Potential of Redox Flow Batteries and Hydrogen as Integrated Storage

Article pubs.acs.org/EF

Potential of Redox Flow Batteries and Hydrogen as Integrated Storage for Decentralized Energy Systems Emma S. Hanley,*,† George Amarandei,†,∥ and Bartek A. Glowacki†,‡,§ †

Department of Physics and Energy, University of Limerick, Plassey Road, Limerick, Ireland Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, U.K. § Institute of Power Engineering, Mory 8, 01-330 Warsaw, Poland ∥ School of Physics, Dublin Institute of Technology, Kevin Street, Dublin 8, Ireland ‡

ABSTRACT: The requirement for low-cost access to energy storage technologies is increasing with the continued growth of renewable energy. The growth of the hydrogen economy is also expected to emerge to improve energy security and meet the growing pressures of environmental requirements. Hydrogen and redox flow batteries (RFB) have promising energy storage characteristics that can allow increased penetration of renewable energy and reduction in grid energy. The strong synergy between natural gas and hydrogen anticipates that new efficient methods of hydrogen production such as microwave plasma processing of natural gas might have a leading role. Additionally, the improvements in the carbon allotropes properties and their cost are expected to influence large-scale energy storage system costs. A technical and economic comparison of vanadium and alliron RFB with hydrogen will be explored on an individual and integrated basis. The findings show hydrogen’s capability for bulk energy storage and highlights the benefits of integrated storage. This proves that integration of various storage systems can play an important role for the future development of the decentralized renewable energy systems.

1. INTRODUCTION Energy storage systems can allow for the decentralization of the energy supply and an increased impact from renewable sources, but energy storage solutions are constrained by site, costs, and efficiency. Currently, pumped hydroelectric energy storage (PHES) makes up 98.3% of total global installed grid storage capacity (127 GW) with less than 10 MW of storage capacity from redox flow batteries (RFB).1,2 Here, we will investigate the use of alternative energy storage technologies to PHES which can allow the continued successful integration of renewable energy into the grid. This research will focus on the potential mitigation of energy problems using storage applications with a comparison of hydrogen and RFB with an emphasis on vanadium (VRFB) and all-iron (all-iron RFB). The conceptualized decentralized system is depicted in Figure 1. The present manuscript will investigate individual and integrated energy storage system response to demand, costs, and efficiencies. In this manuscript we will briefly review the different technologies that will be used in the techno-economic studies. Additionally, for each technology we will test the behavior as an individual system and then combine the hydrogen and VRFB system in an integrated system to investigate the benefits and potential outcomes.

Figure 1. System schematic using hydrogen technologies and vanadium(V) and/or all-iron redox flow batteries (RFB) for storage of intermittent wind energy. The role of liquid hydrogen production for use as a high added value cryogenic is illustrated as a result of the imminent widespread introduction of superconducting technologies.

2. STORAGE SYSTEMS − TECHNOLOGIES, EFFICIENCIES, AND COSTS 2.1. Hydrogen Production. Hydrogen is a well-known energy carrier that can be used for energy storage. Currently steam methane reforming is the main industrial process for hydrogen production. Although the process uses mature technology with high efficiencies it remains associated with relatively high CO2 emissions making this process unsuitable © XXXX American Chemical Society

for the emergence of hydrogen as an energy carrier for decarbonized energy systems, Table 1.3 Alternative lowemission hydrogen production technology must therefore be considered. Received: November 30, 2015 Revised: January 5, 2016

A

DOI: 10.1021/acs.energyfuels.5b02805 Energy Fuels XXXX, XXX, XXX−XXX

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However, the growth of the hydrogen economy is anticipated to emerge to improve the energy security and to meet the growing pressures of environmental requirements. Hydrogen has a low volumetric energy density at standard atmospheric pressure and temperature. Therefore, it must be stored as compressed hydrogen (usually at 35 or 70 MPa) or liquid hydrogen in cryogenic containers at 20 K. Unlike RFB that store energy in already present electrolyte, the hydrogen storage system requires the use of electricity to produce the storage medium (i.e., hydrogen). The energy density of hydrogen has a lower heating value (LHV) of 120 MJ/kg (33 kWh/kg) and a higher heating value (HHV) of 141.80 MJ/kg (39 kWh/kg). Compression is assessed to consume 1.05 kWh/ kg of electrical power.9,10 The compression, storage, and dispensing costs of hydrogen are estimated to range from 2 $/kg to 2.4 $/kg.9 The advantage of compressed hydrogen storage is the high energy density of 780 kWh/m3 at 350 bar and 1309 kWh/m3 at 700 bar, making hydrogen suitable for bulk energy storage.10 In this study the overall levelized cost of electricity produced from hydrogen is considered. 2.1.4. Hydrogen in Electricity Production. Fuel cells are used for the production of electricity from hydrogen gas. A fuel cell uses oxygen and supplied hydrogen gas in reaction to produce electricity, heat, and water vapor with no emission of carbon dioxide. Like RFB (see section 2.2) the power capacity can be increased by stacking the fuel cells in modules. Unlike RFB, hydrogen storage and fuel cells have the ability to be used at high temperature. Solid oxide fuel cells have operating temperatures of 650−1000 °C and electrical efficiencies of 60%. Therefore, the heat can be exploited, and the fuel cell becomes a combined heat-power system with increased efficiencies (between 70% and 80%) that can be employed for decentralized power generation. Other widely used fuel cells are the proton exchange membrane fuel cells (PEMFC) that have lower operating temperatures (50−100 °C) and efficiencies (49−60%).11 They can be employed for backup power, portable power in hydrogen fuel cell vehicles, and small decentralized power systems. PEMFC are the most technologically advanced and developed fuel cell with many commercialized applications and, therefore, the fuel cells are considered within the decentralized system model. Hydrogenics’ HyPM (the fuel cell considered in the study) have produced a selfcontained fuel cell system that is combined with an inverter for AC power and a management system. Efficiencies are 49% with a 20 years estimated lifetime. Its consumption of hydrogen is 780 m3/h.12 The estimated price of PEMFC is in the range of 434−3000 $/kW with a midrange value of 813 $/kW.13 Overall energy efficiency for the system is 32%. This is similar to the overall net efficiency stated in the literature.14 2.2. Redox Flow Batteries. Redox-flow batteries (RFB) have promising storage characteristics, and, as the power and energy capacity of the battery are independent of each other, the RFB can be optimized to maximize the performance and minimize the cost. RFB are modular, and, consequently, the installation is flexible.15 They are rechargeable systems that have the storage medium in the form of electrolyte kept in tanks external to the active cell. The electrochemical reactions and, consequently, the charging and discharging battery cycles are taking place in the battery stack as the electrolyte flows through the two membrane-separated chambers of the active cell (Figure 2).15 Once the electrolyte has been discharged it can be recharged allowing the recovering of the spent electrolyte and its reuse. The energy is stored in the separated

Table 1. Comparison of CO2 Emitted during Different Hydrogen Production Methods hydrogen production method

CO2 (kgCO2/kgH2)

steam methane reforming microwave plasma processing renewable electrolysis

∼8.5 0 (if renewable energy used) 0

For decentralized energy systems based on wind and photovoltaic (PV) energy, hydrogen production either by electrolysis or by microwave plasma processing of natural gas and its storage can represent a viable alternative.4 2.1.1. Electrolysis. Renewable electrolysis can provide near “zero-emission” hydrogen. Alkaline electrolyzers are a mature technology, but improvements in electrolyzers are being researched to enhance the integration of renewable energy with hydrogen production.4 Large scale electrolysis is possible, and it can be implemented for decentralized systems due to the scalability of the hydrogen production. The alkaline electrolyzer efficiency used in the decentralized energy system model for hydrogen production has an efficiency of 66% that corresponds to an electric consumption of 50 kWh/kg of hydrogen produced. The cost of hydrogen from wind electrolysis depends on the wind electricity generation price in a particular region, but it can typically vary from 3.58 to 5.86 $/kg with other sources estimating higher costs of 6−7 $/kg.5 PV electrolysis is more expensive than wind electrolysis with expected current values of 28.19 $/kg and future values of 6.18 $/kg due to expected rapid cost decrease of PV energy.6 2.1.2. Plasma Processing. The scientific interest in processing hydrocarbons, with an emphasis on natural gas and/or methane, for hydrogen generation increased significantly in the last years.7,8 Among different processing methods, microwave plasma processing of natural gas is a “low-emission” method that allows for direct methane decomposition following the reaction described by eq 1.8 The microwave plasma can be generated using the waveguide supplied nozzle-less microwave system operating at 2.45 GHz. Such system allows, depending on the designed configuration, for a microwave output from 0 to 3 or 6 kW at full-wave.8 Thus, for example, a 3 kW test reactor can decompose one mole of methane producing 0.6 mol of carbon and 1.62 mol of hydrogen.8 The resulting hydrogen can be directly used for energy storage (Figure 1). Depending on the experimental conditions the resulting carbonaceous materials can range from carbon black to carbon allotropes.8 CH4 → C(s) + 2H 2

ΔH 0 = +75 kJ/mol

(1)

As for electrolysis, an external source of electricity is required to the plasma reactor in which the methane is decomposed. If the electricity is supplied by grid energy, then the hydrogen production process has associated indirect grid emissions (∼0.450 kgCO2/kWh). The plasma technology is still under development, but methane decomposition with the use of Pt− Rh catalysts leads to hydrogen yields of 72% and carbon yields of 54%.8 This gives the technology a high efficiency with an estimated hydrogen production cost of 1.5 $/kg, noticeably lower than the renewable electrolysis process but dependent on natural gas prices. 2.1.3. Hydrogen as a Storage System. Presently, despite its clear advantages, hydrogen’s employment as an energy carrier is not a widespread storage solution. This is mainly as a result of high costs and lower efficiencies compared to other systems. B


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VRFB is only dependent on the volume of the electrolyte, and it is independent of the plating density. Other advantages include the popularity of the battery with regard to research and also the many installed technology worldwide. Disadvantages include the high cost of vanadium, the sustainability of the supply, and the toxicity of vanadium. Worldwide VRFB with different energy/power (E/P) ratios of ∼2 are implemented to provide storage system solutions.25 Cellcube (the RFB used in this study) is one of the commercial scale VRFB available today.17 2.2.2. All-Iron Redox Flow Batteries. The all-iron RFB, like the VRFB, employs the use of a single chemical element (in this case iron) in several oxidation states on both sides of the active cell (see Table 2), while the electrolyte is kept outside in the storage tanks. The positive electrode of the all-iron battery is the ferric/ferrous redox couple, and the negative electrode involves iron plating from Fe(II).22,23 An advantage of all-iron RFB is the readily available electrolyte with an estimated low cost of 0.23 $/L.21 In the traditional all-iron RFB, in the negative side, the ferrous ions are reduced during charge. Their plating as iron metal onto a graphite electrode of the stack occurs, leading to a coupling between energy and power. On the positive side of the battery ferrous ions are oxidized to ferric ions during charge remaining in the solution. Reactions are opposite on discharge (Table 2). Cheap aqueous electrolytes, inexpensive separators, and the iron widespread availability (∼230 billion metric tonnes of iron) give the all-iron RFB the potential of a reduced cost storage system, while the plating and, consequently, the coupling between the energy and power represent its main disadvantage.21,23 To avoid this disadvantage a slurry electrode containing electrically conductive carbonaceous particles can be made by flowing them in an electrolyte containing the dissolved iron species.21 Such conductive particles can include carbon black and/or carbon allotropes with different surface area and enhanced conductivity, carbon microflakes, nanofibers, nanotubes, etc. Thus, iron is plated onto the carbon particles in the negative side while charging. The carbon particles can then carry the iron metal to be stored in the external tanks allowing for the energy storage capacity and power decoupling, the economic advantages of scaling inherent to RFB being recovered.23 Such carbon particles can be obtained from microwave plasma processing of natural gas allowing for an intimate relationship between the two energy storage systems. The carbons and their properties will influence the electronic conductivity of the slurry electrodes that have to be greater than the ionic conductivity of the electrolyte. This will allow for the iron deposition to occur only onto the slurry particles and not on the current collector leading to a better control of the current distribution.21 As for the VRFB, the solubility and the size of the tanks will determine the amount of stored electrolyte. In the classical configuration of all-iron RFB (or hybrid RFB) the energy capacity will be also limited by the amount of material that can be plated onto the electrodes. Therefore, the electrode surface area plays an important role in determining the all-iron (hybrid) RFB efficiency and lifetime. For slurry all-iron RFB this role becomes secondary. Presently, the slurry all-iron RFB are still in the development stage putting them at a disadvantage with the already commercialized VRFB. The all-iron hybrid RFB and all-iron slurry RFB is suited for area regulation as capacities of 1−40 MW where discharge times between 15 and 30 min are required. Typically, the energy density of the all-iron hybrid battery is 12.7 Wh/L with

Figure 2. Schematic representation of a redox flow battery. The electrolyte is kept in outside tanks and pumped in the two membraneseparated chambers of the active cell.

reactants (electrolytes), while the power is controlled by the stack (Figure 2).15,16 In general, the RFBs share similar flow geometries, and the main differences typically occur in the electrolyte that is used. In this study vanadium and all-iron RFBs were chosen for comparison as both are using similar electrolytes in the two cells, and, therefore, cross contamination is avoided.17−23 The RFBs can operate at low temperatures (from −10 to +45 °C) as long as the electrolytes remain stable and their precipitation does not occur. 2.2.1. Vanadium Redox Flow Batteries. The VRFB has promising energy storage characteristics (e.g., fast response time, low self-discharge characteristics, etc.) that make it a suitable technology for integration with intermittent wind energy. Thus, the VRFB can respond fast to unpredictable changes in wind speed as their response time from shut down is 1 min. The VRFB has a high efficiency in the range of 65− 80%.16 The practical achievable volumetric energy density of VRFB is ∼15−25 Wh/L with the density of vanadium electrolyte approximately 1.3 kg/L. Recently a 50% increase in vanadium ion concentration has been reported, and this will lead to a corresponding increase in the energy density of this storage technology.19,20 Current costs of vanadium electrolyte are 3.22 $/L for small scale production and high purity vanadium; however, projected costs of the electrolyte are estimated to be lower at 2 $/L based on a vanadium pentoxide price of 11 $/kg.21,24 Furthermore, reductions in the cost are also expected if lower purity vanadium oxide will be used. For VRFB the electrolyte is evenly split between the positive and negative tanks. The reactions that occur within the cell during charging and discharging cycles are shown in Table 2. Table 2. Chemical Reactions Occurring at the Negative and Positive Side of the VRFB and All-Iron RFB VRFB positive side negative side

−

all-iron RFB

VO + H2O − e → VO2 + 2H V3+ + e− ⇆ V2+ 2+

+

+

Fe ⇆ Fe3+ + e− Fe2+ + 2e− ⇆ Fe0 2+

There are several advantages of using VRFB for energy storage applications: long cycle life (>10000 cycles), high reliability, deep discharge capability, and high power density. Although the electrodes do not store energy, they are important for charging and discharging of the battery, influencing, together with the electrolyte and separation membrane, the lifetime of the battery, the energy losses, and, consequently, the overall efficiency. As aforementioned, the energy capacity of the C


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Figure 3. Percentage of economic costs associated with 1 MW/4 MWh systems using a) VRFB (where Other represents the cost for tanks − 2%, pumps − 5%, PVC frames − 2%, power conditioning systems (PCS) − 11%, and other materials for battery construction − 3%) and b) all-iron slurry RFB (where Other represents the cost for tanks − 8%, pumps 8%, PVC frames − 4%, flow frames − 5%, and other materials for battery construction − 8%). The patterned segments represent the areas in which carbon price can affect the economic costs of the system when introduced into the battery.23,32

Figure 4. a) Wind-speed energy generated over a 72 h period showing the response of the wind turbine based on the power curve of one 0.5 MW wind turbine. Scenario A with periods of very high wind and Scenario B a typical moderate wind time period are used for determining the system response. b) Solar energy generated in a 40 kW solar PV system taken from data in January and June.34

can potentially influence the different components. As the E/P ratio will affect the RFB costs the different percentages shown in Figure 3 will differ. In VRFB systems the carbons can be used to modify and improve the properties of the membrane electrode assembly (i.e., felt, membrane, and bipolar plates) (Figure 3a), and the impact can be up to 34% of the component’s cost. In all-iron RFB the carbon impact can be large (up to 67%) (Figure 3b), especially when carbon slurries are employed. The costs of the flow frames and for the carbon particles used to form the slurry are both highly uncertain, and these are two of the largest costs considered. The cost of carbon is in a range of 700−2500 $/ton. For a current density of 200 mA/cm2 with a round trip efficiency of 75% and a parasitic loss to pumping the slurry of 0.6% and with the current cost of other cell components the cost of the slurry particles must be kept under 1.1 $/kg to meet the economic goal of 200 $/kW.30

a specific energy 10.9 Wh/kg. Energy efficiency is 56% with operating temperature To = 40 °C.21 2.2.3. Effect of Carbon. The carbon materials have the potential to enhance the efficiencies of the storage technologies. The PEMFC already are reliant on platinum as a material that increases its price. Therefore, platinum replacement with cheaper (carbon) composites would increase the commercial viability of fuel cells and, consequently, of the hydrogen as an energy carrier.26 Carbon allotropes, nanofibers, or nanotubes exhibit physical and chemical potential characteristics that could improve the batteries, making them an ideal material for VRFB solid electrodes.27 This can result in widening the operating potential range and higher chemical stability. Also, all-iron RFB can become an economically viable energy storage technology as a result of the relatively low cost of the electrolyte required. The improvements on the system efficiency offered by the slurry electrodes are making these batteries more attractive.23,28 Carbon felt is another typical electrode material with a wide operating electrode potential range and stability, a high surface area, and a reasonable price, but it possesses poor electrochemical activity. Consequently, much attention has been provided to electrode modification to enhance their electrochemical properties.29−31 The main components where carbon, in its different forms, can affect the RFB economic costs are detailed in Figure 3 (patterned segments).23,32 The percentage breakdown of economic costs are presented in Figure 3 for 1 MW/4 MWh RFB systems to highlight the areas in which the price of carbon

3. DECENTRALIZED INTEGRATED STORAGE SYSTEM Depending on the system sizing and overall load, the decentralized systems can support towns by meeting the domestic demand, and/or they can be used to supply the commercial or industrial buildings and help reduce CO2 emissions by reducing grid energy required. A system dynamic framework was used in this study, and further work will expand on the use of system dynamics to form a more complex system. In this research Vensim is used as the modeling software. This is a professional system dynamic D


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Energy & Fuels simulation software based on selection logic for looking at the performance of complex, real systems.33 The response of the system is based on the state of other variables within the system at a particular instant in time. Exogenous variables within the system are wind speed, system marginal price of electricity, and the system demand along with the technical constants of the storage technology. Scenario analysis is used by employing different energy inputs (Figure 4). The input parameters of the model are set, and the output delivers the valuable characteristic quantities for the characterization and assessment of the system. This framework enables the identification of opportunities for the design rationalization of the integrated energy storage system. The charging and discharging of the energy storage systems form the basis for stocks and flows within the model and the response of the storage system assessed. The wind energy generated from 0.5 MW for two wind scenario over 72 h is shown in Figure 4a. The solar output over 48 h is presented in Figure 4b. Figure 4 highlights the vulnerability of a decentralized system to both wind and PV intermittency indicating the requirement for storage. A town with both domestic and office demand is considered (Figure 5), with the grid price being based on the system

Figure 6. Decentralized wind and individual energy storage systems used in the model. A 0.5 MW wind turbine is integrated with 0.5 MW energy storage technologies. The capital costs for the different technologies are included. These are indicative costs and specific costs for the actual modeled systems, and they can vary as a result of the E/P ratio.37,38

MW fuel cell were chosen for the integration of a 0.5 MW wind turbine into the decentralized energy system. However, RFB with a larger E/P ratio can be advantageous from a capital cost perspective, and the use of redox flow batteries are more cost competitive for storage times of 4 h or more. Further analysis and research must be undertaken when installing a RFB within a particular system to optimize the E/P ratio and the economic costs of the system. In the second study an integrated storage system, with the combination of hydrogen and VRFB storage, is considered to investigate the potential outcomes of an integrated storage system in comparison to the individual system assessment in study 1 (Figure 6). It was reported that adding hydrogen storage to a wind energy system can reduce the total levelized cost of electricity if the storage system can reduce the curtailment by greater than 5%; this is as a result of hydrogen’s ability to act as a bulk energy storage medium and to increase the capacity factor of the system.39 The study aims to look at the viability and stability of the remote system as well as to analyze the results from the integrated hydrogen and VRFB system.

Figure 5. Trend patterns of domestic and office electrical demands. A decentralized integrated energy storage system with wind energy can provide decarbonized electrons to supply the demand, while the grid is maintained as a backup source of electricity.35,36

marginal price to consider the individual operation of the systems. In the first study the response of a decentralized renewable and individual storage system (as depicted in Figure 6) is tested using a system dynamic modeling approach. The investigation of technical and environmental potential of the individual decentralized energy storage systems is the primary objective. Discharging of the storage systems is based on the amount of renewable energy available to meet the demand. Therefore, it is expected that storage system management and operation will be complex due to intermittent renewable energy, different grid energy prices, and load on the system. The role of the storage systems is to maximize the mitigation of grid energy. The costs of the system are based on an average levelized cost for the storage systems. The advantage of RFB includes the independent energy and power capacity. Worldwide installed operational VRFB technologies include many VRFB systems with an E/P ratio of two or less than two. Such systems are installed at Tasumi Sub-Station, Kansai Electric Japan (450 kW/900 kWh), Tottori Sanyo Electric, Japan (1.5 MW/1.5 MWh), Stellenbosch University, South Africa (250 kW/500 kWh), Milan, Italy (45 kW/90 kWh), and Tomamae Hokkaido, Japan (4 MW/6 MWh).25 Here the 0.5 MW/1 MWh redox flow batteries with an E/P ratio of two and a 0.5

4. RESULTS AND DISCUSSIONS One of the objectives is to complete the individual testing of the technologies using different scenarios (Study 1). For this a tornado graph was included to describe how the quantitative analysis responds to reasonable variations in the parameters. Study 2 aims to identify potential advantages of the integration of hydrogen and RFB technologies. 4.1. Study 1 − Comparison of Individual RFB and Hydrogen Technologies. The total demand that resulted from the real domestic and office demand was 10.27 MWh. The demand has to be met by a 0.5 MW wind turbine, a storage system, and the grid as backup over the 72 h period in two different wind scenarios (Figure 4).35,36 To reduce the curtailment the system load can be increased; however, during the night, curtailment can still remain high. The integration of the storage systems is an effective method of reducing E


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“wind to storage”). The hydrogen system acts as a buffer for the wind energy for a long period of time and represents 18.2% of the demand over the 72 h period (Figure 7a). The two RFB systems have a similar response to each other with a better performance from VRFB (here depicted in Figure 7b) due to higher efficiencies. Both the RFB responses are lower than hydrogen as a result of the energy storage being constrained by the lower energy densities. In Table 4 the calculated cost of the storage energy and the grid energy over the period is presented based on levelized energy costs. It should be noted these costs will change as new advanced technologies will be used. Here, these costs are presented only to highlight the lower cost of the redox flow batteries when compared to the hydrogen storage technology. A further study investigates how an integrated VRFB and hydrogen system can reduce grid energy consumption and costs. It can be observed that hydrogen has higher levelized costs per MWh (469 €/MWh) than VRFB (276 €/MWh). As they are still under development the levelized cost of all-iron RFB is not currently available, but it can be anticipated that the use of carbon slurries within the all-iron RFB will allow for a lower levelized cost compared to VRFB. This will put the alliron RFB as a strong candidate for energy storage systems. In the present scenario the levelized cost of all-iron RFB was estimated based on the levelized cost of the electrolyte in the iron−chromium RFB as this technology benefits from the low cost of iron as an electrolyte (230 €/MWh). The cost of hydrogen production is larger than RFB; however, it is capable of meeting more demand in this scenario. This result is one motivation behind the application of an integrated hydrogen and RFB system analyzed in study 2. Table 5 represents the percentage of wind, grid, and storage energy used when the E/P ratio of the RFB is increased to four

curtailment within the system, indicating the importance of storage systems for renewable energy. The extent to which each storage system can respond in the system as well as costs are considered. The model calculates the average use of the wind for different storage systems and the grid energy for both wind scenarios, Scenario A and Scenario B. The results for Scenario A presented in Table 3 indicate an increased capability of hydrogen for bulk energy storage. The Table 3. Average Percentage of Demand Met by Wind, Grid, and Storage Energy for a 72 h Period, Scenario A in Figure 7a direct wind direct grid storage system a

all-iron RFB

VRFB

hydrogen

81.8% 11.3% 6.9%

81.8% 11.1% 7.1%

81.8% 0% 18.2%

0.5 MW/1 MWh.

hydrogen was generated from the renewable electrolyzer system. After compression hydrogen is used by the fuel cells employed (Figure 6). The high curtailment within the system during certain periods can be seen in Figure 7 (represented by

Table 5. Average Percentage of Demand Met by Wind, Grid, and Storage Energy for a 72 h Period, Scenario Aa direct wind direct grid storage system a

all-iron RFB

VRFB

hydrogen

81.8% 4.9% 13.3%

81.8% 4.7% 13.5%

81.8% 0% 18.2%

0.5 MW/2 MWh.

(0.5 MW/2 MWh) for Scenario A. The increase in the E/P ratio can have a positive effect on the system cost. In this case the increases in the energy from the redox flow batteries will allow an improved payback period. Also an increase in the grid energy mitigated exists. The main advantage of RFB includes the fact that the energy and power capacity can be scaled individually, and, therefore, the system can be optimized for different applications. Table 6 represents the results from Scenario B. This scenario shows that the RFB can meet more of the demand compared to

Figure 7. Response of the decentralized a) hydrogen and b) VRFB storage system with Scenario A (Figure 4a). Grid energy, stored energy and wind energy are used for meeting demand. Wind otherwise curtailed is used for the a) production of hydrogen and b) charging of VRFB and is indicated as “wind to storage system” in the graph. Grid energy is used as a backup of the system when the storage system cannot meet demand.

Table 4. Calculated Costs for the Direct Grid Generated and Storage Energy Using Scenario A in Figure 7a all-iron RFB direct grid cost based on system marginal price storage energy cost total cost a

€83.44 estimated f rom iron−chromium battery 0.71 MWh * 230 €/MWh = €163 €246

VRFB

hydrogen

€82.71 0.73 MWh * 276 €/MWh = €202

€0 1.87 MWh * 469 €/MWh = €877

€284

€877

€0.92 = $1. F


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the lower curtailment in the wind energy. Consequently, the additional energy capacity was not used. A standard sensitivity analysis was carried out for Scenario A and Scenario B to further analyze the different advantages and disadvantages of the energy storage technology identified. The wind energy was increased and decreased by 10% in both cases, and the results for Scenario A and Scenario B are shown in Table 8. The results of the sensitivity analysis further highlight

Table 6. Average Percentage of Demand Met by Wind, Grid, and Storage Energy for a 72 h Period, Scenario B in Figure 8 direct wind direct grid storage energy

all-iron RFB

VRFB

hydrogen

71.8% 19.7% 8.5%

71.8% 18.6% 9.6%

71.8% 20.7% 7.5%

the hydrogen system (Figure 8) as a result of higher round-trip efficiencies. From the results, the value of hydrogen as a storage

Table 8. Sensitivity Analysis for Wind Energy ±10% Scenario A all-iron RFB VRFB hydrogen Scenario B all-iron RFB VRFB hydrogen

direct wind (%)

direct grid (%)

storage energy (%)

78.9−84.5 78.9−84.5 78.9−84.5

8.6−14 8.4−13.8 0−7.3

6.9−7.1 7.1−7.3 15.5−16.4

69.1−74.2 69.1−74.2 69.1−74.2

18.5−21.6 18.5−20.1 16.6−25.1

7.3−9.3 7.3−10.8 5.8−9.2

the importance of the amount of wind energy available in the system. The effectiveness of the storage technologies therefore varies with the amount of excess wind within the system. Overall, the storage systems in both scenarios are able to reduce CO2 emissions, equivalent to the amount of grid energy avoided (∼0.45 kgCO2/kWh). Hydrogen storage can mitigate more energy in Scenario A, while the RFB is able to mitigate more in Scenario B. Therefore, the use of decentralized renewable and storage systems will have an important function as stronger environmental policies will be put in place. The uncertainties that contribute to the percentage of storage energy used for meeting demand are presented in Figure 9 and Figure 10 as tornado charts for hydrogen, VRFB, and all-iron RFB technologies for Scenarios A and B, respectively. A sensitivity of 10% was used for total demand, wind energy generated, and wind energy curtailed in the system. In Figure 9a it can be seen that the hydrogen system has a reduction in the amount of energy when wind curtailment is reduced. No change was observed when the curtailed wind increased. The lack of change is a direct outcome of hydrogen’s ability to already provide the maximum energy to the system. The hydrogen reduction with the increase in wind generated energy arises from the fact that wind is used to meet the demand and not the hydrogen. The increase and decrease of demand lead to a reduction of hydrogen present in storage. Lower demand results in less hydrogen requirement. Higher demand leads to an increase of hydrogen required by the system. It can be seen that hydrogen is more sensitive to curtailment in Scenario A than the RFB. The RFB (Figure 9b and Figure 9c) show similar trends. Figure 10 highlights the sensitivities for Scenario B. The wind energy and wind curtailed results highlight the sensitivity of the

Figure 8. Response of the decentralized a) hydrogen and b)VRFB storage system with Scenario B (Figure 4a). Grid energy, stored energy, and wind energy are used for meeting demand. Wind otherwise curtailed is used for the a) production of hydrogen and b) charging of VRFB and is indicated as the “wind to storage” system in the graph.

system is not realized when there is low curtailment within the system. This is as a result of lower efficiencies, and it cannot take advantage of its bulk energy capability (Table 6). Table 7 shows the costs from the energy systems and highlights hydrogen’s higher costs. For the RFB costs, the alliron RFB cost of electricity generation is estimated to be lower by ∼27% compared to the VRFB. Furthermore, their ability to meet similar demand allows the all-iron RFB to become a suitable replacement for VRFB if further developments will be implemented to the present technologies. Like for Scenario A the simulation was repeated for Scenario B with an E/P ratio for the RFB of 4 (0.5 MW/2 MWh). No change was observed in the grid energy mitigated as a result of

Table 7. Calculated Costs for the Direct Grid Generated and Storage Energy Using Scenario B in Figure 8a all-iron RFB direct grid cost based on system marginal price storage system cost total cost a

€155 estimated f rom iron−chromium battery 0.87 MWh * 230 €/MWh = €200 €355

VRFB

hydrogen

€144 0.985 MWh * 276 €/MWh = €272

€163 0.77 MWh * 469 €/MWh = €361

€416

€524

€0.92 = $1. G


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Energy & Fuels

Figure 9. Tornado graph representation of the uncertainties in the percentage of a) hydrogen, b) all-iron RFB, and c) VRFB energy storage used for meeting demand with a sensitivity of 10% in the values of wind curtailed, wind energy generated, and demand in Scenario A.

Figure 10. Tornado graph representing the uncertainties in the percentage of a) hydrogen, b) all-iron RFB, and c) VRFB energy storage used for meeting demand with a sensitivity of 10% in the values of wind curtailed, wind energy generated, and demand in Scenario B.

storage technologies to wind in the system. Unlike RFB, hydrogen has an increased generation of energy when wind energy is increased. This is a result of low hydrogen production in this scenario, and any additional wind to storage will increase hydrogen production. Additional inputs can be incorporated into the decentralized energy system model that can highlight further system responses. The main focus of the model is to investigate the storage system response to wind energy and demand; however, alternatively additional variables such as costs of the storage system could be incorporated into the Tornado diagram investigating further the potential of the storage systems. The results indicate the use of integrated storage systems can potentially allow one storage system to counteract the problems of another (investigated in Section 4.2.1). 4.2. Study 2: Comparison of Integrated Systems. 4.2.1. Study 2a Integrated Hydrogen and VRFB System. Individual systems hold many advantages; however, the use of integrated systems can potentially allow a higher mitigation of grid energy. From Figure 9 and Figure 10 the varying sensitivity to wind curtailment can be identified. To compare the integrated system, wind Scenario A with higher wind curtailment and Scenario B with low wind curtailment were combined to create a model with a time boundary of 144 h with

wind Scenario A for the first 72 h and Scenario B for the next 72 h. This allows a comparison of the benefits of integrated hydrogen and VRFB system. As hydrogen’s value is obtained when excess wind is high, three different levels of wind to hydrogen storage were considered. These were when the system encountered greater than 40% (case 1), 60% (case 2), and 80% (case 3) of curtailment; excess wind was sent to the hydrogen system with the remaining wind and then sent to the VRFB. In the second case when there was >60% curtailment, wind sent to the hydrogen system has the best response (compared to both case 1 and case 3) increasing the amount of storage energy used within the system and, therefore, mitigating grid energy. The percentage of the demand that can be met by the integrated storage systems with the different curtailment function and their comparison to the individual systems is shown in Table 9. The results indicate reduced grid energy use in all three integrated cases compared to the individual systems. It is also interesting to note the operation of hydrogen over the 144 h is capable of reducing overall grid energy by 12.8%, while the VRFB only reduces it by 8.3%; however, from the cost comparison presented in Table 10 the electricity generation cost from the hydrogen system overall is estimated to be ∼50% higher. The integrated system in case 1 is the most expensive case regarding electricity generation as a result of hydrogen H


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Energy & Fuels Table 9. Comparison of Individual VRFB and Hydrogen Storage with Integrated VRFB and Hydrogen Storagea integrated hydrogen and VRFB VRFB individual system

hydrogen individual system

curtailment >40% to hydrogen



76.8% 14.9% 8.3%

76.8% 10.4% 12.8%

76.8% 8.4% 14.8%

76.8% 6.8% 16.4%

76.8% 9.2% 14%

direct wind direct grid storage energy a

If curtailment of >40%, >60%, and >80% the excess wind is sent to the hydrogen system.

The second study considered the integration of hydrogen and VRFB storage, and in this case the capability of hydrogen for bulk energy storage and VRFB with higher efficiencies is complementary. Furthermore, it was found that the effect of the integrated systems depends on the level of excess wind sent to either the hydrogen or VRFB storage system. This emphasizes the need for optimization to ensure the most effective response from the system. The results further highlight the potential that decentralized wind-storage systems can have on CO2 emissions. As a result of the growing awareness and need for future regulatory action the results derived from the study indicate that the analyzed systems can play an important role to meet strengthening environmental regulations. This study compared different technologies and investigated integrated systems. It is clear that such integrated systems can overcome the disadvantages exhibited when each technology is used independently. The use of RFB integrated with hydrogen production systems can be advantageous as a result of hydrogen’s capability for bulk energy storage.

Table 10. Comparison of Individual VRFB and Hydrogen Storage Costs with Integrated VRFB and Hydrogen Storagea integrated hydrogen and VRFB

direct grid cost based on system marginal price storage system cost total cost

VRFB

hydrogen




€227

€163.3

€144

€113

€151

€474

€1231

€1309

€1240

€916

€701

€1394

€1454

€1353

€1067

a

If curtailment of >40%, >60%, and >80% the excess wind is sent to the hydrogen system. Total demand (20.54 MWh).

being used more than the VRFB as a result of more wind energy being sent to the hydrogen system, and this is reflected in the costs of case 2 and case 3 also. From the results shown in Table 9 and Table 10 the advantages of the integrated VRFB and hydrogen system can be identified. It can be seen that there is an optimum percentage to be sent to the hydrogen energy storage when curtailment is higher. From the results it can be identified that a trade-off must be made between mitigation of grid energy or costs. Furthermore, the requirement for grid energy is reduced in the integrated system mitigating further CO2 emissions and strengthening the case for integrated storage systems.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 353 87 311 7431. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

5. CONCLUSIONS The present study investigated the operation of individual and integrated energy storage systems. From the research of the independent systems in study 1, Scenario A was identified as a result of higher available wind energy for storage; the hydrogen system is able to provide more energy to the system due to its capability as a bulk energy storage medium. In contrast the RFB are capable of providing more energy to the system in Scenario B, with reduced availability of energy for storage, as a result of higher efficiencies. Therefore, VRFB or all-iron RFB are more efficient energy storage at times when there are no high periods of curtailment. Continuous VRFB research also allowed an increase in the achievable energy density. Additionally, redox flow batteries are ideally suited to applications with an energy to power ratio of 4 or higher which is evidenced by the cost analyses presented. Furthermore, novel hydrogen production methods (in particular, microwave plasma processing of natural gas) that produce carbon can lead to higher efficiencies in RFB. The most important impact on costs is predicted for all-iron RFB as a result of the use of carbon slurry in the all-iron electrolyte. For VRFB the impact is predicted to be lower as the electrolyte (which is the most expensive component in the system) cost cannot be reduced by carbon.

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ACKNOWLEDGMENTS This work was supported through the Bernal Energy Project research funding at the Bernal Institute, University of Limerick, Ireland.

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REFERENCES

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Potential of Redox Flow Batteries and Hydrogen as Integrated Storage

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