Hydrogen is a propellant of the future in private transport. However, currently there are only a small number of vehicles using this energy carrier. BAM is working on the development of lighter tanks that may accelerate its breakthrough.
Electric cars can be operated not only with lithium batteries but also with hydrogen in an environmentally friendly and quiet way. Fuel cells can convert the chemical energy of gas into electricity. This process produces only water vapour and no nitrogen oxides or CO2. Another advantage of this fuel is that hydrogen can be highly compressed to save space: thus, one kilogram of gas contains almost three times as much energy as one kilogram of crude oil. Hydrogen-propelled vehicles currently on the market provide ranges of about 500 kilometres.
A pressure comparable to the bottom of the Mariana Trench
Hydrogen is a promising energy carrier for replacing combustion engines with more modern engines. Nevertheless, currently there are only about 600 hydrogen cars on Germany's roads. This is partly due to the still sparse network of filling stations. But it is growing steadily. There are more than 80 stations in Germany equipped with this technology – more than in any other country in Europe. Another obstacle is that the vehicles are still relatively expensive.
A major cost factor are the containers that hold hydrogen. Car manufacturers usually install two or three gas storage tanks with a total capacity of around 150 litres and locate them between the front and rear axles. These tanks must meet particularly high safety requirements because they are filled to a pressure of up to 875 bar. This corresponds roughly to the pressure prevailing at the bottom of the sea in the Mariana Trench in the Pacific Ocean which, at about 11,000 metres, is the deepest point in the world. In addition, hydrogen is flammable. “In the event of an accident it must be ensured that the gas storage tanks can withstand the mechanical forces impacting them and that no gas may be released – if possible,” says Georg Mair. Mair, who received a PhD in engineering, leads BAM’s research within the DELFIN project. Cooperation partners from the automobile and supplier industries, carbon fibre manufacturers, companies in the plastics processing industry and scientific institutions have joined forces under this name. “As part of a German government programme, we are jointly researching new hydrogen tanks that meet all mandatory safety requirements but are lighter and therefore more cost-effective to produce”, says Mair.
Together the partners want to develop a prototype gas storage tank in a multi-step process. Up to now, the tanks have been less than one metre long and consist of an inner plastic container, the liner, which must be gastight to store hydrogen. These are wrapped with kilometres of strands, which in turn are made up of thousands and thousands of carbon filaments that are only a few thousandths of a millimetre thick and that give the tank its strength. “The carbon fibres are a decisive factor in the manufacturing of the storage tanks,” explains Mair. “They account for about one third of the costs.” The new prototype should be a tenth of the production cost and save up to 15 percent in weight – without compromising safety.
Testing the limits of the material: different scenes
“First, we need to test the strength limits of the existing state of the art tank,” says Georg Mair. “This provides us with a reference for comparison with the new tank once it is built. We will test these improved storage tanks in a second step and then step three will be a full-size prototype.”
Gas storage tanks are subjected to large forces from the inside and outside. They have to withstand the enormous pressure of the compressed gas inside. BAM scientists carry out a series of special burst tests on several individual reference tanks: they pump a few additional litres of water into the already water filled tanks over several hours. In this way, they increase the internal pressure slowly but steadily and with high precision. “We inflate the tanks like a balloon – only with water. We do this until they burst,” says Mair. The gas storage tanks are enclosed in a protective test chamber during the test, which in turn is placed in a steel container so that there is no danger to the research team when the tank ruptures. Tests to date have shown that the reference tank can withstand an internal pressure of up to 2000 bar.
In addition, the scientists check the tank’s service life: using water, they increase the internal pressure of the storage tank to 875 bar every 15 seconds – and immediately relieve the water pressure again. They carry out 50,000 of these rapid changes within a week. In this way, they simulate the load during refuelling to which the material would be exposed to over many years. The test series provides additional information about the strength of the storage tank.
Every hydrogen tank must also prove that it can withstand external mechanical stresses, such as those that can occur in a rear-end collision. In these scenarios, too, no gas may escape from the storage tanks.
For this test, the scientists drop a steel hemisphere weighing up to 200 kilograms from a drop tower onto test specimens of empty, partially or fully filled reference tanks. They will then examine the plastic of the liner and the carbon fibre winding strands for fine cracks using computer tomography – revealing changes that would not be visible to the eye. They will then use slow burst tests to determine the residual strength of the damaged tanks. Only then it will be possible to assess how safe the test samples are under these extreme conditions and to assess whether a lighter tank would also pass the approval process.
Georg Mair is confident: “If one looks at the world-wide developments around hydrogen, this source of energy will power long-range cars and lorries, trains and ships in a CO2 neutral and low-noise way in a not too far future. We need light, inexpensive and safe storage cylinders that are produced in large quantities in order to be able to actively help shape these developments in Germany.”