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Subcooled liquid hydrogen (sLH2) technology: A CO2-free alternative to diesel

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.