Design and Testing of an Augmented Generator for Solar Fired Absorption Chillers

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This paper presents results from a project sponsored by the California Energy Commission that involved the design and testing of an augmented generator for a solar fired, double effect absorption chiller. Solar powered absorption chillers use water heated by an array of solar collectors to boil a solution of lithium bromide and water. The energy transfer process between pressurized water heated by the solar collectors and the LiBr/H2O solution is the focus of this study. A method of augmenting the heat transfer in the generator was developed, bench tested and implemented in an operating 70kW solar HVAC system. The augmented design involved installing twisted stainless steel inserts in the tubes where the LiBr/H2O solution boils and refrigerant vapor is generated. The inserts increased the overall heat transfer coefficient between the heat medium in the shell side of the generator and the LiBr/H2O solution in the tubes. A solar-fired, double effect absorption chiller requires the collector array and storage tank to operate at temperatures in excess of 150°C. At these temperatures, the heating water must be at a pressure of about 700kPa to prevent it from boiling. This combination of high temperature and high pressure requires that the collectors, storage tanks, pumps, valves and piping be designed according to pressure vessel codes. This increases the initial cost of the system and also requires significant maintenance. The main objective of this work is to develop a method of lowering the requirements for a 150°C heating medium. The ultimate goal is to operate at about 120°C while maintaining the Coefficient of Performance and cooling capacity of the absorption chiller. The results presented in this paper show that the generator with twisted inserts can operate with an average log mean temperature difference of 10°C. The average COP of the chiller is about 1.0 and the chiller provided all of the cooling required by a 743 m2 building. Without the twisted inserts, the generator operated with a temperature difference of 22 to 28°C. The inserts provide significant reduction in the operating temperature of the solar collectors and do not adversely affect the performance of the double effect absorption chiller.