George Karagiannakis

HycycleS: a project on nuclear and solar hydrogen production by sulphur-based thermochemical cycles

The European FP7 project HycycleS focuses on providing detailed solutions for the design of specific key components for sulphur-based thermochemical cycles for hydrogen production. The key components necessary for the high temperature part of those processes, the thermal decomposition of H2SO4, are a compact heat exchanger for SO3 decomposition for operation by solar and nuclear heat, a receiver-reactor for solar H2SO4 decomposition, and membranes as product separator and as promoter of the SO3 decomposition. Silicon carbide has been identified as the preferred construction material. Its stability is tested at high temperature and in a highly corrosive atmosphere. Another focus is catalyst materials for the reduction of SO3. Requirement specifications were set up as basis for design and sizing of the intended prototypes. Rigs for corrosion tests, catalyst tests and selectivity of separation membranes have been designed, built and completed. Prototypes of the mentioned components have been designed and tested.

Development and evaluation of materials for thermochemical heat storage based on the CaO/CaCO3 reaction couple

The current work relates to the development of synthetic calcium oxide (CaO) based compositions as candidate materials for energy storage under a cyclic carbonation/decarbonation reaction scheme. Although under such a cyclic scheme the energy density of natural lime based CaO is high (∼ 3MJ/kg), the particular materials suffer from notable cycle-to-cycle deactivation. To this direction, pure CaO and CaO/Al2O3 composites have been prepared and preliminarily evaluated under the suggested cyclic carbonation/decarbonation scheme in the temperature range of 600-800°C. For the composite materials, Ca/Al molar ratios were in the range between 95/5 and 52/48 and upon calcination the formation of mixed Ca/Al phases was verified. The preliminary evaluation of materials studied was conducted under 3 carbonation/decarbonation cycles and the loss of activity for the case of natural CaO was obvious. Synthetic materials with superior stability/capture c.f. natural CaO were further subjected to multi-cyclic carbonation/d...

Co3O4-based honeycombs as compact redox reactors/heat exchangers for thermochemical storage in the next generation CSP plants

Over the last years, several research groups have focused on developing efficient thermochemical heat storage (THS) systems, in-principle capable of being coupled with next generation high temperature Concentrated Solar Power plants. Among systems studied, the Co3O4/CoO redox system is a promising candidate. Currently, research efforts extend beyond basic level identification of promising materials to more application-oriented approaches aiming at validation of THS performance at pilot scale reactors. The present work focuses on the investigation of cobalt oxide based honeycomb structures as candidate reactors/heat exchangers to be employed for such purposes. In the evaluation conducted and presented here, cobalt oxide-based structures with different composition and geometrical characteristics were subjected to redox cycles in the temperature window between 800 and 1000°C under air flow. Basic aspects related to redox performance of each system are briefly discussed but the main focus lies on the evaluation of the segments structural stability after multi-cyclic operation. The latter is based on macroscopic visual observation and also supplemented by pre– (i.e. fresh samples) and post–characterization (i.e. after long term exposure) of extruded honeycombs via combined mercury porosimetry and SEM analysis.

Thermochemical storage for CSP via redox structured reactors/heat exchangers: The RESTRUCTURE project

The present work provides an overview of activities performed in the framework of the EU-funded collaborative project RESTRUCTURE, the main goal of which was to develop and validate a compact structured reactor/heat exchanger for thermochemical storage driven by 2-step high temperature redox metal oxide cycles. The starting point of development path included redox materials qualification via both theoretical and lab-scale experimental studies. Most favorable compositions were cobalt oxide/alumina composites. Preparation of small-scale structured bodies included various approaches, ranging from perforated pellets to more sophisticated honeycomb geometries, fabricated by extrusion and coating. Proof-of-concept of the proposed novel reactor/heat exchanger was successfully validated in small-scale structures and the next step included scaling up of redox honeycombs production. Significant challenges were identified for the case of extruded full-size bodies and the final qualified approach related to preparati...

A novel thermochemical energy storage and transportation concept based on concentrated solar irradiation-aided CaO-looping

To overcome the temporal and regional gap of surplus solar energy, the concept of thermochemical heat storage is discussed. In this particular case, the application of CaO and CaCO3 as energy carrying compounds for a trans-regional energy distribution concept is analyzed regarding the effective energetic and exergetic storage density. In a comprehensive sensitivity analysis, the influences of reaction temperature, conversion and heat recovery strategies are worked out. It can be seen that the effective storage density is strongly influenced by the preheating of reactants from ambient to reaction temperature. Thus, high conversion rates during forward and reverse reaction as well as improved heat recovery ratios are necessary to achieve a high energetic storage density. In case of effective exergetic storage density, carbonation temperature reaches an optimum. The method presented in this contribution can be applied to similar thermochemical heat storage systems and the results are of great importance for ...

Experimental proof of concept of a pilot-scale thermochemical storage unit

An efficient heat storage system, which allows disposal of the energy independently of the weather conditions, is a key factor on the development of Concentrated Solar Power (CSP). In this respect thermochemical heat storage could play an important role. Despite being still at early development stage, the number of recent studies dealing with thermochemical systems for high temperature storage shows that the interest on this topic is largely increasing. Among the reactive materials studied, certain multi-valent metal oxides seem to be a promising option, especially for air-operated CSP plants.

Solar Hydrogen Production

This chapter summarizes the current status of solar-aided hydrogen production technologies, with special emphasis on high temperature thermochemical concepts. The required high temperatures are achieved via concentrated solar irradiation through the respective systems, e.g., solar towers and solar dishes. Customized, efficient, and robust solar reactor concepts are important to ensure optimum coupling of the thermochemical phenomenon with the solar source. Of fundamental importance for such thermochemical processes is the development of active materials and key components. Some of the most studied and promising active materials are presented in this chapter along with their relevant advantages and challenges. Solar hydrogen (/fuels) production is found to constitute an in principle promising alternative and supplementary solution to currently employed renewables. Nevertheless, further development is required to increase solar-to-fuel efficiencies and to overcome long-term stability issues. Favorable solutions strongly depend on the identification of more active and robust materials as well as on the definition of solar reactor designs that will ensure optimum exploitation of solar irradiation.