Figure 3. DHW storage tank with an internal heating coil (background) and heating mixing shunt (foreground)
a pre-set value is exceeded. While the functionality sounds straightforward, it has certain complexities in determining an appropriate setting. A too high setpoint results in no or limit- ed engagement of the function, and a too low setpoint com- promises the DHW temperature. Further, the optimal setpoint varies over the year due to seasonal variations in the cold-water temperature and the district heating supply temperature. The ratio of the DHW consumption and DHW circulation will also influence the optimal settings. Examples of seasonal variations are given in the figures below: As Figure 4 shows, the cold water varies by 12°C during the year, and the daily DHW demand varies by ~+/-35% of the annual average DHW demand. Based on these variations, it’s under- standable that the setting is not straightforward. An adaptive method was developed to address this challenge. The adaptive return temperature principle continuously adjusts the control- ler settings and adapts to the actual boundary conditions. An example where the adaptive control is compared to the refer- ence control can be seen in Figure 5. In the shown example, a reduction of the district heating re- turn temperature of 3.7°C and 4.6°C was realized, compared to pre- and post-reference periods, respectively. The temperature spikes are related to the disinfection of the DHW system, which has a similar impact on the reference and adaptive control pe- riods. Because of the control principle, the DHW temperature is slightly reduced in short periods during the adaptive control period, engaging the limiter function in an optimal way. In addition to the adaptive district heating return temperature limiting function, a power limiting function was developed. This control principle requires an energy meter to measure the power supplied to the DHW tank, which is becoming more ap- plied due to general energy awareness and heat cost alloca- tion. An example is given in Figure 6, where a reduced district
Often DHW tank applications suffer from high district heat- ing return temperatures and high-power peaks. This typically relates to poor controller settings, wrong control valve sizing, poor heat transfer via the tank coil, poor DHW stratification, and a high share of DHW circulation loss compared to DHW tapping. To improve the performance of DHW tank applica- tions, a new functionality was developed for the Danfoss ECL controllers. The new functionality is an intelligent district heat- ing return temperature limiting function. It works so that when a pre-set return temperature is exceeded, the charging flow to the tank coil is reduced, and thus the district heating return temperature will be kept below the pre-set value.
In the same way, there is the option for a power limiting function, limiting the district heating charging power when
Figure 4. Seasonal variation of cold-water temperature (left) and energy used for preparing DHW (right)
Made with FlippingBook - Online magazine maker