**1. Introduction**

Within the framework of the 2015 United Nations Climate Change Conference (COP 21) in Paris, it has been agreed that global warming should be limited to maximum 2 ◦C above pre-industrial levels. In the IEA 450 Policy Scenario, which can be considered as a consistent pathway for the fulfilment of the climate change target, the set limitation for CO2 concentration is 450 ppm [1]. In order to reach this goal, greenhouse gas emissions in 2035 need to be reduced by 30%, with respect to 2015. Considering the continuous increase of worldwide energy consumption [2], such goal can only be reached by an acceleration in the introduction of renewable energies. However, in such an energy supply system, which is characterized by a high degree of intermittency, some form of storage is required. Among others, conversion of solar energy into chemical fuels such as hydrogen may turn out as an effective form of storage [3], since chemical storage has the key advantage of being free of self-discharge, with respect to thermal and electrical storage systems. Hydrogen can be converted into electricity by means of turbines, generators, and fuel cells, or it can be upgraded to synthetic liquid fuels [4].

#### *1.1. Technology and Literature Review*

Several processes have been considered for the production of solar hydrogen. A comprehensive overview is given in Bozoglan et al. [5]. Five different solar hydrogen techniques can be generally distinguished. One option is to use PV [6], whereas the generated power can be used to run an electrolysis plant [7]. Another option is photoelectrolysis, which consists in the splitting of water in a photoelectrochemical cell powered by solar energy. Despite substantial research effort has been carried out on this topic in the last four decades, substantial problems hinder a wide diffusion of this technology. Solar hydrogen may also be produced by microorganisms such as algae and bacteria (photobiolysis) using solar irradiation as energy source, but this technology is at an early stage of development.

Solar thermolysis has been widely assessed in the past [8]. The feasibility of such a single-step concept is hindered by the high required temperatures (above 2,500 K) as well as by the challenging separation of the produced H2 and O2 gases [9]. Due to these reasons, the techno-economic feasibility of such concept will be low also in the future, according to Kogan [10]. An alternative, as described by Tyner [11], is to use Concentrating Solar Power (CSP) to first generate electrical power, which can be later utilized to run an electrolysis plant. Also a combination of concentrating PV and solar power tower has been recently proposed [12]. Two-step thermochemical cycles offer two key advantages in comparison to direct solar thermolysis [13,14]: first, the gas separation is realized by the absorption of the oxygen into the reactor material, while pure hydrogen is released. Second, the required temperature is significantly lower than for thermolysis. In addition, thermodynamic analysis shows that some of those cycles can achieve—in principle—efficiencies up to 70%, if energy recovery is applied [15].

Several materials have been analyzed both theoretically and experimentally [16]. Among them are zinc, iron, tin, terbium oxides, mixed ferrite, ceria, and perovskite [17]. Cycles based on nonvolatile metals have clear advantages in comparison to sulfur- and bromine-based cycles, in terms of toxicity and corrosivity. In addition, differently from volatile metal oxide-based processes, these materials allow the construction of fixed-bed structural reactors (monoliths) with important structural strength. Among this group mixed ferrites offer the advantage of relatively low reaction temperatures and therefore they have been widely investigated. nickel-ferrite cycles belong to the more general mixed-metal ferrite cycles, whose general reactions are shown in Figure 1. [18].

**Figure 1.** Basic scheme of a 2-step metal-oxide thermochemical process [18].

The first reaction is the endothermic thermal reduction step (TR), in which the metal oxide is reduced by removing part of the oxygen contained in it. The parameter δ is the non-stoichiometric factor that indicates to which extent the reaction occurs. Such non-stoichiometric factor is a function of process temperature and oxygen partial pressure [19]. In the second step—the exothermic water splitting (WS)—the MO is oxidized with water while hydrogen is released. Recently, an industrial-scale plant (750 kWhth) which uses concentrated solar radiation to split water into hydrogen and oxygen using nickel ferrite has been realized at the Plataforma Solar de Almeria (Almeria, Spain) and is expected to produce 3 kg of hydrogen per week [20]. Within those systems, two or more reactors are used in parallel to perform the batch-processes achieving a quasi-continuous hydrogen production. The main drawbacks of this system are material sintering and a relatively low efficiency.

Within the last years, ceria has become the benchmark for non-volatile metal oxide systems. The use of ceria as an interesting material for solar hydrogen applications has risen due to its quick kinetics and its structural stability over a wide range of temperatures [21]. However, higher temperatures with respect to nickel-ferrite, are required in order to achieve satisfying values of the non-stoichiometric factor. In addition, ceria-based cycles simultaneously produce a mixture of hydrogen and carbon monoxide, with the option to further process the syngas in a Fischer-Tropsch reactor to produce liquid solar fuels [22,23]. For those reasons this work focuses on the evaluation of nickel-ferrite and ceria and compares the resulting figures with hydrogen generated by water electrolysis.

With regard to the economic evaluation of hydrogen production by means of solar thermochemical cycles, few studies exist. Graf et al. [24] presented a simplified techno-economic characterization of hybrid-sulfur cycles and a metal oxide based cycles. The calculated price for thermochemical cycles ranged from 3.5 €/kg to 12.8 €/kg in an optimistic and conservative scenario, respectively. Recently, Nicodemus compared again solar thermochemical cycles and solar PV electrolysis [25]. The focus was on the learning curves for each of the main plant components. In that paper, hydrogen production costs of two main routes were presented, i.e., via PV-electrolysis and via solar thermochemical cycles (Zn/ZnO). Depending on the assumptions regarding different policy support mechanisms, the expected LCOH for the solar thermochemical cycle were in the range between 4 €/kg and 4.5 €/kg. The paper further highlighted that the cost projections for PV and electrolysis may lead to a LCOH of 2 €/kg and 3 €/kg, calling for incentive-based policy also for thermochemical cycles in order to improve their long-term economic competitiveness.

#### *1.2. Intention*

The aim of the presented work has been the development of a flexible model for the preliminary assessment and the comparison of different two-step thermochemical cycles for solar hydrogen production, taking into account at the same time technical and economic aspects. We believe that the development of such a simplified model is relevant in order to identify key techno-economic performance parameters of such processes, and can improve the understanding over differences and similarities of different concepts and plant configurations. In addition, the produced results may be useful in order to highlight some promising paths for future research work. Indeed, the main novelty of this work consists in bridging the gap between in-depth technical studies and economic analyses. First ones typically focus on very specific technical aspects such as new materials or component design and do not include economic analysis [20–22]. In the second ones, which to the best of our knowledge assume fixed or reference plant configurations, the inter-dependence between economic input parameters and optimal plant design are neglected [24,25].

The model developed within the framework of this study allows designing an optimal number of modules per reactor for large-scale applications as a function of several technical constraints. In fact, if only one reactor would be used, a large amount of the available solar energy would be wasted. This happens since the solar field power output must be sized to fulfil the peak power demand of the reactor, but a single reactor absorbs its peak power value only for a small fraction of the cycle. A constant power requirement makes a more efficient use of the available solar power. This ideal behavior can be at least approached by running several reactors together in a module, each reactor starting its cycle with a given time displacement. The optimal number of reactors per module allows approaching a constant value for the module power consumption curve.

Moreover, the objective is the analysis of long-term cost perspectives and the comparison of solar thermochemical cycles based on CSP with alternative renewable hydrogen production routes. In order to achieve this goal, a simplified model has been developed based on available literature and in cooperation with the DLR Institute of Solar Research.

The paper is structured as follows: after a literature review, the structure and the details of the technical model are described and discussed. A dedicated technical model has been realized for each of the plant components, i.e., solar field, single reactor, reactor groups. A distinction has been done between the materials used in the reactors. In particular, physical properties and kinetics of both nickel-ferrite and ceria have been considered based on available literature. A simplified economic model has also been implemented. Finally, a case study has been carried out. Technical and economic performances are presented and critically discussed. The technical comparison takes into account the two mentioned reactor materials. The economic analysis includes the consideration of different scenarios and takes into account learning curves for CSP. The hydrogen production cost obtained via solar thermochemical cycles are compared with those for competing systems such as electrolysis powered by photovoltaics (PV).

#### **2. Materials and Methods**

The model consists of following submodules:


The paper first presents the structure of the developed model. In the subsequent analysis the results of the parametric studies are shown and critically discussed. Finally, key findings and suggestions for future research activities are summarized in the conclusions and in the outlook, respectively.

#### *2.1. Technical Model*

As mentioned above, the technical model consists of four sub-models implemented in the Python programming language, which are described in the following sections. The description focuses on the overall structure of the models. Details about single components and validation of the numerical results relative to them can be found in the available literature, which has been taken as a basis for the model development. However, at the plant scale, no validation is available.

#### 2.1.1. Sun Power Evaluation Model

The first step of the calculation evaluates the available solar power. In a CSP plant for hydrogen production (Figure 2), the incoming irradiation is reflected by the heliostat field and concentrates onto the top of a tower, where a receiver is located. Since both the current available direct normal irradiance (DNI) on the solar field and the position of the sun continuously change over the day and over the seasons, an accurate evaluation of the net usable solar power is of foremost importance.

The calculation procedure is performed according to these steps:


solar radiation towards the receiver (i.e., geometrical losses such as cosine losses, shading and blocking), and to the non-ideal behavior of the heliostats (i.e., astigmatism).

• Finally, the net available heat at the receiver is assessed as [20]:

$$\dot{Q}\_{sf} = DNI \cdot \eta\_{sf} \cdot A\_{sf} \cdot (1 - f\_{spill})^{-1}$$

where *Qs f* is further reduced by a non-dimensional spillage factor *fspill*, as not all irradiation reflected by the heliostats hits the receiver [29].

**Figure 2.** CSP heliostat field and solar tower at the Plataforma Solar de Almería, Spain (DLR).
