# Improved Solar Operation Control for a Solar Cooling System of an IT Center
## 5 Conclusions  
For solar cooling systems with an absorption chiller cooling assembly (i.e., including all the supply pumps and the heat rejection device), the possible operating time with solar heat is a key parameter to achieve high solar fractions and thereby high primary energy savings. Hence, for SAC systems, the solar heat operation (SHO) of the absorption chiller should be started as early as possible and should be continued as long as possible in order to reduce the backup heating demand, thereby reducing the main source of the primary energy demand. On the other hand, when the SHO mode is activated too early and/or stopped too late, the available driving temperature from solar collector is low. Consequently, a lower cooling water temperature is necessary to cover the cooling load. It depends on the part load behavior of the chiller and can be calculated by the characteristic equation method. The lower cooling water temperature causes a higher electricity demand in the reject heat device. These theoretical interdependencies were exemplified during a test period with continuous solar heat operation over more than three days at the SAC system of the Federal Environment Agency in Dessau, Germany. It was shown that the resulting electricity demand during solar heat operation might exceed the demand of the reference technology (e.g., a compression chiller cooling assembly) and no primary energy savings are possible anymore.  
In order to extend the solar heat operation period on the one hand, and to ensure a specific electricity demand below a boundary value on the other hand, a model predictive switchover strategy was developed. The combination of an improved characteristic equation method for the part load behavior of absorption chillers with a simple dry-cooler model was used to find a suitable cut-in and cut-off condition for the solar heat operation. The two conditions depend on a minimum solar driving temperature, which has to be reached by the solar collector and storage before solar heat operation is enabled. The minimum driving temperature depends on technical coefficients describing the part load characteristics of the dry cooler and absorption chiller, and also on the load and weather conditions.  
Measurements at a solar cooling system for an IT center with a cooling demand of approximately 20-30 kW\({}_{0}\) around the clock showed that solar heat operation could be started with approximately 67 \({}^{\circ}\)C and was possible down to 49 \({}^{\circ}\)C. The duration of solar heat operation was 21 h in comparison to 13 h of sunshine duration, and the mean specific electricity demand during the whole SHO period was approximately 25% below the average specific electricity demand of the reference cooling technology.  
Although the model predictive switchover strategy utilizes the same method of characteristic equations as the control algorithm of the relevant absorption chiller in the investigated SAC system, it is generally possible to apply the switchover strategy independent of the chiller control. In contrast to the state of the art, where constant (or at least load and weather independent) cut-in temperatures for solar heat operation are used, the new switchover strategy incorporates the actual load and weather condition as well as the part load capability of the absorption chiller and reject heat device. Thus, the new switchover strategy provides the cut-in temperature as a quality of the whole absorption chiller cooling assembly supplied by solar energy.  
This research was funded by the Bundesministerium fur Wirtschaft und Energie (BMWi), grant numbers 03 ET1171A and 03 ET1583.  
The author declares no conflict of interest.