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Re-enacting the Hydrogen Tank Explosion of a Fuel-cell Electric Vehicle: An Experimental Study
May 2023
Publication
With the world-wide decision to reduce carbon emissions through the Paris Agreement (2015) the demand for hydrogen-fuelled vehicles has been increasing. Although hydrogen is not a toxic gas it has a wide flammable range (4e75%) and can explode due to static electricity. Therefore studies on hydrogen safety are urgently required. In this study an explosion was induced by applying fire to the lower part of a fuel cell electric vehicle (FCEV). Out of three compressed hydrogen storage tanks installed in the vehicle two did not have hydrogen fuel and one was filled with compressed gaseous hydrogen of 700 bar and forcedly deactivated its temperature-activated pressure relief device. The side-on overpressure transducers were installed by distance in main directions to measure the side-on overpressure generated by the vehicle explosion. A 10 m-long protective barrier was installed on which reflected overpressure displacement and acceleration were measured to examine the effect of attenuation of explosion damage in the event of an accident. The vehicle exploded approximately 11 min after ignition generating a blast wave fireballs and fragments. The results of the experiment showed that the protective barrier could almost completely block explosive pressure smoke and scattering generated during an explosion. Through Probit function analysis the probabilities of an accident occurring were derived based on peak overpressure peak impulse and scattering. The results of this study can be used to develop standard operating procedures (SOPs) for firefighters as the base data for setting the initial operation location and deriving the safe separation distance.
Hybrid Electric Vehicle: Design and Control of a Hybrid System (Fuel Cell/Battery/Ultra-Capacitor) Supplied by Hydrogen
Apr 2019
Publication
Due to its high efficiency and reduced emissions new zero-emission hybrid electric vehicles have been selected as an attractive challenge for future transport applications. New zero -emission hybrid electric on the other hand has some major drawbacks from the complicated charging process. The hybrid electrical fuel cell system is introduced as the main source to intelligently control multi-source activities. An ultra-capacitor system is selected as the energy recovery assistance to monitor the fuel cell’s fast transient and peak power during critical periods. To regulate energy demand and supply an intelligent energy management system is proposed and tested through several constraints. The proposed approach system aims to act quickly against sudden circumstances related to hydrogen depletion in the prediction of the required fuel consumption basis. The proposed strategy tends to define the proper operating system according to energy demand and supply. The obtained results show that the designed system meets the targets set for the energy management unit by referring to an experimental velocity database.
Design and Analysis of a New Renewable-Nuclear Hybrid Energy System for Production of Hydrogen, Fresh Water and Power
Nov 2021
Publication
This paper investigates an integrated system where solar energy system (with 75MWp bifacial PV arrays) and nuclear power plant (with 2×10MWt HTR-10 type pebble bed reactors) are hybridized and integrated with a 72MWe capacity high-temperature solid oxide electrolysis (SOE) unit to produce hydrogen fresh water and electrical power. Bifacial PV plant is integrated to system for supplying electricity with a low LCOE and zero-carbon system. A Rankine cycle is integrated to generate power from the steam that generated from nuclear heat. According to the available irradiance; the steam is diverted between steam turbine and high-temperature electrolyzer for hydrogen and power generation. Multi-effect desalination unit is integrated to exploit the excess heat to generate fresh water. A system performance assessment is carried out by energy and exergy efficiencies thermodynamically. The bifacial PV plant is analyzed in six selected latitudes in order to assess the feasibility and applicability of the system. Numerous time-dependent analyses are carried out to study the effects of varying inputs such as solar radiation intensity. For 20MWt nuclear 75MWp solar capacity; hydrogen productions are found to be between 0.036 and 0.562kg/s. Among the Northern Hemisphere latitudes the peak daily hydrogen production rate is expected to reach 25.9 tons of hydrogen per day for the 75 °N case mostly with the influence of low temperature and high albedo. The pitch distance change is increased the hydrogen production rate by 28% between 3 m and 7 m tracker spacing. The overall system energy efficiency is obtained between 21.8% and 24.2% where the overall system exergy efficiency is found between 18.6% and 21.1% under dynamic conditions for the 45°N latitude case.
Hydrogen Energy Storage: New Techno-economic Emergence Solution Analysis
Aug 2015
Publication
The integration of various renewable energy sources as well as the liberalization of electricity markets are established facts in modern electrical power systems. The increased share of renewable sources within power systems intensifies the supply variability and intermittency. Therefore energy storage is deemed as one of the solutions for stabilizing the supply of electricity to maintain generation-demand balance and to guarantee uninterrupted supply of energy to users. In the context of sustainable development and energy resources depletion the question of the growth of renewable energy electricity production is highly linked to the ability to propose new and adapted energy storage solutions. The purpose of this multidisciplinary paper is to highlight the new hydrogen production and storage technology its efficiency and the impact of the policy context on its development. A comprehensive techno/socio/economic study of long term hydrogen based storage systems in electrical networks is addressed. The European policy concerning the different energy storage systems and hydrogen production is explicitly discussed. The state of the art of the techno-economic features of the hydrogen production and storage is introduced. Using Matlab-Simulink for a power system of rated 70 kW generator the excess produced hydrogen during high generation periods or low demand can be sold either directly to the grid owners or as filled hydrogen bottles. The affordable use of Hydrogen-based technologies for long term electricity storage is verified.
From Grey to Green and from West to East: The Geography and Innovation Trajectories of Hydrogen Fuel Technologies
May 2023
Publication
Despite the potential of hydrogen as a sustainable energy carrier existing studies analysing the recent evolution of this technology are scattered typically focusing on a specific type of hydrogen technology within a single country or region. In this paper we adopt a broader perspective providing an overview of the evolution of knowledge generation across different types of hydrogen fuel and the leading countries in developing new technologies in this field. Using data from the European Patent Office we map knowledge generation on hydrogen fuel technologies exploring its geographic distribution and its link with environmental sustainability. While the United States leads the generation of new knowledge other Asian and European countries show greater dynamism in growth and specialisation. Our study shows that although hydrogen fuel is considered environmentally friendly most recent technological developments are still related to fossil energy sources. However a faster growth rate is observed in the knowledge of hydrogen fuel from renewable sources pointing to a promising path towards sustainability. Moreover our analysis of the knowledge interconnection between different hydrogen types suggests that those technologies developed for hydrogen based on fossil energy sources have enabled novel applications based on renewable energies.
Material Challenges and Hydrogen Embrittlement Assessment for Hydrogen Utilisation in Industrial Scale
Sep 2023
Publication
Hydrogen has been studied extensively as a potential enabler of the energy transition from fossil fuels to renewable sources. It promises a feasible decarbonisation route because it can act as an energy carrier a heat source or a chemical reactant in industrial processes. Hydrogen can be produced via renewable energy sources such as solar hydro or geothermic routes and is a more stable energy carrier than intermittent renewable sources. If hydrogen can be stored efficiently it could play a crucial role in decarbonising industries. For hydrogen to be successfully implemented in industrial systems its impact on infrastructure needs to be understood quantified and controlled. If hydrogen technology is to be economically feasible we need to investigate and understand the retrofitting of current industrial infrastructure. Currently there is a lack of comprehensive knowledge regarding alloys and components performance in long-term hydrogen-containing environments at industrial conditions associated with high-temperature hydrogen processing/production. This review summarises insights into the gaps in hydrogen embrittlement (HE) research that apply to high-temperature high-pressure systems in industrial processes and applications. It illustrates why it is still important to develop characterisation techniques and methods for hydrogen interaction with metals and surfaces under these conditions. The review also describes the implications of using hydrogen in large-scale industrial processes.
Hydrogen Deep Ocean Link: A Global Sustainable Interconnected Energy Grid<br/><br/><br/>
Mar 2022
Publication
The world is undergoing a substantial energy transition with an increasing share of intermittent sources of energy on the grid which is increasing the challenges to operate the power grid reliably. An option that has been receiving much focus after the COVID pandemic is the development of a hydrogen economy. Challenges for a hydrogen economy are the high investment costs involved in compression storage and long-distance transportation. This paper analyses an innovative proposal for the creation of hydrogen ocean links. It intends to fill existing gaps in the creation of a hydrogen economy with the increase in flexibility and viability for hydrogen production consumption compression storage and transportation. The main concept behind the proposals presented in this paper consists of using the fact that the pressure in the deep sea is very high which allows a thin and cheap HDPE tank to store and transport large amounts of pressurized hydrogen in the deep sea. This is performed by replacing seawater with pressurized hydrogen and maintaining the pressure in the pipes similar to the outside pressure. Hydrogen Deep Ocean Link has the potential of increasing the interconnectivity of different regional energy grids into a global sustainable interconnected energy system.
Inter-Zone Optimal Scheduling of Rural Wind–Biomass-Hydrogen Integrated Energy System
Aug 2023
Publication
To solve the problems of low utilization of biomass and uncertainty and intermittency of wind power (WP) in rural winter an interval optimization model of a rural integrated energy system with biogas fermentation and electrolytic hydrogen production is constructed in this paper. Firstly a biogas fermentation kinetic model and a biogas hydrogen blending model are developed. Secondly the interval number is used to describe the uncertainty of WP and an interval optimization scheduling model is developed to minimize daily operating cost. Finally a rural integrated energy system in Northeast China is taken as an example and a sensitivity analysis of electricity price gas production and biomass price is conducted. The simulation results show that the proposed strategy can significantly reduce the wind abandonment rate and improve the economy by 3.8–22.3% compared with conventional energy storage under optimal dispatch.
Life Cycle Assessment of an Autonomous Underwater Vehicle that Employs Hydrogen Fuel Cell
Aug 2023
Publication
In recent years there has been a significant increase in the adoption of autonomous vehicles for marine and submarine missions. The advancement of emerging imaging navigation and communication technologies has greatly expanded the range of operational capabilities and opportunities available. The ENDURUNS project is a European research endeavor focused on identifying strategies for achieving minimal environmental impact. To measure these facts this article evaluates the product impacts employing the Life Cycle Assessment methodology for the first time following the ISO 14040 standard. In this analysis the quantitative values of Damage and Environmental Impact using the Eco-Indicator 99 methodology in SimaPro software are presented. The results report that the main contributors in environmental impact terms have been placed during the manufacturing phase. Thus one of the challenges is accomplished avoiding the use phase emissions that are the focus to reduce nowadays in the marine industry.
Increasing Energy Efficiency of Hydrogen Refueling Stations via Optimal Thermodynamic Paths
Sep 2023
Publication
This work addresses the energy efficiency of hydrogen refueling stations (HRS) using a first principles model and optimal control methods to find minimal entropy production operating paths. The HRS model shows good agreement with experimental data achieving maximum state of charge and temperature discrepancies of 1 and 7% respectively. Model solution and optimization is achieved at a relatively low computational time (40 s) when compared to models of the same degree of accuracy. The entropy production mapping indicates the flow control valve as the main source of irreversibility accounting for 85% of the total entropy production in the process. The minimal entropy production refueling path achieves energy savings from 20 to 27% with respect to the SAE J2601 protocol depending on the ambient temperature. Finally the proposed method under nearreversible refueling conditions shows a theoretical reduction of 43% in the energy demand with respect to the SAE J2601 protocol.
A Rational Approach to the Ecological Transition in the Cruise Market: Technologies and Design Compromises for the Fuel Switch
Jan 2023
Publication
Supporting policies to achieve a green revolution and ecological transition is a global trend. Although the maritime transport of goods and people can rightly be counted among the least polluting sectors much can be done to further reduce its environmental footprint. Moreover to boost the ecological transition of vessels a whole series of international regulations and national laws have been promulgated. Among these the most impactful on both design and operational management of ships concern the containment of air-polluting emissions in terms of GHG NOx SOx and PM. To address this challenge it might seem that many technologies already successfully used in other transport sectors could be applied. However the peculiar characteristics of ships make this statement not entirely true. In fact technological solutions recently adopted for example in the automotive sector must deal with the large size of vessels and the consequent large amount of energy necessary for their operation. In this paper with reference to the case study of a medium/large-sized passenger cruise ship the use of different fuels (LNG ammonia hydrogen) and technologies (internal combustion engines fuel cells) for propulsion and energy generation on board will be compared. By imposing the design constraint of not modifying the payload and the speed of the ship the criticalities linked to the use of one fuel rather than another will be highlighted. The current limits of application of some fuels will be made evident with reference to the state of maturity of the relevant technologies. Furthermore the operational consequences in terms of autonomy reduction will be presented. The obtained results underline the necessity for shipowners and shipbuilders to reflect on the compromises required by the challenges of the ecological transition which will force them to choose between reducing payload or reducing performance.
Green Steel: Synergies between the Australian Iron Ore Industry and the Production of Green Industry
May 2023
Publication
Green steel produced using renewable energy and hydrogen presents a promising avenue to decarbonize steel manufacturing and expand the hydrogen industry. Australia endowed with abundant renewable resources and iron ore deposits is ideally placed to support this global effort. This paper's two-step analytical approach offers the first comprehensive assessment of Australia's potential to develop green steel as a value-added export commodity. The Economic Fairways modelling reveals a strong alignment between prospective hydrogen hubs and current and future iron ore operations enabling shared infrastructure development and first-mover advantages. By employing a site-based system optimization that integrates both wind and solar power sources the cost of producing green steel could decrease significantly to around AU$900 per tonne by 2030 and AU$750 per tonne by 2050. Moreover replacing 1% of global steel production would require 35 GW of well-optimized wind and solar photovoltaics 11 GW of hydrogen electrolysers and 1000 square kilometres of land. Sensitivity analysis further indicates that iron ore prices would exert a long-term influence on green steel prices. Overall this study highlights the opportunities and challenges facing the Australian iron ore industry in contributing to the decarbonization of the global steel sector underscoring the crucial role of government support in driving the growth and development of the green steel industry.
Synergistic Integration of Hydrogen Energy Economy with UK’s Sustainable Development Goals: A Holistic Approach to Enhancing Safety and Risk Mitigation
Oct 2023
Publication
Hydrogen is gaining prominence as a sustainable energy source in the UK aligning with the country’s commitment to advancing sustainable development across diverse sectors. However a rigorous examination of the interplay between the hydrogen economy and the Sustainable Development Goals (SDGs) is imperative. This study addresses this imperative by comprehensively assessing the risks associated with hydrogen production storage transportation and utilization. The overarching aim is to establish a robust framework that ensures the secure deployment and operation of hydrogen-based technologies within the UK’s sustainable development trajectory. Considering the unique characteristics of the UK’s energy landscape infrastructure and policy framework this paper presents practical and viable recommendations to facilitate the safe and effective integration of hydrogen energy into the UK’s SDGs. To facilitate sophisticated decision making it proposes using an advanced Decision-Making Trial and Evaluation Laboratory (DEMATEL) tool incorporating regret theory and a 2-tuple spherical linguistic environment. This tool enables a nuanced decision-making process yielding actionable insights. The analysis reveals that Incident Reporting and Learning Robust Regulatory Framework Safety Standards and Codes are pivotal safety factors. At the same time Clean Energy Access Climate Action and Industry Innovation and Infrastructure are identified as the most influential SDGs. This information provides valuable guidance for policymakers industry stakeholders and regulators. It empowers them to make well-informed strategic decisions and prioritize actions that bolster safety and sustainable development as the UK transitions towards a hydrogen-based energy system. Moreover the findings underscore the varying degrees of prominence among different SDGs. Notably SDG 13 (Climate Action) exhibits relatively lower overall distinction at 0.0066 and a Relation value of 0.0512 albeit with a substantial impact. In contrast SDG 7 (Clean Energy Access) and SDG 9 (Industry Innovation and Infrastructure) demonstrate moderate prominence levels (0.0559 and 0.0498 respectively) each with its unique influence emphasizing their critical roles in the UK’s pursuit of a sustainable hydrogen-based energy future.
Economic Performance Evaluation of Flexible Centralised and Decentralised Blue Hydrogen Production Systems Design Under Uncertainty
Sep 2023
Publication
Blue hydrogen is viewed as an important energy vector in a decarbonised global economy but its large-scale and capital-intensive production displays economic performance vulnerabities in the face of increased market and regulatory uncertainty. This study analyses flexible (modular) blue hydrogen production plant designs and evaluates their effectiveness to enhance economic performance under uncertainty. The novelty of this work lies in the development of a comprehensive techno-economic evaluation framework that considers flexible centralised and decentralised blue hydrogen plant design alternatives in the presence of irreducible uncertainty whilst explicitly considering the time value of money economies of scale and learning effects. A case study of centralised and decentralised blue hydrogen production for the transport sector in the San Francisco area is developed to highlight the underlying value of flexibility. The proposed methodological framework considers various blue hydrogen plant designs (fixed phased and flexible) and compares them using relevant economic indicators (net present value (NPV) capex value-at-risk/gain etc.) through a detailed Monte Carlo simulation framework. Results indicate that flexible centralised hydrogen production yields greater economic value than alternative designs despite the associated cost-premium of modularity. It is also shown that the value of flexibility increases under greater uncertainty higher learning rates and weaker economies of scale. Moreover sensitivity analysis reveals that flexible design remains the preferred investment option over a wide range of market and regulatory conditions except for high initial hydrogen demand. Finally this study demonstrates that major regulatory and market uncertainties surrounding blue hydrogen production can be effectively managed through the application of flexible engineering system design that protects the investment from major downside risks whilst allowing access to favourable upside opportunities.
Underground Hydrogen Storage (UHS) in Natural Storage Sites: A Perspective of Subsurface Characterization and Monitoring
Jan 2024
Publication
With the long-standing efforts of green transition in our society underground hydrogen storage (UHS) has emerged as a viable solution to buffering seasonal fluctuations of renewable energy supplies and demands. Like operations in hydrocarbon production and geological CO2 storage a successful UHS project requires a good understanding of subsurface formations while having different operational objectives and practical challenges. Similar to the situations in hydrocarbon production and geological CO2 storage in UHS problems the information of subsurface formations at the field level cannot be obtained through direct measurements due to the resulting high costs. As such there is a need for subsurface characterization and monitoring at the field scale which uses a certain history matching algorithm to calibrate a numerical subsurface model based on available field data. Whereas subsurface characterization and monitoring have been widely used in hydrocarbon production activities for a better understanding of hydrocarbon reservoirs to the best of our knowledge at present it appears to be a relatively less touched area in UHS problems. This work aims to narrow this noticed gap and investigates the use of an ensemble-based workflow for subsurface characterization and monitoring in a 3D UHS case study. Numerical results in this case study indicate that the ensemble-based workflow works reasonably well while also identifying some particular challenges that would be relevant to real-world problems.
Economic Modelling of Mixing Hydrogen with Natural Gas
Jan 2024
Publication
As global efforts intensify to transition toward cleaner and more sustainable energy sources the blending of hydrogen with natural gas emerges as a promising strategy to reduce carbon emissions and enhance energy security. This study employs a systematic approach to assess the economic viability of hydrogen blending considering factors such as gas costs and heat values. Various hydrogen blending scenarios are analyzed to determine the optimal blend ratios taking into account both technical feasibility and economic considerations. The study discusses potential economic benefits challenges and regulatory implications associated with the widespread adoption of hydrogen–natural gas mixtures. Furthermore the study explores the impact of this integration on existing natural gas infrastructure exploring the potential for enhanced energy storage and delivery. The findings of this research contribute valuable insights to policymakers industry stakeholders and researchers engaged in the ongoing energy transition by providing a nuanced understanding of the economic dimensions of hydrogen blending within the natural gas sector.
Improvement of SI Engine Combustion with Ammonia as Fuel: Effect of Ammonia Dissociation Prior to Combustion
Mar 2022
Publication
Although recent studies have shown the possibility of running ‘standard’ spark-ignition engines with 6 pure ammonia the operating range remains limited mainly due to the unfavorable characteristics of 7 ammonia for premixed combustion and often requires the addition of a complementary fuel such as H2 8 to extend it. As the best way to add H2 is to crack ammonia directly on-board this paper focuses on 9 the impact of the upstream cracking level of ammonia on the performance and emissions of a single 10 cylinder spark ignition engine. Experiments were performed over several equivalence ratios 11 dissociation rates and load conditions. It is confirmed that only a slight rate of ammonia dissociation 12 (10%) upstream of the combustion considerably enhances the engine's operating range thanks to a 13 better combustion stability. In terms of pollutant emissions the partial dissociation of ammonia 14 especially for slightly lean mixtures induces a very clear trade-off between high NOx and high 15 unburned ammonia level for high and low ammonia dissociation rates respectively. Therefore 16 cracking NH3 does not only improve the operating range of ammonia-fueled spark ignition engines but 17 can also help to reduce NH3. However to reach the same engine output work higher ammonia fuel 18 consumption will be necessary since the global system efficiency is lower using fuel dissociation. In 19 addition the global warming effect is increased with dissociation level since a higher level of N2O is 20 generated by the hydrogen contribution.
Green Hydrogen Production and Liquefaction Using Offshore Wind Power, Liquid Air, and LNG Cold Energy
Sep 2023
Publication
Coastal regions have abundant off-shore wind energy resources and surrounding areas have large-scale liquefied natural gas (LNG) receiving stations. From the engineering perspectives there are limitations in unstable off-shore wind energy and fluctuating LNG loads. This article offers a new energy scheme to combine these 2 energy units which uses surplus wind energy to produce hydrogen and use LNG cold energy to liquefy and store hydrogen. In addition in order to improve the efficiency of utilizing LNG cold energy and reduce electricity consumption for liquid hydrogen (LH2) production at coastal regions this article introduces the liquid air energy storage (LAES) technology as the intermediate stage which can stably store the cold energy from LNG gasification. A new scheme for LNG-LAES-LH2 hybrid LH2 production is built. The case study is based on a real LNG receiving station at Hainan province China and this article presents the design of hydrogen production/liquefaction process and carries out the optimizations at key nodes and proves the feasibility using specific energy consumption and exergy analysis. In a 100 MW system the liquid air storage round-trip efficiency is 71.0% and the specific energy consumption is 0.189 kWh/kg and the liquid hydrogen specific energy consumption is 7.87 kWh/kg and the exergy efficiency is 46.44%. Meanwhile the corresponding techno-economic model is built and for a LNGLAES-LH2 system with LH2 daily production 140.4 tons the shortest dynamic payback period is 9.56 years. Overall this novel hybrid energy scheme can produce green hydrogen using a more efficient and economical method and also can make full use of surplus off-shore wind energy and coastal LNG cold energy.
Drifting toward Alliance Innovation: Patent Collaboration Relationships and Development in China’s Hydrogen Energy Industry from a Network Perspective
Mar 2024
Publication
The hydrogen energy industry as one of the most important directions for future energy transformation can promote the sustainable development of the global economy and of society. China has raised the development of hydrogen energy to a strategic position. Based on the patent data in the past two decades this study investigates the collaborative innovation relationships in China’s hydrogen energy field using complex network theory. Firstly patent data filed between 2003 and 2023 are analyzed and compared in terms of time geography and institutional and technological dimensions. Subsequently a patent collaborative innovation network is constructed to explore the fundamental characteristics and evolutionary patterns over five stages. Furthermore centrality measures and community detection algorithms are utilized to identify core entities and innovation alliances within the network which reveal that China’s hydrogen energy industry is drifting toward alliance innovation. The study results show the following: (1) the network has grown rapidly in size and scope over the last two decades and evolved from the initial stage to the multi-center stage before forming innovation alliances; (2) core innovative entities are important supports and bridges for China’s hydrogen energy industry and control most resources and maintain the robustness of the whole network; (3) innovation alliances reveal the closeness of the collaborative relationships between innovative entities and the potential landscape of China’s hydrogen energy industry; and (4) most of the innovation alliances cooperate only on a narrow range of technologies which may hinder the overall sustainable growth of the hydrogen energy industry. Thereafter some suggestions are put forward from the perspective of an industrial chain and innovation chain which may provide a theoretical reference for collaborative innovation and the future development and planning in the field of hydrogen energy in China.
Economic Evaluation of a Power-to-hydrogen System Providing Frequency Regulation Reserves: A Case Study of Denmark
Mar 2023
Publication
Operating costs are dominant in the hydrogen production of a power-to-hydrogen system. An optimal operational strategy or bidding framework is effective in reducing these costs. However it is still found that the production cost of hydrogen is high. As the electrolysis unit is characterized by high flexibility providing ancillary service to the grid becomes a potential pathway for revenue stacking. Recent research has demonstrated the feasibility of providing such a service but the related economics have not been well evaluated. In this work we propose a comprehensive operation model to enable participation in the day-head balancing and reserve markets. Three types of reserves are considered by using different operational constraints. Based on the proposed operation framework we assess the economic performance of a power-to-hydrogen system in Denmark using plentiful actual market data. The results reveal that providing frequency containment reserve and automatic frequency restoration reserve efficiently raises the operational contribution margins. In parallel by investing in the cash flows net present value and break-even hydrogen prices we conclude that providing reserves makes the power-to-hydrogen project more profitable in the studied period and region.
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