District energy systems are highly efficient solutions to heating and cooling multiple buildings from a central plant. They include networks of steam or hot and chilled water pipes, typically buried underground, that distribute energy to efficiently heat and cool buildings, using less energy than buildings with individual boilers and chillers. They are scalable from a small cluster of buildings to municipal wide solutions. 

In North America, district energy systems are operating in high development density sites, such as college campuses, downtown districts, medical centers, airports, and military bases.  

Some of these systems have been operational for 100 or more years, and, in these cases, are most often distributing steam. There is growing interest in the conversion of district heating networks from steam to hot water distribution — the motivation for which comes from the potential for higher system efficiency and integration of lower temperature heating sources. 

Global, national, and local legislative and regulatory attention to energy efficiency and climate goals, including ambitious goals to transform energy systems toward 100% renewable energy and net-zero carbon emissions, is also becoming a driving force behind investment in district energy systems. In North America, the International District Energy Association (IDEA) tracks investment in district energy systems by the number of buildings and square footage newly connected to district energy systems annually. 

Advantages and challenges of district energy deployment 

District energy systems have been widely recognized as an effective tool for combating global climate change. Cities account for more than 70% of global energy use and 40%-50% of worldwide greenhouse gas emissions. Global advocacy for district energy investment is sufficiently strong to have the backing of the United Nations Environment Programme (UNEP) through its partnership with the Global District Energy in Cities Initiative. Modern district energy systems are unique in their ability to efficiently reduce emissions and primary energy demand on a large scale.

One of the key advantages of district energy is the ability to draw from multiple energy sources. This is often referred to as “fuel flexibility.” Waste heat from industrial processes and renewable energy, such as geothermal, passive solar, biomass, and municipal solid waste, can supplement or completely displace fossil fuels in low-temperature district heating systems.

District energy systems are front-loaded, capital-intensive utilities requiring large central plants and underground piping networks. Return on investment is long and can be further compounded in an environment of relatively low energy costs.

Additional challenges can include:

• Distribution barriers: Roads, rivers, and rights of way can pose challenges to or prevent the installation of buried pipe networks. In dense urban areas that otherwise favor district energy systems, crowded utility infrastructure that is already in the ground leads to extremely high cost of excavation;
   
• Regulatory issues: For a district energy system to work, it must be legal to sell energy to participating parcels in the district; and
   
• Adequate and secure energy demand: Multiple buildings in a close geographic area are needed to achieve the economies of scale in cost and energy efficiency. The long-term participation of these “anchor loads” must be secure.

For private developers, who do not always calculate return on investment in long time periods, and municipalities, where bond funds and/or higher taxes are required to pursue investment, funding the initial investment in district heating and cooling technologies can be difficult. While there is recognition that district energy systems can help meet evolving emissions and efficiency regulations to combat climate change, initial cost remains a sizeable barrier. 

In North America, college and university campuses represent strong current opportunities to capitalize on the value of district energy investment. These campuses tend to have dense building development within a defined border and are often already served by aging stand-alone utilities. Generally, they have greater freedom to make capital investment and operational decisions that can focus on the longer term. Many colleges also have assumed thought leadership roles in the transition to green and net-zero economies. In those cases, there are additional and less tangible reasons for institutional change in their approach to heating and cooling buildings on campus. These interests often include consideration of low temperature district hot water heating networks as well as district cooling systems. 

Energy transfer to the buildings 

District energy systems require a means of transferring energy to the connected buildings. The preferred solution is an indirect connection with a heat exchanger providing energy-efficient heat transfer between fluids. A district heating network has specific operational criteria that must be achieved in order to deliver reliable and efficiently generated energy to connected buildings. One example is achieving a large temperature differential, or Delta T (∆T), across the district’s distribution pipe network. Generally, the goal is to both control and maximize the ∆T across the district’s supply and return pipe network. Achievable ∆T is impacted by how the connected buildings interact with the district energy network. 

Indirect connections ensure that the primary side (district side) and secondary side (building side) remain safely separated. In this way, the building heating or cooling network is decoupled from pressure or temperature variations in the district energy network. Additionally, district energy distribution pipe networks are usually found within a few feet of the surrounding ground level, whereas multistory buildings will have pipe networks well above ground level. This elevation differential can lead to differing pressure requirements. Indirectly connected buildings are also isolated from the danger of district energy pipe breaks or leaks, which could cause considerable property damage.

Indirect building connection by Energy Transfer Stations

Purposefully engineered Energy Transfer Stations (ETS) offer the advantages of indirect network connection while enabling refined energy transfer control. Refined control of energy transfer protects the district operating characteristics to maximize system efficiency while ensuring occupant comfort within the connected building. 

Historically, the North American market for ETS has been too small to support repetitive serial production of equipment in a professionally managed factory environment. The same was true in overseas markets until sufficient scale developed to attract investment. In the European experience, this tipping point came near 30 years ago. Accordingly, indirect building connection via pre-packaged ETS is the norm in Europe and many other global markets, while alternate methods persist alongside pre-packaged ETSs in North America. 

Standards of construction differ for both pressurized pipe networks and electrical safety standards worldwide and these differences impact ETSs. These differences are written into local and national building codes, creating barriers to the import and rapid adoption of pre-engineered and serial produced ETSs in North America.

In the absence of commercially available and serial produced ETSs meeting North America standards required by local and national building codes, district energy utilities, or operators had no choice but to design and produce unique ETSs one at a time. These are often referred to as “stick built” solutions. They can lead to very high and often hidden costs. Furthermore, there is an additional and often overlooked task of addressing the controls of the ETS to ensure both district side performance and building occupant comfort. Too often, these stick-built solutions crafted one at a time lead to high hidden costs as well as long-term performance inadequacy.

These stick-built ETSs increase the cost and risk to owners and developers in transition to energy-efficient, cost-effective district energy. 

Challenges include: 

• Increased engineering cost to prescribe solution;
• Lack of a proven and repeatable controls package;
• Controls with sequences of operation left to a controls contractor, unique and unproven;
• Failure to meet performance expectations; 
• Inexact onsite construction; 
• Time intensive, potential to introduce construction schedule delays; 
• Not reproducible / poorly documented; and
• Difficult to service and maintain.  

New pre-engineered and factory-produced ETS solutions that address the building code requirements in North America are making it easier to address the challenge of indirect building connections within North American district energy networks. Danfoss has adapted its controls solutions to pipe networks meeting North American standards in order to offer the exceptional packaged equipment solutions prominent in global markets to North American customers as well.

These solutions are repeatable, traceable, and configurable. They can be built specifically to ensure easy transport into a building and include factory-fitted and configured controls with sequences of operation that have been proven through decades of experience. These solutions are pressure and leak-tested in the factory environment. Additionally, functional tests ensure correct sequences of operation and controls wiring integrity prior to delivery. The result is a complete energy transfer solution delivered pre-packaged and ready to install and operate.  

Key benefits of pre-engineered, factory-built ETS include: 

1. Reduced engineering cost and risk — Rather than hiring a consultant to design a unique system, a pre-engineered, factory-built ETS can be sized to meet the building’s load and other criteria while benefiting from standardized and proven controls solutions.
    
2. Improved production quality — Fabrication quality is better ensured in a professionally managed factory environment. Pressure and leak detection are carried out in a controlled environment following a prescribed process. Building equipment on-site introduces issues of space, lighting, and welding access that can compromise high quality work. The opportunity to pressure and leak test offsite is lost. 
    
3. Less time onsite — Producing an ETS on-site requires weeks or months of work inside the building and includes services from pipe fitters, electricians, and controls contractors. Producing the unit off-site enables scheduling delivery to coincide with construction project schedules. Construction managers appreciate the ability to minimize activity on already crowded job sites. Installation time is dramatically reduced to as little as one day. Activity on-site is limited to a single electric power connection plus connection to building and district pipe networks. 
    
4. Documented, reproducible, and maintainable installations — Factory-built ETSs are supported by archived engineering design and fabrication documentation, including complete bill of materials that facilitate long-term maintenance needs. “As built” drawings and documentation are unlikely to accompany the completed work of contractors attempting fabrication of a solution onsite. This can lead to long-term maintenance headaches.
    
5. “Plug-and-play” operability — Factory-fitted, configured, and tested controls are integral on factory-built ETSs. They can be configured for the operating conditions at the installation site prior to delivery. As a result, the ETS can be ready to heat or cool its served building upon first start. When a solution is “stick-built” on-site, the controls contractor’s work is just beginning after the pipe fabrication is complete.

Case study: Packaged ETS minimized building downtime, simplified startup 

At Sheridan College in Oakville, Ontario, Canada, for example, updating and converting the building heating network from steam to hot water included the installation of a new energy center and pre-insulated pipe to form a campus-wide district energy network. 

The work proceeded over multiple years. Before the new network could enter service, however, energy transfer to each building needed to be addressed. This would not be trivial, and it also required demolition of some of the existing building heating equipment. There was going to be a period of time when buildings would be without a heating source. Many of the new ETSs would be placed where existing equipment scheduled for demolition was located. In several instances, the problem was compounded by very confined mechanical rooms and/or torturous access routes.

By choosing packaged ETSs from Danfoss with factory-fitted and configured controls, the period when buildings would be without heat would be minimized and multiple buildings could make the transition to the new low-temperature hot water district energy system in quick succession. Careful site survey work and the ability of Danfoss to customize the pipe network on each ETS to facilitate connection to the unique conditions in each mechanical room would minimize delay in connection of the new equipment. It would also enable quick movement of complete energy transfer stations into the tight spaces.

The campus also had little or no space to store these new ETSs. By having the equipment fabricated and tested off-site and held at the factory, delivery could be carefully orchestrated with the small contracting teams on-site. They were able to plan delivery dates, remove equipment from delivery trucks, and quickly move them through the pre-planned access routes and make connections for multiple buildings in quick succession — no construction staging area or yard space was needed.

Confidence in these solutions allowed the Sheridan College team to complete the work in late October and into November of 2019 with the heating season imminent.

After pipe connections and a single electric power connection was made at each unit, the energy transfer stations began heating the buildings in controlled fashion immediately upon startup. Little or no commissioning effort was needed on-site, demonstrating the value of prefabricated ETSs with factory-fitted and configured controls.