Multistage flash steam generation system based on CO2 transcritical heat pump

Abstract

The invention relates to the technical field of CO2 heat pump steam generators, in particular to a multi-stage flash steam generation system based on a CO2 transcritical heat pump, which comprises a CO2 transcritical heat pump with a plurality of heat exchangers which are sequentially connected in series, and each stage of heat exchanger is connected with a flash steam generation circulation system. Working medium water of the flash steam generation circulating system exchanges heat with a high-temperature CO2 working medium generated by the CO2 transcritical heat pump through the heat exchanger to be heated, the heated working medium water enters the flash tank to be subjected to flash evaporation to form steam and water with the temperature and the pressure reduced, and the working medium water with the temperature and the pressure reduced returns to the heat exchanger to be subjected to heat exchange after being pressurized through the booster water pump. Irreversible loss caused by single-stage flash evaporation is effectively reduced, working medium water is subjected to phase change in sequence under different pressures, the average temperature of water in the whole heat absorption process is increased, the heat exchange temperature difference between water and CO2 is reduced, CO2 thermodynamic characteristics are gradually matched, # imgabs0 # loss caused by large heat exchange temperature difference is reduced, and the overall performance of the system is improved.

Description

Technical Field

[0001] The embodiments of the present invention relate to the technical field of CO2 heat pump steam generators, and more particularly to a multi-stage flash steam generation system based on a CO2 transcritical heat pump. Background Art

[0002] With increasing global energy demand and increasingly stringent environmental regulations, traditional energy consumption models face increasing challenges, particularly in industrial processes. Many traditional steam supply systems rely on coal- or gas-fired boilers, which generate heat by burning fossil fuels. This not only results in significant energy waste but also increases greenhouse gas emissions, causing environmental pollution and high operating costs. In the context of pursuing a low-carbon economy and sustainable development, the shortcomings of traditional boilers in terms of energy consumption and environmental protection are becoming increasingly apparent.

[0003] As China's carbon neutrality efforts continue to advance, end-use energy consumption will continue to shift toward electrification. Traditional heating methods, typically based on fossil fuel combustion, will gradually be phased out, and future heating will shift to electric-powered heat pumps. Driven by the goal of carbon neutrality, industrial production and domestic heating will rapidly shift from fossil fuel combustion to renewable electricity-powered heating technologies, making heat pump technology a key solution for this transition.

[0004] Utilizing heat pump circulation systems to generate steam has significant practical significance. As policies restricting traditional fossil energy sources become increasingly stringent, industrial heat generation urgently requires cleaner and more efficient alternatives. Against the backdrop of the growing trend toward electrification, heat pumps not only reduce reliance on fossil energy in traditional production processes but also, powered by renewable energy, efficiently supply high-temperature steam, meeting the production needs of related industries and driving the development of low-carbon, sustainable energy consumption in society.

[0005] At present, the common direct steam generation methods in the industrial field mainly include coal-fired, oil-fired, and gas-fired boilers. Although these methods are technically mature, they are exposing more and more limitations and disadvantages under the background of the "dual carbon" strategy. First of all, gas boilers rely on non-renewable energy. As natural gas resources become scarce, their prices continue to rise and the stability of gas supply decreases, and the operating costs of enterprises continue to rise; at the same time, gas boilers still have flue gas emission problems, which are not in line with current global environmental protection policies. Moreover, boilers are special equipment that involve flammable and explosive gases and have certain safety hazards. Users need to bear additional regulatory costs such as security inspections and insurance. More importantly, these traditional methods generally have large The overall efficiency of the system is low, which makes it difficult to meet the modern energy needs of energy conservation, carbon reduction and system integration optimization. Summary of the Invention

[0006] To this end, an embodiment of the present invention provides a multi-stage flash steam generation system based on a CO2 transcritical heat pump to solve the problems in the prior art.

[0007] In order to achieve the above objectives, the embodiments of the present invention provide the following technical solutions:

[0008] A multi-stage flash steam generation system based on a CO2 transcritical heat pump includes a CO2 transcritical heat pump having a plurality of heat exchangers connected in series. Each stage of the heat exchanger is connected to a flash steam generation circulation system. Working medium water in the flash steam generation circulation system passes through the heat exchanger and exchanges heat with the high-temperature CO2 working medium generated by the CO2 transcritical heat pump to increase its temperature. The heated working medium water flashes to form steam and water with reduced temperature and pressure. The working medium water is pressurized and then flows back to the heat exchanger for further heat exchange.

[0009] The working pressure of each stage of the steam generation cycle system is set according to the temperature of the CO2 working medium in the heat exchanger of that stage, so that the saturation temperature of water at that pressure is lower than and close to the exothermic temperature of the CO2 working medium in the heat exchanger of that stage.

[0010] As a preferred embodiment of the present invention, the flash steam generation circulation system comprises a flash tank and a booster water pump connected in sequence, wherein the inlet of the flash tank is connected to the outlet of the heat exchanger, the outlet of the booster water pump is connected to the inlet of the heat exchanger, and the inlet of the booster water pump is connected to the saturated water area of ​​the flash tank;

[0011] The steam outlet of the flash tank is connected to a back pressure valve, which is used to regulate the pressure of the flash tank outlet pipeline in the current flash steam generation cycle. The pressure of the flash tank inlet pipeline is regulated by the booster water pump.

[0012] As a preferred solution of the present invention, the outlet of the back pressure valve of each stage of the flash steam generation circulation system is connected to a steam compressor, which pressurizes the steam to form steam of uniform target temperature and then converges it for user use.

[0013] As a preferred embodiment of the present invention, the CO2 transcritical heat pump includes an evaporator, a compressor and an expansion valve, the outlet of the evaporator is connected to the inlet of the compressor, the outlet of the compressor is connected to the inlet of the first-stage heat exchanger, the outlet of the last-stage heat exchanger is connected to the inlet of the expansion valve, and the outlet of the expansion valve is connected to the inlet of the evaporator.

[0014] As a preferred solution of the present invention, the temperature of the high-temperature CO2 working fluid passing through the compressor is 50°C to 500°C.

[0015] As a preferred embodiment of the present invention, the saturation temperature of the working medium water in each stage of the flash steam generation circulation system is 2°C to 50°C different from the temperature of the CO2 working medium passing through the corresponding heat exchanger, and the smaller the temperature difference, the higher the overall efficiency of the system.

[0016] The present invention has the following advantages:

[0017] The multi-stage steam generating device provided by the present invention mainly selects CO2 as the heat exchange medium, has high pressure and temperature, can achieve large cooling and heating capacity, has small throttling loss and good heat transfer performance.

[0018] The whole system of the present invention adopts the cooperation of multi-stage heat exchange and multi-stage flash evaporation, so that the phase change temperature (saturation temperature) of water in each stage of heat exchange can be dynamically adjusted and gradually increased as the CO2 working medium continuously cools down in the heat exchanger, thereby achieving a high degree of matching with the characteristics of CO2 in temperature, thereby reducing the energy loss caused by large temperature difference and effectively reducing losses and improve the overall thermal efficiency of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for the embodiments or the description of the prior art. Obviously, the drawings described below are merely exemplary, and those skilled in the art can derive other implementation drawings based on the provided drawings without inventive effort.

[0020] The structures, proportions, sizes, etc. illustrated in this specification are intended only to complement the contents disclosed herein and to facilitate understanding and reading by persons familiar with the art. They are not intended to limit the conditions under which the present invention may be implemented and therefore have no substantive technical significance. Any structural modifications, changes in proportions, or adjustments in sizes, without affecting the efficacy and objectives of the present invention, shall still fall within the scope of the technical contents disclosed herein.

[0021] Figure 1 This is a composition diagram of the steam staged generation system based on the CO2 transcritical heat pump of the present invention;

[0022] Figure 2 This is the temperature entropy diagram of the steam staged generation system based on the CO2 transcritical heat pump of the present invention.

[0023] 1-Evaporator; 2-Compressor; 3-Heat exchanger; 4-Expansion valve; 5-User; 6-Flash tank; 7-Back pressure valve; 8-Steam compressor; 9-Booster pump; 10-Water supply pipe. DETAILED DESCRIPTION

[0024] The following describes the implementation of the present invention using specific embodiments. Those skilled in the art will readily understand the other advantages and benefits of the present invention from the disclosure herein. Obviously, the embodiments described are only a portion of the present invention, not all of it. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are intended to fall within the scope of protection of the present invention.

[0025] like Figure 1 As shown, the present invention provides a multi-stage flash steam generation system based on a CO2 transcritical heat pump, which mainly includes a CO2 transcritical heat pump having a plurality of heat exchangers 3 connected in series. Each stage of the heat exchanger 3 is connected to a flash steam generation circulation system. The working medium water of the flash steam generation circulation system passes through the heat exchanger 3 and exchanges heat with the high-temperature CO2 working medium generated by the CO2 transcritical heat pump to increase its temperature. The heated working medium water enters the flash tank 6 for flash evaporation to form steam and water with reduced temperature and pressure. The working medium water with reduced temperature and pressure is pressurized by the booster water pump 9 and then returns to the heat exchanger 3 to continue heat exchange.

[0026] In this embodiment, the CO2 transcritical heat pump generally includes an evaporator 1, a compressor 2 and an expansion valve 4. The outlet of the evaporator 1 is connected to the inlet of the compressor 2, the outlet of the compressor 2 is connected to the inlet of the first-stage heat exchanger 3, the outlet of the last-stage heat exchanger 3 is connected to the inlet of the expansion valve 4, and the outlet of the expansion valve 4 is connected to the inlet of the evaporator 1.

[0027] The temperature of the high-temperature CO2 working medium passing through the compressor 2 is 50°C to 500°C.

[0028] This embodiment first selects transcritical CO2 as the working fluid, which has high pressure and temperature, can achieve large cooling and heating capacity, has small throttling loss, and has good heat transfer performance. However, due to the high temperature of transcritical CO2, the temperature difference with the working fluid water is relatively large, which has a large Therefore, this embodiment designs a multi-stage heat exchange system for transcritical CO2 as the working fluid to reduce This reduces losses and improves the overall thermal efficiency of the system. (This is not only due to the high temperature of the CO2 working fluid, but also mainly due to the mismatch between the CO2 temperature and the water temperature: the specific heat of CO2 is constantly changing, and there is a large "mismatch" area between its cooling curve and the horizontal saturation temperature line of water. In a multi-stage flash evaporation system, with each stage of heat exchange, the phase change temperature (saturation temperature) of the water can be dynamically adjusted and gradually increased as the CO2 working fluid continuously cools in the heat exchanger, thereby achieving a high degree of temperature matching with the characteristics of CO2.)

[0029] The CO2 working medium passes through the heat exchanger step by step, and the temperature decreases step by step. The temperature of the working medium water after heat exchange decreases step by step synchronously. Therefore, the pressure (saturation temperature) in the flash steam generation circulation system of each stage is set to make the saturation temperature of the working medium water close to the optimal temperature of the CO2 working medium in the corresponding heat exchanger 3.

[0030] Specifically, the saturation temperature of the working medium water is preferably 2° C. to 50° C. lower than the CO 2 working medium in the corresponding heat exchanger 3 .

[0031] During the flash evaporation process, only part of the working water is converted into steam and discharged, and the remaining working water continues to circulate, exchange heat and flash to form steam.

[0032] The flash steam generation circulation system includes a flash tank 6 and a booster water pump 9 connected in sequence, the inlet of the flash tank 6 is connected to the outlet of the heat exchanger 3, the outlet of the booster water pump 9 is connected to the inlet of the heat exchanger 3, and the inlet of the booster water pump 9 is connected to the saturated water area of ​​the flash tank 6;

[0033] The steam outlet of flash tank 6 is connected to a back-pressure valve 7, which is used to regulate the pressure in the outlet pipeline of flash tank 6 during the current flash steam generation cycle. The pressure in the inlet pipeline of flash tank 6 is regulated by a booster water pump 9, and the parameters of each booster water pump 9 are set according to each flash steam generation cycle.

[0034] Each stage of the flash steam generation cycle has a different pressure (saturation temperature) and produces steam at different temperatures. To unify the steam produced at each stage to a target temperature for user use, a steam compressor 8 is connected to the outlet of the back-pressure valve 7 of each stage of the flash steam generation cycle. The steam compressor 8 pressurizes the steam to form steam at a uniform target pressure and then converges it for use by the user 5. The parameters of the steam compressor 8 at each stage are set according to the flash cycle at each stage.

[0035] The working medium water in the flash steam generation circulation system gradually decreases as steam is generated. Therefore, to ensure the circulation, a water supply pipe 10 is connected to the flash tank 6. When the working medium water in the flash steam generation circulation system is lower than the threshold, water is supplied through the water supply pipe 10.

[0036] The following combination Figure 1 , the heat exchange process of the system is described:

[0037] (1) CO2 heat pump cycle

[0038] The outlet of evaporator 1 is connected to the inlet of compressor 2. The low-temperature, low-pressure CO2 working medium absorbs heat in evaporator 1, becomes a gas, and is transported to compressor 2 via a pipeline. The outlet of compressor 2 is connected to the inlet of the first-stage heat exchanger 3. The inlets and outlets of all heat exchangers 3 are connected in sequence, allowing the compressed, high-temperature, high-pressure CO2 to be transported via pipelines to a multi-stage heat exchanger for heat exchange. The outlet of the last-stage heat exchanger 3 is connected to the inlet of expansion valve 4. After multiple stages of heat exchange, the high-pressure CO2 is transported to expansion valve 4 for throttling and pressure reduction. The outlet of expansion valve 4 is connected to the inlet of evaporator 1, and the low-temperature, low-pressure CO2 working medium returns to evaporator 1, completing the CO2 heat pump cycle.

[0039] The following is the CO2 heat pump cycle process:

[0040] Low-temperature, low-pressure CO2 enters compressor 2 from evaporator 1. The compressed, high-temperature, high-pressure CO2 passes through multi-stage heat exchanger 3, exchanging heat with the heat exchange medium (working water). The CO2 releases heat and condenses into a liquid. The condensed, high-pressure liquid CO2 enters expansion valve 4. After throttling and reducing the pressure, the low-temperature CO2 returns to evaporator 1, where it absorbs heat from air, geothermal water, or industrial waste heat, completing the CO2 transcritical cycle.

[0041] (2) Steam generation cycle

[0042] In the primary steam generation cycle, the steam generation side of heat exchanger 3 connects the outlet of heat exchanger 3 to the inlet of flash tank 6. After being heated by heat exchanger 3, the working medium water enters flash tank 6 for flash evaporation. The outlet of flash tank 6 is connected to the inlet of back pressure valve 7 via a pipeline. The outlet of steam compressor 8 is also connected to the outlets of other steam compressors 8 connected in parallel via a pipeline. The compressed steam is ultimately supplied to user 5 through the steam outlet.

[0043] The outlet of flash tank 6 is connected to the inlet of booster water pump 9. Some of the working water returns to primary heat exchanger 3 through booster water pump 9 to continue heat exchange. If the working water in the circulation is insufficient, water is replenished through the inlet of water replenishment pipe 10 connected to flash tank 6 (or between flash tank 6 and booster water pump 9).

[0044] The CO2 after the first stage of heat exchange is piped to the second stage of heat exchanger 3 for further heat exchange with the working water. The connection method is the same as the first stage of the steam generation cycle. The CO2 after the second stage of heat exchange is piped to the next stage of heat exchanger 3 until the CO2 temperature approaches the working water temperature. The connection method is the same for each stage of the cycle.

[0045] The steam generation cycle is as follows:

[0046] The compressed, high-temperature, high-pressure CO2 is first circulated with the primary steam for heat exchange. The circulating working water is heated to a certain temperature by the high-temperature CO2 through the primary heat exchanger 3. A portion of the heated working water enters the flash tank 6, where it is depressurized and evaporated to form steam. After passing through the back-pressure valve 7, which controls the pressure within the pipeline, the steam enters the steam compressor 8 for pressurization. This compression process can be achieved through single-stage supercharging or multi-stage supercharging with intermediate water injection. The resulting steam meets the final process requirements. A portion of the working water returns to the primary heat exchanger 3 via the booster pump 9 for further heat exchange. By the time the portion of working water returns to the primary heat exchanger 3 through the booster pump 9, its temperature will be higher than the initial temperature. For example, after the first heat exchange, the water temperature rises to 50°C. It is then throttled and pressurized to form steam, and the remaining water temperature rises to the saturation temperature at that pressure. Before returning to heat exchanger 3, this remaining water is pressurized by booster pump 9, reaching the saturation temperature at that pressure, typically above 50°C, before entering heat exchanger 3 for the next heat exchange. Therefore, the water temperature gradually increases with each stage of heat exchange. This gradual heating better matches the temperature of the water with that of the CO2, thereby optimizing the heat exchange process and system efficiency. If the working medium water in the cycle is insufficient to meet the flash evaporation process, water replenishment is required through replenishment pipe 10.

[0047] After the first stage of heat exchange, the CO2 enters the second stage heat exchanger 3, where it undergoes further heat exchange with the working water. The steam generation cycle repeats with each subsequent stage until the water temperature gradually approaches the CO2 working medium temperature. All steam compressors 8 are connected in parallel, and the resulting steam is discharged through the steam outlet.

[0048] For the primary steam generation cycle, the evaporation (phase change) of water is carried out at a fixed saturation temperature (determined by the flash tank pressure). When the high-temperature CO2 enters the heat exchanger 3, a large temperature difference is formed with the water, resulting in a significant Loss, at the same time, the specific heat of CO2 is constantly changing, and a large "mismatch" area is formed between its cooling curve and the single horizontal saturation temperature line of water. Therefore, this embodiment adopts the combination of multi-stage heat exchange and multi-stage flash evaporation, so that the phase change temperature (saturation temperature) of water in each stage of heat exchange can be dynamically adjusted and gradually increased as the CO2 working medium continuously cools down in the heat exchanger, thereby achieving a high degree of temperature matching with the characteristics of CO2, thereby reducing energy loss caused by large temperature differences and effectively reducing losses and improve the overall thermal efficiency of the system.

[0049] More specifically, in the multi-stage heat exchange and multi-stage flash evaporation cycle system of this system, the pressure of each stage of the steam generation cycle (specifically reflected in the pressure setting of the flash tank 6) is different, and the pressure setting of each stage is mainly controlled by the back pressure valve 7. The back pressure valve 7 is installed in the steam outlet pipeline of the flash tank 6 to maintain the set pressure in the flash tank 6. The setting of the saturation temperature (pressure) of each stage needs to be set according to the temperature of CO2 at the inlet of each stage of the heat exchanger 3. The goal is to make the heat release curve of CO2 as close as possible to the "multi-stage evaporation temperature line" of water, so that the total Minimize losses as much as possible.

[0050] The following combination Figure 2 The temperature-entropy curve shows that in each stage of heat exchange, the water temperature gradually increases and the entropy increases accordingly. Figure 2 The step-wise change curve of the working fluid water will be closer to the continuous cooling curve of CO2. The phase change temperature of water gradually approaches the temperature of CO2, reducing the energy loss caused by the temperature difference and thus achieving more efficient heat exchange.

[0051] Although the present invention has been described in detail above using general descriptions and specific embodiments, it will be apparent to those skilled in the art that modifications and improvements may be made thereto. Therefore, such modifications and improvements, without departing from the spirit of the present invention, are intended to be within the scope of protection claimed herein.

Claims

1. A multi-stage flash steam generation system based on a CO2 transcritical heat pump, characterized in that: A CO2 transcritical heat pump comprising a plurality of heat exchangers (3) connected in series, wherein each stage of the heat exchanger (3) is connected to a flash steam generation circulation system, wherein working medium water in the flash steam generation circulation system is heated by heat exchange with the high-temperature CO2 working medium generated by the CO2 transcritical heat pump through the heat exchanger (3), and the heated working medium water flashes to form steam and water with reduced temperature and pressure, and the working medium water is pressurized and then flows back to the heat exchanger (3) to continue heat exchange; The working pressure of each stage of the steam generation cycle system is set according to the temperature of the CO2 working medium in the heat exchanger (3) of that stage, so that the saturation temperature of water at that pressure is lower than and close to the exothermic temperature of the CO2 working medium in the heat exchanger (3) of that stage.

2. A multi-stage flash steam generation system based on a CO2 transcritical heat pump according to claim 1, characterized in that: The flash steam generation circulation system comprises a flash tank (6) and a booster water pump (9) connected in sequence, the inlet of the flash tank (6) being connected to the outlet of the heat exchanger (3), the outlet of the booster water pump (9) being connected to the inlet of the heat exchanger (3), and the inlet of the booster water pump (9) being connected to the saturated water zone of the flash tank (6); The steam outlet of the flash tank (6) is connected to a back pressure valve (7), and the back pressure valve (7) is used to regulate the pressure of the outlet pipeline of the flash tank (6) in the current flash steam generation cycle; the pressure of the inlet pipeline of the flash tank (6) is regulated by the booster water pump (9).

3. The multi-stage flash steam generation system based on a CO2 transcritical heat pump according to claim 2, characterized in that: The outlet of the back pressure valve (7) of each stage of the flash steam generation circulation system is connected to a steam compressor (8), and the steam compressor (8) pressurizes the steam to form steam of uniform target pressure and then converges it for use by the user (5).

4. The multi-stage flash steam generation system based on a CO2 transcritical heat pump according to claim 2, characterized in that: The flash tank (6) is connected to a water supply pipe (10), and the water supply pipe (10) is connected to a water storage tank via another booster water pump. When the working medium water in the flash steam generation circulation system is lower than a threshold, water is supplied via the water supply pipe (10).

5. The multi-stage flash steam generation system based on a CO2 transcritical heat pump according to claim 1, characterized in that: The CO2 transcritical heat pump comprises an evaporator (1), a compressor (2) and an expansion valve (4), wherein the outlet of the evaporator (1) is connected to the inlet of the compressor (2), the outlet of the compressor (2) is connected to the inlet of the first-stage heat exchanger (3), the outlet of the last-stage heat exchanger (3) is connected to the inlet of the expansion valve (4), and the outlet of the expansion valve (4) is connected to the inlet of the evaporator (1).

6. A multi-stage flash steam generation system based on a CO2 transcritical heat pump according to claim 5, characterized in that: The temperature of the high-temperature CO2 working medium passing through the compressor (2) is 50°C to 500°C.

7. The multi-stage flash steam generation system based on a CO2 transcritical heat pump according to claim 1, characterized in that: The saturation temperature of the working medium water in each stage of the flash steam generation circulation system differs from the temperature of the CO2 working medium outlet passing through the corresponding heat exchanger (3) by 2°C to 50°C.

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