Modernizing Duluth Energy Systems
Superior Street Project

Closing the Loop – Steam to Hot Water Transition

The City of Duluth is leveraging the major investment of Superior Street reconstruction to modernize its energy system, upgrading from steam to hot water distribution. Updating this energy infrastructure provides Duluth the pathway to a modern, resilient system. This transformation will reduce the energy demands of the system and in customer buildings, also creating opportunities to reduce fossil fuel usage, reduce carbon, and integrate renewable energy.

How will this make Duluth more efficient?

The new closed-loop hot water system will greatly reduce the temperature of the water used to heat buildings along Superior Street. Additionally, energy in a loop can return to the plant in a cycle to get reheated and sent back out to customers. This saves energy and water!

Why are we converting the system?

Timing this project with the Superior Street upgrades will save taxpayers and ratepayers millions of dollars, while gaining significant energy system efficiencies. Additionally, upgrading the system from steam to hot water helps the entire system save energy (and money) in operations and fuel costs, because hot water is a more efficient way to deliver energy. Providing reliable, cost-competitive service is essential to Duluth Energy Systems, and this project will improve operations through reduced fuel input, carbon emissions, chemical treatment, and water use.

What does this mean for customers?

  • Customers will receive reliable and cost-effective energy with the same exceptional customer service they expect from Duluth Energy Systems.
  • We are committed to keeping costs stable as we help you save energy and run your building systems more efficiently.

Customer Benefits

  • Upgrade to building equipment improves building efficiency and reliability.
  • Upgrade to distribution infrastructure reduces system maintenance and associated costs.
  • Simplifies building operations, eliminating steam trap and steam reducing station maintenance and replacement.
  • Automatic building control system improves building efficiency by eliminating the manual interface and provides real-time energy monitoring.
  • Increase in energy capacity to grow the system, which increases rate stability and financial predictability.

Environmental Benefits

  • Increases efficiency = less energy, less water
  • Reduces greenhouse gas emissions
  • Positions the system to integrate renewable energy, such as biomass (wood chips or renewable fuel oil sourced from biomass or agricultural crops)
  • Conserves 20 million gallons of Lake Superior water every year
  • The efficiency of a hot water system makes the integration of additional fuels a more cost-effective option. This past year, a natural gas pilot already proved incredibly successful, saving money on fuel and operations while reducing carbon emissions.

What to expect in the coming months?

  • The Duluth Energy Systems team is moving forward with design of the first phase of the project, which will include buildings between 3rd Avenue West and 7th Avenue West.
  • The distribution and building conversion work for phase one is set to begin in March 2018.
  • The Duluth Energy Systems team will begin meeting with phase one customers in September 2017 to discuss the technical and financial aspects of building conversions.
  • Outreach to phase two customers is slated to begin in late 2017 and continue through early 2018. Phase two includes buildings between Lake Avenue and 4th Avenue East) and third phase (between 3rd Avenue West and Lake Avenue). Please note that conversion for those customers is not slated until 2019-20.
  • Phase three buildings include those between 3rd Avenue West and Lake Avenue.

How does a hot water system work?

Much of Canal Park is already on a hot water distribution system, it 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 are a series of boilers. Hot water is produced by combusting fuels in these boilers (predominantly gas or coal, with future potential for biomass/biofuel).
  • The water is distributed to buildings through a system of underground pipes. This piping loop supplies hot water to buildings and brings cooler water back to the plant to reheat.
  • Once the water reaches the building, it runs through the building heating system to heat the internal spaces.
  • To heat building spaces, hot water is circulated through the building pipes and out to radiators or fan coils that can warm air in rooms and hallways. Additional radiators and coils in air handling units will be installed as part of the project to accommodate the lower water supply temperatures.
  • The thermal energy (via hot water from our system) can be used to heat water for showers, washing hands, or cooking. This is called domestic hot water usage. A heat exchanger will be used to separate the district hot water system from the domestic hot water system.

More information can be found here.

What is more efficient about hot water systems?

Closed-loop hot water systems can 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.

How did 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 treated 40℉ water is heated to 360℉ to create high-pressure steam.
  • The steam is distributed to over 160 buildings downtown.
  • Once these buildings use the steam, it is cooled and condensed to 180℉. The cooled steam condenses to hot water at approximately 180℉ and is then discarded into WLSSD’s wastewater treatment system.
Duluth Energy Systems is operated and managed by Ever-Green Energy.