Thermal Switch

Abstract

The invention concerns a thermal switch, characterized in that said switch comprises a thermal connection element consisting of an integral single part forming a body of thermally conductive homogeneous composition as well as an actuator for opening said switch by mechanical rupture of said thermal connection element.

Description

This invention relates to a thermal switch.

The invention applies in very general terms to any situation in which two mechanically and thermally coupled components must be thermally decoupled.

In this context, the invention is of particularly advantageous application to the cryogenic cooling of space equipment.

At the present time, two types of thermal switches are known:

• • differentially expanding thermal switches based on the principle of the change in the length of materials in response to the temperature of the system. Below a certain temperature, contraction of one of the materials brings the different parts into contact, thus providing the thermal connection. Above this temperature, however, the contact between the different parts is broken, along with thermal connection,
  • phase-change thermal switches based on the change of state of a fluid, the fluid being chosen to suit the operating temperature of the system. If it is a gaseous fluid, it will condense onto the cold part of the system and evaporate on the hot part. Transfer of mass is effected by gravity in a ground-based application and by capillarity in a micro-gravity application. To thermally disconnect the components, the system is purged of its gas, or alternatively the gas is transferred to a similar system in the case of redundancy.

Document U.S. Pat. No. 5,522,226 describes a thermal transfer device consisting of two separate components connected by means of an intermediate layer of iridium.

Document U.S. Pat. No. 4,384,610 describes a thermal coupling system using a member consisting of two different components separated or connected via a metal layer which can be melted by a temperature control.

The main problems with phase-change thermal switches are:

• • the dependency on the operating temperature determined by the materials,
  • the large thermal resistance of the connection,
  • a contact resistance which is large and difficult to control,
  • the need for long components in order to achieve sufficient movement to thermally connect or disconnect the components,
  • significant mechanical constraints in terms of manufacture due to small clearances between components,
  • the risk of the components bonding to the point where they can no longer be separated.

The main problems with phase-change thermal switches are:

• • the dependency on the operating temperature determined by the fluid used,
  • the limited number of fluids in existence, which limits the ranges of use,
  • the problems of pressure containment and sealing,
  • pollution-related problems during evacuation of the fluid from the system,
  • large dimensions due to the pumping ports, safety components and transfer tubes,
  • the difficulty of controlling phase changes in microgravity situations,
  • the fact that the service life and the performance of the connection are limited by the leakage rate of the system,
  • the fact that the thermal resistance of the connection in the non-conducting state, without fluid, is determined by sizing for mechanical strength associated with pressure.

It is therefore an object of the invention to solve all or some of the problems encountered with existing thermal switches, especially those associated with the limited range of operation, size and complexity of implementation.

The thermal connection is thus made by a material having very good thermal conductivity. To thermally disconnect the system, an external action mechanically ruptures this connection.

It is therefore a two-state irreversible device which offers the advantage of providing an excellent thermal link in nominal operation, yet being an excellent heat insulator once the mechanical connection is broken.

The other advantages of the thermal switch in accordance with the invention are:

• • nominal operation is completely passive. No phase change and no moving parts are required to make the initial thermal connection. The connection is naturally heat-conducting. When disconnection becomes necessary, all that is required is to activate the actuator in order to exert the necessary force to rupture the thermal connection element. Once the connection is broken, the force applied by the actuator can be removed and the thermal connection element is thermally insulating and once again passive,
  • it operates independently of the temperature of the system. Specifically, unlike phase-change or differential-expansion thermal switches which rely on the temperature of the change of state of a fluid or on the expansion of a material, the thermal switch of the invention is independent of the operating temperature and can be used over a wide range of temperatures,
  • it is very small, unlike phase-change thermal switches in particular, which require filling ports and safety members. The thermal switch of the invention is compact and low-weight.

Advantageously, said thermal connection element is mechanically resilient, which means that the forces that have to be applied by the actuator to said thermal connection element can be kept low.

In order that the force of the actuator can be removed, the invention provides that it is suitable for being kept open by return means following mechanical rupture of the thermal connection element.

In a first embodiment of the invention, said return means are external to said switch.

In this case, it is provided that said switch comprises a heat-insulating connection member, in order to maintain the mechanical cohesion of its various constituent components.

To limit the range of movement of the components after the thermal connection element has been ruptured, said connection member comprises a mechanical stop.

In a second embodiment of the invention, said return means consist of an elastic member connected to said switch. This member may be a prestressed bellows fitted around the switch.

The description which now follows with reference to the appended drawings, offered by way of non-restrictive examples, will show clearly the nature of the invention and the manner in which it can be implemented.

FIG. 1a is a diagram of a first embodiment of a thermal switch according to the invention in the conducting state.

FIG. 1b is a diagram of the thermal switch as shown in FIG. 1a in the heat-insulating state.

FIG. 2a is a diagram of a second embodiment of a thermal switch according to the invention in the conducting state.

FIG. 2b is a diagram of the thermal switch as shown in FIG. 2a in the heat-insulating state.

FIG. 3 is a diagram showing an example of an application of a switch in accordance with the invention.

FIGS. 1a and 1b show a thermal switch designed to provide thermal contact or, on the other hand, thermal insulation, between two components arranged one on interface 1 and one on interface 2 of the switch. As will be seen later in detail, these components may be, for example, a heat sink and a piece of equipment that must be cooled.

As FIGS. 1a and 1b show, the thermal switch comprises a thermal connection element 10 between the two interfaces 1, 2.

In the nominal conducting state of the switch shown in FIG. 1a, the element 10 has sufficient thermal conductivity to exhibit a shallow thermal gradient between the interfaces 1, 2.

If it is necessary to thermally decouple the interfaces 1, 2, an actuator 20 is activated to apply a force to the thermal connection element 10 sufficient to mechanically rupture said element 10. In order to reduce the force and the size of the actuator 20 and control the break, the thermal connection element 10 is made resilient by means of, for example, a frangible zone 11 in the form of a waist pre-formed into the element 10.

FIG. 1b shows the switch from FIG. 1a in the heat-insulating state, following rupture of the element 10.

To maintain an effective distance between the two remaining pieces of the element 10, return means, such as an external spring (not shown) are provided to exert a return force on the interface 1, for example, and keep the thermal switch open following mechanical rupture of the element 10.

As can be seen in FIGS. 1a and 1b, both the stiffness of the switch assembly and the guidance of the components following rupture are maintained by a mechanically rigid and heat-insulating connecting member 30. A mechanical stop 31 can be used to limit the movement of the components after rupture of the thermal connection element 10.

The embodiment shown in FIGS. 2a and 2b differs from that described above with reference to FIGS. 1a and 1b in the return means employed. In this second embodiment, the return force is exerted by an elastic member 40 connected to the switch. In FIGS. 2a and 2b, this member 40 is a bellows installed in a prestressed condition.

The advantage of such a switch structure is that it produces a completely integrated system that is totally isolated from the external environment.

The materials offering the best compromise between mechanical properties and thermal properties when producing the thermal connection element 10 are:

• • corundums (sapphire, ruby, etc.),
  • ceramics (silicon carbide, tungsten carbide, etc.), and
  • diamond, graphite, silicon, quartz, glass and all metals having a ductile-fragile transition at low temperatures.

These materials have mechanical properties favorable to the operation of the switch of the invention and optimal thermal conductivity at cryogenic temperatures.

The crystalline structure of these materials offers the further advantage of low tensile strength and limited elongation at break.

Unlike other thermal connections which rely on the contact between two or more walls, our thermal switch adds no internal contact resistance because of the use of a one-piece homogeneous connection element 10. The thermal performance of the proposed system is therefore optimal.

The invention thus has the following advantages over the known solutions.

The one-piece homogeneous structure of the connection element 10 avoids the need for an internal junction within the connection element 10. The system can therefore be subjected to higher mechanical loads, a particular advantage for a space application.

Also, the one-piece homogeneous structure has better thermal conductivity than elements with one or more internal connections. The one-piece element does not have the contact resistances found in prior art arrangements, which can be of the same order of magnitude as with the thermal resistance of the complete system.

Again, the homogeneous one-piece structure greatly reduces the problems of assembly, fabrication and hence reproducibility of the switch. These are often major problems in space applications. The connection element 10, once built, requires no other preparation or the use of extra materials necessary to transport the heat in particular.

Further, the one-piece element consisting of a component of homogeneous composition performs the principal function, which is to transport heat, with certainty. The reliability of such a system is much higher than that of any other system based on phase changes of materials or differential expansion.

To control the rupture, a frangible zone is created in the element 10 in the form of a reduced cross section in which the maximum stress is localized.

For reasons of mechanical strength, especially in space applications, the position of the frangible zone can vary along the element 10. This position can be optimized on the basis of the mechanical spectrum.

The actuators employed are preferably piezoelectric actuators. They have the advantage of being compact and of being able to exert large forces. The actuators are actuated by an electrical voltage, generate their force, and then resume their initial state as soon as the electrical power is cut off.

Piezoelectric actuators are moreover compatible with cryogenic operation in a vacuum.

The rigid connecting member 30 gives the components mechanical guidance. However, after rupture, conduction losses through this member must be minimized to ensure that the thermal disconnection is as efficient as possible between the two interfaces 1, 2. Materials which may be used are glass fibers and epoxy resins.

Lastly, the bellows 40 may be made of thin stainless steel. Its developed length should be as long as possible to reduce conduction losses.

One application of the thermal switch according to the invention is thermally to connect cryogenic coolers to equipment for space applications requiring redundancy. The equipment may for example be detectors, filters, amplifiers, bolometers, screens, mirrors, optics, etc.

The problems of redundancy in vehicular applications lead engineers to connect a single piece of equipment to several cryogenic coolers.

In ordinary cryogenic chains, the equipment to be cooled is connected thermally to the cold fingers of two duplicate coolers. The thermal link between the equipment and the cold fingers is provided by a flexible metal mesh, of copper for example. Should one of the coolers fail, the failed cooler not only ceases to participate in the cooling of the equipment but also adds an additional thermal load to the still-functioning cooler.

The thermal switch of the invention can be inserted into the conventional cryogenic chain to disconnect a failed cooler and eliminate the undesired losses which it produces when it is out of action.

FIG. 3 shows a setup for a redundant cooling system incorporating thermal switches in accordance with the invention.

The coolers 100, 200 may for example be Stirling, Gifford-McMahon, or pulse tube machines or any other cooler such as Joule-Thomson, Peltier, adsorption or other coolers. The cold fingers 101, 201 of these coolers are connected to the equipment 300 to be cooled either via a conducting metal mesh 110, 210 and a thermal switch 120, 220 mounted between the mesh and the equipment, or directly on the thermal switch.

When operating nominally, the thermal performance of the switches 120, 220 is optimal, and so the energy budget of the system is not degraded by the presence of these switches. The two coolers run at 50% load (for example), and each supply 50% of the requirement. There is therefore no thermal loss coming from the coolers.

In the event of failure of cooler 200, as indicated in FIG. 3, the thermal switch 220 is activated. The still-active cooler 100 runs at full power and supports the thermal load on its own, while the failed cooler 200 is disconnected thermally by the action of the switch 220. There is therefore no additional load on cooler 100 and the thermal budget is unaffected.

Claims

1-13. (canceled)

14. A thermal switch comprising a thermal connection element that includes a one-piece component forming a heat-conducting body of homogeneous composition and an actuator capable of opening the switch by mechanically rupturing the thermal connection element.

15. The thermal switch of claim 14, wherein the thermal connection element is mechanically resilient.

16. The thermal switch of claim 14, wherein the thermal switch is suitable for being kept open by return means following mechanical rupture of the thermal connection element.

17. The thermal switch of claim 16, wherein the return means are external to the thermal switch.

18. The thermal switch of claim 17, wherein the external means comprises a heat-insulating connecting member.

19. The thermal switch of claim 18, wherein the connecting member comprises a mechanical stop.

20. The thermal switch of claim 16, wherein the return means comprises an elastic member connected to the thermal switch.

21. The thermal switch of claim 14, wherein the thermal connection element is made of a corundum material.

22. The thermal switch of claim 21 wherein the corundum material is selected from sapphire and ruby.

23. The thermal switch of claim 14, wherein the thermal connection element is made of a ceramic.

24. The thermal switch of claim 23, wherein the ceramic is selected from silicon carbide and tungsten carbide.

25. The thermal switch of claim 14, wherein the thermal connection element is made of a material selected from diamond, graphite, silicon, quartz, and glass.

26. The thermal switch of claim 14, wherein the actuator is a piezoelectric component.

27. The thermal switch of claim 14, wherein the thermal switch is utilized in a cooling system.

28. The thermal switch of claim 14, wherein the thermal switch is utilized in a heating system.

29. The thermal switch of claim 14, wherein the thermal switch is utilized in a ground based or on-board cryogenic cooling system.

30. The thermal switch of claim 14, wherein the system is with redundancy.

31. The thermal switch of claim 14, wherein the system is without redundancy.