JPH08316533A - Thermoelectric conversion performance evaluation method and device - Google Patents

Abstract

PURPOSE: To provide a thermoelectric element performance evaluation method and a device, wherein a one-dimensional heat flow condition can be realized so as to accurately evaluate the performance of a thermoelectric element of gradient structure under a condition of large temperature difference. CONSTITUTION: A thermoelectric element arranged between a primary heat source 2 and a cooling source to measure a thermoelectromotive force or the like is surrounded with a heat shield 7, and an auxiliary heat source 3 is provided to heat the outside of the heat shield 7 from across so as to make the temperature distribution of the heat shield 7 nearly equal to a temperature distribution induced in the thermoelectric element due to a temperature difference between the primary heat source 2 and the cooling source. A temperature distribution around the thermoelectric element is set nearly equal to that of the element, whereby the thermoelectric element is kept under a condition of one-dimensional heat flow.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for evaluating the performance of thermoelectric conversion elements used in thermoelectric conversion technology.

[0002]

2. Description of the Related Art Heretofore, thermoelectric cooling and thermoelectric power generation using thermoelectric conversion elements (hereinafter simply referred to as thermoelectric elements) have been proposed. In the thermoelectric conversion technology, in order to improve the conversion efficiency of the thermoelectric cooling and the thermoelectric power generation, it is advantageous to use a high temperature heat source as much as possible and take a large temperature difference. However, the thermoelectric material consisting of a homogeneous material used in conventional thermoelectric conversion technology, its thermoelectric performance depends on the temperature,
Since the conversion efficiency has a peak in a specific temperature range, there is a limit to improving the conversion efficiency over the entire temperature range.

Therefore, in recent years, the carrier concentration inside the thermoelectric element is changed from one end side to the other end side of the thermoelectric element so that the carrier concentration is optimally inclined and distributed so that the maximum thermoelectric conversion efficiency can be obtained. Development of concentration-gradient thermoelectric elements and compounding of multiple thermoelectric element materials with the highest figure of merit in each temperature region with a graded structure to obtain high thermoelectric conversion efficiency over a wide temperature range from low temperature to high temperature Development of a gradient composite thermoelectric element is underway.

When evaluating the thermoelectric conversion characteristics of thermoelectric materials,
The values of the electrical resistance (σ), thermoelectromotive force (α), and thermal conductivity (κ) of the thermoelectric material are mainly measured to obtain the thermoelectric conversion performance index (Z) defined by Z = σα ² / κ. Performance is being sought after. With the conventional homogeneous material, as shown in FIG.
Since the electric resistance, the thermoelectromotive force, and the thermal conductivity are all functions of temperature, there is a peak of the performance at a specific temperature, and the evaluation at each temperature can be performed.

[0005]

However, in the above-mentioned carrier concentration gradient thermoelectric element and gradient structure composite thermoelectric element described above, different portions inside the material have different temperature functions, so that the broken line a, As shown by b and c, the maximum thermoelectric conversion performance can be obtained only under the condition that the optimum temperature distribution showing the maximum characteristics of each part and the optimum temperature difference at both ends of the material are added. Therefore, in such thermoelectric materials and thermoelectric elements formed of these materials, the conventional performance evaluation method of measuring the above-mentioned values at each temperature and evaluating the thermoelectric conversion performance index Z cannot be applied. .

Therefore, in the thermoelectric material and the thermoelectric element having the above-mentioned gradient structure, it is necessary to evaluate the basic performance of the thermoelectric element under the condition that the optimum design temperature difference and the temperature distribution are added. Therefore, the present invention is based on the following principle.
The method of evaluating thermoelectric conversion efficiency and average figure of merit under the conditions of constant temperature difference and temperature distribution was adopted.

That is, as a parameter for evaluating the thermoelectric conversion efficiency, the maximum electric output (Pmax), that is, the internal resistance is used.
The thermoelectric conversion efficiency (η) when (Rm) and the external resistance (Ro) are equal (m = Rm / Ro = 1) is used. Now, P shown in FIG.
In the N-type thermoelectric element, the thermoelectric conversion efficiency (η) can be expressed by the following equation. η = Pmax / Qin (1) where Pmax = Vm ² / 4Rm (2) where Vm is the open circuit voltage, Rm is the internal resistance of the thermoelectric element,
Qin is the amount of heat flowing into the thermoelectric element. In FIG. 5, Qout is the amount of heat flowing out from the thermoelectric element, and Qin and Qout have the same value if there is no heat loss from the element to the surrounding environment. Further, the average thermoelectric conversion performance index can be obtained from the relationship between the conversion efficiency (η) and the average thermoelectric conversion performance index (Z). Therefore, if the electric conductivity, heat flux, and thermoelectromotive force can be measured at the same time, the conversion efficiency of the thermoelectric element and the average performance index can be evaluated.

However, the temperature actually used for the above-mentioned graded composite thermoelectric material and element is generally required to have a temperature drop of several 100 ° C. to 1000 ° C. from room temperature to high temperature. Under such a high temperature drop condition, a large temperature difference exists between the measurement sample and the ambient environment, and between the high temperature part and the low temperature part of the measurement sample. The heat loss was large, and a one-dimensional heat flow environment from the high temperature side to the low temperature side of the thermoelectric element could not be obtained. Therefore, it is difficult to accurately measure the heat flow, and it is difficult to accurately evaluate the thermoelectric element and the thermoelectric material.

Therefore, the present invention seeks to obtain a method capable of accurately evaluating the performance of a thermoelectric conversion material or thermoelectric element having a graded structure, which was difficult with the conventional method. (EN) A thermoelectric conversion element performance evaluation method and apparatus capable of producing a one-dimensional heat flow condition under a temperature drop condition and performing accurate performance evaluation under actual use temperature conditions of a graded structure thermoelectric conversion element and conversion material. The purpose is to

[0010]

To achieve the above object, the thermoelectric conversion performance evaluation method of the present invention provides a thermoelectric conversion method for a thermoelectric element under the condition that a high temperature drop occurs between one end and the other end of the thermoelectric element. In the thermoelectric performance evaluation method for evaluating the conversion efficiency and the average figure of merit, the temperature distribution around the thermoelectric element is kept substantially equal to the temperature distribution of the thermoelectric element so that the thermoelectric element can maintain the one-dimensional heat flow condition. It is a thing.

Further, the thermoelectric performance evaluation apparatus of the present invention is arranged between the main heat source and the cooling source, and surrounds the periphery of the thermoelectric element for measuring thermoelectromotive force with a heat insulating shield, and An auxiliary heat source for heating the outside of the heat shield from the side is provided so that the temperature distribution of the shield becomes substantially equal to the temperature distribution generated in the thermoelectric element due to the heat drop between the main heat source and the cooling unit. . Then, in the thermoelectric performance evaluation apparatus, it is preferable to fill a filler having a low thermal emissivity and a low thermal conductivity between the thermoelectric element and the heat insulating shield, because heat transfer can be prevented.

[0012]

In the thermoelectric conversion performance evaluation method of the present invention, in order to evaluate the thermoelectric performance, the temperature distribution of the environment around the thermoelectric element under the condition that a high temperature drop occurs at both ends of the thermoelectric element to be measured. Is kept substantially equal to the temperature distribution of the thermoelectric element. As a result, the heat flow passing through the thermoelectric element from the high temperature side to the low temperature side maintains the one-dimensional heat flow condition necessary for measuring the heat flow in order to evaluate the performance of the thermoelectric element.

Further, according to the thermoelectric performance evaluation apparatus of the present invention, the auxiliary heat source heats the outside of the heat shield from the side to form a temperature distribution similar to that of the thermoelectric element on the heat shield. The one-dimensional heat flow condition required for the measurement of the composite thermoelectric element having the inclined structure can be realized with high accuracy.

Furthermore, in the thermoelectric performance evaluation apparatus of the present invention, when a filler having a low thermal emissivity and a low thermal conductivity is filled between the thermoelectric element and the heat insulating shield, the thermoelectric element from the high temperature side to the low temperature side. The heat transfer due to the radiant heat toward the is suppressed by the filler, and the one-dimensional heat flow condition is maintained with higher accuracy.

[0015]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a cross-sectional view of a thermoelectric performance evaluation device,
The measuring unit of the thermoelectric performance evaluation apparatus 1 is installed inside a vacuum chamber 2 of cylindrical quartz glass. A main heat source 3 composed of an infrared lamp is arranged at the upper part.
The heat radiated from is reflected by a condenser mirror provided above the main heat source 3 and directed downward and radiated.

On the other hand, on the lower side, the upper heat flow meter 5, the sample 6, and the lower heat flow meter 7 are arranged in a line in this order. Sample 6 is a thermoelectric element for evaluating the performance, is formed in a cylindrical shape, and is provided with a P-N electrode. Further, the upper heat flow meter 5 and the lower heat flow meter 7 have the end surfaces of the sample 6
It is formed in a cylindrical shape that matches the end face of the, and is made using Inconel and copper.

The sample 6 is sandwiched between the upper heat flow meter 5 and the lower heat flow meter 7 and fixed by a force of about 3 kgf by the three alumina rods 10 pulled by the spring 9 on the cooling block 11. Has been done. Further, cooling water 13 circulates inside the cooling block 11 serving as a cooling source to cool the cooling block 11.

The circumference of the sample 6 is surrounded by a heat insulating shield 8, and a plurality of auxiliary heat sources 4 are arranged outside the heat insulating shield 8. Each auxiliary heat source 4
Like the main heat source 3, is constituted by an infrared lamp, and the outside of the heat shield 8 is heated by a reflector whose height can be adjusted. The heat radiation amount of the auxiliary heat source 4 and the height of the reflection plate can be controlled externally. Then, the auxiliary heat source 4 can be controlled so that the axial temperature distribution of the heat insulating shield 8 matches the axial temperature distribution of the sample 6 caused by the temperature difference between the main heat source 3 and the cooling block 11. There is.

In addition, in order to prevent heat transfer from the main heat source 2 side to the cooling block 11 side by radiation between each of the upper heat flow meter 5, the sample 6 and the lower heat flow meter 7 and the heat insulating shield 8, low heat radiation is performed. A bubbler 12 was filled as a filler having a low thermal conductivity and a low thermal conductivity.

FIG. 2 is a thermoelectric performance evaluation device 1 shown in FIG.
It is a schematic diagram which shows the structure of the sample 6 surrounding. As shown in the figure, a thermocouple 15 is attached to each of the upper heat flow meter 4 and the lower heat flow meter 7 and is connected to the thermocouple terminal board 14 (FIG. 1). The thermocouple terminal board 14 is connected to a measuring device (not shown) installed outside the thermoelectric performance evaluation apparatus 1 by a thermocouple compensating lead wire.

Further, as shown in FIG. 2, a nickel foil 16 is interposed between the upper heat flow meter 5 and the sample 6, and a silver foil 17 is interposed between the sample 6 and the lower heat flow meter 7. There is. In the figure, these members are drawn separately for the sake of description, but in reality, the members are in close contact with each other, and from the main heat source 3 of high temperature, as shown in FIG. A heat flow Q is generated toward the low temperature cooling block 11 side cooled by 13. Further, a thermocouple is attached to an arbitrary position of the heat insulating shield 8 surrounding the sample 6 so that the temperature distribution of the heat insulating shield 8 can be measured.

In this embodiment, the shape of the thermoelectric element to be evaluated is basically a columnar element with a P-N electrode, but a rectangular element can be easily dealt with by changing the shape of the upper and lower heat flow meters. . Then, the maximum temperature of the high temperature part of the sample 6 is set to 1500K and the temperature difference between both ends of the sample 6 is set to about 800K, and the temperature distribution of sample 6, electrical conductivity, heat flow velocity, thermoelectromotive force (open end voltage), etc. Was measured. At that time, the temperature distribution in the axial direction of the heat insulating shield 8 is controlled to be the same as the temperature distributions of the sample 6, the upper heat flow meter 5 and the lower heat flow meter 7, respectively, and further, from the main heat source 3 side, the cooling block 11 is provided. In order to prevent heat transfer due to radiant heat to the side,
An alumina bubbler 12 having a diameter of 1 mm to 2.8 mm was used.

Then, the Seebeck thermoelectromotive force and the output from each thermocouple 15 are taken into a computer through a 33-channel multiplexer amplifier and A / D converter for data processing. Furthermore, when measuring the internal resistance of the sample, in order to remove the influence of Seebeck thermoelectromotive force, a pulse current controlled by a computer was used, and the internal resistance was determined by the average value of the voltage values at positive and negative currents. .

Experimental Example Using the apparatus configured as described above, an FeSi2P-N junction element having a diameter of 15 mm and a length of 16 mm was adopted as the sample 5, and the following measurement was performed to evaluate its performance. went. The chamber in which the thermoelectric performance evaluation apparatus 1 is installed is evacuated, the temperature is raised by the main heat source 3 and the auxiliary heat source 4, and the temperatures of the upper heat flow meter 5 and the heat insulating shield 8 are controlled to a predetermined temperature. . After reaching the thermal equilibrium state, the temperature distribution, Seebeck electromotive force, and electric resistance of Sample 6 were measured. Further, the magnitude of the heat flux was obtained from the temperature difference between both ends of the heat flow meter.

As a result, the temperatures of the high temperature portion and the low temperature portion of the device were 1076K and 442K, respectively, and a temperature drop of 634K was obtained. The Seebeck electromotive force and internal resistance (specific resistance) of the device are 270 mV and 0.4, respectively.
It was 7 Ωcm.

Further, since it is very important to realize a one-dimensional heat flow pattern under the high temperature drop condition as in the device of the present invention, in order to confirm and calibrate the heat flow pattern of the device of this embodiment, stainless steel is used. A heat flow calibration experiment was performed using a standard sample made of (SUS304). As a result, when the temperature of the upper heat flow meter 5 was set to 1190K, the upper and lower temperatures of the standard sample were 898K and 787, respectively.
K, and the magnitude of the heat flux through the upper heat flow meter 5 and the standard sample, respectively 21.6W / cm ² and 20.9W / cm ²
And there was a 3% attenuation. This result indicates that a good one-dimensional heat flow pattern can be obtained in the device of this example, and the effectiveness of the thermoelectric conversion performance evaluation method and device according to the present invention was confirmed.

[0027]

As described above, according to the thermoelectric performance evaluation method of the present invention, since the temperature distribution similar to the temperature distribution of the thermoelectric element is formed around the thermoelectric element to be measured, It is possible to prevent heat from entering and exiting from the side surface of the thermoelectric element with respect to the surrounding environment, and to realize a one-dimensional heat flow condition inside the thermoelectric element. As a result, it becomes possible to evaluate the thermoelectric conversion performance under the actual use temperature condition with high accuracy for the composite thermoelectric element having the inclined structure.

Further, according to the thermoelectric performance evaluation apparatus of the present invention, the temperature distribution similar to the temperature distribution of the thermoelectric element measured on the heat insulation shield can be easily obtained by heating the outside of the heat insulation shield from the side by the auxiliary heat source. The one-dimensional heat flow condition required for the measurement of the composite thermoelectric element having the inclined structure can be realized with high accuracy.

Further, in the thermoelectric performance evaluation apparatus of the present invention, when a filler having a low thermal emissivity and a low thermal conductivity is filled between the thermoelectric element and the heat insulating shield, the thermoelectric element from the high temperature side to the low temperature side. It is possible to reduce the heat transfer due to the radiant heat toward, and to maintain the one-dimensional heat flow condition with higher accuracy.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of a thermoelectric performance evaluation device of the present invention.

FIG. 2 is a schematic view showing a structure around a sample of the thermoelectric performance evaluation apparatus of the present invention.

FIG. 3 is a diagram showing the relationship between the temperature and the figure of merit of a conventional thermoelectric element.

FIG. 4 (a) is a schematic view showing different material parts of the gradient structure composite thermoelectric element, and FIG. 4 (b) is a diagram showing a relationship between temperature and performance index of the gradient structure composite thermoelectric element.

FIG. 5 is a schematic view of a PN type thermoelectric element.

[Explanation of symbols]

1 Thermoelectric Performance Evaluation Device 2 Vacuum Chamber 3 Main Heat Source 4 Auxiliary Heat Source 5 Upper Heat Flow Meter 6 Sample 7 Lower Heat Flow Meter 8 Insulation Shield 9 Spring 10 Alumina Rod 11 Cooling Water Block 12 Alumina Bubbler 13 Cooling Water 14 Thermocouple Terminal Board 15 Thermocouple 16 Nickel foil 17 Silver foil

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tatsuo Kumagai, Shibata Town, Shibata District, Miyagi Prefecture

Claims (3)

[Claims] 1. A thermoelectric conversion performance evaluation method for evaluating a thermoelectric conversion efficiency and an average performance index of a thermoelectric conversion element under the condition of causing a high temperature drop between one end and the other end of the thermoelectric conversion element. A thermoelectric performance evaluation method, characterized in that the temperature distribution around the element is kept substantially equal to the temperature distribution of the thermoelectric conversion element so that the thermoelectric conversion element is maintained under a one-dimensional heat flow condition. 2. A thermoelectric conversion element that is arranged between a main heat source and a cooling source and measures thermoelectromotive force is surrounded by a heat insulating shield, and the temperature distribution of the heat insulating shield is the main A thermoelectric performance evaluation device comprising: an auxiliary heat source that heats the outside of the heat shield from the side so that the temperature distribution generated in the thermoelectric conversion element due to the heat drop between the heat source and the cooling unit becomes substantially equal. 3. The thermoelectric performance evaluation device according to claim 2, wherein a filler having a low thermal emissivity and a low thermal conductivity is filled between the thermoelectric conversion element and the heat insulating shield.