Influence of Different Adsorbates on the Efficiency of Energy Storage Using Adsorption

Influence of different adsorbates on the efficiency of energy storage using adsorption

Tobias Kohler, Karsten Müller, Wolfgang Arlt

University of Erlangen-Nuremberg, Erlangen/Germany;

The efficiency and energy density of energy storage using adsorption depends mainly on the degree of desorption that was achieved before, which is determined by desorption temperature, desorption time and the working pair (adsorbate/ adsorbent). The state of the art working pair for adsorptive energy storage is water on zeolite 13X. This working pair needs high desorption temperatures and long desorption times, e. g. 500K for 6 hours, to reach its maximum efficiency. If energy at higher or lower temperature levels has to be stored, the efficiency of the storage is not sufficient to be competitive in the energy storage market.

In order to extend the operation range to different temperature levels, the only possibility is to change the adsorption pair, since desorption temperature and time are fixed by the application.

A huge number of groups work on the optimization of new adsorbents, but most of them aim at maximizing the nominal energy density or the amount of water adsorbed. In this work the goal is to reach high efficiencies of the process at different desorption temperatures. A further peculiarity is the fact that not only the adsorbent is changed but also the adsorbate.

A first examination of the efficiency of an adsorptive energy storage based on literature data of the adsorption of alcohols on BPL activated carbon (Taqvi et al. 1999) shows that the ideal desorption temperature rises, under the assumption that the desorption lasts for six hours, with rising chain length of the alcohol from 70°C for methanol to 147°C for propanol. The efficiencies calculated for these desorption temperatures are significantly higher than for the working pair water on zeolite 13X at the same desorption temperatures.
In order to further underline the effect of different adsorbates on the efficiency of thermochemical energy storage, the possible use of the homologeous series of alcohols on further adsorbents like SAPO and AlPO (Ng,Mintova 2008) are examined. SAPO and AlPO have become more common in recent years and are characterized by a high water and methanol uptake at fixed relative pressures, which corresponds to a high potential in thermochemical energy storage.

In order to examine the efficiencies and energy densities of different working pairs, adsorption isotherms at different temperatures are measured with a Magnetic Suspension Balance. These isotherms are transformed to a temperature independent form, the so called “characteristic curve of adsorption” according to Dubinin and Polányi. This enables the simulation of the strongly non-isothermal process.

References

Ng, E.-P., Mintova, S.: Nanoporous materials with enhanced hydrophilicity and high water sorption capacity. Microporous and Mesoporous Materials 114(1-3), 1-26 (2008).

Taqvi, S.M., Appel, W.S., Le Van, M.D.: Coadsorption of Organic Compounds and Water Vapor on BPL Activated Carbon. 4. Methanol, Ethanol, Propanol, Butanol, and Modeling. Ind. Eng. Chem. Res. 38, 240-250 (1999)