Preheating fuel cells via unused pressure difference with metal hydrides for sub-zero start-up

Kurzfassung

Freezing water degrades fuel cells mechanically. In order to enable cold start below 0°C, one possibility is to preheat the fuel cell. Instead of using additional energy, the approach proposed here uses the pressure difference between the hydrogen pressure tank and the fuel cell, which until now is throttled and lost. Especially mobile applications can benefit from such a system, because they undergo many cold stat-ups and need to be efficient with their available energy. Metal hydrides are metal alloys which absorb and desorb hydrogen in an exo-/endothermic reaction according to the following equation: 〖MH〗_x+y/2 H_2⇆ MH_(x+y)+ ∆_R H The temperature level of the exothermic reaction depends on the given hydrogen pressure according to the equilibrium reaction as shown in the Van’t Hoff-plot in Figure 1. Therefore, high pressure from the storage tank leads to a release of high temperature thermal energy that could be used for preheating. During driving, the fuel cell lowers the pressure, hence the desorption of hydrogen form the metal hydride can take place at ambient temperature. Such a system doesn’t need additional energy since it makes use of the pressure difference in a fuel cell vehicle without consuming any hydrogen. The thermal energy is stored in the metal hydride free of loss for an infinite time and is released only when hydrogen is supplied. Due to the high energy density of about 150-200 kJ/kgMH [1], small storages can be realized. However, such a preheating system has to provide the thermal energy in a very short time requiring high thermal power. Since the thermal conductivity of metal hydrides of around 1 W/(mK) [2] is fairly small, little work has been published yet showing sufficient thermal power [3], [4]. At the DLR, the given approach is investigated with different reactor designs and operation modes, in addition to our previous work in [3]. These investigations include a modular system, which can be easily adjusted to the required storage capacity with the same specific thermal power. This reactor example is shown in Figure 2. Additionally, since the thermal energy is stored as chemical potential, allowing the storage to cool down to ambient temperature, an integrated approach is also possible: In this case the material properties have to be carefully chosen but the thermal storage does not need any additional controlling measures. Therefore, the storage could be integrated directly into the fuel cell. This leads on the one hand to shortened duration for temperature rise inside the fuel cell and on the other hand to a reduced thermal energy demand. The contribution will outline different reactor designs, operation modes and chosen experimental results in order to identify crucial limitations and technological possibilities.