# New Developments and Progress in Absorption Chillers for Solar Cooling Applications
## 5 Research Trends and Opportunities  
In recent years, the research and development of absorption chillers has focused mainly on such critical features as heat rejection using an air-cooled unit, system compactness, large driving heat temperature glides, and high overall system efficiency (i.e., high electrical and thermal COPs). Also, small-capacity chillers may be of interest for use in solar air conditioning applications in residential and small commercial buildings. Although in the past decade, several new developments have been put on the market (Section 3), further significant progress must be made to compete with systems based on a vapor-compression chiller. Accordingly, research into the development of new absorption chillers has been taken three main directions in recent years:  
* _Extension of operational limit: heat rejection and driving heat temperatures_ Heat rejection temperature: the use of water/LiBr in absorption chillers limits the absorber heat rejection temperature because of crystallization [85]. Therefore, increasing this temperature limit means that heat is rejected to the ambient air (without using cooling towers) and air-cooled absorption chillers perform well. Various approaches have been discussed in the literature, among which are solubility enhancement using anti-crystallization additives (e.g., ionic liquids [86; 87] and salts); absorber design improvement (enhanced heat and mass transfer processes); absorption cycle modification (using thermal/mechanical compression booster to increase the absorber pressure); and internal control-via-increasing evaporator temperature. _Driving heat temperature_: the return temperature of the driving heat stream can be reduced (i.e., higher temperature glide) with an improved desorber design and advanced cycle configurations. * _Compactness:_ the absorber and desorber of market available chillers are mostly based on falling film configurations in which the horizontal and vertical tube falling film exchangers are the components that have most been implemented and studied [88]. Nevertheless, this type of design has downsides, including maldistribution of the film on the exchanger surface, which disturbs flow uniformity at low Reynolds number [89] and results in dry patches on the plate surface (non-wetted areas) and thick liquid films (wetted areas). Therefore, it reduces the absorption and desorption rates, and decreases the useful absorber and desorber surfaces, which results in bulky components. This is especially crucial for the absorber since it is the least efficient component of the chiller because of the low mass diffusion coefficient of water vapor in the solution of water/LiBr [90]. Additionally, heat transfer from wetted areas is also lower than from perfectly wetted surfaces because of the greater thickness of the film. Therefore, recently, the use of (i) plate-type falling film absorbers [88] and (ii) membrane Contactors in the absorber and desorber have been proposed and investigated for the development of compact and inexpensive water/LiBr [91; 92; 93; 94; 95; 96; 97] and ammonia/water [98; 99] absorption refrigeration systems, particularly in a small capacity range. The use of a plate-type falling film configuration in the absorber and desorber of the absorption chiller can significantly enhance the absorption and desorption rates, which can make these devices much more compact and more efficient for water/LiBr-based mixtures. * _Control strategy and operation:_ It is common practice in the control and operation of absorption chillers to use fixed volume flow rates for external heat carrier fluids (i.e., driving heat, heat rejection, and chilled water streams). These volume flow rates and the fan speed of the heat rejection unit are fixed at the values for the nominal design condition (i.e., nominal thermal powers). This operation strategy leads to parasitic electricity consumption dominating the chiller's part-load operation.