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The use of photovoltaics (PVs) and/or photo-thermal (PTs) as primary solar-energy solutions is limited by the low solar conversion of PVs due to the spectral mismatch between the incident radiation and/or the PV material. The PTs are curtailed by the limited absorbance and the low thermal conductivity of the working fluid. A possible solution is the use of luminophores able to perform luminescent down-shifting (LDS) conversion and to incorporate them in liquid or solid layers, which act as spectral beam splitters (SBSs). Dispersed in solid polymer layers, luminophores lead to luminescent solar concentrators (LSC). When dispersed in liquid and placed in front of PVs, luminophores act as working fluids and as SBS, leading to hybrid photovoltaic–photo-thermal (PVT) systems. Here, the SBS filters for PV and PVT systems are reviewed. The contribution of luminophores to electrical and thermal energy production is discussed from theoretical, experimental, and economical perspectives. Recent SBS architectural concepts which combine different optical elements are also considered. These architectures can harness the advantageous properties of LSCs, spectral modulators, and hybridisation in a single structure. By combining these different light-management strategies inside of a single structure, an improvement in the electrical and/or thermal energy production can be achieved. [ABSTRACT FROM AUTHOR]

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 applications. [ABSTRACT FROM AUTHOR]

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. Image 1 • Optical absorption of Ag nanostructures tuned through synthesis optimisation. • Absorption range tailored to capture and convert energy within the UV-VIS region. • Particle size-distribution can enhance the nanofluids thermophysical properties. • Particles size and their interactions strongly influence the PTE enhancements. • Anisotropic morphologies and sizes are key factors for nanofluids performance. [ABSTRACT FROM AUTHOR]

Hybridised photovoltaic-thermal (PVT) systems, obtained by merging photo-voltaic (PV) and photo-thermal (PT) technologies, can lead to enhanced conversion efficiencies. Liquid spectral beam splitters (SBSs) offer control of the 'thermal' and 'electrical' components of the solar spectrum. Combining PVT systems with liquid SBSs facilitates enhanced control of the thermal and electrical outputs, further improving the PVT systems efficiency and making them economically viable. Traditionally, liquid SBSs contain nanostructures suspended in a base-fluid to capitalise on the enhanced heat transfer capabilities offered by the nanostructures' surface morphology. These nanofluids have certain disadvantages, such as stability problems, environmental hazards and synthetic cost concerns, which preclude them from becoming a viable substitute for commercial (single-phase) fluids. This study offers an alternative strategy by employing a set of newly developed fluids containing organic and organometallic imidazo[4,5- f ][1,10]phenanthroline derivatives for SBS-PVT systems. These new working fluids were designed to mimic the behaviour of luminescent down-shifting (LDS) molecules while offering enhanced thermophysical characteristics over the host base-fluid. Their optical, fluorescent and thermophysical properties enabled an increased heating rate within the working fluid. Their integration as working fluids in an SBS-PVT system lead to optical efficiencies of ~63% i.e. an 18%–20% improvement over the standalone PV technology. The fluid properties displayed a 61% increase in the economic value of the energy captured when compared to the host base-fluid alone for the same PVT system. Consequently, these novel fluids and the organic molecules they contain can be effectively designed as high performance spectrally selective fluids for PVT applications. Image 1 • The properties of new organic-based fluids were studied for PVT applications. • The fluids demonstrate favorable thermal, optical and fluorescent characteristics. • The fluids can exhibit selective spectral absorbance for hybrid PVT collectors. • When used as LDS optical layers the fluids minimize the losses in PVT systems. • An increased optical collection efficiency and economic value was obtained. [ABSTRACT FROM AUTHOR]