# Improved Solar Operation Control for a Solar Cooling System of an IT Center
## 2 Materials and Methods
### Operating Period with High Solar Fraction
When the cut-in condition became true, the flap near the solar heat storages was opened by the control signal \(\mathrm{C_{PS}}\) and the flaps near to the district heating heat exchanger \(\mathrm{C_{DH}}\) were closed. Afterwards, the discharge pump P35 was switched on and the measured discharge flow rate \(\mathrm{V_{PS}}>0\) was used to supply the absorption chiller with solar heat. The other curves in Figure 3 will be explained later. Figure 2: Continuous solar heat operation extending over more than three days during a test period (i.e., without the improved control strategy): (**a**) global horizontal irradiation, \(\mathrm{q_{gh}}\), temperatures in the hot water circuit, and control signals from RHD; (**b**) flow rates and cooling capacity; (**c**) ambient air temperature and temperatures in the chilled and cooling water circuit; (**d**) specific electricity demand. The examined solar heat operation period was ongoing over more than three days (cf. V\({}_{\rm PS}\) in Figure 2b). Despite the volatile solar irradiation and the resulting volatile driving temperature t\({}_{\rm Di}\) for the chiller, it was possible to control the evaporator outlet temperature t\({}_{\rm Eo}\) with a deviation to the set value t\({}_{\rm Eo}^{\rm set}\) of less than \(\pm 0.5\) K (cf. Figure 2c) by means of the CE controller. Although the flow rates of the absorption chiller were not automatically controlled, they were not constant (cf. Figure 2b). The variation of V\({}_{\rm D}\) is an unwanted effect of hydraulic interdependencies between the discharge pump, heating circuits, and absorption chiller. Therefore, flow rates higher than the design value occurred. In contrast, the cooling water flow rate V\({}_{\rm H}\) was adjusted manually (on the third day in order to avoid the humidifier activation, which was not successful, and afterwards to cover the load and keep the solar heat operation ongoing). During normal operation (i.e., outside the test period for the high solar fraction), SHO is stopped when t\({}_{\rm HO}<\) t\({}_{\rm DI}^{\rm set}\) and the difference \(\Delta\)t\({}_{\rm S}^{\rm offSHO}=\) t\({}_{\rm 2 DI}^{\rm set}-\)t\({}_{\rm PS11}>\) 0 K becomes true. Since this cut-off condition was deactivated during the test period, the desorber inlet temperature t\({}_{\rm DI}\) varied between approximately 85 and 50 \({}^{\circ}\)C and the cooling water inlet temperature to the absorber t\({}_{\rm Ai}\) between 36 and 18 \({}^{\circ}\)C, respectively. To achieve these low cooling water temperatures in the reject heat device, the technical possibility to pre-cool the incoming ambient air by spraying water into the air flow (adiabatic evaporative pre-cooling) was allowed, although this operation mode is normally blocked due to high operating costs and hygienic aspects. By means of two short operating periods of the humidifier on the third day (cf. control signal C\({}_{\rm Hu}\) of the humidifier in Figure 2a), a termination of SHO could be avoided. However, independent of the adiabatic pre-cooling possibility, high air flow rates in the reject heat device were necessary when the storage temperatures were still low (e.g., in the morning) and/or ambient air temperature was high (cf. control signal C\({}_{\rm DC}\) for the ventilators in Figure 2a). Thus, the specific electricity demand \(w\) of the solar thermally driven absorption chiller cooling assembly (ACCA) including the electricity demand of the thermally driven chiller (TDC), the supply pumps (Pu) and reject heat device (RHD):  
\[w_{\rm ACCA}=\frac{\rm P_{el,ACCA}}{\rm Q_{E}}=\frac{\rm P_{el,TDC}+P_{el,Pu}+P_ {el,RHD}+P_{el,RHD}^{\rm aux}}{\rm Q_{E}} \tag{1}\]  
became higher than the specific electricity demand \(w_{\rm CCCA}\) of an electrically driven compression chiller cooling assembly (CCCA) also including the chiller itself, supply pumps, and the reject heat device. In this case, no primary energy saving was possible anymore and the solar heat operation should have been stopped.