HVAC system with predictive free cooling control based on the cost of transitioning into a free cooling state

1. A heating, ventilation, or air conditioning (HVAC) system for a building, the HVAC system comprising:

a chiller configured to provide mechanical cooling for a cooling load in the building when the HVAC system operates in a mechanical cooling state;

a heat exchanger separate from the chiller and configured to provide free cooling for the cooling load in the building when the HVAC system operates in a free cooling state; and

a controller configured to:

predict outside air temperature for a plurality of future time steps;

calculate a minimum free cooling time based on an estimated cost savings resulting from operating the HVAC system in the free cooling state relative to operating the HVAC system in the mechanical cooling state, wherein the minimum free cooling time is a minimum amount of time the HVAC system must operate in the free cooling state for the estimated cost savings to offset a cost of transitioning into the free cooling state:

determine whether the predicted outside air temperature will be less than a free cooling temperature threshold for at least the minimum free cooling time; and

transition the HVAC system from operating in the mechanical cooling state to operating in the free cooling state in response to a determination that the predicted outside air temperature will be less than the free cooling temperature threshold for at least the minimum free cooling time.

2. The HVAC system of claim 1 , wherein the controller is configured to:

determine whether the predicted outside air temperature will be greater than the free cooling temperature threshold for at least a minimum mechanical cooling time; and

transition the HVAC system from operating in the free cooling state to operating in the mechanical cooling state in response to a determination that the predicted outside air temperature will be greater than the free cooling temperature threshold for at least the minimum mechanical cooling time.

3. The HVAC system of claim 1 , wherein the controller is configured to:

identify an actual outside air temperature;

determine whether the actual outside air temperature is above the free cooling temperature threshold; and

transition the HVAC system from operating in the free cooling state to operating in a no cooling state in response to a determination that the actual outside air temperature is above the free cooling temperature threshold.

4. The HVAC system of claim 3 , wherein the controller is configured to transition the HVAC system from operating in the free cooling state to operating in the no cooling state in response to a determination that the actual outside air temperature is currently above the free cooling temperature threshold and predicted to drop below the free cooling temperature threshold within a predetermined amount of time.

5. The HVAC system of claim 1 , wherein the controller is configured to:

identify an actual outside air temperature;

determine whether the actual outside air temperature is below the free cooling temperature threshold; and

transition the HVAC system from operating in a no cooling state to operating in the free cooling state in response to a determination that the actual outside air temperature is below the free cooling temperature threshold.

6. The HVAC system of claim 5 , wherein the controller is configured to transition the HVAC system from operating in the no cooling state to operating in the mechanical cooling state in response to a determination that the actual outside air temperature is currently above the free cooling temperature threshold and predicted to remain above the free cooling temperature threshold for at least a predetermined amount of time.

7. The HVAC system of claim 1 , wherein the controller is configured to calculate the minimum free cooling time by:

identifying the cost incurred as a result of transitioning the HVAC system into the free cooling state;

estimating the cost savings resulting from operating the HVAC system in the free cooling state relative to operating the HVAC system in the mechanical cooling state as a function of an amount of time the HVAC system operates in the free cooling state; and

determining the minimum amount of time the HVAC system is required to operate in the free cooling state for the cost savings to be greater than or equal to the cost incurred.

8. The HVAC system of claim 7 , wherein the controller is configured to estimate the cost savings by:

determining an amount of energy savings resulting from operating the HVAC system in the free cooling state relative to operating the HVAC system in the mechanical cooling state;

identifying a cost per unit energy for each of the future time steps during which the HVAC system will operate in the free cooling state; and

calculating the cost savings by multiplying the amount of energy savings by the cost per unit energy.

9. The HVAC system of claim 7 , wherein the cost incurred comprises at least one of an economic cost of equipment degradation and an increase in electricity cost resulting from stopping and restarting starting the chiller.

10. A method for operating a heating, ventilation, or air conditioning (HVAC) system for a building, the method comprising:

operating the HVAC system in a mechanical cooling state in which a chiller is used to provide mechanical cooling for a cooling load in the building;

predicting outside air temperature for a plurality of future time steps;

calculating a minimum free cooling time based on an estimated cost savings resulting from operating the HVAC system in a free cooling state relative to operating the HVAC system in the mechanical cooling state, wherein the minimum free cooling time is a minimum amount of time the HVAC system must operate in the free cooling state for the estimated cost savings to offset a cost of transitioning into the free cooling state;

determining whether the predicted outside air temperature will be less than a free cooling temperature threshold for at least the minimum free cooling time;

transitioning the HVAC system from operating in the mechanical cooling state to operating in the free cooling state in response to a determination that the predicted outside air temperature will be less than the free cooling temperature threshold for at least the minimum free cooling time; and

operating the HVAC system in the free cooling state in which a heat exchanger separate from the chiller is used to provide free cooling for the cooling load in the building.

11. The method of claim 10 , further comprising:

determining whether the predicted outside air temperature will be greater than the free cooling temperature threshold for at least a minimum mechanical cooling time; and

transitioning the HVAC system from operating in the free cooling state to operating in the mechanical cooling state in response to a determination that the predicted outside air temperature will be greater than the free cooling temperature threshold for at least the minimum mechanical cooling time.

12. The method of claim 10 , further comprising:

identifying an actual outside air temperature;

determining whether the actual outside air temperature is above the free cooling temperature threshold; and

transitioning the HVAC system from operating in the free cooling state to operating in a no cooling state in response to a determination that the actual outside air temperature is above the free cooling temperature threshold.

13. The method of claim 12 , wherein transitioning the HVAC system from operating in the free cooling state to operating in the no cooling state occurs in response to a determination that the actual outside air temperature is currently above the free cooling temperature threshold and predicted to drop below the free cooling temperature threshold within a predetermined amount of time.

14. The method of claim 10 , further comprising:

identifying an actual outside air temperature;

determining whether the actual outside air temperature is below the free cooling temperature threshold; and

transitioning the HVAC system from operating in a no cooling state to operating in the free cooling state in response to a determination that the actual outside air temperature is below the free cooling temperature threshold.

15. The method of claim 14 , wherein transitioning the HVAC system from operating in the no cooling state to operating in the mechanical cooling state occurs in response to a determination that the actual outside air temperature is currently above the free cooling temperature threshold and predicted to remain above the free cooling temperature threshold for at least a predetermined amount of time.

16. The method of claim 10 , further comprising calculating the minimum free cooling time by:

identifying the cost incurred as a result of transitioning the HVAC system into the free cooling state;

estimating the cost savings resulting from operating the HVAC system in the free cooling state relative to operating the HVAC system in the mechanical cooling state as a function of an amount of time the HVAC system operates in the free cooling state; and

determining the minimum amount of time the HVAC system is required to operate in the free cooling state for the cost savings to be greater than or equal to the cost incurred.

17. The method of claim 16 , wherein estimating the cost savings comprises:

determining an amount of energy savings resulting from operating the HVAC system in the free cooling state relative to operating the HVAC system in the mechanical cooling state;

identifying a cost per unit energy for each of the future time steps during which the HVAC system will operate in the free cooling state; and

calculating the cost savings by multiplying the amount of energy savings by the cost per unit energy.

18. The method of claim 16 , wherein the cost incurred comprises at least one of an economic cost of equipment degradation and an increase in electricity cost resulting from stopping and restarting starting the chiller.