HVAC system with free cooling optimization based on coolant flowrate

A method for providing free cooling to a building, the method comprising:

• using one or more pumps to circulate a coolant between a heat exchanger and a cooling tower via a cooling tower circuit, wherein the coolant absorbs heat in the heat exchanger at a first rate of heat transfer dependent upon a flowrate of the coolant in the cooling tower circuit and rejects heat in the cooling tower at a second rate of heat transfer dependent upon a flowrate of air through the cooling tower;
• determining an optimal flowrate of the coolant in the cooling tower circuit to achieve a cooling load setpoint by repeatedly adjusting the flowrate of the coolant to newly calculated values to reduce a difference between the first rate of heat transfer in the heat exchanger and the cooling load setpoint;
• determining an optimal flowrate of air through the cooling tower to balance heat transfer in the cooling tower circuit by repeatedly adjusting the flowrate of the air to newly calculated values to reduce a difference between the second rate of heat transfer in the cooling tower and the first rate of heat transfer in the heat exchanger;
• and
• operating the one or more pumps and the cooling tower to achieve the optimal flowrate of the coolant in the cooling tower circuit and the optimal flowrate of the air through the cooling tower.

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

• a cooling tower configured to remove heat from a coolant in a cooling tower circuit at a rate of heat transfer dependent upon a flowrate of air through the cooling tower;
• one or more pumps configured to circulate the coolant between the cooling tower and a cooling load via the cooling tower circuit, wherein the cooling tower circuit is configured to absorb heat from the cooling load at a rate dependent upon the flowrate of the coolant in the cooling tower circuit;
• and
• a free cooling controller configured to:
• determine an optimal flowrate of the coolant in the cooling tower circuit by minimizing a difference between the rate at which the cooling tower circuit absorbs heat from the cooling load and a cooling load setpoint;
• determine an optimal flowrate of the air through the cooling tower by reducing a difference between the rate of heat transfer in the cooling tower and the rate at which the cooling tower circuit absorbs heat from the cooling load;
• and
• operate the one or more pumps and the cooling tower to achieve the optimal flowrate of the coolant in the cooling tower circuit and the optimal flowrate of the air through the cooling tower.