Abstract:
A method and an arrangement for discharging an energy storage system for electrical energy, particularly in a vehicle having a hybrid drive line, by means of a first discharge resistor, wherein a coolant such as carbon dioxide gas is provided to the first discharge resistor during discharge of the energy storage system for leading off heat as well as a hybrid vehicle comprising such an arrangement.

Description:
BACKGROUND AND SUMMARY 
       [0001]    The present invention relates to a method and an arrangement for discharging an energy storage system for electrical energy, particularly in a vehicle having a hybrid drive line (hereinafter referred to as “hybrid vehicle”), by means of a discharge resistor as well as to a hybrid vehicle comprising such an arrangement. 
         [0002]    In the following the invention is described in connection with hybrid vehicles. However, the invention is not limited to this application. For instance, it can be also used in stationary power generation systems comprising such an energy storage system. The invention is also applicable to stationary and/or movable construction equipments such as construction vehicles or off road machines. 
         [0003]    Hybrid vehicles in general comprise at least two drive engines, wherein a traditional combustion engine is most frequently supported by an additional electrical drive unit. The electric energy for running the electric drive unit is commonly provided by an energy storage system like high voltage capacitors or batteries, wherein the electric energy stored therein is produced by a fuel cell or a generator. 
         [0004]    In case such a hybrid vehicle is involved in an accident there is a potential hazard to a rescue team or other assisting persons or to the driver and the passengers themselves due to the high voltage of the energy stored in the energy storage system. Therefore, the energy storage system has to be discharged in such situations as fast as possible. 
         [0005]    In general, discharging a high voltage energy storage system is preferably performed by a discharge resistor. The discharge resistor limits the discharging current and prevents the energy storage system from exploding or behaving in an uncontrolled dangerous way during the discharge process or thereafter. The resistance of the discharge resistor is chosen to be large enough to keep control of the discharging process and to discharge the energy storage system without damaging it. 
         [0006]    Another discharge process is disclosed in the Japanese patent application JP 2004129367 wherein two discharge methods (discharge by means of an usual discharge resistor and discharge by corona discharge) are performed in parallel. The discharge process itself is initialized in case of an accident. 
         [0007]    Disadvantageously, the use of a large resistance for the discharge resistor or the use of the two discharge methods described above (even if they are performed in parallel) means that the discharge process takes several minutes before it is completed. During that time no safe rescue operation can be performed which might even cost a person&#39;s life. 
         [0008]    It is desirable to provide a method and an arrangement for discharging an energy storage system for electrical energy that reduces the time needed for discharging the energy storage system. It is also desirable to provide a method and an arrangement that reduces the risk exposure of the environment for hazardous side effects caused by the discharge process. 
         [0009]    A method and an arrangement for discharging an energy storage system for electrical energy (hereinafter generally referred to as “energy storage system”), as well as by a hybrid vehicle comprising such an arrangement are disclosed. 
         [0010]    The invention is based, according to an aspect thereof, on the conclusion that one of the major limiting factors for using a resistor with a rather low resistance for discharging an energy storage system is the heat generated during the discharge process. Therefore, according to an aspect of the invention a coolant is provided at the resistor during the discharge process for leading off the heat produced during the discharge process. Advantageously, the discharge resistor can be made of an alloy having a positive temperature coefficient, such as canthal, constantan or tungsten. 
         [0011]    In addition to the fact that its price is rather low, the use of carbon dioxide gas (CO2) as coolant has the advantage that carbon dioxide gas also serves as fire extinguisher. In case the resistor and/or the energy storage system starts burning because of the heat developed by the fast discharge, the coolant also could stop, or prevent, the fire. Therefore, in a preferred embodiment of the invention the coolant is also provided at the energy storage system itself. 
         [0012]    Further, in a preferred embodiment, the coolant is stored under pressure in a pressurized storage unit. Releasing the coolant results in a sudden drop of temperature of the storage unit. The sudden drop in temperature can also be used for further cooling down the energy storage system and/or the discharge resistor. This effect can be maximized by integrating the storage unit into the energy storage system and/or the discharge resistor. Integrating the storage unit has the further advantage that the arrangement size can be reduced. 
         [0013]    Another advantageous embodiment uses at least one energy consumer being in connection with the energy storage system for discharging the energy storage system and/or to support the discharge process. In case a vehicle energy storage system needs to be discharged, the use of an electrical engine for discharging the energy storage system is preferred. 
         [0014]    In another preferred embodiment, the initializing of the discharge process is triggered by a trigger signal. The trigger signal can be transmitted automatically for example by the vehicle and/or manually for example by a remote control operated by a rescue team. 
         [0015]    Preferably, the trigger signal is transmitted by an accident sensing sensor or such system like an anti-collision detection system. The accident sensing sensor, or system, can be a sensor for realizing an accident the moment it happens, correlating the initialization of the discharge process for example to an airbag deployment signal, or an accident prediction sensor calculating the probability of an accident and transmitting the trigger signal in case the probability exceeds a certain threshold. Both solutions have the main advantage that the discharge is already in process when the rescue team arrives or is, in case of the accident prediction sensor, or system, already completed or almost completed. 
         [0016]    It is further advantageous to provide a signal possibility signaling the status of the discharge process, i.e. that the discharge of the energy storage system is completed, or is still in process and/or still needs to be initiated and performed. 
         [0017]    Further advantages and preferred embodiments are disclosed in the figures and the description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    In the following the invention will be further described by means of the Figures. The embodiments shown are merely exemplary and are not intended to limit the claims thereto. 
           [0019]    The Figures show: 
           [0020]      FIG. 1 : a schematic view of a first preferred embodiment of an arrangement for discharging an energy storage system according to the invention. 
           [0021]      FIG. 2 : a schematic view of a second preferred embodiment of an arrangement for discharging an energy storage system according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]      FIG. 1  shows schematically a first preferred embodiment of the inventive arrangement. An energy storage system  2  stores electric energy produced by a fuel cell or a generator (not shown) and is designed for high voltages. Such a high voltage energy storage system stores energy in the range of typically several hundreds volts in contrast to a “normal” vehicle battery providing electric energy at voltage levels of 12 V or 24 V. The energy storage system  2  can comprise a plurality of capacitors or batteries connected in series having a minus pole  4  and a plus pole  6 .  FIG. 1  shows only a single capacitor or battery for the sake of simplicity. 
         [0023]    In case of an accident, the energy storage system  2  is a potential hazard to a rescue team or other assisting persons or to the passengers of the vehicle due to the high voltage of the system. In such situations it is therefore necessary to discharge the energy storage system  2  as fast as possible to zero or to a safe level. 
         [0024]    For discharging the energy storage system  2 , minus pole  4  and plus pole  6  of the energy storage  2  are connected to a first electrical circuit  8  having a first switch  10 . As soon as the first switch  10  is closed the discharge process is started by closing the electrical circuit  8 . For controlling the discharge and preventing the energy storage system  2  from being damaged, the first electrical circuit  8  comprises a first discharge resistor  12  limiting the discharge current I. 
         [0025]    In a further embodiment, an already existing energy consumer can be used as discharge resistance or can be used to support the discharge process by consuming as much energy as possible. For example, in case the energy storage system of a vehicle should be discharged as fast as possible, it can be advantageous to use the electrical engine of the vehicle to consume as much energy as possible. Since this discharge process might take too long time, the energy consumer can also be used to only support the discharge process. The less energy is stored, the faster the discharge process will be. 
         [0026]    The first discharge resistor  12  is for instance a wire resistor having a relatively low resistance and a positive temperature coefficient. In this case “low” is referring to the highest possible discharge current without anything dangerous will happen to the energy storage system  2 , for instance exploding batteries. That also means that if the resistance is “too low” there might be the possibility of unwanted situations caused by the discharge itself like explosion of the battery, burning of the capacitor or emission of toxic gases. Therefore, the discharge current has to be adapted to the battery and capacitor, respectively, used in the system. If the resistance is “too high” the discharge process is too slow. It can be therefore preferable to use a resistor  12  which resistance is heat dependent. That means in case the resistor  12  is cold it provides a relatively low resistance but if the temperature increases the resistance also increases. 
         [0027]    Such resistors are made, for example, from canthal-, constantan- or tungsten-wires. Due to the relatively low resistance the energy storage system  2  can be discharged quite fast, preferably within seconds instead of minutes as with resistors known from the state of the art. Unfortunately, the relatively fast discharge process also produces a lot of heat even if a resistor having a positive temperature coefficient is used. The discharge heat involves the risk of burning and therefore the imminence of fire or/and explosion of the whole system. 
         [0028]    To reduce the risk of burning but to allow a very fast discharge, the inventive arrangement comprises a coolant  14 , as for example carbon dioxide gas (CO2), which is stored in a coolant storage unit  16  such as a tank, preferably under pressure. Releasing the coolant, results in a sudden drop of temperature of the storage unit. The sudden drop in temperature can also be used for further cooling down the energy storage system and/or the discharge resistor. 
         [0029]    Since the storage itself provides a cooling source, when having released the coolant, the storage unit  16  can also be an integral part of the energy storage system  2  and/or of the discharge resistor  12 . This has the further advantage that no coolant loss due to long transport paths can occur. But it is also possible to only incorporate the coolant storage unit into the energy storage system and directing the coolant jet to the discharge resistor, whereby the drop of temperature effect is used to cool down the energy storage system, and the coolant is used to cool the discharge resistor; or vice versa. 
         [0030]    The use of carbon dioxide gas has the additional advantage that in case the system catches fire anyhow, carbon dioxide gas also serves as fire extinguisher. In principle, any electrically non-conducting gas or fluid can be used which is suitable to serve as coolant and which is flame resistant, too, as for instance an inert gas. 
         [0031]    In case the discharge process is started by closing the first switch  10  the coolant  14  is led to the first discharge resistor  12 . This is indicated by arrow  18  in  FIG. 1 . For an optimal cooling of the first discharge resistor  12 , the first discharge resistor  12  comprises a plurality of cooling holes  20  which enable the coolant (or simply air) to cool the resistor wires of the first discharge resistor  12 . 
         [0032]    The coolant  14  can be led to the first discharge resistor  12  by providing means such as guidance channels (not shown). In case the coolant  14 , as for example CO2, is stored under pressure and located in the vicinity of the first discharge resistor  12  such extra providing means are not necessary since opening the pressure tank  16  causes a coolant jet out of the opening of the tank  16  which can be directed towards the first discharge resistor  12 . 
         [0033]    It is further preferable to lead the coolant  14  to the energy storage system  2  as well. Since the fast discharge process has a heat-related impact onto the energy storage system  2  itself, a cooling of the energy storage system  2  would allow an even more faster discharge process. Providing of the coolant  14  to the energy storage system  2  is indicated by arrow  22 . For that, the coolant  14  can be led through the first discharge resistor  12  and after that to the energy storage system  2 , but it is also possible to split the coolant flow and lead one portion to the first discharge resistor  12  and the other portion to the energy storage system  2 . The portions can be of the same size, but it is also possible to provide more coolant at the first discharge resistor  12  and only a small portion of the coolant  14  at the energy storage system  2 , or vice versa. 
         [0034]    It is also possible to place the first discharge resistor  12  in the vicinity of the energy storage system  2  or to integrate the first discharge resistor  12  or part of it into the energy storage system  2 . This simplifies the providing of the coolant  14  to both the first discharge resistor  12  and the energy storage system  2 . 
         [0035]    After cooling the first discharges resistor  12  and/or the energy storage system  2 , the coolant  14  is led to the environment. This is indicated by arrow  24 . 
         [0036]    The first switch  10  is operated by a trigger signal  26  originating from a trigger signal generating source  28  which initializes the discharge process. The trigger signal causes a closure of the first switch  10  and an opening of the coolant storage  16  for providing the coolant  14  to the first discharge resistor  12  and/or to the energy storage system  2  during the discharge process. 
         [0037]    The trigger signal  26  can be transmitted manually and/or automatically. In case the signal is transmitted manually, a person, for example from a rescue team or a passenger, can initiate the discharge process by pressing a button located at the vehicle or operating a remote control. Operating a remote control has the advantage that a direct contact with the vehicle can be avoided. It is also possible that the vehicle itself transmits a signal to a remote control signaling that a discharge is necessary. This might be preferable in case the automatic initiation fails or a further control of the initiation of the discharge process is desired. 
         [0038]    In another preferred embodiment the transmission of the trigger signal is performed automatically. For example, the trigger signal can be transmitted by an accident sensing system or sensor. The accident sensing system or sensor senses whether an accident has happened and then transmits the trigger signal. Since the same principle is applied to the deployment of airbags, the trigger signal can also be correlated to an airbag deployment signal. 
         [0039]    The accident sensing system or sensor can also be part of an accident prediction system. An accident prediction system calculates the probability of an accident and is enabled to transmit the trigger signal in case the calculated accident probability exceeds a certain predefined threshold. Preferably the trigger signal is transmitted a predetermined time before the accident happens so that at the moment of the actual accident the energy storage system is already (almost) completely discharged or at least to a large extent discharged. In order to achieve a discharge of the energy storage system to a (predefined) “wanted” or “safe” level of energy left in the energy storage system the predetermined time period can be correlated to the time necessary to discharge the energy storage system to said “safe” level or to zero. The main advantage of the initiation of the discharge process already before the actual accident happens is that immediately after the happening of the accident a rescue team or other persons can get to the vehicle and provide help without running a risk to be negatively impacted by any hazardous effects caused by energy stored in the energy storage system  2 . 
         [0040]    In other preferred embodiments, an information signal and/or a “safe”-signal can be transmitted in addition to the trigger signal. The information signal—that signals that a discharge process is in process or still needs to be performed—can be transmitted for example to a rescue service center, an accident notification center, an accident recorder or to the outside of vehicle in general. The information signal can also be in form of an acoustic or optical warning signal warning persons not to get near the vehicle in case the discharge process has failed or is still ongoing or still needs to be performed. 
         [0041]    The “safe” signal signals that the discharge process has been completed or the energy storage system is not charged to a hazardous level so that any person coming to an accident scene can be sure that despite the existence of the energy storage system  2  in the vehicle it is not perilous anymore to approach the vehicle and to provide help. 
         [0042]    It is also possible to combine the manual and automatic transmission of the trigger signal so that in case the automatic transmission fails due to damage of the vehicle, the discharge can be initiated anyway. 
         [0043]    In addition to the first discharge resistor  12  and the first electrical circuit  8 , a second (independent) electrical circuit comprising a second discharge resistor having a resistance higher than the resistance of the first discharge resistor  12  can be provided for a controlled discharge process which is slower in time than the discharge process controlled by the first discharge resistor  12 . Instead of using two different discharge resistors comprised in two different electrical circuits, it is also possible to use a single discharge resistor with an adjustable resistance. 
         [0044]    A corresponding second embodiment of the inventive arrangement is shown in  FIG. 2 .  FIG. 2  shows the same components as  FIG. 1 , but additionally illustrates a second electrical circuit  30  comprising a second discharge resistor  32  and a second switch  34 . In principle, it is also possible to use a single  3 -position switch (instead of the two switches  10  and  34  in  FIG. 2 ) which either closes the first electrical circuit  8  or the second electrical circuit  30 . 
         [0045]    The second switch  34  in  FIG. 2  is also operated by a trigger signal  26  generated by a trigger signal generating source. As shown in  FIG. 2 , the trigger signal generating source can be the same as the trigger signal generating source  28  transmitting the trigger signal  26  to the first switch  10 . 
         [0046]    In contrast to the discharge process performed by the first electrical circuit  8  with the first discharge resistor  12 , the discharge process performed by the second discharge circuit  30  and the second discharge resistor  32  is much slower due to the higher resistance of the second discharge resistor  32 . The discharge process using the second discharge resistor  32  is therefore not performed in case of emergency but in all other “non-emergency” cases where a discharge of the energy storage system  2  is needed as well like maintenance, repair or simply parking. The trigger signal  26  is therefore in this case not correlated or associated with an accident related system like the accident sensing sensor described above. 
         [0047]    The trigger signal  26  for the second switch  34  can be transmitted manually by a driver or a repair person by pressing a corresponding button located at the vehicle or by operating a remote control. In principal it is also possible to initiate the trigger signal  26  automatically. This can be done for example by correlating the transmission of the trigger signal  26  to a GPS signal signaling the position of a garage (or any other maintenance place), or by correlating the transmission of the trigger signal  26  with an operation of a central locking system of the vehicle. 
         [0048]    The “slower” discharge process enabled by closing the second electrical circuit  30  has the advantage that it reduces the risk exposure for the environment for hazardous side effects caused by the discharge process even further. 
         [0049]    Preferably, an already existing energy consumer is used as second discharge resistor  32 , particularly the engine of a vehicle. The discharge by an energy consumer is a slower and softer process, ensuring that the energy storage system will not be damaged. 
       Reference Signs 
       [0000]    
       
           2  energy storage system 
           4  minus pole 
           6  plus pole 
           8  first electrical circuit 
           10  first switch 
           12  first discharge resistor 
           14  coolant 
           16  coolant storage 
           18  coolant flow to discharge resistor 
           20  cooling holes 
           22  coolant flow to energy storage system 
           24  coolant flow to environment 
           26  trigger signal 
           28  trigger signal generating source 
           30  second electrical circuit 
           32  second discharge resistor 
           34  second switch