10.07.2015   EUROSUN 2008

Water tank storage

For solar combisystems, water tank storage is the state-of-the-art choice [2]

However there are ways to improve the efficiency of water tank stores and the overall performance of a water based combisystems. Several topics deserve R&D attention to be improved: stratification devices, heat losses, system integration.

Fig. 2: In these detailed simulations from DTU, Denmark, the best performer in Zurich climate is clearly the double stratifier solution.

When there is no stratifier (figure 4), the internal heat exchanger solution performs best.

Fig. 3. Detailed simulation from DTU Denmark showed that model 2 showed slighlty better annual performance than the two others

Stratification devices allow better stratification of hot water in a tank so that less auxiliary will be needed to meet the peak load during DHW draw off for example. There are several solutions on the market but all are somehow complicated and expensive. Pervious fabrics have been investigated in Denmark and the best choice exhibits promising results compared to current devices. It is a two concentric layers system, where water can leave at any level depending on its temperature relative to Denmark investigated 3 configurations of combisystems whose only variation is the stratification scheme in the water tank (figure 2). The alternative with 2 stratification devices, one for the solar loop and one for the return loop perfoms best with a 20% improvement of the annual solar energy delivered to the load in the best case compared to the no stratification device case.