Osmotic Heat Engine Using Thermally Responsive Ionic Liquids

Abstract

The osmotic heat engine (OHE) is a promising technology for converting low grade heat to electricity. Most of the existing studies have focused on thermolytic salt systems. Herein, for the first time, we proposed to use thermally responsive ionic liquids (TRIL) that have either an upper critical solution temperature (UCST) or lower critical solution temperature (LCST) type of phase behavior as novel thermolytic osmotic agents. Closed-loop TRIL-OHEs were designed based on these unique phase behaviors to convert low grade heat to work or electricity. Experimental studies using two UCST-type TRILs, protonated betaine bis(trifluoromethyl sulfonyl)imide ([Hbet][TfN]) and choline bis(trifluoromethylsulfonyl)imide ([choline][TfN]) showed that (1) the specific energy of the TRIL-OHE system could reach as high as 4.0 times that of the seawater and river water system, (2) the power density measured from a commercial FO membrane reached up to 2.3 W/m, and (3) the overall energy efficiency reached up to 2.6% or 18% of the Carnot efficiency at no heat recovery and up to 10.5% or 71% of the Carnet efficiency at 70% heat recovery. All of these results clearly demonstrated the great potential of using TRILs as novel osmotic agents to design high efficient OHEs for recovery of low grade thermal energy to work or electricity.

Phase separation of an ionic liquid mixture assisted by a temperature gradient

“…Last decade, temperature-responsive binary systems made of ionic liquids (IL) and water have shown promising applications such as in metal and rare-earth extraction [1,2], protein stabilisation [3,4], catalytic synthesis [5,6], forward osmosis water desalination [7][8][9] and low grade heat valorization [10,11]. Those techniques are built on the reversible phase separation of the system with temperature loops.…”

Thermally responsive ionic liquids and polymeric ionic liquids: emerging trends and possibilities

“…methanol have been used for hybrid PRO-MD but suffered from high volatility [42]. The advantage of using thermally responsive IL with inherent low vapor pressure, high osmotic pressure, and low crossover due to larger size of ions was first demonstrated by Zhong et al [43] (see Figure 2). As noticeable from figure 2(b) and 2(c) the operation of OHE requires two different working temperature i.e.…”

“…By appropriate choice of IL or IL mixtures and membranes the power density of the OHE can be further improved. Reprinted with permission from [43] Copyright (2015) American Chemical Society.…”

Heat recovery in energy production from low temperature heat sources

“…Novel techniques include thermogalvanic cells, electrochemical heat engines based on batteries or supercapacitors, complexation‐based thermally regenerable batteries, osmotic engines with thermally responsive liquids . Among these techniques, only the electrochemical heat engines can reach efficiencies of the order of 10%; however, in the experiments, the heat sink had to be cooled well below 25°C, in order to avoid parasitic reactions, thus the feasibility in real situations is still questionable or limited only to a few geographical regions.…”