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:

HVAC equipment configured to:

provide mechanical cooling for a cooling load in the building when operating in a mechanical cooling state; and

provide free cooling for the cooling load in the building when operating 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 in the free cooling state relative to operating in the mechanical cooling state, wherein the minimum free cooling time is a minimum amount of time that the HVAC equipment is required to 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;

transition the HVAC equipment 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

wherein transitioning the HVAC equipment from operating in the mechanical cooling state to operating in the free cooling state comprises operating one or more dampers of the HVAC equipment to increase an amount of outside air used by the HVAC equipment to provide cooling for the building.

2. 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 equipment into the free cooling state;

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

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

3. The HVAC system of claim 2 , wherein the controller is configured to estimate the cost savings by:

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

identifying a cost per unit energy for each of the future time steps during which the HVAC equipment 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.

4. The HVAC system of claim 2 , 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 the HVAC equipment.

5. 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 equipment 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.

6. 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 equipment 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.

7. The HVAC system of claim 6 , wherein the controller is configured to transition the HVAC equipment 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.

8. A controller for heating, ventilation, or air conditioning (HVAC) equipment for a building, the controller comprising:

one or more processors; and

one or more non-transitory computer-readable storage media communicably coupled to the one or more processors and having instructions stored thereon that, when executed by the one or more processors, cause the one or more processors to:

operate the HVAC equipment in a mechanical cooling state to provide mechanical cooling for a cooling load in the building;

operate the HVAC equipment in a free cooling state to provide free cooling for the cooling load in the building;

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 in the free cooling state relative to operating in the mechanical cooling state, wherein the minimum free cooling time is a minimum amount of time that the HVAC equipment is required to 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;

transition the HVAC equipment 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

wherein transitioning the HVAC equipment from operating in the mechanical cooling state to operating in the free cooling state comprises operating one or more dampers of the HVAC equipment to increase an amount of outside air used by the HVAC equipment to provide cooling for the building.

9. The controller of claim 8 , wherein the instructions cause the one or more processors to calculate the minimum free cooling time by:

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

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

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

10. The controller of claim 9 , wherein the instructions cause the one or more processors to estimate the cost savings by:

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

identifying a cost per unit energy for each of the future time steps during which the HVAC equipment 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.

11. The controller of claim 9 , 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 the HVAC equipment.

12. The controller of claim 8 , wherein the instructions cause the one or more processors 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 equipment 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.

13. The controller of claim 8 , wherein the instructions cause the one or more processors 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 equipment 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.