Chilled water district cooling is a cleaner and more efficient way of cooling campus buildings. In these systems, a central cooling plant generates chilled water and delivers it to buildings through a system of insulated underground pipes. As the cool water moves through the building, it collects heat and humidity from the indoor air. Upgrades to these systems can result in significant improvements in energy efficiency and cost savings.

These complex systems function by transferring heat through a substance with a very low boiling point, called a refrigerant. Chilled water moves through buildings, collecting heat. Next, the water enters an evaporator, heating the refrigerant to the point of boiling. The refrigerant and heat boil off into a vapor, which then enters a compressor. Meanwhile, the cooled water can cycle through the building again to collect more heat. The compressor pressurizes the vaporized refrigerant into a hot liquid and moves it into a condenser. Subsequently, the hot liquid refrigerant transfers its heat to cool water. After absorbing this waste heat, the water moves into a cooling tower, which expels the waste heat into the atmosphere using evaporation. Finally, cool water returns to the chiller to cool more refrigerant.

Cooling towers not only dispose of waste heat, but also can store water as a sort of battery. When electricity is cheap and clean, operators can have chillers running to cool the campus and store cold water in a storage tank. Then, when the prices increase (which is also when the grid typically pulls older, dirtier energy), controllers can turn off chillers and dispatch the cold water from the tank to cool buildings. 

Existing chilled water district cooling systems can be improved through innovative energy efficiency upgrades. For example, Variable Speed Drive (VSD) Compressors can automatically adjust their operating speed in order to match the cooling demand. This means that the compressor can use less energy at mild temperatures and increase speed with temperature. Additionally, flow rate, water temperature, and overall design optimization can yield significant improvements in overall efficiency. Reducing the temperature of chilled water can reduce the work required by the system. Operating multiple cooling towers can improve the heat transfer efficiency by allowing for more surface area and allowing the condenser water to cool more easily. 

Benefits of Chilled Water District Cooling

• Current equipment can often be upgraded or retrofitted, yielding improvements in energy efficiency. 
• Low cost optimization and control strategies can yield significant results.

Challenges

• Systems are complex, with several interdependent subsystems. Changes to one variable can impact others.
• Control issues, simultaneous heating and cooling, and other burdensome loads on cooling systems must be addressed first. 
• Poorly designed systems may require more extensive overhaul.