Abstract:
A power conversion device includes an enclosure containing one or more drops of a liquid. A capacitive electret transducer is coupled to the enclosure. In response to applied heat at a heating surface, the liquid vaporizes and then condenses on a flexible membrane of the capacitive electret transducer. The flexible membrane is displaced in response to the vaporization-condensation and the capacitive electret transducer generates an output current.

Description:
[0001]    This application claims the priority benefit of French Patent application number 13/58072, the contents of which is hereby incorporated by reference. 
       BACKGROUND 
       [0002]    The present disclosure relates to a device for converting thermal power into electric power, or thermoelectric generator. It more specifically relates to a device using a liquid-to-gas phase change of a fluid. 
       DISCUSSION OF THE RELATED ART 
       [0003]    Devices for converting thermal power into electric power using a liquid-to-gas phase change of a fluid have already been provided. 
         [0004]    In particular, U.S. Pat. No. 8,378,558 describes a device comprising a closed volume delimited by a first wall in contact with a hot source and, in front of the first wall, a second wall in contact with a cold source. The first wall is arranged above the second wall, and a layer of piezoelectric material is horizontally suspended in a closed volume between the two walls, this layer being crossed by vertical openings. The closed volume contains drops of a liquid. The device operates as follows. 
         [0005]    The liquid flows by gravity through the openings towards the second wall. When it comes into contact with the second wall, it abruptly vaporizes, which results in generating mechanical stress which is transmitted to the piezoelectric layer, which turns it into electric signals. 
         [0006]    The vapor passes through the openings in the piezoelectric layer towards the first wall, whereon it condenses. 
         [0007]    The liquid flow is resumed towards the second wall and the cycle starts over. 
         [0008]    French patent application No 1251368 filed on Feb. 14, 2012 describes another device for converting thermal power into electric power using a liquid-to-gas phase change of a fluid. This device enables to do away with the need to direct the cold wall above the hot wall to ensure the flow of liquid towards the hot wall after its condensing on the cold wall. To achieve this, the device comprises a first cavity having a wall in contact with a hot source, a second cavity having a wall in contact with a cold source, a piezoelectric material arranged in at least one of the cavities, a primary channel connecting the first and second cavities, and at least one secondary channel connecting the first and second cavities. A fluid in liquid or gas form is confined in the device. The secondary channel is capable of transporting the fluid in the form of gas. The primary channel is capable of transporting drops of the fluid in liquid form from the second cavity to the first cavity, including when the second cavity is located under the first cavity. To achieve this, the primary channel comprises means for ensuring the displacement of liquid drops from the second cavity to the first cavity. Such means are for example formed by the internal surface of the primary channel, which may comprise sections having different wettabilities along the longitudinal axis of the channel, or may be of electrostatic type. 
         [0009]    It would however be desirable to at least partly improve certain aspects of existing devices for converting thermal power into electric power using a liquid-to-gas phase change of a fluid. 
       SUMMARY 
       [0010]    Thus an embodiment provides a power conversion device, comprising an enclosure containing drops of a liquid and a capacitive electret transducer coupled to this enclosure. 
         [0011]    According to an embodiment, the transducer comprises a flexible electrode forming a wall of the enclosure. 
         [0012]    According to an embodiment, the flexible electrode comprises a grapheme or amorphous carbon film. 
         [0013]    According to an embodiment, the flexible electrode comprises a metal sheet. According to an embodiment, the transducer further comprises a rigid electrode arranged in front of the flexible electrode outside of the enclosure. 
         [0014]    According to an embodiment, the transducer further comprises an electret layer between the flexible electrode and the rigid electrode. 
         [0015]    According to an embodiment, the electret layer is in contact with a surface of the rigid electrode. 
         [0016]    According to an embodiment, the device comprises a first wall intended to be placed in contact with a hot source at a temperature higher than the vaporization temperature of the liquid, and a second wall intended to be placed in contact with a cold source at a temperature lower than the vaporization temperature of the liquid. 
         [0017]    According to an embodiment, the device comprises a first cavity in the vicinity of the first wall, and a second cavity in the vicinity of the second wall, the first cavity being separated from the second cavity by a layer of a thermally-insulating material. 
         [0018]    According to an embodiment, the layer of thermally-insulating material is crossed by openings connecting the first cavity to the second cavity. 
         [0019]    According to an embodiment, at least one of the openings comprises means ensuring the transport of drops of the liquid from the second cavity to the first cavity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The foregoing and other features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, wherein: 
           [0021]      FIG. 1  is a diagram illustrating the operating principle of a capacitive electret transducer; 
           [0022]      FIG. 2  is a simplified cross-section view illustrating an embodiment of a thermoelectric generator; 
           [0023]      FIG. 3  is a simplified cross-section view illustrating another embodiment of a thermoelectric generator; and 
           [0024]      FIG. 4  is a simplified cross-section view illustrating another embodiment of a thermoelectric generator. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    For clarity, the same elements have been designated with the same reference numerals in the various drawings and, further, the various drawings are not to scale. Further, in the following description, unless otherwise indicated, terms “approximately”, “substantially”, “around”, and “in the order of” mean “to within 10%”, and terms referring to directions, such as covering, topping, lateral, above, under, upper, lower, vertical, horizontal, etc. apply to devices arranged as illustrated in the cross-section views of the corresponding drawings. 
         [0026]    An aspect of an embodiment provides a device capable of converting thermal power into mechanical power by means of a liquid abruptly vaporizing when it comes into contact with a hot surface, thus creating a localized overpressure, and of converting this overpressure into electric power by means of a capacitive electret transducer. 
         [0027]      FIG. 1  schematically illustrates an example of a capacitive electret transducer  100 . Transducer  100  comprises a fixed electrode  101 , and, in front of electrode  101 , a mobile electrode  103 . In this example, electrodes  101  and  103  are substantially planar and parallel to each other and electrode  103  is capable of shifting along an axis approximately orthogonal to electrodes  101  and  103 . 
         [0028]    Electrode  101  is coated with an electret layer  105 . Electret here designates an electrically-charged dielectric material, capable of holding its charges or a significant part of its charges for a long period, typically from a few years to a few tens of years. 
         [0029]    In the shown example, a load, schematically shown as a resistor R, is connected between electrodes  101  and  103  of transducer  100 . 
         [0030]    Transducer  100  operates as follows. 
         [0031]    Electret layer  105 , which contains a quantity Q i  of charges of a first polarity for example, negative charges, induces in electrodes  101  and  103  a building up of charges of inverse polarity, positive charges in this example. Calling Q 1  the quantity of charges induced in electrode  101  by layer  105 , and Q 2  the quantity of charges induced in electrode  103  by layer  105 , equilibrium Q i =Q 1 +Q 2  is respected at any time. 
         [0032]    A displacement of electrode  103  relative to electrode  101  causes a reorganization of the charges induced in electrodes  101  and  103  by electret layer  105 . In particular, when electrode  103  moves away from electrode  101 , quantity Q 2  of charges induced in electrode  103  decreases and quantity Q 1  of charges induced in electrode  101  increases. Conversely, when electrode  103  moves towards electrode  101 , quantity Q 2  of charges induced in electrode  103  increases and quantity Q i  of charges induced in electrode  101  decreases. As a result, a current I flows through load R. The relative motion of electrode  103  with respect to electrode  101  is thus converted into electricity. 
         [0033]      FIG. 2  is a cross-section view schematically illustrating an embodiment of a device  200  for converting thermal power into electric power. 
         [0034]    Device  200  comprises a lower wall  201  intended to be in contact with a hot source. The hot source may be an electronic component, for example, an integrated circuit chip. In this case, wall  201  may be an upper surface of the integrated circuit chip. The described embodiments are however not limited to this specific case. As a variation, wall  201  may be a plate of metal or of another material, and the hot source may be any heat source available in the environment, for example, a car exhaust pipe, a duct, a machine wall, etc. 
         [0035]    Device  200  further comprises approximately vertical lateral walls  203  made of a thermally-insulating material, for example, glass, silicon oxide, or any other suitable material. In this example, lateral walls  203  are laid on a peripheral portion of the upper surface of wall  201 . 
         [0036]    Device  200  further comprises, above wall  201 , a layer  205  of thermally-insulating material, for example, made of the same material as lateral walls  203 . In this example, the thickness of layer  205  is smaller than the height of lateral walls  203 , and layer  205  is approximately horizontally suspended above wall  201 , for example, at mid-height of walls  203 . A cavity  207  separates layer  205  from wall  201 , and a cavity  209  is located between layer  205  and the plane comprising the upper surface of lateral walls  203 . As an example, layer  205  and lateral walls  203  may be one piece, for example, obtained by molding or by any other convenient method. As a variation, layer  205  and lateral walls  203  may be separate elements, assembled by any suitable means, for example, by gluing. Layer  205  is crossed by a network of approximately vertical openings  211 , for example, through holes or channels having a circular cross-section. As a non-limiting embodiment, lateral walls  203  may have a height in the range between 100 μm and 1 cm, cavities  207  and  209  may have a height in the range between 100 nm and 5 mm, and openings  211  may have a diameter in the range between 100 nm and 5 mm. 
         [0037]    Device  200  further comprises a flexible electrode  213  suspended above layer  205  and cavity  209 . In this example, a peripheral portion of the lower surface of electrode  213  is attached to the upper surface of lateral walls  203 , for example, by gluing. Flexible electrode  213  for example is a conductive graphene or amorphous carbon film, or a thin metal sheet. 
         [0038]    Device  200  further comprises, above flexible electrode  213 , an electret layer  215  having its upper surface in contact with a rigid electrode  217 . The stack formed by electret layer  215  and by electrode  217  is supported by a ring  219  of a thermally-conductive and electrically-insulating material, for example, resin. Ring  219  itself is supported by a peripheral portion of the upper surface of electrode  213 , above lateral walls  203  of the device. Flexible electrode  213  is thus separated from electret layer  215  by a cavity  221  having its height depending on the thickness of ring  219 . 
         [0039]    Electret layer  215  is for example made of charged TEFLON, of charged parylene, or of any other dielectric material electrically charged, for example, by corona discharge and capable of holding its charges for a long period. As a variation, electret layer  215  may comprise a stack of one or several dielectric layers, for example, made of silicon oxide and/or of nitride, sandwiched between two TEFLON or parylene films. Charges may for example be stored in the central dielectric, for example, by corona discharge, the TEFLON or parylene films having the function of preventing the discharge of the central dielectric. As a variation, electret layer  215  may comprise a stack of one or several dielectric layers, for example, made of silicon oxide and/or of nitride, sandwiched between two films of hexamethyldisiloxane, generally designated as HMDS in the art. It should be noted that HMDS does not intrinsically have electret properties, but the performed tests have shown that, after encapsulation between two HMDS layers, a stack of dielectric, for example, oxide-nitride, may have electret properties, that is, it may hold electric charges for a long period. More generally, electret layer  215  may be made of any material or combination of materials having electret properties. 
         [0040]    Electrodes  213  and  217  and electret layer  215  form a capacitive electret transducer. Electrodes  213  and  217  for example respectively correspond to the mobile electrode and to the fixed electrode of a transducer of the type described in relation with  FIG. 1 . In this example, electrodes  213  and  217  are respectively connected to output nodes or terminals OUT 1  and OUT 2  of conversion device  200 . 
         [0041]    In this example, device  200  further comprises an approximately horizontal upper wall  223  topping electrode  217 , intended to be in contact with a cold source. The cold source for example is a fin-type radiator, or directly ambient air, or any other source having a temperature lower than that of the hot source. It should be noted that in this example, wall  223  is distinct from electrode  217 . As a variation, wall  223  and electrode  217  may be a same element. 
         [0042]    Walls  201  and  223  are preferably made of good heat conductors, to provide a homogeneous temperature distribution in cavities  207  (hot cavity) and  209  (cold cavity) respectively. It should be noted that in the example of  FIG. 2 , cavity  209  is separated from wall  223  by the capacitive electret transducer. However, in practice, the different transducer elements are sufficiently thin and/or good thermal conductors to avoid significantly disturbing the cooling of cavity  209  by the cold source, such a cooling being anyway performed by the thermally-conductive material of ring  219 . As a non-limiting example, flexible electrode  213  may have a thickness in the range between 1 nm and 100 μm, cavity  221  may have a height in the range between 10 nm and 2 mm, electret layer  215  may have a thickness in the range between 50 nm and 20 μm, and rigid electrode  217  may have a thickness in the range between 10 μm and 3 mm. 
         [0043]    Cavities  207  and  209  and openings  211  form a closed volume, preferably tight, delimited by walls  201  and  203  and by flexible electrode  213  which thus define an enclosure  230 . In this volume, a liquid  225  is introduced before sealing. Liquid  225  is selected so that its boiling temperature is lower than the temperature of cavity  207  (hot cavity) or of wall  201  in operation, and higher than the temperature of cavity  209  (cold cavity) or of electrode  213  in operation. As an example, ethanol or methanol having boiling temperatures respectively in the order of 78° C. and 65° C. at the atmospheric pressure may be used. Water or any other liquid may also be used and the closed volume formed by cavities  207  and  209  and by openings  211  may be set to a pressure selected to obtain the desired boiling temperature. 
         [0044]    Device  200  operates as follows: 
         [0045]    When a drop of liquid  225  runs down along an opening  211  and reaches hot wall  201 , it abruptly passes from the liquid state to the gaseous state. Such a fast state change locally generates a strong overpressure. Such an overpressure locally causes a temporary deformation of flexible electrode  213  above opening  211 . Locally, flexible electrode  213  is then capable of moving towards or away from electrode  217 , which causes the occurrence of an electric signal between output nodes OUT 1  and OUT 2  of the device. The electrical power of this signal may either be directly used to power a load, or stored in a battery or another storage system, or recovered by a system for collecting and for shaping the collected electric power. 
         [0046]    After the fast vaporization step, the vapor condenses on the side of electrode  213 , in cold cavity  209 , and flexible electrode  213  locally recovers its initial shape. Electrode  213 , as illustrated in  FIG. 2 , has a liquid accumulation  225  forming thereon. When the mass of liquid  225  increases, drops  227  fall back into openings  211  towards hot wall  201 , and the cycle is resumed. 
         [0047]      FIG. 3  is a simplified cross-section view illustrating an alternative embodiment of the device for converting thermal power into electrical power of  FIG. 2 . Device  300  of  FIG. 3  comprises many elements common with the device of  FIG. 2 . Only the differences between the two devices will be described hereafter. 
         [0048]    Device  300  of  FIG. 3  differs from the device of  FIG. 2  in that it does not comprise thermally-insulating layer  205  horizontally suspended in enclosure  330  containing liquid  225 , between walls  201 ,  213 , and  203 , which define this enclosure. 
         [0049]    In device  300 , drops  227  of liquid displacing from the cold wall (electrode  213 ) to the hot wall (wall  201 ) of the enclosure, and the vapors and overpressures displacing from the hot wall (wall  201 ) to the cold wall (electrode  213 ) of the enclosure are not, as in the device of  FIG. 2 , channeled by openings  211  crossing layer  205 , but may freely move, at any point of the surface of the device in top view. An advantage is that the number of fast vaporization/condensation cycles per surface area unit may be greater than in device  200 . 
         [0050]      FIG. 4  is a simplified cross-section view illustrating another embodiment of a device  400  for converting thermal power into electric power. 
         [0051]    Device  400  comprises a first cavity  401 , a second cavity  403 , a channel  405  or primary channel connecting cavity  401  to cavity  403 , and a channel  407  or secondary channel connecting cavity  401  to cavity  403 . In the shown example, primary channel  405  is a rectilinear channel having a circular cross-section, and secondary channel  407  is a rectilinear channel having a ring-shaped cross-section, its smaller diameter being greater than the diameter of the primary channel, primary and secondary channels  405  and  407  having the same longitudinal axis. The described embodiments are however not limited to this specific case. Primary and secondary channels  405  and  407  thoroughly cross a layer  409  made of a thermally-insulating material, which separates cavity  401  from cavity  403 . 
         [0052]    Cavity  403  is closed by an end wall  411 , substantially parallel to layer  409 , and by lateral walls  413  connecting wall  411  to layer  409 . 
         [0053]    Cavity  401  is delimited by lateral walls  415  and by a flexible electrode  417  suspended above layer  409  and attached to the upper surface of lateral walls  415 . Device  400  further comprises, above flexible electrode  417 , an electret layer  419  having its upper surface in contact with a rigid electrode  421 . The stack formed by electret layer  419  and by electrode  421  is supported by a ring  423  of thermally-conductive and electrically-insulating material, for example, resin. Ring  219  itself rests on a peripheral portion of the upper surface of electrode  417 , above lateral walls  415  of cavity  401 . A cavity  425  thus separates electret layer  419  from electrode  421 . 
         [0054]    Electrodes  417  and  421  and electret layer  419  form a capacitive electret transducer. In this example, electrodes  417  and  421  are respectively connected to output nodes or terminals OUT 1  and OUT 2  of conversion device  400 . 
         [0055]    In operation, wall  411  is intended to be placed in contact with a cold source, and electrode  421  is intended to be placed in contact with a hot source. 
         [0056]    Cavities  401  and  403  and channels  405  and  407  form a closed volume delimited by walls  411 ,  413 , and  415  and by flexible electrode  417 , which thus define an enclosure  430 . A liquid  427  is introduced in this volume. Liquid  427  is selected so that its boiling temperature is lower than the temperature of cavity  401  (hot cavity) in operation, and higher than the temperature of cavity  403  (cold cavity) in operation. 
         [0057]    According to an aspect of the example of  FIG. 4 , primary channel  405  comprises an inner surface such that it ensures the displacements of drops of the liquid from cavity  403  to cavity  401 , independently from the direction of device  400 . For this purpose, the inner surface of the device may comprise sections having different wettabilities, distributed along the longitudinal axis of the channel, to form a surface having a surface energy gradient. 
         [0058]    Device  400  operates as follows. 
         [0059]    When a drop of liquid  427  reaches hot cavity  401 , it abruptly passes from the liquid state to the gaseous state. Such a fast state change locally generates a strong overpressure. Such an overpressure causes a temporary deformation of flexible electrode  417 , which causes the occurrence of an electric signal between output nodes OUT 1  and OUT 2  of the device. 
         [0060]    After the fast vaporization step, the vapor is transmitted by channels  405  and  407  and condenses on the side of cold cavity  403 . A drop then forms in cold cavity  403 . This drop is transported towards hot cavity  401  by channel  405 , and the cycle is resumed. 
         [0061]    Primary channel  405  has a sufficiently large opening to leave way for drops of liquid  427 , and secondary channel  407  has an opening which is too small for drops, but sufficiently large to transmit vaporized liquid or pressure. In the case of water, the diameter of primary channel  405  preferably is in the order of the capillary length, for example between 3 and 5 mm, advantageously in the order of 4 mm, and the diameter of the secondary channel preferably is in the order of the capillary length divided by 10, for example, smaller than 0.5 mm. Secondary channel  407  especially has the function of balancing the pressure between cavities on transfer of the drop from cavity  403  to cavity  401 , to avoid for a depression to create in cavity  403 , which might block the transfer of the drop. 
         [0062]    An advantage of the embodiment of  FIG. 4  is that the device operation is independent from gravity. The device may thus be arranged in any direction. 
         [0063]    An advantage of the above-described embodiments results from the fact that the conversion of a local overpressure into electric power is performed by means of a capacitive electret transducer. This enables, for equivalent overpressures due to the abrupt evaporation of the liquid, to obtain electric signals of much higher amplitudes than in prior art devices where the mechano-electrical conversion is performed by means of a piezoelectric transducer. 
         [0064]    Specific embodiments have been described. Various alterations, modifications, and improvements will readily occur to those skilled in the art. 
         [0065]    In particular, in the examples of  FIGS. 2 and 3 , the capacitive electret transducer is placed on the side of the cold source. As a variation, the transducer may be placed on the hot source side, or two transducers may be provided, one on the hot source side and the other on the cold source side. 
         [0066]    Further, in the example of  FIG. 4 , the capacitive electret transducer is placed on the hot source side. As a variation, the transducer may be placed on the cold source side, or two transducers may be provided, one on the hot source side and the other on the cold source side. 
         [0067]    Further, in the above-described examples, the electret layer of the capacitive electret transducer is formed on a surface of the rigid electrode of the transducer. As a variation, the electret layer may be placed on a surface of the flexible electrode of the transducer. In this case, it should however be ascertained for the stack of the flexible electrode and of the electret layer to remain sufficiently flexible to obtain the desired operation. 
         [0068]    Further, it will be within the abilities of those skilled in the art to adapt the described embodiments to the various alternative embodiments of thermoelectric generators described in above-mentioned U.S. Pat. No. 8,378,558 and in the above-mentioned French patent application No. 1251368 filed on Feb. 14, 2012. The content of these two documents is incorporated herein by reference to the maximum extent allowable by law.