COMPRESSION OF A CRYOGENIC MEDIUM

Abstract

A method for compressing a cryogenic medium, in particular hydrogen, oxygen, nitrogen or argon is described. The cryogenic medium is compressed in two compressor stages from an initial pressure to a final pressure by way of an intermediate pressure, such that the first compressor stage is designed as a cryogenic compressor stage. The cryogenic medium is advantageously compressed to a pressure between 30 and 70 bar in the first compressor stage and in the second compressor stage is compressed to the desired final pressure by means of a hot gas compression.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from German Patent Application DE102012003446.6 filed Feb. 21, 2012.

BACKGROUND OF THE INVENTION

The invention relates to a method for compression of a cryogenic medium.

Generic methods for compressing cryogenic media, such as hydrogen, oxygen, nitrogen or argon, for example, are known from the prior art.

The term “cryogenic medium” is usually understood to refer to a liquefied cryogenic gas which is at a comparatively low temperature. For example, the temperature of cryogenic hydrogen is usually between −253° C. and −245° C.

Generic methods for compression of cryogenic media may be used in filling and refilling storage tanks. For example, various filling methods are used to fill hydrogen storage tanks installed in motor vehicles:

Pressure compensation methods: Several gas buffer storage devices having different pressure levels are filled from a supply system; this may be a stationary storage tank or a pipeline, by means of a compressor or a cryopump. In the case of refueling a motor vehicle, the compressed hydrogen is filled into the storage tank in the vehicle from the gas buffer storage devices by pressure compensation until reaching the final refueling pressure.

Booster Method: In this case, hydrogen from a supply system is compressed by means of a high-performance compressor and then transferred directly into the automotive storage tank.

Combination of pressure compensation and booster methods: Here there is first a partial filling of the vehicle storage tank that is to be filled from the gas buffer storage devices by means of pressure compensation before being filled to the final pressure by the booster method.

In addition, there are refueling methods in which the hydrogen is compressed by means of cryogenic compression to 700 bar with temperature compensation up to maximum 875 bar, such that the boil-off gas formed in cryogenic compression is sent to the automotive storage tank in a first refueling step. A cryogenic compression of liquid hydrogen to supercritical gaseous hydrogen takes place at entrance temperatures between −253° C. and −245° C. Compression of the boil-off gas formed in cryogenic compression requires so-called hot gas compressors, by means of which gas at ambient temperatures is compressed at ambient temperatures, which are understood to be in a temperature range between −20° C. and 40° C. However, such hot gas compressors are comparatively expensive.

Furthermore, this requires a compressor system of at least two to three stages because of the required compression ratio, but such a system would be unfavorable from an energy standpoint. The hydrogen must also be warmed by means of an ambient air evaporator before being fed into the hot gas compressor and therefore it loses the advantage of the high density as a cryogenic gas. So far there are not any cryogenic compression systems that could be used for compression of the boil-off gas to the required pressure level of 400 to 500 bar. However, pressures of 400 to 500 bar are necessary to be able to utilize the boil-off gas for refueling the vehicle according to the current refueling technique, in which the final refueling pressures are between 700 and 800 bar.

The object of the present invention is to provide a method for compression of a cryogenic medium, in particular hydrogen that will avoid the aforementioned disadvantages.

SUMMARY OF THE INVENTION

To solve this problem, a method for compressing a cryogenic medium is proposed, characterized in that the cryogenic medium is compressed in two compressor stages from an initial pressure to a final pressure by way of an intermediate pressure, and the first compressor stage is designed as a cryogenic compressor stage.

The term “cryogenic compressor stage” and/or “cryogenic compression” is to be understood below as a compression process in which a cryogenic liquid medium is converted into a compressed supercritical gas and in which the entrance temperature of the cryogenic medium is below −70° C. Cryogenic compression of liquid hydrogen or other liquefied cryogenic gases is usually performed by using cryogenic piston pumps. The cryogenic medium to be compressed here enters the piston as a liquid and is ejected as a supercritical gas.

According to the invention, the cryogenic medium is then compressed in two compressor stages from an initial pressure to a final pressure by way of an intermediate pressure, such that the first compressor stage is designed as a cryogenic compressor stage. In this process, the cryogenic medium having an initial pressure between 1 and 3 bar is preferably compressed to a pressure between 30 and 70 bar in the cryogenic compressor stage.

The method according to the present invention for compressing a cryogenic medium is advantageously implemented in a combined compressor in which the first cryogenic compressor stage compresses the cryogenic medium from the initial pressure to the desired intermediate pressure. This cryogenic compression makes use of the fact that the medium to be compressed is present with a high density and therefore the compression cylinder can be designed to be relatively small. As a result, the required driving force for the compression is low, so the result is an energetically favorable compression process.

In the first compressor stage the medium is brought to a higher temperature so that the medium in the second compressor stage can be compressed to the final pressure by means of a hot compression process. Because of the pre-compression in the cryogenic compressor stage, the compression space required for the second compressor stage can be designed to be relatively small.

It is proposed that to further refine the method according to the invention for compressing a cryogenic medium, the two compressor stages shall be implemented in piston compressors which are driven via a shared drive. For example, an electric motor with a double gear for operation of two compressor stages may be used as the shared drive.

In addition, according to an advantageous embodiment of the method according to the invention for compression of a cryogenic medium, one single-piston compressor is used for each compressor stage, such that the two single-piston compressors are advantageously operated in as a reciprocating compressor.

In the case of an optimum design of the compression ratio in the two single-piston compressors, i.e., compressor stages, these may be operated as a reciprocating compressor by means of a shared drive. In doing so, the piston here compresses the cryogenic medium first in the first compressor stage. At the same time the piston of the second compressor stage is in the return stroke, and in doing so, draws the pre-compressed medium out of the first compressor stage. When the second stage is compressed, the first compressor stage is in the return stroke and again draws in cryogenic medium.

BRIEF DESCRIPTION OF THE DRAWINGS

The figure is a schematic of a method for compressing a cryogenic medium according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The method according to the invention for compressing a cryogenic medium will be explained in greater detail below on the basis of the exemplary embodiment illustrated in the figure.

The pump arrangement shown in the figure consists of two single-piston compressors, each having a compression space V1 or V2, a piston K1 or K2 driven by a piston rod and a working space A1 or A2, which is required to drive the pistons. One intake valve a or c and one outlet valve b or d is assigned to each compression space V1 and V2.

The medium to be compressed is sent to the first compressor stage and/or to the first compression space V1 through line 1. This is a specially insulated, preferably vacuum-insulated line, which reduces the unwanted heat input to the medium to be compressed to a minimum. The medium to be compressed then flows into the compression space V1 with the intake valve a open. The inflowing medium usually is at a pressure between 1 and 3 bar. Cryogenic compression to a pressure between 30 and 70 bar takes place in the compression space V1. Next the compressed medium is conveyed into an equalizing tank 2 arranged between the two compression spaces V1 and V2 with the outlet valve b open. The medium flows out of the equalizing tank when intake valve c is opened there into the second compressor stage V2 in which compression to the desired final pressure takes place. With the outlet valve d open, the medium compressed to the final pressure is removed through a (high-pressure) line 3.

The figure shows the moment when the piston K1 in the compression space V1 is at top dead center and thus at the end of the intake stroke, while the piston K2 in the compression space V2 is at bottom dead center so the compression stroke is ended.

The drive for the two reciprocating pistons K1 and K2 is provided by means of a hydraulic pump P, which pumps the hydraulic fluid in the working spaces A1 and A2 through the lines 4 through 7, so that it results in an up-and-down movement of the pistons K1 and K2. Instead of the aforementioned hydraulic pump P, an electric motor with a reciprocating double gear may also be used, for example.

The method according to the invention for compression of a cryogenic medium has a number of advantages in comparison with the compression methods that are part of the prior art:

• • favorable compression energetically and in terms of plant technology because only comparatively small compression spaces and thus low drive powers are necessary,
  • no preheating of the cryogenic medium is necessary in contrast with the hot gas compression described above; furthermore there is no need for an intermediate cooler between the two compressor stages,
  • a high final pressure can be achieved at a nominal performance,
  • despite two different compression systems (“cryogenic” and “hot”), only one common drive system is required for optimal design,
  • with respect to design space, complexity and energy efficiency, the method according to the invention for compression of a cryogenic medium constitutes a significant improvement in comparison with the compression methods of the prior art.

Claims

1. A method for compressing a cryogenic medium, characterized in that the cryogenic medium is compressed in two compressor stages from an initial pressure to a final pressure by way of an intermediate pressure, and a first compressor stage is designed as a cryogenic compressor stage.

2. The method according to claim 1, characterized in that the cryogenic medium is compressed to a pressure between 30 and 70 bar in the first compressor stage.

3. The method according to claim 1, characterized in that the two compressor stages are implemented in piston compressors and are driven by means of a shared drive.

4. The method according to claim 3, wherein a single-piston compressor is used per compressor stage, characterized in that the two single-piston compressors are operated as a reciprocating compressor.

5. The method according to claim 1, characterized in that the cryogenic medium is selected from the group consisting of hydrogen, oxygen, nitrogen and argon.