The Hidden Side Of Cities: Using Aquifer Thermal Energy Storage For Energy Saving

Aquifer Thermal Energy Storage (ATES) systems contribute to reducing fossil energy consumption by seasonally storing heat in underground aquifers. Combined with a heat pump, these systems can provide more sustainable heating and cooling for buildings, and they can reduce the use of energy in bigger buildings by more than half. ATES is therefore important for the energy transition in many urban areas in North America, Europe, and Asia.

In the Netherlands, where shallow aquifers make the technology especially cost-effective, ATES is currently used in around 10% of new buildings; while this market share is still modest, the demand of subsurface space for ATES has already grown to congestion levels in many Dutch urban areas.

This problem is to a large extent caused by the current approach for ATES planning and permitting, which uses very wide safety margins between the wells that are used to inject and extract water, and a 2D – rather than 3D – perspective for planning. These policies mean that the available subsurface space is not used optimally, so that cities may not achieve the full potential of ATES technology towards reducing greenhouse gas (GHG) emissions.

The optimal use of subsurface space in dense urban settings can be achieved with a coordinated approach towards the planning and operation of ATES systems, so-called ATES planning. This research identifies and elaborates crucial practical steps towards an optimal use of subsurface space, which are currently missing in ATES planning policies. Starting from an analysis of current ATES plans, coupled agent-based/groundwater simulations were used to demonstrate that an increased demand for ATES requires progressively stricter regulations to minimize GHG emissions. This research resulted in 4 major contributions:

1. Effective guidelines that help prevent the need for ATES planning: Injection/extraction wells should use the full thickness of the aquifer, and should be placed more densely; 2.5 times the average thermal radius of the wells of opposite type (warm vs. Cold) and 1 times the average thermal radius for wells of the same type,  rather than 3 for all wells as under current policies)
2. Thresholds to tell when ATES planning is needed based on the allocation of aquifer space under increased demand for thermal storage. A stepwise approach for planning was identified by simulating a range of allocated aquifer space fractions from 2% to 75%. Self-placement scenarios (in which ATES wells can be placed anywhere, within the guidelines on distances between wells) do not require additional ATES planning below 25% allocated aquifer space fraction, within the current practice and regulatory framework. At allocated aquifer fractions for ATES ranging from 25% to 50%, rules for well design and well spacing foster self-placement. Beyond 50% allocated aquifer space fraction, the highest GHG emission reductions are obtained with a prescribed spatial arrangement of the warm and the cold wells in separate lanes.
3. Effective placement and operation methods for lane placement in the busiest areas. Both the width and the spacing of the lanes must be twice the average thermal radius of the ATES systems in the area. Arrangements on collective systems (i.e. collectively operating a group of smaller individual ATES systems), and an area-wide energy balance between heating and cooling, increase the effective use of aquifer space for ATES even more.
4. An assessment framework to evaluate possible planning strategies. The following assessment parameters were identified: total reduction in GHG emissions, cost for installation, recovery efficiency, and robustness for different future conditions.

We conclude that the improvements of governance, design and planning practices presented in this study can be easily used and implemented in practice in the Netherlands because they fit within the Dutch regulatory framework. Although ATES adoption in other countries is not yet as high as in the Netherlands, the specific problem discussed in this study is also likely to occur in other cities around the world. Countries at the early stage of ATES adoption can take advantage of this research and experience in the Netherlands, by planning and applying ATES according to the methods presented in this study – therefore, ensuring that ATES technology can achieve its full potential for energy savings in cities.

These findings are described in the article entitled Methods for planning of ATES systems, recently published in the journal Applied Energy. This work was conducted by Martin Bloemendal, Marc Jaxa-Rozen, and Theo Olsthoorn from Delft University of Technology.