Ground Temperature Changes Research Articles

A variable water flow rate control method of hybrid geothermal heat pump systems

In the long-term operation of a geothermal heat pump system, when the cooling load is high, the temperature rise in the ground due to heat accumulation occurs. Changes in ground temperature hinder recovery of the temperature on the heat source side of the heat pump, and the performance of the geothermal system deteriorates because the heat exchange efficiency of the circulating water decreases. This problem can be solved by operating a hybrid geothermal system composed of an additional heat source system to the geothermal system. Therefore, in this paper, a method for operating a hybrid geothermal heat pump system was proposed to solve the degradation of system performance due to the temperature rise in the ground due to heat accumulation. The operating conditions for starting the operation of the hybrid geothermal heat pump system were derived, and the coefficient of performance prediction model was proposed for flow rate control considering the energy of the pump and the heat pump system. In addition, the proposed control method was evaluated for energy saving and performance improvement through simulation, and was also evaluated in terms of economic feasibility.

A predictive model of long-term performance assessment of Ground Source Heat Pump (GSHP) systems in Japanese regions

This study predicts the long-term performance of ground source heat pump (GSHP) systems in different climatic regions of Japan. The research model calculates the dynamic heat load of a typical Japanese building in various cities using their meteorological data, considering different schedules for heating and cooling seasons. The model aims to analyze the performance degradation of the GSHP system after 15 years of operation due to the imbalanced heating and cooling loads. Additionally, it compares ground temperature changes, coefficient of performance (COP) variations, and annual operation times based on different thermal imbalance ratios (TIR) for the case studies. The results demonstrate that TIR serves as an appropriate factor for comprehensively evaluating system aspects, including GHE length, ground temperature, and COP degradation, across various climates. It was found that ground temperatures increased by a maximum of up to 4 °C in cooling-dominated regions due to higher thermal imbalance accumulative loads, while it decreased to around 1 °C in heating-dominated regions. The model also revealed that the COP decreased in the dominant operation for each case study due to induced thermal imbalance and soil temperature change. The thermal imbalance caused the system cooling COP decreased by 0.015/year in the worst case (Okinawa City) and 0.006/year in average for other cases.

Assessing the Impact of a Shallow Geothermal System Operation through Multi-Layer Temperature Monitoring in a Mediterranean Climate

Shallow Geothermal Energy (SGE) exchanges heat with the ground. In continuous, long-term operation, the initial temperature field can be disturbed, and subsurface thermal changes can be developed. In this paper, the thermal impact of a SGE system under a Mediterranean climate is handled. Temperature monitoring was conducted on 15 investigation boreholes equipped with a total of 92 thermal sensors placed at specific depths. Investigation boreholes were drilled 1–2 m from SGE system borehole heat exchangers installed in a university building. The analysis handles a one-year monitoring period of SGE system operation. Temperature depth profiles, reaching up to 140 m depth, were registered with a 10 min time step, resulting in a large amount of data. Ground thermal conductivity was estimated experimentally and semi-empirically, allowing us to obtain, using a numerical model, the initial undisturbed ground temperature profiles and compare them with the monitored values. Climate data were recorded by the university meteorological station. Globally, the measured and computed data were coherent, and a non-negligible impact of the SGE system operation in the first year was observed. The building orientation as well as the nearby departments had significant impacts on the shallow ground temperature. Maximum ground temperature changes observed at depths higher than 10–20 m, ranging from 2 to 3 °C as observed in different boreholes, indicate that the system is operating efficiently.

Comparison of different configurations of a solar-assisted ground-source CO2 heat pump system for space and water heating using Taguchi-Grey Relational analysis

This work deals with the comparison of different optimized configurations of solar-assisted ground-source CO2 heat pump systems using the Taguchi method and Grey Relational Analysis (GRA) when used for simultaneous space and water heating in cold coastal climate conditions. The configurations studied include: (1) the solar collectors (SCs) and borehole heat exchangers (BHEs) connected in series, with the working fluid flowing to the SCs first; (2) the SCs and BHEs connected in series, with the fluid flowing to the BHEs first; and (3) the SCs and BHEs connected in parallel to the heat pump. Eight parameters were considered for optimizing the systems’ design, including the BHE length, BHE spacing, BHE number, SC area, BHE-SC mass flow rate, space heating return temperature, heat pump discharge pressure, and heat pump’s outlet temperature. The seasonal performance factor (SPF), levelized cost of heating (LCOH), and the estimated maximum annual ground temperature change (GTC) were chosen as performance indicators to evaluate system performance. The system model was developed using Modelica and 27 simulation runs for every configuration were implemented according to the L27 (93) Taguchi orthogonal array. Single objective optimizations were first performed using the Taguchi method to determine the parameter combinations that would optimize the SPF, LCOH, and GTC, separately. After that, multi-objective optimization was performed using Taguchi-GRA to determine the control factor combination that would give the optimal overall performance when all output variables are considered simultaneously and given equal importance. When the performance indicators were considered separately, simulations show that configuration 2 gave the best SPF (4.025), configuration 3 gave the best LCOH (0.124 USD/kWh), and configuration 1 gave the best GTC (100.257%), respectively. Analysis of variance (ANOVA) showed that the SPF is most sensitive to the heat pump’s discharge pressure and outlet temperature; the LCOH to the BHE length, BHE number, and SC area; and the GTC to all the BHE and SC sizing variables. Multi-objective optimization showed that configuration 1 performs the best, giving a grey relational grade of 0.6875, which is equivalent to an SPF of 3.267, and LCOH of 0.155 USD/kWh, and GTC of 100.021%. BHE length was found to be the most influential parameter to overall performance, irrespective of the configuration.

Direct and indirect effects of fire on parasites in an African savanna

Abstract Fires in grassy ecosystems consume vegetation and initiate high‐quality regrowth, which results in pyric herbivory when mammalian grazers concentrate feeding in recent burns. For environmentally transmitted parasites with transmission mechanisms linked to vegetation structure, fire should exert direct effects on parasites, as well as indirect effects resulting from subsequent enhanced herbivory, which can affect parasite input and exposure to environmental conditions. We combined an experimental manipulation with observational data in the Serengeti National Park to investigate the direct and indirect effects of fire on parasites. We assessed the direct effects of fire by measuring changes in parasitic nematode larvae in the grass layer before and after fire on paired experimental burned and control plots. To investigate indirect effects linked to pyric herbivory, we sampled herbivore dung, grass biomass, ground temperature and larval densities every month for 5 months following fire in seven pairs of burned and unburned monitoring plots. Finally, to assess if fire‐driven changes to larval densities affected host infection burdens, we collected faecal samples from a key host, Grant's gazelle (Nanger granti), each month for 5 months to estimate within‐host parasite burdens. Fire killed all larvae and increased grazer dung inputs by 40% for 2 months following fire. Dung inputs after fire led to larval parasite recolonization of burned patches, but intense herbivory kept grass short and larval densities were associated with changes in ground temperature linked to grass biomass and ambient temperature. Grant's gazelles had lower parasite burdens when sampled in areas with higher compared to lower burned area fraction. Fire and pyric herbivory change the densities of larval parasites in the environment and divide the landscape into burned and unburned regions with distinct infection risks for local herbivores. The indirect effects quantified here represent a novel finding with major implications for all grazing systems impacted by fire.

Modeling and investigation of the performance of a solar-assisted ground-coupled CO2 heat pump for space and water heating

The rise in the popularity of heat pumps should be accompanied by the increase in the utilization of environmentally-safe working fluids. To push for wider uptake of hybrid heat pump systems that use natural working fluids, information about their performance and operating characteristics should be made available. This study models a CO2 solar-assisted ground-coupled heat pump (SAGCHP) system and investigates its performance for space and water heating. Sensitivity analysis, parametric study, and long-term performance simulation were implemented, with the system’s seasonal performance factor (SPF), levelized cost of heating (LCOH), and ground temperature change (GTC) considered as performance indicators. It was seen that ensuring a good combination of design and operating specifications can result in an SPF (∼3.5) and LCOH (0.184 USD/kWh) that are comparable with those of SAGCHP systems that use conventional working fluids. The heat pump’s high-side pressure and output temperature exhibited notable effects on all the performance indicators. Below the optimal operating pressure, a 5% increase in the heat pump’s high-side pressure brought about a ∼7–10% improvement to the SPF, a ∼3–5% reduction to the LCOH, and a ∼6–10% increase to the GTC. Reducing the output temperature by 5% increased the SPF by ∼7–8%, decreased the LCOH by ∼3%, and increased the GTC by ∼6%. Parametric studies identified the presence of optimal heat pump discharge pressure and heat source circulation rate that should be used for operations. Long-term simulation shows that managing the GTC ensures the longevity of the system. Rather than oversizing the borehole heat exchangers (BHEs), it is more practical to reduce ground temperature decline by increasing the BHE spacing or by adding solar collectors.

Investigating the thermal state of permafrost with Bayesian inverse modeling of heat transfer

Abstract. Long-term measurements of permafrost temperatures do not provide a complete picture of the Arctic subsurface thermal regime. Regions with warmer permafrost often show little to no long-term change in ground temperature due to the uptake and release of latent heat during freezing and thawing. Thus, regions where the least warming is observed may also be the most vulnerable to permafrost degradation. Since direct measurements of ice and liquid water contents in the permafrost layer are not widely available, thermal modeling of the subsurface plays a crucial role in understanding how permafrost responds to changes in the local energy balance. In this work, we first analyze trends in observed air and permafrost temperatures at four sites within the continuous permafrost zone, where we find substantial variation in the apparent relationship between long-term changes in permafrost temperatures (0.02–0.16 K yr−1) and air temperature (0.09–0.11 K yr−1). We then apply recently developed Bayesian inversion methods to link observed changes in borehole temperatures to unobserved changes in latent heat and active layer thickness using a transient model of heat conduction with phase change. Our results suggest that the degree to which recent warming trends correlate with permafrost thaw depends strongly on both soil freezing characteristics and historical climatology. At the warmest site, a 9 m borehole near Ny-Ålesund, Svalbard, modeled active layer thickness increases by an average of 13 ± 1 cm K−1 rise in mean annual ground temperature. In stark contrast, modeled rates of thaw at one of the colder sites, a borehole on Samoylov Island in the Lena River delta, appear far less sensitive to temperature change, with a negligible effect of 1 ± 1 cm K−1. Although our study is limited to just four sites, the results urge caution in the interpretation and comparison of warming trends in Arctic boreholes, indicating significant uncertainty in their implications for the current and future thermal state of permafrost.

Quantification of freeze–thaw hysteresis of unfrozen water content and electrical resistivity from time lapse measurements in the active layer and permafrost

AbstractWe quantified seasonal, in‐situ variation in ground electrical resistivity, unfrozen water content, and ground temperature using daily and sub‐daily measurements in active layer and in permafrost on a high‐latitude monitoring site in West Greenland. The in‐situ unfrozen water content in a clayey soil undergoing phase change was 10% higher during freezing than during thawing at the same ground temperature, thus shifting the freezing curve parameterization seasonally. The freeze–thaw unfrozen water content hysteresis was a measurable part of the energy balance of the active layer and corresponds to 12.1% of the total yearly enthalpy change for our field site. We also observed and quantified hysteresis in the relationships between ground electrical resistivity and unfrozen water content, where the inverted resistivities during freezing in the temperature range from 0°C to approximately −4°C were up to one order of magnitude higher than during thawing at the same unfrozen water content. As a result, the petro‐physical models that uniquely derive soil unfrozen water content from bulk soil electrical resistivity measured from ground surface do not fully capture the complexity of the in‐situ processes. The hysteresis should therefore be considered in quantitative interpretation of ground resistivity changes in terms of ground temperature and ice content changes.

Elevation dependency of future degradation of permafrost over the Qinghai-Tibet Plateau

Global warming has caused widespread permafrost degradation, but the geographic regularity of permafrost degradation is unknown. Here, we investigated the three-dimensional features of future permafrost degradation on the Qinghai-Tibetan Plateau. Our findings show that permafrost degradation under shared socioeconomic pathways (SSPs) has obvious three-dimensional characteristics. In comparison to latitude and aridity, permafrost degradation is closely related to elevation, i.e. it slows with elevation, a phenomenon known as elevation-dependent degradation. The pattern of elevation-dependent degradation is consistent across four subzones and is strongly linked to thermal conditions that vary with elevation. Under SSP2-4.5, SSP3-7.0, and SSP5-8.5 scenarios, remarkable elevation-dependent warming (EDW) is observed at 3600–4900 m, but changes in mean annual ground temperature of permafrost and EDW as altitude rises are anti-phase. Under any SSP, the magnitude of mean annual air temperature along altitude belts determines the degree of permafrost degradation (R 2 > 0.90). This research provides new insight on the evolution of permafrost.

Operational strategies to alleviate thermal impacts of the large-scale borehole heat exchanger array in Beijing Daxing Airport

Large-scale ground source heat pump (GSHP) systems are increasingly used for space heating and cooling. In comparison with smaller ones, large GSHP systems are often coupled with much more borehole heat exchangers (BHEs). Because of the intense thermal interactions between BHEs, they are more susceptible to significant ground temperature changes. Meanwhile, they possess the advantage that their operational strategies can be applied with a high degree of freedom, which presents chances to alleviate intense thermal interactions. In this study, we used a new performance indicator to access the effectiveness of GSHP operational strategies on alleviating thermal anomalies. The Daxing Airport GSHP system, contains 10,497 BHEs and is the largest in the world; therefore, it was selected as the test case for performance enhancement through operational strategies. We established a 2D model to predict ground temperature changes during the 50-year operation of the BHEs. First, it was revealed that the most severe thermal anomalies in the study area mainly occurred both within and between the BHE arrays, which should be mitigated. To alleviate the thermal anomalies caused by the thermal interactions of BHEs, operational strategies were applied by adjusting the cooling/heating starting sequence, setting time-dependent thermal loads, and reallocating thermal loads according to the position of the BHEs. Our study demonstrates that only the operation strategy that adjusts the cooling/heating starting sequence is beneficial for different BHE layouts, while the operational strategy that reallocates the thermal loads depending on BHEs position may be only effective for specific BHE layouts. In addition, our new performance indicator can be used to evaluate the effectiveness of the operational strategies and determine the spacing of adjacent BHE arrays. Therefore, it benefits the operation management of BHE array and design of BHE layout, and further guarantees the sustainable operation of the GSHP system.

Influence of wildfire on the rapidly changing features of patchy permafrost, Northeast China

AbstractAs an active ecological agent in the boreal forest, wildfires have many important impacts on the underlying permafrost. However, investigations of post‐fire changes in the features of ecosystem‐protected permafrost are extremely limited in the patchy permafrost regions, Northeast China. Additionally, the impacts of wildfires on patchy permafrost are complicated in this region. In this paper, based on field surveys and observations, permafrost features and their influencing factors were analyzed in the area burned in 2006. The results showed notably lower soil temperatures in shrub‐wetland (NWH1, unburned and NWH2, light burn) in comparison with those in the birch‐larch forest (NWH3, light burn and NWH4, moderate burn). Changes in shallow ground temperatures (0–4 m in depth) were significant. Mean annual soil temperatures (MAST) and annual maximum soil temperature at the light‐burn NWH2 site were 0.3–5.2°C higher than those of unburned NWH1, but annual minimum soil temperatures, 0.3–2.1°C lower. MASTs at the moderately burned NWH4 were 0.6–1.6°C higher than those at the light‐burn NWH3. The active layer thickness (ALT) was greater at burned sites, and; at the moderately burned site, only seasonal frost was found when 13 years after the burn. The contents of soil organic carbon and total nitrogen were the highest at the unburned NWH1 from 0.0 to 0.4 m in depth, followed by those at the lightly‐burned NWH2 and NWH3, and the lowest at the moderately‐burned NWH4. Climate warming, forest fires, and changing local factors (e.g., soil moisture contents, snow cover, soil texture and drainage conditions) worked together to increase ground temperature and ALT. These research results could provide important scientific basis for the protection of boreal forest and wetlands and the ecosystem‐protected permafrost under a warming climate.

Distributed Acoustic Sensing Based on Microtremor Survey Method for Near-Surface Active Faults Exploration: A Case Study in Datong Basin, China.

Active fault detection has an important significance for seismic disaster prevention and mitigation in urban areas. The high-density station arrays have the potential to provide a microtremor survey solution for shallow seismic investigations. However, the resolution limitation of the nodal seismometer and small-scale lateral velocity being inhomogeneous hinder their application in near-surface active fault exploration. Distributed acoustic sensing (DAS) has been developed rapidly in the past few years; it takes an optical fiber as the sensing medium and signal transmission medium, which can continuously detect vibration over long distances with high spatial resolution and low cost. This paper tried to address the issue of near-surface active fault exploration by using DAS. We selected a normal fault in the southern Datong basin, a graben basin in the Shanxi rift system in north China, to carry out the research. Microtremor surveys across the possible range of the active fault were conducted using DAS and nodal seismometers, so as to obtain a shallow shear wave velocity model. Meanwhile, we applied a Brillouin optical time domain reflectometer (BOTDR) and distributed temperature sensing (DTS) to monitor the real-time fluctuation of ground temperature and strain. Our results show that the resolution of the deep structures of the fault via the microtremor survey based on DAS is lower than that via the seismic reflection; whereas, their fault location is consistent, and the near-surface structure of the fault can be traced in the DAS results. In addition, both the BOTDR and DTS results indicate an apparent consistent change in ground temperature and strain across the fault determined by the DAS result, and the combination of surface monitoring and underground exploration will help to accurately avoid active faults and seismic potential assessment in urban areas.

Geothermal dynamic constraints of groundwater source heat pump system in shallow aquifers

Due to the temperature of shallow aquifers being affected by atmospheric temperature, groundwater source heat pumps (GWSHPs) become unstable and the operation efficiency of GWSHP is constrained. In the study, the coupling numerical simulation model of the groundwater flow field and temperature field is established based on the continuous monitoring results in an actual experimental site, and the water and thermal migration of shallow aquifer is simulated under the influence of the atmospheric environment. The influence of the dynamic change in ground temperature is analyzed on a GWSHP. The results indicated that the temperature of the shallow aquifer is affected by the external temperature, and the recharge temperature in the summer cooling period was 33°C, and that in the winter heating period was 6°C in the actual site, to avoid the occurrence of thermal penetration when there is a gap between the actual situation and the design situation, the single cooler can balance the insufficient cooling capacity in summer under the most unfavorable situation. The research results can also provide a reference for the development and utilization of geothermal energy resources in shallow aquifers.

Mock-up experimental study on the performance of a combined cooling-domestic hot water-ground source heat pump system

A ground source heat pump (GSHP) delivers heat from a condenser to the ground when it operates with cooling mode. However, the ground temperature increases when the GSHP system operates for a long time. The increased ground temperature can deteriorate the GSHP’s coefficient of performance (COP). To maintain the balance between the ground temperature and COP, the condensation heat from the cooling GSHP can be used for other heating systems before it is transferred to the ground. This study proposes a combined GSHP system connecting a three RT cooling GSHP with a 1.5 RT domestic hot water (DHW) heat pump, and the performance of the system was evaluated through mock-up experiments. In the combined GSHP system, the condensation heat of the cooling system was used as the heat source of the DHW system. Therefore, the ground temperature could be reduced, and the performances of both the GSHP cooler and DHW heater pump could be enhanced. Mock-up experiments for performance evaluation were conducted with cooling-only, DHW-only, and cooling-DHW operational modes. The results showed that cooling-DHW operation slowed down the change in heat source temperature. In comparison with those of the cooling-only and DWH-only heat pumps, the COPs of the cooling heat pump and DHW heat pump of the combined system were increased by 12.93% and 15.47%, respectively. Moreover, the total COP of the cooling-DHW combined GSHP system increased by 4.4% and 29.55% in comparison with those of the cooling-only GSHP and DHW-only GSHP systems, respectively. Practical application A combined GSHP system was proposed by connecting a GSHP for space cooling with a heat pump for DHW to mitigate the ground temperature changes due to the long-term operation of the GSHP system. The combined system reuses the condensation heat generated by the cooling heat pump as the heat source of the DHW heat pump for ensuring the system performance and building energy saving. Therefore, this system is suitable for buildings with large energy use and heavy hot water demand, especially in summer, such as hospital and hotel buildings.

Cosmogenic 3He paleothermometry on post-LGM glacial bedrock within the central European Alps

Abstract. Diffusion properties of cosmogenic 3He in quartz at Earth surface temperatures offer the potential to directly reconstruct the evolution of past in situ temperatures from formerly glaciated areas, which is important information for improving our understanding of glacier–climate interactions. In this study, we apply cosmogenic 3He paleothermometry to rock surfaces gradually exposed from the Last Glacial Maximum (LGM) to the Holocene period along two deglaciation profiles in the European Alps (Mont Blanc and Aar massifs). Laboratory experiments conducted on one representative sample per site indicate significant differences in 3He diffusion kinetics between the two sites, with quasi-linear Arrhenius behavior observed in quartz from the Mont Blanc site and complex Arrhenius behavior observed in quartz from the Aar site, which we interpret to indicate the presence of multiple diffusion domains (MDD). Assuming the same diffusion kinetics apply to all quartz samples along each profile, forward model simulations indicate that the cosmogenic 3He abundance in all the investigated samples should be at equilibrium with present-day temperature conditions. However, measured cosmogenic 3He concentrations in samples exposed since before the Holocene indicate an apparent 3He thermal signal significantly colder than today. This observed 3He thermal signal cannot be explained with a realistic post-LGM mean annual temperature evolution in the European Alps at the study sites. One hypothesis is that the diffusion kinetics and MDD model applied may not provide sufficiently accurate, quantitative paleo-temperature estimates in these samples; thus, while a pre-Holocene 3He thermal signal is indeed preserved in the quartz, the helium diffusivity would be lower at Alpine surface temperatures than our diffusion models predict. Alternatively, if the modeled helium diffusion kinetics is accurate, the observed 3He abundances may reflect a complex geomorphic and/or paleoclimatic evolution, with much more recent ground temperature changes associated with the degradation of alpine permafrost.

Soil Thermal Response to Temperature Cycles and End Boundary Conditions of Energy Piles

This paper explores the influence of building cover and pile toe boundary conditions on ground temperature distributions surrounding energy piles. Experimental and numerical studies were conducted on two isolated cast-in-place energy piles installed in dense unsaturated sand, one exposed to the atmosphere at the ground surface (diameter=0.6 m and length=16.1 m) and the other installed under a six-story building (diameter=0.6 m and length=10 m). Investigations were conducted for monotonic heating and daily cyclic temperature changes of the piles ranging between 10°C and 35°C. The changes in ground temperature decreased as radial distance increased from the edge of both piles. Cyclic temperatures in both piles induced lower ground temperature changes and decreased the radial thermal influence zone compared with monotonic heating. However, the radial thermal zone in cyclic operating mode can be influenced by different ratios of heating to cooling times and hence should be selected carefully to avoid unexpected ground temperature changes. Atmospheric effects were observed up to a depth of 2 m for the energy pile exposed to the atmosphere. The insulation provided by the building footprint slightly decreased the impacts of ground-atmosphere interaction on the soil temperature distribution with depth near the surface compared to the energy pile exposed to the atmosphere. The ground temperature variations were dominant along the length of the heat exchanger loops for both piles. However, they were negligible near the pile toe below the heat exchanger loops for both piles.

Thermo-energy performance of neighbouring energy piles

Energy piles are often closely spaced such that the thermal response of one pile might affect the response of neighbouring piles through heat transfer in the soil. This paper examines the changes in extracted energy and ground temperature of two cast-in-place bored energy piles installed below a six-storey building, with a diameter of 0.6 m, length of 10 m and 3.5 m centre-to-centre distance. Field experiments were conducted on singular and dual piles for cooling and cyclic temperatures, continuous daily operation of the ground source heat pump (GSHP) and intermittent cyclic operations with stoppage-to-operating ratios of 2:1 and 1:2. Compared to single pile operations, greater energy was extracted from dual piles for all operating modes despite thermal interaction through the soil volume between the dual piles. The larger stoppage-to-operating ratio of the GSHP induced lower pile and ground temperature changes and higher energy extraction than continuous operation for single and dual pile operations. The larger stoppage-to-operating ratio of the GSHP reduced the thermal interaction between the dual piles by imposing lower ground temperature changes compared to continuous operation. The results demonstrate the practical significance of managing the GSPHs operating modes for optimal thermal performance of multiple closely spaced energy piles.

Quantifying the Effect of Uniform and Non-Uniform Changes in Ground Temperature on Outdoor Mean Radiant Temperature with Discrete Ground View Factors of Various Heights

Quantifying the Effect of Uniform and Non-Uniform Changes in Ground Temperature on Outdoor Mean Radiant Temperature with Discrete Ground View Factors of Various Heights

Numerical Investigation of the Influence of Precooling on the Thermal Performance of a Borehole Heat Exchanger

Ground source heat pumps (GSHPs), a high-efficiency and energy-saving air-conditioning technology that utilizes shallow geothermal resources for both heating and cooling, are a vital green energy system for residential and commercial buildings. Improving the performance of such a system was the focus of the current research. As soil temperature and thermal radius are two important aspects that affect the performance of ground source heat pump systems, we conducted a new numerical simulation to capture the changes in sensitive factors and propose the optimized paths. The numerical simulation analyzed the thermal characteristics of a borefield under different pre-cooling times and soil types. The results indicated the following: (1) The rate of the ground temperature change with pre-cooling during the discharging period had a faster rise than in the case without pre-cooling. The longer the precooling time was, the smaller the thermal radius became. In particular, when the precooling time was longer than 14 days, the decrease in the thermal radius rate percentages was less than 4%. (2) Among the three kinds of soils compared, the soils with lower thermal conductivity and thermal diffusivity best suppressed the thermal interference effects. (3) Using a multivariate nonlinear function regression model, a simulation formula was proposed to predict- the thermal radius, which considered the factors of thermal diffusivity, precooling time, and discharging time. The prediction deviation was within 14.8%.

Seasonal deformation monitoring over thermokarst landforms using terrestrial laser scanning in Northeastern Qinghai-Tibetan Plateau

Climate warming has accelerated permafrost degradation over the Qinghai-Tibet Plateau (QTP) over the past several decades. The development of thermokarst landforms is a key indicator of permafrost degradation, while it lacks quantified measurements and comprehensive research over the QTP. The aim of this study is to investigate the development of thermokarst terrains through repeated ground-based elevation observations by using terrestrial laser scanning (TLS) in the northern QTP from April 11, 2016 through June 16, 2018. TLS Time series analysis reveals that the margin of the thermokarst landforms undergoes significant ground subsidence, side materials collapse, and/or uplift and deposit, especially in the southeastern, northwestern part of thermokarst landforms during the middle through the late thaw season. The vertical deformation and headwall retreat of thermokarst landforms reached −3.364 m and 10.66 m from April 11, 2016 to June 16, 2018, respectively. We also generated high-resolution orthophotos based on aerial photos acquired by the built-in 4 K RGB (red, green, blue) camera of DJI Phantom 3 Professional unmanned aerial vehicle (UAV) in April and October of 2016. The UAV images confirmed the TLS observations during the same period and presented the severely deformed area. This study reveals that the seasonal vertical deformation and headwall retreat of the thermokarst landforms in the study site are consistent with seasonal ground temperature change during the observed period. Extreme precipitation event as a key factor triggered the severe deforming of the case study thermokarst. Ground ice, peat layer, and human activity also contributed to the thermokarst landforms formation. The results also illustrate that TLS is an effective method for studying thermokarst development quantitatively.