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]

Spatial Variability and Terminal Density -Implications in Soil Behavior-

Geotechnical engineers often face important discrepancies between the observed and the predicted behavior of geosystems. Two conceptual frameworks are hypothesized as possible causes: the ubiquitous spatial variability in soil properties and process-dependent terminal densities inherent to granular materials. The effects of spatial variability are explored within conduction and diffusion processes. Mixtures, layered systems, inclusions and random fields are considered, using numerical, experimental and analytical methods. Results include effective medium parameters and convenient design and analysis tools for various common engineering cases. In addition, the implications of spatial variability on inverse problems in diffusion are numerically explored for the common case of layered media. The second hypothesis states that there exists a unique terminal density for every granular material and every process. Common geotechnical properties are readily cast in this framework, and new experimental data are presented to further explore its implications. Finally, an unprecedented field study of blast densification is documented. It involves comprehensive laboratory and site characterization programs and an extensive field monitoring component. This full scale test lasts one year and includes four blasting events.Ph.D.Committee Chair: Santamarina, J. Carlos; Committee Member: Frost, J. David; Committee Member: Goldsztein, Guillermo H.; Committee Member: Mayne, Paul W.; Committee Member: Rix, Glenn J

Geothermal energy in a sustainable built environment

Geothermal energy is usually perceived to be about gushing geysers and bubbling mud pools and limited to only the small volcanically active parts of the Earth’s surface. Nothing could be further from the truth. Geothermal energy is in fact an incredible store of energy found in all parts of the world which is beginning to be understood and used for our sustainable future. There are two basic forms of this energy. One form (sometimes referred to as hot dry rocks or enhanced geothermal systems), makes use of the heat (>200°C) in the rocks at depths of up to about 5 kms to produce electricity from extracted (but returnable) hot water. There are several locations around the world where “proof of concept” stage has been or about to be reached suggesting that within the next few years, these systems may be providing a significant proportion of our base-load electricity. The other form makes use of the heat (and the cooling potential) of the soils and rocks within the upper few tens of metres from the surface to heat and cool buildings. It involves the circulation of a fluid through pipes built into building foundations or in specifically drilled boreholes, and back to the surface where heat stored in the fluid is extracted by a heat pump, and used to heat a building. The cooled fluid is reinjected into the ground loops to heat up again to complete the cycle. In cooling mode, the system is reversed with heat taken out of the building transferred to the fluid which is injected underground to dump the extra heat to the ground. The cooled fluid then returns to the heat pump to receive more heat. There are many thousands of these systems installed around the world but many counties have been slow to pick up on their enormous potential. The paper explains how these systems work and looks at some of the issues which require attention in the near future for geothermal energy to become a truly sustainable, renewable and most importantly, continuous, energy source

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

Ground-source heat pump systems: The effects of variable trench separations and pipe configurations in horizontal ground heat exchangers

Ground-source heat pump systems are renewable and highly efficient HVAC systems that utilise the ground to exchange heat via ground heat exchangers (GHEs). This study developed a detailed 3D finite element model for horizontal GHEs by using COMSOL Multiphysics and validated it against a fully instrumented system under the loading conditions of rural industries in NSW, Australia. First, the yearly performance evaluation of the horizontal straight GHEs showed an adequate initial design under the unique loads. This study then evaluated the effects of variable trench separations, GHE configurations, and effective thermal conductivity. Different trench separations that varied between 1.2 and 3.5 m were selected and analysed while considering three different horizontal loop configurations, i.e., the horizontal straight, slinky, and dense slinky loop configurations. These configurations had the same length of pipe in one trench, and the first two had the same trench length as well. The results revealed that when the trench separation became smaller, there was a minor increasing trend (0.5 °C) in the carrier fluid temperature. As for the configuration, the dense slinky loop showed an average that was 1.5 °C lower than those of the horizontal straight and slinky loop (which were about the same). This indicates that, when land is limited, compromises on the trench separation should be made first in lieu of changes in the loop configuration. Lastly, the results showed that although the effective thermal conductivity had an impact on the carrier fluid temperature, this impact was much lower compared to that for the GHE configurations and trench separations.</jats:p

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