# New Developments and Progress in Absorption Chillers for Solar Cooling Applications
## 2 Solar-Powered Absorption Cooling Systems
### Conventional Absorption Chillers--Technical Challenges  
Solar absorption cooling systems are often based on classical/conventional absorption chillers available on the market. However, most current absorption chillers have problems in effectively exploiting solar resources in an energy-efficient (from a primary energy viewpoint) and cost-effective way. Hence, the main hurdle for the broader deployment of such systems is that most commercially available absorption chillers lack key technical features. The limitations of these technical features are the following. * _Heat rejection._ High ambient temperature has two adverse effects: (1) an increased cooling load of the building and (2) a lower potential for heat rejection of the cooling tower. An air-cooled heat rejection system involves increasing the absorber and condenser temperatures which creates the need for high-temperature driving heat in the absorption chillers. In water/LiBr absorption chillers, crystallization is the main obstacle to operating the chiller at high ambient temperature, so various methods have been proposed and applied to limit the risk. These methods include the use of chemical inhibitors, heat and mass transfer enhancement in the absorber, and boosting the absorber pressure using mechanical compression. Consequently, the absorption chillers' potential to reject heat to the atmosphere is hampered, particularly in hot climates. Thereby, evaporative or wet cooling towers are commonly required which is a shortcoming in regions with a scarcity of water. Even in regions where water is not a limited resource, the costs of water and bacteria treatment in evaporative cooling towers of small capacity systems can be as high as 30-40% of the operating costs. Therefore, the absorption systems currently available are relatively expensive in terms of capital expenditure (CAPEX) and operating expenses (OPEX). * _Compactness._ The cost of the heat exchangers is the biggest contribution to the capital investment of any chiller technology. In this regard, absorption chillers always have a downside compared to compression chiller technologies, since they have more heat and mass exchanger components. So, among other factors, such as reducing the consumption of parasitic electricity, if absorption chillers are to be economically competitive, heat exchangers need to be smaller. Conventional absorption chillers are unnecessarily large because of dead spaces that do not have any function (i.e., which are not involved in the cooling process [13; 14; 15]). Moreover, since absorption chillers are large, it is usually impossible or very difficult to retrofit them in existing buildings. Therefore, there is a significant economic incentive to minimize the overall size of the chiller in any new developments. * _Capacity._ The capacity of most commercial absorption chillers is 300 kW or more. In terms of capacity, shell and tube heat exchangers are state-of-the-art technologies; in these chillers, copper tubes are rolled into the heavy front plates to ensure vacuum operating conditions inside the water/LiBr absorption chiller. Using this practice in small-scale chillers (<200 kW) leads to a high specific cost [13]. On the other hand, small chiller machines based on coiled tube heat exchangers are available from some manufacturers, but they have high-pressure drops which lead to high parasitic electricity demands (i.e., they limit the advantages of heat-driven chillers) [13]. * _Temperature glide of the driving heat._ A conventional single-effect water/LiBr absorption chiller typically uses 85-90 \({}^{\circ}\)C hot water as its driving heat and releases it at about 75-80 \({}^{\circ}\)C (i.e., a temperature glide of up to 10 K) [16].