Michael A. Garrabrant

Performance of a Residential-Sized GAX Absorption Chiller

This research effort involved experimentally testing an advanced-cycle, ammonia-water absorption chiller with a cooling capacity of 17.6 kW (5 refrigeration tons (RT)). The system was a generator-absorber heat exchange (GAX) cycle and was sized for residential and light commercial use, where very little absorption equipment is currently used. The components of the cycle were assembled with instrumentation, including flow meters, pressure transducers, and thermocouples. The findings of the research were cycle cooling load and coefficient of performance (COP), as well as many component heat duties and working fluid state points throughout the cycle. The COP of the chiller at essentially full load was measured at 0.68. A simulation of the GAX cycle was performed with a computer program that predicted the heat duties of each component and the cooling load of the cycle. The simulation of the GAX cycle and experimental testing compared closely. Existing market research shows that significant business opportunities exist for a GAX heat pump or chiller with a cooling COP of 0.70 or greater The work performed in this study consisted of testing a GAX cycle with a COP that approached the target value of 0. 70 and identified improvements that must be made to reach the target COP value.

Design of compact microscale geometries for ammonia–water desorption

This article presents the development of two novel compact heat and mass exchange concepts for use as the desorber in small-capacity ammonia–water absorption systems. Both concepts feature a counterflow orientation of the vapor and solution streams, in which the generated vapor rises through small channels within the component, flowing past the heated ammonia–water solution. Heat input is supplied by a hot coupling fluid flowing through a microchannel array adjacent to the vapor/solution channels in a counterflow orientation with the solution stream. This geometry provides high heat transfer coefficients, enabling a significant reduction in component size. This arrangement is also found to produce high-purity ammonia vapor, which leads to reduced rectification loads and a higher coefficient of performance of the system compared to more conventional designs. Additionally, both designs can be configured to include an analyzer and rectification section, leading to more compact integrated system packages and a reduction of external fluid connections and cost. A model of the coupled heat and mass exchange process associated with ammonia–water desorption in these concepts is developed. The proposed design is compact, scalable, versatile, and easily mass-produced. This concept can also be extended for use in other multi-component heat and mass transfer applications.

Development of Ammonia-Water Absorption Heat Pump Water Heater for Residential and Commercial Applications

Approximately half of the water heaters sold in the U.S. and Canada for residential and small commercial applications are natural gas fired storage water heaters, with a maximum theoretical thermal efficiency of 96%. A packaged water heater heated by a 2.9 kW absorption heat pump was designed and demonstrated in this study to achieve performance exceeding these limitations. The modeling and validation of the absorption cycle and of the natural gas-fired combustion system are discussed here. Heat transfer characteristics of the absorption components at expected operating conditions were used to model cycle performance. A single-effect system based on these models was fabricated and yielded a cyclic COP of 1.63, within 3% of predictions. A corresponding GAX cycle-based system yielded performance 20% lower than predicted values, indicating the need for larger heat and mass exchangers to achieve the expected system level performance. The gas-fired burner configuration required for this heat pump is governed by the water heater envelope, desorber geometry and process requirements, coupled with emissions requirements. Parametric CFD analyses were conducted to estimate the impact of chamber design on burner performance, and revealed a beneficial recirculation pattern within the combustion chamber that was strongly influenced by chamber height. Emission reductions depended on chamber diameter, and prototype burners with smaller outer diameter fabricated based on these designs met emission targets.Copyright