Munich, March 13, 2009
High efficiency gas seals for hydrogen powered vehicles
|View of the Magna Steyr plant
Today, as fossil fuels such as crude oil and natural gas are continuing to spiral upwards in price and as, according to the experts, these fuels will not be at our disposal for ever, the call for alternative renewable energy sources is becoming ever more audible in public.
As a result, alongside the synthetic production of SunDiesel from biomass by means of Fischer-Tropsch synthesis, hydrogen is favoured as the cleanest and most environmentally friendly energy source in the energy sector and automotive industry.
In research and development projects, world renowned innovative vehicle manufacturers and their partners as suppliers have already made big efforts to drive hydrogen as an energy source to the point where it is ripe for serial production for environmentally friendly drive systems.
The company BAUER KOMPRESSOREN GmbH, since over 50 years technology and market leader in the high-pressure sector for air and gases, and its subsidiary BAUER POSEIDON in Vienna, was awarded a contract in the second half of the year 2007 by the company MAGNA STEYR Fahrzeugtechnik AG & Co. KG relating to the delivery and installation of a helium high pressure compressor plant. The plant is intended to test prefabricated and piped H.P. vessels for leaks. They will later serve as fuel tanks for gas driven vehicles and will be filled with gaseous hydrogen when they are put into operation.
Who is MAGNA STEYR?
Magna Steyr is an independent company and is the world-wide leading development and manufacturing partner of the vehicle manufacturers. The spectrum of their services ranges from the development and production of components to the development and production of complete vehicles to the customer’s order.
Magna Steyr has a staff of about 10,000 world-wide, over 6,000 of whom are located at its headquarters in Graz. Magna Steyr is a subsidiary of Magna International Inc., the most diversified automotive supplier world-wide.
Hydrogen and its importance
Hydrogen is a colourless, odourless and tasteless gas only traces of which are to be found free in the atmosphere in an uncombined form. It can only be generated with the aid of energy. Moreover, it is combustible when used as filling gas. It is the lightest of all gases and it has the smallest molecular mass. Due to its small molecular mass, hydrogen has the property that it can diffuse through metals which places very high demands on the sealing of the gas pressure vessels and the operating systems.
World-wide about 520 bn Nm³ hydrogen is generated annually. 60 % of it is obtained from fossil fuels, 40 % is generated as a byproduct in the petrochemical industry and during chlor-alcali electrolysis.
As laws regarding CO2 emission are becoming stricter and simultaneously fossil energies becoming rarer, it has been realized during the search for alternative fuels that hydrogen can be viewed as one of the cleanest energy carriers, gaining in both medium and long term importance. As a secondary energy source, hydrogen can be pressurized and so be stored (gaseous up to 700 bar), transported (liquid at -253 °C) and used for generating electricity, heat and power in stationary and mobile applications.
The precondition is that, in the future, it will be obtained from diverse primary energy sources such as solar energy, biomass or water and that it can be integrated economically into the existing energy system.
Helium detects gas leaks
Helium - the lightest element after hydrogen - is chemically inert, has a high diffusibility (very small atomic diameter) and is therefore able to penetrate smallest openings and pores of metals and screw connections. This property is used in practice for checking for leaks on diverse systems where gas losses could occur.
As helium is a very expensive gas (1 l weighs about 130 g and costs about 10 €) and is found in nature only in small quantities (there are 4 relevant helium-rich natural gas sources with a helium content exceeding 2 % world-wide), considerable technical complexity is required to prevent gas leakages. This is especially important during the compression of this gas at higher pressures.
Proven system engineering
In order to keep the leakage rates as low as possible during the compression of helium, the company BAUER KOMPRESSOREN has been intensively engaged on the problems of this gas for many years. That is why all high pressure gas compressor plants of type Vertikus up to the large-scale plant have a closed gastight system and show only minimal leakage rates. Besides the required tightness, the helium compressors have to be better cooled due to the higher heat capacity compared to air, and also require higher quality individual components (e.g. valves).
High-pressure gas compressor station with components
|He high-pressure plant with gas processing system
The plant comprises 3 sectors:
- Helium feed via cylinder racks with pressure reduction 300/40 bar and low pressure storage system.
- High-pressure range with two pressure stages up to 875 bar, overflow lines for rapid and slow filling of test cylinders between 250 and 875 bar.
- Emptying of the test cylinders and helium recovery.
These sectors consist of several single components. They are nearly all installed in an air-conditioned compressor room with the exception of the heat exchanger for the cooling water, which is located on the roof of the compressor house.
The gas is fed via a helium rack (300/40 bar), further reduced in pressure and fed to the low pressure accumulator. As pressure fluctuations of several bars can occur in the low pressure accumulator, the booster has to be configured for a large intake pressure range.
The high pressure range consists of two independent compressors.
The first pressure level is generated by a high pressure gas booster, model GIB 22.11-30, from BAUER KOMPRESSOREN. The booster block is oil-lubricated, has water-cooled interstage and afterstage coolers and compresses via 3 stages to the first pressure level. The gas delivery is 1900-2100 l/min free He gas. The gas-condensate-mixture coming out of the oil and water separators is collected in a condensate collecting tank. The gaseous helium is then re-fed into the intake circuit, as both cylinders are linked to the gas phase.
The BAUER B-CONTROL, a stored program control, controls and monitors all functions of the compressor plant. In the case of any deviation from the normal operating conditions, the BAUER B-CONTROL automatically cuts off the compressor and shows the error source on the display.
The cooling water heated by the booster is recooled by a heat exchanger with frequency-controlled ventilator. The cooling circuit is a closed system with expansion tank for compensating volume fluctuations. An integrated efficient cooling water pump provides the water circulation – configured for winter operation.
Drying and oil separation is performed by the proven BAUER cartridge and filter system P 120 with activated carbon and molecular sieve filling. A downstream deep-bed filter from Zander is used for separating particles and remaining oil condensates. With the aid of a corresponding oil test indicator, the residual oil content of the helium can be checked if required.
The compressed and oil-free helium is stored in an intermediate buffer from which the 2. compressor draws and fills the test cylinders to different test overpressures (875 bar max.).
These pressures of over 300 bar are generated by a water-cooled, hydraulically driven Haskel booster with 22 kW drive power; gas delivery about 2000 Nl/min (depending on ratio of initial and final pressure).
The filling of the tanks up to 250 bar can be carried out slowly via two overflow lines (at low pressures and small containers) or, otherwise, more rapidly. The progress of the different sequences occurs via solenoid valves which are triggered electrically by overriding controls provided on site or manually by control valves.
By means of a buffer container of PN 1000, which is also fed by the Haskel booster, it is possible to fill even smaller container volumes by overflow.
After completion of the filling process and tightness test of the tank, it is necessary to be able to separate it from the filling line without any problems. This is effected via a pneumatically activated ball valve. The compressed gas is expanded into a low pressure tank.
After having found leaks and having recorded them in a coordinate system, the content of the tested containers is emptied. There are two separate, secured strings for low and high pressure tanks. The decompressed helium is then reduced via pressure reducers of different capacities up to 9 - 10 bar and fed to the low pressure container via a volume regulator for flow restriction.
The gas accumulated in the low pressure container has to be brought to a higher pressure level in the two low pressure containers. This is carried out by means of an electrically driven screw compressor which intakes the gas reduced to some mbar and compresses it to 15 bar.
All control and monitor fittings are clearly arranged and easy to operate. They are mounted on different panels and plumbed together.
All pipes – with the exception of the pipes for the water cooling system - are made in stainless steel.
Many years of experience and the detailed expert knowledge gained during handling helium in the high pressure sector are some of the crucial factors which enabled BAUER KOMPRESSOREN and its subsidiary BAUER POSEIDON to be awarded the contract for carrying out this comprehensive order. Since gastight compressor systems with guaranteed low leakage rates result primarily in lower loss rates arising from the compression of the expensive helium during the tightness test of the tanks. Furthermore, by means of a well developed and refined system, after the test, the helium must be re-fed without losses to the intake section. As a result of the higher tightness of the tank modules, it can be assumed that the hydrogen stored within them will diffuse considerably less when using this container system.
L. Kühlwein, graduate engineer