• Energy Research

Increasing use of the ground as a thermal reservoir is expected in the near future. Shallow geothermal energy (SGE) systems have proved to be sustainable alternative solutions for buildings and infrastructure conditioning in many areas across the globe in the past decades. Recently novel solutions, including energy geostructures, where SGE systems are coupled with foundation heat exchangers, have also been developed. The performance of these systems is dependent on a series of factors, among which the thermal properties of the soil play a major role. The purpose of this paper is to present, in an integrated manner, the main methods and procedures to assess ground thermal properties for SGE systems and to carry out a critical review of the methods. In particular, laboratory testing through either steady-state or transient methods are discussed and a new synthesis comparing results for different techniques is presented. In situ testing including all variations of the thermal response test is presented in detail, including a first comparison between new and traditional approaches. The issue of different scales between laboratory and in situ measurements is then analysed in detail. Finally, the thermo-hydro-mechanical behaviour of soil is introduced and discussed. These coupled processes are important for confirming the structural integrity of energy geostructures, but routine methods for parameter determination are still lacking.

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.

Shallow Geothermal Energy (SGE) exchanges heat with the ground. In continuous, long-term operation, the initial temperature field can be disturbed, and subsurface thermal changes can be developed. In this paper, the thermal impact of a SGE system under a Mediterranean climate is handled. Temperature monitoring was conducted on 15 investigation boreholes equipped with a total of 92 thermal sensors placed at specific depths. Investigation boreholes were drilled 1–2 m from SGE system borehole heat exchangers installed in a university building. The analysis handles a one-year monitoring period of SGE system operation. Temperature depth profiles, reaching up to 140 m depth, were registered with a 10 min time step, resulting in a large amount of data. Ground thermal conductivity was estimated experimentally and semi-empirically, allowing us to obtain, using a numerical model, the initial undisturbed ground temperature profiles and compare them with the monitored values. Climate data were recorded by the university meteorological station. Globally, the measured and computed data were coherent, and a non-negligible impact of the SGE system operation in the first year was observed. The building orientation as well as the nearby departments had significant impacts on the shallow ground temperature. Maximum ground temperature changes observed at depths higher than 10–20 m, ranging from 2 to 3 °C as observed in different boreholes, indicate that the system is operating efficiently.

Improving building thermal energy performance is essential to reducing energy consumption, minimizing carbon emissions, and enhancing occupants’ thermal comfort. For this purpose, there is an increasing research interest in this field of building energy performance. This review aims to present a precise and systematic overview of the sensitivity analysis in optimizing the thermal energy performance of buildings. The investigation covers various aspects, including sensitivity analysis techniques, key measures and variables, objectives and criteria, software tools, optimization methods, climate zones, building typology, and climate change effects. The findings reveal that sensitivity analysis is a powerful technique for optimizing energy performance and identifying adaptive strategies such as dynamic shading, reflective coatings, and efficient HVAC set points to address climate change. Most of the study also highlights that the temperature set point is the key influential parameter in both heating-dominant and cooling-dominant climate zones. This review offers critical insights on advancing sustainable building design, informing policy, and guiding future research in energy-efficient building solutions.

Ground heat exchangers in conjunction with shallow geothermal energy system applications have received significant attention in the case of renewable energy. Soil thermal properties such as thermal conductivity and specific or volumetric heat capacity are important aspects for the design of such systems, affecting the performance. They can be obtained with the use of empirical prediction models, laboratory tests and/or in situ tests. Laboratory tests can be performed either under steady-state or under transient conditions and have the advantage of requiring small volumes of soil and producing fast results. There are many types of heat probes commercially available, with limited—though—comparative assessment available in the literature. The current paper deals with the assessment of ground characteristics of seven samples of soil and rock collected from a certain area in the Mediterranean island of Cyprus. Such properties are the thermal conductivity, the thermal diffusivity, the volumetric heat capacity, but there are some other physical properties also. The laboratory testing was done under transient conditions and included measurements taken by two needle probes and one surface probe from two different commercial apparatuses. Comparison of the obtained results for the thermal properties of the samples was made and was also supported by numerical simulations using the COMSOL Multiphysics software through a finite element analysis method on the convection–diffusion equation for heat transfer. Laboratory testing on physical properties of the samples such as moisture content, specific gravity, permeability and particle size distribution was also performed, yielding useful results related to the assessment of the thermal properties.

Shallow geothermal energy systems (SGES) may take different forms and have recently taken considerable attention due to energy geo-structures (EGS) resulting from the integration of heat exchange elements in geotechnical structures. Still, there is a lack of systematic design guidelines of SGES. Hence, in order to contribute towards that direction, the current study aims at reviewing the available SGES modeling options along with their various aspects and practices. This is done by first presenting the main analytical and numerical models and methods related to the thermal behavior of SGES. Then, the most important supplementary factors affecting such modeling are discussed. These include: (i) the boundary conditions, in the form of temperature variation or heat flow, that majorly affect the predicted thermal behavior of SGES; (ii) the spatial dimensions that may be crucial when relaxing the infinite length assumption for short heat exchangers such as energy piles (EP); (iii) the determination of SGES parameters that may need employing specific techniques to overcome practical difficulties; (iv) a short-term vs. long-term analysis depending on the thermal storage characteristics of GHE of different sizes; (v) the influence of groundwater that can have a moderating effect on fluid temperatures in both heating and cooling modes. Subsequently, thermo-mechanical interactions modeling issues are addressed that may be crucial in EGS that exhibit a dual functioning of heat exchangers and structural elements. Finally, a quite lengthy overview of the main software tools related to thermal and thermo-hydro-mechanical analysis of SGES that may be useful for practical applications is given. A unified software package incorporating all related features of all SGES may be a future aim.