(596c) Approach for the Selection of An Innovative Working Pair for Absorption Chillers and Heat Pumps

The huge demand for heating and air conditioning is provided in general under consumption of primary energy. Thermally driven heat pumps work energy efficient and use waste heat streams instead of a mechanical compressor. The performance and working range of absorption chillers and heat pumps is a strong function of the applied working pair. The present work therefore is on the quest for new working pairs which overcome the present restrictions, i.e. corrosion, crystallization, etc. and show favorable working ranges as well as high performances.

As solvent acts a new class of salts called ionic liquids. Ionic liquids are molten salts at temperatures below 100°C. As ionic liquids show an almost negligible vapor pressure the desorption step is simplified. The physical and chemical properties, especially the solution properties of ionic liquids can be tailor-made due to the almost infinite combinations of cation and anion. Ionic liquids are thus a versatile tool for heat pump applications. The refrigerant selection takes into consideration enthalpy of vaporization, heat capacity and boiling conditions as well as aspects of flammability and toxicity. Though, an appropriate working pair cannot be chosen regarding one of those parameters only. This contribution shows an effective method to select an appropriate working pair for absorption heat pumps.

The presented procedure considers in the first instance the solution properties of the chosen working pair. The characteristic diagram called “Dühring Plot” shows the possible working ranges for the working pair. Usually, the Dühring Plot is deduced from experimental data. Here, the prediction tool COSMO-RS was adapted to this task. Thermodynamic prediction models, based on quantum chemistry and statistical thermodynamics enable the user to achieve information concerning the activity coefficient merely based on the molecular structure of a molecule. Concluding from the activity coefficient the saturation pressure above the salt solution can be derived in dependence of temperature and concentration. A theoretical coefficient of performance is derived thereafter. Current working pairs achieve a 10% higher COP than common water / lithium bromide absorption chillers.