Compressors
Introduction
In order to minimise the size of hydrogen storage required a compressor is needed between the electrolyser and storage stages and sometimes before the electrolyser as well. In our case the electrolyser works at atmospheric conditions therefore only one compressor exists between the electrolyser and storage tank.
Hydrogen’s extremely low density results in high pressures, in the region of hundreds of bars, required to keep the volume of storage at a feasible magnitude. Multiple stages of compression are required with staged intercooling to achieve compression from atmospheric pressure (1.01325 bar) to high pressures (350-700 bar). The only compressors that fit this criteria and are suitable for stationary applications are diaphragm and piston compressors. Of the two a piston compressor has been chosen for the hydrogen buffering system in this project. The technical advantages of this compressor over the diaphragm compressor and general technical justification are detailed below.
Compressor Types
The types of compressors available for stationary hydrogen compression are limited to a select few. The two we are concerned with for this project are piston and diaphragm compressors.
Piston Compressor
A piston compressor relies upon the reciprocating action of a hydraulic piston into a cylinder, which houses the gas, to compress the gas in question. The gas is delivered and released by means of valves that often operate via pressure differences. If the gas is to be compressed to high pressures (>200 bar) then it is necessary to use staged compression with intercooling between stages because of the high temperatures reached during compression and the reduction in efficiency brought about by single stage compression to high pressures [1].
Advantages
• High volumetric capacity
• Easy and cheap to maintain
Drawbacks
• Requires oil lubrication for higher pressure ratios
• Leakages are common
Diaphragm Compressor
This type of compressor is ideally suited to compression of toxic or potentially volatile gases such as hydrogen because it is hermetically sealed towards the outside resulting in very low leakage rates in the magnitude of 10-4 mbar/sec. The gas is compressed through the use of an oscillating sandwich diaphragm, which is set in motion by the hydraulic pressure created by a piston. Diaphragm compressors can operate between 250 and 720 rpm depending on the size. Gas is fed in through the suction valve at a suction pressure and is compressed in the gas space and into the discharge valve. It is possible, as with most compressors, to have multiple stages of compression and with a diaphragm compressor stage pressure ratios as high as 1:20 are possible. This allows for a final pressure up to around 3000 bar, limited by allowable stage outlet temperatures.
Advantages
• High efficiencies in the region of 80-85%
• High stage pressure ratio allowing for very high final pressures (up to 3000 bar)
• Hermetically sealed resulting in exceptionally low leakage rates.
• Lubricant free compression chambers eradicating need for lubricant extraction.
• Long diaphragm lifetime of 3000 to 5000 operating hours
Drawbacks
• Low suction capacity (<1000m3/hr)
• Lateral forces caused by the crank drive reduce the lifetime of the sealing components.
References:
[1] www.andreas-hofer.de/E/downloads/piston_compressor-oil-lubricated.pdf
[2] www.andreas-hofer.de/E/downloads/diaphragm-compressors.pdf
