Aquifer Thermal Energy Storage (ATES): Innovations in Renewable Heating and Cooling

Heating and cooling buildings is one of the most energy-intensive activities on the planet, consuming roughly 32% of global energy and contributing significantly to CO₂ emissions. Yet beneath our feet lies a largely untapped resource capable of meeting much of this demand: underground aquifers. Aquifer Thermal Energy Storage, or ATES, is a renewable technology that uses these natural groundwater reservoirs to store and retrieve thermal energy across seasons – cooling buildings in summer and heating them in winter, with minimal fossil fuel use.

What is aquifer thermal energy storage? 🔗

Aquifer Thermal Energy Storage (ATES) is the storage and recovery of thermal energy in subsurface aquifers, achieved by extracting and injecting groundwater through wells. It is classified as an Underground Thermal Energy Storage (UTES) technology – specifically, an “open” system in which groundwater itself acts as the heat carrier, moving between the aquifer and surface heat exchangers.

The basic infrastructure of a standard ATES setup involves two wells drilled into a suitable aquifer – one designated as the “warm well” and the other as the “cold well.” These wells operate in a seasonal cycle, reversing direction every six months to match the building’s shifting demand for cooling and heating.

How ATES systems work: the seasonal cycle 🔗

The operating principle of ATES is elegantly straightforward. In summer, warm air inside a building is absorbed by a fluid circulating through a heat exchanger. This fluid transfers its heat to cool groundwater pumped from the cold well. The now-warmed groundwater is re-injected into the aquifer, where it is stored as a thermal “bubble.” This process simultaneously cools the building and charges the warm well for later use.

In winter, the process reverses. The heated groundwater stored in the warm well – at approximately 18°C – is extracted and used to heat buildings via a heat pump. As it releases its heat, the groundwater cools to around 6°C and is re-injected into the cold well, where it waits to provide cooling again the following summer. This closed loop of thermal exchange requires very little electricity – only enough to run the pumps and, when needed, the heat pump.

Types of ATES systems 🔗

Most operational systems worldwide are low-temperature ATES (LT-ATES), where warm well temperatures stay below 25°C and cold well temperatures remain above 5°C. In most installations, cooling can be delivered directly without a heat pump – a mode known as “direct cooling” – which further reduces energy consumption. A smaller number of high-temperature ATES (HT-ATES) systems exist, storing industrial waste heat or solar thermal energy at higher temperatures for later use in district heating networks.

Bidirectional systems, the most common type, use a well doublet that actively switches pumping direction between seasons. Mono-directional systems, on the other hand, always extract groundwater at its natural temperature without seasonal reversal, and are typically used where thermal recovery rather than storage is the primary goal.

Geological requirements and site suitability 🔗

ATES is not universally applicable – its performance is tightly bound to local hydrogeological conditions. Thick, sandy aquifers greater than 10 meters in depth with high hydraulic conductivity are optimal for ATES. The aquifer must be able to accept and yield water readily, while groundwater flow must be slow enough that injected thermal plumes don’t drift too far from the retrieval well.

Shallow depths – typically up to 200 meters – are preferred for low-temperature systems, as drilling costs rise steeply beyond this. Aquifers should also have consistent geochemical composition throughout; mixing of water from chemically different layers can cause clogging in the wells, reducing efficiency and increasing maintenance. These requirements mean that site assessments involving geological surveys and groundwater modeling are essential before any ATES project begins.

Principles and benefits of ATES 🔗

The fundamental appeal of ATES lies in its ability to shift energy use across time. Buildings need cooling most when outdoor temperatures are highest, and heating when temperatures drop – but these demands are almost exactly out of phase with natural ground temperatures. ATES bridges this seasonal mismatch by storing surplus heat from summer for use in winter, and cold from winter for use in summer.

Energy efficiency gains 🔗

ATES systems can reduce primary energy use by approximately 50% and gas consumption by 80-100% compared to conventional heating and cooling systems. The economics are compelling too: capital costs average around €0.2 million for small systems, with typical payback periods of just 2 to 10 years. Unlike solar panels or wind turbines, ATES requires very little above-ground footprint, making it particularly attractive in dense urban settings where space is limited.

Low operational energy demand 🔗

One of the key engineering advantages of ATES is that the storage medium – the aquifer itself – requires no construction, insulation, or active management. The storage volume is naturally formed in advance and does not require tank construction, making it one of the most cost-effective thermal storage systems available. The only capital infrastructure needed are the wells that access the aquifer, and the surface heat exchangers connected to the building systems.

Role of ATES in sustainable development 🔗

Buildings are a major driver of global carbon emissions. Heating and cooling account for about half of global final energy consumption and are responsible for more than 40% of energy-related CO₂ emissions. Decarbonizing this sector is therefore critical to meeting the Paris Agreement target of net-zero emissions by 2050.

CO₂ emission reductions 🔗

ATES directly addresses this challenge by substituting fossil-fuel-based heating and cooling with a system that relies on stored ground-temperature energy and, at most, a small electrical input. Compared to conventional heating and cooling technologies such as gas boilers and compression chillers, ATES can reduce greenhouse gas emissions by up to 74%. On a larger scale, the installation of 50,000 ATES systems across Europe could reduce annual CO₂ emissions by approximately 3 million tonnes.

Integration with renewable energy sources 🔗

ATES doesn’t have to operate in isolation. It integrates naturally with other renewable energy technologies. Combining ATES with renewable energy sources helps balance energy supply and demand, enhancing system efficiency and reliability, while also alleviating peak energy demands and reducing strain on electricity grids. For example, surplus solar thermal energy collected in summer can be injected into a high-temperature ATES system and recovered in winter for district heating – effectively turning the ground into a seasonal solar energy battery.

ATES can also store industrial waste heat that would otherwise be vented into the atmosphere, channeling it instead into useful building heating. In the Netherlands, researchers at Deltares have been investigating combinations of ATES with aquathermal energy – extracting heat from surface water sources like rivers and canals – to create integrated, sustainable district energy systems.

Alignment with climate policy 🔗

ATES is well-positioned within the broader regulatory landscape driving energy transition. Between 2005 and 2023, greenhouse gas emissions from buildings in the EU fell by 43%, driven by efficiency improvements and heating system decarbonization. The EU’s revised Energy Performance of Buildings Directive (EPBD), adopted in 2024, pushes further, requiring decarbonization of heating systems and improved energy performance. ATES fits directly within this policy direction as a proven, scalable tool for reducing building sector emissions.

Global adoption and applications of ATES 🔗

ATES was first used deliberately for thermal storage in Shanghai around 1960, primarily to cool textile factories. Modern ATES development accelerated in the 1970s and 1980s, with field experiments in France, Switzerland, the US, and Japan. Today, more than 2,800 ATES systems are in operation worldwide, providing over 2.5 TWh of heating and cooling per year.

The Netherlands: a world leader 🔗

No country has embraced ATES more fully than the Netherlands. Approximately 85% of all ATES systems globally are located in the Netherlands, with a further 10% found in Sweden, Denmark, and Belgium. The Dutch success story is rooted in a combination of favorable geology – shallow, sandy aquifers are widespread across the country – and decades of progressive policy support.

The Netherlands now has over 3,000 ATES systems in place, representing around 40 years of accumulated experience. Government-subsidized pilot projects in the late 1980s built early confidence in the technology, and a regulatory streamlining in the early 2010s introduced an eight-week permitting process, company certification standards, and mandatory system monitoring. This policy infrastructure helped ATES scale from just 5 systems in 1990 to thousands today.

Applications in building heating and cooling 🔗

ATES is particularly well-suited to large commercial, institutional, and public buildings that have both significant heating and cooling loads throughout the year. Offices, hospitals, universities, airports, and shopping centers are among the most common applications. ATES is especially suited to large-scale applications because thermal loss per unit of stored energy decreases as the volume of stored water increases relative to its surface area – meaning bigger systems are inherently more thermally efficient.

Beyond individual buildings, ATES is increasingly being deployed in district heating and cooling (DHC) networks, where a centralized ATES installation serves multiple buildings across an urban block or neighborhood. A case study of ATES integration in a district heating network in Southern Finland concluded that combining heating and cooling with seasonally reversible ATES operation had low environmental impact on the aquifer and was economically feasible.

Expanding across Europe and beyond 🔗

The EU-funded E-USE(aq) project brought ATES demonstration sites to Italy, Spain, Denmark, Belgium, and the Netherlands to test the technology’s adaptability across different soil conditions and climates. Project results showed that ATES is economically feasible across these contexts and that social and legislative obstacles can be overcome with appropriate innovation and policy.

Beyond Europe, countries including Germany, Turkey, and the United States are seeing growing interest and investment in ATES. City-scale analysis, such as a 3D heat transport modeling study of Freiburg, Germany, showed that ATES systems can achieve substantial heating and cooling supply rates even under challenging hydrogeological conditions. As aquifer mapping improves globally and policy frameworks catch up with technical readiness, the geography of ATES adoption is expected to widen significantly.

Challenges and considerations 🔗

Despite its clear benefits, ATES is not without constraints. Contaminated aquifers cannot be used, as pumping groundwater can spread existing pollutants. Dense urban areas are particularly vulnerable to this issue, given the slow pace of groundwater remediation relative to the rapid growth of ATES installations. Managing the density of ATES systems in urban subsurfaces also requires careful spatial planning to prevent thermal interference between neighboring installations – where the warm plume from one system migrates into the cold zone of another, reducing efficiency for both.

Public awareness remains a barrier in many regions. The term “aquifer” is unfamiliar to most people, creating the impression of a complex, opaque technology even where the actual operation is straightforward. Expanding ATES adoption globally will require not just favorable geology and strong policy, but also deliberate investment in public education and transparent communication about how the technology works.

What do you think? As cities look to decarbonize their building sectors, should ATES receive the same level of public attention and investment as solar and wind energy? And what role should urban planners play in mapping and protecting underground aquifers as strategic resources for the energy transition?

References

1. https://www.unep.org/resources/report/global-status-report-buildings-and-construction-20242025
2. https://en.wikipedia.org/wiki/Aquifer_thermal_energy_storage
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