Thermochemical energy storage by consecutive reactions for higher 1 efficient concentrated solar power plants ( CSP ) : proof of concept 2 3

Concentrated solar power plants (CSP) combined with thermal energy storage (TES) offers the benefit to provide continuous electricity production by renewable energy feed. There are several TES technologies to be implemented, being the thermochemical energy storage the less studied and the most attractive since its volumetric energy density is 5 and 10 times higher than latent and sensible TES, respectively. Thermochemical energy storage technology is based on reversible chemical reactions, also named thermochemical materials (TCM). One of the main challenges of TCM is to achieve a proper reversibility of the reactions, which in practical conditions leads to lower efficiencies than the theoretically expected. A new concept based on changing from reversible TCM reactions towards TCM consecutive reactions aims to eliminate reversibility problems and therefore improve the overall efficiency. Consecutive TCM reactions can either be based in one cycle, where reactants are needed to feed the reaction, or two coupled cycles which offer the possibility to work without any extra mass reactants input. The plausibility of the implementation of both concepts in CSP is detailed in this paper and case studies are described for each one.

area for the mirrors [4]. 48 49 Energy production is restricted when sun shines, therefore, a system that allows storing solar 50 heat is required. In case of not having problems with sun shine, a producer company might 51 want to store the energy as a function of the price of kWh on the market (to increase company 52 benefits). For this purpose a thermal energy storage system (TES) is essential. TES is becoming 53 particularly important for electricity storage in combination with concentrating solar power 54 (CSP) plants where solar heat can be stored for electricity production when sunlight is not 55 available [5,6]. 56 57 Proposed mechanisms to store thermal energy are based on different physical or chemical 58 principles: sensible heat (molten salts, solid particle materials, etc.), latent heat by means of 59 phase change materials (PCM), and thermochemical heat storage (TCS) using thermochemical 60 materials (TCM) [7][8][9][10][11][12][13]. Nowadays, the use of molten salts is the most viable alternative for 61 TES coming from solar heat to supply intermittent power demand. Nevertheless, molten salts 62 cannot provide a temperature or an energy density as high as the TCS due to the mechanism 63 itself. From a theoretical point of view the use TCS is a challenge that can provide higher 64 storage of energy for longer periods and operate at higher temperatures compared to the other 65 mentioned systems. Consequently, implementing TCS technology would allow increase 66 As an example of this concept, the ZnO/Zn cycle is considered. The first step is the endothermic 125 thermal reduction (ΔH 298K = 350.85 kJ·mol -1 ) of zinc oxide and can be referred as the solar step 126 (eq. 2). The second step, the non-solar step (eq. 3), is the exothermic reduction (ΔH 298K = -67.87 127 kJ·mol -1 ) of CO 2 with zinc to generate CO and ZnO [22,23]. 128

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This approach of storing the TCM and not the solar heat, can be very similar to those proposed 206 for obtaining solar fuels, fuel for fuel cells or via water or CO 2 splitting (WS and CS, 207 respectively) [27, 42-48]. However, the difference is that the material is used for WS and/or CS 208 to produce a chemical to store (H 2 , CO 2 ), and further perform another process such as Fischer-209 Tropsch which allows benefiting solar heat. Instead, in the proposed system, the material is 210 directly stored with the function of storing heat. 211

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In conclusion, a TCS system based in four consecutive reactions divided in two-loops. One of 213 them is exothermic and the other is endothermic, so it can be considered the discharge cycle as 214 the discharge process and the regenerative cycle as the charge cycle; in energy terms. 215

Regenerative cycle
It consists in a discharging cycle and a regenerative cycle. The concept is divided into a main 220 cycle (discharge cycle) which is focused to release heat. The other cycle (regenerative) is 221 focused to close the system. Consequently total enthalpy of the system should be zero, and if 222 discharge cycle is exothermic, regenerative cycle must be endothermic. In this particular 223 concept the involved gases are O 2 and CO/CO 2 . This system of gases and the reactions involved in Eq 4-Eq 7 and drawn in Figure 5.

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As it can be seen, the system consists in four intertwined reactions, but, based on the first 231 reaction (Eq. 4) [31-35]. A second charge reaction (Eq. 5) is necessary to recover BaS, 232 obtaining a closed cycle for TCM. However, to perform the discharge cycle and to maintain the 233 TCS system closed, a second cycle (regenerative cycle) is needed to regenerate/recover the 234 product gases involved in the reactions. A third reaction (Eq. 6) regenerates CO 2 produced in 235 the second reaction, and at the same time, produces CO for BaSO 4 reduction. The last reaction 236 (Eq. 7) serves to close the regenerative cycle for the part of the solid, and at the same time to 237 release O 2 that is needed in the BaS oxidation (Eq. 5). 238 239 Unlike the TCM system concept [36-38, 39-41], in which the system store solar heat is 240 considered, in this novel concept, solar heat is used to achieve the required temperature and the 241 thermal energy for the desired TCM reaction. Therefore, the proposed system store chemical that is easier to operate and, at the same time, CO is easier than C to recover in a regenerative reactor/receiver is not stored as heat; rather it will serve to store CO. 308 309 CO 2 is a very stable gas, so it is not usually considered an oxidizing gas although at elevated 310 temperatures can be reduced and oxidize a reduced metal (or reduced metal oxide). The reaction 311 mechanism is used to WS and CS, where a reduced metal is used as a reaction support (not as 312 catalyst Equations 9 and 10 show the CS mechanism based on the thermal stability of the metal oxide. 317 At a given temperature it favours the reduced or oxidized form. The reduced form always will 318 be more thermally stable, so the reduction reactions will always be at higher temperature than 319 the oxidation reaction. Every metal oxide redox pair has a specific temperature for thermal 320 reduction; consequently, it could control the process by temperature control. On the other hand, 321 the reduction reaction always requires thermal energy (endothermic reaction), but in the case of 322 the metal oxidation (CS) reaction could be exothermic or endothermic, depending of material 323 suggest that it is possible to improve its cyclability. 353

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As the oxidation temperature of the metal oxide is lower than that of the disproportionation, this 355 means that it can work below 1000⁰C, complying with the first premise of the concept. 356 Furthermore, the re-oxidation mechanism usually is controlled by oxygen diffusion inside the 357 particle [67,81]. Although there is a maximum temperature which cannot be overcome, this will 358 not have an important role in the kinetics. To improve kinetics it is essential to control the 359 morphology and particle size of the metal oxide, and also to have a controlled atmosphere. To 360 ensure morphology, minimizing sintering and maintaining surface/volume ratio, it may be 361 advisable to decrease the disproportionation temperature.  Table 2, note that a basis of 100 kg of 373 BaSO 4 in solid state is taken for the balances). Then, operating modes during day and night for 374 BaSO 4 /BaS cycle are detailed and afterwards the balances of the regenerative cycle (see Table  375 3) and the diagram of the whole concept are provided. 376  The mass balance of the regenerative cycle following Eq. 6 and 7 is shown in Table 3, whereas 421 in Figure 8 the diagram of the whole concept implementation is drawn. 422