A low-grade waste heat utilization design method based on medium-temperature compression energy storage system

Abstract

The application relates to a low-grade waste heat utilization design method based on a medium-temperature compressed energy storage system, which comprises the following steps: adding a supercritical CO2 Brayton cycle as a system bottom cycle in the medium-temperature compressed air energy storage system, arranging a simulation analysis model for establishing the low-grade waste heat utilization of the medium-temperature compressed energy storage system; according to the simulation analysis model, a target function is established with the system total cycle efficiency eta en as an optimization target; in the case of meeting preset constraint conditions, an interpolation method is used to solve the target function, so that the parameters of CO2 are obtained, the constraint conditions include energy conservation constraint, heat exchanger two-side temperature constraint and CO2 transcritical state constraint; then, the specific parameters and efficiency of the bottom cycle are determined according to the parameters of CO2, and the maximum efficiency of the total cycle is obtained. The application reduces the use of heat exchangers, and further reduces heat exchange loss, so that energy is efficiently utilized.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of compressed air energy storage system design optimization, in particular to a low-grade waste heat utilization design method based on a medium-temperature compressed energy storage system. BACKGROUND

[0002] Energy storage technology plays an important role in the field of renewable energy generation because it can coordinate the matching between energy supply and demand, smooth the fluctuations on the power supply side, effectively solve the instability of energy supply, and solve the problem of energy recovery and utilization. The research on energy storage technology is of great significance to the future pattern of energy development. Compressed air energy storage has great application potential in large-scale peak regulation of power grids due to its low cost, large capacity, and fast response speed, and is expected to become one of the main development directions of future large-scale energy storage.

[0003] The system electric energy conversion efficiency of compressed air energy storage technology is relatively low in actual engineering. Even the advanced adiabatic compressed air energy storage technology, which is known for its high efficiency, has a cycle efficiency of less than 70%, which is lower than other energy storage methods. In a multi-stage compression system, a part of the low-temperature heat is difficult to utilize, resulting in a large amount of waste heat. At the level of thermal system optimization, the reasonable design of the working process of the thermal system working fluid can effectively improve the system efficiency and reduce the power generation cost, which has become an important research direction in this field. In a medium-temperature compressed air energy storage system, the temperature of the air after primary compression and secondary expansion is about 100℃. Because the temperature is not high, it is difficult to utilize, so it is not recycled. To enhance the efficient utilization of energy, a bottom cycle structure can be added to the medium-temperature compressed air energy storage system to recover the waste heat. In the past, an organic Rankine cycle was used to utilize low-grade heat. According to previous research, the physical properties of CO2 change sharply, which can effectively reduce the compression work and improve the cycle efficiency. Moreover, the temperature curve is smooth, there is no pinch point limitation, the heat transfer loss is smaller, so the supercritical CO2 Brayton cycle is used as the bottom cycle to utilize low-grade waste heat. SUMMARY SUMMARY

[0004] The purpose of the embodiments of the present application is to provide a low-grade waste heat utilization design method based on a medium-temperature compressed energy storage system, which recovers the heat that cannot be utilized during primary compression and secondary expansion in the original medium-temperature compressed air energy storage system, and uses the waste heat in a new transcritical CO2 cycle to improve the system cycle efficiency.

[0005] To achieve the above purpose, the present application provides the following technical solutions:

[0006] The embodiments of the present application provide a low-grade waste heat utilization design method based on a medium-temperature compressed energy storage system, which includes the following steps:

[0007] S1, in a medium temperature compressed air energy storage system, a supercritical CO2 Brayton cycle is added as a system bottom cycle, and a simulation analysis model for establishing low-grade waste heat utilization of the medium temperature compressed energy storage system is arranged;

[0008] S2, according to the simulation analysis model, a target function with the system total cycle efficiency η en as an optimization target is established as follows:

[0009] max (η en ) = f (m T-CO2 , P o_1 , T o_1 , T o_4 )

[0010] Wherein, m T-CO2 is the mass flow of CO2 in the bottom cycle, P o_1 is the pressure of CO2 after being heated by the heat exchanger, T o_1 is the temperature of CO2 after being heated by the heat exchanger, T o_4 is the temperature of CO2 before being heated by the heat exchanger.

[0011] S3, under the condition of meeting the preset constraint condition, the interpolation method is used to solve the target function, to obtain the parameters of CO2, and the constraint condition includes the energy conservation constraint, the temperature constraint of both sides of the heat exchanger, and the CO2 transcritical state constraint; and then the specific parameters and efficiency of the bottom cycle are determined according to the parameters of CO2, to obtain the maximum efficiency of the total cycle.

[0012] The energy conservation constraint is m T-CO2 (h O_1 -h O_4 ) = m oil (h EX4_o_in -h EX4_o_out ), wherein m T-CO2 is the mass flow of CO2 in the bottom cycle, h O_1 is the enthalpy of CO2 after being heated by the heat exchanger, h O_4 is the enthalpy of CO2 before being heated by the heat exchanger, m oil is the mass flow of the heat conducting oil, h EX4_o_in is the enthalpy of the heat conducting oil before entering the heat exchanger, and h EX4_o_out is the enthalpy of the heat conducting oil after entering the heat exchanger.

[0013] The temperature constraint of both sides of the heat exchanger is T o_4 <T EX4_o_out -ΔT and T o_1 <T EX4_o_in -ΔT, wherein T EX4_o_out is the temperature of the heat conducting oil at the outlet of the heat exchanger, ΔT is the end difference of the heat exchanger, and T EX4_o_in is the temperature of the heat conducting oil at the inlet of the heat exchanger.

[0014] The CO2 transcritical state constraint is T o_4 <31.1℃, T o_1 >31.1℃, P o_1 >P c =f(T o_1 )>7.39MPa, wherein, P c is the critical pressure of CO2.

[0015] The CO2 parameters include mass flow, temperature and pressure before and after entering the heat exchanger.

[0016] Compared with the prior art, the application has the beneficial effects that:

[0017] 1. The application has high utilization rate of low-grade waste heat according to the transcritical CO2 Brayton cycle, so the cycle is used as a bottom cycle, and the structure of the system is optimized to reduce the use of heat exchangers, thereby reducing heat exchange loss and realizing efficient energy utilization.

[0018] 2. In the low-grade waste heat utilization design method of the medium-temperature compressed energy storage system, the air side working process and state are the same as those of the medium-temperature compressed air energy storage system, the heat exchanger fluid is changed, and a transcritical CO2 Brayton cycle structure is additionally added as a bottom cycle. The operating parameters of the bottom cycle are optimized for the purpose of optimizing the total cycle efficiency of the system. The transcritical CO2 cycle has certain requirements for the parameters of CO2 working time, and the application sets energy conservation constraints, heat exchanger temperature constraints and CO2 transcritical state constraints to make the results meet the requirements of the transcritical cycle and enable the system to operate safely. BRIEF DESCRIPTION OF DRAWINGS

[0019] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments of the present application. It should be understood that the following drawings only show some embodiments of the present application, and therefore should not be regarded as a limitation on the scope, and for those skilled in the art, other related drawings can also be obtained without creative labor on the basis of these drawings.

[0020] Figure 1 is a typical system structure schematic diagram of a medium-temperature compressed air energy storage system;

[0021] Figure 2 is a medium-temperature compressed air energy storage system structure schematic diagram of the embodiment of the application using low-grade waste heat utilization.

[0022] In the figure: 1 is a first motor, 2 is a second compressor, 3 is a second motor, 4 is a first heat exchanger, 5 is a third compressor, 6 is a second heat exchanger, 7 is a high-temperature heat conducting oil storage tank, 8 is a third heat exchanger, 9 is a fourth heat exchanger, 10 is a first expander, 11 is a second expander, 12 is a generator, 13 is a fifth heat exchanger, 14 is a Brayton cycle compressor, 15 is a sixth heat exchanger, 16 is a condenser, 17 is a third motor, 18 is a low-temperature heat conducting oil storage tank, 19 is a high-pressure gas storage tank, 20 is a Brayton cycle expander, 21 is a seventh heat exchanger, 22 is a first compressor, and 23 is a fourth motor. DETAILED DESCRIPTION

[0023] The technical solutions in the embodiments of the present application will be described below with reference to the drawings in the embodiments of the present application. It should be noted that similar reference numerals and letters represent similar items in the following drawings, and therefore, once an item is defined in one drawing, it does not need to be further defined and explained in subsequent drawings.

[0024] The term "comprising" or "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or apparatus including a series of elements includes not only those elements, but also other elements not explicitly listed, or other elements inherent to such a process, method, article or apparatus. Without more limitations, the element defined by the statement "including a" does not exclude the presence of additional identical elements in the process, method, article or apparatus including the element.

[0025] The terms "first", "second", and the like are only used to distinguish one entity or operation from another entity or operation, and cannot be understood as indicating or implying relative importance, nor can it be understood as requiring or implying any such actual relationship or order between the entities or operations.

[0026] The embodiments of the present application take a typical medium-temperature compressed air energy storage system as an improved object, and the structure of a heat power generation system thereof is as shown in Figure 1 A waste heat utilization cycle taking transcritical CO2 as a working medium is added to the system to improve the system.

[0027] The typical medium-temperature compressed air energy storage system compresses air at normal temperature and pressure into high-pressure air. In the air working medium side, the working components include a first motor, a two-stage compressor, a second motor, a first heat exchanger, a three-stage compressor, a second heat exchanger, a one-stage expander, a two-stage expander, a generator, a third heat exchanger, a high-pressure air tank, a fourth heat exchanger, a fifth heat exchanger, a one-stage compressor, and an engine. In the heat conducting oil side, the working components include the first heat exchanger, a high-temperature heat conducting oil tank, the second heat exchanger, the third heat exchanger, a low-temperature heat conducting oil tank, and the fourth heat exchanger. Wherein:

[0028] The one-stage compressor inlet is air, the outlet is connected with the fifth heat exchanger, the outlet of the fifth heat exchanger is connected with the two-stage compressor inlet, the two-stage compressor outlet, the first heat exchanger, the three-stage compressor, the second heat exchanger, the high-pressure air tank, the third heat exchanger, the one-stage expander, the fourth heat exchanger, and the two-stage expander are sequentially connected, and the outlet of the two-stage expander is air. The arrow in the figure indicates the air flow direction.

[0029] During the operation of the medium-temperature compressed air energy storage system, the temperature of the air rises after the one-stage compression, and the air needs to be cooled by the fifth heat exchanger before being compressed again. The heat exchanger directly uses water for cooling, and a part of the heat is not utilized. During the energy release process, the air is directly discharged into the atmosphere after the two-stage expansion, and the temperature of the discharged air is higher than 90 degrees Celsius, and a part of the waste heat can be utilized.

[0030] The present application improves the medium-temperature compressed air energy storage system, and obtains a medium-temperature compressed air energy storage system using a low-grade waste heat utilization bottom cycle system, as shown in Figure 2 The air side working process and state are the same as those of the medium-temperature compressed air energy storage system, the heat exchanger fluid is changed, and a transcritical CO2 Brayton cycle structure is additionally added as a bottom cycle.

[0031] Specifically, the air after the one-stage compression is directly exchanged with CO2 in the bottom cycle in the heat exchanger. In addition, a heat exchanger is additionally arranged after the two-stage expansion, heat conducting oil is used to exchange heat with the air, the outlet of the heat conducting oil is combined with the heat conducting oil of the heat exchanger after the one-stage expansion, and is connected to the heat exchanger in the bottom cycle to heat the CO2.

[0032] When the above-mentioned medium-temperature compressed air energy storage system is working, the air-side workflow (with bottom circulation) is as follows: air enters the first-stage compressor 22 from the outside, is compressed, enters the seventh heat exchanger 21 for cooling, and then sequentially enters the second-stage compressor 2, the first heat exchanger 4, the third-stage compressor 5, the second heat exchanger 6, the high-pressure gas storage tank 19, the fourth heat exchanger 9, the first-stage expander 10, the third heat exchanger 8, the second-stage expander 11, and the fifth heat exchanger 13, and is finally discharged into the atmosphere. The heat transfer oil side's workflow is as follows: During heat storage, the heat transfer oil from the low-temperature heat transfer oil storage tank 18 enters the heat exchangers after the second-stage compressor 2 and the third-stage compressor 5 for heat exchange, and then is stored in the high-temperature heat transfer oil storage tank 7. During heat release, the heat transfer oil from the low-temperature heat transfer oil storage tank 18 enters the heat exchanger after the second-stage expander 11, and the heat transfer oil from the high-temperature heat transfer oil storage tank 7 enters the two heat exchangers before and after the first-stage expander 10. After heat exchange, it enters the heat exchanger in the bottom circulation system together with the heat transfer oil from the heat exchanger after the second-stage expander 11 for heat exchange. The bottom circulation side's workflow is as follows: CO2 enters the heat exchanger from the expander outlet for heating and is then compressed. The CO2 from the compressor enters the heat exchanger to exchange heat with air during energy storage and enters the heat exchanger to exchange heat with the heat transfer oil during energy release. The CO2 after heat exchange enters the expander to perform work and enters the next cycle.

[0033] Based on the above setup, the bottom circulation system using transcritical CO2 as the working fluid is further designed in detail, including the following steps:

[0034] S1 establishes a simulation analysis model for the utilization of low-grade waste heat in a medium-temperature compression energy storage system.

[0035] Based on the structure and operation data of the above-mentioned intermediate-temperature compression energy storage system and transcritical CO2 Brayton cycle system, a simulation analysis model for low-grade waste heat utilization of the intermediate-temperature compression energy storage system is established using modeling software to clarify the relationship between the overall system efficiency and the operating parameters of the bottom cycle CO2, which is then used to establish the objective function.

[0036] Specifically, the main input parameters of this simulation analysis model include: 1) Air-side related parameters: inlet temperature, inlet mass flow rate, inlet pressure, outlet pressure of each stage of compressor, and outlet pressure of each stage of expander; 2) Heat transfer oil-side related parameters: heat transfer oil mass flow rate in each zone; 3) CO2-side related parameters: CO2 mass flow rate in each zone, CO2 outlet pressure of expander, and CO2 outlet pressure of heat exchanger. The output parameters of this simulation analysis model include: 1) Air-side related parameters: inlet and outlet temperatures of each heat exchanger; 2) Heat transfer oil-side related parameters: outlet temperature of each heat exchanger; 3) CO2-side related parameters: inlet and outlet CO2 temperatures of the heat exchanger; 4) Performance parameters: power of each engine, power of each generator, and overall energy storage efficiency.

[0037] S2 establishes the system cycle efficiency η en The computational model.

[0038] The medium-temperature compressed energy storage system inputs power through the motor and outputs power through the generator, so the cycle efficiency of the system is the ratio of the generator output power to the motor input power.

[0039] S3 The objective function of the bottom cycle expander inlet pressure optimization model with the optimal system cycle efficiency as the target is as follows:

[0040] max(η en )=f(m T-CO2 ,P o_1 ,T o_1 ,T o_4 )

[0041] Wherein, m T-CO2 is the mass flow rate of CO2 in the bottom cycle, P o_1 is the pressure of CO2 after heating by the heat exchanger, T o_1 is the temperature of CO2 after heating by the heat exchanger, and T o_4 is the temperature of CO2 before heating by the heat exchanger.

[0042] S4 Determine the constraint condition related to energy conservation.

[0043] When calculating the bottom cycle CO2 parameters of the medium-temperature compressed energy storage waste heat utilization system, it is necessary to ensure that the heat exchange process of the heat exchanger follows the first law of thermodynamics, and thus the constraint condition related to energy conservation can be obtained: m T-CO2 (h O_1 -h O_4 )=m oil (h EX4_o_in -h EX4_o_out ), wherein m T-CO2 is the mass flow rate of CO2 in the bottom cycle, h O_1 is the enthalpy of CO2 after heating by the heat exchanger, h O_4 is the enthalpy of CO2 before heating by the heat exchanger, m oil is the mass flow rate of the heat transfer oil, h EX4_o_in is the enthalpy of the heat transfer oil before entering the heat exchanger, and h EX4_o_out is the enthalpy of the heat transfer oil after entering the heat exchanger.

[0044] S5 Determine the constraint condition related to the temperature of the two ends of the heat exchanger.

[0045] When calculating the bottom cycle CO2 parameters of the medium-temperature compressed energy storage waste heat utilization system, it is necessary to ensure that the heat exchange process of the heat exchanger follows the first law of thermodynamics, and thus the constraint condition related to the temperature of the two ends of the heat exchanger can be obtained: T o_4 <T EX4_o_out -ΔT and T o_1 <T EX4_o_in -ΔT, wherein To_4 T is the temperature of CO2 before entering the heat exchanger EX4_o_out T is the temperature of the heat transfer oil at the outlet of the heat exchanger, ΔT is the temperature difference of the heat exchanger, T o_1 T is the temperature of CO2 at the outlet of the heat exchanger EX4_o_in T is the temperature of the heat transfer oil at the inlet of the heat exchanger.

[0046] S6 determines the constraint condition related to the transcritical state of CO2.

[0047] In order to ensure the normal operation of the transcritical CO2 bottoming cycle, the pressure and temperature of CO2 need to be constrained, and thus the constraint of the transcritical state of CO2 is T o_4 <31.1℃, T o_1 >31.1℃, P o_1 >P c =f(T o_1 )>7.39MPa, wherein, P o_1 P is the pressure of CO2 at the outlet of the heat exchanger, P c P is the critical pressure of CO2.

[0048] S7 determines the parameters of CO2 during operation

[0049] Under the premise of meeting the constraint condition and the actual operation condition, the interpolation method is used to iteratively calculate the parameters of CO2 in the objective function, so as to obtain the optimal solution, wherein the parameters of CO2 include the mass flow, the temperature before and after entering the heat exchanger and the pressure.

[0050] According to the calculation, as shown in Table 1, the original medium-temperature compression energy storage system is compared with the improved low-grade waste heat utilization system. The total compression work and the total expansion work of the medium-temperature compression energy storage low-grade waste heat utilization system are both increased to a certain extent, the net work of the system is increased, and the total cycle efficiency is improved by 1.52%.

[0051] Table 1 Comparison of main parameters of original system and improved system

[0052]

[0053] The above only describes the embodiments of the present application and is not used to limit the protection scope of the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A design method for low-grade waste heat utilization based on medium-temperature compressed energy storage system, characterized in that, The method comprises the following steps: S1, a supercritical CO2 Brayton cycle is added as a bottom cycle in a medium-temperature compressed air energy storage system to establish a simulation analysis model for low-grade waste heat utilization of the medium-temperature compressed energy storage system; S2, according to the simulation analysis model, establish with system total cycle efficiency η en The objective function with the maximum as the optimization objective is as follows: max(η en ) = f(m T-CO2 , P o_1 , T o_1 , T o_4 ) wherein m T-CO2 is the mass flow rate of CO2 in the bottom cycle, P o_1 is the pressure of CO2 after heating by the heat exchanger, T o_1 is the temperature of CO2 after heating by the heat exchanger, T o_4 is the temperature of CO2 before heating by the heat exchanger; S3, under the condition of meeting preset constraint conditions, an interpolation method is used to solve a target function to obtain CO2 parameters, the constraint conditions include an energy conservation constraint, a temperature constraint on both sides of a heat exchanger and a CO2 transcritical state constraint; and then, specific parameters and efficiency of the bottom cycle are determined according to the CO2 parameters to obtain maximum efficiency of the total cycle.

2. The method according to claim 1, wherein The energy conservation constraint is m T-CO2 (h O_1 -h O_4 ) = m oil (h EX4_o_in -h EX4_o_out ), wherein m T-CO2 is the mass flow of CO2 in the bottom cycle, h O_1 is the enthalpy of CO2 after heating by the heat exchanger, h O_4 is the enthalpy of CO2 before heating by the heat exchanger, m oil is the mass flow of the heat transfer oil, h EX4_o_in is the enthalpy of the heat transfer oil before entering the heat exchanger, and h EX4_o_out is the enthalpy of the heat transfer oil after entering the heat exchanger.

3. The method according to claim 1, wherein The temperature constraint on both sides of the heat exchanger is T o_4 <T EX4_o_out -ΔT and T o_1 <T EX4_o_in -ΔT, where T EX4_o_out is the temperature of the heat transfer oil at the outlet of the heat exchanger, ΔT is the end difference of the heat exchanger, T EX4_o_in is the temperature of the heat transfer oil at the inlet of the heat exchanger.

4. The method of claim 1, wherein, the CO2 transcritical state constraint is T o_4 <31.1 °C, T o_1 > 31.1 °C, P o_1 > P c = f(T o_1 ) > 7.39 MPa, where P c is the critical pressure of CO2.

5. The method of claim 1, wherein, The CO2 parameters include mass flow, temperature and pressure before and after entering the heat exchanger.

Images