Abstract:
An electric power generation system employs a thermoelectric generator placed between an aircraft inner skin and an aircraft outer skin. The thermoelectric generator is configured to utilize a thermal differential between the inner and outer skin to generate an electric current. An electrical interface is provided for access to the electric current generated by said thermoelectric generator.

Description:
BACKGROUND INFORMATION 
       [0001]    Field 
         [0002]    Embodiments of the disclosure relate generally to the field of electrical power generation for aircraft and more particularly to a system for powering aircraft auxiliary system such as sensors employing a thermoelectric generator for generating electricity based on a temperature differential using a thermal capacitor already existing in the vehicle and having a primary function on the vehicle other than as a thermal capacitor. 
       Background 
       [0003]    Modern aircraft employ electrical power for numerous on board systems. Conventional generation of electricity for such usage is accomplished with engine or auxiliary power unit (APU) driven generators located in the aircraft. Power from the generators is then routed through the aircraft for use with standard electrical cabling in numerous wire harnesses. Issues of weight for the extensive wiring systems as well as the potential for undesirable electrical discharge within the circuit system have prompted examination of alternative power routing techniques. 
         [0004]    Thermoelectric generators have been used in aircraft and other vehicles to provide electrical power generation alternatives. However, these uses are typically associated with heat generated by burning fuel. In these applications, location of the heat sources limits the location of the thermoelectric generator and therefore the wiring distance issues may be present even with the alternative power source. Additionally, power is only generated when fuel is being burned. 
         [0005]    Aircraft and other vehicles often have systems which may provide thermal mass reacting to a temperature differential to act as thermal capacitors. Since such systems are primarily for purpose other than the thermal capacitance capability, that capability is wasted and in some cases not enhanced. 
         [0006]    It is therefore desirable to provide an electrical generation system which employs existing thermal capacitors in a vehicle to provide power for auxiliary systems. 
       SUMMARY 
       [0007]    Embodiments disclosed herein provide a power generation system incorporating a principal system having a primary function and having an associated mass providing a thermal capacitor. The embodiments provide for a thermoelectric generator is placed between the thermal capacitor and an external environment. The thermoelectric generator is configured to utilize a thermal differential between the thermal capacitor and the external environment to generate an electric current. An auxiliary system associated with the principal system is connected to operate using the electric current generated by the thermoelectric generator. 
         [0008]    The embodiments provide for a method for generation of electrical power from a thermal capacitor present in a principal system for an auxiliary system such as a sensor associated with the principal system on an aircraft. A thermoelectric generator is mounted to receive heat flow between a thermal capacitor in a principal system on an aircraft and a thermal sink. The aircraft is operated at a cruising altitude providing low temperature air external to the aircraft. Electrical power is then generated by the thermoelectric generator based on the temperature differential between the thermal capacitor and external air as a thermal sink. An auxiliary system may then be operated with the electrical power. 
         [0009]    The features, functions, and advantages that have been discussed can be achieved independently in various embodiments of the present disclosure or may be combined in yet other embodiments further details of which can be seen with reference to the following description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic view of an aircraft as an example vehicle with multiple internal systems which may act as thermal capacitors in addition to their normal functions; 
           [0011]      FIG. 2  is an example embodiment of a fuel tank thermal capacitor arrangement; 
           [0012]      FIG. 3  is an example embodiment of a lavatory water storage tank thermal capacitor arrangement; 
           [0013]      FIG. 4  is an example embodiment of a cargo container thermal capacitor arrangement; 
           [0014]      FIG. 5  is a schematic representation of a thermoelectric generator operating with a thermal capacitor and a battery storage system for power leveling; 
           [0015]      FIG. 6  is a schematic representation of a temperature monitoring application for a thermoelectric generator operating with a thermal capacitor; and, 
           [0016]      FIG. 7  is a flow chart of operation of the thermoelectric generator for distributed generation of power for auxiliary system or sensor use. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Embodiments disclosed herein provide electrical power for an auxiliary system in a vehicle using a thermoelectric generator deriving power from a thermal capacitor existing as a portion of a principal system having a primary function on the vehicle other than as a thermal capacitor. The thermoelectric generator is able to produce electrical power using the potential energy that exists between the temperature of the thermal capacitor and the external cold air or other external environment of the vehicle. 
         [0018]    Using a commercial aircraft as an example vehicle in which the disclosed embodiments may be employed, such aircraft cruise at altitudes above the tropopause and extending well into the stratosphere. Air temperatures in this region of the atmosphere are nominally between −20° and −60° C. Operating altitudes even during climb and descent regularly provide significantly cooler air temperatures. Additionally, aircraft typically have large fuel tanks as a principal system having a primary function to supply fuel to the aircraft engines but which also provide a significant volume of fuel that has thermal inertia, i.e. it tends to remain at an initial temperature and only slowly conforms to a new environmental temperature. This thermal inertia allows the fuel or the fuel tank in which it is stored to act as a thermal capacitor. Additionally other on-board systems which have a primary function not associated with electrical power generation such as water systems for lavatories may also provide materials with sufficient volume and/or mass to provide high thermal inertia to act as thermal capacitors. In aircraft, cargo containers or cargo itself may also provide thermal inertia which may be employed as a thermal capacitor. Each of these principal systems has a primary function in the aircraft; fuel tanks supply fuel for the operation of the aircraft engines, water tanks supply water for galley or lavatory use and cargo containers are designated for the shipment of the cargo contained therein. However, the thermal capacitance offered by each system may provide secondary functionality. 
         [0019]    As shown in  FIG. 1  for an exemplary aircraft system, the aircraft  10  has a fuselage  12  encompassing a cabin  14 . Engines  16  (shown as mounted on the wings  18  for the present example but mounted on the empennage of the aircraft in alternative configurations) typically draw fuel from tanks  20  mounted internal to the wings  18 . Such wing tanks may hold  3900 kg or more of jet fuel which provides a significant thermal capacitance with respect to the exterior temperature of the wing  18  which may act as a thermal sink when the aircraft is in flight at altitude. The fuel tank as a thermal capacitor may be employed in conjunction with a thermoelectric generator to provide electrical power. 
         [0020]    Additionally most aircraft include lavatories, galleys or other systems requiring water storage and one or more water tanks  22  are present, typically in the fuselage  14  of the aircraft. The water tanks  22  also provide a thermal capacitance which may be employed in conjunction with a thermoelectric generator for generation of electrical power. 
         [0021]    Cargo being carried by the aircraft is typically situated in the fuselage  14  and in many cases cargo containers  24  provide significant mass which may be used as a thermal capacitor with external air or, for unheated/unpressurized cargo compartments, air within the compartment. Alternatively, the mass available may only act a thermal capacitance if certain temperature limits in the environment of the cargo container are exceeded which may then provide operation of a thermoelectric generator to power sensors or recorders for that temperature excursion. [Para  21 ] A first exemplary power generation system is shown in  FIG. 2 , wherein a thermoelectric generator  30  is located in thermal contact on a first surface  32  with a wall  34  of the fuel tank  20  as the principal system. Fuel in the tank  20  (generally designated  36 ) provides a mass to act as the thermal capacitor in addition to its primary functionality of providing fuel to the engines. A second surface  38  of the thermoelectric generator  30  is in thermal contact with a wing skin  40  on the surface of the wing  18 . The temperature of the external air (generally designated  42 ) provides a temperature at the wing skin  40  as a reference element which provides a temperature differential between the first surface  32  and second surface  38  of the thermoelectric generator  30  creating a thermoelectric gradient for heat flow (generally designated by arrows  43 ) to provide electrical power to leads  44  which are connected to an auxiliary system such as a sensor  46 . The sensor  46  may be, as examples, a fuel quantity gage, fuel temperature gage or explosive vapor detector to measure a desired physical phenomenon associated with the fuel tank, converting that phenomenon to an instrumentation signal and may provide redundancy to back up a primary system in the aircraft. Output from the sensor may be routed to displays or for other processing through output  48 . While shown as a physical lead, output  48  may be either a wired or wireless communications channel. The operational engagement of the thermoelectric generator  30  between the fuel tank  20  as the thermal capacitor and wing skin  40  in contact with the external air having a different temperature than the thermal capacitor provides the thermoelectric gradient for desired generation of electric current. 
         [0022]    A similar structural arrangement for use with a water tank  22  as the principal system is shown in  FIG. 3 . The thermoelectric generator  30  is positioned with first surface  32  in thermal contact with a wall  50  of the water tank  22  with water in the tank (generally designated  51 ) providing a mass acting as the thermal capacitor in addition to its primary functionality of providing water for use in the galley or lavatory. The second surface  38  of the thermoelectric generator  30  is in thermal contact with a skin  52  on the surface of the fuselage  14 . The temperature of the external air  42  again provides a temperature differential between the first surface  32  and second surface  38  of the thermoelectric generator  30  for heat flow  43  to provide electrical power to leads  44  which are connected to a sensor  54  as the auxiliary system. The sensor  54  may be, as an example, a capacitive level or quantity gauge for the water tank. Output from the sensor  54  may be routed through output lead  56  to a displays  58  located adjacent the water tank location or remotely. 
         [0023]      FIG. 4  shows an embodiment employing a cargo container  24 . The thermoelectric generator  30  is positioned with first surface  32  in thermal contact with a wall  60  of the cargo container  24  with mass of cargo and dead air volume in the container (generally designated  61 ) providing a mass acting as the thermal capacitor in addition to its primary functionality of providing shipment of the cargo. The second surface  38  of the thermoelectric generator  30  is in thermal contact the environment (generally designated  62 ) surrounding the container  24 . The temperature of the environment  62  may provide a temperature differential between the first surface  32  and second surface  38  of the thermoelectric generator  30  for heat flow  43  to provide electrical power to leads  64  which are connected to an auxiliary system  66 . The auxiliary system  66  may be, as examples, a temperature sensor or a GPS location sensor/transmitter. 
         [0024]    While principally described herein as operating between a higher temperature of the thermal capacitor and a lower external or environmental temperature, the embodiments disclosed may also operate where the thermal capacitance of the principal system is at a lower temperature and the external environment is at a higher temperature. In such embodiments, the auxiliary system may be a sensor as described or may be such implementations as a fan to circulate cooling air in a shipping container where the external temperature exceeds a desired value. The auxiliary system may be connected to negatively compensate (i.e. provide negative feedback) for the change in temperature (e.g. fanor cooling device, or a heating device directed at or applied to a particularly sensitive part of the cargo or vehicle to avoid damage or equalize the temperature of an item. This functionality would additionally speed up or delay the thermal inertia in the thermal capacitor. Additionally, the thermoelectric generator may operate when the temperature of the thermal capacitor is either higher or lower and the reference element and/or the environmental sink. While described as an aircraft in principal embodiments herein, the vehicle may be a spacecraft, an aircraft, a helicopter, a lighter-than-air craft, an underwater vehicle, and a missile as alternative examples. 
         [0025]    The operating elements of an exemplary thermoelectric generator  30  are shown in  FIG. 5 . A cold plate  72  and a hot plate  74  are fabricated from alumina ceramic or similar material which may be metalized. The hot plate  72  and cold plate  74  are mounted on opposite sides of a thermoelectric stack  76  which may employ Seebeck effect, the Peltier effect, or the Thomson effect for generation of electrical current. In an example embodiment, the stack is fabricated from bismuth telluride (Bi2Te3) semiconductor p-n junctions that are electrically connected in series or parallel or combination of the two. Electrical power generated by the stack  76  is then provided through leads  78   a  and  78   b . For an exemplary embodiment, a thermoelectric generator  30  may be created using one or an array of single stage operating elements  77  such as the model NL1010T produced by Marlow Industries Inc., Dallas, Tex. The hot and cold plates  72 ,  74  are thermally interfaced to mass of the thermal capacitor and a reference or sink such as the external environment or a skin or wall adjacent the external environment providing the temperature differential as previously described, either directly or on conductive extensions using thermal grease such as that produced by Marlow Industries with part no. #860-3079-001 for optimal thermal conductivity. The cold plate  72  at its outer surface  38  conductively engages the outer skin of the wing or fuselage in the fuel tank and water tank embodiments or the environmental air for the cargo container embodiment for heat transfer and the hot plate  74  interacts at its surface  32  with the thermal capacitor mass through direct conductive engagement of the wall  40 ,  52  or  60 . 
         [0026]    As also shown in  FIG. 5 , the thermoelectric generator  30  may provide generated power to a power conditioning module  80  which converts the generator output to desired current and voltage values (such as 12 volt or 28 volt DC or 110 volt AC) for the operating the associated auxiliary system such as the sensors  46 ,  54 ,  66 . An electrical energy storage device such as battery  82  may be connected to the thermoelectric generator through the power conditioning system for power storage to allow powering of the associated sensors or other auxiliary system when thermal gradients may not be present for operation of the thermoelectric generator  30  or to supplement the power provided by the thermoelectric generator when the temperature differential is small. In alternative embodiments a capacitive storage system may be employed. With full thermal gradients, the power conditioning module  80  may charge the battery  82 . 
         [0027]    A particular application is shown in  FIG. 6  wherein the thermoelectric generator  30  is attached to operate between an environment such as the outside  84  of a container  24  and the interior  86  of the container as the thermal capacitor. The thermoelectric generator would only be activated if a temperature differential of a predetermined amount exists between the outside environment  84  and the interior  86 . A sensor  88  such as a temperature sensor constituting the auxiliary system would only receive current from the thermoelectric generator if the temperature differential was exceeded. The sensor would then activate to indicate the temperature. An actual temperature value may not be required from the temperature sensor but merely an indication that a threshold temperature had been exceeded. A counter  90  for counting the amount of time the temperature differential exceeds the threshold amount may be connected to the sensor  88  and powered by the thermoelectric generator  30  to provide a record for the product in the container  86  (e.g. a dairy product, wine bottle/case, or other product that may be degraded if exposed to an excessive temperature). The counter  90  may be a start-stop timer that can measure a cumulative amount of time in seconds, minutes, hours, or days, as needed. The counter  90  may be initialized based on a cycle of operation. For example if the thermoelectric generator  30 , sensor  88  and/or counter  90  are a self-contained system attachable to the container  86 , the counter may be initialized when the system is attached to the container. Alternatively, in the thermoelectric generator  20 , sensor  88  and/or counter  90  are permanently affixed to the container  86  the counter  90  may be manually reset when the container is loaded and sealed. 
         [0028]    The auxiliary system may also be or include a tripping element  91  such as a fuse or circuit breaker that disconnects the auxiliary system when a temperature differential is exceeded or merely acts as the indicator. 
         [0029]    The thermoelectric generator  30 , sensor  88  and counter  90  could be fabricated from low cost materials to provide a disposable single use system directly associated with or incorporated as a part of the container. Alternatively, the thermoelectric generator  30 , sensor  88  and counter  90  could be a self-contained device which is removably attachable to the container  24 . 
         [0030]    The embodiments disclosed provide a method for generation of electrical power from a thermal capacitor present in a principal system for an auxiliary system such as a sensor associated with the principal system on an aircraft as shown in  FIG. 7 . A thermoelectric generator is mounted to receive heat flow between the thermal capacitor and a thermal sink, step  702 . The aircraft is then operated at a cruising altitude providing a low temperature external to the aircraft, step  704 . Electrical power is then generated by the thermoelectric generator through the temperature differential between thermal capacitor and external air as the thermal sink, step  706 . Generated electrical power from the thermoelectric generator can then be conditioned for proper voltage, step  708 , and provided to operate an auxiliary system such as a sensor, step  710  or charge a power storage system such as a battery or capacitor for power storage to operate the sensor system, step  712 . The battery may then be employed for operation of the auxiliary system if sufficient thermal gradient is not present for the thermoelectric generator to provide sufficient power, step  714 . A counter may be operated when the thermoelectric generator is operating to record a temperature differential above a threshold, step  716   
         [0031]    Having now described various embodiments of the disclosure in detail as required by the patent statutes, those skilled in the art will recognize modifications and substitutions to the specific embodiments disclosed herein. Such modifications are within the scope and intent of the present disclosure as defined in the following claims.