# Improved Solar Operation Control for a Solar Cooling System of an IT Center
## 4 Discussion  
The model predictive switchover strategy for extended solar heat operation (SHO) with improved cut-in and cut-off conditions has been implemented into the industrial programmable logic controller (PLC) of the single-stage \(\rm{H_{2}O}\)/LiBr-absorption chiller of type FM050v0.3 operated at the Federal Environment Agency (UBA) since 2011. Nowadays, this chiller is marketed as type \"Bee\" [19]. Via a Profi-Bus-connection, the information for the control signals of the flaps \(\rm{C_{PS}}\) and \(\rm{C_{DH}}\) is sent to the building management system, where it is put into execution. The switchover from BHO to SHO without a model-based cut-in condition is described in Figure 2. Now, the improved switchover strategy including a model-based cut-in/off condition is described. The measurements and overall results of the strategy are depicted in Figure 6 for an operating period of 2 days in September 2019. In Figures 7 and 8, the cut-in and cut-off time periods are shown separately. On the first day, solar heat operation of the absorption chiller started at 11:00 a.m. and ended on the next day at approximately 08:00 a.m. Thus, the duration of the solar heat operation period is 21 h in comparison to 13 h of sunshine duration, i.e., from 06:30 a.m. until 07:30 p.m. The driving temperature \(\rm{t_{Di}}\) used in SHO started with approximately 67 \({}^{\circ}\)C, increased up to 78 \({}^{\circ}\)C in the afternoon of the first day, and decreased down to 49 \({}^{\circ}\)C in the morning of the next day. The corresponding cooling water temperatures \(\rm{t_{Ai}}\) of 25, 29, and 19 \({}^{\circ}\)C, which are necessary to cover the nearly constant cooling load of 23-29 \(\rm{kW_{0}}\), were supplied by the heat rejection device in dry-cooler mode with a control signal \(\rm{C_{DC}}\) of less the 60%. Accordingly, the mean specific electricity demand during the whole SHO period \(\overline{w}_{\rm{ACCA}}\approx\) 0.18 \(\rm{kW_{el}}\)/\(\rm{kW_{0}}\) is approximately 25% below the average specific electricity demand of the reference cooling technology \(w_{\rm{CCCA}}=\)0.24 \(\rm{kW_{el}}\)/\(\rm{kW_{0}}\) (cf. Figure 6d). Inserting the electrical values \(\rm{P_{el,DC,0}}=\) 13.5 \(\rm{kW_{el}}\) and \(\rm{P_{el,DC,aux}}=\) 1.7 \(\rm{kW_{el}}\) of the reject heat device at UBA into Equation (25) together with \(\rm{w_{CCCA}}=\) 0.24 \(\rm{kW_{el}}\)/\(\rm{kW_{0}}\) and the measured cooling load of about 23 to 29 \(\rm{kW_{0}}\) leads to a maximum allowed control signal for the dry cooler \(\rm{C_{DC}^{on,SHO}}\) of 0.66 to 0.73. Instead of this variable value, a constant value \(\rm{C_{DC}^{on,SHO}}=\) 0.6 was used for test purposes. The resulting necessary driving temperature to start the solar heat operation \(\rm{t^{on,SHO}}\) of about 60 to 64 \({}^{\circ}\)C during the preheating time is plotted in Figure 7. At 10:50 a.m., the cut-in condition became true (cf. Equation (34) and \(\rm{SHO^{on}}\) in Figure 7) and a delay timer of 10 min was started. Since \(\rm{SHO^{on}}=\) 1 was true over the full delay period, the information for the control signal \(\rm{C_{PS}}\) of the flap to be opened and \(\rm{C_{DH}}\) of the flaps to be closed was sent to the building management system via Profi-Bus. Thus, at 11:00 a.m., solar operation started with a lower desorber inlet temperature \(\rm{t_{Di}}\approx\) 67 \({}^{\circ}\)C as before (i.e., \(\rm{t_{Di}}\approx\) 74 \({}^{\circ}\)C during BHO with district heating, cf. Figure 7). If the cut-in condition became false (i.e., \(\rm{SHO^{on}}=\) 0)during the delay period, the timer would have been reset to zero and restarted when \(\mathrm{SHO^{on}}=1\) had become true again. Thereby, a cut-in delay was realized.