posted on 2017-10-12, 00:00authored byAnthony
P. Straub, Menachem Elimelech
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/m2) 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.