Advective Heat Transport Research Articles

Relative contribution of the ocean-air heat exchange and advective heat transport to the increase of the Barents Sea water temperature in the early 21st century

The annual water temperature in the major water masses of the Barents Sea (BS) has significantly increased since the early 2000s. Advective heat transport from the neighboring water areas and heat exchange through the sea surface are the major factors, which shape the hydrological conditions in the BS. The paper estimates the contributions of heat exchange at the sea-atmosphere boundary and advective heat transport to changes in the average water temperature of the BS for the entire sea area. The average annual heat balance of the BS is calculated using atmospheric and oceanic reanalysis data. The change in the average temperature of the BS water is estimated taking into account the heat consumption for ice melting. The average surface heat balance from 1993 to 2018 was negative throughout the entire sea area: –70…–100 W/m2 in the south and –10…–20 W/m2 in the north. The advective heat supply was calculated for 9 straits with neighboring water areas. The determining source of advective heat is the influx of Atlantic waters from the Norwegian Sea between Cape Nordkapp and Bear Island. An average of 40.8 TW of advective heat is supplied through this margin. The calculations showed the predominance of annual heat influx due to advection over heat loss from the sea surface. This excess heat influx resulted in an estimated increase in the water temperature of the BS from 1993 to 2018 at a rate of 0.28 °C per year (taking into account the heat consumption for ice melting). In conclusion, it can be argued that the analysis has validated the hypothesis proposed in the article about compensation of heat losses from the surface of the BS by advective heat flow. The hypothesis is quantitatively confirmed by calculations on a simple box model (with an accuracy of up to an order of magnitude) based on atmospheric and oceanic reanalysis data. The ERA5 and GLORYS12V1 reanalysis data reliably describe the basic patterns of observed variability of ocean, sea ice and atmospheric parameters in the Barents Sea.

Cold seep formation from salt diapir–controlled deep biosphere oases

Deep sea cold seeps are sites where hydrogen sulfide, methane, and other hydrocarbon-rich fluids vent from the ocean floor. They are an important component of Earth's carbon cycle in which subsurface hydrocarbons form the energy source for highly diverse benthic micro- and macro-fauna in what is otherwise vast and spartan sea scape. Passive continental margin cold seeps are typically attributed to the migration of hydrocarbons generated from deeply buried source rocks. Many of these seeps occur over salt tectonic provinces, where the movement of salt generates complex fault systems that can enable fluid migration or create seals and traps associated with reservoir formation. The elevated advective heat transport of the salt also produces a chimney effect directly over these structures. Here, we provide geophysical and geochemical evidence that the salt chimney effect in conjunction with diapiric faulting drives a subsurface groundwater circulation system that brings dissolved inorganic carbon, nutrient-rich deep basinal fluids, and potentially overlying seawater onto the crests of deeply buried salt diapirs. The mobilized fluids fuel methanogenic archaea locally enhancing the deep biosphere. The resulting elevated biogenic methane production, alongside the upward heat-driven fluid transport, represents a previously unrecognized mechanism of cold seep formation and regulation.

The role of short‐term temperature variability and light in shaping the phenology and characteristics of seagrass beds

AbstractSeagrass beds inhabit highly heterogeneous temperature regimes that characterize the marine nearshore. Temperature directly influences seagrasses and also provides indirect information on other ecologically relevant environmental variables. Multiple temperature processes operate on seasonal and sub‐seasonal timescales (i.e., hours to months) and include variation from seasonal air–sea heat fluxes, advective heat transport from upwelling and tidal circulation, and daily heating and cooling of shallow waters. Despite this, seagrass–temperature studies typically only examine a single isolated temperature process, often seasonal heating/cooling or marine heatwaves. Furthermore, elucidation of relationships between different short‐term temperature processes and seagrass metrics could provide insights into biologically relevant temperature metrics for seagrasses. Here, we examine the effects of multiple short‐term temperature processes on Zostera marina beds in Atlantic Canada, by describing the seasonal phenology of bed characteristics, plant morphology, and physiology across different temperature regimes and by identifying relationships between different temperature and seagrass metrics. We also include water depth as a proxy for light availability. Four distinct short‐term temperature processes (median temperature, growing degree day [heat accumulation], daily temperature range, and time in the optimal temperature range [5–23°C]) were used to categorize temperature regimes across our study sites as warm and highly variable, or cool and less variable. We found that both temperature regime and light availability were important for leaf area index (LAI) and shoot density, which both decreased with increasing depth yet had the strongest seasonal differences and maximum rates of increase (shoot density) or lowest values and maximum rates of decrease (LAI) in warm and highly variable temperature regimes. These temperature regimes were also associated with seasonal patterns in number of leaves, reduced leaf lengths and number of leaves per shoot, and thinner rhizomes with less carbohydrate storage. Generalized additive models (GAMs) for each seagrass metric using the four temperature processes indicated that while median temperature displayed expected relationships with almost every seagrass metric, inclusion of other temperature processes revealed additional insights not evident from median temperature alone. Our study shows that different sources of short‐term temperature variation influence seagrass properties and that accounting for these will improve our understanding of the temperature–seagrass interaction.

Leading role of outer-Arctic circulation transport in AMOC response to global warming over a century

Using the Alfred Wegener Institute Climate Model (AWI-CM 1.1 LR), we explored how Arctic and extra-Arctic warming affect the response of Atlantic meridional overturning circulation (AMOC) to quadruple carbon dioxide (4 × CO2) forcing. The results suggest that AMOC weakening is mainly affected by circulation adjustment caused by extra-Arctic warming, while Arctic warming has a limited local impact and a relatively small contribution to AMOC weakening. Due to the warming outside the Arctic, the increase in northward advective heat transport dominates the weakening of deep convection in Nordic Seas. While in the Labrador Sea, the decrease in advection heat transport is compensated by a more significant decrease in ocean heat loss to the atmosphere, leading to an enhancement of the upper ocean stratification. Besides, the weakening of deep convection associated with AMOC response under global warming is more pronounced in Nordic Seas than in Labrador Sea.

The influence of permeability anisotropy in the upper ocean crust on advective heat transport by a ridge-flank hydrothermal system

In this study, we highlight the importance of permeability anisotropy on the hydrogeological regime of a ridge-flank hydrothermal system. Our study site, North Pond, is a marine sediment pond on ∼8 Ma seafloor in the North Atlantic, and represents a low-temperature, end-member ridge-flank hydrothermal system. Previous simulations of North Pond elucidated long-standing hypotheses concerning hydrothermal fluid and heat transport in the upper volcanic crust but failed to fully explain observed patterns of seafloor heat flux in this area. Here we use variography, a geostatistical method, to quantify relations between seafloor heat-flux measurements, and coupled numerical simulations of fluid and heat flow to simulate the hydrogeologic regime. Directional variography shows that heat-flux observations are correlated along-strike of the regional crustal fabric. Three-dimensional simulations that include permeability anisotropy are able to replicate seafloor heat-flux patterns across North Pond. The simulations that result in the best match to thermal data incorporate permeability anisotropy in the horizontal plane. We find that the feedback between permeability anisotropy and the asymmetric geometry of North Pond combine to promote advective removal of heat and mass within the crustal aquifer. These findings suggest that permeability anisotropy in the oceanic crust may influence ridge-flank hydrothermal circulation more broadly.

Influences of Oceanic Processes Between the Indian and Pacific Basins on the Eastward Propagation of MJO Events Crossing the Maritime Continent

AbstractTropical intraseasonal Madden‐Julian oscillation (MJO) is one of the dominant products of ocean‐atmosphere interactions. The relationship between its eastward propagation and atmospheric boundary layer processes has been emphasized. Using the Climate Forecast System Reanalysis (CFSR) daily data, the eastward propagating MJO events are divided into the MJO_C events which pass through the Maritime Continent (MC) and MJO_N that cannot. In further research, the path from Lombok Strait to Timor Sea across the south of MC is identified as the critical movement path of MJO (MJO_SC) events. The differences in the oceanic processes and lower atmospheric fields are then revealed between the MJO_SC and MJO_N events along this critical path. The MJO_SC (MJO_N) events triggered in the eastern (western) side of the tropical Indian Ocean lead to the thermocline deepening (shallowing) and warm (cold) upper ocean in the Lombok Strait‐Timor sea. Subsequently, MJO_N events propagate northeastward over the land of MC and die out. However, MJO_SC events are pulled toward the Timor Sea. During the eastward propagation of the MJO_SC events, an eastward anomalous subsurface warm current is discovered in the Timor Sea, which is prior to the atmospheric convection center. This advective heat transport in the upper ocean dominates the sea surface warming under the MJO atmospheric convection center and its eastern flank. The followed outward turbulent heat flux and vapor cause an increase in moisture and instability in the lower atmosphere east of the MJO convection center, which promotes the growth and continued eastward propagation of the MJO.

Unevenly spatiotemporal distribution of urban excess warming in coastal Shanghai megacity, China: Roles of geophysical environment, ventilation and sea breezes

The complexity and uncertainty of urban excess warming are modulated by the morphology of urban areas, canopy urban heat island (CUHI)–heat wave interactions, the geographical–climatological background and the local circulation. Focusing on the coastal Shanghai megacity, we analyzed the synergistic effects of local climate zones (LCZs), urban ventilation, and sea breezes on the urban excess warming related to the CUHI and heat waves in summer from 2013 to 2018. Over the whole urban areas, the average CUHI intensity (CUHII) increased by 128.91% during heat waves periods relative to non-heat wave periods. Moreover, the spatial-temporal distributions of both CUHII and heat waves exhibited heterogeneously. In dense urban areas (e.g., LCZ 1 and 2), the blocking effect of dense high-rise buildings, and poor ventilation with low wind speeds would exacerbate urban excess warming, which exhibited relatively higher occurrence of heat waves and an increase in the CUHII. By contrast, over open areas of the urban periphery (eg., LCZ5), more heat wave events and stronger CUHII were mainly affected by the horizontal advective transport of urban heat at medium wind speeds. Particularly, the sea breezes could weaken the CUHII and decrease the occurrence frequency of heat wave events during daytime over coastal areas. In addition, the shading effects of high-rise buildings and the evaporative cooling effects of water and vegetation would also mitigate urban warming at the local scale, which would reduce the CUHII as well as the frequency of heat waves.

Heat flow and thermal regime in the Guaymas Basin, Gulf of California: Estimates of conductive and advective heat transport

AbstractHeat flow is estimated at eight sites drilled int the Guaymas Basin, Gulf of California, during the International Ocean Discovery Program Expedition 385. The expedition sought to understand the thermal regime of the basin and heat transfer between off‐axis sills intruding the organic‐rich sediments of the Guaymas Basin, and the basin floor. The distinct sedimentation rates, active tectonics, and magmatism make the basin interesting for scientific discoveries. Results show that sedimentation corrected heat flow values range 119–221 mW/m2 in the basin and 257–1003 mW/m2 at the site of a young sill intrusion, denominated Ringvent. Thermal analysis shows that heat in the Guaymas Basin is being dissipated by conduction for plate ages >0.2 Ma, whereas younger plate ages are in a state of transient cooling by both conduction and advection. Drilling sites show that Ringvent is an active sill being cooled down slowly by circulating fluids with discharge velocities of 10–200 mm/yr. Possible recharge sites are located ca. 1 km away from the sill's border. Modelling of the heat output at Ringvent indicates a sill thickness of ca. 240 m. A simple order‐of‐magnitude model predicts that relatively small amounts of magma are needed to account for the elevated heat flow in non‐volcanic, sediment‐filled rifts like the central and northern Gulf of California in which heating of the upper crust is achieved via advection by sill emplacement and hydrothermal circulation. Multiple timescales of cooling control the crustal, chemical and biological evolution of the Guaymas Basin. Here, we recognize at least four timescales: the time interval between intrusions (ca. 103 yr), the thermal relaxation time of sills (ca. 104 yr), the characteristic cooling time of the sediments (ca. 105 yr), and the cooling of the entire crust at geologic timescales.

How temperatures derived from fluid flow and heat transport models impact predictions of deep geothermal potentials: the “heat in place” method applied to Hesse (Germany)

One key aspect in the energy transition is to use the deep geothermal energy stored in sedimentary basins as well as in igneous and metamorphic basement rocks. To estimate the variability of deep geothermal potentials across different geological domains as encountered in the Federal State of Hesse (Germany), it is necessary to understand the driving processes of fluid flow and heat transport affecting subsurface temperature variations. In this study, we quantify the stored energy in a set of geological units in the subsurface of Hesse with the method of “heat in place” (HIP, sensu Muffler and Cataldi in Geothermics 7:53–89, 1978)—HIP is one proxy for the geothermal potential of these units controlled by their temperature configuration as derived from a series of coupled 3D thermo-hydraulic numerical models. We show how conductive, advective and convective heat transport mechanisms influence the thermal field and thereby the HIP calculations. The heterogeneous geology of the subsurface of Hesse ranges from locally outcropping Paleozoic basement rocks to up to 3.8 km thick Cenozoic, porous sedimentary deposits in the tectonically active northern Upper Rhine Graben. The HIP was quantified for five sedimentary layers (Cenozoic, Muschelkalk, Buntsandstein, Zechstein, Rotliegend) as well as for the underlying basement. We present a set of maps allowing to identify geothermally prospective subregions of Hesse based on the laterally varying thermal energy stored within the units. HIP is predicted to be highest in the area of the northern Upper Rhine Graben in the Cenozoic unit with up to 700 GJ text {m}^{-2} and in the Rotliegend with up to 617 GJ text {m}^{-2}. The calculations account for the variable thicknesses and temperatures of the layers, density and heat capacity of the solid and fluid parts of the rocks as well as porosity.

Long‐Term, High‐Resolution Permafrost Monitoring Reveals Coupled Energy Balance and Hydrogeologic Controls on Talik Dynamics Near Umiujaq (Nunavik, Québec, Canada)

AbstractRising temperatures in the Arctic and subarctic are driving the rapid thaw of permafrost by reducing permafrost cooling, increasing active layer thickness, and promoting talik formation. In this study, the cyrohydrogeology of a permafrost mound located within the discontinuous permafrost zone near Umiujaq (Nunavik, Québec, Canada) is characterized through the analysis of a dataset covering more than two decades of monitoring. This dataset captures a high degree of interannual variability in air temperature and ground thermal conditions, as well as the formation and closure of a supra‐permafrost talik. Data indicate that variable saturation and advective heat transport directly contribute to the expansion and contraction of the talik. Data further indicate the presence of two distinct thermo‐hydrologic settings resulting from differences in surface conditions, as well as subsurface thermal and flow regimes. The first, found at the top of the mound feature, is characterized by very low moisture contents (<0.05 m3/m3), while the second, found at the side of the mound feature, shows higher annual moisture contents that strongly influence the dynamics of heat and groundwater flow. The data were synthesized into a detailed conceptual model of the cyrohydrogeological dynamics that highlights the important role of hydrogeological characterization and long‐term data sets in understanding the effects of groundwater flow on seasonal frost and permafrost dynamics. Specifically, the results presented here show that in the absence of long‐term data sets, longer‐period transient phenomena such as talik opening and closure may be misrepresented as uni‐directional feedback loops, as opposed to highly dynamic temporary phenomena.

Heating or Cooling: Study of Advective Heat Transport in the Inflow and the Outflow of Optically Thin Advection-dominated Accretion Flows

Advection is believed to be the dominant cooling mechanism in optically thin advection-dominated accretion flows (ADAFs). When outflow is considered, however, the first impression is that advection should be of opposite sign in the inflow and the outflow, due to the opposite direction of radial motion. Then how is the energy balance achieved simultaneously? We investigate the problem in this paper, analyzing the profiles of different components of advection with self-similar solutions of ADAFs in spherical coordinates (r θ ϕ). We find that for n < 3γ/2 − 1, where n is the density index in ρ ∝ r −n and γ is the heat capacity ratio, the radial advection is a heating mechanism in the inflow and a cooling mechanism in the outflow. It becomes 0 for n = 3γ/2 − 1, and turns to a cooling mechanism in the inflow and a heating mechanism in the outflow for n > 3γ/2 − 1. The energy conservation is only achieved when the latitudinal (θ direction) advection is considered, which takes an appropriate value to maintain energy balance, so that the overall effect of advection, no matter the parameter choices, is always a cooling mechanism that cancels out the viscous heating everywhere. For the extreme case of n = 3/2, latitudinal motion stops, viscous heating is balanced solely by radial advection, and no outflow is developed.

Impacts of River Discharge on the Sea Temperature in Changjiang Estuary and Its Adjacent Sea

Freshwater plume at the Changjiang River (CR) mouth are essential to the coastal water quality and ecosystem because they can cause estuary stratification and hypoxia, potentially deteriorating the water environment. Furthermore, the advection heat transport is modulated by increasing anthropogenic effects. A comprehensive understanding of the influence of river discharge on the three-dimensional sea temperature, fronts and thermal stratification in the CR estuary remains lacking. A well-calibrated numerical model using Regional Ocean Modeling Systems (ROMS) is used to investigate the impacts of CR discharge on the sea temperature in coastal zones. Model results show that the amplitude and spatial distribution of the heating or cooling rate can be influenced by CR freshwater, especially in frontal areas. Specifically, the large runoff flow will reduce the heating or cooling rate in shallow waters (&lt;20 m) near the CR estuary, whereas it has an opposite effect on the Zhoushan islands region (&gt;20 m). Generally, the effect of the freshwater discharge on the upper layer is greater than on the bottom layer, and the runoff has a positive correlation to the intensity of the frontal zones in the CR estuary, though this relationship is weakened in autumn because of the weak intensity of the frontal zone. Note that seawater thermal stratification and its seasonal variation can be regulated by runoff; thermal stratification will be strengthened in abundant runoff conditions and weakened in scarce runoff conditions.

Groundwater heat transfer and thermal outflow plume modelling in the Alps

Thermal water discharge within the gravity-driven groundwater flow system in the Alps and other similar areas around the world may be hidden in Quaternary deposits which, in these regions, often cover the regional aquifer. When thermal water drains into Quaternary deposits, the mixing of the deep thermal component and the cold shallow groundwater forms a thermal plume that extends parallel to the main groundwater flow in shallow system. In the Bled case study in Slovenia, the thermal water discharges from carbonate rocks into Quaternary glaciofluvial sediments, and as the Toplice spring at a rate of 5 l/s at an average temperature of 21.5 °C. Knowing the spatial extent and intensity of thermal outflow is essential to decision making related to the development and protection of this renewable resource. By approximating the thermal water outflow from a discharge zone as a planar source, a planar advective heat transport model can be used to evaluate its geometry and quantify rates. An analytical procedure follows rough assumptions leading to conservative results. Moreover, a numerical model using the FEFLOW code was applied for comparison with the simulations of the analytical model. The heat transport model was based on measured hydraulic parameters (e.g. groundwater levels) and borehole temperatures as well as on-site and international literature (e.g. dispersivity, thermal conductivity). Nine scenarios were applied accounting for different dimensions of the heat source and compared to the results of numerical simulations. Each scenario was verified by calculating the relative error between the analytical models and the measured borehole temperatures. The results confirm that the main outflow of thermal water can be determined using planar geometry, and is 200–300 m wide. The height of the thermal outflow zone is approximately 25 m, corresponding to the expected thickness of the direct contact between the fractured dolomite and the shallower Quaternary aquifer. Using the proposed widths and depths, the hidden natural thermal outflow rates are estimated at 57–86 l/s.

Graphitization during high-grade metamorphism in the southern Bohemian Massif

Raman spectroscopy on carbonaceous matter reconstructs the progressive path of graphitization during granulite facies metamorphism in the Variscan collisional belt of the southern Bohemian Massif. In this area, heating to 650–750 °C gradually altered the graphite microstructure, attributed to a metamorphic field gradient. Late low-pressure and high-temperature metamorphism at temperatures above 800 °C as well as advective heat transport due to rising granulite diapirs improved the graphite lattice ordering significantly. Numerical parameters derived from the Raman spectra of carbonaceous matter, in particular the G-band width and the S2-band dispersion map metamorphic field gradients in high-grade metamorphic terrains.

New insights into the drainage of inundated ice-wedge polygons using fundamental hydrologic principles

Abstract. The pathways and timing of drainage from the inundated centers of ice-wedge polygons in a warming climate have important implications for carbon flushing, advective heat transport, and transitions from methane to carbon dioxide dominated emissions. Here, we expand on previous research using a recently developed analytical model of drainage from a low-centered polygon. Specifically, we perform (1) a calibration to field data identifying necessary model refinements and (2) a rigorous model sensitivity analysis that expands on previously published indications of polygon drainage characteristics. This research provides intuition on inundated polygon drainage by presenting the first in-depth analysis of drainage within a polygon based on hydrogeological first principles. We verify a recently developed analytical solution of polygon drainage through a calibration to a season of field measurements. Due to the parsimony of the model, providing the potential that it could fail, we identify the minimum necessary refinements that allow the model to match water levels measured in a low-centered polygon. We find that (1) the measured precipitation must be increased by a factor of around 2.2, and (2) the vertical soil hydraulic conductivity must decrease with increasing thaw depth. Model refinement (1) accounts for runoff from rims into the ice-wedge polygon pond during precipitation events and possible rain gauge undercatch, while refinement (2) accounts for the decreasing permeability of deeper soil layers. The calibration to field measurements supports the validity of the model, indicating that it is able to represent ice-wedge polygon drainage dynamics. We then use the analytical solution in non-dimensional form to provide a baseline for the effects of polygon aspect ratios (radius to thaw depth) and coefficient of hydraulic conductivity anisotropy (horizontal to vertical hydraulic conductivity) on drainage pathways and temporal depletion of ponded water from inundated ice-wedge polygon centers. By varying the polygon aspect ratio, we evaluate the relative effect of polygon size (width), inter-annual increases in active-layer thickness, and seasonal increases in thaw depth on drainage. The results of our sensitivity analysis rigorously confirm a previous analysis indicating that most drainage through the active layer occurs along an annular region of the polygon center near the rims. This has important implications for transport of nutrients (such as dissolved organic carbon) and advection of heat towards ice-wedge tops. We also provide a comprehensive investigation of the effect of polygon aspect ratio and anisotropy on drainage timing and patterns, expanding on previously published research. Our results indicate that polygons with large aspect ratios and high anisotropy will have the most distributed drainage, while polygons with large aspect ratios and low anisotropy will have their drainage most focused near their periphery and will drain most slowly. Polygons with small aspect ratios and high anisotropy will drain most quickly. These results, based on parametric investigation of idealized scenarios, provide a baseline for further research considering the geometric and hydraulic complexities of ice-wedge polygons.

Riverbed Temperature and Heat Transport in a Hydropeaked River

AbstractHydropeaking, the alternating storage and release of water from reservoirs for hydropower generation, perturbs the thermal regime of many large rivers. While its effects on river temperature have been long studied, impacts on the thermal regime of riverbeds remain mostly unknown, despite riverbed temperature affecting rates of nutrient cycling and habitat suitability for benthic organisms. This study combines detailed field observations and flow and heat transport modeling to assess the impact of hydropeaking on riverbed temperatures in a large regulated river. The field site was 12 km downstream from a dam that induces large daily flow variations. Vertical thermistor arrays were used to collect riverbed temperature data across the entire 70 m‐wide channel. The riverbed near the left bank was highly dynamic thermally, transitioning between river and groundwater temperatures over daily hydropeaking cycles. In contrast, the rest of the riverbed, including near the right bank, was similar in temperature to the river and had relatively stable temperatures. Modeling showed that the temperatures near the banks are explained by advective heat transport driven by hydrostatic changes in river level, while the temperatures over the rest of the channel can be explained mostly by conductive heating. Gaining groundwater conditions and high sediment hydraulic conductivity favor thermally dynamic zones near banks, while low hydraulic conductivity (below 1 m/d) and neutral or losing groundwater conditions result in muted temperature fluctuations, as observed at the right bank. These spatial patterns can help predict thermally sensitive processes in the riverbeds of hydropeaked or flooding rivers.

Much faster heat/mass than momentum transport in rotating Couette flows

Abstract

A Model of Ice Wedge Polygon Drainage in Changing Arctic Terrain

As ice wedge degradation and the inundation of polygonal troughs become increasingly common processes across the Arctic, lateral export of water from polygonal soils may represent an important mechanism for the mobilization of dissolved organic carbon and other solutes. However, drainage from ice wedge polygons is poorly understood. We constructed a model which uses cross-sectional flow nets to define flow paths of meltwater through the active layer of an inundated low-centered polygon towards the trough. The model includes the effects of evaporation and simulates the depletion of ponded water in the polygon center during the thaw season. In most simulations, we discovered a strong hydrodynamic edge effect: only a small fraction of the polygon volume near the rim area is flushed by the drainage at relatively high velocities, suggesting that nearly all advective transport of solutes, heat, and soil particles is confined to this zone. Estimates of characteristic drainage times from the polygon center are consistent with published field observations.

Experimental assessment of a standing column well performance in cold climates

Standing column wells can provide energy savings at lower first-costs than conventional vertical ground heat exchangers while having a higher potential in dense urban areas. Unfortunately, operating these open wells with groundwater near the freezing point has limited so far their use in northern climates and studies illustrating their successful operation in heating mode are limited. The objective of this study is to provide insights on the various operating conditions affecting the performance of heat pumps linked to standing column wells and demonstrate their potential in cold climates. This work relies on a major research infrastructure designed to operate water-to-air heat pumps connected to a standing column well and its companion injection well under realistic dynamic heating and cooling conditions. During its first operating year, the laboratory was operated continously in heating mode for 26 days. Results show that combined use of a plate heat exchanger and heat pumps allows heat extraction from the ground at significant rates (between 120 W/m and 160 W/m), while keeping the groundwater temperature above 0 °C during peak heating periods. This is approximately two times more than typical values reported for conventional closed loop borehole heat exchangers. Such efficiency was possible owing to the bleed control used, which allows transferring to the injection well part of the groundwater pumped and thus promotes advective heat transport towards the standing column well. Our measurements indicate that bleed was required only 30% of the time and represented 4.6 m3 of groundwater per day on average. These results should dimiss doubts raised in the literature and demonstrate the potential usability of SCWs for cold climates.

Indications of risks in geothermal systems caused by changes in pore structure and mechanical properties of granite: an experimental study

The effects of thermal treatment on the pore structure and mechanical property evolution of granite were experimentally investigated to evaluate the risks of traditional and critical geothermal systems. The microstructure of thermally treated granite was studied using an optical microscope and nuclear magnetic resonance, and the mechanical properties of thermally treated granite were studied under uniaxial compression. X-ray diffraction results showed that the mineralogical phase of granite hardly changed during thermal treatment from room temperature to 800 °C, but the granite experienced first a decrease and then an increase in porosity. Although overall porosity decreased at 200 °C, the proportion of pore throat radium beyond 6.3 μm grew increasingly with the treatment temperature, facilitating advective heat transport in the geothermal system. Intergranular microcracks caused by thermal anisotropic properties started at below 400 °C, and transgranular microcracks first appeared in feldspar at around 400 °C, finally extending to quartz at around 600 °C. The granite was strengthened due to decreased porosity at around 200 °C. Then, the granite was weakened and the failure transited from brittle to ductile due to increased porosity, where a sharp reduction occurred at 400–600 °C for α-β transition of quartz. The change in strength by cyclic heating under 200 °C was unnoticeable for temperature memory effect, but the cumulative strain at peak stress increased with the number of heating cycles. The changes in the pore and mechanical properties caused by thermal treatment favour geothermal systems which are not within the tectonic zone. However, increased porosity and strength weakening destabilize nearby faults due to normal stress release and strength weakening.