WO2011023900A1 - Cooling device for an electronic power system in a vehicle - Google Patents

Abstract

To cool a component of an electronic power system in a vehicle, the device includes: a tank (11) containing a liquid-phase fluid; an evaporator (12) including spheres that constitute a porous medium for pumping the liquid-phase fluid of the tank (11) by capillarity and for converting the fluid into the vapor phase by absorbing a thermal load (2) generated by the component; a condenser (16) connected, at the outlet of the evaporator (12) in order to receive the vapor-phase fluid, to a cold source for converting the fluid from the vapor phase into the liquid-vapor phase, and to the inlet (11) of the tank in order to return the liquid-phase fluid thereto.

Description

 "Cooling device for electronic power system in a vehicle"

The invention relates to a cooling device for an electronic power system in a vehicle, particularly in a motor vehicle.

 An electronic power system is particularly useful in a hybrid or all-electric vehicle to provide power and recovery in electrical energy used to drive and slow the vehicle. Such a system generally comprises many components among which may be mentioned for illustrative and not exhaustive, a battery, super capacitors, thyristors or power transistors.

 The known cooling devices are unsatisfactory.

 The finned panels or radiators are poorly adapted to the new components, the dissipated power of which results in a considerable bulk resulting from the necessary exchange surface.

 Mono-phase coolers of cylindrical type or water plates, pose a problem of integration for large powers to dissipate.

 The immersed two-phase cooling of the components is based on a boiling phenomenon that involves implementation and maintenance constraints that are too high for the automotive sector.

 Two-phase gravity systems, such as for example heat pipes, can only operate vertically, which causes integration difficulties. Their transfer capacities remain limited compared to other two-phase technologies.

The known cooling devices (fins, air loop, fluid loop, etc.) have another disadvantage, that of not absorbing peaks of power in severe stresses with consequence of making difficult the thermal regulation of the power electronics. However, a sudden rise in temperature can cause irreversible damage to electrical components. To avoid this, conventional cooling systems are generally oversized, which implies additional constraints of space and weight.

 To remedy the problems posed by the prior art, the subject of the invention is a device for cooling at least one electronic power system component in a vehicle comprising:

 a reservoir containing a fluid in the liquid phase;

 an evaporator arranged to pump the fluid in the liquid phase of the reservoir by capillarity and to bring the fluid into the vapor phase by absorbing a thermal load generated by the component;

 a condenser connected at the outlet of the evaporator for receiving the fluid in the vapor phase, at a cold source for bringing the fluid from the vapor phase to the liquid phase and at the inlet of the reservoir for returning the fluid to the liquid phase.

 Remarkably, the evaporator comprises a porous medium comprising spheres so as to pump the fluid by capillarity.

 Advantageously, the reservoir comprises a heating element for increasing the fluid temperature in the liquid phase.

 Preferably, the device comprises a regulation module for acting on the heating element so as to homogenize a temperature of the evaporator to a saturation temperature at which the fluid passes from the liquid phase to the vapor phase.

The invention also relates to a motor vehicle comprising at least one component power electronics, characterized in that it comprises a device according to the invention.

 The following explanatory description refers to the accompanying schematic drawings given solely by way of example, illustrating several embodiments of the invention and in which:

 Figure 1 is a block diagram of the device;

 Figure 2 shows in more detail an absorber of the device.

 With reference to FIG. 1, the device according to the invention uses a diphasic fluid loop with thermo-capillary pumping (BFDPT) contiguous to the power electronics housings present in large quantities on hybrid and electric vehicles.

 Among the main elements of the device, there are one or more evaporators 12 for absorbing a thermal load 2 generated by electronic power components (not shown), a condenser 16 for discharging the heat load in the form of a flow of heat 6 to a cold source and a reservoir 11 fed by a fluid in the form of a liquid 1.

 The evaporator 12 is arranged to pump the fluid in liquid form by capillary action from the reservoir 11 by means of capillary tubes or preferably of a porous medium such as that explained hereinafter with reference to FIG. 2.

 Under the effect of the heat load 2 applied to the porous medium included in the evaporator 12, the liquid 1 vaporises and the vapor 5 escapes into a collector 14.

The boundary 3 schematically shows in the form of meniscus the limit at which the fluid pumped by capillarity in the evaporator 12 in the liquid phase 1, reaches the saturation temperature T ₃ from which it boils in a two-phase state. Considering a mass flow M _F of liquid 1 of thermal capacity C _pi which enters the evaporator 12 at a temperature Tn at the outlet of the tank 11, the absorbed heat flux Φ1-3 is given by a formula of the type:

i) Φ _{t 3} = M _F x C _pl (T ₈ -T ₁₁ )

The boundary 4 schematically represents the limit below which the fluid is still in the two-phase state in the evaporator 12 in the form of wet steam at the saturation temperature T ₃ . Considering the mass flow M _F of two - phase fluid of evaporation mass enthalpy H _iv which evaporates in the evaporator 12 at the temperature T ₃ , the absorbed thermal flux Φ3-4 is given by a formula of the type:

 ii) Φ3-4 = MFXHIV

Considering that the thermal load 2 generates a thermal flux Φ2 towards the evaporator 12, the device is dimensioned so as to obtain a mass flow M _F of optimal fluid which satisfies the relation:

Hi) Φ ₂ = 0 ^ + 0 ₃ ^ = M _F x [C _p (T _s -T _u ) + H _lv ] The capillarity and evaporation allow the fluid to move in a completely passive way, so that the mass flow M _F is obtained without the need to use any mechanical pump to ensure the circulation of the fluid. This results in a considerable maintenance gain compared to conventional cooling circuits.

The collector 14 of the evaporator is connected to the condenser 16 via a pipe 15 in which the fluid in the vapor phase 5 flows at a vapor temperature T ₅ close to the saturation temperature T ₃ .

 The pipe 15 extends in the form of a bent tube 17 in the condenser until it opens into a second pipe 18 which connects the condenser 16 to the tank 11.

The condenser 16 is in contact with a cold source 6 at a temperature T ₆ lower than the saturation temperature T ₃ so that the vapor 5 condenses in the tube 17 between two points 7 and 8 of temperature T ₃ . Considering the mass flow rate M _F of two-phase fluid of condensing mass enthalpy H _vi which liquefies in condenser 16 at temperature T ₃ , the exhumed thermal flux Φ ₇ -s is given by a formula of the type:

iv) Φ ₇ _ ₈ = M _F xH _vl

The liquefied fluid continues to cool beyond the point 8 to the outlet of the tube 17 so that the liquid 1 is obtained at the outlet of the condenser 16 at a sub-cooling temperature Ti between the saturation temperature T ₃ and the cold source temperature T ₆ . Considering the mass flow M _F of liquid 1 of specific heat capacity C _pi which enters steady state in the tank 11 at the temperature Ti at the outlet of the condenser 16, the exhumed heat flow Φs-n is given by a formula of the type:

v) O _8-11 = M _p XC _p1 (T ₈ -T ₁ )

Finally, the cold source 6 resorbs a heat flow Φ ₆ of the condenser 16, according to the relation:

It can be seen that the thermal flux Φ ₆ evacuates the thermal flux Φ2 at the cold source, independently of the temperature T ₂ of the thermal load 2 which stabilizes at a value close to the saturation temperature T ₃ and the temperature T ₆ of the cold source 6. The expression of the thermal flux Φ ₆ as a function of the temperature difference (T ₆ - T ₂ ) and of a thermal conductance K _t h, is of the form:

vil) O ₆ = KJT ₆ -T ₂ )

This observation amounts to considering the thermal conductance K _t h as a variable conductance that naturally adapts to the difference in temperature to convey the heat flux. This phenomenon is explained by the length of condensation between points 7 and 8 which naturally increases when the temperature T ₂ or the heat flow Φ ₆ increases and vice versa.

In summary, the steam generated in the evaporator 12 by the heat load 2, then circulates to condenser 16 where the power of the thermal load is dissipated. The subcooled liquid then exits the condenser 16 to return to the evaporator 12 and ensure the cycle. The adaptation of the condensation length allows the device to have a variable conductance so that the cold source temperature has little influence on the maintenance of the temperature level at the evaporator in contact with the electronic power components, provided that of course, the cold source temperature is below the saturation temperature.

 The evaporator 12 is shown in greater detail in FIG. 2 where a body 19 of the evaporator is in contact with an electronic power component 20. The evaporator is filled with spheres 13 which produce a porous medium which sucks the liquid 1 by capillarity.

 The upper face of the highest spheres 13 of the evaporator 12 are in contact with either a solid part of the body 19 which transmits the heat load 2 generated by the electronic power component 20 to the spheres by thermal conduction, or of a opening in the body 19 arranged to constitute the collector 14 which discharges the vapor 5 towards the outlet of the evaporator 12.

 The tank 11 is positioned under the lower part of the evaporator so as to ensure a role of fluid reserve and control of the cycle. A thermal regulation of the tank 11 is incidentally provided to stabilize the saturation temperature in the device and more particularly in the evaporator.

In the tank 11, a heating element 21 such as an electrical resistance, a thermo element or the like, is connected to an electronic unit 22 which regulates the operating temperature of the device so as to maintain the temperature T ₂ at an optimum value at the interface of the control unit. By Moreover, the naturally variable conductance K _t h of the device makes it possible to maintain this temperature whatever the power dissipated, of course insofar as the device is sufficiently sized.

 The regulation is parameterized to act on the behavior of the relation Hi) that we resume here:

Hi) Φ ₂ = M _F x [C _p (T _s -T _u ) + H _lv ]

 In the absence of a heating element 21, the temperature Tn in the tank 11 is substantially equal to the temperature Ti undercooled at the outlet of the condenser 16.

The following table makes it possible to compare the evaporation enthalpy H _iv and the specific heat capacity C _pi in the liquid state of various fluids selected during debugging tests of the device:

Note that the amount of heat absorbed by the enthalpy of vaporization is 150 for hexane to 250 for ammonia times higher than the amount of heat absorbed by heat capacity to raise the temperature of the liquid by 1 Kelvin.

Now the path traveled by the fluid in the evaporator to reach the saturation temperature is then as much lost length to proceed to evaporation which provides the best heat exchange efficiencies. On the other hand, a minimum wetting length of the liquid in the evaporator is necessary to obtain the wetting effect by capillarity. The regulation is parameterized to obtain the best compromise between these two constraints.

 Several variants of the device are possible.

 In a type of architecture CPL ("Capillary Pumped Loop"), the tank 11 is on the pipe 18 while in an architecture type LHP ("Loop Heat Pipe"), the tank is attached to the evaporator.

 The design of the tank 11 may also vary in terms of shapes and volumes. Different designs of the evaporator are possible. Other fluids than those mentioned above can also be used in the device, in particular depending on the operating temperature that is to be achieved.

 Among the numerous advantages of the use of a diphasic fluid loop device with thermocapillary pumping, in particular with thermal regulation of the tank for the cooling of the power electronics, mention may be made of a maintenance gain related to the absence of a pump mechanical, a high transfer capacity (up to 1OkW for Im driving) related to the latent heat of the fluid used, maintaining and regulating the temperature of the electronics for different powers to dissipate thanks to the variable conductance, a possibility of regulating the temperature of the power electronics through the tank and ease of integration resulting from an absence of stress on the position of the cold source relative to the dissipation zone.

Claims

A device for cooling at least one electronic power system component (20) in a vehicle comprising:  a reservoir (11) containing a fluid in the liquid phase;  an evaporator (12) arranged to pump the fluid in the liquid phase from the reservoir (11) by capillarity and to bring the fluid into the vapor phase by absorbing a heat load (2) generated by the component (20);  a condenser (16) connected at the outlet of the evaporator (12) for receiving the vapor phase fluid, at a cold source for supplying the fluid from the vapor phase to the liquid phase and at the inlet of the reservoir (11) to make it returning the fluid to the liquid phase, characterized in that the evaporator (12) comprises a porous medium comprising spheres (13) so as to pump the fluid by capillarity. 2. Device according to claim 1, characterized in that the reservoir (11) comprises a heating element (21) for increasing the fluid temperature in the liquid phase. 3. Device according to claim 2, characterized in that it comprises a control module (22) for acting on the heating element (21) so as to homogenize a temperature of the evaporator (12) to a saturation temperature. at which the fluid passes from the liquid phase to the vapor phase. 4. Motor vehicle comprising at least one component (20) of power electronics, characterized in that it comprises a device according to one of claims 1 to 3 for cooling said component (20).