Abstract
The design of phase-change material (PCM)-based thermal energy storage (TES) systems is challenging since a lot of PCMs have low thermal conductivities and a considerable volume change during phase-change. The low thermal conductivity restricts energy transport due to the increasing thermal resistance of the progressing phase boundary and hence large heat transfer areas or temperature differences are required to achieve sufficient storage power. An additional volume has to be considered in the storage system to compensate for volume change. Macro-encapsulation of the PCM is one method to overcome these drawbacks. When designed as stiff containers with an air cushion, the macro-capsules compensate for volume change of the PCM which facilitates the design of PCM storage systems. The capsule walls provide a large surface for heat transfer and the thermal resistance is reduced due to the limited thickness of the capsules. Although the principles and advantages of macro-encapsulation have been well known for many years, no detailed analysis of the whole encapsulation process has been published yet. Therefore, this research proposes a detailed development strategy for the whole encapsulation process. Various possibilities for corrosion protection, fill and seal strategies and capsule geometries are studied. The proposed workflow is applied for the encapsulation of the salt hydrate magnesiumchloride hexahydrate (MCHH, MgClHO) within metal capsules but can also be assigned to other material combinations.
Highlights
Phase-change materials (PCMs) possess the ability to store large amounts of thermal energy when applied around their phase-change temperature
A gasket made of FKM is applied as sealing element between the PCM and the ambient as this material is compatible with the selected materials and is stable up to the desired temperatures of 150 ◦C [37]
The procedure was discussed in detail with the focus on encapsulation of inorganic PCMs within metal capsules
Summary
Phase-change materials (PCMs) possess the ability to store large amounts of thermal energy when applied around their phase-change temperature. This enables the design of compact thermal energy storage systems (TES). Metals are considered as capsule materials due to their temperature resistance, non-flammability, mechanical strength and high thermal conductivity. The pressure alternation due to phase-change with different amounts of PCM within the capsule has been calculated applying the combined gas law neglecting thermal expansion of the encapsulation material. This research proposes a design strategy with detailed consideration of the whole encapsulation process, starting from the boundary conditions given by the TES application to the material selection and the encapsulation process. The principle is applicable to other materials as well like organic PCMs in metal or plastic containers
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