Assessment of Design Procedures for Vertical Borehole Heat Exchangers

The use of ground source energy systems is a well-established method to provide low cost heating to buildings, diversify the energy mix and help meeting increasingly stricter sustainability targets. However, considerable uncertainties remain over their efficient design, with several standards, guidelines and manuals being proposed over the last few years. This paper aims at providing insight into the implications to the design of a vertical borehole heat exchanger of the adoption of different design procedures. The hypothetical case of a typical dwelling located in London, UK, is analysed in order to highlight the impact on the final design of the chosen methodology. Moreover, a parametric study using an analytical design procedure was performed to point out the influence of various factors, such as borehole characteristics and thermal properties of the ground. It is shown that there are considerable discrepancies between design methods and that uncertainties in some input parameters, such as the thermal properties of the ground, which for relatively small systems are often selected from tables rather than measured in situ, may have a substantial influence on the length of borehole required

Long-term high frequency monitoring of a large borehole heat exchanger array

Borehole heat exchangers are a key technological element of geothermal energy systems and modelling their behaviour has received much attention. The aim in the work reported here has been to produce a reference data set that can be used in analysis of large borehole heat exchanger systems and validation of models of such. A monitoring exercise to collect high frequency data from a large ground heat exchanger array consisting of 56 boreholes over 38 months since the start of operations is reported. The system is associated with a mixed-use university building that has both heating and cooling loads. Ground heat exchange was found to be dominated by rejection of heat over the monitoring period and modest seasonal increases in temperatures. The ground heat exchanger installation has been additionally characterised by analysis of thermal response test data to estimate the effective ground and grout thermal properties. The utility of the measurements as a reference data set by presenting a model validation study is furthermore demonstrated. This has highlighted some features of the data that are more significant in systems of larger scale. These reference data are being made openly available for further work on performance analysis and model validation

Porous materials in building energy technologies—a review of the applications, modelling and experiments

Improving energy efficiency in buildings is central to achieving the goals set by Paris agreement in 2015, as it reduces the energy consumption and consequently the emission of greenhouse gases without jeopardising human comfort. The literature includes a large number of articles on energy performance of the residential and commercial buildings. Many researchers have examined porous materials as affordable and promising means of improving the energy efficiency of buildings. Further, some of the natural media involved in building energy technologies are porous. However, currently, there is no review article exclusively focused on the porous media pertinent to the building energy technologies. Accordingly, this article performs a review of literature on the applications, modelling and experimental studies about the materials containing macro, micro, and nano-porous media and their advantages and limitations in different building energy technologies. These include roof cooling, ground-source heat pumps and heat exchangers, insulations, and thermal energy storage systems. The progress made and the remaining challenges in each technology are discussed and some conclusions and suggestions are made for the future research

Assessment of the influence of shortening the duration of TRT (thermal response test) on the precision of measured values

In this paper the results of testing thermal parameters of the rock environment and measurement of borehole temperature profiles of the newly constructed experimental underground heat storage (BTES (Borehole Thermal Energy Storage)) in Paskov (Czech Republic) obtained with the TRT (thermal response method) and temperature measurement on boreholes at selected depth levels are summarised. The TRT measurement series on eight boreholes has shown the possibility to compare the differences among individual measurements in a practically identical rock environment. The temperature profiling of boreholes enabled studying the dynamics of temperature changes occurring in the rock environment as a reaction to the heat supply during the TRT. The measurement series was performed with the aim to assess the possibility of shortening the TRT duration while maintaining the acceptable precision of the measured results. For this reason the software simulation of shortening the TRT duration to 24 h was performed, and the influence of such shortening to the precision of determination of values λλ and RBRB was studied. The simulation has shown that shortening the test to 24 h in our case would have brought an acceptable amount of inaccuracy with regard to the dispersion of measured values obtained from the real test.Web of Science6412912

Geothermal system optimization in mining environments

Tableau d’honneur de la Faculté des études supérieures et postdoctorales, 2010-2011Les ressources particulières à l’environnement minier, tel que l’eau inondant des galeries et des stériles exothermiques, permettent de diminuer les coûts d’installation des systèmes de pompes à chaleur géothermique. Les particularités de l’environnement minier posent toutefois un défi de taille lors de la conception d’un système puisque l’approche utilisée doit considérer, par exemple, la conductivité hydraulique accrue par les excavations ou la génération de chaleur provenant de l’oxydation de minéraux. L’objectif de ce projet de recherche est de simuler l’opération de systèmes géothermiques issus de sites miniers dans le but de démontrer les économies d’énergie potentielles et faciliter l’installation des systèmes. Des approches numériques sont développées avec le programme HydroGeoSphere pour application à des études de cas réalisées aux Mines Gaspé à Murdochville et à la Halde Sud de la Mine Doyon en Abitibi. Un système de pompes à chaleur d’aquifère aux Mines Gaspé est optimisé avec un modèle numérique où sont superposés des éléments 1D et 3D pour représenter les excavations. Les simulations démontrent que le système peut être opéré à partir d’anciens puits de ventilation afin d’éviter d’effectuer des forages. Les stériles de la Halde Sud sont d’abord caractérisés avec un test de réponse thermique conventionnel, analysé avec l’équation de la ligne-source et le principe de superposition considérant les variations du taux d’injection de chaleur. Une méthode numérique est aussi développée pour analyser l’essai influencé par l’hétérogénéité des matériaux et le gradient géothermique prononcé. Le mort terrain et le roc sous la halde sont caractérisés à l’aide d’un nouveau test de réponse thermique effectué avec des câbles chauffants. Des simulations numériques reproduisent ensuite la distribution de température lors d’essais types, laquelle devient rapidement homogène durant la période de restitution thermique ce qui permet d’analyser la température dans le forage sans connaître la position du capteur. L’opération d’un système de pompes à chaleur couplées au sol aménagé sous la Halde Sud est finalement simulée. L’optimisation des charges de chauffage indique que l’échangeur de chaleur situé sous les stériles peut fournir plus d’énergie thermique qu’un échangeur situé dans un environnent conventionnel, réduisant la longueur de forage à l’installation.Resources associated to mining environments, such as mine water and exothermic waste rock, allow a reduction of installation costs of ground source heat pump systems. Compared to other environments, caution is required when designing systems in mining environments because of enhanced hydraulic conductivity created by mine voids or heat generation due to oxidation of minerals. The objective of this study is to simulate the operation of geothermal systems on mine sites to demonstrate energy savings and promote installation. Numerical modeling approaches are developed with the program HydroGeoSphere applied for case studies conducted at the Gaspé Mines in Murdochville and at the South Dump of the Doyon Mine in Abitibi. A groundwater heat pump system at the Gaspé Mines is optimized with a numerical model, where 1D and 3D elements are superposed to adequately represent the mine voids. The simulations show that the system can be operated using former mining shafts to avoid drilling boreholes. Waste rock of the South Dump is initially characterized with a conventional thermal response test analyzed with the lines-source equation and the superposition principle accounting for variations of heat injection rates. A numerical method is also developed to analyze the test that was affected by the heterogeneity of materials and the strong geothermal gradient. The overburden and the host rock below the dump are characterized with a novel thermal response test using heating cables. Numerical simulations then reproduce the temperature distribution during typical tests, which homogenizes rapidly during the recovery period allowing the analysis of temperature inside the borehole without knowing the position of the sensor. The operation of a ground-coupled heat pump system installed under the South Dump is finally simulated. The optimization of heating loads indicates that the heat exchanger located beneath the waste pile can provide more thermal energy than an exchanger located in a conventional environment, reducing bore length required for a given system

Hydrogeological and Thermal Sustainability of Geothermal Borehole Heat Exchangers

Assessment of the current approach taken by guidelines and design methods of vertical closed loop heat exchangers shows that often groundwater flow is either disregarded or is not methodically incorporated. The state of scientific research in this arena reveals that overlooking the groundwater flow in the design procedure may not always be a correct assumption. The significance of advective heat transport compared to conduction is defined by the groundwater flux or Darcy velocity which heavily depends on the hydraulic conductivity of the ground, followed by the hydraulic gradient which has a relatively limited range. A sensitivity analysis on ground and borehole properties ranks groundwater flux together with the thermal conductivity of the ground and the temperature gradient between the antifreeze and the ground (i.e. inlet and background temperatures) as the key factors defining the heat exchange efficiency. The study confirms that the effect of groundwater advection on an operational borehole heat exchanger (BHE) becomes notable at fluxes ≥10-7 m/s; fluxes ≥10-8 m/s accelerate the returning of ground temperatures to the initial background temperature (i.e. thermal recovery) when the BHE is not operational. Examining the groundwater flow impact on multiple BHEs shows that as increasing the number of boreholes causes larger temperature disturbances, the effect of advective transport becomes more substantial. The thermal interference between BHEs induced by groundwater flow in line arrays can be of higher relevance than square arrays, depending on the flow direction. Although the BHE spacing is a major design parameter, in the long-term groundwater flow may be more critical to improving the thermal performance of the system as it considerably shortens the time to reach steady state. The effect of hydrogeological inhomogeneities, i.e. fractures, depends on their dip angle. Modeling of vertical features up to 10 m away from a BHE with aperture ≥1 mm, which can be recognized through geological investigation techniques but not thermal response testing (TRT), shows long-term impacts. Depending on the openness and distance from the borehole, one major fracture has the most influence on the BHE. For horizontal features, fracture frequency is the key parameter to consider

Novel Wireless Sensor System for Dynamic Characterization of Borehole Heat Exchangers

The design and field test of a novel sensor system based in autonomous wireless sensors to measure the temperature of the heat transfer fluid along a borehole heat exchanger (BHE) is presented. The system, by means of two specials valves, inserts and extracts miniaturized wireless sensors inside the pipes of the borehole, which are carried by the thermal fluid. Each sensor is embedded in a small sphere of just 25 mm diameter and 8 gr weight, containing a transceiver, a microcontroller, a temperature sensor and a power supply. A wireless data processing unit transmits to the sensors the acquisition configuration before the measurements, and also downloads the temperature data measured by the sensor along its way through the BHE U-tube. This sensor system is intended to improve the conventional thermal response test (TRT) and it allows the collection of information about the thermal characteristics of the geological structure of subsurface and its influence in borehole thermal behaviour, which in turn, facilitates the implementation of TRTs in a more cost-effective and reliable way