Seasonal storage of hydrogen in stationary systems with liquid organic hydrides

A comparison of energy storage media for carbon free systems was made on a cost and weight basis for application with renewable energy sources such as hydropower. On a seasonal timescale (summer to winter), storage of hydrogen in liquid organic hydrides was equivalent to other carbon free alternatives and superior to zero emission systems like batteries.Seasonal energy storage is illustrated by the methylcyclohexane-toluene-hydrogen (MTH) system. Low cost summer electricity is used for water electrolysis to yield hydrogen for hydrogenation of toluene. Dehydrogenation in winter gives hydrogen for heat and power generation by fuel cells with an estimated overall electrical efficiency of 41%. Recent laboratory results using commercial, dehydrogenation catalysts in fixed bed reactors show how catalyst efficiency was increased (low by-products) to reduce the carbon emissions to 0.01 kgC/kWh_e. Hydrogen separation membranes and new molecular reactions are being investigated to further increase efficiencies. Economic analyses show that the seasonal storage of hydroelectric power with hydrogen by the MTH system is economically competitive with new hydropower projects.     

Assessment of hydrogen fuel for rotorcraft applications

This paper presents the application of a multidisciplinary approach for the preliminary design and evaluation of the potential improvements in performance and environmental impact through the utilization of compressed (CGH2) and liquefied (LH2) hydrogen fuel for a civil tilt-rotor modelled after the NASA XV-15. The methodology deployed comprises models for rotorcraft flight dynamics, engine performance, flight path analysis, hydrogen tank and thermal management system sizing. Trade-offs between gravimetric efficiency, energy consumption, fuel burn, CO₂ emissions, and cost are quantified and compared to the kerosene-fuelled rotorcraft. The analysis carried out suggests that for these vehicle scales, gravimetric efficiencies of the order of 13% and 30% can be attained for compressed and liquid hydrogen storage, respectively leading to reduced range capability relative to the baseline tilt-rotor by at least 40%. At mission level, it is shown that the hydrogen-fuelled configurations result in increased energy consumption by at least 12% (LH2) and 5% (CGH2) but at the same time, significantly reduced life-cycle carbon emissions compared to the kerosene counterpart. Although LH2 storage at cryogenic conditions has a higher gravimetric efficiency than CGH2 (at 700 bar), it is shown that for this class of rotorcraft, the latter is more energy efficient when the thermal management system for fuel pressurization and heating prior to combustion is accounted for.     

Exergoeconomic and environmental impact analyses of a renewable energy based hydrogen production system

In this study, exergoeconomic and environmental impact analyses, through energy, exergy, and sustainability assessment methods, are performed to investigate a hybrid version renewable energy (including wind and solar) based hydrogen and electricity production system. The dead state temperatures considered here are 10 °C, 20 °C and 30 °C to undertake a parametric study. An electrolyzer and a metal hydride tank are used for hydrogen production and hydrogen storage, respectively. Also, the Proton Exchange Membrane Fuel Cell (PEMFC) and battery options are utilized for electricity generation and storage, respectively. As a result, the energy and exergy efficiencies and the sustainability index for the wind turbine are found to be higher than the ones for solar photovoltaic (PV) system. Also, the overall exergy efficiency of the system is found to be higher than the corresponding overall energy efficiency. Furthermore, for this system, it can be concluded that wind turbine with 60 gCO₂/month is more environmentally-benign than the solar PV system with 75 gCO₂/month. Finally, the total exergoeconomic parameter is found to be 0.26 W/$, when the energy loss is considered, while it is 0.41 W/$, when the total of exergy loss and destruction rates are taken into account.     

Risk-averse stochastic operation of a power system integrated with hydrogen storage system and wind generation in the presence of demand response program

Wind generation (WG) units as renewable energy sources (RESs) are increasing in the world due to environmental functions and lack of conventional energy sources. Also, hydrogen storage system (HSS) as an energy storage system (ESS) is used to cope with variable nature of RESs in which the concepts of power to hydrogen (P2H) and hydrogen to power (H2P) are defined. In this work, a risk-averse stochastic operation of HSS and WG is modeled using a scenario-based stochastic approach by considering price-responsive demand response (DR) program. All uncertainties are modeled via a scenario-based stochastic approach while the risk related uncertainties are modeled via the downside risk constraints (DRC) to capture the risk-averse operation of the HSS and WG. In order to investigate the impact of DRC implementation, a risk-averse strategy is compared versus risk-neutral strategy. Compared results show that the risk-in-cost (RIC) is reduced while the expected operation cost (EOC) is raised to deal with the risk of the uncertainty.     

Optimal design and management for hydrogen and renewables based hybrid storage micro-grids

In an energy sustainability perspective, the renewables penetration is expected to importantly increase over the next decade, requiring modifications in the current electric system in terms of flexibility and reliability. In this respect, storage systems will play a central role and the production of green hydrogen is seen as a promising solution for both short-term and seasonal storage.In this context, the aim of this paper is the development of a methodology for the optimal design of hybrid storage micro-grids based on renewables and hydrogen and the definition of an optimal management strategy in a perspective of hydrogen employment as seasonal storage. In detail, an optimization code – based on mathematical models for each component and on specifically developed optimization strategies for the management of the components interaction – will be presented and applied to a case study. The code optimizes the sizes of the integrated electrolyzer and fuel cell, based on an objective function that maximizes the storage efficiency. It has been applied to the S.A.P.I.E.N.T.E. micro-grid installed at the ENEA Research Centre near Rome (Italy) – composed of photovoltaic panels, batteries, heat pump and thermal storage systems – obtaining the optimal design of the hydrogen section to be integrated as seasonal storage strategy. Furthermore, a parametric analysis on the battery size has been performed. The application of the developed optimization routine resulted in the introduction of a 3.7 kW electrolyzer and 4 kW fuel cell coupled with 36 kWh of battery capacity, enabling a total hydrogen production of about 87.5 kg (corresponding to 1159 kWh of electricity produced during the thermal year).     

A thermally coupled metal hydride hydrogen storage and fuel cell system

This paper examines the ability of metal hydride storage systems to supply hydrogen to a fuel cell with a time varying demand, when the metal hydride tanks are thermally coupled to the fuel cell. A two-dimensional mathematical model is utilized to compare different heat transfer enhancements and storage tank configurations. The scenario investigated involves two metal hydride tanks containing the alloy Ti_0.98Zr_0.02V_0.43Fe_0.09Cr_0.05Mn_1.5, located in the air exhaust stream of a fuel cell. Three cases are simulated: a base case with no heat transfer enhancements, a case with external fins attached to the outside of the tank, and a case where an annular tank design is used. For the imposed duty cycle, the base case is insufficient to provide the hydrogen demands of the system, while both the finned and annular cases are able to meet the demands. The finned case yields higher pressures and occupies more space, while the annular case yields acceptable pressures and requires less space. Furthermore, the annular metal hydride tank meets the requirements of the fuel cell while providing a more robust and compact hydrogen storage system.     

Seasonal energy storage system based on hydrogen for self sufficient living

SELF is a resource independent living and working environment. By on-board renewable electricity generation and storage, it accounts for all aspects of living, such as space heating and cooking as well as providing a purified rainwater supply and wastewater treatment, excluding food supply. Uninterrupted, on-demand energy and water supply are the key challenges. Off-grid renewable power supply fluctuations on daily and seasonal time scales impose production gaps that have to be served by local storage, a function normally fulfilled by the grid. While daily variations only obligate a small storage capacity, requirements for seasonal storage are substantial.The energy supply for SELF is reviewed based on real meteorological data and demand patterns for Zurich, Switzerland. A battery system with propane for cooking serves as a reference for battery-only and hybrid battery/hydrogen systems. In the latter, hydrogen is used for cooking and electricity generation. The analysis shows that hydrogen is ideal for long term bulk energy storage on a seasonal timescale, while batteries are best suited for short term energy storage. Although the efficiency penalty from hydrogen generation is substantial, in off-grid systems, this parameter is tolerable since the harvesting ratio of photovoltaic energy is limited by storage capacity.     

Experimental and theoretical investigation of a solar heating system with heat pump

In this study, the performance of a solar heating system with a heat pump was investigated both experimentally and theoretically. The experimental results were obtained from November to April during the heating season. The experimentally obtained results are used to calculate the heat pump coefficient of performance (COP), seasonal heating performance, the fraction of annual load met by free energy, storage and collector efficiencies and total energy consumption of the systems during the heating season. The average seasonal heating performance values are 4.0 and 3.0 for series and parallel heat pump systems, respectively. A mathematical model was also developed for the analysis of the solar heating system. The model consists of dynamic and heat transfer relations concerning the fundamental components in the system such as solar collector, latent heat thermal energy storage tank, compressor, condenser, evaporator and meteorological data. Some model parameters of the system such as COP, theoretical collector numbers (N_c), collector efficiency, heating capacity, compressor power, and temperatures (T₁, T₂, T₃, T_T) in the storage tank were calculated by using the experimental results. It is concluded that the theoretical model agreed well with the experimental results.     

Integrating hydrogen generation and storage in a novel compact electrochemical system based on metal hydrides

The development of efficient and reliable energy storage systems based on hydrogen technology represents a challenge to seasonal storage based on renewable hydrogen. State of the art renewable energy generation systems include separate units such as electrolyzer, hydrogen storage vessel and a fuel cell system for the conversion of H₂ back into electricity, when required. In this work, a novel electrochemical system has been developed which integrates hydrogen production, storage and compression in only one device, at relatively low cost and high efficiency. The developed prototype comprises a six-electrode cell assembly using an AB₅-type metal hydride and Ni plates as counter electrodes, in a 35-wt% KOH solution. Metal hydride electrodes with chemical composition LaNi_4.3Co_0.4Al_0.3 were prepared by high frequency vacuum melting followed by high temperature annealing. X-ray phase analysis showed typical hexagonal structure and no traces of other intermetallic compounds belonging to the La–Ni phase diagram. Thermodynamic study has been performed in a Sieverts type of apparatus produced by Labtech Int. During cycling, the charging/discharging process was studied in situ using a gas chromatograph from Agilent. It is anticipated that the device will be integrated as a combined hydrogen generator and storage unit in a stand-alone system associated to a 1-kW fuel cell.     

Carbon capture and storage: A possible bridge to a future hydrogen infrastructure for Germany?

For a conceivable fossil-fuelled electricity production strategy with CO₂ capture, the location of available storage options could play a key role for plant siting, as additional CO₂ transport infrastructure might be required in some configurations. The possible spatial separation of electricity generation and centralised fossil hydrogen production with Carbon Capture and Storage (CCS) allows an additional degree of freedom in the system in enabling the transport of hydrogen instead of electricity.This paper analyses energy conversion and transport tasks associated with the plant locations offered by this enhanced scheme. By considering various scenarios for Germany, we describe different gasification/reforming options with CO₂ capture and estimate their cost, including required new infrastructures.The results point out that moderate additional costs could allow the implementation of a first level hydrogen transport infrastructure instead of building a CO₂ transportation network. This could be a smooth way to finance and facilitate the transition to a future larger hydrogen-based energy system. On the long term, this infrastructure would be in place for the transport of non-fossil hydrogen.     

Experimental study of hydrogen storage with reaction heat recovery using metal hydride in a totalized hydrogen energy utilization system

Experimental results for hydrogen storage tanks with metal hydrides used for load leveling of electricity in commercial buildings are described. Variability in electricity demand due to air conditioning of commercial buildings necessitates installation of on-site energy storage. Here, we propose a totalized hydrogen energy utilization system (THEUS) as an on-site energy storage system, present feasibility test results for this system with a metal hydride tank, and discuss the energy efficiency of the system. This system uses a water electrolyzer to store electricity energy via hydrogen at night and uses fuel cells to generate power during the day. The system also utilizes the cold heat of reaction heat during the hydrogen desorption process for air conditioning. The storage tank has a shell-like structure and tube heat exchangers and contains 50 kg of metal hydride. Experimental conditions were specifically designed to regulate the pressure and temperature range. Absorption and desorption of 5,400 NL of hydrogen was successfully attained when the absorption rate was 10 NL/min and desorption rate was 6.9 NL/min. A 24-h cycle experiment emulating hydrogen generation at night and power generation during the day revealed that the system achieved a ratio of recovered thermal energy to the entire reaction heat of the hydrogen storage system of 43.2% without heat loss.     

Design and analysis of stand-alone hydrogen energy systems with different renewable sources

One of the most interesting developments of energy systems based on the utilization of hydrogen is their integration with renewable sources of energy (RES). In fact, hydrogen can operate as a storage and carrying medium of these primary sources. The design and operation of the system could change noticeably, depending on the type and availability of the primary source. In this paper, the results obtained considering a model of a stand-alone energy system supplied just with RES and composed by an electrolyzer, a hydrogen tank and a proton exchange membrane fuel cell are exposed. The energy systems have been designed in order to supply the electricity needs of a residential user in a mountain environment in Italy during a complete year. Three different sources have been considered: solar irradiance (transformed by an array of photovoltaic modules), hydraulic energy (transformed by a micro-hydro turbine in open-flume configuration) and wind speed (transformed by a small-size wind generator). It has been checked that, in that specific location, it is absolutely not convenient to use the wind source; the solar irradiance has a nearly constant availability during the year, and therefore the seasonal storage of the RES in form of hydrogen is the lowest; the availability of the micro-hydro source is less constant than in case of solar irradiance, requiring a higher hydrogen seasonal storage, but its advantage is linked to the higher efficiency of the turbine and the fact that the RES is directly sent to the user with high frequency (for these reasons it is the best plant option).     

TRNSYS simulation models for solar-hydrogen systems

A solar-hydrogen system is a kind of stand-alone power system (SAPS), which can supply low energy dwellings with energy. With TRNSYS (a transient system simulation program) it is possible to perform parametric studies to find possible system configurations for different climates and loads. The systems simulated in this study consist of a photovoltaic (PV) cell array, an electrolyzer, a hydrogen (H₂) storage, a fuel cell, a catalytic burner, a lead-acid battery, DC/DC converters, DC/AC inverters, diodes, a solar collector, and a water storage tank. The main equations for the PV cell, electrolyzer and fuel cell are provided, while the other models are only briefly described. Simulations are performed for conventional low energy dwellings located in northern latitudes and results for different system configurations and operation schemes are given. The results show that the size of the solar-hydrogen system for a conventional low energy house located in Trondheim, Norway (63°N), needs to be quite large. This is mainly due to the somewhat high energy requirements assumed, but also due to the low insolation available. Simulations of the same dwelling located in lower latitudes, in more favorable climates, and/or with lower energy needs (e.g., future dwellings), show how the size of the solar-hydrogen system can be significantly reduced. The study also illustrates the importance of minimizing the thermal and electrical loads before designing a solar-hydrogen system for energy self-sufficient buildings.     

Estimation of system-level hydrogen storage for metal-organic frameworks with high volumetric storage density

Metal organic framework (MOF) materials have emerged as the adsorbent materials with the highest H₂ storage densities on both a volumetric and gravimetric basis. While measurements of hydrogen storage at the material level (primarily at 77 K) have been published for hundreds of MOFs, estimates of the system-level hydrogen storage capacity are not readily available. In this study, hydrogen storage capacities are estimated at the system-level for MOFs with the highest demonstrated volumetric and gravimetric H₂ storage densities. System estimates are based on a single tank cryo-adsorbent system that utilizes a type-1 tank, multi-layer vacuum insulation, liquid N₂ cooling channels, in-tank heat exchanger, and a packed MOF powder inside the tank. It is found that with this powder-based system configuration, MOFs with ultra-high gravimetric surface areas and hydrogen adsorption amounts do not necessarily provide correspondingly high volumetric or gravimetric storage capacities at the system-level. Meanwhile, attributes such as powder packing efficiency and system cool-down temperature are shown to have a large impact on the system capacity. These results should shed light on the material properties that must to be optimized, as well as highlight the important design challenges for cryo-adsorbent hydrogen storage systems.     

Conceptual design of a two-step solar hydrogen thermochemical cycle with thermal storage in a reaction intermediate

This paper presents the conceptual design for a two-step thermochemical cycle producing hydrogen continuously, even off-sun, with the concentrated solar energy as the heat source. For a case study, the two-step iron oxide cycle (Fe₃O₄/FeO) is selected to illustrate the design concept. Two reactors, one storage tank and the solar collector comprise the system. Molten wustite (FeO) is accumulated in the storage tank on-sun. The FeO is not only involved in the reactions but also acts as the heat transfer medium, obtaining the energy from the solar insolation and delivering energy to support the thermal decomposition of magnetite (Fe₃O₄). In this way, the temperature limitation (<800 K) of molten salt is solved, and the intermittency problem of variable insolation is circumvented. A simple feedback scheme is used to control the flow rate between the storage tank and the reactors in order to minimize the temperature fluctuations. For the wustite hydrolysis reaction, the volumetric flow rate of water is regulated to control the temperature in the reactor. We derived the kinetics of the two-step iron oxide cycle from previous experimental reports. We simulated the dynamics of the system over 50 days with mass and energy balances. The simulation results show that the storage tank temperature will be stationary at 2250 K. After five days, the decomposition temperature at 2100 K, and the hydrogen production stabilized at 7 kg/min. Admitting the difficulty of high temperature operation, this design is still promising due to the high efficiency of two-step cycle itself, the process intensification of the FeO acting as the reactant/product/heat transfer medium (no need of heat exchangers), and the continuous operation/production of hydrogen.     

Dynamic optimization of a cogeneration plant for an industrial application with two different hydrogen embedding solutions

Seasonal storage of hydrogen is a valuable option today increasingly considered in order to optimize cogeneration plants under continuous operation in an incentive framework where electricity sale to the national grids is becoming less economically profitable than in the past. The paper concerns the numerical study and optimization of a cogeneration plant installed in an industrial site having an availability of hydrogen over a continuous time scale, to meet the energy needs and mitigating the environmental impact of the plant operation by reducing the energy withdrawal from traditional sources. Two alternatives are analyzed into detail: the former regards energy production through an internal combustion engine, this last properly controlled to be fueled with blends of natural gas and increasing percentages of hydrogen, the latter concerning the addition of fuel cells to the proposed layout to further reduce the electricity integration by the grid. The dynamic response of the cogeneration system under examination is dynamically evaluated to efficiently fulfill the industrial loads to be fulfilled. First, optimization is performed by implementing a PID controller to better track the industrial demand of electric energy. The main results of this solution reveal a ?81% reduction of excess electricity, a ?7% reduction of natural gas consumed but a 47% raise of CO₂ emissions due to the increase in thermal integration. Then, an additional energy generation from fuel cells is assumed. An economic analysis is carried out for each of the implemented configurations. The adoption of fuel cells, despite requiring a greater initial investment, allows obtaining a SPB of 1,4 years (? 16%), 1,17 Mln € of avoided costs (? 18,5%) and 1320 t/year of CO₂ emissions avoided (? 95%) with respect to the initial layout.     

复合储能在微网中的应用研究

微网是接纳分布式能源的有效方式之一,储能技术应用于微网可以改善电能质量、控制功率平衡、提高运行稳定性以及优化能量管理等。复合储能系统将功率型和能量型储能元件有机结合,兼具两类储能元件的优点,可以有效发挥储能技术在微网运行控制中的作用。从复合储能在微网中的应用模式、控制策略以及优化配置三个方面研究其在微网中的应用情况。首先根据储能元件在系统中连接方式的不同,分析集中式与分布式复合储能的应用模式和适用场合;然后基于储能单元改善微网运行特性的控制方法,分析复合储能系统中各储能元件的协调控制以提高系统动态响应特性和运行经济性;最后分析复合储能在微网中的优化配置问题,结合复合储能在微网中的运行控制提出其优化配置建议。     

Developing a mathematical tool for hydrogen production,compression and storage

Among the few lessons learned presented in the literature, authors put in evidence the on-going need to investigate on station components and their integration. The specific power consumption of station units with on-site hydrogen generation is often subject to uncertainty, and it would have been desirable to find more details about the energy contribution of each component. To address this gap, this paper focuses on the development of a mathematical modeling as a dynamic and multi-physical design tool to predict the energy performance of hydrogen production systems. Particularly, the model aims to describe and analyze the energy performance of two different electrolyzer technologies (PEM and Alkaline), integrated with a compressor system and gaseous buffer storage. Multiple tank options and a switching strategy are investigated, as well as a control system to simulate a real infrastructure operation. Auxiliaries and components related to the thermal management system have been also included. A carbon-footprint analysis follows the energy one, focusing on the CO₂ emission reduction. Comparisons between literature data and model show that the hydrogen system proposed model is suitable to evaluate systems with respect to energy efficiency and system performance. The model could be a powerful tool for exploring control strategies and understanding the contributions to the overall energy consumption from the various internal components as a guide to researchers aiming for improved performance.     

Development of hybrid photovoltaic-fuel cell system for stand-alone application

In this paper we present firstly the different hybrid systems with fuel cell. Then, the study is given with a hybrid fuel cell–photovoltaic generator. The role of this system is the production of electricity without interruption in remote areas. It consists generally of a photovoltaic generator (PV), an alkaline water electrolyzer, a storage gas tank, a proton exchange membrane fuel cell (PEMFC), and power conditioning units (PCU) to manage the system operation of the hybrid system. Different topologies are competing for an optimal design of the hybrid photovoltaic–electrolyzer–fuel cell system. The studied system is proposed. PV subsystem work as a primary source, converting solar irradiation into electricity that is given to a DC bus. The second working subsystem is the electrolyzer which produces hydrogen and oxygen from water as a result of an electrochemical process. When there is an excess of solar generation available, the electrolyzer is turned on to begin producing hydrogen which is sent to a storage tank. The produced hydrogen is used by the third working subsystem (the fuel cell stack) which produces electrical energy to supply the DC bus. The modelisation of the global system is given and the obtained results are presented and discussed.     

Numerical simulation and experimental validation of Ti0.95Zr0.05Mn0.9Cr0.9V0.2 alloy in a metal hydride tank for high-density hydrogen storage

In order to achieve rapid hydrogen charging and discharging of high-density hydrogen storage alloys in metal hydride (MH) tank, it is key to optimize the design of MH tank by simulation based on a credible numerical model. Herein, a multi-physical field mathematical model coupled with kinetic equations and heat & mass transfer equations for the de-/hydrogenation of Ti_0.95Zr_0.05Mn_0.9Cr_0.9V_0.2 alloy in MH tank is proposed. According to experimental kinetic curves, appropriate kinetic equations dominated by different rate control steps are chosen for simulating gas-solid reactions. With excellent match between experimental and simulated kinetic results under different pressure and temperature operating conditions, a novel numerical model is established to predict the local temperature variation during the de-/hydrogenation of Ti_0.95Zr_0.05Mn_0.9Cr_0.9V_0.2 alloy in a self-designed hydrogen storage tank with high-density hydrogen storage of 55.1 g H₂/L. Interestingly, further calculated results reveal that the temperature evolution can be accurately matched, which proves the reliability of the designed numerical model and favors to predict the de-/hydriding behaviors. This investigation provides an effective means for subsequent structure optimization and energy & mass transfer performance optimization of high-density hydrogen storage devices, and sheds light on the numerical simulation of heat & mass transfer for advanced energy storage applications.