test-stand for geothermal borehole .grc transactions, vol. 38, 2014 549 test-stand for geothermal

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  • GRC Transactions, Vol. 38, 2014

    549

    Test-Stand for Geothermal Borehole Probes

    Benedict Holbein1, Jrg Isele2, and Luigi Spatafora3

    1M.Sc., Responsible for Cooling System Development 2Dr. Eng., Team Leader Geothermal Group at IAI: ZWERG Project

    3M.Sc., Material Expert of ZWERG ProjectInstitute of Applied Computer Science IAI, Karlsruhe Institute of Technology KIT,

    Herrmann-von-Helmholtz-Platz-1, Germany

    KeywordsDeep geothermal energy, borehole tools, investigation & exploration, geothermal research, engineering development

    ABSTRACT

    The Test-Stand for geothermal borehole probes is an engineer-ing project, aiming at the support of a fast and target-oriented development of new borehole tools, usable in the Geothermal Energy field. Therefore it provides the possibility to test single components and complete systems under realistic conditions.

    It is part of the ZWERG project which aims at the develop-ment of a system-platform for the standardized and affordable engineering of different borehole-tools.

    The project is time-consuming, cost-intensive and challeng-ing in scientific and technical way. One problem is to identify the borehole conditions in the right way, since the geology and thermal condition of wells varies strongly between different regions and depths. This takes effect especially for the chemical composition of thermal waters.

    The Test-Stand is therefor designed in a modular way, which allows the exchange of different components and a step for step enhancement of the complete setup.

    In a first step the surrounding temperature of boreholes up to 250C can be simulated to test a borehole cooling-machine, for the cooling of standard electronics below 70C without time-limitation. In addition to the special cooling-system components, basic modules like casings and borehole-sensors for pressure- and temperature measurement can be tested. The components are heated up, using heating bands and jackets. A big deep fry allows testing components at a hot liquid environment. Like this, an operation in an environment with up to 250C can be simulated, to test the functionality and performance of the cooling-machine under extreme conditions.

    In a next step the borehole surrounding pressure shall be simulated as well. For that reason an autoclave with adequate di-mensions to receive complete borehole-tools, which can generate pressure up to 600 Bar at temperatures up to 250C is planned.

    Thus it will also be possible to test the chemical influence of the borehole water on different materials and functions.

    Introduction

    The Geothermal Energy sector has a high potential, which is often rarely used. Besides the economic difficulties linked to this new technology, there are also problems with the social accep-tance, especially in Europe. In Germany the Geothermal Energy has a hard time competing with wind-, solar- and the growing carbon-based energy. Although it provides the important aspects for a base-load-supplier, its missing the needed political and social support to take an essential role in the energy supply of the future. One basic reason is the lack of knowledge about the conditions and processes in and around the boreholes, deep in the earth crust. This lack is responsible for a big insecurity on the side of investors

    Figure 1. Structure of the ZWERG project.

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    and fears and concerns on the side of the citizens, which block the expansion of this energy supplier. To improve the situation it is evident to realize widespread and affordable possibilities for the investigation of geothermal boreholes. The project ZWERG is committed to provide a way of developing various investigation and interaction devices in a time- and money-saving way.

    The Test-Stand, described in this paper is an important element for further development efforts. It will make the designing pro-cess more effective and increase the reliability of the engineered components.

    The ZWERG project

    The work for ZWERG began in 2011. It follows a system-platform strategy with standardized basic modules and a storage system for the knowledge about engineering solutions and suppli-ers [1]. The target conditions for the developed probes are based on the borehole conditions in a well in Soultz-sous-fret, France at a depth of approximately 5000 m [2].

    As first tools, a development project for the video inspection system GeoKam is currently running, the COBOLD project for the development of a borehole cooling-machine will start soon and a project for taking and conserving samples under original conditions is planned. One important factor of ZWERG is the acquisition of required materials. For the casing i.e. Inconel 718, a nickel-based alloy with yield strength (Rp0.2%) above 1000 MPa at 200 C and high corrosion resistance is needed. Table 1 shows the prices of different suppliers for the dimensions used in ZWERG probes.

    GeoKam

    The first probe GeoKam, a video inspection tool for deep boreholes, is currently being developed within a BMU (Federal Ministry for the Environment, Nature Conservation and Nuclear Safety) -project, in cooperation with the company BRG, which provides well inspection services. The first prototype has been exhibited at the Trade Fair Geotherm in Offenburg, Germany in February 2014. Figure 2 shows the exhibited system connected to a Wireline. GeoKam allows producing life-videos with high

    solutions inside boreholes with surrounding temperatures up to 200C. Different cameras inside, in combination with a specific light-management can film in front and in radial direction. It is possible to focus interesting spots to detect i.e. flaws in the borehole-casing. This could be a starting point for further devel-opments of casing-repair-tools for operations on the spot. The probe-casing, including trans-parent ceramic windows withstands pressures up to 600 Bar [4]. The maximal outer diameter is 95 mm (~3.74 inch), so the probe can be used in 4 inch boreholes. A PCM (Phase Change Material) based cooling-system, including heat-pipes, avoiding the danger of overheated components, allows operation times of 6-8 hours in a 200C environment [5]. The project will be completed with a field test near Mu-nich, Germany in summer 2015.

    Borehole Cooling-Machine

    The borehole cooling-machine will allow the usage of standard electronic components even in hot environments with temperatures above 200C without time limitation. To realize this, it conducts a thermodynamic cycle process. The principle is similar to the one used in most refrigerators but at

    Table 1. Inconel 718 pre-product prices in Germany (Supplier I III) and China (Supplier IV VI) [3].

    type L (m)

    OD (mm)

    t (mm)

    ID (mm)

    m (kg)

    I ()

    II ()

    III ()

    IV ()

    V ()

    VI ()

    Pipe

    170 6 168.3 14.2 139.9 324 74 220 26 602 26 603 21 639

    170 6 168.3 19.05 130.2 43 860

    95 6 101.6 11 79.6 147 36 600 12 115 12 582 10 623

    95 6 101.6 9.5 82.6 20 850

    95 6 101.6 17.5 66.6 218 36 600 17 894 17 894 15 691

    95 6 101.6 19.05 63.5 29 520

    15 9 16 2 12 6 27 000 32 400 525 846 442

    5 90 6 1 4 11 31 500 36 900 14 720 936 2 641 811

    Rod

    170 6 168.3 1064 69 000 35 101 25 427 33 111

    170 6 180 47 800

    95 5 95.25 284 15 000

    95 5 101.6 400.6 12 791 10 135 12 066

    95 5 105 13 221

    15 9 16 14 990 680 476 463 494

    Sum 290 910 225 231 106 439 96 591 94 931

    OD=Outer diameter, t=Wall thickness, ID=Inner diameter, L=Length, m=Weight

    Figure 2. Photo of the exhibited GeoKam.

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    completely different temperature and pressure levels and in an extreme environment. Thats why the used components have to be custom built with appropriate dimensions for the use within borehole probes. The main components, shown in Figure 3 are the inner heat-exchanger (evaporator) where heat from the cooled components and the cooled room is transferred to a refrigerant, the compressor which compresses the gaseous refrigerant to a higher pressure where it condenses in the outer heat-exchanger (condenser), at a temperature above the environment temperature and finally the throttle, where the condensed refrigerant expands back to start -pressure and -temperature. All of them have to fulfill the constraints of mechanical strength, corrosion and temperature -resistance and frame-size corresponding to the borehole-probe operation conditions.

    In Figure 1 the main components can be seen as they are designed for a borehole probe with an outer diameter of 170 mm (~6.69 inch), which shall be usable in boreholes with 8 inch diameters. The casings and outer components like the condenser are made of Inconel 718. Some electronic boards, mounted on the evaporator surface, as example for an application are illustrated too. The cycle process is shown in a logarithmic pressure-enthalpy diagram in Figure 4.

    The example process (Fig. 4) is based on a prognosis program for the setting and validation of the Test-Stand Experiments [6]. It shows the process with acetone as refrigerant conducted at a surrounding temperature of 150 C. The sub-processes are:

    Isothermal evaporation at ~56.5 C Polytropic compression to ~15 Bar Isothermal condensation at ~170 C Isenthalpic expansion to ~1 BarThe cooling capacity, Qc, and the needed compression-work

    effort, Pn, can be estimated graphically, using the enthalpy dif-ferences readable from the diagram.

    Qc =dmdt* hevaporation (1)

    Pn =dmdt* hcompression (2)

    In the equations, dm/dt is the mass-flow controlled by the com-pressor frequency and h are the particular enthalpy-differences. A more exact way to calculate the compressor work is to use the formulation for a polytropi