A model-based study on metal hydride compressor systems and applications for hydrogen refueling stations

In times of noticeable effects of climate change, the energy transition towards environmentally friendly energy sources is becoming increasingly important. The use of hydrogen is one way of realizing such an energy transition. If renewable energies are used for the electrolysis of water, hydrogen can be produced in an environmentally friendly way. The transportation sector in particular, with its high proportion of climate-damaging emissions, offers potential applications. Hydrogen can be used in fuel cells to drive electric motors without producing pollutants. In contrast to battery electric vehicles, the high energy density of hydrogen allows long distances to be covered without interruption for refueling and short refueling times [1]. Another advantage of hydrogen particularly relevant for heavy-duty vehicles is the lower weight compared to battery electric vehicles.
Hydrogen in fuel cell electric vehicles (FCEVs) is usually stored in pressurized gas tanks at a high pressure of 700 bar in the case of passenger cars [2, 3]. For heavy-duty applications, a pressure level of 350 bar is dominantly used [4]. To achieve these high pressures, hydrogen is compressed at a refueling station before the actual refueling. The compression of hydrogen plays an important role in the entire hydrogen infrastructure. At a hydrogen refueling station (HRS), the compressor has the highest energy requirement [5].
Piston compressors are typically used for compression at HRS [6]. These mechanical compressors have a number of disadvantages. Apart from high investment costs, they are potentially susceptible to maintenance and unreliable because of a large number of moving parts. They also have a high space requirement [3, 7, 8]. Metal hydride compressors can be an alternative. These are considered compact, safe and reliable. In addition, they have lower investment costs in comparison [9].
Metal hydride compressors are thermal compressors. This means that compression takes place with the supply and removal of heat. A metal hydride is formed during the exothermic reaction of a metal compound with hydrogen. During this so-called absorption, heat is released. The removal of this heat is necessary for a continuing reaction. A supply of heat is required for desorption. Metal hydrides have the quality that ab- and desorption take place at different pressure levels depending on the temperature. For compression with metal hydrides, hydrogen is first absorbed with the release of heat. Heat is then supplied to the metal hydride in order to reach a higher temperature and thus a higher pressure level. Finally, more heat is supplied to desorb hydrogen at the increased pressure level. In order to achieve the high pressures of HRS application, several compression stages with different metal hydrides are usually connected in series. The periodic supply and removal of heat in several compression stages requires a non-trivial thermal management system.
To date, there have been only few studies on metal hydride compressors on the system level that take a thermal management system and energy demand into account. There is little information on energy demand and energy provision in general and even less regarding the division into electrical and thermal energy demand. The above applies especially for the application at HRS. There is no study known to the author that investigates a complete system suitable for real-life application at a HRS. In order to make a contribution in this context, the use of metal hydride compressor systems at HRS is investigated in a model-based way in this work.
First, an existing laboratory scale system model is refined with a particular focus on material selection to improve performance. Subsequently, an up-scaled compressor system for application at a realistic HRS is developed and modeled. At TU Braunschweig, there is a real-life laboratory for investigation of hydrogen use cases called H2-Terminal. The H2-Terminal includes a HRS for refueling of 350 bar trucks. This HRS and its utilization are taken as the use case of the developed up-scaled metal hydride compressor. By taking into account a thermal management system, the necessary energy demand for this application can be assessed. To reduce the energy demand, several optimization measures are carried out. This includes material selection, optimization of dimensions and metal compound mass as well as investigation of applied temperatures, among other things. Besides, sensitivity of the developed system is analyzed. With the investigations of this work, the viability of metal hydride compressor systems for HRS application is assessed. This way, a reasonable use case can possibly be identified.