Variable refrigerant flow system with pressure optimization using extremum-seeking control

1. A variable refrigerant flow (VRF) system for a building, the VRF system comprising:

a plurality of outdoor VRF units configured to heat or cool a refrigerant for use in heating or cooling the building;

an extremum-seeking controller configured to:

determine a total power consumption of the plurality of outdoor VRF units;

generate a pressure setpoint for the plurality of outdoor VRF units using an extremum-seeking control technique that drives the total power consumption toward an extremum; and

use the pressure setpoint to operate the plurality of outdoor VRF units.

2. The VRF system of claim 1 , wherein generating the pressure setpoint using the extremum-seeking control technique comprises:

perturbing a current value of the pressure setpoint with an excitation signal to generate a perturbed current value of the pressure setpoint;

monitoring the total power consumption resulting from the perturbed current value of the pressure setpoint;

estimating a gradient of the total power consumption with respect to the perturbed current value of the pressure setpoint; and

determining a new value of the pressure setpoint to drive the gradient toward zero.

3. The VRF system of claim 1 , wherein the plurality of outdoor VRF units are configured to deliver the refrigerant to a plurality of indoor VRF units located within the building.

4. The VRF system of claim 1 , wherein:

each of the plurality of outdoor VRF units comprises a compressor and a fan; and

the total power consumption is a combination of a power consumption of the compressor and a power consumption of the fan for each of the plurality of outdoor VRF units.

5. The VRF system of claim 1 , wherein the plurality of outdoor VRF units comprise a master outdoor VRF unit and one or more slave outdoor VRF units, the master outdoor VRF unit configured to:

identify a new value of the pressure setpoint at the master outdoor VRF unit;

use the new value of the pressure setpoint to operate the master outdoor VRF unit; and

communicate the new value of the pressure setpoint from the master outdoor VRF unit to the one or more slave outdoor VRF units.

6. The VRF system of claim 1 , wherein the extremum-seeking controller is configured to determine whether the plurality of outdoor VRF units are operating in a cooling mode or a heating mode.

7. The VRF system of claim 6 , wherein the extremum-seeking controller is configured to control a discharge pressure of the plurality of outdoor VRF units to the pressure setpoint in response to a determination that the plurality of outdoor VRF units are operating in the cooling mode.

8. The VRF system of claim 6 , wherein the extremum-seeking controller is configured to control a suction pressure of the plurality of outdoor VRF units to the pressure setpoint in response to a determination that the plurality of outdoor VRF units are operating in the heating mode.

9. A method for operating a variable refrigerant flow (VRF) system for a building, the method comprising:

determining a total power consumption of a plurality of outdoor VRF units;

generating a pressure setpoint for the plurality of outdoor VRF units using an extremum-seeking control technique that drives the total power consumption toward an extremum; and

using the pressure setpoint to operate the plurality of outdoor VRF units.

10. The method of claim 9 , wherein generating the pressure setpoint for the plurality of outdoor VRF units using the extremum-seeking control technique comprises:

perturbing a current value of the pressure setpoint with an excitation signal to generate a perturbed current value of the pressure setpoint;

monitoring the total power consumption resulting from the perturbed current value of the pressure setpoint;

estimating a gradient of the total power consumption with respect to the perturbed current value of the pressure setpoint; and

determining a new value of the pressure setpoint to drive the gradient toward zero.

11. The method of claim 9 , further comprising:

operating the plurality of outdoor VRF units to heat or cool a refrigerant;

delivering the refrigerant to a plurality of indoor VRF units located within the building.

12. The method of claim 9 , wherein each of the plurality of outdoor VRF units comprise a compressor and a fan, wherein determining the total power consumption of the plurality of outdoor VRF units comprise combining a power consumption of the compressor and a power consumption of the fan for each of the plurality of outdoor VRF units.

13. The method of claim 9 , wherein the plurality of outdoor VRF units comprise a master outdoor VRF unit and one or more slave outdoor VRF units, the method further comprising;

identifying a new value of the pressure setpoint at the master outdoor VRF unit;

using the new value of the pressure setpoint to operate the master outdoor VRF unit; and

communicating the new value of the pressure setpoint from the master outdoor VRF unit to the one or more slave outdoor VRF units.

14. The method of claim 9 , further comprising determining whether the plurality of outdoor VRF units are operating in a cooling mode or a heating mode.

15. The method of claim 14 , further comprising controlling a discharge pressure of the plurality of outdoor VRF units to the pressure setpoint in response to a determination that the plurality of outdoor VRF units are operating in the cooling mode.

16. The method of claim 14 , further comprising controlling a suction pressure of the plurality of outdoor VRF units to the pressure setpoint in response to a determination that the plurality of outdoor VRF units are operating in the heating mode.

17. An extremum-seeking controller for a variable refrigerant flow (VRF) system, the extremum-seeking controller comprising:

one or more interfaces configured to provide control signals to the VRF system; and

a processing circuit configured to:

determine a total power consumption of a plurality of outdoor VRF units;

generate a pressure setpoint for the plurality of outdoor VRF units using an extremum-seeking control technique that drives the total power consumption toward an extremum; and

use the pressure setpoint to operate the plurality of outdoor VRF units.

18. The extremum-seeking controller of claim 17 , wherein generating the pressure setpoint for the plurality of outdoor VRF units comprises perturbing a current value of the pressure setpoint with an excitation signal.

19. The extremum-seeking controller of claim 17 , wherein determining the total power consumption of the plurality of outdoor VRF units comprises combining a power consumption of a compressor and a power consumption of a fan for each of the plurality of outdoor VRF units.

20. The extremum-seeking controller of claim 17 , wherein the processing circuit is further configured to communicate the pressure setpoint from a master outdoor VRF unit to one or more slave outdoor VRF units.