Interpretación del árbol de problemas

The thermal performance of the coaxial BHE has been studied for different borehole depths and with water as the heat carrier. The deep BHE is most efficient for heat extraction, and the potential for cooling decreases as the ground temperature increases with borehole depth. Relative to conventional BHEs, the deep BHE can sustain a higher thermal load during heat extraction which significantly reduces the total drilling length required to cover a given heat load.

The flow direction of the heat carrier affects the thermal performance of the coaxial BHE. During heat extraction, the heat carrier should be injected through the annular space. If used for heat injection, the fluid should enter through the center pipe. This is most important for deeper boreholes, for a shallow borehole with a single and uninsulated center pipe, the flow direction has little importance, as seen in Paper 4.

During heat extraction, the vertical distribution of the thermal load in the borehole follows the thermal gradient; therefore, most of the energy is extracted from the lower part of the borehole. During heat injection, the distribution of the thermal load is reversed such that the upper part of the borehole will be heated up while the lower part is poorly utilized.

The distribution of the thermal load also influences the way deep BHEs should be constructed. The thermal influence is proportional to the thermal load, which during heat extraction is small in the upper part of the boreholes. Several boreholes can, therefore, be placed near each other on the surface, but should have a deviating pattern to create a sufficient distance between the boreholes in the deeper parts of the boreholes.

For a deep BHE with water as the heat carrier, there is a bottleneck problem connected to the thermal output from the borehole. In order to get the heat out from the borehole without lowering the inlet temperature below 0 °C, the mass flow has to be rather high. A high mass flow also increases the thermal performance of the deep coaxial BHE since it reduces the internal heat losses and increases the heat transfer for the surrounding rock. The drawback with a high mass flow rate is that it can cause excessive pressure losses if the internal flow area in the coaxial BHE is too small. It is, however, shown in the parametric study that the increase in system performance with increasing borehole depth significantly outweighs the effect of the increase in pressure losses and pumping power. This indicates that the optimum configuration for a deep coaxial BHE used for heat pump applications is a combination of a thin walled center pipe and a rather high mass flow rate.

PART II

II CLOSED LOOP ENGINEERED GEOTHERMAL

SYSTEMS

II-1

Introduction

In this part of the thesis the focus is on the geothermal resources that are madeupbythethermal

energy stored in the upper few kilometers of the earth 's crust; that is outside the tectonically active areas with naturally occurring hydrothermal resources. The aim with the development of

Engineered geothermal systems(EGS) is to access and extractthisvast energy resource.

EGS has so farmainly been developedthrough aseries of researchand pilot projectswhere the

target has been to use the thermal energy for productionof electricity, which requires a rather

high production temperature.Theseprojectshave as well been initiated ingeological locations

that have more favorable conditions, i.e. higher temperature levels thanwhat can be foundin

Norway (whichis the main focus andorigin for thiswork).

An EGS installation in Norwaywould,however,most likely be used directlyin district heating

systems, (due to the country's high heating demands and large hydropower resources) thus

eliminating the rather low efficiency associated with binary cycles used for electricity

production.The most commonly knownapproach to an EGS is a systemwhere the connectivity

between the productionand injection well is established usingartificiallycreated networksof

fractures.In suchsystemtheheat transfer is less dependent onthe thermalconductionin the

rock (as a large heat transfer area can be created). In the EGS concept studied here the

connectivity between the production and injection well is created by drilling a number of

wellbores. This limits the heat transfer area (which means that the heat transfer is more dependent on thermal conduction). And thus, generally a larger temperature difference is required between the working fluid and the initial temperature of the rock surrounding the wellbores. Interaction with existing fractures, and resident fluids in the rock is not wanted, and the system can, therefore, be referred to as a closed loop EGS.

The concept enables production of hot water in the temperature range required for district heating (60 °C – 90 °C) given what is considered accessible depth (say 5 to 6 km) and a geothermal temperature gradient of about 20 K/km to 30 K/km.

Presently there are no boreholes in the depth range sought onshore in Norway; therefore, the temperature levels at the target depth for an EGS have to be estimated using measurements of temperature and thermal properties from shallower boreholes (< 1000 m).

II-2

Section structure

This section starts with a summary of related previous works. Thereafter follows three parts, the first part is an brief presentation of the closed loop EGS and serves the purpose of demonstrating the performance of the system. This part is summarized in Paper 5 , “A novel

concept to engineered geothermal systems”.

The second part is about the thermal structure of the ground. In Norway there is a lack of deep boreholes ( > 1000 m) onshore. As a consequence there are no direct temperature measurements showing the thermal structure in the depths of interest for EGS. An indirect method to estimate the temperatures is presented, where a numerical steady state thermal model is applied to a geophysical model of the Oslo graben. This part is presented in Paper 6 “Thermal

modeling in the Oslo rift, Norway”.

The third part contains a more detailed study of the closed loop EGS presented in Paper 5. A transient numerical model is developed and presented for the system. The model is then used to study the thermal characteristics of the system. To further explore the usefulness of the EGS as a provider of thermal energy, the model is used in conjuction with measuremens from a district heating network.

II-3

Previous works

Geothermal energy is one of the energy resources that has attracted attention as the search for alternatives to fossil fuels has increased. In the MIT report “Future of Geothermal Energy”, Tester et al. (2006) estimated that 100 GWecould be produced from geothermal systems in the

USA within the next 50 years after a moderate R&D investment. The interest for geothermal energy has increased worldwide, and assessments of geothermal potential are made for many parts of the world, e.g. during the World Geothermal Congress 2010 in total 111 papers were presented in the categories “country updates” and “resource assessments”. Also in Norway, heat flow measurements have been conducted in recent years, indicating an average thermal gradient slightly less than the world average of (30 K/km) Slagstad et al. (2009), Pascal (2010).