Carbon based hybrid materials for thermochemical heat storage

CHP disegno
Autore del lavoro candidato: 
Emanuela Mastronardo

SINTESI CONTENENTE UNA BREVE DESCRIZIONE DEL LAVORO SVOLTO E DEI RISULTATI OTTENUTI: Chemical heat pump (CHP) is a technology for the recovery of waste heat, based on a reversible chemical reaction for heat storage and reuse on demand. Different kind of substances can be used in a CHP system involving reversible gas-gas reactions (NH3/N2/H2, SO3/O2/SO2, CH3OH/H2/CO, cyclohexane/benzene/hydrogen), liquid-gas reactions (isopropanol/acetone/hydrogen) and solid-gas reactions (BaO2/O2/BaO, ZnO/O2/Zn, PbO/CO2/PbCO3, CaO/CO2/CaCO3, CaO/H2O/Ca(OH)2). The choice of storage material depends on the temperature range in which the storage system will to operate. Specifically, this study focuses on the solid-gas MgO/H2O/Mg(OH)2 CHP which operates in the temperature range 200-400 °C: Mg(OH)_2 (s) □(↔) MgO(s)+H_2 O(g) ∆H^0=±81 kJ/mol Dehydration, being endothermic, represents the energy storage step while the exothermic hydration of MgO releases the thermal energy when required. The water vapour produced during the dehydration reaction is condensed in a reservoir and then reused in vapour phase for the hydration reaction closing the pump cycle. Because of the poor heat transfer properties, reactivity and durability to several charging/discharging cycles of the reactants, a more efficient heat storage material for MgO/H2O/Mg(OH)2 CHP needs to be developed for the industrial application of such a technology. In this study hybrid materials made of magnesium hydroxide and carbon (exfoliated graphite and carbon nanotubes) were developed by a deposition-precipitation method. The compatibility between the active phase (Mg(OH)2) and the carbonaceous material is improved by the electrostatic interaction promoted by the different point of zero charge (pHPZC) of the materials. The performance of the resulting hybrid heat storage material was evaluated by thermogravimetric analysis which simulates the CHP cycle. The developed carbon based hybrid materials showed enhanced stability and efficiency under operating conditions with higher heat storage and output capacities at lower reaction temperature and a higher heat output rate.