Transition Metal Ammoniates – A Novel Class of High-Performance Thermochemical Energy Storage Materials

Effective compensation of the temporal differences for peaks in energy production and demand is a major challenge for integration of renewable and sustainable energy sources into the existing energy supply chain. This becomes... [ view full abstract ]

Effective compensation of the temporal differences for peaks in energy production and demand is a major challenge for integration of renewable and sustainable energy sources into the existing energy supply chain. This becomes acute when combining sustainable energy management with fluctuating energy sources as the sun, as they feature non-operational times at e.g. night. To store excess heat for use during low-production times, efficient thermal storage is necessary. Thermochemical energy storage (TCES) is acknowledged as highly efficient, still rather experimental method for thermal energy storage, mainly due to low cycle stabilities and long reaction times.

High-performance TCES materials require maximal volumetric energy storage potentials.¹ Aiming for materials applicable between 25 °C - 400 °C, transition metal ammoniates were so far largely uninvestigated. A systematic study on chlorides and sulphates, investigated for their performance as TCES-materials reversibly forming ammoniate complexes in the presence of gaseous ammonia, is presented. Regarding the energy density and compared to other TCES- materials operated in the temperature window between 25 – 400 °C, transition metal ammoniates are amongst the most attractive energy storage materials reported (fig.1): For the chlorides a maximum of 2.43 MWh m^-3 (NiCl₂), and for the sulphates of 1.72 MWh m^-3 (CuSO₄) was observed. Especially in the case of CuSO₄ a perfect cycle stability was observed.²

CuSO₄ and CuCl₂ were investigated in a laboratory scale reactor (fig.2). After 80 seconds peak temperatures of 312 °C and 238 °C respectively were achieved.³ Careful selection of the temperature profile during the charging and discharging reaction allows for cascadic energy storage, addressing different temperature levels.

The contribution presents and compares experimental results on a widely neglected class of thermochemical storage materials, featuring highly attractive properties combining small scales and high storage densities for e.g. solar heating.

¹ M. Deutsch, D. Müller, C. Aumeyer, C. Jordan, C. Gierl-Mayer, P. Weinberger, F. Winter, A. Werner, Appl. Energy, 2016, 183, 113-120.

² D. Müller, C. Knoll, C. Jordan, J.M. Welch, A. Werner, M. Harasek, P. Weinberger, Appl. Energy, 2017, submitted.

³ D. Müller, C. Knoll, G. Gravogl, A. Werner, M. Harasek, R. Miletich, P. Weinberger, ISES Proceedings,2017, submitted.