Thermal energy network

The core of a TEN is the Ambient Temperature piping loop that conveys thermal fluid between the various thermal sources and sinks. The specific term "Ambient Temperature Loop (ATL)" can also refer to the specific 1-pipe pipe configuration being adopted by some universities, cities, and utilities. One such example is the 1-pipe geothermal network that heats and cools Colorado Mesa University (CMU). Another is the 1st utility installed TEN in Framingham, MA. == Terminology ==

The terminology used for thermal energy networks varies by jurisdiction and source. In engineering literature, similar systems are described as district energy systems, geothermal district energy systems, ambient-temperature loop districts, and 5th Generation district heating and cooling systems. In U.S. media, policy, and utility contexts, such systems have also been described as networked geothermal. == Design and operation ==

Thermal energy networks are designed to serve the goals of reliability, resilience, efficiency/cost-effectiveness, and up front cost reduction. All available thermal loads and sources/sinks are identified, then the most cost-effective sources and sinks are chosen. One typical thermal source/sink is a ground heat exchanger, a.k.a "geothermal". This is because the ground is a huge thermal mass which can be used to as a thermal battery to transfer energy from one season to another. Other sources/sinks might be area industry, flood control ponds, solar thermal, snow melt loops, and both indoor and ground thermal batteries. Thermal energy networks consist of a shared buried pipe loop carrying water or a water-antifreeze solution, a set of geothermal boreholes or other thermal source and link exchangers, pumps and controls, and building-level water-source heat pumps. In many designs, the network loop operates near ground temperature rather than at the higher temperatures used in earlier district heating systems. Buildings needing heat extract it from the loop through heat pumps, while buildings needing cooling reject heat back into the loop. Because heating and cooling loads may occur at different times across a district, some systems are designed to exchange heat between buildings and to use the ground for thermal energy storage or seasonal thermal energy storage. == Ambient Thermal Loop Developments ==

District-scale geothermal heating and cooling systems predate the recent use of the term thermal energy network, but the concept received increased attention in the 2020s as part of building decarbonization policy in the United States. In Massachusetts, Eversource Energy developed a pilot 5th generation geothermal thermal energy network in Framingham, Massachusetts. A 2025 PBS NewsHour report described it as a one-mile system connecting about three dozen homes and municipal buildings to a shared geothermal bore field. == Policy and regulation ==

In the United States, geothermal networks have increasingly been discussed in relation to utility regulation, labor transition, and building electrification. In 2022, New York enacted the Utility Thermal Energy Network and Jobs Act, establishing a legal and regulatory framework for utility thermal energy networks. The legislation described thermal energy networks as ambient-temperature water loops connecting multiple buildings and energy sources, with building owners connecting through water-source heat pumps. Maryland enacted legislation in 2024 authorizing thermal energy network systems. Massachusetts issued state guidance for networked geothermal projects in 2024. == Applications ==

Thermal energy networks have been proposed or deployed in university campuses, mixed-use developments, housing complexes, and neighborhood-scale utility pilots. According to PBS NewsHour, the Framingham pilot served detached houses as well as a school administration building, a fire station, and a public housing development. These systems are used mainly for space heating, air conditioning, domestic hot water, and load balancing between buildings with different thermal demands. == 6th Generation thermal systems ==

The emerging 6th Generation heating and cooling TEN field advances the field with power grid active DER interaction to apply the thermal energy storage elements of the system to both absorb and avoid energy in sync with grid power costs. A key to these systems is the use of "thermal batteries" smaller than the ground thermal battery that is the geothermal ground heat exchanger. A GHEX is a very effective annual cycle thermal storage mechanism. However, it is slow to charge and slow to drain. Grid interactivity requires faster thermal charge and discharge cycling, and thus thermal batteries designed for shorter thermal cycling. The work of ORNL's Thermal Energy Storage Research Group is central to 6th generation TEN systems. == Limitations and considerations ==

The performance and economics of geothermal networks depend on local geology, drilling conditions, load diversity, building retrofit requirements, and financing structure. It is important that careful design of TEN systems is undertaken to avoid over cost or operational issues. == See also ==