(771g) Material Development and Assessment of a Thermochemical Energy Storage Based on CaO/CaCO3

The usage of solar energy for heat and power supply is restricted by the fluctuation of solar radiation. To overcome this and to enable a continuous operation of concentrated solar power plants, thermal energy storages units are essential. In this context, reversible thermal decomposition reactions of solids have been proposed and studied for several years [1]: A solid salt is decomposed into another solid and a gaseous component during daytime. The composition reaction, reverse reaction, occurs during night time or at an overcast sky in order to supply continuously energy for a steam cycle. Investigated reaction schemes contain oxides, hydroxides, carbonates and hydrides [2-5]. Due to high storage densities and discharging temperatures between 400 and 1000 °C, the gas-solid reactions are discussed as a promising concept for increasing the efficiency and operation hours of solar thermal power plants.

In previous works [6], the cyclic carbonation/calcination reaction of CaO is proposed as a thermochemical storage for solar power plants. The energy is stored chemically in CaO after the endothermal calcination reaction of CaCO₃ (eq. R1). The formed energy-rich CaO and CO₂ are stored separately. The energy is released by initiating the exothermal carbonation reaction (eq. RR1) of CaO and CO₂ at temperatures between 600 and 750 °C.

	CaCO3(s) â?? CaO(s) + CO2(g)	 Î?Rh+= 178 kJ mol-1		(R1)
	CaO(s) + CO2(g) â?? CaCO3(s)	 Î?Rh+= - 178 kJ mol-1		(RR1)

The involved components are cheap, non-toxic and in case of CaCO₃ abundantly available. Nevertheless, a profound challenge is the strong cycle-to-cycle degradation of the material due to sintering. A low cyclic stability implies a decrease of active material after several cycles and thus, the efficiency and storage density is lowered.

Within this study, Al-doped materials are synthesized with different Ca/Al molar ratios in order to stabilize the sorbent structure and prevent the mentioned sintering phenomena. The Ca/Al ratio is varied between 75/25 and 95/5. All samples are experimentally characterized in 20 calcination/carbonation cycles by a thermogravimetric analyzer measuring the CO₂ uptake at predefined conditions [6]. Furthermore, based on the results and a performed enthalpy balance of carbonation and calcination reaction the storage density and the energetic efficiency are calculated in order to evaluate the sorbents out of an energetic point of view. All samples are compared to the performance of the benchmark material (natural CaO).

The experiments show that the formed Ca/Al mixed phase stabilizes the sorbent and the CO₂ uptake behavior can thus be maintained. It could be proven that all synthesized sorbents are characterized by an improved cyclic stability and the relative loss of activity from the first to the 20^th cycle is notably reduced. Whereas the benchmark material shows a loss of activity of about 70 % after the 20^th cycle, the degradation of the best prepared sorbent is less than 4 %. In context of storage density and efficiency it can be seen that in general, an increase of Al-content entails a lowered maximum achievable storage density and efficiency due to a decreased initial amount of active material (CaO). Nevertheless, by taking the results of the CO₂ uptake measurements into account, all synthesized samples show an improved performance compared to natural CaO in the 20^th cycle due to a constant CO₂ uptake. In terms of storage density 2 to 3.5 times higher values can be achieved. For the energetic efficiency a factor of up to 1.2 relative to the benchmark material is obtained.

Thus, based on these experimental and theoretical investigations it can be confirmed that the addition of a certain amount of initial inert material is justified in order to optimize the performance of the material. The results of the study lay the foundation for the usage of the CaO/CaCO₃ reaction couple as a high efficient thermochemical heat storage.

[1] W. Wentworth, E. Chen, Solar Energy, 18 (1976), 205-214

[2] A. J. Carrillo, J. Moya, A. Bayón, P. Jana, A. Victor, M. Romero, J. Gonzalez-Aguilar, D. Serrano, P. Pizarro, J.M. Coronado, Solar Energy Materials and Solar Cells, 123 (2014), 47-57

[3] F. Schaube, A. Wörner, R. Tamme, Journal of Solar Energy Engineering, 133 (2011), 1006-1012

[4] M. Aihara, K. Tanaka, M. Watanabe, T. Takeuchi, H. Habuka, Journal of Chemical Engineering of

Japan, 40 (2007), 1270-1274

[5] M. Paskevicius, D. Sheppard, K. Williamson, C. Buckley, Energy, 88 (2015), 469-477

[6] J. Obermeier, B. Müller, K. Müller, W. Arlt, Energy Technology, 4 (2016), 123 â?? 135