Ortho-para hydrogen conversion in the hydrogen liquefaction process is significant for the long-term storage and long-distance transportation of liquid hydrogen. This paper outlines the differences in the properties of orthohydrogen and parahydrogen, reviews the research progress on the physical mechanisms and reaction kinetic models of the ortho-para hydrogen catalytic conversion process, and summarizes the performance of common catalysts. Finally, three mainstream schemes for ortho-para hydrogen conversion are compared. Research on the internal physical mechanisms and reaction kinetic models explores the conversion process from microscopic and macroscopic perspectives, respectively. Owing to the lack of experimental data, scholars have not yet formed a unified explanation for the surface characteristics of catalysts, which must be quantitatively validated. Furthermore, although nickel-based catalysts have higher catalytic efficiency, iron hydroxides and oxide catalysts are the main catalyst choices for ortho-para hydrogen conversion, considering the preparation, activation, and deactivation of catalysts and the characteristics of the liquefier. Among the three mainstream ortho-para hydrogen conversion schemes, the hydrogen liquefaction process with continuous conversion has the lowest energy consumption and is the future direction. Relevant research in China is still in its early stages and has great potential for development. This study provides theoretical guidance for the design and construction of ortho-para hydrogen catalytic conversion test benches.

Ortho-para hydrogen conversion is a key process in hydrogen liquefaction that reduces energy consumption and improves storage efficiency. Understanding the catalytic reaction kinetics and achieving real-time concentration monitoring are crucial for optimizing liquid hydrogen production. Among available techniques, Raman spectroscopy enables rapid, accurate, and efficient in situ measurement of ortho-para hydrogen concentration. Compared with traditional indirect methods that use gas chromatography, this technique meets the dynamic detection needs of the process and provides precise measurements, such as reaction kinetics, making it an advanced measurement technique. This study reviews the Raman scattering spectroscopy-based ortho-para hydrogen concentration measurement technique. Accordingly, the principle and composition of the optical detection system for this measurement technique are systematically explained. This study highlights the advantages and disadvantages of Raman spectroscopy compared with the methods of gas chromatography, nuclear magnetic resonance, and sound velocity by reviewing the development and evolution of the ortho-para hydrogen conversion measurement technique from its early use for macroscopic thermal conductivity measurements to its use for modern, in situ spectroscopic analyses. Finally, the study highlights the advantages of Raman spectroscopy in terms of its self-calibration and nondestructive, in situ, and rapid response capabilities. Overall, this study provides a theoretical basis and technical reference for the high-precision monitoring of ortho-para hydrogen in large-scale applications of hydrogen energy.

Hybrid photovoltaic-thermal (PV-T) collectors, which are capable of cogenerating useful thermal energy and electricity from the same aperture area, have a significantly higher overall efficiency and ability to displace emissions compared to independent, separate photovoltaic panels, solar thermal collectors or combinations thereof. Spectral splitting has emerged as a promising route towards next-generation high-performance PV-T collectors, and nanotechnology plays an important role in meeting the optical and thermal requirements of advanced spectral splitting PV-T collector designs. This paper presents a comprehensive review of spectral splitting technologies based on nanomaterials for PV-T applications. Emerging nanomaterials (nanofluids, nanofilms and nanowires) suitable for achieving spectral splitting based on reflection, diffraction, refraction and/or absorption approaches in PV-T collectors are presented, along with the associated challenges and opportunities of these design approaches. The requirements from such materials in terms of optical properties, thermal properties, stability and cost are discussed with the aim of guiding future research and innovation, and developing this technology towards practical application. Nanofluids and nanofilms are currently the most common nanomaterials used for spectral splitting, with significant progress made in recent years in the development of these materials. Nevertheless, there still remains a considerable gap between the optical properties of currently-available filters and the desired properties of ideal filters. Aiming to instruct and guide the future development of filter materials, a simple generalized method is further proposed in this paper to identify optimal filters and efficiency limits of spectral splitting PV-T systems for different scenarios. It is found that the optimal filter of a spectral splitting PV-T system is highly sensitive to the value of thermal energy relative to that of electricity, which therefore depends strongly on the application and location. The efficiency limit of spectral splitting PV-T collectors is significantly higher than that of standalone PV panels. The stability of nanomaterial filters remains a critical challenge for their long-term employment and also for high-temperature operation in practical applications.