scope:

INTRODUCTION

In the last few decades, due to the increasing level of pollution worldwide and the cost of energy, the search for maximum exploitation of the available energy has lead to the development and use of cogeneration or trigeneration systems. Heating, ventilation, air-conditioning, and refrigeration systems (HVAC-R) play a major role in modern society energy consumption. These systems are mostly based on the vapor compression cycle, due to high efficiency, but the vapor compression cycle needs work input, and high energy consumption is still observed, therefore research efforts have been made to develop intelligent refrigeration systems in order to reduce energy consumption (Vargas and Parise 1995; Buzelin et al. 2005). Hence, alternative HVAC-R systems have been the subject of much recent scientific research. Among these systems, absorption refrigeration is receiving great attention since it may produce energy, heat and cold, using, as energy source, waste heat from industrial processes or, for instance, exhaust gases in automobiles (Temir and Bilge 2004).

The major companies working on this area focus on large capacity absorption systems, i.e. above 100 TR. However, since most refrigeration and air-cooling units are of small capacity and operate based on vapor compression cycle systems, there is still a vast field in which absorption systems could be employed.

An absorption system also allows the direct use of primary energy, particularly solar energy and natural gas, for refrigeration purposes (Ezzine et al. 2004). Although this system is less costly and simpler than vapor compression systems, its comparatively low coefficient of performance has limited its use to few and specific applications. Nevertheless, the absorption refrigeration system may reach a refrigeration capacity higher than that of a vapor compression system when energy sources such as waste (residual) heat from industrial processes, gas or vapor turbines, sunlight or biomass are used instead of electricity (Adewusi and Zubair 2004).

The performance of absorption systems is dependent on an adequate choice of the refrigerant/sorbent working pair, and ammonia-water has been receiving great attention since these fluids do not contribute to the greenhouse effect (Bruno et al. 1999; Lazzarin et al. 1996).

The technical literature is rich in publications on the absorption refrigeration field. Particularly, Abreu (1999) and Villela and Silveira (2005) used as heat source for absorption systems, the combustion of liquid petroleum gas (LPG) and biogas, respectively, studying the design and performing a thermoeconomic analysis of the analyzed systems. Other studies focused on the exergy analysis of absorption refrigeration systems, including Sedighi et al. (2007), Hasabnis and Bhagwat (2007), Khaliq and Kumar (2007), Arivazhagan et al. (2006), and Sencan et al. (2005). Simulation and optimization studies have also been published analyzing the absorption refrigeration system in isolation (Vargas et al. 1996; Vargas et al. 2000a; Vargas et al. 2001). However, the exergy analysis and optimization of an absorption refrigerator to produce cooling and heating, based on a theoretical-experimental model, could not be found in the open literature.

The aim of this work is two-fold: i) to formulate theoretically the absorption system heat transfer interactions using a simplified mathematical model for the energy and exergy analysis of an existing LPG (gas fired) driven absorption refrigeration unit, and ii) based on experimental measurements, to characterize system pull-down times and to carry out an energetic and exergetic optimization for maximum thermodynamic performance of the system, i.e., minimum energy consumption.