• Energy Research

Abstract A number of recent publications have suggested that in order to reproduce thermal testing of energy piles, a finite value for the geo-contact thermal resistance (geo-CTR) at the soil-structure interface needs to be introduced. There is currently no guidance as to what value the geo-CTR should have. The geo-CTR will have two potential impacts in terms of the use of energy geo-structures, (i) reducing heat exchange efficiency, and (ii) increasing temperature changes and associated mechanical impacts within the geo-structure. This article sets out a new experimental method for quantifying the geo-CTR. The proposed method is based on the imposition of a heat flux through the two solid materials that form the contact. Its novelty rests with the acknowledgement that heat loss is inevitable and that the geo-CTR can be more reliably defined based on heat flow measurements at the actual contact. This concept is demonstrated via numerical modelling of a generic test set-up, where the errors induced by not accounting for heat loss, the interpolation of temperatures to the contact and the presence of the heat flow sensor were assessed. Initial test results are then presented that demonstrate how the method works. These results suggest that for a dry medium sand, while the geo-CTR is sensitive to the soil density, it is small and the effect on heat transfer is also likely to be small. Further testing will explore the relative importance of a number of factors and in particular, the soil type, on the geo-CTR.

Heating and Cooling constitute a major part of society's final energy use and a significant contributor to greenhouse gas emissions. The world society ought to mitigate climate change through decarbonisation, which must include the transition to low-temperature, sustainable and renewable heating and cooling technologies. Shallow Geothermal Energy is one of the most energy efficient and least greenhouse gas emitting available alternatives to provide space heating and cooling. The decarbonisation of the heating and cooling sector may have to comprise both individual systems and shared electrified heating and cooling systems from renewable sources of energy, where economies of scale and synergies between different types of consumers can be exploited. To this end, the focus of this paper is on the integration of shallow geothermal energy technologies into district heating and cooling systems. A key contribution of this work is the illustration of a number of practical case studies, highlighting the potential of existing shallow geothermal systems for DHC networks, which, as front runners in adopting such technologies, serve as paradigms for future development. Follows a discussion providing an outlook over the next 25 years. All in all, the future of utilizing shallow geothermal energy for district heating and cooling seems to be promising to play a pivotal role in sustainable urban development and decarbonizing the heating and cooling sector.

Ground source heat pump (GSHP) systems depend on the capacity for heat transfer between the system and the ground, and it is good practice to carry out an in situ thermal response test (TRT) to determine the undisturbed ground temperature, the thermal conductivity of the ground, and the thermal resistance of the borehole. Conventionally, a TRT is undertaken in a replica borehole heat exchanger (BHE); however, alternative methods have been developed that can provide continuous depth-resolved temperature recordings. The enhanced TRT (ETRT) uses a hybrid cable system which incorporates a resistance heating wire to provide a linear heat source and a fibre optic cable to measure the temperature along the length of the borehole. In this paper, a case study is presented in which a TRT and ETRT were carried out in the same BHE to evaluate its thermal response and estimate the thermal characteristics of the ground. After a brief introduction of both methods and their interpretation, a comparison between them is presented regarding their advantages and disadvantages using the results of the performed tests, which revealed an 8% difference in the soil thermal conductivity values, averaged over the length of the BHE.