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!