COMPACT ABSORBER FOR ADVANCED ABSORPTION HEAT PUMPS

Abstract

Almost half of all energy contained in primary energy carriers is discarded as low temperature waste heat. One of few application areas for low temperature waste heat recovery is to drive absorption cooling systems for conversion of waste heat to cooling energy. However, absorption chillers are often not economical due to their bulky, and hence expensive, heat and mass exchangers; the absorber heat/mass exchanger being the largest among them. This dissertation introduces original contributions to advance next generation, more economical absorption chillers by utilizing a novel, highly compact absorber. The novel absorber designed in this work enhances absorption performance by combining rotation of the heat transfer surface for solution-side heat and mass transfer enhancement with innovative high-performance heat transfer technology on the water-side. A numerical model was developed to describe the absorption process and promote design optimization. The replacement of gravitation force by the stronger centrifugal acceleration thins and mixes the solution film and thereby decreases solution-side thermal and mass transfer resistance. The development of an original adaptation of manifold-microchannel technology leads to significant water-side heat transfer enhancement. This dissertation includes the first publication of an experimental characterization of exothermic absorption on a spinning disk. The overall and film-side heat transfer coefficients were 4.7 and 5.5 times higher, respectively, than conventional horizontal tube banks. The absorption rate increased by a factor of 4 to 10 folds over those of the conventional tube absorbers. The power required for spinning the disk was modest and ranged between 1.1% and 2.3% of the cooling capacity. The results suggest that a spinning disk absorber could substantially reduce the size of absorber in the absorption machines. The technology developed in this dissertation can lead to more compact and hence more economical absorption chillers, thereby easing higher market penetration of absorption chillers which in turn can reduce the amount of primary energy spent on cooling applications. Spinning disk absorbers may be especially useful if combined with a new generation of absorbents that promise improved system efficiency and/or expanded application range but exhibit challenging thermophysical properties.