P Schossig, H.-M. Henning, T. Haussmann
Fraunhofer-institute for Solar Energy Systems ISE
Heidenhofstr. 2, 79110 Freiburg
Tel: 0049 761 /4588 5130, Fax: 0049 761 /4588 9000
email: schossig@ise. fhg. de

Abstract

The idea to enhance the thermal comfort of lightweight buildings by integrating Phase — Change-Materials (PCM) into the building structure was targeted in several research projects over decades. Most of these attempts handled macro-capsules or direct im­mersion processes, both techniques with several drawbacks. Due to these problems, none of this PCM-products was successful a wider market. The new possibility to mi­croencapsulate PCMs, a key-technology which overcomes lots of these problems, may open the building industry for PCM-Products.

This paper describes the work at Fraunhofer ISE done in a german government founded project over the last 5 years, from building simulations to first measurements of real size rooms equipped with PCM. First products are now available on the market.

1. Introduction

Since the 1970’s several researchers have tried to use PCMs in buildings to enhance the thermal comfort of lightweight constructions, especially to overcome the overheating prob­lems in summer. They have used techniques like immersion processes ([1],[2]) or macro­capsules (e. g.[3]) to integrate the PCM. Both types have different drawbacks. Unencapsu­lated PCMs may interact with the building structure and change the other properties of the matrix materials or leakage may be a problem over the life time of many years. Macrocap­sules have the disadvantage that they have to be protected against destruction while the building is used (no drilling or nailing in the walls/ceiling). Next the macrocapsules often need much more work at the building site to be integrated into the building structure, mak­ing these systems expensive. Another problem with macrocapsules is the decreasing heat transfer during the solidification process when using PCMs with bad heat transfer coeffi­cients in the solid state like paraffins. This may lead to limitations to discharge the system over the night. Due to these limitations, none of the PCM-products had a big market im­pact.

Recent advances in the technology of microencapsulation may change this situation. The possibility of formaldehyde-free microencapsulation of paraffins allows the straightforward integration of PCMs into conventional construction materials. These only a few microm­eter big capsules can be integrated in special optimised building materials, independent from the phase of the PCM. This solves the above mentioned problems: the capsule shell prevents the interaction between the PCM and the matrix material, there is no extra work

at the building-site to integrate the PCM-products and the capsules are small enough that there is no need to protect them from destruction. The distribution of the small PCM cap­sules in the wall offers a much bigger heat exchange surface, so the heat transfer to charge and discharge the storage is enhanced significantly Fig. 1 shows a schematic drawing of this idea to integrate PCM-microcapules in plaster and fig. 2 a SEM (Scanning electron mi­croscope) picture of these capsules in gypsum plaster.

The Fraunhofer ISE researches in a cooperative project, supported by the german ministry for science and work (BMWA), the opportunities resulting out of the new key-technology of microencapsulation. The industrial partners involved are the Companies BASF, Caparol, maxit and Sto. The work at ISE started with building simulations to identify reasonable ap­plications and useful material parameters. In parallel we did material testing from DSC — measurements over 50 cm x 50 cm wall samples to real size office rooms.