用于光伏电池热管理的硬脂酸和棕榈酸混合物稳定膨胀石墨 (EG)

C Pub Date : 2024-05-15 DOI:10.3390/c10020046
Sereno Sacchet, F. Valentini, Alice Benin, Marco Guidolin, Riccardo Po, L. Fambri
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引用次数: 0

摘要

在这项工作中,开发并分析了用于改造现有硅光伏(PV)电池的被动冷却系统,以减轻高温季节温度升高造成的效率损失。为此,使用了膨胀石墨(EG)来稳定熔化温度接近 53 °C 的相变材料(PCM),以实现热管理系统(TMS),在熔化过程中恒温储存热量,并在结晶过程中释放热量。其中,硬脂酸和棕榈酸混合物(PA-SA)在不同浓度(10、12 和 14 百分之一的比例)的 EG 中进行真空形状稳定,然后在旋转蒸发装置中进行冷压实;由于 PA-SA 在石墨薄片之间的插层作用,减少了 PA-SA 的泄漏;通过粉末冷压实,最大限度地减少了空隙并创建了优先导热模式,提高了热量传递到散装 TMS 所需的热导率。通过差示扫描量热法(DSC)以 1 °C/min 的速度对稳定至 150 °C 的复合材料进行了测试,以精确确定相变温度和焓含量,由于稳定所需的 EG 分数非常低,焓含量仅从纯 PCM 的 196 J/g 略微降低到 169 J/g。尽管 EG 与 PA-SA 的比例仅为 14:100,但该系统的热导率却比纯 PCM 提高了 40 倍(从 0.2 W/(m K) 提高到 8.3 W/(m K),这是现有文献中从未达到的数值),热盘测试和激光闪光分析 (LFA) 所评估的数值趋于一致,并发现与石墨含量和密度都有关联。为了完全避免 PCM 在最终应用过程中泄漏并随之散失到环境中,所有样品都封装在聚乙烯薄膜中。在 30 °C 和 80 °C 下分别进行了压缩试验,模拟了为保持 TMS 在光伏电池背面的位置所需的接触而可能产生的压缩应力,从而评估了机械性能。最后,通过在气候箱中施加热循环来模拟材料的反应,并再现了意大利维罗纳(威尼托,北纬 45°,东经 11°)和杰拉(西西里岛,北纬 37°,东经 14°)两地 2022 年夏季最热和最冷的三天,从而突出了温度上升延迟和日峰值温度平滑的热管理效果。为了在 PCM 的熔化焓和 EG 带来的热导率增强之间实现适当的折衷,EG 对加工和由此产生的复合材料的性能具有重要作用。
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Expanded Graphite (EG) Stabilization of Stearic and Palmitic Acid Mixture for Thermal Management of Photovoltaic Cells
In this work, passive cooling systems for the revamping of existent silicon photovoltaic (PV) cells were developed and analysed in order to mitigate the efficiency loss caused by temperature rise in the hot season. For this purpose, expanded graphite (EG) was used to stabilize a phase change material (PCM) with a melting temperature close to 53 °C in order to realize thermal management systems (TMSs) able to store heat at constant temperature during melting and releasing it in crystallization. In particular, stearic and palmitic acid mixture (PA-SA) was shape-stabilized in EG at different concentrations (10, 12 and 14 part per hundred ratio) under vacuum into a rotary evaporation apparatus followed by cold compaction; PA-SA leakage was reduced due to its intercalation between the graphite lamellae, and the thermal conductivity necessary to maximize the heat transfer to a bulk TMS was improved via powder cold compaction, which minimizes voids and creates preferential thermal conductive patterns. The composite materials, stable till 150 °C, were tested by differential scanning calorimetry (DSC) at 1 °C/min to precisely determine the phase transition temperatures and the enthalpic content, which was only slightly reduced from 196 J/g of the neat PCM to 169 J/g due to the very low EG fraction necessary for the stabilization. Despite only the 14:100 EG-to-PA-SA ratio, the system’s thermal conductivity was enhanced 40 times with respect to the neat PCM (from 0.2 to 8.3 W/(m K), value never reached in works present in the literature), with a good convergence of the values evaluated through hot disk tests and laser flash analysis (LFA), finding correlation with both graphitic content and density. In order to completely avoid leaking with the consequent dispersion of PCM in the environment during the final application, all the samples were encapsulated in a PE-made film. The mechanical properties were evaluated with compression tests at 30 °C and 80 °C simulating a possible compressive stress deriving from the contact needed to maintain the TMS position on the rear of the PV cells. Finally, the material response was simulated by imposing thermal cycles into a climatic chamber and reproducing the three hottest and coldest days of summer 2022 of two Italian locations, Verona (Veneto, 45° N, 11° E) and Gela (Sicily, 37° N, 14° E), thus highlighting the thermal management effects with delays in temperature increase and daily peak temperature smoothing. The role of EG is strategic for the processing and the properties of the resulting composites in order to realize a proper compromise between the melting enthalpy of PCM and the thermal conductivity enhancement given by EG.
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