HT-ATES systems work as a pair of wells, known as a doublet. When excess renewable heat is available, groundwater is extracted from one well, warmed at the surface through a heat exchanger, and injected into the second well, the so-called hot well. During the heating season the direction reverses, and the stored warm water is pumped back up. Ideally, the hot water spreads as an even bubble through the aquifer, and most of it returns through the well screen when it is needed. In practice, however, heat is lost at the thermal front and at the confining layers above and below the aquifer, and buoyancy adds a further complication that has long been recognized: hot, lighter water tends to rise and tilt the thermal front upward during storage.

The new research, led by Stijn Beernink of KWR Water Research Institute and Delft University of Technology together with colleagues, goes a step further. The team realized that density differences do not only drive buoyant flow in the aquifer — they also alter the pressure distribution inside the well itself. When a column of 90-degree water stands next to a column of cool groundwater at roughly 12 degrees, the lighter column gains hydrostatic pressure more slowly with depth. That means the pressure difference between the well and the surrounding aquifer shrinks as you descend, and can even vanish at a certain depth, below which essentially no injection occurs at all.

To quantify the effect, the researchers built axisymmetric numerical simulations using the SEAWATv4 code, which couples groundwater flow with variable density and viscosity, and represented the well as a column of extremely high hydraulic conductivity so that water could distribute itself freely along the screen. They tested injection temperatures of 15, 30, 60 and 90 degrees Celsius in a 40-meter-thick aquifer, running a realistic seasonal cycle of 90 days of injection, storage, extraction and idle time with an annual storage volume of 100,000 cubic meters per well. They also developed analytical equations that describe the same physics, allowing the maximum depth of injection and the peak flow at the top of the screen to be predicted from simple operational parameters.

The results are striking. As injection temperature rises, the flow distribution becomes increasingly lopsided, with the highest inflow concentrated at the top of the well screen — in some scenarios reaching several times the average flow rate, and with essentially no water entering the deepest part of the screen at 90 degrees under low-pressure conditions. The degree of nonuniformity depends on the ratio of pumping rate to aquifer permeability, which controls the injection pressure head. When that ratio is small, the pressure available to push water into the aquifer is modest, the buoyancy term dominates, and hot water infiltrates only through the upper portion of the screen, sometimes just the top 40 to 50 percent.

Viscosity makes matters worse in a self-reinforcing way. Water at 90 degrees is nearly 75 percent less viscous than water at 12 degrees, so the first hot water to enter the top of the aquifer creates a zone of low hydraulic resistance precisely where flow is already strongest. More water rushes through that zone, which heats it further and thins the water even more. In the simulations including both variable density and viscosity, flows at the top of the screen reached up to 3.7 times the average rate. The lowered viscosity also reduces the pressure head needed to inject a given volume, which paradoxically deepens the nonuniformity because less pressure is available to overcome the buoyancy gradient along the screen.

Crucially, the researchers found that the flow distribution is fundamentally asymmetrical between injection and extraction. During injection, buoyancy can choke off flow entirely below the maximum depth of injection. During extraction, however, the pressure difference between the light water column in the well and the denser ambient groundwater grows with depth and can never reach zero, so water is always drawn in across the full length of the screen. The practical consequence is sobering: a well that only injected through its upper half must, during recovery, pull cold ambient groundwater from the bottom half, diluting the extracted heat and directly reducing the thermal recovery efficiency of the system.

The nonuniformity also carries a mechanical warning. Maximum flow velocity at the borehole wall is a critical design parameter for preventing clogging of the screen and gravel pack, and the study shows that assuming uniform flow can underestimate this velocity by a factor of up to 3.7. A well designed on the assumption of even flow distribution might therefore be far more vulnerable to clogging in practice than its designers believed. The team’s screening of published HT-ATES storage conditions, spanning eight earlier studies, indicates that the levels of nonuniformity they quantified overlap substantially with real-world operating conditions, making this a genuinely practical concern rather than a numerical curiosity.

There are also direct implications for how HT-ATES systems are modeled and designed. Many previous simulation studies forced a uniform flow distribution across the well screen as a simplifying assumption. The new work shows this assumption introduces errors of up to about 7 percent in predicted recovery efficiency, in either direction depending on the aquifer’s vertical permeability, and larger errors can be expected in strongly heterogeneous aquifers. Allowing flow to distribute freely, the authors argue, is the safer approach. Where conditions are unfavorable, design remedies exist: operating wells closer to their maximum flow rate, using larger-diameter wells, or deploying multiple partially penetrating screens to spread injection and extraction more evenly across the aquifer.

The findings extend beyond geothermal storage. Similar density-driven nonuniform flow arises in carbon dioxide injection and in aquifer storage and recovery of freshwater in brackish aquifers, where comparable density contrasts of 5 to 25 kilograms per cubic meter occur. The analytical tools developed here — the maximum depth of injection and the maximum normalized flow — could help engineers in those fields as well. As heat networks across Europe and beyond look to the subsurface for seasonal storage at temperatures up to and beyond 90 degrees, this study makes clear that gravity itself must be treated as a design constraint: ignore buoyancy at the well screen, and a significant share of the stored warmth may never come back.

Subject of Research: Buoyancy-induced nonuniform groundwater flow through well screens in high-temperature aquifer thermal energy storage

Article Title: How density and viscosity differences cause nonuniform flow distribution across well screens and impact heat recovery of HT-ATES systems

Article References: Beernink, S., Hartog, N., Vardon, P. J., & Bloemendal, M. (2026). How density and viscosity differences cause nonuniform flow distribution across well screens and impact heat recovery of HT-ATES systems. Hydrogeology Journal. https://doi.org/10.1007/s10040-026-03154-8

Image Credits: AI Generated

DOI: 10.1007/s10040-026-03154-8

Keywords: HT-ATES, aquifer thermal energy storage, well screen, buoyancy flow, groundwater density, viscosity, heat recovery, well hydraulics, thermal energy storage, geothermal energy, SEAWAT, clogging

The research centers on microencapsulated phase change materials, or mPCMs, tiny core-shell particles in which a substance that melts and freezes at a useful temperature is locked inside a polymer wall. Phase change materials absorb large amounts of latent heat as they melt and release that heat as they solidify, acting as thermal batteries that buffer temperature swings. The catch is that the most common PCMs are solid-liquid paraffins that leak when they melt, which is why encapsulation is essential. Microencapsulation solves the leakage problem, dramatically increases the surface area available for heat transfer, and allows the particles to be blended directly into concrete, coatings, textiles, or packaging without any special handling.

What makes the Turkish team’s work novel is the shell material. Most commercial and laboratory mPCMs rely on formaldehyde-based resins such as melamine-formaldehyde or urea-formaldehyde, which can release harmful substances during synthesis and use, or on polystyrene and PMMA, which are flammable. The researchers instead dissolved waste PVC, sourced from a commercial leather company, together with 1-tetradecane, a paraffin that melts at around 5 degrees Celsius, in tetrahydrofuran and used a solvent evaporation technique to precipitate PVC shells around droplets of the paraffin. It is the first time waste PVC has been used this way for a low-temperature, cold-storage PCM, and the choice is doubly clever because PVC is inherently flame retardant, with a limiting oxygen index between 40 and 45, far above the threshold of 26 that separates flame-retardant polymers from ordinary combustible ones.

The synthesis itself is a piece of practical process chemistry. Solution A contained 26 grams of sodium chloride and 9 grams of gelatin dissolved in deionized water, with the salt raising the ionic strength to keep the water from dissolving the THF and the gelatin acting as a natural, biodegradable surfactant that stabilizes the emulsion. Solution B held the waste PVC powder and 1-tetradecane in THF. When B was dripped into the vigorously stirred A, droplets formed whose size depended on how they were added: a Pasteur pipette produced large particles, a micropipette produced small ones. The emulsion was then heated to 56 degrees Celsius so the THF evaporated, leaving behind hardened capsules that were filtered, washed with water and ethanol, and dried. Remarkably, the final particle diameter tracked the initial droplet size, giving the team a simple dial for tuning capsule dimensions, something conventional in-situ polymerization cannot easily achieve.

Two particle populations emerged. The small-particle capsules, designated mPCM/SP, averaged 145 micrometers, while the large-particle capsules, mPCM/LP, averaged 612 micrometers. Both were unimodally distributed, a sign of a well-controlled process. Fourier transform infrared spectroscopy confirmed that the characteristic C-Cl stretching peak of PVC at 1750 per centimeter persisted in the capsules and that the CH2 stretching bands of the paraffin core remained intact, indicating that core and shell coexist without strong chemical interaction, exactly what a good encapsulation should deliver. Differential scanning calorimetry showed that the capsules behave isothermally like the pure paraffin, with melting temperatures of 5.0 and 4.7 degrees Celsius and latent heats of 126.7 and 136.9 joules per gram for the large and small particles respectively. Encapsulation ratios reached 66.5 percent for the large particles and 71.9 percent for the small ones, squarely within the 60 to 90 percent range typical of the best polymer-shelled paraffin microcapsules in the literature.

One subtlety the authors confront head-on is supercooling. The encapsulated paraffin froze at slightly lower temperatures than the bulk material, a well-documented consequence of the shell adding thermal resistance and constraining nucleation within the confined core. Crucially, however, the capsules still solidified above 1 degree Celsius, comfortably inside the operating window for cold-chain and anti-icing service, and the shift is partly an artifact of the relatively fast 5 degrees per minute DSC scanning rate, which is known to exaggerate apparent supercooling. Repeated cycling showed deviations of less than 1 degree Celsius in transition temperatures and about 1.2 percent in latent heat, confirming that the phase change process remains fully reversible and that encapsulation has not degraded the PCM’s intrinsic properties.

Durability testing was equally convincing. The capsules endured 1,000 accelerated thermal cycles between -20 and 30 degrees Celsius, the equivalent of roughly 15 years of daily freeze-thaw service, with DSC signals reproduced almost perfectly after every 100 cycles and FT-IR spectra after 1,000 cycles showing no chemical change. Leak tests were brutal in their simplicity: samples were frozen at -18 degrees Celsius for 12 hours, then baked at 50 degrees Celsius for another 12 hours on filter paper. No visible PCM leakage appeared, and weight losses were a negligible 0.05 to 0.08 percent. Thermogravimetric analysis showed the capsules degrade at higher temperatures than the free paraffin, meaning the PVC shell genuinely protects the core.

The flame retardancy results may be the study’s most distinctive contribution. Because standard limiting oxygen index tests require rectangular samples that microparticles cannot form, the team built a calibration curve by burning reference plastics of known LOI, including expanded polystyrene, polyacrylonitrile, PET, nylon 66, and ABS, and correlating burning times with literature values. Using the resulting equation, they estimated LOI values of 27.6 percent for mPCM/SP and 28 percent for mPCM/LP, above the flame-retardancy threshold and a dramatic improvement over pure 1-tetradecane, whose LOI of 17 means it burns in ordinary air. The mechanism is intrinsic: when PVC decomposes it releases hydrogen chloride gas, which suppresses combustion and promotes a protective char layer. Unlike conventional flame-retardant PCM systems that rely on added ammonium polyphosphate or expandable graphite, this fire resistance comes free with the recycled shell material itself.

The application vision spans two very different worlds. In the sub-5-degree melting range, the capsules are natural candidates for cold-chain packaging, refrigerated transport containers, and insulated panels, where they would absorb heat whenever temperatures rise and release it back as things cool, smoothing out the fluctuations that spoil food and vaccines. In civil infrastructure, mixed into concrete pavements, bitumen binders, or surface coatings, the latent heat released as the capsules freeze could slow the drop of road surface temperature, delay ice nucleation, and reduce ice adhesion, potentially cutting dependence on corrosive chloride deicers. Earlier work by Farnam and colleagues demonstrated the pavement concept with other PCMs, and the Turkish team’s flame-retardant, waste-derived capsules would bring an added safety margin to exactly that use case.

Beyond the immediate applications, the study is a template for what materials scientists call waste valorization: converting an environmental liability into a high-value functional material. PVC is notoriously difficult to recycle through conventional plastic streams because its chlorine content and rheology make it incompatible with aliphatic polyesters and polyolefins, so it is usually landfilled or incinerated. Here, that same chlorine chemistry becomes an asset, delivering flame retardancy that other shell polymers must buy with additives. The process also avoids the nastier solvents common in encapsulation chemistry, using halogen-free THF whose high vapor pressure makes recovery straightforward. As the authors conclude, the work validates a route from a persistent waste problem to thermally reliable, fire-safe, size-tunable microcapsules, and it broadens the reach of phase change material technology into the low-temperature regime where cold chains, winter roads, and a warming world’s cold-storage demands increasingly intersect.

Subject of Research: Flame-retardant microencapsulated phase change materials synthesized from waste poly(vinyl chloride) for low-temperature thermal energy storage and anti-icing applications

Article Title: Novel flame retardant microencapsulated phase change materials from waste poly(vinyl chloride) for maintaining cold and anti-icing applications

Article References: Alkan, C., & Alakara, E. H. (2026). Novel flame retardant microencapsulated phase change materials from waste poly(vinyl chloride) for maintaining cold and anti-icing applications. Journal of Materials Science: Polymers, 1(1), Article 16. https://doi.org/10.1007/s44493-026-00017-2

Image Credits: AI Generated

DOI: 10.1007/s44493-026-00017-2

Keywords: phase change materials, waste PVC recycling, microencapsulation, thermal energy storage, flame retardancy, 1-tetradecane, anti-icing, cold chain, latent heat, solvent evaporation, supercooling, sustainable materials