Modeling and design of a two-stage NH3/H2O absorption chiller

In this work an innovative ammonia-water double-effect GAX absorption system is investigated. The objective is to replace the complex Generator-Absorber heat eXchanger element by commercially-diffused components, decreasing cost and system complexity. A literature review of the most important absorption systems is done: a focus on ammonia-water cycle is conducted as well. As a result, plate heat exchangers are considered the best solution, even if this choice comports the addition of some separation tanks. A physical model is built to study the possible new architectures and it is detailed in the discussion. Numerical simulations are conducted in order to evaluate the cycle performances for both the chiller and heat pump configurations. Also, the inversion curves of the Joule-Thomson coefficient for ammonia-water mixtures are traced to individuate the best points for the poor solution expansion. Results show that plate exchanger-based GAX ammonia-water systems are possible and performing with reference to single-stage cycles. The Coefficient of Performance (COP) is investigated in different operational conditions for both chiller and heat pump, varying each time the Ambient Temperature (Ta), the Cooling Temperature (Te) or the Generation Temperature (Tg) depending on the cases. The chiller was proved to reach a refrigeration COP > 0.5 for a Te=-20°C at Ta=40°C, while the heat pump reached high temperatures (up to 125°C at the hot output) with appreciable performances (heat pump COP =1.6 to 2). An increased stability of GAX system performances and a wider operational range with reference to single-stage ones is observed in both chiller and heat pump configurations.