Department of Chemical and Environmental Engineering, Yale University , P.O. Box 208286, New Haven, Connecticut 06520-8286, United States.
Environ Sci Technol. 2017 Nov 7;51(21):12925-12937. doi: 10.1021/acs.est.7b02213. Epub 2017 Oct 26.
Low-grade heat energy from sources below 100 °C is available in massive quantities around the world, but cannot be converted to electricity effectively using existing technologies due to variability in the heat output and the small temperature difference between the source and environment. The recently developed thermo-osmotic energy conversion (TOEC) process has the potential to harvest energy from low-grade heat sources by using a temperature difference to create a pressurized liquid flux across a membrane, which can be converted to mechanical work via a turbine. In this study, we perform the first analysis of energy efficiency and the expected performance of the TOEC technology, focusing on systems utilizing hydrophobic porous vapor-gap membranes and water as a working fluid. We begin by developing a framework to analyze realistic mass and heat transport in the process, probing the impact of various membrane parameters and system operating conditions. Our analysis reveals that an optimized system can achieve heat-to-electricity energy conversion efficiencies up to 4.1% (34% of the Carnot efficiency) with hot and cold working temperatures of 60 and 20 °C, respectively, and an operating pressure of 5 MPa (50 bar). Lower energy efficiencies, however, will occur in systems operating with high power densities (>5 W/m) and with finite-sized heat exchangers. We identify that the most important membrane properties for achieving high performance are an asymmetric pore structure, high pressure resistance, a high porosity, and a thickness of 30 to 100 μm. We also quantify the benefits in performance from utilizing deaerated water streams, strong hydrodynamic mixing in the membrane module, and high heat exchanger efficiencies. Overall, our study demonstrates the promise of full-scale TOEC systems to extract energy from low-grade heat and identifies key factors for performance optimization moving forward.
低品位热能的热源温度低于 100°C，在全球范围内有大量存在，但由于热源输出的可变性和源与环境之间的小温差，利用现有技术无法有效地将其转换为电能。最近开发的热渗透能量转换（TOEC）过程有可能利用低品位热源的能量，通过使用温度差在膜两侧产生加压液体流，然后通过涡轮机将其转换为机械能。在这项研究中，我们首次对 TOEC 技术的能量效率和预期性能进行了分析，重点研究了利用疏水性多孔蒸汽隙膜和水作为工作流体的系统。我们首先开发了一个框架来分析该过程中的实际质量和热传递，探究了各种膜参数和系统运行条件的影响。我们的分析表明，在分别为 60°C 和 20°C 的热和冷工作温度下，以及 5 MPa（50 巴）的操作压力下，优化后的系统可以实现高达 4.1%的热能-电能能量转换效率（卡诺效率的 34%）。然而，在以高功率密度（>5 W/m）和有限尺寸的热交换器运行的系统中，能量效率会较低。我们确定了实现高性能的最重要的膜特性是不对称的孔结构、高耐压性、高孔隙率和 30 至 100μm 的厚度。我们还量化了利用除气水流、膜组件中的强流体力学混合以及高热交换器效率带来的性能提升。总体而言，我们的研究表明，TOEC 系统有潜力从低品位热能中提取能量，并确定了今后性能优化的关键因素。
