New economical phase change material
Scientists at the University of Bayreuth are optimizing mobile latent heat storages. In the project MALATrans they improve charging and discharging capacity of the heat storages and analyse how the manufacturing costs can be reduced. The phase change materials they are working with are macroencapsulated.
|Project status||Project completed|
|Type of storage||Latent heat storage (several groups of materials)|
|Research objective||Storage material; storage construction (components for charging and discharging)|
|Temperature type||low temperature|
|Storage time||short (hours to days)|
|Number of cycles||> 200 per year|
|Charging temperature||80 - 160 °C|
|Discharging temperature||60 - 140 °C|
|Storage capacity||ca. 4 GJ|
|Energy storage density||300 MJ/m3|
|Project duration||July 2013 until December 2016|
The use of highly efficient mobile thermal storage units is an essential starting point for the expanded use of waste heat. The goal of this project in which the University of Bayreuth is involved with its Department of Engineering Thermodynamics and Transportprocesses together with its Department of Metallic Materials is to expand the current knowledge in the field of latent thermal storage so that more effective mobile latent heat storage systems can be successfully developed upon completion of the project.
Such a storage should have a higher thermal capacity and an increased power for loading and unloading compared to versions which are already available. Furthermore, it should be cheaper to produce and above all develop a more expanded potential for application.
Improve heat transfer
The desired improvements can be achieved by the transition from previous full storage units to macroencapsulated PCM which have a larger effective heat exchange surface and which are easy to produce and to fill. Thus the efficiency of such a heat transport can be improved significantly. The range of applications of mobile latent heat storage is significantly expanded by applying PCM with melting temperatures between 70 °C and 150 °C. Previous storage systems based on sodium acetate trihydrate with its low melting point of 58 °C are suitable only for the supply of few specific heat sinks, e.g. for heating swimming pool water.
Effective means of macroencapsulation
In exploring efficient ways of macroencapsulation, many parameters are to be investigated and optimized, such as geometry, size, material and wall thickness. Systematic parameter variations are performed by numerical simulations. Suitable models are to develop and to validate. In a laboratory storage prospective capsule geometries and fillings are tested in practice. Furthermore, diverse thermodynamic as well as chemical and physical property data must be determined by measurements when identifying suitable PCM. Moreover, the compatibility of PCM and capsule materials will be examined in detail as well as the economic feasibility in a later implementation of the developed approaches. A special feature compared to ordinary storage projects is the holistic view on the whole storage unit including the storage material, capsule material up to the capsule fill.
Significant performance improvement with copper and aluminum
Before varying the capsule geometries, an investigation into various additives for the PCM had been made in order to estimate their potentials for a performance improvement in loading and unloading. The largest performance gains can be achieved with rod-shaped copper and aluminum additives. This resulted in a shortening of a charging cycle by about 40 % by adding 10 % per weight. As part of subsequent work the question shall be adressed whether, from an economic point of view, additives are preferable to an increased capsule surface.
An important aspect of the economy is the number of cycles which should be significantly increased by a higher charge and discharge performance in the framework of this project.
Numerical models for describing the phase change phenomena inside the capsules are developed. Phase change materials (PCM) and encapsulation materials are selected and studied with respect to their suitability. Furthermore, first conceptions for the capsule design are carried out. Next steps are the simulation of the thermal performance of an entire storage unit and the thermal fatigue of the encapsulations. We intend to continue work with the analysis of the compatibility of PCM and encapsulation materials as well as the construction of a storage unit at bench-scale.
Modeling and simulation
The cyclic heating and cooling of the capsules leads to fatigue of the material and over time to stress corrosion. Therefore, Finite-Element-Simulations (FEM) are carried out for prevention during the conception phase. The modeling and simulation of the phase change phenomena is a key aspect regarding the understanding and specific use of the occurred effects. Hence, the enthalpy method has to be adopted in such a manner that the natural convection inside the capsules as well as the settling of the solid phase are respected. Furthermore, the simulation of the entire storage unit is of great interest. The main aspects in this topic are the flow of the heat transfer fluid around the capsules and the thermal performance of the overall storage unit during charging and discharging.
Materials data and compatibility of materials
In this sub-project the most promising PCM and encapsulation materials are identified. A list of requirements which takes into account thermodynamic and technical as well as ecological and economic criteria is the basis for the evaluation of the storage materials. The selection of the encapsulation materials for further investigation includes non-ferrous metals and low and high alloy steel. The potential of coated materials and the compatibility of the encapsulation material and the PCM are studied as well. In addition, all relevant material properties of the encapsulation and storage materials are measured.
Laboratory scale storage unit
The aim of this sub-project is to design a storage unit at laboratory scale including corrosion resistant and highly efficient capsules. The interdisciplinary task, combining thermodynamics, material science, process engineering and economy, calls for an intensive cooperation of the participating departments. The developed capsules will be tested and investigated, in addition to the numerical simulations, in a laboratory scale storage unit
Transfer of results and documentation
The generated knowledge as well as the simulation and measurement results of this project represent an essential contribution to the improvement of storage systems based on macro-encapsulated PCM. Mobile applications and their corresponding requirements are the main focus of this project, but nevertheless a lot of scientific findings can be transferred to stationary storage systems.