Integration of thermo-electrochemical conversion into forced convection cooling

Abstract

Forced convection cooling is important in numerous technologies ranging from microprocessors in data centers to turbines and engines; active cooling is essential in these situations. However, active transfer of heat or thermal energy under a large temperature difference promptly destroys the exergy, which is the free-energy component of thermal energy, and this issue has remained unaddressed. Herein, we describe a thermoelectrochemical conversion to partially recover presently lost exergy in forced convection cooling. We design a test cell in which an electrolyte liquid is forced through a channel formed between two parallel electrodes and the hot-side electrode simulates an object to be cooled. Our investigations show that the narrower interelectrode channels afford higher cooling and power generation performances. The mass transfer resistance is the most dominant type of resistance for all the conditions tested and the charge transfer kinetics is likely to be controlled by viscosity. The dependence of the generated power on the flow rate is caused by the change in the diffusion coefficient of redox species with temperature. As an evaluation measure for such forced-flow thermocells, the gain ()-defined as the ratio of the generated power to the hydrodynamic pumping work required to force the liquid through the cellis introduced.  is above unity in a certain flow rate region. This demonstrates that such a system can generate more electric power than the pump work required to drive the liquid through the cell, suggesting its potential to partly recover presently lost exergy of thermal energy as electricity. Broader Context (maximum 200 words)In our present civilization, forced convection cooling is used in wide-ranging situations from cooling of microprocessors in data centers to that of heat engines including turbines and automobile engines. Active cooling is essential in such situations to avoid thermal failure (for microprocessors) and maximize fuel-to-work conversion efficiencies (heat engines). Here, "active cooling" means the prompt removal of a large quantity of thermal energy in the heat source by a working fluid under a large temperature difference. However, this causes rapid destruction of the exergy, which is the free-energy component of the thermal energy. This issue has remained unaddressed despite the widespread use of forced convection cooling. In this study, to partially recover presently lost exergy in such situations, we integrate thermoelectrochemical conversion, which has been mostly studied for stationary conditions, into forced convection cooling. Through experimental and numerical investigations of a test cell in which the hot-side electrode simulates an object to be cooled, several fundamental properties of such a forced-flow cell are obtained. Our results indicate that such a forced-flow thermocell can generate a larger electric power than the hydrodynamic pumping work required to force the liquid through the cell unit, justifying the concept of this kind of thermocell.

Thermogalvanic energy harvesting from forced convection cooling of 100–200 °C surfaces generating high power density

“…Both the power generation density and cooling efficiency at the heat-releasing surface are important performance measures of such a combined-purpose system. Thus far, previous studies, including ours, reported power densities of 0.25, 22 0.05, 23 0.36, 28 and 0.44 29 W m −2 along with heat transfer coefficients, which are a measure of cooling efficiency (given by eqn (2) later), of 450 23 and 620 29 W m −2 K −1 . While the latter values were moderate, 36 the power densities were too low for this concept to be considered feasible.…”

“…1a) has been unexplored, except for a few previous works that reported limited power densities of only 0.05−0.5 W m −2 . 22,23,28,29 Such a concept is inherently challenging because of the necessity of simultaneously designing the fluid dynamics, heat transfer, and ion transport in the non-isothermal channel region through which the electrolyte passes, where the latter also acts as a coolant. Therefore, optimal design of the flow in the channel region and choice of the electrolyte liquid, as well as optimal choice of materials to favour green and durable systems, are crucial to render this concept feasible.…”

“…3,800 USD/ton 43 for acetonitrile, one of the most common solvents for electrochemical applications 44 ). One of the reasons we optimized the solvent was because, in our preceding studies 22,29 which used 1-ethyl-3-methylimidazolium NTf2 ([C2mim][NTf2]) ionic liquid (IL) as the solvent, the power generation performance was primarily limited by the mass-transfer resistance in the electrolyte, despite the ultrahigh density of solvent ions that led to low solution resistance; this mass-transfer dominance originated from the high viscosity of [C2mim][NTf2] (ca. 52 mPas at 25 °C).…”

“…52 mPas at 25 °C). 29 (Note: In contrast to GBL, [C2mim][NTf2] has a decomposition temperature of ca. 455 C without a boiling point, 45 freezing point of ca.…”

“…Thus, a crucial insight from the present work is that the power-generation performance of non-ionic high-boiling-point solvents, containing high concentrations of redox couple salts, can surpass that of ILs, as presented below. Because of this context, most of the experimental results below will be compared with those acquired in our previous study 29 where the IL was used as the solvent. All experimental details are given in the Experimental section at the end of this article.…”