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The use of geothermal energy in Finland is restricted to the utilization of ground heat with heat pumps. This is due to the geological conditions as Finland is a part of the Fennoscandian (or Baltic) Shield. The bedrock is Precambrian covered with a thin (<5 m) cover of Quaternary sediments. Topography is subdued and does not easily produce advective re-distribution of geothermal heat by groundwater circulation systems. Due to crystalline character of the bedrock, rock porosity and its water content are low. This practically excludes geothermal systems utilizing hot wet rock. The lithosphere is very thick in Finland (150-200 km), and heat flow is mostly below continental average (< 65 mW m -2). Measured heat flow density values in the uppermost 1 km of bedrock range from very low (<15 mW m -2) values to 69 mW m -2 , whereas an average value of 46 sites (53 boreholes) is 37 mW m -2 . Geothermal gradient is typically 8-15 K km -1 , and the annual average ground temperature at the surface ranges from about +5ºC in the southern part to about +2ºC in the northern parts of the country. Temperatures at 500 m below surface are usually between 8 and 14ºC. At 1000 m the temperature ranges from 14 to 22ºC. Values either extrapolated from geotherms or calculated with thermal models suggest that temperatures exceeding 40ºC should be encountered at 1-1.5 km depth. However, in order to reach 100ºC, depths from 6 to 8 km are required. These numbers suggest that Finland is not a good candidate for either wet or dry hot rock systems, although some formations with either anomalously high heat production rate or thermal blanketing effects due to low thermal conductivity should be investigated in more detail. Nevertheless, promising applications can be found for small-scale use of ground-stored heat in all parts of the country. About 10,000 heat pumps have been installed in boreholes, lakes or Quaternary deposits since the early 1980's. About 70 % of them are horizontal ground coupled systems, 20 % are using lake water and 10 % are vertical ground coupled systems. Typical vertical installations are in small family houses using a shallow 100-200 m deep borehole. The order of magnitude of energy extraction from such holes is 50 W/m 3 . The use of ground-heat with heat pumps is currently increasing in Finland.
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GEOTHERMAL ENERGY IN FINLAND
Ilmo T. Kukkonen
Geological Survey of Finland, Address: P.O. Box 96, FIN-02151 Espoo, Finland
e-mail ilmo.kukkonen@gsf.fi
Key words
: Finland, geothermal energy, heat pump.
ABSTRACT
The use of geothermal energy in Finland is restricted to the
utilization of ground heat with heat pumps. This is due to the
geological conditions as Finland is a part of the Fennoscandian
(or Baltic) Shield. The bedrock is Precambrian covered with a
thin (<5 m) cover of Quaternary sediments. Topography is
subdued and does not easily produce advective re-distribution
of geothermal heat by groundwater circulation systems. Due to
crystalline character of the bedrock, rock porosity and its water
content are low. This practically excludes geothermal systems
utilizing hot wet rock.
The lithosphere is very thick in Finland (150-200 km), and
heat flow is mostly below continental average (< 65 mW m
-2
).
Measured heat flow density values in the uppermost 1 km of
bedrock range from very low (<15 mW m
-2
) values to 69 mW
m
-2
, whereas an average value of 46 sites (53 boreholes) is 37
mW m
-2
. Geothermal gradient is typically 8-15 K km
-1
, and
the annual average ground temperature at the surface ranges
from about +5ºC in the southern part to about +2ºC in the
northern parts of the country. Temperatures at 500 m below
surface are usually between 8 and 14ºC. At 1000 m the
temperature ranges from 14 to 22ºC. Values either extrapolated
from geotherms or calculated with thermal models suggest that
temperatures exceeding 40ºC should be encountered at 1-1.5
km depth. However, in order to reach 100ºC, depths from 6 to
8 km are required. These numbers suggest that Finland is not a
good candidate for either wet or dry hot rock systems, although
some formations with either anomalously high heat production
rate or thermal blanketing effects due to low thermal
conductivity should be investigated in more detail.
Nevertheless, promising applications can be found for
small-scale use of ground-stored heat in all parts of the
country.
About 10,000 heat pumps have been installed in boreholes,
lakes or Quaternary deposits since the early 1980's. About 70
% of them are horizontal ground coupled systems, 20 % are
using lake water and 10 % are vertical ground coupled
systems. Typical vertical installations are in small family
houses using a shallow 100-200 m deep borehole. The order of
magnitude of energy extraction from such holes is 50 W/m
3
.
The use of ground-heat with heat pumps is currently increasing
in Finland.
1. INTRODUCTION
Finland is situated between latitudes 60 and 70 N and has a
climate with average annual air temperatures varying from 5ºC
in the southern part to –2ºC in the northern part of Finland.
Because of the climatic conditions, space heating is usually
needed from September to May. The current population of the
country is about 5 million people.
The total annual consumption of energy in Finland in 1998
was 1.29 million TJ, which is divided according to various
energy sources as follows: oil 28.0 %, coal 11.0%, natural gas
10.7 %, nuclear power 17.7%, hydropower 4.1 %, peat 6.2 %,
wood and black liquors 19.2 %, imported electricity 2.6 % and
other sources of energy 0.7 %. About half of the energy (49 %)
is consumed by the industry, 22 % is used in space heating, 18
% in traffic and 11 % in households, agriculture, etc. (Energy
Review, 1999).
Geothermal energy is not used in Finland for electricity
production and there are no direct applications of geothermal
energy either. This situation is due to the Precambrian geology
with thick crust and lithosphere resulting in low geothermal
gradient values. However, there are about 10000 heat pumps
utilizing the ground-stored heat either in bedrock, Quaternary
sediments or water-sources (lakes). Heat pumps seem to
provide a feasible alternative for space heating in small family
houses or country farms. In the official energy statistics
(Statistics Finland, 1998) the consumption of ground heat is
combined with other ‘ambient sources’ of space heating
energy. This number accounts for a total of 1240 TJ in 1997
and it is about 1.2 % of the total energy consumed in space
heating in Finland. The value has more than doubled since
1995 (510 TJ).
This paper reviews the present status and potential of
geothermal energy in Finland, presents basic geothermal data
with temperature and heat flow maps, and reports the history
and development of heat pump applications in Finland.
2. GEOLOGICAL AND GEOTHERMAL CONDITIONS
Finland is a part of the Fennoscandian (also known as the
Baltic) Shield. The bedrock is Archaean (3100 - 2500 Ma) and
Proterozoic (2500 - 1300 Ma) in age, and it is covered by a
thin, usually less than 5 m thick layer of Quaternary sediments.
The crystalline bedrock is characterized by granitoids, gneisses
and other metasedimentary or metavolcanic lithologies.
Heat flow and subsurface temperature data in Finland have
been presented by Puranen et al. (1968), Järvimäki and
Puranen (1979), Kukkonen and Järvimäki (1992) and
Kukkonen (1988, 1989, 1993, 1999). The current geothermal
data is based on the temperature logs on 46 sites and 53
boreholes shallower than 1100 m, as well as laboratory
measurements of thermal conductivity of corresponding drill
core samples. The measurements and the databases are from
the Geological Survey of Finland.
Measured heat flow density (Fig. 1) correlates with the tectonic
age, heat production and lithology of the sites (Kukkonen,
1989, 1993). The lowest values are encountered in the
Archaean and Early Proterozoic areas in eastern and northern
277
Proceedings World Geothermal Congress 2000
Kyushu - Tohoku, Japan, May 28 - June 10, 2000
Kukkonen
Finland (13-30 mW m
-2
), whereas the higher values are
related to Early Proterozoic late-kinematic and anorogenic
(rapakivi) granitoids in southern Finland (40-70 mW m
-2
).
Arithmetic mean of heat flow data is 37±11 mW m
-2
(one
standard deviation).
The climatically controlled average annual ground surface
temperature varies from +6ºC in southern to +2ºC in the
northernmost Finland. The ground temperature can also be
estimated directly from meteorological annual air temperature
in ºC averages as
T (ground) = 0.7, T (air) + 2.93
(Kukkonen,
1987).
Temperature maps are presented for 500 and 1000 m depths
below surface (Fig. 2 and 3). The variation of temperatures
reflects both climatic conditions as well as the crustal
geothermal conditions. Temperature at 500 m is highest in
southern Finland (12-14ºC) and lowest in northern Finland (6-
9ºC). The values at 1000 m are 20-22ºC in the south, and 12-
4ºC in the north, respectively. Extrapolation and calculation of
temperatures at greater depths indicate that the 40ºC isotherm
would be reached at 2-3 km, and the 100ºC isotherm at depths
of 6-8 km (Kukkonen, 1999).
Topography in Finland is subdued and does not easily produce
advective re-distribution of geothermal heat by the
groundwater circulation systems. Due to low hydraulic
permeability and low porosity of crystalline rocks the water
content of bedrock is low (< 1 %), and thus the water content
of the bedrock is low as well. These data indicate that the
prospects for utilizing geothermal energy either in wet or dry
rock systems are not very promising (Kukkonen, 1999).
However, earlier interest in geothermal energy in Finland in
1970-80's was much concentrated in discussing the potential
for such applications (e.g. Kivekäs, 1978, 1979, 1981; Risku-
Norja, 1987, Risku-Norja et al., 1987).
Temperatures in the soil at 1 m depth vary annually between
+2 to +12ºC in southern Finland, and -2 - +12 C in northern
Finland. Temperature in the uppermost (< 200 m) bedrock
below the penetration depth of annual variations is +2 to +8ºC.
Such temperatures are favorable for heat pump systems in the
scale of small family houses, country farms or sometimes in
district heating systems of small communities.
3. USE OF GEOTHERMAL ENERGY: HEAT PUMPS
Due to the cool thermal regime of bedrock, the only type of
geothermal energy used in Finland is ground heat with the aid
of heat pumps installed either vertically in boreholes, or
horizontally in Quaternary sediments as well as lakes and
rivers. The ground heat is considered here as geothermal
energy, although it is a combination of deep geothermal energy
and solar energy stored in the near-surface layers of the earth.
Interest towards such energy sources grew rapidly in the late
1970's after the increase of oil price. Several thousands of heat
pumps were installed in soil, typically in farms in eastern
Finland during 1980's. During the 1980's and 1990's the
relatively low prices of oil and electricity reduced
competitively the heat pump applications, and their popularity
decreased. From 1985 to mid-1990 there were sold only about
100-200 heat pumps annually. However, there is currently an
increasing interest in heat pumps. In 1998 about 800 heat
pumps were sold.
Technological research on the heat pump systems has also
been carried out during the years. Pilot test plant projects and
other studies were carried out in 1970-1980 by the Technical
Research Center of Finland (e.g. Aittomäki and Wikstén,
1978), universities (e.g. Aittomäki, 1983) and by the
governmentally owned electricity producer, Imatran Voima
Company (e.g. Kankkunen, 1985; Tinell et al., 1986).
Unfortunately, there are no detailed statistics available on the
existing heat pump installations, and this branch of business is
divided into a number of small engineering and drilling
companies that makes it difficult to compile such data.
Therefore, the exact statistics of the numbers and properties of
heat pump applications are not easy to obtain, and the present
data are based on the estimates by the specialists working in
the heat pump business. Further, the Finnish Heat Pump
Association (Suomen Lämpöpumppuyhdistys) was established
only in 1999 for promoting the use of heat pumps and
distributing information on such energy systems.
It is estimated that at the end of 1999 there were a total of
about 10,000 heat pumps in Finland, which were utilizing
ground-stored heat in bedrock, soil, lakes or rivers (J.
Hirvonen, The Heat Pump Association of Finland, pers.
comm., 1999). Most of the early installations in 1980's were
made in soil or lakes. About 70% of the heat pumps are
horizontal soil installations, 20% in lakes and 10% in vertical
boreholes (Table 1). Presently there seems to be a trend of
shifting to the vertical ground coupled installations in
boreholes.
A typical small-scale user of a heat pump is a family house
(130-150 m
2
) with an annual demand of heating energy of
about 13,000 kWh/a (including the domestic hot water). This
demand can be satisfied with either a vertical ground coupled
(borehole) installation or horizontal (soil or lake water
coupled) installation, depending on the type and size of
property at use. It is common that the heat pumps work at
about 60 % power of the required maximum heating power
(about 8-9 kW). This is due to the fact that the duration of
extremely cold periods, when the maximum heating power is
required, is only few weeks annually. Thus, the heat pump
satisfies about 90 % of the annual demand of heating energy,
and the remaining heating energy is usually supplied by
electricity.
The vertical ground coupled heat pumps are typically installed
in boreholes 80-130 m deep. Deeper holes (150-200 m) were
preferred in the 1980's. The coefficient of performance (COP),
defined as the ratio of the energy produced to the energy used
by the heat pump, has increased from the values of the early
installations (COP = 2.5) to about 3.3 in the modern
applications. Energy is extracted about 40-60 W/m of
borehole. An ethanol-water solution is used as the heat
exchange fluid and it is circulated in a U-shaped plastic
installed in the borehole.
The horizontal ground coupled systems use pipes that are
buried about 1.0-1.5 m below surface and separated
horizontally by about 1.5 m. In the typical installation for a
130-150 m
2
family house the total length of the pipes is about
150 - 300 m. Horizontal coupled systems in lakes or rivers are
usually dimensioned with slightly shorter pipes than those in
sediments, but no detailed data on the properties of the existing
278
Kukkonen
horizontal installations can be given. Therefore, the data given
in Table 1 are estimates and provide the orders of magnitude
only.
Heat pump technology is utilized in a 0.5 MW district heating
plant in Forssa, southern Finland (Tinell et al., 1986). The
plant provides district heating for a small area with a few
hundred family houses. The heat pump is extracting heat stored
in a shallow (<50 m below surface) aquifer (7ºC) in a
Quaternary esker formation. The water is returned to the
aquifer at a temperature of 2–4ºC. The heat pump is connected
to series with a boiler using heavy fuel oil. Contribution of the
ground heat to the total energy production of the plant amounts
to about 50 %, and the heat pump is operated with a COP value
of 2.1.
Abandoned underground mines provide sometimes an easy
access to utilizable heat sources. Hiiri (1985) investigated the
possibility to use the closed Outokumpu mine in eastern
Finland as a heat source for the district heating plant of the
Outokumpu town. The calculations were based on a heat pump
system with 7 MW heating power. In principle, Hiiri (1985)
found the project technically and economically feasible, but
the sensitivity involving economic and technical parameters
was regarded as considerable. The application was not built,
but the Outokumpu case indicated that the heat pump
applications are worth investigating when a mine is closed.
4. DISCUSSION AND CONCLUSIONS
Geologically Finland represents an environment where the
classical forms of utilizing geothermal energy (hot and dry
rock or steam) are not economically feasible. The remaining
alternative is ground-stored heat extracted with heat pumps
from boreholes, surface sediments as well as lakes and rivers.
At the moment there are about 10,000 vertical or horizontal
ground or lake coupled heat pumps in Finland used for space
heating mainly in family houses and some small district
heating systems in small communities. The majority of the
heat pumps were installed in the 1980's as horizontal ground
coupled systems. The numbers of delivered ground-heat
systems decreased dramatically after 1985 and heat pumps
almost vanished from the heating business. Currently about
800 heat pumps are sold annually, and there has been a slowly
increasing volume of heat pumps sold since 1995. It is
estimated that the total energy produced by heat pumps from
ground heat sources is of the order of 500 TJ/a (Table 1). This
is still less than 1 % of the total consumption of energy in
space heating in Finland.
The major factor retarding the increase of using ground-heat
systems in Finland has been the price of heat pump systems. In
building a typical family house, the cost of installing a heat
pump using ground-heat is about twice the price of installing
systems based on oil or electricity, although the running costs
of ground-heat systems are much lower. It should also be noted
that the dispersed heat pump business may not be very good
against major oil and electricity selling companies in the
country. Additionally, we must also consider the lack of
knowledge on heat pumps among the general audience.
However, the present demand for environmentally better
acceptable and sustainable technologies is constantly
increasing the public interest in this field.
Acknowledgements
J. Hirvonen (the Finnish Heat Pump Association) is
acknowledged for discussions on heat pump business in
Finland.
5. REFERENCES
Aittomäki, A. 1983.
Soil, lake and river systems as sources of
energy.
Tampere University of Technology, Dept. of
Mechanical Engineering, Report 37, 116 p. (in Finnish with
English abstract).
Aittomäki, A. and Wikstén, R.,1978.
Kokemuksia
lämpöpumppulämmityksestä.
Valtion teknillinen
tutkimuskeskus, LVI-tekniikan laboratorio, tiedonanto 36, 40
p. (in Finnish).
Energy Review, No. 2/99. Ministry of Trade and Industry,
Helsinki, 1999.
Hiiri, P., 1985.
Louhostiloista saatavan maalämmön
hyväksikäyttö lämpöpumpun avulla Outokummun kaupungin
lämmöntuottovaihtoehtona.
Master’s thesis, Lappeenranta
University of Technology, Institute of Energy Technology, 68
p. (in Finnish).
Järvimäki, P. and Puranen, M., 1979. Heat flow measurements
in Finland. In:
Terrestrial Heat Flow in Europe
, V. erk and
L. Rybach (editors). Springer, Berlin, pp. 172-178.
Kankkunen, A., 1985.
Boreholes as sources of heat.
Imatran
Voima Oy, Central Laboratory, Research Report, 24/85, 40 p.
(in Finnish with English abstract).
Kivekäs, L., 1978. Prospecting for geothermal energy in
Finland: Geothermal data. In:
Nordic Symposium on
Geothermal Energy, Göteborg, Sweden, May 29-31, 1978,
C.
Svensson and S. Å. Larson (editors), Chalmers University of
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Gothenburg, Sweden, pp. 112-119.
Kivekäs, L., 1979. Geotermisen energian hyödyntäminen.
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Shield.
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Tectonophysics
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Kukkonen, I., 1989. Terrestrial heat flow and radiogenic heat
production in Finland, the central Baltic Shield.
Tectonophysics,
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Kukkonen, I.T., 1993. Heat flow map of northern and central
parts of the Fennoscandian Shield based on geochemical
surveys of heat producing elements.
Tectonophysics,
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13.
Kukkonen, I.T., 1999. Geothermal resources in Finland. In:
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280
281
Kukkonen
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Table 1. GEOTHERMAL (GROUND-SOURCE) HEAT PUMPS AS OF DECEMBER 1999.
Locality Ground or water
temp (°C)
Typical heat
pump capacity
(kW)
Number of
units
Type COP Equivalent Full
load Hr/year
Thermal energy
Used (TJ/year)
Not spec. +2 - +5 8 1000 V 2.5-3.3 4000 50
Not spec -2 - +14 8 7000 H 2.5-3.3 4000 330
Not spec. +1 - +5 8 2000 L 2.5-3.3 4000 95
Forssa S-
Finland
+7 500 1 G 2.1 4900 9
10000 484
Notes: V = Vertical ground coupled, H = Horizontal ground coupled, L = lake or river source, G = groundwater coupled district heating
plant.
282
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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 large-scale geoenergy systems. The first geoenergy studies with measurements with the new TRT equipment have already shown the importance of proper measuring and modelling.
... Correspondingly, the annual average temperature of the ground surface varies from 8°C on the south coast to 2°C in the far north of Finland (GTK, 2019). The thermal conductivity of Finnish rocks is typically over 3 W/(m*K), and the geothermal gradient is 8 -15 K/km (Kukkonen & Peltoniemi, 1998;Kukkonen, 2000). ...
Thesis
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Ground source heat pumps (GSHPs) are an established energy technology, and as such a noteworthy alternative to increase the share of renewable energy in the consumption. At present there are approximately 130 000 geoenergy systems in use in Finland, and in 2018 almost 8000 new GSHPs were sold and installed in the country. The growth rate highlights the significance of good installation practices and public governance of the installations. So far there has been little research into either the construction practices and observed complications of ground heat exchangers (GHEs), or the permit procedures for geoenergy systems in Finland. Therefore, this thesis was designed to examine (1) the management of environmental and quality issues in the construction of GHEs in Finland; (2) the role of public regulation and governance, for example GHE permit systems, in promoting environmental protection and quality control of GHEs in Finland; and (3) ways to develop the capacities of both geoenergy practitioners and public authorities to respond effectively to the environmental and quality challenges. The material for this thesis consisted of a questionnaire study among geoenergy practitioners, interview studies with geoenergy experts and municipal building control officials, heat pump statistics, legal texts, municipal regulations, and permit applications and decisions from municipalities and Regional State Administrative Agencies (AVIs). The questionnaire study asked about the types of complications the practitioners had encountered in their geoenergy projects. The most common types were in order of frequency (1) borehole collapse, (2) discharge of artesian water, (3) harmful spreading of drilling dust and slurry, (4) heat exchanger pipes stuck during installation, (5) flooding caused by artesian water, and (6) heat transfer fluid leakages. Complications resulting from hydraulic connections between separate aquifers and other borehole-related issues were also reported. Competition has been severe within the Finnish geoenergy sector in recent years. A large proportion of the questionnaire respondents referred to the consequent price competition at the expense of quality. Meanwhile, a third of the respondents expressed their concern about quality problems. At present, voluntary training is available for GSHP installers and borehole drillers in Finland, but there are no statutory qualification requirements. Additionally, there are no binding national regulations for the construction of GHEs and geoenergy systems in Finland either. In the questionnaire study, 62% of the respondents were of the opinion that the municipal Action Permit should require BHEs to be built following certain standards. Public authorities may contribute to the quality control of geoenergy systems for example through permit procedures. In Finland there are two permit procedures for GHEs. The municipal Action Permit scheme is applied to almost all geoenergy systems. The Water Permit scheme is administered by the AVIs and it is applied to geoenergy systems on designated groundwater areas. Municipalities have diverse practices in promoting quality control throughout the Action Permit procedure. For example, they may have criteria for the location of the GHE, they possibly require a site manager to be nominated, and building inspectors may control certain details at inspections. The level of expertise varied among building control officials depending on their personal interests and experience. The same applied to the AVIs so that the reasoning in the Water Permit decisions was diversified. As the number of operative geoenergy systems grows, the success and acceptability of the industry depend increasingly on the quality and environmental safety of installations. To promote these, national standards need to be developed for both the construction of GHEs and the Action Permit procedure. Qualification requirements for geoenergy practitioners need to be incorporated into these standards. Sector specific regulations are also needed to clarify the legislation in relation to the Water Permit scheme. Additionally, technical and geological instructions need to be developed to promote geologically sound reasoning in the handling of permits.
... The measured global horizontal solar irradiance in Helsinki is comparable to other cities in Central Europe. The ground conditions are characterised by Precambrian, low-porosity, crystalline bedrock composed mainly of granitic and metamorphic rocks that are typically overlain by a thin layer of sediments of less than 5 m thickness on average (Kukkonen, 2000). The groundwater level is usually found at 1 to 4 metres depth with negligible groundwater flow in the bedrock due to a relatively low fracture frequency. ...
Thesis
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Due to the current issue of global climate change, certain actions have been precipitated in the global energy sector to increase the share of renewable, clean energy. One example of renewable energy is solar thermal energy, which can be utilised for domestic heating purposes in solar communities. However, in countries located at high latitudes, such as Finland, solar thermal energy is most abundant in the summer when the heating demand is low, and less abundant in the winter when the heating demand peaks. The solution to this mismatch is Thermal Energy Storage (TES). TES allows the collection of energy during the summer, which is accumulated in a storage medium, stored seasonally, and extracted in the winter to cover the heating demand. Rock and water in the subsurface are perfect storage media, and a selection of Underground TES (UTES) methods exist which could be utilised as long-term energy storage solutions for solar communities. The goal of this research was to develop a numerical modelling approach for the simulation of the Borehole TES (BTES) systems by first determining which UTES method would be best to apply in a solar community in Finland. Furthermore, through this development, a method was devised to numerically simulate hydraulic fracturing in fractured rock. To select the best UTES method for a Finnish solar community, a criteria-based feasibility study was implemented. It revealed that the BTES method is advantageous in terms of its ease in gaining large storage volumes, feasibility at a small scale, its cost-efficiency and adaptability. Two numerical modelling approaches for the simulation of borehole heat exchangers were proposed, validated by an in situ experiment and used to simulate the BTES systems. Numerical modelling revealed that low thermal diffusivity of the rock is essential for maximising the efficiency of seasonal storage. Furthermore, a fracture mechanics-based numerical model was proposed to simulate the interactions between hydraulic and natural fractures in Fractured TES (FTES) systems. The hydraulic fracturing model indicated that pre-existing discontinuities with low dip angles modify the propagating path of sub-horizontal hydraulic fracture potentially hindering the thermal performance of the FTES systems. The three main conclusions address seasonal TES in hard crystalline rocks. The BTES method is suggested as the most optimal method for a Finnish-based solar community. The two proposed thermal numerical modelling approaches of borehole heat exchangers can aid in the design of BTES systems by efficiently simulating their seasonal performance. Lastly, the proposed hydraulic fracturing model can simulate the construction process of FTES systems. The results of this dissertation contribute towards the development of the state-of-the-art of UTES in hard rocks.
... The use of geothermal energy in Finland is restricted to the utilisation of ground heat with heat pumps. This is due to geological conditions, as Finland is part of the Fennoscandian Shield (Kukkonen, 2000). We call this type of energy 'ground heat' but actually the energy resource we use is heat from bedrock, nowadays up to some 300 metres deep. ...
Book
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The roots of this publication lie in a lecture I gave in 2009 at the World Tunnel Congress in Budapest, Hungary. Following this, the theme has kept me lecturing around the world, mostly in the Far East. Using the City of Helsinki, a forerunner in the field, as a prime example, I have written several papers, given numerous interviews, completed many questionnaires and helped to arrange a number of site visits in order to give inspiration and encouragement to other cities and decision makers on the possibilities of Underground Space Use. Since Budapest, the paper has been elaborated and widened to cover the development of underground space in the urban environment. After that was completed, it was time to release the first edition of this paper to a wider audience in October 2014. This non-commercial publication has been updated and is now available as an independent online publication on the City of Helsinki's website. In my view, the close cooperation that the City of Helsinki has established with the numerous ‘partners’ involved in the planning, financing and designing as well as the actual construction and maintenance of tunnels and underground spaces has perhaps been the crucial factor in sustainable underground property development. As much of this work is also carried out unofficially, trust between the parties is central, particularly when developing processes and sharing risks. I am extremely grateful for the demanding work that so many people have done in the field of Urban Underground Space. My role during the past ten years has been more like an ‘ambassador’ who has strived to advance the long-term sustainable use of underground space. The countless questions, presentations and discussions with colleagues from different countries and cultures have inspired me to write and update this paper ‘Urban Underground Space – Sustainable Property Development in Helsinki’. For this, I thank them all. I also want to thank my own organization and my family for their support and patience during this process, which has lasted much longer than it should have done!
... The bedrock is composed of granitoids and gneisses. Other metavolcanic or metasedimentary lithologies are also present [29]. It is covered by Quaternary soil layers that are less than 5 m thick, on average, due to the advancement and melting of glaciers during the last Ice Age. ...
Article
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Accurate and fast numerical modelling of the borehole heat exchanger (BHE) is required for simulation of long-term thermal energy storage in rocks using boreholes. The goal of this study was to conduct an in situ experiment to validate the proposed numerical modelling approach. In the experiment, hot water was circulated for 21 days through a single U-tube BHE installed in an underground research tunnel located at a shallow depth in crystalline rock. The results of the simulations using the proposed model were validated against the measurements. The numerical model simulated the BHE’s behaviour accurately and compared well with two other modelling approaches from the literature. The model is capable of replicating the complex geometrical arrangement of the BHE and is considered to be more appropriate for simulations of BHE systems with complex geometries. The results of the sensitivity analysis of the proposed model have shown that low thermal conductivity, high density, and high heat capacity of rock are essential for maximising the storage efficiency of a borehole thermal energy storage system. Other characteristics of BHEs, such as a high thermal conductivity of the grout, a large radius of the pipe, and a large distance between the pipes, are also preferred for maximising efficiency.
Article
The thermophysical properties of bedrock are of primary importance when designing borehole thermal energy systems. We present a novel use of the Enhanced Thermal Response Test (ETRT) to determine bedrock thermal conductivity, natural convection, and drill hole thermal resistance as a function of depth in crystalline bedrock. Bedrock was heated with a 228-m-long hybrid cable containing copper wires and fiber optics for temperature monitoring. A reference fiber optic cable was installed along the whole length of the studied drill hole. For groundwater-filled boreholes, the ETRT offers a means to estimate the magnitude of buoyancy-driven natural convection. We estimated that the heating power in the ETRT should not exceed 20 Wm⁻¹ for the thermal conductivities to be determined with sufficient accuracy. According our results, the accuracy of the ETRT can be significantly improved if the test is performed with a hybrid fiber optic cable combined with a reference fiber optic cable. Thermal resistance can be more accurately determined if a reference fiber optic cable is used. The most important achievement of this method is that compared to other measurement methods, the effective thermal conductivity of bedrock can be simultaneously determined along the entire length of the drill hole.
Article
Finland is one of the northernmost countries utilizing ground source heat pumps (GSHPs). In this north European country, GSHPs’ operating conditions are characterized by the cold climate, and hard, crystalline bedrock. Environmental risks and technical problems with ground heat exchangers (GHEs) have been much discussed, but the frequency of complications has not been previously studied in Finland. This article examines the types and construction practices of GHEs, and the range of problems in GHEs experienced by the practitioners. The data was collected through a questionnaire study among Finnish GSHP practitioners, and thematic interviews of Finnish heat pump experts. Borehole heat exchangers (BHEs) proved to be the most popular GHE type in Finland with a share of 85%. The questionnaire responses indicate that the most common complications in BHEs are connected to collapsed boreholes, and artesian or otherwise abundant water yields. Also, issues relating to heat transfer fluids, drilling through multiple aquifers, and design errors are discussed.
Chapter
In Finland, which belongs to the area of the Baltic Shield, 18 heat flow measurements have been carried out in drill holes varying in vertical depth between 389 and 1060 m. The temperature gradient has varied from 9.4 to 24.6 mKm−1, the mean being 12.6 mKm−1. The vertical heat flow has ranged between 21.8 and 50.4 mWm−2, the mean being 35.O±1.6 mWm−2. These figures do not include glacial corrections, which would slightly increase the values of the temperature gradient and the heat flow.
Article
The heat flow-heat production (Q-A) relationship is a useful tool in geothermal research, and it has been widely used for delineating geothermal provinces and determining characteristic parameters of heat production in the continental crust. In this study, a simple but rarely used technique of utilizing the heat flow-heat production relationship is discussed. In central and northern parts of the Fennoscandian Shield extensive geochemical surveys have produced 1483 samples taken from glacial till with a sampling density of 1 sample/300 km2. Heat production values determined from U, Th and K concentrations in these samples were used to calculate a map of heat flow density. A previously determined Q-A relationship, Q = 15.8 + 10.8 · A, was applied. The compiled map covers all Finland, northern Sweden and northern Norway, about 35% of the exposed shield area in Fennoscandia. Heat flow density and heat production increase with decreasing geological age and correlate with granitoid types. The calculated heat flow density values on the map were controlled with 12 drill hole measurements not used in calculating the applied Q-A regression line. Nine of them are from previously unpublished data from the Finnish part of the Shield. The agreement with drill hole measurements and the geochemical estimate is reasonable, although not perfect in all cases. The differences can be attributed to anomalous vertical variation in heat production, reliability of the applied Q-A plot, reliability of till geochemistry in bedrock studies, convective groundwater disturbances or local structural effects. The calculated heat flow map can be used as a data set supplementing drill hole measurements to determine representative values of heat flow density in areas with low numbers of drill hole measurements.
Article
The vertical variation of heat-flow density in the Central Baltic Shield was studied in 17 drill holes (389–1060 m deep). Apparent heat-flow densities calculated in 100 m depth sections, with typical determination errors smaller than 2 mW/m2, showed a variation of up to 15 mW/m2 in single holes. A palaeoclimatic correction for surface temperature variations during the last million years was calculated as a function of depth for each hole with a homogeneous half-space conduction model. If the bedrock temperatures are controlled only by conduction of heat, and the temperature history is accurately known, the palaeoclimatically corrected heat-flow densities should have the same (steady-state) value at all depths throughout a drill hole. In practice, the quality of the correction is indicated by decrease or increase in the standard errors of the drill-hole means of heat-flow density. When the corrections were applied to the measured data, the standard errors decreased in only eight drill holes, and the vertical variation in palaeoclimatically corrected values ranged from a few mW/m2 to 10 mW/m2. In some of the holes, this variation can be attributed to heat-flow refraction at inclined conductivity interfaces, resulting in local heat-flow anomalies. However, the most important cause of the variation seems to be groundwater flow in bedrock, i.e., heat transfer disturbing the conductive regime. This notion is supported by heat-flow density-depth plots and temperature-depth plots and the decrease in observed heat-flow variation below 500 m depth, which is the typical depth of rapidly changing fresh groundwater, below which more saline (and stagnant) groudwaters are usually encountered. The present results indicate the following: 1) Groundwater flow can be fairly common in the upper parts of bedrock in the Baltic Shield, and purely conductive circumstances do not necessarily prevail everywhere; 2) palaeoclimatically corrected heat-flow values must be used with great precaution, especially when signs of groundwater flow are present in the data; 3) the palaeoclimatic correction can be applied to study groundwater flow in bedrock indirectly and to test whether a conductive regime prevails or not.
Article
Finland is part of an ancient, Precambrian area known as the Baltic Shield. During the years 1960–1966, geothermal measurements were carried out at five points in southern and central Finland. In conjunction with these investigations, 212 temperature measurements were made in drill holes and 106 thermal conductivity determinations from core samples. In the temperature measurements, a copper wire resistance or thermistor probe and a portable Wheatstone bridge were used. The thermal conductivities were measured by a steady-state comparison method very similar to that used by birch (1950). The average temperature gradients measured in the different drill holes vary within the range of 11.0–24.6°C/km, the thermal conductivities within that of 4.3–8.0 mcal./cm sec°C and the heat flow values within that of 0.65–1.20 μcal./cm2sec. The average heat flow is 0.90 μcal./cm2-sec. The observed low heat-flow values agree well with the results of measurements made in other Precambrian shield areas. In Finland the geothermal investigations have been carried out by the Geological Survey of Finland and the Outokumpu Company.
Article
Unpublished heat flow measurements from 17 drillholes in Finland (central Baltic Shield) are presented. The holes range from 270 to 1080 m in depth. Sharp vertical changes in the heat flow density and in the temperature gradient indicating groundwater flow disturbances were detected in eight holes. The variations in heat flow were typically of the order of 5–10 mW/m2. In many cases the groundwater flow concept was supported by geological logs and other borehole measurements which indicated the existence of fracture zones at the same depths as the flow. The flow zones were usually encountered at depths of less than 500 m, but flows between fracture zones in the holes were detected even deeper, at as much as 930 m. According to recent groundwater studies in the same holes, the water flowing in the fracture zones seems to be fresh or only slightly saline in composition. This can be taken as an indication of relatively rapidly circulating groundwater, in contrast to the more saline (and more stagnant) groundwater encountered deeper in the holes. The results indicate that disturbances in heat flow induced by groundwater flow may be much more common in areas of crystalline bedrock than has previously been realized. The apparent heat flow densities (mean 35.0 ± 3.0 mW/m2) agree well with values published previously from the central Baltic Shield, but the results from southern and western Finland indicate regionally above-average heat flow values (> 45 mW/m2).
Article
Heat flow density and radiogenic heat production of the bedrock were studied in Finland in part of the central Baltic (Fennoscandian) Shield. Heat flow data were collected from 35 holes 270–1080 m deep. The heat production values at the sites were determined from the drill core samples by gamma ray spectrometry. The areal variation in heat production was studied with the aid of K, U and Th analyses of 1054 glacial till samples collected for the “Geochemical Atlas of Finland”. A heat production map constructed from this data set revealed a strong areal variation that can be attributed to the known lithological, geochemical and tectonic features of the bedrock. In general, heat production seems to increase with decreasing geological age. The presented data strongly suggest that the southern and western parts of Finland are geothermally anomalous (apparent heat flow density 38–68 mW/m², surface heat production > 2.0 μW/m³) in contrast to the other parts of the country (< 42 mW/m², < 2.0 μW/m³). The heat production-heat flow density plots were constructed from apparent and palaeoclimatically corrected heat flow densities; the heat production values were determined from both drill core samples and till samples.
Soil, lake and river systems as sources of energy
  • A Aittomäki
Aittomäki, A. 1983. Soil, lake and river systems as sources of energy. Tampere University of Technology, Dept. of Mechanical Engineering, Report 37, 116 p. (in Finnish with English abstract).
Kokemuksia lämpöpumppulämmityksestä. Valtion teknillinen tutkimuskeskus, LVI-tekniikan laboratorio, tiedonanto 36
  • A Aittomäki
  • R Wikstén
Aittomäki, A. and Wikstén, R.,1978. Kokemuksia lämpöpumppulämmityksestä. Valtion teknillinen tutkimuskeskus, LVI-tekniikan laboratorio, tiedonanto 36, 40 p. (in Finnish).
/99. Ministry of Trade and Industry
Energy Review, No. 2/99. Ministry of Trade and Industry, Helsinki, 1999.
Louhostiloista saatavan maalämmön hyväksikäyttö lämpöpumpun avulla Outokummun kaupungin lämmöntuottovaihtoehtona
  • P Hiiri
Hiiri, P., 1985. Louhostiloista saatavan maalämmön hyväksikäyttö lämpöpumpun avulla Outokummun kaupungin lämmöntuottovaihtoehtona. Master's thesis, Lappeenranta University of Technology, Institute of Energy Technology, 68 p. (in Finnish).