Yang Pengju 1, Zeng Congchao 2, Yao Shan 1, Sun Jian 2
(1. Gansu Engineering Design & Research Institute Co., Ltd., Lanzhou 730030, Gansu, China;
2. Jingdezhen Ceramic University, Jingdezhen 333403, Jiangxi, China)
Extended abstract:
[Background and purposes] Building-integrated photovoltaic/thermal (BIPV/T) systems suffer from severe heat accumulation caused by confined rear ventilation, which triggers obvious attenuation of photovoltaic power generation efficiency and restricts the building thermal comfort. To address the thermal management bottleneck of BIPV/T devices, we proposed a composite PV/T-PCM system integrated with paraffin phase change material (PCM), biomimetic fin structures and serpentine cooling tubes and introduce ceramic-based nanofluids as active cooling media to strengthen heat transfer. It was aimed to clarify the coupling influence of nanofluid types and flow velocity on heat transfer behaviors, thermoelectric output and exergy performance of the integrated system, and provide theoretical guidance for cooling medium selection and operating parameter optimization of BIPV/T-PCM engineering applications.
[Methods] A three-dimensional geometric model of the BIPV/T-PCM composite system with overall dimensions of 478 mm× 316 mm was established. The enthalpy-porosity method was adopted to solve the melting and solidification process of paraffin PCM, while mass, momentum and energy governing equations were constructed for solid components, cooling fluid and PCM regions respectively. Four cooling media, namely pure water, Al2O3/H2O, SiC/H2O and Ag-rGO/H2O ceramic-based nanofluids (volume fraction of nanoparticles=0.1%), were selected as research objects. Transient numerical simulations were carried out via ANSYS Fluent under a solar irradiance of 900 W·m–2 at ambient temperature of 300 K. The finite volume method and PISO algorithm were used for equation discretization and pressure-velocity coupling. Grid independence verification and time step independence verification were implemented to balance simulation accuracy and computational cost, while the reliability of the numerical model was validated against published experimental data. Variations of PV average temperature, PCM liquid fraction, fluid outlet temperature, thermoelectric efficiency and exergy efficiency under flow velocities ranging from 0.01 m·s−1 to 0.10 m·s−1 were comparatively analyzed.
[Results] All three ceramic-based nanofluids possess superior thermal conductivity and cooling performance compared with pure water. The Ag-rGO/H2O delivers the optimal heat dissipation effect. When the flow velocity rises from 0.01 m·s−1 to 0.10 m·s−1, the maximum temperature drop of PV cells reaches 14 K and the average PV temperature drops to 314 K (41 ℃) at 0.1 m·s−1, which is 2.46–3.41 K lower than the value with pure water. Ceramic-based nanofluids accelerate heat transfer from PV panels to cooling loops and thus reduce the heat stored by PCM, leading to a lower PCM liquid fraction under identical flow conditions, and the outlet temperature of Ag-rGO/H2O reaches 305 K at 0.10 m·s−1, 6.4 K higher than that of pure water. The thermoelectric efficiency and total exergy efficiency of the system are markedly promoted by ceramic-based nanofluids, and Ag-rGO/H2O improves the overall system efficiency by 6.2% and total exergy efficiency by 1% relative to pure water; the net total efficiency continuously increases with rising flow velocity, and Ag-rGO/H2O maintains the highest net efficiency across the tested velocity range. Moreover, the cooling improvement induced by velocity increase gradually weakens, once the flow velocity exceeds 0.03 m·s−1, accompanied by continuous reduction of PCM liquid fraction and thermal exergy efficiency, which demonstrates that excessive flow velocity weakens the latent heat storage capacity of PCM and increases the exergy loss of hot cooling fluid.
[Conclusions] Ceramic-based nanofluids can effectively suppress PV temperature rise and enhance the comprehensive energy utilization performance of BIPV/T-PCM systems. The Ag-rGO/H2O outperforms Al2O3/H2O and SiC/H2O in cooling and energy efficiency promotion. Increasing cooling medium flow velocity remarkably reduces PV operating temperature, but weakens the phase change heat storage capacity of PCM simultaneously. Matching appropriate ceramic-based nanofluid and reasonable flow velocity can realize coordinated optimization of heat transfer characteristics, power generation and thermal output of the integrated system. This research offers reliable theoretical support for thermal management design and popularization of building-integrated solar energy equipment.
Key words: BIPV/T-PCM; ceramic-based nanofluid; heat transfer enhancement; thermoelectric performance; exergy analysis