在一含真空管集熱器之吸附式製冷系統之效能分析

Abstract

本研究利用氙弧燈模擬太陽光做為熱源，針對吸附式製冷系統之吸附器進行效能分析，探討影響脫附量、各元件吸熱量、理論COP及真空管熱傳效率的不同因素，包括吸附器出口之管壁溫度、初始壓力及活性碳孔隙率，並進行吸附器及真空管集熱器之熱阻分析。所使用之工作對為活性碳及甲醇，其中初始壓力有三種，分別為6.06 kPa、7.12 kPa、8.07 kPa，活性碳孔隙率有三種，分別為36.9%、21.6%、18.1%，吸附器出口之管壁溫度T3有四種，分別為80.2°C、90.2°C、100°C、110°C，共36種組合。 實驗結果顯示，不論在任何初始壓力及活性碳孔隙率下，理論COP隨吸附器出口之管壁溫度增加而上升，但若吸附器出口之管壁溫度T3過高，則理論COP增加幅度變小甚至下降，當吸附器出口之管壁溫度為100°C時，會有最高之理論COP及真空管熱傳效率。猜測這是因為真空管之環境熱損耗在吸附器出口之管壁溫度T3為110°C時變大，反而造成理論COP下降，導致吸附器出口之管壁溫度 為100°C時，會有最高之理論COP。 當活性碳孔隙率為最小 (18.1%) 時，理論COP及真空管熱傳效率較高，這是因為吸附器內之活性碳量增加，能吸附更多之甲醇，並使甲醇脫附量增加。而活性碳孔隙率下降，造成工作對熱傳導係數上升的影響並不明顯，對於工作對加熱至指定溫度所需之加熱時間影響不大，因此活性碳孔隙率下降，理論COP及真空管熱傳效率上升。 初始壓力的改變，對理論COP及真空管熱傳效率並沒有明顯影響。雖然初始壓力增加會使甲醇之脫附量增加，但也會造成節流閥出口處甲醇之飽和溫度上升、蒸發焓下降，使得製冷量幾近定值，因此初始壓力雖可得到較高之脫附量，但對於理論COP及真空管熱傳效率則幾乎沒有影響。

In this study, we focus on investigating the performance of the solar collector of a evacuated tube in the adsorption refrigeration system. Using methanol and activated carbon as the working pair, the desorbed mass is measured under different initial pressure so that the theoretical COP is calculated. The effects of outlet temperature of the adsorber, the initial presure, and the porosity of the activated carbon are discussed. In addition, we employ the thermal resistance analysis to determine the heat transfer associated with different compoents of the adsorber. The experimental results show that when outlet temperature of the adsorber is 100°C, the maximal theoretical COP and collector efficiency can be reached. The theoretical COP and the collector efficiency increase as the outlet temperature of the adsorber increases. In contrast, once the outlet temperature of adsorber is increased to 110°C, heat loss of the evacuated tube increases and the required heating also augment. Hence, the theoretical COP and the collector efficiency would decrease or be the same value as the outlet temperature of the adsorber is 100°C. When the porosity of activated carbon is decreased to 18.1%, the maximal theoretical COP and the collector efficiency can be achieved. When the amount of activated carbon in the adsorber increases, more methanol can be adsorbed and the amount of desorbed methanol also increases. On the other hand, the effect of the porosity of activated carbon on the effective thermal conductivity of the working pair is negligible. As a result, the effect of the amount of desorbed methanol dominates the role the porosity plays in COP. We also find the theoretical COP and collector efficiency are nearly independent of the initial pressure. Increasing the initial pressure leads to a higher saturation temperature for methanol at the exit of the throttling valve with lower enthanlpy of vaporiation. Consequently, the cooling capacity would almost be constant and The effect of initial pressure is negligible.