Experimental Investigation of Gas Transport in Bulk Materials for Thermochemical Storage

Kurzfassung

Efficient storage technologies for thermal energy are crucial for a sustainable and cost effective energy solution. Thermochemical systems, where heat is stored as reaction enthalpy of reversible chemical reactions, have great potential as an economic option for thermal energy storage. Major advantages of these systems are high storage densities and the possibility of long-term storage. Suitable thermochemical storage materials are generally characterized by high reaction enthalpies, fast dynamics as well as cycling stability. However, for a successful transition from material characterisation to storage solution sufficient heat and mass transfer through the reaction bed and within the particles is vital. The equilibrium of gas-solid reactions depends on the reaction gas partial pressure and the solid temperature. As a consequence, a varying partial pressure within the reaction bed leads to varying reaction temperatures. Therefore, the reaction gas distribution has a strong influence on the storage performance. The control of the reaction temperature (and thereby of the outlet temperature of the reactor) by ensuring a homogeneous and adjustable gas pressure throughout the reaction bed presents one of the main challenges in reactor design. A highly promising reaction system for applications between 400 °C and 600 °C (e.g. thermal solar power or industrial processes) has been found in the hydration of quicklime (CaO). Well known from applications in the building industry the reaction CaO(s) + H2O(g) ⇌ Ca(OH)2(s) + HR is currently being investigated with regard to thermal energy storage. Principle suitability was proven as reversibility, reaction dynamics and cyclability were studied and demonstrated by means of thermogravimetric analysis as well as in lab- and pilot-scale reactors. To achieve fast reaction dynamics a fine powder with a high specific surface area is used as storage material. However, precisely in very fine grained bulks the realisation of sufficient gas transport through the reaction bed is difficult. If the bulk is compacted – e.g. to improve storage density or thermal conductivity in the storage – this problem is even further enhanced. In this contribution experimental methods for the investigation of gas transport at technically relevant conditions are presented. Experimental results are discussed and conclusions in regard to reactor designs and material modifications are drawn.