# Improved Solar Operation Control for a Solar Cooling System of an IT Center
## 2 Materials and Methods
### Operating Period with High Solar Fraction  
During summertime, two operating modes are possible for the absorption chiller:  
* Solar heat operation (SHO); and
* Backup heat operation (BHO). Independent of BHO or SHO, the thermally driven absorption chiller (TDC) is controlled by a model predictive controller based on an improved method of characteristic equations (the so-called CE controller). The CE controller calculates simultaneously the set values in hot and cooling water (i.e., \(t_{\text{DI}}^{\text{set}}\) and \(t_{\text{Ai}}^{\text{set}}\)), which are necessary to cover the cooling load \(\text{Q}_{\text{E}}^{\text{set}}=\text{V}_{\text{E}}\cdot\text{\rho}_{\text{ E}}\cdot\text{\rho}_{\text{E}}\left(t_{\text{EI}}-t_{\text{EO}}^{\text{set}}\right)\). The CE controller is explained in more detail in [15] and in the next section. In summer 2017, several tests were executed in order to extend the duration of SHO and thereby increase the solar fraction and primary energy saving of the SAC system. One of these test periods is depicted in Figure 2, where the cut-in condition for SHO was left unchanged but the cut-off condition was blocked manually in order to maximize the solar fraction. \begin{table}
\begin{tabular}{c c c c} \hline
**Variable** & **Description** & **Unit** & **FM050V0.3** \\ \hline \(t_{\text{DI}}\) & Hot water inlet temperature & \({}^{\circ}\)C & 75 \\ \(V_{\text{D}}\) & Hot water volume flow rate & m\({}^{3}\)/h & 6.0 \\ \(\text{Q}_{\text{D}}\) & Driving heat flow & kW & 44 \\ \hline \(t_{\text{Ai}}\) & Cooling water inlet temperature & \({}^{\circ}\)C & 28 \\ \(V_{\text{H}}\) & Cooling water volume flow rate & m\({}^{3}\)/h & 18.0 \\ \(\text{Q}_{\text{H}}\) & Reject heat flow & kW & 78 \\ \hline \(t_{\text{EO}}\) & Chilled water outlet temperature & \({}^{\circ}\)C & 9 \\ \(V_{\text{E}}\) & Chilled water volume flow rate & m\({}^{3}\)/h & 5.9 \\ \(\text{Q}_{\text{E}}\) & Cooling capacity & kW & 34 \\ \hline COP & Coefficient of performance & - & 0.77 \\ \hline \end{tabular}
\end{table} 
Table 1: Design values of the absorption chiller type FM050V0.3 at the Federal Environment Agency (UBA) in Dessau, Germany. \begin{table}
\begin{tabular}{c c c c} \hline
**Variable** & **Description** & **Unit** & **Dry-Cooler** \\ \hline \(t_{\text{KI,0}}\) & Fluid inlet temperature & \({}^{\circ}\)C & 32 \\ \(t_{\text{K0,0}}\) & Fluid outlet temperature & \({}^{\circ}\)C & 29 \\ \(V_{\text{K,0}}\) & Fluid volume flow rate & m\({}^{3}\)/h & 45.5 \\ x & Mass fraction ethylene-glycol & kg/kg & 0 \\ \hline \(t_{\text{Li,0}}\) & Air inlet temperature & \({}^{\circ}\)C & 21 \\ \(V_{\text{L,0}}\) & Air volume flow rate & m\({}^{3}\)/h & 45.940 \\ \(\text{Q}_{\text{L,0}}\) & Reject heat flow & kW & 158 \\ \(\text{P}_{\text{el,DC,0}}\) & Electrical power & kW\({}_{\text{el}}\) & 13.7 \\ \hline \end{tabular}
\end{table} 
Table 2: Nominal values of the reject heat device in dry-cooler mode at the Federal Environment Agency (UBA) in Dessau, Germany. SHO started on the first day when \(\mathrm{t_{Ho}}>\mathrm{t_{Di}^{set}}\) and the difference \(\Delta\mathrm{t_{S}^{on,SHO}}=\mathrm{t_{2 Di}^{set}}-\mathrm{t_{PS11}}\) was smaller than 3 K for at least 5 min, which was true at approximately 10:30 (cf. Figure 1 for the sensor position and Figure 3 for their chronological progression). At first glance, this condition looks strange because the storage temperature was lower than the necessary driving temperature \(\mathrm{t_{Di}^{min}}=\mathrm{t_{Di}^{set}}\) to cover the cooling load (i.e., the storage was still not hot enough). However, the temperature \(\mathrm{t_{PSO}}\approx\mathrm{t_{Di}}\) available for the chiller is a mixture of \(\mathrm{t_{Ho}}\) and \(\mathrm{t_{PS11}}\) (cf. Figure 1). Hence, the existing cut-in condition for SHO mentioned above (i.e., without the switchover strategy explained later on) is a compromise of \(\mathrm{t_{Ho}}>\mathrm{t_{Di}^{set}}\), which was true at 10:00 a.m., and \(\mathrm{t_{PS11}}>\mathrm{t_{Di}^{set}}\), which was true at 11:50 a.m.