A systematic approach to identify new thermochemical energy storage systems

Abstract

Thermochemical energy storage (TCES) is considered an emerging green technology for increased energy utilization efficiency, thereby achieving a reduction of greenhouse gases. Several reaction systems based on different substance classes (e.g. hydrates, hydroxides, oxides) were suggested and investigated so far. Nevertheless, the number of know reactions which are suitable for TCES is still limited, as the main focus lies on the investigation of a handful known substances, their further improvement or applicability. To find novel promising candidates for thermochemical energy storage and also to allow for a broader view on the topic, this work presents a systematic approach to find new TCES systems. A mathematical search algorithm identifies potential reactions based on thermodynamic databases for different reactive gases. The search results are classified by their applicable temperature range and ranked by storage density. To further assess the potential of the found systems, a novel method for the identification of a temperature and pressure dependent reaction kinetics is proposed. It is an extension of the non-parametric kinetic analysis (NPK) method as it identifies the pressure dependency in addition to the temperature and conversion dependency of the reaction. This is done by analyzing kinetic data in a three-dimensional data space (conversion, temperature, pressure) and attributing the variation of the conversion rate to these independent variables. Thus, a reduction from a three-dimensional problem to three one-dimensional problems is achieved. The derivation of a kinetic model can then be performed for each dependency independently, which is easier than deriving a model directly from the data. This work presents the basic approach of the identification and combination of the three dependencies to build a full kinetic model. Also, the interpretation of the model to achieve a physically motivated model is illustrated. Then the method is applied to identify the complex reaction kinetics of the decomposition of CdCO3 based on a set of thermogravimetric measurements. It is shown that it is possible to identify interaction terms between the dependency terms. One promising application of TCES is its combination with concentrated solar power. Based on the search, the reaction system CuO/Cu2O has been identified as a potential candidate for such a combination. This work studies the reduction of CuO and the oxidation of Cu2O under isothermal and isokinetic conditions. The reaction are analyzed using a simultaneous thermal analysis (STA) and a lab scale fixed bed reactor. To develop kinetic models the NPK approach is utilized. This model free approach is expanded by the Arrhenius correlation to increase the applicable temperature range of the models. The resulting models are evaluated and compared. Furthermore, the cycle stability of the system over 20 cycles is assessed for a small sample mass in the STA and a large sample mass in the fixed bed reactor.