Comparison of hydrogen hydrates with existing hydrogen storage technologies: Energetic and economic evaluations

Abstract

With the development of the hydrogen economy and FCV (fuel cell vehicles), the manner of storing and delivering large quantities of hydrogen arises as a major problem, and increasing research efforts are being targeted to solve this technological issue. Nowadays several hydrogen storage methodologies are available. Technologies are being developed and/or engineered other than the classical compression and liquefaction of hydrogen, which are based on the chemical (metal hydrides, ammonia) and physical (e.g., carbon nanotubes) adsorption of H₂. Also, a novel technology is in progress, which is based on clathrate hydrates of hydrogen. The object of the present work is to evaluate the features and performances of those storing systems with the aim to determine the best available technology throughout the “hydrogen chain”. For each one of the storage solutions presented, we have compared key parameters such as: interaction energy between hydrogen and support, storage capacity, specific energy consumption (SEC). By this work, it is demonstrated that a technology based on clathrate hydrates of hydrogen, while far from being optimized, may be competitive with the other approaches.

Introduction

Hydrogen storage is a main problem hindering the diffusion of the “Hydrogen Economy”. The classical storage methods based on compression and liquefaction are an established and efficient approach, but involve huge problems of safety and the associated costs of compression work and cooling are non negligible. On the other hand, several alternative approaches are currently under investigation. The aim of hydrogen storage technologies is to reduce the volume that hydrogen naturally occupies in its thermodynamically stable state under ambient conditions, i.e., as a gas. Hydrogen gas shows a very low density (0.089 kg/m³), which means that only a little mass is contained within a large volume of gas. However, hydrogen shows a very high energy content by weight, thus being interesting as a fuel or energy carrier. Therefore, it is required to transform hydrogen into an easily handled form, e.g., by compression or liquefaction, or by trapping through interaction with other compounds, by means of strong or weak interactions, such as covalent bonds or van der Waals interactions.

The efficiency of a particular technology of hydrogen storage is not merely a matter of mass or volume capacity, but also of net stored energy. Indeed, energy required to transform hydrogen from the gaseous state to storage conditions, and then the energy required to recover the same from the storage media is a critical issue, which assumes practical implications in particular for metal hydrides. For compressing hydrogen, energy as mechanical work is required; for liquefying the same, on the other hand, cooling energy and compression work are required, plus a certain energy to keep hydrogen under proper thermodynamic conditions in order to maintain the liquid state. When hydrogen is, instead, stored onto (or into) a support, its stability will be higher the stronger its interaction with the support. In that case, a remarkable amount of energy will be required to recover H₂ from the support.

From these considerations, it is clear how the development of a novel hydrogen storage technology is not only a matter of storage capacity but also, and in some cases mainly, a problem of energy efficiency, because what we are storing is substantially energy, and one should be careful to avoid systems with an overall negative energy balance.

By considering a proper combination between stability of the storage system and easiness of hydrogen recovery, it is possible to estimate that an adequate interaction of hydrogen with its storage support should be of about ca. 20 kcal/mol, namely 40,000 kJ/kg of hydrogen. [1] That is an intermediate value between the interaction energy of a covalent bond, typically of about several tens of kcal/mol, and the interaction energy of a weak bond, typically about only a few kcal/mol. Energy efficiency in hydrogen storage and transportation is critical for various reasons. First, in the current demand for energy-efficient systems, [2] storage technologies that waste a remarkable amount of the energy they carry should be replaced with improved technologies, or at least confined to niche applications where other technologies cannot be applied. Secondly, wasting hydrogen is not merely wasting its energy content: hydrogen is not an energy source, i.e., it cannot be mined, but must be produced, e.g., by consuming conventional energy sources. Currently, hydrogen is mainly produced by fossil fuel gasification with an energy recovery efficiency of about 40–60%. This means that if 1 kJ of hydrogen is wasted, roughly 2 kJ of fossil fuel are wasted overall [3].

Third, wasting energy should also be avoided because of Greenhouse Gas (GHG) emissions, being fossil energy mainly based on carbon. Carbon dioxide emissions are an unavoidable and undesired end product, thus technologies that show a lower efficiency contribute to GHG emissions to a higher level than more efficient ones [4].

In the present work, various hydrogen storage systems for which a comprehensive physico-chemical database is available, have been compared in order to give a homogeneous comparison among the various approaches. As classical and well established systems, we chose compression standards at 200 bar into steel cylinders, and compression standards at 350 and 700 bar into the novel, carbon fiber-jacketed aluminum cylinders [5]. Moreover, hydrogen liquefaction has also been analyzed as an established technology [6]. For the class of storage systems based on a “chemisorption” principle, where a hydrogen molecule bond is broken to form new bonds with the storage support, metal hydrides have been analyzed using MgH₂ as representative of High Temperature Hydrides (HTH) [7] and LaNi₅H₆ as representative of Low Temperature Hydrides (LTH) [8]. NaAlH₄, exemplifying a relatively novel class of aluminum alloy-based hydrides (Alanates) [9] has also been analyzed. Single Walled Carbon Nanotubes (SWNTs) were also considered as representative of “physisorption” based systems, where hydrogen interacts with the support at the molecular level through weak interactions [10]. Particular attention has been paid to ammonia as a storage system; this because even though it is considered as a promising hydrogen storage system, it also offers the advantage of having a well established production technology [11].

All these systems have been compared with hydrogen clathrate hydrates, in order to evaluate whether the latter can be competitive as alternative hydrogen storage media. In order to carry out this analysis, first the thermodynamic conditions of the storage systems, with hydrogen stored therein, have been identified, and then the thermodynamic conditions under which the processes of storage and release take place have been evaluated. As a second step, the fundamental data have been collected, such as system gravity, theoretical storage capacity, specific heat, latent heat, etc. Then, the interaction energy between hydrogen and support has been obtained in order to evaluate how a system compares with the above mentioned “optimum” value of roughly 40,000 kJ/kg. Moreover, the processes required for practically carrying out hydrogen storage and release have been devised for each system tested, namely if the process requires warming, cooling, and/or compression. Finally, the actual calculation has been carried out, in order to define the Specific Energy Consumption (SEC), i.e., the portion of stored energy - under hydrogen form - which is required for the operations of storing and releasing hydrogen. By calculating a SEC, it is possible to define the real energy storage capacity for a certain system and thus evaluate its efficiency. Furthermore, also CO₂ emissions related to the particular process of storage and release, have been calculated, in order to evaluate the environmental impact of each strategy.

The final object of the present work is to define and quantify parameters describing the performances of the above storage systems with the aim to determine the best available technology throughout the whole “hydrogen chain”, i.e., from its production to its delivery for final use. Obviously, the SEC is not the only parameter that defines the final efficiency of a storage system: as mentioned above, indeed, also the energy required to keep certain thermodynamic conditions (e.g., boil-off for liquid H₂) should be evaluated jointly with the costs of transportation, and, most importantly, the availability and cost of raw materials. However, to evaluate these latter parameters, further data are required such as storage time or shipping distance. These parameters may differ for different applications, hence they have not been included into the present calculations.

It should be noted also that real process data were available only for well established storage systems, and for all the other, novel experimental systems, storage and release processes have been hypothesized starting from their thermodynamic behaviour. Moreover, parameters such as inefficiency due to irreversibility of certain processes, or due to activation energies, have not been assessed and thus the SEC for those systems should be taken as only indicative and representing a minimum energy cost for a certain experimental system.

The following is a description of the several systems examined herein, and the relevant parameters and processed considered in the calculations.