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.