The Future of Hydrogen Refuelling: Cooling Systems and Design Innovations
Hydrogen presents a viable alternative fuel option for heavy-duty trucks, offering the potential to reduce greenhouse gas emissions and enhance energy efficiency. The RHeaDHy project seeks to demonstrate the effectiveness of a high-flow hydrogen refuelling station designed to accommodate heavy-duty vehicles, including buses and trucks. Its aim is to develop high flow refuelling components and refuelling line to allow refuelling of 700 bar H2 trucks at 100 kg within 10 minutes. However, the efficiency and safety of hydrogen refuelling stations depend heavily on the integration of advanced cooling systems. Here’s why a robust cooling system is critical and how innovative design solutions are addressing the challenges.
The Importance of Cooling Systems in Hydrogen Refuelling
Hydrogen, unlike many other gases, tends to warm up when expanded. Three primary thermodynamic phenomena contribute to a rapid increase in temperature. Firstly, a significant amount of heat is generated during the fast-filling process by converting the kinetic energy of the rapidly flowing hydrogen into internal energy. Secondly, the compression of hydrogen within the tank leads to an increase in temperature, which is the most critical factor contributing to the rise in temperature. Thirdly, the negative Joule-Thomson effect of hydrogen occurs when the flow through the throttle induces a sudden change in pressure, resulting in a temperature fluctuation. At room temperature, most gases experience a slight cooling effect during throttling. Conversely, hydrogen increases in temperature as it expands through the throttle due to its unique negative Joule-Thomson coefficient (the ratio of the temperature of the expanded gas to the pressure). There is a significant temperature difference as the temperature within tank rises.
This characteristic poses a challenge during the refuelling process, where hydrogen is transferred from a high-pressure storage tank to a lower-pressure vehicle tank, resulting in a significant temperature increase. Moreover, pressure drops and energy losses during refuelling generate additional heat. To maintain a rapid refuelling speed without exceeding the maximum allowable temperature of the vehicle tank, pre-cooling the hydrogen is essential. A high-capacity cooling system is thus required to prevent drastic reductions in refuelling speed.
Design Constraints and Solutions in the RHeaDHy Project
One of the main design constraints faced during the assembly of the RHeaDHy project was developing a technical solution capable of supplying up to 310 kW of cooling capacity at -30°C hydrogen target temperature without necessitating a massive chiller for this peak duty.
First, we need to understand what a chiller is. It is a machine that removes heat from a liquid coolant via vapor-compression, adsorption refrigeration, or absorption refrigeration cycles. This liquid can then be circulated through a heat exchanger to cool equipment or another process stream, such as air or process water. Chillers consist of four essential components (Figure 1): an evaporator, a compressor, a condenser, and an expansion unit. The process starts with a low-pressure refrigerant entering the evaporator, where it is heated and undergoes a phase change into a gas. The gaseous refrigerant then goes into the compressor, which increases its pressure.
Figure 1: How a chiller works? Diagram from Swegon.
Simulations based on HyFill
The innovative solution implemented within the RHeaDHy project is to design the cooling system on two chillers and on one pump module that circulates and distributes the liquid coolant. This module includes an integrated coolant buffer tank, serving as cold storage to cover peak loads during the approximately 10-minute refuelling process. This strategy effectively reduces the required permanent capacity of the chillers by 60%.
Cooling system within the high-flow H2 refuelling line
As any part of the high-flow hydrogen refuelling line, the cooling system must be completely aligned with the other components. So far, no significant limitations are expected nor observed in the system due to other components.
Figure 2: Drawing of the chiller developped by LAUDA for the RHeaDHy project.
The design of the cooling system was also meticulously aligned with the required flowrate and coolant inlet temperature of the Printed-Circuit Heat Exchanger (PCHE) to achieve the desired performance.
The cooling system’s noise level is under 85 dB(A), complying with noise protection requirements. Thus, noise is not considered a major issue for the RHeaDHy project. When placed in critical environments with stricter noise requirements, additional sound insulation measures can be met.
As we have seen, hydrogen temperature is closely linked with pressure variation. Hence, it is important to assure a seamless communication between the cooling system and the dispenser to reach an optimal temperature control on the hydrogen side.
Cooling Capacity: Orders of Magnitude
The cooling capacity indicates the amount of heat a system can remove from a refrigerated space over time. It is equivalent to the heat supplied to the evaporator/boiler part of the refrigeration cycle and may be called the « rate of refrigeration » or « refrigeration capacity ».
Products for cooling of hydrogen dispensers that are currently available on the market have a typical peak cooling capacity between 45 to 145 kW. The solution designed by LAUDA for our project is significantly more powerful as it can cover peak power of up to 310 kW at -38°C coolant outlet temperature.
Conclusion
The future of hydrogen refuelling systems looks promising with these advancements. By addressing cooling challenges with innovative design solutions, we move closer to efficient and rapid hydrogen refuelling, crucial for the broader adoption of hydrogen as a clean energy source.
Authors: Felix KORN (LAUDA) and Jean HERISSON (Benkei)