A method for designing the structure of a heat exchange chamber of an automobile exhaust temperature difference generator

Abstract

This invention discloses a structural design method for the heat exchange chamber of an automotive exhaust thermoelectric generator. The method includes: Step 1, measuring the thermal resistance of each part of the automotive exhaust thermoelectric generator; Step 2, measuring and calculating the heat transfer coefficient; Step 3, measuring the cold and hot end temperatures of the thermoelectric generator; Step 4, calculating the output power of the automotive thermoelectric generator; Step 5, calculating the exhaust back pressure loss caused by increasing the expansion ratio of the automotive thermoelectric generator; and Step 6, calculating the net output power P of the tapered thermoelectric generator system. net Step 7: Compare the net output power until ΔP net <0. The beneficial effects of this invention are: by matching and calculating thermoelectric generators under different expansion ratios, the working efficiency of the thermoelectric generator is improved, the net output power of the thermoelectric generator is increased, and the performance of the thermoelectric generator is optimized. Based on the thermal resistance model and the basic theory of thermoelectric effect, the net output power of the thermoelectric generator under different expansion ratios is calculated, and the corresponding optimal expansion ratio is determined.

Description

TECHNICAL FIELD

[0001] The application relates to a structural design method of a car exhaust temperature difference generator, and particularly provides a structural design method of a heat exchange chamber of a car exhaust temperature difference generator. BACKGROUND

[0002] The temperature difference generator is an energy recovery technology developed on the basis of the Seebeck effect, and the principle is that when a temperature difference is generated at both ends of a temperature difference generator sheet, an electric current is generated in a closed loop, so that heat is effectively recovered. Since the temperature difference generator has the characteristics of no moving components, safety, no pollution and simple structure, it can directly convert heat into electricity, which helps to achieve the goal of environmental protection and low carbon. The temperature difference generator applied to the car exhaust directly uses the car exhaust as the hot end, and uses the air cooling or water cooling system as the cold end, and the generated electric energy can be directly used for the in-vehicle electrical system or stored in the battery.

[0003] The temperature difference generator comprises an air inlet, an expansion chamber, a temperature difference generation module installed on the outer surface of the expansion chamber, and an air outlet, and the temperature difference generation module is connected with the expansion chamber and the radiator on both sides.

[0004] The chamber structure of the expansion chamber of the temperature difference generator has a significant influence on the temperature difference generation efficiency. The existing internal structure research of the temperature difference generator is mostly to install fins in the expansion chamber to capture more heat. Generally, the ratio of the distance or diameter between the upper and lower walls in the chamber to the diameter of the air inlet is called the expansion ratio, which also has a great influence on the heat exchange and power generation efficiency of the temperature difference generator. If the diameter of the air inlet is too small relative to the distance or diameter between the upper and lower walls in the chamber, the heat exchange efficiency in the chamber is slow. If the diameter of the air inlet is too large relative to the distance or diameter between the upper and lower walls in the chamber, the exhaust gas flows too fast in the chamber, causing energy waste, and the heat exchange efficiency is also significantly reduced. Therefore, optimizing the expansion ratio to make the temperature difference generator generate heat exchange and power generation with maximum efficiency is a difficult problem to be solved at present. SUMMARY

[0005] The application aims to provide a structural design method of a heat exchange chamber of a car exhaust temperature difference generator, which effectively improves the net output power of the temperature difference generator by changing the expansion ratio, and further optimizes the performance of the temperature difference generator.

[0006] Technical scheme: A structural design method of a heat exchange chamber of a car exhaust temperature difference generator, the specific steps are as follows:

[0007] Step 1, measure the thermal resistance of the heat exchanger bottom plate, the radiator bottom plate, the ceramic plate and the PN junction of the car exhaust temperature difference generator;

[0008] Step 2, measure and calculate the convective heat transfer coefficient of air to the surface of the expansion chamber and the convective heat transfer coefficient of cooling water to the surface of the radiator;

[0009] Step 3, measure the hot end temperature T of the thermoelectric chip h and the cold end temperature T c ;

[0010] Step 4, calculate the output power P of the automobile thermoelectric generator output ;

[0011] Step 5, calculate the exhaust back pressure loss of the automobile thermoelectric generator due to the increase of the expansion ratio;

[0012] Step 6, calculate the net output power P of the tapered thermoelectric power generation system net ;

[0013] Step 7, compare the sizes of the net output power and , if , continue to increase the expansion ratio m, repeat the above steps until .

[0014] Further, the step 1 is specifically as follows:

[0015] The thermal resistance of the expansion chamber bottom plate is:

[0016] (1)

[0017] In the formula, is the thickness of the expansion chamber in contact with the lower ceramic plate, is the thermal conductivity of the heat exchanger bottom plate, is the cross-sectional area of the expansion chamber bottom plate;

[0018] The thermal resistance of the radiator bottom plate is:

[0019] (2)

[0020] In the formula, is the thickness of the radiator bottom plate in contact with the upper ceramic plate, is the thermal conductivity of the radiator bottom plate, is the cross-sectional area of the radiator bottom plate;

[0021] The thermal resistance of the ceramic plate is:

[0022] (3)

[0023] In the formula, is the thickness of the ceramic plate, is the thermal conductivity of the ceramic plate, is the cross-sectional area of the ceramic plate;

[0024] The thermal resistance of the PN junction is:

[0025] (4)

[0026] wherein, is the thickness of the P-type or N-type semiconductor, is the logarithm of the PN junction, is the thermal conductivity of the P-type semiconductor, is the thermal conductivity of the N-type semiconductor, is the cross-sectional area of the single P-type or N-type semiconductor.

[0027] Further, the step 2 is specifically as follows:

[0028] The convective heat transfer coefficient of air to the surface of the expansion cavity and the convective heat transfer coefficient of cooling water to the surface of the radiator are calculated:

[0029] According to the Gnielinski empirical formula, the Nusselt number expression is:

[0030] (5)

[0031] wherein Re is the Reynolds number;

[0032] Reynolds number , wherein is the density of air or water, is the flow rate of air or water, D is the hydraulic diameter, is the dynamic viscosity of air or water;

[0033] The wetted perimeter in the expansion cavity is , the hydraulic diameter is , wherein b is the width of the expansion cavity and H is the height of the expansion cavity;

[0034] Prandtl number , wherein is the dynamic viscosity of air or water, is the specific heat capacity of air or water, is the thermal conductivity of air or water;

[0035] The flow rate of air or water , wherein is the mass flow rate of air or water, is the density of air or water, and A is the flow area of air or water;

[0036] The convective heat transfer coefficient of air to the surface of the radiator or the convective heat transfer coefficient of cooling water to the surface of the radiator is , wherein is the Nusselt number, and D is the hydraulic diameter, thermal conductivity of air or water;

[0037] The convective heat transfer coefficient of air to the surface of the expansion chamber is The relationship between the expansion ratio m and the temperature difference ΔT is: (6)

[0038] Further, the step 3 is specifically as follows:

[0039] The hot end temperature T of the thermoelectric power generation sheet is measured h and the cold end temperature T c :

[0040] The heat absorbed by the heat exchanger from the air is:

[0041] (7)

[0042] In the formula, is the convective heat transfer coefficient of air and the surface of the heat exchanger, is the contact area of air and the surface of the heat exchanger, and are the inlet and outlet temperatures of air, respectively, is the temperature of the bottom plate of the expansion chamber;

[0043] The heat taken away by the cooling water from the radiator is:

[0044] (8)

[0045] In the formula, is the convective heat transfer coefficient of cooling water and the surface of the radiator, is the contact area of cooling water and the surface of the radiator, and are the inlet and outlet temperatures of cooling water, respectively, is the temperature of the bottom plate of the radiator.

[0046] During the operation of the thermoelectric power generation system, the change in the internal energy of air and water is approximately equal to the heat absorbed by the expansion chamber and the heat dissipated by the radiator, respectively, which can be calculated as follows:

[0047] (9)

[0048] (10)

[0049] In the formula, is the specific heat capacity of air, is the mass flow rate of air, is the specific heat capacity of cooling water, is the mass flow rate of cooling water.

[0050] From the total thermal resistance of the thermoelectric power generation system, the heat absorption of the expansion chamber and the heat dissipation of the heat sink can be calculated as follows:

[0051] (11)

[0052] (12)

[0053] wherein, , .

[0054] In the formula, is the hot end temperature of the thermoelectric element, is the cold end temperature of the thermoelectric element, is the thermal resistance of the upper ceramic plate or the lower ceramic plate, is the thermal resistance of the heat exchanger bottom plate, is the thermal resistance of the heat sink bottom plate.

[0055] The heat generated and conducted between the hot end and the cold end of the thermoelectric element includes Fourier heat, Peltier heat and Joule heat, so the heat absorption of the heat exchanger and the heat dissipation of the heat sink can be described as:

[0056] (13)

[0057] (14)

[0058] wherein, (15)

[0059] (16)

[0060] (17)

[0061] (18)

[0062] In the formula, is the Seebeck coefficient of the thermoelectric material, is the current generated under the thermoelectric effect, is the total internal resistance of the thermoelectric module, and N is the logarithm of the PN junction;

[0063] The integral value of the thermoelectric material parameter is taken as the integral mean value, that is:

[0064] (19)

[0065] (20)

[0066] (21)

[0067] There are 9 equations and 9 unknowns in combination of equations (7)-(15), which are: The relationship between the hot end temperature T h , the cold end temperature T c and the expansion ratio m can be solved by iteration under the initial conditions Tei and Twi, i.e.:

[0068] (22)

[0069] Further, the step 4 is specifically as follows:

[0070] The output power P output of the automobile thermoelectric generator is calculated, which is determined by the Seebeck coefficient, internal resistance, two end temperature and load resistance of the thermoelectric generator, i.e.:

[0071] (23)

[0072] wherein, is the gradually increasing m th expansion ratio, is the Seebeck coefficient of the thermoelectric generator, is the hot end temperature of the thermoelectric generator corresponding to the m th expansion ratio, is the cold end temperature of the thermoelectric generator corresponding to the m th expansion ratio, is the total internal resistance of the thermoelectric module, and represents the load resistance.

[0073] Further, the step 5 is specifically as follows:

[0074] The exhaust back pressure loss caused by increasing the expansion ratio of the automobile thermoelectric generator is calculated, which is:

[0075] (24)

[0076] wherein, is the mass flow rate of the exhaust gas, g is the gravitational acceleration, H f is the head loss caused by fluid flow, and H local is the local head loss caused by fluid flow.

[0077] In addition, the head loss H is:

[0078] (25)

[0079] (26)

[0080] wherein, , l, D, v and Re are the frictional resistance coefficient, flow length, hydraulic diameter, average velocity of fluid and Reynolds number, respectively.

[0081] The local head loss can be calculated by the following formula:

[0082] (27)

[0083] (28)

[0084] wherein, ζ is the local resistance coefficient of the tapered pipe, v is the average velocity of the exhaust gas flow, g is the acceleration of gravity, d is the outlet diameter, b is the expansion cavity width, and m is the expansion ratio;

[0085] The relationship between the exhaust back pressure loss and the expansion ratio is obtained by simultaneous solution as:

[0086] (29)

[0087] Further, the step 6 is specifically as follows:

[0088] The net output power P of the tapered thermoelectric generator system is calculated as: net and a curve is drawn, that is:

[0089] P net =P output -P loss (30)

[0090] Further, the step 7 is specifically as follows:

[0091] The sizes of the net output power and are compared, if , the expansion ratio m is continuously increased, and the above steps are repeated until .

[0092] Beneficial effects: by matching the expansion ratio of the expansion cavity with the thermoelectric generator, the net output power of the thermoelectric generator is maximized, and the performance of the thermoelectric generator is optimized. Based on the thermal resistance model and the basic theory of thermoelectric effect, the net output power of the thermoelectric generator under different expansion ratios can be obtained, and the corresponding expansion ratio, that is, the optimal expansion ratio, can be determined. BRIEF DESCRIPTION OF DRAWINGS

[0093] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description only constitute a part of the embodiments of the present application, and for those skilled in the art, other drawings can also be obtained without creative labor based on the provided drawings.

[0094] Figure 1 Flow chart for matching design of expansion ratio of automobile thermoelectric generator.

[0095] Figure 2 Structure diagram of automobile exhaust thermoelectric generator.

[0096] Figure 3 Side view of automobile exhaust thermoelectric generator.

[0097] Figure 4 Schematic diagram of heat dissipation end of automobile exhaust thermoelectric generator.

[0098] Figure 5 Schematic diagram of thermoelectric module.

[0099] Figure 6 Schematic diagram of thermal resistance of each part of thermoelectric system.

[0100] Figure 7 Curve of net output power P of automobile thermoelectric system and expansion ratio m. net DETAILED DESCRIPTION

[0101] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments only constitute a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0102] In the description of the present application, it should be understood that the terms "upper", "lower", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer" and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the purpose of facilitating the description of the present application and simplifying the description, and therefore cannot be understood as indicating or implying that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application.

[0103] ​In the present application, unless otherwise explicitly specified and limited, "on" or "under" of a first feature to a second feature can include that the first and second features are in direct contact, or that the first and second features are not in direct contact but are in contact through another feature between them. Moreover, "on", "above" and "over" of a first feature to a second feature includes that the first feature is directly above and obliquely above the second feature, or only indicates that the first feature is higher in horizontal height than the second feature. "Under", "below" and "underneath" of a first feature to a second feature includes that the first feature is directly below and obliquely below the second feature, or only indicates that the first feature is lower in horizontal height than the second feature.

[0104] As shown in Figure 1 , a method for designing the structure of a heat exchange chamber of a vehicle exhaust temperature difference generator, the specific steps are as follows:

[0105] Step 1, measure the thermal resistance of the expansion chamber bottom plate, the radiator bottom plate, the ceramic plate and the PN junction of the vehicle exhaust temperature difference generator as shown in Figures 2 to 6 ;

[0106] Step 2, measure and calculate the convective heat transfer coefficient of air to the surface of the expansion chamber and the convective heat transfer coefficient of cooling water to the surface of the radiator;

[0107] Step 3, measure the hot end temperature T h and the cold end temperature T c of the temperature difference generator module;

[0108] Step 4, calculate the output power P output of the vehicle temperature difference generator;

[0109] Step 5, calculate the exhaust back pressure loss of the vehicle temperature difference generator due to the increase of the expansion ratio;

[0110] Step 6, calculate the net output power P net of the tapered temperature difference generator system;

[0111] Step 7, compare the sizes of the net output power P and P , if P , continue to increase the expansion ratio m, repeat the above steps until P .

[0112] The step 1 is specifically as follows:

[0113] The thermal resistance of the expansion chamber bottom plate is:

[0114] (1)

[0115] In the formula, is the thickness of the expansion chamber in contact with the lower ceramic plate, the thermal conductivity of the heat sink bottom plate, the cross-sectional area of the expansion chamber bottom plate;

[0116] the thermal conduction resistance of the heat sink bottom plate is:

[0117] (2)

[0118] wherein, the thickness of the heat sink bottom plate in contact with the ceramic plate, the thermal conductivity of the heat sink bottom plate, the cross-sectional area of the heat sink bottom plate;

[0119] the thermal conduction resistance of the ceramic plate is:

[0120] (3)

[0121] wherein, the thickness of the ceramic plate, the thermal conductivity of the ceramic plate, the cross-sectional area of the ceramic plate;

[0122] the thermal conduction resistance of the PN junction is:

[0123] (4)

[0124] wherein, the thickness of the P-type or N-type semiconductor, the logarithm of the PN junction, the thermal conductivity of the P-type semiconductor, the thermal conductivity of the N-type semiconductor, the cross-sectional area of a single P-type or N-type semiconductor.

[0125] The step 2 is specifically as follows:

[0126] The convective heat transfer coefficient of air to the surface of the expansion chamber and the convective heat transfer coefficient of cooling water to the surface of the heat sink are calculated:

[0127] According to the Gnielinski empirical formula, the Nusselt coefficient expression is:

[0128] (5)

[0129] wherein Re is the Reynolds number;

[0130] Reynolds number , wherein is the density of air or water, is the flow rate of air or water, and D is the hydraulic diameter, Dynamic viscosity of air or water;

[0131] Wet perimeter in the expansion chamber is , hydraulic diameter , where b is the expansion chamber width and H is the expansion chamber height;

[0132] Prandtl number , where Dynamic viscosity of air or water, Specific heat capacity of air or water, Thermal conductivity of air or water;

[0133] Flow velocity of air or water , where Mass flow rate of air or water, Density of air or water, A is the flow cross-sectional area of air or water;

[0134] Convective heat transfer coefficient of air to the surface of the heat exchanger or of cooling water to the surface of the radiator is , where Nu is the Nusselt number, D is the hydraulic diameter, Thermal conductivity of air or water;

[0135] By combining the above, the convective heat transfer coefficient of air to the surface of the expansion chamber is The relationship between the expansion ratio m and the convective heat transfer coefficient of air to the surface of the expansion chamber is: (6)

[0136] The step 3 is specifically as follows:

[0137] The hot end temperature T h and the cold end temperature T c of the thermoelectric power generation sheet are measured:

[0138] The heat absorbed by the heat exchanger from the air is:

[0139] (7)

[0140] In the formula, Convective heat transfer coefficient of air and the surface of the heat exchanger, Contact area of air and the surface of the heat exchanger, And are the inlet and outlet temperatures of air, respectively, Temperature of the bottom plate of the expansion chamber;

[0141] The heat taken away by the cooling water from the radiator is:

[0142] (8)

[0143] where, is the convective heat transfer coefficient of the cooling water and the surface of the heat sink, is the contact area of the cooling water and the surface of the heat sink, and are the inlet and outlet temperatures of the cooling water, respectively, is the temperature of the bottom plate of the heat sink.

[0144] The change in internal energy of air and water during the operation of the thermoelectric power generation system is approximately equal to the heat absorbed by the expansion chamber and the heat dissipated by the heat sink, respectively, which can be calculated as follows:

[0145] (9)

[0146] (10)

[0147] where, is the specific heat capacity of air, is the mass flow rate of air, is the specific heat capacity of cooling water, is the mass flow rate of cooling water.

[0148] From the total thermal resistance of the thermoelectric power generation system, the heat absorbed by the expansion chamber and the heat dissipated by the heat sink can be calculated as follows:

[0149] (11)

[0150] (12)

[0151] where, , .

[0152] where, is the hot end temperature of the thermoelectric element, is the cold end temperature of the thermoelectric element, is the thermal resistance of the upper ceramic plate or the lower ceramic plate, is the thermal resistance of the heat exchanger bottom plate, is the thermal resistance of the heat sink bottom plate.

[0153] The heat generated and conducted between the hot end and the cold end of the thermoelectric element includes Fourier heat, Peltier heat and Joule heat, so the heat absorbed by the heat exchanger and the heat dissipated by the heat sink can be described as:

[0154] (13)

[0155] (14)

[0156] where, (15)

[0157] (16)

[0158] (17)

[0159] (18)

[0160] wherein, is the Seebeck coefficient of the thermoelectric material, is the current generated under the thermoelectric effect, is the total internal resistance of the thermoelectric module, and N is the logarithm of the PN junction;

[0161] The integral mean value of the thermoelectric material parameter is taken, i.e.,

[0162] (19)

[0163] (20)

[0164] (21)

[0165] Combining formulas (7)-(15), there are 9 equations and 9 unknowns, respectively: The relationship between the hot end temperature T h , the cold end temperature T c and the expansion ratio m can be obtained by iteration under the initial conditions Tei and Twi, i.e.

[0166] (22)

[0167] The step 4 is specifically as follows:

[0168] The output power P output of the automobile thermoelectric generator is calculated, which is determined by the Seebeck coefficient, internal resistance, two end temperatures and load resistance of the thermoelectric sheet, i.e.

[0169] (23)

[0170] wherein, is the gradually increasing m th expansion ratio, is the Seebeck coefficient of the thermoelectric sheet, is the hot end temperature of the thermoelectric sheet corresponding to the m th expansion ratio, is the cold end temperature of the thermoelectric sheet corresponding to the m th expansion ratio, The total internal resistance of the thermoelectric module, The load resistance is represented.

[0171] The step 5 is specifically as follows:

[0172] The exhaust back pressure loss caused by increasing the expansion ratio of the automobile thermoelectric generator is calculated:

[0173] (24)

[0174] Wherein, The mass flow rate of the exhaust gas, g is the acceleration of gravity, H f The head loss caused by fluid flow, H local The local head loss caused by fluid flow.

[0175] In addition, the head loss caused by fluid flow Is:

[0176] (25)

[0177] (26)

[0178] Wherein, , l, D, v and Re are the head loss caused by fluid flow, respectively, the flow length, the hydraulic diameter, the average velocity of the fluid and the Reynolds number.

[0179] The local head loss can be calculated by the following formula:

[0180] (27)

[0181] (28)

[0182] Wherein, ζ is the local resistance coefficient of the tapered pipe, v is the average velocity of the exhaust gas flow, g is the acceleration of gravity, d is the outlet diameter, b is the expansion cavity width, m is the expansion ratio;

[0183] The relationship between the exhaust back pressure loss And the expansion ratio Is:

[0184] (29)

[0185] The step 6 is specifically as follows:

[0186] The net output power P net Of the tapered thermoelectric generation system is calculated, and the curve is drawn, that is:

[0187] P net =P output -Ploss (30)

[0188] The step 7 is specifically as follows:

[0189] The size of the net output power and is compared, if , the expansion ratio m is continuously increased, and the above steps are repeated until .

[0190] According to the above method, the net output power P net and the change of the expansion ratio m are calculated, as shown in Figure 7 , the curve shows a parabolic trend, that is, the net output power P net of the temperature difference power generation system can reach a maximum value at a certain expansion ratio.

[0191] The load resistance of the tapered temperature difference power generation system in this example is 40 .

[0192] The physical property parameters of the hot air and cooling water used in this example are shown in Table 1.

[0193] The parameters of the temperature difference power generation sheet used in this example and the thermal resistance values calculated by each part are shown in Table 2.

[0194] The polynomial fitting formula of the equivalent thermoelectric parameters of the temperature difference power generation sheet used in this example is shown in Table 3.

[0195] The structural dimensions of the automobile temperature difference power generator used in this example are shown in Table 4.

[0196] Table 1 Physical property parameters of hot air and cooling water

[0197]

[0198] The material used in the expansion chamber of this example is aluminum, and the thermal conductivity is 217.7 W / (m·K), and the materials used in the inlet and outlet are stainless steel.

[0199] Table 2 Specific parameters of temperature difference power generation sheet and thermal resistance values of each part

[0200]

[0201] Table 3 Polynomial fitting formula of equivalent thermoelectric parameters of temperature difference power generation sheet

[0202]

[0203] Table 4 Structural dimensions of automobile temperature difference power generator

[0204]

[0205] The various embodiments described in this specification are implemented in a progressive manner, each embodiment focusing on the differences from other embodiments, and the same or similar parts between embodiments can be mutually referred to. For the apparatus disclosed by the embodiments, since it corresponds to the method disclosed by the embodiments, the description is relatively simple, and the relevant parts can be referred to the description of the method part.

[0206] The above description of disclosed embodiments enables one of ordinary skill in the art to make or use the application. Various modifications to these embodiments will be readily apparent to those of ordinary skill in the art, and the generic principles defined herein can be applied to other embodiments without departing from the spirit or scope of the application. Therefore, the application is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method of designing a heat exchange chamber of a car exhaust temperature difference generator, characterized in that, The specific steps are as follows: Step 1, measure the heat conduction thermal resistance of the heat exchanger bottom plate, radiator bottom plate, ceramic plate and PN junction of the automobile exhaust temperature difference generator; Step 2, measure and calculate the convective heat transfer coefficient of air to the surface of the expansion chamber and the convective heat transfer coefficient of cooling water to the surface of the radiator; Step 3, measuring the hot end temperature T of the thermoelectric sheet h and the cold end temperature T c ; Step 4, calculating the output power P of the automobile thermoelectric generator output ; Step 5, calculate the exhaust back pressure loss caused by increasing the expansion ratio of the automobile temperature difference generator; Step 6, calculating the net output power P of the tapered thermoelectric power generation system net ; Step 7, compare the net output power and , if , then continue to increase the expansion ratio m, repeat the above steps until ; The step 3 is specifically as follows: Measuring the hot end temperature T of a thermoelectric generator sheet h and the cold end temperature T c : The heat absorbed by the heat exchanger from the air is: （7） wherein is the convective heat transfer coefficient for air and heat exchanger surface, is the contact area for air and heat exchanger surface, and are the inlet and outlet temperatures of air, respectively, is the temperature of the expanded cavity floor; The heat taken away by the cooling water from the radiator is: （8） wherein is the convective heat transfer coefficient between the cooling water and the surface of the heat sink, is the contact area between the cooling water and the surface of the heat sink, and are the inlet and outlet temperatures of the cooling water, respectively, is the temperature of the bottom plate of the heat sink; During the operation of the temperature difference power generation system, the change in the internal energy of air and water is approximately equal to the heat absorption of the expansion chamber and the heat dissipation of the radiator, which can be calculated as follows: （9） （10） wherein Cp is the specific heat capacity of air, G is the mass flow of air, Cp is the specific heat capacity of cooling water, G is the mass flow of cooling water; From the total thermal resistance of the temperature difference power generation system, the heat absorption of the expansion chamber and the heat dissipation of the radiator can be calculated as follows: （11） （12） wherein , ; In the formula, T is the hot end temperature of the thermoelectric generator sheet, T is the cold end temperature of the thermoelectric generator sheet, R is the thermal resistance of the upper ceramic plate or the lower ceramic plate, R is the thermal resistance of the heat exchanger bottom plate, R is the thermal resistance of the heat sink bottom plate; The heat generated and conducted between the hot end and the cold end of the temperature difference power generation sheet includes Fourier heat, Peltier heat and Joule heat, so the heat absorption of the heat exchanger and the heat dissipation of the radiator can be described as: （13） （14） wherein (15) （16） （17） （18） wherein S is the Seebeck coefficient of the thermoelectric material, I is the current generated under the thermoelectric effect, Rtot is the total internal resistance of the thermoelectric module, and N is the logarithm of the number of PN junctions. The integral mean value of the thermoelectric material parameter value is taken, that is: （19） （20） （21） There are 9 equations and 9 unknowns in combination of equations (7)-(15), which are: The relationship between the thermal end temperature T h , the cold end temperature T c and the expansion ratio m can be obtained by iteration under the initial conditions Tei and Twi; that is, the thermal end temperature T h , the cold end temperature T c and the expansion ratio m of the thermoelectric sheet are calculated. （22）。 2. The method of designing the heat exchanging chamber of the automobile exhaust thermoelectric generator according to claim 1, characterized in that: The step 1 is specifically as follows: The heat conduction thermal resistance of the expansion chamber bottom plate is: （1） wherein is the thickness of the expansion chamber in contact with the lower ceramic plate, is the thermal conductivity of the heat exchanger bottom plate, is the cross-sectional area of the expansion chamber bottom plate; The heat conduction thermal resistance of the radiator bottom plate is: （2） wherein is the thickness of the heat sink base plate in contact with the upper ceramic plate, is the thermal conductivity of the heat sink base plate, is the cross-sectional area of the heat sink base plate; The heat conduction thermal resistance of the ceramic plate is: （3） wherein is the thickness of the ceramic plate, is the thermal conductivity of the ceramic plate, is the cross-sectional area of the ceramic plate; The heat conduction thermal resistance of the PN junction is: （4） wherein is the thickness of the P-type or N-type semiconductor, is the logarithm of the PN junction, is the thermal conductivity of the P-type semiconductor, is the thermal conductivity of the N-type semiconductor, is the cross-sectional area of the individual P-type or N-type semiconductor.

3. The method of designing the heat exchanging chamber of the automobile exhaust thermoelectric generator according to claim 2, characterized in that: The step 2 is specifically as follows: Calculate the convective heat transfer coefficient of air to the surface of the expansion chamber and the convective heat transfer coefficient of cooling water to the surface of the radiator: According to the Gnielinski empirical formula, the Nusselt coefficient expression is: （5） where Re is the Reynolds number; Reynolds number wherein is the density of air or water, is the flow velocity of air or water, D is the hydraulic diameter, is the dynamic viscosity of air or water; The wetted perimeter in the expanded cavity is , the hydraulic diameter where b is the expanded cavity width and H is the expanded cavity height; Prandtl number where is the dynamic viscosity of air or water, is the specific heat capacity of air or water, is the thermal conductivity of air or water; flow rate of air or water , wherein mass flow of air or water, density of air or water, A is the flow cross-sectional area of air or water; The convective heat transfer coefficient of air to the surface of the heat exchanger or of cooling water to the surface of the radiator is wherein is the Nusselt number, D is the hydraulic diameter, is the thermal conductivity of air or water; Combining the above, the convective heat transfer coefficient of air to the surface of the expansion cavity can be obtained Relationship between the expansion ratio m: （6）。 4. The method of designing the heat exchanging chamber of the automobile exhaust thermoelectric generator according to claim 1, characterized in that: The step 4 is specifically as follows: Calculating the output power P of a thermoelectric generator of an automobile output is determined by the Seebeck coefficient, internal resistance, temperature difference and load resistance of the thermoelectric generator, i.e. （23） wherein, is the i-th expansion ratio, is the i-th expansion ratio, is the Seebeck coefficient of the thermoelectric element, is the i-th expansion ratio, is the hot junction temperature of the thermoelectric element corresponding to the i-th expansion ratio, is the cold junction temperature of the thermoelectric element corresponding to the i-th expansion ratio, is the i-th expansion ratio, is the total internal resistance of the thermoelectric module, denotes the load resistance.

5. The method of designing the heat exchanging chamber of the automobile exhaust thermoelectric generator as claimed in claim 1, wherein: The step 5 is specifically as follows: Calculate the exhaust back pressure loss caused by increasing the expansion ratio of the automobile temperature difference generator: （24） wherein is the mass flow rate of the exhaust gas, g is the acceleration due to gravity, H f is the frictional head loss due to fluid flow, H local is the local head loss due to fluid flow; Further, the head loss along the way is H = 0. （25） （26） wherein, l, D, v and Re are the frictional resistance coefficient, flow length, hydraulic diameter, average velocity of the fluid and Reynolds number, respectively; The local water head loss can be calculated by the following formula: （27） （28） Wherein, ζ is the local resistance coefficient of the tapered pipe, v is the average velocity of the exhaust gas flow, g is the acceleration of gravity, d is the outlet diameter, b is the width of the expansion chamber, and m is the expansion ratio; simultaneously solving for exhaust back pressure loss and expansion ratio the relationship between （29）。 6. The method of designing the heat exchanging chamber of the automobile exhaust thermoelectric generator according to claim 1, characterized in that: The step 6 is specifically as follows: Calculating the net output power P of a tapered thermoelectric power generation system net and plot a graph, namely: P net =P output -P loss (30).

7. The method of designing the heat exchanging chamber of the automobile exhaust thermoelectric generator according to claim 1, characterized in that: The step 7 is specifically as follows: Comparing the net output power and If , then increase the expansion ratio m, repeat the above steps until .

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