Construction method and application of solid-state thermoelectric power generation heat cycle

Abstract

The application belongs to the technical field of solid thermoelectric generation, and discloses a construction method of a solid thermoelectric generation thermodynamic cycle, which decouples the energy coupling conversion relationship of the solid thermoelectric generation into four basic thermodynamic processes, i.e. an equal-mole fraction endothermic process, an isentropic power generation process, an equal-mole exothermic process and an isentropic charging process, to form an ideal solid thermoelectric generation thermodynamic cycle. The application discloses a 'heat-electricity' energy conversion path of the solid thermoelectric generation, and the obtained solid thermoelectric generation thermodynamic cycle can quantitatively analyze key links affecting the energy conversion efficiency, thereby providing a powerful theoretical tool for improving the energy conversion efficiency of the solid thermoelectric generation. Furthermore, the application further provides an actual solid thermoelectric generation thermodynamic cycle with irreversible loss optimization and property constraint optimization, and a thermoelectric material optimization scheme, and clearly defines a thermodynamic optimization path of the energy conversion efficiency of the solid thermoelectric generation. Based on the optimization of the solid thermoelectric generation material according to the application, the energy conversion efficiency can be relatively improved by 10%.

Description

Technical Field

[0001] This invention belongs to the field of solid-state thermoelectric power generation technology, specifically relating to a method for improving the energy conversion efficiency of solid-state thermoelectric power generation under a wide temperature range heat source. Background Technology

[0002] Improving waste heat recovery and utilization rates, and reducing the proportion of fossil fuel combustion in energy supply, will be of great significance to my country's energy conservation, carbon reduction, and energy structure transformation. Solid-state thermoelectric power generation technology can achieve direct heat-to-electricity conversion. Compared with heat-to-work conversion methods such as organic Rankine cycles and Brayton cycles, it has advantages such as no intermediate links, no relatively moving parts, no noise during operation, environmental friendliness, and strong miniaturization and portability. It is a highly usable and reliable thermoelectric conversion technology.

[0003] In recent years, the improvement trend of energy conversion efficiency of solid-state thermoelectric power generation materials has slowed down. Compared with energy conversion technologies such as the Rankine cycle, organic Rankine cycle, and Brayton cycle, the energy conversion efficiency of solid-state thermoelectric power generation materials is relatively low, averaging around 10%, far lower than the Carnot efficiency under corresponding operating conditions, and the thermodynamic perfection is about 15%, while the thermodynamic perfection of existing mature thermoelectric conversion technologies (such as organic Rankine cycle and Brayton cycle) is generally greater than 20%. Therefore, the energy conversion efficiency of solid-state thermoelectric power generation materials urgently needs to be improved, and the improvement of energy conversion efficiency depends on the elucidation of its energy conversion mechanism.

[0004] The study of the energy conversion mechanism of thermoelectric power generation from a thermodynamic perspective provides a possibility for clarifying the universal criteria followed in the design of high-performance thermoelectric materials. The monograph "Chen G. Nanoscale energy transport and conversion: A parallel treatment of electrons, molecules and phonons [M]. New York: Oxford University Press, 2005" derives the energy conservation equation for thermoelectric materials based on the first law of thermodynamics, laying the foundation for the equilibrium statistical thermodynamic analysis of thermoelectric power generation. Thermodynamic cycles originated from Clausius's thinking on combining integral tools with the connotation of thermodynamic entropy, and have become an important tool for the application of classical thermodynamics in engineering. Based on this, irreversible factors such as the Thomson effect, Joule heat, and Fourier conduction were further introduced into the ideal cycle to construct a real cycle that considers irreversible losses (e.g., Chakraborty A., Saha BB, Koyama S., et al. Thermodynamic modelling of a solid state thermoelectric cooling device: Temperature–entropy analysis[J]. International Journal of Heat and Mass Transfer, 2006, 49(19-20): 3547-54).

[0005] However, current research on the thermodynamics of thermoelectric power generation using thermoelectric materials is still in its early stages. The theoretical understanding of energy conversion mechanisms is not yet in-depth, and the coupling and conversion of multiple energy states leads to a lack of clarity regarding the mechanisms for improving energy conversion efficiency. Therefore, developing and analyzing the thermodynamic cycle of energy conversion mechanisms in solid-state power generation is of great significance for guiding further improvements in the energy conversion efficiency of solid-state thermoelectric power generation. Summary of the Invention

[0006] To address the technical problem of unclear energy conversion processes in solid-state thermoelectric power generation materials, this invention provides a thermodynamically self-consistent analytical model with a clear energy conversion path, and proposes a method for constructing a thermodynamic cycle for solid-state thermoelectric power generation, providing theoretical guidance for improving the energy conversion efficiency of solid-state thermoelectric power generation.

[0007] To solve the above-mentioned technical problems, the present invention is achieved through the following technical solution:

[0008] This invention concretizes the energy conversion of solid-state thermoelectric power generation into a thermodynamic energy conversion model, constructing a thermodynamic cycle for solid-state thermoelectric power generation. This model is based on the thermodynamic equilibrium assumption, meaning that the relaxation time for the state parameters of solid-state thermoelectric power generation materials to reach equilibrium is much shorter than the time for external unbalanced disturbances. Considering that the equilibrium time constant of charge carriers during the power generation process of solid-state thermoelectric power generation materials is often on the order of nanoseconds, meaning the equilibrium of the thermodynamic state parameters of solid-state thermoelectric power generation materials is also on the order of nanoseconds, and that in practical applications of solid-state thermoelectric power generation materials, the time constants of unbalanced potential disturbances such as external temperature gradients and heat flows are often on the order of seconds, the aforementioned assumption holds true.

[0009] According to one aspect of the present invention, a method for constructing a solid-state thermoelectric power generation thermodynamic cycle is provided, which decouples the energy coupling conversion relationship of solid-state thermoelectric power generation into four basic thermodynamic processes, constructs an ideal solid-state thermoelectric power generation thermodynamic cycle, thereby revealing the conversion relationship between energy states and completing a direct "thermal-electric" conversion.

[0010] Furthermore, the four basic thermodynamic processes are an equimolar fraction endothermic process, an isentropic power generation process, an equimolar exothermic process, and an isentropic charging process.

[0011] Furthermore, the equimolar endothermic process is as follows: a high-temperature heat source and a low-temperature heat source are applied to both ends of the solid thermoelectric material. The charge carriers of the solid thermoelectric material absorb heat at the high-temperature heat source, their energy increases, their chemical potential increases, and they tend to flow directionally to the low chemical potential region, while the molar concentration of the migrating charge carriers used to do work in the solid thermoelectric material has not changed.

[0012] Furthermore, the isentropic power generation process is as follows: the high chemical potential charge carriers of the solid thermoelectric material spontaneously move directionally to the low temperature heat source to generate current and complete the power generation process.

[0013] Furthermore, the equimolar exothermic process is as follows: the charge carriers that move to the low-temperature heat source in the solid thermoelectric material release heat at the low-temperature heat source, and the chemical potential decreases, while the molar concentration of the migrated charge carriers used to do work in the solid thermoelectric material has not changed.

[0014] Furthermore, the isentropic charging process is as follows: as charge carriers accumulate at the low-temperature heat source, an internal electric field is formed inside the solid-state thermoelectric material; if the charge carriers are holes or other positively charged particles, the direction of the internal electric field is opposite to the migration direction of the charge carriers, thus transporting the charge carriers from the low-temperature heat source to the high-temperature heat source; if the charge carriers are electrons or other negatively charged particles, the direction of the internal electric field is the same as the migration direction of the charge carriers, and the direction of the negatively charged particles is opposite to the direction of the electric field, thus similarly transporting the charge carriers from the low-temperature heat source to the high-temperature heat source.

[0015] Furthermore, after optimizing the ideal solid-state thermoelectric power generation thermodynamic cycle for irreversible losses, further optimization of physical property constraints is performed to obtain the actual solid-state thermoelectric power generation thermodynamic cycle.

[0016] Furthermore, the irreversible losses in the irreversible loss optimization include overpotential losses, heat transfer losses, and transport losses caused by Joule heat, Fourier conduction, Thomson effect, etc.

[0017] Furthermore, the method for optimizing the irreversible loss is as follows: analyze the linear phenomenological law, use the analytical expression of entropy production to graphically represent entropy production in the temperature-entropy thermodynamic cycle, and obtain a quasi-realistic cycle that reduces entropy production.

[0018] Furthermore, the method for implementing the property constraint optimization is as follows: establish the correlation between Seebeck coefficient, thermal conductivity, electrical conductivity and thermodynamic state parameters, construct a surface characterizing the properties of solid-state power generation materials in the thermodynamic temperature entropy coordinate system, and obtain the actual solid-state thermodynamic power generation cycle.

[0019] Specifically, the thermodynamic cycle of the solid-state thermoelectric power generation is constrained by the molar concentration n, Helmholtz free energy F, and temperature T thermodynamic state parameters. These thermodynamic state parameters are closely related to the Seebeck coefficient α, electrical conductivity σ, and thermal conductivity κ of the solid-state thermoelectric power generation material. Therefore, starting from the first law of thermodynamics and based on the Helmholtz characteristic function, an "nFT" property space describing the properties of the solid-state thermoelectric power generation material is further constructed, and the correspondence between "ασκ" and "nFT" is established. By graphically representing the state points, a cycle constrained by actual property properties is constructed, and the state point with the minimum entropy production is searched under these property constraints.

[0020] According to another aspect of the present invention, an application of a solid-state thermoelectric power generation thermodynamic cycle is provided. Based on the actual solid-state thermoelectric power generation thermodynamic cycle, a solid-state thermoelectric power generation material is selected to obtain the combination of physical properties that minimizes the entropy production of the solid-state thermoelectric power generation material, and finally, the material design parameters that optimize the energy conversion efficiency of thermoelectric power generation are quantified.

[0021] Specifically, a promising solid-state thermoelectric power generation material is selected. First, based on the temperatures of the cold and heat sources, an ideal solid-state thermoelectric power generation thermodynamic cycle is constructed. Considering the irreversibility of the process, the ideal solid-state thermoelectric power generation thermodynamic cycle is optimized for irreversible losses (first optimization) according to the thermodynamic fluid dynamics relationships of Joule heat, Fourier thermal conductivity, and Thomson effect, resulting in a quasi-actual thermodynamic cycle. Since the solid-state thermoelectric power generation material is constrained by physical properties α, σ, and κ, physical property curves are introduced based on the quasi-actual thermodynamic cycle to constrain its properties. By searching for the point that minimizes entropy production on the physical property curves, the combination of α, σ, and κ properties that minimizes the entropy production of the solid-state thermoelectric power generation material is obtained (second optimization), resulting in the actual solid-state thermoelectric power generation thermodynamic cycle. Finally, the material design parameters that optimize the energy conversion efficiency of thermoelectric power generation are quantified.

[0022] The beneficial effects of this invention are:

[0023] (i) This invention clarifies the complex energy coupling and conversion of solid-state thermoelectric power generation, and reveals the "thermal-electric" energy conversion path of solid-state thermoelectric power generation; the obtained solid-state thermoelectric power generation thermodynamic cycle can quantitatively analyze the key links affecting energy conversion efficiency, and provide a powerful theoretical tool for improving the energy conversion efficiency of solid-state thermoelectric power generation.

[0024] (II) Based on the ideal solid-state thermoelectric power generation thermodynamic cycle, this invention proposes an optimization scheme for solid-state thermoelectric power generation materials by optimizing irreversible losses and physical property constraints, and clarifies the thermodynamic optimization path for the energy conversion efficiency of solid-state thermoelectric power generation materials. Based on this invention, the optimization of solid-state thermoelectric power generation materials can improve the energy conversion efficiency by 10%. Attached Figure Description

[0025] Figure 1 This is a schematic diagram illustrating the energy conversion principle of solid-state thermoelectric power generation.

[0026] Figure 2 Chemical potential-molar concentration diagram for an ideal solid-state thermoelectric generator Otto cycle;

[0027] Figure 3 Temperature-entropy diagram of the Otto cycle for ideal solid-state thermoelectric power generation.

[0028] Figure 4 Otto cycle temperature-entropy diagram for solid-state thermoelectric power generation considering the irreversibility of the process.

[0029] Figure 5 Otto cycle temperature-entropy diagram for practical solid-state thermoelectric power generation considering material property constraints and process irreversibility. Detailed Implementation

[0030] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. The specific embodiments described are only for explanation and illustration of the present invention and are not intended to limit the present invention.

[0031] like Figure 1 As shown, this invention discloses a method for constructing a solid-state thermoelectric power generation thermodynamic cycle. A physical model of the energy conversion path for solid-state thermoelectric power generation is established, decoupling the complex energy state conversion law into four thermodynamic processes: equimolar heat absorption (1-2), isentropic power generation (2-3), equimolar heat release (3-4), and isentropic charging (4-1), forming an ideal solid-state thermoelectric power generation thermodynamic cycle. The molar concentration-chemical potential diagram and temperature-entropy diagram of this cycle are shown below. Figure 2 , Figure 3 As shown.

[0032] For the equimolar endothermic process of 1-2, charge carriers absorb heat at the high-temperature heat source, and there is no directional migration of charge carriers; only an increase in chemical potential and kinetic energy occurs. Based on statistical thermodynamics, temperature is a macroscopic statistical expression of the kinetic energy of microscopic particles. During the endothermic process, the temperature of the solid-state thermoelectric material increases, as shown in... Figure 2 The molar concentration-chemical potential diagram shown corresponds to an equimolar concentration process; in, as Figure 3 The temperature-entropy graph shown corresponds to the process of endothermic reaction with equimolar concentration and temperature increase.

[0033] For the isentropic power generation process in steps 2-3, charge carriers that have gained energy at the high-temperature heat source undergo directional movement, moving from the high-temperature heat source with a high chemical potential to the low-temperature heat source with a low chemical potential. Due to this carrier migration, the chemical potential decreases as shown in the molar concentration-chemical potential diagram. Since the carrier migration rate is sufficiently fast, typically on the nanosecond scale, it can be considered that the solid-state thermoelectric power generation material has no energy exchange with the external environment, constituting an adiabatic process. Therefore, it appears as an isentropic process in the temperature-entropy diagram.

[0034] For the equimolar exothermic process of 3-4, the carriers migrating to the low-temperature heat source release heat at the low-temperature heat source. The concentration of migrating carriers in the solid-state thermoelectric material remains unchanged. Figure 2 The molar concentration-chemical potential diagram shown corresponds to an equimolar concentration process; in, as Figure 3 The temperature-entropy graph shown corresponds to the process of exothermic reaction with equimolar concentration and decreasing temperature.

[0035] For the isentropic charging process of 4-1, due to the carrier concentration difference between the high-temperature heat source and the low-temperature heat source, an internal electric field is formed inside the solid-state thermoelectric power generation material. This internal electric field transports carriers from the low-temperature heat source back to the high-temperature heat source. Figure 2 The molar concentration-chemical potential diagram shown corresponds to a process where the chemical potential increases and the carrier concentration used for work decreases, as in... Figure 3In the temperature-entropy diagram shown, since the carrier transport process occurs in a very short time, it can be considered an adiabatic isentropic process.

[0036] Due to overpotential losses, heat transfer losses, and transport losses caused by Joule heat, Fourier conduction, and the Thomson effect in the actual cycle process, an Otto cycle for solid-state thermoelectric power generation considering the irreversibility of the process was constructed based on the ideal solid-state thermoelectric power generation thermodynamic cycle. Its temperature-entropy diagram is shown below. Figure 4 As shown.

[0037] For the equimolar endothermic process (1-2) and the equimolar exothermic process (3-4), charge carriers absorb heat at the high-temperature heat source and release heat at the low-temperature heat source. Due to the temperature difference during heat transfer, Fourier conduction causes heat transfer losses. For the isentropic power generation process (2-3) and the isentropic charging process (4-1), charge carriers undergo directional movement, generating current. Overpotential losses occur due to the Joule thermal effect. During the directional movement of charge carriers, a temperature difference exists between the two ends of the solid thermoelectric power generation material, leading to transport losses due to the Thomson effect. By considering the above irreversible losses and analyzing the linear phenomenological law, entropy production is graphically represented in the temperature-entropy thermodynamic cycle using an analytical expression of entropy production, thus constructing a practical cycle. By analyzing the influence mechanism of heat source parameters, physical property parameters, and operating parameters (power generation side voltage, etc.) on the thermoelectric material's thermoelectric power generation performance, the contribution of irreversible factors to entropy production is quantified. Finally, the link with significant potential to reduce entropy increase is identified, and the thermoelectric power generation thermodynamic cycle is optimized to construct a practical cycle that reduces entropy production.

[0038] Since the thermodynamic cycle of solid-state thermoelectric power generation materials is constrained by the Seebeck coefficient, thermal conductivity, and electrical conductivity, which characterize the physical properties of thermoelectric materials, a practical solid-state thermoelectric power generation Otto cycle considering material property constraints and process irreversibility was constructed. Its temperature-entropy diagram is shown below. Figure 5 As shown.

[0039] To reveal the correlation mechanism between physical properties and reflect the underlying characteristics of materials, this study establishes a relationship between the physical property parameters Seebeck coefficient α, thermal conductivity κ, and electrical conductivity σ and the state parameters n, F, and T in the Helmholtz characteristic function, starting from the first law of thermodynamics. Considering irreversibility, material property constraints are introduced to construct a practical solid-state thermoelectric power generation thermodynamic cycle that takes into account both material property constraints and process irreversibility.

[0040] First, the properties of the material, α, σ, and κ, are obtained through simulation or experimentation. Then, the pairwise mathematical correlations between these intrinsic properties are fitted to obtain the α, σ, and κ property correlation curves. Further, discrete curves are used to fit and obtain the α, σ, and κ property surfaces. Since α, σ, and κ are influenced by the combined effects of the three fundamental thermodynamic state parameters n, F, and T, and the material's structural properties, a mathematical method is used to establish the correspondence between "ασκ" and "nFT," transforming the "α-σ-κ" surface into the "nFT" thermodynamic coordinate system. Subsequently, thermodynamic relationships are used... Plot the physical property curves as follows: Figure 5 The temperature-entropy diagram is shown. Since the first-level optimization does not consider actual material properties and is a macroscopic optimization, it often leads to a deviation between the state point and the property curve. Therefore, a second-level optimization is performed by searching for the point with the shortest distance to the first-level optimized state point on the property surface. This is a local optimization, allowing the material to more closely approximate the property curve while maintaining a lower entropy production. Finally, the second-level optimization is completed on the thermoelectric power generation cycle, resulting in the actual solid-state thermoelectric power generation cycle.

[0041] Based on the actual solid-state thermoelectric power generation thermodynamic cycle, promising solid-state thermoelectric power generation materials can be selected, including but not limited to: SnSe, MoS2, black phosphorus, etc. Here, two-dimensional SnSe is taken as an example. First, the cold source temperature and heat source temperature are determined to construct an ideal thermodynamic cycle. Considering the irreversibility of the process caused by Joule heat, Fourier conduction, and the Thomson effect, as well as the material property constraints α, σ, and κ, an actual thermodynamic cycle considering process irreversibility and material property constraints is established. By adjusting the structure of the solid-state thermoelectric power generation material, including but not limited to: introducing interlayer torsion angles, doping, intercalation, etc., material design is carried out under property constraints. Here, interlayer torsion angles are introduced, and property curves of the torsional two-dimensional thermoelectric material SnSe with different numbers of layers (2-4 layers), number of torsion angles (1-3), and torsion angles (0°-60°) are constructed in the temperature-entropy coordinate system. The design parameters, including the number of layers, the number of torsion angles, and the torsion angle, were quantified to optimize the energy conversion efficiency of the torsional two-dimensional thermoelectric material at 300K by searching for the point closest to the property point with the minimum entropy production on the property curve. This ultimately resulted in a 10% relative improvement in the energy conversion efficiency of the solid-state thermoelectric material SnSe at 300K.

[0042] Although the preferred embodiments of the present invention have been described above in conjunction with the accompanying drawings, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many specific modifications under the guidance of the present invention without departing from the spirit of the invention and the scope of protection of the claims, and these modifications all fall within the scope of protection of the present invention.

Claims

1. A method for constructing a solid-state thermoelectric power generation thermodynamic cycle, characterized in that, The energy coupling and conversion relationship of solid-state thermoelectric power generation is decoupled into four basic thermodynamic processes to form an ideal solid-state thermoelectric power generation thermodynamic cycle; the four basic thermodynamic processes are equimolar fraction heat absorption process, isentropic power generation process, equimolar heat release process, and isentropic charging process; After optimizing the ideal solid-state thermoelectric power generation thermodynamic cycle for irreversible losses, further optimization for physical property constraints is performed to obtain the actual solid-state thermoelectric power generation thermodynamic cycle.

2. The method for constructing a solid-state thermoelectric power generation thermodynamic cycle according to claim 1, characterized in that, The equimolar fraction endothermic process is as follows: a high-temperature heat source and a low-temperature heat source are applied to both ends of the solid thermoelectric material. The charge carriers of the solid thermoelectric material absorb heat at the high-temperature heat source, and their energy and chemical potential increase. They tend to flow directionally to the low chemical potential region, while the molar concentration of the migrating charge carriers used to do work in the solid thermoelectric material has not changed.

3. The method for constructing a solid-state thermoelectric power generation thermodynamic cycle according to claim 1, characterized in that, The isentropic power generation process is as follows: high chemical potential charge carriers in the solid thermoelectric material spontaneously move directionally to the low temperature heat source, generating current and completing the power generation process.

4. The method for constructing a solid-state thermoelectric power generation thermodynamic cycle according to claim 1, characterized in that, The equimolar exothermic process is as follows: the charge carriers that move to the low-temperature heat source in the solid thermoelectric material release heat at the low-temperature heat source, and the chemical potential decreases, while the molar concentration of the migrated charge carriers used to do work in the solid thermoelectric material has not changed.

5. The method for constructing a solid-state thermoelectric power generation thermodynamic cycle according to claim 1, characterized in that, The isentropic charging process is as follows: as charge carriers accumulate at the low-temperature heat source, an internal electric field is formed inside the solid-state thermoelectric material; if the charge carriers are holes or other positively charged particles, the direction of the internal electric field is opposite to the migration direction of the charge carriers, thus transporting the charge carriers from the low-temperature heat source to the high-temperature heat source; if the charge carriers are electrons or other negatively charged particles, the direction of the internal electric field is the same as the migration direction of the charge carriers, and the direction of the negatively charged particles is opposite to the direction of the electric field, thus similarly transporting the charge carriers from the low-temperature heat source to the high-temperature heat source.

6. The method for constructing a solid-state thermoelectric power generation thermodynamic cycle according to claim 1, characterized in that, The irreversible losses in the aforementioned irreversible loss optimization include overpotential losses, heat transfer losses, and transport losses caused by Joule heat, Fourier conduction, and the Thomson effect. The implementation method of the irreversible loss optimization is as follows: analyze the linear phenomenological law, use the analytical expression of entropy production to graphically represent entropy production in the temperature-entropy thermodynamic cycle, and obtain the actual cycle that reduces entropy production.

7. The method for constructing a solid-state thermoelectric power generation thermodynamic cycle according to claim 1, characterized in that, The method for implementing the property constraint optimization is as follows: establish the correlation between Seebeck coefficient, thermal conductivity, electrical conductivity and thermodynamic state parameters, and graphically represent the thermodynamic state parameters characterizing the properties of solid-state thermoelectric power generation materials in the thermodynamic temperature entropy coordinate system to obtain the actual solid-state thermoelectric power generation thermodynamic cycle.

8. An application of a solid-state thermoelectric power generation thermodynamic cycle, characterized in that, Based on the actual solid-state thermoelectric power generation thermocycle obtained by the construction method described in any one of claims 1-7, a solid-state thermoelectric power generation material is selected, and the combination of physical properties that minimizes the entropy production of the solid-state thermoelectric power generation material is obtained. Finally, the material design parameters that optimize the energy conversion efficiency of thermoelectric power generation are quantified.

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