Energy-efficient cooling concepts play an important role for numerous applications in the area of comfort and industrial cooling. In this regard, it represents a huge challenge to ensure that the cooling demand of industrialized countries is covered in a sustainable manner with a drastically reduced carbon footprint. In recent years, several innovations in the field of absorption have been introduced. While some of them are based on process improvements, other innovations are based on newly developed chemical system solutions. One example is the use of new working pairs for absorption chillers. In the past, working pair related drawbacks such as crystallization, corrosion, and instability led to a number of innovation barriers. One very promising way to overcome these drawbacks and thereby to allow for new or improved absorption chiller processes is the replacement of the state-of-the-art absorbent LiBr by a suitable ionic liquid (IL). This review aims at discussing new, energyefficient cooling concepts with a special focus on using ionic liquid based working pairs in absorption chillers/absorption heat pumps. The most relevant research and development activities are analyzed, a patent overview is provided, and both new technological opportunities and remaining scientific challenges are identified. Although a large number of ionic liquid papers have been published in this field during the last 10 years, many contributions do not adequately address the interdisciplinary set of application requirements and do not fully recognize that most imidazole-based ionic liquids are not suitable for being used in multieffect absorption cycles since their (in)stability at high temperatures in the presence of water and the resulting cooling capacity do not meet the industrial requirements.

Sorption cooling technologies are well known as energy efficient cooling supplying apparatuses where heat as driving energy is used from waste heat sources, trigeneration systems (engines), solar thermal plants, etc. Especially in the air-conditioning business often a strong correlation exists between insolation and cooling requirement. Therefore, the interest in solar cooling systems prevails since several decades. Some hundred systems have been installed all over the world and operate, some satisfactorily and some others not. However, no really cost-competitive system for widespread application exists. A step forward can only be expected by innovative design of collectors or chillers, by improved system integration and enhanced control strategies, or by expanded application, e.g., the use as a heat pump in the heating period as well. The state of the art was reported several times. Here three important topics are highlighted: the role of collector and process development, the primary energy demand (in combination with the conventional back-up system and auxiliary energy needs), and the role of the double use of the system during summer and winter time (i.e. collectors for both heat and cold, sorption apparatus as chiller and heat pump). The paper is focusing on solar air conditioning rather than solar refrigeration.

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einer Verbesserung führt. Dieses Verhalten lässt sich vor allem auf die veränderte Stoffübergangsfläche und Hydrodynamik der heterogenen Strö-mungsformen zurückführen.[1] J.-U. Repke

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