Assessment of power-to-power renewable energy storage based on the smart integration of hydrogen and micro gas turbine technologies

Assessment of power-to-power renewable energy storage based on the smart integration of hydrogen and micro gas turbine technologies Antonio Escamilla * , David S anchez, Lourdes Garcı ´a-Rodrı ´guez University of Seville (Department of Energy Engineering), Camino de los Descubrimientos s/n, Spain highlights Round-trip e_ciency of P2P energy storage system with micro gas turbines between 22% and 29%. . Literature review of hydrogen electrolysis systems available in the market. Thermodynamic analysis of H2 compression with a speci_c energy consumption of around 10% of H2 energy content. Hydrogen storage in high-pressure vessels, lique_ed and in metal hydrides. Hydrogen transportation by Fuel Cell Electric Truck for an average distance of 200 km per trip. article info Article history: Received 4 January 2022 Received in revised form 22 March 2022 Accepted 23 March 2022 Available online 15 April 2022 Keywords: Power-to-Power Micro-gas turbines Renewable hydrogen Energy storage systems abstract Power-to-Power is a process whereby the surplus of renewable power is stored as chemical energy in the form of hydrogen. Hydrogen can be used in situ or transported to the consumption node. When power is needed again, hydrogen can be consumed for power generation. Each of these processes incurs energy losses, leading to a certain round-trip efficiency (Energy Out/Energy In). Round-trip efficiency is calculated considering the following processes; water electrolysis for hydrogen production, compressed, liquefied or metal-hydride for hydrogen storage, fuel-cell-electric-truck for hydrogen distribution and micro-gas turbine for hydrogen power generation. The maximum achievable round-trip efficiency is of 29% when considering solid oxide electrolysis along with metal hydride storage. This number goes sharply down when using either alkaline or proton exchange membrane electrolyzers, 22.2% and 21.8% respectively. Round-trip efficiency is further reduced if considering other storage media, such as compressed- or liquefied-H 2 . However, the aim of the paper is to highlight there is still a large margin to increase Power-to-Power round-trip efficiency, mainly from the hydrogen production and power generation blocks, which could lead to round-trip efficiencies of around 40%e42% in the next decade for Power-to-Power energy storage systems with micro-gas turbines. ©2022 The Authors. Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/). *Corresponding author. E-mail address: [email protected] (A. Escamilla). Available online at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/he international journal of hydrogen energy 47 (2022) 17505e17525 https://doi.org/10.1016/j.ijhydene.2022.03.238 0360-3199/©2022 The Authors. Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Introduction The interest in Power-to-Power energy storage systems has been increasing steadily in recent times, in parallel with the also increasingly larger shares of variable renewable energy (VRE) in the power generation mix worldwide [1]. Owing to the characteristics of VRE, adapting the energy market to a high penetration of VRE will be of utmost importance in the coming years [2]. Variable renewable energies like wind or solar are characterised for being an intermittent source of energy whose availability for power generation depends mainly on local weather conditions, which can be predicted accurately 24e36 h in advance only [3]. This, along with the very limited options for large-scale energy storage available today and the increasingly larger share of VREs into the energy mix, implies that the grid must still rely on conventional power generation technologies to generate electricity when VRE is not available. These conventional power generation technologies are very reliable but they also feature high emissions of greenhouse gases (GHG) and, in some cases, other hazardous emissions like nuclear waste. Large-scale energy storage is thus one of the most pressing technical challenges to achieve carbonneutrality by 2050. Additionally, and parallel to this, smart energy systems for managing production, distribution and consumption of electricity, heat, and gas are of prime importance to enable a 100% renewable energy scenario [4,5]. Energy Storage Systems (ESS) are usually classified according to the form in which energy is stored: electrical,electrochemical,chemical,mechanical and thermal. An explanation of each of these ESSs is found at [6,7] along with a summary of their main characteristics. Amongst all these, there are only two ESSs which have so far met the requirements for a successful deployment of large-scale energy storage to the market, and both are of the mechanical type: pumped-hydro and compressed-air. Pumped Hydroelectric Storage (PHS) is the oldest and most mature technology. It represents the largest form of storage today, accounting for 96% of the total energy storage capacity amongst all technologies [8]. Nevertheless, PHS is strongly dependent on local geographic features as it requires the presence of both water streams and natural/ artificial basins with large differences in altitude. This poses obvious restrictions on the large-scale deployment of PHS systems. Compressed Air Energy Storage (CAES) has similar limitations since its technical and economic feasibility is limited to locations where large caverns are available [9]. In addition to these two ESS technologies, there are other options for energy storage but they are presently applicable at the small-scale only: electrochemical batteries, flow batteries, Superconducting Magnetic Energy Storage systems (SMES), flywheels, capacitors and supercapacitors. With such limitations, these technologies seem to be more suitable for grid balance when needed than for large-scale energy storage. Nevertheless, with these caveats and despite a lower maturity and somewhat difficult scalability, chemical energy storage has gained interest in recent years. This is partly due to the noteworthy development of larger-scale water electrolysis technologies and to the penetration of Renewable Energy Sources (RES) in the energy mix, which seem to make it technically possible in the near future to produce large amounts of hydrogen using RES only as secondary form of energy [10]. 1 This hydrogen is termed green-hydrogen and the associated energy (hydrogen) storage technology from RES is known as Power-to-Hydrogen (PtG-Hydrogen). The main advantage of this ESS is that hydrogen is an energy carrier which can be stored, transported, and converted into other forms of energy such as mechanical, power, heat, etc. Unlike PHS or CAES, hydrogen can be produced in-situ wherever there are power and water supplies. It can also be stored in different forms (gas distribution network, highpressure tanks, metal hydrides …) and not only can it be delivered in the form of electric power but it can also be converted into mechanical power (mobility), thermal energy (heating networks, process heat …) or even as feedstock for the industry (oil refineries). Thus, even though this work is focused on applications where hydrogen is converted back into electric power through combustion in micro gas turbines, the interest in hydrogen technologies does not limit to electrification but these are also seen as a means to help decarbonise those sectors where electrification is not feasible. The production of hydrogen from water electrolysis using RES and Glossary AEC Alkaline Electrolyser BoP Balance of Plant CA Compressed Air CAES Compressed Air Energy Storage CapEx Capital Expenditure CCS Carbon Capture and Sequestration CHP Combined Heat and Power EC Electrolyser ESS Energy Storage Systems FC Fuel Cell FCET Fuel Cell Electric Truck GHG Greenhouse Gases GT Gas Turbine. LAES Liquid Air Energy Storage LCOH Levelized Cost of Hydrogen mGT micro-Gas Turbine. MH Metal Hydride. OEM Original Equipment Manufacturer OpEx Operational Expenditure P2P Power-to-Power P2X Power-to-X. PEMEC Proton Exchange Membrane Electrolyser PHS Pumped Hydroelectric Storage PtG Power-to-Gas RES Renewable Energy Sources SEC Specific Energy Consumption SMES Superconducting Magnetic Energy Storage SOEC Solid Oxide Electrolyser SOFC Solid Oxide Fuel Cell VRE Variable Renewable Energy 1 Water electrolysis is well established today but massproduction of electrolysers is still a bottle neck for the implementation of this energy storage technology at the large scale. international journal of hydrogen energy 47 (2022) 17505e1752517506 its later use for power generation is termed Power-to-Power (P2P). Numerous research projects on the topic (power-to- hydrogen) have been developed in recent years, both theoretically and experimentally. In this latter group, around 143 PtG projects producing either hydrogen or methane have been operated in the last decades, some of them starting as far back as 1988 [11,12]. Most of these facilities were pilot or demonstration plants with power ratings under 1 MWe, aimed at enabling a more profound understanding of the practical challenges and technical gaps that still need further research. As of 2019, there were 56 active PtG projects based on hydrogen, with an installed electric capacity of 24.1 MWe [11]. Germany hosts the largest number of these plants, with 64 PtG projects, but the stimulus given by governments around the world will expectedly see other countries catching up in the next years [13e16]. The estimated worldwide production of hydrogen by the end of 2018 was 74 Mt, 97% of which was produced through steam reforming of natural gas (methane) and, therefore, with large emissions of carbon dioxide. Most of this hydrogen was actually not traded or distributed but, on the contrary, it was produced locally for refineries and for the production of ammonia [2,17]. From these values, hydrogen production worldwide is expected to reach 240 Mt by 2050 [2], when it will play a key role in the mobility, power and heat sectors even if this production would cover 7.5% of the global energy demand worldwide only. In addition, it goes without saying that significant efforts have to be made to achieve this target, both technical and economic, amongst which the installation of 50 GWe to 60 GWe electrolyser capacity annually. As a consequence of the large effort to scale-up technologies that enable the utilisation of hydrogen as an energy carrier, applications where hydrogen can help to achieve carbonneutrality are flourishing. More than twenty applications where hydrogen can become a cost-competitive low-carbon solution before 2030 have been identified in a recent report by the Hydrogen Council [10], needing a large investment effort if they are to be developed and taken to the market. Approximately EUR 60 billion investment is required by 2030 to achieve cost-effectiveness of hydrogen technologies, which can be facilitated through the following five drivers: reduced market uncertainty, larger improvement-for-investments for technology boost, hybrid solutions, increasing utilisation rates in distribution networks and investments in blue/green hydrogen. 2 Along with these, other actions aimed at setting common policies to nurture a favourable ecosystem for the commercial deployment of hydrogen are needed. In particular, governments are expected to work on the following areas to unfold aligned policies: national strategies, coordination, regulation, standardisation, infrastructures and incentives. From a supply-chain standpoint, the research of P2P solutions can be categorised into hydrogen production, storage and consumption. For production: direct coupling between RES and electrolyser (EC), high temperature electrolysis, and footprint and materials of ECs are currently attracting significant efforts. For storage: higher storage capability at higher pressure ( >60 kgH2at >700 bar), metal hydrides, NG-H 2 blends in gas grids. For power generation: hydrogen combustion, high temperature fuel cells, hybrid systems (not only integration concepts but also control systems, power electronics, and other auxiliary systems). Table 1 contains a summary of the main Power-to-X (P2X) projects that are currently under development, Xbeing any form of energy. Although all of them consider the production of hydrogen through water electrolysis, they differ from one another in the hydrogen storage solution, power rating and target sector (mobility or industry). Indeed, different scales can be observed, from small systems (0.4 MWe) to very large hydrogen infrastructures like the Advanced Clean Energy Storage (ACES) [18] project in central Utah, which is considered the “World's largest”energy storage project planned (1GWe rated electrolyser capacity). ACES will make use of three storage technologies: renewable hydrogen that is then converted into electric power with a hybrid system comprised of solid oxide fuel cells and gas turbines, CAES and large-scale flow batteries. In addition to this, due to the importance of decentralised power generation, there are other projects which cover smaller energy storage capacities, for instance ROBINSON [19] and GREEN HYSLAND [20]. Their main target is to optimise the utilisation of local RES by deploying an integrated, smart and cost-effective energy system coupling thermal and electrical networks. There number of studies in literature dealing with different subjects within the concept of P2X is very large. Heyman et al. present a flexible framework to compare the performance of power-to-gas sites, providing useful indicators about energy conversion technologies, plant size, cost structure, and configuration [23]. Crespi et al. compare the use of hydrogenbased P2P systems, battery systems and hybrid hydrogenbattery systems to supply a constant 1 MW el load with electricity locally generated by a photovoltaic plant [24]. In this work, systems are sized in order to minimize the annual average cost of electricity. Loisel et al. also deal with an Table 1 ePower-to-X projects under development around the world. Project Name Location (X) Product Reinjection Start Power Country City Yes/No Year MWe FLEXnCONFU [21] PT Lisbon H 2 ,NH 3 No 2020 1 HYFLEXPOWER [22] FR Saillat-sur-Vienne H 2 No 2020 12 ROBINSON [19] NW Eigerøy H 2 , Biogas Yes 2020 0.4 ACES [18] US Utah H 2 , CA Yes 2019 1000 GREEN HYSLAND [20] SP Majorca H 2 Yes 2021 e 2 The term blue hydrogen refers to hydrogen that is produced through conventional steam reforming and downstream carbon capture,utilisationandstorage,hencewithnonetcarbonemissions. international journal of hydrogen energy 47 (2022) 17505e17525 17507 economic evaluation to predict the LCOH for different scenarios in France [25]. The authors conclude that, for the different scenarios considered, LCOH would be in the range from 4 to 13 V/kg H 2 . Bexten et al. focus on the economic viability of the on-site hydrogen supply for an industrial gas turbine, analysing the impact of parameters such as number of wind turbines, available EC capacity and hydrogen storage capacity [26]. It is concluded that with the current economic framework, the solution is not economically viable. Focusing more on thermodynamic featuress, Mukelabai et al. deal with power-to-ammonia and ammonia-to-power systems using a reversible solid oxide system, reporting round-trip efficiencies between 41 and 53% for the cases considered [27]. Wang et al. make use of a thermodynamic analysis to compare different fuels in a power-to-X-to-power system using a reversible solid oxide cell [28]. The work reports the following round-trip efficiencies for the different systems considered: 47.5% (methane), 43.4% (syngas), 42.6% (hydrogen), 40.7% (methanol), and 38.6% (ammonia). Ishaq et al. investigate the performance of an integrated wind energy system producing hydrogen and power through a PEMEC and FC [29], reporting energy and exergy efficiencies of the system against wind speed. More complex cycle layouts to increase energy and exergy efficiencies are proposed by other authors. Motahar et al. look into the 2nd Law performance of a hybdrid system comprised of an SOFC and steam injected gas turbine, determining that both the combustion chamber and fuel cell stack are the components with highest exergy destruction [30]. Alirahmi et al. consider an integrated FC/geothermal-based energy system generating cool power and electricity at the same time [31]. The system is driven by geothermal energy and the electricity is mostly produced by a dual organic cycle. An EC is used to produced hydrogen which is later used by a FC to support the grid during peak consumption periods. This yields minimum cost rate and maximum exergy efficiency. Additionally, Tukenmez et al..propose a layout consisting on the following subsystems: gas turbine cycle, Rankine cycle, two organic Rankine cycles, ejector-based cooling, hydrogen production and liquefaction, ammonia production and storage, drying, and hot water generation [32]. It reports overall energy and exergy efficiencies of 62% and 58%, respectively. The current work is aimed at the assessment of power-to- hydrogen-to-power (P2P) energy storage systems as an efficient means to reliably increase the share of renewable energies in the grid. In contrast with most of the works on P2P systems available in literature, focusing more on global techno-economic considerations and disregarding the fundamentals of the technology, the main goal of this analysis is to look into the thermodynamic principles of each process along the value-chain of hydrogen with the aim to characterise the energy balance of this energy storage option. This yields a much more accurate calculation of the round-trip efficiency, figure of merit usually adopted to compare the efficiency of energy storage systems. Additionally, the study is restrained to using micro-gas turbines as a means to produce power from hydrogen. Hence, the P2P system considered here is limited to a maximum power output of around 500 kWe. The paper is organised as follows. Section Introduction provides a comprehensive literature review of P2P systems and technologies with the aim to show that this work comes to fill a gap in the existing body of knowledge. Section Power- to-hydrogen-to-power presents the concept of P2P based on micro gas turbines, and provides the thermodynamic grounds for each of the systems involved. Section Analysis provides an energy balance of all the systems involved in the P2P energy storage section, identifying those systems where there is a larger room for optimisation to attain the highest round-trip efficiency possible. Finally, in Section Discussion of results, the results obtained and the different scenarios of improvement for each of the systems involved are discussed. Power-to-hydrogen-to-power Power-to-Power (P2P) is a process whereby surplus renewable power is curtailed through conversion into chemical energy (in the case of interest for this paper, hydrogen) for storage. This stored energy can later be used in-situ or transported to a consumption node. This can be achieved by using hydrogen as a fuel for power generation in either thermo-mechanical (i.e., heat engines) or electrochemical (i.e., fuel cells) devices or, less often, through direct conversion into heat. Four separate processes can hence be identified to assess energy losses of the entire setup: production, storage, transportation/distribution and power generation (Fig. 1). Each of these processes incurs energy losses, leading to a certain global or round-trip Fig. 1 ePower-to-Power options considered. international journal of hydrogen energy 47 (2022) 17505e1752517508 efficiency representing the fraction of energy originally taken from the grid (or source of electric energy) that is delivered back to it. The following sections deal with the different aspects involved in the calculation of round-trip efficiency of the power-to-power solutions considered in this work. Hydrogen production Hydrogen is not found in free molecular form on Earth but, rather, bounded to other elements in the form of stable compounds, for instance hydrocarbons. Hence, chemical processes are needed to obtain molecular hydrogen, H 2 , and they all require a non-negligible supply of primary energy. Fig. 2 summarises the alternatives that have been investigated so far. The following terminology or colour code is commonly used to refer to different hydrogen production processes [17]: Brown hydrogen: the primary energy source is coal and the specific emissions of the process are estimated at 19 tCO2/tH2. Grey hydrogen: the primary energy source is natural gas and the specific emissions of the process are estimated at 11 tCO2/tH2. Blue hydrogen: same as grey hydrogen but incorporating Carbon Capture and Sequestration (CCS) techniques. Specific emissions decrease to 0.2 tCO2/tH2. Green hydrogen: renewable energy sources RES are used in this case so the associated carbon footprint is potentially null. The vast majority of hydrogen produced in the world today (95%e97%) is grey hydrogen. This is owed to the reliability and scalability of steam reforming to produce hydrogen from natural gas, and to the much lower costs of this technology as opposed to other options to produce green hydrogen. In particular, grey hydrogen is 5e8 times cheaper than green hydrogen according to several techno-economic assessments of the technology [2,10,17]. Nevertheless, despite not being cost-effective today, only green hydrogen will be considered in this document given the target to achieve net-zero emissions by 2050 set forth in the Paris Agreement [33]. Readers interested in other technologies are referred to the bibliography [34e36]. Water electrolysis is the dominant technology for the production of green hydrogen owing to its fairly high conversion efficiency and direct use of electricity, as opposed to other technologies which rely mostly on heat. This is a thermochemical process whereby electricity is consumed to split the water molecule into hydrogen and oxygen. The reaction is carried out in an electrolyser, which is an electrochemical device comprised of two electrodes where the halfreactions of oxidation and reduction take place, and of an ion-conductive electrolyte in between. The overall reaction and the total energy required to split 1 mol of H 2 O at standard pressure and temperature (298.15 K and 1 bar) reversibly are: H2OðlÞ/H2ðgÞþ1 2O2ðgÞ;DH+ðH2OðlÞÞ¼þ285:8kJmol 1 (1) H2OðgÞ/H2ðgÞþ1 2O2ðgÞ;DH+ðH2OðgÞÞ¼þ241:8kJmol 1 (2) The application of Gibbs-Helmhotz equation reveals the constituents of the total energy supply (DH ◦ (H 2 O(l))): DH+ðH2OðlÞÞ ¼ DG+ðH2OðlÞÞ þ T,DS+ðH2OðlÞÞ (3) where: DH ◦ (H 2 O(l)) ¼285.8 kJ mol 1 is the total energy supply to the electrolyser. DG ◦ (H 2 O(l)) ¼237.2 kJ mol 1 is the reversible electric work that must be supplied to the electrolyser to split the water molecule. T,DS ◦ (H 2 O(l)) ¼48.6 kJ mol 1 is the reversible heat (thermal energy) that must be supplied to the electrolyser to split the water molecule if the reaction is to be kept at constant temperature. Fig. 2 eHydrogen production methods. Colour code: green and grey refer to the origin of the primary energy used for hydrogen production, as noted in the figure. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) international journal of hydrogen energy 47 (2022) 17505e17525 17509 The minimum amount of electricity needed by the electrolyser to split the water molecule is DG(H 2 O(l)). Nevertheless, if no additional energy (either electrical or thermal) is supplied, the products will be colder than the reactants and the electrolyser will eventually cool down and come to a halt. Hence, in order to ensure operation at constant temperature, T,DS(H 2 O(l)) must also be supplied to the system. Operation of the electrolyser at constant temperature is called thermoneutral operation and, when carried out reversibly, determines the minimum amount of energy that must be supplied to the electrolyser for sustained operation: 285.8 kJ mol 1 if water is supplied as a liquid or 241.8 kJ mol 1 if the feed is vapour. Based on this energy balance, the energy efficiency of an electrolyser is defined as follows: hLHV ¼_ mH2 ,LHV _ Wel þ_ Q(4) hHHV ¼_ mH2 ,HHV _ Wel þ_ Q(5) Energy efficiency can be defined in terms of the produced chemical energy referred to either the high or low heating values of hydrogen, h HHV and h LHV respectively, though the former case is used more often (h HHV ). As the electrochemical losses of the electrolyser cell increase, the consumption of electrical work increases with respect to the aforesaid value (237.2 kJ mol 1 at standard pressure and temperature) and efficiency decreases accordingly. The electrical work consumed by a cell can also be expressed in terms of voltage (V cell ) and electric current (i cell ): Wel ¼Vcell,icell ¼Vcell,j,Acell (6) In Eq. (6), j and A cell are the current density and active area of the electrolyser cell. The application of Nernst equation links Eqs. (6) and (3) to obtain the relationship between cell voltage and energy exchange: Vrev ¼DG n,F(7) Vtn ¼DH n,F(8) The minimum voltage difference between electrodes splitting the water molecule in a reversible cell is V rev (Eq. (7) 3 ). In these conditions, an amount of thermal energy equal to T ,DS must be supplied to the cell in order to keep temperature constant. Otherwise, a higher voltage must be applied and the excess electricity with respect to the former value is converted into heat. The latter voltage is termed thermal-neutral voltage V tn (Eq. (8)). When the process is not reversible, a higher cell voltage is needed and a voltage efficiency can be defined as follows: hrev ¼Vrev Vcell (9) htn ¼Vtn Vcell (10) Again, this voltage efficiency can be defined with reference to the reversible cell voltage (Eq. (9)) or the thermoneutral voltage (Eq. (10)). There are several electrolyser types, classified according to the nature (composition) of the electrolyte, given that this determines the half-reactions taking place at each electrode and the associated operating temperature and pressure. Nowadays, research is mostly focused on three electrolyser technologies: alkaline (AEC), proton-exchange membrane (PEMEC) and solid oxide (SOEC). Table 2 shows the main characteristics of each electrolyser type. The most mature technology is AEC and it yields the lowest capital cost (CapEx). AEC is more appropriate for stationary applications where the operating conditions remain steady and load changes are slow, owing to their large footprint and difficulty to cope with steep thermal transients. PEMECs are characterised by a smaller footprint and much better response to rapid load changes. However, the Capital Cost (CapEx) of PEMECs is 2e3 times higher than for AEC, even if this figure is expected to decrease to some 600e1300 V/kW in the following decade [17,37,38]. The least mature technology amongst the three is SOEC. The working conditions of this type of electrolyser are totally different to the previous two since water must be provided in the form of steam at around 800C and this requires a significant consumption of energy externally to the electrolyser stack. Interestingly though, the energy required to sustain the water electrolysis reaction is fairly constant in the range from 0C to 1000C[42], meaning that the higher consumption of heat (to produce steam in lieu of hot water) is compensated for by a much lower consumption of electricity. The net result is that the efficiency of SOEC is above 80% referred to the Low Heating Value (LHV) of hydrogen and close to 100% when considering the High Heating Value (HHV), given that the total energy consumption remains approximately constant in the aforesaid temperature range. This stems as an advantage over the other electrolyser technologies but working at such high temperature requires an external heat source (not electricity only as AEC and PEMEC) also brings problems associated with heat management and, of course, more costly materials. Regarding the response time and start-up/shutdown capability (i.e., flexibility) of these types of ECs, both AEC and PEMEC are rather quick and flexible in a wide output range (5e10 to 100% load). Eichman et al. provide information about the response time for different changes in power setting and Table 2 eCharacteristics of different types of electrolyser [37e41]. Parameter Unit AEC PEMEC SOEC TC60e80 50e80 650e1000 PBar 1e30 30e80 1 jA/cm 2 0.5e1.0 2.0e3.0 2.0 VV 1.75e2.40 1.6e2.0 1.2e1.3 SEC kWh=kgH251 70e55 41e40 h HHV %7757e72 98 CapEx V/kW 750 1200e2000 >4500 Regulation speed [order of] seconds Milliseconds hours Minimum load % full load 10e20 0e530 Lifetime Hours 100 000 90 000 25 000 3 F≡Faraday constant ¼96 500 C mol 1 international journal of hydrogen energy 47 (2022) 17505e1752517510 also the time that is needed to start-up and shutdown the system [43]. These studies report that the settling time after a load change is in the order of seconds whereas start-up and shutdown is in the order of minutes. Hence, AEC and PEMEC are eligible for a very wide range of applications, including direct coupling to RES, as well as in the different electricity markets [37]. On the contrary, even if SOEC adapts rather quickly to changes in load setting, both the response and start-up/shutdown times range from minutes to hours to prevent the drastic reduction in the lifetime of components caused by fast temperature gradients [40,41]. Table 3 presents technical specifications taken from Original Equipment Manufacturers (OEMs) of different types of electrolysers, providing realistic information about the current state of the art of the technology. Hydrogen storage As already said, P2P is an energy storage technology with the main advantage to rely on an energy carrier as energy storage medium. This means that hydrogen can be stored in different ways and for as long as it is required, as opposed to other technologies which cannot be used for long-term energy storage. Nevertheless, despite this flexibility, the term-period for which hydrogen energy storage is most attractive is in the order of days to months. For shorter or longer storage periods, other technologies exhibit a better trade-off between discharge time and efficiency of storage [6]. Hydrogen is known to be the fuel with the highest gravimetric density but with the main shortcoming of having one of the lowest densities. Hence, different storage strategies are Table 3 eCommercial availability of electrolysers. Type Label OEM Model P out [bar] H 2 Yield [Nm 3 /h] SEC [kWh/kg H 2 ] AEC A1 SunFire [44] HYLINK 30 1090 52.3 A2 Enapter [45] EL 2.1 35 0.5 53.4 A3 Green Hydrogen [46] A30 35 30 52.2 A4 A60 60 52.2 A5 A90 90 53.6 A6 McPhy [47] Baby 1 0.4 83.5 A7 P 1e2.5 0.5e0.8 66.8e62.6 A8 M 1e2.5 2.4e4.4 64.9e65.7 A9 H 4e83e10 66.7 A10 McLyzer 10-30 30 10 55.6 A11 McLyzer 20-30 20 A12 McLyzer 100-30 100 A13 McLyzer 200-30 200 A14 McLyzer 400-30 400 A15 McLyzer 800-30 800 A16 Nel Hydrogen [48] A150 1e200 50e150 42.3 a - 49.0 a A17 A300 150e300 A18 A485 300e485 A19 A1000 600e970 A20 A3880 2400e3880 PEMEC A21 Nel Hydrogen [48] S10 13.8 0.27 67.9 A22 S20 0.53 A23 S40 1.05 A24 H2 15e30 2 81.2 A25 H4 4 77.9 A26 H6 6 75.7 A27 C10 10 69.0 A28 C20 20 66.8 A29 C30 30 64.5 A30 M100 30 103 50.4 a A31 M200 207 A32 M400 413 A33 M4000 4000 A34 ITM Power [49] HGAS1SP 20 0.99 63.6 A35 HGAS2SP 1.98 63.2 A36 HGAS3SP 3.24 65.3 A37 HGASXMW 15.17 59.7 A38 Hydrogenics b [49] HyLYZER 300 30 300 53.4 A39 HyLYZER 1000 1000 A40 HyLYZER 5000 5000 SOEC A41 SunFire [44] HYLINK 1 750 40.0 a Power consumption provided by manufacturer accounts for the stack only and not for the BoP. b Hydrogenics has been acquired by Cummins. international journal of hydrogen energy 47 (2022) 17505e17525 17511 applied to overcome the low density of hydrogen, be it in gaseous, liquid or solid states; for instance, it is common to liquefy hydrogen through compression with the objective to attain a higher density, but this is very energy intensive. As a representation of this, Fig. 3 shows the energy densities of hydrogen (at different pressures) and other fuels also used for energy storage. It is observed that the energy density of hydrogen at ambient conditions is much lower than that of other gases (e.g., methane, ethane), liquids (e.g., methanol) or solids (e.g., ammonia); moreover, even if the storage pressure of hydrogen were increased to values as high as 300 bar, the energy density would still be significantly lower. Only liquid (cryogenic) or solid (metal hydrides) storage would make a difference but these processes are either highly energy intensive or not suitable for large-scale hydrogen storage. Other substances with higher energy densities, such as ammonia or syn-gas, can be produced using hydrogen as feedstock. This facilitates transportation but incurs higher energy losses due to the incorporation of an additional process where large amounts of energy are consumed (either as heat, mechanical power or both). For instance, the FLEXn- CONFU project (acronym standing for Flexibilize combined cycle power plant through power-to-X solutions using non-conventional fuels) funded by the European Commission studies the use of ammonia as a way to transport hydrogen and produce power [21], a solution that is beneficial to transport large amounts of energy over long distances. This concept falls beyond the scope of this study though, as reported in Section Hydrogen distribution, and hence the conversion of hydrogen into other substances (fuels) is left out of the analysis. Compressed hydrogen gas Hydrogen compression is the most common means to store and distribute hydrogen in relatively small quantities, 5e10 kgH2/tank. The pressure at which hydrogen is compressed depends on the application and it ranges from 50 to 400 bar for stationary applications and to 900 bar for mobility applications (where high density is a critical requirement). This implies large pressure ratios (typically higher than 30:1), enabled by volumetric machines such as reciprocating or diaphragm compressors. Diaphragm compressors compress small to medium flow rates of hydrogen efficiently, attaining high to extremely high pressure ratios of more than 5000:1 [50]. The operating principle of diaphragm compressors ensures oil and leakage free compression with high purity of the outlet flow. However, when operation is discontinuous (frequent start/stop), the lifetime of the diaphragm can decrease substantially, which means more frequent servicing and, therefore, higher maintenance costs. In applications with more frequent transient and/or discontinuous operation, hydraulically-driven dryrunning reciprocating compressors are a viable alternative. They also enable high delivery pressures of up to 3000 bar [50] with also oil, leakage and technically abrasion free compression. With this in mind and given the intermittent nature of RES for the production of green energy (wind and solar photovoltaic are the dominant sources of renewable energy, in addition to hydro), reciprocating compressors with several stages of intercooling are deemed the best solution for hydrogen compression in this work. Delivery pressure from the electrolyser is a key parameter determining compressor power during the downstream compression of H 2 for storage, since this is the pressure (same for temperature) at the intake of the storage compressor. This effect is assessed in Fig. 4a, showing that compression from 1 to 1000 bar requires twice as much work as compression from 30 to 1000 bar, 4 kWh=kgH2and 2 kWh=kgH2respectively. Compression work is influenced not only by inlet conditions and outlet pressure but also by compressor type and, linked to this, how the compression process is carried out. Fig. 4a shows the differences between isentropic (reversible and adiabatic) and reversible, intercooled compressions, the latter of which resembles an isothermal compression process. The different nature of these processes is reflected in Fig. 3 eEnergy density of different fuels. Adapted from Ref. [38]. international journal of hydrogen energy 47 (2022) 17505e1752517512 the value of the polytropic index kin Eq. (12), yielding compression work. When k¼1, compression is at constant temperature. When kequals the ratio of specific heats g(g¼C p /C v ), which is 1.41 for hydrogen, compression is isentropic and compression work increases with respect to the isothermal case. For adiabatic compressors not working reversibly, k>g. Interestingly, volumetric compressors like diaphragm and reciprocating are cooled internally but this does not enable isothermal compression. In practice, 1<k<gand the exact value of kis determined experimentally. Fig. 4a provides an estimate of compression work with and without intercooling between stages. This information is complemented by Table 4 which shows the resulting polytropic index kfor different arrangements of the compression system. Two cases are considered. Case 1 considers individual isentropic compression stages with intermediate cooling; for each individual compression stage, k¼g¼1.41. Case 2 considers that each compression stage is adiabatic but not reversible, hence k>g¼1.41. Two conclusions are drawn from the table: Increasing the number of stages brings about a reduction of compression work because the compression process becomes closer to isothermal. For a given number of stages, non-isentropic compression by the individual stages increases the total work needed to compress hydrogen. In addition to these two conclusions, it must also be noted that a higher number of stages implies higher capital costs. Also, departing from isentropic (i.e. lower efficiency of the individual compression stages) poses a higher demand on the interstage cooling system (coolant pump) because of the higher inlet temperature to the intercooler. That said, and for the purpose of defining the compression energy requirements for the different scenarios in Table 5, pressure ratio per compression stage is calculated according to the next guidelines [51]: 1. Maximum pressure ratio per stage is 4:1. 2. Maximum outlet temperature for all stages is 420 K. Controlling that this temperature does not exceed this upper limit can be achieved either by decreasing inlet gas temperature (more intense cooling) or by increasing stage count. Hence, the number of compression stages (n stages ) and compressor work (W compressor ) can be estimated from Eqs. (11) and (12). nstage ¼log P2 P1 log PRstage (11) Wcompressor ¼_ mH2 ,X nstages i¼1 k k1,R,T1;i,Zi,PRk1 k i1 hcompressor ½W(12) where: _ mH2is the mass flow rate of hydrogen [kg/s]. P 1 and P 2 are the inlet and outlet pressures of the compressor [bar]. PR i is the stage pressure ratio for each compression stage. kis the polytropic index of the gas. Ris the universal gas constant for H 2 , 4124 J kg 1 K 1 . T i is the fluid inlet temperature for each compression stage. Z i is the compressibility factor for each compression stage. Values of the compressibility factor of hydrogen can be found at [52]. h compressor is the efficiency of the compressor. In addition to the specific work of the compressor, the work needed to drive the cooling water pump of the intercooler must also be calculated in order to obtain the total power consumption of the hydrogen compression system. Fig. 4b shows the assumptions made to obtain these performance indicators, which can be estimated with the equations below: _ mH2O¼ _ mH2h1H2h2H2 h2H2Oh1H2Okg s1(13) Wpump ¼X nstages i¼1 g,Hi,_ mi;H2O hpump ½W(14) where: hrefers to the specific enthalpy of H 2 O and H 2 at the inlet (1) and outlet (2) of/from the intercooler [kJ kg 1 ]. _ mi;H2and _ mi;H2Oarethecorrespondingmassflowrates[kgs 1 ] for hydrogen and water for each of the intercooling stages. gis the gravitational acceleration, estimated at 9.81 [m s 2 ]. H i is the head of the pump [m] for each of the intercooling stages. h pump is the efficiency of the pump [%]. The pressure ratio between the hot (hydrogen) and cold (water) sides of the heat exchangers is set to 10. Therefore, pressure on the hydrogen side is ten times higher than on the water side. This assumption is considered when calculating the pump head for each intercooling system. Table 5 shows the power demand of the compression system for different cases, based on the calculations in Eqs. (12) and (14). Liquefied hydrogen (LH 2 ) Hydrogen is commonly produced at close to ambient pressure and temperature, in the range from 1 to 30 bar and from 303 to 333 K, as stated in section Hydrogen production. Nevertheless, in this pressure range, hydrogen is found in the liquid state only at temperatures below 20.37 K (1 bar) to 33.14 K (12.79 bar - critical pressure) [54]. Therefore, a cryogenic plant is needed in order to reduce the temperature of hydrogen to such extremely low values if H 2 is to be stored in liquid state. Fig. 5 shows a generic layout of a facility to liquefy hydrogen. Liquefaction of hydrogen can be divided into four general stages: 1. Precompression (at ambient conditions). 2. Precooling (ambient to about 80 K). international journal of hydrogen energy 47 (2022) 17505e17525 17513 convert surplus renewable electricity into hydrogen and they all are experiencing substantial technical progress leading to better performances, larger scales and lower costs: AEC, PEMEC and SOEC. Hydrogen storage can be enabled in three different states: compressed-H 2 , liquefied-H 2 and in the form of metal hydride. Compressed-H 2 requires high-pressure storage vessels (type III &IV [96]) and a compression system able to handle high pressure ratios ( >10:1), what translates into high capital and operating costs. Nevertheless, if the hydrogen production and consumption nodes are at the same location and sufficient space (land area) is available, lower storage pressures can be used, making this option very attractive. Liquefied-H 2 features a much higher density (z78 kg/m3) but the process to liquefy hydrogen is highly energy intensive, making this option attractive only when long-distance transportation is required, for instance between continents. Metal hydrides enable the highest density among the different H 2 storage systems ( > 115 kgH 2 /m3) but the associated hydrogen release rates are lower than for the other technologies. If this is not a limiting factor and low system weight is not required, then metal hydrides yield the benefit of being a lower energy intensive Fig. 8 eConsidered layout of the Power-to-Power energy storage system based on mGT technology. Colour code: redhydrogen, blue-water, green-power; dark blue-oxygen. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Fig. 9 eRound-trip efficiency for the P2P solutions presented in Fig. 8. international journal of hydrogen energy 47 (2022) 17505e1752517520 process to store H 2 . Yet, given that there exist many types of metal hydrides, it is very important to select the right MH type for each particular application, due to its dependence on heat supply. Regardless of the storage technology, higher H 2 densities lead to lower energy cost of hydrogen transportation/distribution. Therefore, the volume available for transportation of H 2 sets a strong constraint, either in the form of high-pressure tubes, liquefied-H 2 tanks or truck size limitations. If a FCET is considered, around 3840 kg of LH 2 can be transported in a single trip, which is around four times the energy that can be transported when considering compressed-H 2 or MH. Stored energy is eventually discharged in the form of electric power when requested by the grid or end-user. For the concept considered in this work, this is done through hydrogen combustion in a micro gas turbine, which stems as a compact, efficient, economical and very fast-responding power generation technology. Indeed, micro gas turbine are able to run at full capacity just a few minutes after start-up and their electric efficiency ranges from 25% for the smaller units ( <100 kWe) to almost 40% for engines in the larger output range (z500kWe). This efficiency is lower than other power generation technologies such as fuel cells ( >55%) or reciprocating internal combustion engines ( >45%), but mGTs offer additional, beneficial features such as available high grade heat for Combined Heat and Power ( >250C). The paper has discussed the critical figure of merit used to determine the viability of an energy storage system, roundtrip efficiency (h round-trip ), which determines the amount of energy that is given back to the grid in comparison with the energy taken originally. Calculations for various configurations of the selected system have shown that the maximum h round-trip achievable is below 30% and this applies to a system based on the smart integration of a Solid Oxide Elecrolyser (SOEC) for H 2 production, compressed-H 2 storage and micro gas turbine (mGT). If AEC or PEMEC were used in lieu of an SOEC, h round-trip would go down to around 22% at best. This h round-trip is lower than for other energy storage technologies [97e99]: 1. Pumped-hydro: from 65% in older installations to 75e85% in modern facilities. 2. Flywheels: 80%e90%. 3. Batteries: 75%e85%. 4. Electro-thermal: 65%e75%. 5. Compressed air: 45%e70%. 6. Liquid Air: 30%e70%. 4 The aforelisted energy storage technologies have considerably higher h round-trip than power-to-power systems based on micro gas turbines (mGT-P2P). However, only hydro, compressed-air storage (CAES) and liquid air storage (LAES) are comparable to mGT-P2P (or, in general, GT-P2P) in their capacity to store energy for days or even months at a larger scale. Additionally, the critical advantage of the mGT-P2P option over these technologies is that it is independent from geographical constraints and it can be installed almost anywhere where renewable electricity and water are available in addition to the advantage of dealing with an energy vector as it is hydrogen. Hence, despite having h round-trip z29%, it is the only fully flexible (installation wise) large-scale, long-term energy storage solution that can be installed virtually anywhere where there is a need for energy storage. Additionally, it also provides a high-grade heat source which makes it suitable for CHP applications. Finally, it is to note that in spite of the maximum h round-trip of around 30% reported in this work for mGT-P2P energy storage solutions, there is still a large margin for improvement. This potential is found in the hydrogen production and power generation stages of the system. In particular, the operation of an ideal electrolyser with null losses would require 33.33 kWh=kgH2what translates into a potential 13e15% gain in h round-trip . Even if this is a theoretical limit that cannot be achieved in practice, predictions from different world energy organisations [17,37,38] estimate that the energy consumption of electrolysers is expected to decrease by a few kWh/kgH2in the next decade, leading to a Dh round-trip improvement of around 3e5% (one third of the theoretical efficiency gain). Parallel to this, an even larger performance enhancement could come from further improvements in micro gas turbine technology. Indeed, the efficiency of these engines could increase to around 40% [92,100] in the next decade and the combination of SOEC/SOFC and mGT in socalled hybrid systems would boost power generation efficiencies to values as high as 55e60% [101]. The cumulative effect of this foreseen progress on the performance of mGT- P2P technology would translate into h round-trip higher than 40e42% in the next decade. This would take this technology closer to and beyond cost-effectiveness, setting it on the map of essential technologies to enable carbon neutrality by 2050. Conclusions The paper considers that, for power-to-power energy storage solutions, three types of electrolysers can be used to convert surplus renewable electricity into hydrogen: AEC, PEMEC and SOEC. This hydrogen can then be stored in three different states: compressed-H 2 , liquefied-H 2 and in the form of metal hydride; regardless of the storage technology, higher H 2 densities lead to lower energy costs of hydrogen transportation/ distribution. Finally, stored energy is eventually discharged in the form of electric power when requested by the grid or enduser. For the concept considered in this work, this is done through hydrogen combustion in a micro gas turbine (500kWe). The paper has discussed round-trip efficiency (h round-trip )as the critical figure of merit to determine the viability of an energy storage system. The aim of the study is to offer a comprehensive analysis of the options available for each of the systems implied in a P2P-mGT, using round-trip efficiency as the figure of merit for comparison. This leads to certain limitations, mostly coming from the numerous power and heat streams of each subsystem that must be integrated into the optimisation process, which cannot be covered in detail in this work due to length limitations. Therefore, this study is a first step towards achieving highly efficient and optimised layouts for P2P-mGT energy storage systems, whose results 4 if it considers hot/cold recycle [99]. international journal of hydrogen energy 47 (2022) 17505e17525 17521 will narrow the design space for future, refined analysis of components. Calculations for various configurations of the selected system have shown that the maximum h round-trip achievable is below 30%, for the time being, and this applies to a system based on the smart integration of a Solid Oxide Electrolyser (SOEC) for H 2 production, compressed-H 2 storage and micro gas turbine (mGT). If AEC or PEMEC were used in lieu of an SOEC, h round-trip would go down to around 22% at best. Finally, it is to note that in spite of the maximum h round-trip of around 30% reported in this work for mGT-P2P energy storage solutions, there is still a large margin for improvement. The largest potential is found in the hydrogen production and power generation stages of the system. Improvements in these systems could lead to h round-trip of around 40e42% in the next decade. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment This work has been developed in the frame of the Next Generation of Micro Gas Turbines for High Efficiency, Low Emissions and Fuel Flexibility, NextMGT, which has received funding from the European Union's Horizon 2020 research and innovation programme under Marie Sklodowska-Curie grant agreement No 861079. 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