Department of Chemical and Environmental Engineering, Yale University , New Haven, Connecticut 06520-8286, United States.
Environ Sci Technol. 2014 May 6;48(9):5306-13. doi: 10.1021/es405173b. Epub 2014 Apr 24.
We present a novel hybrid membrane system that operates as a heat engine capable of utilizing low-grade thermal energy, which is not readily recoverable with existing technologies. The closed-loop system combines membrane distillation (MD), which generates concentrated and pure water streams by thermal separation, and pressure retarded osmosis (PRO), which converts the energy of mixing to electricity by a hydro-turbine. The PRO-MD system was modeled by coupling the mass and energy flows between the thermal separation (MD) and power generation (PRO) stages for heat source temperatures ranging from 40 to 80 °C and working concentrations of 1.0, 2.0, and 4.0 mol/kg NaCl. The factors controlling the energy efficiency of the heat engine were evaluated for both limited and unlimited mass and heat transfer kinetics in the thermal separation stage. In both cases, the relative flow rate between the MD permeate (distillate) and feed streams is identified as an important operation parameter. There is an optimal relative flow rate that maximizes the overall energy efficiency of the PRO-MD system for given working temperatures and concentration. In the case of unlimited mass and heat transfer kinetics, the energy efficiency of the system can be analytically determined based on thermodynamics. Our assessment indicates that the hybrid PRO-MD system can theoretically achieve an energy efficiency of 9.8% (81.6% of the Carnot efficiency) with hot and cold working temperatures of 60 and 20 °C, respectively, and a working solution of 1.0 M NaCl. When mass and heat transfer kinetics are limited, conditions that more closely represent actual operations, the practical energy efficiency will be lower than the theoretically achievable efficiency. In such practical operations, utilizing a higher working concentration will yield greater energy efficiency. Overall, our study demonstrates the theoretical viability of the PRO-MD system and identifies the key factors for performance optimization.
我们提出了一种新颖的混合膜系统，它可以作为一种热机运行，利用现有的技术难以回收的低品位热能。该闭环系统结合了膜蒸馏（MD）和压力延迟渗透（PRO），MD 通过热分离产生浓缩和纯净的水流，而 PRO 通过水轮机将混合能转化为电能。该 PRO-MD 系统通过耦合热分离（MD）和发电（PRO）阶段的质量和能量流进行建模，热源温度范围为 40 至 80°C，工作浓度为 1.0、2.0 和 4.0 mol/kg NaCl。对于热分离阶段的有限和无限传质和传热动力学，评估了控制热机能量效率的因素。在这两种情况下，MD 渗透物（馏出物）和进料流之间的相对流量被确定为重要的操作参数。对于给定的工作温度和浓度，存在一个最佳的相对流量，可以使 PRO-MD 系统的整体能量效率最大化。在无限传质和传热动力学的情况下，可以根据热力学对系统的能量效率进行分析确定。我们的评估表明，对于热和冷工作温度分别为 60 和 20°C，工作溶液为 1.0 M NaCl 的情况，理论上该混合 PRO-MD 系统的能量效率可以达到 9.8%（卡诺效率的 81.6%）。当传质和传热动力学受到限制时，即更接近实际操作的情况时，实际的能量效率将低于理论上可达到的效率。在实际操作中，使用较高的工作浓度将产生更高的能量效率。总的来说，我们的研究证明了 PRO-MD 系统的理论可行性，并确定了性能优化的关键因素。
