WO2010026266A1

WO2010026266A1 - Thermoelectric device

Info

Publication number: WO2010026266A1
Application number: PCT/EP2009/061661
Authority: WO
Inventors: Ingo Bayer
Original assignee: Bhp Billiton Aluminium Technologies Limited
Priority date: 2008-09-08
Filing date: 2009-09-08
Publication date: 2010-03-11

Abstract

An apparatus (100) for the conversion of thermal energy transferred from a surface (20) of a processing structure or vessel to electrical energy. The thermoelectric device is adapted to engage with the processing structure and be disposed in thermal communication with the surface, the thermal communication being such that thermal energy is transferable from the surface to the thermoelectric device by radiation and optionally convection. The apparatus comprises a first side (30) for receiving thermal energy from the surface by radiation and convection, the first side (30) and the surface (20) defining a first space (72) there between. The apparatus (100) further comprises a second side (40) for receiving thermal energy from the first side (30) by conduction; a body portion (50) positioned between the first side (30) and the second side (40), and having therein at least one thermoelectric element (60) for generating electrical energy from a temperature difference between the first side (30) and the second side (40). A supporting structure comprising a housing having side walls to support the body portion a spaced distance from the radiating surface (20) of the processing structure or vessel is also provided.

Description

Thermoelectric device

Field of the invention

This invention relates to a thermoelectric device for extracting electrical energy from waste heat energy.

Background of the invention

Thermoelectric devices or Seebeck devices are devices which convert temperature differences between opposite sides of the device to electrical energy. Typically made from semi-conducting metals or semi-metals, Seebeck devices are generally wafer- shaped and rely upon the imposition of a temperature difference across their opposing major surfaces as the source of the electrical current they develop. Although berated in the past as being inefficient, recent advances in materials and materials processing have led to significant improvements in efficiency. Moreover, even as historically inefficient power generators, they are still able to access otherwise unavailable energy in an inexpensive, clean and maintenance-free manner.

To date these devices have been primarily used to convert waste heat from automotive exhaust gases to electrical energy. These devices have not been widely used for the recovery of waste heat from higher temperature metallurgical industrial applications due to their relatively inefficient conversion of heat to electrical energy in an industry where few if any attempts have ever been made to recover this energy. Much of this apparent disinterest in waste heat from the industrial metallurgical processes is founded in historically cheap and readily available power. Present processing vessel designs also more readily lend themselves to high production, rather than concentrating on economy of power usage, thereby further discouraging energy recovery attempts. In addition to energy recovery, the use of Seebeck devices on metallurgical vessels can also offer the advantage assisting with the controlling cooling effects required for the chemical processes occurring in the vessel. In the context of the invention, pyrometallurgy consists of the thermal treatment of minerals, metallurgical ores and concentrates to bring about physical and chemical transformations in the materials to enable recovery of valuable metals. Pyrometallurgical processes typically include one or more of the following processes:

• drying

• calcining

• roasting

• fuming

• smelting

• refining

While the invention will be described with reference to the production of aluminium in electrolytic smelting cells, it would be appreciated by those skilled in the art that the invention is applicable to other types of pyrometallurgical vessels.

Processes at temperatures above about 100 ⁰C, have significant energy requirements, for instance, to maintain elevated temperatures. Some specific examples of pyrometallurgical processes having large energy demands include ore sintering, ore reduction/refining, and metal reduction/refining. These energy needs are often provided for by fossil fuel combustion or electricity. In most cases, the energy is not used as efficiently as desirable. A significant loss of energy is through diffuse heat transferred away from the process as part of its operation.

For instance, during reduction of aluminium oxide (alumina) to form aluminium in electrolytic cells only about 30% of the total power consumed is actually used by the reduction process. Much of the remainder of the power is used to maintain the temperature of the process environment, but once generated, this heat is naturally lost to the process by heat flux through the sides of the reduction vessel. A modern aluminium smelting operation may lose in excess of 600 MW of energy due to the continual need to maintain a high temperature process environment. Electrolytic cells for the production of aluminium comprise an electrolytic tank having at least one cathode and at least one anode. The electrolytic tank consists of an outer steel shell having carbon cathode blocks sitting on top of a layer of insulation and refractory material along the bottom of the tank. While the precise structure of the side walls varies, a lining comprising a combination of carbon blocks and refractory material is provided against the steel shell. During the electrolytic process, a large electric current is passed from the anode to the cathode. Aluminium oxide is dissolved in a cryolite bath present in the tank. The operating temperature of the cryolite bath is normally in the range of 930 ^QC to about 970 ^QC. Energy used to maintain this process temperature is lost as natural heat flux through the surfaces of the reduction vessel or through convection with the process off-gases.

This problem is not confined to aluminium electrolysis cells as many pyrometallurgical processes require the thermal balance, or the temperature gradient / environment, within the processing unit to be controlled. Many high temperature metallurgical processing vessels require thermal balance and control of the heat flow at the vessel wall to maintain protective freeze linings to cover the refractory linings within the process vessel walls. Thus, while the invention may be described with reference to aluminium electrolysis cells, it is applicable to a wide range of metallurgical processing vessels used for high temperature treatment and refining of ores, and extraction of valuable metals and their chemical compounds such as their oxides at temperatures generally in excess of 100 °C. This invention may also find application in the conversion of energy contained in the off-gases of these pyrometallurgical processes.

For instance, the heat transfer and subsequent cooling of the cryolite bath at the side walls affects the formation of a layer of 'frozen' cryolite bath on the inside of the side walls of the electrolytic tank. The thickness of this layer / crust / ledge / freeze lining may vary during operation of the cell dependent on, for eg, cryolite bath temperature (which is responsive to electrical current flow) and heat removal from the outside of the side walls. If the crust becomes too thick it will affect the operation of the cell as the crust will grow on the cathode and disturb the cathodic current distribution and thus the magnetic field. If the crust becomes too thin or is absent in some places, the cryolite bath may attack the side wall lining and ultimately result in its failure (necessitating its replacement to avoid damage to the steel shell and possible spillage of cryolite bath from the tank). Thus, controlled ledge formation is essential for good pot operation and long lifetime of the refractory lining within the cell. Thus, controlling the flow of heat from the bath through the side wall lining is essential for controlled ledge formation within the cell.

Accordingly, the present invention provides a means for harvesting heat energy lost from a surface of a processing structure to enhance energy efficiency and preferably to also provide an improved thermodynamic environment within the processing structure.

Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.

Summary of the invention

Thermoelectric materials are known to be able to convert heat energy directly to electrical energy. The inventors have developed an apparatus or thermoelectric device including thermoelectric materials for application with pyrometallurgical processing structures to control and use heat energy normally transferred in a generally uncontrolled manner from the processing structure to the external environment. Specifically, the thermoelectric device of the present invention makes use of both convective and radiant heat energy.

Accordingly, in one aspect of the present invention there is provided a method for controlling and harvesting heat energy transferred from a surface of a processing structure, the method including

- providing an apparatus having at least one thermoelectric element positioned between a first side and a second side of the thermoelectric device, the first side being positioned adjacent to, and in thermal communication with, the surface; the thermal communication being such that thermal energy is transferable from the surface to the first side by radiation;

- passing a first fluid between the surface and the first side such that thermal energy is transferable from the surface to the first side by convection;

- allowing the thermal energy transferred from the surface to the first side to conduct from the first side to the second side; and

- collecting the electric current generated by the thermoelectric element.

The method may further include passing a second fluid over the second side such that thermal energy is transferable from the second side. The second fluid may, but need not be, the same as the first fluid. The temperature of the first fluid is greater than that of the second fluid and both fluid flows might be controlled such that this temperature difference lies in the range 10 °C to 200 °C.

Properties or flow characteristics of the first fluid and/or second fluid may be adjusted to control the thermal balance within the processing structure. For instance, the fluid may be a gas or liquid of varying density. Other possibilities for adjusting the nature of the fluid flows may also be employed. Although not exhaustive, such variations to the fluid flows could include variation of the flow rate of the fluid over the sides of the thermoelectric device, or enhancement of the turbulent character of the fluid flow so as to improve heat transfer. Since the electrical output from the thermoelectric element is at least partially dependent on the thermal gradient, adjusting the properties of the first fluid and/or second fluid also allows control of the electrical output of the thermoelectric device.

The number and arrangement of thermoelectric element(s) provided in the thermoelectric device is dependent on the available surface area, the thermal energy available from the surface, gross heat transfer details of the thermoelectric elements themselves, and cost considerations.

In accordance with the invention, the thermoelectric device may be retrofitted to an existing processing structure or it may be installed as part of a new structure.

In a further aspect of the present invention, there is provided an apparatus for the conversion of thermal energy transferred from a surface of a processing structure or vessel to electrical energy, the apparatus comprising

a thermoelectric device having a first side for receiving thermal energy from the surface by radiation and convection, the first side and the surface defining a first space there between;

a second side for receiving thermal energy from the first side by conduction; and

a body portion positioned between the first side and the second side, and having therein at least one thermoelectric element for generating electrical energy from a temperature difference between the first side and the second side; and

a supporting structure comprising a housing having side walls to support the body portion a spaced distance from the radiating surface of the processing structure or vessel.

The apparatus is provided with a support structure to maintain the body portion a spaced distance from the radiating surface of the processing structure, the first side of the at least one thermoelectric element in the body portion facing towards the radiating surface of the processing structure. The spaced distance between the first side of the body portion and the surface of the processing vessel providing a passage for a first fluid. In some embodiments, the thermoelectric device further includes an optional outer wall, the second side and the outer wall defining a second space there between. The first fluid and the second fluid pass through the first space and the second space, respectively. The first fluid is of a higher temperature than the second fluid.

The first side and second side are preferably made from a conducting material. For instance, a suitable material is aluminium, copper, alloys of such, or other metals or other highly thermally conductive material.

The body portion is preferably made from an insulating material.

The processing structure may be an electrolytic cell.

According to one embodiment of the above aspect, the invention relates to a processing vessel having the above described thermoelectric device fitted thereto.

Accordingly, in another aspect of the present invention there is provided a method for increasing the electrical efficiency, and controlling the thermal balance, of an electrolysis cell, the method including

- providing an apparatus having at least one thermoelectric element positioned between a first side and a second side of the thermoelectric device;

- the first side being positioned adjacent to, and in thermal communication with, a surface of the electrolysis cell; the thermal communication being such that thermal energy is transferable from the surface to the first side by radiation;

- allowing the thermal energy transferred from the surface to the first side to conduct from the first side to the second side; and - collecting the electrical energy thereby generated by the thermoelectric element;

whereby adjustment of the first fluid provides for control of the thermal balance of the electrolysis cell.

Preferably, the electrolytic cell is for the production of aluminium.

The thermoelectric element may be made from any thermoelectric material known in the art to display the Seebeck effect (the generation of an electrical current in response to a thermal gradient) and to be operable at temperatures of from about 100⁰C to about 500 ⁰C.

The thermoelectric device may be retrofitted to an existing electrolysis cell using a mounting means or it may be installed as part of a newly constructed electrolysis cell.

The invention further defines a processing structure and in particular an electrolysis cell having a thermoelectric device as described above positioned adjacent to a surface of the processing structure for recovering and converting thermal energy to electrical energy.

As used herein, except where the context requires otherwise, the term "comprise" and variations of the term, such as "comprising", "comprises" and "comprised", are not intended to exclude further additives, components, integers or steps.

Brief description of the drawings

Figure 1 - A perspective view of a schematic representing an embodiment of the thermoelectric device of the present invention.

Figure 2 - A perspective view of the first side of the thermoelectric device showing fins and the thermoelectric elements in partial cut-away (normally hidden by the first side). Figure 3 - A perspective view of the first side of the thermoelectric device showing alternate fins and the thermoelectric elements in partial cut-away (normally hidden by the first side).

Figure 4 - A perspective view of the second side of the thermoelectric device showing a partial cut-away of the optional outer boundary surface.

Detailed description of the embodiments

A preferred embodiment of the invention will now be described with reference to the above drawings.

The apparatus 100 shown in Figure 1 includes a thermoelectric device having a first side 30 and a second side 40, between which there is positioned body portion 50 and at least one thermoelectric element 60. The thermoelectric elements substantially extend from the first side to the second side within the body portion, but are not normally visible when the device is fully assembled (ie they are hidden by the first and second sides).

The apparatus is adapted to be positioned adjacent to, and in thermal communication with, a surface 20 of a processing structure such as a pyrometallurgical vessel from which thermal energy may be transferred by radiation. The apparatus is further provided with a support structure to maintain the body portion a spaced distance from the radiating surface of the processing structure, the first side of the thermoelectric element or elements in the body portion facing towards the radiating surface of the processing structure. A first space 72 is created between the radiating surface of the processing structure. The spaced distance between the first side of the body portion and the surface of the processing vessel providing a passage for a first fluid.

The supporting structure may comprise a housing having side walls to support the body portion a spaced distance from the radiating surface of the processing structure or vessel. The housing may be provided with fins which direct flow through the first space from an inlet to an outlet. The inlet and outlet to the first space is preferably provided through the side of the housing wall in the direction of fluid flow. The fins for directing fluid flow may completely traverse the first space thus providing separate fluid flow chambers or may extend only partially across the first space to act as guide vanes for the fluid flow.

The housing may further include an outer wall, the second side of the thermoelectric device and the outer wall defining a second space there between. A second fluid 80 may be passed over the second side, optionally through a second space 82 between the second side and an optional outer casing 90. The first fluid and a second fluid pass through the first space and the second space, respectively. The first fluid is of a higher temperature than the second fluid

Also shown in the Figures are the inlets and outlets for the first and second fluid. There is no particular significance regarding the location of these openings or of the net direction of fluid flow through the thermoelectric device. However as stated above it may be preferable to have the inlets and outlets oriented in the direction of flow of the fluids.

The amount of heat energy transferred from the surface, Q, is the sum of the convective, Q_c, and radiative, QR, components. If a first fluid of initial temperature T_1n is passed between the surface and the first side over an area A to result in a final temperature T_out, then Q_c = A.h.(T_0Ut - T_1n), where h is the heat transfer coefficient. If the temperature of the surface is T_s and the temperature of the first side is T_1st, then Q_R = A.ε. (T_1st⁴ - T_s ⁴), where ε is proportional to the emissivity of the affected surfaces..

The material used to construct the surface of first side 30 and second side 40 of the body portion is preferably highly thermally conductive to provide for a more even temperature distribution. To this end, a particularly suitable material is aluminium. The material of the first side also preferably has an emissivity approaching 1 . This may be provided by surface treatment (coating, anodising or other similar technique) of the material of the first side so that Q_R absorbed by the first side approaches Q_R emitted by the surface. The first side may be of any profile; however a particularly preferred profile is one which allows Qc absorbed by the first side to approach Q_c transferred from the surface without adversely affecting the radiative heat transfer through the surfaces. For instance, the first side may include fins 32 (Figures 2 and 3) to increase the surface area available for heat transfer from, and to avoid laminar flow of, the first fluid. The material used to construct the body portion 50 is preferably an insulator to inhibit the flow of thermal energy through the material of the body portion per se and to increase the amount of thermal energy forced to be transferred through the thermoelectric elements. For instance, the body portion may be made from pre-formed ceramic compacts (alumina, magnesia, zirconia, etc) or other material which would impede the flow of heat and electricity through its matrix.

By controlling the type of fluid used as the first and second fluids, and their flow rate through the first and second spaces, it is possible to control (to a degree) the thermal energy being transferred from the processing structure. A greater degree of control may be provided by the incorporation of a heat exchanger type arrangement within the first and/or second spaces. For example, an internal cooling arrangement as described in PCT/AU2005/001617 may be employed (such as shown in Figure 4). The controlled cooling of an external surface of the processing structure of the present invention is superior to that presently known in the art. That is, it provides a greater possible degree of cooling with tighter control.

In relation to an electrolytic cell, this enhanced control of the thermal balance within the cell is significant. Most importantly, the outside temperature of the shell of the electrolytic tank can be controlled so that the formation of the ledge / freeze lining can also be controlled. As an example, the fluid flow rates can be controlled in response to the outside temperature of the shell such that if the outside temperature drops the flow rates can also be slowed to result in a reduced transfer of thermal energy from the shell to the thermoelectric device. The flow rates could be controlled by any means known in the art, for instance, a valve or damper system.

The fluid can be gas or liquid. Preferably, the fluid is a gas as this is cheaper to install and safer to operate. For instance, the fluid may be air. The first fluid flowing through the first space will be of a greater temperature than the second fluid flowing through the second space. In the first space, the first fluid is heated by the surface of the processing structure conductively and transfers its thermal load to the first side convectively. Heat is also passed to the first side from the surface through radiation. The first side may include a series of fins 32 or the like that project into the first space to increase the thermal transfer. In the second space, the second fluid is used to remove heat from the second side. The second fluid is preferably at ambient temperature, but may be cooled. The second side may include a series of fins 42 (Figure 4) or the like that project into the second space to increase the thermal transfer. The fluids may be propelled through the spaces by any means known in the art. For instance, a fan or blower may be used, and may also be powered by electrical energy produced by the thermoelectric device.

In Figure 1 , the thermoelectric elements are shown in a regular spaced pattern. However, the exact positioning of the thermoelectric elements is not critical to the successful operation of the thermoelectric device. Any positioning which allows for the requisite thermal energy transfer through the thermoelectric device and away from the processing structure is suitable. Commercially available thermoelectric elements are generally wafer-shaped and rely upon the imposition of a temperature difference across their opposing major surfaces as the source of the electrical current they develop

The thermoelectric element may be made from any material known in the art to operate at high temperatures. Typically, thermoelectric materials are semi-conducting metals or semi-metals. Ideally, the thermoelectric materials have high electrical conductivity, a high Seebeck coefficient, S, and low thermal conductivity, k. Preferably, the thermoelectric material includes bismuth, lead or gallium compounds which may include bismuth telluride, lead telluride, lead selenide, bismuth antimony, gallium arsenide and gallium phosphide. The main requirement is that the material be able to operate at temperatures of between 100 ⁰C and 450 ⁰C. In one embodiment, bismuth telluride is used as the thermoelectric material.

The wafers are aligned in an insulating support panel, body portion 50, and comprise a sequence of thermoelectric elements or "couples" alternating between p and n type semi-conducting materials electrically connected through the support panel by printed circuits or the like. The matrix of wafers is covered on both the hot side (first side) and cool side (second side) by a layer of diffuser material such as aluminium which assists in providing an even temperature across the heat exchanger and particularly avoids hots spots forming. The layer of diffuser material may be provided with fins or baffles which are preferably arranged in a circuitous path to allow a fluid to flow through the spaces of the device.

Similarly the cool side of the thermomagnetic device (ie the side facing away from the cell walls) is provided with a diffuser sheet and heat exchange channels through which a cooling fluid is passed.

The heat radiating from the surface of the vessel and the temperature difference between the fluids flowing through the heat exchanger channels provides the driving force for the thermoelectric device.

The thermoelectric device is adapted to engage the processing structure. Any mounting means known in the art may be used for this purpose. The thermoelectric device is suitable for retrofitting to an existing processing structure, and thus may be sold as individual components to be assembled or as an assembled module, or the thermoelectric device may be included in the overall design of a new processing structure.

Advantages of the thermoelectric device of the present invention include:

• the recovery of waste thermal energy from high temperature industrial processes as electrical energy at greater efficiency (due to the capture of thermal energy by both convection and radiation);

• the electrical energy is environmentally neutral in that it does not emit carbon;

• the enhanced cooling of, and therefore greater degree of control over, the surface of the processing structure; and

• the ability to stop the cooling of the surface of the processing structure if desired (eg by stopping the fluid flow through the spaces), at which time the thermoelectric device and particular the spaces and the insulating body portion act to retain the thermal balance of the processing structure. It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.

Claims

1. A method for controlling and utilizing heat energy transferred from a surface of a processing structure, the method including

- collecting the electric current generated by the thermoelectric element.

2. A method for increasing the electrical efficiency, and controlling the thermal balance, of an electrolysis cell, the method including

3. The method of claim 1 or 2 further including passing a second fluid over the second side such that thermal energy is transferable from the second side to the second fluid.

4. The method of claim 1 or 2 wherein the temperature of the first fluid is greater than that of the second fluid.

5. The method of claim 4 wherein temperature difference lies in the range 10 °C to 200 °C.

6 The method of claim 1 or 2 wherein the second fluid is the same as the first fluid

7. The method of claim 1 or 2 wherein the electrolytic cell is for the production of aluminium.

8. An apparatus for the conversion of thermal energy transferred from a surface of a processing structure or vessel to electrical energy, the apparatus comprising

a body portion positioned between the first side and the second side, and having therein at least one thermoelectric element for generating electrical energy from a temperature difference between the first side and the second side; and a supporting structure comprising a housing having side walls to support the body portion a spaced distance from the radiating surface of the processing structure or vessel.

9 The apparatus of claim 8 wherein the spaced distance between the first side of the body portion and the surface of the pyrometallurgical vessel provides a first space for the passage for a first fluid.

10. The apparatus of claim 8 or 9 wherein fins are provided in the first space.

1 1 . The apparatus of claim 10 wherein the fins for directing fluid flow completely traverse the first space thus providing separate fluid flow chambers.

12. The apparatus of claim 10 wherein the fins for directing fluid flow extend only partially across the first space to act as guide vanes for the fluid flow or heat convention.

13. The apparatus of claim 8 wherein the housing further comprises an outer wall, the second side and the outer wall defining a second space there between through which a second fluid passes.

14 The apparatus of claim 8 wherein the processing structure is an electrolytic cell.

15. A pyrometallurgical structure incorporating an apparatus of claim 8 .