability to modulate the thermal and electrical power output is significantly influenced by the spectral properties of the heat transfer fluid utilised. In this study, we report on one of the first experimental evaluations of the capability of a multimodal silver nanofluid containing various particle shapes and particle sizes to selectively modulate the solar energy for PVT applications. The diverse set of particle properties led up to a 50.4% enhancement in the solar energy absorbed by the nanofluid over the 300nm—550nm spectral region, where silicon is known to exhibit poor photovoltaic conversion performances. This improved substantially the absorption of solar energy, with an additional 18–129Wm−2 of thermal power being generated by the PVT system. Along with the advancements made in the thermal power output of the PVT system, a decrease of 4.7–36.6Wm−2 in the electrical power generated by the photovoltaic element was noted. Thus, for every∼11Wm−2 increase of thermal power achieved through the addition of the nanoparticles, a reduction of∼3Wm−2 in the ability to generate clean electricity was sustained by the PVT. Despite the energy trade-offs involved under the conditions of the nanofluid, the PVT system cumulatively harvested 405Wm−2 of solar energy, which amounts to a total conversion efficiency of 45%. Furthermore, the economics of the additional energy harvested through merging of the two systems was found to reach an enhancement of 77% under certain European conditions

Development of poly-vinyl alcohol stabilized silver nanofluids for solar thermal applications

Nanofluids offer the potential to address the low thermal conductivities found in conventional heat transfer fluids, through their unique electrical, optical and thermal properties, but their implementation remains restricted due to absorption and stability limitations. Here, we characterize and exploit the distinctive plasmonic properties exhibited by polyvinyl-alcohol stabilized silver nanostructures by tuning their absorption and thermal properties through controlling the nanoparticle size, morphology and particle-size distribution configuration at the synthesis stage. The photo-thermal efficiency of different water-based silver nanofluids under a standard AM1.5G weighted solar spectrum were explored, the influence of each of these components on the resulting fluids performance within a direct absorption solar thermal collection system being considered. Nanofluids, containing an extensive ensemble of particle size-distributions (5 nm–110 nm in diameter) in addition to anisotropic particle morphologies (e.g. prisms, hexagons and other non-spherical geometries), exhibited a significant enhancement in the absorption and photo-thermal energy transfer. Enhancements of 5%–32% in the photo-thermal conversion efficiency were achieved, the enhancement being dependent upon the presence of multiple particle size-distributions and the particle concentration. The enhancement is influenced by the interactions occurring between the individual particle size-distributions but also by the collective behaviour of the particles ensemble. The critical particle diameter, above which the photo-thermal characteristics of the nanofluid become dominated by the larger sized particles present, was identified as 150 nm. The increased performance of these nanofluids, which adopt a more complex particle-size configuration, suggests that they can represent suitable candidates for solar-thermal applications

Combined Experimental and Modeling Analysis for theDevelopment of Optical Materials Suitable to Enhance theImplementation of Plasmonic-Enhanced Luminescent Down-Shifting Solutions on Existing Silicon-Based Photovoltaic Devices

The development of highly efficient solar collectors requires modulating the light interactions with the semiconducting materials. Incorporating luminescent species and metal nanoparticles within a semitransparent polymeric material (e.g., polymethyl methacrylate (PMMA)) leads to the formation of a plasmon-enhanced luminescent down-shifting (PLDS) layer, which offers a retrofittable approach toward expanding the wavelength range over which the conversion process can effectively occur. Adding antireflection coatings (ARCs) further controls the spectral response. However, with each additional component comes additional loss pathways. In this study, the losses related to light interactions with the PMMA and the ARCs have been investigated theoretically using a transfer matrix method and experimentally validated. Two proposed architectures were considered, and the deviations between the optical response of each iteration helped to establish the design considerations. The proposed structure-enhanced (SE) designs generated a predicted enhancement of 37 to 62% for the collection performance of a pristine monocrystalline-silicon solar cell, as inferred through the short-circuit current density (Jsc). The results revealed the synergies among the SE-design components, demonstrating that the spectral response of the SEs, containing a thin polymer framework and an ARC, can be tuned to minimize the reflections, leading to the solar energy conversion enhancement

Nanofluid Development Using Silver Nanoparticles and Organic-Luminescent Molecules for Solar-Thermal and Hybrid Photovoltaic-Thermal Applications

Exploiting solar energy using photo-thermal (PT) and/or hybridised photovoltaic/thermal (PVT) systems can represent a viable alternative to the growing demand for renewable energy. For large-scale implementation, such systems require thermal fluids able to enhance the combined conversion efficiency achievable by controlling the ‘thermal’ and ‘electrical’ components of the solar spectrum. Nanofluids are typically employed for these purposes and they should exhibit high heat-transfer capabilities and optical properties tuned towards the peak performance spectral window of the photovoltaic (PV) component. In this work, novel nanofluids, composed of highly luminescent organic molecules and Ag nanoparticles dispersed within a base fluid, were tested for PT and PVT applications. These nanofluids were designed to mimic the behaviour of luminescent down-shifting molecules while offering enhanced thermo-physical characteristics over the host base fluid. The nanofluids’ conversion efficiency was evaluated under a standard AM1.5G weighted solar spectrum. The results revealed that the Ag nanoparticles’ inclusion in the composite fluid has the potential to improve the total solar energy conversion. The nanoparticles’ presence minimizes the losses in the electrical power component of the PVT systems as the thermal conversion increases. The enhanced performances recorded suggest that these nanofluids could represent suitable candidates for solar energy conversion application

Cantilever-Based Sensor Utilizing a Diffractive Optical Element with High Sensitivity to Relative Humidity

High-sensitivity and simple, low-cost readout are desirable features for sensors independent of the application area. Micro-cantilever sensors use the deflection induced by the analyte presence to achieve high-sensitivity but possess complex electronic readouts. Current holographic sensors probe the analyte presence by measuring changes in their optical properties, have a simpler low-cost readout, but their sensitivity can be further improved. Here, the two working principles were combined to obtain a new hybrid sensor with enhanced sensitivity. The diffractive element, a holographically patterned thin photopolymer layer, was placed on a polymer (polydimethylsiloxane) layer forming a bi-layer macro-cantilever. The different responses of the layers to analyte presence lead to cantilever deflection. The sensitivity and detection limits were evaluated by measuring the variation in cantilever deflection and diffraction efficiency with relative humidity. It was observed that the sensitivity is tunable by controlling the spatial frequency of the photopolymer gratings and the cantilever thickness. The sensor deflection was also visible to the naked eye, making it a simple, user-friendly device. The hybrid sensor diffraction efficiency response to the target analyte had an increased sensitivity (10-fold when compared with the cantilever or holographic modes operating independently), requiring a minimum upturn in the readout complexity

Towards plasmon mapping of SERS-active Ag dewetted nanostructures using SPELS

Thermal dewetting of silver thin flm can lead to SERS-active Ag nanoparticles. Here, we report our progress towards using scanning probe energy loss spectroscopy (SPELS) to map the plasmonic behaviour of SERS-active Ag nanoparticles (NP) by investigating NPs produced through the dewetting study of Ag thin flms on SiO2/Si and Ti/SiO2/Si substrates. The nanoparticles size and spatial distribution were controlled by the deposition and thermal annealing parameters. The results of preliminary SPELS measurements of these structures, alongside SERS data show that there is a correlation between the Raman enhancement and the nanoparticle size and interparticle spacing

Fractal patterning of nanoparticles on polymer films and their SERS capabilities

We demonstrate control, via electro-hydrodynamic (EHD) induced polymer instabilities and nanoparticle mobility, of hierarchical fractal arrangements of gold nanoparticles on patterned thin polymer films. The induced changes in the film curvature enhance fractal formation for high and not for low mobility nanoparticles. The high mobility nanoparticles cluster in circular fractal networks on the rims of a hexagonally ordered array of EHD-induced polymer peaks. These arrangements exhibit plasmonic properties for surface-enhanced Raman scattering (SERS) spectroscopy

The stability of thin polymer films as controlled by changes in uniformly sputtered gold

The stability of polystyrene thin films of low molecular weight on a solid substrate is shown to be controlled by the presence of uniformly distributed gold sputtered at the air–polymer interface. Continuous gold coverage causes the formation of wrinkles. High coverage and Au nanoparticle (NP) density leads to the development of a spinodal instability while low coverage and NP density retards the nucleation dewetting mechanism that beads up the thin polymer film into drops when no coverage is present. Heating at temperature larger than the polymer glass transition temperature for extended periods allows the gold NPs to coalesce and rearrange. The area of polymer surface covered by NPs decreases as a result and this drives the films from unstable to metastable states. When the gold NPs are interconnected by polymer chains a theoretically predicted spinodal instability that patterns the film surface is experimentally observed. Suppression of the instability and a return to a flat film occurs when the polymer interconnections between particles are broken. While the polymer films maintain their physical continuity changes in their chemical surface composition and thickness are observed. The observed film metastability is nevertheless in agreement with theoretical prediction that includes these surface changes

Effect of Au nanoparticle spatial distribution on the stability of thin polymer films

The stability of thin poly(methyl-methacrylate) (PMMA) films of low molecular weight on a solid substrate is controlled by the areal coverage of gold nanoparticles (NPs) present at the air–polymer interface. As the polymer becomes liquid the Au NPs are free to diffuse, coalesce, and aggregate while the polymer film can change its morphology through viscous flow. These processes lead at the same time to the formation of a fractal network of Au NPs and to the development of spinodal instabilities of the free surface of the polymer films. For thinner films a single wavelength is observed, while for thicker films two wavelengths compete. With continued heating the aggregation process results in a decrease in coverage, the networks evolve into disordered particle assemblies, while the polymer films flatten again. The disordering occurs first on the smallest scales and coincides (in thicker films) with the disappearance of the smaller wavelength. The subsequent disordering on larger scales causes the films to flatten