## 4 Identification of Recent Developments and Possible Solutions
### Absorber and Desorber Solutions  
Absorber and desorber are critical components of ACHP cycles to achieve higher system efficiencies and obtain higher sink temperatures [54; 55]. Many studies have been conducted to understand the characteristics of the occurring processes and thermodynamic properties [67; 68]. For use in an ACHP system, various requirements are defined for the heat exchanger properties used. In addition to the general requirements, such as the reduction of size and pressure losses, the suitability for the desired operating parameters must be considered [52]. To reduce installation costs, compact heat exchangers with a high area density (m\({}^{2}\)/m\({}^{3}\)) of the heat transfer surface combined with a high overall heat transfer coefficient (kW/(m\({}^{2}\)K)) are desired [69; 70]. Furthermore, the establishment of effective liquid-vapor mixing and the complete and continual wetting of the heat transfer surfaces are important [71]. Based on the investigations carried out and the data presented in Table 1, the trend from horizontal to vertical shell-and-tube heat exchangers towards vertical PHE for use in ACHP systems can be recognized. The plates can be pressed with different shapes to increase turbulence, fluid distribution and surface area [72]. Due to these features, the design can be much more compact. In addition, PHE can provide high overall heat transfer coefficients, good wettability and liquid-vapor mixing [55]. Various approaches have been and are being investigated to improve the design and operation of absorber and desorber [73]. It was concluded that longer plates combined with good distribution of the mixture and operation in counter-flow with the coolant have positive effects on the performance. [PERSON] et al. (2014) [55] stated that increasing the ratio of plate length to gap (L/D) is more important than the ratio of width to gap (W/D) for more effective operation. Several possibilities are available for the implementation. Due to the similarity of the operating conditions and requirements, the desorber is often designed and operated like a flooded evaporator. The absorber can be operated in falling film mode, where lean solution slides from top to bottom as a thin film on the surface of the vertical plate and vapor filling the free space is absorbed into the liquid film, releasing heat to the coolant. Another approach is to operate the absorber in bubble mode, where the lean solution and vapor are mixed at the bottom before entering the absorber and flow upwards in counter-flow with the coolant [74]. Furthermore, it is possible to operate several PHEs inline using different operating modes [24,54]. Figure 5 shows a schematic representation of a plate heat exchanger operated in counter-flow as absorber in falling film and bubble mode. Bubble mode operation in the absorber of PHE in an ACHP cycle was investigated by [PERSON] et al. (2013) [74]. They concluded that higher pressure is a more significant factor for increasing the heat exchanger performance than size based on experiments with different sized PHE. The maximum overall heat transfer coefficients achieved for different PHE sizes were between 0.96 and 1.61 kW/m\({}^{2}\)K. In contrast, [PERSON] et al. (2002) [76] reported values between 0.3 and 0.55 kW/(m\({}^{2}\)K) at lower temperatures in a PHE test facility. More experimental data from [PERSON] (2005) [77] for bubble and falling film absorber were between 0.6 and 1.4 kW/(m\({}^{2}\)K) and for bubble desorber between 0.4 and 1.5 kW/(m\({}^{2}\)K). [PERSON] et al. (2010) [78] stated for a bubble desorber, that pressure has only a slight influence and mass flux a major influence. Measured overall heat transfer coefficients varied between 2.5 and 4.1 kW/(m\({}^{2}\)K). Due to many parameters affecting the absorption and desorption process, which are not all documented in these sources, a direct comparison or deducing trends becomes impossible.