微太陽能集熱元件之設計與熱傳分析

Design and Thermal Analysis of a Microscale Solar Collector

李育霖(Yu-Lin Li)

指導教授 ： 孫珍理

國立臺灣大學/工學院/機械工程學研究所/碩士(2015年)

https://doi.org/10.6342/NTU.2015.01249

摘要

本研究利用太陽光模擬器作為熱源，探討不同微集熱元件之流道設計、吸收層材質與工作流體對於熱交換效率的影響。我們藉由量測工作流體在流道之出入口溫度差計算單位體積之焓差，並利用熱像儀量測微集熱元件之表面溫度，其中流道設計包括蛇型流道 (serpentine)、雙蛇型流道 (double serpentine)、歧管型流道 (oblique-rib) 與棋盤型流道 (rod-bundle) 四種，工作流體則有水、乙醇、質量分率為0.3與0.001之太古油乳化劑四種。
實驗結果顯示，微集熱元件之吸收層須以高放射率之金屬材料為首選，放射率越高，能使微集熱元件從熱輻射中吸收更多熱。而工作流體的部分，則是水與質量分率為0.001之太古油乳化劑有較好的表現，所造成之工作流體之出入口溫度差、工作流體單位體積焓差與微集熱元件效率皆較高。而質量分率為0.3之太古油乳化劑表現稍微遜色，乙醇則是最不適合使用。在流道設計方面，則是以蛇型流道與雙蛇型流道的表現最佳，更可凸顯工作流體為水與質量分率為0.001之太古油乳化劑時的優勢，棋盤型流道表現較一般，但由於歧管型流道在入出口的對角線位置會造成流體停滯的盲區 (dead zones)，導致工作流體所吸收的太陽熱能最少，表現最差。一般而言，體積流率增加會使工作流體出入口溫度差與工作流體單位體積焓差下降，但微集熱元件效率則是會隨著體積流率增加而上升。本研究所得之結果可協助我們了解微集熱器吸收太陽能熱輻射的機制與主宰之重要參數，以利我們在未來將乳化劑進一步運用在中溫朗肯熱循環系統中。

關鍵字

太陽能 ； 朗肯循環 ； 乳化劑 ； 微流元件設計 ； 熱傳分析

並列摘要

This study focuses on the design of the microscale solar collector for a medium-temperature Rankine Cycle. The influences of the material of the absorption layer, the working fluid, and the microchannel design on the efficiency of the microscale solar collector are discussed. Three different working fluids are tested: water, ethanol, sulfated castor oil/water emulsion with a mass fraction of 0.3 and sulfated castor oil/water emulsion with a mass fraction of 0.001. Experimental results show that the absorption layer of microscale solar collector needs to be a high emissivity metal material, the higher the emissivity, the more heat from the radiation can be absorbed by the microscale solar collector. We measure the temperature difference between inlet and outlet to calculate the enthalpy increase per unit volume, which serves as a reference to evaluate the thermal performance of the collector. From the results, we find that water and sulfated castor oil/water emulsion with a mass fraction of 0.001 lead to the highest efficiency. On the other hand, increasing the mass fraction to 0.3 brings moderate performance, and ethanol results in the poorest efficiency. Among the four microchannel designs, serpentine and double serpentine channel have the best outcome. This is because serpentine and double serpentine channel have higher overall heat transfer coefficient than oblique-rib and rod-bundle channel. In contrast, there exists dead zones in the diagonal corner of the oblique-rib channel design, where working fluid is stagnant and convection is poor, so the performance of oblique-rib channel is the worst. The temperature difference and the enthalpy difference per unit volume both decreases with the increase of the volumetric flow rate and the efficiency of the microscale solar collector is actually enhance by increasing volumetric flow rate. The results of this study help to elaborate the heat transfer mechanisms and the leverage between different important parameters in the solar collector design, which will serve as the foundation to employ emulsions in a medium-temperature Rankine-cycle system in the future.

並列關鍵字

solar ； Rankine cycle ； emulsion ； microchannel design ； thermal analysis

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