T. Bauer*, D. Laing, W.-D. Steinmann, U. Kroner and R. Tamme

German Aerospace Center, Institute of Technical Thermodynamics
Pfaffenwaldring 38-40, 70569 Stuttgart, Germany.

* Corresponding Author, thomas. bauer@dlr. de
Abstract

The utilization of solar generated steam in combination with a suitable storage technology could reduce the fossil fuel dependency significantly. Selection of basic storage concepts strongly depends on the working fluid. In systems using steam as a fluid, most of the heat is

transferred at nearly constant temperature by evaporation and condensation. As a result, latent heat storage systems using phase change materials (PCMs) are suitable, since this storage concept can also operate at nearly constant temperature. The presented work covers material aspects and gives a comprehensive overview of potential organic and inorganic PCMs in the temperature range 120 to 250°C. Measurements of phase diagrams and latent heats of binary alkali nitrate/nitrite systems, previously not published are reported.

Keywords: nitrate, nitrite, phase change material, thermal energy storage, solar process heat

1. Introduction

Thermal energy storage is a vital element in order to improve the energy efficiency of thermal processes and to utilize renewable energy source effectively. Thermal energy storage is based on three major concepts, namely: sensible and latent heat, as well as thermo-chemical storage [1]. Here, the latent heat storage concept is considered. In general, research interests include the qualification of suitable phase change materials (PCMs) and the enhancement of the poor heat transfer caused by the low thermal conductivity of the PCMs. Our ongoing research focuses on high temperature latent heat storage designs for the heat carrier steam [2]. In this storage concept, both the heat carrier fluid (water/steam) and the storage material undergo a phase change at approximately the same temperature. Depending on the steam pressure (2-40 bar), a selection of PCMs with different melting temperatures in the range 120-250°C is required [2].

This paper is directed towards the selection of PCMs in this temperature range. PCMs are classified into two major groups, namely organic and inorganic materials. A further classification criterion is the type of phase change, where melting and solidification (solid-liquid) and the rearrangement of the crystal structure in the solid phase (solid-solid) can be distinguished. Table 1 shows requirements which have to be met for the selection of PCMs, where some can be considered as fundamental and others need to be mainly considered in the design of the storage system.

The equation AH = F • Tm allows an estimation of the heat of fusion of the PCM, where AH is the heat of fusion in J/mol and Tm is the melting temperature in K. For inorganic compounds, the latent heat factor F has typical values between 20 and 50 J/(mol-K) [3]. These values may be also expressed as a multiple of the molar gas constant, e. g. 3R ~ 25 J/(mol-K). This equation shows that the heat of fusion increases with the melting temperature and that compounds with a low molar mass are favorable in terms of large heat of fusion values in J/g.

In literature a variety of PCM options has been proposed [1-9]. First organic and second inorganic PCMs will be discussed (Fig. 1 and 2).