Thermal power distribution system

Abstract

The invention is comprised of one or more variably conductive thermal switches. The thermal switches are sandwiched between two thermal conductors (e.g., heat pipes). Within each thermal switch is an array of micro-switches with a number of finger-like structures that are electrically actuated to form one or more thermal connection between two thermally conductive layers. The net thermal conductance of the switch is scalable based on the number of activated micro-switches. By changing the thermal conductance between the source and the sink, the temperature of the source may be adjusted based on the needs of the system in which it is employed.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional patent application No. 61/204,515, filed on Jan. 8, 2009, entitled “Thermal Power Distribution System” pursuant to 35 USC 119, to which priority is claimed and which application is incorporated fully herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

N/A

DESCRIPTION

1. Field of the Invention

The invention relates generally to the field of thermal management systems.

More specifically, the invention relates to a thermal power distribution system comprising a plurality of thermally conductive switches in a reconfigurable thermal switch array.

2. Background of the Invention

Government and industry have a need for reconfigurable thermal networks for applications such as thermal control and management of space satellite components. A reconfigurable thermal network enables on-the-fly reconfiguration of thermal control systems in response to the needs of the satellite and the requirements of the mission.

A reconfigurable thermal network not only improves a satellite's thermal performance, but can significantly reduce the design time required for developing space thermal control systems. It is estimated a reconfigurable thermal network could provide a reduction of design cycle time from the current one to three years to a few weeks.

The reconfigurable thermal network of the disclosed invention employs a thermal switching system with common heat load interfaces whereby reconfiguring the system for each new satellite simply involves connecting the heat loads or sources to the provided thermal “interface” ports and modifying the related control parameters. The disclosed “plug-and-play” approach works much like connecting different electrical components to a power supply and adjusting each output channel to the desired voltage level.

SUMMARY OF THE INVENTION

The invention is comprised of one or more thermally conductive switch elements. One or more of these thermally conductive switch elements are disposed between two heat conducting members (e.g., heat pipes), preferably in a partial vacuum. The switches may define an array of thermally conductive switch elements with a plurality of finger-like structures that are electrically actuated to form one or more thermal flow paths between the two heat conducting members. The net thermal conductance of the switch array is scalable based on the number of activated thermally conductive switch elements. By selectively creating or removing one or more thermal flow paths between a heat source and the system heat sink(s), the temperature of the heat source may be adjusted based on the needs of the system in which it is employed.

The thermal power distribution system of the invention takes advantage of the fact that while complex systems such as satellites may have many heat sources, each system may have only one heat sink (thermal radiator) element. Typically, a system thermal sink is made up of one or two thermal radiators from which all excess thermal energy in the system must be rejected.

A beneficial aspect of the disclosed invention is the ability to provide independent temperature control of, for instance, various modules within a satellite system (i.e., heat loads or boxes) by inserting a thermal switch array between the heat source and the system's thermal sink. This simple approach enables the described and claimed thermal power distribution system to function as a reconfigurable thermal network.

Further benefits of the above invention include:

1. The ability to autonomously maintain all system components within acceptable temperature ranges,

2. The ability to adapt to a wide range of component sizes, locations, heat loads and operating temperatures,

3. User-defined selection between high thermal conductance and low thermal conductance states as dictated by system components,

4. The ability to route heat from hot components to cold components (i.e., cool and preheat) to maximize efficiency,

5. Allows “plug-and-play” like thermal connectivity between components, panels and radiators.

As further described below, a thermal power distribution system is disclosed comprising a first heat conducting member, a second heat conducting member, a thermally conductive switch element in thermal communication with the first heat conducting member, switch actuator means for selectively positioning the switch element to selectively define a heat flow path between the first heat conducting member and the second heat conducting member which provides the above described benefits.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a preferred embodiment of the invention.

FIG. 2 is a cross-section of an array of thermally conductive switch elements of the invention.

FIG. 2A cross-section of a thermally conductive switch element of the invention in the actuated position and defining a thermal flow path between a first and second heat conducting member.

FIG. 2B cross-section of a thermally conductive switch element of the invention in the unactuated position whereby no thermal flow path is defined between a first and second heat conducting member.

FIG. 3 shows an exemplar cross-section of a frame and insulated standoff of the invention.

FIG. 4 shows plan view of an exemplar switch array of the invention.

FIG. 5 shows thermal model of a preferred embodiment of the invention.

FIG. 6 illustrates a top view of a preferred embodiment of a thermally conductive switch element of the invention and illustrates the switch actuator means and vacuum cavity therebelow.

FIG. 7 is a graph showing the conductance ratio versus the number of thermally conductive switch elements in a switch array of the invention.

FIG. 8 is a graph showing the conductions ratio versus contact conductance.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to the figures wherein like numerals designate like elements among the several views, a block diagram of a preferred embodiment the thermal power distribution system 1 of the invention is illustrated in FIG. 1.

FIG. 1 shows a block diagram depicting the major components in a preferred embodiment of a thermal power distribution system comprising heat radiating means 5, such as a thermal radiator or heat sink, thermal switch control electronics 10, a first heat conducting member 15, a second heat conducting member 20, a thermal interface port 25, a thermally conductive switch element 30 in a switch array 32 and one or more heat loads or “boxes” 35.

Thermal power distribution system 1 provides a standardized, reconfigurable thermal control system for a wide range of instrumentation having thermal management design concerns such as satellites. For instance, reconfiguring thermal power distribution system 1 for a new satellite involves simply connecting heat loads 35 to thermal interface ports 25 and adjusting thermally conductive switch elements 30 on each thermal interface port 25 to achieve the desired temperature.

The terminal end of thermal power distribution system 1 is in thermal communication with thermal radiator 5 whereby excess heat may be discharged to a predetermined location such as a system component or into the environment. In this manner, the overall thermal conductance of switch array 32 using thermally conductive switch elements 30 can be electrically adjusted on the fly through switch control electronics 10.

Thermal power distribution system 1 desirably serves as a “gate keeper” of a satellite's heat rejection system. To ensure a wide range of control, the interconnections have low thermal resistances such as is possible utilizing heat pipes 40. With heat loads 35 connected to thermal ports 25, thermal power distribution system 1 balances a system's heat loads in real time to meet a predetermined temperature profile. This is achieved by actuating one or more switch arrays 32 comprising one or more thermally conductive switch elements 30 which are disposed between each heat load 35 and thermal radiator 5.

In the event a thermal source from a heat load exceeds or falls below a predetermined level, the thermal conductance between heat source 35, first heat conducting member 15 and second heat conducting member 20 is adjusted to achieve the desired temperature by selectively defining a thermal path between first heat conducting member 15 and second heat conducting member 20 in a manner similar to an electrical on-off switch.

FIGS. 2, 2A and 2B illustrate a cross-section of a preferred embodiment of switch array 32 comprising a plurality of thermally conductive switch elements 30.

As shown, thermally conductive switch element 30 is disposed between first heat conducting member 15 and second heat conducting member 20, which are the principal thermal transport connectors. By disposing a switch array 32 comprising a plurality of thermally conductive switch elements 30 at the junction between heat load 35 and thermal radiator 5, thermally conductive switch elements 30 function as a regulators for heat flow, thus regulating the temperature of heat load 35.

As better seen in FIGS. 3 and 4, to ensure low thermal leakage, a thermal insulation material 45 is provided as a standoff between first heat conducting member 15 and second heat conducting member 20. Preferable thermal insulation materials have low thermal conductivity such as glass, polymers, ceramic materials or materials with suitable thermal insulating properties.

A switch array 32 of thermally conductive switch elements 30 performs a key function in the modulation of temperature. With respect to the illustrated preferred embodiment, thermally conductive switch elements 30 are approximately 500 microns in diameter and about 1 mm in pitch. Thus, in an area of 30 mm×30 mm, about 900 thermally conductive switch elements 30 are provided.

A sectioned view of thermally conductive switch element 30 is shown in FIGS. 2, 2A and 2B and illustrates how the thermally conductive switch elements 30 are in thermal communication by physical/mechanical contact between the first heat conducting member 15 and second heat conducting member 20 and define a thermal path between the two conductors.

In its unactuated or “off” state, thermally conductive switch element 30 is in thermal communication only with first heat conducting member 15. Thermally conductive switch element 30 is disposed in a cavity defined between first and second heat conducting members 15 and 20 to allow variable control of heat flow by changing the thermal flow path when in its actuated or “on” state. Alternatively, thermally conductive switch element 30 may be disposed within vacuum cavity 70 so as not to be in thermal communication with either heat conducting member and, when actuated, define a physical/mechanical connection between the two heat conducting members whereby a thermal path between them is defined.

First and second heat conducting members 15 and 20 are desirably fabricated from high thermal conductivity material such as copper or copper beryllium. To provide structural support and thermal isolation, a frame for switch array 32 is preferably provided and supported with a glass thermal standoff. In this manner, an array of thermally conductive switch elements 30 can be electrically and individually turned on or off using suitable thermal switch control electronics 10, allowing the precise adjustment of the thermal conductance in the system.

The small size and large number of thermally conductive switch elements 30 make it possible to achieve a large thermal contrast ratio between the high and low thermal conductance states and permits very small incremental temperature adjustments.

As seen in the preferred embodiment of FIGS. 2, 2A and 2B, each thermally conductive switch element 30 is comprised of a switch actuator means 50, such as a piezoelectric element or disk, over which one or more thermally conductive (e.g., copper, beryllium copper or suitable thermally conductive material) fingers 55 are provided.

FIG. 2A illustrates the thermally conductive switch element 30 with fingers 55 in the “on” position. (i.e., defining a thermally conductive path).

FIG. 2B reflects thermally conductive switch element 30 with fingers 55 in the “off” position. (i.e., no thermally conductive path).

The terminal ends of fingers 55 may be furcated to define a plurality of tines which may further be provided with contact bumps such as a gold bump to provide enhanced thermal contact when thermally conductive switch element 30 is actuated.

The geometry of fingers 55 is preferably patterned such that the deformation of switch actuator means 50, here a piezoelectric disk, urges fingers 55 to and from a first (or “off”) position and a second (or “on”) position.

In a preferred embodiment, piezoelectric actuator material 50 expands or contracts depending on the voltage applied thereto. By disposing two layers of piezoelectric disk material with a flexible membrane therebetween, the contraction of one disk and expansion of the other disk causes the combined disk assembly to deform out-of-plane. This out-of-plane deformation (bending) enables fingers 55 disposed on the piezoelectric disk to make physical/mechanical contact with the adjacent heat conducting member surface, thus thermally bridging the respective heat conducting members.

An alternative preferred embodiment for fingers 55 (not shown) comprises layering a piezoelectric material upon a thermally conductive finger material such as copper whereby fingers 55 operate as cantilevered bimorph elements or alternatively, provided as unimorph elements comprising an active piezoelectric layer and an inactive non-piezoelectric layer

In the embodiment of FIGS. 2, 2A and 2B, switch array 32 of thermally conductive switch elements 30 is preferably approximately 500 microns in diameter. A copper film is deposited and patterned on a surface of a bi-layer piezoelectric switch actuator means 50. Switch actuator means 50 thus deforms out-of-plane when a voltage is applied to the piezoelectric element, urging fingers 55 that are in thermal communication with first heat conducting member 15 to make mechanical contact and be in thermal communication with second heat conducting member 20. The small size and high output force of the piezoelectric switch actuator means 50 results in low thermal contact resistance.

Thermally conductive switch elements 30 are actuated by piezoelectric switch actuator means 50. In an un-deformed state, thermally conductive switch elements 30 do not make contact with the second heat conducting member 20, thus no heat flow occurs between the respective heat conducting members. In a deformed state, thermally conductive switch elements 30 make thermal contact with the heat conducting member surface above it, thus thermally bridging the two heat conducting members 15 and 20.

The amount of deformation is a function of the piezoelectric disk diameter; in this example on the order of about a micron. Very high contact pressure is achievable with piezoelectric disks; on the order of a few hundred KPa.

Fingers 55 are preferably disposed within a vacuum or partial vacuum cavity 70 for thermal insulation between the heat conducting members.

The small features of thermally conductive fingers 55 and the small feature sizes involved in the invention are well-suited for fabrication using Micro-Electro-Mechanical-Systems (MEMS). MEMS processing technology may desirably be used to deposit and pattern sub-micron features on a wide range of materials, including silicon, metal, and dielectrics. Processing of thin-film piezoelectric materials and wafer bonding in vacuum with very small gaps has both been widely performed in the MEMS industry.

Electrical power operates piezoelectric switch actuator means 50. Although the voltage requirement is high, very little current flow occurs in actuation; hence the power consumption is low. In an alternative preferred embodiment, thermally conductive switch elements 30 may be designed to operate in a bi-stable mode. By introducing a built-in stress in the disk during the fabrication, a piezoelectric disk is provided in a deformed state as fabricated. The application of a voltage forces the disk to “snap through” from one deformed state to another and in the second stage. Thermally conductive switch elements 30 make contact with second heat conducting member 20. In this embodiment, no power is required to hold the switch in either of the two states, only for changing the states.

Any switch arrays 32 connected to unused thermal ports 25 or payloads that have been powered down are desirably shut off to minimize parasitic thermal loading.

More complex switch array networks may also be designed to provide additional thermal system reliability and operation flexibility. To achieve best performance, each thermal power distribution subsystem is thermally isolated from the other.

FIG. 6 illustrates an alternative preferred embodiment showing an array of fingers 55 disposed over a piezoelectric disk switch actuator means 50.

The variably conductive thermal switch technology of the invention provides many important advantages including:

1. Low system thermal resistance: Rejection of excess heat on space satellites is accomplished by thermal radiators. Radiators are costly to design and add extra mass to the launch load. To reduce the radiator size, thermal resistance of the network must be kept to a minimum. The thermal power distribution system herein uses a network of low thermal resistance heat pipes to connect heat sources to the sink.

2. Large thermal conductance ratio: To effectively configure a thermal network, the switches must have a large ratio of high to low thermal conductance states. The large ratio is desirable in order to manage a wide range of thermal loads due to different component sizes, distance from the sink, thermal loads, and operating temperatures.

3. Precise temperature control: The large number of thermally conductive switch elements provides the ability to control fine incremental change of thermal conductance; a preferred embodiment shows that approximately 900 contacts packaged in a 3 cm×3 cm thermal switch where the estimated thermal resistance change per contact is 0.015° C./W.

4. High frequency switching: A preferred embodiment of switch array 32 of the invention is actuated by piezoelectric actuator means 50 capable of providing high actuation force and frequency. Typical frequencies achievable are in the range of thousands of Hz. Thermal time constants are generally long (minutes), but for applications involving thermal actuators, a high frequency switch would be desirable.

5. Reconfigurable network: Reconfiguration of the thermal network on the ground or in space is possible to achieve optimal thermal performance of a system.

6. Short design cycle time: Thermal power distribution system 1 is conceived to become a standard thermal control system with a wide range of capabilities. Designing the thermal control systems for spacecraft shifts from designing custom control systems and components to selecting appropriate thermal power distribution system features with capacity for handling the worst-case loads. Once the thermal power distribution system 1 is selected, the operation and control can be very quickly established and tested. Any unexpected thermal loads in the components can be readily accommodated.

7. Optimum thermal performance: The thermal power distribution system can be reconfigured on the ground or in space. Changes in thermal loads can be accommodated by continuous optimization of the thermal power distribution system 1 by simply modifying the thermal conductance characteristics of switch array 32. Changes in thermal loads due to environmental effects or system degradation can be also compensated by closed loop control or ground control.

8. High reliability: Thermal power distribution system 1 is a fault tolerant design; unexpected thermal loads can be accommodated during system testing. Component failures or changes due to unforeseen thermal effects while in space can also be accommodated by rebalancing the thermal conductance of each heat source. For higher reliability, redundant lines and switches may be connected to critical components.

9. Low system cost: Thermal power distribution system 1 is designed to accommodate a wide range of thermal loads. Furthermore, the system can be designed with common interfaces and simple reconfiguration software. The result is a system that may be used in multiple satellites, thus helping to reduce system cost.

10. Plug-and-play connectivity: With a standard interface provided for thermal connections and software control, thermal power distribution system 1 can be easily connected to a wide range of spacecraft. Since each component is independently controlled by dedicated switch arrays 32, adding and removing components to the system can be made in a plug-and-play fashion, without affecting the rest of the system.

11. Use an array of a large number of thermally conductive switch elements 30 provides an increase the thermal conductance ratio, the use of piezo-electric actuator means provides the high force desired for low contact resistance and sealing the switch elements in a partial vacuum provides improved thermal isolation and high reliability.

FIG. 5 shows a thermal lumped model representing the thermally conductive switch elements 30. Each switch array 32 is modeled as a series of resistors consisting of a thermal contact, a conductive finger (e.g., copper), and conductive film (e.g., copper). The thermally conductive switch elements 30 are connected in parallel and the number of switches in the illustrated model is 900. Also in parallel with the thermally conductive switch elements 30 is thermal insulation material 45 (i.e., an insulating standoff ring).

The model for the illustrated standoff ring comprises a standoff ring and vacuum seals (e.g., glass frit). At the top and bottom of the arrays are fingers 55. A substrate (e.g., silicon) is also included in the illustrated thermal model for first heat conducing member 15.

The illustrated thermal model is a steady state conduction model. No convection or radiation heat transfer is included in this model since both effects are considered negligible. A partial vacuum condition is also assumed in the switch. The conduction equations are written as a series of resistors.

The results of the thermal modeling are summarized in Table 1.0.

TABLE 1.0
Summary of switch performance
	Modeling Results
	Thermal Resistance - switch full on	.15°	C./W
	Thermal Resistance - switch full off	13.92°	C./W
	Thermal Resistance Ratio - Roff/Ron	91
	Thermal Resistance Change - minimum	.015°	C./W
	Key Assumptions
	Number of switches	900
	Switch Diameter	500	microns
	Contact Conductance	10,000	W/m2-K

The modeling of a preferred embodiment of the switch array 32 shows that the key design parameters are the total number of thermally conductive switch elements 30 and related contact conductance.

FIG. 7 shows the effect of the number of thermally conductive switch elements 30 on the thermal conductance ratio. This ratio is defined as the thermal resistance of thermally conductive switch elements 30 in the “on” position over the “off” position. In this model, the contact conductance of the switch is assumed to be 10,000 W/m2-K. The results indicate that approximately 1000 thermally conductive switch elements 30 are desired to achieve a conductance ratio of 100. In the illustrated embodiment, in a switch array 32 having a size of 3 cm×3 cm, approximately 900 thermally conductive switch elements 30 can be provided.

FIG. 8 shows the effect of the contact conductance on the conductance ratio and assumes a value for the contact conductance of about 10,000 W/m2-K. Contact conductance for copper ranges from 2,250 to 3,520 W/m2-K in vacuum and when a foil is inserted, the values increases to 13,000 to 17,800 W/m2-K.

FIG. 8 illustrates the effect of contact conductance on the conductance ratio. For this model, a contact conductance value of 10,000 W/m2-K is assumed. The literature on contact conductance suggests a range from 2,250 to 3,520 W/m2-K for the copper operating in vacuum.

FIG. 8 further illustrates that the conductance ratio increases with an increase in contact conductance.

The finished thermally conductive switch elements 30 array is preferably encapsulated in a vacuum cavity 70. In addition, the deformation of the piezoelectric disks is very limited, thus the two heat conducting members should be bonded with very small gap between them.

Reconfigurable thermal networks have applications for thermal control of space, air, marine, and ground vehicles. These networks can potentially be used in industrial power and chemical power plants where the reconfiguration capability could make the plants more flexible and economical to operate. For example, it is possible to use this switch technology in thermally actuated systems. One of the problems with existing thermally actuated systems is a limitation in the rate of alternating heating and cooling. By connecting the thermal actuation system to one or more thermally conductive switch elements 30, the cooling rate can be modulated, thus the temperature of the system can be cycled at relatively high rates.

Many alterations and modifications may be made by those having ordinary skill in the art without departing from the spirit and scope of the invention. Therefore, it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the invention as defined by the following claims. For example, notwithstanding the fact that the elements of a claim are set forth below in a certain combination, it must be expressly understood that the invention includes other combinations of fewer, more or different elements, which are disclosed in above even when not initially claimed in such combinations.

The words used in this specification to describe the invention and its various embodiments are to be understood not only in the sense of their commonly defined meanings, but to include by special definition in this specification structure, material or acts beyond the scope of the commonly defined meanings. Thus if an element can be understood in the context of this specification as including more than one meaning, then its use in a claim must be understood as being generic to all possible meanings supported by the specification and by the word itself.

The definitions of the words or elements of the following claims are, therefore, defined in this specification to include not only the combination of elements which are literally set forth, but all equivalent structure, material or acts for performing substantially the same function in substantially the same way to obtain substantially the same result. In this sense it is therefore contemplated that an equivalent substitution of two or more elements may be made for any one of the elements in the claims below or that a single element may be substituted for two or more elements in a claim. Although elements may be described above as acting in certain combinations and even initially claimed as such, it is to be expressly understood that one or more elements from a claimed combination can in some cases be excised from the combination and that the claimed combination may be directed to a subcombination or variation of a subcombination.

Insubstantial changes from the claimed subject matter as viewed by a person with ordinary skill in the art, now known or later devised, are expressly contemplated as being equivalently within the scope of the claims. Therefore, obvious substitutions now or later known to one with ordinary skill in the art are defined to be within the scope of the defined elements. The claims are thus to be understood to include what is specifically illustrated and described above, what is conceptually equivalent, what can be obviously substituted and also what essentially incorporates the essential idea of the invention.

Claims

1. A thermal power distribution system comprising:

a first heat conducting member,

a second heat conducting member,

a thermally conductive switch element in thermal communication with said first heat conducting member,

switch actuator means for selectively positioning said switch element to selectively define a heat flow path between said first heat conducting member and said second heat conducting member.

2. The thermal power distribution system of claim 1 further wherein said switch actuator means comprises a piezoelectric material.

3. The thermal power distribution system of claim 1 wherein said thermally conductive switch is comprised of copper.

4. The thermal power distribution system of claim 1 wherein said thermally conductive switch is comprised of beryllium copper.

5. The thermal power distribution system of claim 1 further comprising a volume defined between said first heat conducting member and said second heat conducting member.

6. The thermal power distribution system of claim 1 wherein said volume has a partial vacuum disposed therein.

7. The thermal power distribution system of claim 1 wherein at least one of said first or second heat conducting members comprises a heat pipe structure.

8. The thermal power distribution system of claim 1 further comprising thermal switch control electronics.

9. The thermal power distribution system of claim 1 further comprising heat radiating means for the discharge of heat to a predetermined location.

10. A thermal power distribution system comprising:

a first heat conducting member,

a second heat conducting member,

a plurality of thermally conductive switch elements defining a switch array, wherein a plurality of thermally conductive switch elements is in thermal communication with said first heat conducting member, and,

switch actuator means for selectively positioning said selected ones of said switch elements to selectively define one or more heat flow paths between said first heat conducting member and said second heat conducting member.

11. The thermal power distribution system of claim 10 further wherein said switch actuator means comprises a piezoelectric material.

12. The thermal power distribution system of claim 10 wherein said thermally conductive switch is comprised of copper.

13. The thermal power distribution system of claim 10 wherein said thermally conductive switch is comprised of beryllium copper.

14. The thermal power distribution system of claim 10 further comprising a volume defined between said first heat conducting member and said second heat conducting member.

15. The thermal power distribution system of claim 10 wherein said volume has a partial vacuum disposed therein.

16. The thermal power distribution system of claim 10 wherein at least one of said first or second heat conducting members comprises a heat pipe structure.

17. The thermal power distribution system of claim 10 further comprising thermal switch control electronics.

18. The thermal power distribution system of claim 10 further comprising heat radiating means for the discharge of heat to a predetermined location.

1. A thermal power distribution system comprising:

a first heat conducting member,

a second heat conducting member,

a thermally conductive switch element,

switch actuator means for selectively positioning said switch element to selectively define a heat flow path between said first heat conducting member and said second heat conducting member.