Category Archives: EUROSUN 2008

Test results from a Latent Heat Storage developed for a Solar Heating and Cooling System

Harald Mehling*, Stefan Hiebler, Christian Schweigler, Christian Keil, Martin Helm

Bavarian Center of Applied Energy Research (ZAE Bayem), Walther-MeiBner-Str.6, 85748 Garching,

Germany

* ph:++49 89 329442-22, fax: :++49 89 329442-12, e-mail: Mehling@muc. zae-bayern. de

Abstract

In solar thermal installations, full annual utilisation is desirable. This can be achieved with solar space heating during the cold season and solar cooling by means of sorption cooling in the warm season. When low temperature heating and cooling facilities like floor or wall heating systems or activated ceilings are applied for heating and cooling, a low-temperature heat storage using the latent heat of phase change materials (PCM) can be used to significantly improve the system in the cooling and also in the heating mode. After completion of tests on functional models, a full scale storage with about 2 tonnes of CaCl26H2O as PCM and capillary tubes as heat exchanger was built in fall 2006. The storage consists of two modules with a total volume of 1.5 m3 and has a design storage capacity of 120 kWh in the temperature range from 25 °C to 33 °C. At first, the performance of the storage was determined in standalone tests. Then, the storage was integrated into the new system for solar heating and cooling at the ZAE Bayern. This paper reports on the stand alone tests, and the tests performed after the storage was integrated into the system during summer operation in 2007 and winter operation 2007/2008.

Keywords: latent heat storage, solar cooling, absorption chiller, phase change

1. Introduction

In solar thermal installations with large capacity, full annual utilisation is desirable. During the cold season, solar heat serves for space heating. During the warm season, solar heat can be converted into useful cold by means of sorption cooling. A favourable situation is given when low temperature heating and cooling facilities like floor or wall heating systems, or activated ceilings, are applied for heating and cooling. In that case a low-temperature heat storage using the latent heat of phase change materials (PCM) can be used to significantly improve the system in the heating and also in the cooling mode. In the heating mode the heat storage is used to level the highly variable solar gain. In the cooling mode the storage is used to reject waste heat in combination with a dry cooling tower, instead of using a wet cooling tower. To allow the use of a single storage in the heating mode as well as in the cooling mode to reject waste heat, the heat has to be stored in a very narrow temperature range around 30 °C.

Results — Cost Analysis

The results of this analysis are best summarized in Fig.1 below. This figure shows the specific storage capacity [kWh/m3], as well as the specific storage cost [Euro/kWh stored heat], as a function of the ice packing factor (that is, the volume fraction of PCM in the storage). Zero percent IPF is thus equivalent to a conventional stratified water storage. This particular case is assuming a AT for storage of 25 °C.

As shown, when considering the capital (first) cost only, the cost of the PCM-storage is always higher than for a water storage (see IPF=0) although the difference is not very large. However, if the cost of “space requirement” is important, such as in a house, the PCM — solution quickly becomes cost

Fig. 1. A comparison of a PCM thermal energy storage (IPF>0) to a conventional hot water storage (IPF=0) with regards to specific storage capacity (diamonds) and cost (squares).

effective as compared to the hot water storage. Assuming a cost of space of 300 Euro/m3, the specific storage cost levels off at just below 48 Euro/kWh regardless of IPF such that the cost of a PCM storage is the same as that of a conventional stratified water storage. Then the advantage of the PCM storage is clear — approximately one third the space requirement as compared to a water storage.

Fig. 1. also shows that one important attribute affecting the specific storage capacity is of course the Ice Packing Factor so that designing a PCM storage with as high an IPF as possible is good for the technical competitiveness of this technology option for storage. As the IPF increases, the cost- effectiveness of the PCM-storage is also likely to be enhanced. This finding was taken into account when designing the storage prototype presented below.