A photothermal salt-making system and its salt-making method

Abstract

This invention relates to a photothermal salt melting system and its method. The system includes a salt melting furnace, a molten salt heating branch, a photothermal mirror heat exchange branch, a heat transfer oil circulation branch, and a molten salt tank. The salt melting furnace is connected to the molten salt heating branch and the photothermal mirror heat exchange branch via a molten salt pump, and is connected to the molten salt tank via a second molten salt pump and a third regulating valve. Several temperature measuring points are set in the system. The salt melting method involves adjusting the opening of each regulating valve according to the temperature at each measuring point in the system to adjust the amount of molten salt entering the molten salt heating branch and the photothermal mirror heat exchange branch. This allows the mirror and external heating equipment to melt the molten salt together, maximizing the utilization of photothermal energy and reducing the energy consumption of the external heating equipment, thereby effectively reducing fuel costs and carbon emissions.

Description

Technical Field

[0001] This invention relates to the field of primary melting technology for molten salt in solar thermal power plants, and particularly to a solar thermal salt melting system and its salt melting method. Background Technology

[0002] Before a concentrated solar thermal power plant (CSP) is initially put into operation, solid molten salt needs to be heated and melted to the operating temperature. Currently, fossil fuels are mostly used to provide heat for salt melting. However, with tens of thousands of tons of solid molten salt, the melting process consumes a large amount of fuel. The main problems with this fuel-based salt melting method are: firstly, fuel costs increase the initial investment cost of the CSP plant; secondly, fuel combustion causes unnecessary environmental pollution.

[0003] Currently, mainstream solar thermal power plants include tower power plants and parabolic trough power plants. Under the same installed capacity and energy storage market conditions, the theoretical molten salt capacity of parabolic trough power plants is about 2-3 times that of tower power plants. This is mainly because parabolic trough molten salt thermal storage technology still uses heat transfer oil as the heat transfer medium. The lower upper limit of the operating temperature of heat transfer oil (about 390°C) limits the thermal storage temperature of molten salt, which in turn makes it difficult to fully utilize the thermal storage performance of molten salt and further increases the demand for molten salt.

[0004] Therefore, how to effectively utilize the mirror field inherent in parabolic trough solar thermal power plants to participate in salt decomposition is an urgent problem to be solved. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a technical solution capable of utilizing high-temperature heat transfer oil within the mirror field of a parabolic trough solar thermal power plant for the initial melting of solid molten salt:

[0006] A photothermal salt melting system includes a salt melting furnace, a molten salt temperature raising branch, a photothermal mirror field heat exchange branch, a heat transfer oil circulation branch, and a molten salt tank;

[0007] The salt furnace has a built-in electric heater and a level gauge. It is connected to the molten salt temperature raising branch and the photothermal mirror heat exchange branch through molten salt pump one, and is connected to the molten salt tank through molten salt pump two and regulating valve three in sequence.

[0008] The molten salt heating branch includes a regulating valve connected in sequence to a molten salt pump and an external heating device.

[0009] The heat exchange branch of the photothermal mirror field includes a regulating valve two connected in sequence to the molten salt pump one and an oil-salt heat exchanger; the oil-salt heat exchanger forms a heat transfer oil circulation branch with the mirror field through a circulating oil pump and a regulating valve four in sequence; the oil-salt heat exchanger is used to exchange heat between the heat transfer oil from the mirror field and the molten salt in the salt melting furnace.

[0010] When there is sufficient light, most of the heat required for salt melting is provided by the mirror field and a small portion is provided by external heating equipment; when the light is weak, most of the heat is provided by external heating equipment and a small portion is provided by the mirror field.

[0011] Furthermore, the salt melting furnace is equipped with a stirring device to agitate the molten salt inside the furnace and reduce the temperature difference of the molten salt inside the furnace.

[0012] Furthermore, the external heating device is one of an electric boiler, a gas boiler, or an electric heater. The external heating device can be in the form of a boiler or an electric heater, and its main function is to supplement heat when sunlight is insufficient.

[0013] Furthermore, a temperature monitoring subsystem is also included. This subsystem comprises a temperature sensor 1 located at the molten salt outlet of the oil-salt heat exchanger, a temperature sensor 2 located at the point where the molten salt flows into the salt furnace from the molten salt heating branch, a temperature sensor 3 located at the heat transfer oil point inside the mirror field heat absorber tube, and a multi-point temperature measuring instrument extending vertically into the salt furnace. The temperature data detected by temperature sensors 1-3 are T1-T3, respectively, and the average temperature at each measuring point of the multi-point temperature measuring instrument is T4. The temperature monitoring subsystem serves as the basis for automatic system control, adjusting the opening of each regulating valve in real time based on temperature changes at each point.

[0014] A salt-degrading method using the above-described photothermal salt-degrading system includes the following steps:

[0015] S1: Solid molten salt is fed into the salt melting furnace at a constant rate. Before the system reaches the minimum allowable liquid level, the molten salt is melted by an electric heater. The feeding rate is kept constant throughout the salt melting process. Solid molten salt feeding is suspended in case of equipment damage or maintenance to prevent the molten salt from solidifying.

[0016] S2: After reaching the minimum liquid level (the minimum amount of molten salt allowed by the system circulation), the liquid molten salt in the salt furnace is transported to the molten salt heating branch and the photothermal mirror heat exchange branch by molten salt pump one, and the amount of molten salt transported to the molten salt heating branch and the photothermal mirror heat exchange branch is adjusted by regulating valve one and regulating valve two, respectively.

[0017] S3: When the average temperature of the molten salt in the salt melting furnace rises to the set salt melting temperature Ts and the molten salt reaches the set liquid level Hs, start the molten salt pump two to transport the molten salt in the salt melting furnace to the molten salt tank, and adjust the amount of molten salt transported to the molten salt tank through the regulating valve three to keep the liquid molten salt in the salt melting furnace always at the set liquid level Hs.

[0018] S4: After the required solid molten salt is completely added to the salt melting furnace, keep the molten salt pump 2 running and keep the regulating valve 3 at a fixed opening until the required liquid molten salt is completely delivered to the molten salt tank and the photothermal salt melting system is shut down. During this period, the average temperature T4 of the molten salt in the salt melting furnace must always be kept within the range of the set salt melting temperature Ts ± 5℃.

[0019] The advantages of maintaining the liquid molten salt in the salt furnace at the set liquid level Hs in step S3 are: (1) It can effectively prevent the liquid level in the salt furnace from overflowing due to excessively high liquid level; (2) It maintains a constant liquid level in the salt furnace, and at the same time, it maintains a constant salt melting temperature, which means maintaining a constant heat in the salt furnace. This ensures that the salt melting temperature is relatively constant during the continuous salt melting process, avoids large temperature fluctuations causing thermal stress impact on the equipment, and thus ensures more stable system control.

[0020] Specifically, step S2 is as follows:

[0021] When T3 ≥ Ts + 40℃, which is the period of high light intensity, the circulating oil pump is turned on; the opening of regulating valves two and four is adjusted to maintain T1 at [Ts + 25, Ts + 35]℃; at the same time, the opening of regulating valve one is adjusted to maintain T2 at [Ts + 25, Ts + 35]℃; at this time, most of the molten salt exchanges heat with the high-temperature heat transfer oil through the photothermal mirror field heat exchange branch, and a small portion of the molten salt enters the molten salt temperature-raising branch; as the light intensity decreases, the opening of regulating valves two and four is gradually reduced to reduce the proportion entering the photothermal mirror field heat exchange branch, while the opening of regulating valve one is increased to increase the proportion entering the molten salt temperature-raising branch.

[0022] When T3 < Ts + 40℃ and Ts < T1 ≤ Ts + 25℃, that is, when the solar radiation intensity changes from strong to weak, keep the circulating oil pump running, and adjust the opening of regulating valve two and regulating valve four to maintain T1 at [Ts + 25, Ts + 35]℃; at the same time, adjust the opening of regulating valve one to maintain T2 at [Ts + 25, Ts + 35]℃.

[0023] When T3 < Ts + 40℃ and T1 = Ts, shut off the circulating oil pump and adjust the opening of regulating valve two to a fixed, smaller opening. Simultaneously, adjust the opening of regulating valve one to maintain T2 within the range of [Ts + 25, Ts + 35]℃. When the solar radiation intensity weakens to the point where it is insufficient to maintain T1 above Ts, adjust regulating valve two to a fixed, smaller opening to keep the molten salt circulating at a low speed to prevent molten salt condensation in the heat exchange branch of the solar thermal mirror field. Simultaneously, shut off the circulating oil pump, because the heat collection tubes of the mirror field are exposed to the atmosphere. If the heat transfer oil continues to circulate at night without solar radiation, it will accelerate heat dissipation from the mirror field. As the temperature of T1 gradually decreases to Ts, gradually increase the opening of regulating valve one and the power of the external heating equipment to maintain T2 within the target temperature range and T4 at the set salt melting temperature Ts.

[0024] During system regulation, temperature fluctuations within the salt furnace that may occur due to response time can be compensated for by the built-in electric heater. The regulating valves only operate under the aforementioned conditions; otherwise, their states remain unchanged.

[0025] Furthermore, it also includes a controller, and all four regulating valves are electrically operated. The controller automatically controls the opening degree of regulating valves one through four based on the temperature information from the temperature monitoring subsystem, thereby achieving automatic control of the system and improving control efficiency and the accuracy of salt dissolving temperature.

[0026] Furthermore, the electric heater built into the salt-making furnace maintains a constant power throughout the entire salt-making process.

[0027] The beneficial effects of this invention are as follows: by using the high-temperature heat transfer oil circulating in the mirror field of the solar thermal power plant and the external heating equipment to melt solid molten salt, it is possible to effectively save fossil fuels, reduce investment costs, and reduce carbon emissions; and it can achieve automatic control, which is conducive to precise control of the melting temperature. Attached Figure Description

[0028] Figure 1 A schematic diagram of the system of the present invention.

[0029] In the diagram: 100, electric heater. Detailed Implementation

[0030] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples. The following examples are for illustrative purposes only and are not intended to limit the scope of the invention.

[0031] Example 1

[0032] like Figure 1 The illustrated photothermal salt-making system includes a salt-making furnace, a molten salt heating branch, a photothermal mirror field heat exchange branch, a thermal oil circulation branch, and a molten salt tank. The salt-making furnace has a built-in electric heater 100, a level gauge, and a stirring device. In this embodiment, the built-in electric heater 100 is side-mounted and maintains a constant power throughout the salt-making process. It is connected to the molten salt heating branch and the photothermal mirror field heat exchange branch via a molten salt pump, and then connected to the molten salt tank via a second molten salt pump and a third regulating valve. The molten salt heating branch includes a first regulating valve connected to the first molten salt pump and an external heating device (in this embodiment, a gas-fired boiler is used). The photothermal mirror field heat exchange branch includes a second regulating valve connected to the first molten salt pump and an oil-salt heat exchanger. The oil-salt heat exchanger forms a thermal oil circulation branch with the mirror field via a circulating oil pump and a fourth regulating valve. The oil-salt heat exchanger is used to exchange heat between the thermal oil from the mirror field and the molten salt in the salt-making furnace.

[0033] When there is sufficient light, most of the heat required for salt melting is provided by the mirror field and a small part is provided by external heating equipment; when the light is weak, most of the heat is provided by external heating equipment and a small part is provided by the mirror field. The expansion device and other equipment required for the heat transfer oil of the mirror field can be configured by ourselves.

[0034] It also includes a temperature monitoring subsystem; the temperature monitoring subsystem includes a temperature sensor 1 located at the molten salt outlet of the oil-salt heat exchanger, a temperature sensor 2 located at the point where the molten salt temperature-raising branch flows into the salt furnace, a temperature sensor 3 located at the heat transfer oil in the mirror field heat absorption tube, and a multi-point temperature measuring instrument that extends vertically into the salt furnace. Figure 1 T1 to T3 correspond to temperature sensors one to three, respectively, and T4 corresponds to a multi-point temperature measuring instrument. The temperature monitoring subsystem can serve as the basis for automatic control of the system, adjusting the opening of each regulating valve in real time according to the temperature changes at each point.

[0035] Assuming a parabolic trough solar thermal power plant has a set salt-treating temperature Ts of 350℃, the specific steps of the salt-treating method using the above-mentioned solar thermal salt-treating system are as follows:

[0036] S1: Solid molten salt is fed into the salt furnace at a constant rate. Before the minimum liquid level allowed by the system circulation is reached (the length of the molten salt pump, the height of the salt furnace, and the minimum amount of molten salt required for system circulation need to be taken into account), the molten salt is melted by an electric heater. The feeding rate is kept constant throughout the salt melting process.

[0037] S2: After reaching the minimum liquid level, the liquid molten salt in the salt furnace is transported to the molten salt heating branch and the photothermal mirror heat exchange branch by the molten salt pump. The specific control method is as follows:

[0038] When T3 ≥ 390℃, which is the period of high light intensity, the circulating oil pump is turned on; the opening of regulating valve 2 and regulating valve 4 are adjusted to maintain T1 at [375, 385]℃; at the same time, the opening of regulating valve 1 is adjusted to maintain T2 at [375, 385]℃; as the light intensity decreases, the opening of regulating valve 2 and regulating valve 4 is gradually reduced to reduce the proportion entering the heat exchange branch of the photothermal mirror field, while the opening of regulating valve 1 is increased to increase the proportion entering the molten salt temperature-raising branch.

[0039] When T3 < 390℃ and T1 is in (350, 375]℃, it is a period of low light intensity, that is, when solar radiation changes from strong to weak. Keep the circulating oil pump running to adjust the opening of regulating valve 2 and regulating valve 4 to maintain T1 in [375, 385]℃; at the same time, adjust the opening of regulating valve 1 to maintain T2 in [375, 385]℃.

[0040] When T3 < 390℃ and T1 drops to 350℃, shut down the circulating oil pump and adjust the opening of regulating valve 2 to a fixed smaller opening, which can be 10% of the maximum opening; at the same time, adjust the opening of regulating valve 1 to maintain T2 at [375, 385]℃.

[0041] During system adjustment, temperature fluctuations inside the salt furnace that may occur due to response time can be compensated for by the built-in electric heater.

[0042] S3: When T4 is heated to the set salt melting temperature of 350℃ and the molten salt reaches the set liquid level Hs, start the molten salt pump two to transport the molten salt in the salt melting furnace to the molten salt tank, and adjust the amount of molten salt transported to the molten salt tank through the regulating valve three to keep the liquid molten salt in the salt melting furnace always at the set liquid level Hs.

[0043] S4: After the solid molten salt required by the system is completely put into the salt melting furnace, keep the molten salt pump 2 in the start state and keep the regulating valve 3 at a fixed opening until the required liquid molten salt is completely delivered into the molten salt tank and the photothermal salt melting system is shut down. During this period, the average temperature T4 of the molten salt in the salt melting furnace must always be kept within the set salt melting temperature range of [345,355]℃.

[0044] This embodiment also includes a controller, and the regulating valves one to four are all electric regulating valves. The controller automatically controls the opening degree of the regulating valves one to four according to the temperature information of the temperature monitoring subsystem, which can realize the automatic control of the system and improve the control efficiency and the accuracy of the salting temperature.

[0045] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for salt melting of a photo-thermal salt system, characterized in that the photo-thermal salt system comprises a salt melting furnace, a molten salt temperature raising branch, a photo-thermal mirror field heat exchange branch, a heat conducting oil circulation branch, a molten salt tank, and a temperature monitoring subsystem; the salt melting furnace is internally provided with an electric heater and a liquid level meter, and is connected to the molten salt temperature raising branch and the photo-thermal mirror field heat exchange branch through a molten salt pump one, and is connected to the molten salt tank through the molten salt pump two and the regulating valve three in sequence; the molten salt temperature raising branch comprises the regulating valve one and an external heating device connected to the molten salt pump one in sequence; the photo-thermal mirror field heat exchange branch comprises the oil-salt heat exchanger and the regulating valve two connected to the molten salt pump one in sequence; the oil-salt heat exchanger forms the heat conducting oil circulation branch with the mirror field through the circulating oil pump and the regulating valve four in sequence; and the oil-salt heat exchanger is used for heat exchange between the heat conducting oil from the mirror field and the molten salt in the salt melting furnace; the temperature monitoring subsystem comprises the temperature sensor one arranged at the molten salt outlet of the oil-salt heat exchanger, the temperature sensor two arranged at the position where the molten salt in the molten salt temperature raising branch flows into the salt melting furnace, the temperature sensor three arranged at the heat conducting oil in the heat absorption pipe of the mirror field, and the multi-point temperature measuring instrument vertically inserted into the salt melting furnace; the temperature data detected by the temperature sensors one to three are T1 to T3 respectively, and the average temperature of each measuring point of the multi-point temperature measuring instrument is T4; and the salt melting method comprises the following steps: S1: solid-state molten salt is fed into the salt melting furnace at a constant speed, and the electric heater is used to melt the molten salt before reaching the minimum liquid level allowed by the system circulation, and the feeding is kept at a constant speed throughout the whole salt melting process; S2: after reaching the minimum liquid level, the liquid-state molten salt in the salt melting furnace is transported to the molten salt temperature raising branch and the photo-thermal mirror field heat exchange branch through the molten salt pump one, and the amount of molten salt transported to the molten salt temperature raising branch and the photo-thermal mirror field heat exchange branch is adjusted through the regulating valve one and the regulating valve two respectively; the step S2 is specifically as follows: when T3≥Ts+40℃, the circulating oil pump is started, the opening degrees of the regulating valve two and the regulating valve four are adjusted to maintain T1 in [Ts+25, Ts+35]℃, and the opening degree of the regulating valve one is adjusted to maintain T2 in [Ts+25, Ts+35]℃; when T3 when T3 S3: when the average temperature of the molten salt in the salt melting furnace is raised to the set salt melting temperature Ts, and the molten salt reaches the set liquid level Hs, the molten salt in the salt melting furnace is transported to the molten salt tank through the molten salt pump two, and the amount of molten salt transported to the molten salt tank is adjusted through the regulating valve three to keep the liquid-state molten salt in the salt melting furnace at the set liquid level Hs. ​ S4: After the solid-state molten salt required by the system is completely put into the salt furnace, keep the second molten salt pump in the starting state and keep the third regulating valve in the fixed opening until the required liquid molten salt is completely delivered to the molten salt tank, the light heat molten salt system is closed, and T4 needs to be kept in the range of Ts±5℃ at all times during the period.

2. The method of salt disintegration of a photo thermal salt system according to claim 1, characterized in that: The salt furnace is internally provided with a stirring device.

3. The method of salt disintegration of a photo thermal salt system according to claim 1, characterized in that: The external heating equipment is one of an electric boiler, a gas boiler and an electric heater.

4. A method of salt crystallization of a photo thermal salt system according to claim 1, characterized in that: The controller is further included, and the first to fourth regulating valves are electric regulating valves, and the controller automatically controls the opening of the first to fourth regulating valves according to the temperature information of the temperature monitoring subsystem.

5. A method of salting a photo-thermal salt system as claimed in claim 1, characterized in that: The electric heater built-in the salt furnace keeps constant power throughout the salt-making process.

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