Large campuses and district energy systems are evolving from isolated mechanical plants into interconnected thermal ecosystems. These systems now integrate borehole thermal energy storage (BTES), ground heat exchangers, solar thermal arrays, boilers, chillers, heat pumps, domestic hot water (DHW) and process loads into a unified framework.

The central challenge is no longer simply producing heating or cooling, but coordinating how thermal energy is generated, transferred, stored and reused across multiple buildings and subsystems to achieve optimized control for energy efficiency, operating cost reduction, occupant comfort and overall system resiliency.

The goal of this three-part series is to establish the foundation for simplified enterprise control of thermal energy networks. Part 1 defines thermal domains and their relevant characteristics. An optimized system treats each domain as an independent entity optimized for its own operation, while a supervisory layer coordinates overall system performance, which will be addressed in subsequent articles.

What is a thermal domain?

A thermal domain is an operationally bounded subsystem that includes thermal assets, interfaces and control elements participating in energy exchange. Domains are hydraulically separated yet thermally connected, enabling controlled interaction without sacrificing stability or independence. The defining attributes of a thermal domain are summarized in Table 1.

The attributes summarized in Table 1 are not merely descriptive; they define the functional boundaries and interaction rules that govern how thermal energy systems behave at scale. When clearly defined and enforced, each domain can operate within a stable, predictable envelope while still participating in coordinated energy exchange with other domains.

This distinction is critical. Traditional HVAC design often blends systems into a single network, creating unintended interactions. A domain-based approach formalizes boundaries, allowing each subsystem to operate optimally while still participating in a larger energy network.

Supervisory control of thermal domains can be understood by analogy to client-server architecture in the IT world, where system complexity is managed through standardized interfaces and structured communication. In a client-server model, each client and server operates independently with its own software and hardware with local optimization and control logic, yet communicates through a common application programming interface that enables seamless interaction across the enterprise. 

Similarly, in a domain-based thermal system, each domain is locally controlled to maintain performance, safety and efficiency, while a supervisory layer provides a consistent interface for coordination between domains. This approach reduces system complexity by avoiding tightly coupled, custom integrations and instead relying on defined communication and control boundaries. As a result, individual domains can be optimized for their specific function, while enterprise-level visibility, coordination and performance optimization remain fully accessible.

Why thermal domains matter

As campuses grow in size and complexity, diversity of loads and energy sources increases and the complexity of enterprise control systems increases exponentially with the number of network nodes. However, by implementing a supervisory control architecture layered over optimized local domain control, these challenges can be systematically avoided, as summarized in Table 2.

Thermal domains address this by separating complexity into manageable units.

A thermal domain is not simply a piece of equipment. It is a controllable unit of thermal function defined by how it produces, stores, converts, transports or consumes thermal energy within the larger system.

For example, a ground or BTES domain consists of borefields, vertical wells and circulation pumps interacting with the subsurface. Its primary role is not instantaneous heating or cooling, but long-duration storage and seasonal balancing. In summer, excess heat from cooling-dominated buildings can be injected into the ground. 

Months later that same stored energy can be extracted to support winter heating loads at higher efficiencies. The domain behaves as a thermal battery, with performance influenced by long-term energy balance, temperature drift and subsurface conditions rather than short-term control signals.

A hydronic distribution domain serves a completely different function. It is the transportation network of the system, typically composed of hot water, chilled water or ambient temperature loops that move energy between buildings. 

For instance, a campus ambient loop may allow a cooling-dominant office building to reject heat into the loop while a nearby residential building extracts that same heat for space heating. The distribution domain does not create or store energy, but enables shared energy exchange across otherwise independent buildings.

A heat pump domain acts as the translator between temperature levels. Water-to-water or water-to-air heat pumps take low-grade thermal energy and upgrade it to useful temperatures. 

For example, a heat pump may extract 60 F water from an ambient loop and deliver 110 F water for radiant heating, or reject heat from a chilled water system into a warmer loop. This domain is governed by efficiency metrics such as coefficient of performance (COP) and operates best when temperature lifts are minimized through proper domain integration.

A solar thermal domain introduces a renewable and time-dependent energy source. A solar thermal domain brings in a renewable high temperature energy source that changes throughout the day and by season. 

During peak solar conditions, this energy might directly serve DHW or space heating loads. At other times, it may be diverted into a storage domain such as BTES for direct heating use or to increase the efficiency of geothermal heat pumps. The variability of solar input makes this domain highly dependent on coordination with storage and distribution systems.

A boiler or chiller domain provides reliability and peak capacity. These domains provide reliability and cover peak loads. Boilers supply high-temperature heat where lower-temperature systems are not sufficient, such as domestic hot water or legacy heating. Chillers provide additional cooling when ambient or geothermal sources fall short. 

During extreme winter conditions, a boiler can supplement heating as ground temperatures decline or demand rises. During peak summer conditions, a chiller helps maintain required cooling capacity or reduces the size of ground heat exchangers in cooling dominated climates. In practice, these systems are used selectively to support overall operation rather than carry the full load.

A DHW domain operates under strict sanitation and regulatory requirements. It includes storage tanks and heat exchangers designed to maintain temperatures sufficient to prevent Legionella growth. 

Unlike other domains, DHW systems cannot freely fluctuate in temperature or be directly integrated into shared loops. However, they can still participate in energy exchange through indirect interfaces, such as preheating incoming water using recovered heat from other domains or capturing solar thermal heat directly for DHW applications.

Finally, process load domains represent some of the most valuable and often overlooked opportunities for energy reuse. These include systems such as pools, ice rinks, data centers and industrial processes. A data center, for example, continuously rejects heat that can be captured and used elsewhere. 

An ice rink requires constant cooling but simultaneously produces reject heat that can be used to heat adjacent spaces or water systems. In Colorado, for example, the size of the ground heat exchanger is being reduced for the heating dominated Vail Public Library by using heat rejected by the nearby Dobson Ice Arena, which is currently vented to the atmosphere with a cooling tower (https://bit.ly/4dfWI6p).

Using a heat pump (reversing chiller) to chill an ice arena while rejecting recovered heat to radiant-heated floors in the spectator stands provides the highest comfort and system efficiency by effectively doubling the COP when energy is transferred between aligned sources and sinks. Exchanging thermal energy within the same sports complex substantially reduces thermal energy costs and complexity associated with campus wide thermal distribution systems.

This is a clear example of symbiotic process-load thermal domains, where one domain’s waste heat becomes another domain’s useful energy source. Because these domains can function as either thermal sources or sinks depending on operating conditions, they become dynamic participants in a thermal network, continuously shifting roles to maximize overall system performance.

Across these examples, each domain is hydraulically independent but exchanges energy through defined interfaces such as heat exchangers, buffer tanks or controlled loop connections. This separation prevents pressure, flow and control behavior in one part of the system from affecting another.

At the same time, energy can still move between domains, allowing the system to operate as a coordinated network. Maintaining this balance is key to both stability and scalability.

It also allows campus and district systems to expand, adapt and incorporate new technologies without disrupting performance, while supporting shared energy strategies that improve overall efficiency.

Hydraulic separation and thermal coupling

A key feature of domain-based systems is the use of hydraulic separation combined with controlled thermal coupling. This approach allows energy to be transferred between parts of the system without introducing instability from direct hydraulic connection.

Hydraulic separation allows each part of the system to operate with its own pressure, flow and control strategy. In practice, a ground heat exchanger field, a hydronic distribution loop and a heat pump plant can all run at different flow rates, pressures and temperature differentials without affecting one another.

For example, a borefield typically requires steady flow and long circulation cycles, while a building loop sees frequent changes as zones turn on and off. By separating these with heat exchangers or buffer tanks, each can be tuned for its own performance. Using advanced control technologies promoted by Oak Ridge National Laboratories and ASHRAE, optimized flow in the ground loop is controlled by differential temperature, while the distribution side operates at a different flow rate based on differential pressure.

Thermal coupling allows energy to move between hydraulically separate parts of the system. This is typically done with plate-and-frame heat exchangers, shared ambient loops or controlled mixing. 

For example, heat rejected from a cooling-dominated building can be transferred into an ambient loop through a heat exchanger and then used by a heating-dominated building. The Vail Library project follows this approach. The two systems may not share fluid, but they share energy in a controlled and predictable symbiotic manner. 

Local control sits at the equipment and domain level and is separate from any enterprise or supervisory system. Each domain has its own controller that handles real-time operation, safety limits and optimized appliance efficiency.

For example, a heat pump controller manages compressor staging, source and load flow, humidity control and optimal entering water temperature for the lowest lift and highest efficiency, comfort or operating costs. A DHW controller maintains minimum storage temperatures for sanitation and cycles equipment to maintain setpoints. These actions happen continuously and do not rely on higher-level coordination.

Supervisory control can adjust targets or coordinate between domains, but local controllers are responsible for keeping equipment stable and within safe operating limits.

Hydraulic separation, thermal coupling and local control change how the system behaves compared to a traditional central plant. Instead of one large system where disturbances carry through everything, the campus operates as a set of coordinated subsystems. Each part contributes to overall performance but runs on its own terms. 

This approach improves stability and reliability, provides a higher level of system resiliency and makes expansion easier. New systems can be added or modified without disrupting existing systems.

Albert Wallace, is president of Energy Environmental Corporation and executive director of ForgeGEO, Inc., a not-for-profit organization aimed at expanding geothermal infrastructure in the United States. He is a Certified GeoExchange Designer (CGD), Certified Energy Manager (CEM), and IGSHPA/CSA-certified geothermal system designer, instructor and verification inspector. With over 25 years of experience in geothermal and hydronic system design, he has developed supervisory control strategies for advanced thermal network architectures for campuses, districts and large-scale facilities.