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Air heated metal hydride energy storage system design and experiments for microgrid applications
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e9
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Air heated metal hydride energy storage system design and experiments for microgrid applications Aliya Khayrullina a,*, Dmitry Blinov b, Vasily Borzenko b a b
Skolkovo Institute of Science and Technology (Skoltech), Russian Federation Joint Institute for High Temperatures of Russian Academy of Sciences (JIHT RAS), Russian Federation
article info
abstract
Article history:
Emerging technologies of the 21st Century introduced bi-directional flows between a big
Received 15 November 2017
number of uncontrollable and unpredictable generators together with a need for energy
Received in revised form
storage (ES) capable of solving instability issues. With the aim of developing new control
25 May 2018
methodologies, Skoltech developed a Smart Grid laboratory that includes a variety of en-
Accepted 28 May 2018
ergy generators, and storage systems. The capabilities of the grid were expanded with a
Available online xxx
metal hydride (MH) ES and 1 kW fuel cell. MH ES performs at the near ambient temperatures and relatively low pressure, it has adjustable properties, satisfactory gravimetric H2
Keywords:
density, and a simple thermal management. However, existing technologies require an
Low temperature metal hydrides
external heat source, which cannot serve the purpose of autonomous microgrid applica-
PEM FC
tions. The aim of this research was to develop and test an air heated metal hydride energy
System integration
storage system that utilizes the internal waste heat of the system.
Energy storage
Based on low power MH ES system experiments [1] and waste heat investigations [2], an air heated system with 1 m3 H2 MH reactor was developed and tested. The experiments were performed in the system that also includes 1 kW fuel cell and an electrolyzer. Obtained results show higher efficiency rate of the system due to waste heat utilization from the air-cooled polymer electrolyte membrane (PEM) FC, ensure mobility for autonomous applications, and open the opportunity for further research in the field of power system control. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Since renewable energy sources (RES) are highly dependent on the uncontrollable external conditions, it is crucial to balance consumption and production profiles by adding some sources of power that can compensate for the fluctuations. Current energy storage technology offers a variety of solutions for different situations, including the technology applicable for Microgrids (MG), a small electric power system that can be
both interconnected with the grid as well as operate autonomously [3,4]. When running in an autonomous mode possible malfunction of an energy source will cause immediate active and reactive power shortage, which must be instantly compensated for. ES technology for MG lowers the level of power fluctuations and deals with imbalance challenges [5,6]. Hybrid ES combinations system design methodology highly depends on the case-based approach due to an extensive variety of ES technologies.
* Corresponding author. E-mail addresses: [email protected], [email protected] (A. Khayrullina). https://doi.org/10.1016/j.ijhydene.2018.05.145 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Khayrullina A, et al., Air heated metal hydride energy storage system design and experiments for microgrid applications, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.145
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Problem statement The current paper investigates the possibility of replacing diesel engines in more than 900 settlements across Russia located outside of the centralized grid. A typical daily load of these small towns is around 30e40 kW reaching 70 kW at the peak hours in October and January [1]. Taking into consideration solar potential of 3.5e4.5 kWh per m2/day in these areas, a pilot project with solar panels produced 30,000 kWh and saved 11 tons of diesel in a year [7]. Adding ES system to the solar panels has a high potential to increase diesel engine replacement percentage eliminating the need to transport diesel to remote areas, which lowers the price of kWh and increases a reliability of the system.
Proposed solution Proposed solution includes a low temperature MH storage system that is feasible for the kW scale stationary applications [8] and an air-cooled PEM FC. The combination is a reliable system with zero CO2 emissions and no loss of capacity over the time compared to batteries. PEM FC was chosen for the characteristics in maneuverability compared to hightemperature FCs and for the modular design with the absence of moving parts. LaNi5 MH storage works at the near ambient temperatures and relatively low pressures but experience low effective thermal conductivity of the metallic alloy powder and relatively high achievable reaction heat (21.1e44.1 kJ/mol H2) [2,9e13]. The optimization of heat exchange and operation regimes for 5 kW PEM FC and 13 st.m3 hydrogen storage system from the point of mass transfer crisis [14] was successfully shown in [15] as well as the utilization of the heat from an FC to an MH storage. The stabilization was ensured by the use of liquid cooling PEM FC that requires extensive water management processes and results in increased weight, cost, and volume of the entire system. Thus air-cooled PEM FCs replaced water-cooled PEM FCs on the commercial market of 1e10 kW range [2] taking away the possibility to reach comparable levels of heat transfer coefficient (more than 120 W/(m2 K)) from the hot air. In [2], the qualitative experiments resulted in an FC exhaust air successfully heating up the 13 st.m3 maximum capacity hydrogen storage to the level of satisfactory volumetric hydrogen output, which exceeded the values needed for 1.1 kW (e) FC. By properly managing the equilibrium parameters (P-C-T) of the hydrogen absorbing material by modifying the composition of the alloy [13] [16e20] there is a high potential to maintain outlet pressure from the reactor.
Economical verification of the proposed system Capital costs (CAPEX) of the proposed hydrogen energy storage system mainly consist of the cost of an electrolyzer, PEM FC, and AB5 type intermetallic compound. In [21], a feasibility analysis of a 10 kW hydrogen backup power system of similar design showed that with a single unit or small series production the cost of AB5 type intermetallic compound can reach 50% of total system cost, bringing total CAPEX of the system to be 3 times higher than total CAPEX of a conventional lead-acid battery system of the same capacity.
However, the operational costs (OPEX) of the hydrogen system are only 15% of the lead-acid system OPEX. Together with no loss of capacity in hydrogen systems and no need for replacement in 4e5 years, the total cost of ownership becomes lower than for batteries over 4e5 year time [21].
Goal of the research Given all the previous findings and experiments, the goal of the research is to create a 1 kW scale storage system utilizing lowtemperature MH and an FC, where FC exhaust air replaces external heating agent for a hydrogen desorption process and ensures the autonomy of the system. Research objectives are listed below: 1) To perform preliminary FC waste heat investigations by measuring the temperature inside and outside the fuel cell 2) To choose and verify the type of LaNi5 family alloy for MH reactor using PCT diagram basing on the outcome of the preliminary investigations in the form of average FC output air temperature 3) To design the system, chose and verify system components 4) To experimentally test the system and specify working regimes and FC performance peculiarities.
State-of-the-art in MH reactors Unlike conventional hydrogen storage systems, keeping the gas under extremely high pressures of several hundreds of bars, the LaNi5 family alloy MH storage keeps it in a bounded form at the pressure levels around 2 bar [12,22]. MH hydrogen storage uses a reversible chemical reaction forming MH during the process of hydrogen sorption and coming back to alloy or intermetallic compound when hydrogen is released. There is a number of challenges both on the fundamental level and on the level of system design. Also, the direct and reverse processes of metal-hydrogen reaction affect the duration of the hydrogen sorption/ desorption cycle that varies significantly for each MH material and affects MH reactor dynamic performance. Since many MH materials experience fast reaction with hydrogen the process itself from the viewpoint of the system, in general, is often limited by a heat transfer [23e26]. The number of challenges here includes, for example, operation at low temperatures or presence of impurities in hydrogen [18,27e34]. These challenges highly affect kinetic factors that can only be controlled on the systematic level, which means that the modeling of heat and mass transfer to and from an MH material is crucial for the rates of hydrogen uptake and release [23,24]. Besides that, degradation effects [25,35], tolerance to impurities in hydrogen and specifics of structure and morphology are widely known in the literature. However, they are beyond the scope of this research. Summing up, on the fundamental level of MH material, it is important to ensure hydrogen absorption and desorption at the near to equilibrium conditions [24], it is also important to control heat and mass transfer that affects kinetic factors of the reaction. Thus, part of this work investigates the
Please cite this article in press as: Khayrullina A, et al., Air heated metal hydride energy storage system design and experiments for microgrid applications, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.145
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possibility of heat and mass transfer of the hydrogen desorption process initiated by a fuel cell waste heat and finds optimal intermetallic compound capable of performing hydrogen release with limited heat consumption. On the level of system design and engineering, there are two main goals: to ensure fast hydrogen charge/discharge rates and high-efficiency level. These goals are associated with some challenges, including a general layout of the system, design of the reactor elements, heat transfer method, and devices, optimization of MH material using parts of the system, gas connections, and system control. Overview of the general layouts of MH reactors was presented in the patent description by Lototsky, Yartys et al. [26,36]. Recent concepts operating at a medium level of temperatures and generating high pressures include multi-stage reactors created by Ergenics Inc. [37]. Most of the systems use water as a heating/cooling agent [38]. However, intensification of heat supply/removal is still the main problem of the MH system that is highly connected to the low conductivity of a solid state MH in the form of powder [33]. Thus, the challenges of design, such as packing density, heat transfer resistance, a geometry of the system elements [39], require modeling approach with further experimental verification, examples can be found in Refs. [40e45]. Intensification of the heat transfer can be achieved by an increase of the surface area of an exchanger, which is done by using long MH reactors with thick wall, “shell-and-tube” solution [46], fins [47], metal blocks [48] or/and with a parallel connection of the tubes [49]. Studying these aspects [43], issues of modeling and optimization [50] and typical MH reactor systems are presented in [51]. One of the important factors of the heat exchange in the described system is a type of heating/ cooling process: fluid [36,52], electric heating [53,54], convective air cooling [47], gas-gap thermal switch [55,56], Peltier devices [26,57], heat pipes with catalytic combustors [58,59]. Heat transfer was studied in all forms of statement, such as external, internal, and combined. It was also tried in different arrangements, sizes, and transfer matrixes. In this paper, a novel power supply unit utilizing waste air as a heating agent is proposed, described, and experimentally investigated.
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Preliminary investigations Preliminary investigations consist of two separate parts. First, we ran the 1 kW maximum power PEM FC in different working modes from zero load to 900 W and measured an internal temperature by placing a temperature sensor inside the stack. In the second part, we focused on the hot air flow from the coolers of the same FC. The load was steadily increasing, and the measurements of temperature and velocity were taken by the anemometry method. Our goal was to experimentally investigate the temperature and the velocity of the hot air flow of the FC. These measurements will help us to choose the right metallic alloy for the MH storage based on the PCT diagrams.
FC internal temperature measurement In the first experiment, the commercial 1 kW Hoppecke E-1100 PEM FC was used with a temperature sensor inside the stack. The FC was loaded with a variety of loads: zero load, constant load (400e500 W), volatile load (from 0 to 700 W, from 0 to 900 W), maximum load for the experiment (900 W), etc. Two representative graphs of the temperature measurements are shown in Fig. 1. In the first test, the FC started with no load, and the temperature grew steadily to 26 C, on the 50th minute a load of 450 W was introduced, and the temperature grew to 38 C almost immediately. In the 90th minute, the load was switched off again, and the temperature slowly decreased to 25 C. During the second test, on the 10th minute 300 W load was introduced and increased to 700 W, we created a crisis were FC did not receive enough hydrogen pressure for stable performance and shut down on the 20th minute. The FC tried to restart but was kept switched-off until the 30th minute. Regardless of this shutdown, the temperature inside the reactor did not decrease and stayed around 35 C. In the 35th minute, we introduced the peak load of 900 W and continued to switch it between 500 W and 600 W for the rest of the experiment. On the 130th minute we lowered the load down to zero, and the temperature slowly decreased to 28 C until the
Fig. 1 e Temperature inside the 1 kW Hoppecke 1100 PEM FC during different working regimes (a) e zero load, constant load (400 We500 W); (b) e volatile load, peak load (900 W). Please cite this article in press as: Khayrullina A, et al., Air heated metal hydride energy storage system design and experiments for microgrid applications, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.145
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automatic process of powering up the backup battery turned on. To conclude these experiments, we conducted a variety of temperature measurements inside the FC stack during different working modes and regimes, including zero load and crisis shutdown. The findings of these tests are the following: - the FC heats up to 25e30 C even in the absence of load - crisis shut downs do not bring a rapid decrease in the FC, on the contrary, the temperature stays on the levels higher than the zero load temperature - we can estimate 30 C as a lower average FC internal temperature taking into account lower temperatures during the start-up procedures
FC exhaust heat output measurement The goal of the second preliminary investigation was to experimentally measure the output heat flow of the exhaust air from the same commercial 1 kW Hoppecke E-1100 PEM FC with five cooling fans on the back as shown in Fig. 2. Two parameters were measured on each cooling fan separately: - a temperature of the flow, oC - a velocity of the flow, m/s While measuring these two parameters, the load was changed of the system that led to the power level on the FC stack to change. The change of the power was in the range from 70 to 840 W. The measurements were taken by thermal anemometry method using Testo 450. The temperature of the output heat was measured in the dynamic state of airflow. The results are shown in Fig. 3. The first outcome of the airflow measurement states that the output temperature at a given moment of time is almost identical to the temperatures at all cooling fans along the backside of the fuel cell. Second, the temperature of the outflow increases with an increase of the power on the FC stack. The more demand triggers larger amount of reactions happening in the cell; the more heat is being released as the result of these reactions. The 41.92 C temperature point received at 635 W load was measured at the end of the experiment after multiple high-
1 kW FC waste air output temperature 44
Temperature, oC
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Fig. 3 e 1 kW Hoppecke E-1100 PEM FC exhaust air temperature (averaged for 5 cooling fans and scatter). level loads, which means that the FC most likely “collects” the heat closer to the end of the working session. However, the change of the temperature is very limited compared to the change in the output power. The temperature changed from 26.5 C to 44.5 C after the power had changed from 75 W to 835 W (more than 11 times higher). Even though the temperature output went above 25 C almost immediately after the FC came to the stable working mode the measured temperature did not exceed 45 C (318 K) in general, which means that the metallic alloy for the MH storage needs to be chosen with the sorption and desorption plateaus around these temperatures. It limits the variety of the possible metallic alloys that could be used. The velocity measurement is shown in Fig. 4. It can be identified that the velocity is not equal among five coolers for one power output, and there is also no correlation of velocity change with the increase of power output. The central assumption is that the velocity forms non-constant heat output variable that controls the entire amount of released heat. It was also noted that the velocity increases during an intense cooling period that happens almost immediately after a rapid power output decrease. The main outcome of the section states that the alloy for MH storage should be able to give required pressure levels even receiving temperatures below 30 C. We make an
1 kW FC waste air output velocity
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Fig. 2 e The view of 1 kW Hoppecke 1100 PEM FC with five cooling fans on the back.
Fig. 4 e 1 kW Hoppecke E-1100 PEM FC exhaust air velocity (averaged for 5 coolers and scatter).
Please cite this article in press as: Khayrullina A, et al., Air heated metal hydride energy storage system design and experiments for microgrid applications, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.145
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assumption that the average temperature of the exhaust air of the 1 kW PEM FC can be estimated as 30 C and the metallic alloy isotherms should be compared according to this finding.
Alloy composition verification In the works of JIHT RAS [12,14,15,60,61], a variety of LaNi5 family alloys were studied, and P-C-T diagrams for these alloys were obtained. The diagrams show hydrogen pressure inside the reactor depending on the temperature of the heating agent and the concentration of H2. These diagrams provide enough information to choose a proper alloy that will release satisfactory output pressure of hydrogen experiencing relatively low 30 C - 40 C temperatures of heating agent. As an outcome of this study, a La0.9Ce0.1Ni5 was chosen for the designed system. The P-C-T diagram of this alloy obtained experimentally in [60] is shown in Fig. 5. According to the 1 kW Hoppecke E-1100 PEM FC specification, operating pressures (excessive) of hydrogen should be in the range between 0.55 and 0.83 bar, which is 0.055e0.083 MPa respectfully. Experimentally, we noted the low level of hydrogen pressure (excessive) to be around 0.13 MPa for the safe operation. On the PCT diagram, the 20 C desorption plateau is situated on the level of 0.2 MPa, and the 40 C desorption plateau is situated on the level of around 0.45 MPa. It can be concluded, that the required pressure of hydrogen from the MH reactor with this alloy can be reached at the temperatures below 20 C. Thus, the proposed alloy is suitable for the use in this MH system. The lower average temperature level of output air of the FC is 30 C, and the La0.9Ce0.1Ni5 alloy ensures required pressure levels with the heating agent temperatures around 20 C. The difference in 10 C was kept for the losses in the system and possible unstable output air levels.
H2Smart prototype development The goal of the first prototype was to develop and create an experimental power supply system with hydrogen energy storage H2Smart having a power of 1 kW and utilizing FC
waste heat in the system. The technology ensures the quality of electrical energy in micro energy systems that have a load and distributed energy sources through the use of hydrogen energy storage. Hydrogen is produced by electrolysis utilizing waste energy from the renewable sources. Electricity is generated by the PEM fuel cell. This system is designed to be an experimental energy storage system in Smart Grids and to be an essential part of an educational process in Energy Systems programs at Skoltech.
Scheme of the system, working principle The system (Fig. 6) uses waste electrical energy as an input from the renewable sources of energy or the simulation of renewable sources of energy in the Smart Grid laboratory in the Center for Energy Systems of Skoltech. The electricity supplies the electrolyzer and releases hydrogen that gets processed through an H2 dehumidifier due to high-level requirements to the absence of humidity in hydrogen that is sent to a metallic alloy (Fig. 6c). Even considerably low amounts of moisture can irreversibly affect metallic alloy and start the degradation process. Dehumidified hydrogen enters the sorption process inside the reactor with cold water supplied either through an external input or from the radiator. Then hydrogen gets stored in the reactor until the demand load exceeds the production from renewable sources of energy or the simulation of such production. On the next step, the fuel cell enters a startup procedure provided by the backup small capacity battery supply. The reactor releases hydrogen due to a desorption process that is initiated by an external heating agent. Hot air from the fuel cell passes through the radiator and heats up water inside. With the help of a pump, the warm water gets sent to the MH reactor for further desorption process of hydrogen. Output electricity is a 220 V regular outlet through an inverter. The system collects different sets of data and ensures the response to the user changes through the control system based on NI-PXI with a user interface in LabView. Electrolyser H2Box-100 (Fig. 6a) produces electrolytic hydrogen that is being used in industrial and research laboratories to supply chromatographs and gas analyzers with plasma-ionic detectors and other technological processes that
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Fig. 5 e P-C-T (pressure, concentration, temperature) diagram of La0.9Ce0.1Ni5 [60]. Please cite this article in press as: Khayrullina A, et al., Air heated metal hydride energy storage system design and experiments for microgrid applications, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.145
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Fig. 6 e H2Smart experimental setup (a) - rear view; (b) - view of the reactor; (c) - scheme of the H2Smart experimental setup.
require pure hydrogen. The main component of H2Box-100 is an electrolyzer with solid polymer electrolyte with a high proven resource (more than 20000 h). Metal hydride H2 storage is a sectioned type reactor (Fig. 6b) that consists of four containers with metallic alloy, a water heat exchanger and a flange with a sealed high-pressure resistant connector for temperature sensors output signal cables [9]. The heat exchanger is simultaneously a loadbearing outer shell of the module and provides strength, rigidity, and tightness of the structure. At the same time, inside its durable casing, there is a tube that supplies the reactor with the cooling or heating water. The containers consist of the porous metallic alloy that accumulates hydrogen. The inlet walls of the containers are made from the porous material that lets the gas pass through but keeps the metallic alloy from entering the free volume. The reactor is filled with 5 kg of La0.9Ce0.1Ni5, maximum H2 capacity is 1000 st.l, and nominal operating capacity is 720 st.l.
this experiment, the MH reactor was able to maintain required pressure level for the entire duration of the experiment. The graph shows that the pressure rose to 0.7 MPa before the valve was open, then it decreased to 0.25 MPa and started to decrease with the decrease of the amount of hydrogen left in the system. Peaks on the graph are connected with the volatile rise of the heating agent temperature. During the first 4 min the battery was supplying the FC start-up procedures, then the load on the FC was zero. Thus the FC was recharging the backup battery. On the 50th minute, a 450 W load was introduced. It was kept for 40 min, and then went back to zero. During these 40 min, the FC both met the load and powered up the battery. It was noted that in the case of load level being equal to the power FC could provide,
Experimental investigations of MH reactor integration with FC in the H2Smart system Experimental investigations of working regimes The goal of these tests was to experimentally prove the possibility of integration of the proposed MH reactor with a commercial 1 kW PEM FC, explore working regimes of the system, run the system coupled with a backup power supply in the form of a battery. The MH was fully charged before the start of the experiment and was discharged by the desorption process for about 100 min. The pressure in the MH reactor is shown in Fig. 7, power distribution e in Fig. 8. Previously, the pressure level of an FC hydrogen inlet pressure was found experimentally to be around 0.13 MPa. In
Fig. 7 e Pressure in the MH reactor.
Please cite this article in press as: Khayrullina A, et al., Air heated metal hydride energy storage system design and experiments for microgrid applications, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.145
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picture, the blue line and the red line are identical due to the absence of the demand load. In case the FC shuts down or experience a low pressure of hydrogen from the MH back-up battery meets the demand of the load ensuring the reliability of the supply.
Experimental investigations of an FC system response
Fig. 8 e Power distribution during different working regimes.
charging of the battery stops automatically. In the cases of the load being higher than the FC, the battery performs backup supply for the load. It was investigated more in the 6.2 of the current paper. In Fig. 8 the results of the experiments are shown in the two working regimes of the system: 1). zero demand load when all the energy from the FC charges the backup battery 2). increased demand load to 450 W when the FC supplies both the battery and the demand The black line indicates the power requested and received by the load; the red line shows the power received by the battery; the blue line signifies the power output of the fuel cell. The power output is positive on the graph, but the concept of output can be perceived as negative and stand for the sum of both loads (battery and the demand). On the first part of the
Fig. 9 e Power distribution during FC shut down and a system response.
The power distribution for the second part of experimental investigations is shown in Fig. 9. The same color indication is used with the previous graph. For the first 17 min, enough pressure was supplied to the FC, and the load was increasing in a step-by-step process: 300 W, 500 W, and 700 W. Then the pressure for FC was made below operational values of 0.055e0.083 MPa (excessive) and the FC shut down can be noted on the 20th minute. The red lower line indicates that backup battery supply met the demand of the load. The same backup supply performance can be noted on the 125th minute, where FC again experienced a shutdown. On the 22nd minute, rapid peak demand of the load was tested, FC started the operation and met the demand. The lower red peak on the 22nd minute indicates the nuance we noticed further during the operation of the FC. Given different load levels higher and lower than 450 W, the plateau of the FC power output was noted. After multiple additional experiments, we can assume that the voltage level of the battery limits the output of the FC.
Conclusions Preliminary investigations and experimental results show the possibility of using the exhaust air flow from a commercial kW scale PEM FC for the needs of desorption processes in the MH storage system that previously needed a liquid external heating agent and limited the use in the autonomous systems. The proposed novel method of both utilizing waste heat and replacing heating agent in the MH show promising results for further increase of the efficiency. Possible next steps may include the introduction of thermal insulation, further investigation in metallic alloys with the search for higher pressure rates at the low temperatures 20e40 C, heat transfer intensification, radiator replacement by the design of the reactor itself. However, the design of the reactor should take into consideration high humidity level of the outlet FC hot air and eliminate condensation issues. The possibility of using the proposed concept with the high-temperature FCs shows promising future to the technology and the research in this field. The results obtained in this research prove the concept of autonomous energy storage technology feasible for the installation together with solar panels in Far East settlements in Russia. The next step of the project is to introduce a 30 kW hydrogen set up with a new design of the reactor with intensified heat and mass transfer properties. Main outcomes of the research include the following: - 1 kWh hydrogen energy storage prototype was successfully developed and tested
Please cite this article in press as: Khayrullina A, et al., Air heated metal hydride energy storage system design and experiments for microgrid applications, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.145
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- the novel concept of the fuel cell output hot air utilization was proved - the temperature of the FC inside and outside the system was measured - a metallic alloy was chosen suitable for this application - multiple working regimes of the system were shown, including stand by, constant load, volatile loads, peaks, crisis shut down responses - the autonomy of the system was introduced by the replacement of an external heating agent for the metal hydride reactor to the internal waste heat in the system - the energy storage prototype created for the use in renewable energy autonomous power supply systems for the Far East settlements in Russia
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Acknowledgements [16]
The research was supported by Russian Science Foundation grant (project #17-79-20413).
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Air heated metal hydride energy storage system design and experiments for microgrid applications Aliya Khayrullina a,*, Dmitry Blinov b, Vasily Borzenko b a b
Skolkovo Institute of Science and Technology (Skoltech), Russian Federation Joint Institute for High Temperatures of Russian Academy of Sciences (JIHT RAS), Russian Federation
article info
abstract
Article history:
Emerging technologies of the 21st Century introduced bi-directional flows between a big
Received 15 November 2017
number of uncontrollable and unpredictable generators together with a need for energy
Received in revised form
storage (ES) capable of solving instability issues. With the aim of developing new control
25 May 2018
methodologies, Skoltech developed a Smart Grid laboratory that includes a variety of en-
Accepted 28 May 2018
ergy generators, and storage systems. The capabilities of the grid were expanded with a
Available online xxx
metal hydride (MH) ES and 1 kW fuel cell. MH ES performs at the near ambient temperatures and relatively low pressure, it has adjustable properties, satisfactory gravimetric H2
Keywords:
density, and a simple thermal management. However, existing technologies require an
Low temperature metal hydrides
external heat source, which cannot serve the purpose of autonomous microgrid applica-
PEM FC
tions. The aim of this research was to develop and test an air heated metal hydride energy
System integration
storage system that utilizes the internal waste heat of the system.
Energy storage
Based on low power MH ES system experiments [1] and waste heat investigations [2], an air heated system with 1 m3 H2 MH reactor was developed and tested. The experiments were performed in the system that also includes 1 kW fuel cell and an electrolyzer. Obtained results show higher efficiency rate of the system due to waste heat utilization from the air-cooled polymer electrolyte membrane (PEM) FC, ensure mobility for autonomous applications, and open the opportunity for further research in the field of power system control. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Since renewable energy sources (RES) are highly dependent on the uncontrollable external conditions, it is crucial to balance consumption and production profiles by adding some sources of power that can compensate for the fluctuations. Current energy storage technology offers a variety of solutions for different situations, including the technology applicable for Microgrids (MG), a small electric power system that can be
both interconnected with the grid as well as operate autonomously [3,4]. When running in an autonomous mode possible malfunction of an energy source will cause immediate active and reactive power shortage, which must be instantly compensated for. ES technology for MG lowers the level of power fluctuations and deals with imbalance challenges [5,6]. Hybrid ES combinations system design methodology highly depends on the case-based approach due to an extensive variety of ES technologies.
* Corresponding author. E-mail addresses: [email protected], [email protected] (A. Khayrullina). https://doi.org/10.1016/j.ijhydene.2018.05.145 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Khayrullina A, et al., Air heated metal hydride energy storage system design and experiments for microgrid applications, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.145
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Problem statement The current paper investigates the possibility of replacing diesel engines in more than 900 settlements across Russia located outside of the centralized grid. A typical daily load of these small towns is around 30e40 kW reaching 70 kW at the peak hours in October and January [1]. Taking into consideration solar potential of 3.5e4.5 kWh per m2/day in these areas, a pilot project with solar panels produced 30,000 kWh and saved 11 tons of diesel in a year [7]. Adding ES system to the solar panels has a high potential to increase diesel engine replacement percentage eliminating the need to transport diesel to remote areas, which lowers the price of kWh and increases a reliability of the system.
Proposed solution Proposed solution includes a low temperature MH storage system that is feasible for the kW scale stationary applications [8] and an air-cooled PEM FC. The combination is a reliable system with zero CO2 emissions and no loss of capacity over the time compared to batteries. PEM FC was chosen for the characteristics in maneuverability compared to hightemperature FCs and for the modular design with the absence of moving parts. LaNi5 MH storage works at the near ambient temperatures and relatively low pressures but experience low effective thermal conductivity of the metallic alloy powder and relatively high achievable reaction heat (21.1e44.1 kJ/mol H2) [2,9e13]. The optimization of heat exchange and operation regimes for 5 kW PEM FC and 13 st.m3 hydrogen storage system from the point of mass transfer crisis [14] was successfully shown in [15] as well as the utilization of the heat from an FC to an MH storage. The stabilization was ensured by the use of liquid cooling PEM FC that requires extensive water management processes and results in increased weight, cost, and volume of the entire system. Thus air-cooled PEM FCs replaced water-cooled PEM FCs on the commercial market of 1e10 kW range [2] taking away the possibility to reach comparable levels of heat transfer coefficient (more than 120 W/(m2 K)) from the hot air. In [2], the qualitative experiments resulted in an FC exhaust air successfully heating up the 13 st.m3 maximum capacity hydrogen storage to the level of satisfactory volumetric hydrogen output, which exceeded the values needed for 1.1 kW (e) FC. By properly managing the equilibrium parameters (P-C-T) of the hydrogen absorbing material by modifying the composition of the alloy [13] [16e20] there is a high potential to maintain outlet pressure from the reactor.
Economical verification of the proposed system Capital costs (CAPEX) of the proposed hydrogen energy storage system mainly consist of the cost of an electrolyzer, PEM FC, and AB5 type intermetallic compound. In [21], a feasibility analysis of a 10 kW hydrogen backup power system of similar design showed that with a single unit or small series production the cost of AB5 type intermetallic compound can reach 50% of total system cost, bringing total CAPEX of the system to be 3 times higher than total CAPEX of a conventional lead-acid battery system of the same capacity.
However, the operational costs (OPEX) of the hydrogen system are only 15% of the lead-acid system OPEX. Together with no loss of capacity in hydrogen systems and no need for replacement in 4e5 years, the total cost of ownership becomes lower than for batteries over 4e5 year time [21].
Goal of the research Given all the previous findings and experiments, the goal of the research is to create a 1 kW scale storage system utilizing lowtemperature MH and an FC, where FC exhaust air replaces external heating agent for a hydrogen desorption process and ensures the autonomy of the system. Research objectives are listed below: 1) To perform preliminary FC waste heat investigations by measuring the temperature inside and outside the fuel cell 2) To choose and verify the type of LaNi5 family alloy for MH reactor using PCT diagram basing on the outcome of the preliminary investigations in the form of average FC output air temperature 3) To design the system, chose and verify system components 4) To experimentally test the system and specify working regimes and FC performance peculiarities.
State-of-the-art in MH reactors Unlike conventional hydrogen storage systems, keeping the gas under extremely high pressures of several hundreds of bars, the LaNi5 family alloy MH storage keeps it in a bounded form at the pressure levels around 2 bar [12,22]. MH hydrogen storage uses a reversible chemical reaction forming MH during the process of hydrogen sorption and coming back to alloy or intermetallic compound when hydrogen is released. There is a number of challenges both on the fundamental level and on the level of system design. Also, the direct and reverse processes of metal-hydrogen reaction affect the duration of the hydrogen sorption/ desorption cycle that varies significantly for each MH material and affects MH reactor dynamic performance. Since many MH materials experience fast reaction with hydrogen the process itself from the viewpoint of the system, in general, is often limited by a heat transfer [23e26]. The number of challenges here includes, for example, operation at low temperatures or presence of impurities in hydrogen [18,27e34]. These challenges highly affect kinetic factors that can only be controlled on the systematic level, which means that the modeling of heat and mass transfer to and from an MH material is crucial for the rates of hydrogen uptake and release [23,24]. Besides that, degradation effects [25,35], tolerance to impurities in hydrogen and specifics of structure and morphology are widely known in the literature. However, they are beyond the scope of this research. Summing up, on the fundamental level of MH material, it is important to ensure hydrogen absorption and desorption at the near to equilibrium conditions [24], it is also important to control heat and mass transfer that affects kinetic factors of the reaction. Thus, part of this work investigates the
Please cite this article in press as: Khayrullina A, et al., Air heated metal hydride energy storage system design and experiments for microgrid applications, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.145
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possibility of heat and mass transfer of the hydrogen desorption process initiated by a fuel cell waste heat and finds optimal intermetallic compound capable of performing hydrogen release with limited heat consumption. On the level of system design and engineering, there are two main goals: to ensure fast hydrogen charge/discharge rates and high-efficiency level. These goals are associated with some challenges, including a general layout of the system, design of the reactor elements, heat transfer method, and devices, optimization of MH material using parts of the system, gas connections, and system control. Overview of the general layouts of MH reactors was presented in the patent description by Lototsky, Yartys et al. [26,36]. Recent concepts operating at a medium level of temperatures and generating high pressures include multi-stage reactors created by Ergenics Inc. [37]. Most of the systems use water as a heating/cooling agent [38]. However, intensification of heat supply/removal is still the main problem of the MH system that is highly connected to the low conductivity of a solid state MH in the form of powder [33]. Thus, the challenges of design, such as packing density, heat transfer resistance, a geometry of the system elements [39], require modeling approach with further experimental verification, examples can be found in Refs. [40e45]. Intensification of the heat transfer can be achieved by an increase of the surface area of an exchanger, which is done by using long MH reactors with thick wall, “shell-and-tube” solution [46], fins [47], metal blocks [48] or/and with a parallel connection of the tubes [49]. Studying these aspects [43], issues of modeling and optimization [50] and typical MH reactor systems are presented in [51]. One of the important factors of the heat exchange in the described system is a type of heating/ cooling process: fluid [36,52], electric heating [53,54], convective air cooling [47], gas-gap thermal switch [55,56], Peltier devices [26,57], heat pipes with catalytic combustors [58,59]. Heat transfer was studied in all forms of statement, such as external, internal, and combined. It was also tried in different arrangements, sizes, and transfer matrixes. In this paper, a novel power supply unit utilizing waste air as a heating agent is proposed, described, and experimentally investigated.
3
Preliminary investigations Preliminary investigations consist of two separate parts. First, we ran the 1 kW maximum power PEM FC in different working modes from zero load to 900 W and measured an internal temperature by placing a temperature sensor inside the stack. In the second part, we focused on the hot air flow from the coolers of the same FC. The load was steadily increasing, and the measurements of temperature and velocity were taken by the anemometry method. Our goal was to experimentally investigate the temperature and the velocity of the hot air flow of the FC. These measurements will help us to choose the right metallic alloy for the MH storage based on the PCT diagrams.
FC internal temperature measurement In the first experiment, the commercial 1 kW Hoppecke E-1100 PEM FC was used with a temperature sensor inside the stack. The FC was loaded with a variety of loads: zero load, constant load (400e500 W), volatile load (from 0 to 700 W, from 0 to 900 W), maximum load for the experiment (900 W), etc. Two representative graphs of the temperature measurements are shown in Fig. 1. In the first test, the FC started with no load, and the temperature grew steadily to 26 C, on the 50th minute a load of 450 W was introduced, and the temperature grew to 38 C almost immediately. In the 90th minute, the load was switched off again, and the temperature slowly decreased to 25 C. During the second test, on the 10th minute 300 W load was introduced and increased to 700 W, we created a crisis were FC did not receive enough hydrogen pressure for stable performance and shut down on the 20th minute. The FC tried to restart but was kept switched-off until the 30th minute. Regardless of this shutdown, the temperature inside the reactor did not decrease and stayed around 35 C. In the 35th minute, we introduced the peak load of 900 W and continued to switch it between 500 W and 600 W for the rest of the experiment. On the 130th minute we lowered the load down to zero, and the temperature slowly decreased to 28 C until the
Fig. 1 e Temperature inside the 1 kW Hoppecke 1100 PEM FC during different working regimes (a) e zero load, constant load (400 We500 W); (b) e volatile load, peak load (900 W). Please cite this article in press as: Khayrullina A, et al., Air heated metal hydride energy storage system design and experiments for microgrid applications, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.145
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automatic process of powering up the backup battery turned on. To conclude these experiments, we conducted a variety of temperature measurements inside the FC stack during different working modes and regimes, including zero load and crisis shutdown. The findings of these tests are the following: - the FC heats up to 25e30 C even in the absence of load - crisis shut downs do not bring a rapid decrease in the FC, on the contrary, the temperature stays on the levels higher than the zero load temperature - we can estimate 30 C as a lower average FC internal temperature taking into account lower temperatures during the start-up procedures
FC exhaust heat output measurement The goal of the second preliminary investigation was to experimentally measure the output heat flow of the exhaust air from the same commercial 1 kW Hoppecke E-1100 PEM FC with five cooling fans on the back as shown in Fig. 2. Two parameters were measured on each cooling fan separately: - a temperature of the flow, oC - a velocity of the flow, m/s While measuring these two parameters, the load was changed of the system that led to the power level on the FC stack to change. The change of the power was in the range from 70 to 840 W. The measurements were taken by thermal anemometry method using Testo 450. The temperature of the output heat was measured in the dynamic state of airflow. The results are shown in Fig. 3. The first outcome of the airflow measurement states that the output temperature at a given moment of time is almost identical to the temperatures at all cooling fans along the backside of the fuel cell. Second, the temperature of the outflow increases with an increase of the power on the FC stack. The more demand triggers larger amount of reactions happening in the cell; the more heat is being released as the result of these reactions. The 41.92 C temperature point received at 635 W load was measured at the end of the experiment after multiple high-
1 kW FC waste air output temperature 44
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Fig. 3 e 1 kW Hoppecke E-1100 PEM FC exhaust air temperature (averaged for 5 cooling fans and scatter). level loads, which means that the FC most likely “collects” the heat closer to the end of the working session. However, the change of the temperature is very limited compared to the change in the output power. The temperature changed from 26.5 C to 44.5 C after the power had changed from 75 W to 835 W (more than 11 times higher). Even though the temperature output went above 25 C almost immediately after the FC came to the stable working mode the measured temperature did not exceed 45 C (318 K) in general, which means that the metallic alloy for the MH storage needs to be chosen with the sorption and desorption plateaus around these temperatures. It limits the variety of the possible metallic alloys that could be used. The velocity measurement is shown in Fig. 4. It can be identified that the velocity is not equal among five coolers for one power output, and there is also no correlation of velocity change with the increase of power output. The central assumption is that the velocity forms non-constant heat output variable that controls the entire amount of released heat. It was also noted that the velocity increases during an intense cooling period that happens almost immediately after a rapid power output decrease. The main outcome of the section states that the alloy for MH storage should be able to give required pressure levels even receiving temperatures below 30 C. We make an
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Fig. 2 e The view of 1 kW Hoppecke 1100 PEM FC with five cooling fans on the back.
Fig. 4 e 1 kW Hoppecke E-1100 PEM FC exhaust air velocity (averaged for 5 coolers and scatter).
Please cite this article in press as: Khayrullina A, et al., Air heated metal hydride energy storage system design and experiments for microgrid applications, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.145
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assumption that the average temperature of the exhaust air of the 1 kW PEM FC can be estimated as 30 C and the metallic alloy isotherms should be compared according to this finding.
Alloy composition verification In the works of JIHT RAS [12,14,15,60,61], a variety of LaNi5 family alloys were studied, and P-C-T diagrams for these alloys were obtained. The diagrams show hydrogen pressure inside the reactor depending on the temperature of the heating agent and the concentration of H2. These diagrams provide enough information to choose a proper alloy that will release satisfactory output pressure of hydrogen experiencing relatively low 30 C - 40 C temperatures of heating agent. As an outcome of this study, a La0.9Ce0.1Ni5 was chosen for the designed system. The P-C-T diagram of this alloy obtained experimentally in [60] is shown in Fig. 5. According to the 1 kW Hoppecke E-1100 PEM FC specification, operating pressures (excessive) of hydrogen should be in the range between 0.55 and 0.83 bar, which is 0.055e0.083 MPa respectfully. Experimentally, we noted the low level of hydrogen pressure (excessive) to be around 0.13 MPa for the safe operation. On the PCT diagram, the 20 C desorption plateau is situated on the level of 0.2 MPa, and the 40 C desorption plateau is situated on the level of around 0.45 MPa. It can be concluded, that the required pressure of hydrogen from the MH reactor with this alloy can be reached at the temperatures below 20 C. Thus, the proposed alloy is suitable for the use in this MH system. The lower average temperature level of output air of the FC is 30 C, and the La0.9Ce0.1Ni5 alloy ensures required pressure levels with the heating agent temperatures around 20 C. The difference in 10 C was kept for the losses in the system and possible unstable output air levels.
H2Smart prototype development The goal of the first prototype was to develop and create an experimental power supply system with hydrogen energy storage H2Smart having a power of 1 kW and utilizing FC
waste heat in the system. The technology ensures the quality of electrical energy in micro energy systems that have a load and distributed energy sources through the use of hydrogen energy storage. Hydrogen is produced by electrolysis utilizing waste energy from the renewable sources. Electricity is generated by the PEM fuel cell. This system is designed to be an experimental energy storage system in Smart Grids and to be an essential part of an educational process in Energy Systems programs at Skoltech.
Scheme of the system, working principle The system (Fig. 6) uses waste electrical energy as an input from the renewable sources of energy or the simulation of renewable sources of energy in the Smart Grid laboratory in the Center for Energy Systems of Skoltech. The electricity supplies the electrolyzer and releases hydrogen that gets processed through an H2 dehumidifier due to high-level requirements to the absence of humidity in hydrogen that is sent to a metallic alloy (Fig. 6c). Even considerably low amounts of moisture can irreversibly affect metallic alloy and start the degradation process. Dehumidified hydrogen enters the sorption process inside the reactor with cold water supplied either through an external input or from the radiator. Then hydrogen gets stored in the reactor until the demand load exceeds the production from renewable sources of energy or the simulation of such production. On the next step, the fuel cell enters a startup procedure provided by the backup small capacity battery supply. The reactor releases hydrogen due to a desorption process that is initiated by an external heating agent. Hot air from the fuel cell passes through the radiator and heats up water inside. With the help of a pump, the warm water gets sent to the MH reactor for further desorption process of hydrogen. Output electricity is a 220 V regular outlet through an inverter. The system collects different sets of data and ensures the response to the user changes through the control system based on NI-PXI with a user interface in LabView. Electrolyser H2Box-100 (Fig. 6a) produces electrolytic hydrogen that is being used in industrial and research laboratories to supply chromatographs and gas analyzers with plasma-ionic detectors and other technological processes that
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Mass % H2
Fig. 5 e P-C-T (pressure, concentration, temperature) diagram of La0.9Ce0.1Ni5 [60]. Please cite this article in press as: Khayrullina A, et al., Air heated metal hydride energy storage system design and experiments for microgrid applications, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.145
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Fig. 6 e H2Smart experimental setup (a) - rear view; (b) - view of the reactor; (c) - scheme of the H2Smart experimental setup.
require pure hydrogen. The main component of H2Box-100 is an electrolyzer with solid polymer electrolyte with a high proven resource (more than 20000 h). Metal hydride H2 storage is a sectioned type reactor (Fig. 6b) that consists of four containers with metallic alloy, a water heat exchanger and a flange with a sealed high-pressure resistant connector for temperature sensors output signal cables [9]. The heat exchanger is simultaneously a loadbearing outer shell of the module and provides strength, rigidity, and tightness of the structure. At the same time, inside its durable casing, there is a tube that supplies the reactor with the cooling or heating water. The containers consist of the porous metallic alloy that accumulates hydrogen. The inlet walls of the containers are made from the porous material that lets the gas pass through but keeps the metallic alloy from entering the free volume. The reactor is filled with 5 kg of La0.9Ce0.1Ni5, maximum H2 capacity is 1000 st.l, and nominal operating capacity is 720 st.l.
this experiment, the MH reactor was able to maintain required pressure level for the entire duration of the experiment. The graph shows that the pressure rose to 0.7 MPa before the valve was open, then it decreased to 0.25 MPa and started to decrease with the decrease of the amount of hydrogen left in the system. Peaks on the graph are connected with the volatile rise of the heating agent temperature. During the first 4 min the battery was supplying the FC start-up procedures, then the load on the FC was zero. Thus the FC was recharging the backup battery. On the 50th minute, a 450 W load was introduced. It was kept for 40 min, and then went back to zero. During these 40 min, the FC both met the load and powered up the battery. It was noted that in the case of load level being equal to the power FC could provide,
Experimental investigations of MH reactor integration with FC in the H2Smart system Experimental investigations of working regimes The goal of these tests was to experimentally prove the possibility of integration of the proposed MH reactor with a commercial 1 kW PEM FC, explore working regimes of the system, run the system coupled with a backup power supply in the form of a battery. The MH was fully charged before the start of the experiment and was discharged by the desorption process for about 100 min. The pressure in the MH reactor is shown in Fig. 7, power distribution e in Fig. 8. Previously, the pressure level of an FC hydrogen inlet pressure was found experimentally to be around 0.13 MPa. In
Fig. 7 e Pressure in the MH reactor.
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picture, the blue line and the red line are identical due to the absence of the demand load. In case the FC shuts down or experience a low pressure of hydrogen from the MH back-up battery meets the demand of the load ensuring the reliability of the supply.
Experimental investigations of an FC system response
Fig. 8 e Power distribution during different working regimes.
charging of the battery stops automatically. In the cases of the load being higher than the FC, the battery performs backup supply for the load. It was investigated more in the 6.2 of the current paper. In Fig. 8 the results of the experiments are shown in the two working regimes of the system: 1). zero demand load when all the energy from the FC charges the backup battery 2). increased demand load to 450 W when the FC supplies both the battery and the demand The black line indicates the power requested and received by the load; the red line shows the power received by the battery; the blue line signifies the power output of the fuel cell. The power output is positive on the graph, but the concept of output can be perceived as negative and stand for the sum of both loads (battery and the demand). On the first part of the
Fig. 9 e Power distribution during FC shut down and a system response.
The power distribution for the second part of experimental investigations is shown in Fig. 9. The same color indication is used with the previous graph. For the first 17 min, enough pressure was supplied to the FC, and the load was increasing in a step-by-step process: 300 W, 500 W, and 700 W. Then the pressure for FC was made below operational values of 0.055e0.083 MPa (excessive) and the FC shut down can be noted on the 20th minute. The red lower line indicates that backup battery supply met the demand of the load. The same backup supply performance can be noted on the 125th minute, where FC again experienced a shutdown. On the 22nd minute, rapid peak demand of the load was tested, FC started the operation and met the demand. The lower red peak on the 22nd minute indicates the nuance we noticed further during the operation of the FC. Given different load levels higher and lower than 450 W, the plateau of the FC power output was noted. After multiple additional experiments, we can assume that the voltage level of the battery limits the output of the FC.
Conclusions Preliminary investigations and experimental results show the possibility of using the exhaust air flow from a commercial kW scale PEM FC for the needs of desorption processes in the MH storage system that previously needed a liquid external heating agent and limited the use in the autonomous systems. The proposed novel method of both utilizing waste heat and replacing heating agent in the MH show promising results for further increase of the efficiency. Possible next steps may include the introduction of thermal insulation, further investigation in metallic alloys with the search for higher pressure rates at the low temperatures 20e40 C, heat transfer intensification, radiator replacement by the design of the reactor itself. However, the design of the reactor should take into consideration high humidity level of the outlet FC hot air and eliminate condensation issues. The possibility of using the proposed concept with the high-temperature FCs shows promising future to the technology and the research in this field. The results obtained in this research prove the concept of autonomous energy storage technology feasible for the installation together with solar panels in Far East settlements in Russia. The next step of the project is to introduce a 30 kW hydrogen set up with a new design of the reactor with intensified heat and mass transfer properties. Main outcomes of the research include the following: - 1 kWh hydrogen energy storage prototype was successfully developed and tested
Please cite this article in press as: Khayrullina A, et al., Air heated metal hydride energy storage system design and experiments for microgrid applications, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.145
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- the novel concept of the fuel cell output hot air utilization was proved - the temperature of the FC inside and outside the system was measured - a metallic alloy was chosen suitable for this application - multiple working regimes of the system were shown, including stand by, constant load, volatile loads, peaks, crisis shut down responses - the autonomy of the system was introduced by the replacement of an external heating agent for the metal hydride reactor to the internal waste heat in the system - the energy storage prototype created for the use in renewable energy autonomous power supply systems for the Far East settlements in Russia
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Acknowledgements [16]
The research was supported by Russian Science Foundation grant (project #17-79-20413).
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Please cite this article in press as: Khayrullina A, et al., Air heated metal hydride energy storage system design and experiments for microgrid applications, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.05.145