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

Deadline for manuscript submissions: closed (31 December 2017) | Viewed by 51069

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Prof. Dr. Simon Rees

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School of Civil Engineering, University of Leeds, Leeds, West Yorkshire LS2 9JT, UK
Interests: building energy; geothermal heating and cooling; energy geotechnics; thermal energy storage; thermal energy networks; building simulation methods
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Special Issue Information

Dear Colleagues,

We are inviting contributions to a Special Issue of Energies with the theme of “Geothermal Heating and Cooling”.

Exploiting the ground as a heat exchange system offers many possibilities for highly efficient applications in building heating, cooling, and thermal storage at a wide range of scales. Research in this technology has grown substantially in the last decade, and continues to grow as markets develop rapidly outside of Europe and North America. Although there are known to be more than four million ground source heat pump systems in operation, research initiatives continue, for example, to improve design methods, system integration and control, operational performance, as well as improve economics through approaches, such as novel heat exchangers and system hybridization strategies. Contributions on these and other topics are welcomed from academic and industrial researchers, as well as practitioners and system operators.

Geothermal heating and cooling technologies cross traditional disciplinary boundaries between Building Physics, Energy, Geotechnics, and Geology. A noticeable trend in the research literature is the increasing engagement of geotechnical engineers as opportunities to combine foundation elements and geothermal heat exchange systems are studied and realized. Papers addressing both thermal and geotechnical aspects of site investigation, design methods and operational performance are accordingly welcome.

Dr Simon Rees
Guest Editor

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Keywords

Ground heat transfer
Thermal Resistance
Borehole heat exchangers
Energy Piles and walls
Ground Thermal Properties
Design Methods
Energy storage
System Integrration
Heat Pump developments
Operational Performance
Hybrid systems
System control.

Published Papers (11 papers)

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Research

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21 pages, 22211 KiB

Open AccessArticle

Suitability Evaluation of Specific Shallow Geothermal Technologies Using a GIS-Based Multi Criteria Decision Analysis Implementing the Analytic Hierarchic Process

by Francesco Tinti, Sara Kasmaee, Mohamed Elkarmoty, Stefano Bonduà and Villiam Bortolotti

Energies 2018, 11(2), 457; https://doi.org/10.3390/en11020457 - 22 Feb 2018

Cited by 26 | Viewed by 6350

Abstract

The exploitation potential of shallow geothermal energy is usually defined in terms of site-specific ground thermal characteristics. While true, this assumption limits the complexity of the analysis, since feasibility studies involve many other components that must be taken into account when calculating the effective market viability of a geothermal technology or the economic value of a shallow geothermal project. In addition, the results of a feasibility study are not simply the sum of the various factors since some components may be conflicting while others will be of a qualitative nature only. Different approaches are therefore needed to evaluate the suitability of an area for shallow geothermal installation. This paper introduces a new GIS platform-based multicriteria decision analysis method aimed at comparing as many different shallow geothermal relevant factors as possible. Using the Analytic Hierarchic Process Tool, a geolocalized Suitability Index was obtained for a specific technological case: the integrated technologies developed within the GEOTeCH Project. A suitability map for the technologies in question was drawn up for Europe. Full article

(This article belongs to the Special Issue Geothermal Heating and Cooling)

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16 pages, 4963 KiB

Open AccessArticle

Detailed Theoretical Characterization of a Transcritical CO₂ Direct Expansion Ground Source Heat Pump Water Heater

by Parham Eslami-Nejad, Messaoud Badache, Arash Bastani and Zine Aidoun

Energies 2018, 11(2), 387; https://doi.org/10.3390/en11020387 - 07 Feb 2018

Cited by 13 | Viewed by 3750

Abstract

A new avenue in modern heat pump technology is related to the use of natural refrigerants such as carbon dioxide (CO₂). The use of CO₂ in direct expansion ground source heat pumps (DX-GSHP) has also gained significant interest as it offers opportunities for cost reduction of the ground loop, albeit some challenges remain in their development, design and use. To address these challenges and to characterize CO₂-DX-GSHP performance for water heating applications, a detailed theoretical model and a fully-instrumented test apparatus was developed and built at CanmetENERGY Research Laboratory. The theoretical model was validated against a set of experimental results and adopted to investigate the performance of the system over a wide operating range. Validation results showed that the model predicts the experimental results within the measurement uncertainty. A detailed system performance analysis was also performed using the theoretical model to understand the system behavior and explore the actions required for performance improvement in future installations. The results of the analysis showed that improper design and control of some components, such as the gas cooler and ground heat exchanger can degrade the system performance by up to 25%, and the heat pump heating capacity by 7.5%. Full article

(This article belongs to the Special Issue Geothermal Heating and Cooling)

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22 pages, 7520 KiB

Open AccessArticle

Applying Petroleum the Pressure Buildup Well Test Procedure on Thermal Response Test—A Novel Method for Analyzing Temperature Recovery Period

by Tomislav Kurevija, Kristina Strpić and Sonja Koščak-Kolin

Energies 2018, 11(2), 366; https://doi.org/10.3390/en11020366 - 04 Feb 2018

Cited by 12 | Viewed by 6296

Abstract

The theory of Thermal Response Testing (TRT) is a well-known part of the sizing process of the geothermal exchange system. Multiple parameters influence the accuracy of effective ground thermal conductivity measurement; like testing time, variable power, climate interferences, groundwater effect, etc. To improve the accuracy of the TRT, we introduced a procedure to additionally analyze falloff temperature decline after the power test. The method is based on a premise of analogy between TRT and petroleum well testing, since the origin of both procedures lies in the diffusivity equation with solutions for heat conduction or pressure analysis during radial flow. Applying pressure build-up test interpretation techniques to borehole heat exchanger testing, greater accuracy could be achieved since ground conductivity could be obtained from this period. Analysis was conducted on a coaxial exchanger with five different power steps, and with both direct and reverse flow regimes. Each test was set with 96 h of classical TRT, followed by 96 h of temperature decline, making for almost 2000 h of cumulative borehole testing. Results showed that the ground conductivity value could vary by as much as 25%, depending on test time, seasonal period and power fluctuations, while the thermal conductivity obtained from the falloff period provided more stable values, with only a 10% value variation. Full article

(This article belongs to the Special Issue Geothermal Heating and Cooling)

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19 pages, 1646 KiB

Open AccessArticle

Economic Optimal HVAC Design for Hybrid GEOTABS Buildings and CO₂ Emissions Analysis

by Damien Picard and Lieve Helsen

Energies 2018, 11(2), 314; https://doi.org/10.3390/en11020314 - 01 Feb 2018

Cited by 8 | Viewed by 4506

Abstract

In the early design phase of a building, the task of the Heating, Ventilation and Air Conditioning (HVAC) engineer is to propose an appropriate HVAC system for a given building. This system should provide thermal comfort to the building occupants at all time, meet the building owner’s specific requirements, and have minimal investment, running, maintenance and replacement costs (i.e., the total cost) and energy use or environmental impact. Calculating these different aspects is highly time-consuming and the HVAC engineer will therefore only be able to compare a (very) limited number of alternatives leading to suboptimal designs. This study presents therefore a Python tool that automates the generation of all possible scenarios for given thermal power profiles and energy load and a given database of HVAC components. The tool sizes each scenario properly, computes its present total cost (PC) and the total CO

_{2}

emissions associated with the building energy use. Finally, the different scenarios can be searched and classified to pick the most appropriate scenario. The tool uses static calculations based on standards, manufacturer data and basic assumptions similar to those made by engineers in the early design phase. The current version of the tool is further focused on hybrid GEOTABS building, which combines a GEOthermal heat pump with a Thermally Activated System (TABS). It should further be noted that the tool optimizes the HVAC system but not the building envelope, while, ideally, both should be simultaneously optimized. Full article

(This article belongs to the Special Issue Geothermal Heating and Cooling)

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19 pages, 6093 KiB

Open AccessArticle

Numerical Investigation on the Heat Extraction Capacity of Dual Horizontal Wells in Enhanced Geothermal Systems Based on the 3-D THM Model

by Zhixue Sun, Ying Xin, Jun Yao, Kai Zhang, Li Zhuang, Xuchen Zhu, Tong Wang and Chuanyin Jiang

Energies 2018, 11(2), 280; https://doi.org/10.3390/en11020280 - 24 Jan 2018

Cited by 32 | Viewed by 4682

Abstract

The Enhanced Geothermal System (EGS) constructs an artificial thermal reservoir by hydraulic fracturing to extract heat economically from hot dry rock. As the core element of the EGS heat recovery process, mass and heat transfer of working fluid mainly occurs in fractures. Since the direction of the natural and induced fractures are generally perpendicular to the minimum principal stress in the formation, as an effective stimulation approach, horizontal well production could increase the contact area with the thermal reservoir significantly. In this paper, the thermal reservoir is developed by a dual horizontal well system and treated as a fractured porous medium composed of matrix rock and discrete fracture network. Using the local thermal non-equilibrium theory, a coupled THM mathematical model and an ideal 3D numerical model are established for the EGS heat extraction process. EGS heat extraction capacity is evaluated in the light of thermal recovery lifespan, average outlet temperature, heat production, electricity generation, energy efficiency and thermal recovery rate. The results show that with certain reservoir and production parameters, the heat production, electricity generation and thermal recovery lifespan can achieve the commercial goal of the dual horizontal well system, but the energy efficiency and overall thermal recovery rate are still at low levels. At last, this paper puts forward a series of optimizations to improve the heat extraction capacity, including production conditions and thermal reservoir construction design. Full article

(This article belongs to the Special Issue Geothermal Heating and Cooling)

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17 pages, 1738 KiB

Open AccessArticle

Explicit Multipole Formulas for Calculating Thermal Resistance of Single U-Tube Ground Heat Exchangers

by Johan Claesson and Saqib Javed

Energies 2018, 11(1), 214; https://doi.org/10.3390/en11010214 - 16 Jan 2018

Cited by 21 | Viewed by 4127

Abstract

Borehole thermal resistance is both an important design parameter and a key performance characteristic of a ground heat exchanger. Another quantity that is particularly important for ground heat exchangers is the internal thermal resistance between the heat exchanger pipes. Both these resistances can be calculated to a high degree of accuracy by means of the well-known multipole method. However, the multipole method has a fairly intricate mathematical algorithm and is thus not trivial to implement. Consequently, there is considerable interest in developing explicit formulas for calculating borehole resistances. This paper presents derivation and solutions of newly derived second-order and higher-order multipole formulas for calculating borehole thermal resistance and total internal thermal resistance of single U-tube ground heat exchangers. A new and simple form of the first-order multipole formula is also presented. The accuracy of the presented formulas is established by comparing them to the original multipole method. The superiority of the new higher-order multipole formulas over the existing formulas is also demonstrated. Full article

(This article belongs to the Special Issue Geothermal Heating and Cooling)

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17 pages, 3638 KiB

Open AccessArticle

Experimental Study on the Heat Release Operational Characteristics of a Soil Coupled Ground Heat Exchanger with Assisted Cooling Tower

by Weibo Yang, Binbin Yang and Rui Xu

Energies 2018, 11(1), 90; https://doi.org/10.3390/en11010090 - 01 Jan 2018

Cited by 4 | Viewed by 3137

Abstract

Hybrid ground source heat pump systems (HGSHPS) with assisted cooling towers is one of the most efficient cooling and heating technologies for buildings with cooling-dominated loads. For the system, the coupled heat release mode between the ground heat exchanger (GHE) and cooling tower is vital for underground soil temperature recovery characteristics and system operation performance. In order to obtain the heat release operation characteristics with different coupled modes of the GHE and cooling tower, a set of multi-functional heat release experimental systems of soil coupled GHE with assisted cooling tower was constructed. The experimental investigations on the system heat release operation characteristics operated in the separate GHE heat release mode, combination heat release mode and day and night alternate heat release mode were undertaken based on the experimental system. The results show that for the separate GHE heat release mode, the heat release rate of GHE rises rapidly during the first two hours of operation, then, gradually tends to be steady, and the soil excess temperatures at various depths gradually rise with time. For the combination heat release mode with continuous operation of cooling tower, in view of reducing soil heat accumulation and accelerating soil temperature recovery, it is more conducive to the heat release by opening the cooling tower on sunny days. For the combination heat release mode with intermittent operation of cooling tower, when the total time ratio of cooling tower running to stop is constant, the intermittent time is longer, the better the effect of soil temperature recovery. Additionally, the soil temperature recovery rate can be improved greatly by the release heat operation of cooling tower during night, and the longer the cooling tower runs, the closer the soil temperature is to the initial temperature. Full article

(This article belongs to the Special Issue Geothermal Heating and Cooling)

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4617 KiB

Open AccessArticle

Evaluation of the Internal and Borehole Resistances during Thermal Response Tests and Impact on Ground Heat Exchanger Design

by Louis Lamarche, Jasmin Raymond and Claude Hugo Koubikana Pambou

Energies 2018, 11(1), 38; https://doi.org/10.3390/en11010038 - 25 Dec 2017

Cited by 16 | Viewed by 3681

Abstract

The main parameters evaluated with a conventional thermal response test (TRT) are the subsurface thermal conductivity surrounding the borehole and the effective borehole thermal resistance, when averaging the inlet and outlet temperature of a ground heat exchanger with the arithmetic mean. This effective resistance depends on two resistances: the 2D borehole resistance (R_b) and the 2D internal resistance (R_a) which is associated to the short-circuit effect between pipes in the borehole. This paper presents a field method to evaluate these two components separately. Two approaches are proposed. In the first case, the temperature at the bottom of the borehole is measured at the same time as the inlet and outlet temperatures as done in a conventional TRT. In the second case, different flow rates are used during the experiment to infer the internal resistance. Both approaches assumed a predefined temperature profile inside the borehole. The methods were applied to real experimental tests and compared with numerical simulations. Interesting results were found by comparison with theoretical resistances calculated with the multipole method. The motivation for this work is evidenced by analyzing the impact of the internal resistance on a typical geothermal system design. It is shown to be important to know both resistance components to predict the variation of the effective resistance when the flow rate and the height of the boreholes are changed during the design process. Full article

(This article belongs to the Special Issue Geothermal Heating and Cooling)

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3553 KiB

Open AccessArticle

Internal Force Response of a Pile in an Inhomogeneous Temperature Field

by Dan Zhang, Yian Wang and Jian Cheng

Energies 2018, 11(1), 18; https://doi.org/10.3390/en11010018 - 22 Dec 2017

Cited by 32 | Viewed by 2931

Abstract

An inhomogeneous temperature field was built in an experimental model of sand with an embedded pile. The temperature of the soil, as well as the temperature and strain on opposite sides of the pile were investigated in the process of temperature balance. The effect of the inhomogeneous temperature field on the internal force of the pile was analyzed. The experimental results show that the inhomogeneous temperature field will cause a bending deformation in the pile body according to the FBG (fiber Bragg grating) strain sensors. The distribution of the bending moment along the length of the pile is related to the temperature difference. The maximum bending moments reached −25.7 N·m when the temperature difference was about 1.3 °C. Therefore, the influence of the inhomogeneous temperature field o· the internal force of the foundation pile should be taken into account in the applications of a ground source heat pump system. Full article

(This article belongs to the Special Issue Geothermal Heating and Cooling)

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4338 KiB

Open AccessArticle

Heat Conduction in Porous Media Characterized by Fractal Geometry

by Zilong Deng, Xiangdong Liu, Yongping Huang, Chengbin Zhang and Yongping Chen

Energies 2017, 10(8), 1230; https://doi.org/10.3390/en10081230 - 18 Aug 2017

Cited by 25 | Viewed by 5200

Abstract

Fractal geometry (fractional Brownian motion—FBM) is introduced to characterize the pore distribution of porous material. Based on this fractal characterization, a mathematical model of heat conduction is presented to study heat conduction behaviors in porous material with a focus on effective thermal conductivity. The role of pore structure on temperature distribution and heat flux is examined and investigated for fractal porous material. In addition, the effects of fractal dimension, porosity, and the ratio of solid-matrix-to-fluid-phase thermal conductivity (k_s/k_f) on effective thermal conductivity are evaluated. The results indicate that pore structure has an important effect on heat conduction inside porous material. Increasing porosity lowers thermal conductivity. Even when porosity remains constant, effective thermal conductivity is affected by the fractal dimensions of the porous material. For porous material, the heat conduction capability weakens with increased fractal dimension. Additionally, fluid-phase thermal conduction across pores is effective in porous material only when k_s/k_f < 50. Otherwise, effective thermal conductivity for porous material with a given pore structure depends primarily on the thermal conductivity of the solid matrix. Full article

(This article belongs to the Special Issue Geothermal Heating and Cooling)

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Review

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17 pages, 1032 KiB

Open AccessFeature PaperReview

Assessment and Public Reporting of Geothermal Resources in Germany: Review and Outlook

by Thorsten Agemar, Josef Weber and Inga S. Moeck

Energies 2018, 11(2), 332; https://doi.org/10.3390/en11020332 - 02 Feb 2018

Cited by 24 | Viewed by 5431

Abstract

Any geothermal resource assessment requires consistent and widely accepted terminology, methods, and reporting schemes that facilitate the comparison of geothermal resource estimates. This paper reviews common resource assessment methods, as well as reporting codes and terminology. Based on a rigorous analysis of the portrayed concepts and methods, it discusses the appropriateness of the existing reporting codes for sustainable utilization of geothermal resources in Germany. Since the last quantitative geothermal resource assessment in Germany was done 15 years ago, a revised report is overdue. Unlike fossil energy commodities, geothermal energy replenishes naturally and heat recuperation increases in created heat sinks. This replenishment process offers the opportunity for sustainable reservoir management in the case of moderate production rates or cyclic operation. Existing reporting codes, however, regard geothermal resources in a similar way to fossil resources or focus too much on field development rather than on the whole assessment process. In order to emphasize the renewability of geothermal energy, we propose the reporting of geothermal capacities (per doublet or per km²) instead of recoverable heat energy which depends very much on project lifetime and other factors. As a first step, a new classification scheme for geothermal resources and reserves is outlined. Full article

(This article belongs to the Special Issue Geothermal Heating and Cooling)

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