Challenges and Prospects for Solar Thermochemical Reactor Engineering for Renewable Resource Utilization and Poly-Generation
Authors: Konstandopoulos Athanasios , Dimitrakis Dimitrios , Kakosimos Konstantinos 
Published: July 2019

In the present work we examine how recent progress at our collaborating laboratories, over various solar thermochemical technologies, can be brought to bear on the advancement of renewable resource utilization in Qatar.

Specific examples of such technologies include solar water splitting for H2 production, solar CO2 splitting thermochemical energy storage in reduced metal oxides, solar cracking of CH4 for production of carbon nanoparticles and hydrogen, and production of metal nanoparticles to be employed as renewable fuel. Where feasible the potential of using locally sourced materials is stressed and particular examples are given (e.g. the use of Qatar's CaCO3). Enabling technologies over all scales (from the lab to the demonstration scale) include advanced instrumentation and testing platforms (e.g, solar simulators, solar furnaces, etc). Reactor concepts and system designs are formulated and analyzed and promising designs from the user's perspective are identified for advancing to the next level. Here we describe results and future directions from three such examples.

A low energy, zero-effluent system for clean water production, employing a concentrated solar thermal aquatic stream processing module is described. The projected specific electric energy consumption of the system does not exceed 1 kWh/m3, which in fact can be largely produced by the system itself. The system can exhibit an unprecedented (>99%) clean water recovery from any aquatic stream (seawater, brines, wastewater). At the same time the salt/solids content of the aquatic stream is separated in dry-form, thus being transformed from a waste-to-be-managed into a raw material ready for further commercial exploitation. The system is based on a modular design hence it is applicable over a range of scales, from standalone applications to large scale industrial water plants as well as an add-on to existing large scale producers and/or users of water, for further recovery of water and residual solids (salts, valuable metals from brine/wastewater effluents).

Production of hydrogen can be achieved via photo- and electro-chemical water splitting processes. In order for such processes to be feasible, they employ sacrificial electron donors which also bind oxygen molecule. Then, they have to be regenerated, via thermochemical reactions, and also release the oxygen. So the whole process operates in a cyclic mode, thus called water-splitting cycle. The use of solar energy for the realization of the thermochemical steps increases the overall solar-to-hydrogen efficiency. The Hybrid Sulfur Ammonia cycle, described here, uses ammonium sulfate as the sacrificial donor and a mixture of alkali-metal sulfates and pyrosulfates, which help to recover oxygen at lower temperatures and higher efficiency. We present the of thermal analysis experiments conducted for the calculation of the respective thermodynamic properties (heat capacity and enthalpy). In addition, with the use of advanced thermodynamic tools (FacTSAGE etc), and experimental thermal analysis, we completed the thermodynamic analysis, and selected the reaction temperatures in a range from 350°C to 900°C.

The crucial drawback of solar energy is its intermittent character. At the same time, pollution caused by the release of CO2, mainly in power plants and cement production, results in a series of problems such as global warming and poor air quality. Thermal Energy Storage (TES) can provide an escape route for both the aforementioned problems. TES can be used in order to store heat during periods of high solar irradiation and release it during the night or periods of low activity. Carbonate systems, such as calcite and dolomite, can be utilized for TES. In this study we focus on the reversible calcination/carbonation cycle of (CaMg)CO3, because it is a very strong candidate as a high efficiency thermochemical material. Its energy density is the highest among other materials. It can be found in vast quantities in Qatar, according to its geological formation, thus can be easily used as low cost/high added value material. Furthermore, it is a non-toxic and safe material with easy separation of the phases. In addition, the cycle involves CO2 capture during the discharge stage. This ends up in both energy release and greenhouse gases elimination. So far, we have characterized thermally, chemically and morphologically local raw surface carbonates. Results show that dolomitic powder, (CaMg)CO3 from Northern Qatar and subsurface rocks have an average capacity of 6 mmol CO2/g, but sintering deactivates material significantly after 20 cycles. Currently, we are designing a potential TES process coupled to cement production. In parallel, we are investigating experimentally the available methods to enhance the capacity of the raw material and improve its stability.

Present research contributes towards the solution of the Global Challenges that our societies face today, and in particular it provides access to safe and clean water and energy. These can be achieved with improved performance, energy efficiency and ease of usability.