An energy-saving control strategy for VRF and VAV combined ...
Transcript of An energy-saving control strategy for VRF and VAV combined ...
An energy-saving control strategy for VRF and VAV combined air conditioning system in heating mode
Yonghua Zhu, Xinqiao Jin, Zhimin Du, Xing Fang, Bo Fan
Shanghai Jiao Tong UniversityP. R. China
For Refrigeration and Air conditioning Conference
Background
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VRF system: Merits:simple structure &
maintenance; high part-load efficiency…;
Demerits:no outdoor air intake; strongly coupled, difficult to optimal control;
Challenge: stronger demand of IAQ (fresh air) nowadays.
VRF+VAV combined system: Dedicated outdoor air supplying
; Perspective of joint (intelligent )
control;
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I Research objective
VRF unit
OAP unit
VAV part
OA flow control (DCV)
Static pressure control
OA supply temperature
control
OAP capacity control
VRF part
Indoor temperature
control
VRF capacity control
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I Research objective
Structure and control designing
dynamic simulator developing
control and operation performances
Energy saving control method
Generic simulation model of multi-evaporator variable refrigerant flow air conditioning system for control analysis. International Journal of Refrigeration, 2013, 36:1602-1615.
Simulation of variable refrigerant flow air conditioning system in heating mode combined with outdoor air processing unit. Energy and Buildings, 2014, 68:571-579.
Control and energy simulation of variable refrigerant flow air conditioning system combined with outdoor air processing unit. Applied Thermal Engineering, 2014, 64:385-395.
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II Motivation
Changing OA supply temperature results in a reciprocal relationship between
the VRF unit and the OAP unit;
Energy consumption is affected.
It is possible to decrease energy consumption of the combined system by finding the best OA supply temperature.
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II Motivation
COP of the VRF (OAP) unit attains its maximum when PLR locates in the
range from 0.4 to 0.7.
(proved by experimental and simulation results)
PLR
It is possible to improve COP of the combined system by stopping the low-efficient unit.
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General idea of the energy-saving
control method:
OA supply temperature optimizing;
OAP unit stopping and restarting.
III Energy saving control method
Restarting the OAP unit when it is off by the restarting threshold; /load is larger than θ1 of total capacity more than θ2 minutes/
Finding the best OA supply temperature by load allocation optimization method
Stopping the OAP unit by the stopping threshold/OAP allocated load is smaller than θ3 of its capacity more than θ4 minutes/
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Load allocation optimization method:
Making use of the reciprocal relationship;
Total load prediction;
Energy consumption prediction of the
units.
III Energy saving control method
, , , ,,
loads OAP set s OAP meas
OAP meas
Q LRT TQ
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Comparative control method:
•Benchmark:Fixed OA supply
temperature without OAP stopping;
PAR θ1 θ2 θ3 θ4
Value 1/4 5 1/6 5
IV Control method evaluation
IV Control method evaluation
Total load changes widely in the test day, leading to significant efficiency variation of the system;
OAP allocated load has similar changing law to the total load; OAP unit is stopped from early afternoon and restarted after the load is high
enough in the evening.Page 10
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Daily total heating capacity is almost the same in the two control methods;
The proposed control method consumes 5.17% less of energy; Feasibility of the proposed method is proved.
IV Control method evaluation
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Impacts of stopping and restarting thresholds
IV Control method evaluation
Control method BenchmarkProposed
θ3=1/10 θ3=1/8 θ3=1/6 θ3=1/5
Energy consumption (kWh) 39.28 38.42 38.35 38.34 38.25
Energy saving (%) - 4.97 5.14 5.17 5.39
Stopping threshold affects energy consumption of the combined system;
Too small stopping threshold is not helpful for energy saving; Large stopping threshold is preferable, but too large stopping
threshold will cause frequent on-off switch of OAP unit .
Similarly, too large or too small restarting threshold is not helpful for energy saving or safety operation of the OAP unit.
Conclusion
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An energy-saving control method is proposed and evaluated for the VRF-VAV combined AC system in heating conditions;
The control method makes a coordination of the VRF unit and the OAP unit by combining outdoor air supply temperature optimization and on-off switching of OAP unit;
The control method is proved to be feasible and superior to conventional fixed set-point control method;
The control method can be easily extended to cooling conditions.
THANKS FOR YOUR
ATTENTION!