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MgH2-based hydrogen storage materials are promising candidates for solid-state hydrogen storage allowing efficient thermal management in energy systems integrating metal hydride hydrogen store with a solid oxide fuel cell (SOFC) providing dissipated heat at temperatures between 400 and 600 °C. Recently, we have shown that graphite-modified composite of TiH2 and MgH2 prepared by high-energy reactive ball milling in hydrogen (HRBM), demonstrates a high reversible gravimetric H storage capacity exceeding 5 wt % H, fast hydrogenation/dehydrogenation kinetics and excellent cycle stability. In present study, 0.9 MgH2 + 0.1 TiH2 +5 wt %C nanocomposite with a maximum hydrogen storage capacity of 6.3 wt% H was prepared by HRBM preceded by a short homogenizing pre-milling in inert gas. 300 g of the composite was loaded into a storage tank accommodating an air-heated stainless steel metal hydride (MH) container equipped with transversal internal (copper) and external (aluminium) fins. Tests of the tank were carried out in a temperature range from 150 °C (H2 absorption) to 370 °C (H2 desorption) and showed its ability to deliver up to 185 NL H2 corresponding to a reversible H storage capacity of the MH material of appr. 5 wt% H. No significant deterioration of the reversible H storage capacity was observed during 20 heating/cooling H2 discharge/charge cycles. It was found that H2 desorption performance can be tailored by selecting appropriate thermal management conditions and an optimal operational regime has been proposed.

In energy systems, multi-generation including co-generation and tri-generation has gained tremendous interest in the recent years as an effective way of waste heat recovery. Solid oxide fuel cells are efficient power plants that not only generate electricity with high energy efficiency but also produce high quality waste heat that can be further used for hot and chilled water production. In this work, we present a concept of combined cooling, heating and power (CCHP) energy system which uses solar power as a primary energy source and utilizes a reversible solid oxide fuel cell (R-SOFC) for producing hydrogen and generating electricity in the electrolyser (SOEC) and fuel cell (SOFC) modes, respectively. The system uses “high temperature” metal hydride (MH) for storage of both hydrogen and heat, as well as “low temperature” MH's for the additional heat management, including hot water supply, residential heating during winter time, or cooling/air conditioning during summer time. The work presents evaluation of energy balances of the system components, as well as heat-and-mass transfer modelling of MH beds in metal hydride hydrogen and heat storage system (MHHS; MgH2), MH hydrogen compressor (MHHC; AB5; A = La + Mm, B Ni + Co + Al + Mn) and MH heat pump (MHHP; AB2; A = Ti + Zr, B Mn + Cr + Ni + Fe). A case study of a 3 kWe R-SOFC is analysed and discussed. The results showed that the energy efficiencies are 69.4 and 72.4% in electrolyser and fuel cell modes, respectively. The round-trip COP's of metal hydride heat management system (MHHC + MHHP) are close to 40% for both heating and cooling outputs. Moreover, the tri-generation leads to an improvement of 36% in round-trip energy efficiency as compared to that of a stand-alone R-SOFC.

In this work, we report about development of computer model of hydrogen Thermal Desorption Spectra (TDS) using a combination of modified Kissinger approach and combined form of differential kinetic equation. For an individual TDS peak, the developed model allows to adequately fit the experimental TDS data using up to 6 parameters, including the activation energy of desorption and the pre-exponent factor. The fitting procedure also allows visualizing the reaction model function using the experimental TDS data. The model was verified for the nanostructured magnesium hydride prepared by high-energy ball milling of magnesium in hydrogen, including its composites with carbon and titanium.

Mg-based materials have become the cornerstone of hydrogen storage applications and have shown recently promises in thermal energy storage applications. However, synthesis of some materials including Mg2FeH6 ternary hydride poses a number of challenges. In spite a variety of reports on various synthesis routes yielding Mg2FeH6, none of them produces a 100% pure Mg2FeH6. In this communication, we have attempted to synthesize Mg2FeH6 by high energy reactive ball milling (HERBM) performed in hydrogen gas. Prior to the ball milling, the starting stoichiometric 2:1 mixture of the initial materials Mg+Fe was heat treated through a judicious temperature program. It was found that the yield of Mg2FeH6 is related to the conditions of heat treatment, time and temperature, and the milling parameters such as ball to powder mass ratio (BPR) and rotation speed of the planetary mill. Moreover, it was found that the synthesis took place in less than 5 hours of milling with a maximum yield of Mg2FeH6 of 84 wt%, which is a noticeable improvement as compared to the reference data.

In order to uncover the inner working mechanism and performance of solid oxide fuel cell (SOFC) with biomass gasification syngas as fuel, a two dimensional SOFC multi-physical field model is established. This study makes up for the deficiency that the previous studies of coupling biomass gasification unit and SOFC stack mostly stay at the system level. The results show that the SOFC fueled by the syngas produced from gasification of biomass with steam as the agent has the best performance. The peak power density could achieve approximately 10240 W m−2. With the improvement of operating temperature, the peak power density of SOFC will be increased. At the temperature of 1123 K, the peak power density could achieve about 15128 W m−2. The average reaction rate of water gas shift (WGS) reaction is −29.73 mol m−3 s−1 when the operating temperature is 1123 K. This indicates that the WGS reaction will proceed in reverse direction at high temperatures, thereby reducing the hydrogen concentration. In addition, increase in the anode flux and decrease in the cell length lead to the increase of SOFC current density. In general, this work could provide guidance for the optimization and practical application of SOFC using biomass syngas as fuel.

The effects of a small amount of nonmetal elements (N, F and Cl) with high electronegativity interstitially doping on improving the hydrogen storage characteristics of Mg-based metal hydrides were systematically investigated by first-principle calculations in this paper. The interstitial positions which the doping elements easily occupied were firstly determined. The calculation results showed that these elements are most likely to hold the center position of octahedral sites with two Ni and four Mg atoms. Based on this, the crystal structures, thermal stability, dehydrogenation energy and electronic structures of all the crystals, including Mg2Ni, Mg2NiN0.5, Mg2NiF0.5, Mg2NiCl0.5 and their hydrides, were further investigated. The nonmetals with high electronegativity exhibit the favorable effects on the characteristics of Mg-based metal hydrides. Doping F significantly reduces the dehydrogenation energy of Mg2NiH4 by about 25%, because of the strong hybridization between F and H atoms. When doping Cl into Mg2Ni and Mg2NiH4, the formation enthalpies decrease respectively by 0.047 and 0.024 eV atom−1, due to the reduction of integral intensity of the bonding electron. Among the three elements, N has the best effects on improving both kinetics and thermodynamics. Doping N not only causes the formation enthalpies of Mg2Ni and its hydride to decrease by 0.215 and 0.141 eV atom−1 respectively, but also reduces the dehydrogenation energy of Mg2NiH4.

In order to predict heat and mass transfer characteristics of metal hydride reactors accurately, a novel three-dimensional multiphysics model was presented. In the newly established model, the velocity field of the heat transfer fluid was obtained by solving the Navier–Stokes equations or the k-e turbulence model. The model was numerically solved using the commercial software package COMSOL Multiphysics V3.5a. Two traditional models were also solved for the reactors under the same set of conditions. A dimensionless parameter N was defined to assess the effects of neglecting the variation of heat transfer fluid temperature on the hydrogen absorption rate. The results from numerical simulation indicated that when N is greater than 0.01, the variation of heat transfer fluid temperature cannot be neglected. In this case, the newly established model was valid while the other two models were not. Moreover, it was found that the effective thermal conductivity of the metal hydride, the flowrate of the heat transfer fluid and the contact resistance were crucial factors for improving the performance of the metal hydride reactors.

Metal hydrides show great potential for hydrogen storage. However, for efficient hydrogen storage, thermal management is the technical barrier. Among the different heat exchangers proposed in the literature, finned tube heat exchangers are of great technological interest due to their adaptability to wide range of practical applications, high compactness and high heat transfer efficiency. In the present paper, the optimization of finned heat exchanger considering both enhanced heat transfer and vessel volume efficiency is conducted. A semi-analytical expression of heat transfer rate from a single fin is derived. The effects of fin dimension (fin thickness and radius) on the heat exchanger performance are studied. It was shown that the thermal resistance of the whole heat exchanger can be reduced by increasing the fin radius and decreasing the fin thickness, while the fin volume is kept fixed. In the second part of the study, a 2-D numerical simulation was performed in order to validate the results of the analytical study. The effects of two parameters (cooling tube diameter, the fin length) on the hydrogen charging time were highlighted. The increasing in the tube diameter from 2.5 mm to 5 mm results to 25% reduction of the charging time, which is very noticeable. On the other hand, given a reactor radius, increasing the length of fin reduces the overall thermal resistance of the reactor-heat exchanger. The results showed that the decreasing of the thermal resistance of 13% leads to a decreasing in charging time of 42%. Finally, it was found that the results of the numerical simulation agreed qualitatively with those of analytical study. Therefore, the analytical solution presented can be used for a quick assessment of the finned tube heat exchanger design without significant errors.

Metal hydride systems are utilized in many practical applications such as hydrogen storage, heat pumps, etc. The establishment of a metal hydride system requires the expression of reaction rate for the first step design, from an engineering standpoint. However, improper experimental determination of intrinsic kinetics usually leads to significant errors in the kinetic data. This paper presents a novel methodology of estimating these errors based on the entropy generation analysis. For this purpose, numerical simulation is performed taking into account the experimental conditions for a real case. The results showed that if the operating conditions (i.e. hydrogen pressure, heat transfer coefficient) and the size of the experimental setup are not chosen properly, the kinetic data obtained from the experiments will be largely misled. Therefore it should be carefully taken into account in order to minimize the relative errors induced on the kinetic data.