Thermopower and harvesting heat

doi:10.1126/science.abf3342

GSTDTAP > 气候变化

DOI	10.1126/science.abf3342
	Thermopower and harvesting heat
	Xun Shi; Jian He
	2021-01-22
发表期刊	Science
出版年	2021
英文摘要	Harvesting and directly converting the enormous, yet largely untapped, reservoir of low-grade heat into electricity has the potential for immense economic and environmental impacts. To this end, thermoelectrics ([ 1 ][1]), thermionic capacitors ([ 2 ][2]), and thermocells ([ 3 ][3]) are among the state-of-the-art options. Despite distinct working principles of the internal circuit, the trio are heat engines in a general sense because they convert a temperature difference input Δ T into a device voltage output Δ V D that empowers the electrons in the external circuit to do work. The unit of the Δ V D/Δ T ratio, a numeric performance index of the energy conversion device, is the same as the thermopower. However, it is misleading to equate or compare this ratio to the thermopower, an intrinsic physical property of material described by a rank-2 tensor ([ 2 ][2]–[ 7 ][4]). The concept of thermopower originates in the Seebeck effect. When a Δ T is applied across an electronic conductor, charge carriers diffuse from the hot end to the cold end until the material's resultant open-circuit voltage difference (Δ V M) counterbalances further diffusion. The Δ V M measures the electrochemical potential difference due to the Δ T -induced carrier concentration gradient. The thermopower of an electronic conductor is encoded in the material-specific Δ V M/Δ T ratio with the sign denoting the type of the majority carriers and the magnitude relating to the average entropy per carrier charge (on the order of µV/K) ([ 8 ][5]). Likewise, the concept of ionic thermopower can be defined through the Soret effect of an ionic conductor (on the order of mV/K) ([ 9 ][6]). Although the thermoelectric device and thermionic capacitor build on the material's Seebeck effect and ionic Soret effect, respectively, a device is far more than the constituent active materials. Not only is the Δ V M of the constituent active materials and the electrodes the physical cause of the Δ V D, chemical reactions can also contribute to the voltage difference. The Δ V D of the thermocell, a Δ T -driven battery with a redox couple–containing electrolyte ([ 3 ][3]), includes the thermodiffusive and thermogalvanic voltages. The Δ V D/Δ T ratio of the thermocell hardly reflects the constituent active materials' thermopower. The Δ V D/Δ T ratio is therefore distinct from the thermopower in concept and in practice. It is faulty to reappropriate the formula of thermoelectric figure of merit of a material to assess the performance of a device, or to compare the Δ V D/Δ T ratios at face value. With the Δ T acting as the common driving force of the energy conversion process, the specific cause of the Δ V D dictates the device's working mode. The thermoelectric device and thermocell work in a continuous mode, and the power density is the primary performance metric. Meanwhile, the working mode of the thermionic capacitor is intermittent, and its performance ought to be assessed by the power density and energy density, like the performance of regular capacitors. ![Figure][7] Making heat work for us Differences in temperature can be used to drive current with a variety of different devices. Thermoelectrics rely on a p-type and an n-type material that drives electrons (e) or holes (h) from source to sink. Thermionic capacitors rely on separation of positive from negative ions. Thermocells require temperature difference–specific ion and electron cycling. The relative effectiveness of these devices requires care when comparing performance metrics. GRAPHIC: V. ALTOUNIAN/ SCIENCE The high Δ V D/Δ T ratios, on the order of mV/K, that have been reported for thermionic capacitors ([ 2 ][2], [ 5 ][8]) and thermocells ([ 3 ][3], [ 6 ][9], [ 7 ][4]) are vital to the application of wearable electronics, but with certain trade-offs. For thermionic capacitors and thermocells (see the figure), the normalized maximum power densities, PA−1 Δ T−2 , where P is the maximum power output and A is the module's cross-sectional area perpendicular to the heat flow direction, are at least one order of magnitude lower than for thermoelectric devices ([ 10 ][10]–[ 12 ][11]). This difference is due to large internal resistances. Shortening the interelectrode spacing to reduce the internal resistance falls short as it will diminish the driving force, Δ T . A Δ T = 20 K typical in low-grade heat harvesting energetically equals 1.72 meV, compared to eV-level driving forces in regular capacitors and batteries. The power density and energy density of thermionic capacitors ([ 2 ][2]) are orders of magnitude lower than those of regular capacitors and batteries ([ 13 ][12]) because of weak driving forces. The multifold nature of the energy conversion process in the device further explains these performance discrepancies, where the challenges and opportunities lie. The efficiency of a multifold energy conversion process is subject to the efficiency of each subprocess and the coupling of the subprocesses. Thermoelectricity is a single-fold process with the simplest working principle. The working media in the internal and external circuits are both electrons. The energy conversion process of the thermionic capacitor is twofold. The ion diffusion is driven by Δ T and the interactions between the accumulated ions and the electrodes ([ 2 ][2]). The energy conversion process of the thermocell includes the ionic diffusion driven by Δ T and by the chemical reaction–induced ionic concentration gradient, the redox reactions at the electrodes, and the interactions between the electrolyte and the electrodes ([ 3 ][3]). 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领域	气候变化 ; 资源环境
URL	查看原文
引用统计
文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/312352
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Xun Shi,Jian He. Thermopower and harvesting heat[J]. Science,2021.
APA	Xun Shi,&Jian He.(2021).Thermopower and harvesting heat.Science.
MLA	Xun Shi,et al."Thermopower and harvesting heat".Science (2021).