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Energy piles are a consolidated underground heat exchanger alternative to traditional boreholes in ground source heat pump (GSHP) systems. Previous works focused on assessing the differences between piles and boreholes, but few assessed small piles in operational conditions. Moreover, most of these studies centered around cylindrical concrete piles, overlooking short screw piles. Using in-situ testing, established analytical methods, and advanced three dimensional (3D) finite element model simulations, this work assesses three thermal response tests (TRT) executed in different energy pile structures, one being a unique group of eight short energy screw piles connected in series, located in the same site in Melbourne, Australia. Detailed numerical analysis provided reliable soil and structure thermal parameter predictions and detailed computations allowed the study of thermal effects for the energy screw piles steel components. The results show limited impact of the steel components on effective thermal conductivity, but a reduction in thermal resistivity that may provide a speedier thermal exchange in short term GSHP operation. In addition, the more traditional TRT rigs and analytical interpretation provided reasonable results for the pile group in series, and show a similar performance to a borehole heat exchanger of similar pipe length; however, the short piles engage only the upper soil layers, with potentially lower thermal conductivity. TRT in single short screw piles require careful consideration, because common rigs may be unable to cater for the required low fluid flow rates and heating power. Thus, for the cases assessed herein, the pile group TRT proved to be more reliable than individual pile testing, due to their short length. [ABSTRACT FROM AUTHOR]

Embedding heat exchangers into a screw pile can form a cost-effective energy pile with a fast installation capability. However, better solutions to handle thermal waves and thermal interferences among energy piles are still required. This work aims to solve the issues by proposing a novel concept of an energy screw pile filled with mixtures of phase change materials (PCM) such as paraffin and "solid" grouting. Numerical simulations are conducted to investigate the utilisation of PCM energy screw piles on reducing fluid and ground temperatures, considering different constituents in PCM-solid mixtures, moisture conditions, phase change temperatures T pc and operation schemes. Results demonstrate that a mixture with higher effective thermal conductivity λ eff slows down the PCM phase transition and results in a lower fluid temperature feeding the ground source heat pump (GSHP). Dry mixtures with higher PCM content benefit the fluid temperature reduction, while a lower PCM content in a wet mixture is also satisfactory. Higher specific heat capacity allows pile back-filling to absorb more heat and reduce the thermal radius of influence, while over-enhancing its λ eff might worsen thermal interference. The selection of the PCM phase change temperature T pc should comply with the GSHP system operation scheme and the project aim. • A novel concept of PCM energy screw pile is proposed. • PCM energy screw pile can reduce fluid temperature and thermal interference. • The phase change can be engineered by controlling the λeff via PCM-solid mixtures. • A phase change temperature with high utilisation ratio increases efficiency. • The selection of PCM-solid mixture should comply with operation scheme and aim. [ABSTRACT FROM AUTHOR]

Thermal response tests (TRT) are an essential in situ test for the sizing of ground source heat pump systems. Its application on energy piles is becoming an established practice, involving key differences in relation to testing boreholes. Recently, group TRTs emerged as an alternative to testing single short energy pile elements. This work assesses the execution and interpretation of those tests through different analytical heat transfer models implemented in a unit-response calculation methodology. To assess the suitability of the method, a comprehensive parametric analysis using a validated numerical model is undertaken, considering different energy pile sizes, thermal properties for both concrete and soil and group pile arrangements. The analytical methodology is validated against experimental and numerical results, and then tested to interpret the numerically simulated TRTs using a curve fitting algorithm. The broad parametric investigation and the evaluation of the capacity of different analytical methods on modelling each scenario provide directions to execute and interpret group TRTs. The soil effective thermal conductivity and energy pile thermal resistance are obtained with less than 5% precision error for most scenarios and less than 10% considering all scenarios. These results can be further improved with specific individual G-functions for the pile elements, that can be implemented on the proposed methodology. [ABSTRACT FROM AUTHOR]