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As ZnO nanoparticles had the advantages of high thermal conductivity and low cost, the possibility of using ZnO nanoparticles in spectral splitting photovoltaic/thermal (PV/T) systems was initially studied from the perspective of optical properties. Water–ZnO and glycol–ZnO nanofluids were prepared via a two-step method and used for model validation and stability testing. The scheme employed to investigate the optical properties and radiative transfer of the nanofluids was developed using Mie scattering theory combined with the Monte Carlo ray tracing (MCRT) method. The overall effective spectral transmittance coefficients of PV cells were utilized for comprehensive evaluation of the spectral transmittances of the nanofluids in spectral splitting PV/T systems. The overall effective spectral transmittance of a PV cell water-ZnO nanofluids was 21.54% higher than that those of cells containing water–polypyrrole and water–Cu9S5 nanofluids, respectively. The effects of the nanoparticle diameter, mass concentration and the optical length of the nanofluid on the spectral transmittance of glycol–ZnO nanofluid were also investigated.

The ultrasonic-assisted solar photochemical splitting of water had been explored in recent years to enhance hydrogen production efficiency. In this study, a photon-absorption-based study was conducted to investigate the mechanism of the ultrasonic-assisted solar photochemical splitting of water. An elaborate test bench for temperature-controlled, ultrasonic-assisted solar photochemical water splitting was designed, set up, and tested. A comparison of the hydrogen production between the ultrasonic-assisted and conventional solar photochemical splitting of water was carried out. The effective nanoparticle size before and after ultrasonic vibration, as well as after solar photocatalysis, was analyzed. Furthermore, the spectral absorptivity of the nanofluids before and after ultrasonic vibration, as well as after solar photocatalysis, was investigated by both experimental and numerical methods. The investigation indicated that the improved particle dispersion in the solution prepared by ultrasonication allowed the absorbance of more incoming sunlight. The amount of hydrogen produced by the ultrasonic-assisted hydrogen production was 3.45 times that of conventional solar photochemical splitting of water without pre-ultrasonicated. Besides, an effective spectral absorptivity coefficient was proposed as a modified measure of spectral absorptivity. In addition, the optimal particle diameter was optimized using the Monte Carlo ray tracing method to identify the best light absorption performance.

Radiative transfer is the dominant mode of heat transfer at high temperature for an absorbing-scattering media. Due to the complex solving of radiative transfer equation, radiative transfer is usually omitted in conventional heat transfer analysis of molten salt. In this study, the radiative, conductive and laminar convective coupled heat transfer of molten salts was numerically analyzed by the codes compiled based on finite element method (FEM) coupled with commercial Fluent software. The effects of radiative transfer on temperature distribution of molten salts were analyzed. Besides, the effects of scattering albedo, refractive index and fluid inlet velocity on temperature distribution of molten salts were also investigated. The numerical results indicated that the maximum elevated temperature deviation can reach up to 47.4% (101 K) when radiative transfer was not considered.

Advanced solar energy utilization technology requires high-grade energy to achieve the most efficient application with compact size and least capital investment recovery period. Concentrated solar power (CSP) technology has the capability to meet thermal energy and electrical demands. Benefits of using CSP technology with parabolic trough collector (PTC) system include promising cost-effective investment, mature technology, and ease of combining with fossil fuels or other renewable energy sources. This review first covered the theoretical framework of CSP technology with PTC system. Next, the detailed derivation process of the maximum theoretical concentration ratio of the PTC was initially given. Multiple types of heat transfer fluids in tube receivers were reviewed to present the capability of application. Moreover, recent developments on heat transfer enhancement methods for CSP technology with PTC system were highlighted. As the rupture of glass covers was frequently observed during application, methods of thermal deformation restrain for tube receivers were reviewed as well. Commercial CSP plants worldwide with PTC system were presented, including those that are in operation, under construction, and announced. Finally, possible further developments of CSP plants with PTC system were outlined. Besides, suggestions for future research and application guidance were also illustrated.

Thermochemical reactions involved in endothermic reactions can make use of concentrated solar irradiation as the energy source for high-temperature processed heat. Over the past few decades, tremendous progress has been made in the development of solar thermochemical reactors. The maximum temperature in a solar thermochemical reactor can reach 1500 K or higher, and heat is transferred to the particles in the reactor predominantly by radiative heat transfer. The present work provides a comprehensive review of radiative heat transfer in particle solar thermochemical reactors. The review first covers the commonly used catalysts in solar thermochemical reactions, types of solar thermochemical reactors, and the theory of radiative transfer. Further, recent developments in obtaining the spectral radiative properties of particles, the progress of radiative heat transfer research, and radiative heat transfer coupled with solar thermochemical reactions are reported. Finally, further developments and future prospects for obtaining the spectral radiative properties of high-temperature particles and radiative heat transfer in solar thermochemical reactor are outlined.

Photocatalysts currently in use are only able to utilize very small part of the solar spectrum that arrives at the earth's surface (mainly ultraviolet light). Most of the photons that are not absorbed by the photocatalysts are converted to heat. However, there is no consensus on the effect of reaction temperature on photo-thermo chemical synergetic catalysis, which has been studied herein using experimental investigations combined with thermodynamic analysis. An elaborate photo-thermo chemical reaction test rig was initially designed and set up that can test experimental variable while the other influence factors were kept constant. The effects of ultrasonic and operation temperature on Pt/TiO2 particle cluster distribution during the photo-thermo chemical synergetic catalytic water splitting process were analyzed by an upright microscope for the first time. The results indicated that the H2 production rate varies with reaction temperature, and 55 °C is the optimum temperature for the photo-thermo chemical synergetic catalytic water splitting process studied here. A maximum H2 production rate of 11.934 mmol/(h g) could be achieved using a classical Pt/TiO2 catalyst, when operating under the optimum reaction conditions.

Because of the ultra-high operation temperature in solar thermochemical reactors, radiative transfer accounts for the majority of the total heat transfer. Determining the radiative characteristics of catalysts during solar thermochemical reactions is critical for numerical calculations. Optical constant of material is only determined by the intrinsic properties of materials and not related to the micro-structural morphology. In this study, optical constants of four typical types of metal oxides often used as catalysts during solar thermochemical reactions were obtained by using infrared variable angle spectroscopic ellipsometry. In addition, the variation of optical constant of quartz window with temperature was also investigated using infrared variable angle spectroscopic ellipsometry in the far infrared spectral region. The experimental results indicated that the peak values of the refractive index and extinction coefficient of the optical window decreased with increasing temperature, and the degree of polarization of optical window decreased with increasing temperature and already decreased to 40% at 673 K in the spectral range of 7.2–7.5 μm. In addition, the refractive index and extinction coefficient varied sharply with increasing wavelength for both the catalysts and optical window.

Solar driven CO 2 reforming of methane can store extra 20% concentrated solar energy to chemically bonded energy. With the aim to evaluate key operational parameters on energy storage efficiency, a finite volume method (FVM) coupled with thermochemical kinetics was developed to analyze the solar driven CO 2 methane reforming performance in a metallic foam thermochemical reactor. The LH kinetic model of CO 2 reforming of methane was compiled by user defined functions (UDFs) to exactly predict the temperature distribution and mole fractions of gas species. The numerical results were validated with experimental results which were tested in Niigata University to guarantee the accuracy of the developed model. Effects of gas mixture inlet velocity, porosity of metal foam reactor, CH 4 /CO 2 ratio and heat flux distribution on energy storage efficiency were analyzed for providing theoretical guideline to the application of solar driven CO 2 methane reforming. The numerical results indicated that maximum energy storage efficiency of CO 2 reforming of methane was found when the CH 4 /CO 2 ratio is 0.67.

With the aim to increase the overall heat transfer performance and reliability of tube receiver for parabolic trough solar collector system, asymmetric outward convex corrugated tube is introduced as the metal tube of parabolic trough receiver. An optical-thermal-structural sequential coupled method was developed to study the heat transfer performance and thermal strain of tube receiver for parabolic trough solar collector system. Heat transfer performance and thermal strain comparisons between conventional tube receiver and asymmetric outward convex corrugated tube receiver are conducted. The researches indicated that the usage of asymmetric outward convex corrugated tube as receiver can enhance the heat transfer performance and reduce the thermal strain effectively. By using asymmetric outward convex corrugated tube as receiver, the maximum enhancement of overall heat transfer performance factor is 148% and the maximum restrain of von-Mises thermal strain is 26.8%.