What is District Energy?
District heating systems produce hot water, steam, or chilled water at a central plant and then distribute the energy through underground pipes to buildings connected to the system. Individual buildings connected to district heating no longer need boilers or to maintain redundancy for heating needs. Customers use the thermal energy provided by Duluth Energy Systems to meet their space heating, water heating, and heat processing needs.
How does the steam system work?
The City’s district heating system is 85 years old, when it was designed as a once-through steam system. Energy in the steam system follows these steps:
- 90 million gallons per year of 40℉ water is heated to 360℉ to create high-pressure steam.
- The steam is distributed to over 160 buildings downtown through a series of underground pipes, most of which are also 85 years old.
- In many buildings, the same steam produced by the plant is circulated through the building to heat office spaces and living areas.
- As these buildings use the steam, it cools and condenses back into water at a temperature of approximately 180℉.
- The condensate is then discarded into either the storm sewer or sanitary sewer.
- Although once state of the art, engineering advancements have been made and today’s systems have transitioned to the use of hot water as the heating medium, replacing steam.
How will the hot water system work?
Much of Canal Park is already on a hot water distribution system and serves as a local example of how the hot water system can service a wide mix of buildings.
- Inside the main Duluth Energy Systems plant, boilers produce steam by combusting fuels (predominantly gas or coal, with future potential for biomass/biofuel).
- Hot water is created by transferring the energy in the steam to the hot water closed loop system in an energy transfer station located in the plant.
- This hot water is then distributed to buildings through a system of underground pipes.
- Once the water reaches the building, it transfers heat to the buildings separate hot water system through an energy transfer station located in their building.
- Hot water from the buildings energy transfer station is then used to heat domestic hot water and internal building spaces.
- The water from the plant that feeds the customers energy transfer station in then returned back to the plant to replenish the heat that was transferred to the building.
- This system operates on a continuous basis.
How will this make Duluth more efficient?
The new closed-loop hot water system will operate at a much lower temperature than the existing steam system. The lower the temperature of the heating medium, the lower the heat losses. This is due to the temperature difference between the heating medium and the surrounding air temperature and the rate at which the heating medium cools. The greater the difference, the faster the heating medium will give up heat, which is energy.
In a closed loop hot water system, the hot water from Duluth Energy Systems heats water in your building in an energy transfer station. The hot water that will flow through offices or living spaces in your building is separate from the hot water in the DES hot water loop. Any heat that is not absorbed by your buildings hot water energy transfer system to heat your building is returned to the DES plant to be reheated and pumped back into the hot water loop. The recycling of this water results in water savings. In an open loop steam system, no water or steam is returned to the plant, it is all wasted to the storm or sanitary sewer.
The water that is returned to the plant in a closed loop hot water system is significantly warmer than the make-up water that comes from Lake Superior. This also increases the efficiency of the system as it takes far less energy to bring this water back up to operating temperature than it does to bring the ice-cold water of Lake Superior to operating temperature.