(453b) 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)