The evolution of district energy systems
District energy has undergone tremendous changes since the first commercial operations began in New York during the year 1877 with the Holly Steam Combination Company (Collins 1959). In those earliest days, steam distribution was advantageous because it did not require pumping and was later a byproduct of many communities’ electric generators. The high operating temperatures and pressures, however, posed a great risk to system engineers and consumer safety (Lund et al. 2014). Therefore, a more than century long decrease in operating temperatures and pressures began (Fig. 1).
Lower operating temperatures for a district energy network also make more resources applicable as a source and sink for heating or cooling (Lund et al. 2014; Zeh et al. 2021). By 2022, an interest in replacing existing gas grids with networked water systems of lower but varying temperature regimes resulted in the phrase “thermal energy networks” being codified or proposed in law across several US states (Cordes et al. 2023; Parker et al. 2022; Williams et al. 2023). This district energy evolution, taking the technology from high temperatures with fossil fuel sources to low temperatures and low emissions with multiple sources, has made the TEN a fundamental part of building stock decarbonization strategies.
Many of these new sources for a TEN are waste heat or byproducts of other processes, including other buildings, across a community (Walker et al. 2017). While one building in a community is demanding heating another may be demanding cooling, introducing the potential for load sharing. This load sharing or waste heat valorization can be a signature feature of a TEN (Lund et al. 2014; Wirtz et al. 2020). Waste heat valorization is simply the recycling of heat byproducts for a useful purpose. Load sharing allows simultaneous heating and cooling processes to exchange their heat byproduct with another across the TEN, thereby reducing primary energy consumption. Byproduct or waste heat are typically of little value to the primary generator and, therefore, may be purchased at low- or no-cost. These heat sources may decrease consumer costs on a TEN, reducing one of the financial barriers to the broader adoption of utility-style heating and cooling systems.
Thermal energy network configurations
Since Lund et al. (2014), district heating generations have been defined by their operating temperature regimes. Some newcomers to district energy are calling fifth generation district heating and cooling (5GDHC) thermal energy networks (Home Energy Efficiency Team 2024). The TEN, however, references all generations of district energy systems—as all are a thermal network. It may be more appropriate to consider the TEN as a ‘network of networks’ at the city scale. The TEN may serve to connect several different topologies of district energy systems (Fig. 2), offering hydraulic separation with energy transfer stations or substations which modulate several different operating temperature regimes or ‘generations’ at a large scale. Examples of the TEN include projects in Leuven, Belgium (Pattijn and Baumans 2017) and Technical University of Berlin, Germany (Stanica et al. 2021).
The hydraulic design of the pipes in a TEN usually has two options: single pipe (Sommer et al. 2020), or two-pipe distribution (Boesten et al. 2019; Li and Wang 2014; von Rhein et al. 2019). Recent literature explores the advantages of disadvantages of one configuration over the other in the context of the district heating and cooling generations (Gudmundsson et al. 2022; Lund et al. 2021; Zeh et al. 2021). A main finding of this review, however, is that the inputs for a TEN are temperature agnostic, allowing good engineering practice and other socio-economic needs to dictate the thermo-hydraulic design and phasing out of other system operating conditions across communities. In-building equipment selection is also outside of the scope of this review, though countless numbers of heating, ventilation, air conditioning, and refrigeration (HVACR) technologies can reject or extract heat from a TEN.
In a two-pipe distribution system (Fig. 3) a cold line and a hot line are used. When a building is rejecting heat, the fluid travels from the cold line, into the structure for heat exchange and upgrading (usually with a heat pump) if necessary, then back to the hot line. The primary advantage of this configuration is that heat sources and sinks with higher fluid temperature differentials can be separated to avoid mixing, with the potential for end-use performance improvements (Boesten et al. 2019). One of the disadvantages of the two-pipe configuration is that heat losses and gains may be incurred that degrade performance improvements (Averfalk and Werner 2020; Sulzer and Hangartner 2014).
In a single pipe portion of a TEN, one pipe distributes all the heat of fluids from the sources and sinks. The advantages include simplicity in consumer node connections, modularity, and a reduction in heat losses. Individual circulating pumps at consumer nodes reduce the primary distribution pumping requirements. Single pipe hydraulic distribution is designed for a large bandwidth of temperature drift. Temperature drift is the variation in continuous operating temperatures for the working fluid across the network. Most often the temperature range is near ambient, or near ground temperatures, not exceeding 30°C (86°F) (Lindhe et al. 2024; Sommer et al. 2020), though existing high-temperature systems can integrate, given proper controls. Lower operating temperatures make the connection of far more sources and sinks possible while reducing unwanted heat losses or gains (Sommer et al. 2020). The single pipe design is also in use for “last mile” heat valorization of high-temperature networks when the working fluid is changed from a water-based solution to CO2 (Noreskar 2022). The disadvantages include higher electricity draws for pumping and larger diameter pipe requirements than previous district heating and cooling generations (Jebamalai 2023, Chapter 6; Sommer et al. 2020).
Underground thermal energy storage review
Underground Thermal Energy Storage (UTES) represents an array of techniques for storing thermal energy within subsurface geological formations over a long period—usually a heating or cooling season, though it may be diurnal. Storing thermal energy in the subsurface leverages the rock medium beneath the built environment of a city or facility, providing a space-saving technique that improves the energy efficiency of heating and cooling processes. If a surface process is rejecting heat or extracting heat, it will inevitably be emitting a higher or lower temperature waste heat. Capturing that cool-th or heat-th waste heat, often available from intermittent renewable resources or other industrial processes, is possible with UTES. This section reviews a spectrum of UTES approaches, including aquifer thermal energy storage (ATES), reservoir thermal energy storage (RTES), and various engineered solutions such as borehole thermal energy storage (BTES), ground heat exchangers (GHX) and cavern thermal energy storage (CTES). Overall, UTES applications can support a TEN, mitigating society’s reliance on fossil fuels and reducing greenhouse gas emissions while providing flexible and cost-effective solutions for heating, cooling, and industrial processes.
Aquifer thermal energy storage (ATES)
ATES first took root in Shanghai, China, during the 1960s (Gao et al. 2009). By 1975, a government-funded ATES project was underway in Mobile, Alabama, USA (Tsang 1978). Storing thermal energy from existing power plants and solar was a focus from the beginning (Rabbimov et al. 1971; Tsang 1978), where the characterized hot water temperatures were often in excess of 120 °C (248 °F). It was not widely understood, however, that temperatures below 25 °C (77 °F) would be adequate for large-scale thermal energy applications within the same aquifers. Today ATES systems are largely in use for applications in building heating and cooling.
ATES systems are popular in Europe, where high adoption rates exist in the Netherlands (Bloemendal 2018; Fleuchaus et al. 2018; Nielsen et al. 2019), often with characteristically low temperatures between about 5–20 °C (40–68 °F) (Fig. 4). This lower-temperature storage is sufficient when supplemented with a ground-source heat pump (GSHP). In those building heating and cooling applications, ATES can more appropriately be thought of as an energy efficiency measure that requires seasonal balance. Seasonal balance of the warm and cold plume is possible by combining mechanical supplements such as dry coolers and reversing valves (Bloemendal 2018; Dickinson et al. 2009).
The most common configuration for building heating and cooling is a pair of wells, each receiving warm or cool water for storage in a highly permeable formation. The depth of drilling is less important in ATES than both the water quality and the salinity (Bloemendal 2018). With a seasonal switch, the thermal plume of one well is extracted, with fluids being heat exchanged at the surface before reinjection. Where seasonal load imbalance exists, a supplemental dry cooler or other mechanical solution may be used to rebalance the thermal plumes in the subsurface. Heat recovery efficiency in heating mode may range from 50 to 80%, while cold plume recovery may approach 100% (Matos et al. 2019; Van Lopik et al. 2016).
In many cases, the ambient temperatures of the aquifer may be used for passive cooling without a heat pump (Fig. 5). When free (passive) cooling can be introduced to the mechanical system, significant improvements in the coefficient of performance (COP) are possible. The COP is the ratio of useful heating or cooling provided to the work input required, typically electricity. When COP improves, the imported electricity for the vapor compression cycle of a heat pump is reduced or eliminated, and the heating and cooling system becomes far more sustainable.
Reservoir thermal energy storage (RTES)
Hot and cold storage in deeper reservoirs is increasingly distinguished in literature from ATES (Pepin et al. 2021), more widely referred to as RTES. Although RTES and ATES both use subsurface pore space for fluid storage, Pepin et al. (2021) delineates RTES by describing the reservoirs as those containing slower-moving fluids with mature geochemical characteristics. More simply, less saline fluids—those often used for drinking water—are found in the same formations useful for ATES, whereas RTES fluids are brackish and are not useful for drinking water without additional treatment. The regulatory burden for drinking water uses other than consumption is high, perhaps making RTES more useful for the large-scale collection of waste heat and industrial process loads (Matos et al. 2019; Pepin et al. 2021; Zhang et al. 2023).
Heat recovery efficiencies in these systems are thought to improve over time, much like ATES, as the reservoir equilibrates to the hot or cold plume. Van Lopik et al. (2016), referring to deep, high-temperature (HT), brackish reservoir thermal storage as HT-ATES, found recovery efficiencies approach 70% while accounting for fluid density differences, increasing up to 78% without free convection. Each instance was run over 4 cycles for 80°C injection temperatures. Pepin et al. (2021) found that cooling recovery efficiency was the most effective means of thermal recovery in RTES, with values ranging from 96.3 to 99.3% over a 5-year cycling period.
Other UTES approaches
Other methods of engineered UTES include BTES and the GHX. These are the most prevalent forms of UTES across the world, making up 72% of worldwide geothermal (ground source) heating and cooling applications (Lund and Toth 2021). Some literature segregates borehole arrays or geoexchange from BTES (Liu et al. 2021). In this sense, an individual vertical borehole, typically reaching 100’–850′ (30 m–260 m), may provide a medium with which to store heat seasonally (Kavanaugh and Rafferty 2014; Nordell 1993; Zymnis and Whittle 2021).
Fundamentally, the same mechanisms of heat transfer are taking place in the subsurface for both BTES and GHX, albeit at different scales and with a different magnitude of influence on the surrounding soil or rock material. BTES is conceptualized in a slightly different manner than GHX. In BTES, no balance may be required on the load side to achieve the unambiguous goal of supplying thermal energy at times of demand that vary from different times of thermal energy production (Reuss 2015). Furthermore, BTES is appropriate for high-temperature thermal storage, as is the case in Drake’s Landing, Alberta, Canada (Sibbitt et al. 2012).
For a discussion of other GHX variants the reader is encouraged to reference Kavanaugh and Rafferty (2014) or Minea (2022). Geoexchange or closed-loop borehole thermal storage systems connected to a heat pump, have been used since 1946 (Kemler 1947). Often, a single u-tube made of plastic pipe, typically 25–32 mm (1″–2″) in diameter, is inserted into a borehole of about 60–260 m (200–850′) depth to extract and reject heat from a connected load. The pipe and fluid properties are selected to optimize heat transfer to cost ratios, minimize friction losses, accommodate tremie lines for grouting, and prevent pipe bursting, among other reasons (Gagné-Boisvert and Bernier 2017; McCartney et al. 2017; Proffer 2022).
Without thoughtful load balancing on the borehole arrays, the heat recovery efficiency may be unacceptably low (Schincariol and Raymond 2023). Where the preferable narrow temperature bandwidth cannot be achieved, as described by Wang et al. (2021), because the heating and cooling demands are imbalanced, the GHX is supplemented by additional sources or sinks. One possible way to compensate for load profiles with a significant seasonal imbalance (Fig. 10) is multivalency using only low-temperature resources (those below ~ 35 °C, or 95°F) on the ground loop. This prevents pipe damage, avoids tripping ground-source heat pumps at high temperatures, and provides an opportunity for the designer to increase the seasonal performance factor for the system.
Integrating UTES with thermal energy networks
A TEN is a piped network of working fluids, usually water, which can connect geothermal sources with geoexchange sources and sinks, or other thermal resources (solar thermal, heat rejection from cooling operations, electric-to-thermal conversions, or many others) across a geographic area. Demands for both heating and cooling across the TEN will have a diurnal variation and seasonal variation which provides the opportunity to implement different scales of UTES. Storing large amounts of hot or cold fluids in UTES allows the energy system to produce from subsurface resources at a more convenient time, charge the storage, and release the energy when the demand arises. This increased capability over conventional district heating and cooling is often referred to as demand-side management (DSM) (Nielsen et al. 2019).
The benefits of combining conventional district heating and surface thermal storage is widely known (Bertelsen and Petersen 2017; Jebamalai et al. 2020; Lake et al. 2017). Therefore, drawing a line from UTES to the point of energy consumption is made possible using a TEN. Optimization potential increases for a variety of metrics in a TEN when coupled with UTES (Buonocore et al. 2022; Lake et al. 2017; Oh and Beckers 2023). As a matter of economics, this scalable DSM solution may also alleviate the capital-intensive nature of individual geothermal and geoexchange systems. Communities, including commercial and residential, or industrial process heat users, may benefit from higher heating and cooling efficiencies with the savings of a utility-scale product (Oh and Beckers 2023).
Challenges and opportunities for UTES and TEN integration
Despite the many advantages that UTES may offer for sustainable city-scale heating and cooling, implementation barriers remain, including public awareness, inconsistencies in subsidy, and complex regulatory frameworks that only sometimes comport with good engineering or hydrogeological practices. Understanding the influence of initial investment costs on sustainable energy system adoption is an important starting point. In terms of first costs, the expenses associated with drilling are significantly higher than conventional combustion-driven equipment and solutions (Hanova et al. 2007; Li et al. 2023; Robins et al. 2021). Furthermore, fewer than ¼ states provide owner–operator credits for realized avoided energy costs from geothermal system installations.
Regulatory structures often impair the development of UTES through groundwater law, mineral and petroleum law, or public service monopolization (Matos et al. 2019; NY Governor’s Press Office 2023; Strauss 2022). Public perception of UTES—much like geothermal—reflects education level, socio-economic status
