Periodic Low-Grade Heat Energy High-Efficiency Recovery and Conversion System and Method

Abstract

This invention provides a periodic low-grade heat energy high-efficiency recovery and conversion system and method, comprising: a thermally regenerated flow battery system and a heat transfer module; the thermally regenerated flow battery system includes a battery module and two electrolyte storage tanks and a regenerative heat exchanger connected thereto; the heat transfer module includes a periodic heat source heat exchange unit and a radiative cooling unit. The TREC-FB and the heat transfer module are connected via a flow pump to control the flow state and valves to control the flow direction, forming a periodically changing electrolyte circulation loop. The electrolyte in the electrolyte storage tank is preheated or precooled by the electrolyte flowing out of the battery module through the regenerative heat exchanger, and then, after heat exchange through the heat transfer module, enters the battery module for chemical energy and electrical energy conversion. Based on the temperature dependence of electrochemical reactions, this invention utilizes RC (electrical resistance) to significantly improve the operating temperature difference of the TREC-FB. The regenerative design enables indirect regeneration between periodic cold and heat sources, and the battery's output power and efficiency are improved by adjusting the flow rate.

Description

Technical Field

[0001] This invention relates to the field of thermo-electrochemical conversion technology, and in particular to a periodically varying heat source recovery and conversion system and method. Background Technology

[0002] The Earth's rotation causes the alternation of day and night. Objects on Earth absorb solar radiation during the day and passively radiate heat into outer space at night, resulting in periodic temperature changes. This creates a large number of sustainable periodic sources of low-grade cold and heat in the environment.

[0003] Current research on waste heat recovery mainly focuses on technologies such as the Kalina cycle and the organic Rankine cycle. However, their applicable temperature range is more suited to medium- and low-temperature industrial waste heat between 100°C and 200°C. Furthermore, their large size and associated maintenance requirements make them unsuitable for small-scale, distributed low-grade heat recovery scenarios. Semiconductor thermoelectric technology offers advantages in compactness and distributed operation, but the high cost of thermoelectric materials limits its application in low-grade heat recovery. In addition, the thermoelectric potential (Seebeck coefficient) of semiconductor thermoelectric materials is often only on the order of 0.1 mV / K, making it difficult to utilize the small temperature difference between the low-temperature heat source and the environment to provide sufficient voltage and power to supply the device.

[0004] In recent years, electrochemical devices that directly convert heat to electricity, such as thermally diffused ion capacitors, thermochemical batteries, and thermal regenerative electrochemical cycles (TREC), have provided new ideas for the utilization of low-grade thermal energy. Compared with the thermoelectric potential of semiconductor thermoelectric devices, the ion thermoelectric potential of electrochemical systems is often 1-2 orders of magnitude higher (1-10 mV / K). Therefore, the use of electrochemical devices to recover low-grade thermal energy has received widespread attention in recent years. However, single thermoelectric ion capacitors and thermochemical batteries have limitations such as difficulty in continuously and stably outputting current and relatively low thermoelectric potential, respectively. In addition, both utilize the temperature difference between hot and cold electrodes to generate electricity, which poses a challenge in maintaining the temperature difference across the battery during long-term operation.

[0005] Compared to thermoelectric technology based on temperature gradients, TREC batteries are isothermal devices whose energy conversion process is based on the overall temperature change of the device, and they have periodic operating characteristics, making them more feasible for recovering periodic low-grade heat sources in the environment. However, traditional TREC batteries also have many problems, such as no electrolyte flow within the battery, poor mass transfer of oxidized active materials, large overpotential losses, battery capacity limited by battery volume, and their operating mode requiring alternating cold and hot cycles, making it difficult to achieve heat recovery between low-grade cold and hot sources. These limitations restrict the output power and efficiency of TREC batteries, thus limiting their application in the recovery of periodic low-grade cold and hot sources generated in the environment. In addition, in terms of working principle, TREC utilizes the temperature dependence of the anodic and cathode couple reactions, and its open-circuit voltage ( ) is defined as: ,in The temperature coefficient of the battery is numerically equal to positive ( ),burden( The difference in the temperature coefficient of the electrolyte: ΔT represents the battery's operating temperature difference. Clearly, increasing the battery's operating temperature difference is one of the important ways to increase the battery's output voltage and power. However, the amplitude of temperature fluctuations in real-world heat sources is usually small, which further limits the practical application of TREC in environmental heat recovery. Summary of the Invention

[0006] This invention provides a periodic low-grade heat energy high-efficiency recovery and conversion system and method to solve the defects of the existing technology of difficult periodic low-grade cold and heat source recovery and utilization and low conversion rate, and to achieve high-efficiency recovery and conversion of periodic low-grade cold and heat sources.

[0007] To achieve the above objectives, the present invention provides a periodic low-grade heat energy high-efficiency recovery and conversion system, comprising: a thermal regeneration flow battery system and a heat transfer module.

[0008] The thermal regenerative flow battery system (TREC-FB) includes a battery module and two electrolyte storage tanks connected to it. The heat transfer module includes a periodic heat source heat exchange unit and a sky radiation cooling unit. An electrolyte circulation loop is formed between the electrolyte storage tanks, the heat transfer module, and the battery module, allowing the electrolyte in the storage tanks to exchange heat through the heat transfer module before entering the battery module for chemical and electrical energy conversion. A flow pump and control valve are installed in the electrolyte circulation loop to control the flow state and direction of the electrolyte. Based on the temperature dependence of electrochemical reactions, this invention designs a thermal energy recycling and conversion system using the periodic heat source heat exchange unit and the sky radiation cooling unit as coupled heat exchange objects. The electrolyte undergoes reversible redox reactions sequentially in periodically changing low-temperature and high-temperature environments, converting thermal energy into chemical energy, and finally into electrical energy, thereby achieving continuous recycling and conversion of periodic low-grade heat sources.

[0009] Furthermore, the periodic heat source heat exchange unit includes a periodic heat source and a heat exchanger; the periodic heat source includes a periodically changing low-temperature range and a high-temperature range; the electrolyte circulation loop is connected to the heat exchanger. The electrolyte in the electrolyte storage tank flows through the electrolyte circulation loop and then through the heat exchanger, where it is heated or cooled, before flowing into the battery module to undergo a redox reaction for charging or discharging. In practical applications, the battery module is also connected to an external load or power supply.

[0010] Furthermore, the heat transfer module also includes a sky radiation cooling unit (RC) connected to the periodic heat source heat exchange unit. This unit radiates heat into the sky when the periodic heat source is in a low-temperature range to further reduce the electrolyte temperature. The sky radiation cooling unit is a device that cools itself by radiating heat into the cold sky. It faces the sky during operation to increase the operating temperature difference of the thermal regeneration flow battery system. This device can adapt to the characteristics of the periodic heat source in the environment described in this invention, improving the conversion efficiency in the low-temperature range.

[0011] Furthermore, the sky radiation cooling unit includes a radiation cooler and a cooling heat exchanger; the radiation cooler is selected from materials with an emissivity ≥0.8 for the mid-infrared band above 4 micrometers, such as polyester films, black paint surfaces, glass, etc.; preferably, it is a material with an emissivity ≥0.8 for the atmospheric window wavelength range of 8-13 micrometers and an emissivity ≤0.1 for other mid-infrared bands, such as polyethylene terephthalate films, silicone films, etc.; the cooling heat exchanger is connected to the electrolyte circulation loop.

[0012] The electrolyte flow channels inside the refrigeration heat exchanger can be structurally designed according to actual needs to improve the cooling and heat exchange effect, such as serpentine flow channels, comb-shaped flow channels, and spiral flow channels. The radiant coolant can be placed directly on the surface of the refrigeration heat exchanger, or the cooling area of ​​the radiant refrigeration device can be increased by adding various geometrically extended surface elements between the radiant coolant and the refrigeration heat exchanger, such as trapezoidal fins, triangular fins, and rectangular fins.

[0013] Passive sky radiative cooling is a phenomenon of spontaneous cooling that occurs by radiating heat into the cold sky through the long-wave infrared transmission window (8-13 μm) of the atmosphere. It has been shown to achieve approximately 100 W / m² in dark environments compared to natural cooling. 2 The net cooling power density is high. Therefore, by utilizing ambient heat sources during the day and coupling radiative cooling devices at night, it is expected to increase the operating temperature difference of TREC batteries, thereby increasing the battery's output voltage and power.

[0014] Furthermore, the electrolyte circulation loop is equipped with a heat source control valve and a cold source control valve, and the cold source control valve is located on the circulation loop where the sky radiation cooling unit is located.

[0015] Furthermore, the battery module includes a positive electrode and a negative electrode, a current collector, and an ion exchange membrane; the two electrolyte storage tanks are respectively connected to the positive and negative electrodes of the battery module, and the electrolytes stored in the two electrolyte storage tanks are two different redox couples with opposite temperature coefficients, and the half-reaction equation is O + ne - ⇋R, where O is an oxide, R is a reducing agent, and n is the number of electrons transferred per unit electrode reaction. For example, OR(+)||OR(-) includes, but is not limited to, Fe(CN)6. 3- / Fe(CN)6 4- (+)||Cu(NH3)4 2+ / Cu(NH3)2 + (-), V 3+ / V 2+ (+)||Fe(CN)6 3- / Fe(CN)6 4- (-) and I - / I3 - (+)||Fe(CN)6 3- / Fe(CN)6 4- OR(+) represents the redox couple of the positive electrolyte, and OR(-) represents the redox couple of the negative electrolyte.

[0016] The battery module converts the chemical energy of the electrolyte into electrical energy. It can operate in various series and parallel configurations, such as a single cell or multiple cells connected in series and parallel in a stack, or multiple stacks connected in series and parallel. After heat exchange in the heat transfer module, the electrolyte flows to the positive and negative electrodes to carry out the electrochemical reaction. Electrolyte flow channels are provided at the positive and negative electrodes to ensure efficient electrolyte flow, such as serpentine, comb-shaped, or spiral channels. The positive and negative electrodes collect the current generated by the electrochemical reaction and can be electrodes of various working types, such as inert electrodes, electrodes participating in the chemical reaction, or electrodes with catalytic functions, including but not limited to materials such as platinum sheets, graphite felt, carbon paper, carbon cloth, carbon nanotubes, Prussian blue, zinc, or copper. Ion exchange membranes are used to ensure charge balance during the charging and discharging of positive and negative electrolytes. They can include various types, such as, but not limited to, cation exchange membranes, anion exchange membranes, amphoteric exchange membranes, embedded ion exchange membranes, or polyelectrolyte composite membranes.

[0017] Furthermore, the thermally regenerated flow battery system also includes a regenerative heat exchanger disposed between the battery module and the electrolyte storage tank, used to preheat and precool the electrolyte flowing out of the electrolyte storage tank via the regenerative heat exchanger. This configuration adapts to the temporal nature of the periodic heat source and the characteristics of electrolyte circulation, achieving temporal regeneration between the periodic heat and cold sources.

[0018] Furthermore, the periodic heat source is an environmental heat source, a building heat source, or an industrial application heat source with alternating hot and cold temperatures, such as the diurnal temperature fluctuations of metal building roofs, or diurnal temperature changes in remote or off-grid areas.

[0019] Furthermore, the periodic low-grade heat energy high-efficiency recovery and conversion system also includes a heat energy prediction system and an electrolyte parameter control system, which are used to predict the temperature change trend of the periodic heat source and the sky radiation cooling unit in the next cycle, and to control the concentration, storage volume and flow rate of the electrolyte through the electrolyte parameter control system to optimize the energy conversion rate.

[0020] In particular, the thermal regeneration flow battery system in the periodic heat source high-efficiency recovery and conversion system provided by the present invention operates in an adiabatic environment to eliminate heat loss of the system. The pipelines, battery modules, electrolyte storage tanks, flow pumps, control valves, heat exchangers and regenerative heat exchangers are all designed with insulation.

[0021] A method for efficient recovery and conversion of periodic low-grade heat energy, employing any one of the periodic low-grade heat energy efficient recovery and conversion systems described above, includes the following steps:

[0022] Based on the temperature change pattern of the periodic heat source heat exchange unit or the sky radiation cooling unit, the electrolyte parameters in the electrolyte storage tank are set; then, the flow pump is turned on to allow the electrolyte to flow out of the electrolyte storage tank, and after heat exchange by the periodic heat source heat exchange unit or the sky radiation cooling unit, it flows into the battery module to undergo an oxidation-reduction reaction, completing charging or discharging; the electrolyte after the reaction is completed preheats or precools the electrolyte from the electrolyte storage tank for the next cycle, and then flows back into the electrolyte storage tank in a cycle;

[0023] Once the reaction within a temperature range of the periodic change is complete, the electrolyte undergoes heat exchange again in the electrolyte circulation loop, resulting in a reversible redox reaction, thus realizing the periodic recovery and conversion of low-grade thermal energy.

[0024] Specifically, taking the case where the battery electrolyte temperature coefficient is greater than 0 as an example, under high-temperature conditions, the valve flowing to the cold source is closed, and the valve flowing to the heat source is open. The low-temperature electrolyte in the electrolyte storage tank flows through the heat source via a micro-flow pump and exchanges heat with it to form a high-temperature electrolyte. Subsequently, it is discharged within the battery, and the electrolyte in the storage tank after discharge becomes a high-temperature electrolyte. During this process, the low-temperature electrolyte in the electrolyte storage tank is preheated by the high-temperature electrolyte from the battery module through a regenerative heat exchanger, achieving effective heat recovery. Conversely, under low-temperature conditions, the valve flowing to the heat source is closed, and the valve flowing to the cold source is open. The high-temperature electrolyte in the electrolyte storage tank flows through the cold source via a flow pump and exchanges heat with it to form a low-temperature electrolyte. Subsequently, it is charged within the battery, and the electrolyte in the storage tank after charging becomes a low-temperature electrolyte. During this process, the high-temperature electrolyte in the electrolyte storage tank is precooled by the low-temperature electrolyte from the battery module through a regenerative heat exchanger, also achieving effective heat recovery. The portion of electrical energy that is higher during high-temperature discharge than during low-temperature charging during the entire cycle is the total electrical power output of the battery.

[0025] Furthermore, the determination of the concentration range of the redox couple in the positive and negative electrode electrolytes in this invention can simultaneously consider electrolyte concentration and solubility, and be determined based on the relationship between electrolyte concentration and battery capacity. Typically, the maximum soluble concentration of the electrolyte is selected at a specific temperature to improve battery capacity.

[0026] Furthermore, the determination of the flow range of the positive and negative electrolytes in this invention can be based on parameters such as the current density during the charging and discharging process, the concentration of the redox couple, and the pump power consumed. Typically, the electrolyte flow rate that meets the maximum battery power density or efficiency is selected.

[0027] The beneficial effects of this invention are as follows:

[0028] 1. The periodic low-grade heat energy high-efficiency recovery and conversion system and method provided by this invention uses a periodic heat source as the heat exchange object and utilizes the temperature dependence of electrochemical reactions. Through the heat transfer effect of the periodic heat source heat exchange unit, a heat energy recycling and conversion system is designed. The electrolyte undergoes reversible redox reactions in periodically changing low-temperature and high-temperature environments, converting heat energy into chemical energy, and finally into electrical energy, thereby realizing the continuous recovery and conversion of periodically changing low-grade heat sources. This solves the problems of difficult recovery and utilization of periodic low-grade cold and heat sources and low conversion rates in existing technologies.

[0029] 2. The periodic low-grade heat energy high-efficiency recovery and conversion system provided by the present invention can effectively recover periodic low-grade heat energy in the environment by coupling a periodic low-grade heat source with a radiative cooling thermal regenerative flow battery system. By coupling the radiative cooling device and the periodic low-grade heat source as the cold and heat sources for the operation of the electrochemical system, respectively, compared with a single periodic low-grade heat source, the battery operating temperature difference is increased, thereby improving the output voltage and energy conversion efficiency of the device.

[0030] 3. This invention uses a TREC flow battery to solve the problem of poor mass transfer of active materials in the battery, thereby improving the battery's output current; the use of a storage tank increases the battery capacity and energy density, and through a regenerative design, indirect regeneration between cold and hot electrolyte fluids is achieved, thereby improving thermal energy utilization and solving the problems of poor mass transfer of active materials and difficulty in regeneration in traditional TRECs; based on the above advantages, this invention improves the battery's output power and efficiency, and achieves effective recovery and utilization of periodic low-grade thermal energy. Attached Figure Description

[0031] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0032] Figure 1 This is a flowchart of the periodic low-grade heat energy high-efficiency recovery and conversion system provided by the present invention;

[0033] Figure 2 This is a schematic diagram of the structure of the periodic low-grade heat energy high-efficiency recovery and conversion system provided by the present invention;

[0034] Figure 3 (a) is a graph showing the relationship between the temperature and cooling power density of the sky radiation cooling unit in Example 1; (b) is a graph comparing the cold source temperatures with and without the sky radiation cooling unit.

[0035] Figure 4Temperature-entropy relationship curve during the operation of the periodic low-grade heat energy high-efficiency recovery and conversion system provided by the present invention;

[0036] Figure 5 (a) shows a comparison of the charge and discharge processes of thermal regenerative flow battery systems with and without sky radiation cooling units; (b) shows a comparison of the average power density and actual thermo-electric efficiency of a single charge and discharge cycle of thermal regenerative flow battery systems with and without sky radiation cooling units.

[0037] Figure label:

[0038] 10: Heat transfer module; 11: Periodic heat source heat exchange unit; 111: Heat source end control valve; 112: Cold source end control valve; 12: Sky radiation cooling unit;

[0039] 20: Thermally regenerated flow battery system; 211: First electrolyte storage tank; 212: Second electrolyte storage tank; 221: Electrode; 222: Current collector; 223: Ion exchange membrane; 23: Regenerative heat exchanger; 24: Load or power supply; 25: Flow pump. Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0041] The calculation method for battery-related performance parameters in the periodically varying heat source recovery and conversion system provided by this invention is as follows:

[0042] Battery terminal voltage during discharge ( ): ;

[0043] Battery terminal voltage during charging ( ): ;

[0044] in, and These are the battery temperatures during discharge and charging, respectively. and These are the battery open-circuit voltages during discharge and charging at standard temperatures, respectively. The total temperature coefficient of the battery. and These represent the total overpotential of the battery during discharge and charging, respectively. and These are the discharge and charging currents, respectively. This represents the battery's internal resistance.

[0045] The average power density of the battery ( ):

[0046] ;

[0047] in, The duration is for each day.

[0048] Thermoelectric conversion efficiency of the battery ( ):

[0049] ;

[0050] in, The net work done within a single cycle of the system. This refers to the heat absorbed by the battery under reversible conditions. For heat recovery loss, This represents the total heat capacity of the electrolyte in the system.

[0051] Example 1

[0052] Please see Figure 1 and 2 As shown, the system includes a thermally regenerating flow battery system 20 and a heat transfer module 10 connected in a loop; the thermally regenerating flow battery system 20 includes a battery module and two electrolyte storage tanks connected thereto (e.g., ...). Figure 2 The battery module 10 includes a first electrolyte storage tank 211 and a second electrolyte storage tank 212. The heat transfer module 10 comprises a periodic heat source heat exchange unit 11 and a sky radiation cooling unit 12. An electrolyte circulation loop is formed between the positive and negative electrolyte storage tanks, the periodic heat source heat exchange unit 11, and the battery module. This allows the electrolyte in the storage tanks to exchange heat through the periodic heat source heat exchange unit 11 before entering the battery module for chemical and electrical energy conversion. The electrolyte circulation loop includes two heat source control valves 111 and two cold source control valves 112.

[0053] The periodic heat source heat exchange unit 11 includes a periodic heat source and a heat exchanger, which is connected to the electrolyte circulation loop. The periodic heat source includes periodically changing low-temperature and high-temperature ranges. The high-temperature range is the daytime temperature of the metal building roof, which is 60°C. The nighttime temperature of the sky radiation cooling unit 12 is 8.1°C. Figure 3 As shown in (b).

[0054] In the sky radiation cooling unit 12, the substrate is made of aluminum, and the radiation cooler material is an organosilicon thin film with high emissivity (>= 0.8) in the atmospheric window band of 8-13 micrometers and low emissivity (<= 0.1) in the remaining mid-infrared band.

[0055] The thermally regenerated flow battery system 20 includes a battery pack, positive and negative electrolyte storage tanks, a load or power supply 24, two micro-flow pumps 25, and two regenerative heat exchangers 23. Each cell in the battery pack contains positive and negative electrolytes, two electrodes 221, two current collectors 222, and an ion exchange membrane 223. In this embodiment, the battery pack contains only one cell. The electrodes are made of graphite felt, the current collectors are titanium foil, the electrolytes in the positive and negative electrolyte storage tanks are 0.3 M KI3 / 0.8 M KI and 0.3 M K3Fe(CN)6 / 0.3 M K4Fe(CN)6, respectively, and the supporting electrolytes in the positive and negative electrolyte storage tanks are KCl with concentrations of 1.1 M and 0.35 M, respectively. The temperature coefficient of the entire battery is 2.44 mV / K, the ion exchange membrane is a cation exchange membrane, and the efficiency of the regenerative heat exchanger 23 is 70%. The entire thermal regenerative flow battery system operates in an insulated environment, with the pipelines, battery packs, electrolyte storage tanks, micro flow pump 25, control valves, heat exchangers, and regenerative heat exchangers 23 all being insulated.

[0056] The system's single-cycle operation is as follows: Under high-temperature conditions, the valve flowing to the cold source (radiant cooling device) is closed, while the valve flowing to the heat source (the temperature of a metal building roof during the day) is opened. The low-temperature electrolyte in the electrolyte storage tank flows through the heat source via a micro-flow pump, exchanging heat with it to form a high-temperature electrolyte. The electrolyte flow rate is 10 ml / min. Subsequently, the system discharges within the battery, with a discharge current density of 5 A / m. 2 After discharge, the electrolyte in the storage tank becomes a high-temperature electrolyte. During this process, the low-temperature electrolyte in the storage tank is preheated by the high-temperature electrolyte from the battery module through a regenerative heat exchanger, achieving effective heat recovery. Conversely, under low-temperature conditions, the valve flowing to the heat source side is closed, and the valve flowing to the cold source side is open. The high-temperature electrolyte in the storage tank flows through the cold source by a pump and exchanges heat with it to form a low-temperature electrolyte. The flow rate of the electrolyte is the same as the former. Subsequently, it is charged in the battery, where the charging rate and discharging rate are equal. After charging, the electrolyte in the storage tank becomes a low-temperature electrolyte. During this process, the high-temperature electrolyte in the storage tank is precooled by the low-temperature electrolyte from the battery module through a regenerative heat exchanger, also achieving effective heat recovery. The portion of electrical energy that is higher during high-temperature discharge than during low-temperature charging during the entire cycle is the total electrical work output by the battery, i.e. Figure 5 (a) 1 ’ -2 ’ -3 ’ -4 ’ The area enclosed by the figure in the middle.

[0057] The thermal regenerative flow battery system obtained in this embodiment has a relatively large operating temperature difference, approximately 52°C. Figure 5 (a) 1 ’ -2 ’ -3 ’ -4 ’ The voltage-charge relationship during the actual charge-discharge process of the system is given, and the energy density of a single charge-discharge cycle is approximately 2.77 J / cm². 3 The absolute energy conversion efficiency is 3.58%.

[0058] Example 2

[0059] The difference between the periodic low-grade heat energy recovery and conversion system in this embodiment two and the one in embodiment one is that the cold source in this embodiment two is only the temperature of the roof of the metal building at night, that is, the sky radiation cooling unit 12 is not coupled.

[0060] The operating temperature difference of the thermal regenerative flow battery system obtained in this second embodiment is 40°C. Figure 5 Figure (a) 1-2-3-4 shows the voltage-charge relationship during the actual charging and discharging process of the system. The average power density of a single charge-discharge cycle is approximately 2.01 J / cm³. 3 The absolute energy conversion efficiency is 3.15%.

[0061] Compared to the periodic low-grade heat energy recovery and conversion system without a sky radiation cooling unit in Example 2, the periodic low-grade heat energy recovery and conversion system with a sky radiation cooling unit in Example 1 has an energy density and absolute energy conversion efficiency that are increased by 36.8% and 13.7% respectively per charge-discharge cycle.

[0062] Example 3

[0063] In this third embodiment, the periodic low-grade heat energy recovery and conversion system consists of a periodic heat source coupled with radiative cooling thermal regenerative flow battery system, which is basically the same as in the first embodiment. The difference is that the electrolyte flow rate used in the third embodiment has multiple parameters, namely 0.1, 1, 10, and 100 ml / min, and the current density during the charging and discharging process also has multiple parameters, ranging from 5, 10 / 20, 40, and 80 A / m. 2 .

[0064] The operating temperature difference of the thermally regenerated flow battery system obtained in this embodiment is approximately 52 °C. By controlling the current density during the charging and discharging process and adjusting the system's flow rate, the maximum average power density range for a single charge-discharge cycle is approximately 0.30-1.63 W / m³. 2 The corresponding maximum absolute energy conversion efficiency ranges from 1.16% to 3.58%, and the maximum Carnot efficiency ranges from 7.42% to 22.96%.

[0065] Example 4

[0066] The periodic low-grade heat energy recovery and conversion system in this embodiment four differs from that in embodiment one in that the regenerative heat exchanger 23 is not coupled; otherwise, it is the same as that in embodiment one.

[0067] Example 4: The energy density of a single charge-discharge cycle is approximately 0.28 kJ / cm². 3 The absolute energy conversion efficiency is 1.29%.

[0068] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A system for efficient recovery and conversion of periodic low-grade heat energy, characterized in that, The application relates to a heat regenerative liquid flow battery system (20) and a heat transfer module (10); the heat regenerative liquid flow battery system (20) comprises a battery module and two electrolyte storage tanks connected with the battery module; the heat transfer module (10) comprises a periodic heat source heat exchange unit (11) and a sky radiation refrigeration unit (12); an electrolyte circulation loop is formed between the electrolyte storage tanks, the heat transfer module (10) and the battery module, so that the electrolyte in the electrolyte storage tanks is subjected to heat exchange through the heat transfer module (10) and then enters the battery module to realize conversion between chemical energy and electric energy; the periodic heat source heat exchange unit (11) comprises a periodic heat source and a heat exchanger; the periodic heat source comprises a periodically changed low-temperature interval and a high-temperature interval; the electrolyte circulation loop is connected with the heat exchanger; the periodic heat source is an environmental heat source, a building heat source or an industrial application heat source with the characteristics of cold and heat alternation; the sky radiation refrigeration unit (12) is connected with the periodic heat source heat exchange unit (11) and is used for radiating heat to the sky when the periodic heat source is in the low-temperature interval to further reduce the temperature of the electrolyte; the heat regenerative liquid flow battery system (20) further comprises a regenerative heat exchanger (23) arranged between the battery module and the electrolyte storage tanks respectively, which is used for preheating and precooling the electrolyte flowing out of the electrolyte storage tanks through the regenerative heat exchanger (23). The sky radiation refrigeration unit (12) comprises a radiation refrigeration body and a refrigeration heat exchanger; the radiation refrigeration body is selected from a material with an emission rate of greater than or equal to 0.8 to a medium infrared wave band of greater than or equal to 4 microns or an emission rate of greater than or equal to 0.8 to an atmospheric window wavelength range of 8-13 microns and an emission rate of less than or equal to 0.1 to other medium infrared wave bands; the refrigeration heat exchanger is connected with the electrolyte circulation loop. The battery module comprises positive and negative electrodes, a current collector (222) and an ion exchange film (223); two electrolyte storage tanks are connected with the positive and negative electrodes of the battery module respectively; the electrolytes stored in the two electrolyte storage tanks are two different oxidation-reduction couples and have opposite temperature coefficients. A flow pump (25) is arranged in the electrolyte circulation loop to realize flow control of the electrolyte; a heat source end control valve (111) and a cold source end control valve (112) are further arranged on the electrolyte circulation loop, and the cold source end control valve (112) is arranged on the circulation loop where the sky radiation refrigeration unit (12) is located. The heat regenerative liquid flow battery system (20) operates in an adiabatic insulation environment, and the pipeline, the battery module, the electrolyte storage tank, the flow pump, the control valve, the heat exchanger and the regenerative heat exchanger are all subjected to insulation treatment. The periodic low-grade heat energy efficient recovery and conversion system further comprises a heat energy prediction system and an electrolyte parameter control system, which are used for predicting the temperature change trend of the next period of the periodic heat source and the sky radiation refrigeration unit in advance and adjusting the concentration, the storage capacity and the flow of the electrolyte through the electrolyte parameter control system to optimize the energy conversion rate.

2. The system for efficient recovery and conversion of periodic low-grade heat energy according to claim 1, wherein, ​ 3. The system for efficient recovery and conversion of periodic low-grade heat energy according to claim 1, wherein, ​ 4. The system for efficient recovery and conversion of periodic low-grade thermal energy according to claim 1, wherein, ​ 5. The system for efficient recovery and conversion of periodic low-grade thermal energy according to claim 1, wherein, ​ 6. The system for efficient recovery and conversion of periodic low-grade thermal energy according to claim 1, wherein, ​ 7. A method for efficient recovery and conversion of periodic low-grade heat energy, characterized by, The periodical low-grade heat energy efficient recovery conversion system according to any one of claims 1 to 6, comprising the following steps: According to the temperature variation law of the periodical heat source heat exchange unit (11) or the sky radiation refrigeration unit (12), the electrolyte parameters in the electrolyte storage tank are set; then the flow pump is opened, the electrolyte flows out from the electrolyte storage tank, and after heat exchange through the periodical heat source heat exchange unit (11) or the sky radiation refrigeration unit (12), flows into the battery module to occur oxidation-reduction reaction, complete charging or discharging; the electrolyte after the reaction is completed preheats or pre-cools the electrolyte from the electrolyte storage tank in the next cycle, and then circulates into the electrolyte storage tank; After the reaction in one temperature interval in the periodical variation is completed, the electrolyte is again heat exchanged in the electrolyte circulation loop to occur reversible oxidation-reduction reaction, so that the periodical recovery conversion of periodical low-grade heat energy is realized.

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