Patent Publication Number: US-2009217664-A1

Title: Submerged Geo-Ocean Thermal Energy System

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This case claims priority to: U.S. Provisional Patent Application Ser. No. 61/033,415, filed Mar. 3, 2008 (Attorney Docket: 711-136US); and U.S. Provisional Patent Application Ser. No. 61/042,185, filed Apr. 3, 2008 (Attorney Docket: 711-189US); each of which is incorporated herein by reference. 
     If there are any contradictions or inconsistencies in language between this application and one or more of the cases that have been incorporated by reference that might affect the interpretation of the claims in this case, the claims in this case should be interpreted to be consistent with the language in this case. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to energy systems in general, and, more particularly, to geothermal energy systems. 
     BACKGROUND OF THE INVENTION 
     Non-petroleum-based energy sources are desirable. Geothermal and Ocean Thermal Energy Conversion (OTEC) systems represent two attractive such sources. Each can provide electrical energy through the exploitation of a naturally occurring temperature differential. In the case of geothermal systems, this temperature differential is between a naturally-occurring hot spot well below the earth&#39;s surface and the ambient temperature at the location of the geothermal system. For OTEC systems, this differential is between the temperature of ocean water at a deep level (e.g., &gt;100 meters) and the temperature of water at the ocean&#39;s surface. 
     Geothermal systems have been in operation for many years. A conventional geothermal system typically uses a gas-driven turbine to turn an electrical generator. The electrical generator, in response, provides output electrical energy. The blades of the turbine are driven by either hot gas that come directly from the geothermal heat source or working fluid that is vaporized by the hot gas at a heat exchanger. 
     There are several problems with conventional geothermal systems that have thus far limited their use. First, the hot gas from the geothermal source is highly corrosive. As a result, the lifetime of the turbine and other system components can be compromised. Second, atmospheric temperature acts as the heat sink for conventional geothermal systems. The power generation capacity of a conventional geothermal system decreases as the ambient temperature at the turbine increases. This is due to the fact that the power generation is directly related to the temperature differential of the system. To further exacerbate matters, the reduction in power generation capacity tends to occur at times when such power is needed most (e.g., when it is hot out and air conditioning demand increases, etc.) Further, latitude and seasonal temperature variation cause variability in the power generation capability of these systems. 
     In a typical OTEC system, electrical energy is also generated by a generator that is driven by a turbine. The turbine is driven by means of a heat engine that forces vapor forced across its blades. The heat engine results from the temperature differential between deep ocean water and surface water. Conventional OTEC systems can either be open-cycle or closed-cycle. In an open-cycle system, warm seawater flows into a low-pressure container, wherein it boils and creates steam that drives the blades of the turbine. In closed-cycle system, warm surface seawater is pumped through a first heat exchanger where its heat vaporizes a working fluid. The vaporized working fluid then drives the turbine blades. Cold water is pumped from a deep water level through a second heat exchanger, where the working fluid is condensed to complete a closed cycle. 
     Like geothermal systems, OTEC systems also have several problems in practice. At most latitudes, seasonal temperature variations cause variations in their power generation capability. As a result, the deployment of OTEC systems is substantially limited to tropical regions. Further, the daily solar cycle induces variations in the temperature of the uppermost surface seawater. As a result, OTEC system deployment is practical primarily only in areas that have a thermocline with a deeper hot surface region so that the OTEC systems are not subject to diurnal fluctuations. Still further, localized weather conditions can temporarily obstruct sunlight or strengthen winds at the location of the OTEC system. Decreased solar energy and wind-driven evaporation will lower surface water temperature in the region. 
     SUMMARY OF THE INVENTION 
     The present invention provides an energy generation system that avoids or mitigates some of the problems of prior-art energy generation systems. The present invention is an energy generation system that is based on a temperature differential between a geothermal heat source and a deep-water layer. Some embodiments of the present invention are particularly well-suited for deployment in deep-ocean environments that comprise a geothermal heat source, such as near a volcanic island. 
     In some embodiments, an energy generation system comprises an energy conversion unit that includes a first heat exchanger having a hot zone, a second heat exchanger having a cold zone, and a thermoelectric system for converting a temperature difference between the hot zone of the first heat exchanger and the cold zone of the second heat exchanger into electrical energy. 
     In some embodiments, the hot zone of the first heat exchanger and the geothermal heat source are thermally coupled through a closed-loop fluid system. In some embodiments, the hot zone of the first heat exchanger and the geothermal heat source are thermally coupled through a hydrothermal vent. An open-loop conduit conveys at least a portion of the hydrothermal vent through the first heat exchanger where it is thermally coupled with the hot zone. In some embodiments, the hot zone of the first heat exchanger and the geothermal heat source are thermally coupled through an earth crust penetrating rod. The penetrating rod is inserted into the geothermal heat source and conducts heat from the geothermal heat source to the first heat exchanger where the rod is thermally coupled with the hot zone. 
     The second heat exchanger comprises a cold zone whose temperature is regulated by the fact that it is thermally coupled to a deep-water layer. Preferably, the deep-water layer exhibits a high heat capacity and a temperature that is substantially constant regardless of latitude, weather conditions, the annual solar cycle, or even the daily solar cycle. The thermoelectric system interposes the first heat exchanger and the second heat exchanger. 
     In some embodiments, the hot zone of the first heat exchanger and cold zone of the second heat exchanger are interposed by a thermoelectric element having a first surface and a second surface. This thermoelectric element generates electrical energy as a function of temperature difference between the first and second surfaces. In some embodiments, this thermoelectric element comprises an element that generates electrical energy by means of the Peltier effect. In some embodiments, this thermoelectric element comprises a quantum-well thermoelectric element. 
     In still some other embodiments, the energy conversion unit is operatively coupled to a Rankine-cycle engine. 
     An embodiment of the present invention comprises: an energy conversion unit wherein the heat exchanger comprises a hot zone that is thermally coupled to a subterranean geothermal heat source, and a cold zone that is thermally coupled to a region of a body of water; a thermoelectric element that generates electrical energy based on a thermal differential between the hot zone and the cold zone; and a chamber for enclosing the heat exchanger and the thermoelectric element, wherein the chamber comprises a physical adaptation for withstanding external pressure that exceeds 1 atmosphere. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a schematic diagram of details of a prior-art dry steam geothermal energy conversion system. 
         FIG. 2  depicts a schematic diagram of details of a binary-cycle geothermal energy conversion system. 
         FIG. 3  depicts a schematic diagram of a portion of a representative OTEC power generation system in accordance with the prior art. 
         FIG. 4  depicts a schematic diagram of details of an energy conversion system in accordance with an illustrative embodiment of the present invention. 
         FIG. 5  depicts a method for generating electrical energy in accordance with the illustrative embodiment of the present invention. 
         FIG. 6  depicts calculated efficiencies of energy conversion cycles versus temperature differential for an energy conversion system in accordance with the present invention. 
         FIG. 7  depicts a schematic diagram of details of an energy conversion system in accordance with a first alternative embodiment of the present invention. 
         FIG. 8  depicts a schematic diagram of details of an energy conversion system in accordance with a second alternative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts a schematic diagram of details of a prior-art dry steam geothermal energy conversion system. Energy system  100  comprises inlet conduit  106 , turbine  108 , generator  110 , condenser  112 , conduits  114  and  116 , cooling tower  118 , and outflow conduit  122 . Turbine  108  and generator  110  collectively define a turbogenerator. In cases, a water pump is coupled to the outflow of condenser  112  to drive the return water to greater depths or to facilitate flow in a very long or narrow outflow conduit  122 . 
     Inlet conduit  106  is inserted through earth crust  102  into geothermal reservoir  104 . Inlet conduit  106  conveys steam from geothermal reservoir  104  to turbine  108 . Turbine  108  is operatively coupled to generator  110 . 
     The steam from geothermal reservoir  104  drive turbine  108 , which turns generator  110  to produce electrical energy. The generated electrical energy is conveyed to an end user or storage facility on output cable  120 . 
     Conduit  114  receives the steam that passes through turbine  108  and conveys it to condenser  112 . At condenser  112 , the steam is cooled by heat transfer fluid that circulates between cooling tower  118  and condenser  112  via conduit  116 . The heat transfer fluid acts as a heat sink for the steam, inducing the steam to condense into water in condenser  112 . Outlet conduit  122  conveys the condensate back to geothermal reservoir  104 . Cooling tower  118  removes the absorbed heat in the heat transfer fluid by vaporizing water into the surrounding atmosphere prior to circulating the heat transfer fluid back to condenser  112 . 
       FIG. 2  depicts a schematic diagram of details of a binary-cycle geothermal energy conversion system. Energy system  200  comprises inlet conduit  202 , heat exchanger  204 , closed-loop conduit  206 , turbine  108 , generator  110 , condenser  112 , conduit  208 , and cooling tower  118 . 
     In operation, inlet conduit  202  conveys hot water and/or steam from geothermal reservoir  104  to heat exchanger  204 . At heat exchanger  204 , the heat of the hot water/steam in inlet conduit  202  heats a working fluid contained within closed-loop conduit  206 . This working fluid evaporates into a pressurized vapor, which drives through turbine  108  to the condenser  112 . The flow of pressurized vapor through turbine  108  causes the turbine to turn generator  110 . In response, generator  110  generates electrical energy, which is conveyed to an end user on output cable  120 . 
     After passing through the turbine, the depressurized vapor enters condenser  112 . At condenser  112 , the vapor is thermally coupled with cold water that flows through conduit  208 , which induces the vapor to condense back into a liquid working fluid. Conduit  208  conveys cold water to and from cooling tower  118 . Condensate pumps and filters are often included in the working fluid circuit to facilitate flow between condenser  112  and heat exchanger  204 . 
       FIG. 3  depicts a schematic diagram of a portion of a representative OTEC power generation system in accordance with the prior art. OTEC system  300  comprises platform  302 , surface water conduit  304 , deep water conduit  308 , turbogenerator  312 , closed-loop conduit  314 , heat exchanger  318 , pump  320 , and condenser  322 . 
     Platform  302  is a conventional floating energy-plant platform. Platform  302  is anchored to the ocean floor by mooring line  334 , which is connected to anchor  336 . Anchor  336  is embedded in the ocean floor. In some instances, platform  302  is not anchored to the ocean floor and platform  302  is allowed to drift. Such a system is sometimes referred to as a “grazing plant.” 
     Surface water conduit  304  is a large-diameter conduit suitable for pumping water, by means of pump  328 , from surface region  332  into heat exchanger  318 . 
     Closed-loop conduit  314  is a closed-circuit loop of conduit that contains working fluid  316 . Ammonia is commonly used as such a working fluid; however, there are many other fluids that can be used as working fluid  314 . 
     Closed-loop conduit  314  and surface water conduit  304  are thermally coupled at heat exchanger  318 . As a result, working fluid  316  and surface water  306  are also thermally coupled at heat exchanger  318 . This enables the heat of surface water  306  to vaporize the working fluid  316 . The expanding vapor turns turbogenerator  312 , which generates electrical energy. The generated electrical energy is provided on output cable  120 . 
     After passing through turbogenerator  312 , the vaporized working fluid enters condenser  322 , which comprises heat exchanger  324 . At heat exchanger  324 , closed-loop conduit  314  and deep water conduit  308  are thermally coupled, which enables the thermal coupling of the vaporized working fluid  316  and cold water  310 . Cold water  310  is drawn from deep water region  330  by pump  326 . Typically, deep water region  330  is 1000+ meters below the surface of the body of water. Water at this depth is at a substantially constant temperature of a few degrees centigrade. After passing through heat exchanger  324 , cold water  310  is ejected into mid-level region  338  to avoid cooling the surface water near platform  302 . 
     Cold water  310  acts as a heat sink for vaporized working fluid  316  at heat exchanger  324 . As a result, the hot vaporized working fluid  316  is cooled by cold water  310  and condenses back into its liquid state. Once working fluid  316  is condensed, pump  320  recycles it back into heat exchanger  318  where it can be vaporized again to continue the cycle that drives turbogenerator  312 . 
     Conventional OTEC systems have several drawbacks. First, it is difficult and energy intensive to pump cold water up from depths of 1000+ meters. This challenge is further exacerbated by the fact that cold water is more dense than warm water, which increases the energy required to draw it up to the surface. This significantly increases the cost and reduces the benefits of using an OTEC approach for power generation. 
     Second, for an OTEC generation system capable of generating 10&#39;s to 100&#39;s of megawatts, deep water conduit  308  typically has a diameter within the range of 3-10 meters and a length greater than 1000 meters. Such a conduit is difficult and expensive to manufacture. 
     Third, the size and length of deep water conduits makes them susceptible to damage from environmental conditions, such as strong currents, storms, and wave action. As a result, complicated and expensive infrastructure is required to protect these conduits from damage. For example, numerous recent efforts have been made to improve the reliability of cold water conduits. These include the development of flexible conduits, inflatable conduits, rigid conduits made from steel, plastics, and composites, and gimbal-mounted conduits. Even with such proposed innovations, long cold water conduits remain a significant reliability and cost issue. 
       FIG. 4  depicts a schematic diagram of details of an energy conversion system in accordance with an illustrative embodiment of the present invention. Energy conversion system  400  comprises close-loop conduit  402 , optional pump  408 , energy conversion unit  410 , pressure hull  416 , and chimney  420 . One skilled in the art will recognize, after reading this specification, that the present invention is suitable for operation in any suitable body of water, including, without limitation, oceans, seas, lakes, straights, gulfs, and bays. 
     Energy conversion unit  410  is a solid-state thermoelectric energy conversion system. Energy conversion unit  410  comprises hot zone  412 , thermoelectric element  418 , and cold zone  414 , which is a portion of pressure hull  416 . 
     Hot zone  412  comprises a substantially thermally conductive plate that enables the transfer of heat from close-loop conduit  402  to surface  422  of thermoelectric element  418 . 
     Cold zone  414  is a portion of pressure hull  416  that is substantially thermally conductive. Cold zone  414  enables the thermal coupling of surface  424  of thermoelectric element  418  and cold water outside pressure hull  416 . 
     Thermoelectric element  418  is a solid-state device that generates an open-circuit voltage based on a temperature difference between surfaces  422  and  424 . In some embodiments, thermoelectric element  418  comprises a bismuth-telluride alloy. Commercial examples of thermoelectric element  418  include HZ modules available from Hi-Z Technology, Inc. 
     In some embodiments, thermoelectric element  418  is a solid-state element that generates electrical energy by means of the Peltier effect. 
     In some embodiments, energy conversion unit  410  comprises a classic Rankine-cycle engine instead of a solid-state thermoelectric element. In such embodiments, hot zone  412  and cold zone  414  are included in heat exchangers, such as heat exchangers  318  and  324  described above and with respect to  FIG. 3 . As a result, in these embodiments, hot zone  412  and cold zone  414  enable vaporization and condensation, respectively, of a working fluid as part of a Rankine cycle. 
       FIG. 5  depicts a method for generating electrical energy in accordance with the illustrative embodiment of the present invention. Method  500  is described herein with continuing reference to  FIG. 4 . 
     Method  500  begins with operation  501 , wherein working fluid  404  is circulated through closed-loop conduit  402 . Working fluid  404  is analogous to working fluid  316 , described above and with respect to  FIG. 3 . In some embodiments, working fluid  404  is pumped through closed-loop conduit  402  by optional pump  408 . Closed-loop conduit  402  passes through bore hole  406  into geothermal reservoir  104 . 
     In some embodiments, pump  408  is not necessary since the temperature gradient between energy conversion unit  410  and geothermal reservoir  104  can inherently induce the flow of working fluid  404  through closed-loop conduit  402 . As working fluid  404  loses its heat at energy conversion unit  410 , it cools and naturally descends toward geothermal reservoir  104 . This creates a natural convective flow (clockwise, as depicted in  FIG. 4 ) around closed-loop conduit  402 . 
     At operation  502 , hot zone  412  is heated by virtue of thermally coupled working fluid  404 . Hot zone  412  and a portion of closed-loop conduit  402  collectively define heat exchanger  426 . Heat exchanger  426  thermally couples geo-thermal reservoir  104  and surface  422  of thermoelectric element  418 . 
     At operation  503 , cold zone  414  is cooled by cold water in deep water region  330 . Cold zone  414  is cooled by virtue of heat exchanger  428 , which comprises a portion of pressure hull  416  and is, therefore, in direct contact with the deep-level water. Heat exchanger  428  thermally couples surface  424  of thermoelectric element  418  with the deep-level water. Although in the illustrative embodiment cold zone  412  is a portion of pressure hull  416 , it will be clear to one skilled in the art, after reading this specification, how to make and use alternative embodiments wherein cold zone  412  is a separate component from pressure hull  416 . In some embodiments, one or more bore-hole fittings are used to couple closed-loop conduit  402  to a permanent conduit that resides within bore hole  406 . In such embodiments, therefore, a substantial portion of energy conversion system  400  is detachable and removable. 
     Pressure hull  416  is a shell of structural material with sufficient strength to withstand the pressures that exist at deep water levels. The specific design of pressure hull  416  is based upon the intended application and deployment depth. For example, a pressure hull intended to be deployed at a depth of 1000 meters must be able to withstand water pressure that exceeds 100 atmospheres. In addition, pressure hull  416  comprises an electrical feed-through to enable generated electrical energy to be conveyed on cable  120 . 
     In some embodiments, pressure hull  416  is a vessel that is filled with an incompressible, electrically non-conductive fluid. As a result, the need for the walls of pressure hull  416  to withstand externally applied high pressure is mitigated. 
     In some embodiments, energy conversion system  400  is suitable for deployment in environments wherein the water in surface region  332  has a substantially constant temperature of a few degrees centigrade, such as arctic regions. In such embodiments, the need for pressure hull  416  to withstand high external pressures is mitigated and pressure hull  416  can be replaced by a less robust, substantially water-tight chamber. 
     It is an aspect of the present invention that the water at a deep level of an ocean or similar body of water provides a heat sink with sufficient heat capacity to enable it to maintain a substantially constant temperature at all times. It is well-known that ocean temperatures drop with depth. For example, tropical and semi-tropical ocean temperatures at depths of 400, 500, and 1000 meters remain substantially constant at 12, 8, and 4° C., respectively. Deep water levels, therefore, have a heat-sink capability that is well-suited to the present invention. 
     At operation  504 , thermoelectric element  418  generates electrical energy based on the temperature difference between hot zone  412  and cold zone  414 . 
       FIG. 6  depicts calculated efficiencies of energy conversion cycles versus temperature differential for an energy conversion system in accordance with the present invention. Plot  600  depicts the percentage of the Carnot cycle conversion efficiency for a range of temperature differentials that are based on water depths and geothermal heat source temperatures. As one skilled in the art will recognize, the Carnot cycle represents the most efficient cycle possible for converting a given amount of thermal energy into work. 
     Conversion cycle  602  depicts the efficiency for a thermoelectric energy conversion system based on the temperature differential between surface water (i.e., 1 m deep having a temperature of approximately 28° C.) and a relatively cool geothermal source (having a temperature of 100° C.). Although the systems in accordance with the present invention are operable for smaller temperature differentials, the temperature differential for conversion cycle  602  represents the smallest reasonable temperature cycle commonly available using a geothermal heat source. The energy conversion efficiency of conversion cycle  602  is a modest 19% of the Carnot cycle. 
     Conversion cycle  604 , on the other hand, represents the largest temperature cycle commonly available using a geothermal heat source. Conversion cycle  604  is based on the temperature difference between water at 1000 m depth (having a temperature of approximately 4° C.) and a hot geothermal source (having a temperature of approximately 200° C.), the energy conversion efficiency is approximately 41% of the Carnot cycle. Conversion cycle  604 , therefore, is characterized by a conversion efficiency that is 22% greater than that of conversion cycle  602 . This represents an efficiency improvement of more than 100%. 
     At operation  505 , the generated electrical energy is conveyed to the end-user via cable  120 . In some embodiments, cable  120  terminates at a land-based installation. In some embodiments, cable  120  terminates at an open ocean vessel, such as an anchored barge, ship, spar platform, oil rig, dedicated power production platform, and the like. An open ocean vessel could be moored to nearby sea mounts, pinnacles, or the ocean floor. 
     At optional operation  506 , the convective flow of cold water across cold zone  414  is constrained by chimney  420 , thereby increasing the heat flow through the heat exchanger and seawater. The length of the chimney  420  is a matter of design choice, but is based on the temperature difference between the convecting seawater as it passes by cold zone  414  and the depth of the average thermocline at that temperature. The heat sink included in cold zone  414  would be designed to exhibit less head loss than the pressure difference (inside and outside) at the bottom of the chimney  420 , minus the fluid drag up the chimney  420 . In some embodiments, chimney  420  is not used since the rate at which the convective flow of cold water flows across cold zone  414  is sufficient to ensure that the ambient temperature of the water in the local area of cold zone  414  does not substantially increase during operation of energy system  400 . 
       FIG. 7  depicts a schematic diagram of details of an energy conversion system in accordance with a first alternative embodiment of the present invention. Energy conversion system  700  comprises close-loop conduit  402 , optional pump  408 , energy conversion unit  410 , pressure hull  416 , chimney  420 , and rod  702 . 
     Rod  702  is a crust-penetrating rod suitable for insertion into the hot material of geothermal reservoir  104 . Rod  702  comprises structural material that is substantially thermally conductive. As a result, rod  702  conducts heat from geothermal reservoir  104  into heat exchanger  704 , which comprises rod  702  and hot zone  412 . 
     At heat exchanger  704 , heat from geothermal reservoir  104  is absorbed by hot zone  412 . Operation of system  700  is analogous to the operation of system  400 , described above and with respect to  FIGS. 4 and 5 . 
       FIG. 8  depicts a schematic diagram of details of an energy conversion system in accordance with a second alternative embodiment of the present invention. Energy conversion system  800  comprises close-loop conduit  402 , optional pump  408 , energy conversion unit  410 , pressure hull  416 , chimney  420 , and conduit  802 . 
     Conduit  802  receives water of hydrothermal vent  804 . This water has been heated by the hot material of geothermal reservoir  104 . As a result, conduit  802  conducts heat from geothermal reservoir  104  into heat exchanger  806 , which comprises a portion of conduit  802  and hot zone  412 . 
     At heat exchanger  806 , therefore, heat from water of hydrothermal vent  804  is absorbed by hot zone  412 . After the water of ocean thermal vent  804  exits heat exchanger  806 , conduit  802  exhausts it to a region of the body of water that is sufficiently displaced from cold zone  404  to avoid heating the water outside of pressure hull  416 . 
     Operation of system  800  is analogous to the operation of system  400 , described above and with respect to  FIGS. 4 and 5 . 
     It is to be understood that the disclosure teaches just one example of the illustrative embodiment and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.