The Impact Of Storage Conditions On The Recovery Efficiency Of ATES Systems

Aquifer Thermal Energy Storage (ATES) systems provide sustainable heating and cooling for buildings by seasonally storing heat in underground aquifers. Combined with a heat pump, ATES systems prevent fossil energy consumption for space heating and cooling of buildings and reduce the use of energy in typical utility buildings by more than half.

Climates with a heating and cooling season, together with the presence of aquifers, are favorable conditions for ATES application. Many urban areas across the world may benefit from the sustainable energy of ATES systems, but currently, ATES is mainly used in the Netherlands. The Dutch apply ATES in around 10% of new buildings; despite this modest market share, many Dutch urban areas suffer from a scarcity of space for ATES in the subsurface.

Figure 1: ATES working principle. Credit:Ā Martin Bloemendal

Before increasing the number of ATES systems in local districts, it is first required to identify which ATES-well design approaches result in the highest efficiency and the most optimal use of subsurface space. In this research, tools were developed to take account for energy losses under various geohydrological conditions. New practice suggests, compared to current practice, that both geohydrologic, as well as ATES-specific operational conditions, are taken into account. Data from Dutch operational ATES systems was used to verify and illustrate the developed concepts.

The results show that in the subsurface dispersion, losses can be neglected because they are an order of magnitude smaller than conduction losses and advection when there is a high ambient groundwater flow velocity. Dispersion causes the spreading of heat due to differences in flow velocity of the groundwater in the pores in between the sand particles when infiltrated or extracted from the well. When there is no ambient groundwater flow only heat conduction causes losses of the stored heat. This then makes the thermal recovery efficiency dependent on the ratio between the circumference (A) and the total volume (V) of the stored heat in the thermal cylinder around the ATES well.

The smaller the A/V-ratio, the less conduction occurs and higher the recovery efficiency is: it is shown that there is a negative linear relation between recovery efficiency and A/V-ratio. At a given storage volume, related to the size of the building, the screen length of the ATES well determines the A/V-ratio and is, therefore, an important design parameter for optimal performance of the ATES system. However, in current practice, the screen length is mostly determined to meet the required well discharge. Well capacity is also important; data from existing ATES systems shows that in general screens should be designed longer to meet the optimal A/V-ratio, so this requirement does not jeopardize the maximum capacity.

Figure 2: Lines: the relation between specific well discharge and radial distance at which the radial flow velocities where conduction and dispersion are equal for the outer-bounds of the range of thermal dispersivity (Ī±) regularly applied in literature. Open circles: the thermal front of the ATES systems in the data at different storage capacities related to their specific well discharge. Credit:Ā Martin Bloemendal
Figure 3: Simplified presentation of the resulting subsurface thermal and hydrological storage cylinder for an ATES system for homogeneous aquifer conditions. Credit:Ā Martin Bloemendal

In case of ambient groundwater flow in the storage aquifer, advection of the stored heat is also an important loss factor. For low rates of groundwater flow (<25 m/y), an analytical relation is derived between recovery efficiency and well design. Under conditions of higher groundwater flow velocities, it is shown how multiple wells in line with the groundwater flow direction can counteract the advection losses. Infiltration is done in the upstream well, while extraction of heat occurs in the downstream well where the heat has traveled to.

Key aspects are 1) the mutual distance between the wells and 2) the storage cycle/energy demand pattern to ensure recapturing of the heat at the downstream well. Under typical seasonal ATES conditions, it is shown that the distance between the wells should be around 0.4 times the yearly groundwater flow velocity.

These findings are described in the articles entitledĀ Analysis of the impact of storage conditions on the thermal recovery efficiency of low-temperature ATES systems and ATES systems in aquifers with high ambient groundwater flow velocity, recently published in the journalĀ Geothermics. This work was conducted by Martin Bloemendal and Theo Olsthoorn fromĀ Delft University of Technology and Niels Hartog from Utrecht University.

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