## 2 Materials and Methods  
The objective of this work was the design of the components of a solar air conditioning facility for the Foxa 3 Cantos hotel located in Madrid. The hotel owner wanted to take advantage of an existing field of flat-plate solar collectors to carry out a demonstration project of solar cooling. The flat-plate solar collector (Vitosol 100), its orientation (south) and inclination (60\({}^{\circ}\)) are starting data, as well as the cooling capacity of the absorption machine Yazaki WFC SC10 (35 kW), which accounts for only 3% of the installed cooling capacity. As mentioned above, the design of a solar air conditioning facility basically involves the design of the solar collector field and the storage unit. That is why this study has been aimed at evaluating the fraction of incident solar radiation that is capable of being used by the absorption chiller for the production of cold water at 7 \({}^{\circ}\)C. To this end, the effect of the following parameters on the performance of the system was evaluated: solar collector field surface, hot water storage volume, and heat storage temperature required to drive the absorption machine when it is idle. The inclusion of the drive temperature analysis is considered essential since it determines both the efficiency of the solar collector field and the EER of the absorption chiller. The available solar radiation and the rest of the climatic variables of the locality in which the installation is located have been obtained through the METEONORM program [14]. Figure 1 shows an outline of the model developed in the TRNSYS program [6] for the design of the facility. Its operating principle is as follows. A solar collector field (whose surface is one of the study variables) transfers the collected energy to a storage tank (whose volume is another of the study variables). When the water temperature in the storage tank reaches a certain value (set-point: operating temperature plus 2.5 \({}^{\circ}\)C), a power of 50 kW is extracted from it, which is the nominal operating power of the absorption chiller. The energy extraction stops when the temperature of the tank decreases below the set-point temperature minus 2.5 \({}^{\circ}\)C.  
The TRNSYS model integrates the different energy flows. In this case, the energy extracted from the storage tank has been calculated for each combination of the design parameters. This will allow us to express the potential of each combination as the fraction of the solar radiation incident in the collector field that can be used for driving the absorption chiller. Figure 1: Scheme of the system model built in Transient System Simulation Tool (TRNSYS). Type 1 of TRNSYS models the efficiency of the flat-plate solar collector using a quadratic equation. The coefficients provided to the model were those corresponding to the collector model installed in the hotel (Vitosol-100): a\({}_{0}\) = 0.84; a\({}_{1}\) = 3.36 (W/m\({}^{2}\)K) and a\({}_{2}\) = 0.013 (W/m\({}^{2}\)K\({}^{2}\)). This efficiency depends on the temperature of the fluid circulating through the flat-plate collector (T) according to Equation (1). \[\eta_{\rm col}\mathrm{=a_{0}-a_{1}\frac{(T-T_{amb})}{I}-a_{2}\frac{(T-T_{amb})^ {2}}{I}} \tag{1}\]  
Type 158 was used for the heat storage tank. This model simulates the operation of a cylindrical tank that has been assigned a loss coefficient of 0.77 W/m\({}^{2}\)\({}^{\circ}\)C. The tank has been divided into a total of 10 nodes in order to adequately model their degree of stratification. The tank interacts with two water flows, one that connects it to the heat exchanger (where the energy from the solar collector field comes from) and the other that connects it to the load (absorption machine) and through which the nominal drive power (50 kW) is extracted when the temperature of the water in the tank is higher than the set-point temperature.