Wind-driven device for cooling permafrost

Permafrost warming threatens the stability and bearing capacity of polar and high-altitude infrastructures. Conventional passive cooling—ventilation ducts, crushed-rock embankments, and thermosyphons—relies on air convection and often fails to deliver sufficient cooling for high-demand applications such as expressways. Thermosyphons are sensitive to installation angle and air-gas systems suffer from the low specific heat of air. High-albedo pavements offer surface temperature reduction but face durability and UV aging issues. To overcome air-convection limits, the study proposes a wind-driven, liquid-based forced-convection device that automatically engages in cold weather and hibernates in warm weather, aiming to efficiently extract heat from permafrost strata while preventing summer heat ingress.

Prior approaches to preserve permafrost subgrades focus on enhancing convective heat loss using ambient cold air: ventilation ducts transfer heat from subgrade to cold air during cold seasons; crushed-rock revetments promote reverse convection cycles but can lose effectiveness if voids clog; thermosyphons remove heat via evaporation-condensation loops but require near-vertical installation, with performance degrading at low angles. High-albedo pavements were explored since the 1970s and revisited with synthetic materials, but coatings age under UV in 1–2 years. Hybrid passive techniques (e.g., crushed rock plus ducts or thermosyphons) partially improve performance but still rely on air convection limits. Electrically powered active cooling (e.g., vapor compression) has been proposed but raises durability and power-dependence concerns. A related windmill-driven air device (Zhu et al.) uses forced air convection in open conduits, differing from the present sealed liquid-coolant concept.

Device concept: A wind-driven, sealed-loop, liquid-circulation heat exchanger coupled to a temperature-activated mechanical clutch using a phase change material (PCM). In cold air, PCM solidifies to engage the clutch and transmit wind power to a pump via non-contact magnetic coupling; in warm air, PCM melts to disengage the clutch, stopping circulation to prevent heat ingress. Components and materials: Windmill with 3 blades, 0.8 m diameter, starts at wind speed >1.8 m/s; no-load rotational speed ~1200 r/min at 6 m/s. Mechanical clutch PCM: 2% ethanol in deionized water, freezing at −4 °C. Sealed, non-contact magnetic gear couples windmill shaft to pump shaft to avoid leakage. Fluid-circulation heat exchanger: double-tube with inner PVC tube (low thermal conductivity) and outer Q235 steel tube (enhanced heat transfer). Geometry and properties (Table 1): inner tube thickness 3 mm, diameter 33 mm, density 1380 kg/m³, Cp 900 J/g/K, k 0.14 W/(m·K); outer tube thickness 4 mm, diameter 88 mm, density 7850 kg/m³, Cp 470 J/g/K, k 52.34 W/(m·K); coolant density 1058 kg/m³, Cp 3121 (J/g/K as reported), k 0.34 W/(m·K), phase-change temperature −45 °C (for coolant). Cross-sectional areas sized so inner tube velocity is higher (fast return), annulus flow is slower (longer residence time for soil-to-coolant heat transfer). Pump discharge rate ~200 mL/s at 10 m/s wind. Operation principle: Coolant descends along inner PVC tube, exits via bottom openings, and ascends through the outer annulus exchanging heat with surrounding soils; heat is ultimately rejected to cold ambient air at the exposed upper outer tube via forced convection when engaged. Engagement detection uses separation of temperature profiles between inner tube and outer annulus; power computed as q = c ρ v ΔT. Field site and installation: Beiluhe Permafrost Station, Qinghai-Tibet Plateau, China (34.8° N, 92.9° E). Stratigraphy: 0–1.0 m loose silt clay, 1.0–2.0 m silt clay, 2.0–8.0 m icy clay, underlain by weathered mudstone; permafrost table ~2.0 m; mean annual ground temperature −1.0 to −0.5 °C at 10–15 m. A 12 cm borehole was drilled to 8.1 m; device centered and overhung so lower end at 8.0 m depth; annulus backfilled with dry fine sand. Four additional instrumented boreholes at 0.6, 1.5, 3.0, and 5.7 m radii with PVC tubes and thermistors, backfilled with dry fine sand. Instrumentation and data: Inside exchanger, thermometers at 1.0 m intervals for both inner pipe and outer annulus along a 10 m height (total 20). External skin thermometers mounted at 0.5–2.0 m intervals. Sensor precision 0.1 °C. Data logged at 60 min intervals with Campbell CR3000 and AM16/32B multiplexer; logging from 09/01/2020 to 09/01/2022 with remote transfer. A low-reflective coating was applied to mitigate solar effects on PCM. Engagement/hibernation controlled by ambient temperature relative to PCM freezing point (−4 °C).

• The device operated effectively in the field for over two years, with clear daily and seasonal engagement patterns inferred from inner–outer tube temperature separation.
• Cooling extent: The surrounding soil column up to a 1.5 m radius and 8.0 m depth was effectively cooled by 0.6–1.0 °C over two years. At larger radii (3.0 and 5.7 m), the device impact was minimal.
• Temporal evolution: On 10/15/2020 (1.5 months after installation), the cooled radius (−1.0 °C isotherm) was ~0.5 m. By 01/15/2021 it expanded to ~1.2 m; by 04/15/2021 reached ~1.5 m. In 2022, the −1.0 °C isotherm extended further, though the rate of outward expansion diminished.
• Power performance: Peak instantaneous power reached ~300 W; annualized average power was 68.03 W. Operational metrics depend on PCM freezing temperature, pump parameters, and ambient temperatures.
• Operational window: Predominantly mid-October to mid-April; device fully disengaged early July to early October. In 2021, annual operational duration was 105.15 days (~28.8% of the year).
• Thermal behavior: Inner tube temperatures showed negligible along-depth variation, indicating minimal energy loss during the downward flow (PVC effectiveness). Outer annulus temperatures increased from bottom upward to ~1.0 m depth due to heat uptake. Skin temperatures near the device (r = 0.04 m) exhibited larger winter variability corresponding to coolant circulation.
• Ground conditions: Despite substantial cooling between 2–8 m depth, the permafrost table position did not change, mitigating concerns of shallow ground frost heave.
• Comparative performance: The device’s annual cooling power (~68 W) exceeds reported active thermosyphon instantaneous power (<25 W) and expected annual output (<25 W), highlighting the efficiency of liquid forced convection over air/vapor systems.

The findings demonstrate that a wind-driven, liquid-based forced-convection system can overcome the low heat capacity limitations of air-based passive cooling, achieving substantive and controlled heat extraction from permafrost strata while avoiding summer heat ingress via automatic hibernation. The observed 0.6–1.0 °C soil cooling within a 1.5 m radius to 8.0 m depth over two years corroborates the device’s efficacy and targeted influence. Operational analytics (engagement seasonality and duration) align with local air temperature and wind regimes, validating the PCM-activated clutch strategy. Compared with thermosyphons, the device offers higher heat transfer efficiency, remote operability verification via Hall-effect sensor and magnetic floater, and flexible deployment geometries including separable hot/cold ends and adaptable tube configurations. Maintaining the permafrost table while cooling deeper ground suggests potential to stabilize subgrades without inducing frost heave. The diminishing rate of radial expansion in year two indicates approaching thermal equilibration in the affected soil volume and highlights the importance of design optimization (PCM setpoint, exchanger geometry, pump characteristics) to match site-specific climate and infrastructure needs.

This study introduces and field-validates a wind-driven, PCM-clutched, sealed liquid-circulation device for permafrost cooling. Over two years at the Qinghai-Tibet Plateau, the device cooled a cylindrical soil volume (radius up to 1.5 m, depth 8.0 m) by 0.6–1.0 °C with an average annualized power of 68.03 W and peak instantaneous power ~300 W. The system autonomously engages in cold weather and hibernates in warm periods, preventing reverse heat flow. It demonstrates advantages over air-based passive systems and thermosyphons in efficiency, monitoring, and deployment flexibility, with potential applications to embankments, pile foundations, airstrips, and pipelines. Future work should assess long-term durability (30–50 years), optimize PCM setpoints and exchanger geometry (length, radius), refine pump design for high discharge at low RPM, simplify magnetic coupling, conduct multi-region tests across diverse wind and temperature regimes, and perform full-device numerical simulations and scale-up studies for varied infrastructures.

• Mechanical complexity: Two magnetic couplings and multiple bearings may affect long-term reliability; simplification to a single magnetic coupling is proposed.
• Pump durability: Potential impeller fatigue at high rotational speeds over extended operation; design optimization toward high discharge at lower RPM is needed.
• Operational sensitivity: Performance depends on PCM freezing temperature, pump parameters, and ambient wind/temperature conditions; suboptimal PCM setpoints can inadvertently warm already-cooled zones.
• Limited field duration and scope: Two-year single-site test; broader validation across permafrost regions with different wind and thermal regimes and extended (30–50 year) durability testing are required.
• Cooling extent: Effective radius observed to ~1.5 m, with diminishing expansion rate in year two; scaling strategies and exchanger configurations need evaluation to cover wider subgrade areas.