Compressed air energy storage system and method based on near-isothermal compression and waste heat utilization

Abstract

A kind of compressed air energy storage system and method based on near isothermal compression and waste heat utilization, system includes compressor unit and expander unit, the air inlet of first stage compressor is connected, the exhaust port of compressor unit is connected oil separator, the high-pressure air obtained is transported to gas storage tank by first gas path valve, the exhaust port of gas storage tank is connected the air inlet of expander unit by second gas path valve again;The number of stages of expander unit is equal to the number of stages of compressor unit, and the last stage of expander unit is divided into two branches by three-way valve, the expander of first branch is connected cold air as heat exchange working substance, the expander of second branch and other stages of expander are all connected hot oil obtained by oil separator as heat exchange working substance, and isothermal expansion or near isothermal expansion is realized by three-way valve control.This application system has simple structure, less power consumption components, according to the peak period and low period of electricity to select different system operation mode, through switching operation mode, the whole system can be low in energy consumption, save cost.

Description

Technical Field

[0001] This invention belongs to the field of compressed air energy storage technology, specifically relating to a compressed air energy storage system and method based on near-isothermal compression and waste heat utilization. Background Technology

[0002] The basic principle of compressed air energy storage is as follows: during the energy storage process, an electric motor drives a compressor to generate high-pressure air, which is then stored in the storage device. During the energy release process, the expansion of the air in the storage device drives a generator to produce electricity. Compressed air energy storage technology has advantages such as large energy storage capacity, long energy storage cycle, high system efficiency, long operating life, and low specific investment, and is considered one of the most promising large-scale energy storage technologies.

[0003] The main technical routes for compressed air energy storage systems include: afterburning compressed air energy storage, adiabatic compressed air energy storage, isothermal compressed air energy storage, and liquid compressed air energy storage. The advantages of isothermal compressed air energy storage are low operating parameters and high theoretical efficiency. It avoids the problems associated with heat / cold storage, such as expensive phase change materials, difficulty in storing the heat storage medium, and high thermal resistance. Currently, isothermal compression technology mainly uses a liquid or solid piston mechanism to drive the compression and expansion process. During compression, water mist (premixed water foam) or a liquid piston is injected to achieve large-area heat exchange, thus achieving isothermal compression. This heat is stored in water or water foam and released during expansion. Because it requires a sufficient heat exchange process, high-frequency operation is not possible. The single-stage volumetric flow rate is limited, and the injection volume affects the piston stroke. The injection volume is difficult to control precisely, resulting in a large clearance volume and decreased efficiency. Furthermore, to achieve periodic intake and exhaust, vulnerable valves are required, leading to decreased reliability. Meanwhile, the piston-type isothermal compressed air device that uses water spray cooling has a low heat exchange efficiency because the water mist sprayed from the bottom of the cylinder cannot completely occupy the cylinder volume in the initial stage of compression, and the contact area between the water mist and the air is limited, which is not conducive to two-phase heat exchange between air and water mist. Summary of the Invention

[0004] The purpose of this invention is to address the problems in the prior art by providing a compressed air energy storage system and method based on near-isothermal compression and waste heat utilization. This system reduces the operating parameters of the compressor, improves the efficiency of the energy storage system, can utilize waste heat to further enhance system efficiency, increases the energy storage density of the system, and reduces overall system energy consumption and saves costs through two operating modes.

[0005] To achieve the above objectives, the present invention provides the following technical solution: A compressed air energy storage system based on near-isothermal compression and waste heat utilization includes a compressor unit consisting of multiple compressors connected together, and an expander unit consisting of multiple expanders connected together. The inlet of the first-stage compressor is supplied with air drawn in from the external environment. The outlet of the compressor unit is connected to an oil separator. The high-pressure air obtained from the oil separator is delivered to an air storage tank via a first air passage valve. The outlet of the air storage tank is then connected to the inlet of the expander unit via a second air passage valve. The number of stages in the expander unit is equal to the number of stages in the compressor unit. The last-stage expander unit is divided into two branches by a three-way valve. The first branch expander uses cold air as the heat exchange medium, while the second branch expander and all other expanders use hot oil obtained from the oil separator as the heat exchange medium. When the three-way valve is opened upwards, no hot oil is supplied to the expanders, achieving isentropic expansion. When the three-way valve is opened downwards, hot oil is supplied to the expanders, achieving near-isothermal expansion. The outlets of the second branch expander and all other expanders are connected to the oil separator, which then delivers the separated oil to the compressor unit.

[0006] As a preferred embodiment, the inlet of the first-stage compressor is equipped with a first air filter, and the outlet of each stage compressor is connected to an oil separator. The oil separator transports the separated oil to a second oil storage tank for storage. The outlet pipeline of the second oil storage tank is connected to the last stage second branch expander and other stages of expanders in the expander unit via a second oil circuit valve and a second oil pump.

[0007] As a preferred option, a heat exchanger is installed on the outlet pipeline of the second oil storage tank, and the heat exchanger is connected to the waste heat.

[0008] As a preferred option, a minimum pressure valve, a check valve, and a dryer are installed between the oil separator connected to the last stage compressor and the first gas path valve.

[0009] As a preferred embodiment, each compressor in the compressor unit is connected to an electric motor, and the electric motor is connected to a power source via a first switch; each expander in the expander unit is connected to a generator, and the generator is connected to a power source via a second switch.

[0010] As a preferred embodiment, the exhaust ports of the last stage second branch expander in the expander unit and the other stages of expanders are all connected to an oil separator. The oil separator transports the separated oil to the first oil storage tank for storage. The outlet pipeline of the first oil storage tank is connected to each compressor in the compressor unit via the first oil circuit valve and the first oil pump.

[0011] As a preferred embodiment, a second air filter is provided between the first-stage expander and the second air circuit valve in the expander unit.

[0012] A control method for a compressed air energy storage system based on near-isothermal compression and waste heat utilization: Different system operating modes are selected based on peak and off-peak electricity consumption periods; The system operating modes include compression energy storage mode and energy utilization mode; In the compressed energy storage mode, air from the outside environment is introduced into the compressor unit and compressed by the compressor unit to become room temperature and high pressure air, which is then stored in the air storage tank. In energy utilization mode, room temperature high-pressure air stored in the gas storage tank enters the expander unit to release energy. When cooling is required, the three-way valve is controlled to select the connection mode of the expander unit according to the amount of cooling required. When the three-way valve is opened upward, hot oil is not introduced into each expander to achieve isentropic expansion. When the three-way valve is opened downward, hot oil is introduced into each expander to achieve near isothermal expansion. The cold air obtained after the expander unit releases energy is introduced into the place that needs cooling.

[0013] As a preferred embodiment, in the compression and energy storage mode, the first switch connecting the compressor to the power supply is closed, the second switch connecting the expander to the power supply is disconnected, the first gas circuit valve is opened, the second gas circuit valve is closed, the first oil pump connecting the first oil tank to the compressor unit is turned on, the second oil pump connecting the second oil tank to the expander unit is turned off, the first oil circuit valve at the outlet of the first oil tank is opened, and the second oil circuit valve at the outlet of the second oil tank is closed.

[0014] As a preferred embodiment, in energy utilization mode, the first switch connecting the compressor to the power supply is disconnected, the second switch connecting the expander to the power supply is closed, the first gas path valve is closed, the second gas path valve is opened, the first oil pump connecting the first oil tank to the compressor unit is closed, the second oil pump connecting the second oil tank to the expander unit is opened, the first oil path valve at the outlet of the first oil tank is closed, and the second oil path valve at the outlet of the second oil tank is opened.

[0015] Compared with the prior art, the present invention has at least the following beneficial effects: By thoroughly mixing oil and air, the contact area is increased, resulting in more efficient heat exchange and a significant improvement in the energy storage system efficiency. Since the working fluid used in this energy storage cycle is air, a non-toxic, harmless, and pollution-free natural working fluid that can be directly obtained from the environment and is inexhaustible, air can be directly introduced into the cooling environment without the need for recycling. This invention's system is safe, pollution-free, energy-efficient, and highly effective. The system has two operating modes: a compression energy storage mode during off-peak electricity demand and an energy utilization mode during peak electricity demand. The connection method of the expander units is selected by controlling the three-way valve according to the required cooling capacity. When the three-way valve is open upwards, hot oil is not introduced into the expanders, achieving isentropic expansion; when the three-way valve is open downwards, hot oil is introduced into the expanders, achieving near-isothermal expansion. The cold air obtained after the expanders release energy is then introduced into the cooling environment, thus achieving peak shaving and valley filling of electricity, resulting in energy saving and cost reduction. Overall, the system of this invention has a simple structure, few power-consuming components, and can reduce the energy consumption of the entire system and save costs through two operating modes.

[0016] Furthermore, the system of the present invention can utilize the already mature twin-screw oil injection technology. Twin-screw compressors have good variable operating condition performance, which can greatly improve the shortcomings of poor variable operating condition performance of compressor units currently used for compressed air energy storage.

[0017] Furthermore, a heat exchanger is installed on the outlet pipeline of the second oil storage tank of the present invention. When there is waste heat in the area where the system is located, the waste heat can be connected to improve the efficiency of the system and increase the energy storage density of the system.

[0018] Furthermore, during off-peak electricity demand periods, the system employs a compression energy storage mode. The gas system stores high-pressure, ambient-temperature air compressed by the compressor unit in a gas storage tank, while the oil system absorbs the heat of compression and stores it in a first oil storage tank. During peak electricity demand periods, an energy utilization mode is used. The gas system allows the high-pressure, ambient-temperature air from the storage tank to enter the expander unit for energy release and power generation. The oil system absorbs the heat of compression to ensure near-isothermal expansion. Attached Figure Description

[0019] Figure 1 Schematic diagram of the compressed air energy storage system based on near-isothermal compression and waste heat utilization in Embodiment 1 of the present invention; Figure 2 Schematic diagram of the compressed air energy storage system based on near-isothermal compression and waste heat utilization in Embodiment 2 of the present invention; Figure 3 The present invention relates to a PT diagram of a compressed air energy storage system based on near-isothermal compression and waste heat utilization when no waste heat source is connected. Figure 4 This invention is based on the PT diagram of a compressed air energy storage system that utilizes near-isothermal compression and waste heat utilization when cold energy is generated; Figure 5 This invention relates to the PT diagram of a compressed air energy storage system based on near-isothermal compression and waste heat utilization when a waste heat source is connected; Figure 6 Flowchart of the control method for a compressed air energy storage system based on near-isothermal compression and waste heat utilization according to an embodiment of the present invention; In the attached diagram: 1-First air filter; 2-Compressor; 3-Oil separator; 4-Minimum pressure valve; 5-Check valve; 6-Dryer; 7-First air circuit valve; 8-Air tank; 9-Second air circuit valve; 10-Second air filter; 11-Expander; 12-First three-way valve; 13-First switch; 14-Power supply; 15-Motor; 16-Generator; 17-Second switch; 18-First oil pump; 19-First oil circuit valve; 20-First oil tank; 21-Second oil tank; 22-Second oil circuit valve; 23-Heat exchanger; 24-Second oil pump; 25-Second three-way valve; 26-Third liquid circuit valve. Detailed Implementation

[0020] The present invention will now be described in further detail with reference to the accompanying drawings.

[0021] See Figure 1This invention relates to a compressed air energy storage system based on near-isothermal compression and waste heat utilization. The system includes a compressor unit consisting of multiple compressors 2 connected together, an expander unit consisting of multiple expanders 11 connected together, a first air filter 1, a second air filter 10, an oil separator 3, a minimum pressure valve 4, a check valve 5, an air tank 8, expanders 11, a heat exchanger 23, and several valves. Air drawn in from the external environment is introduced into the compressor unit's inlet. The exhaust port of each stage compressor 2 is connected to the oil separator 3. The high-pressure air passing through the oil separator 3 is connected to the inlet of the air tank 8 via the minimum pressure valve 4, check valve 5, dryer 6, and first air path valve 7. The exhaust port of the air tank 8 is connected to the inlet of the expander unit via the second air path valve 9 and second air filter 10. The exhaust port of each stage expander 11 is connected to the oil separator 3. The number of stages in the expander unit is equal to the number of stages in the compressor unit. The number of stages in the compressor unit is determined by the pressure ratio. The last stage expander unit is divided into two branches by the first three-way valve 12. The expander in the first branch uses cold air as the heat exchange medium, while the expander 11 in the second branch, along with the other expanders 11 in each stage, uses hot oil obtained from the oil separator 3 as the heat exchange medium. The opening state of the first three-way valve 12 is determined based on whether cooling is required. When cooling is required, the first three-way valve 12 opens upwards, the oil circuit is not connected to the expander unit, and the cooling generated during the expansion process is supplied to the area requiring cooling. When the first three-way valve 12 opens downwards, it is in normal power generation mode. Depending on whether there is waste heat, the heat exchanger 23 can be connected to the oil circuit system to further increase the temperature of the oil before it enters the expander unit, thereby increasing power generation. The heat exchanger 23 is connected to the waste heat source, and the oil flowing out of the second oil storage tank 21 enters the expander unit after exchanging heat with the waste heat. The power supply 14 is connected to the motor 15 through the first switch 13. The motor 15 is connected to and drives the compressor unit to work, compressing the air. The expander unit is connected to and drives the generator 16 to work, and the generator 16 recovers energy by supplying electricity.

[0022] See Figure 2 In the system structure given in Embodiment 2 of the present invention, the liquid sprayed is oil. At this time, the outlet temperature of the compressor is relatively high. Therefore, cold oil is introduced into the compressor exhaust end through the third liquid circuit valve 26 for cooling and to absorb the heat of compression.

[0023] like Figure 3 , Figure 4 and Figure 5 As shown, the compressed air energy storage system based on near-isothermal compression and waste heat utilization of this invention has three working modes: no waste heat source access, cold energy generation, and waste heat source access. Points a to g in the figure represent the compression energy storage mode, points h to n represent the energy utilization mode with cold energy release, and points h to o represent the normal energy utilization mode.

[0024] This invention relates to a control method for a compressed air energy storage system based on near-isothermal compression and waste heat utilization: Select the system operating mode according to the peak and off-peak electricity consumption periods.

[0025] The system operates in two modes: compression and energy storage, and energy utilization. In compression and energy storage mode, atmospheric air is filtered through the first air filter 1 to remove impurities, then drawn in through the compressor unit's intake port and compressed to room temperature and high pressure. This compressed air then passes through the minimum pressure valve 4, check valve 5, and dryer 6 before being stored in the air tank 8. In energy utilization mode, the room temperature and high pressure air stored in the air tank 8 passes through the second air filter 10 and enters the expander unit for energy release. Energy is recovered by connecting to the generator 16 to supply electricity. When cooling is required, depending on the amount of cooling needed, the last few stages of the expander unit 11 are not connected to the oil system and perform expansion and cooling, allowing the cooled air to be supplied to the area requiring cooling.

[0026] like Figure 1 As shown in the diagram, points a to b to c represent the compression energy storage mode, points c to e represent the energy utilization mode with cold energy release, and points c to d represent the normal energy utilization mode. Figure 6 As shown, the control method of this invention controls the operating mode of the energy storage system according to peak and off-peak electricity consumption. The compressor unit is the main power-consuming component in this invention system. Since the electricity price is lower during off-peak hours and higher during peak hours, the control principle of this system is to start the compressor unit during off-peak hours and shut it down during peak hours. The expander unit starts to release energy.

[0027] This invention solves the problem of the unit's inability to operate continuously, achieving high flow rate, eliminating clearance volume, and realizing high efficiency and high reliability. However, due to insufficient heat exchange within the cavity, the pressure ratio is too high and leakage is relatively high. To address this, a multi-stage compression method is adopted, with interstage liquid injection heat exchange and high-temperature liquid recovery to enhance the output power during the expansion process while fully utilizing the heat exchanged.

[0028] like Figure 1As shown, the first switch 13 controls the start and stop of the drive motor 15, thereby controlling the start and stop of the compressor 2. The compressor 2 compresses gas, and the oil separator 3 filters out the oil from the compressed high-pressure air. The minimum pressure valve 4 establishes the internal pressure of the unit. The check valve 5 prevents the high-pressure gas in the gas storage tank 8 from flowing back into the compressor unit. The gas storage tank 8 stores high-pressure gas, and the expander unit expands the gas to generate electricity and provide refrigeration. The generator 16 converts the expansion work into electrical energy. The first gas valve 7 and the second gas valve 9 control the flow of gas in the pipeline. The first oil valve 19 and the second oil valve 22 control the flow of oil in the pipeline. In the compression and energy storage mode, the first switch 13 is closed, the second switch 17 is open, the first gas valve 7 is opened, the second gas valve 9 is closed, the first oil pump 18 is opened, the second oil pump 24 is closed, the first oil valve 19 is opened, and the second oil valve 22 is closed. At this time, the motor 15 and the compressor unit are in working condition. The air in the atmosphere is compressed into room temperature and high pressure air by the compressor unit and then stored in the gas storage tank 8. In energy utilization mode, the first switch 13 is open, the second switch 17 is closed, the first gas valve 7 is closed, the second gas valve 9 is opened, the first oil pump 18 is closed, the second oil pump 24 is opened, the first oil valve 19 is closed, and the second oil valve 22 is opened. At this time, the generator 16 and the expander unit are in working condition, and the expansion work generated by the expander unit is used to generate electricity through the generator 16.

[0029] Figure 1 Point a represents air in the atmosphere, point b represents the outlet state of the compressor unit, point c represents the inlet state of the expander unit, point d represents the outlet state of the expander unit, point e represents the outlet state of the expander unit when there is a cooling demand, point f represents the state of oil entering the compressor unit, point g represents the state of oil leaving the compressor unit, point h represents the state of oil entering the expander unit, and point i represents the state of oil leaving the expander unit. Ideally, from point a to point b, it is isothermal compression, during which the air temperature remains constant while the pressure increases. From point c to point d, it is isothermal expansion, during which the air pressure decreases while the temperature remains constant. From point c to point e, part of it is isothermal expansion and part isentropic expansion, during which the air pressure and temperature decrease, and the temperature drops below the ambient temperature, which can be used for refrigeration. The process from point a to point b to point c represents the compression energy storage mode, while the processes from point c to point d and from point c to point e represent the energy utilization mode.

[0030] like Figure 6 As shown, peak electricity consumption refers to the period with the highest electricity price, and off-peak electricity consumption refers to the period with the lowest electricity price. Considering that the compressor is the main power-consuming component of this system, the control method of this system is as follows: During peak electricity demand, the system adopts energy utilization mode, while during off-peak electricity demand, the system adopts compressed energy storage mode.

[0031] In the compressed energy storage mode, the first switch 13 is closed, the second switch 17 is opened, the first air valve 7 is opened, the second air valve 9 is closed, the first oil pump 18 is opened, the second oil pump 24 is closed, the first oil valve 19 is opened, and the second oil valve 22 is closed.

[0032] In energy utilization mode, the first switch 13 is turned off, the second switch 17 is closed, the first gas valve 7 is closed, the second gas valve 9 is opened, the first oil pump 18 is turned off, the second oil pump 24 is opened, the first oil valve 19 is closed, and the second oil valve 22 is opened.

[0033] As can be seen, this invention is a compressed air energy storage system and control method based on near-isothermal compression and waste heat utilization. Utilizing the already mature twin-screw oil injection technology, it ensures thorough mixing of oil and air, increasing their contact area and achieving more efficient heat exchange. This allows the compression and expansion processes to proceed at near-isothermal temperatures, improving the system efficiency of compressed air energy storage and reducing energy loss. It can also provide a certain cooling capacity through expansion cooling. Furthermore, the system is simple, has few power-consuming components, and can switch between two modes based on electricity prices, ultimately achieving energy and cost savings.

[0034] The above description is merely a preferred embodiment of the present invention and is not intended to limit the technical solution of the present invention in any way. Those skilled in the art should understand that, without departing from the spirit and principles of the present invention, the technical solution can be modified and replaced in several simple ways, and these modifications and replacements are all within the scope of protection covered by the claims.

Claims

1. A compressed air energy storage system based on near-isothermal compression and waste heat utilization, characterized in that, The system includes a compressor unit consisting of multiple compressors (2) connected together, and an expander unit consisting of multiple expanders (11) connected together. The inlet of the first-stage compressor (2) is supplied with air drawn in from the external environment. The outlet of the compressor unit is connected to an oil separator (3). The high-pressure air obtained from the oil separator (3) is transported to the air storage tank (8) via the first air passage valve (7). The outlet of the air storage tank (8) is then connected to the inlet of the expander unit via the second air passage valve (9). The number of stages of the expander unit is equal to the number of stages of the compressor unit, and the last stage of the expander unit is divided into three stages by a three-way valve. The system has two branches. The first branch's expander is supplied with cold air as the heat exchange medium, while the second branch's expander (11) and other expanders (11) at all levels are supplied with hot oil obtained from the oil separator (3) as the heat exchange medium. When the three-way valve is opened upwards, no hot oil is supplied to each expander, achieving isentropic expansion. When the three-way valve is opened downwards, hot oil is supplied to each expander, achieving near-isothermal expansion. The exhaust ports of the second branch's expander (11) and other expanders (11) at all levels are connected to the oil separator (3), and the oil separator (3) will then transport the separated oil to the compressor unit. The first air filter (1) is installed at the air inlet of the first stage compressor (2). The exhaust port of each stage compressor (2) is connected to the oil separator (3). The oil separator (3) transports the separated oil to the second oil storage tank (21) for storage. The outlet pipeline of the second oil storage tank (21) is connected to the last stage second branch expander (11) and other stages of expanders (11) in the expander unit via the second oil circuit valve (22) and the second oil pump (24). A heat exchanger (23) is installed on the outlet pipe of the second oil storage tank (21), and the heat exchanger (23) is connected to the waste heat. Each compressor (2) in the compressor unit is connected to an electric motor (15), and the electric motor (15) is connected to a power source (14) through a first switch (13); each expander (11) in the expander unit is connected to a generator (16), and the generator (16) is connected to a power source (14) through a second switch (17). The exhaust ports of the last stage second branch expander (11) and the other stages of expanders (11) in the expander unit are all connected to the oil separator (3). The oil separator (3) transports the separated oil to the first oil storage tank (20) for storage. The outlet pipeline of the first oil storage tank (20) is connected to each compressor (2) in the compressor unit through the first oil circuit valve (19) and the first oil pump (18).

2. The compressed air energy storage system based on near-isothermal compression and waste heat utilization according to claim 1, characterized in that, The oil separator (3) connected to the last stage compressor (2) is equipped with a minimum pressure valve (4), a check valve (5), and a dryer (6) between it and the first gas path valve (7).

3. The compressed air energy storage system based on near-isothermal compression and waste heat utilization according to claim 1, characterized in that, A second air filter (10) is provided between the first-stage expander (11) and the second air valve (9) in the expander unit.

4. A control method for a compressed air energy storage system based on near-isothermal compression and waste heat utilization as described in any one of claims 1 to 3, characterized in that: Different system operating modes are selected based on peak and off-peak electricity consumption periods; The system operating modes include compression energy storage mode and energy utilization mode; In the compressed energy storage mode, air from the outside environment is introduced into the compressor unit and compressed by the compressor unit to become normal temperature and high pressure air and stored in the air storage tank (8). In the energy utilization mode, the ambient temperature high pressure air stored in the gas storage tank (8) enters the expander unit to release energy. When cooling is required, the three-way valve is controlled to select the connection mode of the expander unit according to the amount of cooling required. When the three-way valve is opened upward, hot oil is not introduced into each expander to achieve isentropic expansion. When the three-way valve is opened downward, hot oil is introduced into each expander to achieve near isothermal expansion. The cold air obtained after the expander unit releases energy is introduced into the place that needs cooling.

5. The control method according to claim 4, characterized in that: In the compression storage mode, the first switch (13) connecting the compressor (2) to the power supply (14) is closed, the second switch (17) connecting the expander (11) to the power supply (14) is disconnected, the first gas valve (7) is opened, the second gas valve (9) is closed, the first oil pump (18) connecting the first oil tank (20) to the compressor unit is opened, the second oil pump (24) connecting the second oil tank (21) to the expander unit is closed, the first oil valve (19) at the outlet of the first oil tank (20) is opened, and the second oil valve (22) at the outlet of the second oil tank (21) is closed.

6. The control method according to claim 4, characterized in that: In energy utilization mode, disconnect the first switch (13) connecting the compressor (2) to the power supply (14), close the second switch (17) connecting the expander (11) to the power supply (14), close the first gas valve (7), open the second gas valve (9), close the first oil pump (18) connecting the first oil tank (20) to the compressor unit, open the second oil pump (24) connecting the second oil tank (21) to the expander unit, close the first oil valve (19) at the outlet of the first oil tank (20), and open the second oil valve (22) at the outlet of the second oil tank (21).

Images