Increasing hydrogen liquefaction in Europe
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An excellent energy carrier with respect to weight, hydrogen is widely considered to be the transport fuel of the future. One kilogramme of hydrogen contains 33.33 kWh of usable energy, compared to about 12 kWh/kg for petrol and diesel. In terms of volumetric energy density however, hydrogen is outperformed by liquid fuels. This poses a challenge when transporting hydrogen from where it is produced to the refuelling stations from which it will be distributed to consumers.
In the absence of a pipeline network, liquefaction may be the most economical way of distributing large quantities of hydrogen fuel. However, current H2 liquefaction technology lacks the necessary capacity and infrastructure to meet potential demand. Funded under the European Union's Seventh Framework Programme, the Integrated Design for Demonstration of Efficient Liquefaction of Hydrogen (IDEALHY) project, which ran from 2011 to 2013, aimed to enable the development of an economically viable hydrogen liquefaction capacity in Europe.
The IDEALHY project brought together world experts to design a generic hydrogen liquefaction process and plan for a large-scale demonstration of efficient hydrogen liquefaction in the range of up to 200 tonnes per day. This represents a substantial scale-up compared to existing and proposed plants worldwide. One drawback of hydrogen liquefaction is the amount of energy the process requires. This is partly due to the very low temperatures required to condense it into its liquid state (about -253°C). The IDEALHY project investigated the various steps in the liquefaction process in detail, using innovations and greater integration in an effort to roughly halve the specific power consumption for hydrogen liquefaction compared to the state of the art (i.e. reduce it to about 6 kWh/kg) while simultaneously reducing investment costs. Essentially, the project aimed to support a viable, economic liquefaction capacity in Europe, to increase economic transportation of hydrogen to refuelling stations and, in so doing, to accelerate investment in hydrogen infrastructure.
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The IDEALHY liquefaction process uses two successive Brayton cycles1 with a common compressor train. The refrigerant is a helium/neon mixture (‘nelium’) selected for optimum compressibility and refrigeration efficiency. The hydrogen is pre-cooled to 130K using a mixed refrigerant (MR), and this MR cycle provides the additional cooling needed for the two Brayton cycles. The current state of the art hydrogen liquefaction technology has a power consumption of 12 kWh/kg, which is equivalent to 36% of the useable energy contained in 1 kg of hydrogen. Based on technology analysis, conceptual work and process optimisation, the IDEALHY project has showed that 6.4 kWh/kg can be achieved, but there is potential to further reduce specific power consumption, pending tests of appropriate components, such as turbo machinery, and operation of a demonstration plant.
The task of the IDEALHY project was to identify the best process for the liquefaction of hydrogen and the components needed to build a high-efficiency large-scale plant. This plant should have a low investment cost, be safe and easy to operate and have a positive cash flow over its life cycle. The project benchmarked existing and proposed processes for hydrogen liquefaction at large scale (>50 tonnes per day) via detailed simulations. The most promising concept was developed further, during which the process was optimised to ensure the lowest possible energy consumption. Investment cost was a consideration, so the amount and complexity of equipment was kept to a minimum, without compromising efficiency. In parallel with this work, discussions were held with equipment manufacturers to ensure the availability of components. The researchers stressed that close cooperation with manufacturers was crucial if the right equipment is to be available for plant construction at a later date.
According to a report produced as part of the project, currently about one commercial hydrogen liquefier is built per year worldwide with a capacity of 5-10 tonnes per day (tpd). Plants with a capacity of up to about 20 tpd will probably be built in the foreseeable future, and will be based on the technology currently available. The lowest capacity for which the IDEALHY technology would be fully usable is about 40 tpd. There is currently no market for a liquid hydrogen production rate in this order of magnitude. Such a plant would however have a lifetime of at least 30 years, during which time the market size will grow. In the meantime, the part-load efficiency of the proposed IDEALHY process means that production will be profitable down to about 25% of the nominal capacity.
According to conclusions reached in the Assessment of Complete Plan report, the area which has seen least discussion in the project, and in which the most work remains to be done, is in the trade-off needed between capital expenditure, operating expenses and efficiency advantages. For future work leading to an actual plant design, it will be crucial to carry out detailed cost engineering on the proposed design and to ensure that the process ultimately selected is economically feasible. Plans are already underway to implement a hydrogen refuelling infrastructure in Europe in preparation of the commercialisation of fuel cell vehicles, and the research conducted as part of this project will make a significant contribution towards the roll-out of this infrastructure.
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1A thermodynamic cycle composed of two adiabatic and two isobaric changes in alternate order. Fuel and a compressor are used to heat and increase the pressure of a gas; the gas expands and spins the blades of a turbine, which, when connected to a generator, generates electricity.