Publication: Eyector-absorbedor adiabático como potenciador de un ciclo híbrido para refrigeración por absorción basado en la disolución amoniaco-nitrato de litio
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2015-07
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2015-07-24
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Abstract
Las instalaciones de producción de frío emplean mayoritariamente máquinas de compresión mecánica accionadas mediante energía eléctrica; contribuyendo por tanto a la emisión indirecta de gases de efecto invernadero (CO₂) y a la emisión directa de gases limitados por el Protocolo de Montreal. Una alternativa a estas máquinas son las de refrigeración por absorción, las cuales son accionadas mediante energía térmica (que puede ser calor residual o de origen renovable, reduciendo así las emisiones). Sin embargo, las máquinas de absorción adolecen actualmente de un coste inicial relativamente alto, una eficiencia moderada y un gran tamaño. Con el objetivo de avanzar en el desarrollo de las máquinas de absorción, en esta Tesis se presenta un nuevo ciclo de absorción y un nuevo dispositivo asociado. Esta Tesis propone un ciclo híbrido en el que se tiene un pequeño aporte extra de energía eléctrica que permite aumentar la eficiencia y la potencia. El ciclo de partida es el ciclo de absorción de simple efecto (es decir, con dos niveles de presión de refrigerante) y está basado en la tecnología de la absorción adiabática. Una única bomba impulsa toda la disolución (la dirigida al generador y la recirculada al absorbedor) hasta la máxima presión del ciclo. La disolución es sub-enfriada antes de ser introducida como fluido motriz en un dispositivo tipo eyector (EAA) con triple función: absorbedor adiabático, compresor de refrigerante y válvula reductora de presión de la disolución. Los tres niveles de presión de refrigerante permiten tener un rango de funcionamiento más amplio que el ciclo de simple efecto de partida. Además, se logra la ventaja de contar con un menor número de componentes y de menor complejidad que otros ciclos híbridos equivalentes. En este trabajo la disolución utilizada es amoniaco-nitrato de litio (NH₃/LiNO₃), aunque puede ser utilizada otra. Se lleva a cabo un análisis teórico del ciclo propuesto mediante una simulación numérica donde se compara con otros ciclos de absorción; se estudian sus límites de operación y se establecen relaciones entre las condiciones de funcionamiento del ciclo y los parámetros constructivos del EAA. Posteriormente se analiza experimentalmente el ciclo y, en particular, el EAA. Para ello se ha diseñado y construido un prototipo de EAA para poder modificar sus parámetros geométricos principales así como para visualizar el flujo en su interior. Los resultados experimentales han demostrado un correcto y prometedor funcionamiento del EAA y han permitido determinar los límites de operación del ciclo propuesto
Cooling facilities employ mainly mechanical-compression machines driven by electricity; thus contributing to indirect emission of greenhouse gases (CO₂) and direct emission of gases limited by the Montreal Protocol. Alternatives to these machines are the absorption refrigeration ones, which are driven by means of thermal energy (it can be waste heat or from renewable sources, thereby reducing emissions). However, absorption machines suffer from a relatively high initial cost, moderate efficiency and large size. In order to progress in the development of absorption machines, in this Thesis a novel absorption cycle and a new associated device are presented. This Thesis proposes a hybrid cycle with an extra consumption of electricity for increasing efficiency and capacity. The starting cycle is the single-effect absorption cycle (ie, with two refrigerant pressure levels) and it’s based on the adiabatic absorption technology. A single pump drives all the solution (the directed to the generator and the recirculated to the absorber) to the maximum pressure of the cycle. The solution is sub-cooled before being introduced as motive flow into an ejector-type device (EAA) with triple purpose: adiabatic absorber, pressure booster for refrigerant vapor and solution expansion valve. The three refrigerant pressure levels allow a broader operating range than the single-effect cycle. Furthermore, it is achieved the advantage of having a lower number of components and of lesser complexity than equivalent hybrid cycles. In this work, the solution used is ammonia-lithium nitrate (NH₃/LiNO₃), although it can be used another one. A theoretical model compares the proposed cycle with other absorption cycles, showing its operating limits and the relations between operating conditions of the cycle and construction parameters of the EAA. Later, the cycle and, particularly, the EAA are experimentally analyzed. For this purpose a prototype of EAA has been designed and manufactured allowing to modify its main geometric parameters and to visualize the inside flow. The experimental results have shown the proper and promising operation of the EAA and have allowed identifying the operating limits of the proposed cycle.
Cooling facilities employ mainly mechanical-compression machines driven by electricity; thus contributing to indirect emission of greenhouse gases (CO₂) and direct emission of gases limited by the Montreal Protocol. Alternatives to these machines are the absorption refrigeration ones, which are driven by means of thermal energy (it can be waste heat or from renewable sources, thereby reducing emissions). However, absorption machines suffer from a relatively high initial cost, moderate efficiency and large size. In order to progress in the development of absorption machines, in this Thesis a novel absorption cycle and a new associated device are presented. This Thesis proposes a hybrid cycle with an extra consumption of electricity for increasing efficiency and capacity. The starting cycle is the single-effect absorption cycle (ie, with two refrigerant pressure levels) and it’s based on the adiabatic absorption technology. A single pump drives all the solution (the directed to the generator and the recirculated to the absorber) to the maximum pressure of the cycle. The solution is sub-cooled before being introduced as motive flow into an ejector-type device (EAA) with triple purpose: adiabatic absorber, pressure booster for refrigerant vapor and solution expansion valve. The three refrigerant pressure levels allow a broader operating range than the single-effect cycle. Furthermore, it is achieved the advantage of having a lower number of components and of lesser complexity than equivalent hybrid cycles. In this work, the solution used is ammonia-lithium nitrate (NH₃/LiNO₃), although it can be used another one. A theoretical model compares the proposed cycle with other absorption cycles, showing its operating limits and the relations between operating conditions of the cycle and construction parameters of the EAA. Later, the cycle and, particularly, the EAA are experimentally analyzed. For this purpose a prototype of EAA has been designed and manufactured allowing to modify its main geometric parameters and to visualize the inside flow. The experimental results have shown the proper and promising operation of the EAA and have allowed identifying the operating limits of the proposed cycle.
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Keywords
Refrigeración por absorción, Ciclo híbrido