Patent Application
20140000288
The invention relates to an apparatus for storing hydrogen, comprising an inlet line for compressed hydrogen as well as an outlet line for hydrogen at a lower pressure, and for storing magnetic energy.
The increasing share of fluctuating regenerative energy sources used to supply electrical power requires additional measures for balancing feed-in fluctuations. Liquefied hydrogen which has a high volumetric energy density is suitable for this as energy store and as fuel. Compared to the compressing of hydrogen, however, a higher amount of electrical energy is required for liquefying hydrogen which typically amounts to approximately 30% of the upper heating value of hydrogen as compared to approximately 10% for the compressing to approximately 15 MPa to 20 MPa.
According to H. Quack in “DIE SCHLÜSSELROLLE DER KRYOTECHNIK IN DER WASSERSTOFF-ENERGIEWIRTSCHAFT” [the Key Role of Cryogenics in the Hydrogen Energy Management], T U Dresden, http://images, energie-portal 124.de/dateien/downloads/verfluessigung-dresden.pdf, industrial-size hydrogen liquefying apparatuses normally operate with several continuous gas flows, with heat exchangers (recuperators) through which gas flows simultaneously in counter-flow directions, with several gas return lines, with expansion machines for the refrigeration at intermediary temperatures, and with separate loops for the ortho-para conversion required with the hydrogen. The hydrogen which is at last liquefied in a relief valve can then be removed and transported to the location of application. A large share of the electrical energy needed for the liquefying is not used during the warming up at the location of application.
Processes where gas flows alternately from the cold and the warm side through a regenerator or regenerative heat exchanger are known from small cooling devices; see also R. Radebaugh, Cryocoolers: the state of the art and recent developments, J. Phys.: Condens. Matter 21 (2009) 164219. For the Stirling process, a stationary reactor and, in the cold section, a mechanically operated expansion piston is used. With the Gifford-McMahon process, a mechanically operated regenerator is used as displacement piston. Pulse tube coolers also require the gas to pass through cycle processes, wherein the mechanical piston or displacer in that case is replaced by a pulsing gas column. These methods have in common that they are restricted to maximum structural dimensions that extend to the kW range, that they operate at frequencies of 50 Hz to 1 Hz, and that they are primarily used for the refrigeration, but not for the liquefying of gas.
In the case of hydrogen, it is necessary to take into consideration the two spin states of the hydrogen molecule which differ energetically by a conversion heat at the level of approximately 1.5 times the liquefying enthalphy. In practical operations, the hydrogen must therefore be as close as possible to the ortho-para balance during each processing step that is connected to cooling, storing and reheating. To meet this requirement, catalysts are normally used for the ortho-para conversion during the cooling process, as disclosed in R. Gross, W. Otto, A Patzelt, M. Wanner, “FLÜSSIGWASSERSTOFF FÜR EUROPA—DIE LINDE ANLAGE IN INGOLSTADT” [Liquid Hydrogen for Europe—the Linde Plant at Ingolstadt], Linde reports from the magazine TECHNIK UND WISSENSCHAFT [Technology and Science] 71/1994, pages 36-42. However, this requires gas return lines and the availability of additional cooling or refrigeration capacity are needed at intermediate temperatures.
The DE 10 2007 042 711 A1 discloses the use of liquefied hydrogen as energy store for longer storage periods in combination with a superconducting magnetic energy store (SMES), used therein for the short-term storage.
The DE 38 43 065 A1 discloses the use of a magnetic coil for a magneto-caloric liquefying stage. The magnetic field in this case functions to successively magnetize and demagnetize paramagnetic and ferromagnetic materials in a cyclical process during which the magneto-caloric effect generates cold which can be used to liquefy gases such as hydrogen.
The DE 196 00 936 A1 discloses a membrane fuel cell, surrounded by a HTSL coil/hollow-cylinder storage device, as well as a semiconductor HTSL photo-voltaic cylinder. Gaseous hydrogen and oxygen, separated by catalyst and a membrane, flow from both sides into a porous distributor. During the hydrogen/oxygen reaction, an electric current is generated in the current collector for which the voltage can be tapped at the poles, as well as water which escapes at the outlet. The HTSL coil that surround the membrane fuel cell and the HTSL hollow cylinder are cooled with liquid hydrogen and function as additional second/minute storage devices. The p/n doped semiconductor HTSL photo-voltaic cylinder generates additional solar current. The hydrogen which evaporates as a result of sun irradiation is liquefied once more with the aid of the heat exchanger and the magneto-caloric HTSL regeneration stages and is returned to the HTSL cooling system in the liquid state or is returned in the gaseous state to the fuel cell. During the reversal of the process of the fuel cell reaction, meaning the electrolysis, the same membrane arrangement functions to generate hydrogen from water, which is then liquefied magneto-caloric as cooling agent HTSL. A share of the electrical energy for the electrolysis can be generated by the p/n semiconductor HTSL photo-voltaic cylinder.
The DE 101 06 483 A1 discloses circumventing at least one catalyst for the ortho-para conversion during the liquefying process in order to avoid the refrigeration capacity. This method is advantageous in cases where only short storage times are required for the liquefied hydrogen, in particular when it is used in a hydrogen-operated vehicle shortly after being generated.
Starting with this premise, it is the object of the present invention to provide an apparatus that is reliable and is configured with the fewest possible components for the intermediate storage of hydrogen and magnetic energy, as well as a method for operating said apparatus, which overcome the disadvantages and restrictions of the prior art.
In particular, an apparatus is to be made available which balances the known efficiency advantage of compressed hydrogen and, together with the additional function of the electrical energy storage in the superconducting magnetic energy store (SMES), provides an energetically advantageous type of storage for hydrogen in the liquefied form.
This object is solved regarding the apparatus with the features disclosed in claim 1 and regarding the method for operating said apparatus with the method steps disclosed in claim 5. The dependent claims respectively describe advantageous embodiments of the invention.
An apparatus according to the invention comprises in particular:
On the one hand, the apparatus according to the invention is based on the use of regenerators, meaning thermal mass storage devices, instead of recuperators which are counter-current heat exchangers while, on the other hand, the magnetic field of the at least one magnetic coil of the superconducting magnetic store (SMES) makes it possible to completely omit catalysts for the ortho-para conversion as well as for the para-ortho reconversion. Apart from simple shut-off valves, moving masses and gas return lines can thus be omitted in the cold region.
The at least one regenerator which, according to one favorable embodiment, is located at least partially in a chamber under vacuum conditions, is configured with one or several storage media that absorb heat or release heat. Since the regenerator is operated in a strong magnetic field that changes and since the heat inputs and the cooling losses resulting therefrom are to be kept low, mostly non-magnetic and electrically insulating storage media with simultaneously high heat capacity are especially suited for this.
The at least one regenerator is typically operated between the ambient temperature and a temperature which can reach no more than 35K. For that reason, a subsequent expansion of the hydrogen, which is compressed to a sufficiently high pressure, to a pressure of approximately 0.1 MPa already makes possible a far-reaching liquefying. A direct liquefying at approximately 0.1 MPa, on the other hand, would require that the complete liquefying enthalpy (evaporation heat) would have to be extracted precisely at the boiling point for hydrogen, which is not only hard to realize but would also be quite inefficient. It is therefore advantageous if the hydrogen is initially compressed to supercritical pressures at the ambient temperature and is then liquefied with the aid of a relief valve.
The liquefying and cooling enthalpy of the hydrogen is extracted during the heating phase, following the storage phase, from the at least one regenerator which can later remove these enthalpies over a broad temperature range from the compressed hydrogen during the renewed cooling.
According to a special embodiment, the at least one regenerator is spatially arranged such that it is positioned above the storage tank for the liquefied hydrogen, wherein respectively the cold end of the at least one regenerator faces the liquid surface of the liquefied hydrogen bath (LH2 bath) and, in this way, facilitates a release of the liquefied hydrogen into the storage tank.
To thermally stabilize the at least one regenerator at its cold end, one preferred embodiment provides for a thermal contact between the LH2 bath and the cold end of the at least one regenerator, by means of which a portion of the at least one regenerator can also be held in place mechanically.
To reduce in particular the heat input into the storage tank for liquefied hydrogen (LH2), the tank according to one especially preferred embodiment is mostly surrounded by a so-called radiation shield that is cooled with liquid nitrogen (LN2). At the same time, this bath of liquid nitrogen (LN2 bath) also serves to provide a better thermal stabilization of the at least one regenerator during changing operating conditions, wherein this thermal contact to the LN2 bath can furthermore also be used for mechanically holding in place at least parts of the at least one regenerator. According to an alternative embodiment, liquid oxygen LO2 is used in place of the liquid nitrogen LN2.
A method according to the invention comprises the steps a) to d).
According to step a), the operating current for a superconducting magnetic energy store (SMES) that is configured with at least one magnetic coil is adjusted via current supply lines. As a result, electrical energy is supplied to, is stored in, or is drawn from the connected magnetic coil. With this step already, energy can thus be supplied for a short time.
To supply energy over a longer period of time, the stored hydrogen is additionally used, which is preferably generated in an electrolyser and is once more converted to electrical current in a fuel cell.
It is essential for the operating mode according to the invention that the magnetic field intensity at all locations of the at least one regenerator, of the (Joule Thomson) relief valve, and of the storage tank for the liquefied hydrogen does not drop below a fixed lower limit. Only in this way is it ensured that the magnetic field supports the ortho-para conversion and/or the para-ortho reconversion of the hydrogen in such a way that for each temperature a far-reaching thermal balance adjusts between the two spin states.
According to step b), compressed hydrogen is supplied at the ambient temperature via the warm end of the at least one regenerator, is cooled in the at least one regenerator through releasing heat to the at least one storage medium therein, is then allowed to expand to a lower pressure in the (Joule Thomson) relief valve and, in the process, is mostly liquefied, and is finally in step c) caught and stored in the storage tank in the form of liquefied hydrogen as LH2 bath.
According to step d), the liquefied hydrogen (LH2) is removed as needed from the LH2 bath in the storage tank and is supplied to the cold end of the at least one regenerator, is warmed up therein through heat absorbed from the at least one storage medium, and is discharged via the warm end of the at least one regenerator in the form of gaseous hydrogen (GH2).
To ensure a reliable operation, the at least one regenerator is furthermore stabilized thermally through contact with cold baths.
For this, the cold end of the at least one regenerator is stabilized to the temperature level of the LH2 bath according to a first embodiment.
According to a different embodiment, at least a portion of the at least one regenerator is stabilized to the temperature level of a LN2 bath or a LO2 bath.
In the case where oxygen is to be used for the thermal stabilization of the at least one regenerator, the oxygen is generated together with the hydrogen in a high-pressure electrolyser, is then initially also cooled through heat transfer, the pressure is allowed to expand and, in the process, the oxygen cooled down further and/or is partially liquefied. The use of a high-pressure electrolyser reduces the electrical energy that is otherwise required for the compression.
The proposed combination of liquid hydrogen and superconducting magnetic storage device (SMES) makes available control power for the electrical mains along time scales ranging from seconds, to hours, and days, thereby contributing to buffering the increased share in the power supply of fluctuating wind power and sun power. In particular, this approach supports the incentive to use liquefied hydrogen instead of compressed hydrogen.
The invention is explained in further detail in the following with the aid of exemplary embodiments and the Figures.
The Figures show in detail:
An alternative embodiment uses an expansion machine, a turbine in this case, in place of a Joule-Thomson relief valve 40. A cold LH2 pump is additionally installed at the outlet for discharging the LH2 from the storage tank. However, as a result of the masses rotating in the strong magnetic field, this arrangement has the disadvantage of inducing eddy currents in the cold region which lead to additional heat input and thus a cooling loss.
The superconducting, magnetic energy store 10 is operated at the temperature of the liquefied hydrogen LH2. The cryogenic tank 15 of the energy store 10 represents a section of the LH2 storage tank 20 that is open toward the top, is supplied via the Joule Thomson relief valve 40 with liquefied hydrogen LH2, and is connected by means of a LH2 overflow line 29 to the main section of the storage tank 20. This configuration ensures that the energy store 10 is still cooled completely, even with a partial emptying of the LH2 tank 20 and the resulting lowering of the liquid level 22.
The exemplary embodiment shown in
The principle of the method according to the invention is illustrated in
The operating mode according to the invention is introduced in
Particularly advantageous embodiments are obtained when selecting the following parameters:
The superconducting magnetic energy store 10 is operated locally with magnetic fields in the range of 1-30 T, preferably of 4-11 T, while the operating current is changed in the range of 30% to 100%, wherein the magnetic field intensity at all locations of the at least one regenerator, the (Joule Thomson) relief valve, and the storage tank for the liquefied hydrogen preferably does not fall below a minimum limit of 0.1 T, especially preferred not below 1 T.
Superconducting materials which can be used for generating magnetic fields at the temperature of the liquefied hydrogen include, in particular, BiSrCaCuO, YBaCuO or MgB2. A direct synergy between the storage of LH2 and the operation of the superconducting magnetic energy store 10 thus results if, as shown in
According to
The storage media of the at least one regenerator 30 advantageously have a magnetic susceptibility with an absolute value below 10−5, an electrical conductivity of less than 10−8 S/m and a volume-referenced specific thermal capacity above 20 MJ/M3·K. These properties can be found in particular in Teflon or other materials on the basis of solid hydrocarbons.
The compressed hydrogen, which is supplied via the warm end of the at least one regenerator 30, is preferably compressed to a pressure above 1.3 MPa and, following the passage through the at least one regenerator 30, is then expanded in the (Joule Thomson) relief valve 40 to an expansion pressure below 0.1 MPa. Since the pressure levels depend on the hydrogen pressure desired for the discharge, the pressure in the storage tank 20 is adjusted via the pressure control, such that it exceeds the expansion pressure during the storage phase by at most 0.2 MPa. The temperature of the liquefied hydrogen is also adjusted via the pressure control and the cooling device 21 attached to the storage tank 20, such that it is maintained in the range of 18-35K. When exiting the at least one regenerator 30 following the storage phase, the hydrogen has a temperature of no more than 30K below the ambient temperature.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2011 013 577.4 | Mar 2011 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP12/00985 | 3/6/2012 | WO | 00 | 9/10/2013 |
