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Analytical and Numerical Methods for a Preliminary Assessment of the Remediation Time of Pump and Treat Systems
Get PDFSeveral remediation technologies are currently used to address groundwater pollution. “Pump and treat” (P&T) is probably one of the most widely applied, being a process where contaminated groundwater is extracted from the subsurface by pumping and then treated before it is discharged or reinjected into the aquifer. Despite being a very adaptable technology, groundwater remediation is often achieved in long and unsustainable times because of limitations due to the hydrogeological setting and contaminant properties. Therefore, the cost–benefit analysis over time results in an inecient system and a preliminary evaluation of the clean-up time is crucial. The aim of the paper is to compare, in an integrated manner, the application of some models to estimate the time to compliance of a P&T system in relation to the specific hydrogeological condition. Analytical
solutions are analyzed and applied to an industrial site and to a synthetic case. For both cases, batch flushing and the advection-dispersion-retardation (ADR) model underestimate remediation times comparing the results to real or simulated monitoring data, whereas the Square Root model provided more reliable remediation times. Finally, for the synthetic case, the reliability of analytical approaches and the eects of matrix diusion are tested on the basis of a numerical groundwater transport model specifically implemented, which confirm the results of the analytical methods and the strong influence of the matrix diusion on the results
Borehole Heat Exchangers: heat transfer simulation in the presence of a groundwater flow
Get PDFThe correct design of the Borehole Heat Exchanger is crucial for the operation and the energy performance of a Ground Source Heat Pump. Most design methods and tools are based on the assumption that the ground is a solid medium where conduction is the only heat transfer mechanism. In turn in regions rich in groundwater the groundwater flow influence has to be assessed, by including the convection effects. In this paper a numerical model of a 100 m U-pipe in a saturated porous medium is presented. The model is created adopting MT3DMS coupled to MODFLOW. A Darcy flow is imposed across the medium. The typical operation of a Borehole Heat Exchanger operating both in winter and in summer is simulated for two years, under different groundwater velocities. The energy injected to and extracted from the ground is derived as a function of the Darcy velocity and compared with the purely conductive case. Temperature fields in the ground at key moments are shown and discussed. From both the energy and the aquifer temperature field points of view, the velocity ranges for respectively negligible and relevant influence of the groundwater flow are identified
Energy performance and thermal impact of a Borehole Heat Exchanger in a sandy aquifer: Influence of the groundwater velocity
No full textIn a saturated soil, the groundwater flow affects both the energy performance and the thermal impact on
the surrounding soil of Borehole Heat Exchangers linked to Ground-Source Heat Pumps. In this paper a
numerical model in MODFLOW/MT3DMS of a single U-pipe in a sandy aquifer is proposed in order to
investigate the two issues in a coupled approach. After validating the model, the typical yearly operation
of a Borehole Heat Exchanger extracting and injecting heat into the ground is simulated. For 0.1 6 Pe 6 1
cold and warm plumes develop and the heat rate increases non linearly from 11% to 105%
Borehole Heat Exchangers in aquifers: simulation of the grout material impact
No full textIn Ground Source Heat Pump installations the U-shaped pipes are located in boreholes filled with grouts that are enhanced from the thermal conductivity point of view and usually hydraulically impermeable. Here two cases are considered: (1) a numerical finite-difference model of a single U-pipe in a sandy aquifer, implemented using the MT3DMS code, where the grout material surrounding the Borehole Heat Exchanger is not simulated and (2) the same numerical model where, in turn, the presence of the grout is accounted for in a small volume around the Borehole Heat Exchanger, as in real in situ conditions. Simulations were carried out first keeping the heat rate constant, as when simulating a Thermal Response Test, and then maintaining the inlet temperature constant in order to reproduce the yearly operation of the Ground Source Heat Pump system.
Then, fitting the numerical ground response with analytical solutions and
comparing the two models, the effect of the grout material was assessed in
terms of exchanged energy and temperature distribution in the subsoil.
Results show that the presence of the grout around the Borehole Heat
Exchanger is negligible from the exchanged energy and temperature
distribution point of view. Whereas, looking at the Darcy velocities it was
noticed that the ones returned by the analytical solutions (that necessarily
neglects the presence of the grout) are at least 30% smaller than the
velocities implemented in the grouted model. So when comparing the
MT3D simulations, or using MT3D for TRT interpretation, the presence of
the grout around the BHE can at a first approximation be disregarded in
terms of both exchanged energies and temperature distribution in the
subsoil. In turn when Darcy velocity, obtained by Thermal Response Test
interpretation through analytical solutions (MLS), is used in a numerical
model it must be corrected specially for advection-dominated cases
Borehole geothermal exchanger simulation in aquifers: Modflow code performances compared to linear source analytical solution
No full textGeothermal energy, and especially the use of low enthalpy resources (T<20°C), has a rising importance worldwide; ground-coupled heat pump (GCHP) systems have been used increasingly because they are among the cleanest and most energy efficient air-conditioning systems for buildings. Simulation models can be applied for a more effective use of the subsoil for geothermal purposes. In fact they are useful tools for the design of efficient systems which consider also the need to not create abnormal temperature distributions in soil and aquifers. In the hydrogeology field the Modflow/MT3DMS codes are the most wide spread used programs to face environmental problems and forecast quantity and quality impacts on groundwater resources. Although Modflow/MT3DMS are used to represent open circuit heat pumps, they had never been used to represent borehole heat exchangers (BHE). Aim of this study is to simulate BHEs through Modflow/MT3DMS and compare its results with the analytic solution known as linear source for a case where ground water flow can be neglecte
Borehole Heat Exchangers: how flow velocity influence and dispersion influence heat transfer
No full textThe heat pumps coupled to geothermal systems likely to use low enthalpy resources (T<20°C) are gradually spreading, representing one of the most efficient and lower environmental impact technologies for cooling and heating of buildings. Most common geothermal systems are formed by closed loop boreholes (Borehole Heat Exchangers or BHEs) buried into the ground, typically 100 m deep, where a thermal-carrier fluid is circulated into polyethylene U-pipes, extracting heat from the ground in winter and/or injecting heat into the ground in summer.
The energy performance of these systems depends on the heat transfer process between the BHEs and the ground. In many applications the ground can be considered as a purely conductive medium: in fact this hypothesis is at the base of the most commercially availabele tools used to design BHEs, such as GHLEPRO or EED (Hellstrom 2001). Therefore some efforts have recently been carried out to include the effects of the presence of a groundwater flow into the BHEs modeling (Diao 2004). In this case the heat is transported not only by conduction but also by advection. To consider this extended problem could change both the correct prediction of the energy performance of the BHEs and their design and also the investigation of the thermal impact, in other words the temperature perturbation produced by the BHEs operation in surrounding aquifer. The aim of this work is the evaluation of these two aspects, varying the rate of groundwater flow velocity and dispersion coefficient using a numerical model realized through Modflow/MT3D (Angelotti 2014), already validated respect to the Moving Line Source (Molina-Giraldo 2011), demonstrating that both advection and dispersion play an important role in the heat transfer
Borehole Heat Exchanger simulations in aquifer: the borehole grout influence in thermal response test modeling
No full textGeothermal energy, and especially the use of low enthalpy resources, has a rising importance; ground-coupled heat pump (GCHP) systems have been used increasingly because they are among the cleanest and most energy efficient heating and cooling systems for buildings. Simulation models can be applied for a more effective use of the subsoil for geothermal purposes. In fact they are useful tools for the design of efficient systems which consider also the need to avoid abnormal temperature distributions in soil and aquifers. In the hydrogeology field the Modflow/MT3DMS codes are the most widely used programs to face environmental problems and forecast quantity and quality impacts on groundwater resources. Although Modflow/MT3DMS are used to represent open circuit heat pumps, they had never been used to represent borehole heat exchangers (BHE). Aim of this study is to simulate a Thermal Response Test (TRT) through Modflow/MT3MS codes implementing into the model all the components of a Ground Heat Exchanger system: from the U-shaped BHE to the grout material surrounding it. For GCHP systems, the TRT is commonly used to determine the heat transport parameters of the subsurface as thermal conductivity and thermal diffusivity. It also allows to determine the BHE thermal resistance which mainly depends on the geometry of the dug area and on the thermal properties of the surrounding grout material. Starting from a model implemented in a previous work (Angelotti et. al., 2014) Modflow has been used for the simulation of a TRT. Two cases have been analysed and compared: in the first the cells around the BHE are assigned the aquifer parameters while in the second they have been assigned the characteristics of the grout material (hydraulically impermeable but highly conductive from the thermal point of view). A 10-6 m/s groundwater Darcy velocity case is analysed applying a 40 W/m specific heat rate to the fluid circulating into the BHE. Considering the temperature distribution in the ground, the differences between the two simulated cases (with/without-grout) result essentially negligible presenting a maximum value equal to 0.07 °C in the proximity of the BHE (around 20 cm from the centre of the U-pipes). A slightly more relevant difference (0.55°C) in simulated temperature is registered very close to the BHE, into the area occupied by the grout (only 6 cm from the centre of the U-pipes). This is due to the absence of the advective term in the zone where the grout is present and the heat transfer is linked just to conduction term. By the way there are almost no differences in the temperature of the heat-carrier fluid in the two cases (0.02 °C). These results lead to conclude that, simulating a TRT, the effect of the grout is negligible, therefore is advantageous to avoid modeling its geometry and thermal/physical parameters so as to save time and energies in the model implementation phase
Thermal impact in aquifers and energy performance evaluation of borehole heat exchanger by the use of numerical modelling
Get PDFGround-Source Heat Pump (GSHP) are among the cleanest and most energy efficient systems for heating or cooling buildings as alternative to traditional methods. The study verified the possibility both to use GSHP systems in Zootechnical area and to numerically model these systems for thermal impact evaluations in aquifer and energy performances.
A numerical model of a U-shape pipe of BHE in a saturated sandy porous medium was implemented in MODFLOW/MT3DMS codes, although this implied strong grid refinement and extensive computational resources. The first goal of the modelling phase was to simulate a BHE and compare its results with an analytic solution (generally used for thermal response test interpretation), known as linear source, for a case where groundwater flow was neglected (Gehlin et al., 2002). In most cases the groundwater flow influence on the energy performance of BHEs needs to be assessed and a Moving Line Source (MLS) solution was proposed by Molina Giraldo et al. (2011) in order to examine the interaction between BHE and groundwater flow. The temperature distributions in aquifer, outputs of the numerical model run in a constant heat rate mode, was then fitted with respect to MLS solution and the results proved that the numerical model was sufficiently accurate (Angelotti et al., 2014).
After the validation, the numerical model was adapted to a constant inlet fluid temperature case, in order to reproduce a typical operation of a GSHP. In particular, the thermal-carrier fluid was circulated into U-pipes of BHE, extracting/injecting heat from/into the ground. The results achieved in these simulations concerned predicted exchanged energy and temperature distribution: groundwater flow velocity, varied to represent a wide ensemble of hydrogeological systems, and thermal dispersivity coefficient (advection and dispersion terms usually neglected by most of energy codes) played an important role in the heat transfer (Angelotti et al., 2014).
At last, the numerical model was adapted to a real case, EcoZoo project, funded by Lombardy Region and Italian Ministry of Research and Education and supervised by Tethys Srl. The system was composed of five BHEs 60 meters deep, a heat pump, an air handling unit with heat recovery system; it provided heating, cooling and air exchange to a piglet room in Experimental Didactic Zootechnical Centre of the University of Milan. The monitoring system, set up in LabVIEW (an object base programming code), comprised the measurements of a set of different parameters, concerning energy and temperature in subsoil, to enable the numerical modelling of the environmental impact on groundwater system. The acquired parameters allowed the validation of numerical model through the comparison with the monitored data. Through the calibration of hydraulic and thermal parameters, a good agreement between numerical and real data was found. Therefore, the numerical model correctly reproduced the heat transfer in aquifer. It was also possible to thermally typify the aquifer and to reproduce the real length of thermal plume, after a heating operation. The presence of experimental data allowed to evaluate the capability of MODFLOW/MT3DMS, providing for the lack of experimental data in literature
Zoo-technical application of Ground Source Heat Pumps: a pilot case study
Get PDFGround Source Heat Pumps are energy-efficient HVAC systems usually adopted in residential
and commercial buildings. However the control of the thermal environment is required not only
in spaces occupied by people, but also in intensive breeding farms, in order to maintain healthy
conditions and to increase productivity. In the Italian livestock breedings, heating is usually
provided by means of gas or Diesel burners directly installed in the stable. An important part of
the heating load is due to the large ventilation rates required for the livestock wellbeing.
Cooling is either absent or achieved by evaporative systems that also increase the humidity level
in the stables, thus requiring even larger ventilation rates. Therefore the applicability of
geothermal heating and cooling in breeding farms was analysed in a research project co-funded
by the Lombardy Region and the Italian Ministry of Research and Education. A pilot system for
heating, cooling and ventilation was designed and installed in a piglets room at the
Experimental and Didactic Zoo-technical Center of the University of Milan. Five Borehole Heat
Exchangers (BHEs), installed down to a depth of 60 meters into an alluvial aquifer, were
coupled with a Ground Source Heat Pump. The heat pump provides heating and cooling to an
Air Handling Unit, including a Heat Recovery system. A monitoring system was installed in
order to measure comfort conditions in the piglet room, operating conditions and energy
consumption of the HVAC system, together with the spreading of the thermal plume in the
ground. In this paper the results of a monitoring campaign carried out in a typical winter period
are presented and discussed. The overall energy efficiency of the system, expressed in terms of a
COP, results to be equal to 4.04. A comparison between the pilot HVAC system and a
traditional one is also carried out, showing that the proposed solution can provide over 40%
primary energy saving. Following, cost savings in energy bills for farmers are found, although
the ratio between electricity cost and fuel cost is a key parameter
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