Analysis of an Innovative Microchannel Condenser Design for Modular Chillers and Unitary Rooftop Air Conditioners

An innovative way of integrating microchannel condensers in modular chillers and rooftop air conditioners is presented in this paper. The geometrical characteristics of the heat exchanger and how it can be incorporated into commercial air conditioners is discussed. Details are given on the performance analysis of the heat exchanger under at typical chiller operating conditions. Effect of multiple factors such as air entrance losses, fan performance and refrigerant/air mal distribution is analyzed. Analysis shows that the new heat exchanger increases the capacity of a typical chiller condenser by 20%. At same capacity the new heat exchanger resulted in around 2K drop in condensing temperature thereby increasing the chiller efficiency by 5.5%

Modeling, Verification and Optimization of Hybrid Ground Source Heat Pump Systems in Energyplus

Hybrid Ground Source Heat Pump system simulation capability was added to EnergyPlus. Program was modified to handle multi-year simulation capability. New supervisory controls were added and a plate heat exchanger model developed to configure realistic systems. The models were verified by comparing to the published results in literature by Yavuzturk et al. (2000). GenOpt was used to investigate the optimal design, control strategies and configuration. Life cycle cost of the system was used as objective function. Effects of using first year operating cost and 20th year operating cost on optimization results were studied. The modifications and addition of new models successfully modeled HGSHP systems in EnergyPlus. The results obtained from EnergyPlus model agreed well within the modeling differences with Yavuzturk results. Optimization proceeded to decrease the GLHE length to the minimum possible, since it was the highest contributor towards cost. Optimization of HGSHP systems was found to be system specific and highly affected by the load imbalance. Use of 20th year operating cost instead of first year operating cost to calculate the life cycle cost affected the results of optimization.Mechanical & Aerospace Engineerin

Study of Frost Growth on Heat Exchangers Used as Outdoor Coils in Air Source Heat Pump Systems

During winter heating operation, the outdoor coil of a heat pump acts as an evaporator and when the ambient temperature is near freezing, the moisture in the atmospheric air freezes on the coil surface. The frost growth affects the air flow and also adds resistance to heat transfer thus reducing the capacity and the efficiency of the heat pump. An experimental facility is designed and built to test a small scale heat exchanger working in frosting conditions. Tests are carried out using small scale fin-tube and microchannel heat exchanger over a range of glycol inlet temperatures, air velocities and ambient air RH. A semi-empirical frost model based on a scaling approach is developed and applied to both fin-tube heat and microchannel heat exchangers. The model is developed to handle non-uniformities in both refrigerant and air side. The frost model is integrated with a segment-by-segment heat exchanger calculation algorithm and is validated against experimental data. Frost growth model is also incorporated in a quasi-steady state system simulation algorithm.Findings and Conclusions: From the experimental study it was observed that the temperature of the surface and air RH affected the rate of frost growth significantly while the air velocity did not have a great influence. It is demonstrated that a dry heat transfer correlation can be used during frost growth period by correctly accounting for frost thickness in the hydraulic diameter calculations. Ignoring the phenomenon of air redistribution was found to result in errors in the range of 20% to 50% in predicted frost thickness. Frost thickness predicted by the frost model is within 5% of the measured values for most of the cases. Frost mass accumulation predicted by the simple model is higher than the measured values due to a uniform thickness assumption in the model. An improved frost model based on 1-D finite volume discretization is also presented which improved the frost mass prediction to within 13% of measured values. The system simulation is validated against experimental results and found to match reasonable well. The discrepancy between simulation and experiments were due to the effects of system transience in the initial stages.Mechanical & Aerospace Engineerin