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A special issue of Energies (ISSN 1996-1073).

Deadline for manuscript submissions: closed (31 July 2017) | Viewed by 61536

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Prof. Dr. Rajandrea Sethi

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Guest Editor

DIATI—Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy
Interests: ground water engineering; shallow geothermal energy
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Dr. Alessandro Casasso

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Guest Editor

Politecnico di Torino—Department of Environment, Land and Infrastructure Engineering (DIATI), Corso Duca Degli Abruzzi 24, 10129 Torino, Italy
Interests: shallow geothermal energy; groundwater engineering; carbon footprint
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Special Issue Information

Dear Colleagues,

Geothermal energy is an important renewable energy source with a wide range of uses, such as the production of electricity, process heat, greenhouses, and building heating and cooling. It is also one of the most multi-disciplinary fields of energy engineering, encompassing a large number of sciences, such as heat transfer, geology, hydrogeology, chemistry, and economy. The focus of this Special Issue is to provide an overview of the ongoing research on low enthalpy geothermal resources, which is an emerging branch of geothermal energy. Research papers on innovative plants (both ground source heat pumps and direct heat uses) and on the assessment of low enthalpy geothermal resources are welcome. The presentation of international collaborative projects of research, technology transfer, and policy development is also appreciated. In addition, statistic and economic studies on shallow geothermal technologies are of particular interest, along with the assessment of environmental impacts of the exploitation of such geothermal resources.

Prof. Rajandrea Sethi
Dr. Alessandro Casasso
Guest Editors

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Keywords

Ground Source Heat Pump (GSHP)
Ground Water Heat Pumps (GWHP)
Low enthalpy
Energy economics
Geothermal Energy

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Published Papers (10 papers)

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Research

5348 KiB

Open AccessEditor’s ChoiceArticle

Factors Influencing the Thermal Efficiency of Horizontal Ground Heat Exchangers

by Eloisa Di Sipio and David Bertermann

Energies 2017, 10(11), 1897; https://doi.org/10.3390/en10111897 - 18 Nov 2017

Cited by 36 | Viewed by 6587

Abstract

The performance of very shallow geothermal systems (VSGs), interesting the first 2 m of depth from ground level, is strongly correlated to the kind of sediment locally available. These systems are attractive due to their low installation costs, less legal constraints, easy maintenance and possibility for technical improvements. The Improving Thermal Efficiency of horizontal ground heat exchangers Project (ITER) aims to understand how to enhance the heat transfer of the sediments surrounding the pipes and to depict the VSGs behavior in extreme thermal situations. In this regard, five helices were installed horizontally surrounded by five different backfilling materials under the same climatic conditions and tested under different operation modes. The field test monitoring concerned: (a) monthly measurement of thermal conductivity and moisture content on surface; (b) continuous recording of air and ground temperature (inside and outside each helix); (c) continuous climatological and ground volumetric water content (VWC) data acquisition. The interactions between soils, VSGs, environment and climate are presented here, focusing on the differences and similarities between the behavior of the helix and surrounding material, especially when the heat pump is running in heating mode for a very long time, forcing the ground temperature to drop below 0 °C. Full article

(This article belongs to the Special Issue Low Enthalpy Geothermal Energy)

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4603 KiB

Open AccessArticle

Dynamic Modeling and Simulation of Deep Geothermal Electric Submersible Pumping Systems

by Julian Kullick and Christoph M. Hackl

Energies 2017, 10(10), 1659; https://doi.org/10.3390/en10101659 - 21 Oct 2017

Cited by 7 | Viewed by 7182

Abstract

Deep geothermal energy systems employ electric submersible pumps (ESPs) in order to lift geothermal fluid from the production well to the surface. However, rough downhole conditions and high flow rates impose heavy strain on the components, leading to frequent failures of the pump system. As downhole sensor data is limited and often unrealible, a detailed and dynamical model system will serve as basis for deeper understanding and analysis of the overall system behavior. Furthermore, it allows to design model-based condition monitoring and fault detection systems, and to improve controls leading to a more robust and efficient operation. In this paper, a detailed state-space model of the complete ESP system is derived, covering the electrical, mechanical and hydraulic subsystems. Based on the derived model, the start-up phase of an exemplary yet realistic ESP system in the Megawatt range—located at a setting depth of 950

m

and producing geothermal fluid of 140

^{\circ} C

temperature at a rate of 0.145

m^{3} s^{- 1}

—is simulated in MATLAB/Simulink. The simulation results show that the system reaches a stable operating point with realistic values. Furthermore, the effect of self-excitation between the filter capacitor and the motor inductor can clearly be observed. A full set of parameters is provided, allowing for direct model implementation and reproduction of the presented results. Full article

(This article belongs to the Special Issue Low Enthalpy Geothermal Energy)

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9608 KiB

Open AccessArticle

Development and Experimental Validation of a TRNSYS Dynamic Tool for Design and Energy Optimization of Ground Source Heat Pump Systems

by Félix Ruiz-Calvo, Carla Montagud, Antonio Cazorla-Marín and José M. Corberán

Energies 2017, 10(10), 1510; https://doi.org/10.3390/en10101510 - 28 Sep 2017

Cited by 28 | Viewed by 7261

Abstract

Ground source heat pump (GSHP) systems stand for an efficient technology for renewable heating and cooling in buildings. To optimize not only the design but also the operation of the system, a complete dynamic model becomes a highly useful tool, since it allows testing any design modifications and different optimization strategies without actually implementing them at the experimental facility. Usually, this type of systems presents strong dynamic operating conditions. Therefore, the model should be able to predict not only the steady-state behavior of the system but also the short-term response. This paper presents a complete GSHP system model based on an experimental facility, located at Universitat Politècnica de València. The installation was constructed in the framework of a European collaborative project with title GeoCool. The model, developed in TRNSYS, has been validated against experimental data, and it accurately predicts both the short- and long-term behavior of the system. Full article

(This article belongs to the Special Issue Low Enthalpy Geothermal Energy)

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14744 KiB

Open AccessArticle

Thermal Impact Assessment of Groundwater Heat Pumps (GWHPs): Rigorous vs. Simplified Models

by Bruno Piga, Alessandro Casasso, Francesca Pace, Alberto Godio and Rajandrea Sethi

Energies 2017, 10(9), 1385; https://doi.org/10.3390/en10091385 - 12 Sep 2017

Cited by 38 | Viewed by 5219

Abstract

Groundwater Heat Pumps (GWHPs) are increasingly adopted for air conditioning in urban areas, thus reducing CO₂ emissions, and this growth needs to be managed to ensure the sustainability of the thermal alteration of aquifers. However, few studies have addressed the propagation of thermal plumes from open-loop geothermal systems from a long-term perspective. We provide a comprehensive sensitivity analysis, performed with numerical finite-element simulations, to assess how the size of the thermally affected zone is driven by hydrodynamic and thermal subsurface properties, the vadose zone and aquifer thickness, and plant setup. In particular, we focus the analysis on the length and width of thermal plumes, and on their time evolution. Numerical simulations are compared with two simplified methods, namely (i) replacing the time-varying thermal load with its yearly average and (ii) analytical formulae for advective heat transport in the aquifer. The former proves acceptable for the assessment of plume length, while the latter can be used to estimate the width of the thermally affected zone. The results highlight the strong influence of groundwater velocity on the plume size and, especially for its long-term evolution, of ground thermal properties and of subsurface geometrical parameters. Full article

(This article belongs to the Special Issue Low Enthalpy Geothermal Energy)

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2709 KiB

Open AccessFeature PaperArticle

On the Influence of Operational and Control Parameters in Thermal Response Testing of Borehole Heat Exchangers

by Borja Badenes, Miguel Ángel Mateo Pla, Lenin G. Lemus-Zúñiga, Begoña Sáiz Mauleón and Javier F. Urchueguía

Energies 2017, 10(9), 1328; https://doi.org/10.3390/en10091328 - 3 Sep 2017

Cited by 19 | Viewed by 4092

Abstract

Thermal response test (TRT) is a common procedure for characterization of ground and borehole thermal properties needed for the design of a shallow geothermal heat pump system. In order to investigate and to develop more accurate and robust procedures for TRT control, modelling, and evaluation in semi-permeable soils with large water content, a pilot borehole heat exchanger was built in the main campus of the Universitat Politècnica de València. The present work shows the results of the experiments performed at the site, analysing the improvements that have been introduced both in the control of the heat injected during TRTs and in the methods to infer the ground thermal parameter. Three models are compared: two based on the infinite-line source theory and one based on the finite-line source scheme. The models were tested under two possible configurations of the equipment, i.e., with and without strict control of injected heat. Our results show the importance of heat injection control for a robust parameter assessment and the existence of additional heat transfer processes that the used models cannot completely characterize and that are related to the presence of significant groundwater flow at the site. In addition, our experience with the current installation and the knowledge about its strengths and weaknesses have allowed us to design a new and more complete test-site to help in the analysis and validation of new ground heat exchanger geometries. Full article

(This article belongs to the Special Issue Low Enthalpy Geothermal Energy)

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8202 KiB

Open AccessArticle

Thermal Behaviour under Service Loads of a Thermo-Active Precast Pile

by Borja Badenes, Teresa Magraner, Cristina De Santiago, Fernando Pardo de Santayana and Javier F. Urchueguía

Energies 2017, 10(9), 1315; https://doi.org/10.3390/en10091315 - 1 Sep 2017

Cited by 5 | Viewed by 3979

Abstract

A research project was developed in Spain to undertake some studies on the geothermal use of pile foundations (PITERM PROJECT). The experiment consists of a specifically designed, constructed and fully monitored geothermal precast pile driven at Polytechnic University of Valencia. An important distinctive feature of the developed pile was the fact that it was assembled from two identical sections connected with a specific joint, developed by Rodio-Kronsa. This allows the installation of much longer precast piles into the ground. The pile is under two types of loads: mechanical and thermal. The mechanical load was applied by means of a mechanical frame anchored to the ground and three additional anchors used to induce an active compressive force. The thermal load was produced by means of a thermal rig able to inject heat or extract heat from the pile at any desired programable heat injection/extraction rate. One of the features of this precast pile is its geometry, similar to a single U borehole heat exchanger (BHE) which is not common in thermoactive piles, usually equipped with probes attached to the armatures. In our study, we have characterized the thermal behaviour of the precast pile experimentally and simulated its temperature response by means of a TRNSYS model. This article describes part of a test series carried out where the mechanical and thermal behaviour of a pile subjected to thermal and mechanical loads simulating a real pile in a building was studied. Therefore, this publication has only focused on the thermal performance of the pile and its thermal modelling by computer. From this model, the thermal parameters of the soil–pile system have been extracted and compared with those of a single standard single U BHE. In essence, our assessment points to a quite similar thermal behaviour of the studied precast pile compared to a conventional single U borehole heat exchanger of the same length and equivalent diameter, while the installation costs of such elements would be substantially lower due to its double, structural and thermal, function. Full article

(This article belongs to the Special Issue Low Enthalpy Geothermal Energy)

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6089 KiB

Open AccessFeature PaperArticle

Thermal Response Testing Results of Different Types of Borehole Heat Exchangers: An Analysis and Comparison of Interpretation Methods

by Angelo Zarrella, Giuseppe Emmi, Samantha Graci, Michele De Carli, Matteo Cultrera, Giorgia Dalla Santa, Antonio Galgaro, David Bertermann, Johannes Müller, Luc Pockelé, Giulia Mezzasalma, Davide Righini, Mario Psyk and Adriana Bernardi

Energies 2017, 10(6), 801; https://doi.org/10.3390/en10060801 - 13 Jun 2017

Cited by 37 | Viewed by 6087

Abstract

The design phase of ground source heat pump systems is an extremely important one as many of the decisions made at that time can affect the system’s energy performance as well as installation and operating costs. The current study examined the interpretation of thermal response testing measurements used to evaluate the equivalent ground thermal conductivity and thus to design the system. All the measurements were taken at the same geological site located in Molinella, Bologna (Italy) where a variety of borehole heat exchangers (BHEs) had been installed and investigated within the project Cheap-GSHPs (Cheap and efficient application of reliable Ground Source Heat exchangers and Pumps) of the European Union’s Horizon 2020 research and innovation program. The measurements were initially analyzed in accordance with the common interpretation based on the first-order approximation of the solution for the infinite line source model and then by utilizing the complete solutions of both the infinite line and cylinder source models. An inverse numerical approach based on a detailed model that considers the current geometry of the BHE and the axial heat transfer as well as the effect of weather on the ground surface was also used. Study findings revealed that the best result was generally obtained using the inverse numerical interpretation. Full article

(This article belongs to the Special Issue Low Enthalpy Geothermal Energy)

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2485 KiB

Open AccessArticle

A Numerical Study on the Impact of Grouting Material on Borehole Heat Exchangers Performance in Aquifers

by Luca Alberti, Adriana Angelotti, Matteo Antelmi and Ivana La Licata

Energies 2017, 10(5), 703; https://doi.org/10.3390/en10050703 - 17 May 2017

Cited by 30 | Viewed by 5023

Abstract

U-pipes for ground source heat pump (GSHP) installations are generally inserted in vertical boreholes back-filled with pumpable grouts. Grout thermal conductivity is a crucial parameter, dominating the borehole thermal resistance and impacting the heat exchanger efficiency. In order to seal the borehole and prevent leakages of the heat carrier fluid, grouting materials are also hydraulically impermeable, so that groundwater flow inside the borehole is inhibited. The influence of groundwater flow on the borehole heat exchangers (BHE) performance has recently been highlighted by several authors. However groundwater impact and grouting materials influence are usually evaluated separately, disregarding any combined effect. Therefore simulation is used to investigate the role of the thermal and hydraulic conductivities of the grout when the BHE operates in an aquifer with a relevant groundwater flow. Here 3 main cases for a single U-pipe in a sandy aquifer are compared. In Case 1 the borehole is back-filled with the surrounding soil formation, while a thermally enhanced grout and a low thermal conductivity grout are considered in Case 2 and Case 3 respectively. Simulations are carried out maintaining the inlet temperature constant in order to reproduce the yearly operation of the GSHP system. For each of the 3 cases three different groundwater flow velocities are considered. The results show that a high thermal conductivity grout further enhances the effects of a significant groundwater flow. The conditions when neglecting the grout material in the numerical model does not lead to relevant errors are also identified. Full article

(This article belongs to the Special Issue Low Enthalpy Geothermal Energy)

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3118 KiB

Open AccessArticle

Efficiency Analysis of the Main Components of a Vertical Closed-Loop System in a Borehole Heat Exchanger

by Cristina Sáez Blázquez, Arturo Farfán Martín, Ignacio Martín Nieto, Pedro Carrasco García, Luis Santiago Sánchez Pérez and Diego González-Aguilera

Energies 2017, 10(2), 201; https://doi.org/10.3390/en10020201 - 10 Feb 2017

Cited by 25 | Viewed by 9262

Abstract

In vertical closed-loop systems, it is common to use single or double U-tube heat exchangers separated by longitudinal spacers. In addition, the helical-shaped pipe is another configuration that requires lower drilling lengths but it is less used. The aim of the present research is to study the influence of these components on the total efficiency of a borehole heat exchanger (BHE). Thus, the differences between using single/double U-tubes (with or without spacers) and helical pipes are analysed in terms of efficiency. Through different laboratory tests, a small vertical closed-loop system was simulated in order to analyse all these possible configurations. The grouting materials and the temperatures of the ground were modified at the same time in these tests. Regarding the heat exchange process between the ground and the heat carrier fluid, it must be highlighted that the best results were obtained for the helical-shaped pipe configuration. Some of the improvements offered by this heat exchanger typology with respect to the vertical configuration is that a lower drilling depth is required even it requires a larger diameter. This leads to significant economic savings in the performing drilling process. Finally, it is also worth noting the importance of using spacers in vertical U-tubes and that no improvements have been found regarding the use of single or double configuration of U-tubes. Thanks to the laboratory results derived from this study it is possible to establish the optimum behaviour pattern for the entire vertical closed-loop systems. Full article

(This article belongs to the Special Issue Low Enthalpy Geothermal Energy)

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2311 KiB

Open AccessArticle

Dimensionless Maps for the Validity of Analytical Ground Heat Transfer Models for GSHP Applications

by Paolo Conti

Energies 2016, 9(11), 890; https://doi.org/10.3390/en9110890 - 29 Oct 2016

Cited by 19 | Viewed by 4747

Abstract

This article provides plain and handy expressions to decide the most suitable analytical model for the thermal analysis of the ground source in vertical ground-coupled heat pump applications. We perform a comprehensive dimensionless analysis of the reciprocal deviation among the classical infinite, finite, linear and cylindrical heat source models in purely conductive media. Besides, we complete the framework of possible boreholes model with the “hollow” finite cylindrical heat source solution, still lacking in the literature. Analytical expressions are effective tools for both design and performance assessment: they are able to provide practical and general indications on the thermal behavior of the ground with an advantageous tradeoff between calculation efforts and solution accuracy. This notwithstanding, their applicability to any specific case is always subjected to the coherence of the model assumptions, also in terms of length and time scales, with the specific case of interest. We propose several dimensionless criteria to evaluate when one model is practically equivalent to another one and handy maps that can be used for both design and performance analysis. Finally, we found that the finite line source represents the most suitable model for borehole heat exchangers (BHEs), as it is applicable to a wide range of space and time scales, practically providing the same results of more complex models. Full article

(This article belongs to the Special Issue Low Enthalpy Geothermal Energy)

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