CH715547B1 - Heat transfer medium for solar thermal power generation and process for its production. - Google Patents

Abstract

The present invention discloses a heat transfer medium for solar thermal power generation comprising a molten nitrate salt heat storage medium and carbon nanotubes added to the molten nitrate salt heat storage medium; wherein the carbon nanotubes is 0.10% to 1.00% and the diameter of the carbon nanotubes is d ≥ 50 nm. The present invention further discloses a method for producing the heat transfer medium for solar thermal power generation. Compared to the molten nitrate salt heat storage medium, the heat transfer medium according to the invention for solar thermal power generation under conditions in which the decomposition temperature is similar has a significantly higher coefficient of thermal conductivity. Therefore, when the heat transfer medium is used, the efficiency of using solar energy can be improved. Compared to other similar media materials in the prior art, the compatibility of the molten nitrate salt heat storage medium of the heat transfer medium according to the invention with carbon nanotubes is better than that with other added components.

Description

Field of the invention

The present invention belongs to the technical field of solar thermal power generation and relates in particular to a heat transfer medium for solar thermal power generation and a method for its production.

State of the art

Environmental and energy problems are two of the most important issues in the world today. The effective use of solar energy is an important means of developing new energy sources, protecting the environment and ensuring sustainable development. Therefore it is strongly supported by national policies. In recent years, solar thermal power generation has become one of the most important methods of using solar energy on a large scale. The key technology of solar thermal power generation is the technology of heat transfer and heat storage at high temperatures, at which the heat transfer and heat storage medium is essential for improving the efficiency and reducing the cost of the system.

Currently widely used heat transfer and heat storage media are mainly high pressure water, steam, thermal oil, liquid metals, etc. In comparison, molten salt has the advantages of a wide operating temperature, a high thermal stability, a low viscosity, a low saturation vapor pressure and a low price and is therefore the first choice for solar thermal energy storage media. According to the various anions, the molten salt heat storage materials used in solar thermal power generation mainly include molten nitrate, molten carbonate, molten chloride salt, molten fluoride salt, etc. Among them, nitrate is the first choice for molten salt applications because of its low melting point, low cost and low corrosion.

Solar energy resources are very abundant in China, particularly in the west and south of the Qinghai-Tibet Plateau. In Qinghai Province there are good conditions such as: B. grid connection, cooling water sources and a large amount of desert land, which is very suitable for the construction of large solar thermal power plants. At the same time, Qinghai Province has a large number of salt lakes, large salt reserves and abundant salt mines, and is one of the main production sites in China for salt chemicals extracted from salt lakes. Among them, sodium chloride, magnesium chloride, potassium salt, lithium, thenardite, and other minerals rank first in China. The abundant inorganic salts such as B. potassium, sodium and magnesium salts are important raw material sources for the main components of the molten nitrate salt heat storage medium (potassium nitrate and sodium nitrate). By combining this with the advantages of local resources, the operating costs of systems for solar thermal use can be reduced, while at the same time solving the problem of poor area-wide use of salt lake resources and increasing the value of salt lake resources.

As examples of the use of binary molten nitrate heat storage medium as a heat transfer and heat storage medium, two systems, namely the centrally controlled 10 MW CSP system in Delingha and the 10 MW CSP system from Dunhuang Shouhang Energy Saving New Energy. Co., Ltd. may be mentioned. The molten salt has good thermal stability and is low in costs. However, it has a low coefficient of thermal conductivity of only 0.5 W · m <-1> · K <-1>, which has an unfavorable effect on the heat transfer performance of the system.

[0006] There is currently related research to improve the thermal conductivity of the molten nitrate salt heat storage medium. The main purpose is to add other components like silica sand, metal oxide (or non-metal oxide) nanoparticles, and sodium silicate to it to form a multiple system. Although this improved its thermal conductivity, no specific values were given in these reports. The improvement is mainly aimed at the thermal stability of the molten nitrate salt heat storage medium, meaning the improvement achieved by increasing the decomposition temperature. However, when other components are added to form multivariate nitrate salt melts, the coefficient of thermal conductivity tends to decrease as the melting point decreases. Therefore, there is still a great challenge to improve the thermal conductivity of the molten nitrate salt heat storage medium, which is important and practical.

Object of the invention

To eliminate the above problems, namely that the thermal conductivity of the existing molten nitrate salt heat storage medium is relatively small, which affects the heat exchange efficiency of the heat transfer method for molten salt in actual operation and thus the power generation and energy use efficiency, the present invention provides a heat transfer medium for solar thermal power generation and a process ready for its manufacture. In the heat transfer medium for solar thermal power generation, the thermal conductivity of the molten nitrate salt heat storage medium can be effectively improved by using carbon nanotubes.

To achieve the above object, the following technical solutions are used in the present invention: An embodiment of the present invention provides a heat transfer medium for solar thermal power generation, which comprises a molten nitrate salt heat storage medium and carbon nanotubes added to the molten nitrate salt heat storage medium; wherein the mass percentage of the carbon nanotubes is 0.10% to 1.00% and the diameter of the carbon nanotubes is d ≥ 50 nm.

Further, the molten nitrate salt heat storage medium comprises sodium nitrate and potassium nitrate which are homogeneously mixed.

Further, in the heat transfer medium, the mass percentage of sodium nitrate is 59% to 60% and the mass percentage of potassium nitrate is 39% to 40%.

It is another object of the present invention to provide a method for producing the heat transfer medium for solar thermal power generation, comprising the steps of: S1: grinding and mixing the molten nitrate salt heat storage medium with carbon nanotubes to obtain a raw material powder, wherein the raw material powder the mass percentage of the carbon nanotubes is 0.10% to 1.00% and the diameter of the carbon nanotubes is d ≥ 50 nm; S2: heating and melting the raw material powder for at least 1 hour to obtain a melt mixture; S3: Cooling the melt mixture to room temperature and pulverizing it to obtain a heat transfer medium for solar thermal power generation.

Further, the molten nitrate salt heat storage medium comprises sodium nitrate and potassium nitrate which are homogeneously mixed.

Further, in the raw material powder, the mass percentage of sodium nitrate is 59% to 60% and the mass percentage of potassium nitrate is 39% to 40%.

Furthermore, in step S2, the temperature holding time is 1 to 3 hours.

Further, in step S2, the raw material powder is melted at temperatures of 300 ° C to 400 ° C.

The advantageous effects of the present invention achieved in comparison with the prior art are described below: In the present invention, carbon nanotubes with a composite diameter d ≥ 50 nm are added to a molten nitrate salt heat storage medium and the amount of added carbon nanotubes is regulated to to obtain a heat transfer medium for solar thermal power generation. Compared to the molten nitrate salt heat storage medium, the heat transfer medium for solar thermal power generation under conditions in which the decomposition temperature is similar has a significantly higher coefficient of thermal conductivity and an improved heat transfer performance of the binary molten nitrate salt heat storage medium. Therefore, when the heat transfer medium is used for solar thermal power generation, the efficiency in using solar energy can be improved. Compared to other similar media materials in the prior art, the compatibility of the molten nitrate salt heat storage medium with carbon nanotubes is better than that with other added components.

Brief description of the drawing

For a better understanding of the technical solutions according to the exemplary embodiments of the invention, a drawing according to the exemplary embodiments of the invention is briefly described below. It will be understood that the drawings illustrate only a few exemplary embodiments of the present invention and that one skilled in the art can obtain other drawings on the basis of these drawings without creative effort. 1 shows a schematic comparative diagram of the thermal conductivity coefficients of the heat transfer media for solar thermal power generation, which were obtained in comparative example 1 and in exemplary embodiments 1 to 5 of the present invention.

Detailed description of the exemplary embodiments

Exemplary embodiments of the present invention are described in detail below. However, this invention can be embodied in many different forms and should not be viewed as limited to the embodiments set forth herein. Rather, these embodiments are provided to explain the principles and practical applications of the invention, thereby enabling those skilled in the art to understand various embodiments and various modifications of the invention that are suitable for a particular intended application.

The present invention provides a heat transfer medium for solar thermal power generation, which comprises a molten nitrate salt heat storage medium and carbon nanotubes added to the molten nitrate salt heat storage medium.

In particular, in the heat transfer medium, the mass percentage of the carbon nanotubes is 0.10% to 1.00% and the diameter of the carbon nanotubes is d ≥ 50 nm.

Further, in the heat transfer medium, the mass percentage of the carbon nanotubes is preferably 0.10% to 0.50%.

In particular, the molten nitrate salt heat storage medium comprises sodium nitrate and potassium nitrate which are homogeneously mixed. As a rule, the mass percentage of sodium nitrate can be set to 59% to 60% and the mass percentage of potassium nitrate to 39% to 40% in the heat transfer medium.

In the heat transfer medium according to the invention, a significant increase in the coefficient of thermal conductivity can be achieved under similar conditions with regard to the decomposition temperature of the molten nitrate salt heat storage medium.

The present invention further provides a method for producing the heat transfer medium, in particular comprising the following steps: In step S1, the molten nitrate salt heat storage medium and the carbon nanotubes are ground and mixed homogeneously to obtain a raw material powder.

In particular, in the raw material powder, the mass percentage of the carbon nanotubes is set to 0.10% to 1.00% and the diameter of the carbon nanotubes is set to d 50 nm.

Further, in the raw material powder, the mass percentage of the carbon nanotubes is preferably 0.10% to 0.50%.

Further, the molten nitrate salt heat storage medium comprises sodium nitrate and potassium nitrate mixed homogeneously with the mass percentage of sodium nitrate set to 58% to 60% and the mass percentage of potassium nitrate to 38% to 40% in the raw material powder.

In step S2, the raw material powder is heated and melted and the temperature is maintained for at least 1 hour to obtain a melt mixture.

In particular, heating and melting of the raw material powder can only be achieved when the temperatures of the raw material powder are not lower than the melting temperature and not higher than the decomposition temperature. At the same time, taking energy consumption factors into account, the heating temperature should generally be set to 300 ° C to 400 ° C.

After the raw material powder is melted, it is necessary for the subsequent homogeneous mixing to hold the temperature for a time. The time for which the temperature is to be maintained is preferably set to 1 to 3 hours.

In step S3, the melt mixture is cooled to room temperature and pulverized to obtain a heat transfer medium for solar thermal power generation.

The performance of the heat transfer medium according to the invention for solar thermal power generation and the method for its production are described below with reference to specific exemplary embodiments, but the present invention is not limited to the following exemplary embodiments. The following exemplary embodiments accordingly only represent specific exemplary embodiments of the invention.

Embodiment 1

The present embodiment provides a method for producing a heat transfer medium for solar thermal power generation. The specific procedure is described below: First, 59.94% sodium nitrate, 39.96% potassium nitrate and 0.10% carbon nanotubes were homogeneously mixed in a crucible to obtain a powder mixture. Together, sodium nitrate and potassium nitrate can be referred to as a molten nitrate salt heat storage medium. The powder mixture was then heated and melted in a muffle furnace at 400 ° C. After the powder mixture was melted, it was kept at a constant temperature for 60 minutes to obtain a melt mixture. Finally, the melt mixture was cooled to room temperature and mechanically crushed in order to obtain a nitrate salt melt heat storage medium doped with carbon nanotubes, namely a heat transfer medium for solar thermal power generation.

In this way, in the present exemplary embodiment, a heat transfer medium was obtained by the above production process, which consists of 99.90% molten nitrate salt heat storage medium and 0.10% carbon nanotubes, the molten nitrate salt heat storage medium consisting of 59.94% sodium nitrate and 39.96 % Potassium nitrate.

In particular, the diameter of the carbon nanotubes is d 50 nm.

In the present embodiment, the melting point and the decomposition temperature of the heat transfer medium for solar thermal power generation were tested using the simultaneous thermal analysis. The test result is shown in Table 1. At the same time, in the present exemplary embodiment, the coefficient of thermal conductivity of the heat transfer medium for solar thermal power generation was measured using the TPS method (Transient Plane Source Method) at 300 ° C. The test result is shown in Table 1.

Embodiment 2

In the description of embodiment 2, the same points as were listed in embodiment 1 are not repeated here and only the difference to embodiment 1 is described. The difference between embodiment 2 and embodiment 1 is that in the production method of embodiment 2 59.88% sodium nitrate, 39.92% potassium nitrate and 0.20% carbon nanotubes (d ≥ 50 nm) to obtain a powder mixture homogeneously in a crucible were mixed, at the same time the powder mixture was heated and melted in a muffle furnace at 340 ° C. After the powder mixture was melted, it was kept at a constant temperature for 150 minutes to obtain a melt mixture. The rest necessary for obtaining a heat transfer medium for solar thermal power generation will be described with reference to Embodiment 1. FIG.

Thus, the difference between the heat transfer medium for solar thermal power generation of the present embodiment and the heat transfer medium of embodiment 1 is that the heat transfer medium for solar thermal power generation of the present embodiment consists of 99.80% molten nitrate salt heat storage medium and 0.20% carbon nanotubes, wherein the molten nitrate salt heat storage medium consists of 59.88% sodium nitrate and 39.92% potassium nitrate.

The same method as in Embodiment 1 was used to measure the melting point and the decomposition temperature of the heat transfer medium for solar thermal power generation. The test result is shown in Table 1. At the same time, in the present exemplary embodiment, the coefficient of thermal conductivity of the heat transfer medium for solar thermal power generation was measured using the TPS method at 300 ° C. The test result is shown in Table 1.

Embodiment 3

In the description of embodiment 3, the same points as were listed in embodiment 1 are not repeated here and only the difference to embodiment 1 is described. The difference between embodiment 3 and embodiment 1 is that in the production method of embodiment 3 59.82% sodium nitrate, 39.88% potassium nitrate and 0.30% carbon nanotubes (d ≥ 50 nm) to obtain a powder mixture homogeneously in a crucible were mixed, at the same time the powder mixture was heated and melted in a muffle furnace at 300 ° C. After the powder mixture was melted, it was kept at a constant temperature for 180 minutes to obtain a melt mixture. The rest necessary for obtaining a heat transfer medium for solar thermal power generation will be described with reference to Embodiment 1. FIG.

Thus, the difference between the heat transfer medium for solar thermal power generation of the present embodiment and the heat transfer medium of embodiment 1 is that the heat transfer medium for solar thermal power generation of the present embodiment consists of 99.70% molten nitrate salt heat storage medium and 0.30% carbon nanotubes, wherein the molten nitrate salt heat storage medium consists of 59.82% sodium nitrate and 39.88% potassium nitrate.

The same method as in Embodiment 1 was used to measure the melting point and the decomposition temperature of the heat transfer medium for solar thermal power generation. The test result is shown in Table 1. At the same time, in the present exemplary embodiment, the coefficient of thermal conductivity of the heat transfer medium for solar thermal power generation was measured using the TPS method at 300 ° C. The test result is shown in Table 1.

Embodiment 4

In the description of exemplary embodiment 4, the same points as were listed in exemplary embodiment 1 are not repeated here and only the difference from exemplary embodiment 1 is described. The difference between the present exemplary embodiment and exemplary embodiment 1 is that, in the production method of the present exemplary embodiment, 59.76% sodium nitrate, 39.84% potassium nitrate and 0.40% carbon nanotubes (d 50 nm) are homogeneous for obtaining a powder mixture in a crucible were mixed, at the same time the powder mixture was heated and melted in a muffle furnace at 300 ° C. After the powder mixture was melted, it was kept at a constant temperature for 180 minutes to obtain a melt mixture. The rest necessary for obtaining a heat transfer medium for solar thermal power generation will be described with reference to Embodiment 1. FIG.

Thus, the difference between the heat transfer medium for solar thermal power generation of the present embodiment and the heat transfer medium of embodiment 1 is that the heat transfer medium for solar thermal power generation of the present embodiment consists of 99.60% molten nitrate salt heat storage medium and 0.40% carbon nanotubes, wherein the molten nitrate salt heat storage medium consists of 59.76% sodium nitrate and 39.84% potassium nitrate.

The same method as in Embodiment 1 was used to measure the melting point and the decomposition temperature of the heat transfer medium. The test result is shown in Table 1. At the same time, in the present exemplary embodiment, the coefficient of thermal conductivity of the heat transfer medium for solar thermal power generation was measured using the TPS method at 300 ° C. The test result is shown in Table 1.

Embodiment 5

In the description of exemplary embodiment 5, the same points as were listed in exemplary embodiment 1 are not repeated here and only the difference from exemplary embodiment 1 is described. The difference between the present exemplary embodiment and exemplary embodiment 1 is that, in the production method of the present exemplary embodiment, 59.70% sodium nitrate, 39.80% potassium nitrate and 0.50% carbon nanotubes (d 50 nm) are homogeneous to obtain a powder mixture in a crucible were mixed, at the same time the powder mixture was heated and melted in a muffle furnace at 300 ° C. After the powder mixture was melted, it was kept at a constant temperature for 180 minutes to obtain a melt mixture. The rest necessary for obtaining a heat transfer medium for solar thermal power generation will be described with reference to Embodiment 1. FIG.

Thus, the difference between the heat transfer medium for solar thermal power generation of the present embodiment and the heat transfer medium of embodiment 1 is that the heat transfer medium for solar thermal power generation of the present embodiment consists of 99.50% molten nitrate salt heat storage medium and 0.50% carbon nanotubes, where the molten nitrate salt heat storage medium consists of 59.70% sodium nitrate and 39.80% potassium nitrate.

The same method as in Embodiment 1 was used to measure the melting point and the decomposition temperature of the heat transfer medium. The test result is shown in Table 1. At the same time, in the present exemplary embodiment, the coefficient of thermal conductivity of the heat transfer medium was measured using the TPS method at 300 ° C. The test result is shown in Table 1.

In order to verify the performance of the heat transfer medium for solar thermal power generation of the above embodiments of the present invention, the following comparative tests were carried out.

Comparative example 1

In this comparative example, the heat transfer medium provided comprises a molten nitrate salt heat storage medium consisting of 60% sodium nitrate and 40% potassium nitrate without the presence of carbon nanotubes.

The same method as in Embodiment 1 was used to measure the melting point and the decomposition temperature of the heat transfer medium. The test result is shown in Table 1. At the same time, in the present exemplary embodiment, the coefficient of thermal conductivity of the heat transfer medium was measured using the TPS method at 300 ° C. The test result is shown in Table 1.

Table 1: Comparison of the performance test results of Embodiments 1 to 5 with the performance test results of Comparative Example 1

Comparative example 1 230.1 540.9 working example 1 226.2 577.2 working example 2 225.4 573.9 working example 3 225.0 559.4 working example 4 222.0 557.1 working example 5 222.2 551, 3

As can be seen from Table 1, in comparison with the molten nitrate salt heat storage medium without carbon nanotubes, the melting point and the decomposition temperature of the heat transfer medium according to the invention for solar thermal power generation are essentially similar, but the coefficient of thermal conductivity is higher. At the same time, with the increase in the amount of added carbon nanotubes, a significant increase in the coefficient of thermal conductivity can be observed.

While the present invention has been described in connection with particular embodiments, it should be understood that various modifications and variations can be made in form and details without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations come within the scope of the invention and the appended claims.

Claims (10)

1. Heat transfer medium for solar thermal power generation, characterized in that it comprises a nitrate salt melt heat storage medium and carbon nanotubes added to the nitrate salt melt heat storage medium; wherein the mass percentage of the carbon nanotubes is 0.10% to 1.00% and the diameter of the carbon nanotubes is d ≥ 50 nm. 2. Heat transfer medium for solar thermal power generation according to claim 1, characterized in that the mass percentage of the carbon nanotubes is 0.10% to 0.50%. 3. Heat transfer medium for solar thermal power generation according to claim 1, characterized in that the nitrate salt melt heat storage medium comprises sodium nitrate and potassium nitrate, which are mixed homogeneously. 4. Heat transfer medium for solar thermal power generation according to claim 3, characterized in that the mass percentage of sodium nitrate is 59% to 60% and the mass percentage of potassium nitrate is 39% to 40%. 5. A method for producing a heat transfer medium for solar thermal power generation according to any one of claims 1-4, characterized in that it comprises the following steps: S1: grinding and mixing the molten nitrate salt heat storage medium with carbon nanotubes to obtain a raw material powder, wherein in the raw material powder, the mass percentage of the carbon nanotubes is 0.10% to 1.00% and the diameter of the carbon nanotubes is d ≥ 50 nm; S2: heating and melting the raw material powder for at least 1 hour to obtain a melt mixture; S3: Cooling the melt mixture to room temperature and pulverizing it to obtain a heat transfer medium for solar thermal power generation. 6. The manufacturing method according to claim 5, characterized in that the mass percentage of the carbon nanotubes in the raw material powder is 0.10% to 0.50%. 7. The manufacturing method according to claim 5, characterized in that the molten nitrate salt heat storage medium comprises sodium nitrate and potassium nitrate, which are mixed homogeneously. 8. The manufacturing method according to claim 7, characterized in that, in the raw material powder, the mass percentage of sodium nitrate is 59% to 60% and the mass percentage of potassium nitrate is 39% to 40%. 9. Production method according to one of claims 5 to 8, characterized in that in step S2 the temperature holding time is 1 to 3 hours. 10. Production method according to one of claims 5 to 8, characterized in that the raw material powder is melted at temperatures of 300 ° C to 400 ° C in step S2.