Description

The invention relates to a sorption store 1 for the storage of gas comprising at least two gas adsorbent media distinguishable from each other by different adsorption behaviors with respect to the stored gas. The invention additionally relates to a drive system and a vehicle comprising such a sorption store.

Owing to the increasing scarcity of oil resources, research is increasingly being made to unconventional fuels such as methane, ethanol or hydrogen for operating an internal combustion engine or a fuel cell. For this purpose, vehicles comprise a storage vessel for keeping a stock of the fuel. For the storage of gas in stationary and mobile applications, the gas is stored in pressure vessels, often referred to as compressed natural gas (CNG) technique or in sorption stores, often referred to as adsorbed natural gas (ANG) technique. Sorption stores are also known as ANG tanks. ANG has the potential to replace compressed natural gas CNG in mobile storage applications such as in vehicles. Although a substantial research effort has been devoted to ANG, very few studies evaluate the impact of heat of adsorptions on system performance. In turn, in ANG- applications a micro powder solid, such as activated carbon, is packed in a vessel to increase the storage density, enabling lower pressure operation with the same capacity. Adsorption is an exothermic process. Any adsorption or desorption is accompanied by a temperature change in an ANG-storage system. The heat of adsorption has a detrimental effect on performance during both filling- and discharge cycles. A temperature increase as high as 80°C can occur during the filling cycle. A filling cycle normally will be performed in a fuel station, at least for mobile applications, where the released adsorption heat can be removed. Contrary to the filling cycle, the rate of discharge is dictated by the energy demand of the application. The filling time cannot be widely varied to moderate the impact of cooling during the use of ANG storage vessels.

Such sorption stores comprise in particular adsorbent media having a large internal surface area on which the gas is adsorbed. The gas is stored by the adsorption on the adsorbent medium, in the cavities between an individual particle of the adsorbent medium and in parts of the vessel, which are not filled with adsorbent medium. The filled sorption store 1 can be pressurized and non-pressurized. Selection of a suitable vessel depends on the applied maximum pressure. The higher the storage pressure the more gas can be stored per volume. Adsorption describes the attachment of atoms or molecules of a gaseous or liquid fluid onto the surface of a solid material, which is referred to as adsorbent medium for the purpose of the present invention. Terms like adsorbent, adsorber and adsorption medium are equally known for the denomination of the said solid material. The adsorption capacity of the adsorbent media, defined by the ratio of the mass of the adsorbed gas or liquid to the mass of the adsorbent me- dium, strongly depends on temperature and is reduced with increasing temperature. In the aim of a maximal exploitation of the storage space, the temperature profile established in the adsorbent media during the filling procedure has to be taken into consideration. Furthermore, an efficient adsorption allows a reduced filling time as the same amount of gas can be stored in a shorter time period. Hence, the maximum amount of stored gas can be increased when the available filling time is limited. During filling the sorption store with gas two sources are relevant for a temperature increase in the vessel. These are the heat due to the compression of the gas and the heat liberated as a result of the exothermic adsorption. The generated heat directly depends on the amount of adsorbed gas. The more gas is adsorbed on the adsorbent medium, the more heat is liberated. And with increasing adsorbed amount of gas on the adsorbent medium, the adsorption rate, defined as amount of gas adsorbed per unit of time, is reduced.

Besides, desorption is an endothermic process and heat has to be supplied when gas is taken from the store. Heat management is therefore of great importance when sorption stores are used.

US 2005/0178463 discloses a method for quick filling a vehicle hydrogen storage vessel with hydrogen according to the conventional compressed natural gas (CNG) technique. The method and system disclosed compensate the temperature increase in the vessel during charging. The gas storage is conducted stepwise according to a particular algorithm.

US 2009/0261 107 describes a vehicle having a fuel cell system and/or an internal combustion engine and at least one gas tank to be filled with gaseous fuel. A comparatively high storage density is obtained with a gas tank which comprises metal-organic framework (MOF) as a stor- age material and which is embodied as a compressed gas tank for storing the gaseous fuel under pressure.

DE10 2009 000 508 describes a sorption store equipped with a cooling jacket. This approach addresses the temperature dependency of the filling and the discharging procedure in order to exploit the storage space to a maximum.

DE 10 2008 043 927 describe an apparatus for the storage of gas and a process for discharging a gas from a sorption store, wherein the gas is withdrawn at a constant temperature and afterwards compressed to a given working pressure. The process allows a complete emptying of the storage vessel without an energy consuming heating of the gas.

US 8,100,151 discloses a heat adsorbent medium which is provided in a predetermined length of a polygon or curvilinear shape and which allows an improved heat transfer from the adsorbent to the outside of the tank during the refill.

US 2010/0181212 describes a combination of metal-organic framework (MOF) material with open-pored polymer foams (poly-HIPE) which posses transport and storage pores and can be installed in tanks in the form of blocks or cylinders. The supported metal-organic framework (MOF) material enables a high storage capacity, an improved gas transport and, therefore, an accelerated filling and discharging of the tank.

US 2010/0133280 and US 7,641 ,715 disclose materials which comprise both, a solid adsorbent medium and a phase change component. The composite material is used for the adsorption and desorption of gas in pressure sorption stores. The composite material compensates the inhibiting effect due to heat liberation and heat consumption during gas adsorption and desorption, respectively. A disadvantage of known solutions for the heat management in sorption stores is that additional components are needed for the conduction and introduction of heat, and these create further costs and take up further construction space. In addition, the capacity of the store volume cannot be fully exploited. These disadvantages are particularly serious in mobile applications, for example in motor vehicles. Therefore, there is continuing interest in providing efficient heat management concepts for such storage systems.

It is an object of the invention to provide a sorption store 1 that compensates the limiting effect on the adsorption capacity due to an elevated temperature during filling the vessel with gas. The temperature increase is caused by the generated heat of adsorption and a limited heat conduc- tivity of the adsorbent media. The invention addresses the temperature profile established in the adsorbent media during and after filling, which is characterized by increasing temperature from the bottom to the top of the vessel. Hereby, the approach is of a simple construction and energy saving. A fast and efficient filling of the sorption store 1 is enabled. The object is achieved by the sorption store 1 , comprising at least two gas adsorbent media distinguishable from each other by different adsorption capacities for the stored gas at equal adsorption conditions and, which is in particular suitable for the use in a drive system of a vehicle. The invention further provides a drive system and a vehicle equipped with a sorption store 1 according to the invention. Apart from vehicles as cars and trucks, the sorption store 1 of the invention can also be used in other mobile applications, for instance in drive systems of boats. In addition, the sorption store 1 of the invention is suitable for stationary applications, for example in gas stations.

The sorption store 1 of the invention comprises at least two gas adsorbent media disposed within an at least one vessel, said at least two gas adsorbent media distinguishable from each other by different adsorption capacities, said adsorption capacities defined by the ratio of the mass of the adsorbed gas to the mass of the adsorbent medium, for the stored gas at equal adsorption conditions, said adsorption conditions defined by temperature, pressure and gas phase composition. The sorption store 1 combines at least two gas adsorbent media within at least one vessel, wherein the two gas adsorbent media show different adsorption behaviors with respect to the gas to be stored. The adsorption behaviors are characterized by the fact that the adsorption capacity of a first gas adsorbent medium is less reduced by elevated temperatures than the adsorption capacity of a second gas adsorbent medium.

Gas inside a storage vessel heats up during filling as heat is generated by compression and adsorption. Dependent on the flow conditions and heat transfer condition in the vessel and in the gas adsorbent media, the temperature distribution is not homogeneous. Due to gravity and a difference in density, the warmer gas fraction in a storage vessel tends to locate in a vertical direction closer to the top of the storage vessel compared to a cooler gas fraction. Therefore, a temperature profile is established with increasing temperature from the bottom to the top of the vessel.

Temperature in the vessel increases in vertical direction from the bottom to the top of the vessel during and / or after the filling with gas. In the context of the invention, a higher temperature in the upper part of the vessel is understood to be between 40°C to 60°C and the temperature in the lower part is understood to be between 20°C to 40°C. The temperature in the vessel during and directly after filling is higher than the temperature of the gas at the inlet of vessel, whereas the temperature in the lower part of vessel is rather close to the ambient temperature and the temperature difference between the upper and the lower part of the vessel can be approximately 15°C.

As a consequence the applied at least two gas adsorbent media are preferably not homogeneously mixed but rather spatially separated. The at least two gas adsorbent media are spatially separated from each other in a way that a first gas adsorbent medium, characterized by a greater adsorption capacity for the stored gas compared to a second gas adsorbent medium at a first temperature is located in the vessel in vertical direction closer to the top of the vessel than the second gas adsorbent medium, whereas the first temperature is the current average temperature of the first adsorbent medium 10 during and / or after the filling with gas. The elevated adsorption capacity at elevated temperatures allows the storage of a greater amount of gas in the same sorption store vessel and, therefore, a higher energy density in the sorption store 1 .

For the purposes of the invention, sorption stores are stores which comprise an adsorbent medium having a large surface area in order to adsorb gas and thereby store it. Sorption stores can store gas by means of adsorption and compression of gas. Thus, heat is liberated during filling of the sorption store 1 , while the desorption is activated by introduction of heat.

In an embodiment of the invention the stored gas contains hydrocarbons and / or water, and combinations thereof. The stored gas contains preferably gas selected from a group comprising of methane, ethane, butane, hydrogen, propane, propene, ethylene, water and / or methane, and combinations thereof, in particular natural gas. In particular preferred is stored gas which comprises methane as a main component.

Fuels can be stored in the sorption store 1 of the storage unit of the invention and be provided by desorption to an internal combustion engine or a fuel cell for example. Methane is particularly suitable as fuel for internal combustion engines. Fuel cells are preferably operated using methanol or hydrogen.

In an embodiment of the sorption store 1 of the invention, the at least two gas adsorbent media being spatially separated from each other by a separator 14 which is permeable for gas and not permeable for the at least two adsorbent media. The separator 14 can be made of various materials which are preferably inert with respect to the gas to be stored and with respect to the gas adsorbent media. The separator 14 can be made for example of the material the vessel is made of or of any metal, steel, fabric, fiber, plastic or composite material. The form of the separator 14 and its fixation in the vessel is not limited. It can be provided in form of grid or in form of a perforated plate for example. The purpose of the installation of a separator 14 is to prevent the different gas adsorbent media from mixing.

In a further embodiment, the total mass of adsorbent media disposed in the vessel comprises between 5% and 100%, preferably between 30% and 90% or more than 30% and in particular preferably between 50% and 80% by weight of the first adsorbent medium 10 and between 0% and 95%, preferably between 10% and 70% or less than 70% and in particular preferably between 50% and 20% by weight of the first adsorbent medium 12. The remaining volume in the vessel which is not filled with the first adsorbent medium 10 can be occupied by none, one, two and more other gas adsorbent media. The number and types of different gas adsorbent media is not limited. The gas adsorbent media are preferably asserted in the storage space according to their adsorption behavior. In a further embodiment of the sorption store 1 according to the invention the volume in the vessel, predetermined for the dis- position of the first adsorbent medium 12, is kept free of adsorbent media and used for gas compression without adsorption.

Various materials are suitable as adsorbent medium for the sorption store 1 . At least one gas adsorbent medium applied in a sorption store 1 according to the invention is a porous and/or microporous solid. The at least two gas adsorbent media are selected from a group comprising activated charcoals, zeolites, activated aluminia, silica gels, open-pore polymer foams and metal-organic frameworks, and combinations thereof. The adsorbent medium preferably comprises metal-organic frameworks (MOFs). Zeolites are crystalline aluminosilicates having a microporous framework structure made up of AI04- and Si04 tetrahedra. Here, the aluminum and silicon atoms are joined to one another via oxygen atoms. Possible zeolites are zeolite A, zeolite Y, zeolite L, zeolite X, mordenite, ZSM (Zeolites Socony Mobil) 5 or ZSM 1 1 . Suitable activated carbons are, in particular, those having a specific surface area above 500 m2 g-1 , preferably above 1500 m2 g-1 , very particularly pref- erably above 3000 m2 g-1. Such an activated carbon can be obtained, for example, under the name Energy to Carbon or MaxSorb.

Metal-organic frameworks (MOF) are known in the prior art and are described, for example, in US 5,648,508, EP-A-0 790 253, M. O'Keeffe et al., J. Sol. State Chem., 152 (2000), pages 3 to 20, H. Li et al., Nature 402, (1999), page 276, M. Eddaoudi et al., Topics in Catalysis 9, (1999), pages 105 to 1 1 1 , B. Chen et al., Science 291 , (2001 ), pages 1021 to 1023, DE-A-101 1 1 230, DE-A 10 2005 053430, WO-A 2007/054581 , WO-A 2005/049892 and WO-A 2007/023134. The metal-organic frameworks (MOF) mentioned in EP-A-2 230 288 A2 are particularly suitable for sorption stores. Preferred metal-organic frameworks (MOF) are M IL-53, Zn-tBu-isophthalic acid, AI-BDC, MOF 5, MOF-177, MOF-505, MOF-A520, H KUST-1 , IRMOF-8, IRMOF-1 1 , Cu-BTC, AI-NDC, AI-AminoBDC, Cu-BDC-TEDA, Zn-BDC-TEDA, AI-BTC, Cu-BTC, AI-NDC, Mg-NDC, Al-fumarate, Zn-2-methylimidazolate, Zn-2-aminoimidazolate, Cu-biphenyldicarboxylate-TEDA, MOF-74, Cu-BPP, Sc-terephthalate. Greater preference is given to MOF-177, MOF-A520, HKUST-1 , Sc-terephthalate, AI-BDC and AI-BTC.

Apart from the conventional method of preparing the MOFs, as described, for example, in US 5,648,508, these can also be prepared by an electrochemical route. In this regard, reference may be made to DE-A 103 55 087 and WO-A 2005/049892. The metal organic frameworks pre- pared in this way have particularly good properties in respect of the adsorption and desorption of chemical substances, in particular gases.

A particularly suitable material for the adsorption in the lower temperature range is the metal- organic framework material MOF Z377. Suitable for the adsorption at the higher temperature range is especially the metal-organic framework material MOF A520. The reduction of the adsorption capacity of MOF A520 due to an increase in temperature is smaller compared to MOF Z377.

The costs for the materials MOF A520 and MOF Z377 differ significantly. Depending on the raw materials, the embodiment and the purchased amount, currently the price for MOF Z377 is by a factor of 10 to 100 higher than the price for MOF A520. Therefore, it is a further advantage of the invention to combine different materials as adsorbent medium and to apply expensive adsorbent media only selectively in spatially limited zones, where the technical advantage of a higher adsorption capacity can be fully exploited.

MOF A 520 is based on aluminium fumarate. The specific surface area of a MOF A520, measured by porosimetry or nitrogen adsorption, is typically in the range from 800 m ^A 2/g to 2000 m ^A 2/g. The adsorption enthalpy of MOF A520 with regard to natural gas amounts to 17kJ/mol. Further information on this type of MOF may be found in "Metal-Organic Frameworks, Wiley- VCH Verlag, David Farrusseng, 201 1 ". The pellets have all a cylindrical shape with a length of 3 mm and diameter of 3 mm. Their permeability is preferably between 1 ·10 ^Λ -15 m ^A 2 and 3·10 ^Λ -3 m ^A 2. The porosity of the bed, which is defined as the ratio of the void volume between the pellets to the total volume of the vessel without considering the free volume within the pellets, is at least 0.2, for example 0.35.

MOF Z377, in literature also referred to as MOF type 177, is based on zinc-benzene- tribenzoate. The specific surface area of a MOF Z377, measured by porosimetry or nitrogen adsorption, is typically in the range from 2000 m ^A 2/g to 5000 m ^A 2/g. The MOF Z377 typically posses an adsorption enthalpy between 12 kJ/mol and 17 kJ/mol with respect to natural gas. MOF C300 is based on copper benzene-1 ,3,5-tricarboxylate and for example commercially available from Sigma Aldrich under the tradename Basolite® C300.

Generally, a variety of materials can be applied and be combined for gas adsorbent media, in- dependency of their characteristics regarding their impact on the gas flow in the vessel, their packing density and their heat capacity. The adsorbent media are preferably applied as pellets but can likewise be applied as powder, monolith or in any other form.

In an embodiment the porosity of the adsorbent medium is preferably at least 0.2. The porosity is defined here as the ratio of hollow space volume to total volume of any subvolume in the vessel of the sorption store 1. At a lower porosity, the pressure drop on flowing through the adsorbent medium increases, which has an adverse effect on the filling time.

In a preferred embodiment of the invention, the adsorbent medium is present as a bed of pellets and the ratio of the permeability of the pellets to the smallest pellet diameter is at least between 1*e ^A -1 1 m ^A 2/m and 1*e ^A -16 m ^A 2/m, preferably between 1 *e ^A -12 m ^A 2/m and 1 *e ^A -14 m ^A 2/m, and most preferably 1*e ^A -13 m ^A 2/m. The rate at which the gas penetrates into the pellets during filling depends on the rapidity with which the pressure in the interior of the pellets becomes the same as the ambient pressure. With decreasing permeability and increasing diameter of the pellets, the time for this pressure equalization and thus also the loading time of the pellets increases. This can have a limiting effect on the overall process of filling and discharging.

The sorption store 1 for storing the gaseous fuel can comprise a closed vessel. When gas is taken from the store, rapid and constant provision of gas has to be ensured. The sorption store 1 can be equipped with a feed device which comprises at least one passage through the vessel wall through which a gas can flow into the vessel. The feed device can comprise, for example, an inlet and an outlet which can each be closed by means of a shutoff device.

The feed device can comprise means to vary the gas stream for example throttle valves or con- trol valves, which can be located inside or outside of the vessel. The vessel can further comprise more than one passage through the vessel wall for example in order to lead the gas stream in optional subdepartments of the vessel or in order to provide separate passages for the filling and the discharge of the gas. Preferably, the same passage or the same passages are used for both, the discharge of the gas and the filling of the vessel.

Depending on the installation space available and the maximum permissible pressure in the vessel, different cross-sectional areas are suitable for the cylindrical vessel, for example circular, elliptical or rectangular. Irregularly shaped cross-sectional areas are also possible, e.g. when the vessel is to be fitted into a hollow space of a vehicle body. For high pressures above about 100 bar, circular and elliptical cross sections are particularly suitable. The vessel size vary according to the application. Diameters of the vessel of approximately 50 cm are typical for tanks in trucks and approximately 20 cm for tanks in cars, respectively. In cars fill volumes between 20 L and 40 L are provided whereas tanks of a volume between 500 L and 3000 L can be found in trucks. In a further embodiment, the at least one vessel is substantially mounted horizontally. The vessel can be characterized by an elongated form and it can be installed in a horizontal position. Besides a vessel substantially horizontally mounted vessel, a vertical installation is likewise fea- sible. In a further embodiment, the vessel of the sorption store 1 has a cylindrical shape and optionally a dividing element is arranged essentially coaxially to the cylinder axis.

The choice of the wall thickness of the vessel and of the dividing elements is dependent on the maximum pressure to be expected in the vessel, the dimensions of the vessel, in particular its diameter, and the properties of the material used. In the case of an alloy steel vessel having an external diameter of 10 cm and a maximum pressure of 100 bar, for example, the minimum wall thickness has been estimated as 2 mm (in accordance with DIN 17458). The gap width of the double walls is selected so that a sufficiently large volume flow of the refrigerant can flow through them. It is preferably from 2 mm to 10 mm, particularly preferably from 3 mm to 6 mm.

In an embodiment of the invention, the at least one vessel is a pressure vessel for the storage of gas at a pressure in the range up to 500 bar, preferably in a range of 1 bar to 400 bar, most preferably in a range of 1 bar to 250 bar and in particular preferably in a range of 1 bar to 100 bar.

The vessel is usually cooled during filling and/or heated during discharging. As a result, larger amounts of gas can be adsorbed or desorbed in the same time.

An improvement in heat transfer can be achieved when not only the vessel wall but also option- al at least one dividing element, or in the case of a plurality of dividing elements one or more thereof, are cooled or heated. For this purpose, the at least one dividing element or a plurality of dividing elements, in particular all dividing elements present, can be configured as double walls so that a refrigerant can flow through them. A configuration with double-walled channel walls has the advantage that for switching from cooling to heating, it is merely necessary for the coolant to be changed or its temperature to be altered appropriately. Thus, this embodiment is, in mobile use, equally suitable for filling with fuel and for the traveling mode. A pump can convey the refrigerant in the cooling circuit. A pumping power of the pump can be varied as a function of a fill level of the sorption store 1 .

Depending on the temperature range, which is appropriate for the cooling or heating of the gas in the sorption store 1 , different heat carrier media may apply, for example water, glycol, alcohols or mixtures thereof. Corresponding heat carrier media are known by a person skilled in the art.

The time required for filling is substantially influenced by the properties of the adsorbent medium, and particularly by its adsorption kinetic. An important influencing factor is the expected maximum temperature during filling, which likewise depends on the material properties, especially on the adsorption enthalpy. The choice of particular pressure levels during filling as well as the course of pressure increase is preferably adapted to the particular adsorption kinetic, adsorption enthalpy and the heat transfer to the walls of the vessel. In case of a fast heat transfer of the liberated adsorption heat, higher initial pressures are advantageous in order to minimize the total required filling time. The time required for filling can further be reduced by feeding a cooled gas.

Brief description of the drawings:

The invention is illustrated below with the aid of drawings. However, the examples described and the aspects emphasized therein merely illustrate the principles and do not constitute a restriction of the invention. Rather, many modifications of the type which a person skilled in the art would routinely make are possible.

The figures show:

Figure 1 A sorption store 1 according to the invention;

Figure 2 Temperature profile established in the sorption store 1 according to the invention; Figure 3 Isotherms of two different adsorbent media relating to the stored gas;

Figure 4 Isotherms of MOF A520 and MOF Z377 for the adsorption of natural gas;

Figure 5 A drive system for vehicles with sorption store 1 according to the invention;

Figure 6 A vehicle with a driving system with sorption store 1 according to the invention.

Embodiments: Figure 1 shows a sorption store 1 comprising a cylindric vessel 2 installed in horizontal position. The cylindric vessel 2 is equipped with a feeding pipe 4, a valve 6 and a double jacket 8. The upper part of the cylindric vessel 2 is filled with a first adsorbent 10, whereas the lower part of the cylindric vessel 2 is filled with a second adsorbent 12. The two adsorbent media are separated from each other by a perforated plate as a separator 14. In the embodiment of figure 1 , the sorption store 1 is filled with two types of metal-organic framework (MOF) material, whereby the adsorption capacity of the first adsorbent 10 is less reduced at higher temperature, established in the upper part of the cylindric vessel 2, compared to the second adsorbent 12. The sorption store 1 is filled with a fuel gas 16 which is partly adsorbed on the two adsorbent media having a large internal surface area. Heat is liberated as a result of adsorption when filling the sorption 1 store with the gas 16, consequently a temperature profile is established in the cylindric vessel 2 with increasing temperature from the bottom to the top of the cylindric vessel 2.

Figure 2 shows the temperature profile over the height of the cylindric vessel 2 during and / or after filling. The temperature 20 increases from a temperature 22 at the inlet of the cylindric vessel 2 with the height 24 of the cylindric vessel 2. The temperature increase can be linear, exponential or follow any other function in dependency of the applied adsorbent medium, the gas and the flow conditions in the cylindric vessel 2. The adsorbent medium present at the corresponding height of the cylindric vessel 2 changes at a height 26 from the second adsorbent medium 12 to the first adsorbent medium 10, whereas the first adsorbent medium 10 is located in the upper part of the cylindric vessel 2. This temperature profile has to be addressed in order to ensure an efficient adsorption in the space higher than height 26 of the cylindric vessel 2. The presence of the first adsorbent medium 10 allows an improved adsorption capacity and therefore storage capacity at higher temperatures, which are higher than a temperature 28, the tem- perature at the height 26.

Figure 3 illustrates the adsorption isotherms of the stored gas relating to the adsorbent media 10 and 12. The adsorption capacity 30, defined by the ratio of the mass of the adsorbed gas 16 to the mass of the adsorbent medium 10 or 12, is shown over the pressure 32. Isotherms are shown for both adsorbent media 10 and 12 for a temperature 34 T=20°C and the temperature 36 T=60°C, respectively. In this illustrative configuration, all adsorption isotherms show the characteristics of a Langmuir isotherm. Thus, the steep slope of the graph for small pressures decreases with increasing pressure 32 until a maximum adsorption capacity is reached. The isotherms for the higher temperature 36 T=60°C is characterized by overall smaller adsorption capacities compared to the isotherms for smaller temperatures 34, here T=20°C. In the embodiment of figure 3, the reduction of the adsorption capacity with higher temperature is more pronounced for the second adsorbent medium 12 compared to the first adsorbent medium 10. This effect is exploited in order to obtain a higher energy density in the adsorption store 1 , enabled by a higher amount of gas stored in the same volume of the cylindric vessel 2. For this purpose, adsorption store 1 according to the invention provides at least two gas adsorbent media distinguishable from each other by different adsorption capacities for the stored gas. In this illustrative example the adsorbent medium 10 is located in the upper part of the cylindric vessel 2, where elevated temperatures are predominant. Figure 4 illustrates the adsorption isotherms of MOF Z377 at 20°C 35, of MOF A520 at 20°C 37, of MOF Z377 at 80°C 38 and of MOF A520 at 80°C 39 for natural gas. MOF Z377 shows at 20°C adsorption capacities which are clearly superior to the adsorption capcities of MOF A520. At 80°C the adsorption isotherms of MOF Z377 and MOF A520 are very narrow and the adsorption capacities are similar. The reduction of the adsorption capacity of MOF A520 due to an in- crease in temperature is smaller compared to MOF Z377. Consequently, at lower temperatures approximating 20°C the application of MOF Z377 is advantageous in the aim of an effective adsorption of natural gas, which is not the case for elevated temperatures. At elevated temperatures approximating 80°C, there is no noticeable difference in the adsorption capacities of the two compared materials and the material which is less expensive can be selected without a technical disadvantage in this regard.

Figure 5 shows a drive system 40, for instance for a hybrid vehicle, having a storage unit 42 which comprises a battery 44, a fuel tank 46 configured as sorption store 1 according to the invention and optionally a further fuel tank 48. The drive system 40 of figure 5 is equipped with a motor unit 50 which comprises an internal combustion engine 52 and an electric motor 54. Such drive systems 40 are particularly suitable for hybrid vehicles in which both combustion energy and electric energy are utilized for power- ing the vehicle. Thus, the internal combustion engine 52 can supply energy to the drive axle 56 of the hybrid vehicle by combustion of a fuel from a fuel tank 46, 48 and/or the electric motor 54 can supply energy to the drive axle 56 of the hybrid vehicle by means of electric energy stored in a battery 44. Figure 6 shows a vehicle 60 comprising a drive system 40 which is equipped with a sorption store 1 according to the invention. In the embodiment of figure 5, the sorption store 1 is installed in the back of the vehicle in a horizontal position.

Overall, efficient adsorption and an improved energy density in the filled fuel store can be real- ized by means of the proposed sorption store 1 . In particular, limited space in a vehicle and, therefore, limited space in a vessel can be optimally exploited by combining at least two adsorbent media distinguishable from each other by different adsorption capacities for the stored gas. The amount of stored gas per volume can be increased without the requirement of further energy or construction efforts. The aim of a higher energy density in the storage vessel is obtained without complex control system. Therefore, the existing technique for filling and discharging on the vehicle and at the gas station can be used and remain unchanged.

A sorption store 1 according to the invention can be matched to the circumstances in mobile and stationary applications, for example integrated into hybrid vehicles or into combined heating and power stations.

Results of a simulation calculation which compares, by way of example, the application of only one adsorbent medium and the application of at least two spatially separated adsorbent media are presented below.

Comparative Example

A vessel which has a fill volume of 536 liters and is filled with pellets of the metal-organic framework (MOF) material A520 as adsorbent medium. The vessel is assumed as horizontally mounted sorption store. The vessel is filled with natural gas from 1 bar to 250 bar within 16 minutes. Due to adsorption, buoyancy and flow effects the upper part of the vessel is characterized by higher temperatures compared to the lower part of the vessel. After filling, the inlet valve is closed. The dominant flow fields and the temperature distribution in the vessel are calculated by Computational Fluid Dynamics (CFD). Cool gas from the inlet flows to the bottom of the ves- sel. Here, the adsorption generates heat, whereby the temperature of the gas increases and a gas flow of warmer gas ascends to the upper part of the vessel. The buoyancy effect lets ascend gas, which is warmer and therefore less dense, upwards. Consequently, the overall average temperature of 50°C is not homogeneously distributed, but the lower part of the vessel volume, representing one third of the total vessel volume, is characterized by an average tempera- ture of 10°C, whereas the remaining upper part of the vessel, representing two thirds of the total vessel volume, possesses an average temperature of 60°C.

By an above describe filling of the vessel with only MOF A520 as adsorbent medium, 76 kg of the of natural gas can be stored in a vessel with a volume of 536 liters.

Example

The filling procedure was effectuated according to the comparative examples using the same gas and tank. The only difference distinguishing the example from the comparative example is the selection of the adsorbent media. In the case of the example, the vessel is not filled with only one type of adsorbent media but with two types of adsorbent media. The upper third of the volume of the vessel is filled with the MOF material A520, already applied in the comparative example. The remaining lower part, representing two thirds of the total vessel volume, is filled with a MOF material Z377.

In this configuration, the temperature distribution is as described for the comparative example. When the adsorbent media A520 and Z377 are applied as two layers in the same vessel, the resulting mass of stored gas amounts to 34 kg in the lower third of the vessel volume, adsorbed on the surface and compressed in the cavities of the material A520, and to 60 kilogram in the remaining upper part of the vessel volume, adsorbed on the surface and compressed in the cavities of the material Z377. In total, a mass of 94 kg of the gas can be stored in a vessel of 536 liters by the combination of the adsorbent media. Consequently, an improvement of 24 % by weight of stored gas per volume was reached.

Overall, by the replacement of a part of one gas adsorbent medium by a second type of gas absorbent media an improvement of 24 % in energy density stored in a vessel can be obtained.

Further, by replacing only a selected part of the material MOF A520 by the cost intensive mate- rial Z377 in this example an economic benefit is achieved in comparison to the alternative of filling the sorption store completely with the material Z377.

Reference numerals

1 sorption store

2 cylindric vessel

4 feeding pipe

6 valve

8 double jacket

10 first adsorbent

12 second adsorbent

14 separator

16 gas

20 temperature

22 temperature at the inlet of the vessel

24 height of the vessel

26 height of the vessel between two adsorbent ι

28 temperature at the height 26

30 adsorption capacity

32 pressure

34 temperature T=20°C

35 isotherme of MOF Z377 at 20°C

36 temperature T=60°C

37 isotherme of MOF A520 at 20°C

38 isotherme of MOF Z377 at 80°C

39 isotherme of MOF A520 at 80°C

40 drive system

42 storage unit

44 battery

46 fuel tank

48 further fuel tank

50 motor unit

52 combustion engine

54 electric motor

56 drive axle