Optimizing adsorbents for heat storage applications : estimation of thermodynamic limits and Monte Carlo simulations of water adsorption in nanopores

This thesis starts out with a review of low temperature heat storage technologies. A reference application of seasonal solar heat storage for space heating is considered. The principles of adsorptive heat storage utilizing reversible water adsorption in microporous solids are discussed. Thermodynamic limits to the energy density achievable in such a system are analyzed through statistical mechanical adsorption models (Langmuir, Bragg-Williams lattice gas). It is shown that an energy density of 300 kWh/m3 would be achievable under the chosen cycle conditions with an ideal adsorbent having a porosity of 40% (corresponding to the value for real silica gels). Adsorption data of real silica gels and zeolites are compared to this thermodynamic limit, showing that there is plenty of room for a targeted optimization of adsorbents for this application. <br> <br>Understanding the relationship between adsorbent microstructure and water adsorption equilibria is a prerequisite for a targeted materials design. To this end, molecular Monte Carlo simulations of water adsorption in microporous model systems are employed in the second part of this thesis. Simulations based on the Metropolis algorithm are carried out in the grand canonical ensemble, yielding the number of water molecules adsorbed in thermodynamic equilibrium under given conditions of temperature and pressure. The simulation makes use of configurational bias (CBMC) methods to improve sampling efficiency at high adsorbate densities. The model of the water molecule employed here is one developed in the group of K. Gubbins at Cornell, featuring tetrahedrally arranged square-well interaction sites and a Lennard-Jones core. A slit pore geometry is considered for the microporous solid. In a computationally demanding parametric study, the influence of pore width and the distribution of hydrophilic surface sites on adsorption equilibria of the model system are analyzed. It is shown that the results obtained are consistent with recent theoretical findings on the phase equilibria of water under geometrical confinement. <br> <br>Based on the adsorption properties of the model system, various hypotheses relevant to a targeted design of adsorbents for heat storage applications are developed. The findings of this thesis are considered to be equally relevant to the improvement of adsorbents for heat pumps and cooling machines.

Author dc:creator

• Schmidt, Ferdinand Paul

Contributors dc:contributor

• Luther, Joachim

• Wärmespeicher
• Adsorptionswärmespeicher
• adsorption
• heat storage
• monte carlo simulation
• water
• micropores

Repository record source_url

https://freidok.uni-freiburg.de/data/1506

OAI identifier oai:identifier

oai:freidok.uni-freiburg.de:1506