Uncertainties in the design of ground heat exchangers

Mikhaylova, Olga; Johnston, Ian; Narsilio, Guillermo A.

doi:10.1680/jenge.15.00033

Cited by 17 publications

(5 citation statements)

References 10 publications

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“…The more reliable way to obtain this information is via an in-situ technique known as thermal response testing (TRT) [12,13]. While cheaper investigations maybe carried out in the laboratory, the TRT is preferred for all but the smallest of systems [14,15] due to the ability to test the ground at full scale and to provide information on the as-constructed GHE conditions. Since energy piles are at prima facie similar to borehole GHEs, there has been a desire to apply the same in-situ testing technique.…”

Section: Introductionmentioning

confidence: 99%

Thermal Response Testing of Large Diameter Energy Piles

2019

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Energy piles are a novel form of ground heat exchanger (GHE) used in ground source heat pump systems. However, characterizing the pile and ground thermal properties is more challenging than for traditional GHEs. Routine in-situ thermal response testing (TRT) methods assume that steady state conditions in the GHE are achieved within a few hours, whereas larger diameter energy piles may take days or even weeks, thereby incurring significant costs. Previous work on pile TRTs has focused on small diameters up to 450 mm. This paper makes the first rigorous assessment of TRT methods for larger diameter piles using field and laboratory datasets, the application of numerical and analytical modelling, and detailed consideration of costs and program. Three-dimensional numerical simulation is shown to be effective for assessing the data gathered but is too computationally expensive for routine practice. Simpler fast run time steady state analytical models are shown to be a theoretically viable tool where sufficient duration test data is available. However, a new assessment of signal to noise ratio (SNR) in real field data shows how power fluctuations cause increased uncertainty in long duration tests. It is therefore recommended to apply transient models or instead to carry out faster and more cost-effective borehole in-situ tests for ground characterization with analytical approaches for pile characterization.

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Section: Introductionmentioning

confidence: 99%

Thermal Response Testing of Large Diameter Energy Piles

2019

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“…The second reason can be the specific assumptions of the models which might not be accurate for particular GHEs. The differences between actual values of borehole thermal resistances and the values of these parameters estimated during design can lead to oversizing or undersizing of GHEs (Mikhaylova, et al 2016). …”

Section: Comparison Of Operational and Modelled Borehole Thermal Resimentioning

confidence: 99%

Assessment of Effective Borehole Thermal Resistance from Operational Data

Mikhaylova¹,

Johnston²,

Narsilio³

2017

Proceedings of the IGSHPA Technical/Research Conference and Expo 2017

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Ground source heat pump (GSHP) systems use the ground as a source of sustainable thermal energy for heating and cooling of buildings. Efficient design of the ground heat exchangers (GHEs) for these systems is important so that long-term operation is adequate, efficient and cost-effective. Several design methods have been developed to size GHEs, and many of these methods, including the widely used ASHRAE method, use an effective borehole thermal resistance to model thermal processes in boreholes. A correct estimation of this parameter is crucial for an adequate sizing of borehole GHEs. This study estimates an experimental effective borehole thermal resistance of the borehole GHEs of an operating GSHP system based on monitoring data collected during the Elizabeth Blackburn School of Sciences full-scale shallow geothermal operational study in Melbourne, Australia. The experimental resistance is compared with the resistances predicted using several analytical and numerical methods. It was found that the experimental resistance can be significantly different from the resistances predicted by these other methods. The paper discusses possible reasons for such differences.

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“…Despite their importance, in small projects, soil thermal properties are normally assumed based on recommended values for geological materials or estimated using predictive models [21]. While it is possible to satisfactorily calculate c s by adding the heat capacities of the different soil constituents according to their volume fraction [22], an accurate estimation of the thermal conductivity is complicated.…”

Section: Introductionmentioning

confidence: 99%

Determination of Thermal Conductivity Properties of Coastal Soils for GSHPs and Energy Geostructure Applications in Mexico

2021

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The thermal conductivity of soils is a fundamental parameter for the design of ground-source heat pump systems (GSHPs) and energy geostructures. This paper presents a comprehensive evaluation of the physical, mineralogical, and thermal characteristics of typical coastal soils from Tabasco, Mexico. Twenty-five soil samples from four different strata were studied using the thermal needle probe method, X-ray diffractometry, scanning electron microscopy, and standard geotechnical soil classification tests. The results showed a significant correlation between the dry density and porosity with the thermal conductivity of the studied samples, which ranged between 1.17 and 2.32 W m−1 K−1. The performed statistical analyses indicated that coarse-grained soils had larger thermal conductivities and higher variability than fine-grained soils. Additionally, the performance of six models to estimate the thermal conductivity of soils was validated against the experimental data. All models provided accurate estimations for fine-grained soils, but only the effective medium theory (EMT) showed an adequate fit for coarse-grained soils. The results represent one of the first datasets for the thermal properties of Mexican soils. They will contribute to the implementation of GSHPs and energy geostructures in the country and locations with similar subsoil conditions, especially where time and resources are not available for their experimental determination.

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Uncertainties in the design of ground heat exchangers

Cited by 17 publications

References 10 publications

Thermal Response Testing of Large Diameter Energy Piles

Thermal Response Testing of Large Diameter Energy Piles

Assessment of Effective Borehole Thermal Resistance from Operational Data

Determination of Thermal Conductivity Properties of Coastal Soils for GSHPs and Energy Geostructure Applications in Mexico

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