M. Hadjieva1*, M. Bozukov1, Ts. Tsacheva2

1 Central Laboratory of Solar Energy & New Energy Sources, Bulgarian Academy of Sciences,

72 Tzarigradsko Schosse blvd., 1784 Sofia, Bulgaria

2 Institute of Physical Chemistry, Bulgarian Academy of Sciences, G. Bonchev, bl. 11, 1113 Sofia, Bulgaria

* Corresponding Author, sthermal@phys. bas. bg

Abstract

The modified phase change materials (PCM) are the promising new source for cost effective heat storage systems integrated in industrial process solar/heat technology. Application of the modified PCM storage units to heating/cooling technology allows a multiple utilization of solar/thermal energy by repeatable heat cycling process. After years of development and use of typical PCM with diverse heat returns in practice, the modified PCM, which succeeded over some PCM drawbacks or/and add a new function, are a practical choice for heat storage and recovery of solar/thermal energy. Novel modification of the inorganic salt eutectics absorbed within a graphite matrix to produce a highly thermally conductive multifunctional composite was designed for thermal storage at a high temperature cooling technology (up to 250oC). The (Na/K)NO3 eutectics/graphite composite improved thermal conductivity of typical PCM like inorganic salts, (Na/K)NO3 eutectics, which temperature range of phase transition and heat storage capacity are appropriate for solar/thermal storage at the industrial solar steam process. Detailed analysis and control of the specific structuring of salt/graphite composites for long-term thermal stability and function at high temperature heat storage technology of solar concentration electrical plants were in the special focus of this study. Results showed elemental separation and forming the Na-rich spherical masses and the K-rich layered structure over the graphite plane as the effect of the eutectics solidification mechanism that influence strongly thermal storage behaviour of the (Na/K)NO3 eutectics/graphite composite.

Keywords: Phase change material, (Na/K)NO3 eutectics, PCM/graphite composite, latent heat storage technology

1. Introduction

The modification of phase change materials responds to new ideas of solar/thermal industrial technologies to ensure complete energy utilization through heat storage process. Integration of thermal storage system in novel technologies is an ultimate need for well-organized economical functioning and saving heat/energy. Knowledge, collected on typical PCM and thermal storage applications with a varied income in a past century, lets to meet requests of new thermal technology demands. Progress in advanced PCM modification for improving typical PCM behaviour or use additional PCM functions for technology perspectives need a multidisciplinary innovative work to produce efficient storage materials according next generation PCM requirements. The successful paraffin modifications with graphite were developed for achievement of a quality new property (thermal conductivity) and advantage of the PCM heat

storage systems for an industrial process technology. [1,2]. The PCM composites, based on (Na/K)NO3 eutectics and graphite, were elaborated in the framework of EU project DISTOR [3].