*Case Study*

The developed model is finally used to assess the LCOH in a case study. The selected site for the analysis is the Plataforma Solar de Almería (Almeria, Spain). Figure 5 shows a DNI distribution map of Spain. One can observe that South-East Spain is characterized by highest annual DNI values (around 2100 kWh/m2/y). The plant is assumed to have a heliostat field with 3000 heliostats with each 40.96 m2 surface. The nominal receiver thermal power is of 90 MW.

**Figure 5.** DNI distribution map of Spain [37].

First, key technical plant characteristics such as optimal reactor number for a module, overall energy balances and typical production profiles are shown. A comparison between Nickel-Ferrite and Ceria is carried out. Second, the economic results for the base case are presented and a sensitivity analysis on selected parameters is carried out. Finally, a best case scenario is evaluated and its results are reported.

A view on the energy balance for a single reactor helps understanding the main loss mechanisms of such systems. A Sankey plot for a full reactor cycle is shown in Figure 6, which represents the time integral of the power fluxes during the whole cycle. The enthalpy term is related to the efficiency. The resulting reactor efficiency in the base case is 6.4% for nickel-ferrite and 13.4% for ceria. Such a difference is mainly due to the faster kinetics of ceria, which leads to a shorter cycle duration time. In both cases, the re-radiation losses through the window are very high and accounts for 59.4%–67.0% of the incoming solar energy. The geometry of the absorber and of the window has been taken as a given input and has not been optimized within the framework of this work.

**Figure 6.** Sankey diagrams for nickel-ferrite (**left**) and ceria (**right**) for a full cycle.

The heat losses through the insulation have a minor impact on the system performance. A larger energy amount (from 14.0% up to 22.7%) is required for the heat-up of the working fluid entering the reactor.

As discussed above, a smart choice of the number of reactor in each module allows to dramatically reduce the non-used share of available solar power. Figure 7 shows the results of the optimization procedure. In both cases, the single reactors within one module are activated with such a time displacement that the peak power requirement of different reactors do not overlap. Then, the optimal number of modules per reactor is mainly a function of the duration of the heat-up phase. In principle, additional gains could be reached by the optimization of the heat-up profile. However, this would have been out of the scope of this preliminary work. In ceria both the temperature level of the TR phase as well as the spread between TR and WS temperature is higher than for nickel-ferrite. This leads to longer heat-up duration in ceria cycles. As a consequence, in ceria a lower number of reactors per module can be obtained than for nickel-ferrite. In the figure one can also observe that the cycle duration for nickel-ferrite is almost twice the cycle time for ceria. This is due to the different kinetics and temperature levels. In addition, the module peak power for ceria is roughly 40% higher than for nickel-ferrite.

The previous results show the behavior of single reactors and single modules under the assumption of constant available solar power. In reality, available sun power varies during the day and more than one module can be run at the same time, as shown in Figure 8.

For a given number of modules in a plant, only a certain fraction can be operated at the same time, depending of the currently available solar power. When the Sun power is equal or higher than the sum of the nominal power of a certain number of modules, such modules are heated up. Within each single module, the reactors are successively activated taking into account the time displacement between reactors. This module activation trend is visible in Figure 8, where during the morning hours the number of productive cycles is lower than the number of activated modules, since the heat-up procedure needs a certain time. Due to the fact that the cycle time in ceria is lower than in nickel-ferrite, ceria is more reactive than nickel-ferrite to variations in the available solar power.

**Figure 7.** Solar power requirement for a single reactor and for a module (**left**: nickel-ferrite, **right**: ceria).

**Figure 8.** Plant daily performance (ceria, typical day, base case).

The total investment for the case is 35.5 Mio. € for nickel-ferrite and 32.5 Mio. € for ceria. The higher investment for nickel-ferrite is due to the higher number of reactors needed, which in turn is a result of the slower kinetics. However, since solar field investment (which is the same in both cases) accounts for ca. two thirds of the total investment, the difference is relatively small. The resulting LCOH are 38.83 €/kg for nickel-ferrite and 13.06 €/kg for ceria. Figure 9 shows the LCOH structure. In both cases, the instalment accounts for more than 60% of the LCOH, while the substitution of the active material represents ca. another 20% of the specific production cost. The operation and maintenance cost amount to ca. 15% of LCOH.

**Figure 9.** LCOH shares for the base case (**left**: nickel-ferrite, **right**: ceria).

The conclusion is that solar field and tower cost are—less surprisingly—the most important cost factors. Also, the reduction of the substitution of metal oxide can significantly enhance the economic figures. Such considerations are confirmed by the sensitivity analysis for the case with ceria on economic parameters in Figure 10 (the case with nickel-ferrite is similar and it is reported in the Appendix A). The reduction of the cost of the solar field turns out to be by far the most important cost driver. In addition, absorber cost and life time play an important role. On the contrary, the impact of nitrogen cost or electricity cost for compressors on LCOH is almost negligible.

**Figure 10.** Sensitivity analysis of LCOH (ceria, base case).

Finally, a best case scenario for ceria has been evaluated. Nickel-ferrite has been neglected due to the higher LCOH in the base case. This scenario takes into account a reduction of the key economic parameters. In particular, heliostat field as well as tower cost are assumed to be reduced by 40%. The active material cost is halved and the absorber life time has been extended from 1 year to 20 years. These assumptions, in particular those regarding the absorber life time, are very optimistic. However, this assumption is taken as this work aims at the evaluation of the long term potentials of this technology. The LCOH of ceria in the best case accounts to 6.68 €/kg. The results compare well with the figures recently presented by Nicodemus [25].

#### **4. Conclusions**

Within this paper a techno-economic model for the evaluation and comparison of different materials for solar thermochemical hydrogen production has been developed. The novelty of the model consists in the simplified but still flexible approach, which considers at the same time technical and economic aspects. In this way, the model is able to take into account the inter-dependence between economic input parameters and optimal plant design. In addition, the model makes possible evaluating key design and operation parameters of such plants, and comparing different materials (i.e., nickel-ferrite and ceria). Furthermore, sensitivity analyses over a wide range of techno-economic parameters can be easily realized.

The technical model has been developed based on available literature and in close collaboration with the DLR Institute of Solar Research. It consists of four sections including sun power evaluation, single reactor model, kinetics for both the considered materials, and complete plant model including underground hydrogen storage facilities. The model has been used to optimize the design of a large-scale solar hydrogen production plant (90 MWth solar reactor power at design). In particular, the design optimization procedure consists on the minimization of the solar power dumping which typically occurs due to the different heat requirements of the thermal reduction and water splitting phases, respectively. For both nickel-ferrite and ceria an optimal number of reactors per module has been found. Each of the reactors within one module is then operated with a suitable time displacement relative to the other reactors of the same module. The comparison demonstrated that ceria achieves higher efficiency than nickel-ferrite (13.4% instead 6.4%), which is mainly due to the different kinetics. This difference leads to a lower LCOH for ceria (13.06 €/kg instead of 38.83 €/kg for nickel-ferrite, both in the base case). The analysis of the cost structure highlighted the importance of reducing investment cost for solar field in order to improve the plant economics. A best case scenario with optimistic assumptions regarding investment cost and absorber life-time has been considered. LCOH in this case is 6.68 €/kg for ceria. Finally, the integral evaluation of the thermal balances over a full cycles showed that around two third of the incoming solar radiation are lost due to re-radiation through the window. The main competitor of solar thermochemical cycles for hydrogen production remains photovoltaics-powered electrolysis, which is expected to reach specific production cost between 2 €/kg and 3 €/kg in the long term perspective.

**Author Contributions:** Conceptualization, M.M.; Formal analysis, M.M. and M.P.; Methodology, M.M., M.P. and T.F.; Software, M.P.; Supervision, M.M.; Writing—original draft, M.M.; Writing—review & editing, M.M. and M.P.

**Funding:** This research was funded by the Helmholtz Association within the framework of the Program-Oriented Funding (DLR basic funding: "Future Fuels" Project).

**Acknowledgments:** A special thank goes to Matteo Pecchi for his extraordinary engagement during his stay at DLR.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
