利用施加壓差來提升滲透能源轉換與利用溫度梯度來改進電動能源轉換和除鹽表現

Abstract

由於對水資源和用電需求越來越高，採用奈米流體裝置來得到乾淨且永續的能源轉換以及淡水，是一種不錯的方法。具有離子選擇性這種特性的薄膜，擁有把天然梯度，像是濃度差、壓力差等，轉換成電能的潛力，還有乾淨的水也可以透過除鹽技術來生產。而想辦法提升能源轉換效率和除鹽表現，是具有效益的。 第一章節中，濃鹽差發電，是透過具有離子選擇性的孔洞，把混合的自由能轉換成電能，而這有不錯的潛力。針對實際應用層面，發展以薄膜為規模的多孔洞性材料是極其需要的。不幸的是，嚴重的濃度極化會使得所能獲得的功率明顯下降，尤其在孔密度很高的時候。為了緩解這個問題，我們提出，在具有多個奈米孔洞的薄膜兩端，施加額外壓差的方法，以及也有考慮所需消耗的有關功率。得到的結果顯示，施加負壓差可以提升所能獲得的功率，因為選擇性有所提升。此外，如果薄膜的孔密度很高的話，為了使所能獲得的能源具有經濟效益，增長孔道長度是必須的。舉例來說，如果是孔道長度為2000奈米與孔密度為2.5×〖10〗^9 pores/cm2的情況下，可以得到213 mW/m2的功率密度提升，藉著施加-1 bar的壓差，在pH 11和323 K下，成功獲取到正的淨功率密度。還有在不同條件下的系統表現以及相關機制亦會被仔細探討檢視。以上的研究成果已發表於國際期刊Physical Chemistry Chemical Physics。 第二章節中，由於對水資源和用電需求越來越高，藉由利用廢熱來施加溫度梯度，以達到提升電動能源轉換以及除鹽表現，貌似是個不錯的方法。此外，用一個對稱因子的參數，來表示不同孔道形狀，而不同孔道幾何的影響是有被考慮的。另外，有提出一個除鹽指標，用來評估除鹽表現，主要用以檢視除鹽率和體積流率之間的權衡。再者，針對產生的功率和能源轉換效率，NaCl的表現要比KCl來得好，這是因為鈉離子的擴散係數比鉀離子來得小。另一方面，因為溫差所造成的熱擴散影響對NaCl比較明顯，所以溫差對NaCl的除鹽率之影響，是比KCl來得顯著。然後由於選擇性的因素，施加正溫差會降低除鹽率，施加逆溫差則會提升除鹽率。此外，正壓差下的除鹽率會稍微比負壓差下的除鹽率來得大，因為從tip端施壓會有較大的進入電場強度。最後根據所提出之除鹽指標的評估，正溫差擁有比逆溫差還要好的除鹽表現，而這和電動能源轉換之表現的情況正好相反，也就是說，逆溫差的能源轉換效率是優於正溫差的能源效率。

As the demands for water resources and electricity are increasingly significant, adopting nanofluidic device for obtaining clean and sustainable energy conversion as well as freshwater is a great way. The membrane having the property of ion selectivity has a potential to transform natural gradient (e.g., salinity gradient, hydraulic pressure gradient) into electricity, and the clean water can also be produced through desalination. It is beneficial to improve the energy conversion efficiency and desalination performance. In chapter 1, salinity gradient power, which converts Gibbs free energy of mixing to electric energy through an ion-selective pore has a great potential. Towards practical use, developing membrane-scaled nanoporous materials is desirable and necessary. Unfortunately, the presence of a significant ion concentration polarization (ICP) lowers appreciably the power harvested, especially at a high pore density. To alleviate this problem, we suggest applying an extra pressure difference △P across a membrane containing multiple nanopores, taking account of the associated power consumption. The results gathered reveal that applying a negative pressure difference can improve the power harvested due to the enhanced selectivity. In addition, if the pore density of a membrane is high, raising its pore length is necessary to make the energy harvested economic. For example, if the pore length is 2000 nm and pore density 2.5×〖10〗^9 pores/cm2, an increment in power density of 213 mW/m2 can be obtained by applying △P=-1 bar at pH 11 and 323 K, where a net positive power density can be retrieved. The performance of the system considered under various conditions is examined in detail, along with associated mechanisms. In chapter 2, as the demands for freshwater and electricity are increasingly significant, exploiting waste heat to apply temperature gradients for improving the electrokinetic energy conversion (EKEC) and desalination performances seems a great way. In addition, the effect of different pore geometries, characterized by a symmetric factor, is taken into consideration. A desalination index is proposed to evaluate the performance of desalination, examining the trade-off between the ion rejection rate and the volumetric flow rate. Besides, the power production and efficiency of NaCl are better than those of KCl because the diffusivity of Na+ is smaller than that of K+. On the other hand, since the thermo-diffusion induced by temperature difference ∆T for the case of NaCl is more significant, ∆T influences rejection rate of NaCl more appreciably than that of KCl. Due to the selectivity, applying a positive (negative) ∆T lowers (raises) rejection rate. In addition, the rejection rate at positive pressure difference (∆P>0) is slightly larger than that at ∆P<0 because the former has a stronger inlet electric field. According to the assessment of desalination index proposed, a positive ∆T manifests a better desalination performance than a negative ∆T, which is opposite to the EKEC performance, that is, energy efficiency of negative ∆T is better than that of positive ∆T.