Latent heat type functional fluid for cooling fuel cell and preparation method of latent heat type functional fluid

Abstract

The invention discloses a latent heat type functional fluid for fuel cell cooling and a preparation method thereof, and belongs to the technical field of fuel cell cooling. The latent heat type functional fluid for cooling the fuel cell comprises a base fluid and an additive, the additive is a boron nitride and paraffin phase change microcapsule with a core-shell structure, and porous / flaky boron nitride is a shell and wraps the paraffin core. The base solution is an ethanol water solution with the mass fraction of 50%, and the mass fraction of the additive is 2-30%. The preparation method of the functional fluid comprises the following steps: preparing an emulsifier solution, preparing an oil / water phase emulsion, preparing a flaky / porous boron nitride dispersion liquid, mixing the flaky / porous boron nitride dispersion liquid with the oil / water phase emulsion, adjusting the pH value, filtering, drying to obtain a phase change microcapsule, and mixing the phase change microcapsule with an ethanol water solution. By adopting the latent heat type functional fluid for cooling the fuel cell and the preparation method of the latent heat type functional fluid, the problems of weak heat exchange capability and high conductivity of the existing fuel cell cooling liquid can be solved.

Description

Technical Field

[0001] The present invention relates to the technical field of fuel cell cooling, and in particular to a latent heat functional fluid for fuel cell cooling and a preparation method thereof. Background Art

[0002] In recent years, affected by the energy crisis and environmental pollution, energy transformation has become an inevitable choice, especially in the field of transportation. The development of internal combustion engines that use traditional fossil energy as the main fuel to new energy has become an irreversible trend. Proton exchange membrane fuel cells have become one of the main power sources for new energy vehicles due to their high energy density, low operating temperature and fast start-stop response. However, proton exchange membrane fuel cells are highly sensitive to operating temperature. Too high or too low operating temperature will reduce the battery output performance and even damage key components. Therefore, it is particularly important to achieve effective cooling of the fuel cell and ensure temperature uniformity.

[0003] Generally, the cooling system of fuel cells mainly adopts liquid cooling or air cooling. The cooling medium directly passes through the bipolar plate or flows through the cooling channel. When selecting, four factors, including fuel cell power, system complexity, economic cost, and application scenario, need to be considered comprehensively. For fuel cell vehicles, the required battery power is about 100kW. Liquid cooling has high thermal conductivity and heat capacity, high heat dissipation rate, and is not limited by system size, weight and complexity, so it has become the preferred cooling method. At present, the traditional cooling medium adopts a mixed solution of water and additives such as ethylene glycol, which has the characteristics of non-toxicity, antifreeze and chemical inertness. However, after a period of use, the cooling medium will be oxidized or contaminated by metal ions collected by the bipolar plate, resulting in enhanced conductivity, which in turn causes a series of problems such as current shunting, coolant degradation and accelerated corrosion. In addition, the cooling efficiency of traditional coolants is low, and it is impossible to control the temperature of high energy density fuel cell stacks. It can be seen that the coolant used in the fuel cell cooling system needs to take into account both cooling capacity and electrical safety, and the development of new coolants is imperative.

[0004] As a special coolant, latent heat functional fluid consists of a base fluid mainly composed of water and a phase change material additive. During the flow heat transfer process, a "micro-convection" effect is generated near the phase change material particles, which effectively enhances the heat transfer. At the same time, when the phase change material reaches the phase change temperature, a phase change will occur, absorbing a large amount of latent heat, thereby achieving a higher cooling efficiency. Therefore, it has gradually emerged in various cooling systems. For example, patent CN202210148199.7 provides a vehicle-mounted vaccine transport box temperature control system using latent heat functional fluid, which can effectively preserve vaccines during long-distance transportation; patent CN202210249763.4 uses latent heat functional fluid to recover waste heat to invent a refrigerator defrost system, which achieves the purpose of intelligent defrosting and energy saving; patent CN201910316146.X invented a floor radiation air-conditioning system based on latent heat transfer fluid, which can reduce room temperature fluctuations and overall energy consumption; patent CN201821695200.3 has developed a cold chain temperature control system packaging system based on latent heat phase change fluid, which can keep warm for a long time and realize long-distance transportation.

[0005] In order to further enhance latent heat storage and improve thermal stability, researchers have proposed a microcapsule structure, which usually uses a two-dimensional material as a shell to encapsulate the phase change material to prevent leakage of the core material. However, there are few reports on the application of latent heat functional fluids in fuel cell thermal management, and the thermal conductivity of ordinary microcapsule composite phase change materials is small. Although adding high thermal conductivity materials to the microcapsules for modification can improve their thermal conductivity, it often also brings about the problem of high electrical conductivity, and it is impossible to take into account both high thermal transfer and electrical safety. Therefore, the development of a latent heat functional fluid with strong heat exchange capacity and high electrical safety for use in fuel cell cooling systems is of great significance to effectively improve the thermal management and output performance of fuel cells. Summary of the invention

[0006] The purpose of the present invention is to provide a latent heat functional fluid for fuel cell cooling and a preparation method thereof, so as to solve the problems of weak heat exchange capacity and high conductivity of existing fuel cell coolant.

[0007] To achieve the above-mentioned object, the present invention provides a latent heat functional fluid for fuel cell cooling, comprising a base liquid and an additive, wherein the additive is a core-shell structured boron nitride@paraffin phase change microcapsule, wherein the porous / flaky boron nitride is a shell wrapped around the outside of the paraffin core.

[0008] Preferably, the base liquid is an ethanol aqueous solution with a mass fraction of 50%.

[0009] Preferably, the mass fraction of the additive is 2%-30%.

[0010] The method for preparing the latent heat functional fluid for fuel cell cooling comprises the following steps:

[0011] S1, dissolving styrene-maleic anhydride copolymer particles and sodium hydroxide powder in deionized water, and stirring to obtain a yellow transparent emulsifier solution;

[0012] S2, adding paraffin and hexadecyltrimethylammonium bromide to the emulsifier solution, stirring evenly to obtain a stable oil / water phase emulsion;

[0013] S3, ultrasonically dispersing the porous boron nitride and the flaky boron nitride in anhydrous ethanol to form a stable flaky / porous boron nitride dispersion without precipitation;

[0014] S4, adding the flaky / porous boron nitride dispersion dropwise into the oil / water phase emulsion, and stirring evenly to obtain a phase change microcapsule suspension;

[0015] S5, adjusting the pH value of the phase change microcapsule suspension to be acidic, filtering, washing, and drying to obtain the phase change microcapsules;

[0016] S6. Add the phase change microcapsules into the ethanol aqueous solution and stir evenly, and obtain the latent heat functional fluid after ultrasonic dispersion.

[0017] Preferably, in S1, the mass ratio of styrene-maleic anhydride copolymer particles to sodium hydroxide powder is 1:1-3, the mass ratio of styrene-maleic anhydride copolymer particles to deionized water is 1:10-20, and stirring is carried out at 60°C-90°C for 2h-4h.

[0018] Preferably, in S2, the mass ratio of paraffin to hexadecyltrimethylammonium bromide is 5-8:1, the volume ratio of paraffin to emulsifier is 1:3-8, the stirring temperature is 60°C-90°C, and the stirring time is 0.5h-2h.

[0019] Preferably, in S3, the mass ratio of the flaky boron nitride to the porous boron nitride is 1:1-3, and the ultrasonic dispersion time is 10 min-50 min.

[0020] Preferably, in S4, the stirring temperature is 60°C-90°C, and the stirring time is 2h-4h.

[0021] Preferably, in S5, citric acid is used to adjust the pH value to 4-5, and the drying temperature is 30°C-50°C. Preferably, in S6, the mass fraction of the ethanol aqueous solution is 50%, the ethanol aqueous solution is prepared by anhydrous ethanol and deionized water, the stirring temperature is 20°C-40°C, the stirring time is 10min-50min, and the ultrasonic time is 10min-50min.

[0022] The advantages and positive effects of the latent heat functional fluid for fuel cell cooling and the preparation method thereof of the present invention are:

[0023] 1. The present invention adopts ethanol as the base liquid of the latent heat functional fluid, which effectively improves the heat exchange capacity of the fluid.

[0024] 2. The composite phase-change microcapsules were obtained by modifying paraffin with porous / flaky boron nitride. The melting point of paraffin is 58°. The latent heat and heat transfer coefficient of the fluid are increased through the phase change of paraffin, and the microcapsules have lower electrical conductivity. The porous boron nitride is used as the framework for adsorbing paraffin, and the flake boron nitride is used for supplementary encapsulation. The self-assembly of the two is achieved by the interaction of π-π bonds, forming a hybrid shell of flake / porous boron nitride, and obtaining a new phase-change microcapsule with high thermal conductivity and good stability.

[0025] 3. By adjusting the amount of composite phase change microcapsule additives added, latent heat fluids with different thermal conductivity properties can be prepared to meet different thermal conductivity requirements.

[0026] 4. The fluid described in the present invention can achieve efficient cooling, which helps to improve the temperature uniformity inside the fuel cell and enhance the output performance.

[0027] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a preparation flow chart of an embodiment of the present invention;

[0029] Figure 2 The conductivity test graph of the latent heat type functional fluid prepared in Examples 1-5 of the present invention and the ethanol aqueous solution prepared in Comparative Example 1 at 40° C. to 80° C.;

[0030] Figure 3 Thermal conductivity test diagram of latent heat type functional fluids prepared in Examples 1-5 of the present invention and ethanol aqueous solution prepared in Comparative Example 1;

[0031] Figure 4 Convective heat transfer coefficients of latent heat type functional fluids prepared in Examples 1-5 of the present invention and ethanol aqueous solution prepared in Comparative Example 1 at different Reynolds numbers;

[0032] Figure 5 Viscosity test results of latent heat functional fluids prepared in Examples 1-5 of the present invention and ethanol aqueous solution prepared in Comparative Example 1 within the working temperature range;

[0033] Figure 6 The water pump power curves of the latent heat functional fluids prepared in Examples 1-5 of the present invention and the ethanol aqueous solution prepared in Comparative Example 1;

[0034] Figure 7 The stack polarization curves of the latent heat functional fluids prepared in Examples 1-5 of the present invention and the ethanol aqueous solution prepared in Comparative Example 1. DETAILED DESCRIPTION

[0035] The technical solution of the present invention is further described below through the accompanying drawings and embodiments.

[0036] Example 1

[0037] Figure 1 1 is a preparation flow chart of an embodiment of the present invention. Figure 1 As shown, the preparation method of latent heat type functional fluid for fuel cell cooling includes:

[0038] S1. Dissolve 10 g of styrene-maleic anhydride copolymer particles and 20 g of sodium hydroxide powder in 150 mL of deionized water, and stir at 80° C. for 3 h to obtain a yellow transparent emulsifier solution.

[0039] S2. Stir 30 g of paraffin, 5 g of hexadecyltrimethylammonium bromide and 150 mL of emulsifier solution at 80° C. for 1 h to obtain a stable oil / water phase emulsion.

[0040] S3. Ultrasonic dispersion of 7.5 g of flaky boron nitride and 12.5 g of porous boron nitride in 100 mL of anhydrous ethanol for 30 min to form a stable flaky / porous boron nitride dispersion without precipitation.

[0041] S4. Add the boron nitride dispersion dropwise into the oil / water phase emulsion, and stir at 80° C. for 3.5 h to obtain a phase change microcapsule suspension.

[0042] S5. Adjust the pH value of the phase change microcapsule suspension to 4.5 with citric acid, wash and filter with deionized water for 3 times, and dry at 40° C. to obtain phase change microcapsules.

[0043] S6. Take 50g of phase change microcapsules and dissolve them in 50% ethanol solution, stir them at 30°C for 30min, and then ultrasonically disperse them for 30min to obtain a latent heat functional fluid with a mass fraction of 5% of boron nitride@paraffin phase change microcapsules.

[0044] Example 2

[0045] This embodiment adopts the same method as that of embodiment 1 to prepare the latent heat type functional fluid, except that the mass fraction of boron nitride@paraffin phase change microcapsules in the latent heat type functional fluid of this embodiment is 10%.

[0046] Example 3

[0047] This embodiment adopts the same method as that of embodiment 1 to prepare the latent heat type functional fluid, except that the mass fraction of boron nitride@paraffin phase change microcapsules in the latent heat type functional fluid of this embodiment is 15%.

[0048] Example 4

[0049] This embodiment adopts the same method as that of embodiment 1 to prepare the latent heat type functional fluid, except that the mass fraction of boron nitride@paraffin phase change microcapsules in the latent heat type functional fluid of this embodiment is 20%.

[0050] Example 5

[0051] This embodiment adopts the same method as that of embodiment 1 to prepare the latent heat type functional fluid, except that the mass fraction of boron nitride@paraffin phase change microcapsules in the latent heat type functional fluid of this embodiment is 25%.

[0052] Comparative Example 1

[0053] This comparative example uses only 50% ethanol aqueous solution as the base liquid, and no boron nitride@paraffin phase change microcapsules are added to the base liquid. 100 mL of anhydrous ethanol is dispersed in an equal amount of deionized water, and stirred thoroughly to obtain 50% ethanol aqueous solution.

[0054] Figure 2 The conductivity test diagram of the latent heat type functional fluid prepared in Examples 1-5 of the present invention and the ethanol aqueous solution prepared in Comparative Example 1 at 40°C to 80°C. Figure 2 As shown, since the increase in temperature will accelerate the ion migration rate, the conductivity of the latent heat functional fluids in Examples 1-5 shows an increasing trend to varying degrees with the increase in temperature. The ethanol solution is a mixture of anhydrous ethanol and deionized water, both of which have high insulation properties, so no charged particles will be generated after mixing. Even if the temperature increases to 80°C, the conductivity is extremely low, and its value is less than 0.0006mS / cm. For Examples 1-5, since the solid phase change microcapsule itself has a higher conductivity than the liquid material, the conductivity of the latent heat functional fluid is positively correlated with the amount of phase change microcapsules added. When the temperature of Example 1 is 60°C, its conductivity reaches 0.0016mS / cm. Under all test temperature conditions, the maximum conductivity of Examples 1-5 is only 0.0023mS / cm, which is lower than the 0.005mS / cm specified in the technical indicators. Therefore, the latent heat functional fluid described in the present invention as a coolant fully meets the insulation requirements of the fuel cell cooling system and has reliable electrical safety.

[0055] Figure 3 The thermal conductivity test diagram of the latent heat type functional fluid prepared in Examples 1-5 of the present invention and the ethanol aqueous solution prepared in Comparative Example 1. Figure 3As shown, the thermal conductivity of the latent heat functional fluids of Examples 1-5 and the ethanol solution of Comparative Example 1 increases with increasing temperature. This is because the increase in temperature will lead to intensified molecular vibration, which further causes the increase in heat transfer speed. The change law of the thermal conductivity of Examples 1-5 in the non-phase change temperature range is the same as that of Comparative Example 1, but in the phase change temperature range of 50℃~60℃, the thermal conductivity decreases to varying degrees, and the decrease increases with the increase in the content of phase change microcapsules. This is because the phase change material in the microcapsule begins to melt at 50℃~60℃, and gradually changes from solid to liquid. Therefore, the amount of phase change microcapsules added is a key factor affecting the thermal conductivity of the latent heat functional fluid. In the non-phase change temperature range, compared with the ethanol solution, the thermal conductivity of Example 1 can be increased by up to 38%. Even when the phase change material is completely liquid, its thermal conductivity can be stabilized at 0.506W / (m·K), which is 1.27 times that of Comparative Example 1.

[0056] Figure 4 The convective heat transfer coefficients of the latent heat type functional fluids prepared in Examples 1-5 of the present invention and the ethanol aqueous solution prepared in Comparative Example 1 at different Reynolds numbers. Figure 4 As shown, the convective heat transfer coefficient of the latent heat functional fluid of Examples 1-5 is significantly greater than that of the ethanol aqueous solution of Comparative Example 1. Although the mass percentage of the phase change microcapsules in Example 1 is only 5%, its convective heat transfer capacity has been significantly improved. Because the phase change microcapsules will begin to change phases once they reach the phase change temperature, thereby absorbing a large amount of latent heat, the heat transfer coefficient of the latent heat functional fluid increases linearly with the increase of the phase change microcapsules, and the heat transfer capacity of Examples 1-5 is also improved. Especially under high Reynolds number conditions, the convective heat transfer coefficients of Examples 1 and 5 reached 468.58W / (m 2 K) and 806.05W / (m 2 ·K), which are 1.1 times and 3.61 times of those in comparative example 1 under the same working conditions, respectively. This indicates that the prepared latent heat functional fluid has strong heat exchange capacity and excellent cooling performance.

[0057] Figure 5 The viscosity test results of the latent heat functional fluids prepared in Examples 1-5 of the present invention and the ethanol aqueous solution prepared in Comparative Example 1 within the working temperature range are shown in FIG. Figure 5As shown, the viscosity changes with temperature in the same way as general thermal fluids, that is, a negative correlation trend. When the amount of phase change microcapsules added is less than 15%, the viscosity of Examples 1-3 does not change much. When Example 3 is increased from 40°C to 80°C, its maximum difference with Comparative Example 1 only decreases from 6mPa·s to 3.2mPa·s, and the effect on the fluid flow characteristics can be ignored. However, when the amount of phase change microcapsules added is further increased, the viscosity of Examples 4 and 5 changes significantly. The viscosity of Example 5 at 40°C is 15 times that of Comparative Example 1, reaching 30.2mPa·s. Although the viscosity decreases after a period of temperature increase, it is still at a relatively high level. Excessively large viscosity values ​​will cause the flow resistance to increase, thereby increasing the power consumption of the power component water pump during system operation.

[0058] Figure 6 The water pump power curves of the latent heat functional fluids prepared in Examples 1-5 of the present invention and the ethanol aqueous solution prepared in Comparative Example 1, the change of the Reynolds number is achieved by changing the flow rate of the cooling medium. Figure 6 As shown, the power consumption of the water pump driving different cooling media is positively correlated with the content of phase change microcapsules, and the overall change pattern is related to the viscosity of phase change microcapsules of different concentrations. The pump power consumed by Examples 1, 2 and Comparative Example 1 is almost linearly related to the Reynolds number, and the change in pump power is small with the increase of Reynolds number. When the content of phase change microcapsules in the latent heat fluid increases, the power consumption of the water pump increases exponentially with the increase of Reynolds number, and the high-viscosity latent heat fluid requires higher driving power consumption of the water pump. When the Reynolds number increases from 18 to 90, the water pump power of Example 5 loses 18.1W more. Therefore, considering the material cooling efficiency and system energy consumption, the latent heat functional fluids prepared in Examples 2-4 are more conducive to the operation of the fuel cell stack.

[0059] Generally, the output performance of a fuel cell stack is reflected by the change of voltage with current density, i.e., the polarization curve. High-performance coolant can maintain the normal and stable operation of the stack under high current density. The stack was tested under different cooling medium conditions. The stack parameters and related test parameters are shown in Table 1.

[0060] Table 1 Stack parameters and test conditions

[0061]

[0062]

[0063] Figure 7 The stack polarization curves of the latent heat functional fluids prepared in Examples 1-5 of the present invention and the ethanol aqueous solution prepared in Comparative Example 1 are shown in FIG. Figure 7 As shown, at the open circuit voltage and current density less than 150 mA / cm 2When the current density is increased gradually, the heat generation of the stack increases, which leads to dehydration of the membrane electrode and a decrease in proton conductivity. Therefore, the polarization curve gradually shows a difference. When the current density is 900mA / cm 2 When the mass fraction of phase change microcapsules is higher than 15%, the maximum difference in the voltage between Example 4 and Comparative Example 1 is 0.291V, which indicates that the output performance can be improved by 8.6%. 2 Under high load conditions, the proportion of stack output performance improvement gradually decreases. When the current density increases to 900mA / cm 2 When the maximum voltage difference between the stacks corresponding to Example 4 and Example 5 is only 0.052 V. Therefore, considering the output performance of the stack, the operating cost and the power consumption, the latent heat functional fluid prepared in Example 4 is further preferred, which can better meet the working requirements of the fuel cell stack.

[0064] Therefore, the use of the latent heat functional fluid for fuel cell cooling and the preparation method thereof described in the present invention can solve the problems of weak heat exchange capacity and high conductivity of existing fuel cell coolants.

[0065] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A latent heat functional fluid for cooling a fuel cell, characterized in that: It includes a base liquid and an additive. The additive is a boron nitride@paraffin phase change microcapsule with a core-shell structure. The porous / flaky boron nitride is a shell wrapped around the outside of the paraffin core.

2. A latent heat type functional fluid for fuel cell cooling according to claim 1, characterized in that: The base liquid is an ethanol aqueous solution with a mass fraction of 50%.

3. The latent heat functional fluid for fuel cell cooling according to claim 1, characterized in that: The mass fraction of the additive is 2%-30%.

4. A method for preparing a latent heat functional fluid for fuel cell cooling according to any one of claims 1 to 3, characterized in that: The following steps are involved: S1, dissolving styrene-maleic anhydride copolymer particles and sodium hydroxide powder in deionized water, and stirring to obtain a yellow transparent emulsifier solution; S2, adding paraffin and hexadecyltrimethylammonium bromide to the emulsifier solution, stirring evenly to obtain a stable oil / water phase emulsion; S3, ultrasonically dispersing the porous boron nitride and the flaky boron nitride in anhydrous ethanol to form a stable flaky / porous boron nitride dispersion without precipitation; S4, adding the flaky / porous boron nitride dispersion dropwise into the oil / water phase emulsion, and stirring evenly to obtain a phase change microcapsule suspension; S5, adjusting the pH value of the phase change microcapsule suspension to be acidic, filtering, washing, and drying to obtain the phase change microcapsules; S6. Add the phase change microcapsules into the ethanol aqueous solution and stir evenly, and obtain the latent heat functional fluid after ultrasonic dispersion.

5. The method for preparing a latent heat functional fluid for fuel cell cooling according to claim 4, characterized in that: In S1, the mass ratio of styrene-maleic anhydride copolymer particles to sodium hydroxide powder is 1:1-3, and the mass ratio of styrene-maleic anhydride copolymer particles to deionized water is 1:10-20, and stirring is performed at 60° C.-90° C. for 2 h-4 h.

6. The method for preparing a latent heat functional fluid for fuel cell cooling according to claim 4, characterized in that: In S2, the mass ratio of paraffin wax to hexadecyltrimethylammonium bromide is 5-8:1, the volume ratio of paraffin wax to emulsifier is 1:3-8, the stirring temperature is 60°C-90°C, and the stirring time is 0.5h-2h.

7. The method for preparing a latent heat functional fluid for fuel cell cooling according to claim 4, characterized in that: In S3, the mass ratio of the flaky boron nitride to the porous boron nitride is 1:1-3, and the ultrasonic dispersion time is 10 min-50 min.

8. The method for preparing a latent heat functional fluid for fuel cell cooling according to claim 4, characterized in that: In the step S4, the stirring temperature is 60°C-90°C, and the stirring time is 2h-4h.

9. The method for preparing a latent heat functional fluid for fuel cell cooling according to claim 4, characterized in that: In S5, citric acid is used to adjust the pH value to 4-5, and the drying temperature is 30°C-50°C.

10. The method for preparing a latent heat functional fluid for fuel cell cooling according to claim 4, characterized in that: In S6, the mass fraction of the ethanol aqueous solution is 50%, the ethanol aqueous solution is prepared by anhydrous ethanol and deionized water, the stirring temperature is 20°C-40°C, the stirring time is 10min-50min, and the ultrasonic time is 10min-50min.

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