H2 bus refueling uncooled and yet safe - scope for warmer days and larger refueling volumes
The environmental crisis is becoming more and more noticeable and we therefore need to reduce our CO2 emissions. This makes the use of public transportation, such as buses, all the more important. If these are also powered by green hydrogen instead of diesel or gasoline, this is definitely a step in the right direction. However, if the refueling process is made more complex by an elaborate pre-cooling process, this not only increases the investment sum, but also leads to higher running costs, higher CO2 emissions and additional land consumption.
But what does this have to do with refueling a hydrogen bus? In order to increase the efficiency of the drive train compared to a hydrogen combustion engine, a hybrid hydrogen system is usually used. This means that the bus not only has a pressurized hydrogen tank, but also a battery on board to store energy. The vehicle is powered by an electric motor which, depending on its load profile, is either supplied solely by the converted energy from the hydrogen by means of a fuel cell or additionally from the battery.
There are different types and pressures of hydrogen pressure tanks. A Solaris bus was refueled during a hydrogen bus test run by an Austrian mobility provider. This bus uses a type IV storage tank with a nominal storage pressure of 350 bar and a maximum pressure of 438 bar. Until now, buses have usually used a type III accumulator, which consists of an aluminum housing encased in carbon fiber.
The big difference to a type IV accumulator is that the aluminum housing is no longer used and a plastic inliner is used instead. This tank is therefore more temperature-sensitive than a type III storage tank. The permissible operating temperature is between -40 °C and +85 °C. The tank system of the Solaris bus consists of several individual tanks and has a theoretical capacity of 1,560 liters. This corresponds to 37 kg of hydrogen at 350 bar and 15 °C.
The Solaris bus is also equipped with an infrared interface that supplies information about the actual tank pressure, tank temperature and SOC1) to the filling station.
No detailed "heavy-duty" refueling protocol available
For light-duty vehicles, specifications for the refueling process are described in SAE2) J2601. Part 2 of this standard deals with the process for heavy-duty vehicles, i.e. buses and trucks. However, it only lists the operating pressures and temperatures already mentioned as well as three refueling speed classes. However, there are no detailed tables to derive the pressure increase ramp required for standardized refuelling and the target final pressure, as is common practice worldwide for light-duty vehicles.
The pressure increase ramp of the refueled Solaris bus is shown in Figure 1 as a dark blue curve. This is the rising pressure at the fuel dispenser outlet and then the pressure increase in the vehicle, which is shown as a light blue curve. But why is the choice of pressure ramp important for refueling? Two different effects occur during the filling of the tank.
Firstly, hydrogen in the pressure range used has the effect of becoming warm when the pressure is reduced (expansion). This is called the reverse Joule-Thompson effect. Depending on the pressure difference and level, other gases usually become cold when they are depressurized. This means that when we let the hydrogen flow from the filling station system into the tank, the pressure is initially reduced and the hydrogen becomes warm. As the hydrogen flows into the tank until it reaches its final pressure, the hydrogen is compressed at the same time.
This releases heat as a result of compression. How warm the tank actually gets after refueling depends on the pressure increase per unit of time, the starting pressure of the vehicle tank, the tank size, the tank structure and the ambient temperature. This is not a problem as long as there is enough material to absorb heat and time for the tank system to release heat to the environment. Opinions are now divided as to the time and, in particular, the pressure ramp and media temperature at which hydrogen buses may be refueled. Since, as described in the previous paragraph, there are no detailed specifications, the final approval of the permissible refueling parameters is currently still up to the bus manufacturers. However, not all of them are in agreement.
Robert Adler, CEO of Maximator Advanced Technology GmbH, has developed his own refueling table with a similar structure to "light-duty" refueling, which specifies a pressure increase per unit of time and a target final pressure at different starting pressures and ambient temperatures. The aim of this table is to fill the vehicle tank safely, i.e. not to overheat despite the minimum refueling time. In contrast to the refueling tables developed by Maximator Advanced Technology GmbH, there are simulations that derive a need for pre-cooling the hydrogen, as the tank could exceed its maximum operating temperature of 85 °C if the filling speed or pressure increase is too high.
Based on these simulations, it was suggested that a future standard for heavy-duty vehicles should stipulate cooling above the maximum hydrogen temperature at the dispenser outlet. However, for a simulation of the refueling process, many parameters are assumed to the best of our knowledge and compromises are also made in relation to reality, which means that the result deviates from reality. Based on these theoretical considerations, a refrigeration system would be required to pre-cool the hydrogen to a specified temperature. As already mentioned, this means increased investment and operating costs as well as increased space requirements.
In order to prove that the refueling protocol developed by Robert Adler also works without cooling, refueling tests had to be carried out under real conditions.
Simulation versus reality
These were carried out in consultation with the bus manufacturer Solaris. They started with a very slow pressure increase ramp and gradually increased it until the pressure increase ramp corresponded to that of the developed refueling table. During refueling, the tank temperature and tank pressure were monitored via the infrared interface and also by a Solaris technician. The temperature in the tank system was measured for a total of five storage bottles with three temperature transmitters per bottle. The filling station used was supplied by a 200 bar trailer and consisted of a 1,000 bar compressor ("MAX-Compression") and a 1,000 bar high-pressure storage tank with three storage banks. Filling was carried out in cascades, first from the 200 bar trailer and then from the three high-pressure storage banks. Normally, a bus refueling system has lower storage pressures of around 500 bar. Due to the reverse Joule-Thompson effect, this results in a lower heat input than the 1,000 bar high-pressure accumulators used by Maximator. The increased accumulator pressure and the ambient temperature of 23.5 °C on the day of refueling resulted in the best conditions for a worst-case scenario test.
The refueling process can be seen in Figure 1. The pressure of the hydrogen tank at the start of refueling was 42 bar. The start pressure is detected at the beginning via a pressure surge, recognizable as the first pressure peak (dark blue line in Figure 1).
In addition, the tank pressure (light blue line) and the temperature (red line) are read in via the IR interface. The light green line shows the current mass flow rate during refueling, the orange line shows the upper temperature limit of the type IV tank. Pressure and flow peaks are always clearly visible in between. These are the points at which the tank was switched from one storage bank to the next higher one. It can also be seen that the outlet pressure of the dispenser is almost 100 bar higher than the actual tank pressure. This is referred to as dynamic pressure loss, which increases with the flow rate and drops to zero again at a lower flow rate towards the end of refueling. This is caused by the refueling equipment, but also by components in the tank system. The refueling process took a total of 10 minutes and 26 kg of hydrogen was refueled. The refueling was carried out with a pressure increase of 38 bar/min, whereby the final tank pressure was 330 bar.
No cooling necessary
This test refueling has shown that it is not necessary to cool the hydrogen before refueling. How can you tell? The maximum temperature measured during refueling was approximately 65 °C. This is 20 °C below the maximum operating temperature of the tank system of 85 °C. Now you could argue that the outside temperature on the day of the test refueling was only 23.5 °C. However, refueling is permitted up to 40°C. As already mentioned, there is still a delta of 20 °C up to the limit of the maximum operating temperature of the hydrogen tank.
This means that there is still enough leeway for warmer days and larger refueling quantities.
1) SOC = State of Charge 2) SAE = Society of Automotive Engineers
Technical publication: 09/2020
Author: Robert Adler, Sahra Gruber (Maximator Advanced Technology)