# New Developments and Progress in Absorption Chillers for Solar Cooling Applications
## 4 Prototypes of Absorption Chillers
### Single-Stage Absorption Chillers
#### 4.1.2 Water/LiBr Working Pair  
Water/LiBr is the most commonly used working pair in absorption chillers because of its superior properties. However, water/LiBr absorption chillers have some well-known drawbacks that ammonia/water absorption chillers do not such as crystallization and operation under vacuum. The thermophysical properties of the water/LiBr mixture can be found in the literature [65; 66; 67; 68].  
A 2 kW air-cooled single-effect water/LiBr chiller was developed and experimentally tested at the Heat and Mass Transfer Technological Center (CTTC) at the Universitat Politecnica de Catalunya (UPC) in Terrassa, Spain [69]. To define the chiller's control strategy [69], a numerical model was developed to predict the thermal behavior of this absorption chiller under the transient mode of operation and its interaction with other system components (e.g., solar thermal collectors and circulation pumps). The model developed was validated using performance data (i.e., thermal COP and cooling capacity) obtained from the experimental tests carried out at CITC-UPC [70]. They also carried out numerical simulations using the NEST (Numerical Edifice Simulation Tool) platform (an in-house developed object-oriented numerical tool) which focused on the performance and key drawbacks associated with water/LiBr absorption chillers (e.g., crystallization) and geometrical aspects. The outcomes of these numerical simulations and experimental studies also helped to understand and design the control system's strategies.  
On the basis of the investigation of the 2 kW air-cooled chiller, a pre-industrial single-effect water/LiBr prototype with a cooling capacity of 7 kW was designed and developed at the CITE-UPC  
Figure 15: Air-cooled, single-effect ammonia/LiNO\({}_{3}\) absorption chiller laboratory prototype developed by [PERSON] and [PERSON] [64]. (**a**) Schematic diagram; (**b**) Prototype internal and external views.  
laboratory. This prototype has an absorber and a condenser that are directly air-cooled. Both prototypes, with capacities of 2 kW and 7 kW, were part of a virtual laboratory, and all experimental data showed that there was little risk of LiBr crystallization. Furthermore, the dynamic numerical model was modified to improve its accuracy and reliability [71]. The model was improved largely by mass transfer calculations based on the author's experience of the falling film absorption process. The heat and mass transfer parameters used in the model developed were validated using the steady-state experimental data obtained from the laboratory prototype with a cooling capacity of 2 kW, and there was reasonable agreement among the thermal COP values (with the capacity being slightly under-predicted). The numerical model was used to predict the performance of the 7 kW air-cooled water/LiBr prototype. However, the model predicted the cooling capacity to be 81% (i.e., 5.7 kW) of the expected cooling capacity even though the heat exchangers of the chiller had been designed to provide the nominal capacity of 7 kW [71].  
The transient model of the chiller was validated using the experimental data obtained under steady-state conditions. For the 2 kW air-cooled chiller, there was good agreement between the numerical and experimental results with small discrepancies in semi-empirical information used in the model. The 7 kW chiller was also experimentally validated at UPC-BarcelonaTech [72]. The model was used to evaluate the absorption chiller as well as the various control strategies (e.g., on/off versus fuzzy logic). The price of the air-cooled water/LiBr chiller developed (with the collaboration of Termo Fluids S.L., Spain) with a capacity in the range 7-11 kW and a thermal COP of \(\sim\)0.7, and the rest of installation, without the solar collector field, is estimated to be about 15,000 Euro (1800-2150 Euro per kW of cold).