This Is Part 2/2 of Building a Sustainable Future with Thermal Energy Networks by Marc Miller, Egg Geo, LLC

Modern energy demands are growing, and the need for sustainable, efficient solutions to heat and cool our homes and businesses has never been greater. Enter Thermal Energy Networks (TENs)—innovative systems that distribute and reuse energy to reduce costs, cut emissions, and optimize performance.

 

This article continues to explores the implementation of TENs, their benefits, and how one would connect their home or building to a TEN, helping you understand their potential to transform energy systems for the better.

 TENs Connected to Buildings

Regardless of whether the stakeholder is a home, a light commercial building, or an industrial building, the fundamental connection to the ATL remains the same: the decoupled secondary loop. This loop features a circulator pump sized to handle the maximum load of the system. As a result, there’s no need for a balancing valve, as the flow rate (GPM) delivered to the intermediary heat exchanger is regulated by maintaining a delta T (temperature difference) across the heat exchanger.

The pump automatically adjusts to sustain this delta T, with its rated capacity based on the maximum flow required to meet the stakeholder’s demands. For instance, in heating mode, if the ATL circulating heat transfer medium is of a low temperature, compared to normal operating conditions, the secondary pump increases speed to deliver a higher rate of flow with a lower temperature fluid. Conversely if the temperature of the fluid increases, then the flow rate can be reduced.

This is because of the following formulas:

BTU/hour = Flow Rate (GPM) × ΔT (°F) × 500

Flow Rate (GPM) = BTU/hour ÷ (ΔT (°F) × 500)

ΔT = The temperature difference between the inlet and outlet water temperature of the heat exchanger

500 = a constant that accounts for the weight of water (in pounds per gallon), the specific heat of water, and a time conversion factor.

For instance, if a stakeholder requires 120,000 BTUs to be delivered to their building while maintaining a 10°F delta T across the heat exchanger inlet and outlet water temperatures, we can determine the required GPM (gallons per minute) using the following formula:

GPM = BTU / (ΔT × 500)

Now, plug in the values:

GPM = 120,000 / (10 × 500)

GPM = 120,000 / 5,000

GPM = 24

So, the GPM required is 24 gallons per minute.

Now, if the temperature in ATL drops because of thermal diversity and temperature cascade effect, the water in the ATL will contain less thermal energy. To compensate, we would need to either increase the size of the heat exchanger to provide more surface area for thermal energy transfer or increase the volume of water that has fewer BTUs to continue to deliver 120,000. Because there are fewer BTUs the delta t will decrease to 5℉-6℉. To determine the required flow rate (GPM), we use the same formula:

GPM = BTU / (ΔT x 500)

Now, plug in the values:

GPM = 120,000 / (6 x 500)

GPM = 120,000 / 3,000

GPM = 40

So, the GPM becomes 40.

In most two-pipe systems, heat exchangers are designed to operate at a specific flow rate (gpm) and a fixed temperature differential (delta T), typically around 10°F. However, one-pipe systems are can be engineered for a varying delta T’s, which necessitates the use of a larger heat exchanger for worst case scenario.

The steady-state heat transfer equation: Q = U x A x ∆T is used applied to heat exchangers to determine its thermal energy transfer capabilities.

• Q: The total heat transfer rate (measured in BTU/hr or similar units).

• U: The overall heat transfer coefficient, which measures how effectively heat is transferred through a material or system (units: BTU/hr·ft²·°F).

• A: The surface area through which heat is being transferred (measured in square feet).

• ∆T: The temperature difference between the two sides of the material or system (measured in degrees Fahrenheit).

To calculate the heat transfer rate (Q) for a plate and frame heat exchanger made of copper/nickel using the formula Q = U x A x ∆T, we can substitute the known values directly.

Here’s the example:

Given:

• A (surface area) = 24 ft²

• ∆T (temperature difference) = 10°F Between the house water loop and the ATL

• U (overall heat transfer coefficient for copper/nickel) ≈ 500 BTU/hr·ft²·°F

Q = U x A x ∆T

120,000BTU/hr = 500 x 24 x 10°F

Result:

The heat transfer rate (Q) for this copper/nickel plate and frame heat exchanger is 120,000 BTU/hr.

This equation calculates the heat transfer rate across a surface by incorporating the area, the thermal properties of the material, and the temperature difference driving the heat flow. Widely applied in engineering disciplines such as thermodynamics and HVAC system design, it serves as a foundational tool for optimizing energy systems.

In an Ambient Temperature Loop (ATL) system, as the ATL temperature drops and the delta T narrows to just 5°F or 6°F between the inlet and outlet water temperatures to meet the required BTU transfer, the pump operates efficiently without significantly increasing its speed. This approach minimizes head pressure and reduces energy consumption for pumping. The pump’s maximum speed is calibrated to manage worst-case scenarios with a 5°F–6°F delta T, ensuring consistent and energy-efficient performance under demanding conditions.

Conversely, as the ambient loop temperature rises, the delta T proportionally increases due to the higher thermal energy present in the water. In response, the pump slows down—potentially to 50% of its capacity—to adhere to the industry-standard 50/90 design principle. This ensures both energy efficiency and system reliability across varying load conditions.

The ATL system is specifically designed to align with the 50/90 guideline, a cornerstone in HVAC system efficiency. This principle prioritizes sustainable energy use by setting pumps to operate at 50% capacity under normal conditions, reducing energy consumption while maintaining peak performance. During maximum load conditions, the system scales up to 90% capacity, leveraging pump affinity laws to ensure reliable operation without over-sizing equipment. The integration of variable speed drives further enhances this strategy, enabling precise flow control and optimizing thermal energy transfer for maximum efficiency.

Additionally, the heat exchanger acts as a hydraulic separator, ensuring no physical mixing of water between different stakeholders while facilitating efficient energy exchange. This design isolates the hydraulics of each stakeholder connected to the loop, maintaining system efficiency and providing seamless, independent energy sharing.

Why TENs Are the Future

Thermal Energy Networks represent the next evolution in energy systems—offering a path to decarbonization while improving efficiency and reducing costs. Whether adopted by urban centers, industrial complexes, or residential neighborhoods, TENs can dramatically reduce energy dependencies and support long-term climate goals.

It’s not just about scaling renewable energy; it’s about harnessing it smarter. By integrating geothermal technology, heat pumps, and innovative distribution systems, TENs set a precedent for sustainable energy solutions we can all count on.

Interested in how this technology can benefit your community or business? Start exploring implementation options today to join the growing movement toward a sustainable energy future.

Part I of this story can be found here: https://mechanical-hub.com/building-a-sustainable-future-with-thermal-energy-networks/

Marc Miller is a Mechanical Systems SME – Educator – Technical Writer – Author – Construction Management Consultant with Home – Egg Geo.  He is presently the Lead Author on two textbook projects with Egg Geo.  He may be reached at marcm@egggeo.com.

Did you know that it has been a tradition to dye the Chicago River green? In fact, the Chicago Journeymen Plumbers Local 130, or the Chicago Plumbers Union, have been dyeing the river since 1962. The process includes an orange, vegetable-based powder that is dumped into the river. The powder is spread by two motorboats (one for dumping, one for stirring the water). The dye is designed to be environmentally safe and does not pose a threat to the river’s ecosystem. We talk about that, AHR videos, thermal energy networks and much more.

This Is Part 1/2 of Building a Sustainable Future with Thermal Energy Networks by Marc Miller, Egg Geo, LLC

Modern energy demands are growing, and the need for sustainable, efficient solutions to heat and cool our homes and businesses has never been greater. Enter Thermal Energy Networks (TENs)—innovative systems that distribute and reuse energy to reduce costs, cut emissions, and optimize performance.

This article explores the basics of TENs, their benefits, and real-world applications, helping you understand their potential to transform energy systems for the better.

What Are Thermal Energy Networks?

At their core, TENs are district energy systems that use ambient temperature loops (ATLs) to share heating and cooling between buildings. These loops typically circulate a heat transfer fluid, such as water or a water-propylene glycol mixture, at a temperature range of 50°F to 85°F. By tapping into diverse thermal sources, such as geothermal wells, waste heat recovery, or natural water bodies, TENs provide sustainable, cost-effective alternatives to traditional heating and cooling systems.

Crucially, TENs optimize thermal sharing between stakeholders:

• Consumers exclusively use thermal energy.

• Prosumers both produce and consume energy (e.g., offices that generate heat and reuse it internally).

• Generators supply thermal energy into the network (e.g., geothermal wells or industrial processes).

 How TENs Are Different?

Unlike traditional systems like two-pipe or four-pipe setups of district heating and cooling scenarios, TENs use an ambient one-pipe loop that operates at variable fluid flow and temperatures instead of relying on constant temperature and variable flows. Here’s what sets TENs apart:

• One-Pipe Systems manage heating and cooling directly by varying the flow of the fluid of varying temperature circulating the loop.

• Efficiency in Pumping reduces energy waste with self-balancing, simpler designs that eliminate the need for balancing valves. This results in significantly lower operating and installation costs.

• Pump Controlled Instead of Valve Controlled. No balance valves or control valves are used. Flow through decoupled secondary loops is managed by a pump sized to deliver the optimal flow.

 Key Benefits of TENs

Energy Efficiency

TENs enable efficient thermal load sharing and shedding. For example:

• Unused heat from one building can be redistributed to another, maximizing energy use across the network.

• Water source heat pumps (WSHPs) leverage the stable temperature of ATLs, operating with coefficients of performance (COP) ratings of 3–5, which translate to lower energy consumption.

Cost Savings

Compared to traditional systems, TENs offer significant financial benefits:

• Lower Installation Costs: With single-pipe systems, there’s less piping, no balancing or control valves, and reduced labor for installation because of less fittings, less tooling and overall less pipe. Additionally there is a single pipe size for the entire loop.

• Reduced Operating Costs: Circulating water at ambient temperatures requires much less pumping power.

 Scalability and Flexibility

TENs are highly scalable—there’s no limit to the number of stakeholders that can connect to an ATL. The caveat is that when sizing the loop, it must be sized for the total load to include anticipated future expansion. The more buildings added, the better the system operates due to improved thermal diversity, which counterbalances the temperature cascade effect (where fluid temperatures progressively change as they circulate).

 Environmental Impact

Integrating geothermal energy with TENs significantly reduces greenhouse gas emissions. By shifting away from fossil-fuel-based heating systems, TENs pave the way for a green and sustainable future.

Real-World Example

The Eversource Geothermal Pilot in Framingham, Massachusetts, is a first-of-its-kind utility-scale networked geothermal project. Launched in 2024, this system connects 135 residential and commercial buildings, providing ground-sourced heating and cooling through an ambient temperature loop.

Key takeaways from this pilot include:

• Reduced reliance on fossil fuels, replaced with renewable energy from earth’s natural thermal stability.

• Positive feedback from participating stakeholders due to noticeably reduced energy costs and environmental impact.

• A replicable model for other communities to follow.

“This project… is enabling our team to see how we can provide services in a completely new way,” shared Bill Akley, President of Gas Distribution at the project’s groundbreaking.

https://www.eversource.com/content/residential/save-money-energy/clean-energy-options/geothermal-energy/geothermal-pilot-framingham

Challenges and Considerations

No system is without challenges, but TENs offer practical solutions for common concerns.

1. Temperature Cascade Effect: Concerns about progressive temperature changes are resolved by system diversity. More connected stakeholders create a smoothing effect that distributes thermal loads more evenly.

1. Initial Investment: Modern legislative incentives and community grants, like those funded by the U.S. Department of Energy, are now making upfront costs more affordable for communities adopting TENs.

How TENs Function, a Simple Explanation

In the first illustration below, a TEN utilizing its one-pipe Ambient Temperature Loop (ATL). The TEN consists of a geothermal source/sink, an intermediary heat exchanger, a main loop distribution pump, and three stakeholders: a cooling-dominant data center, a heating-dominant industrial plant, and an apartment building with relatively balanced heating and cooling loads.

During the summer, the network manages a total heat rejection of 540,000 BTU/h from the apartment building and the data center. However, only 140,000 BTU/h is transferred to the geothermal sink via the heat exchanger (HX-1). This reduction occurs because the network redirects excess heat to the industrial plant, utilizing it for productive purposes rather than rejecting it to the ground.

In winter, the network requires a total of 640,000 BTU/h to meet the heating demands of the apartment building and the industrial plant. Of this, 400,000 BTU/h is supplied by the geothermal source through HX-1, while the remaining 240,000 BTU/h is provided by the data center, offsetting the geothermal demand. This demonstrates the network’s ability to efficiently balance loads by redistributing energy among stakeholders.

 

 

In the second example below, we introduce a cooling-dominant load of approximately 10 tons (120,000 BTU/h) to the loop. The system is designed to maintain a leaving heat exchanger temperature of 65°F and a return temperature of 75°F, preserving a 10°F delta T. This consistency is achieved through pump control. By increasing the pump speed and actively managing water flow through the heat exchanger based on temperature, the system sustains the desired 10°F delta T. foundational principles of a TEN.

 

The performance of such a network improves with greater load diversity, i.e., the inclusion of more stakeholders with varying thermal profiles. Additionally, since the system is pump-controlled and does not rely on balancing valves the system is self-balancing. In fact, the temperature cascade effect is shown to improve with an increasing number of thermally diverse stakeholders connected to the loop.

In the previous scenario, the pump’s flow rate was 28 GPM. With the added load, it increases to 54 GPM. Additionally, the inlet temperature to the industrial plant rises from 56.4°F to 69.6°F. Even with the extra 10 tons of cooling load, the pipe size for both the geothermal source/sink loop and the ambient temperature loop remains 3 inches. Importantly, the return temperature to the heat exchanger remains consistent at a 10°F delta T with a 75℉-return design temp.

By examining the ambient temperature loop (ATL), we observe that the temperature range improves significantly, aligning with the target range of 55°F–85°F. Previously, when there were only three loads, the loop’s temperature immediately after the industrial plant was much lower at 36.4°F. Furthermore, the heat transfer through HX-1 and the geothermal well field increases substantially, from 140,000 BTU/h to 260,000 BTU/h, reflecting the additional 10 tons of cooling load.

It is worth noting that this theoretical scenario assumes no other consumers of waste heat, providing just a snapshot in time. In practice, there will always be various heat consumption requirements, such as domestic hot water production, pool heating, or snow melting. Thermal energy networks (TENs) are designed not only for heating and cooling but also for these additional purposes. As a result, the BTUs from cold storage will be utilized across multiple applications beyond simple heating or cooling thus keeping the amount of heat to be rejected by HX-1 to a minimum

This simplified explanation underscores the system’s ability to adapt to increased loads while maintaining efficiency and highlights the versatility of thermal energy networks in meeting diverse energy demands. In part 2 of this article, we will dive into the specifics of how to connect your home or building to the TEN with a decoupled secondary loop and talk about how to control the pump to account for Thermal Diversity and Temperature Cascade Effect.

Marc Miller is a Mechanical Systems SME – Educator – Technical Writer – Author – Construction Management Consultant with Home – Egg Geo.  He is presently the Lead Author on two textbook projects with Egg Geo.  He may be reached at marcm@egggeo.com.

On this week’s update, we’ll talk U.S. infrastructure and what the current state of that looks like, World Plumbing Day, we’ll feature some of our AHR visits and a new contractor spotlight with Shawn Ziegler, Accurate Mechanical out of Lancaster, Ohio. Plus, we’ll talk new, weekly podcasts. Oh, and I almost forgot, Gate Lice.

Grundfos Alpha Comfort System

Rheem Endeavor Universal Heat Pump Video

Contractor Spotlight: Shawn Ziegler—Rising Up

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