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Explore the performance limit of a solar PV – thermochemical power generation system
Author: Li Wenjia, Hao Yong | Print | Close | Text Size: A A A | 2018-11-30

Performance limit of a solar hybrid power generation system integrating efficient photovoltaic (PV) cells and methanol thermal (T) decomposition is explored from a thermodynamic perspective within the capability of state-of-the-art technologies. This type of PVT system features potentially high “net solar-to-electric efficiency” in general, primarily resulting from a key difference in the design of the thermal part compared with conventional PVT systems, i.e. replacing heat engines by a thermochemical power generation module for thermal energy utilization. Key design parameters of the system, including PV cell type, emissivity, solar concentration ratio and solar concentrator type, are individually studied. A system combining all such optimized aspects is projected to achieve net solar-to-electric efficiencies up to 51.5%, after taking all major (e.g. optical, radiative) losses into consideration. This study reveals important insights and enriches understanding on design principles of efficient PVT systems aimed at comprehensive and effective utilization of solar energy.

Conclusions

Comprehensive analysis and optimization of a PVT system integrating solar photovoltaic cells and a methanol thermochemistry module for efficient power generation with solar input are carried out. Results show that PV cells in general promote net solar-to-electric efficiency of the hybrid system regardless of their type, and the reason mainly lies in exergy loss reduction during solar collection by partially converting solar energy into electricity first and the remainder into low-/mid-temperature heat (instead of directly converting all solar energy into heat) for subsequent thermal-to-power conversion. We also show that the net solar-to-electric efficiency can be increased from 43% to 45% by lowering emissivity (down to 0.2), or from 43% to 49% by increasing the concentration ratio (up to 1000). Such improvements partially result from PV efficiency increase, and partially from the minimization of radiative and/or convective heat losses compared with the total solar energy input, although through different ways. In particular, high surface emissivity of PV cells results in a significantly higher portion of radiative heat losses compared with the solar energy input in the hybrid system (e.g. 8% at 250 °C) than that in the reference system (e.g. 3% at 250 °C, in the absence of a PV module), underlining the necessity of modifying surface energy-exchange properties of PV cells in their PVT applications in consideration of system level performance maximization. Key considerations for efficient PVT system designs are summarized, and a theoretical net solar-to-electric efficiency of 51.5% is projected for an optimized system. In addition, the necessary replacement of trough solar collector by Fresnel collector (due to concentration ratio limitation) in the pursuit of higher performance could further bring down the PV material cost and bring potentials in device miniaturization and distributed power generation.

The results have been published on Applied Energy 206 (2017) 843-850.

Fig.1. Illustration of the solar PV-thermochemical hybrid system; (a) flowchart, in which shaded rectangles represent PV cells, while the reference system does not have such PV cells; (b) cross-section view of the hybrid system based on solar trough collector; (c) cross-section view of the reference system based on solar trough collector.

 
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