Analysis of the Storage Capacity of An Uncooled MH Tank
Abstract
The need for cooling of metal hydride (MH) tanks during the hydrogen absorption process is a well-known fact, especially when it is necessary to achieve full absorption of the alloy with hydrogen within a short time frame. However, effective cooling of the tank is not always possible. The absence of cooling significantly extends the absorption time or reduces the absorption capacity of the alloy, which is crucial for the competitive operation of a metal hydride tank. This article focuses on the effect of the absence of an external cooling system on the change in the absorption capacity of the MH alloy within a defined filling time frame.
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Introduction
Hydrogen is considered one of the most promising energy carriers, as it is renewable and has the potential to significantly contribute to decarbonization and the reduction of greenhouse gas emissions not only in transportation but also in the energy sector and industry. However, efficient and safe storage of hydrogen presents a major challenge for the further development of hydrogen energy.
One of the main obstacles in hydrogen storage, whether for short-term or long-term use, is its very low energy density, small molecular size, and wide explosive concentration range when mixed with air. These negative factors significantly complicate its safe storage and distribution within the hydrogen infrastructure. This is evident not only in its use at refuelling stations for hydrogen-powered vehicles but also in heating systems, where previously unforeseen complications can arise.
Conclusion
Even when using internal elements to improve heat transfer (intensifiers), an external cooling system remains essential to maintain optimal hydrogen absorption conditions in MH tanks if the filling process is time-limited. Otherwise, heat accumulates, leading to an increase in alloy temperature and a consequent decrease in absorption capacity.
Experimental results show that at a pressure of 10 bar, the stored hydrogen capacity at 40 °C is approximately 65.06% lower than at 20 °C; at 20 bar, it is 77.31% lower; and at 30 bar, the difference between the ideal storage capacity and the capacity of the uncooled MH tank with limited filling time is 66.77%.
By extending the tank filling time at a given pressure, the actual storage capacity would gradually approach the ideal capacity, and after a sufficiently long period, the difference between ideal and real filling times would completely disappear.
The development of systems for adequate cooling of MH tanks during the absorption process is crucial for their wider adoption, along with research into heat dissipation systems from the tank core, as the thermal conductivity of powdered alloys significantly complicates this process.
Alongside cooling research, the development of new alloys with improved properties such as higher capacity, faster kinetics, and better thermal conductivity must also proceed. This progress could significantly expand the usability of metal hydrides and contribute to the broader spread of the hydrogen economy.