Nanofluids and phase-change materials for thermal management in photovoltaic/thermal systems: A critical review of performance, reliability, sustainability, and scale-up

Photovoltaic/thermal (PV/T) collectors recover heat while limiting temperature-related photovoltaic losses, but reported nanofluid and phase-change material (PCM) benefits remain difficult to compare because formulations, boundary conditions, parasitic loads, test durations and performance definitions vary. This critical review evaluates device-level studies on mono- and hybrid nanofluids, spectral beam splitting, PCMs, nano-enhanced PCMs and thermoelectric-generator (TEG) integration using a structured, non-meta-analytic evidence appraisal. Thermal mechanisms, numerical assumptions, experimental quality, ageing, economics and life-cycle implications are compared. Selected studies report panel-temperature reductions up to 16.6 °C for hybrid-nanofluid cooling, collector efficiency near 58% for an Ag–SiO₂/carbon spectral filter and a 10.4 °C local reduction with an optimized PV–TEG interface. These are condition-specific outcomes, not universal effect sizes, and depend on irradiance, flow, concentration, optical path, heat rejection and reference configuration. Hybrid nanofluids should not be labelled synergistic unless they outperform matched-concentration mono-fluid predictions while maintaining acceptable pressure drop and stability. PCM conductivity enhancement can improve heat spreading but may reduce latent capacity or prevent full daily regeneration. Multi-component systems broaden functionality but accumulate contact-resistance, control, maintenance, toxicity and cost risks. The review contributes a condition-aware framework linking gross performance to net energy, reliability, sustainability and application-specific scale-up. Standardized reporting of material characterization, uncertainty, parasitic power, cycling and end-of-life pathways is required to translate laboratory gains into deployable PV/T performance.