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Spectral radiative transfer between particles in a semitransparent medium (i.e., water, glass, and air) needs to be considered when photo-transmission or high-temperature phenomenon is involved. However, the particle size distribution during application is generally an un-continuous change and conventional simplified treatments of particle size distributions would induce large radiative transfer differences between the calculated results and experimental data. In this study, the differential-integration method is developed and put forward based on measured non-uniform particle size distribution couppled with Mie scattering codes by the authors to calculate the spectral radiative properties of non-uniform size particles, which can obtain higher calculation accuracy. The Monte Carlo ray tracing (MCRT) method is adopted to solve the radiative transfer equation (RTE) of a semitransparent medium with non-uniform size particles. The research results indicated that the maximum relative transmittance error between experimental data and calculated data can decrease from 53.1% to 3.8%, when the optical properties of the particles with non-uniform size distribution were calculated using the differential-integration method to replace convectional effective uniform size distribution.

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.