1.1. Results of the laboratory measurements [2]

All storage solutions dealt with in Subtask C were only laboratory prototypes. Measured results and projected heat storage densities for units of 70 and 1000 kWh storage for single family houses are reported. The prototypes use either paraffins or sodium acetate trihydrate, but all of them had a phase change at about 58°C in order to provide space heating and domestic hot water. The system from HEIG-VD additionally uses a PCM with phase change at 27°C in the preheating zone of the buffer store.

The prototypes are intended for different applications. While the stores from HEIG-VD, Switzer­land and University of Lleida, Spain are short term heat storages for solar combisystems, the store from the Technical University of Denmark is used as seasonal storage by making use of the sub­cooling effect in hydrated salts. The work of Graz University of Technology is dealing with very short term storage for boilers, to reduce start-stop cycles and emissions. For small short term storages one decisive factor is to deliver enough thermal power for the domestic hot water demand (26 kW e. g. for filling a bath tub of a single family residential building). This means high specific power and therefore either high thermal conductivity of the solid PCM and/or small distances for the heat transfer from PCM to the heat carrier. For larger stores this problem is far smaller due to the lower necessary specific power. The projects were financed partly from national and partly from European Union projects.

The storage density compared to water is strongly dependent on the temperature lift in the storage tank. For small temperature differences (50 — 70 °C) and a bulk PCM tank with immersed heat exchanger (like the store used at the Graz University of Technology), the store can be theoretically sized about 1/3 of the volume compared to water, if sodium acetate trihydrate is used as PCM.

With this layout additionally about 20 kW thermal power can be delivered for the DHW produc­tion with less than 8 K heat loss. For the same PCM-material but macro-encapsulated and for a temperature lift from 25 to 85°C or 20 to 70°C in solar combisystems the store has the same size as a water store. For such cases there is even theoretically little benefit from PCM with respect to the store size.

In terms of material cost, all materials are expensive compared to water, ranging from pure sodium acetate with about 1 €/kg, paraffin with about 2 €/kg (including nucleation enhancer) to sodium acetate trihydrate with graphite and nucleation enhancers with about 3 — 4 €/kg. The cost for the whole storage system has not been estimated here.