EXPERIENCE FROM THE FIRST THERMAL RESPONSE TEST (TRT) EQUIPMENT IN FINLAND. N. Leppäharju, I. Martinkauppi, M. Nousiainen

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1 EXPERIENCE FROM THE FIRST THERMAL RESPONSE TEST (TRT) EQUIPMENT IN FINLAND N. Leppäharju, I. Martinkauppi, M. Nousiainen Geological Survey of Finland P.O. Box 97, FI Kokkola, Finland ABSTRACT The exploitation of ground heat has traditionally been very small scaled in Finland. Due to recent rises in energy prices and the increasing awareness of environmental issues a growing interest in ground heat has been revealed. Today Finland is one of the countries with fastest growing number of heat pumps. The Geological Survey of Finland (GTK) has invested in research, development and promotion of ground heat in Finland. GTK focuses on large-scale commercial projects in cooperation with companies and leading research institutes. GTK has recently acquired a Thermal Response Test (TRT) equipment which is the first of its kind in Finland. In addition to TRT measurements GTK offers detailed geological and geophysical studies which are needed for the accurate modelling and planning of especially largescale geoenergy systems. The first geoenergy studies with measurements with the new TRT equipment have already shown the importance of proper measuring and modelling. 1. INTRODUCTION The Geological Survey of Finland (GTK) is Finland s national geological research centre established in GTK s activities are guided by two strategic main programmes which are bedrock and raw materials and land use and environment. In Energy Programme GTK has three research subjects: geoenergy from bedrock and sediments, bioenergy from peat (existing national resource) and nuclear power (focusing on safe waste storage, site assessment, prospecting and expert leasing). A national goal set for Finland by EU Commission and agreed by Finnish Government for year 2020 is to raise the share of renewable energy to 38 % (now 28 %) of the total consumption and cut down the CO 2 emissions with 16 %, from activities like traffic, heating and agriculture. Therefore GTK aims at increasing the awareness of geoenergy and utilization of ground heat in Finland. In practice this means for example increasing the number of borehole heat exchanger (BHE) systems. GTK has strongly invested in research and method development of ground heat exploitation. At the moment, GTK focuses mostly on large scale commercial geoenergy projects, like office buildings, shopping centres and industrial buildings, including hybrid solutions (solar-/bio-/wind-

2 ) and energy storage systems. There is close cooperation with private companies from designers and financiers to manufactures of equipments as well as with the governmental and other public authorities. GTK offers geological and geophysical field studies, Thermal Response Tests (TRT), modelling and dimensioning of the boreholes and follow-up temperature measurements with Distributed Temperature System (DTS). 2. GEOENERGY IN FINLAND The number of heat pumps in Finland increases rapidly. The European Heat Pump Association EHPA classifies Finland as one of the European countries with most significant growth in the larger markets (among Italy, France, Norway and Austria). Number of ground source heat pumps sold in 2007 was about The total number of ground source heat pumps in Finland in years is presented in Figure 1. As a comparison, in Sweden there were almost ground source heat pumps sold in Bedrock is relatively similar in Finland and Sweden as they both belong to the Fennoscandian Shield. So, from the geological point of view, there s lots of unused geoenergy potential in Finland. (EHPA, s.a) Figure 1: Total number of ground source heat pumps in Finland in year (Sulpu, 2008) Crystalline bedrock in Finland consists typically of granitoids, gneisses and other metasedimentary or metavolcanic rocks. The water content and rock porosity of bedrock are low. The bedrock is covered by Quartenary sediments and the depth of soil layer is on average 8,6 meters, while 3-4 meters is a typical value (Salonen et al., 2006). The depth of groundwater is usually about 2-4 meters. Average ground surface temperatures are presented in Figure 2. Figure 3 is a line chart of the temperature in the ground at different seasons in Finland on average. Average heat flow in Finland is 0,037 W/m 2, which is far below continental average 0,065 W/m 2. Geothermal gradient is typically 8-15 K/km. The low geothermal gradient is due to Precambrian geology with very thick lithosphere ( km). (Kukkonen, 2000)

3 Figure 2: Average ground surface temperatures in Finland. (Leppäharju, 2008) Temperature ( o C) January November May July 4 March September Depth (m) s = 1*10-6 m 2 s -1 Figure 3: The temperature in the ground at different seasons in Finland on average. (Leppäharju, 2008)

3. TRT EQUIPMENT The first Finnish Thermal Response Test (TRT) equipment was constructed in spring 2008. It was designed by Geological Survey of Finland (GTK) in co-ordination with Norwegian NGU.

The Finnish TRT uses heat injection in order to determine the effective thermal conductivity and the thermal resistance of the borehole. The heater has a stepwise adjustment from 3 kw to 18 kw.

4 3. TRT EQUIPMENT The first Finnish Thermal Response Test (TRT) equipment was constructed in spring It was designed by Geological Survey of Finland (GTK) in co-ordination with Norwegian NGU. The response test equipment is based essentially on the Swedish TED. The Finnish TRT uses heat injection in order to determine the effective thermal conductivity and the thermal resistance of the borehole. The heater has a stepwise adjustment from 3 kw to 18 kw. The heat carrier fluid is circulated with a continuously adjustable pump. The normal measurement flow speed during a measurement is kept between 0,6 l/s and 1,0 l/s to ensure turbulent flow. The TRT rig uses three-phase current but there is also a 7 kw gasoline generator in the rig for small scale measurements. Temperature is measured with five sensitive sensors. Fluid temperatures are measured from ingoing and outgoing pipes. The air temperature is measured outside the rig and in two places inside the rig (the control and engine sections). Temperatures, fluid flow, electric power and pressure are recorded by an advanced data logger. In the beginning of a measurement the data is recorded every 10 seconds and after a few hours every 10 minutes. Heating is normally continued at least for 72 hours. Remote control and data collection is possible via GPRS. The equipment is built onto a single axel covered trailer (Figure 4). To ensure thermal insulation during a measurement the rig is split into three sections. The control, engine and generator sections are isolated from each other so that one can work in the control or generator section without disturbing the measurement. The pipes between the rig and borehole are also insulated from changes in ambient air temperature. Figure 4: The Finnish thermal response test rig. Photo by I. Martinkauppi.

5 The heat exchanger boreholes in Finland are mostly closed systems. Typical collector type is single U-pipe, but double U-pipes and coaxial pipes are used more or less, too. The boreholes are normally filled with groundwater. Heat carrier fluid is typically 28 % ethanol. 4. CASE STUDIES GTK has conducted two large, commercial geoenergy projects which are presented as Case 1 and Case 2. In addition, GTK coordinates a national GEOENER project which aims at concept development of large scale geoenergy systems. Many Finnish companies participate in GEOENER so the project has a strong economic aspect besides of technical and geoscientific viewpoints. At the moment there are four GEOENER subprojects and GTK will conduct geological and geophysical measurements and design a geoenergy heating and/or cooling system for each of them. Case 1: Heating and cooling of a residential area in Southern Finland GTK conducted an extensive geoenergy study in Southern Finland in a commercial project in summer The results were used to plan and model a borehole heat exchanger system for a residential area with 250 detached houses. The houses will use ground heat energy for both heating and cooling. The study included geological bedrock mapping, bore powder analysis, fracturing study, borehole geophysics and TRT measurements. The study provided detailed information about the geology of the area. Geologically the area of the study is quite homogeneous, consisting of solid granite with a thin cover of soil on top. Four 200 meters deep test boreholes were drilled at different parts of the area. Heat exchanger pipes (single U-pipes) were placed in three of the boreholes and one was left open for geophysical follow-up surveys. TRT measurements showed that the effective thermal conductivity in the area is a bit higher than the average thermal conductivity of granite. Also, the measurements suggested good repeatability of TRT measurements with the new Finnish TRT equipment. Case 2: Heating and cooling of a large industry hall in Southern Finland In autumn 2008 GTK carried out a geoenergy study in Southern Finland in an area where a large industry hall is planned to be built. The building will use ground heat for both heating and cooling, and GTK will use the results of the study in modelling the borehole heat exchanger system. A geological bedrock mapping revealed a diagonal contact of two different rock types, diorite and granite gneiss, in the middle of the area. In addition, the granite gneiss was much more fractured than the diorite. Therefore, test boreholes were drilled on both sides of the contact and an additional borehole was placed near the contact. TRT measurements were done in order to get information about the effective thermal conductivity, groundwater movements and the thermal resistance in the boreholes. Two of the boreholes were measured both with and without spacers.

6 The spacers keep the distance (from centre to centre) between the two shanks of the U-pipe in 75 mm. The TRT measurements showed that there is a significant difference in the effective thermal conductivity on the diorite and the granite gneiss sides of the contact. The granite gneiss side has almost 1 W/(mK) higher effective thermal conductivity than the diorite side. The tests also implied that in this case the spacers have a considerable effect on the thermal resistance of the boreholes, but further studies have to be made on this. Both of these results have significant effect on the modelling and dimensioning of the borehole system. REFERENCES European heat pump association EHPA, s.a. European Heat Pump Statistics: Outlook 2008, version 1.5. [Pdf-file]. [ ]. < Kukkonen, I., Geothermal energy in Finland. In: Proceedings World Geothermal Congress 2000 in Kyushu Tohoku, Japan, May 28 June 10. International Geothermal Association, Leppäharju, N., Kalliolämmön hyödyntämiseen vaikuttavat geofysikaaliset ja geologiset tekijät. Master thesis, University of Oulu, 79 pp. Salonen, V.-P., Eronen, M. & Saarnisto, M., Käytännön maaperägeologia. Keuruu, 237 pp. (in Finnish). SULPU (Finnish heat pump association), Tilastoja maalämpöpumppu. [Web page]. [ ]. <

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