CN111546852A - Transcritical carbon dioxide electric vehicle thermal management system and control method thereof - Google Patents
Transcritical carbon dioxide electric vehicle thermal management system and control method thereof Download PDFInfo
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- CN111546852A CN111546852A CN202010364584.6A CN202010364584A CN111546852A CN 111546852 A CN111546852 A CN 111546852A CN 202010364584 A CN202010364584 A CN 202010364584A CN 111546852 A CN111546852 A CN 111546852A
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B60—VEHICLES IN GENERAL
- B60H—ARRANGEMENTS OF HEATING, COOLING, VENTILATING OR OTHER AIR-TREATING DEVICES SPECIALLY ADAPTED FOR PASSENGER OR GOODS SPACES OF VEHICLES
- B60H1/00—Heating, cooling or ventilating [HVAC] devices
- B60H1/00357—Air-conditioning arrangements specially adapted for particular vehicles
- B60H1/00385—Air-conditioning arrangements specially adapted for particular vehicles for vehicles having an electrical drive, e.g. hybrid or fuel cell
- B60H1/00392—Air-conditioning arrangements specially adapted for particular vehicles for vehicles having an electrical drive, e.g. hybrid or fuel cell for electric vehicles having only electric drive means
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B60—VEHICLES IN GENERAL
- B60H—ARRANGEMENTS OF HEATING, COOLING, VENTILATING OR OTHER AIR-TREATING DEVICES SPECIALLY ADAPTED FOR PASSENGER OR GOODS SPACES OF VEHICLES
- B60H1/00—Heating, cooling or ventilating [HVAC] devices
- B60H1/32—Cooling devices
- B60H1/3204—Cooling devices using compression
- B60H1/3205—Control means therefor
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B60—VEHICLES IN GENERAL
- B60H—ARRANGEMENTS OF HEATING, COOLING, VENTILATING OR OTHER AIR-TREATING DEVICES SPECIALLY ADAPTED FOR PASSENGER OR GOODS SPACES OF VEHICLES
- B60H1/00—Heating, cooling or ventilating [HVAC] devices
- B60H1/32—Cooling devices
- B60H1/3204—Cooling devices using compression
- B60H1/3205—Control means therefor
- B60H1/3211—Control means therefor for increasing the efficiency of a vehicle refrigeration cycle
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B60—VEHICLES IN GENERAL
- B60H—ARRANGEMENTS OF HEATING, COOLING, VENTILATING OR OTHER AIR-TREATING DEVICES SPECIALLY ADAPTED FOR PASSENGER OR GOODS SPACES OF VEHICLES
- B60H1/00—Heating, cooling or ventilating [HVAC] devices
- B60H1/32—Cooling devices
- B60H2001/3269—Cooling devices output of a control signal
- B60H2001/327—Cooling devices output of a control signal related to a compressing unit
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B60—VEHICLES IN GENERAL
- B60H—ARRANGEMENTS OF HEATING, COOLING, VENTILATING OR OTHER AIR-TREATING DEVICES SPECIALLY ADAPTED FOR PASSENGER OR GOODS SPACES OF VEHICLES
- B60H1/00—Heating, cooling or ventilating [HVAC] devices
- B60H1/32—Cooling devices
- B60H2001/3269—Cooling devices output of a control signal
- B60H2001/328—Cooling devices output of a control signal related to an evaporating unit
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Abstract
The invention discloses a transcritical carbon dioxide electric vehicle thermal management system and a control method, wherein the transcritical carbon dioxide electric vehicle thermal management system comprises the following steps: the system comprises a compressor, a gas cooler, a first electronic expansion valve, a liquid storage device, a second electronic expansion valve and an evaporator; the compressor adopts a 3+1 cylinder air-supplementing piston type compressor, an air suction port a of the compressor is connected with 3 main cylinders for main cycle compression, an air suction port b of the compressor is connected with 1 auxiliary cylinder for air-supplementing compression, two paths of refrigerants are mixed after compression is completed, and the compression frequencies of the two paths of refrigerants are consistent; the outlet of the compressor is connected with the liquid storage device through the gas cooler and the first electronic expansion valve; the gas outlet of the liquid storage device is connected with a gas suction port b of the compressor; the liquid outlet of the liquid storage device is connected with the air suction port a of the compressor through the second electronic expansion valve and the evaporator. The invention solves the problem of insufficient refrigeration performance of transcritical carbon dioxide and promotes the green refrigerant CO2Stepping into the process of the actual application, for environmental protection andand the energy conservation makes great contribution.
Description
Technical Field
The invention belongs to the technical field of transcritical carbon dioxide systems, and particularly relates to a transcritical carbon dioxide electric vehicle thermal management system and a control method thereof.
Background
The conventional refrigerants have been gradually facing a tendency to be eliminated due to poor environmental protection. And CO2The refrigerant has obvious advantages as a natural refrigerant. First, CO2The self-body is non-toxic and non-flammable, the quality is low when the self-body is used as a heat pump system or an air conditioning system, even if leakage occurs, the concentration is low, and the self-body cannot cause fatal damage to human bodies. Second, CO2Is pure natural gas, can not damage the environment and the atmosphere, is green and environment-friendly, meets the requirement of sustainable development, and not only can CO2The content is rich in natural environment, the price is low, and the economical efficiency is good. Finally, CO2The transcritical cycle has unique advantages in heating, and the temperature of the heat release process is higher and a considerable temperature slip (about 80-100 ℃) exists. The research shows that: transcritical CO2When the evaporation temperature of the heat pump water heater is 0 ℃, the water temperature can be heated from 0 ℃ to 60 ℃, the COP of the heat pump water heater can reach 4.3, and the energy consumption is reduced by 75 percent compared with that of an electric water heater and a gas water heater. In cold regions, the heating capacity and efficiency of the conventional air source heat pump decrease rapidly with the decrease of ambient temperature, and the use of the heat pump is limited. And CO2The heat pump system can maintain higher heat supply amount and higher water outlet temperature in a low-temperature environment, and energy consumed by the auxiliary heating equipment is greatly saved.
However, according to the existing transcritical CO2The cyclic study found that transcritical CO is present2When the air conditioner is circularly used, the refrigerating performance at high ambient temperature (above 35 ℃) is not ideal, the refrigerating performance is obviously deteriorated along with the rise of the ambient temperature, the requirement of cold quantity cannot be met, the purpose of air conditioning is achieved, and the requirement of comfort level of a human body cannot be met. This disadvantage severely hampers transcritical CO2The comprehensive popularization and application of the refrigerant involve the development of green refrigerants.
Disclosure of Invention
The invention aims to provide a transcritical carbon dioxide electric vehicle thermal management system and a control method thereof, which can improve the refrigeration performance of a transcritical carbon dioxide system at high ambient temperature (above 35 ℃) and meet the requirement of human comfort.
In order to achieve the purpose, the invention adopts the technical scheme that:
a transcritical carbon dioxide electric vehicle thermal management system, comprising: the system comprises a compressor, a gas cooler, a first electronic expansion valve, a liquid storage device, a second electronic expansion valve and an evaporator;
the compressor adopts a 3+1 cylinder air-supplementing piston type compressor, an air suction port a of the compressor is connected with 3 main cylinders for main cycle compression, an air suction port b of the compressor is connected with 1 auxiliary cylinder for air-supplementing compression, two paths of refrigerants are mixed after compression is completed, and the compression frequencies of the two paths of refrigerants are consistent;
the outlet of the compressor is connected with the liquid storage device through the gas cooler and the first electronic expansion valve; the gas outlet of the liquid storage device is connected with a gas suction port b of the compressor; the liquid outlet of the liquid storage device is connected with the air suction port a of the compressor through the second electronic expansion valve and the evaporator.
A control method of a transcritical carbon dioxide electric vehicle thermal management system comprises the following steps:
the refrigerant is compressed to a high-temperature high-pressure supercritical state of a state point 2 in the compressor, enters a gas cooler for medium-pressure heat release to be a state point 3, is subjected to isenthalpic throttling by a first electronic expansion valve to be in a two-phase state point 4, enters a liquid storage device, directly enters a compression cavity through a b air suction port of the compressor to be in a state point 7, participates in the next refrigerant compression process, flows out of the liquid storage device, becomes a saturated liquid state point 6, is subjected to throttling again by a second electronic expansion valve, enters a two-phase state point 8 from the saturated liquid state, enters an evaporator for medium-pressure heat absorption, becomes a saturated gas state or a superheated gas state of the state point 1, finally enters the compression cavity of the compressor through an a air suction port of the compressor to participate in the next compression process.
Further, the stability of the system depends on the dryness of the refrigerant at the state point 5, if the gas state at the state point 5 is too much, the amount of the liquid refrigerant in the accumulator will gradually decrease, the refrigerant entering the evaporator is seriously insufficient, the suction gas of the compressor is seriously overheated, and the system performance is increasingly deteriorated; if the liquid refrigerant at the state point 5 is too much, the refrigerant in the air supply loop is less and less, even the sucked liquid refrigerant directly enters the compressor, the refrigerant entering the evaporator after secondary throttling is more and more, the outlet of the evaporator is still in a two-phase state, the compressor sucks air and carries liquid, and the service life of the compressor and the system performance are affected badly; the control method controls the flow of the refrigerants in a main gas suction path and an auxiliary gas suction path of the compressor, and adjusts the mass flow of two paths of refrigerants so that the dryness of the refrigerants entering the liquid accumulator after the refrigerants converged at the final gas discharge side are throttled for one round just meets the system requirement, and the liquid level of the liquid accumulator is always kept constant;
pressure sensors and temperature sensors are arranged at the front and the back of the first electronic expansion valve, and data acquisition is carried out on the temperature and pressure states of the two points, namely: pre-valve temperature T4Front pressure P of valve4Post-valve temperature T5Pressure P after valve5(ii) a Then:
h4＝f(T4，P4)
the compression process from state point 4 to state point 5 is equal enthalpy, so
h5＝h4
Dryness x of state point 55Determined by the following equation:
x5＝f(P5，h5)
the dryness value of the state point 5 is reflected on the liquid level change of the liquid storage device, and when the liquid level of the liquid storage device is kept constant all the time, the system can be considered to run stably; under any condition, after the system is operated, the density of the refrigerant at the state point 7 is calculated according to the parameter at the state point 5:
P7＝P5
ρ7＝(P7，X7＝1)
the density calculation for state point 1 yields:
ρ1＝(P1，T1)
the volumes of the auxiliary suction cavity and the main suction cavity of the compressor are respectively set as V1And V2Then twoThe mass flow of the refrigerant in the path satisfies the following conditions:
m1＝ρ1·V1
m7＝ρ7·V7the mass flow ratio of the two paths of refrigerants is
When the mass flow ratio n of two paths is adjusted to x5/(1-x5) The system is stabilized.
Furthermore, the flow of the refrigerant at the auxiliary cylinder air suction port and the main cylinder air suction port of the compressor is controlled by controlling the pulse opening of the air suction valve plate at the auxiliary cylinder air suction port and the main cylinder air suction port of the compressor in the exhaust process, and negative feedback control logic is adopted.
Furthermore, the pulse opening control of the air suction valve plate adopts a current pulse form; when two paths of suction flow of the system are regulated, after a control signal is input, a suction valve plate is opened, and t0The electromagnetic valve is automatically closed after s, automatically opened after t's, and t0s, automatically closing, and repeating the steps in such a way, wherein each Ts is one regulation period; wherein: t ═ T0+ t'; t s when the solenoid valve is opened, the system performs an automatic flow regulation process, t's when the solenoid valve is closed is a process of dynamic stabilization of the system again, and whether the mass flow ratio of the sucked air is matched with the dryness of the state point 5 is judged more accurately again; if the system is gradually operated stably, the mass flow ratio n of the two paths is equal to x5/(1-x5) And if the liquid level of the liquid accumulator is stable, a signal for closing the electromagnetic valve is input, the electromagnetic valve enters a closed state all the time, and the adjusting process is finished.
Furthermore, the time interval of the pulse signals is set to be related to the mass flow ratio of the two paths of sucked refrigerants after the system runs stably; namely, it is
t＝f(n)
Wherein: t is the opening time interval of the air suction valve plate in unit of second;
n is the mass flow ratio of the two suction refrigerants, and has no unit.
Furthermore, under the same environmental working condition,performance of system operation, i.e. COP value, with intermediate pressure PmidThe increase in (b) shows a tendency to increase first and then decrease; there is an optimum intermediate pressure PoptThe performance of the system is optimized, and the optimal middle exhaust pressure value is maintained at P along with the change of the ambient temperatureoptThe recommended value of the optimal intermediate pressure is 5MPa, i.e. PoptAnd 5, specifically determining according to the experimental calibration of the actual equipment.
Furthermore, under the same environmental condition, the COP of the system tends to increase firstly and then decrease along with the increase of the exhaust pressure, namely, the optimal exhaust pressure P existsdis_optValue of optimum exhaust pressure and ambient temperature TenvEvaporation temperature TevaIntermediate pressure PoptAnd the temperature T of the air supplyair(ii) related; namely, it is
Pdis_opt＝f(Tenv,Teva,Popt,Tair)
The specific control relation is related to the size and the capacity of the actual equipment, and is obtained through limited experiments according to the quantitative relation and the function weight.
Furthermore, the intermediate pressure, the exhaust pressure and the evaporation pressure of the system are controlled, so that the system always operates under the optimal performance on the basis of meeting the refrigeration requirement; the intermediate pressure and the exhaust pressure have optimal values corresponding to different working conditions one by one under different operating conditions, the evaporation pressure is determined by the requirements of the refrigerating capacity and the air supply temperature, and different air supply temperatures correspond to different evaporation pressures. The system adopts multi-negative feedback PID coupling control logic, the rotating speed of the compressor controls the evaporation pressure of the evaporator, the second electronic expansion valve controls the intermediate pressure, and the first electronic expansion valve controls the exhaust pressure value of the system; controlling the intermediate pressure, wherein the instantaneous intermediate pressure value is the input quantity of the PI controller, and the opening degree of the second electronic expansion valve is the output quantity; the specific control logic is as follows: when the intermediate pressure value is higher, i.e. Pmid≥PoptWhen the opening degree of the second electronic expansion valve is increased, the intermediate pressure is reduced, at the moment, the evaporation pressure also slightly rises, the rotating speed of the compressor is reduced to control the evaporation pressure to be constant, meanwhile, the exhaust pressure is reduced, and then the exhaust pressure is reducedAnd the opening degree of the first electronic expansion valve is increased, the exhaust pressure value is increased, the operation is repeated until the intermediate pressure and the exhaust pressure reach the optimal values corresponding to the working conditions, and the evaporation pressure is stabilized at the required value corresponding to the air supply temperature. The whole system is maintained to operate properly under the optimal working condition; when the intermediate pressure value is lower, i.e. Pmid≤PoptWhen the pressure of the air is increased, the opening degree of the second electronic expansion valve is reduced, the intermediate pressure is increased, the evaporation pressure is slightly reduced, the rotating speed of the compressor is increased to control the evaporation pressure to be constant, meanwhile, the exhaust pressure is increased, the opening degree of the first electronic expansion valve is increased, the exhaust pressure value is reduced, the operation is repeated until the intermediate pressure and the exhaust pressure reach the optimal values corresponding to the working conditions, and the evaporation pressure is stabilized at the required value corresponding to the air supply temperature; the whole system is maintained to be operated under the optimal working condition.
Compared with the prior art, the invention has the following beneficial effects:
firstly, the refrigerant adopted by the invention is natural working medium CO2The environment-friendly type solar energy water heater is environment-friendly, low in price, good in economy and outstanding in heating performance, and meets the modern development requirement of green economy sustainable development.
Furthermore, the existing transcritical carbon dioxide system always faces the difficult problems that the refrigerating performance is poor at high ambient temperature (above 35 ℃), air conditioning cannot be realized, and the requirement of human comfort is met. Aiming at the problems, firstly, the invention provides a function of improving the refrigeration performance of a transcritical CO2 system by adopting a 3+1 cylinder parallel compression system; secondly, the invention provides a method for controlling the stable operation of the system and specific operation steps, and the first step of improving the refrigeration performance is realized. Finally, the invention provides the optimal intermediate pressure of the system, provides the intermediate pressure value, and simultaneously provides the control logic and the operation method for controlling the system to operate under the optimal working condition, so that the transcritical CO is subjected to2The refrigeration performance of the system is further improved, and CO is promoted2The application process of the refrigerant realizes comprehensive replacement of the traditional refrigerant as early as possible, and makes a great change for environmental protection and energy conservation.
Drawings
FIG. 1 is a schematic structural diagram of a transcritical carbon dioxide electric vehicle thermal management system according to the present invention;
FIG. 2 is a schematic diagram of a system cycle P-h of a transcritical carbon dioxide electric vehicle thermal management system of the present invention;
FIG. 3 is a logic block diagram of the suction valve plate control of the thermal management system of the transcritical carbon dioxide electric vehicle.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings.
Referring to fig. 1, the present invention provides a transcritical carbon dioxide electric vehicle thermal management system, including: a compressor 1, a gas cooler 2, a first electronic expansion valve 3, a liquid reservoir 4, a second electronic expansion valve 5, and an evaporator 6.
The outlet of the compressor 1 is connected with a liquid storage device 4 through a gas cooler 2 and a first electronic expansion valve 3; the gas outlet of the liquid storage device 4 is connected with a b air suction port of the compressor 1; the liquid outlet of the liquid reservoir 4 is connected to the suction port a of the compressor 1 via the second electronic expansion valve 5 and the evaporator 6.
The refrigerant is compressed to a supercritical state 2 with high temperature and high pressure in the compressor, enters a gas cooler for medium pressure heat release to be a state point 3, is subjected to isenthalpic throttling by a first electronic expansion valve and then is in a two-phase state point 4, enters a liquid storage device, a gaseous refrigerant directly enters a compressor compression cavity through a b air suction port of the compressor to be a state point 7 to participate in the next refrigerant compression process, a liquid refrigerant state point 5 flows out of the liquid storage device to be a saturated liquid state point 6, is subjected to throttling by a second electronic expansion valve again, enters a two-phase state point 8 from the saturated liquid state again, then enters an evaporator for medium pressure heat absorption to be a saturated gas state or superheated gas state point 1, finally enters the compression cavity of the compressor through an a air suction port of the compressor to participate in the next compression process, and the pressure-enthalpy diagram of a circulating system is shown in figure.
Furthermore, a compressor of the system adopts a 3+1 cylinder air-supplementing piston type compressor, an air suction port a of the compressor is connected with 3 main cylinders for main cycle compression, an air suction port b of the compressor is connected with 1 auxiliary cylinder for air-supplementing compression, two paths of refrigerants are mixed after compression is completed, and the compression frequencies of the two paths of refrigerants are consistent.
Further, the stability of the thermal management system depends on the dryness of the refrigerant at the state point 5, if the gas state at the state point 5 is too much, the amount of the liquid refrigerant in the accumulator will gradually decrease, the refrigerant entering the evaporator is seriously insufficient, the suction gas of the compressor is seriously overheated, and the system performance is increasingly deteriorated; if the liquid refrigerant at the state point 5 is too much, the refrigerant in the air supply loop is less and less, even the sucked liquid refrigerant directly enters the compressor, the refrigerant entering the evaporator after secondary throttling is more and more, the outlet of the evaporator is still in a two-phase state, the compressor sucks air and carries liquid, and the service life of the compressor and the system performance are affected badly. Therefore, the flow of the refrigerants in the main suction gas circuit and the auxiliary suction gas circuit of the compressor needs to be strictly and timely controlled, the mass flow of the two paths of refrigerants is adjusted, so that the dryness of the refrigerant entering the liquid storage device after the refrigerant finally converged at the exhaust side passes through one-time throttling just meets the system requirement, and the liquid level of the liquid storage device is always kept constant.
Further, in order to achieve the above control purpose, a pressure sensor and a temperature sensor are respectively installed before and after the first electronic expansion valve 3, and data acquisition is performed on the temperature and pressure states of the two points, that is, the temperature T before the valve is obtained4Front pressure P of valve4Post-valve temperature T5Pressure P after valve5. Then:
h4＝f(T4，P4)
the compression process from state point 4 to state point 5 is equal enthalpy, so
h5＝h4
Dryness x of state point 55Determined by the following equation:
x5＝f(P5，h5)
the dryness value of the state point 5 is reflected on the liquid level change of the liquid storage device, and when the liquid level of the liquid storage device is kept constant all the time, the system can be considered to be stable in operation. Under any condition, after the system is operated, the density of the refrigerant at the state point 7 can be calculated according to the parameters of the state point 5:
P7＝P5
ρ7＝(P7，X7＝1)
the density of state points 1 can also be calculated:
ρ1＝(P1，T1)
the volumes of the auxiliary suction cavity and the main suction cavity of the compressor are respectively set to be V1And V2Then, the mass flow of the two paths of refrigerants can be obtained:
m1＝ρ1·V1
m7＝ρ7·V7
when the mass flow ratio n of two paths is adjusted to x5/(1-x5) The system is stable.
Furthermore, the flow of the refrigerant in the auxiliary suction path and the main suction path of the compressor is controlled by controlling the pulse opening of the two suction valve plates of the compressor in the exhaust process, and negative feedback control logic is adopted, and the control logic block diagram is shown in figure 3.
Furthermore, the pulse opening control of the air suction valve plate adopts a current pulse form, namely when two paths of air suction flow of the system need to be regulated, after a control signal is input, the air suction valve plate is opened, and t0The electromagnetic valve is automatically closed after s, automatically opened after t's, and t0s is then automatically closed, and so on, one regulation cycle per Ts. Wherein: t ═ T0+ t'. And t s, opening the electromagnetic valve, automatically adjusting the flow rate of the system, and closing t's to dynamically stabilize the system again so as to judge whether the dryness fraction value of the state point 5 reaches the required value more accurately. If the system is gradually operated stably, namely the dryness value n of the state point 5 is equal to x5/(1-x5) And if the liquid level of the liquid accumulator does not change basically, inputting a signal for closing the electromagnetic valve, enabling the electromagnetic valve to enter a closed state all the time, and finishing the adjusting process.
Furthermore, the time interval of the pulse signal is set to be related to the mass flow ratio of the two paths of sucked refrigerants after the system is stably operated. Namely, it is
t＝f(n)
Wherein: t is the opening time interval of the air suction valve plate, and the unit of s is s;
n is the mass flow ratio of the two paths of suction refrigerants, and has no unit;
in certain parallel compression systems, the value of t is constant.
Furthermore, under the same environmental working condition, the running performance of the system, namely COP value, is along with the intermediate pressure PmidShows a tendency to increase first and then decrease, i.e. there is an optimum intermediate pressure PoptThe performance of the system is optimal, and the optimal intermediate exhaust pressure value is basically kept unchanged along with the change of the ambient temperature and is always maintained at PoptAbout, the recommended value of the optimal intermediate pressure is 5MPa, i.e. PoptThe specific value is also determined by the actual experimental system, and a fine correction is made on the reference value. It is therefore desirable to control the intermediate pressure of the system so that the system is always operating at the optimum intermediate pressure.
Further, the COP of the system tends to increase and decrease with the increase of the exhaust pressure, i.e. there is an optimum exhaust pressure Pdis_optValue of optimum exhaust pressure and ambient temperature TenvEvaporation temperature TevaIntermediate pressure PoptAnd the temperature T of the air supplyairIt is related. Namely, it is
Pdis_opt＝f(Tenv,Teva,Popt,Tair)
The specific control relation is related to the size and the capacity of the actual equipment, and can be obtained through limited experiments according to the quantitative relation and the function weight.
Furthermore, the intermediate pressure and the exhaust pressure of the thermal management system have optimal values corresponding to the working conditions one to one under different operating conditions, the evaporation pressure is determined by the requirements of the refrigerating capacity and the air supply temperature, and different air supply temperatures correspond to different evaporation pressures. The system adopts multi-negative feedback PID coupling control logic, the rotating speed of the compressor controls the evaporation pressure of the evaporator 6, the second electronic expansion valve 5 controls the intermediate pressure, and the first electronic expansion valve 3 controls the exhaust pressure value of the system. For the control of the intermediate pressure, the instantaneous intermediate pressure value is the input quantity of the PI controller, and the opening degree of the second electronic expansion valve 5 is the output quantity.
Further, the specific control logic is as follows: when the intermediate pressure value is higher, i.e. Pmid≥PoptAnd increasing the opening degree of the second electronic expansion valve 5 to reduce the intermediate pressure, slightly increasing the evaporation pressure at the moment, reducing the rotating speed of the compressor to control the evaporation pressure to be constant, simultaneously reducing the exhaust pressure, reducing the opening degree of the electronic expansion valve 3, increasing the exhaust pressure value, repeating the steps until the intermediate pressure and the exhaust pressure reach the optimal value corresponding to the operation working condition, stabilizing the evaporation pressure at the value corresponding to the air supply temperature required by refrigeration, and maintaining the whole system to operate moderately under the optimal working condition and reach stability. When the intermediate pressure value is lower, i.e. Pmid≤PoptWhen the temperature of the air supply reaches the optimal value corresponding to the operation working condition, the evaporation pressure is stabilized at the value corresponding to the air supply temperature required by refrigeration, and the whole system is maintained to operate moderately under the optimal working condition all the time. The adjustment of the three influences each other, and the three can not be independent from each other when the design is controlled and adjusted, and need to be coupled and adjusted.
It will be appreciated by those skilled in the art that the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The embodiments disclosed above are therefore to be considered in all respects as illustrative and not restrictive. All changes which come within the scope of or equivalence to the invention are intended to be embraced therein.
Claims (9)
1. A transcritical carbon dioxide electric vehicle thermal management system, comprising: the system comprises a compressor (1), a gas cooler (2), a first electronic expansion valve (3), a liquid storage device (4), a second electronic expansion valve (5) and an evaporator (6);
the compressor (1) adopts a 3+1 cylinder air supply piston type compressor, an air suction port a of the compressor is connected with 3 main cylinders for main cycle compression, an air suction port b of the compressor is connected with 1 auxiliary cylinder for air supply compression, two paths of refrigerants are mixed after compression is completed, and the compression frequencies of the two paths of refrigerants are consistent;
the outlet of the compressor (1) is connected with a liquid storage device (4) through a gas cooler (2) and a first electronic expansion valve (3); the gas outlet of the liquid storage device (4) is connected with a gas suction port b of the compressor (1); the liquid outlet of the liquid storage device (4) is connected with the air suction port a of the compressor (1) through a second electronic expansion valve (5) and an evaporator (6).
2. The control method of the transcritical carbon dioxide electric vehicle thermal management system of claim 1, comprising the steps of:
the refrigerant is compressed to a high-temperature high-pressure supercritical state of a state point 2 in the compressor, enters a gas cooler for medium-pressure heat release to be a state point 3, is subjected to isenthalpic throttling by a first electronic expansion valve to be in a two-phase state point 4, enters a liquid storage device, directly enters a compression cavity through a b air suction port of the compressor to be in a state point 7, participates in the next round of refrigerant compression process, flows out of the liquid storage device, becomes a saturated liquid state of a state point 6, is subjected to throttling again by a second electronic expansion valve, enters a two-phase state point 8 from the saturated liquid state, enters an evaporator for medium-pressure heat absorption to be in a saturated gas state or a superheated gas state of a state point 1, finally enters the compression cavity of the compressor through an a air suction port of the compressor to participate in the next round of compression process.
3. The control method of the transcritical carbon dioxide electric vehicle thermal management system according to claim 2, wherein the stability of the parallel compression system depends on the dryness of the refrigerant at the state point 5, if the gas state at the state point 5 is too much, the amount of the liquid refrigerant in the accumulator will gradually decrease, the refrigerant entering the evaporator is seriously insufficient, the suction gas of the compressor is seriously overheated, and the system performance will be more and more deteriorated; if the liquid refrigerant at the state point 5 is too much, the refrigerant in the air supply loop is less and less, even the sucked liquid refrigerant directly enters the compressor, the refrigerant entering the evaporator after secondary throttling is more and more, the outlet of the evaporator is still in a two-phase state, the compressor sucks air and carries liquid, and the service life of the compressor and the system performance are affected badly; the control method controls the flow of the refrigerants in a main gas suction path and an auxiliary gas suction path of the compressor, and adjusts the mass flow of two paths of refrigerants so that the dryness of the refrigerants entering the liquid accumulator after the refrigerants converged at the final gas discharge side are throttled for one round just meets the system requirement, and the liquid level of the liquid accumulator is always kept constant;
pressure sensors and temperature sensors are arranged in front of and behind the first electronic expansion valve (3), and data acquisition is carried out on the temperature and pressure states of the two points, namely: pre-valve temperature T4Front pressure P of valve4Post-valve temperature T5Pressure P after valve5(ii) a Then:
h4＝f(T4，P4)
the compression process from state point 4 to state point 5 is equal enthalpy, so
h5＝h4
Dryness x of state point 55Determined by the following equation:
x5＝f(P5，h5)
the dryness value of the state point 5 is reflected on the liquid level change of the liquid storage device, and when the liquid level of the liquid storage device is kept constant all the time, the system can be considered to run stably; under any condition, after the system is operated, the density of the refrigerant at the state point 7 is calculated according to the parameter at the state point 5:
P7＝P5
ρ7＝(P7，X7＝1)
the density calculation for state point 1 yields:
ρ1＝(P1，T1)
the volumes of the auxiliary suction cavity and the main suction cavity of the compressor are respectively set to be V1And V2Mass flow of refrigerant in two pathsThe amount satisfies:
m1＝ρ1·V1
m7＝ρ7·V7
the mass flow ratio of the two paths of refrigerants is
When the mass flow ratio n of two paths is adjusted to x5/(1-x5) The system is stabilized.
4. The control method of the transcritical carbon dioxide electric vehicle thermal management system according to claim 2, wherein the flow rates of the refrigerants at the auxiliary cylinder air suction port and the main cylinder air suction port of the compressor are controlled by controlling the pulse opening of the air suction valve plates at the auxiliary cylinder air suction port and the main cylinder air suction port of the compressor in the exhaust process, and negative feedback control logic is adopted.
5. The control method of the trans-critical carbon dioxide electric vehicle thermal management system according to claim 4, wherein the pulse opening control of the air suction valve plate is in a current pulse form; when two paths of suction flow of the system are regulated, after a control signal is input, a suction valve plate is opened, and t0The electromagnetic valve is automatically closed after s, automatically opened after t's, and t0s, automatically closing, and repeating the steps in such a way, wherein each Ts is one regulation period; wherein: t ═ T0+ t'; the ts of the electromagnetic valve is opened, the system performs an automatic flow regulation process, the t's of the system is closed, and the system is dynamically stabilized again so as to judge whether the mass flow ratio of the air suction is matched with the dryness of the state point 5 more accurately; if the system is gradually operated stably, the mass flow ratio n of the two paths is equal to x5/(1-x5) And if the liquid level of the liquid accumulator is stable, a signal for closing the electromagnetic valve is input, the electromagnetic valve enters a closed state all the time, and the adjusting process is finished.
6. The control method of the transcritical carbon dioxide electric vehicle heat management system according to claim 4, wherein the time interval of the pulse signal is set in relation to the mass flow ratio of the refrigerants sucked in the two paths after the system is stable in operation; namely, it is
t＝f(n)
Wherein: t is the opening time interval of the air suction valve plate in unit of second;
n is the mass flow ratio of the two suction refrigerants, and has no unit.
7. The control method of the transcritical carbon dioxide electric vehicle thermal management system according to claim 4, wherein under the same environmental condition, the performance of system operation, namely COP value, is along with the intermediate pressure PmidThe increase in (b) shows a tendency to increase first and then decrease; there is an optimum intermediate pressure PoptThe performance of the system is optimized, and the optimal middle exhaust pressure value is maintained at P along with the change of the ambient temperatureoptRecommendation PoptAnd 5, specifically determining according to the experimental calibration of the actual equipment.
8. The control method of the transcritical carbon dioxide electric vehicle thermal management system according to claim 2, wherein under the same environmental condition, the COP of the system tends to increase first and then decrease along with the increase of the exhaust pressure, that is, the optimal exhaust pressure P existsdis_optValue of optimum exhaust pressure and ambient temperature TenvEvaporation temperature TevaIntermediate pressure PoptAnd the temperature T of the air supplyair(ii) related; namely, it is
Pdis_opt＝f(Tenv，Teva，Popt，Tair)
The specific control relation is related to the size and the capacity of the actual equipment, and is obtained through limited experiments according to the quantitative relation and the function weight.
9. The control method of the transcritical carbon dioxide electric vehicle thermal management system according to claim 2, wherein the exhaust pressure, the intermediate pressure and the evaporation pressure of the system are control targetsThe intermediate pressure, the exhaust pressure and the evaporation pressure of the system are controlled, so that the system always operates under the optimal performance on the basis of meeting the refrigeration requirement; controlling the intermediate pressure of the system to enable the system to always operate under the optimal intermediate pressure; the system adopts multi-negative feedback PID coupling control logic, the rotating speed of the compressor controls the evaporation pressure of the evaporator, the second electronic expansion valve controls the intermediate pressure, and the first electronic expansion valve controls the exhaust pressure value of the system; controlling the intermediate pressure, wherein the instantaneous intermediate pressure value is the input quantity of the PI controller, and the opening degree of the second electronic expansion valve is the output quantity; the specific control logic is as follows: when the intermediate pressure value is higher, i.e. Pmid≥PoptWhen the system is in operation, the opening degree of the second electronic expansion valve is increased, the intermediate pressure is reduced, the evaporation pressure is slightly increased at the moment, the rotating speed of the compressor is reduced, the evaporation pressure is controlled to be constant, meanwhile, the exhaust pressure is reduced, the opening degree of the first electronic expansion valve is reduced, the exhaust pressure value is increased, the operation is repeated until the intermediate pressure and the exhaust pressure reach the optimal value corresponding to the operating environmental working condition, the evaporation pressure reaches the refrigeration working condition requirement value, and the whole system is maintained to operate moderately under the optimal working condition; when the intermediate pressure value is lower, i.e. Pmid≤PoptAnd meanwhile, the opening degree of the second electronic expansion valve (5) is reduced to increase the intermediate pressure, the evaporation pressure is slightly reduced at the moment, the rotating speed of the compressor is increased to control the evaporation pressure to be constant, meanwhile, the exhaust pressure is increased, the opening degree of the first electronic expansion valve (3) is increased, the exhaust pressure value is reduced, the operation is repeated until the intermediate pressure and the exhaust pressure reach the optimal values corresponding to the operating environmental working conditions, the evaporation pressure reaches the refrigeration working condition requirement value, and the whole system is maintained to operate moderately under the optimal working conditions.
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