Study on solar liquid desiccant dehumidification and regeneration for central air-conditioning applications

Abstract

Dehumidification of air conditioning is traditionally accomplished with vapor refrigeration equipment. The disadvantage of this air handling process is that the temperature of the air must be cooled below its dew-point which is deemed to be energy expensive. An open cycle liquid desiccant dehumidification system driven by solar energy was emerged as a potential alternative to a conventional vapor refrigeration system for humidity control in an air conditioning system. This system can utilize solar energy and can provide cooling more efficiently. Application of the open cycle solar collectors/regenerators (C/R) is an effective way to utilize low-grade solar thermal energy to concentrate desiccant solution for air-conditioning. This project is to study how efficiently the solar desiccant dehumidification and regeneration can be used for central air-conditioning in hot-humidity weather area like Hong Kong. This thesis, firstly, presents the numerical simulation results of an open cycle liquid desiccant dehumidification system, attempting to obtain the optimum configurations of the solar assisted air-conditioning system and to validate the feasibility for using the liquid desiccant dehumidification system to handle the latent load and improve energy efficiency of central air-conditioning systems in a building. Secondly, a simplified simulation model for analyzing the coupled heat and mass transfer process on an open cycle solar C/R is introduced. Within narrow ranges of operating conditions, linear approximations were made to find the dependence of an equilibrium humidity ratio on the solution temperature, constant properties and coefficients. New parameters, which represent the heat and mass transfer driving forces, were defined and the basic differential equations were rearranged to obtain two coupled non-homogeneous linear equations. The climate conditions, such as solar radiation, air temperature, humidity ratio and wind speed, were treated as non-homogenous parameters for direct evaluation. In order to measure the performance of an open cycle and forced flow solar regenerator over a hot summer day for simulation validation purpose, an experiment using H2O/LiBr solution as working fluid was performed. The effects of ambient temperature, ambient relative humidity, forced air velocity and solar radiation intensity on the regeneration of the liquid desiccant have been experimentally observed. The simulation results were used to validate the simplified simulation model and develop the heat and mass transfer coefficient correlations. Unlike other types of solar utilization methods for which solar radiation is the only dominant weather parameter, the performance of the open cycle solar desiccant dehumidification system is also closely related to the ambient humidity and temperature significantly. To better predict its performance, a stochastic simulation approach is adopted to syntactically generate a series of meteorological parameters by using 15 years of real weather conditions of Hong Kong (between 1979-1988 and 1996-2000). Specifically, a first-order multivariate Markov chains model was developed to represent ambient temperature, humidity and solar radiation. The static properties of real weather data, like mean value, standard variance and persistence of a certain climate, are well preserved in the syntactically generated weather data and the inter-dependence of the solar radiation, ambient temperature and humidity are well reflected. Using the synthetic series of weather conditions, the annual performance of the open cycle liquid desiccant dehumidification system was analyzed. The energy saving, compared with a conventional vapor compression system, is in the range of 40-50%. More energy saving can be achieved if the ventilation load contains higher portion of a latent load. Besides, the economical aspect for using the open cycle solar desiccant dehumidification/regeneration system in Hong Kong is also discussed based on the system simulation and experimental results.