Published December 15, 2025
By What Is a Thermal Energy Network and Why It Is Gaining Momentum in US Cities
The start of construction on a geothermal thermal energy network serving New Haven’s Union Station marks an important moment in the evolution of building decarbonization strategies in the United States. Rather than relying on individual building retrofits or stand‑alone heat pump installations, the project applies a shared underground thermal system to support heating and cooling across a major public facility and surrounding buildings. As cities face mounting pressure to reduce emissions from buildings, which account for roughly one third of US energy related carbon dioxide output, this project provides a timely entry point for explaining how thermal energy networks work and why they are attracting growing attention from policymakers, utilities, and climate planners.
A thermal energy network is a system that uses the stable temperature of the ground to provide heating and cooling through a shared infrastructure. Underground pipes circulate water through boreholes drilled deep into the earth, where temperatures remain relatively constant throughout the year. Buildings connected to the network use electric heat pumps to exchange heat with this loop. In winter, heat is drawn from the ground and upgraded for space heating. In summer, excess heat from buildings is transferred back into the ground. Unlike traditional district heating systems that distribute steam or hot water, thermal energy networks operate at near ambient temperatures, which reduces heat loss and improves efficiency.
The operation of a thermal energy network depends on several core components working together.
Key physical and operational elements typically include:
Vertical boreholes are typically drilled hundreds to more than one thousand feet underground, depending on geology and system size. These boreholes contain closed loop piping filled with water or a water antifreeze mixture. The fluid circulates continuously, carrying thermal energy between the ground and connected buildings. Each building uses its own heat pump to meet interior heating or cooling demand. Because different buildings often have different thermal needs at the same time, waste heat from cooling dominant buildings can be reused by heating dominant buildings, improving overall system performance. In the case of Union Station, early test drilling confirmed that local subsurface conditions could support a large scale system capable of serving a busy transportation hub with high year round energy demand.
Several converging trends are driving renewed interest in thermal energy networks. Space heating remains one of the hardest sectors to decarbonize, particularly in colder regions where natural gas has long dominated. At the same time, many states and cities have adopted climate targets that require deep emissions reductions from buildings by 2030 and 2050. Federal incentives introduced under the Inflation Reduction Act and complementary EPA climate grant programs have improved the financial viability of shared geothermal infrastructure. Utilities are also beginning to view thermal networks as a potential alternative to maintaining and expanding aging gas distribution systems. The Union Station project reflects this shift, showing how public funding and policy support can help move geothermal networks from pilot concepts to active construction.
| System type | Primary energy source | Cold climate performance | Infrastructure model | Emissions profile |
|---|---|---|---|---|
| Thermal energy networks | Electricity with ground sourced heat | High and stable due to constant ground temperatures | Shared underground thermal loop with building level heat pumps | Very low when paired with clean electricity |
| Air source heat pumps | Electricity and ambient air | Moderate to high with efficiency declines in extreme cold | Individual building systems | Low to moderate depending on grid mix |
| Natural gas heating | Fossil gas combustion | High but fuel dependent | Distributed gas pipeline network | High direct carbon emissions |
| District steam systems | Centralized fossil or mixed fuel sources | High but with significant distribution losses | Central plant with hot water or steam pipes | Moderate to high depending on fuel mix |
Despite their potential, thermal energy networks face several challenges that will influence how quickly they scale. Upfront capital costs for drilling and underground installation remain high compared with conventional systems, even though long term operating costs are typically lower. Construction can be disruptive in dense urban environments, requiring careful coordination with transportation and public works agencies. Regulatory frameworks for utility ownership, rate design, and customer participation are still evolving. Projects anchored by large public buildings, universities, or transit facilities can help address some of these barriers by providing stable initial demand. The New Haven Union Station project illustrates how such anchor sites can serve as catalysts for broader neighborhood scale deployment.
Thermal energy networks are moving from niche applications toward a more prominent role in urban decarbonization strategies. The geothermal system now underway in New Haven demonstrates how shared ground source infrastructure can deliver efficient, electric heating and cooling at a scale suited to complex city environments. While technical and regulatory hurdles remain, growing policy support and real world project experience are strengthening the case for wider adoption. As cities seek practical pathways to reduce building emissions without compromising reliability, thermal energy networks are likely to become an increasingly important part of the clean energy transition.
