Groundwater Heat Pump Systems Research Articles

Mechanisms of well iron clogging in groundwater heat pump systems: Insights from video imaging, hydrogeochemical analysis, and geochemical modeling

Groundwater heat pump (GWHP) systems are increasingly popular as low-carbon and environmentally friendly technologies, but well clogging induced by iron remains a significant issue. This study investigated the clogging characteristics and biogeochemistry of three typical wells (pumping, injection, and observation wells) in an operating GWHP system using video imaging, sampling, and analysis of hydrogeochemical and microbial data. The results revealed that iron-induced well clogging is a complex process involving physical, chemical, and microbial factors. Pumping wells experience clogging due to water mixing with varying redox conditions, resulting in hematite-based iron oxide deposits. Injection wells exhibit higher clogging severity, with transformed oxidation and accumulation of reduced iron minerals at the solid-liquid interface, resulting in darker colored clogs with magnetite. Clogging in both extraction and injection wells is closely related to iron-rich aquifer sections, where severe clogging occurs. Shallow clogging due to iron oxide is limited and attributed to the oxidation of zero-valent iron in well casing material. Iron-oxidizing bacteria and iron-reducing bacteria were detected in the consolidated deposits of clogged wells, indicating their involvement in the clogging formation process. Moreover, a strong correlation was observed between the presence of nitrate-reducing bacteria in the water phase and the severity of clogging, suggesting a possible link between iron oxidation and nitrate reduction in the system. Geochemical modeling results further supported the observed clogging severity in GWHP systems and confirmed varying clogging mechanisms in different wells and depths. These findings contribute to the understanding of clogging in GWHP operations, aiding in robust water utilization and energy-saving efforts, and supporting global carbon reduction initiatives.

Feasibility study on the optimal design method of ground-water source hybrid heat pump system applied to office buildings

In order to reduce the energy consumption of buildings, a ground source heat pump (GSHP) system was introduced to reduce the building energy consumption. However, general GSHP systems are not as popular as photovoltaic and thermal systems owing to soil imbalances and performance degradation during long-term operation. To solve this problem, we propose a ground-water source hybrid heat pump (GWHP) system. GWHP systems can simultaneously use ground and water sources to avoid soil imbalance and degradation. In this study, we proposed a GWHP system and investigated its feasibility as a replacement for a GSHP system in terms of performance and heat source temperature. In addition, an optimal capacity design method for the GWHP system is presented to evaluate its economic feasibility. The performance and economic analysis show that the cooling and heating performance of the proposed GWHP was 15 % and 3 % better, respectively, compared to the existing GSHP system. Our results also showed that the proposed design method was approximately 5.3 % less expensive than the existing GSHP systems. The GWHP system could be a suitable alternative to the GSHP system, as confirmed by both the energy simulation and cost analysis.

Influence of Geology, Hydrogeology, and Climate on Ground Source Heat Pump Distribution in Slovenia and Selected European Countries

Shallow geothermal energy (SGE) is a renewable energy that could contribute to the decarbonatization of the heating and cooling sector. SGE is predominantly harnessed through ground source heat pump (GSHP) systems. The choice of which type of GSHP system depends on various factors. Understanding these factors is crucial for optimizing the efficiency of GSHP systems and fostering their implementation. In this paper, we have analysed the spatial distribution of GSHPs in Slovenia. We identified 1073 groundwater and 1122 ground-coupled heat pump systems with a total heat pump capacity of almost 30 MW. We quantitatively assessed the influence of geological, hydrogeological, and climate conditions on their spatial distribution. Using the χ2 test and information value method, we identified hydrogeological conditions as the most influential factor for the GSHP systems’ spatial distribution. We also performed the spatial analysis of geological and hydrogeological data in 22 European countries, including Slovenia. We collected the reported numbers of installed GSHP units in 2020 and were able to distinguish the shares of groundwater and ground-coupled heat pump systems for 12 of these countries. The analysis showed that ground-coupled heat pumps predominate in most countries, even if the natural conditions are favourable for groundwater heat pumps.

Numerical investigation of a district scale groundwater heat pump system: A case study from Colchester, UK

Nearly 40% of Europe’s total energy consumption is dedicated to buildings and heating/cooling make a significant part of this consumption. Groundwater heat pumps (GWHP) are highly efficient, and low-carbon technology that can supply heating/cooling to buildings on small or large scales. Thus, they contribute to achieving European targets of net-zero greenhouse gas emissions by 2050. In the literature, studies on the utilisation of GWHP at a district scale, particularly in chalk aquifers, are relatively rare. The implementation of district-scale geothermal heat pump (GWHP) systems poses several challenges, including dealing with the scale and complexity of the systems, addressing geological variability, managing high initial investments, balancing energy demand and supply, ensuring proper maintenance and monitoring, and mitigating potential environmental impacts. These challenges require careful consideration and strategic planning to ensure the successful deployment and sustainable operation of these systems., This study numerically investigates a district-scale GWHP system and analyses the thermal plume development created due to the heating operation, offering insights into system performance. A good match was found between field results and simulation results for water level increase and drawdown. However, there is a difference of approximately 11% in system efficiency between field tests and simulations due to the lower abstraction temperature detected in the simulation. The simulation results show that cooler water injection into the fractured chalk aquifer creates a thermal plume radially spanning out to 50 m. The thermal plume has no effect on the abstraction temperature and system performance. This result can be attributed to the large distance between injection and abstraction wells and the low hydraulic gradient.

Sustainability assessment of hydrochemical and microbiological disturbances in shallow aquifer by thermal utilization

The use of geothermal systems with shallow groundwater to achieve global carbon neutrality has increased substantially. However, they not only have hydraulic and thermal effects on the environment, but also induce hydrochemical and microbial changes that can worsen the sustainability of water resources. This study focused on evaluating groundwater mixing and pollution in a surrounding shallow aquifer induced by operation of a groundwater heat pump (GWHP) system. To explain these phenomena, hydrogeochemistry, multiple isotopes (O, H, and Sr), Rn, and microbial data were combined with geochemical modeling of the test system. Few studies have been conducted on field measurements using these combined approaches. The hydrogeochemical and microbial data showed specific characteristics accompanying the thermal use of groundwater. The calculated saturation index values and PHREEQC modeling results indicated a high probability of clogging effects in wells located near the boundary of the thermal plume. There was also a decrease in the microbial diversity index. Rn and stable isotopes indicated active mixing between circulating water and ambient groundwater along the main groundwater flow direction during the operation. This study demonstrated that subsurface hydrological and microbial environments, such as those of the GWHP system, responded to impacts of moderate intensity. Because the applied data were able to derive possible environmental and operational problems of shallow geothermal systems, hydrological and microbiological signatures should be considered for sustainable management of groundwater in geothermal sites. Our study can have implications for energy reuse from storage within an aquifer, such as aquifer thermal energy storage (ATES) based on field tests, using combined analysis.

Numerical Investigation and Optimization of a District-Scale Groundwater Heat Pump System

Groundwater heat pump (GWHP) systems are acknowledged as renewable and sustainable energy sources that can effectively fulfill the heating and cooling requirements of buildings on a district level. These systems harness geothermal sources available at shallow depths. To ensure the long-term sustainability of the system, the thermally used water is generally reinjected into the aquifer, creating a thermal plume starting from the injection well. Over time, this thermal plume may reach the abstraction well in the long term, potentially leading to a reduction in system efficiency. The operation types have a significant impact on this matter, and their effects have not been extensively studied in the existing literature. Therefore, this study aims to determine the optimal operating configurations for the Northern Gateway Heat Network, a GWHP system established in Colchester, UK. In this study, four distinct operation types are considered: (1) continuous heating (actual system), (2) heating and recovery, (3) heating and cooling, and (4) aquifer thermal energy storage (ATES). The results indicate that ATES operation yields the highest thermal energy output due to its ability to benefit from stored energy from the previous operation. However, implementing the ATES system may encounter challenges due to factors such as well development, hydraulic conductivity, and hydraulic gradient. On the other hand, implementing heating and cooling operations does not require additional considerations and offers not only free cooling to buildings but also a delay in thermal feedback time.

Laboratory Investigation of Impact of Injection–Abstraction Rate and Groundwater Flow Velocity on Groundwater Heat Pump Performance

Using low-temperature (shallow) groundwater as a heat source or heat sink is a common practice to supply space heating or cooling, especially in the United States, Canada, China, and several European countries. The groundwater heat pump (GWHP) system has been extensively studied in recent decades using numerical approaches, which have some limitations in understanding the soil’s thermal behavior. Therefore, a laboratory-scale experimental study involving cooling tests was carried out to investigate the impact of GWHP on system performance and sustainability with varying groundwater flow velocities and injection and abstraction rates. The results demonstrated that groundwater flow velocity, as well as injection and abstraction rates, significantly impact thermal plume development. Higher injection and abstraction rates create a larger thermal plume, thereby decreasing abstraction temperature. However, groundwater flow prevents heat development around the well by dispersing the heat in the groundwater flow direction. Furthermore, the results indicate that the energy gain only increased by 81% and 107%, with a respective increase of 100% and 200% in injection and abstraction rates.

Drilling Deeper in Shallow Geoexchange Heat Pump Systems—Thermogeological, Energy and Hydraulic Benefits and Restraints

In the last decade, due to climate change concerns and new environmental regulations in the EU, there was a tremendous rise in installed heat pump systems in new homes and buildings. The majority of these installed units are related to air-source heat pumps, as they offer a good trade-off between capital and operating expenses. However, when analysing heating and cooling heat pump systems from the primary energy consumption and ecological aspects, groundwater and shallow geothermal heat pump systems offer superior efficiency, compared to all market-available thermo-technical systems today. In the last decade, ground-source systems have seen some technological improvement by employing new borehole heat exchanger designs, such as piping with internal fins and a wider diameter (so called Turbocollector) to enhance the heat transfer between fluid and rock, as well as to reduce the pressure drop in the system. Furthermore, the process of drilling deeper offers higher ground temperatures and consequently higher seasonal performance factors in the heating cycle, due to the effect of the geothermal gradient. Nevertheless, although deeper boreholes provide better heat extraction rates per meter during the heat pump heating cycle, at the same time, it reduces heat rejection rates during the heat pump cooling cycle. The objective of this paper is to analyse and evaluate benefits and downsides of a new approach in the heat pump system design with deeper borehole heat exchangers of up to 300 m, comparing it to the traditional design of double-loop exchangers with 100 m depth. The geothermal borehole grid design simulation model, along with heat extraction and rejection, is performed on a yearly basis. The results are showing that the benefits of shallow geothermal boreholes, from the hydraulic and thermodynamic point of view, still dominate over deeper solutions.

Optimizing the spatial arrangement of groundwater heat pumps and their well locations

Increasing numbers of groundwater heat pump systems in urban areas can lead to negative thermal interference between neighboring systems. These systems and their associated wells can be optimally positioned so as to minimize such interactions and maximize the use of the shallow geothermal potential. This paper presents a new method that determines the optimal positioning of groundwater heat pumps and their wells by concurrently considering relevant technical and regulatory constraints. The method is based on an improved analytical model for estimating thermal anomalies in groundwater, i.e. thermal plumes, and integer linear programming optimization. The method is tested using a neighborhood with 56 parcels, i.e. potential systems, in two optimization cases: steady and dynamic. In both cases, the method successfully determines both the systems to be installed and the optimal locations of the wells such that relevant spatial regulations are satisfied and the energy extracted from the aquifer is maximized. The dynamic case is somewhat more restrictive, as 43 of 56 potential systems are installed, compared to 44 in the steady case. This method represents a significant improvement over existing ones, since it is able to efficiently optimize the locations and well layouts of a large number of systems simultaneously.

A Review of Groundwater Heat Pump Systems in the Italian Framework: Technological Potential and Environmental Limits

For new buildings in densely urbanised cities, groundwater heat pump systems (GWHPs) represent a concrete, effective solution for decarbonising existing energy systems. Environmental factors must be considered to limit the GWHP system’s impact on the subsurface. Particular attention must be given to the long-term sustainability of groundwater abstraction modalities and the development of a thermally affected zone around re-injection wells. Simplified solutions and numerical models have been applied to predict subsurface heat transport mechanisms; these simulations allow researchers to consider site-specific geological conditions, transient heat and groundwater flow regimes, and anisotropies in the subsurface media. This paper presents a comprehensive overview of the current research on GWHPs and discusses the benefits and limitations of their diffusion in Italy. The sources used provide information on and examples of the correct methodological approaches for depicting the induced variations while avoiding the overestimation or underestimation of the impact that GWHPs have on exploited aquifers.

Field studies on the energy consumption and thermal comfort of a nZEB using radiant ceiling panel and open-loop groundwater heat pump system in a cold region

Field studies were conducted in an office building in a cold region equipped with a radiant ceiling panel (RCP) and open-loop groundwater heat pump (GWHP) system. First, the indoor thermal comfort in a target office was evaluated based on the results obtained from the physical measurements and survey questionnaire. The results showed that the suspended RCP system could achieve comfortable indoor thermal conditions throughout the year. The indoor temperature was maintained within the range of 22–26 °C, the humidity was within 30–60%, and CO2 concentration was mostly below 1000 ppm. Second, the energy performance was tested simultaneously, and intermittent and continuous operation methods were compared. Under the continuous operation method, a stable indoor environment was maintained, and the COP of the GWHP system increased by 10%. The results showed the potential to further reduce energy consumption and maintain a high level of thermal comfort through the optimization of system operations. Finally, the annual energy consumption was calculated based on the recorded measurements. The results illustrate that this building can reach the ZEB-ready level in Japan. The primary energy consumption can be decreased by approximately 67% compared to the reference by applying the combined system.

Spatial migration and distribution of suspended particles in saturated sand layer under the impact of temperature

AbstractThe migration, deposition and detachment of suspended particles in saturated sands are used in many fields. Temperature is an important influencing factor in the transport and deposition process of particles in porous media, and it plays an important role in ground‐water heat pump (GWHP) projects. This article discusses the independent development of a three‐dimensional percolation sandbox test device and the design of a particle migration test based on the Reynolds number in the context of the backfilling process of GWHP systems as the experimental background. The effects of various temperatures (10, 25, and 40°C) on the movement of suspended particles in porous media were studied. The experimental results showed that as the temperature rose, more particles were deposited in the sand layer, the uneven flow of particles in the sand layer accelerated, and the particles spread out in the direction opposite to the non‐percolation direction with a wider distribution range in the sandbox. Increased temperature also promoted the percolation of smaller particles, causing the diffusion zone to terminate sooner and the deposition zone to last longer, resulting in the capture of more particles on the surface and in the pore channels of the sand layer.

How groundwater flow field change affects heat transfer in groundwater heat pumps based on physical experiments

A groundwater heat pump (GWHP) is a form of ground-source heat pump that utilizes groundwater as its heat source and offers great efficiency, significant energy savings, and little pollution. As a GWHP system runs, thermal breakthrough is a crucial feature of system design that can result in a progressive shift in pumping water temperature and negatively impact GWHP performance. Variation in the groundwater flow field is crucial for heat transfer in terms of pumping and recharging well groups, especially when high porosity and permeability aquifers are taken into account, as this has a direct impact on the development of the groundwater temperature field. The primary goal of this study was to evaluate the impact of groundwater flow field variation due to groundwater velocity and recharge rate allocation changes on heat-transfer characteristics using laboratory physical experiments, to study the evolution trends of the groundwater flow field and groundwater temperature field, and to compare and determine various pumping and recharge well-group layout schemes to improve GWHP operation efficiency. The test results showed that the rise rate of pumping water temperature and the thermal breakthrough occurrence time increase with the increase of groundwater velocity, and when the groundwater velocity is the same, the thermal breakthrough occurrence time and the rise rate of pumping water temperature in the vertical well layout scheme are 0.48–0.76 times and 1.36–4.0 times that of the parallel well layout scheme, respectively. In addition, the recharge rate allocation has a greater impact on the parallel well layout scheme but a smaller impact on the vertical well layout scheme. The physical test results can provide a scientific basis for future research.

Frontier Research of Engineering: Geothermal Energy Utilization and Groundwater Heat Pump Systems

Geothermal energy is a near-inexhaustible and multi-purpose resource capable of satisfying global energy demand while lowering the reliance on fossil fuels for primary energy [...]

The efficiency loss in groundwater heat pump systems triggered by thermal recycling

Thermal recycling occurring in groundwater heat pump systems is a major cause of efficiency loss in groundwater heat pump systems and its understanding is critical for systems design. However, the response of the systems performance to these phenomena is not easy to predict given that a negative feedback is created, as the rejected heated/cooled water injected back into the aquifer will tend to negatively affect the production well(s) of the doublet(s) starting the negative loop. This work approaches the nonlinearity of the thermal recycling problem for the first time, by making use of distributed numerical models of groundwater flow and heat transport combined with thermodynamic models of geothermal heat pumps. Simulation results showed that thermal interference between production and injection was unavoidable for the scenarios investigated. Different scenarios were found where the impact of thermal recycling was not sustainable during cooling seasons, thus resulting in the inoperability of the heat pump. Thermal recycling was found to be responsible of an increase in operation costs of up to 38.4 k€·year−1, as well as of an environmental impact due to the release of up to 76.2 t eqCO2·year−1 for a 402 kWth installation. The results of this article will allow to improve the understanding of thermal recycling as a nonlinear complex and a transient process responsible for an efficiency loss of groundwater heat pump systems.

Optimizing submersible pump regulation of a clogging prone groundwater heat pump system in Melhus, Norway

The groundwater heat pump (GWHP) system at Lena terrasse in Melhus, Norway has suffered from clogging by iron oxide incrustations, sediment deposits, organic slimes and gas entrapment. This has necessitated annual rehabilitations of the injection well and heat exchanger, and questioned how the submersible pump should be regulated to maximize the seasonal performance factor (SPF = heat output/power input) and reduce the clogging risk. The submersible pump operates with variable speed drive to yield groundwater pumping rates (Q) between 4 and 16 l/s. The GWHP system has been continuously monitored by pressure, temperature, flowmeter and energy meter sensors for 2.5 years, and with semimonthly step-discharge tests. During three step-discharge tests hydrochemistry and suspended solids content were also measured. Monitoring data show that the monthly performance factor (SPFM) is negatively correlated with Q. At Q > 12-13 l/s, suspended solids concentrations increase. Heat exchanger clogging rates increase at Q < 6 l/s. Hydrochemistry is stable, regardless of pumping rate. All clogging rates appear to be independent of pumping rate in the range 6-12 l/s. Therefore, pumping rates are advised to be kept between 6-12 l/s, and minimized but adjusted to the heating demand to avoid freezing of the groundwater.

An integrated strategy for the improvement of thermo-economic performance of a GWHP system

The operation management has been often used to reduce energy consumption of a Groundwater Heat Pump (GWHP) system and Aquifer Thermal Energy Storage (ATES) has also been successfully applied for heat storage over the past decades. However, there lacks of understanding in combining these strategies to improve thermo-economic performance of a GWHP system. In this paper, an integrated strategy was proposed to manage the operation of a GWHP system installed in a greenhouse in Hubei, China. On the one hand, the running of the pumping wells controlled by the hourly and daily thermal load was compared. The analysis of energy consumption shows that 25% amount of energy can be saved using the hourly-load model to maintain suitable temperature-band of the greenhouse. On the other hand, to maximize the extractable heat from the subsurface, the pumping and recharging wells were interchanged between heating and cooling seasons. The aquifer was then used as a ATES and thermal efficiency of heat pumps was increased up by 18% compared with a traditional mode. Conclusively, the Coefficient of Performance (COP) of the GWHP system increased from 3.8 to 6.2 in the cooling period and from 3.4 to 5.1 in the heating period. It suggests that this proposed approach was an efficient method to enhance thermo-economic performance of a GWHP system.

Well layout optimization for groundwater heat pump systems using the adjoint approach

Groundwater heat pump systems cause thermal anomalies in the aquifer that can impact upon downstream systems and reduce their efficiency. Therefore, it is important to optimally position the extraction and injection wells of such systems to avoid negative interactions and maximize the thermal potential of the aquifer. This paper presents a new method to determine optimal well layouts of groundwater heat pumps using the adjoint approach, which is an efficient way to solve the underlying PDE-constrained optimization problem. An integral part of the method is the numerical groundwater simulation, which here is based on the finite element method. In addition, a multi-start initialization strategy is introduced in an attempt to better reach the global optimum. The method is applied to a real case study with 10 groundwater heat pumps, i.e. 20 wells, and two optimization scenarios with different natural groundwater temperatures. In both scenarios, the optimization method successfully determines a well layout that maximizes groundwater temperatures at all extraction wells. Comparing the results from these scenarios demonstrates that hydro-geological conditions can have a significant impact on the optimal well layout. The proposed method is equally applicable to systems with multiple extraction and injection wells and can be extended to various other shallow geothermal applications, such as combined heating and cooling systems.

Seepage and heat transfer characteristics of sandstone thermal reservoir under aquitard damage

Flow and thermal breakthroughs are the main reasons for the reduction in the operational efficiency of a single-well groundwater heat pump (SWGWHP) system. Furthermore, a large initial pumping flow rate, attributed to the large initial heating/cooling load of the building, can consolidate and damage the aquifer. In this study, an aquitard damage test bench was constructed to investigate the formation and development of flow and thermal breakthroughs during the operation of the SWGWHP system by varying the initial recharge rate (Q), aquifer thickness (H), and the time difference between pumping and recharge (Δt); and to investigate the aquifer damage, seepage, and heat transfer patterns. The results indicated that pressure was the main factor that damaged the aquitard. Aquitard damage can cause or aggravate the flow breakthrough phenomenon. The flow breakthrough strength is the main factor affecting the thermal breakthrough strength. The results of the orthogonal test analysis indicate that the order of the primary and secondary influencing factors of the flow breakthrough strength and operating time efficiency is aquitard thickness (H) > initial recharge rate (Q) > pumping and recharging time difference (Δt). The existence of the aquitard reduced the flow breakthrough strength by 15.2–33.2% and prolonged the efficient operation time of the system by 2.5–5 times. Based on quantitative analyses of the flow and thermal breakthrough strengths, and the variation in the system coefficient of performance, this study reveals the reasons for the formation of thermal breakthrough and provides theoretical suggestions to ensure a safe and stable operation of the SWGWHP system. The results of the study can guide practical installations as well as future research on SWGWHP systems.

Flow and heat transfer characteristics of seepage-channel wall to injected suspended particles in a groundwater heat pump system

Well and aquifer clogging caused by deposition of particles is a serious problem that restricts the application and heat transfer efficiency of groundwater heat-pump systems. Mixed flow of particles and fluid in a porous medium was simplified to pipe flow based on a capillary model. A migration test bench for suspended particles was set up to explore the flow and heat transfer response of the seepage channel to the injecting suspension by varying suspended particle size (Dp), roughness of pipe wall (K), and injection pressure (Pin). The results show that the pressure drop of the suspension was influenced by the factors in the order Pin > Dp > K. Suspended particles were deposited at the entrance of the seepage channel, and deposition increased with increasing particle size. The temperature variation of the channel wall was sensitive to particle size. The temperature gradient of the suspension with a particle median size of 17.62 μm was 0.049 °C/min when the injection pressure was 1.8 kPa and wall roughness was 1.65 mm, which was lower than at 0.148 °C/min, when the particle median size was 25.12 μm. This research provides insights into the design and optimization of injection projects to prevent particle clogging.