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.
In our present civilization, forced convection cooling is used in wide-ranging situations from cooling of microprocessors in data centers to that of power turbines and automobile engines. Active cooling is essential in such situations. However, active transfer of heat or thermal energy across a large temperature difference promptly destroys the free-energy component of the thermal energy called “exergy”. This issue has remained unaddressed despite widespread use of forced convection cooling. In this study, to partially recover presently lost exergy in such situations in the form of electric power, we integrate thermo-electrochemical conversion, which has been mostly studied for stationary conditions, into forced convection cooling. The proposed concept is graphically illustrated in Fig. 1. To investigate this concept, we design a test cell (Fig. 2a) in which an electrolyte liquid is forced through a channel formed between two parallel electrodes (Fig. 2b) and the hot-side electrode simulates an object to be cooled. We use cobalt redox couple CoII/III(bpy)3(NTf2)2/3 (Fig. 2c) dissolved in an ionic liquid as the coolant (electrolyte) for this purpose. Our experimental 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 controlled by electrolyte 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 thermo-electrochemical cells, the “gain”—defined as the ratio of the generated power to the hydrodynamic pumping work required to force the liquid through the cell—is introduced. The gain is found to be above unity in a certain flow rate region. This demonstrates that such a system can generate more electric power than the hydrodynamic pump work required to drive the liquid through the cell, supporting our concept to address aforementioned unresolved issue in the situations of forced-convection cooling. Reference: Ikeda, K. Fukui, and Y. Murakami, Phys. Chem. Chem. Phys., 2019, in press, DOI: 10.1039/c9cp05028k; arxiv.org/abs/1908.08646. Figure 1
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