2016 ASHRAE Annual Conference

The core components of vapor absorption refrigeration systems (VARSs) are the absorber, generator, condenser and evaporator. A pump is a critical component of a VARS to circulate the refrigerant–absorbent solution from the low pressure absorber to the high pressure generator. High quality mechanical/electrical energy is required to run this pump. Furthermore, the electrical pump is exposed to the high temperature corrosive solution. A thermally-driven bubble pump, which can be powered by waste heat or solar thermal energy, offers a simple and efficient technique for lifting a liquid from lower to higher levels, after which it can flow by gravity. In the vapor absorption refrigeration cycle, a bubble pump can be used to lift the solution from the absorber to the generator and also desorb the refrigerant vapor for achieving the necessary cooling effect. The performance of a VARS strongly depends on the bubble pump parameters. So the proper modelling and analysis of bubble pump parameters is a crucial to maximize the cycle performance. Extensive theoretical and experimental research has been performed in order to use the bubble pump for VARS. Some analyses have been developed based on air-lift pumps, some did not consider friction factor effects, two-phase flow, or the gas void fraction. Laminar flow is assumed and heat loss was not included for these analytical models. Beside these factors, thermophysical properties (such as specific heat, heat of vaporization, density, viscosity, surface tension) of the solution are also important for the evaluation of bubble pump performance as well as the overall performance of it in refrigeration systems. In this study, an analytical model of a bubble pump characteristics was developed and experimental work was conducted in order to use this pump in a VARS. In the simulation model, two-phase turbulent flow with heat loss, friction, surface tension effects and other thermophysical properties was considered. The model was validated by operating the bubble pump with water at atmospheric conditions. The bubble pump performance was investigated with tube diameters of 6 to 10 mm and lifting ratios (the ratio of the height of the liquid in the tube to the tube length) of 0.6 to 0.8, and at different heat inputs. Experimental results agreed with theoretical within 14%. The maximum liquid flow rate was obtained during slug flow at 180 watts heat input, a lifting ratio of 0.8, and tube diameter of 10 mm.

Absorption chillers represent a promising alternative to traditional vapor compression chillers, especially for the air conditioning systems. Indeed, they provide the same cooling supply without such a high electrical consumption, being driven by a low-temperature heat source. On the other side, they are not yet competitive with compression chillers because of their large size and high costs of investment. Their optimization becomes then a fundamental task to perform. Among the several components of these chillers, the absorber has been identified as the limiting one. The main obstacle is the low wetting on its tube bundle, which limits the absorption process. One way to overcome this problem is by the mean of additives. Indeed, small quantities of alcoholic surfactants in the working fluid lower the surface tension, promoting local turbulence at the vapor-liquid interface (Marangoni convection), which in turn leads to higher heat and mass transfer coefficients in the absorber. Nevertheless, only two kinds of additives are mostly used in these chillers and the experimental results available in literature are not in good agreement. The surface tension of water and aqueous lithium-bromide solution with different surfactants is experimentally investigated in this work. Common additives (e.g. 2-Ethylhexanol, 1-Octanol) as well as new additives are used. Their concentration in the solutions is varied in a wide range. The surface tension is measured according to the Pendant Drop Method. Several parameters are varied during the experiments, such as pressure, temperature and surrounding conditions. All the measurements are performed in a closed vacuum cell, in order to have the vacuum condition that occurs in the absorber, and to produce reproducible data. Results are discussed and compared with the available literature. The current study is carried out in the framework of the ITN Marie Curie “SHINE” research program financed by the European Union.