WO2018100075A1

WO2018100075A1 - Two-phase cooling for an electric drive system

Info

Publication number: WO2018100075A1
Application number: PCT/EP2017/081009
Authority: WO
Inventors: Gunter Freitag; Andreas REEH; Guillermo ZSCHAECK
Original assignee: Siemens Aktiengesellschaft
Priority date: 2016-12-01
Filing date: 2017-11-30
Publication date: 2018-06-07

Abstract

The invention relates to highly-efficient cooling of an electric drive system. For this purpose, a coolant which comprises a primary liquid cooling medium to which a secondary cooling medium is added in a solid state, is provided. The secondary cooling medium is selected such that it changes during cooling, brought about by heat absorbed by one of the components which is to be cooled, during a phase exchange from a solid to a liquid state of aggregation.

Description

description

Two-phase cooling for an electric drive system The invention relates to a highly efficient cooling of an electric drive system.

For mobile applications, for example for the propulsion of aircraft such as airplanes or helicopters or for electrically powered watercraft, etc., concepts based on electric drive systems are being investigated and used as an alternative to the conventional internal combustion engines. Such an electric drive system, which can be designed as a purely electric or as a hybrid electric drive system, generally has at least one electric machine which is operated to drive the propulsion means of the vehicle as an electric motor. Furthermore, a corresponding source of electrical energy for supplying the electric motor is provided, which may be formed, for example, in the form of a battery or in the form of a further electrical machine designed as a generator. Furthermore, the electric drive system usually includes power electronics, with the help of the

Electric motor is operated. In the case of a hybrid electric drive system, furthermore, an internal combustion engine is provided which is integrated serially or in parallel into the drive system and, for example, drives the already mentioned generator, which in turn provides electrical energy stored in the battery

and / or the electric motor can be supplied.

Such systems are described, for example, in WO2015106993A1,

WO2015128121A1 or also in WO2017025224A1 described. The components of such electrical drive systems, ie in particular the electrical energy source, for example the generator and / or the battery, as well as the power electronics and the electric motor, generate waste heat during operation due to loss of power. Services. This waste heat must be dissipated to prevent overheating of the respective component. If these components become too hot, the lifetimes of the components and / or damage can be reduced, the latter having an effect on the reliability of the overall system and, in particular, in the case of an application of the electric drive system in an aircraft in the event of a fault, signifi- cant risk for passengers as well as significant property damage.

Therefore, cooling systems are needed to reduce the risk of overheating. However, typical cooling systems are relatively heavy, which generally proves to be a significant disadvantage for mobile applications. The aim is therefore to make the required cooling systems as compact as possible or lightweight and efficient. To achieve this, a large heat transfer coefficient and a high heat capacity of the cooling medium of the cooling system are necessary. Various approaches for cooling the components of such an electric drive system are known, bpsw. active or passive air cooling systems, direct cooling systems such as oil cooling systems, liquid cooling systems or boiling medium cooling systems.

Air has the lowest heat transfer coefficient as well as the lowest heat capacity of the generally eligible cooling media, i. the volume of the cooling medium air required for a given cooling capacity is greatest. Consequently, the air used as a cooling medium devices. the required space and weight largest, so that the power density of the entire system is the lowest.

Oil cooling has the advantage for electrical machines that is cooled directly at the place where the heat loss occurs, namely at the stator windings. The appropriately equipped electric machines build compact and are comparatively light, so the power density of these electrical machines is comparatively large. However, this does not apply to the power electronics, because heat capacity and heat transfer coefficient of the medium oil are lower than, for example, those of water.

The cooling medium water has a larger heat transfer coefficient than oil and the largest among the eligible cooling media heat capacity. However, water can not be routed directly to the live parts of the machine.

The boiling medium cooling is based on the utilization of the phase transition between liquid and gaseous state of the cooling medium. The liquid medium is conducted to the component to be cooled and evaporates there, resulting in cooling of the component. A disadvantage is the volume increase in the formation of the gas and the technical treatment of the two-phase flow of the cooling medium. Also, the mounting position of these systems is critical and has a significant impact on efficiency because the gas phase is lighter than the liquid phase, making these cooling problems especially for aerospace applications.

It is therefore an object of the invention to specify a possibility for cooling components of an electric drive system, in particular in a mobile application.

The approach presented here pursues the concept of using a coolant for cooling a component, in particular of an electric drive system, wherein the coolant comprises a liquid primary cooling medium to which a secondary cooling medium is added, wherein the secondary cooling medium in the ground state in the solid state is present in the coolant. The coolant therefore comprises the primary cooling medium and the secondary cooling medium. The ground state is the state in which the coolant has not yet absorbed heat from the component to be cooled. In other words, the coolant for cooling the component thus comprises a liquid primary cooling medium to which a solid is added as a secondary cooling medium. The secondary cooling medium is selected such that in the ground state it is present as a solid and, when a certain temperature is exceeded, a phase transition to a liquid takes place. In this phase transition, the secondary cooling medium absorbs heat from the environment, thus causing a cooling. The secondary cooling medium is therefore selected such that it performs the phase change from the solid to a liquid state of matter during the cooling of the component caused by a heat absorbed by the component. In practice, this means that the apparent heat capacity of the coolant is increased.

The secondary cooling medium is in particular with regard to. the particular temperature selected and adapted to the component to be cooled of the electric drive system, that the temperature of the component to be cooled in normal operation is in a temperature range in which the component to be cooled has a maximum efficiency. The normal operation describes the operation of the drive system and in particular those of the component to be cooled under normal conditions, i. For example, there is no error case.

For example, water or oil can be used as the primary, liquid cooling medium. The use of water is at least advantageous in that water has a very high heat transfer coefficient and a very high heat capacity. In the case of using oil as the primary cooling medium, live components can be cooled directly

As a secondary cooling medium paraffin can be used. The secondary cooling medium may, for example, comprise a multiplicity of paraffin volumes, in particular paraffin spheres, encased in a surfactant. The Tensidummantelung the paraffin balls causes, for example, that the paraffin balls after melting do not agglomerate and accordingly solidify again in the original form as before liquefaction. This also causes the coolant to still be pumped through the appropriate conduit system without clogging it.

A corresponding cooling system uses this coolant with primary and secondary cooling medium. In this case, both the cooling system itself and the component to be cooled may each be part of an electric drive system, for example for an electrically driven aircraft.

The component to be cooled may be, for example, a component of an electrical machine, in particular an electric motor or a generator, or the machine itself, as well as a power electronics or a battery. For example. the cooling system is suitable for cooling a rotor or a

Stators of an electric machine. A corresponding electric drive system, in particular for an aircraft, typically comprises a plurality of components to be cooled and a cooling system for cooling at least one of the plurality of components as described above.

The cooling method for cooling a component with such a coolant comprises three steps in a basic process: In a first step, the coolant with the solid state in the secondary cooling medium is fed to the component to be cooled. In a second step, the coolant, and with it the secondary cooling medium, absorb heat from the component to be cooled, whereupon the secondary cooling medium changes to a liquid state. In a third step, the coolant with the secondary cooling medium in the liquid state is led away from the component to be cooled. The coolant can, for example, circulate in a circuit, wherein the base process is followed by a fourth step, in which the coolant guided away from the component to be cooled releases heat with the secondary cooling medium in the liquid state to a heat exchanging device, so that the secondary cooling medium goes back to the solid state. After this fourth step, the basic process becomes the first, the second and the third

Step again.

Alternatively, the coolant after the third

Step out of the cooling system.

Thus, the liquid primary cooling medium transports the initially solid state secondary cooling medium to the waste heat source, i. to the component to be cooled. The phase transition of the secondary cooling medium from solid to liquid, which takes place there in operation due to the temperature of the component to be cooled during operation, allows a higher absorption of heat and thus an increased cooling effect compared to the use of the primary cooling medium alone.

Technically, a liquid is transported here. The amount of the coolant to be used can be reduced because the total heat capacity of the coolant is larger than that of the primary cooling medium alone. The size of the cooling system and thus the weight are reduced, which has a positive effect on the power density of the drive system.

Due to the described use of the particular coolant with the secondary cooling medium, which carries out a phase transition from solid to liquid and thereby causes cooling of the component to be cooled, the advantage initially results that a smaller amount of coolant must be used than if the primary coolant alone would be used. This results in the cooling system, including the coolant, being lighter in weight and more compact can be built. In addition, the cooling system must transport the coolant only in the quasi-liquid state and, for example, in contrast to boiling medium cooling, not partially in the liquid and partly in the gaseous state. Furthermore, it has an advantageous effect that it is fundamentally possible to resort to existing cooling systems, for example to those designed as water cooling systems.

Further advantages and embodiments will become apparent from the drawings and the corresponding description.

In the following the invention and exemplary embodiments will be explained in more detail with reference to drawings. There, the same components in different figures are identified by the same reference numerals. It is therefore possible that in the description of a second figure for a particular reference, which has already been explained in connection with another, the first figure, no further explanation. In such a case, it can be assumed in the embodiment of the second figure that the component identified there by this reference symbol also without further explanation in connection with the second figure has the same properties and functionalities as explained in connection with the first figure , Furthermore, for the sake of clarity, not all the reference signs in all figures are shown in part, but only those which are referred to in the description of the respective figure. It shows:

1 shows a drive system for an aircraft,

2 shows a detail of a conduit system of a cooling

tems with coolant,

3 shows details of the secondary cooling medium.

The term "electromagnetic interaction" is the interaction known from an electrical machine between see the magnetic fields of the magnetic means of the rotor, for example. Permanent magnets, and the magnetic means of the stator, for example. Current-carrying coils in one or more Statorwicklungssystemen meant, on the basis of which the electric motor develops its torque or due to which a generator provides an electric current.

A "non-rotatable" connection of two components, for example a rotor with a shaft, should be distinguished by the fact that a rotation of one of the components basically transfers to the other component, as well as the case where one of the components is decelerated In this case, the other component is also braked due to the non-rotatable connection It can be assumed that the rotational frequencies or rotational speeds of two components which are connected to each other in a rotationally fixed manner are always identical.

FIG. 1 shows by way of example and only schematically a hybrid-electric drive system 1, as can be used, for example, in an aircraft which is driven by a propulsion means or propeller 10. Other mobile applications of this electric drive system 1 are naturally conceivable, for example for driving a rail or watercraft.

The drive system 1 comprises the already mentioned propeller 10, which is driven by a drive 20, wherein the drive 20 comprises an electric motor 100. The propeller 10 is rotatably connected via a shaft 130 to a rotor 110 of the electric motor 100, wherein permanent magnets 111 are arranged on the rotor 110. The electric motor 100 further includes a stator 120 having a stator winding system 121. The electric motor 100 and in particular its

Stator winding system 121 is connected via power electronics 60 to an electrical power supply 30 of the drive system 1, which is set up and controlled by a control / controller 40 to feed an electric current into the stator winding system 121. This leads to in that the stator winding system 121 generates magnetic fields which interact electromagnetically with the magnetic fields of the permanent magnets 111, so that in consequence the rotor 110 and with it the shaft 130 and the propeller 10 are set in rotation. It can be assumed that this concept of the electric motor 100 is well known and therefore need not be explained in detail at this point. In the example illustrated here for illustration, the energy supply 30 comprises a battery 310 and a further electrical machine 200, which in this case is designed as a generator 200. The generator 200 has a rotor 210 with permanent magnets 211 and a stator 220 with at least one stator winding system 221. The rotor 210 is rotatably connected to a shaft 230. Furthermore, the shaft 230 is likewise non-rotatably connected to a combustion power unit 50, for example a turbine, and can be driven by the internal combustion engine 50. In the case of the shaft 230 driven by the internal combustion engine 50, the rotor 210 rotates with the permanent magnets 211 which coincide with the rotor

Stator winding system 221 of the generator 200 such electromagnetically interact in such a way that in the stator winding system 221, an electrical voltage is induced. It can be assumed that this concept of the generator 200 is well known and therefore need not be explained in more detail here.

The controller 40 controls the power supply 30, for example, depending on the state of charge of the battery 310 and / or the electrical energy required by the electric motor 100 such that the motor 100, the required energy from the battery 310 and / or from the generator 200 is supplied , The controller / controller 40 may also cause an electrical energy provided by the generator 200 to be at least partially supplied to the battery 310 to charge it. A possibly remaining part of the electric energy provided by the generator 200 can be supplied to the electric motor 100 are supplied. The described routing of the electrical energy between generator 200, battery 310 and electric motor 100 or power electronics 60 is realized by means of switches 31, 32, which are actuated by the controller 40 as needed. The possible flow directions of the electrical energy are indicated by corresponding arrows.

The electrical energy provided by the generator 200 and / or by the battery 310 is converted by the power electronics 60 into the form required for operating the electric motor 100.

During operation of the drive system 1 with propeller 10, drive 20, power electronics 60, electrical power supply 30 and internal combustion engine 50 heat will be generated at various components of the system 1, which must be dissipated as described in the introduction. For this purpose, a cooling system 70 is provided, which uses a coolant 73 which is supplied to the respective component to be cooled via a line system 71. In the following, it is purely exemplary assumed that the component to be cooled is the stator 120 of the electric motor 100. Accordingly, FIG. 1 shows only a connection with the stator 120 with regard to the line system 71, but not with other components of the drive system which may have to be cooled. However, it should be clear that, for example, the rotor 110 of the motor 100, the power electronics 60 , Rotor 210 and stator 220 of the generator 200, the battery 310 as well as other components which generate heat during operation of the drive system 1, can be understood as "component to be cooled" in each case, that means that the coolant 73 also flows through these components Accordingly, it is not intended to be understood that the solution described here should be used only for cooling the stator 120. The embodiment of the line system 71 does not show this development. The conduit system 71 may, for example, be a pipe system in which the coolant 73 is guided as indicated by the arrows. The basic mode of operation of the cooling system 70 is based on the concept that heat from the component to be cooled, that is to say from the stator 120 in the present example, passes to the coolant 73 flowing past the stator 120. In the example of the stator 120, this may be realized such that the coolant 73 (not shown)

Statorbleche the stator 120 flows around and thus is in direct contact with the stator laminations. The warmed up

Coolant 73 continues to flow and thus leaves the region of the component to be cooled, for example in order to discharge the absorbed heat elsewhere, for example to a heat exchanger 72 of the cooling system, for example to the environment. In the event that the cooling system 70 is designed as a circuit, the thus cooled coolant 73 can then be reused and, for example, again supplied to the same or another component to be cooled. The FIG 2 shows a section of the conduit system 71 in not true to scale representation and in particular the coolant 73. The coolant 73 for cooling the respective component comprises a primary, liquid and homogeneously distributed cooling medium 73-1, which is represented by the indicated waves len , A secondary cooling medium 73-2 is added to the primary cooling medium 73-1. Specifically, the secondary cooling medium 73-2 is selected to be in a solid state as long as the coolant 73 has a first, low temperature Tl, and that it is caused by the heat absorbed by the component in the cooling of the component corresponding increase in temperature of the coolant 73 to a second, higher temperature T2> Tl performs a phase change from the solid to a liquid state of matter.

The cooling approach used here is therefore based on the concept of particularly efficient two-phase cooling. In this case, the secondary cooling medium 73-2 lies special then in a solid state, when the coolant 73 has not yet absorbed heat from the component to be cooled. This is at least the case when the coolant 73 is on the way from the heat exchanger 72, in which it was possibly cooled down, to the component to be cooled. After sufficient thermal contact with the component to be cooled and the corresponding transition of the heat from the component to be cooled to the coolant 73 and the associated phase transition of the secondary cooling medium 73-2 from solid to liquid, the coolant 73 comprising the now liquid secondary cooling medium 73 flows -2 and the already liquid primary cooling medium 73-1 through the conduit system 71 to the heat exchanger 72, there to release heat to the environment as already described, wherein the secondary cooling medium 73-2 passes from the liquid to the solid state. It should be noted that this

Transition from liquid to solid does not have to take place first in the heat exchanger 72, but possibly also on the way through the line system 71 between the component to be cooled and the heat exchanger 72.

The quasi-carrier used as the primary cooling medium 73-1, which is typically constantly in liquid form, may be, for example, an oil or alternatively water. For the exemplary case in which the component to be cooled is the stator 120, a coolant 73 should be used which is non-conductive, in which case oil would be preferable as the primary cooling medium 73-1.

For example, paraffin may be used as the secondary cooling medium 73-2, which when heated carries out the phase transition from solid to liquid. Preferably, the paraffin is in a plurality of small balls 73 -2a with diameters of, for example. L-ΙΟμιη before, which in each case as shown in FIG 3 by a surfactant 73 -2b are sheathed, wherein the secondary cooling medium 73-2 total, for example, a weight fraction of 10-40% of the coolant 73 can make up. The optional

Surfactant sheathing 73 -2b causes the paraffin spheres 73-2a to not agglomerate with each other, so that the process is reversible. remains flexible and the so-designed coolant 73 can be repeatedly performed in a circuit between the heat exchanger 72 and the component to be cooled, wherein the secondary cooling medium 73-2 performs phase transitions from solid to liquid and liquid to solid in each circulation.

Suitable surfactants are commercially available surfactants, for example so-called "Tween40" or else "Tween 60", if appropriate mixed with so-called "SpanöO", but other surfactants would also be suitable.

The cooling method used can therefore be summarized in such a way that for cooling the component to be cooled, for example for cooling the stator 120, with the coolant 73 comprising the primary 73-1 and the secondary cooling medium 73-2 in a basic process comprising three steps in the first

Step the coolant 73 is performed with the solid state secondary cooling medium 73-2 to the component to be cooled, in the second step, the coolant 73 and with it the secondary cooling medium 73-2 absorbs heat from the component to be cooled, and whereby the secondary cooling medium 73-2 goes into a liquid state, and in the third step, the coolant 73 is guided away with the liquid state located in the secondary cooling medium 73-2 away from the component to be cooled.

Preferably, the coolant 73 circulates in a circuit. This is followed by a fourth step in the basic process, in which the coolant 73 guided away from the component to be cooled releases heat with the secondary cooling medium 73-2 still in the liquid state at a heat exchanging device 72, for example at the heat exchanger. so that the secondary cooling medium 73-2 reverts to the solid state. After this fourth step, the basic process with the first, the second and the third step can be executed again.

Alternatively, the coolant 73 could be drained from the cooling system 70 after the third step. It should be noted that the various connections of the controller / controller 40 with the different components of the drive system 1 are not shown for clarity. However, it follows from the respective context that and how the control / control 40 must be connected to the respective component. For example. For example, the controller 40 is connected to the battery 310 at least via a data connection to determine its state of charge. Likewise, the controller / controller 40 must be connected to the power supply 30 in a corresponding manner in order to be able to adjust the electric current which is supplied to the electric motor 100, in order to ultimately adjust the propeller 10 in the desired manner to produce a specific propulsion, and to confirm, on the other hand, the switches 31, 32 so that the flow of electrical energy is also adjusted within the power supply 30.

reference numeral

1 hybrid electric drive system

10 propellers

20 drive

100 electric machine, electric motor

130 wave

110 rotor

111 permanent magnet

120 stator

121 stator winding system

60 power electronics

30 electrical power supply

40 control / control

310 battery

200 electric machine, generator

210 rotor

211 permanent magnets

220 stator

221 stator winding system

230 wave

50 combustion engine

31 switches

32 switches

70 cooling system

73 Coolant

71 pipe system

72 heat exchangers

73-1 primary cooling medium

73-2 secondary cooling medium

73-2a paraffin

73 -2b Surfactant coating

Claims

claims

1. Coolant (73) for cooling a component, in particular for cooling a component (120) of an electric drive system (1), wherein the coolant (73) comprises a primary, liquid cooling medium (73-1), which is a secondary cooling medium (73-2) is added in the solid state.

2. Coolant (73) according to claim 1, characterized in that the secondary cooling medium (73-2) is selected such that when cooling the component (120) caused by the component (120) heat absorbed a phase change from solid into a liquid state of aggregation.

3. Coolant (73) according to claim 1 or 2, characterized in that the secondary cooling medium (73-2) is a paraffin-based cooling medium.

4. coolant (73) according to claim 3, characterized in that the secondary cooling medium (73-2) comprises a plurality of respectively coated paraffin volumes (73 -2a), in particular paraffin spheres.

5. Coolant (73) according to claim 4, characterized in that the sheath (73 -2b) of a respective paraffin volumes (73-2a) of a surfactant (73-2b) is formed.

6. Coolant (73) according to one of claims 1 to 5, characterized in that the primary cooling medium (73-1) is water or oil.

7. cooling system (70) for cooling at least one component (120) with a coolant (73) according to one of claims 1 to 6.

8. Cooling system (70) according to claim 7, wherein both the cooling system (70) and the component to be cooled (120) are each part of an electric drive system (1).

9. cooling system (70) according to one of claims 7 to 8, characterized in that the component to be cooled (120) is a component of an electrical machine (100, 200), in particular an electric motor (100) or a generator (200), a Power electronics (60) or a battery (310) is.

10. Cooling system (70) according to claim 9, characterized in that the component to be cooled is a stator (120, 220) or a rotor (110, 210) of the electric machine (100, 200).

11. An electric drive system (1), in particular for an aircraft, comprising a plurality of components (60, 100, 110, 120, 200, 210, 220, 310) and a cooling system (70) according to one of claims 7 to 10 for cooling at least one (120) of the plurality of components (60, 100, 110, 120, 200, 210, 220, 310).

12. A method for cooling a component (120) with a coolant (73) according to any one of claims 1 to 6, character- ized in that in a base process

in a first step, the coolant (73) with the secondary cooling medium (73-2) in the solid state is guided to the component (120) to be cooled,

- In a second step, the coolant (73) and with him the secondary cooling medium (73-2) heat from the to be cooled

Receives component (120) and thereby transitions into a liquid state,

- In a third step, the coolant (73) with the located in the liquid state secondary cooling medium (73-2) of the component to be cooled (120) is guided away.

13. The method according to claim 12, characterized in that the coolant (73) circulates in a circuit, wherein - Connected to the base process, a fourth step, in which the away from the component to be cooled (120) guided coolant (73) with the liquid state in the secondary secondary cooling medium (73-2) at a heat exchanging device (72) emits heat so that the secondary cooling medium (73-2) returns to the solid state, and

after the fourth step, the basic process with the first, the second and the third step is executed again.

14. The method according to claim 12, characterized in that the coolant (73) after the third step from the cooling system (70) is derived.