Conceptualizing novel CH₃OH-based thermochemical energy storage routes via a modeling approach

Article Conceptualizing novel CH 3 OH-based thermochemical energy storage routes via a modeling approach ACH 3 OH-to-CH 4 storage system via syngas represents a novel approach for energy conversion using concentrated solar power. Here, Rodriguez-Pastor et al. propose a storage system where the resulting CH 4 is of high molar fraction and can be utilized in existing technologies, offering greater energy storage density due to the liquid state of CH 3 OH. Diego Antonio Rodriguez-Pastor, Andre´s Carro, Giuseppe Masci, Carlos Ortiz, Vittorio Verda, Ricardo Chacartegui [email protected] (D.A.R.-P.) [email protected] (R.C.) Highlights ACH 3 OH-to-CH 4 storage system is proposed Thermal efficiencies greater than 40% are obtained through solar integration CH 4 is produced in greater than 70% molar fraction, usable in existing technologies Competitive levelized storage costs with other thermal energy storage systems are shown Rodriguez-Pastor et al., Cell Reports Physical Science 4, 101357 April 19, 2023 ª2023 The Author(s). https://doi.org/10.1016/j.xcrp.2023.101357 ll OPEN ACCESS Article Conceptualizing novel CH 3 OH-based thermochemical energy storage routes via a modeling approach Diego Antonio Rodriguez-Pastor, 1, *Andre ´s Carro, 1 Giuseppe Masci, 1,3 Carlos Ortiz, 2 Vittorio Verda, 3 and Ricardo Chacartegui 1,4,5, * SUMMARY Thermal energy storage systems are an emerging option for efficient energy conversion and storage, especially if they can concentrate solar energy. This work studies a flexible CH 3 OH-to-CH 4 conversion system from intermediate conversion to synthesis gas. The design is based on a combination of processes already tested experimentally and applied in industry. The concept we develop integrates the decomposition of CH 3 OH and methanation processes, providing different pathways for energy use, such as natural gas, direct heat, and power supply, or storage in chemical bonds. This flexibility in adapting the operation of the system to different energy availability and energy needs makes the concept appealing for changeable application. Thermal efficiencies of 39% are possible for the CH 3 OH decomposition phase and of 26% for the overall system for CH 4 production. Thus, from the high energy density of CH 3 OH, levelized storage costs of V134.8/MWh can be obtained, which is lower than systems based on molten salts. These results should spur interest in further advances for the proposed flexible concept. INTRODUCTION Increasing the energy storage capacity of renewable energy systems is currently a key challenge. 1 Costs of photovoltaics (PVs), wind, and even concentrating solar power (CSP) are currently competitive with fossil fuels, 2 although dispatchability is a major issue. According to IRENA, 3 the global energy storage capacity should increase from 30 GWh today to more than 9,000 GWh by 2050 to meet the COP21 agreement. 4 To achieve this ambitious goal, several technologies and energy storage paths must be developed on a large scale. Among the suitable options, CSP is particularly attractive due to its dispatchability potential, low cost, and scalability, 5 based on the possibility of storing thermal energy. Interestingly, thermal energy storage (TES) costs are one order of magnitude lower than battery electrochemical energy storage ($300–400/kWh). 6 Thermochemical energy storage (TCES) is based on reversible chemical reactions in which energy is stored and released by forming and breaking chemical bonds or by adsorbing or releasing thermal energy. Usual processes have three stages: endothermic dissociation, product storage, and exothermic reaction. TCES systems generally have higher energy densities than sensible and latent TES. 7 Energy losses during storage time are negligible for those systems that allow products to be stored at ambient temperature, making it a technology with great potential for long-term storage applications. 8 Several TCES systems are being thoroughly investigated, 1 University of Seville, Escuela Te ´cnica Superior de Ingenieros, Camino de los Descubrimientos s/n, 41092 Seville, Spain 2 Universidad Loyola Andalucı ´a, Av. de Las Universidades S/n, Dos Hermanas, 41704 Seville, Spain 3 Department of Energy Engineering, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy 4 University of Seville, Laboratory of Engineering for Energy and Environmental Sustainability, Seville 41092, Spain 5 Lead contact *Correspondence: [email protected] (D.A.R.-P.), [email protected] (R.C.) https://doi.org/10.1016/j.xcrp.2023.101357 Cell Reports Physical Science 4, 101357, April 19, 2023 ª2023 The Author(s). This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 1 ll OPEN ACCESS from laboratory prototypes to pilot scale, 9 based on hydroxides, 10 metal redox, 11 carbonates, 12–14 ammonia, 15,16 or CH 3 OH 17 systems. The turning temperature at which a reversible reaction occurs is of interest. A low endothermic reaction temperature allows the use of CSP solar receivers with low concentration ratios, with consequent advantages in costs and maturity. Even renewable energy sources, such as PV and wind power, could be integrated. In contrast, high-temperature exothermic reactions ensure high-exergy heat fluxes during the discharge phase. CH 3 OH decomposition is a promising candidate for TCES for the following reasons: (1) the endothermic reaction occurs with a high conversion rate at <350C, which involves a suitable integration with low-cost Fresnel or parabolic trough receivers when integrating the energy storage system in CSP plants; (2) the exothermic reaction (energy release) occurrs at high temperature (>500C); (3) once CH 3 OH-derived syngas is stored, several routes can be followed depending on operation strategies: syngas to CH 3 OH, syngas to CH 4 , or syngas to CH 4 to CH 3 OH; (4) catalysts are well known at both laboratory and industrial scales 18 ; (5) well-known plug-flow packed-bed reactors can be used for the reactions; (6) unlike gas-solid TCES, the system is more straightforward, avoiding the use of immature systems such as solar particle receivers, heat exchangers, or high-temperature lock hoppers; (7) a high enthalpy of reaction, which, in addition to the possibility of storing the products in liquid and pressurized gas, guarantees a high energy density of the system; (8) CH 3 OH is clean and cheap, and it can be stored and delivered conveniently 19 ; (9) fast kinetics to rapidly absorb solar energy and release the stored heat 20 ; (10) the streams of gas and liquids can be handled with well-known commercial technologies; and (11) no side reactions occur if the reaction temperatures lock. According to IRENA, 21 current CH 3 OH production will increase from 100 to 500 Mt by 2050, of which 385 Mt will be produced from renewable energy. On the contrary, the prospects for natural gas tend to increase very smoothly, 22 mainly by its widespread use in high-temperature heating processes. Therefore, the proposed conversion in this article is of economic and environmental interest for the predicted 2030–2050 scenarios, offering a pathway to keep up with gas demand in the future through its production using renewable CH 3 OH. Several recent studies have analyzed CH 3 OH TCES systems at the laboratory scale or based on process simulations. A comprehensive review of reactions and methods to produce syngas from CH 3 OH was carried out by Garcia. 23 A detailed kinetic model for CH 3 OH synthesis was carried out on a commercial Cu/ZnO/Al 2 O 3 catalyst bed. 24 Bai 17 evaluated the technical feasibility of a 20 kW CH 3 OH-based TCES system, including a remodeled parabolic trough (filled with Cu/ZnO/Al 2 O 3 as a catalyst) used as an endothermic reactor in which the fed gas, CH 3 OH, is successfully decomposed in the receiver at 3–5 bar and also an internal combustion engine (ICE) is directly fed with the syngas produced. In this work, a larger-scale (1 MWe) system with the combustion of the produced syngas was also simulated, estimating annual energy efficiency values without solar side losses of 33.78%. The same concept was tested on a 100 kWe prototype scale by Liu 25 where the CH 3 OH conversion rate exceeded 80% and a solar-to-electric efficiency of 24.73% was achieved. In these configurations, a high-temperature exhaust gas exiting the ICE could be used to preheat CH 3 OH entering the reactor or for other high-temperature applications, that is, to produce cooling and heating energies through a double-effect LiBr-H 2 O absorption refrigerator. 26 CH 3 OH would also be produced using H 2 and CO in a solar thermal cycle of cerium oxide. 27 Hong 28 proposed the combustion of CH 3 OH decomposition products in an open syngas-fueled combined cycle with a net solar-to-electric efficiency of 35%. The performance of hybrid thermochemical solar PV systems operating in a full solar spectrum, taking ultraviolet-visible light photons for direct solar PV production and ll OPEN ACCESS 2Cell Reports Physical Science 4, 101357, April 19, 2023 Article the visible-infrared spectrum for thermochemical decomposition of CH 3 OH, is a relevant research line with solar-to-electric efficiencies of up to 38%–45%. 20,29–33 This article proposes a novel TCES process configuration based on different routes to obtain CH 4 and CH 3 OH based on endothermic and exothermic reactions. It is based on a novel integration of reactions studied and experimentally validated in the literature. The system can operate in an open loop or as a closed-cycle TCES system. CH 4 , the main component of natural gas, is used in various industrial processes ranging from heating to electricity generation and as a raw material in the chemical industry. 34 CH 3 OH is widely used as a feedstock to produce other chemicals. 21 Also, it is considered fuel for ICEs 35 and biodiesel production. Comparing CH 3 OH and CH 4 as fuels, CH 4 presents a higher energy density (50 MJ/kg CH 4 > 19.9 MJ/kg CH 3 OH 36 ) and a higher industrial maturity. On the other hand, the costs associated with transporting CH 4 are higher due to a lower energy density (0.0364 MJ/L CH 4 < 15.6 MJ/L CH 3 OH 36 ) than CH 3 OH, which is liquid under ambient conditions, and it can have favorable characteristics for transport applications and regarding product storage The concept provides different pathways for energy use, either as the feedstock for natural gas, direct heat, and power supply or stored in chemical bonds. This flexibility in adapting the operation of the system to different energy availability and energy needs makes the novel concept highly interesting. This article presents the concept by integrating concentrated solar energy for CH 3 OH decomposition (route 1). Renewable electricity, from PV or wind power, is considered for the compression of the syngas produced to achieve an optimum high pressure in the methanation reaction. High-pressure syngas is transferred to the methanation reaction, which releases heat at high temperatures, 28 comprising route 2b. The high-temperature stream at the methanation reactor outlet can be used in an expansion process that can be used to reduce the required compression work by 75%. The CH 4 stream removes the H 2 O and is a commodity used in industries. The heat released at high temperatures in the methanation reaction can be integrated with a power system. This high versatility of the direct route allows the integration and operation of the system to be adapted to different external and energy availability requirements. The conversion of CH 4 to CH 3 OH closes the cycle to provide additional flexibility if required by the operation. This article is structured as follows. First, a description of the proposed systems is given, describing the routes and details of the conversion reactions to syngas from CH 4 and CH 3 OH, after which the synthesis processes of such syngas and possible commercial catalysts and reactors are discussed. An analysis of the conversion yields for both routes is carried out, and the overall energy yields are presented, considering the charging and discharging stages. Finally, the analyses associated with the reaction temperatures of each process and the reaction pressure are carried out, proposing a techno-economic optimization for decision-making on the operating parameters of the plant. RESULTS AND DISCUSSION System description The system uses renewable heat from CSP for the endothermic decomposition of CH 3 OH into syngas. There are several alternatives to perform decomposition, although all occur at atmospheric or slightly pressurized pressure (<10 bar) and at temperatures generally below 350C, 37 which guarantees optimal integration with medium-temperature solar systems. The present work evaluates the TCES system in open-loop operation mode. In an open-loop mode, CH 3 OH is decomposed ll OPEN ACCESS Cell Reports Physical Science 4, 101357, April 19, 2023 3 Article into syngas (route 1 in Figure 1) and later converted to CH 4 (route 2b; syngas or CH 4 can be stored, providing flexibility to the system). In the open-loop operation, CH 3 OH is fed constantly to the system, which produces heat, electricity, and CH 4 in different stages. This approach was proposed in the patent by Chacartegui. 38 The closed-loop operation mode involves the regeneration of CH 3 OH after energy storage occurs (syngas storage), either directly from the syngas (route 2a in Figure 1) or through the path syngas-CH 4 -CH 3 OH (route 3-2a), which would avoid a constant supply of CH 3 OH to the system. This route adds flexibility to the system integration and operation modes if required due to system constraints. CH 3 OH to syngas The first step is the conversion of CH 3 OH to syngas, which stores energy. CH 3 OH thermochemical conversion can occur via different routes: direct decomposition of CH 3- OH, 17,25 steam reforming, 39 partial oxidation, 40,41 and auto thermal reforming. 42,43 CH3OH /CO +2H2DH298 =90:7kJ =mol (Equation 1) Direct decomposition of CH 3 OH is a simple process for carrying out the conversion, as it does not require the management of O 2 and steam. It usually occurs at temperatures lower than 350C 44 and has lower kinetics than steam reforming and oxidation. 23 According to Brown, 45 operating at 315C and ambient pressure, complete CH 3 OH conversion can be achieved using Pt/Al 2 O 3 -based catalyst. According to Bai, 17 CH 3 OH conversion is around 95% for a solar flux using a Cu/ ZnO/Al 2 O 3 porous catalyst. The higher the pressure (up to 20 bar), the lower the temperature to reach 100% conversion to CH 3 OH. A complete conversion would be obtained at atmospheric pressure and 315C. 17 Using palladium supported on a ceria-zirconia catalyst, a complete conversion could be achieved at 200C. 46 Plug-flow packed-bed reactors are usually proposed to perform the decomposition. Several works have proposed a packed bed-like reactor inside the parabolic trough receiver to improve solar integration to carry out endothermic reactions. 17,20,26,47 Figure 2 illustrates the proposed conceptual scheme for the conversion of CH 3 OH to syngas. Decomposition can be obtained using CSP at 300C within a parabolic Figure 1. CH 3 OH-to-CH 4 -analyzed routes A schematic outlining the routes for CH 3 OH-to-CH 4 via syngas are outlined, connecting the interplays between each process. ll OPEN ACCESS 4Cell Reports Physical Science 4, 101357, April 19, 2023 Article trough configuration. The solar field would indirectly provide the heat required to keep the fixed bed reactor at 300C and produce the endothermic decomposition of CH 3 OH. CH 3 OH enters the reactor as a compressed liquid driven by pump P1, and the reaction occurs at 10 bar to reduce the energy consumption in compressor C1. A part of the heat provided by the solar collector will be used to increase the temperature of CH 3 OH and for its evaporation (170C). The complete conversion is assumed at the design operation. The syngas produced is further compressed to 40 bar for storage. A renewable electricity-driven intercooler compression (C1) is considered. Thus, a CSP-PV hybridization is considered for the energy storage step. If the reaction is not completed under off-design conditions, a flash separator will return the unconverted fraction of CH 3 OH to the reactor in a closed loop. Before entering the endothermic reactor, CH 3 OH (stream m30) is preheated in HXA using the hot products (s1) that exit the receiver after the reaction takes place (Figure 2). Syngas to CH 4 A relevant novelty of the concept is the approach to converting syngas to CH 4 (route 2b). Therefore, instead of using syngas directly for power conversion or CH 3 OH production for energy storage (route 2a), the stored syngas can be upgraded to CH 4 for various applications. Depending on the application (CH 4 production or energy storage), it is possible to find the best trade-off between the efficiency of renewable energy storage (CSP, PV, wind) storage efficiency and the quality and quantity of syngas or CH 4 produced. This section evaluates the conversion of syngas to CH 4 .Figure 3 shows the conceptual scheme for the transformation of syngas to CH 4 . CO +3H2/CH4+H2ODH298 =206 kJ=mol (Equation 2) Exothermic methanation converts syngas into CH 4 (Equation 2). Syngas methanation is typically carried out at temperatures between 250Cand700 C, 48 predominantly in fixed-bed reactors. 49 The energy discharge stage by this route (2b) begins the flow of syngas from the storage (stream s13) to send it to the methanation reactor. Energy can be recovered with a turbine in the outlet stream through the HXF and HXE preheaters (Figure 4). During the exothermic methanation reaction, the heat previously stored in chemical form is completely released at a high temperature (approximately 700C) in an adiabatic reactor. 50 The gas exiting the reactor (stream c1) removes the heat released in methanation and drives a gas turbine to produce electricity (turbines T3–T4) that → Figure 2. Conceptual scheme of the conversion of CH 3 OH to syngas Scheme outlining the route for CH 3 OH to syngas as discussed in the text, where m is CH 3 OHandsissyngas(CO/H 2 ). ll OPEN ACCESS Cell Reports Physical Science 4, 101357, April 19, 2023 5 Article canbeusedtopowercompressionontheCH 3 OH-to-syngas route, reducing the required renewable electricity. The CH 4 produced (c8) is stored, and it could be later used as direct feedstock for other industries, for direct combustion in high-efficiency power blocks (i.e., combined cycles), or within the thermochemical system for energy storage, evolving again to syngas (route 3) or CH 3 OH (3-2a). In the direct CH 4 combustion case, the overall plant operates in an open cycle, in which the raw material is pure liquid CH 3 OH under ambient conditions, and the CH 4 is ultimately burned for power production. The advantage of using CH 4 as a direct feedstock for other industries or to generate power is the use of already existing commercial technologies without requiring additional changes. Model of the system The proposed models are simulated in the commercial software EES 51 and the integration of the NIST-JANAF thermochemical tables. 52 The sum of all species and their respective chemical potential is shown in Equation 3, which expresses the value of the Gibbs free energy of the current in each case. 53 → Figure 3. Conceptual scheme of syngas-to-CH 4 conversion Outline of the process for syngas converting to CH 4 , where C is CH 4 ,sissyngas(CO/H 2 ), and w is H 2 O. AB Figure 4. Mole fractions as a function of reaction temperature Graphs depicting (A) CH 3 OH decomposition (route 1) and (B) methanation (route 2b). ll OPEN ACCESS 6Cell Reports Physical Science 4, 101357, April 19, 2023 Article G=X n k nkmk(Equation 3) Analytical modeling involves the minimization of the objective function, where a conservation matrix C, a vector nof constituents, and the vector bof molar quantities are expressed at the output. min GðnÞCTn=b;nkR0ck(Equation 4) In this case, the concept of Lagrangian is applied to solve the previous problem, where ljis the Lagrange multiplier. Where NC is the number of components, arefers to the phase and Uto the total number of phases. L=GXNC jlj"XU aXNa kckjna kbj#(Equation 5) The equilibrium constant for the corresponding reactions is calculated from the Gibbs free energy of the chemical species in each case from Equation 6. lnkeq=DG RT (Equation 6) Several considerations assumed for both configurations are shown in Table 1.All equipment is known in the industry and can be modeled with reasonable accuracy from zero-dimensional (lumped volume) approaches. 54,55 Models with more detailed kinetics and higher resolution may give more accurate results. 56 The definition of the performance of the system and processes yields a global conversion efficiency, hg,asdefinedinEquation 7: hg= mdis oXkLHVk+PiWT i+P i Qex mch iLHVj+P i WC i+PiQendo =mdis oXkDhdis o;k mch iDhch j;i ;(Equation 7) where LHV represents the lower heating value of the fuel at the inlet of the charge step, Dho/Dhiis the energy contained in the outlet/inlet stream, Wx iis the compressor (C) and turbine (T) power, and Qiis the heat of the reaction. The conversion efficiencies hkof any chemical species k,wherejis the chemical specie at the reactor inlet (Equation 8): hk=mk oXkDhk o mj iDhj i ;(Equation 8) Table 1. Thermodynamic considerations for the proposed conversion routes, according to the charge or discharge step Step Variable Value Endothermic CH 3 OH/CH 4 inlet molar flow of the load process 100 mol/s CH 3 OH storage temperature/pressure 64.67C/1 bar syngas CO/H 2 storage pressure 40 bar heat exchangers approach temperatures 20 K isentropic efficiency of liquid CH 3 OH pump 65% isentropic efficiency of compressors 89% Exothermic CH 3 OH synthesis conversion 80% discharge pressure 1 bar energy storage time 10 h heat exchangers approach temperatures 20 K isentropic efficiency of turbines 92% ll OPEN ACCESS Cell Reports Physical Science 4, 101357, April 19, 2023 7 Article where the mass fraction Xkof the component kat the outlet of the corresponding reactor is considered. The charge hch and discharge efficiencies will be given by the amount converted in each phase between the required or exothermic energy in the processes involved (Equations 9 and 10): hch = mch o1Xch iDhch o mch iDhch i (Equation 9) hdis = mdis o1Xdis iDhdis o mdis iDhdis i ;(Equation 10) where Xch iand Xdis iare the mass fractions of the components at the inlet of the charging and discharging processes, respectively. Economic models are subject to the evolution of both component and product markets. The approximations used are based on widely used formulations for estimating early-stage technologies. The economic analysis is carried out based on the expressions shown in Table 2, which have been included in the EES programming code. Global economic indicators are proposed, such as the levelized cost of storage (LCOS). The LCOS has been defined considering as values of the parameters a discount rate (r) of 5% and a useful life of the plant (n) of 20 years, considering the energy available Qdis iat the exit of the discharge step of both processes: LCOS = CAPEX +P n i=1 OPEXi ð1+rÞi P n i=1 Qdis i ð1+rÞi :(Equation 11) CAPEX is the initial investment cost of the plant, OPEXiis the annual maintenance cost, and Qdis iis the energy produced in the turbines in the exothermic phase of the proposed systems. On-design values The evaluation of the efficiencies described above is shown in Table 3 for the proposed operating parameters, which arise from the techno-economic optimization of the processes involved. The efficiency results are higher in the endothermic phase than in the exothermic phasebecauseofthecompleteconversionofCH 3 OH and a stoichiometric defect in methanation. In any case, the proposed overall efficiency of the system as a storage system reaches 19% for its cost-benefit optimization and 22.2% for Table 2. Expressions of the economic analysis of the installation Equipment Expression Reference Compressors ICC=643:15$_ WC 0:9142 Carlson et al. 57 Turbines ICT=9923:7$_ WT 0:5886 Carlson et al. 57 Pump ICP=3531:4$_ WP 0:71 $1+10:8 1hi;P3Michalski et al. 58 Heat exchangers CHE =2546:9$A0:67 HE $p0:28 HE $106Michalski et al. 58 Endothermic reactor ICDr =13140$_ Qr 0:67 Michalski et al. 58 Exothermic reactor ICMr =19594$_ Qr 0:5Tesio et al. 59 Tanks ICTank =83$VBayon et al. 60 ll OPEN ACCESS 8Cell Reports Physical Science 4, 101357, April 19, 2023 Article