# New Developments and Progress in Absorption Chillers for Solar Cooling Applications
## 4 Prototypes of Absorption Chillers
### Advanced Absorption Chillers
#### 4.2.2 Mechanical Compressor Assisted Absorption Chillers  
The application range of conventional absorption chillers is restricted by the properties of the working fluid (e.g., water/LiBr) used in the chiller. For instance, the driving heat temperature needs to be higher than the required minimum activation temperature, and the chiller has a limited temperature lift. Accordingly, for applications such as solar cooling, the absorption chiller needs to be oversized to adjust to a low-grade driving heat source [79]. Moreover, since the temperature lift of water/LiBr-based chillers is limited, dry cooling towers cannot be used in this type of absorption chiller. Therefore, to extend the operating range of absorption chillers, a hybrid refrigeration cycle is formed by combining the absorption cycle and mechanical compression process in a series or parallel flow arrangement. The hybrid mechanical compression combined with the absorption cycle  
Figure 17: Double-lift (half-effect) ammonia/water absorption cycle of the University of Imshruck prototype. Adapted from Reference [77]. Note: CON = condenser; EVA = evaporator; HABS = high-temperature absorber; HDES = high-temperature desorber; HSHX = high-temperature solution heat exchanger; LABS = low-temperature absorber; LDES = low-temperature desorber; LSHX = low-temperature solution heat exchanger; RHX = refrigerant heat exchanger. benefits to overcome the restrictions of the working fluid characteristics. These benefits include higher heat rejection temperatures for dry cooling under hot climate conditions, lower driving heat inlet temperatures so that low-temperature solar or other types of heat can be used (e.g., waste heat), and greater cooling capacity during peak load conditions [79; 80; 81; 82; 83]. The integration of a mechanical compressor (either between EVA and ABS (Figure 18a) or between DES and CON (Figure 18b)) leads to operational flexibility and boosts the cooling capacity for a single-effect water/LiBr absorption chiller. Thereby, operating conditions could be more extreme (e.g., high heat rejection temperature, \(>\)35 \({}^{\circ}\)C) and chiller designs could be more compact and cost-effective using the compressor-assisted absorption chiller. To date, the lack of a suitable compressor for refrigerant water vapor has been a bottleneck for implementing such a hybrid water/LiBr absorption-compression chiller. The properties of water (i.e., low specific density, high-pressure ratio, and high vapor outlet temperature) represent a significant challenge for the design of the compressor. However, high-speed turbo vapor compressors for refrigerant water have recently been put on the market by some companies in Europe and Asia because of significant progress in new materials and manufacturing techniques which have improved geometries and increased impeller tip speed (up to 600 m/s) [83]. Accordingly, the technical proof of concept of the mechanical compressor-assisted single-stage water/LiBr absorption chiller unit has become feasible, and a laboratory prototype of the unit has been built and tested at the Munich University of Applied Sciences (Germany) [82]. This hybrid chiller unit with a size of 2.6 m\({}^{3}\), depicted in Figure 19, was developed for a capacity of 15 kW with the chilled water supply at 6 \({}^{\circ}\)C and tested under various operating conditions. The maximum driving heat source and cooling water inlet temperatures are 90 \({}^{\circ}\)C and between 35-40 \({}^{\circ}\)C, respectively. The main components of this prototype (i.e., absorber, condenser, desorber, and evaporator) were built using falling film over the side-by-side horizontal tube bundle heat exchangers [82]. This type of heat exchanger ensures superior thermodynamic and fluid dynamic performances as well as scalability to higher chiller capacities [82]. Besides, the unit is equipped with a purge system for the non-condensable gases to ensure continuous equilibrium test conditions.