S. Gschwander, P. Schossig, H.-M. Henning Fraunhofer-Institut fOr Solare Energiesysteme Heidenhofstr.2, 79110 Freiburg Tel.: 0761 /4588 5291, Fax: 0761 /4588 9000 Email: stefan. gschwander@ise. fhg. de

Abstract

Phase-Change-Slurries (PCS) are mixtures of a Phase-Change-Material (PCM) and a carrier-fluid. Such PCS of microencapsulated paraffin as PCM and water as carrier — fluid are investigated at Fraunhofer ISE. The shell of the microcapsule prevents an interaction between the paraffin and the water.

At ISE a test-facility was built to study the stability of the capsules while pumped with conventional pumps through common used heating components like pipes, heat exchangers, volume-flow measurement instruments, pressure relief valves etc. To analyze the stability of the capsules SEM-pictures are taken after pumping them sev­eral weeks to control the optical state of the capsules. The specific heat of fusion is checked by DSC-Measurements.

Thermal measurements are carried out to investigate the thermal behavior of the Slurry while pumped through heat exchangers. The results show that the PC-Material can be melted and frozen while flowing through the heat exchangers. The presented results illustrate that microencapsulated PC-Slurries can enhance the heatcapacity of a heat-carrier-fluid and they are also stable enough to be used with common heating or cooling devices.

Introduction

Phase-Change-Materials (PCM) can store heat approximately isothermical in a very small temperature bend. A suspension of microencapsulated paraffin and water is a pumpable Phase-Change-Slurry (PCS). With these kind of slurries it is possible to charge a pCm at one place with heat and to discharge it again at an other place.

At Fraunhofer ISE this kind of PCS-Fluids are studied within a European project for two years now. The aim of this project is to develop PCS, which offer a high specific heat of fusion and also the capability to be used in common pipeworks. In Japan PC-Slurries, espe­cially Ice-Slurries, are already in use, particularly in the sector of building climatisation. With Ice-Slurries the phase transition from water to ice or reverse is used to store high quantities of heat. Ice-Slurries are suspensions of water and ice-particles, additives prevent the aggre­gation of ice in the slurry. With these kind of slurries high quantities of heat can be stored at 0°C. Temperatures below the point of freezing of water has to be reached to generate such an Ice-Slurry.

Comfortable temperatures for humans during summertime reaching from 22 to 24°C [4], so it is not very reasonable to cool down to temperatures below 0°C. Especially chillers are running with a bad efficiency at low evaporator temperatures. PCM with higher melting points could noticeable improve this situation.

These kinds of slurries could also be interesting for heating and storage applications if the melting-enthalpies are high enough. Heat exchangers would operate with smaller
temperature differences thus the heat exchange area could be minimized. Another bene­fit, compared to water in heating applications, would be the lower operating temperatures which will reduce heat losses. Lower mass-flow-rates would be possible because of the high storage density of PCS, that causes lower hydraulic performance and for that less electrical energy is necessary.