WO1994019833A1

WO1994019833A1 - Thermoelectric devices with recuperative heat exchangers

Info

Publication number: WO1994019833A1
Application number: PCT/US1993/001837
Authority: WO
Inventors: Aharon Zeev Hed
Original assignee: Aharon Zeev Hed
Priority date: 1993-02-16
Filing date: 1993-02-16
Publication date: 1994-09-01

Abstract

A thermoelectric device, in one embodiment, may be a surgical probe or catheter (310) and has a cylindrical structure with a hollow central annulus member (212) in which a liquid is pumped so that the heated fluid is pumped from the center of the structure and discharged on the outer surface of an outer annulus member (213). A plurality of thermoelectric cells (211a-211h) are positioned in the space between the inner and outer annulus member (212, 213) with the cells being radially directed relative to the axis of the inner annulus member (212). In another embodiment, the thermoelectric cells are arranged in plates (71) and liquid is pumped along one side of each plate (71) and then its opposite side.

Description

"THERMOELECTRIC DEVICES WITH RECUPERATIVE HEAT EXCHANGERS "

Background of the Invention

1. Field of the Invention

The present invention relates to thermoelectric based heat pumps and refrigerators and some of their specific applications.

2. Related Art

Thermoelectric cells are well known in the prior art for the conversion of electric power to heat or to convert heat to electric power. These cells, whose operation is based on the Seebeck effect or the Peltier effect, are used in their simplest form to measure temperature (as in many thermocouples) , in more complex structures to pump heat between a cold and hot reservoir when an external electrical power is supplied, or to generate electrical power when an external thermal gradient is provided.

An elementary thermocouple consists of two dissimilar materials connected electrically at one end and having a thermal gradient between their connected ends and their respective opposing ends. Such a thermal gradient induces a voltage which varies with the thermal gradient imposed and depends on the relative electronic properties of the materials of the thermocouple (the Seebeck effect) . Conversely, when a voltage is applied to the thermocouple it causes a thermal gradient to appear whose direction depends on the polarity of the applied voltage (the Peltier effect) . Heat pumps using the thermoelectric effect usually involve two different semiconductor materials, one a p-type semiconductor (conductivity due to positive charge carriers or holes) and the other an n-type semiconductor (current carried by negative charge carriers or electrons) . It is preferred that these semiconductors are capable of sustaining a large thermal gradient and therefore materials having low thermal conductivity are chosen. Typical materials used in thermoelectric cells are bismuth telluride (p and n type) , lead telluride and various compounds of silicon and germanium.

When the thermoelectric cells are used as heat pumps, or to transfer heat from a cold reservoir to a warmer reservoir, several thermocouples ("couple") are connected in series. Namely, the hot end of the p "leg" of one couple is connected to the hot end of the n "leg" of the next couple. Since all the cells are equal in composition, and dimensions, the voltage drop on each couple is the same (the total voltage divided by the number of cells) and a single thermal gradient is developed on the assembly between the cold junctions (between each pair's cold legs) and the hot junctions (between neighboring pairs) . Examples of such products are well known in the prior art and are available, for instance, from Thermoelectron Corporation of Waltham, Massachusetts. In the prior art, thermoelectric cells for heat pumps and for power generation units have generally been built in a planar geometry. Namely, a multiplicity of thermoelectric couples are assembled between two planes with all the intercouple junctions on one plane and all the intracouple (between the two members of a couple) junctions on the opposing plane. The couples are electrically connected in series and are thermally in parallel. Heat is transferred from one plane to the other plane when an appropriate DC voltage is applied to the assembly. The maximum temperature gradient achievable with a given couple depends on the properties of materials used in the couple and is generally limited to about 70 degrees Celcius. The temperature gradient of such planar devices can be further increased by cascading a number of devices in series thermally (but insulated electrically) , so that the hot side of one device serves as the thermal cold side of the next device in the cascade. The heat pumping capacity can be increased by connecting additional devices thermally in parallel.

The planar structure of the thermoelectric cells and cascades of the prior art imposes a limitation on the quantity of heat that can be extracted through the cells' cold face because the cells' efficiency decreases with increasing thermal gradient. With a given type of thermocouple, and within the limitations imposed by maximum current that can be passed through such couples, only an increase in the cold surface area (and thus additional thermocouples) can increase the rate of heat extraction from the cold reservoir. Thus, when the device to be cooled is relatively small, and the heat that needs to be extracted from that device is large, a thermoelectric cell cannot be used.

In the instant invention, the use of a recuperative fluid heat exchanger in conjunction with thermoelectric couples uniquely configured allow for higher temperature gradients and higher thermodynamic efficiencies.

Summary Of The Invention In the instant invention a plurality of thermoelectric couples are assembled as flat strips. All the intracouple junctions are on one side (the "cold side") and all the intercouple junctions are on the other side (the "warm side") . A large number of such strips are assembled side-by-side to form a flat plate with all their cold sides on one plane and their warm sides on the opposing plane. Neighboring strips are essentially thermally isolated from each other. The plate is then inserted into a vessel to divide the vessel space into a "cold" side and a "warm" side. These two spaces are separated by the structure of thermoelectric cells, except that at the distal end the two spaces are connected by a fluid path for the flow of a heat exchanging liquid. A DC current is passed through the assembly of thermoelectric cells so that all the intracouple junctions are cooled. Simultaneously, a continuous flow of heat exchanging liquid is pumped, or flows, in a direction orthogonal to the thermoelectric strips, with the fluid flow being toward the distal end of the plate (on the cold side) and the same flow of fluid returning on the warm side. In this configuration, the same heat exchanging liquid is progressively warmed in its flow over the warm side. The thermal gradient on each thermoelectric couple (and on each strip) between their respective cold and warm sides is very small, to keep the couples' efficiency high. However, the change in temperature of the heat exchanging liquid, during its flow to the distal end, may be very large and depends on the number of strips used in a particular device. Thus, the instant thermoelectric heat pump normally operates with a small thermal gradient on the individual thermoelectric cells and therefore operates at a high thermoelectric efficiency, for the device as a whole.

This unique feature overcomes a major shortcoming of traditional thermoelectric devices: that the efficiency declines very rapidly with the thermal gradient applied on the devices. In one of the embodiments of the invention a plurality of thermoelectric couples are positioned circumferentially on a cylinder so that all the intercouple junctions are on the inner surface of the cylinder and all the intercouple junctions are on the outer cylinder surface.

When a voltage is applied in one direction, the inner surface of the cylinder (inner core) cools off and heat is withdrawn from the core and rejected at the periphery (the outer surface of the cylinder) . When the voltage is applied in the reverse direction, heat is pumped into the inner core. This configuration allows for a number of novel thermoelectric devices. For instance, a number of cylindrical cells can be assembled side-by-side on a long cylindrical structure in which liquid flows through the inner core and is progressively cooled, resulting in a cold tip at the distal end of the structure. Since the quantity of heat extracted from the liquid is proportional to the length of the structure, or the area of the cylindrical structure inner surface, and this length can be made large relative to the cell's diameter, the flowing liquid can be used as a heat exchange medium to produce very cold heat pump tips which may be used to efficiently extract heat from very small objects. Objectives Of The Invention

It is the primary objective of the instant invention to provide a thermoelectric heat pump having a plurality of thermoelectric couples assembled as a number of independent strips which is operated with a recuperative heat exchanging medium so that the thermal gradient on each couple is small, yet the temperature lift on the assembly is large.

It is another objective of the instant invention to provide a cylindrically shaped thermoelectric cell having a plurality of thermocouples and to provide assemblies of such devices operating as heat pumps or refrigerators.

It is yet another objective to increase the efficiency of a multi-element thermoelectric refrigerator by operating each thermocouple of the device at a minimal thermal gradient.

It is yet another objective of the instant invention to provide an assembly of such cells assembled side-by-side on a common hollow structure in which a heat exchange fluid flows within the hollow core and returns in a path between the periphery and an enclosing cylindrical structure essentially concentric with the cell, to provide optimal heat removal at the distal end of the assembly.

It is yet another objective of the instant invention to provide thermoelectrically cooled surgical catheters and surgical probes. Brief Description Of The Drawings

Other objectives and features of the present invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings.

In the drawings:

Figure 1 is a perspective view of the thermoelectric heat pump of the present invention;

Figure 2A is a side cross-sectional view of a thermoelectric cell strip of the present invention, a cross-section of the thermoelectric refrigerator taken along line B-B of Figure 1;

Figure 2B is a cross-section taken along line A-A of Figure i;

Figure 3 is a cross-section of an alternative thermoelectric refrigerator of the present invention, along a plane similar to Figure 2A;

Figure 3A is a cross-section of the alternative thermoelectric refrigerator of Figure 3, taken along a plane similar to Figure 2B;

Figure 4 is a cross-section of an alternative thermoelectric refrigerator of the present invention along a plane similar to Figure 2A;

Figure 5 is a cross-section of an alternative thermoelectric refrigerator of the present invention along a plane similar to Figure 2A; Figure 6 is a cross-section of an alternative thermoelectric refrigerator of the present invention along a plane similar to Figure 2A;

Figure 7 is a cross-section through a cylindrical thermoelectric cell of the instant invention;

Figures 8A and 8B are cross-sections showing certain details of the structure of the cylindrical thermoelectric cell prior to assembly into stacked modules;

Figure 9 is a side cross-sectional view showing stacked assemblies of cylindrical thermoelectric cells in a different thermoelectric refrigerator;

Figures 10A, 10B and IOC are wiring schemes for assembling a multitude of cylindrical thermoelectric cells;

Figure 11 is a perspective view, partly broken away, of an embodiment of a thermoelectric refrigerator of the present invention;

Figure 12 is a front cross-sectional view of the refrigerator of Figure 11, taken along line A-A of Figure 1;

Figure 13 is a cross-sectional view of the refrigerator of Figure 11 taken along line B-B of Figure 7;

Figure 14 is a schematic diagram of a catheter system utilizing the refrigerator of the instant invention;

Figure 15A shows the geometry of prior art thermoelectric cells; and Figure 15B shows the geometry of prior art cascaded thermoelectric cells.

Description Of The Preferred Embodiments Figure 1 shows a perspective of a typical thermoelectric refrigerator 10 of the instant invention.

Figure 2A shows a cross-section through the refrigerator 10 along the plane B-B of Figure 1. Within the refrigerator thermoelectric couples 11 are connected to each other in series to form a strip 20. Each couple consists of a p-leg 13 and an n-leg 12; for example, the couple is of doped bismuth telluride. The p and n legs within each couple are connected by conducting elements 14, forming the intracouple junction. The p and n legs of neighboring couples are connected on the opposing plane by intercouple conducting elements 15. All the intracouple junctions 14 of a strip form a plane 14a ("cold plane") and all the intercouple junctions 15 form an opposing plane 15a ("warm plane") . When a DC current, in a given direction, is passed through the strip 20, a thermal gradient is developed on the thermocouples and heat will flow from the cold plane 14a to the warm plane 15a. By reversing the direction of the DC current the direction of that heat flow will be reversed as well. The strip 20 is terminated by feedthrough electrodes 16 and 17 on opposing ends of the strip 20 which are adapted to be connected to a DC power source 18. In the prior art, shown in Figures 15A and 15B, the thermocouples are mounted on ceramic substrates 9; but such substrates are not required in the present invention.

The spaces 23 between the couples and between the legs can be evacuated, in which case electrically insulating support structures, which are good thermal conductors, are provided as the cold and warm planes respectively. Alternatively, as shown in Figure 2A, the spaces 23 are filled with an electrical and thermal insulating material which also provides mechanical support to the strip 20. This eliminates the need for the insulating substrate (alumina) traditionally used on both sides of the cells, and thus further increases the efficiency of the thermoelectric cell, as further detailed below.

The multiple strips 20 are aligned and assembled adjacent to one another in a direction perpendicular to the cross-section shown in Figure 2A, forming a plate having a first side (cold side) and second side (warm side) . The plate of strips 20 are enclosed within a closed vessel 19 and the plate divides the vessel inner space into two volumes (spaces). The space 21 ("cold space") is adjacent to the plane containing the intracouple junctions 14 and the space 22 ("warm space") is adjacent the plane containing the intercouple junctions 15.

Examples of assembly methods that can be utilized for such structures are described in more detail below. To assure optimal operation of the system, the opposing sides of strip 20 (the electrodes 16 and 17 sides) have seals 24 and 25 within the vessel's walls to assure that there is no fluid flow around the strip from the space 21 to space 22.

A cross-section of the refrigerator in plane A-A is shown in Figure 2B. Here the view reveals that the refrigerator consists of many strips 20 positioned side-by-side forming the plate, each strip 20 being as described in connection with Figure 2A. The plate separates the vessel into two spaces, the cold space 21 and the warm space 22. The vessel 19 is terminated on the cold end by a distal cold plate 126. The cold plate 126 is in thermal contact with a thermal load 128. the opposing end (not shown in Figures 2A and 2B) includes means to pump a heat exchanging liquid to the cold space 21 and means to withdraw that liquid from the warm space 22. The fluid may be a gas or liquid and preferably is a silicone liquid. A thermally insulating barrier 127 provides separation of the cold and warm spaces 21 and 22 up to their respective means of heat exchanging liquid introduction and withdrawal.

The external walls of the vessel 19 are of an insulating material to assure the adiabatic development of a thermal gradient in the direction of flow of the heat exchanging liquid. The end wall 126 should, however, be made of a thermally conductive material to assure good heat transfer between the cold load and the recuperative heat exchanging liquid at the distal end of the device.

The operation of the refrigerator is as follows. DC power is applied to all the thermocouple strips and the heat exchanging liquid is pumped so that it flows in the space 21 toward the cold end 126. As the heat exchanging liquid flows, the thermoelectric strips withdraw heat from the heat exchanging liquid in the cold space 21 and transfer that heat to the warm space 22. As a result, the heat exchanging liquid is cooled to a temperature which essentially scales with the sum of the small thermal gradients on all the strips. When the heat exchanging liquid reaches the cold plate 12b, which is in contact with the thermal load 128, it is at a temperature somewhat lower than the thermal load's temperature, and thus withdraws heat from the cold thermal load. As the heat exchanging liquid returns through the warm space 22, it is heated from the heat output from each of the thermoelectric strips. The heat exchanging liquid exits the vessel at a temperature higher than its entry temperature and is directed to a warm heat exchanger (not shown) where the extracted heat is disposed of to the ambient environment. If desired, the liquid may then be recycled through the refrigerator. While the thermal gradient on each strip is kept low, a large thermal gradient can be achieved in the direction of flow of the heat exchanging liquid, the purpose of the refrigerator being to extract heat from the thermal load 128 and ultimately dispose of it.

Because each thermoelectric couple is exposed to a thermal gradient which is smaller than (T_A - T_c) ; where T_A and T_c are the exiting heat exchanging liquid's temperature and the cold plate temperature, respectively, the device operates at higher efficiency than in the prior art planar geometry thermoelectric couples. In such prior art planar thermoelectric cells (as shown in Figure 15A) , Δ T = T, - T. is limited to about 60°C to 70°C and the typical efficiency is in the range of 5% to 15%, for AT = 60°C and under 1% for 70°C. In the prior art, for higher Δ T, a cascade of a number of planar cells (see Figure 15B) with progressively larger areas is required. These conventional solutions are expensive and limit the heat pumping capacity of the devices. In contrast, the recuperative heat exchanger thermoelectric cell of the instant invention allows for higher Δ T and device efficiencies that are 3 to 10 times larger. Furthermore, the elimination of the alumina substrates on both sides of the cells further increases the efficiency. One can show that a thermal gradient inverse to the gradient on the active thermoelectric couples exists on the external substrates. This gradient is proportional to the thickness of the substrate and the heat transfer rate through the substrate and inversely proportional to the substrate's thermal conductivity and its area. For example, using alumina for such a substrate, 25 mils thick and 1.25" on each side (for instance Tellurex model # Cl-2.8-127-2) the inverse gradient Δ = W/7.5, where W is the rate of heat transfer through the substrate. Using the data for the same cell at a nominal temperature gradient of 60 C on the cell, operating with a current of 5 amperes (and about 12 volts) , the inverse gradient on the cold side isΔT ^?C_ = 1.43°C and on the hot side Δ T_u = 10.56 C. Thus the true gradient on the active thermocouples (bismuth telluride in this case) is 72 C. An approximation of the substrate-less cell's coefficient of performance (COP) is determined for an observed normal cell temperature gradient of 48 C or about 28%, which is about 80% higher than the COP of 15.65% of the same cell at a gradient of 60 C. For smaller gradients of 10°C to 30°C (on the couples) which are more typical to the operation of the recuperative heat exchanger assisted cells of the instant invention, the gains in COP due to the elimination of the alumina substrates are in the range of 30% to 50%.

Preferably all the thermocouples within a strip are of the same geometry and have the same electrical characteristics. Consequently, the thermal gradient on all the couples within a strip is the same (the same current passes through all the cells within a strip) . Once steady state is established, the temperature profile within the moving heat exchanging liquid remains constant with time and the heat exchanger is therefore a recuperative heat exchanger. We therefore term this refrigerator a "recuperative, heat-exchanger-assisted thermoelectric refrigerator."

Figure 3 shows the cross-section of a refrigerator 30 in which the inner space is divided into three parts by two plates of thermoelectric strips 40 and 50 respectively. The cross- section in the lane A-A of Figure 3 is shown in Figure 3A. As in Figure 2A, each strip consists of a plurality of thermocouples 31 having their intracouple junctions 34 forming cold planes facing the cold spaces 41 and 51 respectively, and their intercouple junctions 35 facing the warm space (52 and 42 jointly) . In this case the warm planes of the two groups of strips face each other, but by reversing the current, or reversing the orientation of the strips, the inner spaces 42,52 can be made the cold spaces and the two outer spaces may be made the warm spaces.

Referring to Figures 3 and 3A, in the configuration depicted, the heat exchanging liquid flows through the spaces 41 and 51 toward the cold plate 146, which is in thermal contact with a thermal load 148. As the heat exchanging liquid moves toward the cold plate 146, it is progressively cooled by each strip of thermocouples and thus reaches a low temperature at the cold plate 146, which is somewhat lower in temperature than the thermal load's temperature. Heat is withdrawn from the load 148 and the heat exchanging liquid returns through the common warm space 42 until it reaches a temperature above its original entry temperature to the refrigerator. The liquid then passes between the insulating plates 147 and 157 where it is directed to a warm heat exchanger (not shown) where the heat is rejected to ambient and the liquid is recycled to flow back in the spaces 41 and 51.

In Figures 4, 5 and 6 we show other embodiments of the instant invention where the geometry is essentially axial symmetric.

Figure 7 shows a cross-section through a thermoelectric cell 200 of the instant invention. In the following discussion it is assumed that the cold side of the couples (thermocouples) 211 (211a-211h) points toward the inner annulus 212 (inner core, tube or cylinder) of the cylindrical cell 210 and the warm side is in contact with the external retaining annulus 213. However, by reversing the polarity of the DC power source 214 (shown as a battery) the direction of heat flow and thus of the thermal gradient on each couple 211 will be reversed as well.

Each couple 211 (211a-211h) consists of two dissimilar elements, the p-type leg 215 (215a-215h) and the n-type leg 216 (216a-216h) . The two elements of each couple are electrically connected at their cold side with a conductor 217 (217a-217h) and the hot side of the p-type leg 215 (215a-215h) is electrically connected via another conductor 218 (218a-218h) to the hot end side of the n-type leg 216 (216a-216h) of a neighboring couple 211. Similarly, the hot side of the n-type leg 216 (216a-216h) of couple 211 is electrically connected to the hot side of the p-type leg 215 (215a-215h) of the neighboring couple on the opposing side by conductor 218 (218a-218h) . This arrangement continues between all neighboring couples except that at couple 211c the circuit is broken to allow connection to the power source 214 via a pair of electrodes 219 (wires) . When DC power is applied to the electrodes 219, a current passes through all the couples, and since the couples are electronically equivalent (same materials and dimensions) , the voltage of the power supply is equally distributed between the couples. The central annulus 212 and the external retaining annulus 213 are both made of heat conductive and electrically insulating materials. Alternatively, the cold and heat electrodes 217 and 218 are coated with a thin insulation to maintain the voltage differential between the various couples in the assembly and the annuli 212 and 213 are metal. The space between the insulating annulus 212 and annulus 213, which is not occupied by the couples, can be evacuated, left with ambient air or filled with a dielectric thermally insulating substance, for instance an epoxy, or a liquid, for example a trimethylsiloxy terminated polydimethylsiloxane (available as "PS040" TM from Petrach Systems, Bristol, Pennsylvania) .

For temperatures near ambient, the inner and outer annuli 212 and 213 are preferably made from a suitable thin electrically insulating plastic material. When manufacturing such a cell, the elements of the couples are preferably assembled on a flexible substrate which is later deformed to form a cylindrical structure as described in Figures 8A and 8B. Figure 8A shows a number of identical couples 221, each of the two legs are connected electrically with each other by conductors 227 and the couples are connected in series with each leg in one couple connected to the opposing leg of the neighboring couple by electrical connections 228. These connections, for example, of copper, are preferably deposited on the flexible plastic substrates 223 or made from thin and flexible metallic (copper) conductors fastened on the substrate 223. The opposing end elements are terminated with electrodes 229, allowing connection of the assembly to a power supply. By deforming the substrates 223 into a cylindrical structure, a cell is obtained as described in connection with Figure 7. Depending on the application and desired structure, the substrate 223 may be the external annulus 223 or the internal annulus 212.

In Figure 8B a similar structure is shown as in Figure 8A, except the legs have a cross-section which increases in the direction of increasing temperature, when the high temperature side of the cell is the external periphery of the cell. This geometry allows for somewhat more efficient operation of the cell. Axial structures as described herein with less symmetry, for instance, cylindrical structures having polygonal cross-sections, or even partially elliptical, are feasible as well.

For higher temperatures, appropriate structural metals for the inner and outer annuli are preferred and are coated with insulation at their point of contact with the electrodes 217 and 218. Alternatively annuli 212 and 213 may be of ceramic, like alumina or aluminum nitride (for good thermal conductivity) . In this case alternative assembly methods are used including fastening of each individual element on the inner annulus and sleeving the external annulus onto the assembly.

A plurality of these cylindrical thermoelectric strips are then assembled on a single tubular structure resulting in an elongate cylindrical refrigerator 240 as described in Figure 9. The individual strips 231 are assembled on a common hollow 242. A recuperative heat exchanger assisted thermoelectric refrigerator results when we insert the assembly 240 into a containing cylindrical structure 241 having one closed end as described in Figure 9.

As with the rectilinear refrigerator of the instant invention previously described, a heat exchange liquid is pumped through the inner hollow 242 toward the closed end plate 245 (cold plate) of the external cylinder 241 and returns in the space between the bracing structure 243 and the outer cylinder 241. Ring seals 244 are fitted at the opposing ends of the thermoelectric refrigerator 240 to prevent access of the heat exchanging liquid to the thermoelectric cells between the bore and the bracing structure. The recirculating pump and the specific plumbing associated with the inner flow and outer flow away from the refrigerator are not shown and these are well known in the prior art; some of these elements are, however, detailed in Figure 3.

When the power on the device described in Figure 9 is off, the temperature of the exchange fluid and the end plate 245 ("cold plate") is in equilibrium with the environment. Power is applied to the refrigerator 240 and simultaneously the flow of the recirculating liquid in the direction of the arrows 246 and 247 is started. When looking at a steady state flow of the heat exchanging liquid, a declining temperature gradient is established within the hollow 242.

We have marked three planes A, B and C perpendicular to the heat exchanging fluid flow in Figure 9. When moving from A to C the incoming heat exchanging liquid is gradually cooled until it reaches a temperature somewhat under the heat load's temperature. When returning on the outer periphery, the heat exchanging liquid is heated, first by absorbing heat from the load. Secondly, it is heated as it moves on the outer surface of the thermoelectric cells. The liquid is further heated by the heat pumped from the inner space (when cooling the flowing heat exchanging liquid) and by the resistive heating losses (vi) of the thermoelectric cells which equals the product of the current (i) through the cell by the voltage drop (v) on each cell:

The temperature gradient on thermoelectric cells will therefore increase in the direction form C to A, as each successive stage receives a somewhat hotter fluid. However, the thermal gradient on any thermoelectric cell will always be lower than the total temperature difference between the temperature of the exiting liquid at A and the temperature of the liquid at its maximum cooling inside the inner tube at C. Therefore, for this temperature difference the present device will generally provide a more efficient heat pump than traditional planar thermoelectric cells.

The cylindrical refrigerator structure of Figure 9 may be incorporated into a surgical probe or catheter 310, as shown in Figure 14. In Fig. 14 a catheter 310 is constructed from flexible tubing, extruded, for instance, from a silicone polymer (from Petrack Systems, Bristol, PA) or from "Peek", a polyetherketone (T.M. Uptech, Oak Harbor, Washington) , capable of keeping its flexibility to -30°C. The catheter is terminated at its distal end (inner end) with a tip 311 (head portion) , as described in Figure 9, and without a substrate. The catheter body portion 9 consists of two concentric tubular structures (double lumen structure) , an external lumen 313 and an internal lumen 334, each of which is an elongated flexible tube. The two lumens are not necessarily made of the same materials. The external lumen withstands fluid pressure in the range of about 150 to about 200 psi but need not be flexible to -30°C. , since the external lumen 313 is always at temperatures above freezing. The internal lumen 314, on the other hand, is made of a material capable of keeping its flexibility to -30°C. but is not required to withstand the same high pressures as the outer lumen. At the proximal end (outer end) the catheter 310 body portion 309 is terminated with a plug-like connector 312 that allows easy insertion to the fixture 315. Fixture 315 can have any of many possible shapes, including, without limitations, a cylinder, a handle or a paralleliped.

The fixture 315 is hollow and has an entry portion 316 connectable to the inner lumen 314 of the catheter 310 on one end and to a liquid containable tube 324. A circulating chilled fluid is chilled in an external chiller 323 connected to the tube 324, although such external cooling may not be required in all cases. The flow direction of the chilled fluid is toward the distal end of the catheter through the inner lumen 314 of the catheter 310. The fluid first cools the target tissues and then carries heat of fusion from the tissue. The return of the liquid to the chiller is in the tubular space between the inner and outer tubes of the catheter. The outgoing cooling fluid flows through the hollow of the fixture 315 to an exit port 317 and through tube 325 to the chiller 323 for cooling and recirculation. The connection of the fixture 315 with the plug¬ like termination of the catheter is self sealing so that, when the plug 312 is inserted in the fixture 315, the fluids can flow in their respective paths, but when the plug is disconnected, the fluid cannot escape. This can be achieved either by valves on the respective tubes or by spring loaded seals. The fluid conduits 324 and 325, as well as the electric lead pairs, are preferably consolidated into a single multi-element "cable" ported to the fixture 315.

In Figure 10 we show a number of ways by which the independent thermoelectric strips 20 of Figure 2 or 231 of Figure 9 can be connected electrically into a single thermoelectric refrigerator. Each thermoelectric strip is represented by the open circles (251, 251' and 251" for Figures 10A, 10B and IOC respectively) terminated by two electrodes (250, 259' and 259") for Figures 10A, 10B and IOC respectively.

Figure 10A shows the standard and preferred parallel wiring, in which opposing electrodes 259 of a single unit are connected to opposing conductors 258 and 257, connecting each unit to the power source 254 directly. The refrigerator is actuated when the switch 256 is closed.

This arrangement applies the same voltage on each strip and the current may vary slightly depending on the average temperature of the strip which will be a function of the position of the strip within the refrigerator. The heat extraction capability of each strip will also vary depending on the position of the strip within the refrigerator, strips close to the cold end usually having a smaller temperature gradient imposed on them (higher heat extraction capabilities) than strips near the hot end where the temperature gradient is larger. One of the advantages of the parallel wiring is its simplicity of implementation. Another advantage is that the failure of a given strip would not cause failure of the total refrigerator.

Described below is an embodiment of the instant invention of a large-scale heat pump in which a plurality of modules, such as are described in Figures 2B, 3A or in Figure 9, can be used.

Figure 11 shows the general structure of the refrigerator 70, without the warm heat exchanger and the piping leading the recuperative heat exchanging liquid in and out of the refrigerator working zone (these are shown in Figure 12) . Within the working zone are a number of parallel plates 71, extending from the top of the vessel 75 to the bottom, except for an open space 72 at the bottom of the plates which allows for fluid movement between the compartments 73 and 74 created between two neighboring plates. The bottom of the vessel is constructed of a thermal conductor and is in thermal contact with the thermal load 76. All other parts of the vessel 75 are preferentially thermally insulating. The top of each plate 71 is terminated with an insulating element 77 which preferably is an integral part of the plates 71. The edges 78 of the plates 71 form a seal with the side walls of the vessel 75. These seals need not be perfect as long as massive flow of the heat exchanging fluid in the space between the plate and the vessel wall, and thus between adjacent compartments, is prevented.

To better illustrate the structure of the refrigerator, Figures 12 and 13 respectively show cross-sections of the refrigerator through plane A and plane B. Plane A passes through all the plates 71 and is perpendicular to the plates, while plane B is through the surface of the rightmost plate 71.

Figure 12 is a cross-sectional view through the thermoelectric plates 71. These plates 71 consist of strips of thermoelectric couples 88 connected in series and embedded within the plate's matrix 71. This matrix is made of a higher thermally and electrically insulating material, for instance a closed pores polyurethane foam. In this specific embodiment each plate 71 consists of 17 such strips assembled side-by-side from the bottom of the plate 71 to the top of the plate; but to allow space for fastening and collection of leads, a segment 77 at the top of the plate is preferably left devoid of thermocouple strips.

A space 72 is left between the bottom of the plates and the bottom part of the enclosing vessel 75 to allow for return flow of the heat exchanging liquid. The bottom plate of the vessel is in thermal contact with the thermal load 76. In the present example, we have eight thermoelectric plates 71 separated by compartment spaces 83 and 84. The top of the vessel is equipped with a liquid flow manifold 180 capable of distributing a heat exchanging liquid at ambient temperature, for instance, from the warm heat exchanger to all compartments 84. Another liquid flow manifold 181 collects the heat exchanging liquid that has been heated, for instance, above ambient, from the compartment 83 and delivers the heat exchanging liquid to the warm heat exchanger 89. A variable speed recirculating pump 182 controls the flow rate of the heat exchanging liquid through the system. The heat exchanging liquid's direction of flow is indicated by the arrows. Specifically, this liquid flow is toward the thermal load in the compartments 84 and toward the warm heat exchanger in the compartments 83.

Figure 13 shows a cross-section of the side of the rightmost plate 71 of Figure 7. The electrical contacts 93 connect adjacent legs within a thermocouple strip and are preferably in thermal contact with the flowing dielectric recuperative heat exchanging fluid, namely, the thermocouples are substrate-less (without substrates) . The cross-section of Figure 13 shows the cold and warm manifolds 180 and 181, respectively.

The operation of the device is as follows: The strips of thermoelectric elements are powered with a DC current source and the liquid heat exchanging medium is pumped to flow down the compartments 84. The electrical current's direction is such that the cold planes of the strips in each plate face the compartments 84 and the warm planes of the strips face the compartments 83. The plates are arranged in "pairs" so that warm planes always face warm planes (and the external enclosure for the end plates) , while cold planes always face cold planes.

As the liquid moves down the compartments 84, heat is withdrawn from the heat exchanging liquid in that compartment and transferred to the heat exchanging liquid in the neighboring compartments 83, which liquid is moving in the opposite direction (in this case upward) . When the heat exchanging liquid reaches the bottom of the vessel, its temperature has been lowered below the temperature of the thermal load, and thus it can withdraw heat from that load. As the liquid flows from compartments 84 to the compartments 83, and in the process, reverses its flow direction, it absorbs additional heat from the thermocouples in the plates and thus at each point, the liquid temperature in the compartments 83 is higher than at the opposing side of the plates in the compartments 84.

In the presence of a gravitational field (most earthbound installations) when the heat exchanging liquid is a liquid whose density increases with decreasing temperature, the refrigerator may be operated without the pump 182. When operated in this mode, the cold heat exchanger is positioned on the bottom of the installation and the warm heat exchanger on the top of the installation. A flow of the heat exchanging liquid will be induced by density differences once the thermoelectric couples are powered because, as the liquid is cooled within the compartments 84, it will increase in density and tend to sink toward the cold plates, and as the liquid in the compartments 83 is heated it will decrease in density and thus tend to rise toward the warm heat exchanger. After a short time, a steady- state situation is set up in the system, causing flow of the heat exchanging liquid in the correct direction and pumping heat from the thermal load to the warm heat exchanger in a continuous manner.

There are two major advantages of the heat pumps of the present invention relative to the classical compression cycle based refrigerators using chlorofluorocarbon compounds (CFC's or Freons) . The first advantage is that the heat exchanging liquid can be any of many low cost unchlorinated and unfluorinated compounds, like water, mixtures of water and glycol, silicone fluids etc. , thus reducing the environmental hazard of this refrigeration technology relative to current CFC based compression devices. The second advantage is that the efficiency of the heat pump of the presently invention is only weakly load- dependent when the temperature gradient is constant, because the power to the system can be controlled continuously to respond to thermal load changes. In compression cycles, however, the full efficiency of the heat pump is achieved only at full capacity. When lower thermal loads are desired, the heat pump must operate in the on-off mode, often reducing the overall thermodynamic efficiency by a factor of 2. Two examples of temperature distribution in recuperative heat exchanger based thermoelectric devices, according to the present invention, are set forth below.

Example 1

The device described in Figure 2A is constructed with only nine strips 120 The incoming heat exchanging fluid follows the arrows and is cooled by the thermoelectric strips from an entry temperature of 30°C on the entry side of the insulating barrier 127 to -10°C, when cooled by the last (leftmost) strip in the device. It is assumed that each strip can withdraw the same amount of heat form the heat exchanging liquid, and that the specific heat of the liquid is temperature independent. Under these conditions, and considering that the flow rate, dV/dt, of the heat exchanging liquid is the same throughout, the heat exchanging liquid's temperature will decrease monotonically by the same increment of 5 C for each consecutive strip within the cold space 122. The temperature of the thermal load is set to be above the lowest temperature of the heat exchanging liquid by the same increment 5 C.

Each strip will transfer heat to the heat exchanging liquid flowing in the warm space 122, equal to q . for that strip plus the power dissipated within the cell, Ri. This heat can be calculated if the COP (coefficient of performance) is known for each temperature gradient on the strips. The COP, by definition, is the ratio of the rate of heat withdrawal from the cold side to the power required to withdraw the same heat, Ri. The rate of heat discharging on the hot side of the strip is q .. = q . + Ri.

For each strip, q .. = q . (1+1/COP) .

In the following Table I the temperature distribution for the 9 strips device is calculated using the COP given in an article entitled "Thermoelectric Heat Pumps Cool Packages Electronically" by Dale A. Zeskind (Electronics, July 31, 1980 by McGraw-Hill Inc.)

TABLE I

1

Cl Cl dT _i 1+1/COP ^q hi hi 9 -10 5 5 1.24 6.2 -5 8 -5 5 6.2 1.26 6.3 1.2 7 0 5 7.5 1.28 6.4 7.5 6 5 5 8.9 1.30 6.5 13.9 5 10 5 10.4 1.34 6.7 20.4 4 15 5 12.1 1.40 7.0 27.1 3 20 5 14.1 1.50 7.5 34.1 2 25 5 16.6 1.62 8.1 41.6 1 30 5 18.7 1.88 9.4 49.7 outgoing temperature 59.1 In TABLE I the following terms are used: i, is the order of the strip; T . , the liquid "incoming" temperature at strip i; q . the heat withdrawn from strip i (up to a constant Cdv/dt, the liquid specific heat multiplied by the liquid's flow rate); dT . the temperature gradient on each strip; 1 + 1/COP, the ratio <! . * / <l_ci _> ^for strip i; q, . , the heat discharged to the liquid by strip i; and T, . the liquid "outgoing" temperature on the hot side at strip 1.

In this example, for instance, the heat withdrawn from the cold load Q is 5Cdv/dt, while the heat discharged at the hot heat exchanger is (59.1-30)CdV/dt, or Q_h = 29.1 CdV/dt.

The overall COP of the device is Q /W, where W is the power input to the system and equal the sum of Ri on all strips is as follows:

COP = QC/W = Q_/ (Q.-Q Ϊ = 5 = 20.75% ^{C n C} 24.1

The total temperature lift is at least 64.1 of Q (59.1+5), and for such a lift the normal COP of a thermoelectric device is under 5%. The device as configured in this Example I thus performs 4 times better than a single transverse thermoelectric device. Example 2

One group of applications for which the instant invention is particularly suited involve the removal of heat from a relatively small area. For instance, when it is desired to cool or freeze tissues at the distal end of a surgical probe or at the distal end of a catheter. Such probes can be used in various surgical procedures, for instance in the treatment of a brain tumor, where the tumor is cooled to cause tissue necrosis, with minimal post¬ operative bleeding. Similarly, a catheter for ablating arrhythmogenic tissues in the heart that can be inserted via an artery or vein and have only its distal end cold is another application of the instant invention.

Such a catheter system is schematically described in Figure 14 where we show the catheter 212 which is constructed from flexible tubing, extruded, for instance, from a silicone polymer (from Petrach Systems, Bristol, Pennsylvania) or from Peak, a polyetheretherketone (TM, Upchurch, Oak Harbor, Washington) , capable of keeping its flexibility to -30°C. The catheter is terminated with a cylindrical thermoelectric cell 211 as described, for instance, in Figures 7 and 9. The catheter body portion 209 consists of two concentric tubular structures defining two lumens. An external tube 213 and an internal tube 214, each of which is an elongated tube. These two tubes are not necessarily made of the same material. The external tube 213 is made to withstand pressures in the range of about 150 to 200 psi, but need not be flexible to -30° since its temperature is always kept above freezing. The internal tube, on the other hand, needs to keep its flexibility to temperatures if -30 C but is not required to withstand the same high pressures as the external tube. At the proximal end the catheter 210 body portion 209 is terminated with a plug-like connector 212 that allows easy insertion to the fixture 215. The fixture 215 can have any of many possible shapes including, without limitations, a cylinder, a handle or a paralleliped.

The fixture 215 is hollow and has an entry portion 216 connectable to the inner tube 214 of the catheter 210 on one end and to a liquid containable tube 224. A circulating heat exchanging liquid which discharges the heat removed from the distal end in a chiller 223 is circulated from the chiller in the tube 224 and back from the distal end through the external lumen via tube a connection 217 which redirects the warm heat exchanging liquid through a tube 225 to the chilled 223. The flow direction of the chilled fluid is toward the distal end in the inner tube 214 of the catheter 210 and the heat exchanging fluid is further cooled in the cylindrical thermoelectric cell 211. The chiller thus acts as the warm heat exchanger of the instant invention and also to precool the entering liquid to allow cooling of the catheter tip beyond what the thermoelectric cells can do. The heat exchanging liquid removes heat from the distal end and is further heated during return by passing on the hot side of the couples in the thermoelectric cell 211. As a result, the outer tube temperature is kept near the body temperature and freezing of the catheter along its length to the artery is avoided.

The heat exchanging liquid is any low melting point (under -25°C) liquid like for instance a silicone fluid PS40 (Petrach System, Bristol, Pennsylvania) , a trimethylsiloxy terminated polydimethylsiloxane. The fluid can be first prechilled to about -10 C on the external chiller and then further cooled to about - 25°C by the stack of cylindrical thermoelectric cells 211. The system also contains a DC power supply source 219, a temperature monitor 221 and an electronic controller 222.

Claims

CLAIMS :

1. A thermoelectric device operating as a heat pump comprising:

(i) a vessel forming a liquid-tight container and having a cold end portion thereof adapted to be in thermal contact with an external heat source from which heat is to be withdrawn;

(ii) a plurality of thermocouples joined into a continuous plate having a first side and a second side opposite thereto, said plate being positioned in the vessel;

(iii) each thermocouple comprising a leg of p-semiconductive material and a leg of n-semiconductive material, each leg having two ends and each thermocouple having an intra-couple electrical connection means to electrically connect one leg to the other leg of its thermocouple at one end thereof; said intra-couple means being adjacent the first side of the plate;

(iv) a group of inter-couple connection means to electrically connect adjacent couples; said inter-couple means being adjacent to the second side of the plate;

(iii) a DC power source connected to each thermocouple adapted to thereby create thermal gradients from the first side of the plate to the second side of the plate; (vi) said plate dividing said vessel into a warm space on the first side of the plate and a cold space on the second side of the plate; and

(vii) means to flow liquid within the vessel over both sides of the plate from the cold space to the warm space in a continuous and recirculating closed-loop liquid flow path to cool the liquid by contact with the first side of the plate and warm the liquid by contact with the opposite second side of the plate and to flow the cooled liquid into thermal contact with the cold end portion and thereby withdraw heat from the external heat source.

2. A thermoelectric device as set forth in claim 1 wherein said flow means includes pump means to flow a stream of liquid over said sides of the plate.

3. A thermoelectric device as in claim 1 wherein the thermocouples are connected together in subsets and the subsets of thermocouples within the plate are connected in series.

4. A thermoelectric device as in claim 1 comprising a plurality of said thermocouple subsets within the plate arranged side-by-side.

5. A thermoelectric device as in claim 1 wherein the thermocouples within the plate are electrically and physically equivalents.

6. A thermoelectric device as in claim 1 wherein portions of the thermocouples are in direct contact with the liquid in the warm space and the liquid in the cold space without a substrate therebetween.

7. A thermoelectric refrigerator comprising:

(i) a plurality of thermoelectric plates each plate having a plurality of thermocouples positioned in the plate with opposite first and second sides;

(ii) each thermocouple comprising a leg of p-semiconductive material and a leg of n-semiconductive material, each leg having two ends adapted to have its heat flow directed from said plate first side to said plate second side, each leg being in thermal contact with the plate first side at one of its ends and being electrically connected to the other leg of its thermocouple at its opposite end and adjacent the plate second side, the thermocouples being electrically connected into subsets and subsets of thermocouples within each plate being connected in series;

(iii) DC power source means connected to each thermocouple to furnish DC power thereto; (iv) vessel means to contain said plates and adapted to contain a liquid which flows in a continuous closed-loop recirculating liquid path over said first side of each plate and over said second side of each plate, wherein said vessel means has at least one closed end in thermal contact with an external heat source from which heat is to be withdrawn; and

(v) flow means to flow said liquid over each of said plates, the flow being in a direction about orthogonal to said direction of thermocouple heat flow, said flow being over one side of each plate toward said closed end and then over the opposite side of each plate in the reverse direction, thereby withdrawing heat form the heat source.

8. A thermoelectric device as set forth in claim 7 wherein said flow means includes pump means to pump said liquid over said sides of each plate.

9. A thermoelectric refrigerator as in claim 7 wherein the thermocouples within each plate are connected in series.

10. A thermoelectric refrigerator as in claim 7 comprising a plurality of thermocouples within each plate arranged side-by- side and wherein the thermocouples within each plate are electrically and physically equivalent.

11. A thermoelectric refrigerator as in claim 7 wherein portions of the thermocouples are in direct contact with the liquid flowing on both sides of the plate without a substrate therebetween.

12. A recuperative heat exchange thermoelectric device comprising a plurality of thermoelectric cells each cell comprising:

(i) an inner annulus member having an imaginary axis; (ii) an outer annulus member coaxial with the inner member and having an inner wall separated from the inner member by an annular space;

(iii) a plurality of thermocouples positioned in the annular space, each thermocouple comprising a leg of p-semiconductive material and a leg of n-semiconductive material, each leg having two ends and being radially directed relative to said axis, each leg being in thermal contact with the inner annulus member at one of its ends and being electrically connected to the other leg of its thermocouple at the same end, the thermocouples within each cell being connected in series; with the p-type leg of one couple connected to the n-type leg of an adjacent couple at the ends nearest the outer annulus member; (iv) the device further including DC power means connected to each thermocouple to furnish DC power thereto and pump means to flow liquid in a continuous and recirculating stream through said inner annulus member and then over said outer annulus member; and

(iv) a housing means to house the cells and a liquid flow space around the outer annulus member, said housing means having at least one closed end in thermal contact with an external heat source from which heat is to be withdrawn and the pump means includes means for pumping the liquid first within the housing means through the inner annulus member and toward the closed end and then through the said space around the outer annular member in reverse direction thereby withdrawing heat from the closed end and exuding heat at an external heat exchanger;

(v) wherein the thermocouples are in direct contact with the liquid within the inner annulus member and the liquid within the outer annulus member without a substrate therebetween.

13. A thermoelectric device as in claim 12 wherein the housing means is part of a surgical probe or catheter.

14. A thermoelectric device as in claim 12 wherein the inner member of each cell is a round elongated tube adapted for the flow of fluid therein and the outer member is an elongated tube adapted for the flow of fluid thereover and wherein the axii of the cells are aligned along a common axis.

15. A thermoelectric device as in claim 12 wherein in each cell the inner member is an electrical insulator and thermal conductor.

16. A thermoelectric device as in claim 12 wherein

(i) within each cell the thermocouples are electrically and physically about the same;

(ii) the cells are arranged with their axii along a common axis; and

(iii) the cells are arranged in a sequential order relating to the progressive increase selected from the group of an increase in the size of the thermocouple of the cell and an increase in the number of thermocouples of each cell.

17. A recuperative heat exchange thermoelectric device comprising a plurality of thermoelectric cells, each cell comprising:

(iii) a plurality of thermocouples positioned in the annular space, each thermocouple comprising a leg of p-semiconductive material and a leg of n-semiconductive material, each leg having two ends and being radially directed relative to said axis, each leg being in thermal contact with the inner annulus member at one of its ends and being electrically connected to the other leg of its thermocouple at the same end, the thermocouples within each cell being connected in series; with the p-type leg of one couple connected to the n-type leg of an adjacent couple at the ends nearest the outer annular member;

(iv) the device further including DC power means connected to each thermocouple to furnish DC power thereto and pump means to flow liquid in a continuous and recirculating stream, first over said outer annulus member and then through said inner annulus member; and

(v) a housing means to house the cells and a liquid flow space around the outer annulus member, said housing means having at least one closed end in thermal contact with an external heat source from which is to be withdrawn and the pump means includes means for pumping the liquid first within the housing means through the space around the outer annulus member toward the closed end and then through the inner annulus member in reverse direction thereby withdrawing heat from the closed end and exuding heat at an external heat exchanger;

(vi) wherein portions of the thermocouples are in direct contact with the liquid within the inner annulus member and the liquid within the outer annulus member without a substrate therebetween.

18. A thermoelectric device as in claim 17 wherein the housing means is part of a surgical probe or catheter.

19. A thermoelectric device as in claim 17 wherein the inner member of each cell is a round elongated tube adapted for the flow of fluid therein and the outer member is an elongated tube adapted for the flow of fluid thereover and wherein the axii of the cells are aligned along a common axis.

20. A thermoelectric device as in claim 17 wherein in each cell the inner member is an electrical insulator and thermal conductor.

21. A thermoelectric device as in claim 17 wherein

(ii) the cells are arranged with their axii along a common axis; and