In order to optimize a tank-in-tank system, simulations have been carried out with different DHW profiles and space heating loads. For tank-in-tank systems, the height and diameter of the inside tank might play an important role in the tank design. Therefore, in the current study, the effect of height and diameter of the upper and lower part of the inside tank is investigated by means of 7 different tank-in­tank designs. Model 1-4 has the same DHW tank volume of 120 litre. In Model 2 and 4, the upper part of the inside tank is located at the same height above the level where the outlet to the auxiliary boiler (middle of the tank) is located. But in Model 1 and 3, the upper part of the inside tank is located at the same level as the inlet of the auxiliary boiler. Model 1 and 2 has larger diameter than Model 3 and 4 in the upper part but smaller diameter at the bottom. In Model 5, 6 and 7, the DHW tank volume is increased to 160 litre and have the same diameter. In Model 5, the upper DHW volume of the inside tank is located above the outlet to the auxiliary boiler, in Model 6, it is located at the same level as the outlet to the auxiliary boiler and in Model 7, it is located below the outlet to the auxiliary boiler.

Fig. 5 shows the net utilized solar energy versus the space heating load for 6 m2 solar collector area and different tank-in-tank designs with the Task 26 domestic hot water profile. The net utilized solar energy is the space heating demand + the domestic hot water consumption — the auxiliary energy supply to the top of the heat storage tank. It can be seen that Model 7 has the highest thermal performance compared to the other models followed by Model 6, 5, 2, 1, 4 and 3 respectively. That is: The lower the set point temperature, the higher the thermal performance. It is also seen that the thermal performance of the system is increasing for increasing the space heating demand. With the Task 26 domestic hot water profile, the thermal performance is increasing for increasing auxiliary volume of the hot water tank.

Fig.5. Net utilized solar energy versus space heating load for different tank-in-tank designs with Task 26 DHW

profile

The same tank-in-tank systems are also investigated with the different DHW profiles: High, medium and low flow rates as shown in Fig. 4. Fig. 6 shows the net utilized solar energy versus the space heating load for the seven different models and the high flow rate DHW profile. Model 4 has the highest thermal performance compared to the other models. After model 4 follows Model 1, 6, 5, 7, 3 and 2. The thermal performance of the systems are increasing for increasing space heating load and that the thermal performances of the systems are much higher with the same average daily hot water consumption than with the Task 26 profile. This is caused by the high required set point temperature for the Task 26 profile, in turn due to the fact that the flow rate is even higher than in the high flow

DHW profile and that two discharges can follow one another with short or no interval. For both low and medium flow rate DHW profile, the trend of the net utilized solar energy for the different models is the same. The difference of thermal performance between the best and worst model is about 2-3%.

The differences of the thermal performances caused by the three different DHW profiles are small.

Fig. 6. Net utilized solar energy versus space heating demand for different tank-in-tank designs with high flow

DHW profile