High-pressure steam and chilled water are essential for campus building air temperature regulation. Their applications also include water heating, food warming and aiding in more complex research loads such as cooling equipment in laser research.
Heating Process
In our boilers, steam is produced at very high pressure with more heat content, very much like a residential pressure cooker. Under the pressure of ongoing steam generation, steam fills the void in the miles of underground supply mains leading to buildings across campus without the need of being pumped. In these boilers, a delicate mixture of air and fuel is needed to achieve the optimum flame combustion for heating up water.
As steam releases its heat to the buildings, it condenses back into water. This condensate is collected and pumped back, sometimes with the help of gravity, to the boiler plant.
Because some steam is naturally lost and no large system can be perfectly efficient, water is routinely treated and added to the boiler water to make up for losses.
Boiler
Dual fuel boiler using clean natural gas & diesel fuel oil
Inside Boiler
Clean natural gas burning inside a boiler
Cooling Process
The cooling principles behind UNL’s large‑scale district energy system are fundamentally similar to how a household air conditioner works—just on a vastly greater scale. In a home, an air conditioner uses a refrigerant to absorb heat from indoor air. As the refrigerant absorbs this heat, it changes from a liquid to a gas and moves to the outdoor condenser unit, where the heat is expelled. The refrigerant is then compressed and condensed back into a liquid, allowing the cycle to repeat.
UNL’s campus district energy system operates on the same basic concept, but instead of cooling individual rooms, it provides comfort cooling and equipment cooling for the entire campus. Rather than circulating refrigerant through every building, the system uses chilled water as the primary medium for absorbing heat. The utility plants supply this water at 42°F through an extensive underground distribution network. As the chilled water flows through air‑handling units in campus buildings, it absorbs heat and returns to the utility plants at approximately 54–56°F.
At the utility plants, this warmed water is routed through large centrifugal chillers—massive machines that function similarly to household air conditioners but with far greater capacity and efficiency. In the evaporator section of each chiller, the warm return water passes through thousands of copper tubes surrounded by a refrigerant. As the refrigerant absorbs heat, it boils, cooling the water back down to 42°F before it is pumped back out to campus. The refrigerant, now carrying the absorbed heat, moves to the condenser section, where the heat is rejected into the cooling tower system.
This cycle operates continuously—24 hours a day, 365 days a year—to ensure a reliable and stable source of cooling for the entire university.
For perspective, while a typical household air conditioner is rated at roughly 2 tons of cooling capacity, UNL’s utility plant chillers range from 1,800 to 5,000 tons each—a dramatic illustration of the scale required to meet campus‑wide demand.
Cooling Towers
The heat that is absorbed by the refrigerant is then passed on to another water loop that goes to the cooling tower outside of the utility plant. The cooling towers are tall open-air structures with giant suction fans at the top. The hot water travels to the top of the tower and is then sprayed downwards into a multiple layers of honeycomb mesh that helps break apart the water droplets. The suction fans then draws in cool air from the bottom of the tower to cool down the water and the heat is then expelled from the top of the tower.
