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Abstract
A modular variable-speed heat pump model is developed from first principles. The system model consists of steady-state evaporator, compressor, and condenser component sub-models. The compressor model currently implemented accounts for re-expansion, valve pressure drop, and back leakage using empirical coefficients obtained from tests covering a wide range of pressure ratio and shaft speed. Variations in heat transfer coefficients with refrigerant and secondary fluid flow rates are modeled over a wide range of capacity. Pressure drops in piping and heat exchangers are also modeled. The resulting heat pump model is flexible and fast enough for use in finding optimal compressor, fan, and pump speeds and optimal subcooling for any specified capacity fraction and operating condition. To confirm the model's accuracy, simulation results are compared to experimental data with condenser inlet air temperatures ranging from 15°C to 45°C, evaporator inlet air (dry) from 14°C to 34°C, and cooling capacity from 1.1 kW to 3.9 kW. The refrigerant charge balance has not been modeled; instead, it is assumed that a liquid receiver maintains the necessary charge balance. Over this wide range of conditions, the coefficient of performance prediction errors are found to be ±10%. An example of configuring the HPM with a different evaporator demonstrates the benefits of a modular approach.
Acknowledgments
The authors would like to acknowledge Masdar Institute (Abu Dhabi, UAE) and MIT Energy Initiative (Cambridge, MA, USA) for supporting this work.
Tea Zakula, Student Member ASHRAE, is doctoral candidate. Nicholas Thomas Gayeski, Associate Member ASHRAE, is Principal. Peter Ross Armstrong, Member ASHRAE, is Associate Professor. Leslie Keith Norford, Member ASHRAE, is Professor.