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Abstract
The multi-functional variable refrigerant flow system enables simultaneous heating and cooling of the zones it serves by allowing the heat exchangers of both indoor units and outdoor units to operate as evaporators or condensers. To realize automatic and smooth switching between different modes, a mode switching strategy is proposed for a multi-functional variable refrigerant flow system. Whether to turn on or off an indoor unit is determined by the zone temperature and a preset hysteresis band about the temperature set-point. Based on a thermodynamic analysis, a decision variable for determining outdoor unit-heat exchangers mode switching is proposed as the outdoor unit-heat exchangers airside temperature differential normalized by the dimensionless outdoor unit fan speed. Two bumpless transfer strategies are applied to produce smooth mode switching. To evaluate the proposed strategy, a simulation study is performed with a Modelica-based dynamic simulation model of a four-zone multi-functional variable refrigerant flow system. Simulation results validate the effectiveness of the proposed strategy and the performance of the bumpless transfer strategies.
Nomenclature
| EEV | = | electronic expansive valve |
| BPV | = | bypass valve |
| HX | = | heat exchanger |
| COP | = | coefficient of performance |
| SH | = | superheat |
| PCS | = | compressor suction pressure (bar) |
| PCD | = | compressor discharge pressure (bar) |
| H | = | heating mode |
| C | = | cooling mode |
| O | = | outdoor unit |
| M | = | mode change unit |
| I | = | indoor unit |
| comp | = | compressor |
| Ti | = | i-th zone temperature (°C) |
| = | heat transfer rate (W) | |
| = | shaft power (W) | |
| = | mass flow rate (kg/s) | |
| h | = | specific entropy (kJ/kg) |
| cp | = | specific heat in constant pressure (J/kg·K) |
| σ | = | indicating variable of ODU-HX mode switch |
| ϵ | = | preset airside temperature difference (°C) |
| nf | = | ODU fan speed (Hz) |
| nfr | = | rated ODU fan speed (Hz) |
| τdw | = | preset dwell time of ODU-HX mode switch |
| Ω | = | compressor speed (Hz) |
| rPCS | = | compressor suction pressure set-point (bar) |
| rPCD | = | compressor discharge pressure set-point (bar) |
| Subscripts | ||
| min | = | minimum |
| max | = | maximum |
Acknowledgments
The authors are grateful of TLK Thermo for their permission of access to the products of TIL Suite for the simulation study presented in this study, as well as their patient help. The authors appreciate Dr. Zhigang Wu of Johnson Controls—Hitachi Joint Venture for the valuable inputs to this work.
Funding
This work is supported in part by Johnson Controls, Inc.
Notes
1 For a VRF system that can realize simultaneous heating and cooling operation, the cooling dominant mode refers to the scenario where the collective cooling load for all IDUs in cooling operation is greater than the collective heating load for all IDUs in heating operation; while the heating dominant mode is defined likewise.
Additional information
Notes on contributors
Liujia Dong
Liujia Dong is a PhD Student. Yaoyu Li is Member ASHRAE and an Associate Professor. John M. House, PhD, is Member ASHRAE. Timothy I. Salsbury, PhD.
Yaoyu Li
Liujia Dong is a PhD Student. Yaoyu Li is Member ASHRAE and an Associate Professor. John M. House, PhD, is Member ASHRAE. Timothy I. Salsbury, PhD.
John M. House
Liujia Dong is a PhD Student. Yaoyu Li is Member ASHRAE and an Associate Professor. John M. House, PhD, is Member ASHRAE. Timothy I. Salsbury, PhD.
Timothy I. Salsbury
Liujia Dong is a PhD Student. Yaoyu Li is Member ASHRAE and an Associate Professor. John M. House, PhD, is Member ASHRAE. Timothy I. Salsbury, PhD.