CN104696029A - Organic Rankine cycle system and method for switching operation modes thereof - Google Patents

Abstract

An organic Rankine cycle system and a method for switching an operation mode thereof. The switching method comprises the step of judging whether an actual pressure value of a working fluid of the organic Rankine cycle system before entering the inlet of the expander is larger than or equal to a critical pressure value of the working fluid. If yes, the organic Rankine cycle system is operated in a transcritical operation mode. If not, the organic Rankine cycle system is operated in the primary critical operation mode.

Description

Organic Rankine Cycle System and Its Operation Mode Switching Method

technical field

The present invention relates to an organic Rankine cycle system, in particular to an organic Rankine cycle system and a switching method between a subcritical operation mode and a transcritical operation mode.

Background technique

Organic Rankine Cycle (ORC) uses organic substances with a low boiling point at normal pressure as the working fluid, which is the most efficient and economical among current medium and low temperature thermal power generation technologies. The organic Rankine cycle is divided into a sub-critical cycle system or a transcritical cycle system according to the internal circulating working fluid operating below or above the critical point. Generally speaking, the transcritical cycle system has higher system thermal efficiency and heat extraction rate of heat source, so it generates more electricity under the same cold and heat source conditions.

The heat sources applied to transcritical organic Rankine cycle systems are diverse and wide-ranging, including variable heat sources such as medium and low temperature waste heat, geothermal heat, hot springs, and solar thermal energy. However, since the temperature of the above-mentioned variable heat source and the supply amount per unit time have periodic, intermittent or irregular variations, if the transcritical ORC faces the above-mentioned variable heat source, it may be caused by the heat source The condition of the organic rankine cycle changes beyond the operation condition of the transcritical organic rankine cycle, which leads to the shutdown of the organic rankine cycle system, thereby limiting the utilization rate of the organic rankine cycle system. In this way, the cumulative power generation of the organic Rankine cycle system will be reduced. Therefore, how to increase the power generation of the organic Rankine cycle system in the face of variable heat sources, thereby improving its economic benefits, will be one of the problems that researchers should solve one.

Contents of the invention

The present invention provides an organic Rankine cycle system and a method for switching between a subcritical operation mode and a transcritical operation mode, so as to improve the operable range of the organic Rankine cycle system when facing a variable heat source.

In order to achieve the above object, the present invention provides a method for switching the operating mode of an organic Rankine cycle system, comprising the following steps: judging whether an actual pressure value of a working fluid of an organic Rankine cycle system before entering an expander inlet is greater than Equal to a critical pressure value of the working fluid: if yes, make the organic Rankine cycle system operate in a critical operation mode. If not, the organic Rankine cycle system is operated in a primary critical operation mode.

In the method for switching the operation mode of the above-mentioned organic Rankine cycle system, the operation process of the transcritical operation mode includes the following steps: searching a supercritical working fluid pressure and temperature database to obtain an operating temperature range corresponding to the actual pressure value and judging the relationship between an actual temperature value before the working fluid enters the expander inlet and the working temperature range: if the actual temperature value is lower than the lower limit of the working temperature range, then lower a driving force of the working fluid An output frequency value or an output flow value of the pump; if the actual temperature value falls within the working temperature range, maintain the output frequency value or the output flow value of the pump; and if the actual temperature value is higher than the working temperature If the upper limit of the temperature range is not reached, then increase the output frequency value or the output flow value of the pump.

The method for switching the operation mode of the above-mentioned organic Rankine cycle system, wherein if the actual temperature value is lower than the lower limit value of the working temperature range, the step of lowering the output frequency value or the output flow value of the pump , further comprising switching the operation mode of the organic Rankine cycle system from the transcritical operation mode to the subcritical operation mode if the actual pressure value before entering the expander inlet is lower than the critical pressure value.

The above-mentioned method for switching the operation mode of the organic Rankine cycle system, wherein in the operation process of the transcritical operation mode, if the actual temperature value is higher than the upper limit value of the working temperature range, the output of the pump is increased After the step of determining the frequency value or the output flow value, it also includes that if the actual pressure value before entering the expander inlet is higher than the allowable maximum pressure value of the organic Rankine cycle system, then the organic Rankine cycle system is stopped. .

In the method for switching the operation mode of the above-mentioned organic Rankine cycle system, in the operation process of the transcritical operation mode, the supercritical working fluid pressure temperature database has multiple sets of pressure and working temperature range data, and each set of pressure and working temperature Range data has a pressure value and a corresponding operating temperature range.

In the above-mentioned method for switching the operation mode of the ORC system, the operation process of the subcritical operation mode includes the following steps: calculating an actual superheat value of the working fluid according to the actual pressure value and the actual temperature value; and Judging the relationship between the actual superheat value before the working fluid enters the expander inlet and a preset superheat range: if the actual superheat value is less than the lower limit of the preset superheat range, lower the pump’s The output frequency value or the output flow value; if the actual superheat value falls within the preset superheat range, maintain the output frequency value or the output flow value for driving the pump; and if the actual superheat value is greater than The upper limit value of the preset superheat range increases the output frequency value or the output flow value of the pump.

The above-mentioned method for switching the operation mode of the organic Rankine cycle system, wherein in the operation process of the subcritical operation mode, if the actual superheat value is less than the lower limit value of the preset superheat range, the pump is lowered After the step of the output frequency value or the output flow value, if the actual pressure value before entering the expander inlet is lower than the allowable minimum pressure value of the organic Rankine cycle system, making the organic Rankine cycle The system stops functioning.

In the method for switching the operation mode of the above-mentioned organic Rankine cycle system, if the actual superheat value is greater than the upper limit value of the preset superheat range, the output frequency value or the output flow value of the pump is increased After the step, if the actual pressure value before entering the expander inlet is higher than the critical pressure value, switching the operation mode of the organic Rankine cycle system from the subcritical operation mode to the transcritical operation mode.

The method for switching the operation mode of the above-mentioned organic Rankine cycle system, wherein the pump is a variable flow pump or a constant flow pump with a variable frequency motor.

The method for switching the operation mode of the above-mentioned organic Rankine cycle system, wherein the expander is a turbine, a screw expander, a scroll expander, a positive displacement expander or a reciprocating expander.

The method for switching the operation mode of the above-mentioned organic Rankine cycle system, wherein when the temperature or flow rate of a heat source or a cold source supplied to the organic Rankine cycle system changes, if the organic Rankine cycle system is in the transcritical operation mode , then execute the step of judging whether an actual temperature value of the working fluid before entering the expander inlet falls within a working temperature range; A step of whether an actual superheat value before entering the expander inlet falls within a preset superheat range.

To achieve the above object, the present invention provides an organic Rankine cycle system, which includes a heat source heat exchanger, an expander, a condenser, a pump, a pressure sensor and a control element. The expander is connected with the heat source heat exchanger through pipelines. The condenser is connected to the expander through pipelines. The pump is connected with the condenser and the heat source heat exchanger through pipelines. The pump is used to drive a working fluid to flow sequentially through the heat source heat exchanger, expander and condenser to form an organic Rankine cycle. The working fluid has a critical pressure value. The pressure sensor is used to detect an actual pressure value of the working fluid at the inlet of the expander. The control element is used to judge the relationship between the actual pressure value of the working fluid and the critical pressure value. If the actual pressure value of the working fluid is greater than the critical pressure value, the control element controls the organic Rankine cycle to operate in a transcritical operation mode. If the actual pressure value of the working fluid is less than the critical pressure value, the control element controls the organic Rankine cycle to operate in the primary critical operation mode.

The above-mentioned organic Rankine cycle system also includes a temperature sensor, which is used to detect an actual temperature value of the working fluid before entering the inlet of the expander, and the control element stores a supercritical working fluid pressure temperature database, and the supercritical The working fluid pressure temperature database has multiple sets of pressure and working temperature range data, and each set of pressure and working temperature range data has a pressure value and a corresponding working temperature range. When the organic Rankine cycle system is in the transcritical operation mode , if the actual temperature value is greater than the upper limit value of the working temperature range, the control element increases an output frequency value or an output flow value of the pump; if the actual temperature value is less than the lower limit value of the working temperature range, the The control element reduces the output frequency value or the output flow value of the pump.

The above-mentioned organic Rankine cycle system, wherein in the transcritical operation mode, if the actual pressure value is greater than the allowable maximum pressure value of the organic Rankine cycle system, the control element stops the operation of the organic Rankine cycle system, if When the actual pressure value is less than the critical pressure value, the control element switches the organic Rankine cycle system from the transcritical operation mode to the subcritical operation mode.

In the organic Rankine cycle system described above, the control element stores a superheat calculation module, and the superheat calculation module of the control element calculates an actual superheat of the working fluid based on the actual pressure value and the actual temperature value value, when the organic Rankine cycle is in the subcritical operation mode, if the actual superheat value is less than a lower limit value of a preset superheat range, the output frequency value or the output flow value of the pump is lowered, If the actual superheat value is greater than the upper limit of a preset superheat range, then increase the output frequency value or the output flow value of the pump.

The above-mentioned organic Rankine cycle system, wherein in the subcritical operation mode, if the actual pressure value is less than the allowable minimum pressure value of the organic Rankine cycle system, the control element stops the operation of the organic Rankine cycle system, if When the actual pressure value is greater than the critical pressure value, the control element switches the organic Rankine cycle system from the subcritical operation mode to the transcritical operation mode.

In the organic Rankine cycle system mentioned above, the working fluid is an organic refrigerant, and the organic refrigerant is one selected from the group consisting of HFCs, mixed refrigerants, HCs, and FCs.

In the organic Rankine cycle system mentioned above, the pump is a variable flow pump or a constant flow pump with a variable frequency motor.

The organic Rankine cycle system above, wherein the expander is a turbine, a screw expander, a scroll expander, a positive displacement expander or a reciprocating expander.

The above-mentioned organic Rankine cycle system also includes a generator connected with the expander to convert the rotational kinetic energy of the expander into electrical energy for output.

According to the organic Rankine cycle system and its subcritical operation mode and transcritical operation mode switching method disclosed in the present invention, the operation mode of the organic Rankine cycle system is switched through pressure conditions, so that when the condition of the working fluid changes, the organic Rankine cycle The Ken cycle system can be continuously adjusted to an appropriate operation mode in order to improve the operable range and utilization rate of the organic Rankine cycle system and increase its cumulative power generation, thereby improving the economic benefits of the organic Rankine cycle system.

The above descriptions about the contents of the present invention and the following descriptions of the embodiments are used to demonstrate and explain the principles of the present invention, and provide further explanations of the patent application scope of the present invention.

Description of drawings

Fig. 1 is a system schematic diagram of an organic Rankine cycle system according to an embodiment of the present invention;

2 is a schematic diagram of the temperature-entropy performance of the organic Rankine cycle system of FIG. 1 in a transcritical operation mode;

3 is a schematic diagram of the temperature-entropy performance of the organic Rankine cycle system of FIG. 1 in a subcritical operation mode;

Fig. 4 is the flowchart of the switching method of subcritical operation mode and transcritical operation mode of the organic Rankine cycle system of Fig. 1;

Fig. 5 is the operation flowchart of the organic Rankine cycle system of Fig. 1 in the transcritical operation mode;

6 and 7 are schematic diagrams of the temperature-entropy performance of the temperature variation of the heat source supplying the organic Rankine cycle system of FIG. 1;

FIG. 8 is a flowchart of the operation of the ORC system in FIG. 1 in a subcritical operation mode.

Among them, reference signs:

Detailed ways

Please refer to Figure 1 to Figure 3. FIG. 1 is a system diagram of an organic Rankine cycle system according to an embodiment of the present invention. FIG. 2 is a schematic diagram of the temperature-entropy performance of the organic Rankine cycle system in FIG. 1 in a transcritical operation mode. FIG. 3 is a schematic diagram of the temperature-entropy performance of the organic Rankine cycle system of FIG. 1 in a subcritical operation mode.

As shown in Figure 1, the organic Rankine cycle system 10 operable in the subcritical and transcritical states of the present embodiment comprises a heat source heat exchanger 100, an expander 200, a condenser 300, a pump 400, a pressure A sensor 510 , a temperature sensor 520 , a generator 600 and a control element 700 .

The heat source heat exchanger 100 is used for an external heat source to flow in. The heat source exchanges heat with the organic Rankine cycle system 10 to provide heat energy for the organic Rankine cycle system 10 . The heat source is, for example, medium and low temperature waste heat, geothermal heat, hot spring, solar thermal energy and the like. The expander 200 is used to release fluid pressure. The condenser 300 is used for an external cold source to flow in, and the cold source exchanges heat with the ORC system 10 to take away the heat energy of the ORC system 10 . The cold source is, for example, a water-cooled cooling tower, an air-cooled cooler, river water, sea water, or other low-temperature fluids. The pump 400 is used to drive the flow of the working fluid of the ORC system 10 and to adjust the pressure of the working fluid.

The heat source heat exchanger 100, the expander 200, the condenser 300, and the pump 400 are connected to each other through pipelines, and the pump 400 is used to drive a working fluid to flow through the heat source heat exchanger 100, the expander 200, and the condenser sequentially from the pump 400. The device 300 constitutes an organic Rankine cycle to convert thermal energy into mechanical energy. The working fluid has a critical pressure value PC (as shown in FIG. 2 and FIG. 3 ). If the actual pressure value of the working fluid before entering the inlet of the expander 200 after passing through the heat source heat exchanger 100 is greater than the critical pressure value PC, it means that the working fluid has only a single phase at a single time point. If the actual pressure value of the working fluid before entering the inlet of the expander 200 after passing through the heat source heat exchanger 100 is lower than the critical pressure value PC, it means that the working fluid has more than two phases at a single time point. The working fluid is an organic refrigerant. This embodiment takes R-134a organic refrigerant as an example, but it is not limited thereto. The organic refrigerant can also be selected from HFCs (such as: R134a, R245fa, R32, R23, R41, R125, R152a, R236fa, etc.), mixed Refrigerants (such as: R404A, R407C, R507A, R410A, etc.), HCs (such as: R116, R218, RC318, n-pentane, etc.), FCs (such as: butane, isobutene, propane, methane, etc.) one. In addition, the pump 400 is a variable flow pump or a constant flow pump with a variable frequency motor. The expander 200 is a turbine, a screw expander 200, a scroll expander 200, a displacement expander 200, a reciprocating expander 200, and the like.

In this embodiment, the pump 400 is a constant flow pump. The control element 700 changes the output frequency value or the output flow value of the pump 400 through a frequency conversion device 800 . However, the frequency conversion device 800 is not an essential component. If the pump 400 uses a variable flow pump, the frequency conversion device 800 does not need to be provided.

The pressure sensor 510 is used to detect an actual pressure value P _r of the working fluid after passing through the heat source heat exchanger 100 and before entering the inlet of the expander 200 .

The temperature sensor 520 is used to detect an actual temperature value T _r of the working fluid after passing through the heat source heat exchanger 100 and before entering the inlet of the expander 200 .

The generator 600 is connected to the expander 200 . The expander 200 is used to drive the generator 600 to convert mechanical energy into electrical energy.

The control element 700 is used to judge the relationship between the actual pressure value P _r of the working fluid and the critical pressure value P _C , if the actual pressure value P _r of the working fluid is greater than the critical pressure value P _C , the control element 700 controls the organic Rankine cycle The system 10 operates in a transcritical operation mode (the temperature-entropy performance diagram of the organic Rankine cycle system 10 in the transcritical operation mode is shown in FIG. 2 ). If the actual pressure value P _r of the working fluid is less than the critical pressure value P _C , the control element 700 controls the organic Rankine cycle system 10 to operate in the primary critical operation mode (the temperature-entropy of the organic Rankine cycle system 10 in the subcritical operation mode The performance diagram is shown in Figure 3).

When the organic Rankine cycle system 10 is installed, it can be determined according to the historical data of the heat source, the temperature/flow variation situation and the operating parameters of the organic Rankine cycle system 10. The operating temperature range corresponding to the pressure value and the preset suitable superheat range when operating in the subcritical operation mode, and record the above information in a supercritical working fluid pressure temperature database and a superheat calculation module of the control element 700, In order to enable the control element 700 to automatically adjust the output frequency value or the output flow value of the pump 400 through the supercritical working fluid pressure temperature database and the superheat calculation module.

The supercritical working fluid pressure and temperature database has multiple sets of pressure and working temperature range data of the organic Rankine cycle system 10 operating in the subcritical operating mode, and each set of pressure and working temperature range data has a pressure value and a corresponding working temperature scope.

See Figures 4 through 5. FIG. 4 is a flowchart of a method for switching between the subcritical operation mode and the transcritical operation mode of the organic Rankine cycle system in FIG. 1 . FIG. 5 is an operation flowchart of the organic Rankine cycle system in FIG. 1 in a transcritical operation mode.

As shown in Figure 4. First, the control unit 700 will judge whether the actual pressure value P _r of the working fluid of the organic Rankine cycle system 10 after passing through the heat source heat exchanger 100 before entering the inlet of the expander 200 is greater than or equal to a critical pressure value PC of the working fluid (such as step S100 shown).

If yes, the control unit 700 will make the organic Rankine cycle system 10 operate in a transcritical operation mode (as shown in step S200 ).

If not, the control unit 700 will make the organic Rankine cycle system 10 operate in the primary critical operation mode (as shown in step S300 ).

As shown in FIG. 5 , in the operation mode of the critical operation mode, first, the control unit 700 will search a supercritical working fluid pressure and temperature database to obtain an operating temperature range corresponding to the actual pressure value _Pr (as shown in step S210 ).

Next, the control unit 700 judges the relationship between an actual temperature value T _r of the working fluid before entering the inlet of the expander 200 after passing through the heat source heat exchanger 100 and the working temperature range T _R (shown in step S220 ).

If the temperature sensor 520 detects that the actual temperature T _r of the working fluid before entering the inlet of the expander 200 is lower than the lower limit T R _,MIN of the working temperature range, the control element 700 will lower the output frequency of the pump 400 Or output the flow value (as shown in step S230). If the pressure sensor 510 detects that the actual pressure value P _r of the working fluid before the inlet of the expander 200 is lower than the critical pressure value PC, the control element 700 will change the operating mode of the organic Rankine cycle system 10 from the transcritical operating mode Switch to the sub-critical operation mode (as shown in step S260 ), so that the organic Rankine cycle system 10 can continue to generate power and increase the cumulative power generation.

If the temperature sensor 520 detects that the actual temperature value Tr of the working fluid entering the inlet of the expander 200 _falls within the working temperature range, then maintain the output frequency value or the output flow value of a pump 400 that drives the working fluid (as in step shown in S240).

If the temperature sensor 520 detects that the actual temperature T _r of the working fluid before entering the inlet of the expander 200 is higher than the upper limit T R _,MAX of the working temperature range, the control element 700 will increase the output frequency of the pump 400 Or output the flow value (as shown in step S250). If the pressure sensor 510 detects that the actual pressure value P _r of the working fluid before entering the inlet of the expander 200 is higher than the allowable maximum pressure value P _A,MAX of the organic Rankine cycle system 10, the control element 700 will make the organic Rankine cycle The Ken cycle system 10 is stopped (as shown in step S270 ), so as to avoid damage to the organic Rankine cycle system 10 .

The following examples illustrate the operating conditions of the organic Rankine cycle system 10 in the transcritical mode. Please refer to Figure 6 and Figure 7. 6 and 7 are schematic diagrams of the temperature-entropy performance of the temperature variation of the heat source supplying the organic Rankine cycle system of FIG. 1 .

Assume that under the condition of a stable heat source (temperature Ts1, flow rate Ms1) and a heat sink, the ORC system 10 operates stably in a transcritical operation mode. Before the working fluid enters the expander 200, the pressure, temperature and mass flow rate of the working fluid are respectively the first pressure value P1, the first temperature value T1 and the first mass flow rate M1 (and the output frequency value or output flow rate of the pump 400 value is proportional to). When the cooling source is fixed and the flow rate of the heat source is fixed, but the temperature increases, the first temperature value T1 of the heat-exchanged working fluid will increase to the first transient temperature value T1-1. At this time, the control element 700 can gradually increase the output frequency value or output flow value of the pump 400, thereby increasing the pressure of the working fluid from the first pressure value P1 to the second pressure value P2 (the mass flow rate of the working fluid will also be corresponding from the first mass flow rate M2 to the second mass flow rate M2). Since the mass flow rate of the working fluid increases (M2>M1) and the external heat source is limited, the temperature of the working fluid before entering the inlet of the expander 200 will drop from the first transient temperature value T1-1 to the second temperature value T2. At this time, the control element 700 will judge whether the second temperature value T2 falls within the working temperature range of the second pressure value P2, and if not, continue to adjust the output frequency value or output flow value of the pump 400 until the working fluid The temperature value falls within the operating temperature range. In addition, assuming that the adjusted second pressure value P2 of the working fluid is higher than the allowable maximum operating pressure value _{PA,MAX of} the system, the control element 700 will stop the organic Rankine cycle system 10 .

Assume that under the condition of a stable heat source (temperature Ts1, flow rate Ms1) and a heat sink, the ORC system 10 operates stably in a transcritical operation mode. Before the working fluid enters the expander 200, the pressure, temperature and mass flow rate of the working fluid are respectively the first pressure value P1, the first temperature value T1 and the first mass flow rate M1 (and the output frequency value or output flow rate of the pump 400 value is proportional to). When the cooling source is fixed and the flow rate of the heat source is fixed, but the temperature decreases, the first temperature value T1 of the heat-exchanged working fluid will decrease to the second transient temperature value T1-2. At this time, the control element 700 can gradually lower the output frequency value or the output flow value of the pump 400, thereby reducing the pressure of the working fluid from the first pressure value P1 to the third pressure value P3 (the mass flow rate of the working fluid will also be corresponding from the first mass flow rate M2 to a third mass flow rate M3). Since the mass flow rate of the working fluid decreases (M3<M1) and the external heat source is limited, the temperature of the working fluid before entering the inlet of the expander 200 will rise from the second transient temperature value T1-2 to the third temperature value T3. At this time, the control element 700 will judge whether the second temperature value T2 falls within the working temperature range of the second pressure value P2, and if not, continue to adjust the output frequency value or output flow value of the pump 400 until the working fluid The temperature value falls within the operating temperature range. In addition, assuming that the adjusted third pressure value P3 of the working fluid is lower than the critical pressure value PC of the working fluid, the control element 700 will switch the operation mode of the organic Rankine cycle system 10 from the transcritical operation mode to the subcritical operation mode . Assuming that the adjusted third pressure value P3 of the working fluid is lower than the allowable minimum operating pressure value _PA,MIN of the system, the control element 700 will stop the organic Rankine cycle system 10 .

In addition, if the temperature of the heat source is fixed and the flow rate is changed, the operation procedure of the ORC system 10 is also the same as above, so it will not be repeated here.

As mentioned above, the method for the control element 700 to adjust the output frequency value or output flow value of the pump 400 can be set through PID control.

See Figure 8. FIG. 8 is a flowchart illustrating the operation of the ORC system in FIG. 1 in a subcritical operation mode.

As shown in Figure 8, in the operation mode of the subcritical operation mode, firstly, the superheat calculation module of the control element 700 will calculate the actual pressure value P _r and the actual temperature value T _r before the working fluid enters the inlet of the expander 200 An actual superheat value OS _r (as shown in step S310).

Next, the control unit 700 judges the relationship between the actual superheat value OS _r of the working fluid before entering the inlet of the expander 200 and a preset superheat range OS _R (shown in step S320 ).

If the temperature sensor 520 detects that the actual superheat value OS _r of the working fluid before entering the inlet of the expander 200 is less than the lower limit value OS _R,MIN of the preset superheat range, the control element 700 will lower the output of the pump 400 Frequency value or output flow value (as shown in step S330). If the pressure sensor 510 detects that the actual pressure value P _r before entering the inlet of the expander 200 is lower than the allowable minimum pressure value _PA,MIN of the organic Rankine cycle system 10, the control element 700 will make the organic Rankine cycle system 10 stops running (as shown in step S360 ), so as to avoid damage to the organic Rankine cycle system 10 .

If the temperature sensor 520 detects that the actual temperature value T _r of the working fluid entering the inlet of the expander 200 falls within the working temperature range T _R , then maintain an output frequency value or an output flow value of a pump 400 that drives the working fluid (as shown in step S340).

If the temperature sensor 520 detects that the actual superheat value OS _r of the working fluid before entering the inlet of the expander 200 is greater than the upper limit value OS _R,MAX of the preset superheat range, the control element 700 will increase the pump 400 Output frequency value or output flow value (as shown in step S350). If the pressure sensor 510 detects that the actual pressure value _Pr of the working fluid before entering the inlet of the expander 200 is higher than the critical pressure value _Pc , the control element 700 will make the operating mode of the organic Rankine cycle system 10 change from subcritical to The operation mode is switched to the transcritical operation mode (as shown in step S370 ), in order to obtain better power generation efficiency.

According to the organic Rankine cycle system and its subcritical operation mode and transcritical operation mode switching method disclosed in the present invention, in addition to the pressure condition, the organic Rankine cycle system is switched by the working temperature range condition and superheat condition. The operation mode enables the organic Rankine cycle system to continuously adjust to the appropriate operation mode when the conditions of the external cold and heat sources change, so as to improve the operable range and utilization rate of the organic Rankine cycle system and increase its cumulative power generation, and then Improving the economics of the Organic Rankine Cycle system.

Claims (20)

1. A switching method of an operating mode of an organic Rankine cycle system, characterized in that, comprising the following steps: Judging whether an actual pressure value of a working fluid in an organic Rankine cycle system before entering an expander inlet is greater than or equal to a critical pressure value of the working fluid: If so, operating the ORC system in a transcritical mode of operation; and If not, the organic Rankine cycle system is operated in a primary critical operation mode. 2. The switching method of the operating mode of the Organic Rankine cycle system as claimed in claim 1, wherein the operation process of the transcritical operating mode comprises the following steps: Searching a supercritical working fluid pressure and temperature database to obtain a working temperature range corresponding to the actual pressure value; and Judging the relationship between an actual temperature value of the working fluid before entering the inlet of the expander and the working temperature range: If the actual temperature value is lower than the lower limit of the working temperature range, lowering an output frequency value or an output flow value of a pump driving the working fluid; maintaining the output frequency value or the output flow value of the pump if the actual temperature value falls within the operating temperature range; and If the actual temperature value is higher than the upper limit of the working temperature range, the output frequency value or the output flow value of the pump is increased. 3. The method for switching the operation mode of the Organic Rankine cycle system as claimed in claim 2, wherein if the actual temperature value is lower than the lower limit of the working temperature range, the output of the pump is lowered After the step of determining the frequency value or the output flow value, if the actual pressure value before entering the expander inlet is lower than the critical pressure value, the operation mode of the organic Rankine cycle system is changed from the cross-critical operation mode Switch to the subcritical operating mode. 4. The method for switching the operation mode of the Organic Rankine cycle system as claimed in claim 2, wherein, in the operation process of the transcritical operation mode, if the actual temperature value is higher than the upper limit of the working temperature range value, then after the step of increasing the output frequency value or the output flow value of the pump, if the actual pressure value before entering the expander inlet is higher than the allowable maximum pressure value of the organic Rankine cycle system, Then the organic Rankine cycle system is stopped. 5. The switching method of the operating mode of the Organic Rankine cycle system as claimed in claim 2, characterized in that, in the operation process of the transcritical operating mode, the supercritical working fluid pressure temperature database has multiple sets of pressure and operating temperature Range data, each set of pressure and working temperature range data has a pressure value and a corresponding working temperature range. 6. The switching method of the operating mode of the Organic Rankine cycle system as claimed in claim 1, wherein the operation process of the subcritical operating mode comprises the following steps: calculating an actual superheat value of the working fluid according to the actual pressure value and the actual temperature value; and Judging the relationship between the actual superheat value before the working fluid enters the expander inlet and a preset superheat range: If the actual superheat value is less than the lower limit value of the preset superheat range, lower the output frequency value or the output flow value of the pump; If the actual superheat value falls within the preset superheat range, maintaining the output frequency value or the output flow value for driving the pump; and If the actual superheat value is greater than the upper limit of the preset superheat range, then increase the output frequency value or the output flow value of the pump. 7. The method for switching the operation mode of the Organic Rankine cycle system as claimed in claim 6, wherein in the operation process of the subcritical operation mode, if the actual superheat value is less than the preset superheat range After the step of lowering the output frequency value or the output flow value of the pump, it also includes if the actual pressure value before entering the expander inlet is lower than the allowable minimum pressure of the organic Rankine cycle system When the value is reached, the organic Rankine cycle system is stopped. 8. The method for switching the operation mode of the organic Rankine cycle system as claimed in claim 6, wherein if the actual superheat value is greater than the upper limit value of the preset superheat range, the pump is increased After the step of outputting the frequency value or the outputting flow value, if the actual pressure value before entering the expander inlet is higher than the critical pressure value, making the operation mode of the organic Rankine cycle system change from the subcritical The operation mode is switched to the transcritical operation mode. 9 . The method for switching the operating mode of an organic Rankine cycle system according to claim 1 , wherein the pump is a variable flow pump or a constant flow pump with a variable frequency motor. 10. The switching method of the operating mode of the Organic Rankine cycle system as claimed in claim 1, wherein the expander is a turbine, a screw expander, a scroll expander, a positive displacement expander or a reciprocating expander machine. 11. The method for switching the operation mode of the Organic Rankine cycle system as claimed in claim 1, characterized in that, when the temperature or the flow rate of a heat source or a cold source supplied to the Organic Rankine cycle system changes, if the Organic Rankine cycle system If the organic Rankine cycle system is in the transcritical operation mode, then perform the step of judging whether an actual temperature value of the working fluid before entering the expander inlet falls within a working temperature range, if the organic Rankine cycle system is in the subcritical In the operation mode, a step of judging whether an actual superheat value of the working fluid before entering the expander inlet falls within a preset superheat range is executed. 12. An organic Rankine cycle system, characterized in that it comprises: a heat source heat exchanger; An expander connected to the heat source heat exchanger through pipelines; a condenser connected to the expander through pipelines; A pump is connected to the condenser and the heat source heat exchanger through pipelines, and the pump is used to drive a working fluid to flow through the heat source heat exchanger, the expander and the condenser in sequence to form an organic Rankine cycle, the working fluid has a critical pressure value; a pressure sensor, used to detect an actual pressure value of the working fluid before entering the inlet of the expander; and A control element, used to judge the relationship between the actual pressure value of the working fluid and the critical pressure value, if the actual pressure value of the working fluid is greater than the critical pressure value, the control element controls the organic Rankine cycle system It operates in a transcritical operation mode, and if the actual pressure value of the working fluid is less than the critical pressure value, the control element controls the organic Rankine cycle system to operate in a primary critical operation mode. 13. The organic Rankine cycle system as claimed in claim 12, further comprising a temperature sensor for detecting an actual temperature value of the working fluid before entering the inlet of the expander, and the control element stores a super Critical working fluid pressure and temperature database, the supercritical working fluid pressure and temperature database has multiple sets of pressure and working temperature range data, each set of pressure and working temperature range data has a pressure value and a corresponding working temperature range, the Organic Rankine When the circulation system is in the transcritical operation mode, if the actual temperature value is greater than the upper limit value of the working temperature range, the control element increases an output frequency value or an output flow value of the pump; if the actual temperature value is less than the If the lower limit of the working temperature range is reached, the control element reduces the output frequency value or the output flow value of the pump. 14. The organic Rankine cycle system according to claim 13, characterized in that, in the transcritical operation mode, if the actual pressure value is greater than the allowable maximum pressure value of the organic Rankine cycle system, the control element makes The organic Rankine cycle system stops running. If the actual pressure value is less than the critical pressure value, the control element switches the organic Rankine cycle system from the transcritical operation mode to the subcritical operation mode. 15. The organic Rankine cycle system as claimed in claim 13, characterized in that there is a superheat calculation module stored in the control element, and the superheat calculation module of the control element is based on the actual pressure value and the actual temperature value Calculate an actual superheat value of the working fluid, and when the organic Rankine cycle is in the subcritical operation mode, if the actual superheat value is less than a lower limit value of a preset superheat range, the pump’s If the output frequency value or the output flow value is greater than the upper limit of a preset superheat range, the output frequency value or the output flow value of the pump is increased. 16. The organic Rankine cycle system according to claim 15, characterized in that, in the subcritical operation mode, if the actual pressure value is less than the allowable minimum pressure value of the organic Rankine cycle system, the control element makes The organic Rankine cycle system stops running, and if the actual pressure value is greater than the critical pressure value, the control element switches the organic Rankine cycle system from the subcritical operation mode to the transcritical operation mode. 17. The organic Rankine cycle system according to claim 12, wherein the working fluid is an organic refrigerant, and the organic refrigerant is selected from the group consisting of HFCs, mixed refrigerants, HCs, and FCs . 18. The organic Rankine cycle system according to claim 12, wherein the pump is a variable flow pump or a constant flow pump with a variable frequency motor. 19. The organic Rankine cycle system according to claim 12, wherein the expander is a turbine, a screw expander, a scroll expander, a positive displacement expander or a reciprocating expander. 20 . The organic Rankine cycle system according to claim 12 , further comprising a generator connected to the expander to convert the rotational kinetic energy of the expander into electric energy for output. 21 .