System Efficiency Improvement for a Variable-Capacity/Variable-Fan-Speed Residential Heat Pump System with Ductwork
By: Sreenidhi Krishnamoorthy
Advisor: Professor Mark Modera
The purpose of this research effort was to understand and optimize the operation of variablecapacity
/ variable-fan-speed air-conditioning systems for single and multi-zone residential
applications. This was accomplished through experimental study and mathematical modeling of
the interactions between thermodynamic, aerodynamic and heat transfer impacts of the variable
speed air-conditioning equipment connected to a residential duct system, so as to maximize System
COP at different outdoor conditions.
The interactions between delivery effectiveness (ratio of cooling delivered at the grilles to cooling
delivered at the equipment), cooling capacity and indoor airflow was investigated by performing
full-scale laboratory testing using a heat-pump equipment connected to a “typical-design”
residential duct system, located within a climatic chamber. A range of summer climate conditions
were simulated within the chamber at different capacities and evaporator fan speeds.
Measurements were made to characterize the efficiency of the cooling equipment and the airtight
duct system capable of being zoned. The results indicated that when no zoning is employed,
conduction losses cause the efficiency of the distribution system to decrease as the compressor and
evaporator-fan speeds are reduced, the effect being more pronounced at hotter duct-zone
temperatures. Delivery efficiency at part load changes disproportionately in different cooling
zones, compromising occupant comfort through elevated supply air temperatures. An
appropriately zoned airflow control mechanism can significantly reduce duct heat gain and
produces the highest System COP at all climates tested.
A first-principles based mathematical model was developed to analyze the performance of the airconditioning
equipment and the duct system. The model predictions on delivery effectiveness and
equipment COP were validated using the experiments, and the combined system model was then
used to run additional simulations that would explain the sensitivity of design choice parameters
such as the insulation resistance of the duct system, and the impact of duct-zone temperatures
being higher than the outdoor air temperature.
Date(s) - 07/19/2018
11:00 am - 12:00 pm
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