Prototype PCM Storage Evaluation

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.