Experimental and Numerical Study of Pulsating Water and Nanofluid Flow in Photovoltaic-Thermal Solar Collectors

Abstract

Photovoltaic thermal (PVT) systems are designed to generate electricity and useful heat from the same surface, but their performance is often limited by high photovoltaic (PV) cell temperature and weak heat transfer under laminar cooling conditions. Most previous work has used steady water flow and geometric modifications to improve PVT cooling, which can increase complexity and cost. Pulsating flow and nanofluids have shown heat transfer benefits in other applications, but their effect on full-scale flat-plate PVT collectors has not been clearly quantified. This work investigates the influence of controlled pulsating flow on the thermal and electrical performance of a flat-plate water-cooled PVT system under laminar conditions. Two experimental campaigns were carried out using the same indoor solar simulator with an average light intensity (I) between 700 and 800 W/m2. In the first part, water was used as the working fluid and the PVT system was tested under uncooled, continuous (steady)-flow and pulsating-flow operation. Flow rates of 1-4 L/min were examined with pulsation frequencies of 0.25, 0.5, 1 and 2 Hz. System performance was evaluated against uncooled and continuous-flow reference cases. Pulsating operation reduced the PVT surface temperature and increased thermal efficiency compared with continuous flow, while electrical efficiency showed a smaller but consistent improvement. The frequency of 0.5 Hz obtained the best performance, with thermal efficiencies above 50% at higher flow rates and electrical efficiency around 9.8% without a measurable increase in average pressure drop. In the second part of the thesis study, a 0.1 vol.% Al2O3/water nanofluid was used at a fixed flow rate of 4 L/min under continuous and pulsating flow. Frequencies from 0.25 to 2 Hz were tested and supported by a three-dimensional (3D) transient Computational Fluid Dynamics (CFD) model of the PVT channel. Pulsating nanofluid cooling further reduced PVT surface temperature, with the 2 Hz case giving the lowest measured value of about 30 degrees C and a maximum thermal efficiency increase of roughly 22% compared with continuous nanofluid flow. Electrical efficiency gains remained modest, on the order of 1%, which confirms that the main benefit lies in improved thermal recovery. The CFD predictions reproduced the experimental trends closely. Overall, the results show that pulsation frequency can be used as a control parameter to enhance the thermal performance of flat-plate PVT systems while maintaining laminar flow and only small changes in electrical efficiency.