Patent Publication Number: US-2022238895-A1

Title: Use of a heat source for generation of electricity and aircraft comprising a cooling system

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefit of the German patent application No. 10 2021 101 987.7 filed on Jan. 28, 2021, the entire disclosures of which are incorporated herein by way of reference. 
     FIELD OF THE INVENTION 
     The invention relates to a cooling system for a two-phase refrigerant with integrated generation of electricity and to an aircraft comprising such a cooling system. In particular, the invention relates to a cooling system for a heat source comprising an expander which uses gaseous refrigerant from an evaporator in order to drive a generator and to an aircraft comprising such a cooling system. 
     BACKGROUND OF THE INVENTION 
     In the description which follows and the attached claims, a “two-phase refrigerant” refers to a medium (fluid) which changes its state of matter in the case of cold transfer or heat transfer. Generally, gaseous refrigerant is cooled in a condenser and thereby changes into the liquid state. As a result of heat exchange with an object or medium to be cooled, the liquid refrigerant can vaporize and can cool the object or medium to be cooled by withdrawing therefrom the energy required for vaporization of the refrigerant (enthalpy of vaporization). 
     In aircraft, fuel cells are increasingly being used for providing electricity in the aircraft. Especially in aircraft flying at an altitude between 15 km and 25 km, such as a pseudo-satellite (high-altitude pseudo-satellite—HAPS) for example, electricity is used for propulsion of the aircraft. This is produced during the day by solar cells and used for propulsion, and remaining energy is stored chemically. For example, the electricity generated is used in an electrolyzer for splitting water. In order to be able to propel the aircraft at night as well, the chemically stored energy is converted into electrical energy in a fuel cell. For example, the fuel cell can generate water and electricity through the reaction from hydrogen and oxygen, stored separately. The combination of this alternating storage of electricity and generation of electricity is also referred to as a regenerative fuel-cell system. 
     Since these highflying aircraft are designed to fly for several days, weeks or even months, a lowest possible weight is important. 
     Furthermore, in such aircraft and also in other apparatuses, heat-generating components are present that must be cooled during operation. However, this excess heat is often only released to the environment. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a cooling system, and an associated aircraft, which works in an energy-efficient manner, yet has a low weight. 
     According to a first aspect for better understanding of the present disclosure, a cooling system containing a two-phase refrigerant comprises a condenser which is configured to cool the two-phase refrigerant and to convert gaseous refrigerant into liquid refrigerant and an evaporator which is integrated in a heat source or thermally coupled thereto and is configured to heat the two-phase refrigerant, wherein at least some of the refrigerant vaporizes to form gaseous refrigerant. The cooling system furthermore comprises a conveying device which is configured to convey the two-phase refrigerant from the condenser to the evaporator. 
     Waste heat of the heat source is therefore absorbed by the refrigerant and taken away by the cooling system. In other words, heat exchange with the device to be cooled (part or other component) or a medium heated by the heat source takes place in the evaporator, and it is especially the enthalpy of vaporization of the refrigerant that serves for absorption of heat energy into the refrigerant. 
     In the present disclosure, a liquid heat transfer fluid is preferably dispensed with for weight reduction of the cooling system. Therefore, a two-phase refrigerant is used, at least some of the refrigerant changing into the gaseous state in the evaporator. As a result, the cooling system is altogether lighter compared to a system containing solely liquid heat transfer fluid. For example, the cooling system has to be designed for solely liquid refrigerant only in a region between condenser outlet and evaporator inlet, whereas other sections of the cooling system are designed for gaseous refrigerant. Compared to conventional cooling circuits with purely liquid operation, the weight of the liquid refrigerant is not applicable in the region of gaseous refrigerant, and refrigerant lines can be smaller and thus lighter in the region of liquid refrigerant, since altogether less liquid refrigerant is required. 
     The condenser can be thermally coupled to a heat sink; for example, what can flow through the condenser is a cold fluid (gas or liquid) which absorbs heat and cools the refrigerant in the condenser and, in doing so, converts gaseous refrigerant into the liquid state. 
     The cooling system moreover comprises a generator which is configured to generate electricity and an expander which is fluidically connected to the evaporator downstream of the evaporator and is configured to expand the gaseous refrigerant from the evaporator, to obtain mechanical energy and to drive the generator. The cooling system thus makes it possible to recover energy from the waste heat of the heat source that would otherwise be released to the heat sink of the condenser. Heat energy can be converted into electricity by the expander and generator and used energetically, especially when the heat energy is withdrawn from the heat source by means of the enthalpy of vaporization of the refrigerant. 
     The expander can, for example, be a turbine which is driven by the gaseous refrigerant. In the cooling system, the gaseous refrigerant is present under a higher pressure than the liquid refrigerant, especially the liquid refrigerant downstream of the condenser. This higher pressure can be used to drive the turbine and thus to recover energy in the form of electricity. 
     In one configuration variant, the cooling system can furthermore comprise a fuel cell, wherein the evaporator is integrated in the fuel cell or thermally coupled to the fuel cell. The waste heat of the fuel cell can therefore be used by the expander and generator to recover energy. In particular, more electricity can be generated than by the fuel cell alone. 
     The fuel call can be a reversible fuel cell, i.e., a fuel cell which can both generate electricity and use electricity for chemical storage (electrolysis). Alternatively or additionally, the fuel cell can also be an electrolyzer. Therefore, the fuel-cell cooling system is not restricted to a fuel cell for generation of electricity, but can also be used only for an electrolyzer. Likewise alternatively or additionally, the cooling system can cool a fuel cell and a separately arranged electrolyzer. Here, for example, the fuel cell and the electrolyzer can each constitute an evaporator which is supplied with refrigerant from the conveying device as desired, or the fuel cell and the electrolyzer can have a common thermal coupling to the evaporator. 
     For example, in the case of use in an electrically powered aircraft, additional electricity can therefore be obtained. This concerns phases in which electricity is generated by means of a fuel cell, but also phases in which electricity is generated via other means (e.g., solar cells) and is chemically stored by means of an electrolyzer. 
     Alternatively or additionally, the heat source can be any device or component of an apparatus which produces waste heat. For example, electrical parts can generate waste heat and can constitute a heat source. Furthermore, the evaporator can be thermally coupled to a battery, an electronic component, an electric motor and/or a drive or integrated therein, and they can likewise be cooled by the cooling system described. In addition, a combustion engine, a gas turbine or a similar apparatus having a high waste heat load, and also the waste gases thereof, etc., can be cooled using the cooling system, with energy being recovered by the expander and generator. The waste heat of this device/component alone or in combination with the waste heat of a fuel cell and/or an electrolyzer can be used for additional generation of electricity by the expander and generator. 
     In one configuration variant, the cooling system can furthermore comprise a first collection vessel which is configured to collect the liquid and gaseous refrigerant from the evaporator, and a first discharge line. The collection vessel can, for example, be fluidically connected to the evaporator just after the evaporator (comprise a short cooling-system line) or be integrated in the evaporator. Alternatively, a cooling-system line can also have, downstream of the evaporator, a widening which serves as a collection vessel. The first collection vessel serves for separation of gaseous and liquid refrigerant which leaves the evaporator. 
     The first discharge line can be configured to discharge the gaseous refrigerant from the first collection vessel. In particular, the first discharge line can fluidically connect the first collection vessel to the expander. For example, the first discharge line can be connected to the collection vessel in an upper region thereof for separation and discharge of the upwardly flowing gaseous refrigerant from the liquid refrigerant by means of gravity. 
     Since only gaseous refrigerant is supplied to the expander, the expander can have a relatively simple construction, since it need not be designed for refrigerants in droplet form (partially liquid). Therefore, the energy recovery system can be implemented very easily. 
     In a further configuration variant, the cooling system can furthermore comprise a first regulating valve which is configured to regulate a flow rate of the gaseous refrigerant through the first discharge line. In particular, the control system can be configured to control the first regulating valve in such a way that the gaseous refrigerant supplied to the expander is superheated. In this connection, the first regulating valve establishes and maintains a pressure of the gaseous refrigerant in the first discharge line (downstream of the first regulating valve) that prevents condensation of the gaseous refrigerant. In particular, the pressure of the gaseous refrigerant in the first discharge line is reduced compared to a pressure upstream of the first regulating valve (e.g., in the first collection vessel). This too avoids formation of droplets in the gaseous refrigerant, and the structure of the expander is simpler and thus lighter as a result. 
     Here, the control system is especially configured to ensure superheating of the refrigerant, but to maximize a pressure difference with respect to a section of the refrigerant downstream of the expander. The reduction of the pressure in the gaseous refrigerant for superheating should have a minimum reduction of the mechanical energy to be obtained by the expander. 
     Alternatively or additionally, the refrigerant in the first discharge line can also be heated by supplying heat energy. Any available heat source can be used for this purpose. For example, the refrigerant can be heated between 5 K and 15 K, preferably by 10 K, above the boiling temperature (the dew point) in order to achieve sufficient superheating of the refrigerant. 
     In yet a further configuration variant, the cooling system can furthermore comprise a second discharge line which fluidically connects the first collection vessel to a part of the cooling system downstream of the condenser and is configured to discharge liquid refrigerant from the first collection vessel. For example, the second discharge line can be connected to the first collection vessel in a lower section thereof for discharge of the downwardly flowing liquid refrigerant by means of gravity. 
     The liquid refrigerant flowing through the second discharge line does not require an additional conveying device, the resulting being that the energy recovery system can likewise be of a light design and does not require energy for such an additional conveying device. Owing to the connection between first collection vessel (directly or just after the evaporator) and the part of the cooling system downstream of the condenser, there is a pressure difference between start and end of the second discharge line. As a result of the pressure difference, the liquid refrigerant is pushed or conveyed through the second discharge line into the part of the cooling system downstream of the condenser without additional devices and/or without the aid of gravity. The pressure in the part of the cooling system downstream of the condenser is significantly determined by the cooling and condensation of the refrigerant in the condenser. Conversely to the increasing pressure in the evaporator, the pressure is lower at the outlet side of the condenser than at the inlet side thereof. 
     By means of the first regulating valve, the pressure in the first collection vessel can moreover especially be controlled in such a way that the pressure difference between first collection vessel and the part of the cooling system downstream of the condenser is formed and maintained. 
     In another configuration variant, the cooling system can furthermore comprise a supply line which fluidically connects the conveying device to the evaporator and a second regulating valve which is arranged in the supply line and is configured to regulate a flow rate of the refrigerant through the supply line. In this connection, the control system can be configured to control the second regulating valve in such a way that the evaporator is operated in a wet vaporization process. In the supply line, liquid refrigerant is conducted from the conveying device to the evaporator. 
     With the aid of the second regulating valve, it is then firstly possible to control the quantity of refrigerant in the evaporator that is required for taking away the heat energy arising there. Secondly, it is possible for the control system to conduct a certain excess of liquid refrigerant into the evaporator, so that a wet vaporization process takes place therein. In other words, more liquid refrigerant is conducted into the evaporator than can be vaporized by the supply of heat from the fuel cell and/or the electrolyzer. The wet vaporization process causes liquid refrigerant to be present throughout the evaporator. This allows an even cooling effect along the flow direction of the refrigerant and thus an even heat distribution within the evaporator. 
     For example, fuel cells and electrolyzers work most efficiently when certain operating parameters are adhered to, especially when a certain operating temperature is held. For example, the operating temperature can be between 70° C. and 90° C. or between 80° C. and 90° C. 
     Since these components comprise a stack of many individual cells, it is furthermore necessary to achieve an even temperature distribution within the stack, i.e., across all the cells. Owing to the wet vaporization process, what is available within the fuel cell and/or the electrolyzer at any point is liquid refrigerant which can vaporize there with absorption of heat energy. Regions having strong heat development (so-called hotspots) can therefore be avoided. It is self-evident that the same applies to any other heat source which is cooled using the cooling system. 
     Merely by way of example, at least 20% more liquid refrigerant can be conducted into the evaporator than is vaporized therein. In other words, in the flow direction of the refrigerant downstream of the evaporator, 20% of the (formerly liquid) refrigerant is (continues to be) in liquid form, whereas the rest of the refrigerant has changed into the gaseous state in the evaporator. 
     According to one configuration variant, in order to allow the relatively narrow temperature range of the operating temperature of the heat source (e.g., fuel cell/electrolyzer) in a lasting mariner, the control system of the cooling system can furthermore be configured to capture operating conditions of the heat source, the fuel cell and/or the electrolyzer, to ascertain a cooling demand on the basis of the operating conditions, and to operate the cooling system in such a way that the cooling demand of the heat source, the fuel cell and/or the electrolyzer is covered. 
     For this purpose, the control system can, by of example, ascertain a demand for electrical energy which is to be generated by the fuel cell or is available to the electrolyzer for chemical cleavage (e.g., on the basis of connected electrical loads or electricity generation devices, such as solar cells for example). The electrical energy is in relation to the quantity of heat generated by the heat source, the fuel cell and/or the electrolyzer. It is this quantity of heat that is to be absorbed and taken away by the cooling system. 
     In another configuration variant, the control system can furthermore be configured to control the conveying device and/or the condenser in such a way that the temperature of the refrigerant supplied to the evaporator substantially corresponds to the temperature of the evaporator. The control system can control a delivery rate of the refrigerant through the conveying device and/or the outlet temperature of the liquid refrigerant from the condenser in order to determine the desired temperature of the refrigerant which is supplied to the evaporator. 
     Thus, in one configuration variant, the heat source, the fuel cell and/or the electrolyzer can have a section which forms the evaporator. This can avoid excessively strong cooling or at least sectional cooling of the heat source, the fuel cell and/or the electrolyzer. Such cooling can, for example, affect the operation of individual cells in the stack of a fuel cell or electrolyzer and/or have altogether an adverse effect on efficient operation of the heat source. By approximation (or equalization) of the temperature of the refrigerant and the evaporator, the operating temperature of the heat source, the fuel cell and/or the electrolyzer can be kept constant. Cooling of the heat source, the fuel cell and/or the electrolyzer is then effected (virtually exclusively) via the enthalpy of vaporization of the refrigerant. 
     Commonly, a fuel cell and an electrolyzer have channels, through which a heat transfer fluid is guided in order to cool them. According to some of the present variants, the fuel cell and/or the electrolyzer form the evaporator. For example, the refrigerant of the cooling system can flow through the heat transfer fluid channels which are already present and which therefore form the evaporator. As a result, existing fuel cells and/or electrolyzers can be used with the cooling system described here. 
     Alternatively or additionally, the cooling system can furthermore comprise a cooling circuit which thermally couples the heat source to the evaporator. The cooling circuit can comprise its own heat transfer fluid (fluid or other medium for transfer of heat energy). This has the advantage of optimizing the heat transfer fluid for the heat source, whereas the refrigerant in the cooling system has other properties which would have disadvantages in the heat source (e.g., electrical conductivity of the refrigerant in the case of a fuel cell, an electrolyzer or some other electrically conductive heat source). 
     Merely by way of example, the heat transfer fluid can be enclosed in a heat pipe which is arranged between the evaporator and the heat source. In this connection, the heat transfer fluid independently ensures heat exchange between evaporator (refrigerant of the cooling system) and the heat source. In other words, what takes place independently is the cooling circuit in the heat pipe. This allows fluidic separation of the cooling system and the heat source. 
     Alternatively or additionally, the cooling circuit can comprise a further conveying device which conveys a heat transfer fluid through lines of the cooling circuit and especially from the heat source to the evaporator. The additional lines of the cooling circuit mean that the heat source and evaporator of the cooling system can be spatially separate. 
     In another configuration variant, the cooling system can furthermore comprise a bypass line which is connected to the first regulating valve and is configured to conduct gaseous refrigerant from the first regulating valve to a part of the cooling system downstream of the expander. In this connection, the regulating valve can be designed in such a way that it supplies between 0% and 100% of the gaseous refrigerant to the expander, whereas the rest of the gaseous refrigerant (100% to 0%) is conducted through the bypass line. As a result, the output of the expander and thus of the generator can be adapted to certain load cases, for example if the total achievable electricity cannot be consumed or stored. On the other hand, in a starting phase of the cooling system, the gaseous refrigerant can also be conducted through the bypass line in order to reach the necessary operating conditions of the cooling system as rapidly as possible and not to withdraw energy from the refrigerant cycle which is building up. 
     In another configuration variant, the cooling system can furthermore comprise a second collection vessel which is configured to collect the liquid refrigerant from the condenser. In other words, the second collection vessel is fluidically connected to the condenser downstream thereof, so that condensed (now liquid) refrigerant in the condenser flows into the second collection vessel. The second collection vessel serves as a reservoir of liquid refrigerant for the conveying device, so that gaseous refrigerant does not reach the conveying device and, as a result, damage to the conveying device can be avoided. 
     For example, the second discharge line can fluidically connect the first collection vessel to the second collection vessel. The central task of the second collection vessel is to form a reservoir of liquid refrigerant for the section of the cooling system leading up to the evaporator. Therefore, liquid refrigerant which is separated from the gaseous refrigerant in the first collection vessel can be directly conducted into the second collection vessel. The second discharge line thus forms a bypass of the condenser. 
     In another configuration variant, the second regulating valve can be connected to a further refrigerant line which fluidically connects the second regulating valve to the second collection vessel. For example, the second regulating valve can be implemented in such a way that between 0% and 100% of refrigerant is conducted from the conveying device into the evaporator via the supply line, whereas the correspondingly remaining quantity of refrigerant (between 100% and 0%) is conducted from the conveying device into the collection vessel. As a result, the quantity of refrigerant which is supplied to the evaporator can be determined independently of the rate of change of the delivery rate of the conveying device. Moreover, the conveying device can also be operated in a more constant manner, and it is treated with care as a result. Altogether, this can reduce the total quantity of liquid refrigerant in the cooling system and optimize and reduce the size of the first and/or second collection vessel. This can further save weight. 
     In yet a further configuration variant, the cooling system can furthermore comprise a supercooler which is configured to supercool refrigerant downstream of the condenser and upstream of the conveying device. Supercooling of the refrigerant prevents cavitation in the conveying device, since the refrigerant has been cooled further below its boiling point. For example, the supercooler can reduce the temperature of the refrigerant leaving the condenser by 2 to 10 K, preferably by 5 K, (below the outlet temperature at the condenser or below the boiling point). 
     Furthermore, the supercooler can be integrated in the condenser, form part of the condenser or be a section of the condenser. As a result, the condenser and the supercooler can share a heat sink (e.g., a cold fluid, cold air stream, etc.). Alternatively, the supercooler can also be implemented separately from the condenser and have its own heat sink. 
     In a further configuration variant, the cooling system can comprise at least one pressure sensor and/or temperature sensor which measures the pressure and/or the temperature of the refrigerant at a relevant position in the cooling system. The control system can access the signals or data of the at least one sensor in order to control the conveying device, the first regulating valve and/or the second regulating valve. 
     Merely by way of example, the at least one sensor can be arranged in the first collection vessel and/or in the second collection vessel. As a result, the pressure and/or the temperature downstream of the evaporator and/or downstream of the condenser can be determined. Furthermore, fuel cells or electrolyzers are already equipped with such sensors, which can be coupled to the control system in order to determine the pressure and/or the temperature of the refrigerant in the evaporator. 
     In the case of a known volume of the first and/or second collection vessel (or of the entire cooling system), the control system can determine the volume of the liquid and/or gaseous refrigerant on the basis of the sensor signals or data and taking into account the laws of thermodynamics Especially in the case of constant operation of the cooling system and thus a substantially homogeneous temperature distribution of the refrigerant within the cooling system, the entire (liquid) quantity of refrigerant can be calculated. As a result, the control system can also be configured to detect a leak or a loss of refrigerant. 
     Alternatively or additionally, fill-level sensors can also be provided in the first and/or the second collection vessel in order to determine the quantity of liquid refrigerant. 
     Furthermore, the control system can control the cooling system (or its components) in such a way that the pressure in the cooling system downstream of the evaporator is about 3.2 bar and that downstream of the condenser is 2.4 bar. The pressure difference between 3.2 and 2.4 bar is normally sufficient for conveying the liquid refrigerant from the first collection vessel to the section of the cooling system downstream of the condenser. Furthermore, the pressure difference, which is also present between the first collection vessel and a section of the cooling system upstream of the condenser, is sufficient for converting the refrigerant after exit from the evaporator, where it is a saturated vapor, into superheated vapor (e.g., with the aid of the first regulating valve). 
     In one configuration variant, the refrigerant can be R1336mzz(Z), R134a, methanol or CO2 (carbon dioxide). It is self-evident that any other two-phase refrigerant can be used. However, R1336mzz(Z) is especially suitable in a cooling system having a fuel cell or electrolyzer as the heat source, the fuel cell or electrolyzer being optimally operated at a temperature of approx. 85° C., since R1336mzz(Z) vaporizes at a pressure between 2.4 and 3.2 bar and a temperature between 70° C. and 90° C. This can achieve optimal cooling of the fuel cell or the electrolyzer at optimal operating temperature. In addition, R1336mzz(Z) is electrically nonconductive, meaning that it can be directly introduced into the fuel cell or the electrolyzer. 
     In a further configuration variant, the condenser can be operated in such a way (be cooled by the heat sink in such a way) that the pressure difference in relation to the pressure of the refrigerant in the first collection vessel is sufficient for conveying the gaseous and the liquid refrigerant to the condenser and/or to the section of the cooling system downstream of the condenser and/or for superheating the gaseous refrigerant. Thus, a flow rate of the cooling medium of the heat sink can be controlled in such a way that a desired condenser temperature and thus temperature profile of the refrigerant from the inlet to the outlet of the condenser is achieved. 
     In yet a further configuration variant, the cooling system can comprise a third regulating valve which is arranged in the second discharge line and is configured to regulate a flow rate of the liquid refrigerant through the second discharge line. The third regulating valve allows not only the regulation of the flow rate (e.g., by the control system), but also regulation of the pressure in the first collection vessel. Especially in the starting phase of the cooling system, a higher pressure in the evaporator and in the cooling system on the outlet side of the evaporator can be built up as a result. 
     In another configuration variant, the first discharge line or at least a section thereof can be designed as a double-wall pipe. In this connection, an inner pipe of the first discharge line can guide liquid refrigerant, for example refrigerant from the conveying device. For example, the inner pipe of the first discharge line can form the supply line from the conveying device to the evaporator. In the outer pipe of the first discharge line, gaseous refrigerant can be guided from the evaporator or the first collection vessel to the condenser, i.e., can form the first discharge line. As a result, the gaseous refrigerant in the first discharge line is heated by the liquid refrigerant, thereby preventing condensation in the first discharge line. 
     In yet a further configuration variant, at least a section of the first discharge line can have a fixed flow resistance which is predetermined such that the refrigerant in the first collection vessel has a higher pressure than the refrigerant in the part of the cooling system downstream of the condenser. In other words, instead of or in addition to the first regulating valve, the flow rate of the gaseous refrigerant from the first collection vessel to the condenser is restricted by the first discharge line itself, so that pressure builds up in the first collection vessel. Owing to the pressure difference now present between the first collection vessel and the outlet side of the condenser, the liquid refrigerant can be conveyed from the first collection vessel to the outlet side of the condenser without additional devices and/or without the aid of gravity. 
     For example, the first discharge line can have a section which has a smaller diameter than other lines of the cooling system, certain flow-impeding fittings and/or a short narrowing of the flow cross section in the flow direction. This can save weight, especially since a first regulating valve is not necessary. Moreover, control complexity can be reduced. The pressure difference between first collection vessel and outlet side of the condenser can, in this case, be exclusively controlled via the conveying device and the conveyed quantity of refrigerant in the evaporator. Especially in cooling systems having low control complexity, for example in the case of even heat generation by the heat source to be cooled using the evaporator, constant operation of the cooling system is possible even without a first regulating valve. 
     According to a further aspect for better understanding of the present disclosure, an aircraft can comprise an electrical drive and a cooling system according to the first aspect (or one of its configuration variants and/or examples). The electrical drive can, for example, be an electrically powered engine or a motor with a propeller. In particular, the generator driven by the expander and possibly the fuel cell of the cooling system can supply the electrical drive with electricity. As a result, the size of the fuel cell and thus its weight can be reduced. 
     Alternatively or additionally, the electricity generated by the generator can also be used to supply other electrical components in the aircraft. As a result, a fuel cell, battery or other power supply can be optimized in terms of its size (and thus its weight) with respect to the electrical drive of the aircraft. 
     The aircraft can be a transport aircraft, passenger aircraft, light aircraft or a pseudo-satellite (high-altitude pseudo-satellite—HAPS). For example, the aircraft can be powered by electricity which is generated in the fuel cell and the generator, or certain electrical components in the aircraft can be powered by the electricity thus generated. Alternatively or additionally, the aircraft can also comprise an electrolyzer which is cooled by the cooling system. Any weight reduction is hugely important for an aircraft, since less energy is required for propulsion. 
     It is self-evident that the above-described aspects, configurations, variants and examples can be combined without this having been explicitly described. Each of the described configuration variants and each example are thus to be regarded as optional with regard to each of the aspects, designs, variants and examples or even combinations thereof The present disclosure is consequently not limited to the individual configurations and configuration variants in the sequence described, or to a particular combination of the aspects and configuration variants. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred exemplary embodiments of the invention will now be explained in more detail with reference to the appended schematic drawings, in which: 
         FIG. 1  schematically shows a first variant of a cooling system; 
         FIG. 2  schematically shows a second variant of a cooling system; 
         FIG. 3  schematically shows a third variant of a cooling system; 
         FIG. 4  schematically shows a fourth variant of a cooling system; 
         FIG. 5  schematically shows a fifth variant of a cooling system; 
         FIG. 6  schematically shows a sixth variant of a cooling system; and 
         FIG. 7  schematically shows an aircraft. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  schematically shows a first variant of a cooling system  10  which comprises a condenser  110 , an evaporator  130  and a conveying device  120 . The evaporator  130  is, for example, connected to the conveying device  120  via a supply line P 1 , so that a refrigerant of the cooling system  10  can be conducted from the conveying device  120  into the evaporator  130 . In the evaporator  130 , the refrigerant, which is a two-phase refrigerant, can absorb heat energy and, as a result, at least some of the refrigerant vaporizes. In the further course of the cooling system  10 , which will be described in more detail, the refrigerant flows to the condenser  110 , in which it is cooled and condensed and is subsequently resupplied to the conveying device  120  in a liquid state. For this purpose, the condenser  110  can be thermally coupled to a heat sink A 2 , for example in the form of a cold fluid stream (outside air in the case of a vehicle). 
     In the variant depicted, the evaporator  130  is integrated in a heat source  130  (fuel cell and/or electrolyzer). In other words, the heat source  130  has sections, through which the liquid refrigerant flows and vaporizes therein, at least in part, thereby acting as an evaporator. In this connection, the enthalpy of vaporization is especially used for cooling the heat source 130 . 
     Downstream of the evaporator  130  (or integrated therein or connected thereto) is a first collection vessel  135  which is configured for collection and separation of the liquid and gaseous refrigerant from the evaporator  130 . For example, the first collection vessel  135  can be fluidically connected to an outlet of the evaporator  130  via a line P 15 . 
     The cooling system  10  has a first discharge line P 21  which fluidically connects the first collection vessel  135  to a part of the cooling system  10  upstream of the condenser  110 . As a result, gaseous refrigerant can be discharged from the first collection vessel  135  and be conducted into the condenser  110  for condensation. In the first discharge line P 21 , there can be arranged a first regulating valve  137  which is configured to regulate a flow rate of the gaseous refrigerant through the first discharge line P 21 . In this connection, the first discharge line is formed by the line sections P 21  and P 25 . By means of the first regulating valve  137 , it is possible, firstly, to set/achieve a pressure drop of the gaseous refrigerant in the line P 25  for superheating of the gaseous refrigerant and, secondly, to control pressure in the first collection vessel  135 . 
     Alternatively, a section of the first discharge line, for example the line section P 21  directly after the first collection vessel  135 , can have a fixed flow resistance. In this connection, the fixed flow resistance can be adapted to the entire system in order to build up a higher pressure in the first collection vessel  135  than in the line section P 25  and also in the part of the cooling system  10  downstream of the condenser  110 . As a result, the first regulating valve  137  can be dispensed with. 
     Furthermore, the cooling system  10  has a second discharge line P 22  which fluidically connects the first collection vessel  135  to a part of the cooling system  10  downstream of the condenser  110 . By means of the second discharge line P 22 , liquid refrigerant can be discharged from the first collection vessel  135  and be resupplied to the cooling system in a region which guides liquid refrigerant, i.e., downstream of the condenser  110 . From there, it can be resupplied to the conveying device  120 , for example via the lines P 30  and P 5 . The liquid refrigerant in the second discharge line P 22  can be brought about solely on the basis of a pressure difference between the first collection vessel  135  and the section of the cooling system  10  downstream of the condenser  110 . An additional conveying device is not necessary. Optionally, a further regulating valve  139  ( FIG. 3 ) can be integrated in the second discharge line P 22  in order, for example, to build up the pressure difference (the higher pressure in the first collection vessel  135 ) and/or a fill level of liquid refrigerant in the evaporator  130  more rapidly. 
       FIG. 1  shows, moreover, an expander  160  which is fluidically connected to the evaporator  130  downstream of the evaporator  130 . For example, the expander  160  can be arranged in the first discharge line P 21 , preferably in the section P 25  of the first discharge line downstream of the first regulating valve  137 , and expand gaseous refrigerant from the evaporator  130 . For example, the expander  160  can be implemented in the form of an (expansion) turbine, a multistage turbine, a Wankel engine or a piston engine. A droplet-resistant expander  160  is preferred, since refrigerant droplets in the gaseous refrigerant cannot be ruled out in all operating phases of the cooling system  10 . However, a simpler, and hence lighter, structure of the expander  160  is advantageous for an aircraft. 
     The expander  160  converts the energy of the gaseous refrigerant upon expansion into mechanical energy, for example into a rotating movement. The mechanical energy can be used to drive a generator  162  and, as a result, it generates electricity. For example, the expander  160  and the generator  162  can be arranged on a common shaft, as is schematically depicted in  FIG. 1 . The mechanical energy which can be obtained by the expander  160  depends, firstly, on the pressure difference between first collection vessel  135  and inlet side of the condenser  110  and, secondly, on a possible flow rate of the gaseous refrigerant in the first discharge line P 21 , P 25 . 
     Lastly, the cooling system according to  FIG. 1  comprises a second collection vessel  115  which is configured to collect the liquid refrigerant from the condenser  110 . In this connection, the second discharge line P 22  can fluidically connect the first collection vessel  135  to the second collection vessel  115 . The second collection vessel  115  can also be integrated in the condenser  110  or be a (widened) section of a refrigerant line P 28 , P 30  downstream of the condenser  110 . 
     If gaseous refrigerant accumulates in the second collection vessel  115 , the second collection vessel  115  can be fluidically connected to an inlet side of the condenser  110  via a return line P 29 . For example, the return line P 29  can open into a line section P 27  of the cooling system upstream of the condenser  110 . In order to avoid a bypass of the condenser  110 , a check valve can be provided at the end of the return line P 29 . 
     In order to prevent gaseous refrigerant from getting into the conveying device  120 , a supercooler  117  can be provided in a section of the cooling system between condenser  110  and conveying device  120 , for example between second collection vessel  115  and conveying device  120 . The supercooler  117  can have its own heat sink A 1  (air stream or other cold fluid) and not that of the condenser  110 . Alternatively, the supercooler  117  and the condenser  110  can share a heat sink (not depicted) and/or the supercooler  117  and the condenser  110  form a unit (not shown), i.e., are mutually integrated. 
     Moreover,  FIG. 1  additionally depicts a second regulating valve  132  which is arranged in the supply line P 1 , P 2  and is configured to regulate a flow rate of the refrigerant through the supply line P 2 . In particular, the second regulating valve  132  can determine the quantity of refrigerant that is supplied to the evaporator  130 . As a result, the cooling performance of the evaporator  130  is controlled, and thus also the pressure in the evaporator  130  and in the sections of the cooling system  10  downstream of the evaporator  130 . 
     Optionally, the second regulating valve  132  can also be a branch of the supply line P 1  and conduct at least some of the refrigerant conveyed through the line P 1  by the conveying device  120  back into a section of the cooling system  10  downstream of the condenser  110  via a line section P 16 . For example, the line section P 16  can open into the second collection vessel  115 . As a result, the conveying device  120  can be operated continuously, whereas the inflow into the evaporator  130  is controlled via the second regulating valve  132 . 
     The cooling system  10  has furthermore a control system  150  (or control unit, controller, processor or computer) which is configured to control the conveying device  120  and especially its delivery rate of liquid refrigerant through the lines P 5  and P 1 . Furthermore, the control system  150  can also determine and control the opening and closing and also a degree of opening of the regulating valves  132 ,  137 . In addition, the control system  150  is configured to regulate the operation of the condenser  110  and/or the supercooler  117 , for example by control of the supply of cold fluid as heat sink A 1 , A 2 . 
     Furthermore, the cooling system  10  can have sensors, especially pressure sensors and temperature sensors (not depicted). By means of the sensors, the control system  150  can ascertain the pressure and/or the temperature of the refrigerant at the relevant section of the cooling system  10  and control the conveying device  120  and/or regulating valves  132 ,  137  and/or heat sinks A 1 , A 2 . In this connection, the control system  150  is especially designed to ensure a temperature in the evaporator  130  (in the heat source, the fuel cell and/or the electrolyzer) that is as constant as possible. Moreover, the pressure difference between first collection vessel  135  and second collection vessel  115  can be built up and held by means of the control system  150  and, as a result, efficient operation of the cooling system  10  is made possible in a rapid and lasting manner 
     In particular, the control system  150  can bring about superheating of the gaseous refrigerant in the first discharge line P 21 , P 25  with the aid of the first regulating valve  137 . Depending on the refrigerant used, a very small pressure drop between line P 21  and line P 25  can already lead to superheating of the refrigerant. The pressure drop can be very small, for example between 10 and 1000 mbar, for example in the case of refrigerant which is guided in the cooling system within a range between 2 bar and 4 bar. Merely as an example, the refrigerant used can be R1336mzz(Z), which is run between 2.4 bar and 3 bar in the cooling system. 
     Merely by way of example, the control system  150  can carry out various procedures in order to start the cooling system  10 . For example, the second regulating valve  132  can be controlled in such a way that only a connection between line P 1  and bypass P 16  is present, whereas the first regulating valve  137  is open. Now, the heat sink A 2  of the condenser  110  is put into operation in order to allow a temperature and pressure of the refrigerant for operation of the evaporator  130 . If sufficient liquid refrigerant is present in the section downstream of the condenser  110 , for example in the second collection vessel  115 , the control system  150  starts the conveying device  120 . 
     The control system  150  can be configured to determine the cooling demand of the evaporator  130 . For example, the control system  150  can be supplied with signals or data which reflect an operating state of the device to be cooled. For example, on the basis of the consumed or generated electricity of a fuel cell or an electrolyzer, it is possible to ascertain how high the cooling demand of the fuel cell or the electrolyzer is. Accordingly, the control system can control the second regulating valve  132  in such a way that the necessary quantity of liquid refrigerant gets into the evaporator  130  from the line P 1 . As a result of the pressure now rising in the first collection vessel  135 , the control system  150  can (at least partially) close the first regulating valve  137  in order to establish the above-described pressure difference between first and second collection vessel  135 ,  115 . 
     Here, the control system  150  can limit the pressure in the first collection vessel  135  and thus in the evaporator  130  to a maximum. For example, the pressure in a fuel cell or an electrolyzer should be limited to a certain value, 3.5 bar by of example, in order to ensure the reliable operation thereof By means of the first regulating valve  137 , the pressure in the evaporator  130 , but also the quantity of liquid refrigerant in the evaporator  130 , is controllable. Therefore, optimal operation of the fuel cell or the electrolyzer can be ensured. 
     The control system  150  can furthermore be configured to calculate (by means of pressure and temperature sensors) or measure (by means of a fill-level sensor) a fill level of liquid refrigerant in the evaporator  130 . If a sufficient fill level has been reached, the control system  150  can lessen the degree of opening of the second regulating valve  132  and/or reduce the delivery rate of the conveying device  120 . In particular, the control system  150  can now operate the evaporator  130  in a wet vaporization process. 
     Furthermore, the control system  150  is configured to regulate the operation of the condenser  110  and/or the supercooler  117  in order to provide sufficient liquid refrigerant on the inlet side (upstream) of the conveying device  120 . In particular, the heat sink A 1  or A 2  can be regulated here by the control system  150  in order to condense (liquefy) more or less refrigerant, and to hold it available in the second collection vessel  115 , for example. 
     Lastly, the control system  150  can prevent the line P 25  of the cooling system  10 , which line guides gaseous refrigerant, from being flooded with liquid refrigerant. For this purpose, the quantity of liquid refrigerant which flows through the supply line P 1  can be controlled by closure of the second regulating valve  132  and can, for example, be diverted into the bypass P 16 . 
     In a further exemplary case, the control system  150  can also be designed to control the device to be cooled (heat source). This is, for example, necessary if the cooling system  10  cannot achieve sufficient cooling performance in the evaporator  130 . In the event of a leakage of the refrigerant from the cooling system  10  or an excessively high temperature of the heat sink A 1 , A 2 , it may be necessary to reduce the output of the device to be cooled and the associated heat quantity generated. In particular, the control system  150  is configured to capture the operating parameters of the device to be cooled and of the cooling system  10  and to ascertain in advance whether sufficient cooling of the device to be cooled can be achieved or whether the output (heat generation) of the device to be cooled must be reduced. Here, the control system  150  can take into account the maximum permissible pressure in the evaporator  130  and also minimum fill levels in the first and/or second collection vessel  135 ,  115  and in the evaporator  130 . 
     It is self-evident that the control system  150  can also switch off the device to be cooled and the entire cooling system  10  in order to avoid damage to the device to be cooled and/or the cooling system  10 . Here, the control system  150  can be configured to open the first regulating valve  137  in order to supply as much gaseous refrigerant as possible to the condenser  110 . As a result, sufficient liquid refrigerant can be held available, for example in the second collection vessel  115 , for later renewed starting of the cooling system. 
     If the refrigerant downstream of the conveying device  120  is too cold to be conducted into the evaporator  130  (e.g., the operation of a fuel cell or an electrolyzer may be hindered or stopped in the event of excessively strong cooling), the refrigerant in the line P 1  can be heated. In the simplest case, a separate heater (not depicted) can be provided in order to provide the optimal temperature of the refrigerant for the evaporator  130 . 
     In any case, the control system  150  is configured to conduct gaseous refrigerant, via the first regulating valve  137 , through the first discharge line P 25  to the expander  160  if, for example, a sufficient pressure difference is present between first and second collection vessel  135 ,  115 . The greater the pressure difference, the greater the necessary cooling demand at the condenser  110  in order to liquefy the refrigerant and to reduce the pressure of the refrigerant at the same time. Since the pressure difference largely arises through the vaporization of the refrigerant in the evaporator  130 , this energy can be recovered by the expander  160  and generator  162  by means of generation of electricity. 
     To simplify what is depicted in  FIGS. 2 to 6 , the generator  162  and the common shaft with the expander  160  is depicted only in  FIG. 1 . 
     A further variant of a cooling system  10  for a heat source  130  is depicted in  FIG. 2 . The cooling system  10  shown comprises a multiplicity of components which are also comprised in the cooling system  10  according to  FIG. 1 , and so the description thereof is not repeated here. 
     In the cooling system  10  according to  FIG. 2 , what is shown is a bypass line P 25   a , through which gaseous refrigerant can be conducted past the expander  160 . For example, the first regulating valve  137  can be configured in such a way that it conducts between 0% and 100% of the gaseous refrigerant into the first discharge line P 25  toward the expander  160  and conducts the remaining quantity of the gaseous refrigerant (100% to 0%) through the bypass line P 25   a  past the expander  160 . As a result, the gaseous refrigerant can, for example, be directly conducted to the condenser  110  during the starting phase of the cooling system  10 . 
     In the variant according to  FIG. 3 , the cooling system  10  has a further regulating valve  139  which is provided in the second discharge line P 22 . The cooling system  10  shown comprises a multiplicity of components which are also comprised in the cooling system  10  according to  FIGS. 1 and 2 , and so the description thereof is not repeated here. 
     For example, the control system  150  can close the further regulating valve  139  if the pressure in the first collection vessel  135  is (still) insufficiently high for regular operation of the cooling system  10 . Equally, the control system  150  can close the further regulating valve  139  if insufficient liquid refrigerant is present in the first collection vessel  135 . What is thereby avoided is that gaseous refrigerant gets into the section of the cooling system  10  downstream of the condenser  110  (e.g., into the second collection vessel  115 ) through the second discharge line P 22 . 
       FIG. 4  shows a further variant of a cooling system  10  which, however, can be operated not only in a wet vaporization process, but also in a dry vaporization process. The cooling system  10  shown comprises a multiplicity of components which are also comprised in the cooling system  10  according to  FIGS. 1 to 3 , and so the description thereof is not repeated here. 
     In contrast to the cooling system variants according to  FIGS. 1 to 3 , the evaporator  130  is thermally coupled to the heat source  172  in the variant according to  FIG. 4 . For example, the cooling system  10  has a cooling circuit  170  which thermally couples the heat source  172  to the evaporator  130  via a heat transfer fluid. In this connection, the heat transfer fluid, and the refrigerant used in the cooling system  10 , can be different media which have been optimized and/or adapted for the cooling circuit  170  and the cooling system  10 , respectively. 
     For example, the heat transfer fluid in the cooling circuit  170  can be a liquid heat transfer fluid and/or one-phase heat transfer fluid. It brings about a uniform temperature distribution within the heat source  172 , since no sections of the heat source  172  are wetted with gaseous heat transfer fluid with possible formation of hotspots as a result. 
     Furthermore, a refrigerant which completely vaporizes in the evaporator  130  can be used in the cooling system  10 . In other words, the cooling system  10  and the evaporator  130  thereof is operated in a dry vaporization process. This is possible because a uniform temperature (even temperature distribution) now need not prevail in the evaporator  130 , but only in the heat source  172  with the heat transfer fluid of the cooling circuit  170 . The energy from the enthalpy of vaporization, which energy is now increased in the refrigerant of the cooling system  10 , can be used for greater conversion into mechanical energy at the expander  160  and thus for higher generation of electricity. 
     The dry vaporization process furthermore makes it possible to dispense with the first collection vessel  135  and the second discharge line P 22  ( FIGS. 1 to 3 ). The cooling system is lighter as a result. 
     Furthermore, the cooling circuit  170  can comprise a further conveying device  174  which conveys the heat transfer fluid through lines of the cooling circuit  170  and especially from the heat source  172  to the evaporator  130 . Alternatively, the heat transfer fluid can also be moved without a conveying device  174  by heating at the heat source  172  and cooling at the evaporator  130 . 
     This variant has furthermore the advantage that the heat source  172  can be arranged at a spatial distance from the cooling system  10  if necessary. 
     The heat source can likewise be a fuel cell, an electrolyzer, a battery, an electronic component, a motor, a gas turbine or any other heat-generating device. 
     In the variant according to  FIG. 5 , a further bypass line P 41  is depicted. 
     The cooling system  10  shown comprises a multiplicity of components which are also comprised in the cooling system  10  according to  FIG. 4 , and so the description thereof is not repeated here. 
     In particular, this variant of the cooling system  10 , like the variant according to  FIG. 2 , makes it possible to directly guide gaseous refrigerant from the first regulating valve  137  to the condenser  110  through the bypass line P 41 . This means that, firstly, the proportion of energy to be recovered that is available to the expander  160  and thus to the generator  162  can be regulated. Secondly, gaseous refrigerant can be re-provided in the evaporator  130  directly via the condenser  110 . 
     According to the variant in  FIG. 6 , the cooling system  10  can be yet further simplified. Because of the dry vaporization process in the evaporator  130 , the first regulating valve  137  can be dispensed with. The control system  150  can, for example, regulate the circuit in the cooling system  10  merely via the supply of liquid refrigerant through the supply line P 1 , P 2  into the evaporator  130 , and absorb sufficient heat energy from the cooling circuit  170  at the same time. 
       FIG. 7  schematically shows an aircraft  1  which, for example, has an electrical drive. The electrical drive can, by way of example, be implemented with an electric motor  5  and with a propeller  7  coupled to a motor shaft. It is self-evident that other electrical drive systems can also be provided in the aircraft  1 . The aircraft  1  furthermore comprises a cooling system  10 , for example a cooling system  10  according to any of  FIGS. 1 to 6 . 
     In the aircraft  1 , what can be provided is a fuel cell  130 ,  172  for generation of electricity that, for example, supplies the electric motor  5  with electrical energy. The fuel cell  130 ,  172  is cooled by a cooling system  10 . For example, a cooling system  10  according to any of  FIGS. 1 to 3  can be used for cooling of the fuel cell  130 . Alternatively, the fuel cell  172  can be part of a cooling circuit  170 , as is shown in  FIGS. 4 to 6 , for example, together with a cooling system  10 . 
     In this connection, the condenser  110  of the cooling system  10  can be arranged in the aircraft  1  in such a way that it is thermally coupled to the surroundings of the aircraft  1  (e.g., air). The ambient air of the aircraft  1  can therefore be used as a heat sink A 1  or A 2 . 
     While at least one exemplary embodiment of the present invention(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). In addition, in this disclosure, the terms “comprise” or “comprising” do not exclude other elements or steps, the terms “a” or “one” do not exclude a plural number, and the term “or” means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority.