Patent Application
20050284154
The invention relates generally to the production and storage of hydrogen and, more particularly, to a system and method for storing hydrogen at reduced temperature and increased pressure to improve the available storage energy density of the hydrogen.
Hydrogen is typically produced in bulk in steam methane reforming plants. The hydrogen storage energy density is then typically improved through compression or liquefaction. In a hydrogen liquefaction plant, feed gas is cooled and liquefied using multiple heat exchangers. Liquefaction plants are typically extremely large, with a correspondingly large investment with respect to fixed cost and operating cost. An important contributor to the large cost of operating a liquefaction plant is the large amount of electricity needed to liquefy hydrogen.
As interest in hydrogen as an alternative fuel to oil and natural gas has increased in recent years, hydrogen storage has been the subject of intensive research. Hydrogen is a promising alternative fuel because it creates less pollution than fossil fuels and can readily be produced from renewable energy resources, thus eliminating the net production of greenhouse gases. Because of its relatively high availability and low cost, hydrogen is a candidate for, among other things, alternative fuel vehicles.
Hydrogen contains more chemical energy per weight than any hydrocarbon fuel, but it is also the lightest existing substance. Hydrogen is, therefore, problematic to store effectively in small containers. One approach to storing hydrogen involves the use of metals and alloys, which are reacted with hydrogen to form metal hydrides. However, these storage methods have several disadvantages. For example, the use of metal hydrides adds undesirable weight to storage tanks. Other disadvantages may include undesirably large volume or weight, boiling loss or energy loss during charging and discharging with hydrogen.
Furthermore, if hydrogen is to be used as a fuel for transport, it must be stored in a cost-effective manner. The lack of a convenient and cost-effective hydrogen storage system makes it difficult to introduce hydrogen on a large scale for use in vehicles and other applications. Storage of 5 kg of gaseous hydrogen (equivalent in terms of energy to about 16 liters or about 4.2 gallons of gasoline) may be considered a minimum practical requirement for a general-purpose vehicle because that amount of fuel could provide an approximate 448 km (about 278 miles) range at a consumption rate of 28-km/liter. The external volume for a pressure vessel storing 5 kg of hydrogen at 13.8 megapascals (Mpa) or 2,000 pounds per square inch (PSI) and room temperature (20° C. or 68° F.) is at least 500 liters. This volume is too large to be practically used in many applications, such as light duty vehicles. If hydrides are used, the weight of the overall storage container is problematic. A hydride storage system may weigh 300 kg for 5 kg of hydrogen, resulting in a substantial reduction in vehicle fuel economy and performance.
The alternative of low-pressure liquid hydrogen storage provides the advantages of reduced weight and compactness. However, liquid hydrogen typically has high boiling losses in a non-insulated vessel. It is also relatively expensive to maintain liquid hydrogen at a sufficiently low temperature. Evaporative losses that occur during periods of inactivity because of environmental heat transfer add to system inefficiency.
The aforementioned problems with uninsulated containers for low temperature storage have led to experimentation with insulated storage containers. Insulated storage containers offer improved performance relative to uninsulated storage containers, but insulated containers are still not effective enough to be practical in widespread use. Low-pressure storage vessels tend to have high evaporation losses because of evaporation and leaks. Also, losses from a hydrogen vessel in a vehicle tend to grow rapidly as the daily driving distance drops. In addition to these disadvantages, low temperature storage typically requires a high-pressure pump for effective delivery of hydrogen to an engine. The high-pressure pump adds significant cost to the fuel delivery system.
Liquefaction of hydrogen is a very energy-intensive process. Even at a very large scale (several tons per day) using state-of-the-art technology, the liquefaction process consumes at least about 30% of the energy content of the hydrogen itself, as measured by the lower heating value (LHV).
There is a need, therefore, for an improved technique for storing hydrogen in an efficient and cost-effective manner. In particular, a need exists for a technique that can be employed to facilitate the efficient and cost-effective storage of hydrogen for use as a vehicle fuel.
In accordance with one aspect of the present technique, a system and method are illustrated for producing and storing hydrogen at low temperature and high pressure. Such a system may comprise at least a compressor for compressing hydrogen from an atmospheric pressure to a storage pressure, at least an intercooler coupled to the compressor for cooling the hydrogen, a cooling system for cooling the hydrogen at the storage pressure and a temperature equivalent to about liquid nitrogen temperature for storage, and a storage vessel to store the hydrogen for end application.
In accordance with another aspect of the present technique, a system for storing hydrogen in a motor vehicle at liquid nitrogen temperature is illustrated, wherein the hydrogen is utilized as a fuel for a vehicle engine.
In accordance with yet another aspect of the present technique a method is described for processing hydrogen at liquid nitrogen temperature.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
The present technique discloses a system and method for storing hydrogen at cryogenic temperatures.
The problems to be solved by the present technique are further explained with reference to the Figures, wherein like features are designated with like reference numerals.
Turning now to the drawings,
Heat generated during the compression of hydrogen in the compressor 12 is removed in the heat exchanger 16 and the cooled hydrogen 30 flows to the liquid nitrogen vessel 24. As a fundamental feature of storing compressed hydrogen, the amount of hydrogen stored correlates directly with the storage temperature. An effective way of enhancing the storage of hydrogen is to reduce the temperature of storage. Liquid nitrogen 32 is relatively inexpensive and widely available. Therefore it is a practical cooling media for cooling hydrogen gas. However, the coolant used in both the heat exchangers 16 and 20 could be air or any other cryogenic coolant wherein temperature of the cooling media can vary from about 77 degrees Kelvin to about 4 degrees Kelvin. In one such embodiment of the present technique, liquid nitrogen 32 is used as a cooling media for the heat exchangers 16 and 20 to cool the compressed hydrogen. The temperature of liquid nitrogen is at about 77 degrees K.
In the arrangement illustrated in the
In one embodiment of the present technique, a Linde cycle is utilized to cool the gaseous hydrogen to liquid nitrogen temperature. In another embodiment of the present technique, the cooling can be achieved using a mixed refrigerant cycle comprising a single compressor, at least two gaseous refrigerant mixed together, at least two valves, which may comprise Joule Thompson valves, and at least two heat exchangers to cool the hydrogen. In the Brayton cycle mentioned above, the cooling mechanism cools the hydrogen directly using the hydrogen, where the hydrogen is compressed beyond the delivery pressure, cooled with air or water, and the expanded to the delivery pressure and temperature. In the Claude cycle, the hydrogen is compressed in at least a compressor with at least one stage, cooled in an evaporative cooler and then passed through at least a heat exchanger wherein the coolant is liquid nitrogen and then finally expanded through an expansion valve. Hence through this process, the hydrogen is cooled to liquid nitrogen temperature. In yet another embodiment of the present technique, the cooling mechanism is adapted to cool the hydrogen using a magnetic refrigerator comprising a magnetocaloric regenerator and a superconducting magnet. Magnetic refrigeration is based on the magnetocaloric effect (MCE), an intrinsic property of all magnetic materials that peaks in the vicinity of the magnetic ordering temperature. The magnetocaloric effect depends on the way a material's atomic spins align themselves. All materials store heat in the form of atomic vibrations. An applied magnetic field forces the atoms into alignment, reducing the system's heat capacity and causing it to expel energy, which the water or any coolant carries away. When the field is removed, the atoms randomize again and can absorb energy from their surroundings, creating a cooling effect. In the case of a ferromagnetic material, it is the warming as the magnetic moments of the atoms are aligned on the application of a magnetic field, and the cooling when the magnetic moments become randomly oriented on removing the magnetic field. The warming and the cooling of a magnetic material in response to a changing magnetic field is similar to the warming and the cooling of a gaseous medium in response to compression and expansion. Therefore, magnetic refrigeration operates by magnetizing/demagnetizing the magnetic material.
Referring to
In practice, depending on the heat generated, multiple heat exchangers could be employed to cool the compressed hydrogen. The number of heat exchangers used for cooling the compressed nitrogen 22 in the compressor 20 may also vary depending on system design requirements. In an exemplary embodiment of the present technique, diaphragm compressors are employed to compress hydrogen. The basic design of a diaphragm compressor may help to provide leak resistant and non-contaminating gas compression. However, those of ordinary skill in the art will appreciate that other types of compressors may be employed in embodiments of the present technique. Additionally, the nitrogen gas compressor 18 need not be of the same type or size as the hydrogen compressor 12.
As described above, weight is an important consideration in the design of hydrogen storage containers, especially for vehicular applications. In the embodiment illustrated in
The storage vessel 42 may also include a thermal insulator 72 surrounding the inner vessel 44 in the evacuated space 66. The thermal insulator 72 serves to inhibit radiative heat transfer to the storage volume 64. One exemplary embodiment of the thermal insulator 72 comprises an external multilayer super insulation (MLSI) 74. The MLSI 74 reduces the heat radiation thereby reducing vapor losses, especially during cryogenic operation. The outer vessel 46 operates to keep a vacuum around the inner vessel 44, which is helpful for effective operation of the MLSI 74. The MLSI 74 exhibits good thermal performance when under a relatively high vacuum, for example, at a pressure lower than about 10−5 millibar (Mbar).
The hydrogen, which is cooled by the nitrogen storage vessel 24 (
In embodiments of the present technique, it has been observed that, at a storage temperature of about 77 degrees Kelvin, the equilibrium of hydrogen is about 60% ortho hydrogen, indicating that only a small amount of ortho hydrogen is converted to para hydrogen. Accordingly, less heat is released. Thus, storing hydrogen at about liquid nitrogen temperature removes or reduces the desirability of expending additional energy to facilitate the ortho-para conversion that is otherwise needed if a lower storage temperature is desired. Based on the energy density data illustrated in
A pressure of about 6,000 PSI at about liquid nitrogen temperature (point 110) has been found to be a good choice for balancing temperature and pressure considerations as it provides only about 20 Kg/M3 less storage density than liquid hydrogen at 2,000 PSI (see line 104). The point 110 indicates the estimation of the energy required to get the required state of hydrogen. In other words, point 110 indicates that in order to produce compressed, cryogenic hydrogen by the present technique, about 15.4% of the total energy content of the hydrogen, as measured by the lower heating value (LHV), is required at a pressure of 6,000 PSI and volumetric storage density of 60 Kg/M3. Similarly, as indicated in line 106, 13.9% of the LHV is required to store hydrogen at 2,000 PSI and volumetric storage density of 40 Kg/M3. Even though storage of hydrogen at about liquid nitrogen temperature and a pressure of about 6,000 PSI is desirable, pressures in the range of about 2,000 PSI to 10,000 PSI are believed to be acceptable for the present technique.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
