Novel single-double-effect LiBr-H?O absorption prototype with a highly efficient direct air-cooled adiabatic absorbercharacterization, simulation and experimental results

Resumen

Due to unsustainable growth of air conditioning market, a great interest in solar cooling technologies has emerged. The coincidence between availability of solar irradiation and peaks of cooling demand makes solar cooling a very attractive option to replace conventional refrigeration machines based on electricity. What is more, solar cooling systems normally use natural refrigerants that are not harmful to the environment. However, an improvement of the current technology is needed for solar cooling systems to compete with electricity?powered air conditioning systems. In this work, a novel air?cooled single–double?effect LiBr/H?O absorption prototype is proposed as a solution to improve the viability of solar cooling systems. This prototype presents the following distinguishing features: firstly, it is directly air?cooled, which means that no cooling tower is needed; secondly, it is made up by compact heat exchangers, which allows for a reduced size of about 1 m?; thirdly, it incorporates an adiabatic absorber operating with flat?fan solution sheets, which permits the working solution not to crystallize at high ambient temperatures; lastly, it can be powered by solar heat in its single?effect mode (4.5 kW), or by an alternative source such as fuel or waste heat in its double?effect stage (7 kW). In this way, 100% of the cooling demand may be supplied by a single absorption machine using solar energy as far as possible or, when it is not available, efficiently utilizing a fuel or even waste heat, for instance in a trigeneration system. This thesis includes a detailed description of that single–double?effect absorption prototype as well as the fundamentals for its numerical simulation. Likewise, experimental results from a testing campaign carried out in Madrid during 2010 are presented and discussed. A solar facility with evacuated flat?plate collectors was used to test the single-effect operation mode of the prototype. In turn, the double?effect stage was fired by a thermal oil facility with electrical resistances. As relevant results of the whole experimental campaign it is worth mentioning that the single?effect stage was able to work with COP values around 0.6, whereas the double?effect mode permitted to achieve values of about 1.0. The chilled water temperatures mostly ranged between 14°C and 16°C in single?effect operation mode, while they were around 12°C for the double?effect stage. Besides, it is highly noteworthy that after some 125 hours of operation under a wide range of conditions (outdoor temperatures up to 39.5°C), no solution crystallization was noticed. On the other hand, this work includes an in?depth description of the absorber assembled in the single–double?effect prototype. Furthermore, a mathematical model is developed for simulation of air?cooled flat?fan sheets adiabatic absorbers. As far as we know, there is not any numerical modeling for this kind of absorbers in the literature. Based on that model, which was as well experimentally validated in this study, the capacity of the prototype absorber is optimized as a function of the energy consumption of its ancillary equipment (solution pump and fan). Finally, the positive results derived from this work may be regarded as an important contribution to the development of air?cooled LiBr/H?O absorption technology. Even though a few improvements in the prototype are still required, it seems that the proposed system represents a feasible alternative to overcome some of the major obstacles concerning solar air conditioning. -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------