Optimal design of a cooperated energy storage system to balance intermittent renewable energy and fluctuating demands of hydrogen and oxygen in refineries
Introduction
The oil refining and petrochemical industries require large amounts of hydrogen and oxygen. Hydrogen plays an irreplaceable role to meet the growing demand and stringent environmental regulations in the production of high-quality fuels in refineries, in which the steam methane reforming (SMR) technology for hydrogen production is usually used (Lemus and Martínez Duart, 2010; Olateju et al., 2014), despite of the intensive carbon emissions (Acar and Dincer, 2019; Schmidt Rivera et al., 2018). Oxygen is commonly used to improve catalyst regeneration and sulfur recovery in refineries, which is mainly produced by an air separation unit (ASU), consuming a lot of electricity through the conversion of fossil fuel (Tafone et al., 2018). Nowadays, the refineries are confronting with the challenges of growing demands for hydrogen and oxygen under restrictions of greenhouse gas emissions.
To reduce the environmental impact of chemical production and increase the penetration of intermittent renewable energy sources, the power-to-gas (PtG) technologies are attached more importance for decarbonizing the energy supply and improving the flexibilities of energy systems (Daiyan et al., 2020; Maggio et al., 2019; Yang et al., 2020). The excessive electrical energy is usually converted into hydrogen and oxygen by water electrolysis processes (Parra et al., 2019), where the produced hydrogen and oxygen can be utilized as feedstock for oil refining, methanol production, ammonia synthesis (Allman et al., 2019; Giddey et al., 2017), and iron and steel industry. These clean and renewable sources play an important part in the greenization of process industries (Elsholkami and Elkamel, 2018).
Recent advances in electrolytic hydrogen production showed that the introduction of hydrogen by renewable energy to the crude upgrading and oil refining industries is one of the viable options to reduce the carbon intensity of fuel production. However, there are some limitations that affect the extensive use of electrolysis for hydrogen and oxygen production in process industries. The limitations include the intermittent nature of renewable energy, the expensive cost of hydrogen/oxygen, the uncertainty of the system, the integration of the hydrogen network. In particular, as the nature of intermittency and variability of renewable energy, it is imperative to ensure stable matches between demand and supply at both temporal and spatial scales before the hydrogen produced by renewable energy can be directly utilized by industrial processes (Chen et al., 2019; Nicita et al., 2020; Shi et al., 2020). Moreover, the demands of hydrogen and oxygen usually fluctuate in practical refineries due to variations of feed properties, degradation of catalyst activity, changes in the market, seasonal shift, and so on (Liang et al., 2016). Subsequently, it is essential to establish a cooperated energy storage system and relevant operational strategies to effectively coordinate the intermittent renewable energy supply and the fluctuating hydrogen and oxygen demands in practice.
For the purpose of increasing the penetration of renewable energy in industrial systems, there have been some approaches to deal with renewable energy supply and improve the flexibility of the power systems. However, when the renewable energy is converted into hydrogen and oxygen through electrolyzers to meet the requirement of a refinery, the renewable energy is converted into materials, i.e., two energy carriers. Furthermore, in a refinery, the fluctuating demands of hydrogen and oxygen make the energy storage more complicated than sole electricity storage. In this context, the storage of energy carriers and electricity should be considered simultaneously. Thus, the energy storage mode in the refinery should be different from the power systems in which electricity storage is the major need to overcome the intermittent renewable energy supply. Hence, although the methods to damp the fluctuation of renewable energy in the power systems can be used as reference, these methods should be extended to adapt to the energy storage system that couples the renewable energy with the demands in the refineries.
Electrolyzers are core devices to convert renewable electricity into hydrogen and oxygen, which are modular processing units wherein electricity is applied to split water molecules into hydrogen and oxygen. When powered by renewable electricity sources, the electrolyzers produce hydrogen and oxygen at the same time without emissions. The produced hydrogen is attached more importance for the applications, converted to stable electricity in fuel cells, for example, and the oxygen is usually sold to the market (Nicita et al., 2020; Olateju et al., 2016; Rivarolo et al., 2016). In fact, a large amount of oxygen is needed in refineries provided by using ASU, sulfur recovery, and catalyst regeneration, for examples. Therefore, if the produced oxygen by the electrolyzers is used to meet the oxygen demand in the refineries, the capacity and carbon emissions of the ASU could also be reduced. Therefore, the key issue is to match the fluctuating demands of hydrogen and oxygen in the refineries simultaneously. In addition, the excessive electricity from renewable sources should be steadily dispatched to the grid if the produced electricity from the renewable energy resources is connected to the grid. Thus, the stability of the power grid connection should also be considered to alleviate the impact on the grid (Olateju et al., 2016; Van der Roest et al., 2020). However, these aspects are paid less attention so far when renewable energy is integrated with the industrial processes.
The major objective of this work is to solve the problems of optimal design and operations of a cooperated energy storage system to balance the intermittent supply of renewable energy and the fluctuating demands of hydrogen and oxygen in the refineries, where the coordinated storage of hydrogen, oxygen, and electricity are highlighted. A mathematical programing model for the cooperated energy storage system was proposed to investigate the optimal design of the system to meet the fluctuating demands and effects of stable output flowrates of hydrogen and oxygen on the system.
The rest of this paper is organized as follows. The existing studies on the applications of electrolytic hydrogen production for oil refining and the solution method are reviewed in Section 2. The structure of an electricity-hydrogen-oxygen cooperated energy storage system is introduced in Section 3. A mathematical programing model to optimize the cooperated energy storage system is proposed in Section 4, and case studies are presented in Section 5. The conclusions are drawn in Section 6.
Section snippets
Literature review
In recent, the potential applications of electrolytic hydrogen production for oil refining have been investigated. Olateju et al. (2014), (2016) developed a techno-economic analysis framework to assess the hydrogen production by wind turbines applied in the bitumen upgrading industry. Pelaez-Samaniego et al. (2014) investigated the potential application of hydrogen produced by the excessive hydropower in Ecuador for oil refining industries. Al-Subaie et al. (2016) studied the environmental
An electricity-hydrogen-oxygen cooperated energy storage system
Fig. 1 shows an electricity-hydrogen-oxygen cooperated energy storage system that couples the intermittent supplies of renewable energy with the fluctuating demands of hydrogen and oxygen in a refinery. There are five subsystems in the system, i.e., the renewable energy generation subsystem, the energy conversion subsystem, the energy storage subsystem, the utility subsystem, and the subsystem of a refinery. The system in detail can be found in Wang et al. (2021).
The components of the system
Objective function
The objective is to minimize the total cost (TC) of the system. The TC includes the investment cost and operation cost of the system minus the revenue, which can be expressed as (Wang et al., 2021)
The investment cost is the annualized cost including the capital cost and the replacement cost of each component during the lifespan of the system. The operation cost includes the maintenance cost of each component and the cost of hydrogen and oxygen from the utilities in a year.
Fundamental data
In this case study, the typical demands of hydrogen and oxygen are adopted from a crude refinery in China, where the hydrogen is consumed by the hydrogenation units and the oxygen is consumed by the Claus system for sulfur recovery. The capacity factor of wind turbines (Staffell and Pfenninger, 2016) and photovoltaics (Pfenninger and Staffell, 2016) are given. The time horizon is one year with hour resolution, and the operation time of the electricity-hydrogen-oxygen cooperated energy storage
Conclusions
For the purpose of increasing the penetration of renewable energy in process industries with attenuating the impacts of intermittent power supply on the industrial systems, the energy storage systems are usually constructed to buffer the fluctuating power supply with the stable flowrates of multiple energy carriers. In this work, in order to solve the design and operation problems of a cooperated energy storage system with multiple energy carriers for matching the intermittent renewable energy
CRediT authorship contribution statement
Jing Wang: Methodology, Software, Formal analysis, Visualization, Writing – original draft, Writing – review & editing. Lixia Kang: Data curation, Software, Formal analysis, Visualization, Investigation, Funding acquisition. Yongzhong Liu: Conceptualization, Methodology, Visualization, Validation, Writing – review & editing, Supervision, Project administration, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors gratefully acknowledge funding by the projects (Nos. 21878240 and 21808179) sponsored by the National Natural Science Foundation of China (NSFC) and Key Research and Development Program of Shaanxi Province (2019GY-139).
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