Overall statement:

The paper presents an investigation into the energy and exergy performance of a nanofluid-based PV/T system. The paper aims to predict the transient response of the metrics. The PV/T uses a trapezoidal-shaped pipe as it covers more area of the collector; improve the heat transfer by increasing the area of contact. The authors used energy balance equations, which then were solved using the MATLAB ODE solver, to derive the transient temperatures. Two types of nanofluids were used to observe their effect on the energy and exergy performance of the system and to compare them to a water-based PV/T.

The strengths:

1) The introduction presents a good research background and the problem statement of the study. 2) The authors describe the difference between the circular and trapezoidal pipes effectively. 3) The system design and specifications are well laid-out in detail. 4) The mathematical model is well displayed; providing assumptions, validation source and Model calibration. 5) The results and discussion section provide a description of the findings and a thorough discussion.

The weaknesses:

1. Although irreversibility is discussed briefly in the paper, it is preferable to show the irreversibility throughout the day.

Discussion:

The CuO nanofluid-based PV/T with trapezoidal pipe exhibits higher pumping losses, due to higher pressure drop, when increasing the concentration of the nanofluid, compared to the other configurations. In either case, the trapezoidal pipe shows more pressure drop than the circular pipe, although by small amount. Essentially, the CuO nanofluid shows higher thermal conductivity than the Al2O3 nanofluid. Moreover, the findings suggest that maximum cooling was achieved when using the CuO nanofluid in the system.

• This is a post-publication review. The adherence to journal guidelines is not considered as the paper has already been processed by the editorial team of the journal