Analysis of a thermochemical energy storage system based on the reversible Ca(OH)2/CaO reaction

Depósito de investigación de la Universidad de Sevilla https://idus.us.es/ This is an accepted manuscript of a paper published by Elsevier in Energy, Vol. 261 Part A, available at: https://doi.org/10.1016/j.energy.2022.125064 © 2022 Elsevier. En idUS Licencia Creative Commons CC BY-NC-ND Analysis of a thermochemical energy storage system based on the reversible Ca(OH)2/CaO reaction A. Carro a*, R. Chacartegui a,c, C. Ortiz b. J.A. Becerraa,c a Dpto. Ingeniería Energética. Universidad de Sevilla, Camino de los Descubrimientos s/n, 41092, Sevilla, Spain b Materials and Sustainability Group, Department of Engineering, Universidad Loyola Andalucía, Avda. De las Universidades s/n, 41704 Dos Hermanas, Seville, Spain c Universidad de Sevilla, Laboratory of Engineering for Energy and Envorinmental Sustainability, 41092 Seville, Spain * Corresponding author Abstract The development of novel energy storage technologies is crucial for the massive deployment of large-scale renewable energy systems. This paper presents the conceptual study of an integrated system for the large-scale storage of solar thermal energy in the form of thermochemical energy based on calcium hydroxide. Calcium oxide-based storage media are very promising because they are an abundant and inexpensive resource with high energy density and the possibility of storage under ambient conditions. The equilibrium temperature of the reaction is around 500°C, so the integrated system operates in a temperature range similar to that of concentrating solar power plants and molten salt storage, being able to reach higher temperatures in power production depending on the partial pressure of the steam generated in the reaction. The study discusses the technological challenges of the system, highlighting the importance of recovering the latent heat from condensation of the steam generated in the dehydration reaction, which represents 38% of the solar thermal energy entering the reactor during the charging phase, and proposes recovery mechanisms. The analysis shows the potential competitiveness of the technology as a large-scale energy storage system. By considering the full recovery of the latent heat of the steam, the system achieves a round-trip thermal efficiency of more than 80%, an overall thermal-to-electrical efficiency of 40% and LCOE values of 100 USD/MWhe. Keywords Thermochemical Energy, Energy Storage, Calcium Hydroxide, Calcium Oxide, CSP. Nomenclature Subscripts e/ele Electric th Thermal Nomenclature and letters CAES Compressed Air Energy Storage CaH Calcium hydroxide based storage system CaL Calcium-Looping. CaCO3 Calcium Carbonate - Limestone CaO Calcium Oxide – Quick lime Ca(OH)2 Calcium Hydroxide – Slaked lime CO2 Carbon Dioxide CSP Concentrated Solar Plant ΔHr Reaction enthalpy EES Engineering Equation Solver EPC Engineering Procurement and Construction H2O Water HTF Heat transfer Fluid HX Heat Exchanger LCOE Levelised Cost of Electricity LHRS Latent Heat Recovery System NH3 Ammonia NPV Net Present Value O&M Operation & Maintenance PHS Pumped Hydro Storage PCM Phase Change Material ST Solid Transport T-Q Temperature-Heat T-s Temperature-Entropy TCES ThermoChemical Energy System TES Thermal Energy Storage TGA Thermogravimetric Analysis TIT Turbine Inlet Temperature 1. Introduction Energy storage systems continue to grow in importance with the steady increase in renewables within the framework of the global energy transition [1]. The different forms of renewable energy and their applications and, above all, the mismatch between resource availability and power demand highlight the need for new energy storage technologies [2]. Low-cost and low-impact sustainable energy storage systems are required to improve the dispatchability of renewable energy systems [3]. Among the energy storage technologies commercially available on the market, the electrochemical battery may be the most widely distributed, with a price of 100-1000 USD/MWhe [4] and a discharge capacity limited to approximately 40 MW [5]. On a large scale, there are few options available on the market. Pumped Hydropower Systems (PHS) and Compressed Air Energy Systems (CAES), electrical energy storage systems with a discharge capacity that can reach in the order of GW [6,7], are the most developed and widely used storage options, but both have constraints in terms of location [6]. They require large extensions or certain land formations and can have a large associated environmental impact [8]. They are priced around 120-210 and 350-400 USD/MWhe [4], respectively. In terms of thermal energy storage systems, the currently commercially available option is the molten salt system [9]. Molten salt technology can be found in the majority of Concentrated Solar Power (CSP) plants with storage, with a low price of 10-13 USD/MWhe [4,9]. However, it has major limitations in terms of operating temperature, lower than 600ºC due to salt degradation [10], and drawbacks in operation and maintenance [11]. Thermochemical energy storage (TCES) systems are emerging as an alternative to molten salt systems [12]. Thermochemical storage systems use chemical reactions to absorb/produce heat in a process that typically has two parts [13]. In the charging phase, heat from a thermal source, such as a CSP plant, is used to perform an endothermic reaction, and the products of that reaction are stored separately. In the discharge phase, the previously stored products are used to trigger an exothermic reaction, in which a large part of the previously stored heat is recovered, which can be used in a power block to produce electricity [14]. Among the thermochemical storage technologies, the one based on Calcium-Looping (CaL) stands out [15]. CaL is a type of thermochemical storage system based on a reversible solid-gas reaction [16], which uses calcination/carbonation of CaCO3/CaO- rich (calcium carbonate/calcium oxide) solids stream [17]. As an energy storage system, it has recently been tested in a relevant environment, achieving a Technological Readiness Level of 5 (TRL-5), through the SOCRATCES project [18,19]. The CaCO3/CaO equilibrium temperature at atmospheric pressure is around 900°C [20], which allows the production of power at high temperature and high efficiency, which can reach values of 45% [21]. However, it still presents technological challenges, such as the multicyclic deactivation of CaO, which limits the conversion rate [22]. Some steps behind in terms of development is the storage system based on the reversible Ca(OH)2/CaO (calcium hydroxide/calcium oxide) dehydration/hydration reaction [23], which is the focus of this work. In this thermochemical storage system, which also consists of a reversible solid-gas reaction [16], during the charging phase, thermal power from an external source is used to trigger the endothermic dehydration reaction of calcium hydroxide (Ca(OH)2). In the discharging phase, heat is recovered by the exothermic hydration reaction of calcium oxide (CaO) obtained in the dehydration process [24], as described in Equation 1. 𝐶𝑎(𝑂𝐻)2(𝑠)+∆𝐻𝑟 ↔𝐶𝑎𝑂(𝑠) + 𝐻2𝑂(𝑔) (1) The equilibrium temperature of the reaction (Equation 1) at atmospheric pressure is around 500°C [25], a temperature that is easy to reach in CSP plants [26] and which can be exploited in existing high-impact electrical and industrial thermal power plants, which operate at temperatures of the order of 300-500°C. Higher discharge temperatures are possible depending on the partial pressure of the steam in the reaction [27]. Being able to deposit reaction products at room temperature [28], the calcium hydroxide-based system (CaH) allows long-term storage and has a significantly higher energy density than other thermal energy storage systems, such as phase change materials (PCMs) [29] or sensible heat storage systems [30]. The energy storage density using calcium hydroxide as a storage medium is estimated to be approximately 100 kWhel∙t-1 [31]; this value is about seven times higher than that of existing solar thermal power plants with sensible heat storage using molten salts (approximately 13.1 kWhel∙t-1) [31]. In terms of raw material, limestone is used as a CaO precursor, a widely available, cheap (10$/tonne) and non-polluting material [15]. This paper presents an integrated system for large-scale solar thermal energy storage based on the reversible dehydration/hydration reaction of the Ca(OH)2/CaO system. The study discusses the technological challenges of the system, highlighting the importance of recovering the latent heat of condensation of the steam generated in the dehydration reaction, which represents 38% of the solar thermal power that reaches the reactor. Extreme cases where all latent heat is recovered and where this heat is given up to the environment are analysed, and different recovery mechanisms are proposed, such as an ammonia Rankine cycle or pressurised steam storage. The analysis shows that the technology can be the basis for competitive energy storage, with thermal efficiencies between 75-98%. Overall thermal-electrical efficiency of 40% and Levelized Cost of Electricity (LCOE) value of about 100 USD/MWhe are reached by considering the full recovery of latent heat from steam. The structure is as follows. After the introduction to the thermochemical storage system based on calcium hydroxide technology, a section is dedicated to describing the characteristics of the chemical reactions involved in the process (Ca(OH)2 dehydration and CaO hydration). Experimental studies that have investigated the characterisation of the reaction are presented. This is followed by a description of some of the main studies on the integration of storage systems using calcium hydroxide technology for energy storage in large-scale plants. Subsequently, a section is dedicated to describing the modelling characteristics of the TCES system integrated in a CSP plant, which is analysed afterwards. It has a subsection focusing on the charging phase, another on the discharging phase and a third on the details of heat recovery in the system. The analysis and results are then presented. The work ends with an economic analysis and conclusions. 2. Dehydration/hydration reaction of Ca(OH)2/CaO This section is devoted to introducing the reactions involved in the TCES system in more detail. It describes aspects such as the enthalpy of the reaction, the equilibrium curve of the system, conversion rates, and temperature and pressure conditions. The process begins with the dehydration of Ca(OH)2 (slaked lime), in which the solid reactant decomposes while absorbing heat at a temperature slightly above 500°C [25] yielding solid CaO (quicklime) and steam (Equation 1). These products are cooled and stored separately under atmospheric conditions. When heat recovery is desired, the previously stored CaO is used in solid form, which reacts in the presence of steam to form solid calcium hydroxide again in the exothermic hydration reaction, releasing heat at a temperature close to 500°C if the process takes place at partial steam pressures of 1 atm [25]. The reaction enthalpy (Equation 1) value has been characterised in a range from 94.6-109.2 kJ/mol in studies such as those by Samms and Evans [32] and by André and Abanades [16]. In this study, an intermediate value of 104.2 kJ/mol is considered, widely used in other works as [25]. The equilibrium curve determines the theoretical limit at which the endothermic or exothermic reaction occurs, taking into account that reaction conditions too close to the equilibrium curve should be avoided in practice in order for reactions to proceed faster [33], which reduces the residence times and the size of the reactors. The most common way to approximate equilibrium lines is through the equation of Clausius-Clapeyron [34], which is shown in equation 2. 𝑙𝑛(𝑝𝑒𝑞)=− ∆𝐻 𝑅∙𝑇𝑒𝑞 + 𝑐𝑡𝑒 (2) Equilibrium curves have been experimentally characterised by different authors, such as Barin [35] studying thermochemical data for pure substances, or Criado et al.[36], with papers about reaction kinetics, Schaube et al. [27,37], with a thermodynamic and kinetic study of the reaction, or Samms and Evans [32], studying the thermal dissociation of Ca(OH)2. Table 1 shows the equations of the equilibrium curves of the reactions studied by different authors. Table 1. Equilibrium lines of the Ca(OH)2/CaO system Author Ref Barin 𝑝𝑒𝑞[kPa]=2.3∙108∙exp(−11607 𝑇𝑒𝑞[𝐾] ) [35,36] Schaube 𝑝𝑒𝑞[kPa]=100∙exp(−12845 𝑇𝑒𝑞[𝐾] +16.508) [27] Samms 𝑝𝑒𝑞[bar]=exp(−11375 𝑇𝑒𝑞[𝐾] +14.574) [32,38] The processes have been characterised at different partial pressure values and temperatures. Schaube et al. [25] tested the hydration process in the range from 17.6 kPa to 95.6 kPa, with temperatures of 252°C for pressures far from equilibrium and temperatures in the range of 410- 452°C at pressures close to equilibrium. Risthaus et al. [39] performed tests with steam pressures between 8.7 kPa and 470 kPa. Schmidt et al. [40] studied the effect of temperature in the range 131ºC to 391 °C on hydration with a steam pressure of 8.7 kPa. Funayama et al. [41] performed cyclic tests ranging from 350°C to 490°C for dehydration and temperature and partial steam pressure conditions of 350°C and 57.9 kPa for hydration. Huang et al. [42] performed dehydration from 450°C to 550°C, while hydration was carried out at 250°C and 50 kPa steam partial pressure. Figure 1 shows a graph with tests by different authors and the characterisation of the equilibrium curve by different works. Figure 1: Equilibrium lines (Table 1) and pressure and temperature values of different tests Different conversion percentages are obtained for the reactions from the test conditions and the time considered. Schaube et al. [25] reached CaO conversion percentages of 100% after 120 seconds at a temperature of 252°C and steam partial pressure of 95.6 kPa, while at higher temperatures (432-445 °C), conversions of over 95% were achieved after 300 seconds and steam partial pressure values of 74.1 and 95.6 kPa. Furthermore, in their full-cycle tests, they maintain a conversion rate close to 100% at a temperature of 448 °C in the N2 atmosphere during dehydration. In hydration, complete conversion was reached after 120 seconds under conditions of 252°C and 95.6 kPa, while it was extended to 600 seconds when the temperature increased to 452°C. Schmidt et al. [40] achieved the best CaO conversion results at low temperatures (131- 209°C), reaching 100% before 200 seconds, while in their complete cycle tests, they were able to maintain the degree of Ca(OH)2 conversion above 90% from the first to the tenth test cycle. In the thermogravimetric analysis (TGA) experiments of Xia et al. [43], 70% Ca(OH)2 conversion in the reaction was achieved earlier by lowering the relative amount of Ca(OH)2 in the solids stream from 709 seconds for the Ca(OH)2 richest stream (90% wt%) to 618 seconds for the stream of 80% wt. Huang et al. [42] achieved complete conversions of Ca(OH)2 at a temperature of 550 °C and values above 80% for CaO at 250°C and 50 kPa. In this study, considering the high conversion rates published in the literature, full conversion is assumed for both reactions. Furthermore, considering the equilibrium curve of Barin [35] described in Table 1, and a 50°C jump in temperature between reaction and equilibrium temperatures (aimed at increasing reaction rate and decreasing reactor size [33]), dehydration and hydration temperatures of 569.2°C and 469.2°C, respectively, are assumed when the process is carried out at a partial steam pressure of 1 atm. The equilibrium temperature, according to Barin, at a partial pressure of steam of 1 atm is 519.2ºC. 3. Calcium hydroxide-based technology as an energy storage system This section delves into technology based on calcium hydroxide as a thermochemical energy storage system and subsequent thermal energy recovery and power production. Several works have been published at the laboratory level in which steam is stored/generated by cooling/heating from a reservoir external to the reactor. Funayama et al. [41] used a packed bed of Ca(OH)2 granules, in which steam is injected from a reservoir with a heater to evaporate liquid phase water for hydration and a cooler to condense the incoming steam for dehydration. Takham and Tippayawong [44] used a similar configuration but with a different tank for steam and water. The schematic used in these laboratory tests is shown on the left-hand side of Figure 2. Figure 2: Scheme of laboratory tests in the CaO/Ca(OH)2 hydration/dehydration reaction [41,44] (left) and conceptual scheme of the large-scale CSP-TCES integrated system (right). Focusing on large-scale energy storage, the thermochemical system would harness thermal energy from an external source, such as a CSP plant, to trigger the dehydration reaction in a Ca(OH)2- rich solids stream, generating CaO and steam as reaction products. The products, previously stored at room temperature, react in the hydration reaction, producing heat and forming Ca(OH)2 again, closing the storage/heat recovery cycle. The heat from the hydration reaction is used in a power block to produce electricity. A schematic of the CSP-TCES integration using the Ca(OH)2/CaO system is shown on the right-hand side of Figure 2. Some works that discuss the integrated CSP-CaH system for large-scale storage and its main aspects are mentioned below. Bayon et al. [45] present a techno-economic evaluation of solid-gas thermochemical storage systems for solar thermal energy applications, considering three plant models with a reference power of 100 MWe: a reference plant with molten salt storage, a closed gas cycle, and an open cycle. It assumes 100% conversion rates in the reactions and thermal power output at the receiver of 294 MWth. A wide range of thermochemical and molten salt storage systems are compared, with a storage capacity of 4 hours. In the Ca(OH)2/CaO system, charging and discharge temperatures of 700°C and 505°C are considered, respectively, achieving a thermal efficiency of 98.5% and a capital cost of 4.78 $/MJ, compared to 9.61-16.78 $/MJ for molten salts. Criado et al. [24] studied a thermochemical energy storage process based on CaO/Ca(OH)2 in a single circulating fluidised bed reactor coupled to large solid storage silos. For the proposed base case, with a maximum thermal output of 100 MWth during hydration, a thermal efficiency (thermal to thermal) of the process of 63% is achieved, with CaO and Ca(OH)2 storage volumes in the range of 390-460 m3 per operating hour and a conservative reference value of the hydration conversion of 60%. The steam generated in dehydration only releases part of its heat in preheating the Ca(OH)2-rich solids stream entering the reactor. Tapaches et al. [46] propose a CSP storage system based on the Ca(OH)2/CaO reaction, using a single fixed-bed reactor, depending on the mode of operation, charging or discharging, dehydration or hydration reaction takes place. In their study, an intermediate HTF is used to transfer the thermal power between the solar receiver, the reactor, and the power block. The power block uses a Rankine steam cycle with three turbine bodies. The steam generated in the dehydration reaction exchanges heats in the second superheater and the Rankine steam cycle preheater. During the charging phase, the heat in the Rankine cycle condenser provides heat to the thermochemical storage evaporator. The authors underline the importance of the recovery of this heat, which covers 23-26% of the total energy consumption of the power block so that thermochemical storage not only adds electricity production during the discharge phases but also enables higher electricity production during the storage phases. The economic analysis shows prices in the range of 12-60 €/MWh, depending on the location considered in the plant. Pelay et al. presented the energetic and exergetic analysis [31] and the LCA [47] of the CSPTCES integration with the Rankine cycle as a power cycle through the conceptual study of three different configurations based on a base case plant of a 100 MWel solar tower plant. The conventional regenerative Rankine cycle includes a steam generator, a turbine, a condenser, an open feed water heater, and pumps. Aspects such as water-steam management are highlighted as key issues for integrating the TCES in the Rankine cycle. The feasibility of integration is carefully determined by the flow circuit configurations and the operating conditions for the charging and discharging stages. The overall efficiency (thermal to electric) of the installation under ideal conditions is evaluated at 0.358 for the thermal concept, 0.349 for the mass concept, and 0.392 for the turbine concept. One of the key aspects of the energy storage and recovery process based on the Ca(OH)2/CaO reaction is the condensation of the steam generated during dehydration. The increase in the enthalpy of the vapour condensation at atmospheric pressure is 40.66 kJ/mol, while the enthalpy of reaction is around 104.2 kJ/mol. This is equivalent to almost 40% of the stored energy. Special care must be taken in the recovery of sensible and latent heat from these streams since the sensible heat that the solids stream entering/leaving the reactor can absorb/deliver, considering the temperature drop (from 25ºC to 500ºC approximately) is similar to the total sensible heat that can be delivered/absorbed by the solids stream and the steam leaving/entering the reactor, depending on whether the charging/discharging phase is considered. If the latent heat of the steam leaving the reactor during charging is not recovered, a non-negligible amount of energy would be delivered. This work performs a conceptual study of the integration of CSP-CaH for large-scale energy storage, in which different options for the recovery of the latent heat of condensation of the steam leaving the reactor during charging are proposed and compared, and power production is based on a supercritical CO2 Brayton cycle. 4. Integrated CSP-TCES system based on calcium hydroxide modelling In this section, the system configuration of the integrated CSP-CaH system for large-scale energy storage studied is presented. First, the common characteristics of the charging phase are shown, and second, the common characteristics of the discharging phase are shown. Finally, different latent heat recovery mechanisms are presented, depending on whether or not the heat of condensation is recovered during charging and how it is recovered. To perform system modeling, the commercial software EES (Engineering Equation Solver) [48] has been used to solve the mass and energy balances of the system presented in this section. Several databases have been accessed to calculate the properties of substances, both for water [49,50], solids [51,52], carbon dioxide (CO2) [53,54] and ammonia (NH3) [55]. The equations used to determine the energy balance in the thermal reactors and simulate their behaviour are the same as those published in other works in which TCES system models are presented, such as those of Chacartegui et al. [15], based on the reversible calcination/carbonation reaction of the CaCO3/CaO system, or that of Criado et al. [56] based on the same reversible reaction of the Ca(OH)2/CaO system, which is used in this work. 4.1 Charging phase Figure 3 shows the principle of operation of the system during the charging phase, where energy storage takes place in the CSP plant. Departing with a stream of Ca(OH)2-rich solids from a tank at ambient temperature (purple lines ), the stream is split between a solid-solid exchanger (HXS) and a solid-fluid exchanger (HXC). These exchangers raise the temperature of the solid stream entering the reactor, where the Ca(OH)2 decomposes into CaO and steam through the consumption of heat from solar thermal energy. The steam generated in the reactor (red lines) releases heat into the solid-fluid exchanger (HXC), and the CaO-rich solids stream leaving the reactor (orange lines) passes to the solid-solid exchanger (HXS). The solid-solid heat exchanger (HXS) is considered as two interconnected solid-gas heat exchangers, with an intermediate HTF bridging the heat transfer between the solids. Figure 3: System configuration in the charging phase When complete conversion does not occur in the reactors, both the solid stream entering and leaving the reactor are mixtures of Ca(OH)2 and CaO. CaO that enters the reactor does not react, although it does absorb heat and raises its temperature. At the exit of the reactor (orange lines), the stream is composed of the Ca(OH)2 mixture that has not reacted and the CaO in the inlet achieved. The proximity in the lines that characterise the phase change of the Ammonia Rankine cycle in a T-s diagram marks the efficiency of the process. Finally, a pressurised steam storage system is presented. It is not an LHRS since the change of state of the water does not take place. After losing sensible heat in HXC, the steam, with a temperature close to the saturation temperature, is compressed and stored. During the discharge phase, the previously stored steam expands in a turbine, recovering part of the compression work before entering the HXE exchanger. A schematic of the process is shown in Figure 8c. Only sensible heat is transferred in the exchangers, and steam phase change does not occur. A good integration in the heat exchanges is ensured. Furthermore, compression of the steam increases internal energy, which is later recovered in the expansion in the turbine. Main assumptions in the LHRS systems are shown in the Table 4 Table 4: Assumptions made in the modelling of LHRS systems. Equipment Description Units Value NH3 Rankine cycle Minimum temperature difference ºC 5 Pressurized Steam Storage pressure bar 40 Isentropic efficiency Compressors % 89 Turbines % 91 5. SYSTEM ANALYSIS AND RESULTS In this section, the effect of different parameters on the performance of the system is analysed. First, the analysis of the streams that compose the system is presented, showing in detail the heat exchanges and the results of the mass and energy balances. Second, an optimisation of the system efficiency is shown, using the parametric analysis of the mole fraction of solids circulating through each heat exchanger during the charging and discharging phases. Finally, the impact of the conversion rate in the reactors is analysed through the analysis of the different efficiency indicators defined in the previous sections. 5.1 Analysis of heat transfer and properties of the streams involved in the TCES This section shows an analysis of the characteristics of the energy storage system and heat and power exchanges in the components. Table 5 shows the properties of the currents forming the system during the charging phase. The values are maintained in spite of the different latent heat recovery system options, except for the steam stream when pressurised steam is used. Table 5. Charging phase streams. Ref Type Phase Temperature [ºC] Pressure [bar] Flow rate [kmol/h] Flow rate [kg/s] h1 Ca(OH)2 Solid 25 - 1537 31.62 h2 Ca(OH)2 Solid 84.6 - 1537 31.62 h3 Ca(OH)2 Solid 554.2 - 1537 31.62 h1’ Ca(OH)2 Solid 25 - 1720 35.25 h3’ Ca(OH)2 Solid 490.9 - 1720 35.25 h4 Ca(OH)2 Solid 520.8 - 3257 67.02 c1 CaO Solid 569.2 - 3257 50.73 c2 CaO Solid 45 - 3257 50.73 w1 H2O Steam 569.2 1 3257 16.03 w2 H2O Steam 164.7 (151.8)* 1 (40)* 3257 16.03 w3 H2O Sat. Water 99.6 (-)* 1 (-)* 3257 16.03 w4 H2O Water 47.1 (-)* 1 (-)* 3257 16.03 w5 H2O Water 35 (542)* 1 (40)* 3257 16.03 n1 NH3 Sat. Gas 94.6 56.2 - 50.79 n2 NH3 Gas 111.7 56.2 - 16.93 n3 NH3 Sat. Liquid 104.6 68.4 - 16.93 n4 NH3 Biphase 94.6 68.4 - 50.70 * Pressurised steam conditions when considered as an LHRS In HXC heat exchange, where the steam leaving the reactor is condensed in different stages, saturated steam is not achieved at the exit of the first cooling stage. The rest of the heat is released in the condenser. Figure 9 shows the T-Q diagrams of the heat exchange of the charging phase. Figure 9: T-Q diagrams of the heat exchange of the charge phase for conversion rates of 100% in both reactors: left) HXS and right) HXC. Figure 9-right shows the significant difference between sensible heat and latent heat of the HXC heat exchanger. It highlights the vital importance of the latent heat recovery in the storage system based on the Ca(OH)2/CaO reaction, which is more than half of the heat of the steam stream formed in the dehydration. Table 6 shows the different power values for the components in the charging phase, considering an operating period of 8 hours. Table 6: Power and balance in the charging phase over an operating period of 8 hours. Power [MW] LHRS Ideal Non Rank Steam Termochemical Reactor 100 100 100 100 HXC Precooling 21.08 21.08 21.08 18.4 HXC Condensator 36.79 36.79 36.79 0 HXC Postcooling 2.67 2.67 2.67 0 HXC 60.55 60.55 60.55 18.4 CT1 0.82 37.61 0.82 1.17 HXS 23.4 23.4 23.4 22.79 C1 - - 1.35 17.8 P1 0 0 0 0 ST1 0.56 0.56 0.56 0.54 ST2 0.74 0.74 0.74 0.72 Volume [%] Solids Storage 873 873 873 873 Water/Steam tank 470.7 470.7 470.7 37855 NH3 tanks 0 0 26413 0 Efficiency [%] Charging efficiency 95 95 95 92.5 As expressed in previous sections, the sensible heat in the heat exchanges is similar, which allows a good integration in the temperature profiles. The latent condensation heat corresponds to more than half of the energy available in the steam leaving the reactor, so it is not negligible. In the cases where the heat of condensation is not recovered, the cooling tower transfers this heat to the environment. This energy loss is not evaluated in the efficiency indicator of the load phase, but it will affect the discharge phase. In Rankine and pressurised steam systems, a large storage volume is required. The choice of these condensing heat recovery systems may limit the size of the storage plant. Based on the assumptions made in Table 2 for the charging phase and Table 3 for the discharging phase, the results for the discharging phase are presented. The different properties of the different currents are shown in Table 7. Table 7. Discharging phase streams. Ref Type Phase Temperature [ºC] Pressure [bar] Flow rate [kmol/h] Flow rate [kg/s] h5 Ca(OH)2 Solid 469.2 - 510.7 10.67 h6 Ca(OH)2 Solid 114.6 - 510.7 10.67 h7 Ca(OH)2 Solid 40 - 510.7 10.67 h5’ Ca(OH)2 Solid 469.2 - 566.9 11.67 h6’ Ca(OH)2 Solid 86.4 - 566.9 11.67 h8 Ca(OH)2 Solid 64.3 - 1086 22.34 c3 CaO Solid 449.2 - 1086 16.91 w6 H2O Steam 25 (542.2)* 1 (40)* 1086 5.43 w7 H2O Steam 90.34 (-)* 1 1086 5.43 w8 H2O Sat. Steam 99.6 (113.7)* 1 1086 5.43 w9 H2O Steam 406.7 (370.7)* 1 1086 5.43 w10 H2O Steam 406.7 (370.7)* 1 1086 5.43 w11 H2O Steam 464.2 (370.7)* 1 1086 5.43 w12 H2O Steam 469.2 1 1086 5.43 s1 CO2 Gas 35 148 - 335.3 s2 CO2 Gas 44.1 225 - 335.3 s2* CO2 Gas 399.1 225 - 335.3 s3 CO2 Gas 464.2 225 - 335.3 s4 CO2 Gas 414.7 148 - 335.3 s4* CO2 Gas 49.1 148 - 335.3 * Pressurised steam conditions when considered as an LHRS As in the charging phase, in HXE heat exchange, where the steam entering the reactor evaporates in different stages, no saturated liquid is obtained at the exit of the first heating stage. The rest of the heat is provided by the evaporator. Figure 10 shows the T-Q diagrams of the heat exchange in the charging phase. Figure 10: T-Q diagrams in the discharge phase heat exchange for 100% conversion rate in both reactors: left) HXF and right) HXE. Again, the important difference between sensible and latent heats at the heat exchanger, HXE, is shown. The evaporation heat is almost two thirds of all heat exchanged in HXE. When the heat of condensation is not recovered during charging, this energy is taken out of the reactor during unloading, losing part of the usable heat in the power block, as shown in Table 8, which shows the power values of the different equipment in the discharge phase. Table 8: Power balance in the discharging phase over an operating period of 24 hours and a pressure of 1 bar at the hydrator. Power [MW] LHRS Ideal Non Rank Steam Thermochemical reactor 31.14 31.14 31.14 30.33 HXE Preheating 1.11 1.11 1.11 4.3 HXE Evaporator 12.26 12.26 12.26 0 HXE Reheat 5.28 5.28 5.28 0 HXE 18.65 18.65 18.65 4.3 HXF 6.23 6.23 6.23 6.06 Thermal power to Power Block 27 14.7 27 25.9 T2 0 0 0.06 0.41 P2 0 0 0 0 ST3 0.18 0.18 0.18 0.19 ST4 0.25 0.25 0.25 0.24 Power Block Electricity 13.6 7.43 13.6 13.1 Volume [%] Solids Storage 873 873 Water/Steam tank 470.7 470.7 470.7 37855 NH3 tanks 0 0 3201 0 Efficiency [%] Discharging th efficiency 86.7 47.3 86.7 85.4 Global th efficiency 81 44.22 81 77.73 Electrical efficiency 38.21 19.7 37 32.14 When the condensation heat is released into the environment, the overall thermal efficiency of the system drops from 81% in the ideal system to 47.3% in the system where no heat is recovered. Latent heat recovery systems using the NH3 Rankine cycle and pressurised steam show high efficiency. The LHRS-Rankine system is very close to ideal recovery, with 37% overall electrical efficiency, compared to 38.21% for the ideal system. 5.2 Splitting of the solids stream The charging and discharging phases involve splitting the Ca(OH)2-rich solids stream entering or leaving the reactor. The chosen criteria to calculate the mole fraction of solids passing through each exchanger is an equal division of the difference between the estimated heat that could be absorbed by the total Ca(OH)2-rich solids stream and the sum of the sensible heat that could be absorbed by the CaO-rich solids stream and the steam. The temperature drop between the reactor and the storage is considered for the temperature increase in the calculation. In this section, the value of the mass fraction that optimises the process is studied by means of parametric analysis, considering the ideal LHRS. A parametric analysis of the molar fraction of solids passing through each heat exchanger is performed. Figure 11¡Error! No se encuentra el origen de la referencia. (up) shows the temperature variation in the heat exchangers HXS and HXC and the thermal efficiency in the charging stage with the mole fraction of solids flowing through the HXS exchanger, while Figure 11¡Error! No se encuentra el origen de la referencia. (down) shows the same analysis but in the HXF and HXC heat exchangers and the discharging thermal efficiency, where the mole fraction of solids passing through HXF varies. Figure 11: Sensitivity analysis of the molar fraction of solids for each exchanger in: up) charging; down) discharging; considering ideal conditions for the latent heat of condensation recovery. The two analyses show a peak efficiency close to 0.46 in the splitting of the solids stream. Here, the outlet temperature of the exchangers is practically the same, and the temperature of the Ca(OH)2-rich stream mixture reaches a maximum during charging or a minimum during discharging, indicating that the heat transfer is at a maximum. Under these conditions, the thermal efficiency of the charging and discharging phases reaches the values of 98.15% and 87.66%, respectively. The inlet temperature of the Ca(OH)2-rich solid stream entering the dehydrator reactor reaches a temperature of 551.9ºC, close to the 569.2ºC considered in the dehydration, reducing the heat absorbed in the reactor. In the discharge phase, the CaO-rich solid stream temperatures and the steam entering the hydrator reactor are 439.2ºC and 446.7ºC, minimising the sensible heat absorbed in the reaction to reach the considered reaction temperature of 469.2ºC. The overall thermal efficiency is 84.6%, while the electrical efficiency considering the supercritical CO2 Brayton cycle as power block reaches 39.94%. 5.3 Efficiency Indicators In order to analyse the efficiency of the charge and discharge stages and the overall efficiency of the plant, Equations 9 to 12, three parametric analyses are proposed, in which the conversion percentages vary and considering ideal conditions for the latent heat of condensation recovery. In the first analysis, the conversion percentages are the same in both reactors, varying between 50- 100%. In the second, the conversion percentage in hydration is 50%, while that of dehydration increases from 50-100%. The third analysis keeps the conversion rate in dehydration fixed at 50%, while in hydration, it increases from 50-100%. Table 9 shows values for the Ca(OH)2 molar fraction of the solids stream entering dehydration, the CaO mole fraction of the solids stream entering hydration and the efficiency of the charging, discharging and overall phase at three points in each analysis. Table 9: Efficiency as a function of conversion in reactions considering ideal conditions for LHRS. Dehydration Conversion Hydration Conversion Ca(OH)2 Composition CaO Composition Charging efficiency Discharging efficiency Global efficiency Electrical efficiency 1 1 1 1 95.0 86.7 81.0 38.2 0.8 0.8 0.83 0.83 94.3 87.1 80.7 36.8 0.6 0.6 0.71 0.71 93.1 82.2 75.3 32.0 0.6 0.5 0.62 0.75 92.8 78.7 71.8 28.0 0.8 0.5 0.55 0.89 93.6 79.6 73.3 31.4 1 0.5 0.5 1 94.1 80.2 74.2 32.6 0.5 0.6 0.75 0.62 92.5 81.5 74.1 30.5 0.5 0.8 0.89 0.55 92.8 85.4 78.0 33.3 0.5 1 1 0.5 93.1 84.5 77.4 33.5 In the charging phase, the efficiency is relatively stable at around 92-95%, obtaining the worst results when the conversion rate is low in dehydration. In discharge, efficiency values fall below 80% when the hydration conversion rate is low. The overall efficiency is a sum of the above effects, showing lower values when the reaction rate drops in hydration, as the drop in discharge efficiency is greater than that of the charging phase. Figure 12 shows the efficiency results of the three analyses as a function of the mole fraction of Ca(OH)2 in the initial solids stream. Figure 12: Efficiency map for conversion values in the range of 50-100% and pressure of 32 bar It shows how the percentage in the hydration reaction has a major impact on the efficiency of the plant. When the conversion rate in hydration is kept constant and equal to 50%, the lowest efficiency zone of the plant is obtained, with values that do not exceed 75% of the overall thermal efficiency. As the hydrate conversion rate increases, regardless of whether the hydrate conversion remains fixed at 50% or increases at the same rate as the hydrate conversion rate, the overall thermal efficiency increases to above 75 and stabilises at 77% when the conversion rate in the dehydration is 50%, and at 81% when the conversion rate of the two reactors is above 80%. The analysis shows how it is possible to achieve an overall thermal efficiency of almost 78% when the conversion rates are 50% and 80% in dehydration and hydration, respectively. 6. ECONOMIC ANALYSIS OF THE SYSTEM This section contains a brief economic analysis based on the preliminary approach of the investment and operating costs of the proposed concept. It is expressed in terms of the Levelized Cost of Electricity (LCOE) (eq. 13) [9], defined as the normalised cost of energy that provides a Net Present Value (NPV) equal to capital and operating expenses over the lifetime of the system. In this analysis, the integration of a steam cycle is considered responsible for power production. Table 10 shows the estimated capital costs for the main components of the plant. 𝐿𝐶𝑂𝐸= 𝐼+∑𝑂&𝑀𝑖 (1+𝑟)𝑖 𝑛 𝑖=1 ∑𝐸𝑖 (1+𝑟)𝑖 𝑛 𝑖=1 (14) Where I relates to investment costs (USD), O&M is the maintenance cost (USD/year), E corresponds to the annual energy produced and sold to the grid (MWh/year), r is the discount rate, and n is the considered lifetime. In this preliminary analysis, the ideal condensation heat recovery of the integrated CSP-CaH system is considered. The estimated investment cost of the equipment is summarised in Table 10. In the estimation of the investment cost of the reactors, the equations used by Michalski et al. [59] and Criado et al. [60] for circulating fluidized bed reactors are considered. For heat exchangers, the equations of Michalski et al. [59] and Tesio et al. [61] for gas-gas and fluid-gas heat exchangers are used. Solid-solid heat exchangers consist of two solid-gas heat exchangers. Table 10: Estimated capital costs of the main components of the plant. Component Scaling parameter Capital cost (IC) in MUSD Ref Solar field Heliostats field area [m2] 𝑐𝑆𝐹 𝑢=3856∙𝐴ℎ𝑒𝑙𝑖𝑜𝑠𝑡𝑎𝑡𝑠−0.2976 𝐼𝐶𝑆𝐹 𝐻𝑒𝑙𝑖𝑜𝑠𝑡𝑎𝑡𝑠 =𝑐𝑆𝐹 𝑢∙𝐴ℎ𝑒𝑙𝑖𝑜𝑠𝑡𝑎𝑡𝑠 [62,63] Solar tower and receiver Receiver design thermal power [kWth] 𝐼𝐶𝑡𝑜𝑤𝑒𝑟 𝑠𝑜𝑙𝑎𝑟 =57.07∙𝛷𝑅𝑒𝑐𝑒𝑖𝑣𝑒𝑟 [4] 𝐼𝐶𝑐𝑙𝑐 𝐶𝑃𝐶 =44.91∙𝛷𝐶𝑃𝐶 Reactors: Dehydrator and hydrator Thermal Power (kWth) 𝐼𝐶𝑐𝑙𝑐 𝐶𝐵𝐹 =13140∙𝑄󰇗𝑟𝑒𝑎𝑐𝑡𝑜𝑟0.67 [59,60] 𝐼𝐶𝑐𝑎𝑟𝑏 𝐶𝐵𝐹 =85900∙𝑄󰇗𝑟𝑒𝑎𝑐𝑡𝑜𝑟0.5 Solid-Fluid heat exchanger UA [W/K] and Pressure [bar] 𝐼𝐶𝐻𝑋 𝑠𝑜𝑙𝑖𝑑−𝑓𝑙𝑢𝑖𝑑 = 18.48∙𝐴𝐻𝑋 0.67∙𝑝𝐻𝑋 0.28 [59,62] Fluid-fluid heat exchanger Area [m2] and Pressure [bar] 𝐼𝐶𝐻𝑋 𝑓𝑙𝑢𝑖𝑑−𝑓𝑙𝑢𝑖𝑑 = 3197 ∙ 𝐴𝐻𝑋 0.67∙𝑝𝐻𝑋 0.28 [59,62] Cooling towers Thermal power (kWth) 𝐼𝐶𝐶𝑜𝑜𝑙𝑖𝑛𝑔 𝑡𝑜𝑤𝑒𝑟 = 32.3∙𝑄󰇗 [59,64] Large storage tanks Volume (m3) 𝐼𝐶𝑡𝑎𝑛𝑘 𝑙𝑎𝑟𝑔𝑒 =83∙𝑉𝑡𝑎𝑛𝑘 [45] sCO2 Compressor Power [kW] 𝐼𝐶𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑜𝑟 𝑠𝐶𝑂2 =7331∙𝑊󰇗𝐶𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑜𝑟0.67 [61,65] sCO2 Turbine Power [kW] 𝐼𝐶𝑡𝑢𝑟𝑏𝑖𝑛𝑒 𝑠𝐶𝑂2 =8279∙𝑊󰇗𝑇𝑢𝑟𝑏𝑖𝑛𝑒0.684 [61,65] Electric generator Gross power output [kW] 𝐼𝐶𝐸𝐺 =84.5∙𝑃𝐺 0.95 [59,61] For the CSP plant, the methodology described by Ho et al. [4] is used for the estimation of the tower and the solar receiver. It is considered an efficiency of the 50% in the solar receiver. The cost of the solar field is based on an estimate of the heliostat area required to achieve the thermal power in the reactor, assuming a design point of 850 W/m2 [66] and an efficiency of 60% for the solar field. For the power block, the methodology described by Tesio et al. [61] and Carlson et al. [65], in the estimation of power equipment using supercritical CO2 as working fluid, is used. The total investment cost is the sum of the module/equipment costs. Labour and facility support costs are 20% of the equipment investment cost, while engineering services are at 4.5% when applying these to form the Engineering, Procurement and Construction cost (EPC). Contingencies are estimated at 10% of the EPC. Table 11 summarises the costs taken into account in the calculations. Table 11: Summary of the plant cost estimate Units Storage capacity h 24 Energy generated MWh/day 354.6 Investment cost Solar receiver M USD 4.49 Solar tower M USD 5.71 Solar field M USD 32.71 Heat exchangers & Reactors M USD 46.32 Tanks M USD 0.19 Power Block M USD 12.25 Total M USD 101.6 Additional cost Labour M USD 20.32 Facility Support M USD 20.32 Engineering services M USD 6.4 Contingencies M USD 14.86 Owner M USD 3.27 O&M costs Fixes M USD/year 0.1 Variable M USD/year 0.21 O&M costs M USD/year 0.31 Total Investment cost M USD 166.8 LCOE USD/MWh 106.2 Compared to other technologies, the normalised cost of energy falls within a competitive range since it is 100-130 USD/MWhe for molten salt systems [4], 150-220 USD/MWhe for pumped hydro [4], 120-210 USD/MWhe or for CAES [4]. For thermochemical storage systems, studies evaluated the cost of the Calcium Looping process at 70 USD/MWhe [15]. 7. CONCLUSIONS In this work, an integrated CSP-TCES system based on the hydration/dehydration reaction of calcium oxide is analysed. Solar thermal energy triggers the dehydration reaction in a calcium hydroxide stream, storing calcium oxide and water separately. The calcium oxide hydration reaction is triggered when the stored energy is recovered, releasing the previously consumed heat. A model in EES has been developed to analyse the system. The equations and operating conditions were established on the basis of existing literature, studies, and tests on the material. With the system configuration adopted, recovery of the thermal energy available in the products leaving the reactors by the reactants entering is vital to minimise the reactor energy required by the reactants to increase their temperature to reach the reaction temperature. In each phase, the sensible heat of the Ca(OH)2-rich solids stream is similar to the sum of the sensible heat of the CaO-rich solids stream and steam. The criterion chosen in the heat exchanges between these streams is that of only sensible heat exchange, so that the heating/cooling of the steam is carried out in three stages. In this way, the reactants enter the reactor at a temperature close to the reaction temperature. Optimisation of the mole fraction of solids that pass through each exchanger as a function of the mole fraction of Ca(OH)2 in the solids stream that enters or leaves the reactors during charging and discharging, respectively, shows that it is still possible to improve sensible heat transfer, reaching the maximum global thermal efficiency of the system. Due to the energy requirements of the water phase changes, the sensible heat exchanged by the solids stream entering the reactor during charging or leaving the reactor during discharging is not sufficient to achieve condensation or total evaporation of the fluid. Several options are considered to recover the latent heat of condensation of the steam generated in the dehydration reaction: ideal recovery, non-recovery, a Rankine cycle using ammonia as the working fluid, and a pressurised steam storage system instead of condensation.