Performance prediction of a coupled metal hydride based thermal energy storage system
Graphical abstract
Introduction
Thermal energy storage (TES) is essential for efficient utilization of energy resources as it can balance the mismatch between energy consumption and the supply. Thermo-chemical heat storage promises high gravimetric energy densities due to the large enthalpy of reaction associated with chemical reactions [1].
Metal hydrides (MH) are chemical compounds formed when a metal/alloy reversibly reacts with hydrogen gas. One can store thermal energy in MH while decomposing the hydride into metal and hydrogen. This charging process involves an endothermic reaction. The formation of MH is an exothermic reaction associated with the release of heat (discharging process). The operation of MH involves both sensible heat and thermochemical heat simultaneously. The supply of high temperature HTF to MH bed during charging process raises the temperature of the bed (sensible heating) and decomposes MH into metal and hydrogen (heat of reaction). This decomposition of hydride is an endothermic reaction, for which the energy required is supplied by flowing heat transfer fluid (HTF) which loses sensible heat to MH bed. While discharging process, thermochemical heat is released by the formation reaction between metal and hydrogen gas which raises the bed temperature and is recovered by supplying HTF at a lower temperature. The rise in temperature of HTF indicates the sensible heating. The salient feature of MH based TES systems (MH-TES) over the conventional storage modes (sensible and latent heat storage) is that they offer long-term energy storage at near ambient conditions without thermal losses. These systems are of interest in several thermal engineering applications due to their large gravimetric heat storage capacities and wide range of operating conditions [2]. MH are best known for their compactness, durability and safety.
Extensive numerical and experimental research works have been reported on MH for hydrogen storage applications [3,4]. MH have also been used for several applications like heating, cooling, heat transformation, hydrogen compression and hydrogen purification [5,6]. The use of MH for thermal storage applications originated in early 1970s and reported first in a U.S patent by Winsche [7] for the application a steam power generation system. For MH-TES applications, hydrides with high heat of reaction and hydrogen storage capacity are desirable [8].
Magnesium based hydrides have been preferred for TES applications due to their large enthalpy of reaction, good thermal conductivity and easy availability. Reiser et al. [9] presented Mg-based alloys as potential candidates for high temperature TES and discussed various features and feasibility of Mg, Mg2Ni, Mg2Fe and Mg–Co based systems. These materials were experimentally shown to have high thermal energy densities of about 2257 kJ kg−1 operating between 230 °C and 550 °C. It was observed that Mg2NiH4 is less stable compared to MgH2 and has minimum hydrogenation temperature of 230 °C. At hydrogen storage capacity of 3 wt%, Mg2Ni alloys store 916 kJ kg−1 of thermal energy. The reaction of Mg2Ni with hydrogen is as follows:
Several research efforts have been reported on integrating MH-TES with concentrated solar power (CSP). Harries et al. [10] reviewed the current status of MH based solar thermal energy storage systems and suggested Mg based hydrides for TES applications due to their large hydrogen storage capacity and dissociation temperature ranges that match the CSP requirements. Paskevicius et al. [11] investigated the viability of energy dense hydrogen storage materials for CSP applications. A prototype apparatus was tested with a sample of 19 g of MgH2. The hydride sample was thermally cycled up to 420 °C for more than 20 times without any noteworthy loss in hydrogen storage capacity. Corgnale et al. [12] presented a selection procedure for MH for CSP based TES systems. They carried out a techno-economic analysis to choose the most promising MH and evaluated the performance using simplified system models. The models with NaMgH3, TiH2 and CaH2 as the high temperature alloys were able to achieve high energy storage densities at 600 °C operating temperature.
One of the challenges associated with MH-TES systems is the storage of hydrogen gas released during charging process until it is required again to release heat. The use of a secondary hydriding alloy for hydrogen storage enables the TES system to be compact. This requires two dynamically coupled hydride beds; a high temperature hydride bed for thermal energy storage and a low temperature hydride bed for hydrogen storage. Studies on such coupled hydride beds are rather scarce in literature. Mellouli et al. [13] developed a 2-D mathematical code to study the feasibility and, heat and mass transfer in coupled hydride thermal energy storage system. The system comprised of high temperature hydride (Mg2FeH6) and low temperature hydride (Na3AlH6). Their results demonstrated energy efficiency of 96% with energy density of 90 kWh m−3. Gambini et al. [14] developed a numerical model to analyze the dynamic heat and mass transfer in thermal storage systems. They used a lumped-parameters model to simplify the dynamics of MH energy systems. The analysis revealed that the heat exchanger designs and HTF flow conditions play crucial roles in the transfer of hydrogen between dynamically coupled MH beds and subsequently, in the energy transfer. Recently, Nyamsi et al. [15] performed a comparative study for the selection of paired alloys for MH-TES in two steps. Four alloys namely, Mg, Mg2Ni, Mg2FeH6 and LaNi5 were selected and a detailed 2-D analysis of heat and mass transfer revealed that the Mg based alloys possess high energy density (>1 GJ m−3) due to their high reaction enthalpy, with 30% additional energy density due to substantial sensible heat storage. The Mg based alloys paired with LaNi5 provided high energy storage efficiency of about 80%. d’Entremont et al. [16] presented the technical feasibility and the dynamics of a MH-TES system based on the coupled Mg2FeH6–Na3AlH6 hydride materials. They simulated a small scale model using COMSOL Multiphysics® imposing constant temperature boundaries. The simulations were performed at operating ranges of 450 °C–500 °C temperature and 30–70 bar hydrogen supply pressure. The results showed volumetric energy storage density of about 132 kWh m−3. d’Entremont et al. [17] also developed a simulation model using COMSOL Multiphysics® based on coupled NaMgH2F and TiCr1.6Mn0.2 for solar driven steam power plant applications and achieved a volumetric energy storage density of 226 kWh m−3.
The selection of thermodynamically compatible MH pairs is pivotal for an efficient design of MH-TES system. The objective of the present work is to discuss the thermodynamic compatibility criteria for the selection of MH pairs for cyclic operation and to present a 3-D numerical model to analyze the performance of the system. To accomplish this, van't Hoff equation is used to evaluate the thermodynamic compatibility of hydride materials. COMSOL Multiphysics® is used to develop a 3-D simulation model of MH-TES. COMSOL Multiphysics® is a commercial finite element-based software which facilitates design and simulation of various engineering applications, especially multiphysics or coupled phenomena. A simulation model of MH-TES system consisting of dynamically coupled Mg2Ni and LaNi5 hydrides is developed using COMSOL 5.4. Mg2Ni is referred as the energy storage bed (ESB) and LaNi5 as the hydrogen storage bed (HSB). The numerical model comprises of the chemical reactions between hydride materials and hydrogen gas with mass, momentum and energy balances in the free and porous media. For LaNi5, well established kinetics equations are adopted while for Mg2Ni, suitable kinetics equations are developed using data from literature.
Section snippets
Thermodynamic compatibility and temperature limits for coupled MH-TES
The selection of material pairs is the first step in the design of MH-TES system. In addition to storage capacity and cost, it is also essential to focus on the interoperability between the two hydrides for feasible cyclic operation. The MH-TES system works based on hydrogen flow between the hydride beds. Each hydride bed will have a plateau pressure corresponding to its temperature. The pressure of hydrogen gas in each reactor depends upon the plateau pressure of the hydride bed it contains.
Physical model
A geometric configuration of two concentric tubular MH reactors connected by a tube for hydrogen transfer is chosen for the present study. The reactor (R1), containing Mg2Ni is used for energy storage and the other reactor (R2), containing LaNi5 is used for hydrogen storage. The geometry of one of the reactors is shown in Fig. 3. The two reactors differ in length but they have similar geometry. As shown in Fig. 3, the reactors are designed with outer jackets with HTF flow through the annulus
Mathematical model
In the following section, the governing equations used for thermal modeling are given.
Results and discussion
The simulation is performed on MH-TES for a few consecutive cycles and it is observed that the variations in results are insignificant from the third cycle onwards. Therefore, results of the first three consecutive cycles are presented in this section. The charging and discharging processes are simulated for 20000 s each. The system would completely charge/discharge if the reaction time is increased further. However, the long duration of reaction time could be a drawback. Hence, the
Conclusions
The thermodynamic compatibility criterion to select a feasible hydride pair for cyclic operation of a coupled metal hydride based thermal energy storage system is presented. The MH pair of Mg2Ni and LaNi5 is chosen to analyze the energy storage performance of coupled hydride system in which Mg2Ni is used for energy storage and LaNi5 is used for hydrogen storage. Three consecutive cycles of operation of the coupled Mg2Ni–LaNi5 based TES simulated using COMSOL 5.4 are analyzed. 3-D geometries of
Acknowledgement
The work reported in this paper is supported by the grant (Grant No. DST/TM/SERI/FR/178(G) dated 30-06-2015) from the Department of Science and Technology, Government of India.
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