Thermal Energy Storage (TES)
Thermal Energy Storage (TES) aims to develop new materials and systems for heat storage for domestic and industrial applications.
Sustainable energy sources play an increasingly important role in the supply of energy. At present the Dutch government policy focuses on a net energy neutral built environment in 2050. To reach this goal newly built houses should generate energy to compensate for the existing built environment.
The mission of Energy Technology is to develop new methods and tools for the extraction, conversion, transportation, storage and use of energy, targeted towards yielding highly efficient (sustainable) energy systems whilst mitigating side effects on humans, nature and the environment.
Therefore our research is focused on seasonal heat storage using thermo-chemical materials (TCM), geothermal energy, photovoltaic thermal (PVT) panels and (individual) thermal comfort.
Within the next 20 years the supply of fossil fuels, mainly oil and natural gas, will not be sufficient any more to provide the world's economies. At present the Dutch government policy focuses on a net energy neutral built environment in 2050. Seasonal heat storage and geothermal energy are key issues to achieve this goal.
A promising concept for seasonal heat storage in the built environment is based on the reversible sorption process of water vapor. An important class of sorption materials constitutes of the salt hydrates. In winter time the salt can be hydrated releasing heat. In summer the salt is dehydrated by using solar heat. Dependent on the material properties this cycle can be repeated as many times as needed. Examples of salt hydrates suitable to be used in a seasonal heat storage system are magnesium sulfate and magnesium chloride, which are cheap, non-toxic, non-corrosive and available in large quantities. One of the bottlenecks in the application of sorption materials in seasonal heat storage systems is the rate at which the material can be hydrated or dehydrated. This rate is highly determined by the transport properties of vapor and heat in the matrix-structure of the material on the one hand and in the pore-structure of the material on the other hand. Another bottleneck in the application of compact heat storage is the reactor design. Reactors using solid materials as ‘working fluid’ are much more difficult to control with respect to efficient heat and mass transfer rates. The goal of this research line is to gain more physical insight into the limiting transport properties of vapor and heat on both the micro- and macro-scale, which will be used to set up design guidelines for new solid sorption materials and compact heat storage systems.
The research is carried out in close co-operation with ECN, the Energy research Centre of the Netherlands. The work is included in IEA (International Energy Agency) Task 42, Compact Thermal Energy Storage: Material Development for System Integration.
Part of the work is financed by the European Graduate School on Sustainable Energy Technologies, which is a network of excellence in a new European University Alliance, at the moment consisting of Danmarks Tekniske Universitet (DTU), Technische Universität München (TUM) and the Technische Universiteit Eindhoven (TU/e).
The research on heat storage focuses on the following topics:
- Development of macro-scale models of heat and vapor transport at reactor level
An experimental setup is developed to investigate the performance of solid sorption materials at reactor level. Hydration and dehydration experiments can be conducted in which the temperature, the relative humidity and the flow rate of the incoming air stream can be controlled. Temperature measurements in the reactor show the propagation of the reaction front and give an indication of the maximum temperature which can be reached. The relative humidity measurements at the outlet of the reactor give insight into the progression of the hydration/dehydration reaction. The results are used to validate a macro-scale model which is developed within COMSOL.
- Development of micro- and meso-scale models for heat and vapor transport
Micro- and meso-scale models will be developed for hydration/dehydration of powdery samples. The micro-scale model will be based on a Monte-Carlo (MC) simulation technique, making use of a stochastic model for the nucleation positions on crystal and grain level followed by isotropic growth of the hydration/dehydration regions. In the next phase this model will be coupled to the vapor and heat transport within the pore-structure of a powdery sample making use of a so-called Coarse-Grained Monte-Carlo (CGMC) technique. The developed micro- and meso-scale models will be validated using experimental data from a parallel running project at ECN. After validation the meso-scopic model can be used to study the transport properties of vapor and heat as function of structure properties of the sorption materials like porosity, grain size (distribution) and the presence of local defects in the matrix-structure (cracks, melts).
- Characterisation of hydration/dehydration reactions and heat and vapor transport in sorption materials
Experimental research is carried out on the level of individual particles and powdery samples to obtain insight into the heat and vapor transport processes on the one hand and to indentify the hydration/dehydration reactions and their accompanying reaction rates and enthalpies on the other hand. At ECN various experimental apparatus are available to study the hydration/dehydration processes at grain and powder level, like SEM, XRD, TGA-DSC, and so forth. Processes will be studied as function of temperature, pressure and the presence of local defects and cracks on grain level, and as function of grain size (distribution), layer thickness, porosity, and again temperature and pressure on powder level. Special attention will be given to the optimization of vapor transport on both levels. The knowledge gained within this project will be used in the development of optimized sorption materials.
- Molecular dynamics to enhance efficient compact heat storage
Here the focus is on the development of a Molecular Dynamics simulation tool for the dynamical behavior of water molecules in the interlayer and on the surface structure of a salt-hydrate grain over a range of temperatures. Molecular Dynamics simulations are especially well suited to address on an atomistic scale questions related to the poorly understood processes of (de)hydration, hydrogen bonding, ion-pair formation, diffusion of dissolved species, adsorption processes and phase separation. Understanding these processes is essential for the identification of the ranges of temperature, pressure and density and of specific material characteristics for optimal heat and mass transfer properties.