Simulations

The simulation tool allows the user to choose different parameters related with the simulation process: time-step, accuracy, etc. The smaller the time-step, the longer the simulation time but the more precise are the results concerning the PCM interaction. This statement has to be taken into account when a PCM simulation is carried out. It was checked in previous simulations that bigger time-steps (to perform faster simulations) hid the operation of the PCM.

The length of the simulation can be chosen by the user going from daily simulations to two years simulation. Four different climates can be selected representing the four most common climates in Europe: Stockholm (Sweden) for a moderate northern climate, Zurich (Switzerland) for a moderate central climate, Barcelona (Spain) for a coast Mediterranean climate with high humidity and temperature in summer but a mild winter, and Madrid (Spain) with a continental Mediterranean climate with high temperature but low humidity in summer. The climate chosen for the simulations carried out was Madrid.

The building simulated is a two 70 m2 storey single family house facing south. Four different buildings can be selected and their differences depend on the energy demand of the building, existing an energy demand of 15, 30, 60 o 100 kWh/m2. Even a 100 kWh/m2 without shading can be chosen in order to simulate cooling demand. The 60 kWh/m2 was the selected house for the simulations performed.

The auxiliary system had a nominal power of 10 kW and the water introduced into the store was heated up to 63 °C (set point). The collectors field is 10 m2 of flat plate collectors facing south with a slope of 45 °C. The storage tank volume is 800 L following the recommendations of W. Weiss [6].

Two different types of simulations were carried out. In the first one, the aim was to observe the influence of some parameters in the result of the simulations. The parameters changed were the different inlets and outlets of the store and the position of the temperature sensors that operates the auxiliary system. Annual simulations were carried out and the results were evaluated for every month
and every week. Temperatures into the store and in every inlet and outlet of the tank as well as mass flow rates were the values checked to observe the influence of the PCM in the system behaviour. Fig. 2 shows the relative position of each sensor into the store.

image010 image011 image012 Подпись: Variable Description Tfluid4 Fluid temperature at sensor 4 Tfluid3 Fluid temperature at sensor 3 Tfluid2 Fluid temperature at sensor 2 Tfluid1 Fluid temperature at sensor 1 Tpcm PCM temperature Tpcm PCM temperature TSA Water temperature from store to auxiliaiy TAS Water temperature from auxiliary to store TSB Water temperature from store to space heating TBS Water temperature from space heating to store TSD Water temperature from store to DHW TXdS Water temperature from DHW heat exchanger to store TSC Water temperature from store to solar collectors TXsS Water temperature from solar collectors heat exchanger to store Tssa1 First on/off auxiliary temperature sensor Tssa Second on/off auxiliary temperature sensor

The second set of simulations was mainly focused on the influence of the PCM. Three different values of the Parea ratio were simulated: 0.25, 0.5 and 0.75. Also the geometry of the module was changed, different lengths of the PCM modules and different diameters were simulated. For a given PCM mass, the thinner the module was, the higher the number of modules into the storage. Also two different kind of PCM were simulated into the store focused in the two different demands. The sodium acetate — graphite compound for the DHW demand and placed at the top of the tank and the RT48 paraffin for the space heating demand and placed in the middle of the tank.

TSC

Подпись: TXdS

Fig. 2. Position of the monitored sensors inside the storage tank