On-board hydrogen storage
By next year (2016), an interested environmentally conscious consumer will already be able to choose between three different models of fuel cell vehicles, as Hyundai, Toyota and Honda are bringing hydrogen powered cars to the market. Several other major automakers, including Mercedes, Nissan, Ford and BMW are also developing zero-emission fuel cell electric vehicles (FCEV). The biggest challenge faced by the vehicle manufacturers is to ensure a driving range able to meet customer expectations, with above 500 km driving autonomy and refuelling times and costs similar to those of conventional ICE cars.
Due to the size and weight constraints in vehicles and the physical properties of hydrogen, on-board hydrogen storage is particularly challenging. Although hydrogen has a high gravimetric energy density (120 MJ/kg), its volumetric energy density is the lowest of all common fuels. To increase its volumetric energy density, hydrogen is compressed and/or cooled down, both for transport and storage purposes. Another characteristic of hydrogen is its small molecular size, which allows permeation through most materials. Selection of the right material for the storage system is therefore also challenging and needs further research.
For fuel cell cars to achieve a driving range above 500 km, similar to today's passenger vehicles, around 5 kg of hydrogen should be stored on-board. As mentioned above, in order to store this amount of hydrogen in a reasonable volume, the density of the gas has to be increased. Currently, the most mature technology for storing hydrogen is in compressed form, within high-pressure steel cylinders at around 20 MPa and 50 L volume, but these tanks would be far too heavy and bulky for on-board storage.
The highest gravimetric energy storage densities can be achieved through liquefaction or cryo-compression of hydrogen; however these storage solutions have a high technical complexity and boil-off losses may be incurred. Therefore compressed hydrogen (CGH2) has become the industry standard for on-board storage. 70 MPa high pressure systems are the most common solution in light-duty vehicles, whereas 35 MPa systems are used for buses. Research is being carried out to optimize the on-board storage system, so that customers can be offered a compact, safe, reliable, inexpensive and energy efficient method of hydrogen storage.
The tanks for CGH2 have cylindrical shape and are made of carbon fibre reinforced epoxy resin (CFRE) with an internal liner made of steel or aluminium (type 3) or plastic (type 4). The type 4 vessels are cheaper and lighter than type 3 tanks, but they have relatively low thermal conductivity, which is an issue during fast refuelling (5 kg H2 in 3-5 minutes) since the heat released due to compression during refuelling increases the temperature of the gas inside the tank. The temperature should be kept below 85°C to avoid overheating the liner and the composite wrapping. Overheating can reduce the structural resistance of the tank and thus jeopardise the safety of the storage system. The FCH JU funded HyTransfer project aims to develop and experimentally validate a practical approach for optimizing means of temperature control during fast transfers of compressed hydrogen to keep the gas temperature below the specified temperature limit (gas or material) taking into account the container and system‘s thermal behavior.
Several projects of the FCH JU have the aim to reduce costs and ensure safety of composite tanks. The project HyComp conducted research on whether it is possible to reduce the safety factor of these vessels, which could help make this technology cheaper. In addition they proposed testing procedures adapted to specific features of composite materials, for type approval, manufacturing quality assurance and in-service inspection. Pre-normative research on resistance to mechanical impact of composite-overwrapped pressure vessels is being conducted by the consortium of the project HyPactor. The project aims at strengthening the knowledge on the influence of mechanical impact with respect to full composite (type-4) high pressure cylinders integrity. The COPERNIC project aims at increasing the maturity and competitiveness of CGH2 tank manufacturing processes evolving from classical automotive manufacturing technologies or concepts. It also targets costs while improving composite quality, manufacturing productivity and using optimized composite design, materials and components. The project Mathryce focusses on hydrogen compatibility of materials and the development of a methodology for the design and lifetime assessment of hydrogen high pressure metallic vessels that takes into account hydrogen-enhanced fatigue.
Liquid hydrogen storage
Hydrogen in liquid form has a much higher energy density than in its gaseous form at ambient temperatures, even when at high pressures. Liquid hydrogen can be stored at ambient pressures, but the process of liquefaction is energy intensive as it currently consumes more than 30% of the energy content of the liquefied gas. Further optimisation is however possible, as proven by the project IdealHy which developed a highly efficient liquefaction plant concept. In a liquefaction plant, hydrogen is compressed and cooled in a multi-step heat exchange process. Liquid hydrogen (LH2) is formed below 21.2K, therefore special cryogenic tanks are needed for storage in stationary tanks, for example at a refuelling station, or for distribution of liquid hydrogen in tankers. Liquid hydrogen storage is also implemented for industrial use, and maritime large scale transport of liquid hydrogen is being considered. BMW's prototype hydrogen 7 series vehicles were equipped with a specially designed tank for liquid hydrogen as well as regular gasoline, with a capacity of eight kilograms of hydrogen. Liquid on-board storage in passenger vehicles is no longer being developed by the automotive industry, mainly due to high venting losses associated with boil-off and limited availability of liquid hydrogen.
Cryo-compressed hydrogen storage
Higher density hydrogen storage at lower pressures can be achieved with cryogenic gaseous hydrogen. Thermally insulated pressure vessels with an operating pressure of up to 32 MPa have been developed for hydrogen. Because of the higher maximum pressure, venting losses are much lower than those encountered for liquid hydrogen storage. This storage technology has a high system complexity and additional monitoring of vacuum stability of the thermal insulation is needed to assure continuous performance. Tests are continuing to establish the technical feasibility and safety of this storage technology. Several projects have been supported in Germany by NOW for the validation of this technology.
Materials based hydrogen storage
Hydrogen can be stored in a variety of materials and liquids, based for example on metal hydrides, complex chemical compounds such as sodium boron hydride or liquid organic hydrogen carriers, such as N-ethylcarbozol. Daimler had been investigating another type of storage material, metal organic frameworks, which offer a large surface on which hydrogen can be adsorbed at low temperature. The advantage of chemical storage is that these systems operate at much lower pressures than conventional gaseous hydrogen storage, thereby improving the fundamental safety of the storage system. At present, no solid storage material fulfils the typical targets set for automotive applications. The main issues linked to solid state hydrogen storage solutions are a significant penalty in terms of gravimetric density, poor reversibility, costs, unfavourable kinetics and thermodynamic properties. Some classes of materials perform better than others in different fields, but no material is able to fulfil all the requirements for on-board storage. Reversibility in particular is the biggest discriminating factor in practical applications of solid state hydrogen storage materials. This field remains an active area of academic research. Whereas the goal of finding a solid state material suitable for on-board storage on a car seems elusive, other applications have been identified. The FCH JU has supported research into solid hydrogen storage (a magnesium amide/lithium hydride mixture) to power a 5 kW auxiliary power unit in the project SSH2S. There are few commercially available products utilizing solid state storage and these are commonly based on metal hydrides. Metal hydrides tend to have low gravimetric density, but this is not necessarily a drawback for some uses, such as supplying hydrogen to materials handling vehicles and stationary storage systems. A magnesium hydride (MgH2) based system has been deployed by the French company McPhy; higher efficiency is achieved due to the use of a phase-change material, which utilizes the heat released during hydrogenation. Small scale storage cartridges are also available for specific portable applications, for example for military use, which the FCH JU project Hyper is developing further. Storage in liquid organic compounds has also been proposed for the distribution of large quantities of hydrogen over long distances with active research being conducted in Germany and Japan by the Chiyoda Corporation.
The on-board storage of hydrogen as a compressed gas can be considered a safe and mature technology, but the biggest obstacle to fuel-cell vehicles deployment remains the lack of hydrogen filling stations. Automakers have signed a joint letter of understanding, addressed to the oil and energy industries, and government organisations, urging for the development of hydrogen infrastructure to allow for a market introduction. There are plans in place to open a signification number of refuelling stations in the next few years, and by 2020 it may well be possible to drive a FCEV all the way across Europe.