Thermo-hydrodynamic constraints of the thermal compressor on NH3/H2O absorption chiller performance

Heat supply system based on absorption heat pump have great potentials on energy saving and emission reduction due to it can be powered by waste thermal energy and solar energy, reducing notable electricity consumption. The most common working fluid in absorption refrigeration cycles able to achieve subzero cooling temperatures is the ammonia/water pair. The European ASTEP project is being developed within this framework, and one of its main objectives is to demonstrate the feasibility of the application of solar thermal energy to partially cover the cooling demands at an industrial site, the MANDREKAS dairy company, with a cooling demand at 5ºC. Within that project, the present work aims to study the feasibility and main constrains to use a commercial air-cooled ammonia/water absorption chiller driven by a patented solar concentrator (SunDial) to satisfy the MANDREKAS’s cooling demand at different outdoor conditions.

A key element of the air-cooled ammonia/water absorption refrigeration cycle is the thermal compressor, where different mass and heat transfer processes take place. The thermal compressor consists of a reboiler, a distillation column, a liquid-cooled rectifier, and an air-cooled absorber. The set of the first three devices (desorber) must provide ammonia vapor with a high degree of purity. Otherwise, the coefficient of performance will decrease because of the temperature glide, what will make not possible to cool down the medium of interest, and the water accumulation in the evaporator, with the consequent heat transfer process degradation. The air-cooled absorber must absorb the whole ammonia vapor production of the desorber.

The performance of a thermal compressor operating under different conditions are studied considering the different limitations, mainly, of hydrodynamic nature, due to the limited capacity of the trays includes in the distillation column, of mass and heat transport nature, both liquid-vapor interface area or coupling fluid-solid surface area.