130 research outputs found
Site investigation for energy geostructures
No full textEnergy geostructures are structure or infrastructure foundations used as heat exchangers as part of a ground source heat pump system. While piles remain the most common type of energy geostructure, increasingly infrastructure projects are considering the use of other buried structures such as retaining walls and tunnels for heat exchange. To design and plan for construction of such systems, site investigations must provide appropriate information to derive analysis input parameters. This paper presents a review of what information regarding the ground, and also the structures themselves, would be required for the ground energy system design process. Appropriate site investigation methods for energy gesotructures are reviewed, from desk study stages through in situ testing to laboratory testing of samples recovered. Available methods are described and critically appraised and guidance for practical application is given
Observation of thermally driven water flow in soils via micro-focus X-ray Computed Tomography
No full textIt is well-known that moisture movement and heat transfer often happen simultaneously in unsaturated soils, therefore forming a coupled flow pattern. This process is relevant for ground source heat pump systems, nuclear waste disposal or other areas of energy geotechnology. However, studies on the analysis of water flow in response to thermal variations are still required, especially in terms of quantitative analysis in three-phase unsaturated soil systems. This paper presents a study conducted using advanced microfocus X-ray Computed Tomography (micro-XCT) techniques, which enables both the visualisation of moisture progression and quantification of water change within the soil system. Heat was applied to the soil specimen, inducing heat transfer accompanied with the water flow under the thermal gradient. A series of short scans were operated at different temporal stages during the heating process, enabling the acquisition of representative image data for quantitative analysis. The results in terms of moisture distribution during the heating process have been obtained and interpreted. The study shows that the micro-XCT is able to assess the imposed coupled thermal-moisture flow processes in soils, which will help understand fundamental soil processes and provide quantitative data for the relevant model validation
Error analysis of the thermal cell for soil thermal conductivity measurement
No full textSoil thermal conductivity is an important factor in the design of energy foundations and other ground heat exchanger systems. Laboratory tests in a thermal cell are often used to determine the thermal conductivity of soil specimens. Two interpretation methods have been suggested. Analysis can be based on the assumption of one-directional heat flow and the thermal conductivity calculated using Fourier's law. Alternatively the lumped capacitance method can be employed, using results generated as a specimen cools. In this study, six samples of London Clay were tested using a thermal cell. A finite-element model of the tests was then used to determine the validity of the assumptions made in analysis. The model showed substantial heat loss through the sides of the specimens, which would have a significant impact on the calculated thermal conductivity. The conditions required for the lumped capacitance method to be valid were also found not to be met. Consequently neither analysis method is recommended. A better approach would be to pursue apparatus with fewer heat losses or transient testing techniques
Comparison of two different models for pile thermal response test interpretation
No full textThermal response tests (TRTs) are regularly used to characterise the thermal resistance of borehole heat exchangers and to assess the thermal conductivity of the surrounding ground. It is becoming common to apply the same in situ testing technique to pile heat exchangers, despite international guidance suggesting that TRTs should be limited to hole diameters of 152 mm (6 in.). This size restriction arises from the increased thermal inertia of larger diameter heat exchangers, which invalidates the assumption of a steady state within the concrete needed to interpret the test data by traditional line source analysis techniques. However, new methods of analysis for pile heat exchangers have recently been developed that take account of the transient behaviour of the pile concrete. This paper applies these new methods to data from a multi-stage TRT conducted on a small diameter test pile. The thermal conductivity and thermal resistance determined using this method are then compared with those from traditional analytical approaches based on a line source analysis. Differences between the approaches are discussed, along with the observation that the thermal resistance may not be constant over the different test stages
Study of short-term evaporation in sand specimens via micro-focus Xray computed tomography
No full textWater flow and heat transfer often occur simultaneously in soils, in coupled flow, for example in applications such as ground heat thermal storage and disposal of nuclear waste. It is proposed that micro-focus X-ray computed tomography (micro-XCT) can be used to investigate this phenomenon. A natural thermally-driven water flow, evaporation from an exposed soil specimen, is studied to assess the applicability of micro-XCT techniques to other problems in soil heat / moisture transfer. This paper presents studies of short-term evaporation from a sand. A series of scans were conducted on a specimen of sand during evaporation, to provide data for comparison with simultaneous gravimetric measurements made using a sensitive balance in the scanner. Both qualitative and quantitative data from the micro-XCT are presented. Drying of the specimen during each stage can be clearly seen in the image data. However, quantification of the change in water content by image analysis is more challenging, owing to the compromise between the image quality and the scanning time. To overcome this, an alternative approach to segmentation of the image data using Gaussian curve fitting is proposed and validated against global gravimetric measurements. It is concluded that this approach to the analysis of the rapid movement of moisture within the soil is promising, and it will be applied in future experiments
Data on coupled hydrothermal flow in fine sands based on X-ray CT imaging and numerical simulation
No full textCoupled hydrothermal flow can occur in soils, for example in applications such as ground heat storage and nuclear waste disposal. Therefore, approaches to quantitative analysis of water transfer in response to imposed thermal gradients are required, especially in unsaturated conditions. Analysis methods also require validation by laboratory and field data, which can be hard to obtain. This dataset includes the processed results from X-ray µCT experiments where specimens of a fine sand and a fine silty sand were subjected to heating from their base. Repeated scans, set up to balance image quality and scan duration, were carried out during the heating process, on specimens between 20% and 50% saturation. Gaussian decomposition techniques were used to determine the changing soil phase proportions throughout the experiments, which are reported here. The CT data is accompanied by COMSOL files for simulation of the experiments on the fine sand. The dataset is to accompany the paper at https://doi.org/10.1016/j.gete.2022.100380 and provides the processed information used to construct the key results figures in that paper.</span
A radial model for fast analysis of thermal pile heat exchangers
No full textEnergy piles offer an effective way to decarbonise heating and cooling via exchange of heat through the deep foundations of a building or other structure. However, efficient analysis methods are required to allow energy performance to be determined in a way compatible with building energy design and energy system design approaches. We test a fast run time analytical model (the Claesson-Javed radial model) that retains a physical basis linked to the energy pile geometry, while making simplifications to permit a radial approximation to be used. We also present validation G-functions from the model development processes that have been derived from 2D and 3D numerical simulation.</span
A fast approximate method for simulating thermal pile heat exchangers
No full textGround source heat pump systems, operating in conjunction with vertical ground heat exchangers, will play a key role in decarbonising heating and cooling of buildings. Design of traditional borehole heat exchangers relies on tools which implement routine analytical relationships between heat transferred and the temperature change in the ground and circulating thermal fluid. However, for novel piled foundations used as ground heat exchangers, there are few such analytical solutions available that are practical for routine implementation. This paper examines the use of a radial approximation to simulate the dynamic thermal behaviour of pile heat-exchangers. Originally developed for small diameter and high aspect ratio borehole heat exchangers, the approach is more challenging for piles since unsteady heat transfer within the pile material is more significant over typical timescales. Nonetheless, we demonstrate that for pile diameters between 300 mm and 1200 mm, generally the error is <1 °C with centrally placed heat transfer pipes or four or more pipes placed near the edge with circumferential spacing less than 550 mm. The radial model is therefore practical for most pile configurations. The strong performance of the model is demonstrated for a year of hypothetical heating and cooling cycles, and also against a field-scale thermal response test
Economics of geotechnical asset deterioration, maintenance and renewal
No full textTransport and other infrastructure systems are supported by, adjacent to and retained by extensive systems of earthworks of varying (and increasing) age, and of variable quality of original construction. These earthworks are subject to natural deterioration, which can be accelerated and complicated by the effects of climate change. The ACHILLES research program is providing improved understanding of earthworks behavior, performance and deterioration, and is developing methods and tools to provide decision support for the construction, maintenance and renewal of earthworks, with particular emphasis on the management of existing, deteriorating assets. Conventional cost-benefit analysis methods, of the type used for new infrastructure projects, do not directly provide the decision support needed for the maintenance and renewal of existing earthworks assets, and an alternative approach is proposed and demonstrated. The handling of the uncertainty associated with earthworks behavior, deterioration rates and times to failure is also considered, as is the extension of the single-asset approach to the management of multiple earthworks assets
The thermal performance of foundation piles used as heat exchangers in ground energy systems
No full textPile heat exchangers are expected to make a significant contribution to meeting UK and EU renewable energy and carbon dioxide reduction targets. However, design for the thermal capacity of pile heat exchangers has to date been largely based on methods developed for borehole heat exchangers. Piles have a much smaller aspect (length to diameter) ratio than boreholes and consequently their thermal behaviour is different in a number of important ways. This thesis explores these differences and makes recommendations for improved assessment of pile heat exchanger thermal capacity.Traditionally vertical heat exchanger design assumes separation of the thermal effects in the ground and in the pile. A transient temperature response function is used to assess temperature changes in the ground and a steady state resistance is applied to the pile concrete. In this thesis existing approaches to temperature response functions are critically assessed for use with thermal piles. It is important to take into account the larger pile diameter, which causes increased temperature changes in the short term. In the long term, the shorter pile length will result in reduced temperature changes as steady state is reached more quickly.Simple 2D numerical modelling has been carried out and the results used to derive a new method for determining pile thermal resistance. However, for large diameter piles, the time taken for the pile to reach steady state suggests that the use of a constant thermal resistance in design is not always appropriate. In these cases it is recommended that a transient temperature response function is used to assess the response of the ground and the concrete together.The applicability of short duration thermal response testing for pile heat exchangers has been examined. Modelling and case study data has shown that the technique is only reliable for piles of 300mm diameter or less. For the special case of large diameter piles with centrally placed heat transfer pipes then it is possible to use the test to determine the thermal conductivity of the pile concrete, but not pile thermal resistance
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