As part of a demonstration project within the framework of the German “Solarthermie 2000 plus” Programme, a pilot installation of the innovative solar heating and cooling system was installed in the R&D building of the ZAE Bayern, division “Technology for Energy Systems and Renewable Energy”, located in Garching (close to Munich / Germany). The building houses offices and
laboratories for about 75 researchers. The pilot installation will simultaneously serve as field test project for a recently developed compact water/LiBr absorption chiller with 10 kW nominal capacity. The chiller has been designed for chilled water supply/return temperature 15 / 18 °C and allows for utilization of a rather moderate driving hot water temperature 85 / 75°C when operated with an open wet cooling tower. In the pilot installation however, the new latent heat storage (LHS) is used in combination with a dry cooling tower, thereby avoiding the typical disadvantages of a wet cooling tower. The driving heat source for the chiller and for the solar heating system is a solar collector field with an area of about 40 m2, located on the roof of the building. The heating and cooling system is designed to supply about 50 % of the building space with heating and cooling.

A 15 kWh PCM-based storage prototype was designed, built, and experimentally evaluated with regards to capacity and power properties. Below, details and results from this work are presented and discussed.

3.1. Design Features

In order to reach as high an IPF as possible while minimizing the cost of the PCM storage, a novel design was incorporated into the prototype named HEATPACK. In the HEATPACK, the PCM is enclosed in a cylindrical tank without further packaging material (e. g., balls or bags), and an extended surface tube heat exchanger is submerged in the PCM. The challenge of the concept is to in addition to high capacity achieving a high power of heat transfer, while maintaining low cost. A similar configuration is under examination in e. g. Austria, and has shown excellent power properties as reported within the framework of the IEA Solar Heating and Cooling implementing agreement [4].

One difference though is the cylindrical configuration of HEATPACK, and that the Austrian concept used finned tubes. Other alternative configurations excluding packaging materials are e. g.: the spiral cylinder concept examined by Banaszek et al [5]; or the flat plate heat exchanger concept used in a free-cooling application by Zalba et al [6].

Here, the HEATPACK prototype was built with a volume of close to 140 liters, the specific storage capacity was around 100 kWh/m3, and the IPF was close to 80%. The Austrian concept mentioned above had a storage capacity estimated to 76 kWh/m3 [4]. For a water storage with the same volume, the capacity is approximately 5 kWh assuming a 25 °C temperature difference. The PCM used was, as mentioned before, a commercial salt-based PCM with a theoretical melting point at 58 °C.