4.3.1 Interface design proposals

Generally, an MH vessel consists of a gas-proof containment in which the MH material is placed, of a thermal management system (TMS) with a heat transfer fluid (HTF) to remove or supply the reaction heat and of means for the input, output and distribution of hydrogen [Reference Pohl11, Reference Lototskyy, Tolj, Pickering, Sita, Barbir and Yartys34]. The essential interfaces of the MH cartridge are schematically illustrated in Fig. 7. To keep the exchangeable MH cartridge as simple as possible, the identifier can be a passive device like a barcode or an RFID chip, which is analysed and communicated by a stationary reading device. The temperature sensor of the cartridge could also be part of the stationary infrastructure.

Figure 7.

Essential interfaces of the cartridges and overarching management system for control.

While there are various ways to remove and to supply the reaction heat, the use of liquid, such as water-glycol mixtures, or air as HTF are commonly applied to reject heat to the ambient atmosphere and to transfer waste heat to the MH material [Reference Lototskyy, Tolj, Pickering, Sita, Barbir and Yartys34, Reference Nguyen and Shabani37]. Figure 8 presents the TMS principles that are considered for the heat transfer out of and into the MH cartridge. It should be noted, that the sensible heat capacity of the cold BOG potentially assists the removal of the absorption heat [Reference Rosso and Golben12]. Thereby, the hydrogen would not only act as a reactant, but also as a HTF. However, the use of hydrogen as a HTF is not considered in this study.

Figure 8.

Potential TMS interfaces (a), (b) and (c) for heat transfer out of/into the MH cartridge.

Although Fig. 8 depicts fans to drive the air flow, both convection types, natural and forced, may be used. However, natural convection only allows low specific absorption and desorption rates [Reference Davids, Lototskyy, Malinowski, van Schalkwyk, Parsons, Pasupathi, Swanepoel and van Niekerk38, Reference Bürger, Dieterich, Pohlmann, Röntzsch and Linder39]. For higher hydrogen mass flows, for example the boil-off losses during refueling operations, slow specific desorption rates would lead to unreasonable system sizes as large amounts of MH material would be required. The use of natural convection is especially challenging for the BOG recovery application with its operating conditions close to the MH material’s equilibrium conditions.

Although forced air convection is more effective than natural convection [Reference Dornheim, Baetcke, Akiba, Ares, Autrey, Barale, Baricco, Brooks, Chalkiadakis and Charbonnier40], it is expected to be more limiting in terms of implementation than the heat transfer by liquids. Firstly, consumers with air cooled PEMFCs will be required to apply heat supply by air. However, liquid cooling is the most common cooling method for FCs with more than 5 kW of power [41]. Secondly, for heat supply by air, the MH cartridges require external fins and have to be installed in the cooling air ducts of the PEMFCs, which will lead to limitations regarding potential consumers due to design space restrictions. For waste heat transfer through liquid media, positioning of the MH cartridges is less restricted, as the HTF lines could be placed more versatilely. Fast couplings in the HTF lines allow easy and quick replacement of the MH cartridges.

For an MH cartridge with liquid heat transfer, internal fins are important to utilise the high thermal performance of the liquid HTF, while additional external fins are not reasonable. For heat transfer by air, external fins are of a greater importance than internal fins, but a combination of both shows the highest performance [Reference Dornheim, Baetcke, Akiba, Ares, Autrey, Barale, Baricco, Brooks, Chalkiadakis and Charbonnier40]. These conclusions can be derived from the terms of the total heat transfer coefficient ${U_{{\rm{total}}}}$ , which is part of the following Equations (1), (2) and (3) to calculate the thermal power $P$ of the TMS of the MH cartridge [Reference Kölbig, Weckerle, Linder and Bürger42, 43]:

(1) \begin{align}{\rm{P}} = {U_{{\rm{total}}}} \cdot {\rm{A}} \cdot {\rm{\Delta }}T = {U_{{\rm{total}}}} \cdot {\rm{A}} \cdot \left( {{T_{{\rm{HTF}}}} - {T_{{\rm{MH}}}}} \right)\end{align}

(2) \begin{align}{U_{{\rm{total}}}} = \frac{1}{{{R_{\rm{total}}}}}\end{align}

(3) \begin{align}{R_{{\rm{total}}}} = \frac{{{s_{{\rm{MH}}}}}}{{{\lambda _{{\rm{MH}}}}}} + \frac{1}{{{\alpha _{{\rm{wall}}}}}} + \frac{{{s_{{\rm{wall}}}}}}{{{\lambda _{{\rm{wall}}}}}} + \frac{1}{{{\alpha _{{\rm{HTF}}}}}}\end{align}

The factor $A$ describes the available area for heat transfer. The terms $s$ and $\lambda $ represent the travel distance through the material and its thermal conductivity, while $\alpha $ is the term of the heat transfer coefficient. The terms are illustrated in Fig. 9, which displays a schematic cross section resembling a unit cell of an MH cartridge.

Figure 9.

Schematic of a unit cell of the MH cartridge to illustrate the heat transfer path.

To keep the MH material in isothermal condition for not deteriorating the reaction rate by heating up or cooling down, the thermal power $P$ should correspond to the heat of reaction $\dot Q$ for absorption and desorption as shown in Equation (4):

(4) \begin{align}{\rm{P}} = \dot Q\end{align}

The heat of reaction $\dot Q$ can be calculated with the molar mass flow of hydrogen ${\dot n_{{\rm{H}}2}}$ and the reaction enthalpy $\Delta H$ according to the following Equation (5) [Reference Chabane, Ibrahim, Harel, Djerdir, Candusso and Elkedim44, Reference Weckerle, Nasri, Hegner, Linder and Bürger45]:

(5) \begin{align}\dot Q = {\dot n_{{\rm{H}}2}} \cdot \Delta H = {\dot m_{{\rm{H}}2}} \cdot \frac{{\Delta H}}{{{M_{\rm{H2}}}}}\end{align}

It has to be noted, that setting the thermal power $P$ equal to the heat of reaction $\dot Q$ is an idealistic scenario. This equivalence assumes steady-state conditions, a uniform temperature distribution in the MH bed and heat transfer only in thickness direction within the unit cell. Nevertheless, this equivalence is used in the following paragraphs to point out the sensitivity of the terms of the heat transfer coefficient ${U_{{\rm{total}}}}$ to derive conclusions for the conceptual design.

The average mass flow rate ${\dot m_{{\rm{H}}2}}$ , which results from the target absorption time from Table 2, leads to an average power requirement of 20 kW to reject the heat of reaction $\dot Q$ of a single cartridge. Applying this value for the cooling power demand $P$ , according to the idealistic equivalence described above, leads to unreasonable large dimensions of external fins for heat rejection by air, especially at hot day conditions when the temperature difference for heat rejection becomes low. A respective heat transfer coefficient ${\alpha _{{\rm{HTF}}}}$ of natural convection of 5 W/(m²K) and a temperature difference ${\rm{\Delta }}T$ of 5 K would already lead to an external fin area requirement of 800 m², even when neglecting the other terms of ${U_{{\rm{total}}}}$ [Reference Dornheim, Baetcke, Akiba, Ares, Autrey, Barale, Baricco, Brooks, Chalkiadakis and Charbonnier40]. As the heat transfer coefficient ${\alpha _{{\rm{HTF}}}}$ of liquid cooling is two orders of magnitude higher, with values in the range of 300 to 1 000 W/(m²K), its area requirement is correspondingly smaller [Reference Dornheim, Baetcke, Akiba, Ares, Autrey, Barale, Baricco, Brooks, Chalkiadakis and Charbonnier40, Reference Feng, Xiangming, Ouyang, Languang, Wu, Kulp and Prasser46]. Hence, liquid heat transfer is recommended to not deteriorate the compact nature of the MH cartridges.

When using liquid heat transfer, the heat conduction through the MH material drives the total heat transfer coefficient ${U_{{\rm{total}}}}$ . As MH powder material shows a low effective thermal conductivity ${\lambda _{{\rm{MH}}}}$ in the range of 0.1–1 W/(mK), the thermal resistance of the cartridge wall can usually be neglected [Reference Hahne and Kallweit30, Reference Nguyen and Shabani37, Reference Dornheim, Baetcke, Akiba, Ares, Autrey, Barale, Baricco, Brooks, Chalkiadakis and Charbonnier40, Reference Weckerle, Nasri, Hegner, Bürger and Linder47]. Hence, the travel distance through the MH material ${s_{{\rm{MH}}}}$ should be minimised during the sizing process of the cartridge either by a thin MH bed design or by implementing internal fins as proposed above. Another approach is to increase the effective thermal conductivity of the MH material ${\lambda _{{\rm{MH}}}}$ by compressing the MH powder to pellets or by adding materials with high thermal conductivity, e.g. aluminum, expanded natural graphite or carbon fibers [Reference Lototskyy, Tolj, Pickering, Sita, Barbir and Yartys34, Reference Nguyen and Shabani37, Reference Kölbig, Weckerle, Linder and Bürger42, Reference Bae, Tanae, Monde and Katsuta48]. While adding passive materials increases the overall mass of the cartridge, the utilisation of externally formed pellets limits the design possibilities of the cartridge and may lead to hydrogen mass transfer limitations [Reference Nguyen and Shabani37, Reference Kölbig, Weckerle, Linder and Bürger42].

4.3.2 Metal hydride cartridge design proposals

Besides the properties of the MH material and the means for thermal management, the outer shape of the vessel of the cartridge influences the performance of the system. Figure 10 introduces different designs for vessel shapes and summarises their specific properties.

Figure 10.

Overview of different vessel shapes and their properties according to Refs [Reference Pohl11, Reference Rosso and Golben12, Reference Lototskyy, Tolj, Pickering, Sita, Barbir and Yartys34, Reference Kölbig, Weckerle, Linder and Bürger42, Reference Yang, Wang, Zhang, Meng and Rudolph49–Reference Röver, Roth, Hoffmann, Baetcke and Herzog51].

All cartridge cross sections of Fig. 10(a)–(c) are illustrated with two types of internal channels: arteries for hydrogen transfer and lines to accommodate the liquid HTF. While a filter tube is often used as a hydrogen artery for MH powder beds, a drilled hole is sufficient for compact MH pellets [Reference Yang, Wang, Zhang, Meng and Rudolph49]. The design of the HTF lines depends on the sizing of the TMS as described in the previous section. The shape, size and number of HTF lines is driven by the intention to reduce the travel distance through the MH material ${s_{{\rm{MH}}}}$ . The acceptable travel distance depends on the target value for the thermal power $P$ , on the available area $A$ , on the available temperature difference $\Delta T$ and on the applied means to enhance the effective thermal conductivity of the MH bed ${\lambda _{{\rm{MH}}}}$ . Thus, the overall design is also driven by the dimensions of the cartridge. While a compact design requires numerous HTF lines and preferably contains internal fins as well, a long and thin cartridge with a thin MH bed might work with only a few HTF lines or even with none at all, when external fins become promising. Hence, the final design choice depends on the available design space and requires a dedicated conceptual design study.

Another point that has to be considered for the cartridge design is the expansion of the MH material during hydrogenation, which can lead to stresses and failure of the cartridge [Reference Kölbig, Weckerle, Linder and Bürger42, Reference Weckerle, Bürger and Linder52]. To compensate for the expansion, the MH powder may be filled into the cartridge up to a porosity of approximately 67% [Reference Kölbig, Weckerle, Linder and Bürger42]. However, this leads to a tradeoff between a dense filling to increase the storage capacity and an effective thermal conductivity of the MH bed on the one hand and larger stresses in the cartridge on the other hand [Reference Lototskyy, Tolj, Pickering, Sita, Barbir and Yartys34, Reference Kölbig, Weckerle, Linder and Bürger42]. Besides limiting the filling density, the addition of expanded natural graphite can also protect the cartridge from stresses, as it compensates the expansion of the MH [Reference Lototskyy, Tolj, Klochko, Davids, Swanepoel and Linkov53].

4.3.3 Proposals for metal hydride cartridge logistics

Potential locations to place the cartridges during BOG capture are shown in Fig. 11. Although option (a) enhances flexibility between actual BOG rates and hydrogen demands by allowing to refill the MH cartridges also with hydrogen from the grid, this solution requires higher infrastructure demands and thereby offsets the benefits of bowser distribution. Hence, it may be beneficial to implement option (b), (c) or (d) in combination with only a single, separate facility for intermediate storage. This intermediate storage acts as buffer during times with high amounts of boil-off and low hydrogen demand of consumers or vice versa. The storage could be positioned underground or could be a common storehouse as in Fig. 6. The storage is preferably equipped with connections to the stationary GH2 pipeline grid. This enables to refill the MH cartridges in phases with low boil-off occurrence and high demand of the consumers. Moreover, the MH cartridges could also deploy the hydrogen into the grid for re-liquefaction and thereby serve as a buffer to translate the rapid bursts of BOG from the refueling operations to a constant hydrogen mass flow, which is suited for the liquefier [Reference Rosso and Golben12].

Figure 11.

Potential positioning of MH cartridges during LH2 refueling.

Option (c) and (d) may be beneficial compared to (b), as the systems of the bowser truck could be used to supply electrical power for sensors, for communication or to drive the HTF pump. As described in Section 2.1, the LH2 trailer offers a weight margin of approximately 7 tons that can be used for the integration of the cartridges and further subsystems. However, the hydrogen capacity of 50 kg suggested by Mangold et al. could not be fully achieved by retrofitting a conventional LH2 trailer, as this capacity would require 25 cartridges in total which leads to a total cartridge weight of 7.5 tons. Nevertheless, the mismatch may be solved by modifying the LH2 trailer or by slightly reducing the total capacity of the recovery system onboard of the refueling truck.

Besides weight, the total capacity that is installed in the LH2 trailer might also lead to challenging heat rejection demands if multiple cartridges are absorbing in parallel. Based on the estimations of Mangold et al. shown in Section 2.1 [Reference Mangold, Silberhorn, Moebs, Dzikus, Hoelzen, Zill and Strohmayer6], an amount of 12.5 kg of BOG forms during the chill-down prior to refueling. Assuming that no depressurisation of the bowser truck is necessary in the environment of an airport, no BOG needs to be vented after refueling. During refueling, the transfer losses may be as low as 0.1% of the delivered hydrogen as also described in Section 2.1. Based on exemplary predictions of LH2 fuel masses onboard of a future hydrogen-powered regional, narrow-body and wide-body aircraft of approximately 1, 5 and 20 tons [Reference Mangold, Silberhorn, Moebs, Dzikus, Hoelzen, Zill and Strohmayer6, Reference Brewer8, Reference Clemente54], the BOG losses during transfer are 1, 5 and 20 kg, respectively. Together with the 12.5 kg of BOG prior to refueling, the total losses sum up to 13.5, 17.5 and 32.5 kg. Assuming that all aircraft categories are refueled within the target absorption time of 30 minutes from Table 2, the heat rejection demands result to approximately 135, 175 and 325 kW based on the equation in Section 4.3.1. To limit this cooling power demands, it may be reasonable to reduce the hydrogen mass flow rates by operating multiple LH2 trailers in parallel, especially if larger aircrafts are refueled.

However, option (b) can still be promising as it offers high independency and flexibility. Especially for lower hydrogen mass flow rates, when natural convection is sufficient to reject the heat of absorption, this positioning can be beneficial.

Besides the positioning of the MH cartridges, potential means for replacement and transportation have to be considered. Due to the high weight of the MH cartridges, devices like industrial robots, cranes, conveyor belts, staff with exoskeletons, hand lift trucks, forklifts or towing tractors with trailers may be suitable. In addition, the cartridges should offer means for handling like handles, lugs, threads, wheels, guide rails or guide pins and poka-yoke elements. The cartridges may also be equipped with bumper devices like rubber slipovers to protect the vessel or the interface components from damage during the handling operations. Although MHs are described as an intrinsically safe hydrogen storage form due to the moderate pressures and the endothermic nature of the hydrogen release, a connection tear-off or a vessel rupture still implies a safety risk due to the pyrophoric nature of MHs [Reference Lototskyy, Tolj, Pickering, Sita, Barbir and Yartys34, 36, Reference Zalosh55].