Reversible solid oxide systems for energy and chemical applications – Review & perspectives

Journal of Energy Storage 24 (2019) 100782

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Journal of Energy Storage journal homepage: www.elsevier.com/locate/est

Reversible solid oxide systems for energy and chemical applications – Review & perspectives

T

Vikrant Venkataramana, , Mar Pérez-Fortesb, Ligang Wangb, Yashar S. Hajimolanaa,1, Carlos Boigues-Muñozd, Alessandro Agostinid, Stephen J. McPhaild, François Maréchalc, Jan Van Herleb, P.V. Aravinda ⁎

a

Process & Energy, Delft University of Technology, Leeghwaterstraat 39, 2628 CB, Delft, the Netherlands Group of Energy Materials, Swiss Federal Institute of Technology in Lausanne, Valais Wallis, Rue de l’Industrie 17, 1951, Switzerland c Industrial Process and Energy Systems Engineering, Swiss Federal Institute of Technology in Lausanne (EPFL), Rue de l’Industrie 17, Sion 1951, Switzerland d ENEA, Energy Technologies Department, Hydrogen and Fuel Cells, Via Anguillarese 301, 00123, Rome, Italy b

ARTICLE INFO

ABSTRACT

Keywords: Solid oxide reversible cell Power-to-X-to power Energy storage Chemicals Process chains

This paper presents a mini-review in the field of energy storage using reversible solid oxide cells (rSOCs) for development of energy storage systems for the future. Such energy storage systems fall under the category of power-to-X-to power systems where excess electrical energy produced through renewables is stored in the form of chemicals and the same chemicals are used for conversion back to power. The main competitors of energy storage systems based on rSOC are pumped hydro storage, compressed air storage and batteries and it is envisioned that with better heat integration techniques, the round trip efficiency of rSOC systems can be improved to reach the target value of 80% as specified in the joint EASE-EERA report for European energy storage technology.

1. Introduction With increasing penetration of renewables into the power market and the fact that they are intermittent in nature, there is an increasing need for energy storage with elevated capacity combined with high charge/discharge periods. The intermittent nature of renewables, when integrated with existing centralised energy generation systems, brings a set of challenges such as grid overloading, grid frequency mismatch and difficulty in scheduling operational times of conventional power plants. In order to take care of these challenges, the concept of smart grids along with Internet of Things (IoT) has been proposed and is being developed [1,2]. The smart grid will automatically carry out a power

balance, thereby making the supply-demand ratio to be always equal to one. Future smart grids however need regulations, standards, control strategies and ad-hoc tools to deal with fluctuating power. In EU28, Switzerland and Norway, 950 R&D and demonstration projects have been identified in the field of smart grids alone. The percentage of R&D projects were more when compared to demonstration projects (57% vs 43%), but with lower share of funding (32% vs 68%). R&D projects are still needed for further development of the respective technology, investigation of newer and advanced options, and for integration and interoperability [3]. A micro-grid, which is a part of the smart grid, can include a set of energy conversion systems, energy storage devices and

Abbreviations: BoP, balance of plant; CAES, compressed air energy storage; CAPEX, capital expenditure; CDU, carbon dioxide utilisation/CO2 utilisation; CHP, combined heat and power; DME, dimethyl ether; EASE, European association for storage of energy; EDLC, electrical double layer capacitor; EERA, European energy research alliance; FES, flywheel energy storage; FU, functional unit; FT, Fischer tropsch; GHG, greenhouse gas; GWP, global warming potential; IoT, Internet of Things; LCA, life cycle analysis; LCIA, life cycle inventory analysis; OS, operational scenario; PEM, polymer electrolyte membrane; PHS, pumped hydro storage (Wikipedia says storage); Power-to-X-to power, power to commodity chemicals to power; RFB, redox flow battery; SMES, superconducting magnetic energy storage; SOC, solid oxide cell; SOFC, solid oxide fuel cell; SOEC, solid oxide electrolyser cell; rSOC, reversible solid oxide cell; TES, thermal energy storage; TRL, technology readiness level ⁎ Corresponding author at: Vrije Universiteit Brussel (VUB), Electrical Engineering and Energy Technology (ETEC), Vrije Universiteit Brussel, Pleinlaan 2, 1050, Elsene, Brussels, Belgium. E-mail address: [email protected] (V. Venkataraman). 1 This author now currently works at the Department of Thermal & Fluid Engineering, Faculty of Engineering Technology, University of Twente, Drienerlolaan 5, 7500 AE Enschede, the Netherlands. https://doi.org/10.1016/j.est.2019.100782 Received 17 February 2019; Received in revised form 18 May 2019; Accepted 18 May 2019 2352-152X/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Schematic showing working principle of rSOC in individual modes.

consumers that works independently of the centralised grid. Within the context of energy storage, systems based on reversible solid oxide cells (rSOC) are gaining increased attention and interest. An rSOC is both a fuel cell and an electrolyser combined together in a single device, converting fuels to electricity and heat in the fuel cell mode and vice versa. The working principle of the rSOC is similar to the SOFC in the fuel cell mode and to the SOEC in the electrolysis mode, the main advantage being the ability of the device to carry out both operation in both modes. The schematic for the same is shown in Fig. 1. The terms anode and cathode have been avoided when describing the rSOC operation. This is because the annotations for the electrodes change when the operational mode changes. Hence the terms fuel electrode and air electrode (also possible to use oxygen electrode if pure oxygen is used) have been used. rSOC systems may be commercialised in a micro-grid or a larger context as a unique standalone solution or as a hybridised solution (with other storage solutions). The market opportunity for rSOC exists because the current energy storage technologies have not been able to fully meet the requirements for energy storage applications by themselves on a standalone basis [4–6], some of them being flexible in terms of deployment, capability of seasonal storage and decoupling the power producing aspect from the energy storage aspect. Some of the key requirements for energy storage technologies are ease of scalability, ease in adaptability, long term discharge, good dynamic response and low environmental impact and it is envisaged that rSOC based systems can solve/overcome many of these challenges, details of which will be listed out in the forthcoming sections. Energy storage is essential in order to act as a source or sink for power, taking the role of the former when demand exceeds supply from power plant and vice versa. In short, it strives to keep the ratio between power demand and supply at one at all times. The interest in energy storage using hydrogen and other chemicals (such as methanol or ammonia etc.) as a storage medium is increasing due to the fact that more energy (in terms of volume) can be stored in the form of hydrogen (and other chemicals) when compared to storing it in batteries [7]. This interest has in turn translated to employing fuel cells and electrolysers in energy storage system technologies.

electrolysis is a process with low efficiency due to high operating voltage required. The operating voltage for splitting an H2O molecule can be significantly reduced at high temperature. This makes the Solid Oxide Electrolysis Cell (SOEC) an attractive and efficient option. Unfortunately, SOEC currently has a low TRL (Technology Readiness Level), still being at the laboratory scale [13,14] with the exception of some field demonstrations by Sunfire GmbH. Renewable energy sources, such as biomass, solar and wind, are usually coupled with electrolysis technologies in order to absorb power (in times of excess production) thereby helping with power levelling and assisting in synthesis of a renewable product [15]. Wind is currently the most cost effective source among renewables but is also the least predictable [16]. There has been a lot of emphasis of late on the use of SOCs (Solid Oxide Cells) as electrolysers and also the use of the same cell/stack for both modes of operation (rSOCs). This is because rSOCs have lower activation losses at lower current densities (when compared to alkaline or PEM electrolysers) which in turn translate to higher power production during the fuel cell mode and lower power consumption in the electrolyser mode. Alkaline and PEM based electrolysers are currently commercial but expensive technologies. They are not so efficient and there is a need to have two separate components - one operating in the fuel cell mode and the other in the electrolysis mode, when such systems are interconnected for power balancing and energy storage. Another advantage in employing rSOC technology lies in the fact that the same unit can be used both for electricity generation and fuel production thereby reducing CAPEX (capital expenditure). Also it has very high theoretical efficiency (> 60% electrical efficiency in fuel cell mode and > 80% energy efficiency in electrolysis mode) and –being reversible - is located in the same place where hydrogen is produced whereas other technologies (standalone fuel cell/electrolyser units) might involve producing hydrogen at a different place and would probably require transportation and delivery and thus additional energy. 1.2. Knowledge gap There are some papers on modelling of rSOC cells and systems [17–22], mostly with H2 and methane as fuels in the fuel cell mode, combined with either steam electrolysis or co-electrolysis. Besides the two papers [18,22] where a detailed system level study along with BoP is carried out, the remaining papers either model i-V curves of single cells and validate the results with experiments or carry out a control strategy to optimise the energy management of the whole system. Hence, there is limited literature in the field of electrical energy storage systems employing rSOC units. This paper focuses on how rSOC systems can be integrated into the energy storage network by listing out several prospective process chains that could be developed and that are attractive from a power-to-X-to power perspective. This work first outlines the different energy storage technologies that are currently available and then positions rSOC systems as a potential candidate for future energy systems. Similar and past projects where fuel cells and electrolysers were employed for energy storage are discussed thereafter, followed by techno-economic and environmental impact analyses. This is then followed by potential

1.1. SOFCs and SOECs Current stationary SOFC systems mainly transform hydrogen-rich gaseous fuels into electricity and heat; however, the main advantage of SOFC systems is their fuel flexibility; utilisation of even liquid fuels is a possibility. However a reformer is almost always required to operate on fuels other than hydrogen and this can either be an external reformer or there are possibilities to have internal reforming within the cell(s) [8,9] as well. The development of anode microstructures to prevent carbon formation and liquid fuel customisation (addition of additives) to reduce coking are examples of research targeted towards liquid fuel utilisation [10]. Hydrogen from water electrolysis can be produced by either alkaline or proton exchange membrane electrolysers. The most commercial electrolysers up to date are of the alkaline class, with sizes ranging between 0.6–125 MW of produced H2 [11,12]; however, water 2

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V. Venkataraman, et al.

Fig. 2. Classification of different electrical energy storage technologies. (*Hydrogen is not the only chemical that can be produced. Other chemicals such as methanol, dimethyl ether - DME, methane, syngas can also be produced and stored. However they need to be reformed prior to use in fuel cell mode).

benefits and challenges and the potential applications of these systems in the future energy storage scenario.

2.1. Mechanical energy storage Mechanical energy storage is represented by pumped hydro system (PHS), compressed air energy storage (CAES) and flywheel energy storage (FES). In 2017, PHS represented over 96% of worldwide bulk storage capacity of over 168 GW [23]. The five largest PHS plants operational in the world are listed down in Table 2. PHS is a mature energy storage technology with advantages of high storage capacity, high efficiency and relatively low capital cost per unit of energy. However, it requires a geographically suitable site (like hilly or mountainous region) in order for effective implementation which may lead to a host of social and ecological issues [24]. Besides appropriate site selection, PHS

2. Different energy storage technologies and positioning of rSOC based systems There exist different energy storage technologies and these can be broadly classified under five main categories- mechanical, electrochemical, electrical, chemical and thermal. The different technologies falling under these categories are summarised in Fig. 2. The technical characteristics of these energy storage technologies at different TRL are summarised in Table 1. Table 1 Technical characteristics of different energy storage technologies. System

PHS

CAES

FES

Battery Lead-acid Battery Li-ion SMES TES Reversible fuel cells Flow batteries

Super Capacitors

Primary application management • Energy and seasonal • Backup reserves service also • Regulation available through variable speed pumps

management • Energy and seasonal • Backup reserves integration • Renewable leveling • Load regulation • Frequency shaving and off peak • Peak storage stability • Transient leveling and • Load regulation stabilization • Grid quality • Power regulation • Frequency quality • Power regulation • Frequency leveling and • Load regulation stabilization • Grid management • Energy levelling • Load energy storage • Seasonal • Ramping Shaving • Peak Shifting • Time regulation • Frequency quality • Power quality • Power • Frequency regulation

Energy Density (W h/L)

Daily selfdischarge (%)

Response time

Life time (years)

Cycle life (cycles)

Round trip efficiency

Initial investment cost (USD/kW) [33]

0.5–2 [34,35]

0 [5]

3–6 min [5]

30–60 [34,36]

10,000–30,000 [34]

70–85 % [5]

500–4600

2–6 [37]

< 0.3 [24]

3–6 min [37]

20–40 [34]

8,000–12,000 [38]

42–55 % [39]

500–1500

20–80 [40]

> 20% per hour [32]

< 4 sec [41]

20–30 [34] [42]

> 20,000 [41]

85–90 % [43]

130–500

50–90 [34]

0.1–0.3 [34]

< 1 sec [38]

5–15 [34]

1,800 [44]

80–90 % [45]

200–500 [34]

0.1–0.3 [34]

< 1 sec [38]

1,000–20,000 [34]

90–95 % [46]

300–3500

0.2–2.5 [34]

10–15 [34]

< 1 sec [38]

20,000 [41]

< 95 % [47]

13–515

80–500 [34]

5–30 [34]

< 15 min [34]

5–15 [38] [34] 20–30 [34] [41] 5–20 [34]

–

72–80 % [47]

500–3000 [34]

Almost zero [34]

Seconds [38]

5–20 [34] [48]

1,000–20,000 [34] [48]

16–35 [34] [41]

Almost zero [41]

< 1 sec [41]

15–20 [41]

12,000 [41]

84 % [41]

–

10–30 [34]

10–20 [34] [49] [50]

< 1 sec [41]

10–30 [51]

> 100,000 [34]

< 95 % [47]

130–515

3

500–750

the the the the

electricity electricity electricity electricity

needs needs needs needs

of of of of

homes homes homes homes

and and and and

businesses. businesses. businesses. businesses.

4 Germany

Hybrid energy storage system Braderup (Lithium-ion and Vanadium redox flow batter), Shleswig-Holstein Braderup, developed by Sony and Vanadis Power GmbH

Country Germany Japan USA Korea Japan USA

SMES station

Principle Grid Test Nosso Power Station Upper Wisconsin Hyundai Chubu electric power Co Battery and supercapacitor, LIRR Malverne WESS station

First commercial HTS-SMES System stability and power quality Power quality and reactive power support Power supply quality Provide comparison to transient voltage To perform regenerative braking, charging and discharging in 20-second time periods

Application

10 MWh 10 MWh 3 MW/0.83kWh 3MWh 5 MWh 0.04 MWh

[75] [75] [75] [75] [75] [75]

References

References [73] [74]

[72]

[71]

[70]

[69]

References

[66] [67] [68]

[65]

References

[64]

[64]

[63]

[61] [62]

References

[31]

[59,60] [31] [31] [31]

References

(continued on next page)

Storage Capacity (MWh)

Storage Capacity (MWh) 4MW output capacity and 20 MWs storage capacity 277 kW/8 kW h

3 MW h (2 MW h Lithium-inon and 1 MW h vanadium redox fellow battery)

80 MWh

120 MWh

300 MWh

Storage Capacity (MWh)

2 MW (for 15 min) 20 MW / 80 MWh 5.25 MWh

200

4 hr X 135 MW

9 hr X 300 MW

145 110

Storage Capacity (MWh)

1,872

3,003 2,400 2,400 1,932

Storage Capacity (MWh)

5 MWh (20 MW over 15 min.)

The storage is used to stabilize the network to compensate for fluctuations caused by renewable energies. The storage is used to stabilize the network to compensate for fluctuations caused by renewable energies. To store energy at a low network load and then to feed this energy back into the grid at peak load The storage system is connected to the local community wind farm (18 MW installed capacity) to save a possible electricity surplus.

Application Supercapacitors integrated with diesel generator, to improve efficiency of original plant Supercapacitors integrated with battery system to provide peak demand response, load shifting and support for 1.2 MW PV array

USA

Litium-ion battery, Southern California Edison developed by Tesla

Country Spain USA

USA

Lithium-ion battery, Escondido Substation

Super Capacitor Station Canary Islands North Carolina

Japan

Sodium-sulphur battery, Buzen Substation

Application

Canada Canada USA Country

Grid balancing Grid balancing Spacecraft energy storage

USA

FES station in Stephentown, New York Developed by Beacon Power FES station in Minto, Ontario, developed by NRStor FES station in Fresno, Developed by Pacific Gas Electric FES Developed by NASA

Grid Battery Station

Grid balancing

Country

FES Station

Storage Capacity (MWh)

USA

Application

Utilizes nuclear-sourced night-time power for compression and produces peak power during the day via a natural gas turbine

USA

Norton CAES station Ohio Project developed by Haddington Ventures Inc. Markham CAES plant, Texas Project developed jointly by Ridege Energy Services EI Paso Energy CAES plant, Iowa Project developed by the Iowa Association of Municipal Utilities USA

Utilizes nuclear-sourced night-time power for compression and produces peak power during the day via a natural gas turbine The plant utilizes nuclear-sourced night-time power for compression and then produces peak power during the day by releasing the compressed air into a gas-fired combustion turbine Excess electricity generated at night by base load coal and nuclear power plants is used to compress air that is stored underground and then released in the daytime to fuel the plant’s turbines and produce energy Utilizes nuclear-sourced night-time power for compression and produces peak power during the day via a natural gas turbine

Germany USA

CAES plant, Huntorf CAES plant, McIntosh, Alabama

Application

Country

balances balances balances balances

CAES Station

quietly quietly quietly quietly

This process is designed to level the load of nearby nuclear power plants on the grid. It also replaces the need to build natural gas peak power plants used only during high demand.

station station station station

USA

generate generate generate generate

To To To To

USA China China Japan

Bath Country pumped storage station Guangdong Pumped Storage Station Huizhou Pumped Storage power station Okutataragi Pumped Storage power station Ludington Pumped Storage power station

Application

Country

PHS station

Table 2 List of developed operational energy storage system plants throughout the world.

V. Venkataraman, et al.

Journal of Energy Storage 24 (2019) 100782

Journal of Energy Storage 24 (2019) 100782

technology also requires a long construction time as well as high capital investment. Therefore PHS is generally not a versatile energy storage technology which can be employed everywhere. For CAES, the charging process is realised by compressing air to desired pressure and storing the compressed air with or without the heat produced; while in the discharging process, the stored air is first heated up with stored or external heat and then expanded to export power [25–27]. The advantages of CAES include high storage capacity (capable of providing over 100 GW h with a single unit), relatively low capital cost and high efficiency compared to other energy storage technologies. Similar to PHS technology, CAES is also geographically restricted. The other disadvantages with CAES include consumption of fossil fuels for external heat source and consequently generation of pollutants from combustion processes which makes them less attractive [28]. Due to these reasons, the applications of CAES are rather limited. The developed CAES plants throughout the world are listed out in Table 2. Flywheel Energy Storage FES systems are meant mainly for high power/short duration applications with the advantage of having long charge-discharge cycles. FES systems have already been applied in many fields such as transportation (electric cars, buses, light trains, ferries and underground transport), aircraft launchers, grid energy storage and wind turbines. Some of the systems developed with FES are listed out in Table 2. The main advantages of this technology include: fast start-up response (from seconds with standard FES systems to minutes with micro high speed flywheel), high round-trip efficiency (up to 90%) high power and energy densities, long service life for several million full depth discharge cycles [29], environmental friendliness and reduced volume footprint [30]. This technology, however, suffers from high maintenance costs, bulkiness and increased capital expenditure for high speed types [31], idling losses during standby which leads to relatively high discharge losses up to 20% of the stored capacity per hour [32].

[77]

1,030.5

1,680

12

A thermal storage system absorbs part of the daytime heat absorbed by the solar field. The heat is used to drive a turbine-generator when direct sunlight is not available,

Solar plant with thermal storage

Ice Bear units within the northern California territory aims to reduce peak electricity load demand by up to 6 MW

Spain

USA

USA

TES, molten salt, Granada, station developed by Andasol Solar Power Station TES, molten salt, Solana Generating Station, Arizona, Gila Brand TES-Ice, California, Redding

[78]

[76]

Storage Capacity (MWh) Application Country TES station

Table 2 (continued)

References

V. Venkataraman, et al.

2.2. Electrochemical energy storage Batteries store electrical energy in the form of chemical energy. They offer some important operating benefits to the electrical utility system and can respond very fast to load changes thereby improving system stability. The largest operational grid battery plants are listed out in Table 2. Batteries typically have low standby losses over short periods and high energy efficiency. However, the disadvantages associated with batteries, for large scale energy storage, include small power capacity, low energy densities, a limited discharge capability, a short cycle life and high maintenance cost [52]. In addition, most batteries contain toxic and hazardous materials which have a potentially high environmental impact at the disposal phase. Taking all this into consideration they do not offer the best solution for large scale energy storage. Among all battery types, lithium ion batteries are the most widely used rechargeable battery, that exhibit higher power and energy densities, relatively long lifetime (6–8 years), non-memory effect and low self-discharge rate [53]. Lithium has the lowest reduction potential of any element, allowing Li based batteries to have the highest possible cell potential [54]. Current Li-ion technology is based on insertionreaction electrodes, with vast majority of anodes being comprised of graphite and cathodes (mainly lithium iron phosphate and metal oxides, depending on the manufacturer). The main shortcomings of this technology are the tendency to overheat (thermal runaway), susceptibility to over voltages due to overcharging if proper voltage protection circuitry is not incorporated, unequal cell balance, limited cycling life and increased costs. The capital costs are mainly due to processing of cobalt in cathodes. Redox flow batteries (RFBs) have the advantages of a flexible layout, long cycle life, rapid response times and safety advantages over lithium ion and sodium batteries [52]. Some types also offer easy state 5

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of charge determination (through voltage dependence on charge), low maintenance and tolerance to overcharge and over discharge. However current RFBs have low energy density and low charge and discharge rates compared to other industrial electrode processes [52]. Compared to non-reversible fuel cells or electrolysers using similar electrolytic chemistries, flow batteries generally have somewhat lower efficiency.

molten salts or rocks) with water being the cheapest option. SHS has two main advantages: it is cheap and without the risks associated with toxic materials. The boiling point (100 °C at 1 bar) limits the use of water as SHS medium for high temperature applications, requiring enhancement in system pressures [57]. Underground thermal energy storage (UTES) is also a widely used thermal storage technology, which makes use of the ground (eg soil, sand, rocks and clay) as a storage medium for both heat and cold storage. The possibility of using phase change materials (PCMs) in solar system applications can offer a high storage capacity that is associated with the latent heat of the phase change [57]. Thermo-chemical storage can offer even higher storage capacities. Thermo-chemical reactions such as adsorption can be used to accumulate and discharge heat and cold on demand and to control humidity in a variety of applications using different chemical reactants [57]. Cool or cold thermal energy storage has recently attracted interest for its industrial refrigeration applications such as process cooling, food preservation and building air conditioning systems [57]. However, TES suffers from its low energy efficiency [58] and seasonal storage capability. Table 2 provides a list of developed TES units for commercial use.

2.3. Electrical energy storage Supercapacitors also known as ultra-capacitors or electrochemical capacitor offers large power densities, in the range of 0.5–10 kW/kg while exhibiting relatively low energy densities, 0.2–5 W h/kg. There are currently two types of categories: electric double layer capacitors (EDLCs) and pseudo-capacitors. In the former, capacitance is generated from the accumulation of charges at the electrode-electrolyte interface, while in the latter, highly reversible surface redox reactions (Faradaic reactions) drive the operation of the device. EDLC electrodes are made of carbon in its various manifestations: carbon fibre, graphite, graphene and nanotubes. As for pseudo capacitors, MnO2 and RuO2 are typical materials used for the electrodes. Overall, supercapacitors exhibit excellent roundtrip efficiencies (90–95%), outstanding power densities (0.5–10 kW/kg) and long operation lifetimes even in harsh environments [55]. It must be noted that while pseudo capacitors exhibit higher specific capacitances and energy densities than EDLCs [56], the number of charge-discharge cycles is limited by the relatively poor electrical conductivity of the electrodes. The main drawback of this technology is the low energy density, leading to an operational window which extends only in the ‘seconds’ range. Furthermore, the low cell voltages of the individual elements call for a series connection of the supercapacitors, thereby increasing the risk of overvoltage [53]. Although there are a large number of leading companies and developers for capacitors and supercapacitors around the world such as SAFT (France), Power System Co and Chubu Electric Power Co (Japan), ELIT (Russia), NESS (Korea) and Powercache (USA), the capital cost of these energy storage systems is still too high to be suited for large-scale and long-term applications. Two demonstration projects in which supercaps were installed are given in Table 2. SMES (Superconducting magnetic Energy Storage) employs the magnetic field generated by the flow of DC current in a coil in its superconducting state (temperature below its superconducting critical temperature), to have virtually zero ohmic resistance. SMES have a fast response time (in the order of mS) and can easily switch between charge and discharge states. They are therefore used for applications that need power quality correction or voltage drop correction and are hence used for short duration systems e.g UPS (Uninterrupted Power Supply) units. The typical power rating for this type of application is up to 40 MW. The different small and large scale SMES units available for commercial use are listed out in Table 2. The major issues facing implementation of SMES units are high costs of the cooling unit and the environmental impact associated with the strong magnetic field.

2.5. Positioning of rSOC systems in comparison with other energy storage technologies After reviewing the above energy storage technologies, the main competing energy storage technologies with rSOC based energy storage systems are: i PHS (Pumped Hydro Storage) ii CAES (Compressed Air Energy Storage) and iii Batteries (mainly flow batteries) The reasons as to why the above energy storage technologies are considered as competitors to rSOC technology is as follows:

• PHS, CAES and flow batteries are all meant for seasonal energy • •

storage where energy can be stored to cater to a long term demand and not for short bursts or impulses. These energy storage technologies (of course depending on plant size and capacity) can either supply the electrical power requirements (to meet the load demand) on their own or can supplement a conventional power plant by adding electrical power to the grid. These can also be used for absorbing excess electrical power from the grid, thereby assisting in grid balancing.

All these technologies are quite mature and have been commercialised but have their own drawbacks as listed in the previous sections. Due to thermal inertia of high temperature heat exchangers, rSOC based systems are not suitable for frequency regulation of the electric grid but more for energy management, load levelling, peak shaving and seasonal energy storage. The above three competing energy storage technologies are active players in these categories as well however they cannot be deployed at any location with the exception of batteries with poor economic scaling (volume and weight increases linearly with increase in scale of the energy storage). rSOC systems are envisaged to be deployed almost anywhere – in both grid dependent and independent areas. In places where the rSOC is connected with the grid, it will assist in grid stability and load levelling and in grid independent areas, the rSOC can be directly coupled with the renewable energy sources but this will be a challenge from a control and electrical point of view and will warrant a sophisticated energy management strategy. Energy storage based on rSOC technology has the following advantages when compared to other conventional energy storage technologies:

2.4. Thermal energy storage (TES) TES systems have been employed in a wide range of applications. Some of the advantages of TES systems include long cycle life, low capital cost per kWh and almost zero impact on the environment. Depending on the particular technology, it stores the excess thermal energy which can then be recovered for electricity generation at scales ranging from individual processes to entire regions. TES technologies are becoming important for electrical energy storage in combination with concentrated solar power plants where solar heat can be stored for electricity production when sunlight is not available. Sensible heat storage (SHS) is the simplest method based on storing thermal energy by heating or cooling a liquid or solid storage medium (eg. Water, sand,

a) Flexibility – Flexibility here can be defined in two ways, one with 6

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regard to system size and the other with regard to use of different fuels. rSOC systems can easily be scaled up or down without loss in thermodynamic performance. This is because unlike conventional thermodynamic devices such as steam and gas turbines which are usually more efficient at a large scale, electro-chemical performance of the rSOC is not affected by size. They are also able to operate on different types of carbonaceous fuels (during the fuel cell mode) b) Adaptability – The system can easily be integrated (electrically, thermally or chemically) with other renewable energy technologies, certain industries and a host of different applications, thereby making it adaptable to any situation c) Capability – Power sizing and energy capacity can be done independently for the two modes of operation in rSOC. This gives rSOC systems an edge over other storage technologies. The operational times between the fuel cell mode and the electrolyser mode can be tailored independently and this along with a robust system control mechanism makes energy storage systems based on rSOC extremely capable of catering to different power and energy storage capacities. d) High efficiency – Not only the US DOE (United States Department of Energy) but also the European Association for Storage of Energy have set an ambitious target of 80% round trip (RT) efficiency for grid energy storage [6,79]. It is envisaged that EES (Electrical Energy Storage) systems based on rSOC can achieve up to 70% roundtrip efficiency (albeit with a specific set of operating conditions) and this is quite close to the ambitious US DOE target which gives rSOC systems an edge over the other energy storage technologies when all the above advantages are also taken into consideration.

easy deployment of this technology in Europe. Such an existing grid storage is not an option for other energy storage technologies which again gives rSOC systems an edge over other energy storage technologies. It is also believed through some preliminary testing [80] that reversible operation might slow down performance degradations which otherwise would be accelerated in single mode operation, but more experimental data is needed to prove this. This gives rSOC systems an edge over using two separate units because a single unitised stack is able to perform both operations (fuel cell & electrolysis). This automatically translates to lower investment costs in the form of CAPEX (Capital expenditure) not only at the stack level but also perhaps at the BoP (Balance of Plant) level. rSOC systems can play a significant role in absorbing excess renewable power, thereby converting it to hydrogen or other chemicals downstream (in combination with other feedstocks). This way the hydrogen is definitely produced in a sustainable and green way but the same cannot be said for the downstream chemicals because that would depend on the other feedstocks. It is however envisaged that the emissions from production processes of downstream chemicals will be minimised due to the fact that at least hydrogen is produced in a completely clean manner. All these points position rSOC systems as a flexible energy conversion and storage technology when compared to conventional systems. Even within electrochemical electrolyser technologies, SOC technology is potentially superior to alkaline and PEM based electrolysers [81]. This is because high-temperature electrolysis leads to reduced overpotentials at low current densities and also part of the energy required for electrolysis can be met by the heat produced within the stack (during exothermic operation) or even by waste heat from industrial processes. This results in considerable savings in input energy required for the whole electrolysis process. Co-electrolysis, which is using CO2 and H2O(g) to generate syngas, is an option only in high temperature electrolysers and not in low temperature electrolysers. The generated syngas can have various H2/CO compositions which can then be used in Fischer-Tropsch processes [81] to generate synthetic fuels. The comparison of rSOC with the main competing energy storage technologies for a set of crucial characteristics is mentioned in Table 3.

One of the most important challenges for any energy storage system is to store energy at a large scale and be able to meet the energy demand for a relatively long period of time. Therefore, key aspects that have to be met by any energy storage technology include: cost, efficiency, storage capacity and widespread availability. Out of these four aspects, rSOC technology scores high on the last three aspects and that is why it is predicted to be an important contributor in the energy storage field. The only point where rSOC technology does not have an advantage over the other three technologies is the category of affordability. This is mainly for two reasons – cost of manufacturing of the SOC stack and the fuel source needed at the fuel electrode side. The capital cost of rSOC is relatively high, due to the early stage commercialisation of the solid oxide cell stack; however it is predicted that the stack cost can be largely reduced once massive production is established and economics of scale are achieved. The cost of fuel source can be brought down if the rSOC stack is able to produce its own fuel (while operating in electrolysis mode). Thermal energy storage systems can be used in conjunction with rSOC systems because heat integration and optimisation plays a crucial role in rSOC systems, that are most beneficial if waste heat is utilised together with the electric energy to be stored and incorporating thermal energy storage systems into rSOC systems is envisaged to boost system efficiency. The main characteristics of different energy storage technologies is shown in Fig. 3. The absolute numbers are not shown instead these are normalised to ‘one’ because the sole purpose of this graph is to provide a quick positioning of the different energy storage technologies with one another. A key advantage of rSOC systems over other energy storage technologies is that chemicals/ hydrocarbon fuels can be produced from renewable hydrogen which in turn may be produced by renewable electricity. In that case, the production of these fuels/chemicals takes place in a more sustainable way. rSOC systems working on hydrogen and synthetic methane can use the existing gas grids (which are present in almost all European countries) for storage as well as a source of fuel if the generated hydrogen is used for different purposes. This allows

3. Power-to-X to power projects, overall context and learnings rSOC based systems have not been widely tested apart from a project carried out by Sunfire GmbH [82]; hence practical data for such rSOC based systems are sparse. Until now there have been several integrated renewable electricity storage systems in Europe, and some of them are even combined with a fuel cell. A combination of electrolyser and fuel cell (or H2 engine), as separate modules, has been used in many cases and they were used for the power-to-H2-to power cycle. An overview of the relevant renewable hydrogen systems installed in Europe is given in Table 4. The installations in Utsira, Prague, Puglia and Corsica use electricity generated by wind, solar or biomass systems in conjunction with an electrolyser-fuel cell unit and a corresponding H2 storage system. Electrolysisfuel cell systems compete with the benchmarked technology scheme based on wind-diesel engines. It is to be noted that the identified systems in Utsira, Leicestershire, Prague, Puglia and Corsica do not consider a reversible fuel cell technology; therefore, the electrolysers and fuel cells are dimensioned separately for electricity storage and demand needs. As pointed out in [83], the Utsira offshore experience successfully proved the concept of integrating wind turbines with electrolyser-fuel cell systems. From the four years of operation of the system in the Utsira project [83], the lessons learnt point out among others that in order to have a competitive system, wind energy utilisation must be increased by installing load following electrolysers and that the plant size can be beforehand optimised (size of the main components, hydrogen storage). Overall additional technical improvements and cost reductions were needed for commercial capability. The lessons learnt from HARI project 7

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Fig. 3. Comparison of main characteristics of different energy storage technologies. (For each characteristic, a certain technology has the maximum number and the remaining have numbers below that. Hence the normalisation, for each characteristic, is done by dividing the actual number (for that technology) with the maximum number).

[84] (installation in Leicestershire) highlight the need for better efficiency improvement during the design phase while obtaining the appropriate equipment size, and a better performing electrolyser. The European companies IHT2, Nel3, McPhy4 ITM power5, Siemens6, HZI Etogas7 and Hydrogenics8 are active players in the field of commercialisation of electrolysers (polymer and alkaline based). AREVA Greenenergy box combines an electrolyser, a fuel cell and the storage of O2 and H2 in modules of 5-500 kW9 . The company Sunfire has developed an rSOC module, in partnership with Boeing, capable of generating 50 kWe and 42 Nm3/hour of H210 . Sylfen commercialises rSOC systems, in combination with batteries, for buildings to provide heat and electricity needs11, the company still being a very young one. Several relevant European projects related to solid oxide technology are GrINHy (high-temperature electrolysis in an integrated iron and steel plant)12, INGRID (hydrogen storage systems for power supply and demand balancing in power grids with high penetration of renewable energy – linked to the plant being installed in Puglia, in Table 4)13, STORIES (penetration of renewable energy systems in islands)14, PRISMI (penetration of renewable energy systems in Mediterranean islands)15, MefCO2 (synthesis of methanol from captured CO2 using surplus electricity)16 and HyUnder (underground H2 storage in Europe)17 . The recently concluded project TWENTIES18 was one of the largest demonstration projects for integration of renewables (wind) into

the electricity grid. The SOLETAIR project aimed at demonstrating the whole power-to-liquid chain in a demonstration unit that included renewable electricity generation, H2 production from a PEM electrolyser, CO2 from direct air capture, reversible water gas shift process and Fischer-Tropsch process for hydrocarbon production19 . In Europe, Germany has been making efforts in including power-togas into its energy portfolio. It has been increasing its energy storage technologies in the field of lithium-ion batteries, other battery technologies and power-to-gas technologies, from 2012. The total installed capacity of power-to-gas technologies has increased from 0 MW up to about 20 MW; however, this is not yet comparable with the total installed capacity of PHS (9.3 GW) [86]. Overall, rSOC systems can be relevant in both grid dependent and grid independent (small size < 100 kW) systems. Offshore islands would benefit from such a technology, avoiding imports of fossil fuelbased raw materials. For instance, four small islands have been identified in [87], which have or aim at having completely autonomous and renewable systems: Utsira (NO) (described before), Samso (DK), El Hierro (ES) and Agios Efstratios (EL). Orkney Islands in Scotland also have big plans in creating a fully integrated model comprising hydrogen production, storage, transportation and utilisation of heat. Most of the above mentioned hydrogen-fuel cell projects mainly use alkaline or polymer electrolyte membrane cells, and thus electrolysis and fuel cell operations are carried out in different components altogether, contrary to rSOC systems, where electrolysis and fuel cell operations happen in the same component. On-field case studies include; the Faroe Islands case study [88], where an electrolyser-fuel cell system has been tested as a complete autonomous system, with an installed capacity of 50 kW. It uses the wind energy and replaces the well-known diesel generators. Storage of H2 at 30 bar is taken into account and the lifetime of the system was estimated as 15 years. The case study reported in [89] is based on the telecom market in Italy. Solar energy was used to power the electrolyser-fuel cell (5 kW) and batteries system. Thus, as seen from above, the use of rSOC in grid management is a relatively new concept and to the best of our knowledge, limited literature can be found in the conceptual design of such installations.

2

www.iht.ch/technologie/electrolysis/industry/about.html. nelhydrogen.com/product/electrolysers/. 4 mcphy.com/en/our-products-and-solutions/electrolyzers/. 5 www.itm-power.com/about. 6 www.industry.siemens.com/topics/global/en/pem-electrolyzer/silyzer/ pages/silyzer.aspx. 7 www.hz-inova.com/cms/en/home?page_id=4896. 8 www.hydrogenics.com/. 9 www.areva.com/EN/operations-4501/the-greenergy-box.html. 10 www.sunfire.de/en/. 11 sylfen.com/en/home/. 12 www.green-industrial-hydrogen.com/home/. 13 www.ingridproject.eu/. 14 ec.europa.eu/energy/intelligent/projects/sites/iee-projects/files/projects/ documents/stories_maximization_of_the_penetration_of_res_in_islands.pdf. 15 www.researchgate.net/project/PRISMI-Promoting-RES-Integration-forSmart-Mediterranean-Islands. 16 www.mefco2.eu/ 17 hyunder.eu/. 18 windeurope.org/about-wind/reports/twenties-project. 3

4. Techno-economics of rSOC technology In this section, the authors have provided a preliminary technoeconomic analysis of rSOC technology. It is to be kept in mind that 19

8

soletair.fi/technical-specifications/mobile-synthesis-unit/.

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Table 3 Comparison of rSOC based energy storage system with other competing energy storage technologies. Characteristic

PHS

CAES

Batteries

rSOC based energy storage system

Geographic location Scale Lifetime RT cycle efficiency Storage duration Power-commodity-power capability

Limited MW > 30 years 85 % max Hours/months NO

Limited MW > 20 years 85 % max Hours/months NO

Can be deployed anywhere kW 10 years 95 % maxa Minutes/Days NO

Can be deployed anywhere kW, scalable to MW after technology maturation > 5 years 70 % max Hours/months YES

a

For Li-ion batteries.

Table 4 Non-exhaustive list of renewable hydrogen systems installed in Europe [83,85]. Description

Location

Off-grid test: Two wind turbines (2 × 600 kW) + 5 kW flywheel + 100 kVA master synchronous machine to balance voltage and frequency + 55 kW hydrogen internal combustion engine + 10 kW fuel cell + 10 Nm3/h electrolyser (peak load 48 kW) + hydrogen compression (200 bar) and storage (2400 Nm3 pressure vessel). It was the first full scale wind-H2 plant, nowadays dismantled. Power-to-H2 projects developed in Germany, principally synthesising H2 which is later injected into the natural gas grid. Hydrogen retail stations with onsite electrolysers to provide H2 for fuel cell vehicles. A wind-to-hydrogen system, with 6 MW of wind power. Provision of electricity to the rail network. The synthesised H2 is (i) mixed with biogas and fed into cogeneration plants (heat for district heating, and electricity injected into the grid), (ii) used in a HRS (in Berlin and Hamburg). Test facility that uses electricity from wind and solar into a mixture of H2 and methane, which is stored underground. Power-to-H2 pilot plant that uses a high pressure proton exchange membrane electrolyser to store wind generated electricity. Storage of H2 in large caverns for a large power installation. A 36 kW electrolyser, a pressurised gas storage tank and fuel cells of 2 and 5 kW. Electricity from photovoltaics, wind and micro-hydroelectric (50, 13 and 3.2 kW, respectively). A 25 kW electrolyser, with H2 buffer, connected to a wind park. A combination of wind turbine and 1 MW electrolyser. Photovoltaic electricity storage in a 7 kW electrolyser and a 4 kW fuel cell. Smart grid: electricity supply and demand optimisation (smart balancing support), in a system with 39 MW h of energy storage, using a H2-based solid state storage + 1.2 MW hydrogen electrolyser + fuel cell. It will store renewable electricity from the already installed 3500 MW of solar, wind and biomass power. Jointly with the Prenzlau (GE) plant, the project combines a 560 kW solar plant with electrolysers, H2 storage and fuel cells.

Utsira (NO)

techno-economics is a complicated subject and appropriate boundaries have to be drawn when studying a particular technology. rSOC technology itself is limited to the stack (or cell) level however it is usually implemented to be a part of the bigger system and thus sometimes it becomes necessary to analyse the techno-economics of the larger system where rSOC technology is employed. When considering techno-economics of rSOC it is be understood that materials employed to manufacture rSOCs also plays a crucial role in deciding the costs of the stack (or cell).The typical materials for SOC are yttria stabilized zirconia (YSZ) (often 8 mol% YSZ), Scandia stabilized zirconia (ScSZ, usually 9 mol% Sc2O3 - ScSZ) and gadolinium doped ceria (GDC) for the electrolyte, NiO-YSZ for the fuel electrode and lanthanum-strontium-manganite (LSM) or lanthanum-strontiumcobalt-ferrite (LSCF) for the oxygen (or air) electrode. Therefore, the rSOC employs non-precious elements, such as yttrium, cerium, and lanthanum which have been identified as crucial materials in the performance of SOC [90]. Yttrium availability may be critical for both near and medium term. Scandium has emerged as a possible alternative to yttrium [91]. The assessment of poly-generation (heating and/or cooling and/or fuels) systems is complicated due to the fact that multiple useful products [92] are produced. Design and operating strategies should give priority to one of the demands. In the best case scenario, all the electricity and all the heating/cooling produced are assumed to be consumed, but this is rarely possible due to non-coincidence of electrical and thermal demands. Other design criteria may maximise the return of investment, or optimise other decision-making criteria; in which case peak demand for any particular product may not be satisfied. Thus, many strategies exist to deal with the fulfilment of the demand. The technical and economic features of SOFC/SOEC/rSOC are summarised in Table 5. When operating at a system level, different operational alternatives

DE DE Prenzlau (DE) Pilsbach (AT) Auersthal (AT) Teesside (UK) Leicestershire (UK) Athens (GR) Shapinsay (UK) Prague (CZ) Puglia (IT) Corsica (FR)

are available for rSOC technology, i.e. (i) steam electrolysis, (ii) coelectrolysis, (iii) fuel cell operation powered only by H2, and/or (iv) fuel cell operation powered by carbon-based compounds or ammonia. The selected approach should match with the decided market need; i.e. fulfilment of electricity demand, fulfilment of the chemical demand, wind profile following, among others. The decision is not trivial and different tools and methodologies can be applied to take decisions and diminish the associated risks, particularly in the development and implementation of systems at low TRL, where uncertainty is high. This is the case of rSOC systems where chemicals synthesis processes are combined as well. The aim of process design in rSOC systems should be to specify the most sustainable (economic, environmental, technical and social points of view) operating conditions and flowsheet configurations. Aspects to take into account in this exercise are, for instance:

• Split of operation in SOFC and SOEC modes • Purity of the CO source; further purification or treatment technology that may be needed • Collection of CO (small sources to a pipeline) • CO capture from conventional power plants • CO use from heavy industry • Need of local heat storage • Heat availability from renewables • Heat availability from conventional power plants • Heat availability from heavy industry • Excess of electricity from solar • Excess of electricity from wind • Water cycle in power-to-commodity-to power • Quantification of water consumption • Others 2

2

2 2

In an attempt to homogenise and to allow comparability among 9

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Table 5 Summary of relevant technical features and economics of SOFC, SOEC and rSOC. Development status

SOFC Commercial

Typical stack size Preferred reactant type or possible product

1 kW–25 kW 3–15 kW – Diverse fuels (H2, natural gas, biogas, ethanol, methanol, Hydrogen (steam electrolysis) or Technically all possible fuel cell fuels propane, LPG, diesel, DME, ammonia, formic acid, syngas (co-electrolysis) hydrazine, etc.) S (< 1.0 ppm) S (< 1.0 ppm) S (< 1.0 ppm) External reformer, high-temperature heat exchangers, water and fuel handling equipment, post-burner or off-gas burner (only used during SOFC mode operation) 0.7–0.9 0.9–1.6a – 0.1–0.4 Usually 0.2–1.0, up to 4 possible – [93] Usually atmospheric Usually atmospheric; pressure up Atmospheric to 20 bar investigated [94] 60 % > 80 % [95], 43–45 % [95,96] b,c 82–86 % [95,96] and even 90 % (CHP possible but heat storage from SOFC for (HHV) obtained [97] SOEC preferred) b 80–90 % [98] (CHP) 77–90% [95,96] large potential to 60–80 % [99,100] 0/100, depending on the system design (e.g., 50–120% 0/100, depending on the system – [60]) design (e.g., 50-120% [60]) > 1 hour > 1 hour > 1 hour > 4 hours > 4 hours > 4 hours Typically, 5 years (45,000 h), < 90,000 h also recommended in [101];up to 10-year continuous operation reported by industry; durability may be improved by reversible operation [102] Stationary power supply or CHP, range extender for Renewable-to-chemicals Electrical storage for balancing renewable electrical mobility, mobile/military/civil use (e.g., for profiles, fully- or partially autonomic distributed robot or telecom systems) energy supply 2300 €/kW (2.5 MWd) and 526 €/kW (50 MWd) for micro1000 €/kW (prediction for 2030), Similar to SOFC/SOEC CHP units (kW scale) [103] ; 1500-1830 €/kW [95], d d 481–379 $/kW (10–30 MW ) and 280 $/KW (> 50 GW ) 700 €/kW for 5 kW and 500 for large-CHP units (100–250 kW scale) [104] €/kW for 20-1000 kW [97] d d 6600 €/kW (2.5 MW ) and 1956 €/kW (50 MW ) for – Higher than SOFC/SOEC system, depending on micro-CHP units (kW scale) [103]; system design and storage capacity 1500–1200 $/kW (10–20 MWd) and 1000–800 $/kW (> 5 GWd) for large-CHP units (100–250 kW scale) [104] Slow start-up and load shifting, high-temperature corrosion, thermal isolation, thermal expansion compatibility, ceiling difficulty, high CAPEX due to early commercialization

Poison Balance of plant Operating voltage (V) Current density (A/cm2) Operating pressure (bar) Electrical efficiency (LHV) Total system efficiency Min/max load Hot start time Cold start time Lifetime (h) Applications Stack cost

System capital cost

Challenges

SOEC Laboratory and demo

rSOC R&D

a

Cell technology with ultra-high current density over 4 A/cm2 at 1.6 V and 750 C and 6 A/cm2 at 800 C developed, stack with 3 A/cm2 at 1.5 V demonstrated [93]. b Fuel cell’s efficiency is for methane as the fuel, while electrolysis’s Efficiency is for H2 production. c Round-trip power-to-power efficiency. d Annual production and sell.

different H2 systems, and to allow comparability of H2 systems with conventional energy systems (to accelerate H2 implementation), the International Energy Agency (IEA) ‘Hydrogen Implementing Agreement’ [105] has proposed in its Task 18 (Integrated Systems Evaluation) the needed tools for the design and optimisation of H2 energy systems. The following subsection provides an overview of the different studies found about evaluation of rSOC systems, mainly dealing with H2 and/or methane.

hydrogen. The round trip efficiency achieved was about 50%, at 3 bar [106]. The focus was on the thermal management strategy in [6]; exothermic methanation provided the needed heat during the endothermic co-electrolysis mode. Moreover, methane and fuel cell exhaust were stored in tanks to be used in the reversible operation. The purpose was to elucidate the best size of storage, gas compositions, system pressure, stack temperature, pressure and fuel utilisation. A round trip efficiency above 70% was reported, at 20 bar [100]. The rSOC hydrogen system performance with a non-optimised system at atmospheric pressure showed a lower round trip efficiency of 30% in [107]. It was indicated that, when working at a pressure of 10 bar and working in SOFC mode, with the selected system size, it was not possible for the system to provide the needed power to compress its own SOFC fuel. The self-sustained rSOC system configuration in [18] accounts for heat storage as well. The generated heat in fuel cell mode was used in the electrolysis mode after being stored in latent heat storage tanks. Methanation was also incorporated in SOEC mode; in SOFC mode the reactor was used as a steam-methane reformer. Fuel storage and air tanks were used as well. The optimised system worked at 25 bar, having a round trip efficiency of 54–60 % (with SOEC working in endothermic mode) and operating at a current density of 0.25 A/cm2. Pressure and current density in this case, are pointed out as key parameters. Temperature, pressure, reactant composition, utilisation factor and current density appear as the main variables that control the rSOC system efficiency in [107]. Inlet gas composition, temperature, operating voltage

4.1. Techno-economic analysis of rSOC systems based on H2 and CH4 In the design of rSOC systems, the management of heat is crucial. The size of the system, and particularly, the quantity of heat electricity and/or hydrogen/chemical storage will depend on the selected market strategy for a given flowsheet proposal. In [106], an rSOC is studied in a distributed system in combination with wind power. The extreme capacity cases are determined by a fulfilment of a specific demand of electricity, with (i) maximum wind power output and (ii) zero wind. Taking into account the total system efficiency, the best strategy turned out to be the one where wind penetration was maximised; i.e. SOFC was used when the wind power was not enough to fulfil the electricity demand, and SOEC mode was employed to store surplus wind energy. However, when considering a compromise between power supply and demand, the best option was to partially supply an amount of wind power to the users, while the remaining was strategically stored as 10

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were influencing variables in [106]. In [100] for the rSOC system, larger capital cost contribution comes from the storage tanks and the stacks. Overall, the importance of a preliminary design step, optimising the system’s operating conditions, as well as the size of the auxiliaries, and particularly the size of the different storage systems, according to the market strategy selected, appears as key. As summarised in Table 5, the cost of a system based on a SOFC can vary between € 700–6600/kWe, depending on the heat integration and the scale. The stack cost for an SOEC is € 500–1830/kWe, depending on the scale. In the sensitivity analyses from [108], the electrolyser’s capital expenditure is varied with figures between € 1000/kWe and € 300/kWe, while the H2 price varies from € 50/MWhH2 down to zero (up to year 2050). One may wonder why is the price of hydrogen going down to zero by 2050? It is assumed that by 2050, the share of renewables across the world will increase significantly that hydrogen will entirely be produced by renewable electricity – either via low temperature water electrolysis or high temperature steam electrolysis. Thus in a way the price is virtually zero as the hydrogen is entirely produced by excess renewable energy which would otherwise be wasted. Another range of values is reported in [109] for an electrolyserbased plant (alkaline/PEM), ranging from € 600/kWe to € 2000/kWe, down to € 300–500 /kWe (by year 2030) depending on the implemented scale. As an example, the same source [109] reports a fuel cell (PEM) plant investment cost of € 3500/kWe, going down to € 2000/kWe by year 2030. For comparison with other storage technologies, PHS installation cost is approximately € 1000/kWe going down to € 500/kWe (by year 2030), CAES installation cost is ˜€ 1000/kWe and going down to € 700/kWe (by 2030) and a complete H2 storage system (electrolysis, fuel cell, storage, gas turbine) ranges between € 1500–2000/kWe, expected to go down to € 500–800/kWe (by 2030), thus becoming competitive with flow batteries, which have a capital cost of € 1000–1500/kW, going down to € 600–1000/kW (by 2030). Lithium-ion batteries have a significantly lower installation cost of € 150–200/kW, going down to € 35–65/kW by year 2030 [110]. The power-to-X systems have a larger investment cost, and particularly in [110], these costs are not yet expected to decrease by year 2030: a power-to-methane case may have a power installation cost of € 1000–2000/kWe. This subsection has focussed on hydrogen and methane as the two most common chemicals used in SOEC and SOFC systems. Similar approaches can be taken when other fuels are used. The following subsection presents an overview of the market situation of not only hydrogen and methane but also of other potential chemicals that can be used and synthesised/used in rSOC systems.

trends and needs in the CDU field, the following points can be addressed within the rSOC systems framework:

• Evaluation of the net amount of fossil fuel that can be avoided with the rSOC technology to synthesise selected fuels and chemicals. • Optimisation of the system through the design of heat integrated plants. • Study of the plant flexibility for combining fuel synthesis with •

electricity storage and grid balancing, working at full and partial loads. Evaluation of the CO2 emitted by the whole supply chain by customised life cycle assessments [113]. Identification of the best CO2 sources in Europe, according to concentration and impurity conditions of the different CDU routes [114].

The CDU industry will have to face competition with already existing products, which are synthesised from fossil fuel at lower production cost. The market penetration of CO2-based products creates different issues regarding their business models [115,116], depending on whether CDU will displace or coexist with fossil fuel exploitation:

• Risk of market saturation. For example, for current "expensive" • •

products such as formic acid, larger availability would decrease their price. The recognition of CO2 as a "raw material" will generate a price that may differ from the current CO2 prices: commercial CO2, EU ETS prices or cost of CO2. Cost effective hydrogen production may rely on “low priced” renewable electricity available in periods of over-supply. If the demand for such electricity increases e.g. by extended electricity storage capacity, its price may also increase.

The dimension of the CDU plant, coupled with a renewable source of power, will depend on the renewable electricity produced and on the power strategy followed (base-load, load-following), rather than on the demand of the synthesised product or the current product synthesis (fossil fuel–based) scale [117]. Again, the CDU plant dimension will depend on the market strategy suggested for the rSOC system, which is case specific. As previously mentioned, one of the characteristics of SOFC systems, is that not only gaseous fuels can be used, but also liquid fuels. Taking into account (i) a circular approach of rSOC systems, i.e. the fuel or chemical synthesised during electrolysis mode is used during fuel cell mode, and (ii) that alternative economies are emerging and needed to decrease global CO2 emissions, the following products are identified as relevant for future energy applications: hydrogen, methane, methanol, syngas and ammonia. These chemicals have a growing use for energy and fuel applications. Indeed both, the electricity and transport sectors are relevant for the rSOC technology. The following paragraphs briefly summarise the main market characteristics of each of the chemicals.

4.2. Potential chemicals and fuels in rSOC systems During electrolysis mode, the solid oxide system can produce H2 which can be stored and later used during fuel cell mode, or can be further combined with CO2 to produce other chemicals. The SOEC mode has the potential for generating hydro-carbon based fuels/chemicals from the co-electrolysis of H2O and CO2, especially at high pressure [106]. rSOC systems can therefore produce hydro-carbon based chemicals (i) electrochemically, (ii) via syngas or (iii) through CO2 + H2 reactions. Carbon dioxide utilisation (CDU) is currently a new opportunity for the chemical sector which considers CO2 as a valuable raw material [111]. Further development of the CDU market will depend upon the availability of CO2 (i.e. amount and quality of the CO2 from power plants, industries and captured from the atmosphere) and the dissemination of CO2-based products. Certain technologies require a set of favourable circumstances to be applied on a large and replicable scale [112]. CDU, as an option for CO2 abatement, contributes to the current climate change mitigation actions by reducing the use of fossil resources, increasing the life cycle of fossil CO2 and generating net reduction of CO2 emissions by harvesting sustainable CO2. Among the

4.2.1. Hydrogen Global demand for H2 is projected to increase in the next years due to regulations in transport fuel desulphurisation (in road, air and maritime transport) [118] and due to GHG emission reduction in all energy related sectors. Transport and stationary sectors will be crucial for the hydrogen economy. It is estimated that global H2 demand will increase by 5–6 % till 2020 [118]. Almost 96% of all the H2 currently is derived from fossil fuels. Natural gas is the main fossil fuel used followed by liquid hydrocarbons (together 77%), coal (18%), water electrolysis (4%) and other sources (1%) [119]. This preference of production via hydrocarbons is due to the lower energy required to generate it. Hydrogen is a crucial raw material for the chemical industry, mostly used for hydrogenation and reduction processes. The annual H2 production worldwide (in terms of energy) is estimated at 6 EJ (2012) [120]. European H2 production was estimated at 92 billion 11

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Nm3 in year 2007, with 98% of it produced in EU28 and the remaining 2% produced in Iceland, Norway and Switzerland [121]. In Europe, large quantities are consumed in refineries, followed by the ammonia industry, in methanol synthesis and metallurgy [122]. Use of captive H2 accounts for more than 90% of the used H2 (for ammonia or methanol production) in comparison to merchant sales of H2, i.e. the volume of imports and exports cancelling each other out [122]. The calculated merchant H2 production is about 630 kt/year (for year 2007). European distributed installations produce (i) up to approximately 2500 t/year of H2, most of them from reforming of natural gas20 and (ii) 10-600 m3/h (equivalent to 7.5–447 t/year) from electrolysis, assuming continuous operation [119].

Western European plants have an average size of 450 kt of methanol per year [116]. 4.2.4. Syngas Synthesis gas, called syngas, is a mixture of mainly H2 and CO, with different proportions of water and CO2. Usually, the term producer gas is used to describe a syngas with H2, CO and CH4 [133]. Syngas is the basis for most of the H2 and CO produced. It is also used as a fuel in gas turbines and it can generate saturated hydrocarbons via the Fischer–Tropsch (FT) process [134]. Syngas is also used for the synthesis of bulk chemicals like acetic anhydride, vinyl acetate, ethylene, and ethanol23 . Production of syngas is very flexible since it is not restricted to a single fuel source. It can be obtained from natural gas, coal, petroleum refinery fractions, biomass and organic wastes. Traditionally, natural gas and petroleum fractions have been the largest sources for producing syngas worldwide, the choice depending on the trade-off between costs and availability; however, because of global economic, energetic and environmental contexts, the share of syngas production via coal and biomass gasification is increasing as well. In rSOC systems, water and CO2 are of interest to produce syngas through co-electrolysis. The proportion of H2/CO in the syngas depends on the source, the syngas generation process and its corresponding performance parameters [135]. The global syngas production was about 117 GWth in 2014 and is projected to grow at a compound annual growth rate of 9.5% between 2015 and 2020, reaching 213 GWth by 2020 [39]. The compound annual growth rate is expected to increase to 17.8% by 2022 [39]. The increasing demand for fuels, electricity and fertilisers will motivate further use of syngas.

4.2.2. Methane Natural gas is the cleanest fossil fuel as of today, having the lowest CO2 emissions per kWh of fuel. It is being perceived as the complementary fuel for energy generation to renewable energy sources in the European future energy mix21 . The power plants commissioned after year 2000 are mainly advanced combined cycle plants and principally fuelled by natural gas [123]. The amount of natural gas consumed worldwide was 3314 billion m3 in 2012 and in the same year, the amount produced in Europe22 was 143 billion m3. Thermochemical production of synthetic natural gas (SNG) from syngas is technically feasible and is commonly used after solids (coal, biomass, waste) gasification [124]. Biogas (a mixture of methane and CO2) is produced by anaerobic digestion of (wet) organic matter in dry gas holders of 1-1000 m3 [125], and can be upgraded to biomethane (> 95% CH4) if the CO2 is separated out or further converted to methane. Biogas is also a well-studied feed option for fuel cells, thus increasing the share of biomass energy sources at the same time [126]. Indeed, stationary fuel cell systems efficiently convert hydrogen, biogas and (bio) methane/ natural gas into electricity and heat.

4.2.5. Ammonia There are two grades of ammonia: commercial (also known as technical) 99.5 wt% and refrigeration grade 99.98 wt %, respectively.24 Ammonia is synthesised from N2 and H2 by Haber Bosch process. The synthesis of ammonia is divided into three steps: (i) syngas production, (ii) compression and (iii) synthesis and purge gas management [136,137]. Ammonia synthesis can use either natural gas or coal as the feedstock. In 2008 about 80% of the NH3 synthesised worldwide used natural gas as raw material [138]. The average age of NH3 plants in Europe is 37 years, with the newest plant commissioned in 2007; thus hardly any new plants are foreseen in the near future [139]. About 48% of the global NH3 produced is used in the production of urea [140] and almost 90% of the urea produced worldwide is consumed as fertiliser [141], however this value is close to 50% in Europe [142]. Worldwide ammonia production capacity was 211 Mt in 2013, with an average capacity utilisation factor of 83% [143]. In EU27, there are a total of 39 ammonia plants, producing 19.8 Mt/year (2013) [132]. According to the Ecofys report [139], the yearly production of ammonia in EU28 in 2014 was about 17 Mt. Urea can be used in the chemical industry: about 10% of the global urea consumption is in the production of adhesives, melamine, protein supplement in animal feed, formaldehyde, resins, environmental applications and others [144]. The use of urea, by means of SCR (Selective Catalytic Reduction) for mitigating air pollution from diesel fuelled cars, buses and trucks and in stationary applications is increasing. SCR aids in reducing up to 80% of NOx emissions by converting it into N2 and H2O [145] and hence is important in current automobile industry. According to [142], water electrolysis represented 0.5% of the world capacity of NH3 in 1990.

4.2.3. Methanol About 70% of the worldwide production of methanol is used in the chemical industry. The main chemical derivatives produced are (in order of importance): formaldehyde, methyl tertiary-butyl ether (MTBE), acetic acid, DME, propene, methyl methacrylate and dimethyl terephthalate (DMT). “Methanol-to-X” stands for synthesis of gasoline, olefins, propene and aromatics from methanol [127]. Formaldehyde required 31% of the world methanol demand in 2012 [128]. The uses of methanol in direct fuel applications include its conversion into MTBE/ tert-Amyl methyl ether (TAME), biodiesel, DME and gasoline blending, accounting in total for 37% of global methanol demand [129]. Methanol can be used directly in gasoline blending and other methanol derivatives such as MTBE, DME and biodiesel could also be blended with gasoline and diesel (FQD). Methanol can be also transformed into ethanol and thus blended with gasoline. The methanol economy has attracted the attention of the US [130], China and Brazil [131]. Until the end of 2018, the global demand of methanol for production of formaldehyde is expected to grow at an average rate of 5% whereas the demand for methanol for fuel applications is expected to rise slightly more, at a rate of about 6.5% [128] or even 12.5% [132]. The global installed production capacity for methanol was 95.5 Mt in 2012 [129], with the European contribution being 3% [132]. China dominates not only the global methanol production capacity but also the world methanol consumption – ca. 50% [128]. Methanol is mainly produced nowadays from natural gas [127]. The feedstocks used in the European industry are mainly natural gas and residual fuel oil with a ratio 3:7. 20

iet.jrc.ec.europa.eu/about-jec/sites/about-jec/files/documents/report_ 2013/wtt_report_v4_july_2013_final.pdf. 21 www.fch.europa.eu/sites/default/files/ FCHJU_FuelCellDistributedGenerationCommercialization_0.pdf. 22 eippcb.jrc.ec.europa.eu/reference/BREF/REF_BREF_2015.pdf.

23 ihsmarkit.com/products/chemical-technology-pep-bulk-chemicals-fromsynthesis-gas-1982.html. 24 pubchem.ncbi.nlm.nih.gov/compound/ammonia#section=FormulationsPreparations.

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5. Potential challenges

to be optimised, currently it is around 50% [153]. For air electrodes (or oxygen electrodes), there are stability and operational differences when the solid oxide cell is working in fuel cell mode and electrolysis mode. Operation at high current densities causes structural and chemical changes due to large polarisation losses at these current densities. Electrode delamination is a major problem due to high oxygen partial pressure at the electrode-electrolyte interface and high oxygen partial pressures are caused during electrolysis operation. There are stability issues for cells operated at VEL > 1.3 V. Operation at current densities greater than 1 A cm-2 leads to mechanical degradation of the reversible solid oxide cell, due to oxygen evolution at the air electrode which then causes structural and chemical changes [153]. Low current densities lead to lower power production and also lower hydrogen production. These challenges have to be solved to obtain high and reliable rSOC performance. There can also be potential challenges from the sealing perspective when the same device is used for both fuel cell and electrolysis operation [157,158]. The issue of seals becomes important because the gas compositions are entirely different in both modes (if fuels such as methane or ammonia or methanol are used in the fuel cell mode). When hydrogen is used as the fuel for fuel cell mode and steam for electrolysis mode, the gas compositions will be different at the entry and exit points of the cell. This may have an effect on the seals and needs to be investigated. Also the corrosion effects on seals from reducing and oxidising environments is something to be investigated. Other challenges for the seals include – mechanical and chemical stability and bonding capability with the respective electrode materials.

The main challenges foreseen for implementation of rSOC technology for energy storage are three-fold, ranging from cell level to macro level. These are explained below. 5.1. From a material point of view Power and efficiency losses in SOFC can result from changing composition and microstructure of cell components during operation [146,147]. Specifically, performance degradation of the SOFC can be a consequence of: microstructural changes in the electrodes, reactions between materials to form new phases and contamination of active interfaces [148,149]. For example, Ni particle coarsening, which decreases the active triple-phase-boundary density and transport network connectivity in the fuel electrode, is one well-extended phenomenon observed [146,147,150,151]. The materials applied and investigated for SOFC can work with almost no adaptation in SOEC mode, which indeed makes research on feasibility and durability faster than for SOFC mode. However, degradation strongly affects the efficiency of the SOEC stack. For instance, in the fuel electrode, progressive nickel relocation and depletion in the proximity of the electrolyte has been observed [152]. Since the same stack is operated both in the fuel cell and electrolysis mode, the electrode materials used should be able to perform both reliably and effectively in both modes without undergoing degradation. Some of the main issues faced are [153]: i) ii) iii) iv) v) vi) vii) viii) ix) x)

Electrode stability Electrode delamination Thermomechanical stress Material costs Nickel oxidation Nickel coarsening Interconnect oxidation Carbon deposition Fuel starvation Catalyst poisoning

5.2. From a system point of view Different systems need to be developed when different fuels and electrolysis options are used in the reversible solid oxide cells. So far, besides the limited knowledge available in literature (as mentioned earlier in Section 1), there is no mention of the optimised system design and the kind of BoP (Balance of Plant) that needs development in order to realise integration of rSOC systems into the energy storage network. For example, even the air blower has varying power requirements in the fuel cell mode and in the electrolysis mode and thus selecting the right blower will be a challenge. The operating current density in both modes plays a vital role in deciding the thermal management strategies to be employed for the rSOC stack (and system in general). Different thermal management strategies and systems need to be developed when i) different fuels are used in fuel cell operation mode and ii) when different electrolysis methods are employed. These different strategies have not yet been explored and there is no information on how optimum heat integration can be achieved. Storage of heat during fuel cell operation mode of the rSOC is a challenge. Also high current density operation will generate sufficient waste heat for process gas heating requirements but might reduce the roundtrip efficiency of the system because the rSOC stack will operate in a low efficiency region. The possibilities of operating the rSOC at low current densities in the fuel cell mode and at high current densities in the electrolysis mode need to be explored but this might lead to low power production in the fuel cell mode and might have other challenges. Achieving a 100% circular approach is quite a challenge from a system perspective considering the storage aspects and the different reactant utilisations that each operating mode may have. Circular approach here is defined as the products (exhaust/ outlet streams) produced from one operational mode being used as inputs (or reactants) for the other operational mode. Under many operating scenarios it might be more efficient to use the exhaust streams for chemical production or to transport it to other locations rather than store them onsite for use in the other operational mode. For example during extended periods of electrolysis mode, the outlet streams from the fuel electrode can be partially stored for the fuel cell operation mode and the rest can be used for chemical production.

There have been some suggestions to make the electrolyte of an rSOC cell to be a mixed ionic and electronic conductor rather than keep it as a pure ionic conductor. This is because during electrolysis operation, the high oxygen partial pressures on the air electrode side might tend to reverse the oxygen ion conduction in the electrolyte [154,155]. High operating temperatures between 750–900 °C are mostly favourable for SOFC operation but also place very high constraints on the material aspects. Intermediate operating temperatures (600–700 °C) are getting attractive however the typical electrolyte material loses its conductivity at such low temperatures. Hence alternate electrolyte materials [153] are also being looked into but not yet commercialised. For electrodes, two techniques namely: infiltration and use of nanostructures have shown some improvements in electrode performance. However, the key challenge still remains over the stability of these electrodes over an extended period of time and at high temperatures [156]. For fuel electrodes, the overall conductivity target is around 100 S cm−1 and this has not yet been achieved. The main reason for fuel electrode degradation is partially related to the applied current when operating in different modes (fuel cell and electrolyser) and mainly to the adsorbed impurities on the electrode when operating on fuels that contain impurities [153]. When operated on clean fuels the fuel electrode degradation occurs due to Ni loss or Ni agglomeration. Also the low catalytic activity of the fuel electrode might affect the steam electrolysis and co-electrolysis performance when the cell is operating in electrolyser mode and thus in turn might affect the stack and system performance. The porosity percentage is also a crucial factor when the same cell is being used for both operational modes and hence this needs 13

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5.3. From a macro-level point of view One of the main challenges for rSOC technology from a macro level perspective is the switching between the two modes of operation. How fast is the single device able to switch from one mode to another and what implications will this have on the energy storage and grid balancing aspects? This is something that has to be looked in detail and researched further. This challenge will also have profound effects when such systems are connected on a macro scale to the electricity grid and with different industries that generate by-products which serve as reactants for either rSOC stacks or for chemical processes downstream. Detailed dynamic studies on rSOC systems are one way to glean information on time lags involved and thus suitable measures can be designed to be taken during those periods. For example, the periods of time lags can be taken over by batteries or supercapacitors or any other such devices. As an example, four main operational scenarios are described when rSOC systems are integrated with the electricity grid. These scenarios represent metastable conditions that can occur within the same system application. Certain pre-conditions are assumed for these operational scenarios, which are:

•

•

The above are some of the operational challenges that warrant further investigation. One solution could be to trigger an automatic indication system so that hydrogen trucks can be sent to that location and transport the excess hydrogen that is being produced (for the rSOC system operating on steam electrolysis). This can then be used at another location where a similar system is operating in the fuel cell mode. Operational scenario 3 (OS 3): Load demand (kW) > Base load (kW) & there is no renewable energy generation

• In this scenario, the load demand is greater than the base load but

i The electricity grid is assumed to be balanced all the time. ii The power flowing in the grid, to meet the base load, is from a centralised power generating station. iii The additional input power to the grid is either from renewables (solar, wind or any other source) or from the fuel cell operation mode of the rSOC and not from anywhere else. iv The base load is met either by a conventional or a futuristic power plant connected to the grid.

there is no renewable energy generation. ○ The rSOC system operates in the fuel cell mode, generating electrical energy to supplement the base load in order to meet the total load demand. ○ The fuel cell operation mode will last depending on the size of the fuel storage tank. What should be done if the storage tanks runs out and there is still no renewable energy being fed in? ○ A proper technical study needs to be carried out for the area/ place where such a system will be installed so that the storage tanks can be sized accordingly. ○ If the storage tanks are running out then a message/communication must be sent for hydrogen delivery (or for other fuel that the fuel cell mode uses). As the rSOC operates only in the fuel cell mode, the dynamics involved in fuel cell operation needs to be studied and evaluated in detail.

Operational scenario 1 (OS 1): Load demand (kW) = Base load (kW) & no renewable energy production

• In this scenario, the entire load demand is taken care of just by the • •

•

power supplied from the conventional power plant (or a futuristic power plant connected to the grid) As there is no renewable energy being fed to the grid, there is no problem of excess power absorption. The rSOC system remains non-operational during this period because there is no necessity for additional power nor is there a necessity for additional power absorption.

Operational scenario 4 (OS 4): Load (kW) > Base load (kW) & there is renewable energy generation

• In this scenario, the load demand is greater than the base load and

Operational scenario 2 (OS 2): Load demand (kW) = Base load (kW) & there is renewable energy generation

•

• In this scenario, the entire load demand is taken care of by the •

the maximum operating point (on the polarisation curve) so that way at least a part of the excess renewable energy is used. Once again a way has to be found to use the excess renewable energy. The rSOC operates only in the electrolyser mode as there is no need for the fuel cell mode. Hence only the dynamics involved in operating the electrolysis mode needs to be studied. If the storage tanks get full during electrolysis operation mode and there is still renewable energy being fed in to the grid, a way needs to be found to absorb the excess power. This could be one of the challenges in this operational scenario and might warrant a hybrid energy storage system.

power generated from the conventional or futuristic power plant but there is power being fed into the grid from renewables. The power that is fed into the grid from the renewables can be used to supply energy for electrolysis operation mode of the rSOC system, thereby absorbing the excess power and keeping the grid stable. There can be three situations here: ○ The renewable energy being fed meets the energy needs of the electrolyser (in this case electrolysis mode of rSOC). The renewable energy is then converted to useful chemicals/fuels and thus stored as chemical energy. ○ The renewable energy being fed is less than the needs of the electrolyser. If this situation happens then the operating point of electrolysis (on the polarisation curve) should be adjusted to whatever that can be achieved with that quantity of electrical energy. If the renewable energy being fed is still less than the minimum energy requirements for electrolysis then there has to be a way to use this renewable energy. ○ The renewable energy being fed is greater than the needs of the electrolyser. In this situation, the electrolyser can be operated at

•

• • 14

there is electrical energy being fed to the electricity grid from renewables. The load demand in this case is met by the base load and the renewable energy being fed in and if needed also from the fuel cell operation mode of the rSOC system. The renewable energy being fed to the grid can be used in the following ways: ○ Meet the demand of the load alone. ○ Meet the demand of the load and feed energy for electrolyser operation mode of the rSOC. This can happen only if renewable energy generated is sufficient to cater to both. ○ Meet only the demand of the electrolyser operation mode of the rSOC. This may not be the preferred way because using the electrical energy from the renewables directly to meet the load demand is more efficient than converting it to hydrogen (or any other chemical) first and then converting it back to electricity. The rSOC system will have to operate in fuel cell mode if the load demand is not met by the base load and the renewable energy generated together. Once again the electrolysis operation mode and the fuel cell operation mode is limited by the storage capacity of the system. However this can be solved by a proper technical assessment of the area/venue where the system is installed.

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• The switching between the fuel cell mode and the electrolyser mode

for FC operation mode or as a fuel for transport applications. The remaining process chains are envisaged when a complete hydrogen economy might not be feasible. Ammonia is not only a valuable commodity chemical but can also be used as a fuel in the FC operation mode, after carrying out an ammonia cracking step. With steam electrolysis, H2 can be produced which serves as one of the raw materials needed for the Haber-Bosch process, thereby generating ammonia which can then be used back in the FC mode or can be sold as a commodity chemical in the market. The third process chains considers methane as a fuel for use in the rSOC stack. Use of methane as a fuel results in a syngas mixture at the outlet stream (from the fuel electrode) during FC operation mode which can be used to synthesise useful chemicals. If the rSOC is connected to the natural gas network then the generated outlet streams (from both FC & EL modes) can solely be used for chemical production. The fourth process chain is akin to the third with the added advantage of CDU by means of co-electrolysis. The fifth process chain considers methanol as a fuel. Just like ammonia, this is not only an attractive commodity chemical but can also be used as a fuel in FC operation mode after carrying out reforming. Steam electrolysis in this process chain produces H2 which is one of the raw materials for methanol synthesis. The sixth process chain is similar to the fifth but with the added advantage of CDU. Finally the last process chain considers syngas a fuel in FC operation mode and uses co-electrolysis in the EL operation mode. A range of chemicals can be produced by Fischer Tropsch process when syngas is the feedstock. This process chain, if commercially developed, can also be used for synthesis of jet fuels and thus the transportation sector (including aviation) can be decarbonised. For each fuel and electrolysis combination used in the operation of the rSOC, there will be an ad-hoc system design. Each process chain will encompass different systems such as the electrochemical system of the rSOC, the chemical synthesis system of the chemical to be produced, the post processing system for refinement of chemical produced etc. Every process chain designed is envisaged to work in a ‘circular approach’ or ‘circular mode’. Circular mode has been defined in the previous section. If there is still some quantity of products (fuel/chemical) left (after using it for fuel cell mode operation) then it can be sold into the market. The process chains shown in Fig. 5 are interesting because not only do they allow rSOC systems to be flexible as far as large scale energy storage (in order of MW) is concerned but also facilitate the production of vital chemicals from renewable energy (considering that excess renewable power is completely absorbed by the rSOC system during electrolysis). They also consider a range of fuels besides hydrogen which then enables a smooth transition from the current day energy mix to the futuristic hydrogen based economy. The main idea of developing such process chains is to convert power (excess power to be more specific) to commodity chemicals and converting these chemicals back to power. Another interesting aspect about these process chains is that one can decide on the ratio of renewable power that can be converted to chemicals and that can be used directly to meet electrical needs. This gives total flexibility for energy storage systems based on rSOC technology in deciding the most economically viable strategy to be adopted for operation of the process chains depending on the price of electricity, the price of chemicals and other relevant factors. For example, in some scenarios it might be more economically attractive to convert all the renewable power to chemical and use the chemical for other purposes and in some scenarios it might be attractive to convert only 70% of the renewable power to chemicals and use the remaining directly for use elsewhere. Syngas for example is an important product that serves as a feed stock for synthesis of several chemicals. However when rSOC is used with steam electrolysis, only H2 can be produced at the fuel electrode outlet. CO2 has to be supplied externally in order to synthesise chemicals such as methane and methanol. Of course each process chain is unique and warrants a unique operating strategy. This will depend on a range of factors such as i) rSOC

will be very crucial for this operating strategy. The switching will definitely not be instantaneous between the two modes and hence suitable strategies need to be developed. How should the system cope/respond while the system is switching from one mode to another? Should there be an additional back up in the form of batteries/ capacitors which will come into play only during the switching?

• •

The different conditions that need to be accounted for when considering the above scenarios are:

• Delay in switching between fuel cell and electrolyser operation modes. • Delay in fuel cell mode reaching steady state conditions after switching. • Delay in electrolyser mode reaching steady state conditions after switching. • In the event where the fuel cell mode doesn’t reach steady state

conditions, how and from where should the additional power be supplemented? The same goes for the electrolyser mode. How should the excess power be absorbed if the steady state conditions are not reached?

These are some of the challenges that need to be addressed from an operational point of view. They are definitely solvable but there has to be a complete understanding of what the system constraints are and then tailor these systems accordingly. Fig. 4 shows the state of the rSOC stack under different operational scenarios and a hypothetical load curve. 6. Broad spectrum of power-to-X to power concept – with different process chains The entire power-to-X to power block diagram is shown in Fig. 5. This figure captures the broad spectrum of the concept, the way in which different industries along with the electricity grid and the rSOC system are integrated. On the left of the schematic, one can see the integration with different industries or gas networks (mainly H2 and natural gas/CH4) which will supply the feedstock needed for rSOC operation. Even steam and CO2 can be made available for free for rSOC stack operation provided these by-products from corresponding industries are connected appropriately. At the core of the schematic, the rSOC stack and system plant is present which supplies electrical power and heat and regenerates the different chemicals/fuels. Finally on the right side there is the market – transport or commodity, where these different chemicals are finally used. Based on a thorough market study on the fuels/chemicals, as detailed out in Section 4, the following process chains if developed with rSOC at the core can make a significant impact in future energy storage solutions and also for generation of these chemicals in a more sustainable way. Process chain 1 – Hydrogen + steam electrolysis Process chain 2 – Ammonia + steam electrolysis Process chain 3 – Methane + steam electrolysis Process chain 4 – Methane + co-electrolysis Process chain 5 – Methanol + steam electrolysis Process chain 6 – Methanol + co-electrolysis Process chain 7 – Syngas + co-electrolysis Note: For each of the above process chains, the fuel mentioned will be used in the FC mode and the corresponding electrolysis option is mentioned thereafter. A brief description of these process chains is given below. The first process chain is the simplest and most straightforward. This process chain if developed commercially will produce hydrogen during electrolysis mode and this can either be used directly as the fuel 15

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Fig. 4. State of rSOC stack, A) Operational scenario 1, B) Operational scenario 2, C) Operational scenario 3, D) Operational scenario 4; E) Operational state of the rSOC stack for a hypothetical load curve.

location site ii) renewable energy share and profile iii) market demand for such commodity chemicals and iv) techno-economics. Detailed thermodynamic analysis of each process chain will provide information on best operating strategies for the same, both from the perspective of heat integration and from the perspective of the second law of thermodynamics. Such an analysis will also provide information on complete plant design and the operating conditions/operating window for which these process chains will be attractive but these calculations are currently out of the scope of this paper and will be detailed out in a separate one.

i Island areas (with and without grid connection) ii Residential complexes The motivation behind choosing these areas for initial deployment is because these areas are on a relatively smaller scale where pilot testing of rSOC technology can be carried out. One would not want to test out new technologies where conventional systems are already well established because that would mean possibilities of interruption with the service if something goes wrong with the new technology. These areas/ regions chosen should already be supplied with sufficient renewable energy else it does not make sense for rSOC systems to be employed there. The reason why these places will be interesting for initial deployment is because the load profile and the maximum renewable power input for these places are precisely known. For these applications, a process chain based on H2/steam or methane/steam will be most suitable as other process chains are a little more complicated,

7. Potential applications are

Some of the areas where rSOC technology can be initially deployed

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Fig. 5. Power-to-X-to power block diagram.

Fig. 6. Applications and deployment of rSOC energy storage systems (PC = Process Chain).

considering additional BoP that might need installation. Also other process chains might involve production of chemicals and these might require transportation. For the initial deployment it will be relevant if the outlet streams are stored only in storage tanks without worrying about transportation. Using hydrogen and methane as fuels along with steam electrolysis simplifies the system design when compared with using other fuels and co-electrolysis options. Residential complexes (in Europe) are mostly connected to a natural gas grid, so that makes employing the process chain based on methane/steam easy. Another reason for choosing these places is that a small scale demonstration of the rSOC system will provide sufficient confidence for deployment on a much larger scale. Also when these fuels are used, they can be directly fed to the fuel electrode (direct internal reforming is a possibility when using methane) but other fuels such as ammonia and methanol cannot be directly fed without consequences of extensive degradation and performance loss. A second deployment for the rSOC systems could be adjacent to industries that can provide surplus heat and/or that generate CO2 as the by-product. This will help in the former case to increase round-trip efficiency of the rSOC and in the latter the co-electrolysis operation of rSOC systems by providing the required CO2 feed for free. This will bring integration of energy storage (with rSOC systems) with industries and chemical production thereby making the whole energy storage more holistic. This also incorporates CDU which will aid in reducing

carbon dioxide emissions, opening up the scope for chemical production in a more sustainable manner. A future deployment could be on a much larger scale (say hundreds of kW). This could again either be large rSOC power plant which is centrally located or a network of different rSOC power plants connected together. In this scenario, the rSOC power plant could also be integrated with gas turbines, to boost the electrical efficiency of the power plant. As this is on a much larger scale, any of the process chains can be implemented. Fig. 6 shows the idea of implementation of energy storage systems based on rSOC technology. 8. Conclusions With increasing global energy demand, not only is the need for energy generation crucial but also the need for efficient and flexible energy storage. If energy generation capacity increases manifold without a corresponding increase in energy storage capacity then there will arise a scenario where the supply will overshoot the demand. Hence it is vital to develop solutions for energy storage technology that meet different scales and which are also flexible. Just like energy production which is making a transition to sustainable ways, energy storage also needs to be sustainable. In this paper we have given an introduction to rSOC technology and compared it with other energy storage technologies. Energy storage 17

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systems based on rSOC technology are suitable for storing excess electricity produced in the form of chemicals. They can also be connected to the grid for grid stabilisation purposes. This dual feature makes them unique when compared to other energy storage technologies. The main competitors to rSOC technology are pumped hydro storage (PHS), compressed air energy storage (CAES) and flow batteries. The key advantages of rSOC over its main competitors are i) deployment at any geographical location (provided there is either some renewable energy source or a power grid), ii) easy scalability from kW range to MW range and iii) capability to generate commodity chemicals. Round trip efficiency of rSOC systems is close to 70% and it is envisaged that this number can go higher up to 80% (with better heat integration strategies). A techno-economic review of rSOC systems was conducted and it was found that the cost per kW for rSOC systems is still significantly greater when compared to its main competitors. The primary reason for this is the low TRL of rSOC technology which is in the range of 3–4. Besides the rSOC stack, most of the remaining components in the system are off-the-shelf components available in the market and thus mass manufacturing of stacks is envisaged to bring down the costs. It was also highlighted how energy storage systems with rSOCs can be developed by listing out the different process chains and also chalking out reasons as to why these process chains will be interesting. Every process chain is unique and the development of the system should be carried out based on the tailored needs for a specific location. Only the broad concept was showcased here and the individual plant design for different process chains can be customised locally depending on a number of factors such as i) availability of renewable energy ii) heat streams iii) energy storage needs and many other factors. It is believed that if rSOC technology is taken to a higher TRL level then the chemicals and fuels listed out in the process chains will be of great interest both from a market and sustainability perspective.

[7] [8]

[9] [10] [11] [12]

[13] [14] [15] [16] [17] [18]

[19]

Disclaimer

[20]

The content of this document reflects only the author’s view and the European Commission is not responsible for any use that may be made of the information it contains.

[21]

Acknowledgements

[22]

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 731224.

[23] [24] [25]

Appendix A. Supplementary data

[26]

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.est.2019.100782. [27]

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