# Improved Solar Operation Control for a Solar Cooling System of an IT Center
## 4 Discussion
Figure 6: Solar heat operation over 21 h with the improved control strategy: (**a**) global horizontal irradiation, \(\mathrm{q_{gh}}\), temperatures in 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. Figure 7: Start-up of solar heat operation with the improved control strategy. Due to the lower driving temperature \(\mathrm{t_{D}}\approx 67\)\({}^{\circ}\)C (which is a mixed temperature of \(\mathrm{t_{H}}\approx 70\)\({}^{\circ}\)C from the solar heat exchanger and \(\mathrm{t_{PS11}}\approx 65\)\({}^{\circ}\)C from the storage), the set value for the cooling water \(\mathrm{t_{Ai}^{set}}\) was decreased by the CE controller for a while in order to match the cooling load and chilled water set value, \(\mathrm{t_{Eo}}\approx\mathrm{t_{Eo}^{set}}\). Therefore, the controller of the dry-cooler increased the control signal \(\mathrm{C_{DC}}\) to adjust \(\mathrm{t_{Ko}}\) to \(\mathrm{t_{Ai}^{set}}\) (cf. Figure 6a). Shortly after 11 a.m., \(\mathrm{C_{DC}}\) also increased above the cut-off value \(\mathrm{C_{DC}^{off,SHO}}\) = 0.5 and the cut-off condition \(\mathrm{SHO^{off}}\) according to Equation (35) became true. Thus, for a definite result, \(\mathrm{SHO^{on}}\) and \(\mathrm{SHO^{off}}\) have to be used as set and reset inputs in a bistable function block. With the rising ambient air temperature until 04:00 p.m., the necessary driving temperature \(\mathrm{t^{on,SHO}}=\mathrm{t_{Di}^{on,SHO}}\) also increases according to Equation (28). Since the available temperature from the solar collector and storage is high enough, \(\mathrm{SHO}\) is ongoing. At nighttime, the available storage temperature \(\mathrm{t_{PS11}}\) falls below \(\mathrm{t^{on,SHO}}\). Nevertheless, the second part of the cut-off condition in Equation (35) is not true. Due to the low ambient air temperature, a low cooling water temperature is also available to counterbalance the low driving temperature. Thereby, the chilled water set value is still matched without an increase of the specific electricity demand \(\mathrm{w_{ACCA}}\) (cf. Figure 6d). From approximately 06:30 a.m. of the second day on, the cooling water temperature \(\mathrm{t_{Ai}}\) is limited by the boundary value \(\mathrm{t_{Ai}^{min}}=19\)\({}^{\circ}\)C (cf. Figure 8). With a continuously decreasing driving temperature \(\mathrm{t_{D}}\approx\mathrm{t_{PS11}}\) below 50 \({}^{\circ}\)C, the evaporator outlet temperature \(\mathrm{t_{Eo}}\) starts to increase and the cooling load is not matched anymore. Consequently, \(\mathrm{SHO}\) has to be stopped approximately at 07:30 a.m. The rapid increase of \(\mathrm{t^{on,SHO}}=\mathrm{t_{Di}^{min,SHO}}\) at approximately 7:45 a.m. is an effect of a switching action in one of the heating circuits, which causes a much lower hot water flow rate \(\mathrm{V_{D}}\) for some minutes. For flow rates close to zero, the slope coefficients \(\mathrm{K_{4}^{*}}\) and \(\mathrm{K_{5}^{**}}\) (i.e., the possible capacity of the chiller or its 'thermal size') become very small and consequently the necessary driving temperature \(\mathrm{t^{on,SHO}}\) high. Unfortunately, this interference happened in parallel or at the end of the cut-off delay for \(\mathrm{SHO}\). Nevertheless, it did not switch off the solar heat operation (although it looks like it) and did not have any effect on the end of \(\mathrm{SHO}\) at all. The cut-off condition (cf. Equation (35) or \(\mathrm{SHO^{off}}\) in Figure 8) was true since 07:10 a.m. With the cut-off delay time of 20 min, \(\mathrm{SHO}\) was stopped at 07:30 a.m. A very few minutes later (i.e., after communication between PLC and the building management system), the discharge pump P35 was switched off and the flow rate \(\mathrm{V_{PS}}\) reached zero. Now, \(\mathrm{SHO}\) also stopped hydraulically. Incidentally, at the same moment, \(\mathrm{V_{D}}\) increased after the short interruption.