Abstract:
An electric generator may utilize the thermoacoustic effect to extract usable energy from the natural thermal gradient present between hydrothermal fluid issuing from an undersea hydrothermal vent and ambient sea water by creating an acoustic pressure wave in a working fluid inside a thermoacoustic resonance chamber. The acoustic energy is converted to electric energy by a piezoelectric transducer that resonates with the acoustic wave, and is then transported to land through a network of power cables. This provides a robust system that reduces the need for frequent maintenance visits, and therefore enables the extraction of deep sea hydrothermal energy into a viable and cost effective renewable energy alternative.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to and the benefit of provisional patent application No. 61/264,339, filed in the United States Patent and Trademark Office on Nov. 25, 2009, the entire content of which is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    The following description relates generally to a system for converting thermal energy into electric energy, and in particular, a system for utilizing the thermoacoustic effect to convert a thermal gradient into electric energy. 
         [0004]    2. Background 
         [0005]    Hydrothermal vents are common natural phenomena where geothermally heated water emanates from the earth. Some types of hydrothermal vents are located on the sea floor, frequently at mid-ocean ridges at the boundaries between tectonic plates.  FIG. 1  is a simplified illustration showing a certain type of hydrothermal vent. Here, ambient sea water  2  is drawn into the sea floor  4 . Due to subsurface magma, at depths the sea floor includes heated layers  6 , wherein the seawater drawn into the sea floor is heated to high temperatures and expelled through a hydrothermal vent  8  back into the ocean. In some hydrothermal vents, due to a buildup of minerals that the seawater picked up while being drawn through the sea floor, a chimney  10  may occur. 
         [0006]    Due to this natural phenomenon, a large, naturally occurring temperature gradient is created where superheated hydrothermal fluid  12  issues into cold sea water. For this reason, there has been interest in harvesting energy from undersea hydrothermal vents. However, a number of practical difficulties have prevented any large-scale generation of energy at undersea hydrothermal vents. For example, the vents are highly inaccessible, typically occurring at remote locations under thousands of meters of water. Further, when expelled from the vents, the hydrothermal fluid generally contains a concentration of minerals and various compounds and is frequently acidic and corrosive, potentially destroying most types of conventional electric generating equipment in short order. Moreover, even the primary feature that makes these vents desirable, that is, the extreme thermal gradient between the hydrothermal fluid and the surrounding sea water (e.g., a temperature change from about 350° C. to 2° C. in a distance of just a few feet) is so great as to make the use of conventional heat pumps, stirling engines, or steam turbines difficult or impossible. In addition, these devices each have many moving parts and other issues that reduce their reliability, resulting in the need for relatively frequent maintenance, which is much more difficult to perform at the deepwater locations where the hydrothermal vents occur. 
       SUMMARY 
       [0007]    In various representative aspects, the instant disclosure provides for a system, an electric generator, and a method for generating and/or distributing electric energy based on hydrothermal energy at undersea hydrothermal vents. 
         [0008]    In one example, an electric generator utililzes the thermoacoustic effect to organize and extract usable energy from the natural heat gradient present between hydrothermal fluid issuing from a hydrothermal vent and ambient sea water by creating an acoustic wave in a working fluid inside a thermoacoustic resonance chamber. The acoustic energy is converted to electric energy by a piezoelectric transducer that resonates with the acoustic wave, and is then transported to land through a network of power cables. Unlike other the heat engines like steam turbines or sterling engines, the thermoacoustic generator disclosed herein may have no moving parts, in the sense that there are generally no spinning or sliding components. As the system is more robust, it is less likely to require frequent maintenance visits, and therefore enables the extraction of deep sea hydrothermal energy into a viable and cost effective renewable energy alternative. 
         [0009]    In one aspect, the disclosure provides an electric generator including a channel for directing a flow of a first fluid, e.g., hydrothermal fluid issuing from an undersea hydrothermal vent, and a thermoacoustic resonance chamber that penetrates the channel. The thermoacoustic resonance chamber is configured to have a first heat exchanger inside the channel for absorbing heat energy from the superheated hydrothermal fluid, and a second heat exchanger outside the channel for moving the heat energy, e.g., into ambient sea water. 
         [0010]    Another aspect of the disclosure provides a method of generating electric energy. Here, a thermoacoustic resonance chamber is provided at an undersea hydrothermal vent. The thermoacoustic resonance chamber includes a first heat exchanger thermally coupled to a hydrothermal fluid emanating from the hydrothermal vent, and a second heat exchanger thermally coupled to ambient sea water. A standing acoustic wave is set up in the thermoacoustic resonance chamber, and the acoustic energy corresponding to the standing acoustic wave is converted into electric energy utilizing a piezoelectric transducer. 
         [0011]    Another aspect of the disclosure provides an apparatus for generating electric energy. The apparatus includes means for generating a standing thermoacoustic wave between a hydrothermal vent and sea water, and means for converting acoustic energy from the thermoacoustic wave into electric energy. 
         [0012]    These and other aspects are more fully comprehended upon review of this disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The accompanying drawings, together with the specification, illustrate exemplary embodiments of the present invention, and, together with the description, serve to explain the principles of the present invention. 
           [0014]      FIG. 1  is a simplified diagram illustrating a natural undersea hydrothermal vent. 
           [0015]      FIG. 2  is a perspective view showing an electric generator in accordance with an aspect of the disclosure. 
           [0016]      FIG. 3  is a cross-section of the electric generator of  FIG. 2 . 
           [0017]      FIG. 4  is a partial cross-section showing a detail of a stack in a thermoacoustic resonance chamber in accordance with an aspect of the disclosure. 
           [0018]      FIG. 5  is a diagram illustrating a heat exchanger in accordance with an aspect of the disclosure. 
           [0019]      FIG. 6  is a diagram illustrating a heat pipe in accordance with an aspect of the disclosure. 
           [0020]      FIG. 7  is a simplified schematic diagram of a system for generating and distributing electric energy in accordance with an aspect of the disclosure. 
           [0021]      FIGS. 8A and 8B  are schematic diagrams illustrating a transmission line in accordance with an aspect of the disclosure. 
           [0022]      FIG. 9  is a flow chart illustrating a process of generating electric energy in accordance with an aspect of the disclosure. 
       
    
    
       [0023]    Elements and steps in the figures are illustrated for simplicity and clarity and have not necessarily been rendered according to any particular sequence. For example, steps that may be performed concurrently or in different order are illustrated in the figures to help to improve the understanding of various aspects of the disclosure. 
       DETAILED DESCRIPTION 
       [0024]    In the following detailed description, only certain exemplary embodiments of the present invention are shown and described, by way of illustration. As those skilled in the art would recognize, the invention may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Also, in the context of the present application, when an element is referred to as being “on” another element, it can be directly on the other element or be indirectly on the other element with one or more intervening elements interposed therebetween. Further, in the context of the present application, when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or be indirectly connected or coupled to the other element with one or more intervening elements interposed therebetween. Like reference numerals designate like elements throughout the specification. 
         [0025]    A detailed description of an exemplary application, namely a system for converting hydrothermal energy at an undersea hydrothermal vent into electric energy, is provided as a specific enabling disclosure that may be generalized to any application of the disclosed system, device and method for converting thermal energy into electric energy in accordance with various embodiments of the present invention. As generally depicted in  FIG. 2 , an exemplary embodiment of the present invention provides a channel  102  for directing the flow of a fluid  104 , and a thermoacoustic resonance chamber  106  that penetrates the channel  102 . The thermoacoustic resonance chamber  106  includes at least one first heat exchanger  108  inside the channel  102  and at least one second heat exchanger  110  outside the channel  110 . In this way, thermal energy from the hot fluid  104  flowing through the channel  102  may be transferred from the first heat exchanger  108  to the second heat exchanger  110 , and thereby, into an ambient fluid  112  coupled to the second heat exchanger  110 . 
         [0026]    The vent channel  102  directs the hydrothermal fluid  12  to the hot side heat exchangers  108 . As the hydrothermal fluid  12  comes in contact with the ambient seawater  2 , it cools quickly. To prevent a loss of heat, the vent channel  102  may substantially seal up against the hydrothermal vent  8 , to reduce or prevent mixing of the sea water  2  with the hydrothermal fluid  12 . In an exemplary embodiment, the vent channel  102  may be heavily insulated to reduce or prevent heat from being lost to the sea water  2 , ensuring that it retains its heat as it passes the hot side heat exchangers  108 . The outer shell of the vent channel  102  may be made of a suitable material to give it strength and resist the corrosive sea water, while the inside may be coated in a corrosion resistant material such as a nickel alloy to protect it from the hydrothermal fluid  12 . The vent channel  102  may be open at both ends, to allow the hydrothermal fluid  12  to flow past the hot side heat exchanger  108  and reach the sea water  2  without substantially obstructing the flow. 
         [0027]    The thermoacoustic resonance chamber  106  may enclose a working fluid  114  capable of carrying an acoustic pressure wave to transfer the heat energy from the first heat exchanger  108  to the second heat exchanger  110 . That is, the thermoacoustic resonance chamber  106  may have a toroidal shape configured to create a resonance of an acoustic pressure wave within a working fluid  114  in the chamber. A “stack”  138  may partition the thermoacoustic resonance chamber  106  into a number of generally tubular channels  118 , arcing around the inside of the toroid. At least one piezoelectric transducer  120  may be suitably placed within the thermoacoustic resonance chamber  106  to harness the vibration energy generated by the acoustic pressure wave and convert it into usable electric energy. 
         [0028]    The thermoacoustic effect is a physical phenomenon resulting from the thermal properties of a gas such as the working fluid  114  and the geometry of the thermoacoustic resonance chamber  106 . In general, in a fixed volume, heating causes a gas to expand, and cooling causes the gas to contract or compress. By the same token, compression of a gas causes it to heat, while expansion or decompression of a gas causes it to cool. Thus, within the thermoacoustic resonance chamber, when heat transfers from the first heat exchanger  108  to the working fluid  114 , a localized expansion of the working fluid  114  occurs. This localized expansion travels through the working fluid  114  as an acoustic pressure wave. Because pressure is ideally proportional to temperature, there is a small temperature fluctuation in the working fluid  114  where the acoustic pressure wave passes. During the time t=1/(πf), where f is the frequency of the acoustic pressure wave, the distance that heat is able to diffuse through the working fluid  114  is determined by the thermal penetration depth δk, defined as δk=√(2κ/(πfρcρ)), where κ is the thermal conductivity of the working fluid  114 , ρ is its density, and cρ is its specific heat at a constant pressure. As this thermal penetration depth δk is usually quite small, the purpose of the stack  138  is to partition the large cross-sectional area of the thermoacoustic resonance chamber  106  into a number of relatively narrow tubular channels  118  extending around the thermoacoustic resonance chamber  106  in order for the thermoacoustic effect to manifest itself. 
         [0029]    When the working fluid  114  is confined to a chamber having suitable dimensions for a resonance effect, a standing acoustic pressure wave may be generated in the working fluid  114 . That is, as a temperature gradient across the stack  138  is created, a self-sustaining oscillation is formed in the channels of the stack  138 , which carries heat from the hot side to the cold side, and in turn is amplified. As this wave travels through the thermoacoustic resonance chamber, it resonates until it becomes a powerful acoustic pressure wave. The frequency of the wave depends on the length of the resonator, with the specific harmonic(s) of the wave determined by the length and placement of the stack  138  in relation to other components inside the resonator, and in part by the non-linear effects that may begin to exhibit themselves at higher amplitudes. Here, as heat continues to be transferred from the first heat exchanger  108  to the working fluid  114 , the pressure wave moves the heat within the working fluid  114  and the stack  138  away from the first heat exchanger  108 . Suitable placement of the second heat exchanger  110  enables this heat to be removed from the working fluid  114  and the stack  138 . Moreover, a plurality of first heat exchangers  108  and/or second heat exchangers  110  may be suitably located to improve the transfer of the heat through the resonance chamber  106 . 
         [0030]    In an exemplary embodiment, a portion of the stack  138  may extend around one-eighth of the circumference of the toroidal thermoacoustic resonance chamber  106  to induce the second harmonic. 
         [0031]    The stack  138  may be constructed out of any suitable material capable of being formed into one of the above-described shapes and withstanding the heat gradient (such as, but not limited to stainless steel or ceramic). 
         [0032]    In some examples, in addition to or in the place of the hexagonal or generally round tubular channels  118  described above, the stack may include a set of parallel plates to form the stack&#39;s channels, with the separation distance between the plates being a multiple of the thermal penetration depth. Here, the exact distance between the plates may vary in accordance with other parameters of the machine (for example the frequency, the choice of the working fluid, etc). In some examples, the stack may include a set of parallel solid tubes or pins, with an open space between them within the thermoacoustic resonance chamber  106 . Those skilled in the art will understand that there are numerous equivalent configurations of the stack within the spirit and scope of the instant disclosure. 
         [0033]    The thermoacoustic resonance chamber  106  includes the ring-shaped portion of this embodiment of the generator. In an exemplary embodiment, the chamber is a generally hollow, toroidal structure filled with a suitable working fluid  114  (such as, but not limited to, helium or other inert gas) in which the acoustic pressure waves may propagate. In an exemplary embodiment, there may be two sets of stacks  138  and heat exchangers  108 ,  110 , with each set comprised of a stack with one heat exchanger on either side, as illustrated in cross-section in  FIG. 3 . These sets may be positioned inside of the thermoacoustic resonance chamber  106  so that the hot side heat exchangers  108  of each set are approximately opposite each other. The thermoacoustic resonance chamber  106  may intersect the vent channel  102  so that the hot side heat exchangers  108  are inside the vent channel  102 , while the cold side heat exchangers  110  are outside the vent channel  102 . The piezoelectric transducers  120  may be placed opposite each other, approximately halfway between the two stacks  138 , as seen in  FIG. 3 . Along with containing the working fluid  114 , the thermoacoustic resonance chamber  106  may also act as a pressure vessel. While the static pressure of the working fluid  114  is at equilibrium with the pressure of the outside environment, the acoustic pressure wave may create large pressure fluctuations at the pressure antinodes of the system. As a pressure vessel, the thermoacoustic resonance chamber  106  may be made of a material that is strong enough to withstand those forces, as well as resist the corrosive effects of the ambient sea water  2  and hydrothermal fluid  12 , such as, but not limited to, one of various titanium alloys. In addition, the thermoacoustic resonance chamber  106  may include a layer of insulation to ensure that heat from inside the vent channel  102  or the hot side of the stacks  138  is not lost through conduction. 
         [0034]      FIG. 5  is an illustration of an exemplary heat exchanger. The first and second heat exchangers  108 ,  110  are in charge of moving heat  116  to and from the ends of the stacks  138 . In an exemplary embodiment, each heat exchanger  108 ,  110  may include two portions: a first, internal portion  122 , and a second, external portion  124 . The internal portion  122  may resemble the geometry of the stack  138 , so that it can make contact without blocking the channels. For example, when the stack  138  includes channels  118  having hexagonal cross-section as illustrated in  FIG. 4 , the internal portion  122  may have pores  126  having a hexagonal cross-section that may align with the tubular channels  118 . The internal portion  122  of the heat exchanger may connect through the shell of the thermoacoustic resonance chamber  106  to the outside portion  124 , which may be a series of fins  128  over which either the hydrothermal fluid  12  or the ambient sea water  2  washes over, depending on whether the respective heat exchanger is a hot side heat exchanger or a cold side heat exchanger. 
         [0035]    In an exemplary embodiment, the heat exchangers may be constructed of a solid piece of a suitable material (e.g., one with a high thermal conductivity, such as, but not limited to, copper), and the fins  128  may be coated with a corrosion resistant material  130  (such as, but not limited to, nickel alloys). 
         [0036]    The hot side heat exchangers may be subjected to further stress, as the hydrothermal fluid  12  may carry a bit of sediment and may have a large dissolved mineral content, both of which could gum up the heat exchangers. In an exemplary embodiment, to ensure that these heat exchangers stay clean, they may be slightly less mechanically damped than the cold side heat exchangers, causing them to vibrate slightly as some of the acoustic power is dissipated through them. This, along with the non-reactive coating, may make the heat exchangers substantially self-cleaning and reduce or eliminate clogging. 
         [0037]    Returning to  FIG. 3 , the piezoelectric transducer  120  may be located within the thermoacoustic resonance chamber  106 , for example, at a pressure antinode of the standing pressure waves in the working fluid  114 . Here, the acoustic pressure waves create a vibration, causing a strain across the piezoelectric transducer  120  and generating an electric potential that may be harvested and sent by a transmission line as useful electric energy. 
         [0038]    A piezoelectric crystal produces electric power in response to changes in stress along the crystal&#39;s face. The placement of a piezoelectric transducer  120  at a pressure antinode, the region of the largest pressure differences in the thermoacoustic waveform, allows for a direct transition from acoustic to electric power, without having to resort to systems with moving parts. The electric power may then be rectified, and sent to the power line system. The piezoelectric transducer  120  may be constructed of most piezoelectric materials, such as, but not limited to, quartz, barium titanate, lead zirconate titanate, etc. 
         [0039]    In an exemplary embodiment, at least one of the heat exchangers  108 ,  110  may include a heat pipe  132 .  FIG. 6  illustrates an exemplary heat pipe  132  including a thermally conductive material  134  that may undergo a phase change, e.g., from a liquid to a gas or from a gas to a liquid, to efficiently transfer heat. For example, the heat pipe  132  may include a porous portion  136  and/or one or more channels into which the thermally conductive material  134  may be placed in its liquid phase. At the hot side of the heat pipe  132 , this thermally conductive material  134  may evaporate to its gaseous phase, and move along an internal chamber  140  towards the cold side of the heat pipe  132 . At the cold side, the gaseous thermally conductive material  134  may condense back into its liquid phase, entering back into the porous portion  136  or the channels. Here, the porous portion  136 /channels may be configured to draw the thermally conductive material  134  back towards the hot side of the heat pipe  132  by way of a capillary action, where the process may repeat itself. In this way, a transfer of heat from a hot side of a heat exchanger to its cold side may be improved. 
         [0040]      FIG. 7  is a simplified diagram illustrating a power distribution system  200  in accordance with an exemplary aspect of the disclosure. Here, the system  200  includes a plurality of electric generators  100  in accordance with the above description, and a power line network  202  to transmit the electric energy along a transmission line  206  from the electric generators  100  to a remote location  204 . 
         [0041]      FIGS. 8A and 8B  are simplified illustrations of the transmission line  206 . In an exemplary embodiment, the transmission line  206  may include a number of main lines  216  (e.g., two main lines  216 ) and a number of sub-connectors  208  for coupling together at least two of the plurality of main lines  216 . 
         [0042]    Utilizing this configuration, the power line system  200  may provide a plurality of alternate paths for power to flow should any individual line be damaged. For example, at each junction  212  between a main line  216  and a sub-connector  208 , a monitoring device  210  may be arranged. Here, in the event of a broken line  214  as illustrated in  FIG. 8B , these monitoring devices  210  may report the segment at which the break  214  occurred. One example of a monitoring device  210  capable of these operations is the NI USB-6009 Data Acquisition (DAQ) unit, together with a compatible computer system. Utilizing this device, the computer system, together with the DAQ unit is then placed in a pressurized, sealed capsule with water-tight, high-power connections for the transmission line  206  to attach. The capsule may include a power converter to convert the line voltage to the operating voltage of the computing system. The DAQ unit may connect to a small data line, attached to the transmission line  206  such that the transmission line  206  connects a data port on one end of the line to another data port on the opposing end. Each DAQ unit may transmit pulses at regular intervals, and at the same time, monitor the data line for incoming pulses. Thus, when the lines break at any point, the pulses are be interrupted, and the monitoring devices  210  may transmit an error code through the network containing information detailing the segment of the break. 
         [0043]    In an exemplary aspect of the disclosure, the system  200  may be a modular system, easily reconfigured with the drop-in addition of new generators  100 , the movement of generators  100  from one hydrothermal vent  8  to another if the natural venting changes, or removal or replacement, e.g., of nonfunctional generators. 
         [0044]    Of course, those skilled in the art will recognize that the generator  100  may be utilized in other suitable environments other than an undersea hydrothermal vent  8 , and the generator  100  and the power transfer network  200  may be utilized separately or in conjunction. For example, the generator  100  may be utilized in any of innumerable systems as a substitute for a steam turbine, a heat engine, a stirling engine, etc. 
         [0045]    The present disclosure may include a description of functional block components and various processing steps. Such functional blocks may be realized by any number of hardware or software components configured to perform the specified functions and achieve the various results. For example, the various heat exchangers may be employed, e.g., as a shell and tube, one or more conductive plates and/or fins, fluid heat exchangers, phase change heat exchangers such as a heat pipe, and the like, which may carry out a variety of functions. In addition, aspects of the disclosure may be practiced in conjunction with any number of heat engines, and the system described, utilizing a thermoacoustic resonance chamber, is merely one exemplary application. Further, any number of conventional techniques may be employed for generating electric energy from thermal energy in accordance with a temperature difference between the hydrothermal fluid and the ambient sea water, and the like. 
         [0046]      FIG. 9  is a flow diagram of a process of generating electric energy in accordance with an exemplary aspect of the disclosure. In some embodiments the process is performed by circuitry or a network processor. In some embodiments the process is performed by the generator  100 . In some embodiments the process is performed by the transmission system  200 . 
         [0047]    In block  300  the process does provides a thermoacoustic resonance chamber at a hydrothermal vent. In block  310  the process sets up a standing acoustic wave, for example, within the thermoacoustic resonance chamber. By way of the standing acoustic wave, in block  320 , the process transfers heat from the hydrothermal vent to ambient sea water through the thermoacoustic resonance chamber. In block  330 , the process converts acoustic energy into electric energy with a piezoelectric transducer, e.g., placed at a suitable location, e.g., at a pressure antinode, within the thermoacoustic resonance chamber. In block  340 , the process transmits the electric energy to a remote location. In some embodiments the process transmits the electric energy to the remote location utilizing the transmission system  200  illustrated in  FIG. 7 . 
         [0048]    In block  350 , the process determines whether a transmission line has been broken, potentially compromising the transmission of the electric energy to the remote location. If the transmission line has been broken, in block  360 , the process reroutes the electric energy to a non-broken portion, and may notify a utility that the line is in need of repair. 
         [0049]    In the foregoing specification, the invention has been described with reference to specific exemplary embodiments. Various modifications and changes may be made, however, without departing from the scope of the present invention as set forth in the claims. The specification and figures are illustrative, rather than restrictive, and modifications are intended to be included within the scope of the present invention. Accordingly, the scope of the invention should be determined by the claims and their legal equivalents rather than by merely the examples described. 
         [0050]    For example, the steps recited in any method or process claims may be executed in any order and are not limited to the specific order presented in the claims. Additionally, the components and/or elements recited in any apparatus claims may be assembled or otherwise operationally configured in a variety of permutations and are accordingly not limited to the specific configuration recited in the claims. 
         [0051]    Benefits, other advantages and solutions to problems have been described above with regard to particular embodiments; however, any benefit, advantage, solution to a problem, or any element that may cause any particular benefit, advantage, or solution to occur or to become more pronounced are not to be construed as critical, required, or essential features or components of any or all the claims. 
         [0052]    As used herein, the terms “comprise,” “comprises,” “comprising,” “having,” “including,” “includes” or any variation thereof, are intended to reference a non-exclusive inclusion, such that a process, method, article, composition or apparatus that comprises a list of elements does not include only those elements recited, but may also include other elements not expressly listed or inherent to such process, method, article, composition, or apparatus. Other combinations and/or modifications of the above-described structures, arrangements, applications, proportions, elements, materials, or components used in the practice of the present invention, in addition to those not specifically recited, may be varied or otherwise particularly adapted to specific environments, manufacturing specifications, design parameters, or other operating requirements without departing from the general principles of the same.