Effects of surface air temperature on thermal performance of vertical ground heat exchangers

Peer Reviewe

Carrier fluid temperature data in vertical ground heat exchangers with a varying pipe separation.

The dataset in this article is related to shallow geothermal energy systems, which efficiently provide renewable heating and cooling to buildings, and specifically to the performance of the vertical ground heat exchangers (GHE) embedded in the ground. GHEs incorporate pipes with a circulating (carrier) fluid, exchanging heat between the ground and the building. The data show the average and inlet temperatures of the carrier fluid circulating in the pipes embedded in the GHEs (which directly relate to the performance of these systems). These temperatures were generated using detailed finite element modelling and comprise part of the daily output of various one-year simulations, accounting for numerous design parameters (including different pipe geometries) and ground conditions. An expanded explanation of the data as well as comprehensive analyses on how they were used can be found in the article titled "Ground-source heat pump systems: the effect of variable pipe separation in ground heat exchangers" (Makasis N, Narsilio GA, Bidarmaghz A, Johnston IW, 2018) [1]

Assessment of effective borehole thermal resistance from operational data

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

Financial assessment of ground source heat pump systems against other selected heating and cooling systems for Australian conditions

Ground source heat pump (GSHP) systems can provide cost-effective space heating and cooling for buildings while using less fossil fuel compared to many conventional systems. Despite these benefits, they typically have higher upfront costs and longer payback periods than other heating and cooling systems. These costs are often seen as potential roadblocks for property owners to install GSHP systems over conventional systems. The financial attractiveness of GSHP systems can be increased by adopting a hybrid ground source heat pump (HGSHP) system where GSHP systems provide the baseload thermal energy with the balance provided by conventional systems. This paper assesses the lifetime costs of GSHP and HGSHP systems designed for seven major cities in Australia and compares these costs with the lifetime costs of conventional systems. The results indicate that adopting HGSHP systems in Australian climatic conditions (from tropical to cool temperate) and under current Australian installation and fuel costs can lower the normalised lifetime costs of heating and cooling compared to adopting a GSHP or a conventional system only for the same applications

Thermal Response Testing of Large Diameter Energy Piles

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

A Laboratory Study on Non-Invasive Soil Water Content Estimation Using Capacitive Based Sensors

Soil water content is an important parameter in many engineering, agricultural and environmental applications. In practice, there exists a need to measure this parameter rather frequently in both time and space. However, common measurement techniques are typically invasive, time-consuming and labour-intensive, or rely on potentially risky (although highly regulated) nuclear-based methods, making frequent measurements of soil water content impractical. Here we investigate in the laboratory the effectiveness of four new low-cost non-invasive sensors to estimate the soil water content of a range of soil types. While the results of each of the four sensors are promising, one of the sensors, herein called the “AOGAN„ sensor, exhibits superior performance, as it was designed based on combining the best geometrical and electronic features of the other three sensors. The performance of the sensors is, however, influenced by the quality of the sensor-soil coupling and the soil surface roughness. Accuracy was found to be within 5% of volumetric water content, considered sufficient to enable higher spatiotemporal resolution contrast for mapping of soil water content

Impact of Geometrical Misplacement of Heat Exchanger Pipe Parallel Configuration in Energy Piles

Peer reviewed: TrueAcknowledgements: Support received from The University of Melbourne’s Research Computing Services is appreciated. This research was made feasible through the utilisation of the Nectar Research Cloud, an Australian collaborative research platform supported by the NCRIS-funded Australian Research Data Commons (ARDC). The first author acknowledges the unwavering support received from supervisors at The University of Melbourne, Australia, and The University of Cambridge, UK.Publication status: PublishedShallow geothermal or ground source heat pump (GSHP) energy systems offer efficient space heating and cooling, reducing greenhouse gas emissions and electrical consumption. Incorporating ground heat exchangers (GHEs) within pile foundations, as part of these GSHP systems, has gained significant attention as it can reduce capital costs. The design and optimisation of GHEs connected in parallel within energy piles have been researched widely, considering symmetrical placement, while the potential misplacement due to construction errors and the optimal placement remain mostly unexplored. This study utilises 3D finite element numerical methods, analysing energy piles with diameters from 0.5 m to 1.4 m, equipped with parallelly connected U-tube and W-tube GHEs. The impact of GHE loop placement is analysed, considering the influence of the ground and concrete thermal conductivities, pile length, fluid flow rate, GHE pipe diameter, and pile spacing. Results indicate a marginal impact, less than 3%, on the overall heat transfer when loops deviate from symmetry and less than 5% on the total heat transfer shared by each loop, except for highly non-symmetric configurations. Symmetrical and evenly spaced loop placement generally maintains favourable thermal performance and ease of installation. This study underscores the flexibility in GHE design and construction with a low risk of thermal yield variations due to uncertainties, particularly with a separation-to-shank distance ratio between 0.5 and 1.5 in a symmetrical distribution.</jats:p

Estimating vertical and lateral pressures in periodically structured montmorillonite clay particles

Given a montmorillonitic clay soil at high porosity and saturated by monovalent counterions, we investigate the particle level responses of the clay to different external loadings. As analytical solutions are not possible for complex arrangements of particles, we employ computational micromechanical models (based on the solution of the Poisson-Nernst-Planck equations) using the finite element method, to estimate counterion and electrical potential distributions for particles at various angles and distances from one another. We then calculate the disjoining pressures using the Van't Hoff relation and Maxwell stress tensor. As the distance between the clay particles decreases and double-layers overlap, the concentration of counterions in the micropores among clay particles increases. This increase lowers the chemical potential of the pore fluid and creates a chemical potential gradient in the solvent that generates the socalled 'disjoining' or 'osmotic' pressure. Because of this disjoining pressure, particles do not need to contact one another in order to carry an 'effective stress'. This work may lead towards theoretical predictions of the macroscopic load deformation response of montmorillonitic soils based on micromechanical modelling of particles.<br>Dada uma argila montmorilonítica de alta porosidade e saturada por counteríons monovalentes, investigamos as respostas da argila ao nível de partículas para diferentes cargas externas. Como soluções analíticas não são possíveis para arranjos complexos de partículas, empregamos modelos computacionais micro-mecânicos (baseados na solução das equações de Poisson-Nernst-Planck), utilizando o método de elementos finitos, para estimar counteríons e distribuições de potencial elétrico para partículas em diversos ângulos e distâncias uma da outra. Nós então calculamos as pressões de separação usando a relação de Van't Hoff e a tensão de cisalhamento de Maxwell. À medida que a distância entre as partículas de argila diminui e as duplas camadas se sobrepõem, a concentração de counteríons nos microporos entre as partículas de argila aumenta. Este aumento reduz o potencial químico do fluido nos poros e cria um gradiente de potencial químico no solvente, que gera a chamado pressão 'osmótica' ou de 'separação'. Devido a esta pressão de separação, as partículas não precisam de contato entre si, a fim de exercer uma 'tensão efetiva'. Este trabalho pode conduzir a previsões teóricas da resposta macroscópica a carga de deformação em solos montmoriloníticos baseado na modelação micromecânica das partículas