The scope, in terms of general system aspects, for Subtask C was the same as that for the whole of Task 32, namely solar heating and cooling systems for residential buildings, principally detached houses for one up to a few families. Buildings with a larger specific heat load (>100 kWh/(m2a) for Zurich climate) are not considered. The main focus was to find storage solutions sized to achieve a significant solar fraction but also for other applications in the heat storage field for domestic housing, especially to reduce the cycling rate of conventional boilers.

Some solutions using PCM have already been tested in full scale pilot plants and some durable commercial products are already on the market for special applications (Cristopia, Rubitherm among others).

Detailed activities included

• the selection of suitable materials,

• the development of storage prototypes and

• the optimization of existing solutions in an integrated system such as the reference combisystem defined by Subtask A.

All solutions with PCM stores were compared to pure water stores.

Fig. 1 shows a classification of processes for PCM storage of heat, in Subtask C only the paraffins (analytical grade) and hydrated salts have been addressed.

In terms of temperature, the storage solutions have been limited to temperatures < 85°C, because the maximum needed temperature for the domestic applications with low temperature heating systems is the DHW demand with around 50°C. The phase change temperature of the materials chosen (mainly sodium acetate trihydrate, partly embedded in a graphite matrix to increase the thermal conductivity) is at about 58°C. For some other tests additional PCM with a lower phase change temperature was chosen (paraffin).

Simulation models of the PCM storage component were developed for different types of PCM heat store philosophies, as no validated models were available for the simulation software TRNSYS at the beginning of the task. These models were validated by laboratory tests before being integrated into a system model within TRNSYS. Each Subtask was responsible to develop an appropriate tool, in order to enable an estimation of the performance of a system with the proposed storage concept.