T. SCHAEFER, Linde Engineering, Pullach, Germany

Safe, simple and fast, hydrogen (H₂) filling stations must work in the same way as diesel filling stations to be a practical, carbon dioxide (CO₂)-free alternative to today’s global diesel standard. Advanced cryogenic H₂ pumps are now taking efficiency to a new level, and a completely new refueling process for liquid H₂ can already compete with the diesel standard.

To make the transportation of heavy goods CO₂-free, there is no way around H₂. The advantages are immense: on the one hand, the vehicles do not produce any local CO₂ emissions and, by using green H₂, the goal of zero CO₂ emissions can be achieved across the entire value chain, from the production of fuel to its consumption. Secondly, the refueling time of around 10 min–15 min for a truck is comparable to conventional diesel refueling. The range of vehicles is also comparable: more than 1,000 kilometers (km).

A pioneer in H₂ technology. One focus is on H₂ refueling stations (HRSs), including the necessary services. In recent years, the author’s company has implemented numerous projects in this area. These include, for example, the world's first refueling system for H₂ trains, the refueling infrastructure for the world's first H₂-powered ferry, the first refueling system for commercial vehicles in South Korea and the most powerful HRS for trucks in Europe. In short, the author’s company’s technologies have already carried out over 1-MM refueling operations.

In 2011, the company built the world’s largest liquid H₂-based filling stations for road vehicles in Berlin. Liquid hydrogen (LH₂) was refueled in compressed gaseous form with no intermediate vaporization. This increased the capacity of each of the two fuel pumps to up to 100 kilograms (kg)/hr of H₂. Now the same technology is powering the world’s largest HRS network based on LH₂ supply in California.

Truck manufacturers are now very interested in LH₂ because of its high energy density. The tanks in the vehicles can be smaller than those used in systems that fill up with H₂ in the gaseous form. The vehicle tanks are also lighter as a result. The refueling process itself is also completed faster, the resulting ranges are greater and energy efficiency at the refueling stations is reaching new groundbreaking dimensions.

The heart of LH₂ refueling. The key component of a refueling station providing LH₂ is the compression unit. The cryogenic pump converts the LH₂ into gaseous H₂ and compresses it from 2 bar to up to 900 bar. By using cryogenic liquid hydrogen, compression is possible with much less energy required by a gas compressor, which equates to 1.5 kWhr/kg. In addition, the technology used stands out for its dependability, and very high capacity of up to 200 kg/hr for the new versions. HRSs from the company are the first to be based on liquidly supplied cryogenic pump technology.

An energy-efficient cryogenic pump makes the difference. The cryogenic pump works with LH₂ at -253°C. Since H₂ cannot simply be sucked in at this temperature, the pump uses a two-chamber system that is completely immersed in the cryogenic liquid. By using cryogenic LH₂, only 10%−20% of the energy of a gas compressor is required for compression and there is no need for a cooling system in the thermal management process. Low maintenance requirements ensure additional cost savings.

A new liquid technology, subcooled LH₂. A new refueling process for LH₂, developed together with Daimler Truck, significantly increases the efficiency of H₂ technology in commercial vehicles once again. With the so-called subcooled LH₂ (sLH₂) technology, the H₂ is dispensed, also liquidly, into the vehicle and thus a higher storage density can be realized onboard, as well as greater range, faster refueling and much higher energy efficiency can be achieved.

One of the key effects is that the pressure and temperature are selected in such a way that the LH₂ does not contain any gaseous parts, and thus, there are no back gases during the refueling process. In conventional LH₂ systems, part of the H₂ evaporates when it is filled into the tank. This gas must be extracted through a second line at great expense, a process that also requires complex communication between the fuel pump and vehicle.

Both are no longer necessary with the new system, reducing the investment costs for the refueling unit and speeding up the refueling process even more. Another advantage of the new technology is that the energy required for the entire refueling process is reduced. With sLH₂, the total energy requirement is 0.05 kWhr/kg of H₂. Compared to the already rather energy-efficient systems, this corresponds to a reduction down to one-thirtieth. Another great advantage is the connecting power required for such a sLH₂ station that can deliver up to 500 kg/hr is only 50 kW.

The future of H₂ in heavy goods transport. Diesel may be the established standard in heavy goods transport, but H₂ is an alternative that cannot be ignored. With the new refueling systems, the infrastructure required for professional freight transport can be provided without significant compromises in terms of safety, simplicity and speed. This includes the nationwide supply of LH₂. The only thing this eliminates is local CO₂ emissions.