Determination of the thermal characteristic of the ground in Cyprus and their effect on ground heat exchangers

This thesis was submitted for the award of Doctor of Philosophy and was awarded by Brunel University London.Since the ancient years, human beings were using holes and caves to protect themselves from weather conditions making it the first known form of exploiting ground’s heat, known as Geothermal Energy. Nowadays, geothermal energy is mainly used for electricity production, space heating and cooling, Ground Coupled Heat Pump (GCHP) applications, and many other purposes depending on the morphology of the ground and its temperature. This study presents results of investigations into the evaluation of the thermal properties of the ground in Cyprus. The main objectives were i) to determine the thermal characteristics of the ground in Cyprus, ii) investigate how they affect the sizing and positioning of Ground Heat Exchangers (GHE) and iii) present the results for various ground depths, including a temperature map of the island, as a guide for engineers and specifiers of GCHPs. It was concluded that there is a potential for the efficient exploitation of the thermal properties of the ground in Cyprus for geothermal applications leading to significant savings in power and money as well. Six new boreholes were drilled and two existing ones were used for the investigation and determination of i) the temperature of the ground at various depths, ii) its thermal conductivity, iii) its specific heat and iv) its density. The thermal conductivity was determined by carrying out experiments using the line source method and was found to vary in the range between 1.35 and 2.1 W/mK. It was also observed that the thermal conductivity is strongly affected by the degree of saturation of the ground. The temperature of the undisturbed ground in the 8 borehole locations was recorded monthly for a period of 1 year. The investigations showed that the surface zone reaches a depth of 0.25 m and the shallow zone 7 to 8 m. The undisturbed ground temperature in the deep zone was measured to be in the range of 18.3 °C to 23.6 °C and is strongly dependent on the soil type. Since the ground temperature is a vital parameter in ground thermal applications, the temperature of the ground in locations that no information is available was predicted using Artificial Neural Networks and the temperature map of the island at depths of 20 m, 50 m and 100 m was generated. Data obtained at the location of each borehole were used for the training of the network. Data for the sizing of GHEs based on the ground properties of Cyprus were presented in an easily accessible form so that they can be used as a guide for preliminary system sizing calculations. With the aid of Computational Fluid Dynamics (CFD) software the capacity of the GHEs in each location and the optimum distance between them was estimated. Additionally, the long term temperature variation of the ground was investigated. For the first time since a limited study in the 1970’s, a research focusing on the determination and presentation of the thermal properties of the ground in Cyprus has been carried out. Additionally, the use of Artificial Neural Networks (ANNs) is an innovative approach for the prediction of data at locations where no information is available. The publication of this information not only contributes to knowledge locally but also internationally as it enables comparison with other countries with similar climatic conditions to be carried out.Research Promotion Foundation of Cypru

Investigation and determination of the geothermal parameters of the ground in Cyprus for the exploitation of geothermal energy and the impact of the results in the design of the geothermal systems

The Energy Service of the Ministry of Commerce, Industry and Tourism has the overall responsibility for Energy matters in Cyprus and specifically for preparing and implementing programmes for energy conservation, the promotion of renewable energy sources (RES) and the development of technologies for the utilization of RES. The Government of Cyprus being aware of the benefits of geothermal energy and in order to increase the share of energy from renewable sources consumed in heating and cooling in 2020, promotes the geothermal energy systems through a Scheme that provides financial incentives for the utilization of RES for heating and cooling. However, the lack of valid data for the ground thermal properties in Cyprus was one of the main obstacles for the design of efficient geothermal systems, the implementation of the Schemes in the field of geothermal energy and the calculation of the share of energy from renewable sources for heating and cooling according to the methodology defined by the E.U in the directive 2009/28/EC. In an effort to identify suitable energy efficient systems for heating and cooling of buildings and the correct calculations of their contribution to the national targets, the Energy Service participated in a project founded by the Research Promotion Foundation of Cyprus to investigate and determine the geothermal parameters of the ground of Cyprus at six representative sites in Cyprus, for use in the design of ground heat exchanger applications and ground thermal storage. The paper presents the importance of the Isothermal map that helps consultants to design efficiently geothermal energy systems, calculate effectively heat losses of buildings to the ground and design the thermal energy storage equipment. The importance’s of the results are analyzed by national authorities’ experts’ point of view for evaluating geothermal applications bridging in this way the gap between technical output and commercial reality

Measurements of ground temperatures in Cyprus for ground thermal applications

The knowledge of the thermal behaviour of the ground and the factors affecting it are important for many construction projects and for using the ground as heat storage or a heat exchange media. Not enough information exists regarding the thermal behaviour of the ground in Cyprus and this is one of the main interests of this study. The ground temperature distribution was recorded for the period between May 2006 and May 2007, at the Athalassa region in Nicosia. For this purpose a borehole was drilled and a U-tube heat exchanger made of 32 mm external diameter polyethylene pipe and thermocouples were placed up to the depth of 50 m. All data were recorded using an Omega OMB-DAQ 55/65 USB data acquisition module for a typical day each month at 15 min intervals. The surface zone in the area, reaches the depth of 0.5 m while the shallow zone extends up to 7 m. Below the 7 m depth the temperature remains almost unchanged at 22.6 °C and close to the mean annual ambient air temperature of 19.5 °C. Ground temperatures up to the depth of 7 m and 7.7 m were also recorded at the Ariel and Ayia Phyla locations in Limassol, respectively. The two boreholes were drilled to study and compare the thermal behaviour of the ground in other lowland places of the Island of Cyprus as well. The comparison of the data collected showed almost no differences between the three data collection points in the ground temperature distribution regardless of the differences in the composition of the groun

Comparison of calculated and measured ground thermal properties

This paper presents a way of calculating the in-situ borehole ground thermal properties. For this procedure a U-tube ground heat exchanger embedded in a borehole is utilized, allowing hot water to circulate in the tube. The water is heated with constant power and its temperature is recorded in constant time intervals. Then the results are analyzed using the line source method. The formulas are solved using a simple excel program to derive the values of the thermal properties with a fitting technique. These results are compared with measured values of samples taken from the same area where the borehole is located. The comparison shows that only the experimental results analyzed with the theoretical model can give reliable values because the data concern the actual stage of compaction and moisture content of the soil

A practical method for computing the thermal properties of a Ground Heat Exchanger

The aim of this paper is to show a practical way of estimating the thermal ground properties, namely the ground thermal conductivity, and in particular the thermal diffusivity and the volumetric heat capacity in a reliable manner, for sizing Ground Heat Exchangers (GHEs). A well-known thermal model, proposed by Blackwell in 1954, is applied and is validated both in the heating mode and in the cooling mode, using a GHE as a probe. The value of the thermal conductivity can be easily determined by the model but the procedure also requires knowledge of the ground specific heat capacity and density, which are normally deduced from the (non-accurate) geological data of the site. In addition to the above, the thermal model is also solved analytically –based on the actual parameters used in the experiment–leading to the computation of the ground thermal diffusivity, the volumetric heat capacity and the thermal resistance of the GHE. The possible errors and drawbacks of the whole method are then discussed and finally a complete set of guidelines is provided to the field Engineer for estimating the ground thermal properties from a single test, rendering the use of the geological data of the side unnecessary

Single and double u-tube ground heat exchangers in multiple-layer substrates

The efficiency of ground-source heat pumps depends on the correct sizing of the associated ground heat exchangers (GHEs). Multiple ground layers with regard to GHE and the presence of more than one loops in each borehole (multiple U-tubes) are sparsely raised in the literature. Therefore the accurate prediction of the performance of the GHE under these conditions is of fundamental importance. A newly developed and validated model is applied to a multiple layer ground regime with different properties and the effect of the layer sequence on the outlet temperature is examined. The model is also modified to allow the study of a double U-tube GHE in a single borehole and the assessment of its efficiency with regard to its building cost. Various configurations are then compared to show that an in-series double GHE is the most efficient. The numerical model developed for energy flows and temperature changes in and around a borehole, when a fluid circulates through single and double U-tubes, is validated upon comparing its results with established experimental results for a single GHE. This model is an improvement of a previous one, based on the time-dependent convection-diffusion equation

An Analysis of Heat Flow Through a Borehole Heat Exchanger Validated Model

Earth heat exchangers are essential parts of the ground-source heat pumps and the accurate prediction of their performance is of fundamental importance. This paper presents the development and validation of a numerical model for the simulation of energy flows and temperature changes in and around a borehole heat exchanger when a fluid circulates through a U-tube. Based on the time-dependent convection-diffusion equation, the FlexPDE software package is employed to solve the resulting boundary value problem that model a heat exchanger. First, the mathematical model is validated through a comparison with data obtained from experiments with real borehole set-ups in Cyprus. Then the validated model is used to study the heat flow and the temperature variation in the heat-exchanger. Finally conclusions are extracted on how various parameters like the U-tube diameter, the variation of the ground thermal conductivity and specific heat and the borehole filling material affect the temperature of the inlet and outlet fluid

Application of the European Directive for the Energy Performance of Βuldings in Cyprus

It is a fact that today in Cyprus there are no building-regulations concerning thermal insulation of the building envelope and consequently no restriction exist regarding energy performance. After joining the EU in May 2004, Cyprus has to comply with various directives of the Union. One of them is the Directive on the Energy Performance of Buildings 2002/91/EC. In order to comply with this requirement, a special committee was formed for the development of the required methodology and software to be used for this purpose. The objective of this paper is to present the methodology and program developed and its characteristics together with the type of output the user is expected to get. Considering that Cyprus is fully dependant on imported energy, it is believed that this new directive will lead to energy efficient buildings, and in the long run it will benefit the economy of the island

Measurements of ground temperatures in Cyprus for ground thermal applications

The knowledge of the thermal behaviour of the ground and the factors affecting it are important for many construction projects and for using the ground as heat storage or a heat exchange media. Not enough information exists regarding the thermal behaviour of the ground in Cyprus and this is one of the main interests of this study. The ground temperature distribution was recorded for the period between May 2006 and May 2007, at the Athalassa region in Nicosia. For this purpose a borehole was drilled and a U-tube heat exchanger made of 32 mm external diameter polyethylene pipe and thermocouples were placed up to the depth of 50 m. All data were recorded using an Omega OMB-DAQ 55/65 USB data acquisition module for a typical day each month at 15 min intervals. The surface zone in the area, reaches the depth of 0.5 m while the shallow zone extends up to 7 m. Below the 7 m depth the temperature remains almost unchanged at 22.6 °C and close to the mean annual ambient air temperature of 19.5 °C. Ground temperatures up to the depth of 7 m and 7.7 m were also recorded at the Ariel and Ayia Phyla locations in Limassol, respectively. The two boreholes were drilled to study and compare the thermal behaviour of the ground in other lowland places of the Island of Cyprus as well. The comparison of the data collected showed almost no differences between the three data collection points in the ground temperature distribution regardless of the differences in the composition of the groun