Thermodynamic development of a novel integrated air-conditioning system with DOAS using liquid desiccant

Abstract

A dedicated outdoor air system (DOAS) used for air-conditioning has recently attracted many attentions because it can significantly improve both energy efficiency and indoor air quality. Cooling-based dehumidification has inherent weaknesses because the process air has to be cooled down below its dew-point temperature for dehumidification and reheated thereafter. Currently available DOASs with solid or liquid desiccants have their own thermodynamic weaknesses. These obstruct the wide applications of DOAS with desiccant dehumidification in practice. Therefore, thermodynamic development and study of a novel integrated air-conditioning system with DOAS using liquid desiccant is conducted in this thesis. Mathematical models of primary components of the proposed system are developed. Simplified numerical model for internally cooled/heated dehumidifier/regenerator and a robust numerical solution are developed, which is characterized by linearization of the non-linear term in the model and multi-gridding approach to guarantee convergence in a wide range of component size and operation conditions. A semi-empirical model for chiller with a recovery exchanger installed between the compressor and the condenser is developed. All the models are validated by either experimental or manufacturers' data, providing a basis for this study. New exergy analysis method is adopted to develop air-conditioning system with high thermodynamic perfection by introducing concepts of dry-, wet-, cold- and heat-exergies, beneficial and unbeneficial exergies to quantify real exergy gains and destructions/losses and to identify each factor causing exergy loss. Application of the newly proposed concepts and method to analysis of the standard liquid desiccant system with DOAS results in the proposed system. It mainly consists of a DOAS subsystem for dehumidification, heat recovery chiller for cooling, and heat pump utilizing waste condensing heat from chiller for desiccant regeneration. The DOAS subsystem is characterized by effectively harvest the sensible and latent load of the exhaust air by an enthalpy exchanger and two sensible heat recovers. The rational exergy efficiency of the proposed system is 8.0% as compared to 3.1% of the standard system. The proper design methods of the proposed system are then developed. They consist of 1) a new RTS-based design cooling load calculation method for sizing of chiller; and 2) a ε-NTU method for design of liquid desiccant dehumidifier. The new design cooling load calculation method overcomes the weakness of RTS method for intermittent cooling by accounting for additional cooling load due to intermittent operation. The ε-NTU method for dehumidifier is characterized by two performance indices of enthalpy effectiveness and mass transfer effectiveness. The iterative design procedure of the system is presented and a design tool is provided for engineer. The thermodynamic study of the proposed system indicates that energy efficiency is enhanced by more than 60% for air-conditioning in a typical office building of Hong Kong, with the average COP reaching 4.21. The thermal analysis also reveals the indispensable role of total heat exchanger to minimize and balance the dehumidification load on dehumidifier. The effects of weather conditions on the performance of the system are analyzed in detail. The sensitivity analysis of key independent operation parameters of the proposed system are carried out to identify their optimal ranges to provide guidelines for the optimal design of the proposed system. The operation parameters studied are condensing temperature of chiller, cooling and hot water flow rates and solution flow rate. The results indicate that: 1) condensing temperature of chiller affects the energy consumption of the system the most and should be as low as possible; 2) the effects of cooling and hot water flow rates are consistent and their optimal value should be about 0.5~1.0 times of fresh air flow rate; 3) the effect of solution flow rate is small and it optimal value is limited by the largest flow rate without desiccant carry-over.