Identifier: TDX:1207
Authors: Álvarez Bermúdez, Maria Eugenia
Abstract:
Despite its huge potential for energy savings and low environmental impact, the use of absorption refrigeration systems is well below its technical and economic potential. The conventional working fluid H2O/LiBr has serious problems regarding corrosion and thermal decomposition at temperatures above 150°C. For this reason, we propose the development of absorbents suitable to triple-effect absorption refrigeration cycles. These absorbents should be non-corrosive, thermally stable, and allow taking advantage of the thermal potential of high temperature heat sources. The objective of this work is to study the solutions of alkaline nitrates, LiNO3+KNO3+NaNO3 with a percentage mass composition in salts of 53, 28 and 19%, respectively, called “Alkitrate”, as a new working fluid for absorption chillers using water as refrigerant in triple-effect configurations that can operate up to temperatures of about 260°C. In a first stage, experimental measurement, estimation and modelling of the thermophysical properties of aqueous alkaline nitrate and nitrite solutions were carried out in order to create an experimental database of thermophysical properties. Subsequently, the absorption cycle configuration called 'Alkitrate topping cycle' was simulated using the ABSIM (Advanced Modular Simulation of Absorption Systems) program. The Alkitrate topping cycle showed a slightly higher efficiency as compared to the LiBr/H2O triple-effect cycle at generator temperatures above 180ºC, without the problems related to thermal stability and corrosion of H2O/LiBr at temperatures higher than 150ºC. Subsequently, an experimental set-up was designed and built-up to study the absorption process with Alkitrate as working pair in a horizontal falling film absorber at operating conditions of interest for absorption refrigeration cycles driven by high temperature heat sources. Then, a sensitivity study was conducted and the trends of absorber efficiency parameters were evaluated. At the operating conditions used in the experiments, the values achieved for the absorption mass flux were in the range 2.83-6.55 g.m- 2.s-1, the solution-side heat transfer coefficient ranged from 631.9 to 1715.8 W.m-2.ºC-1, while the mass transfer coefficient was in the range (2.1-6.0)x10-5 m.s-1. Based on the experimental data base achieved from the experimental set-up, a model that uses artificial neural networks (ANN) for predicting the absorber efficiency parameters in a horizontal falling film absorber was developed. The results showed a good agreement between experimental and calculated data, indicating that the use of ANN is an effective technique for predicting the absorber efficiency parameters. Finally, a physical model of a horizontal tubes falling film absorber with Alkitrate, involving coupled heat and mass transfer processes was developed. The mathematical model integrates three solution flow regimes (drop formation, droplet fall, and falling film) and predicts the absorber performance at different operating conditions.
Repositori URV