Abstract:
A hybrid energy storage system (ESS) includes a first energy storage device including a battery having an impedance for providing a substantially constant power output, and a second energy storage device linked to the first energy storage and including a high power electrochemical double layer capacitor (EDLC) for providing intermittent bursts of high voltage output in a range of 1.5 to 3.0 volts, wherein an operation rating of the second energy source is within a temperature range between 75 degrees Celsius and 330 degrees Celsius while exhibiting a leakage current less than 1 amp per liter of volume over the range of operating temperatures and at a voltage up to a rated voltage.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention generally relates to electrochemical double-layer capacitors, and more specifically to a battery-capacitor hybrid energy storage system for high temperature applications. 
         [0002]    A battery-only energy storage system (BESS) is typically designed to meet certain required specifications in total energy capacity, power handling capability and physical size and weight. The selection of suitable battery technologies becomes increasingly limited as the required power handling capability increases. This problem is exacerbated as the required operating temperatures increase. 
         [0003]    Conventional solutions utilize so-called high-rate lithium cells. In applications requiring high power or pulsed power, conventional solutions may require a BESS that is significantly oversized with respect to total energy capacity in order to meet the requirement in power handling capability. Additionally, an overall safety of the energy storage system may be improved if a high-rate lithium cell is not needed. 
         [0004]    Accordingly, what is desired is an energy source which is capable of providing a suitable combination of total energy capacity, power handling capability per unit weight and volume and that operates at high temperature without the need for a high-rate lithium cell. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention discloses a battery-capacitor hybrid energy storage system (HESS) for high temperature applications. 
         [0006]    In general, in one aspect, the invention features an HESS including a first energy storage device including a battery having an impedance for providing a substantially constant power output, and a second energy storage device linked to the first energy storage and including a high power electrochemical double layer capacitor (EDLC) for providing intermittent bursts of high power output in a range of 1.5 to 3.5 volts, wherein an operation rating of the second energy source is within a temperature range between 75 degrees Celsius and 330 degrees Celsius while exhibiting a leakage current less than 1 amp per liter of volume over the range of operating temperatures and at a voltage up to 3.5 volts. 
         [0007]    Other features and advantages of the invention are apparent from the following description, and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The invention will be more fully understood by reference to the detailed description, in conjunction with the following figures, wherein: 
           [0009]      FIG. 1  is a block diagram. 
           [0010]      FIG. 2  is a block diagram. 
           [0011]      FIG. 3  is a block diagram. 
       
    
    
       [0012]    Like reference numbers and designations in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0013]    As shown in  FIG. 1 , an exemplary circuit diagram of a battery-ultracapacitor hybrid energy storage system (HESS)  10  includes a first energy source  12  having a high energy density, and a high capacity, but which may not be able to provide high power as required by certain applications. The first energy source  12  may thus be one or more battery cells, examples of which include, but are not limited to, a lead-acid battery, a nickel-cadmium battery, a nickel-metal hydride battery, a lithium ion battery, a lithium polymer battery, a lithium ion polymer electrolyte battery, a zinc air battery, and combinations thereof. The battery cell or cells may also be a primary battery, such as a conventional alkaline battery or even a compact portable fuel cell. The battery cell or cells may be disposed in a battery pack. 
         [0014]    The battery-ultracapacitor HESS  10  includes a second energy source  14  connected electrically in parallel to the first energy source  12 . The second energy source  14  is capable of delivering a burst of high power, as may be required by a particular application. As such, the second energy source  14  is preferably an electrochemical double layer capacitor (EDLC). 
         [0015]    At high application temperatures, parasitic leakage current levels are generally elevated. In parallel combinations, energy sources share leakage currents, so, for instance, an elevated leakage current in the EDLC could in theory drain the stored energy from the primary energy source at a prohibitive rate when exposed to high application temperatures. As such, the second energy source  14  is preferably an EDLC designed specifically to exhibit low-leakage current at nominal working voltages when exposed to high application temperatures (a High-temperature EDLC). 
         [0016]    The battery-ultracapacitor HESS  10  includes first and second resistors  16  and  18  to provide a measure of the impedance to the first energy source  12  and second energy source  14 , respectively. In should be noted that these resistors  16 ,  18  are circuit model elements that represent an non-ideal battery and capacitor output impedances, not actual resistors that are added to the system  10 . 
         [0017]    As shown in  FIG. 2 , in a specific example, the HESS  10  includes electrochemical double layer capacitor (EDLC)  100 . The EDLC  10  includes two electrodes  112 ,  114 , each with a double layer of charge at an electrolyte interface, supported on conductive metal plates  116 ,  118 . The electrodes  112 ,  114  are the two electrically conducting parts of the EDLC  100 . 
         [0018]    When the HESS  10  incorporating a high-temperature EDLC replaces a battery-only energy storage system (BESS), the power handling requirements of the battery in that HESS are reduced. As such, the HESS  10  having equivalent performance in power handling capability and energy capacity to a high-rate lithium BESS can include a low-rate rather than a high-rate lithium battery. An overall safety of the HESS  10  is increased when an HESS having a low rate lithium battery replaces a high-rate lithium BESS 
         [0019]    In addition, the HESS  10  incorporating a low-rate lithium battery and having equivalent performance in power handling capability and energy capacity exhibits lower volume and weight compared to a high-rate lithium BESS. 
         [0020]    The EDLC  100  includes charge collectors  120 ,  122 , respectively, an electrolyte  124  and a separator  126 . In general, the electrolyte  124  is a chemical compound (salt) that disassociates into electrically charged ions. A solvent that that dissolves the ions might be present. The resulting electrolytic solution is an ionic liquid that conducts electricity by ionic transport. 
         [0021]    The exemplary EDLC  100  is either a wound or prismatic and then packaged into a cylindrical or prismatic enclosing body. The enclosing body is hermetically sealed. In one example, the package is hermetically sealed by laser, ultrasonic, or welding. 
         [0022]    In general, the separator  126  is a thin structural material (usually a sheet) used to separate the electrodes  112 ,  114 , of a divided electrochemical cell into two or more compartments. The electrolyte  124  occupies pores in the electrodes  112 ,  114  and separator  126 . 
         [0023]    In the EDLC  100 , the electrodes  112 ,  114  can be provided by and include activated carbon, carbon fibers, rayon, graphene, aerogel, carbon cloth, and/or carbon nanotubes. Activated carbon electrodes can be manufactured, for example, by producing a carbon base material by carrying out a first activation treatment to a carbon material obtained by carbonization of a carbon compound, producing a formed body by adding a binder to the carbon base material, carbonizing the formed body, and finally producing an active carbon electrode by carrying out a second activation treatment to the carbonized formed body. Others methods may also be used. 
         [0024]    Carbon fiber electrodes can be produced, for example, by using paper or cloth preform with high surface area carbon fibers. Other methods may be used. 
         [0025]    In one specific example, multiwall carbon nanotubes (MWNT) on a variety of substrates using chemical vapor deposition (CVD) are fabricated for use in the electrodes  112 ,  114  of EDLC  100 . In a preferred embodiment, low-pressure chemical vapor deposition (LPCVD) is used. The fabrication process uses a gas mixture of acetylene, argon, and hydrogen, and an iron catalyst deposited on the substrate using electron beam deposition and or sputtering deposition. 
         [0026]    The electrolyte  124  includes a pairing of a cation  130  and an anion  132  and may include a solvent. Various combinations of each are used. In the exemplary EDLC  100 , the cation  130  can include 1-(3-Cyanopropyl)-3-methylimidazolium, 1,2-Dimethyl-3-propylimidazolium, 1,3-Bis(3-cyanopropyl)imidazoliu, 1,3-Diethoxyimidazolium, 1-Butyl-1-methylpiperidinium, 1-Butyl-2,3-dimethylimidazolium, 1-Butyl-3-methylimidazolium, 1 -Butyl-4-methylpyridinium, 1-Butylpyridinium, 1-Decyl-3-methylimidazolium, 1-Ethyl-3-methylimidazolium, 3-Methyl-1-propylpyridinium, and combinations thereof. 
         [0027]    In the EDLC  100 , the anion  132  can include bis(trifluoromethanesulfonate)imide, tris(trifluoromethanesulfonate)methide, dicyanamide, tetrafluoroborate, hexafluorophosphate, trifluoromethanesulfonate, bis(pentafluoroethanesulfonate)imide, thiocyanate, trifluoro(trifluoromethyl)borate, and combinations thereof. 
         [0028]    The solvent can be Acetonitrile, Amides, Benzonitrile, Butyrolactone, Cyclic Ether, Dibutyl carbonate, Diethyl carbonate, Diethylether, Dimethoxyethane, Dimethyl carbonate, Dimethylformamide, Dimethylsulfone, Dioxane, Dioxolane, Ethyl formate, Ethylene carbonate, Ethylmethyl carbonate, Lactone, Linear Ether, Methyl formate, Methyl propionate, Methyltetrahydrofuran, Nitrile, Nitrobenzene, Nitromethane, N-methylpyrrolidone, Propylene carbonate, Sulfolane, Sulfone, Tetrahydrofuran, Tetramethylene sulfone, Thiophene, Ethylene glycol, Diethylene glycol, Triethylene glycol, Polyethylene glycols, Carbonic acid ester, y-Butyrolactone, Nitrile, Tricyanohexane, or any combination thereof. 
         [0029]    The separator  126  can be fabricated from non-woven glass. The separator  126  can also be fabricated from fiberglass, flouro-polymers, Telfon®, PTFE, and ceramics. For example, using non-woven glass, the separator  126  can include main fibers and binder fibers each having a fiber diameter smaller than that of each of the main fibers and allowing the main fibers to be bonded together. 
         [0030]    Once the EDLC  100  is fabricated, it may be used in high temperature applications with little or no leakage current. The EDLC  100  described herein can operate efficiently at temperatures from −20 C. to 300 C. with leakage currents normalized over the volume of the device less than 1 amp per liter (A/L) of volume of the device within the entire operating voltage and temperature range. One key to this performance at −20 C. to 300 C. with leakage currents less than 1 A/L is the assembly process itself, which produces a finished EDLC  100  having a moisture concentration in the electrolyte of less than 500 parts per million (ppm) over the weight and volume of the electrolyte and an amount of impurities less than 1000 ppm. 
         [0031]    More specifically, the activated carbon, carbon fibers, rayon, carbon cloth, and/or nanotubes making up the two electrolytes  112 ,  114 , is dried at elevated temperature in a vacuum environment. The separator  126  is dried at elevated temperature in a vacuum environment. The electrolyte  124  is dried at elevated temperature in a vacuum environment. Once the electrodes  112 ,  114 , the separator  126 , and electrolyte  124  are dried under vacuum, they are packaged without a final seal or cap in an atmosphere with less than 50 parts per million (ppm) of water. The uncapped EDLC  10  is dried under vacuum over a temperature range of 100 C. to 300 C. Once this final drying is complete, the EDLC  100  is sealed in an atmosphere with less than 50 ppm of moisture. 
         [0032]    In addition, impurities, such as halide ions (chloride, bromide, fluoride, iodide), free amines (ammonia), sulfate, metal cations (Ag, Al, Ba, Br, Ca, Cd, Co, Cr, Cu, Fe, K, Li, Mg, Mn, Mo, Na, Ni, Pb, Sr, Ti, Zn), and so forth, are kept to below 1000 ppm. Further, impurities from solvents and precursors used in the synthesis process are kept below 1000 ppm and can include, for example, bromoethane, chloroethane, 1-bromobutane, 1-chlorobutane, 1-methylimidazole, ethyl acetate, methylene chloride and so forth. 
         [0033]    By reducing the moisture content in the EDLC  100  to less than 500 part per million (ppm) over the weight and volume of the electrolyte and the impurities to less than 1,000 ppm, the EDLC  100  can efficiently operate over a temperature range of −20 C. to 300 C. with a leakage current less than 1 Amp per Liter within the temperature and voltage range. 
         [0034]    As shown in  FIG. 3 , a small signal circuit diagram  20 , which is equivalent to the circuit in  FIG. 1 , includes a battery current (i bat )  22 , a first resistor (R b )  24 , a capacitor (C)  26 , a second resistor (R c )  28 , a capacitor current (i c )  30  and a load current (i load )  32 . Analysis of the circuit diagram  20  leads to the following transfer functions between the small-signal load current (i load )  32 , the battery current (i bat )  22  and capacitor current (i c )  30 : 
         [0000]        i   b   /i   load =( R   c   sC+ 1)/( sC ( R   c   +R   b )+1) 
         [0000]        ic/i load= RbsC /( sC ( Rc+Rb )+1) 
         [0035]    Given a desired operation of the ESS  10 , the system parameters are constrained as follows: 
         [0000]        R   c /( R   b   +R   c )≦( I   b,max   −I   b,DC )/ I   load,max  
 
         [0000]      1 /CR   c ≦ω p  
 
         [0036]    wherein the DC component of the battery current I b, DC  equals a time-averaged load current i load  and ω p  is the frequency component of the load current. 
         [0037]    High-Rate Lithium BESS Example 
         [0038]    Using an Electrochem Industries high-rate cell MWD150 3B3900 DD-size: 
         [0039]    Temperature range: 0° C. to 150° C. 
         [0040]    Cell weight: 220 grams (g) 
         [0041]    Cell volume: (1/2)*(32.6 mm 2 )*π*127.5 m*1e−6=106 ml 
         [0042]    Open ckt. Voltage voc=3.67 V 
         [0043]    Capacity=24 Ah 
         [0044]    i max =500 mA 
         [0045]    To achieve VW=40 V, what is needed is 40/3.67=11 cells in parallel 
         [0046]    6*11=66 cells leading to a capacity of 66*24=1,584 Ah 
         [0047]    BESS weight=66*220 g=14.5 kg 
         [0048]    BESS volume=66*106 ml=7 liters (l) 
         [0049]    High Temperature EDLC and Low-Rate Lithium Battery Hess Example 
         [0050]    Temperature range: 0° C. to 150° C. 
         [0051]    Cell weight: 95 g 
         [0052]    Cell volume: (1/2)*(24.6 mm 2 )*π*101.6 m*1e−6=48 ml 
         [0053]    Open ckt. Voltage voc=3.67 V 
         [0054]    Capacity=12 Ah 
         [0055]    i max =500 mA 
         [0056]    ESR b =1.5Ω 
         [0057]    To achieve V w =40 V what is needed is 11 cells in series. To achieve capacity greater tan or equal to 350 Ah requires three such strings in parallel 
         [0058]    3*11=33 cells leading to I max =3*68 mA and 33*12 mAh=396 mAh capacity 
         [0059]    Battery weight: 33*95 g=3.1 kg 
         [0060]    Battery volume: 33*48 ml=1.6 liter 
         [0061]    In a D-size example: 
         [0062]    Temperature range: 0° C. to 330° C. 
         [0063]    Cell weight: 84 g 
         [0064]    Cell volume: 56 ml 
         [0065]    Open ckt. Voltage voc=2 V 
         [0066]    Capacity: 12 Ah 
         [0067]    i max =4.3 A 
         [0068]    ESR c =14 mΩ 
         [0069]    Capacitance: 141 F 
         [0070]    Leakage current: less than 1 amp per liter of volume over the range of operating temperatures and at a voltage a rated voltage 
         [0071]    To achieve Vw=40 V, 20 cells in series are needed. To check the design constrains it is given: 
         [0000]        R   b =11/3 *ESR   b =5Ω
 
         [0000]        R   c =20*14 mΩ=280 mΩ
 
         [0000]        I   b,max =3*68 mA=204 mA 
         [0000]        C= 1/20*141 F=7 F 
         [0000]        i   load,max =3A 
         [0000]        i   b,DC =0.01*3 A=30 mA 
         [0000]      ( i   b,max   −i   b,DC )/ l   load,max =0.058 and ( R   C   +R   B )=0.053 satisfy  R   c /( R   b   +R   c )≦( I   b,max   −I   b,DC )/ I   load,max .
 
         [0000]      Ω p =1.24 rps and 1 /CR   c =0.45 satisfies 1 /CR   c ≦ω p  
 
         [0072]    The capacitor string discharges the battery at a rate of 3 mA/396 mAh=0.7%/hour. 
         [0073]    EDLC weight=20*84 g=1.68 kg 
         [0074]    EDLC volume=20*56 ml=1.12 liters 
         [0075]    HESS weight=3.1 kg+1.68 kg=4.8 kg 
       CONCLUSION 
       [0076]    The HESS  10  effectively buffers 3 A pulses every 5 seconds without discharging the battery more than 1% per hour. Further, the HESS  10  achieves lower weight and volume compared to the BESS while incorporating a low-rate rather than a high-rate primary. 
         [0077]    It is to be understood that the foregoing description is intended to illustrate and not to limit the scope of the invention, which is defined by the scope of the appended claims. Other embodiments are within the scope of the following claims.