Distributed combined cooling heating and power system based on series screw expander structure

Abstract

The utility model discloses a distributed combined cooling heating and power system based on a series screw expander structure, and belongs to the field of solar thermal power generation. The system comprises a water vapor Rankine cycle loop, a hot water heating cycle loop and a lithium bromide absorption refrigeration cycle loop, and a first screw expander and a second screw expander are arranged in series. By controlling switching of the three three-way valves and starting and stopping of the two screw expanders, the system can be flexibly switched to three working modes of single power generation, combined heat and power supply or combined cooling and power supply. In a single power generation mode, the two expansion machines are connected in series for operation, and the generating capacity is maximized; in the heating or cooling mode, only the first expansion machine operates, and heating or cooling circulation is driven through high-grade exhaust waste heat of the first expansion machine. According to the utility model, the problem of waste of exhaust waste heat of a single screw expander is effectively solved, and gradient utilization and on-demand flexible distribution of solar heat energy are realized.

Description

TECHNICAL FIELD

[0001] The utility model belongs to solar thermal power generation and cold heat and power cogeneration technical field, specifically related to a kind of distributed cold heat and power cogeneration system using series screw expander structure. BACKGROUND

[0002] Cold heat and power cogeneration system is a kind of system using cascade energy, can provide three kinds of energy of cold, heat and electricity simultaneously.It has received extensive attention in the past decade due to its potential in reducing greenhouse gas emissions and improving overall thermal efficiency.In many technical paths, the initial investment of fuel cell driven system is high and the technology is less mature, while internal combustion engine and gas turbine system has inherent defects such as high dependence on fossil fuels, large amount of combustion pollutant emissions and inflexible thermal power ratio adjustment.Compared with the above, the system based on organic Rankine cycle or water vapor Rankine cycle shows high technical reliability, good operation flexibility and suitability for distributed energy supply, etc., and becomes a more promising choice.The driving equipment of Rankine cycle system mainly includes speed type turbine and volumetric expander represented by screw expander.Although the peak efficiency of single screw expander may be lower than that of turbine, its series of inherent characteristics make it have advantages in distributed, especially off-grid type cold heat and power cogeneration applications.The market power range of screw expander is extensive, from several kilowatts to several megawatts, which can well match the needs of small and medium-sized distributed power generation, which is different from the characteristics that turbine usually needs to be economical at high power.The limited import and export pressure difference makes the exhaust steam of screw expander maintain relatively high temperature and pressure, unlike turbine pursuing the highest isentropic efficiency and maximizing power generation, screw expander can actively sacrifice part of power generation to obtain higher quality exhaust heat, so as to provide heat source for subsequent heating or refrigeration cycle.When the actual operation pressure ratio reaches three times of built-in pressure ratio, the isentropic efficiency of screw expander only decreases by 10%.In addition, the excellent variable working condition operation characteristics and the resistance of screw expander to the fluctuation of working medium state enhance the adaptability of system to seasonal and time variation of energy demand.

[0003] However, when the screw expander is applied to a combined cooling heating and power system, there is still a key problem. In a single power generation mode, due to the lower built-in specific volume ratio of the screw expander itself, the outlet steam still carries a large amount of available heat energy (i.e. higher enthalpy), which will cause significant high-grade energy waste if directly discharged to the condenser. In the prior art, although there is a scheme of using multiple screw expanders for waste heat recovery, the core requirement of how to realize flexible and efficient switching among the three modes of power generation, heat supply and cooling in a combined cooling heating and power system with serial screw expanders has not been designed. Specifically, how to dynamically adjust the system operation according to user requirements, realize switching of the operation mode under the premise of ensuring energy cascade utilization, and overcome the problem of waste of exhaust heat of the screw expander in a single power generation mode, which has not been fully solved in the prior art.

[0004] Therefore, it is of great significance to develop a combined cooling heating and power system with serial screw expanders which can integrate energy cascade utilization and flexible switching of operation modes, for improving the efficiency of solar energy utilization and the economy of system operation. Content of the utility model

[0005] The technical problem to be solved by the utility model is to overcome the problem of excessive exhaust heat in a single power generation mode and waste of high-grade energy in the prior art combined cooling heating and power system which only uses one screw expander.

[0006] To achieve the above object, the utility model adopts the following technical scheme.

[0007] The utility model provides a kind of distributed combined cooling heating and power system based on the structure of serial screw expander, including first loop (water vapor rankine cycle loop), second loop (hot water heating cycle loop) and third loop (lithium bromide absorption refrigeration cycle loop).

[0008] The first loop includes solar field, high-temperature storage tank, first screw expander, first three-way valve, second screw expander and the inlet of mixed heat exchanger connected in series by pipeline.

[0009] The second loop flows into heat user from second three-way valve and returns mixed heat exchanger by second working medium pump by switching the third three-way valve.

[0010] The third circuit is connected to the generator from the mixed heat exchanger by switching the second three-way valve. The outlet of the generator is divided into three paths: the first path is connected to the condenser, the first throttling valve, the evaporator and the absorber in sequence; the second path is connected to the second working medium pump; and the third path is connected to the solution heat exchanger, the second throttling valve and the concentrated solution inlet of the absorber in sequence. The dilute solution outlet of the absorber returns to the generator through the solution heat exchanger and the third working medium pump.

[0011] The system can be switched between the single power generation mode, the combined heat and power mode and the combined cooling and power mode by controlling the first three-way valve, the second three-way valve, the third three-way valve and the start-stop of the first screw expander and the second screw expander.

[0012] In the single power generation mode, the first three-way valve guides the fluid from the outlet of the first screw expander to the inlet of the second screw expander, and the second three-way valve guides the fluid to the cooling tower, so that the two screw expanders are connected in series to realize maximum power generation. In the combined heat and power mode, the first three-way valve guides the fluid from the outlet of the first screw expander to the mixed heat exchanger, and the third three-way valve guides the fluid to the heat user, so that only the first screw expander operates and the exhaust heat is used for heating. In the combined cooling and power mode, the fluid from the outlet of the first screw expander flows into the mixed heat exchanger through the first three-way valve, and the second three-way valve guides the fluid to drive the lithium bromide refrigeration cycle of the generator, so that only the first screw expander operates and the exhaust heat is used for cooling.

[0013] The first screw expander can be connected to the generator through the first gearbox, and the second screw expander can be connected to the same generator through the second gearbox, and the transmission ratios of the two gearboxes can be independently adjusted to optimize the respective operating conditions.

[0014] The series connection of the two screw expanders fundamentally solves the problem of high exhaust heat grade and direct condensation of the single screw expander in the single power generation mode, which leads to energy waste. The system can be flexibly and reliably switched between the single power generation, the combined heat and power and the combined cooling and power modes by controlling the valves and the start-stop of the expanders, so as to accurately match the energy demand of different seasons and users. In addition, the system also fully utilizes the excellent performance of the screw expander under variable operating conditions, significantly enhances the adaptability and operating stability of the system in response to the fluctuation of the input fluid state and the change of the load, and finally realizes the step-by-step utilization and flexible distribution of the heat source energy, so that the system can maintain high energy utilization efficiency in the three modes, thereby improving the overall thermal efficiency and economy of the system throughout the year. BRIEF DESCRIPTION OF DRAWINGS

[0015] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed in the following embodiment or prior art description will be briefly introduced.

[0016] Figure 1 is the whole structure schematic diagram of the distributed combined cooling heating and power system based on the series screw expander structure provided by the embodiment of the utility model;

[0017] Figure 2 is the working medium flow direction and working principle schematic diagram of the system in single power generation mode provided by the embodiment of the utility model;

[0018] Figure 3 is the working medium flow direction and working principle schematic diagram of the system in combined cooling heating and power mode provided by the embodiment of the utility model;

[0019] Figure 4 is the working medium flow direction and working principle schematic diagram of the system in combined cooling and power mode provided by the embodiment of the utility model;

[0020] Figure 5 is the first connecting structure schematic diagram of the series screw expander provided by the embodiment of the utility model;

[0021] Figure 6 is the second connecting structure schematic diagram of the series screw expander provided by the embodiment of the utility model;

[0022] Figure 7 is the third connecting structure schematic diagram of the series screw expander provided by the embodiment of the utility model;

[0023] In the drawing: 1, solar heat collection field; 2, high temperature heat storage tank; 3, first screw expander; 4, first gearbox; 5, generator; 6, first three-way valve; 7, second screw expander; 8, second gearbox; 9, mixed heat exchanger; 10, second three-way valve; 11, third three-way valve; 12, cooling tower; 13, second working medium pump; 14, first working medium pump; 15, heat user; 16, generator; 17, condenser; 18, first throttling valve; 19, evaporator; 20, cold user; 21, absorber; 22, solution heat exchanger; 23, third working medium pump; 24, second throttling valve; 25, fourth throttling valve. DETAILED DESCRIPTION

[0024] As Figure 1 shown, the distributed combined cooling heating and power system based on the series screw expander structure provided by the embodiment, mainly by three core cycle coupling, specifically can include the water vapor Rankine cycle loop for power conversion, the hot water heating cycle loop for satisfying the winter or low temperature season heating demand, and the lithium bromide absorption refrigeration cycle loop for satisfying the summer or high temperature environment refrigeration demand. The system collects heat energy through the solar heat collection field 1, and stores heat energy by using the high temperature heat storage tank 2, so as to maintain the stability of the heat source working medium (usually water / steam).

[0025] like Figure 2 As shown, in single power generation mode, the system mainly relies on a steam Rankine cycle to maximize power output. In this cycle, water is the working fluid, undergoing a phase change cycle between liquid water and steam. Specifically, liquid water absorbs heat and evaporates into high-temperature, high-pressure steam in the solar collector field 1, which then enters the high-temperature heat storage tank 2 via pipeline. The outlet of the high-temperature heat storage tank 2 is connected to the inlet of the first screw expander 3, which, as a high-pressure stage expansion component, converts the thermal energy of the high-temperature, high-pressure steam into mechanical energy. The exhaust port of the first screw expander 3 is connected via pipeline to the upper side of the common inlet of the first three-way valve 6. In single power generation mode, the left side of the first outlet of the first three-way valve 6 is connected to the inlet of the second screw expander 7. At this time, the medium-pressure steam discharged from the first screw expander 3 enters the second screw expander 7 for secondary expansion, fully utilizing the pressure difference. The exhaust port of the second screw expander 7 is connected to the steam inlet of the mixing heat exchanger 9. The working fluid (gas or gas-liquid two-phase state) entering the mixing heat exchanger 9 is collected here, and its outlet is connected to the inlet of the second three-way valve 10. One outlet of the second three-way valve 10 is connected to the upper side of the third three-way valve 11. The third three-way valve 11 is used to switch the flow direction, and in single power generation mode, it is connected to the cooling tower 12. The working fluid is completely condensed into liquid water in the cooling tower 12, releasing waste heat. The outlet of the cooling tower 12 is connected to the inlet of the second working fluid pump 13. The second working fluid pump 13 transports the liquid water back to the mixing heat exchanger 9, and the working fluid outlet of the mixing heat exchanger 9 is connected to the first working fluid pump 14. Finally, after being pressurized by the first working fluid pump 14, the liquid water returns to the solar collector field 1, completing the cycle.

[0026] In this mode, two screw expanders are set to operate in series, and the internal specific volume ratio of both can be configured to 5 (i.e., a 5&5 combination). At this point, the inlet and outlet pressure ratio and isentropic efficiency of the first screw expander are 17.92 and 66.37%, respectively, while those of the second screw expander are 21.97 and 64.17%, respectively. When the temperature at the solar collector outlet state point a reaches 250℃, the optimized maximum thermal efficiency of the system can reach 21.59%. The thermodynamic parameters of each key state point in the steam Rankine cycle loop are shown in Table 1. Table 1 shows that the temperature at state points a and b is 250℃, the pressure is 3.98 MPa, and the enthalpy is 2800.93 kJ / kg. After two stages of expansion, the exhaust pressure drops to 0.01 MPa (state point e), at which point the temperature is lower (only 46℃), and the enthalpy drops to 2205.52 kJ / kg. This portion of low-grade heat is discharged through cooling tower 12, while the system utilizes the large enthalpy drop (from 2800.93 kJ / kg to 2205.52 kJ / kg) to generate electricity.

[0027] Table 1. State parameters at various points when thermal efficiency is maximized under a single power generation mode.

[0028]

[0029] As Figure 3 shown, in the heat and power cogeneration mode, the system mainly operates the water vapor Rankine cycle loop and the hot water heating cycle loop, which is suitable for scenarios requiring heating such as winter. In this mode, the working medium is water, and its circulation process starts from the solar heat collection field 1, where the liquid water absorbs solar radiation heat energy and changes into high-temperature and high-pressure water vapor. According to the data shown in Table 2 below, the high-temperature steam (state point a) flowing out from the solar heat collection field 1 and stored in the high-temperature heat storage tank 2 has a temperature of 192°C, a pressure of 1.31 MPa, an enthalpy value of 3204.74 kJ / kg, and a dryness of 100%. The high-temperature and high-pressure steam then enters the first screw expander 3 for one-stage expansion to do work, converting heat energy into mechanical energy to drive the generator 5 to generate electricity. The expanded steam (state point f) has a pressure of 0.05 MPa and a temperature of 80°C, and at this time, the working medium changes to wet steam with a dryness of 91.13% and an enthalpy value of 2438.24 kJ / kg, still retaining a large amount of high-grade latent heat of vaporization. Subsequently, by controlling the first three-way valve 6 to switch to the lower side, the wet steam is not introduced into the second screw expander 7, but is directly guided to flow into the mixed heat exchanger 9. The wet steam flowing out from the mixed heat exchanger 9 flows through the upper side of the second three-way valve 10 and the upper side of the third three-way valve 11 in turn, and finally flows into the heat user 15. In the heat user 15, the wet steam releases the latent heat of vaporization for heating, and the working medium itself changes phase to condense into liquid, at which time the temperature is maintained at 80°C, but the enthalpy value is greatly reduced to 753.01 kJ / kg. The liquid working medium after heat release then flows out of the heat user 15 and enters the second working medium pump 13. Under the action of the second working medium pump 13, the liquid water is transported back to the mixed heat exchanger 9, and then enters the first working medium pump 14 for pressurization, and finally reflows into the solar heat collection field 1 to complete the cycle. In this working condition, the pressure ratio and isentropic efficiency of the first screw expander 3 are 27.65 and 61.74% respectively, the thermal efficiency of the system is 12.79%, and the heat and power cogeneration efficiency reaches 99.13%.

[0030] Table 2 State parameters of each point when the thermal efficiency is maximum in the heat and power cogeneration mode

[0031]

[0032] As Figure 4As shown, in the combined cooling and power mode, the system synchronously operates the water vapor Rankine cycle loop and the lithium bromide absorption refrigeration cycle loop, which is suitable for summer cooling scenarios. The working fluid flow in this mode contains the water vapor flow path in the Rankine cycle side and the lithium bromide flow path and the water working fluid flow path in the refrigeration side. First, in the Rankine cycle side, the liquid water is heated to high-temperature and high-pressure steam in the solar heat collection field 1. According to the data in Table 3 below, the temperature of the steam (state point a) entering the first screw expander 3 is 250°C, and the pressure is 3.98 MPa. After the steam does work in the first screw expander 3, the temperature of the steam (state point f) discharged is 80°C, the pressure is 0.05 MPa, and the dryness is 91.13%. The steam enters the mixed heat exchanger 9 through the first three-way valve 6, and then, by switching to the second three-way valve 10 on the right side, the steam is guided to flow into the heat source channel of the generator 16. Inside the generator 16, the steam releases heat to the lithium bromide solution and condenses into liquid water, which then flows out of the generator 16 outlet, is pumped back to the solar heat collection field 1 in sequence through the second working fluid pump 13, the mixed heat exchanger 9, and the first working fluid pump 14, and completes the cycle.

[0033] Table 3 State parameters of the water vapor Rankine cycle loop in the combined cooling and power mode when the thermal efficiency is maximum

[0034]

[0035] At the same time, as shown in Table 4 below, inside the generator 16 and in the refrigeration cycle side, the lithium bromide solution boils after absorbing heat, separating water vapor (state point q, temperature 94.75°C) and concentrated lithium bromide solution (state point u, concentration 63.5%). The water vapor flows into the condenser 17 and condenses into liquid (state point r), and then enters the evaporator 19 after being throttled and reduced in pressure by the first throttling valve 18. In the evaporator 19, the water vapor evaporates by absorbing heat from the cold user 20, achieving external cooling supply. The evaporated water vapor enters the absorber 21. On the other hand, the concentrated lithium bromide solution (state point u) generated by the generator 16 flows through the solution heat exchanger 22 and the second throttling valve 24 to be cooled to state point w (48.5°C), and then enters the absorber 21. In the absorber 21, the concentrated lithium bromide solution absorbs water vapor from the evaporator 19 to recover into a dilute solution. The dilute lithium bromide solution is then preheated by the solution heat exchanger 22 and pressurized by the third working fluid pump 23 to flow back to the generator 16 to restart the cycle. Under this working condition, the maximum thermal efficiency of the system is 13.64%, the lithium bromide refrigeration cycle COP is 0.713, and the combined cooling and power efficiency of the system reaches 88.64%. At this time, the inlet and outlet pressure ratio and the isentropic efficiency of the first screw expander 3 are 20.01 and 65.17%, respectively.

[0036] Table 4 State parameters of the lithium bromide absorption refrigeration cycle loop in the combined cooling and power mode when the thermal efficiency is maximum

[0037]

[0038] Further, regarding the mechanical connection and driving mode of the screw expander and the generator, the embodiment provides three optional structural forms in combination with the drawings to meet different power output requirements.

[0039] As shown in the first structure: Figure 5 The first screw expander 3 and the second screw expander 7 are independently driven by double gearboxes. Specifically, the first screw expander 3 is connected to one end shaft extension of the generator 5 through the first gearbox 4, and the second screw expander 7 is connected to the other end shaft extension of the generator 5 through the second gearbox 8. In this structure, the two expanders are mechanically coupled through their respective shaft couplings and gearboxes. The core advantage is that the controller can independently adjust the transmission ratio of the first gearbox 4 and the second gearbox 8 according to the inlet and outlet pressure difference in single power generation mode, ensuring that the first screw expander 3 and the second screw expander 7 can work within their respective optimal speed ranges, preventing overexpansion and underexpansion, and ensuring that each expansion screw operates at the highest efficiency.

[0040] As shown in the second structure: Figure 6 The system adopts a single gearbox coaxial driving arrangement to realize the working mode of one expander running and the other expander standby. In this structure, the first three-way valve 6 is controlled to switch the working fluid path, and a disconnectable structure (such as a clutch / mechanical decoupling component) is provided at the shafting between the two screw expanders, so that the two-stage expanders can be decoupled when needed. When the system only needs to work by the first screw expander 3, the shafting is disconnected, the second screw expander 7 remains standby (or idles without doing work), and only the first screw expander 3 drives the generator 5 through the single gearbox to output power; when two-stage series power generation is needed, the shafting is re-engaged to realize the common work of the two machines. Since this structure operates under the condition of single gearbox synchronous speed, to ensure that the screw expander has sufficient carrying capacity and matching under synchronous speed, the rotor of the low-pressure screw expander in the two is usually designed to be larger in size, making the rotor and bearing structure more robust to withstand greater torque.

[0041] As shown in the third structure: Figure 7 The system also adopts a single gearbox coaxial driving arrangement, and the working mode can be adjusted to only the first screw expander 3 running and the second screw expander 7 standby. In this structure, the working fluid path is still selected by controlling the first three-way valve 6 and the newly added fourth throttle valve 25, and the shafting between the two screw expanders is disconnected to mechanically decouple the two expanders. When the system needs to work only by the first screw expander 3, the shafting is in a disconnected state, the second screw expander 7 is standby, the working fluid enters the first screw expander 3 after being switched by the valve, and the first screw expander 3 drives the generator 5 through the single gearbox to output electric energy. Compared with the second structure, the third structure has the advantages ofFigure 6 Under the premise of single gearbox synchronous speed design, the rotors of the low-pressure screw expander in both structures are usually of larger size and are supported by higher strength bearings to improve the torque-carrying capacity and operation reliability.

[0042] In summary, the distributed combined cooling, heating and power system provided by the embodiment successfully solves the problems of excessive reduction of isentropic efficiency and waste of exhaust heat under variable working conditions of a single expander through the innovative series connection structure of double screw expanders and multi-loop switching. Data show that the system realizes high thermal efficiency through energy cascade utilization in single power generation mode, and has excellent thermal performance and economic benefits.

Claims

1. A distributed combined cooling, heating and power system based on a structure of series screw expander, characterized in that, The system comprises a first circuit, a second circuit and a third circuit. The first circuit comprises a solar field (1), a high-temperature heat storage tank (2), a first screw expander (3), a first three-way valve (6), a second screw expander (7) and an inlet of a hybrid heat exchanger (9) connected in series by pipelines, and an outlet of the hybrid heat exchanger (9) connected to a second working medium pump (13) through a second three-way valve (10), a third three-way valve (11) and a cooling tower (12) in sequence, and then returned to the hybrid heat exchanger (9), and the hybrid heat exchanger (9) is provided with a flow path returned to the solar field (1) through a first working medium pump (14); The second circuit flows the fluid from the second three-way valve (10) into a heat user (15) through switching the third three-way valve (11), and then returns to the hybrid heat exchanger (9) through the second working medium pump (13); The third circuit flows the fluid from the hybrid heat exchanger (9) into a generator (16) through switching the second three-way valve (10); an outlet of the generator (16) is connected to a condenser (17), a first throttling valve (18), an evaporator (19) and an absorber (21) in sequence; a second path is connected to the second working medium pump (13); a third path is connected to a solution heat exchanger (22), a second throttling valve (24) and an inlet of a concentrated solution of the absorber (21) in sequence; a dilute solution outlet of the absorber (21) is returned to the generator (16) through the solution heat exchanger (22) and a third working medium pump (23); The system is switched between a single power generation mode, a combined heat and power mode and a combined cooling and power mode by controlling the first three-way valve (6), the second three-way valve (10) and the third three-way valve (11), and starting and stopping of the first screw expander (3) and the second screw expander (7).

2. The distributed combined cooling heating and power system based on the structure of the series screw expander machine according to claim 1, characterized in that: In the single power generation mode, the first three-way valve (6) flows the outlet of the first screw expander (3) into the inlet of the second screw expander (7), the second three-way valve (10) flows the outlet of the hybrid heat exchanger (9) into the third three-way valve (11), the third three-way valve (11) flows the fluid into the cooling tower (12), and the first screw expander (3) and the second screw expander (7) both operate.

3. The distributed combined cooling heating and power system based on the structure of the series screw expander machine according to claim 1, characterized in that: In the combined heat and power mode, the first three-way valve (6) flows the outlet of the first screw expander (3) into the inlet of the hybrid heat exchanger (9), the second three-way valve (10) flows the outlet of the hybrid heat exchanger (9) into the third three-way valve (11), the third three-way valve (11) flows the fluid into the heat user (15), and only the first screw expander (3) operates.

4. The distributed combined cooling heating and power system based on the structure of the series screw expander machine according to claim 1, characterized in that: In the combined cooling and power mode, the first three-way valve (6) flows the outlet of the first screw expander (3) into the inlet of the hybrid heat exchanger (9), the second three-way valve (10) flows the outlet of the hybrid heat exchanger (9) into the generator (16), the generator (16) drives a lithium bromide refrigeration cycle to operate as a cooling user (20) for cooling by using heat of the flowing fluid, and only the first screw expander (3) operates.

5. The distributed combined cooling heating and power system based on the structure of the series screw expander machine according to claim 1, characterized in that: The first screw expander (3) is connected to a generator (5) through a first gearbox (4), the second screw expander (7) is connected to the generator (5) through a second gearbox (8), and the transmission ratios of the first gearbox (4) and the second gearbox (8) can be independently adjusted.

6. The distributed combined cooling heating and power system based on the structure of the series screw expander machine according to claim 5, characterized in that: The first screw expander (3) and the second screw expander (7) are respectively connected to the first gearbox (4) and the second gearbox (8) through couplings.

7. The distributed combined cooling heating and power system based on series screw expander structure according to claim 1, characterized in that: The heat user (15) and the cooling tower (12) are arranged in parallel on a flow path downstream of the third three-way valve (11).

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