Field of invention

This invention relates to a heat transfer device that extracts electric energy from a heat source, more specifically to a device for controlling the extraction of heat from a heat source by transforming a controlled amount of heat into electric energy by means of a MHD generator.

Background

Heat tubes using phase change and kinetic energy to transport large amounts of heat efficiently for conversion elsewhere are known. Heat from a source causes a working fluid to undergo phase change from liquid phase to gas phase at a heat absorbing end and liquid in the gas phase is then transferred to a heat extracting end where the heat is transferred out of the heat tube to a secondary heat circuit causing the working fluid to undergo a phase change from the gas phase to the liquid phase.

Such heat tubes transfer heat but leave the problem of energy recovery to the secondary heat circuit which adds to the complexity while reducing effective temperature which in turn reduces efficiency of heat extracted as useful energy rather than energy in the form of low temperature waste heat.

Magnetohydrodynamic (MHD) generators generating electric energy from a conductive fluid flowing through a magnetic field are known. These often operate as topping up circuits in thermal power stations and rely on ionized conductive gases that can be chemically aggressive. A M HD generator uses a conducting fluid flowing through a magnetic field to generate an electric current. A typical

M HD generator consists of MHD electrodes, magnetic field and fluid flow substantially perpendicular to each other. Typically the magnetic field is set up using permanent magnets with N and S poles placed opposite each other across the fluid flow. Preferably a horseshoe arrangement or a Halbach arrangement is used. Also electromagnets can be used. A heat pipe is a heat-transfer device that combines the principles of both thermal conductivity and phase transition to efficiently manage the transfer of heat between two solid interfaces (no secondary heat circuit necessary). At the hot interface of a heat pipe a liquid in contact with a thermally conductive surface turns into a vapor by absorbing heat from that surface. The vapor then travels along the heat pipe to the cold interface and condenses back into a liquid - releasing the latent heat. The liquid then returns to the hot interface through either capillary action, centrifugal force, or gravity, and the cycle repeats.

Closed Loop Pulsating Heat Pipe (CLPHP) or Pulsating Loop Heat Pipe (PLHP) is a heat transfer device based on heat pipes comprising a bubble pumping action in the heat absorbing end, which causes up to as much as 97 % of the energy absorbed by the working fluid to be transformed into kinetic energy. The PLHP is well described in 'On the definition of Pulsating Heat Pipes: An Overview' by Khandekar and Groll 2003.

US 7554223 Bl describes a magnetohydrodynamic energy conversion device using a heat exchanger. A shutter M HD circuit includes a section for extracting heat from a heat exchanger in order to raise the temperature in a liquid electrically conductive material, in the M HD circuit. The liquid material is further capable of being ionized by the heat being extracted from the heat exchanger.

US 2013321111 Al describes a method using MHD in electrochemical processes, for example by use of permanent magnets for magnetizing the electrodes or current collectors through the electrodes. The process industry produces large quantities of waste heat which typically escape into the atmosphere or is transported away from the process by some kind of cooling system comprising a liquid working fluid (water) operating at a low temperature. This invention describes a device and method for transforming a part of the waste heat into useable electric energy without any moving parts, wherein the electricity can be fed back into the process or to the grid after being processed electronically to conform to the electric standards in use.

Summary of the invention

The invention describes a device for controlling the extraction of heat from a heat source by transforming a controlled amount of heat into electric energy, the device comprising a closed heat tube with a conducting working fluid, wherein the heat tube comprises a heat absorbing end for absorbing heat from a heat source and transforming a part of the heat to kinetic energy in the working fluid, and a heat extracting end for extracting electric energy, characterized in that at least one MHD generator is mounted onto the heat tube on the heat extracting end , transforming a part of the kinetic energy in the working fluid into electric energy. The device also comprises at least one sensor connected to one or both of the heat source and the heat extracting end, a variable load connected to the M HD generator, and a processor processing the signals from the at least one sensor, and being connected to a user interface.

Furthermore the invention describes a method for transforming heat into electricity without any moving parts characterized by the steps of: transforming at least a part of the heat from a heat source into kinetic energy in a working fluid in a heat tube; transforming at least a part of the kinetic energy in the working fluid into electricity by means of a M HD generator; measuring the temperature of one or both of the heat source and the heat extracting end; and adjusting a variable load connected to the M HD generator up if the temperature is too high compared to a user defined threshold and down if the temperature is to low compared to a user defined threshold.

Brief description of the drawings The invention will be further described below in connection with exemplary embodiments which are schematically shown in the drawings wherein like numerals in different drawings represent the same feature:

Fig. 1 shows a first embodiment of a heat tube provided with a MHD generator

Fig. 2 shows a second embodiment of a heat tube provided with a MHD generator with an ionizer and a condensation unit.

Fig. 3 shows a third embodiment of a heat tube provided with a first M HD generator for working fluid in the gas phase and a second MHD generator for working fluid in the liquid phase, connected in series, and also a third generator provided on the cooling circuit of the condenser. Fig. 4 shows a cross sectional view of a MHD generator viewed along the direction of the fluid flow.

Fig. 5 shows an embodiment wherein a heat absorbing panel drives a working fluid through a M HD generator,

Fig. 6 shows an embodiment wherein a heat absorbing panel is provided with two output connectors connected to a manifold, the output of which drives a working fluid through a M HD generator,

Fig. 7 shows an embodiment wherein a heat absorbing panel is provided with two output connectors and two input connectors, wherein each pair of input and output connector drives a working fluid through a MHD generator,

Fig. 8 shows a spiral evaporation section of a thermosiphon wherein the MHD generator is located on the condensation part of the thermosiphon,

Fig. 9 shows a ferrofluid pulsating loop heat pipe system with an induction unit,

Fig. 10 shows a liquid metal pulsating loop heat pipe system with a M HD generator, Detailed description of the invention

The essential part of the energy extraction device, according to this invention, is a heat circuit 200 shown in figure 1, consisting of one or more heat tubes containing a working fluid 202 having a heat absorbing end 212, which is in thermal contact with a heat source, i.e. excess heat from an industrial process, and a heat extracting end 214, where energy is extracted from the heat tube by one or more extraction means. Examples of extraction means are a condenser 500 with a cooling circuit, a MHD generator 400, and an induction unit 413 for use with ferrofluid, all three of which do not have any moving parts apart from the flow of the working fluid 202.

The device first converts heat from a heat source into kinetic energy in a working fluid 202 of a heat tube and then converts the kinetic energy of the working fluid 202 into electricity by means of a MHD generator 400 or induction unit 413, which are mounted onto the heat extracting end as different devices. Thus an energy extraction device without any moving parts is established, and the energy from the M HD generator 400 or induction unit 413 is extracted in the form of electricity.

In order to make the electric energy extraction as efficient as possible when using a MHD generator 400 or an induction unit 413, it is advantageous to transform as much as possible of the heat from the heat source into kinetic energy in the working fluid 202. The effect of the M HD generator is most importantly determined by the square of the magnetic field and the square of the velocity of the working fluid. Fig. 4 shows a cross sectional view of a MHD generator viewed along the direction of the fluid flow. An electric field is set up perpendicular to the magnetic field and the flow velocity between two electrodes 408 and may be extracted by electrical connections 409. Obviously it is important that the conduits and the M HD generator are made of electricaly non-conducting materials.

As can be easily understood, it is important to cool the magnets 406 in the MHD generator in order to keep them below their Curie Temperatures. Therefor the magnets 406 must be provided with magnet cooling units 404 and an insulating layer 402 between the working fluid conduit in the M HD generator and the magnets 406, as is shown in figure 4.

By choosing an appropriate working fluid 202 and an appropriate design of the heat tube according to the temperature levels and amount of heat available, up to 97 % of the sensible heat absorbed by the heat tube may be transformed into kinetic energy of the working fluid 202. The diameter of the tube and the design of the heat tube are important to achieve optimal function of the device. The remaining latent heat in vapours of the working fluid 202 (downstream from the M HD generator 400), condensate into liquid in a condenser 500.

In one embodiment the heat tube is a heat pipe. Many different types of heat pipes are available, such as centrifugal heat pipes, heat pipes based on wicks, diode heat pipes, thermosiphons etc. For the purpose of this invention a thermosiphon present itself as a usable solution because it circulates a fluid without the necessity of a mechanical pump. It is driven by gravity and the differences in density that arise when the working fluid is heated. Consequently the thermosiphon functions best when the system is positioned substantially vertically with the heat extracting end 212 at the bottom.

In a preferred embodiment the design of the heat tube is a so called 'pulsating loop heat pipe' (PLHP). As can be seen in fig. 9 and 10, PLHP's comprises a number of loops/bends or more specifically a sequence of 180 degree turns where a region around every second turn is in thermal contact with the heat source, preferably the lower part. PLHP is the best known design for transferring heat into kinetic energy in a working fluid 202. The reason for this is that the surface tension of the liquid enhances formation of gas bubbles and liquid plugs that spans the entire cross section of the pipe, thus the liquid plugs are pushed forward by the expanding gas bubbles, creating a bubble agglomeration and pumping action the diameter of the pipe must be below a critical size for the bubble pumping action to take place. The critical size is primarily determined by the surface tension of the working fluid 202 and tension and specific weight of the vapour and liquid. As those parameters are depending strongly on fluid and vapour temperatures appropriate critical diameter must be selected according to the operational condition of the heat tube. The bubble pumping action is utilized in the PLHP to maximize the kinetic energy in the working fluid 202.

The PLHP is working optimally in a vertical position with the heat absorbing end 212 at the lower end. In this position gravity and the bubble pumping action is working in the same direction. The number of loops in a PLHP is limited. At least if it is desirably to keep the percentage of kinetic energy high. The amount of heat a given PLHP can absorb and still have a high percentage of heat from the heat source being transformed into kinetic energy is also limited. As the amount of heat increases the surface tension of the liquid part of the fluid will not be able to withstand the heat and a larger portion of the heat from the heat source will be absorbed in the working fluid 202 as latent heat. Then the PLHP start working as a thermosiphon

Choice of working fluid 202 is essential. In an embodiment where a MHD generator 400 is used for extracting electric energy from the kinetic energy of the working fluid 202, the fluid must be electrically conductive and have a boiling point well below the temperature of the heat source. Other important properties when choosing a working fluid 202, are surface tension, heat capacity, specific weight, specific latent heat of vaporisation, ability to ionize and corrosive/poisonous properties. Alkali metals are possible alternatives for high temperature extraction in the industry. The MHD generator 400 is also described in numerous publications. Essentially, it transforms kinetic energy into electricity by moving a conducting material through a magnetic field to generate electric current. In general the working fluid 202 of the M HD generator 400 can be ionised vapours at high temperature or electrically conductive liquids like NaCI solutions, other chemical solutions, molten salts and liquid metals. The MHD generator 400 is positioned in the heat extracting end of the heat tube and constitutes the most important part of the extracting end of the heat tube. In order to make the MHD generator 400 as efficient as possible the magnetic field across the flow of the working fluid 202, working fluid 202 velocity and the electrical conductivity must be as high as possible. The conductivity of the working fluid 202 may be enhanced by installing an ionizer 205 prior to the MHD generator 400. The vapours may be ionised by radiation by for example ultraviolet light or by high voltage discharge impulses from an electrode 207. Preferably the ionizer 205 is located where the fluid is substantially in the gas phase.

Downstream of the MHD generator 400 a condenser 500 may be positioned as shown in figure 2 and 3, to condensate the remaining gas pockets in the working fluid 202 before it is returned in the liquid phase to the heat absorbing end 212. In its simplest form the condensation unit 500 may be a length of tubing exposed to air.

In another embodiment a first MHD generator for working fluid in the gas phase and a second MHD generator 450 for working fluid 202 in the liquid phase are connected in series with a condensation unit 500 between the MHD Generators 400, 450. Additionally the condensation unit may power a third MHD generator 641 in a secondary heat circuit 600. Secondary heat circuits 600 are particularly useful when the temperature of the heat source/working fluid is high and may be attached to the condenser 500 as shown in fig 2 and 3 for extracting latent heat, or may be attached to a unit for extracting sensible heat (not shown).

A check valve may be positioned somewhere on the heat tube for quicker establishment of a stable rotation of the working fluid 202 when starting up the system.

Applications for the process industry

In industrial applications like the process industry the most preferred embodiment will typically be an embodiment which is able to transfer large amount of excess process heat to the working fluid 202 in a short time span. Two important characteristics of a system for extracting heat from an industrial process cell is good thermal contact between the side walls of the cells and a working fluid 202, and capacity for extracting large amounts of heat.

Good thermal contact between the side walls of process cells and the heat tubes is best achieved by having conduits integrated in the side walls of the process cell. A more practical solution might be flat heat absorbing panels 350 comprising two or more metal plates provided with corresponding mating grooves being pressed together by means of diffusion bonding or bracing to form conduits inside/between the two or more metal plates and thus forming the flow channels of the device within a heat absorbing panel 350. Examples of such solutions are shown in figure 5, 6 and 7. The configuration of the conduits inside the heat absorbing panel 350 may vary from one weaving conduit to multiple parallel conduits. The heat absorbing panels are then fitted onto the sides of the process cell - metal against metal, for the best possible thermal contact. Figure 5 shows a simple embodiment of the device according to this invention comprising a heat absorbing panel 350, a MHD generator, an ionizer 205, an electrode 207, and an input 420 and output 421 connector, between which the MHD generator 400 is mounted onto a heat tube. When the heat absorbing panel 350 is in thermal contact with a heat source, it will drive the working fluid 202 through the MHD generator 400.

Figure 6 shows an embodiment wherein a heat absorbing panel 350 is provided with two output connectors 421 connected to a manifold, the output of which drives a working fluid through a M HD generator 400,

Figure 7 shows an embodiment wherein a heat absorbing panel 350 is provided with two output connectors 421 and two input connectors 420, wherein each pair of input 420 and output 421 connectors drives a working fluid 202 through a MHD generator 400, 400',

Capacity for extracting large amounts of heat can be achieved by increasing the number of conduits, and also the number of panels beside each other. It is also possible to increase the diameter of the conduits and flow velocity, but this reduces the thermal contact per volume of working fluid 202 and is not so effective. Also, if the heat absorbing panel is to function as a PLHP the conduits must below a critical size.

In one embodiment the heat absorbing panel function as a thermosiphon. The panel may comprise conduits as described above or the panel 350 may be a panel as described in Pat. No. 20140845 of Goodtech.

For processing plants in general and aluminium electrolysis cells in particular a large heat transport capacity makes it possible to freeze out a side layer directly on a steel shell lining the cell with a much thinner layer of silicon carbide (SiC) between the side layer and the steel shell. Another important advantage is that it avoids the use of water as a working fluid 202 in close proximity of the liquid aluminium or other process materials.

Ideally all of the excess process heat should be taken up by the device according to this invention. In order to achieve this, the heat tubes, preferably in the shape of one or more panels with integrated conduits must be sandwiched between the heat source and an isolating layer.

In another embodiment the heat absorbing panel functions as a PLHP. Then the heat absorbing panels must be constructed differently compared to the thermosiphon. As seen in fig. 9 and 10, and explained above, the conduit must comprise several loops weaving in and out of thermal contact with the heat source, preferably in a near vertical position. In an industrial cell environment this can be achieved by providing the heat absorbing panel with a cold region 215 preferably above a heat absorbing region 213 in the heat absorbing panel, wherein the cold region 215 is not in contact with and preferably isolated from the heat source.

By varying the load of the M HD generator 400 mounted on the heat extracting end a variable heat extraction of the working fluid 202 is achieved. This means heat extraction can be quickly varied without the use of valves, pumps and other heat dissipating means that is slower. This is a more cost effective solution since it is a simpler solution than prior art. The system has no moving parts apart from the flow of the working fluid 202 and therefor requires less maintenance. In another embodiment shown in fig. 8 heat tubes are spiralling around a cylindrical object, which is a typical shape of a furnace in the steel industry.

In numerous industrial processes it is desirable to be able control the temperature / heat exchange in process cells. The device according to the invention comprises one or more sensors connected to the heat source or heat extracting end 214. The device also comprises at least one processor processing the signals from the sensors using a computer program and a user interface connected to the processor so that the desired temperatures / heat exchange predetermined by a user can be achieved by varying a load 411, such as an resistive load, connected to the M HD generator 400 as shown I figure 10.

The method associated with the device according to this invention comprises measuring the temperature of one or both of the heat source and the heat extracting end (214) and then adjust a variable load connected to the M HD generator (400) up if the temperature is too high compared to a user defined threshold and down if the temperature is to low compared to a user defined threshold. Again all this is done without any moving parts, making the device less vulnerable to wear and technical breakdown.

Extracting electric energy by Induction, using ferrofluid as working fluid

In another embodiment the working fluid 202 is a ferrofluid and the energy extracting unit is an induction unit 413. The induction unit comprises a magnet 406 which magnetizes the ferrofluid and a coil that is positioned downstream from the magnet 406 through which the magnetized ferrofluid is flowing, thus inducing a current in the electrical coil 412, while slowing and cooling the working fluid 202. A ferrofluid is a fluid that becomes strongly magnetized in the presence of a magnetic field. It is a liquid comprising nanoscale ferromagnetic or ferrimagnetic particles suspended in a carrier fluid. The ferrofluid must remain below its Curie Temperature. Therefore the temperature span of the induction unit 413 is limited to lower temperatures compared to the MHD generator 400 and as was the case with the M HD generator, magnet cooling units 404 must be provided to keep the magnets 406 below their Curie Temperatures.

In order to maximize the amount of electricity taken out of the induction unit 413, the strength of the magnetic field and velocity of the working fluid 202 are important factors. The need for velocity in the working fluid 202 together with the need for keeping the working fluid 202 below its Curie temperature clearly favours PLHP over other heat tube devices.

The following reference numbers and signs refer to the drawings

200 Heat circuit

202 Working fluid

205 Ionizer

207 Electrode

212 Heat absorbing end

213 Heat absorbing region

214 Heat extracting end

215 Cold region

350 Heat absorbing panel

400 MHD generator, first MHD generator

400' MHD generator, second branch

402 Thermal insulation for MHD generator

404 Magnet cooling unit

406 Magnet

408 MHD electrode

409 Electrical connection

411 Load

412 Electrical coil

413 Induction unit

420 Input connector

421 Output connector

450 Second MHD generator

500 Condensation unit

600 Secondary heat circuit

641 Third M HD generator