ARC FAULT SUPPRESSION IN ENERGY STORAGE SYSTEMS Technical Field The present disclosure relates to an energy storage system for power grid applications. More particularly, the present disclosure relates to an energy storage module/rack for an energy storage system, an energy storage system comprising said energy storage module, and a power grid system comprising said energy storage system. Background In recent times, there has been a desire to transition power grids towards using renewable sources of electricity such as solar panels, wind farms, and the like. However, such sources may be unreliable in that they do not provide a constant amount of electrical energy from hour to hour, or from day to day. Thus, sometimes, power generation may exceed demand, while at other times, power generation may not be enough to satisfy demand. Therefore, in achieving a more sustainable power grid, there is a desire to storage electrical energy generated by renewable sources during times of excess generation such that the stored electrical energy may then be released and distributed among the power grid during times of excess demand. Energy storage systems are used in electrical power grids for storing electrical energy through different kind of storage systems depending on the purpose of the storage. An energy storage system may be defined by the quantity of energy it is capable of storing or by the power it is able to output. For example, modern capacitor-based storage systems may store a large amount of energy and be able to provide a high power output quickly. This makes such capacitor-based systems suitable for sustainable power grid applications. However, when storing large amounts of electrical energy, there is a risk of failures of faults, which may cause severe damage to the energy storage system. There is a desire to reduce the risk of fault-induced damage. Summary It is realized as a part of the present disclosure that one of the main causes of damage to energy storage systems, ESSs, is an arc fault. The high energy results in a fault with a high current. Electrical arcs carrying a high current may not only damage the capacitors through the arc but also through the heat caused by the arc that may cause explosions and plasma. Hence, the arc fault is one of the most dangerous faults for the ESS. Therefore, the arc fault is preferably suppressed as quickly as possible to reduce the potential damage. The arc fault may occur in the capacitor when one of the capacitors are exposed for a short circuit, between two parts with different voltage potential. Therefore, it is further realized as a part of the present disclosure that there is a desire to suppress such arc faults quickly and in a manner that renders the system safe such that faulty components can be removed and replaced. It is an object of the present disclosure to mitigate, alleviate or eliminate one or more of the above-identified deficiencies in the art and disadvantages. This object is at least partly met by the present disclosure, the full scope of which is defined in the independent claims. Preferred embodiments of the present disclosure are set out in the dependent claims. According to a first aspect of the present disclosure, there is provided an energy storage module for an energy storage system, the energy storage module comprising at least one capacitor for storing electrical energy and a diode arranged anti-parallel to the capacitor. The diode is configured to impede current flow through the diode when the energy storage module is in a normal operation, the normal operation being where the energy storage module stores electrical energy and provides stored electrical energy to a power grid, and the diode is further configured to allow current flow through the diode during an arc fault in the capacitor. Put another way, the normal operation of the energy storage module is where the energy storage module is charging or discharging. Hence, when charging, the energy storage module stores energy (e.g., excess generated energy) from the power grid and, when discharging, the energy storage module provides energy to the power grid, e.g., when the power grid has a deficit of energy. During such a normal operation, the diode may operate in a first mode where it impedes current flow through the diode. In the case of an arc fault in the capacitor, the diode may operate in a second mode where the diode allows current flow through the diode. The diode may enter the second mode when there is an event that causes a change of polarity in the energy storage module. That is, for example, an event that causes a voltage across the diode to change from positive to negative. One example of such event may be an arc fault. The diode is arranged in anti-parallel with the capacitor, meaning that the diode is connected with its polarities reversed to the polarities of the capacitor. By connecting the diode anti-parallel to the capacitor in this way, the diode can block a reverse voltage through the capacitor. Therefore, the capacitor can be prevented from an accelerated aging. The diode blocks reverse voltage both during charging and discharging of the capacitor. A reverse voltage through the capacitor may in addition to damaging it also cause a short circuit in the capacitor. A short circuit may cause an arc fault. The anti-parallel diode decreases the risk that an arc fault occurs. Should an arc fault nevertheless arise in the capacitor, the anti- parallel diode may then allow current flow from said capacitor, through the diode. When the current of the arc fault commutates to the diode, the arc fault is suppressed. Thus, the arcing time of the arc fault may advantageously be reduced. By reducing the arcing time, the arc fault will not have as long time to, for example, increase the temperature within and around the energy storage module, and the risk of plasma as well as an explosion will be advantageously decreased. Hence, the diode not only decreases the risk of an arc fault but also reduces the damage to the capacitor if an arc fault would occur. The diode may be arranged in a fault branch (which may also be referred to as a ‘fault circuit’). The fault branch may have a positive connection point connected to a positive terminal of the capacitor and a negative connection point connected to a negative terminal of the capacitor. The fault branch may therefore be how the diode is connected to the capacitor. That is, to connect the diode in anti-parallel with the capacitor, the diode may be connected with its negative terminal to the positive connection point of the fault branch, and with the positive terminal of the diode to the negative connection point of the fault branch. The diode may be configured to allow current flow through the fault branch when there is a change in polarity of the potential difference between the positive connection point and the negative connection point. That is, the change in polarity may occur when the voltage at one of the connection points is changed such that it passes the level of the voltage at the other connection point. For example, if the negative connection point surpasses the positive connection point in voltage, there is a change of polarity as the voltage difference between these points changes from negative to positive. A short circuit in the capacitor (e.g., leading to an arc fault) may cause such a change of polarity. The energy storage module may further comprise a bypass branch connected in parallel to the at least one capacitor and configured to bypass the capacitor. The bypass branch may therefore be used for redirecting the current flow in the energy storage system such that the current does not pass through the capacitor of the energy storage module if said capacitor is experiencing a fault. If the capacitor is broken in this way, it is advantageous to be able to bypass it such that further faults may not be caused due to the broken capacitor, and such that the rest of the energy storage system may continue to carry out normal operations. The bypass branch may be used to partially or preferably fully discharge the capacitor to thereby render the energy storage safe for repair or replacement. When the capacitor is fully (or partially or mostly) discharged, the current may then flow past either the diode or the bypass branch depending on the current directions. The capacitor will be fully bypassed. The at least one capacitor may be a supercapacitor. The supercapacitor is suitable to be used in energy storage systems. The higher capacitance value of the supercapacitor makes it possible to store higher amounts of energy. By storing higher amounts of energy, the energy storage system may be suitable for a wider range of different applications such as power grid applications. The diode may be a press-packed diode. The press-packed diode advantageously enters a short circuit failure mode when excessive current flows through. In the short circuit failure mode, the press-packed diode still provides a reliable path for either short circuit or operational current to flow through. The bypass branch may further comprise a thyristor. The thyristor can be used for controlling when the current can flow through the bypass branch. That is, the thyristor may provide a switch operation that can be operated for a long period with a minimum error possibility. Thyristors do not contain moving parts and, therefore, a thyristor is able to quickly switch current paths through the energy storage module. The lack of moving parts also results in that the bypass branch may break the current without a physical gap which reduces the risk of arcs, which further reduces the risk for damage of the energy storage module. The bypass branch may further comprise a resistor configured to limit a current through the bypass branch. High currents may damage components in the bypass circuit, e.g., thyristors and/or switches. By having the resistor in the bypass branch, the high current passing the bypass branch can be limited. The size of the resistor may be chosen depending on the expected fault current. The resistance of the resistor may be of the same size as the equivalent series resistance, ESR, of the energy storage module. In other embodiments the resistor may be higher than the ESR of the energy storage module. By reducing the fault current the resistor may prolong the life span of the other components in the energy storage module. The energy storage module may further comprise a switch connected in parallel to the capacitor and configured to bypass the capacitor and/or the bypass branch. The switch may be a mechanical switch or a power electronic switch. The switch may provide a further means for redirecting current through the energy storage module in the event of a fault, for example if the bypass branch is destroyed or damaged. By activating the switch, the current is provided with an alternative path for the current to flow. This may be used as a further bypass path that provides the possibility to bypass a damaged capacitor. The switch may also provide a method for the personnel to alternate the path of the energy. Moreover, if a resistor is included in a bypass branch, it may not be preferred to keep the resistor in the current path of normal operations as it may affect current flow through the energy storage module and through the wider energy storage system during charging or discharging. The diode may have a current integral rating of at least 300 kiloampere ² second, kA ² s. The current integral rating is a value of the energy or heat that may be passed through the diode without damage to the diode. The current integral rating of the diode may preferably be adapted for the energy storage system that the diode operates in. With a current integral rating of at least 300 kA ² s, for example, the diode may be adapted to be suitable for use in an energy storage system in a power grid system. The at least one capacitor may comprise a plurality of capacitors connected in parallel with each other. With a plurality of capacitors that are connected in parallel, the storage capacity of the energy storage module may be further increased while retaining a level of modularity such that a failure of one of the capacitors does not render the energy storage module entirely non-operational. The number of capacitors that are connected in parallel may vary accordingly with the needs of the energy storage system, for example. According to a second aspect of the present disclosure, there is provided an energy storage system comprising at least one energy storage module substantially as described above. The energy storage module may be used in an energy storage system and controlled by the energy storage system, e.g., by a controller thereof. With such a modular energy storage system, the number and configuration of energy storage modules may be advantageously adapted according to the needs of the application in which it is used. There may be a bypass branch for each energy storage module or for two or more of the energy storage modules. The energy storage system may comprise a plurality of energy storage modules and the energy storage system may further comprise a rack bypass branch connected in parallel to said plurality of energy storage modules and configured to bypass said plurality of energy storage modules. The plurality of energy storage modules may be connected in series. A plurality of energy storage modules connected together in series may be referred to as a ‘rack’. By connecting a plurality of energy storage modules together in the energy storage system, the energy storage capacity is further increased. By each energy storage module having a plurality of capacitors connected in parallel and each energy storage system having a plurality of said energy storage modules, the flexibility and modularity of the energy storage system can be further adapted to the application where it is used. By providing a rack bypass branch that is connected in parallel to the plurality of energy storage modules the full energy storage system or a full rack may be bypassed, e.g., when it is damaged. This may advantageously protect the energy storage system from further damage. This may be preferable in a building where a plurality of energy storage systems are connected together, for example. With the rack bypass branch and the bypass branch of the module, a flexibility is provided where a part of the energy storage module or the full energy storage system may be disconnected, e.g., from a wider power grid system. This may provide an increased resilience of the energy storage system. According to a third aspect of the present disclosure, there is provided a power grid system comprising the energy storage system substantially as described above. Effects and features of the second and third aspects may be largely analogous to those described above in connection with the first aspect. Embodiments mentioned in relation to the first aspect may be at least largely compatible with the second aspect and third aspects. It is further noted that the present disclosure relates to all possible combinations of features unless explicitly stated otherwise. A further scope of applicability of the present disclosure will become apparent from the detailed description given below. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the disclosure, are given by way of illustration only, since various changes and modifications within the scope of the disclosure will become apparent to those skilled in the art from this detailed description. Brief Description of the Drawings The present disclosure will by way of example be described in more detail with reference to the appended drawings, which shows presently preferred embodiments of the disclosure. Fig.1 illustrates a circuit of an energy storage system. Fig.2 illustrates a circuit of an energy storage module. Fig.3 illustrates a circuit of the energy storage module of Fig.2 with a short circuit fault. Fig.4 illustrates a circuit of the energy storage module with a bypass branch. Fig.5 illustrates a circuit of the energy storage module with a bypass branch, a resistor and a switch. Fig.6 illustrates a circuit of the energy storage module with two capacitors. Fig.7 illustrates a circuit of the energy storage system. Detailed Description The present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments of the disclosure are shown. This disclosure may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the disclosure to the skilled person. Fig.1 shows an example embodiment of an energy storage system 100. Energy storage systems such as that illustrated are used for storing energy when there is excess of power on a power grid. The stored energy may later be distributed back to the power grid at moments where there is lack of power in the power grid system. For example, a lack of sunlight in a solar powered power grid or a lack of wind in a wind powered power grid may cause a lack of electrical power in the power grid system. The energy storage system 100 comprises a plurality of energy storage modules 1. The energy storage system 100 also controls the energy storage modules 1, e.g., deciding when they should charge or discharge. In Fig.1 an energy storage system 100 with a plurality of energy storage modules 1 is illustrated. The number of energy storage modules 1 may depend on the application where the energy storage system 100 is used. A power grid system is an example of such application. The energy storage modules 1 are connected both in series and parallel to each other, as illustrated. The number of energy storage modules 1 in series can be adapted to the voltage requirement of the application. Each series of energy storage modules 1 may be referred to as a rack, and each rack is connected in parallel to each other, and the number of racks may be decided depending on the storage capacity requirement of the energy storage system 100. In Fig.1 an example of an energy storage module 1 is shown in an enlarged view. The enlarged view illustrates an energy storage module 1 where a series of sets of two capacitors 10 are connected in parallel and a diode 30 connected is connected in anti-parallel to each set of two capacitors 10. In each module, and as with the connection of modules 1, there may be capacitors connected in series as well as in parallel. The capacitors 10 are used to store electrical energy. The capacitor 10 may be a supercapacitor, i.e., a type of high-capacity capacitor 10. The higher capacitance value of the supercapacitor makes it possible to store higher amounts of energy. By storing higher amounts of energy, the energy storage system may be suitable for a wider range of different applications, e.g., in power grid systems. Each of, or some of, the energy storage modules shown in Fig.1 may be provided with a bypass branch 50 that is connected in parallel with the capacitors 10 and the diodes 30. In some embodiments, the energy storage system 100 may further comprise a rack bypass branch connected in parallel to the plurality of energy storage modules 1 and is configured to bypass said plurality of energy storage modules 1. The diode 30 in anti-parallel, the bypass branch and the rack bypass branch can be used for protection of the capacitors 10. Each of the features will be further described when they are introduced in Fig.2-7. Turning to Fig.2, an energy storage module 1 for an energy storage system 100 is provided. The energy storage module 1 comprises a capacitor 10 for storing electrical energy and a diode 30 arranged in anti-parallel to the capacitor 10. The diode 30 is arranged in a fault branch 20, and wherein the fault branch 20 has a positive connection point 22 connected to a positive terminal 12 of the capacitor 10 and a negative connection point 24 connected to a negative terminal 14 of the capacitor 10. It will be appreciated that the labels ‘positive’ and ‘negative’ referring to the connection points are not intended to functionally limit these connection points but merely refer to the polarity of the terminal of the capacitor to which the connection point may be connected. That the diode 30 is arranged anti-parallel with the capacitor 10 means that the diode 30 is connected with its polarities reversed to the capacitor 10. In other words, to connect the diode 30 in anti-parallel. The diode 30 is connected with its negative terminal 34 to the positive connection point 22 of the fault branch 20 and with the positive terminal 32 of the diode 30 to the negative connection point 24 of the fault branch 20. When the diode 30 is connected in anti-parallel, it is configured to impede current flow through the diode 30 when the energy storage module 1 is in a normal operation, where the energy storage module 1 is configured to store electrical energy and provide stored electrical energy to a power grid. The diode 30 is further configured to allow current flow through the diode 30 during an arc fault 40 in the capacitor 10. The normal operation is where the energy storage module 1 is charging or discharging. Hence, when charging, the energy storage module 1 stores energy (e.g., excess generated energy) from the power grid and, when discharging, the energy storage module provides energy to the power grid, e.g., when the power grid has a deficit of energy. The diode 30, in an energy storage system 100 for a power grid system, has preferably a current integral rating of at least 300 kiloampere ² second, kA ² s. The current integral is a value of the amount of energy or heat that may pass through the diode 30 without risking that the diode 30 melts or get damaged. The current integral rating of the diode 30 may be adapted for the energy storage system 100 that the diode 30 operates in. With a current integral rating of at least 300 kA ² s the diode 30 may be adapted to work in the energy storage system 100 in a power grid system 200. One type of diode 30 that is suitable for a power grid system 200 is a press-packed diode. The press-packed diode enters into a short circuit failure mode when excessive current/energy passes therethrough, in which mode it still provides a reliable path for either short circuit or operational current to flow through. Turning to Fig.3, the energy storage module 1 from Fig.2 is illustrated with an arc fault 40. The arc fault 40 may occur in the capacitor 10 when it is exposed to a short circuit, between two parts with different voltage potential. The high energy of the energy storage module 1 may then result in a fault with a high current. Electrical arcs carrying a high current may not only damage the capacitors 10 through the arc but also through the heat caused by the arc that may cause explosions and plasma. Therefore, it may be preferred that the arc fault 40 is suppressed as quickly as possible to reduce the potential damage. The arc fault 40 generates an increased voltage on the side of the arc fault 40. This causes a change in polarity of the potential difference between the positive connection point 22 and the negative connection point 24 of the fault branch 20. The change of polarity in the fault branch 20 results in a reversal of the direction of potential difference, and therefore current flow. In this state, the diode 30 will allow current to flow through the fault branch 20. When the current of the arc fault 40 is allowed to commutate through the diode 30, the arc fault 40 will be suppressed rapidly. Therefore, the damage caused by the arc fault 40 on the energy storage module 1 can be significantly reduced. Turning to Fig.4 and 5, optional embodiments of the energy storage module 1 with a bypass branch 50 are illustrated. The bypass branch 50 is connected in parallel to the capacitor 10. In cases where the capacitor 10 is broken or damaged, it is advantageous to be able to bypass it such that further faults may not be caused due to the damaged capacitor 10. Thus, the bypass branch 50 may be used for redirecting the current such that the current does not pass through the capacitor 10. By allowing a discharge current to flow through the bypass branch 50, the capacitor 10 may be fully discharged. When the capacitor 10 is fully discharged, the current will flow through either the diode 30 or the bypass branch 50 depending on the type of bypass branch 50 and if the energy storage system 100 is charging or discharging. Either way, the damaged capacitor 10 of the energy storage module 1 can be fully bypassed. By bypassing the capacitor 10 the rest of the energy storage module 1 and the energy storage system 100 may operate as usual. If a module 1 comprises a plurality of capacitors (e.g., connected in series and/or in parallel to each other), then the bypass branch 50 may be provided in parallel thereto, so as to bypass all of the capacitors 10 in the module 1. The bypass branch 50 may comprise different components, e.g., that may be used for controlling the bypass branch 50. In Fig.5 an embodiment where the energy storage module 1 comprises a thyristor 52, a resistor 54 and a switch 60 is illustrated. The energy storage module 1 may comprise of any combination of the components and also other components that may provide the bypassing function. For the sake of simplicity, not all combinations are disclosed in the figures, although it will be appreciated by a person skilled in the art that components in Fig.5 may be added or removed or provided in different combinations. The thyristor 52 may be used for controlling when the current can flow through the bypass branch 50. The thyristor 52 may be triggered by a voltage, a gate signal or any other method for triggering the thyristor 52. When the thyristor 52 is triggered, the current will be allowed to flow through it. The thyristor 52 is a preferred bypassing unit due to its lack of moving parts which reduces the risk of arcs and the ability of the thyristor 52 to quickly switch the path of the current. When there is a short circuit fault, an arc fault 40 may occur at the same time, and a high peak current may arise in the capacitor 10. To limit the peak current from the short circuit fault, or other types of peak currents, a resistor 54 may be connected into the bypass branch 50. The size of the resistor 54 can be selected according to the size of the current expected in the storage system 100 that the energy storage module 1 operates in. In an embodiment where the energy storage system 100 consist of few energy storage modules and a thyristor 52 in the bypass branch 50 it may be cost effective to use a design where the resistor 54 is of the same resistance as the equivalent series resistance, ESR, of the energy storage module 1. In other embodiments the resistance of the resistor 54 may be higher than the ESR of the energy storage module 1. The resistor 54 advantageously protects the other components in the energy storage module 1. The switch 60 is connected in parallel to the capacitor 10 and is configured to bypass the capacitor 10 and/or the bypass branch 50. The switch 60 may be a mechanical switch or a power electronic switch. The switch 60 provides an extra safety feature that may be used if the bypass branch 50 is destroyed or damaged. By activating the switch 60 the current is provided with an alternative path, e.g., a lower impedance path (especially if there is an additional resistor in the bypass branch), for the current to flow. This may be used as a further bypass unit that provides the possibility to bypass a damaged capacitor 10. Turning to Fig.6 and 7, different embodiments of an energy storage module 1 with combinations of capacitors 10 are illustrated. In Fig.6 the energy storage module 1 consists of two parallel capacitors 10 and a diode 30 connected in anti-parallel. The energy storage module 1 in Fig.6 may in some embodiments be an energy storage system 100 or form part of a larger energy storage module 1 where a plurality of energy storage modules 1 in Fig.6 is connected in series as in Fig.7. The energy storage system 100 of Fig.7 comprises a rack bypass branch 110 that is connected in parallel with the plurality of series-connected modules 1. The energy storage modules 1 of Fig.7 are connected in series to form a rack, and said rack may form part of a larger energy storage system 100 as in Fig.1 or act as an energy storage system 100 by itself. Some energy storage modules 1 may comprise a bypass branch (such as bypass branch 50 discussed in relation to Figs.4 and 5) and/or the energy storage system 100 may comprise a rack bypass branch 110. The rack bypass branch 110 may be connected in parallel to bypass all of the modules 1 in the rack. By providing a bypass branch that is connected in parallel to the one or more capacitors in a module 1 (e.g., as discussed above in relation to Figs.4 and 5), and/or a rack bypass branch 110 that is connected in parallel to the plurality of energy storage modules 1 in a rack, the full energy storage system 100 may be bypassed when it is damaged, or just parts thereof, in an advantageously modular manner. The person skilled in the art realizes that the present disclosure by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.