GRETA is co-financed by the European Regional Development Fund through the Interreg Alpine Space programme. Send us an email at [email protected] and see more about GRETA at www.alpine-space.eu/projects/greta.
1/18
Catalogue of technical and operational criteria
for shallow geothermal systems in the Alps
This document was written in the course of the ERDF funded project GRETA (2015 – 2018). It is the final
deliverable of the technical work package 3 “Operational criteria for the utilization of Near-Surface
Geothermal Energy in the Alpine environment”. The Geological Survey of Austria, as responsible partner,
elaborated this catalogue with the contribution from the involved project partners
The preliminary studies for this deliverable were carried out mainly within the activities 3.1 “Assessment of
existing techniques and best practices for the utilization of Near-Surface Geothermal Energy” and 3.2
“Operational criteria and constraints for shallow geothermal systems in the Alpine environment” from
16.12.2015 to 15.06.2017.
This deliverable will be taken into account by WP4 “Operative constraints and thresholds for the assessment
and mapping of Near-Surface Geothermal potential” and WP5 “Elaboration of technical concepts on existing
and possible future uses of NSGE for its integration into the energy plans of pilot areas”. This document was
conducted interactively with the relevant stakeholders and through a feedback loop with the observers.
Deliverable D.3.3.1 – Multi-language catalogue of technical and operational criteria for the
shallow geothermal systems in the Alps
16/09/2017 – 15/09/2018: This catalogue summarizes all relevant proven and estimated concepts
for the use of geothermal methods in the Alpine environment. It represents a joint document
available in the involved national languages French, German, Italian and Slovenian.
GRETA is co-financed by the European Regional Development Fund through the Interreg Alpine Space programme. See more about GRETA at www.alpine-space.eu/projects/greta.
2/18
Introduction
This document was written in the course of the ERDF funded project GRETA (2016 – 2018) and represents a
summary of information gained throughout the project´s duration. It is the final deliverable of the technical
work package 3 “Operational criteria for the utilization of Near-Surface Geothermal Energy in the Alpine
environment”. The preliminary studies for this deliverable were carried out mainly within the activities of work
package 3, but equally importantly, with collaboration of other technical work packages.
The aim of GRETA’s workpackage 3 was to formulate a multi-language
guideline of operational criteria and state of the art of the utilization of
shallow geothermal energy in the Alpine environment. This catalogue shall
be used to raise the level of knowledge for implementation of shallow
geothermal systems both for planners and other interest groups.
The content of this document includes:
the range of installations in the Alpine space, giving an overview of
commonly used techniques and the suitability of these systems for
application… see page 4
the specifics of shallow geothermal use in the Alpine region, showing
usage profiles for different climatic conditions and building types… see
page 8
operational criteria and their constraints, describing parameters
relevant for the shallow geothermal use and those used for potential
mapping … see page 11
possible environmental challenges… see page 17
Shallow geothermal is applicable nearly every-where. Still, the huge potential of shallow geothermal is scarcely known and used. The GRETA project wants to unlock the potential of this energy source in the alpine space. Among renewable energy sources, geothermal is one of the few technologies capable of reducing the demand of primary energy and can therefore play a significant role facing the challenges of the on-going energy transition.
SH ALLO W GEOT HE RMAL AT A GL ANCE
GRETA is co-financed by the European Regional Development Fund through the Interreg Alpine Space programme. See more about GRETA at www.alpine-space.eu/projects/greta.
3/18
Principle of shallow geothermal installations
Shallow geothermal systems use the ground as a heat source (and/or sink) to provide heating and domestic
hot water (and/or cooling) to buildings. In a few cases, the temperature of the source/sink allows a direct heat
exchange with the ground: it is the case of warm shallow aquifers exploited for heating, and for cooling of the
so-called “free cooling” systems. In heating mode, the shallow geothermal heat source is too cold for a direct
utilisation. Therefore, shallow geothermal systems use a heat pump to generate the demanded temperature
level. Geothermal heat pump systems feature three main components:
- The ground side, to extract/inject heat out of or into the ground (ground water / solid underground /
artificial ground like buildings foundations);
- The heat pump itself, to transfer heat from a cold source to a hot sink;
- The building side, i.e. the equipment inside the building that transfers the heat or cold into the rooms
(radiant heating / floor heating / etc.).
In heating mode The heat pump consumes electricity to shift heat from the ground (inlet temperature) to the
building side. Typical inlet temperatures range between 0 – 5 °C for BHEs and 10 – 15 °C for groundwater use.
Temperature levels at the building side range between 25 – 35 °C for floor heating in modern buildings and up
to 45 – 60 °C for domestic hot water. The ratio between the heat provided to the building and the electricity
consumed by the heat pump is called coefficient of performance (COP) or, when calculated over the whole
season, the seasonal performance factor (SPF). These coefficients strongly depend on the temperature levels
of the heat source and the heat sink: the smaller the temperature spread between ground and building side,
the higher the efficiency of the system.
GRETA is co-financed by the European Regional Development Fund through the Interreg Alpine Space programme. See more about GRETA at www.alpine-space.eu/projects/greta.
4/18
The range of shallow geothermal installations in the Alpine space
Table 1 shows a qualitative summary of the range of installations for the whole Alpine Space. In inner-alpine
environments, the existence of productive groundwater bodies usable for open-loop systems is mostly bound
to the valley bottoms, while closed-loop systems can be installed nearly everywhere, even in remote locations.
Hereafter, the suitability, the advantages and the disadvantages of the most commonly used shallow
geothermal systems are assessed.
Table 1: Comparison of the suitability of different shallow geothermal energy technologies.
Borehole Heat Exchanger
Thermal Groundwater Use
Shallow Heat Collector
Thermal Surface Water Use
Geostructures
System type CLOSED-LOOP OPEN-LOOP CLOSED-LOOP OPEN-LOOP CLOSED-LOOP
Free cooling potential MEDIUM HIGH MEDIUM HIGH MEDIUM
Need of drillings? YES YES NO (ground excavation)
NO NO
Need of an aquifer? NO YES NO SURFACE WATER NO
Depth of installation Normal: 50-150 m Maximum: up to 400 m
Normal: 10-20 m Maximum: up to 100 m
Normal: 1,2-1,5 m Maximum: up to 3 m
Deep point in the surface water reservoir
Depending on building construction
Installation costs HIGH MEDIUM LOW LOW MEDIUM
Influenced by seasonal temperature variation?
LOW LOW - MEDIUM depending on depth of aquifer
HIGH HIGH MEDIUM
Efficiency MEDIUM HIGH LOW MEDIUM – HIGH MEDIUM
Space demand LOW LOW HIGH LOW No extra space demand
Specific heat extraction (VDI 4640)
20-100 W/m 0,25 m3/h per kW evaporator capacity
8-40 W/m2 0,25 m3/h per kW evaporator capacity
Constraint: System shut down at 0°C
Size of thermal plume LOW HIGH LOW – MEDIUM HIGH LOW – MEDIUM
Main advantages + Can be installed almost everywhere + Constant performance + Easy upscaling for applications sizes
+ High efficiency, especially in free cooling + Medium installation costs
+ Low installation costs
+ High efficiency, especially in free cooling + Low installation costs
+ Can be integrated in many new buildings + No extra space demand
Main disadvantages - Relatively expensive
- Need of a productive aquifer
- Influenced by outside air temperature - Lower efficiency
- Influenced by outside air temperature - Constant performance cannot be guaranteed
- Relatively low efficiency
Awareness of possible challenges
! Anhydrite layers in the underground ! Karstic rocks in the underground
! Groundwater chemistry ! Groundwater temperature
! Rocky ground ! Low ground temperatures at high altitudes
! Water chemistry
GRETA is co-financed by the European Regional Development Fund through the Interreg Alpine Space programme. See more about GRETA at www.alpine-space.eu/projects/greta.
5/18
CLOSED LOOP SYSTEMS
Borehole Heat Exchangers (BHE) are composed of one or two U-pipes where a heat carrier fluid is circulated
to exchange heat between the ground and the building. They are the most common solution for shallow
geothermal energy throughout the whole Alpine Space. They are used both for small plants such as detached
houses (single BHE or a few ones) and for larger plants like hotels (BHE fields). BHE systems are fully efficient
if used for combined heating and cooling purposes. The heat injected into the ground during summer in cooling
periods can be reused for heating in winter. Hence, BHE fields are very efficient in buildings with a relatively
high cooling demand, like offices or hospitals.
The other two main typologies of closed-loop systems are the so-called thermally active geostructures and
the shallow heat collectors (SHC) or ground heat collectors, respectively. Besides its structural function,
thermally activated geostructures can be used as ground heat exchangers, thus avoiding drilling BHEs on
purpose. For example, geothermal piles are diffused for multi-storey office buildings, where foundation piles
are necessary.
Shallow heat collectors are typically installed at a depth of about one to two meters and are frequently used
for single detached houses since they require a large unbuilt area for their installation. This area has to remain
unsealed for the infiltration of rainwater for thermal regeneration in summer and to avoid damage. Thus, a
large number of other uses (tree planting, car parking etc.) are prevented.
A general drawback of closed loop systems compared to open systems is the slightly lower source
temperature. Moreover, heat exchange and transport in soil or solid rock is much slower than in moving
groundwater. The biggest advantage of this type of installation: it is possible almost everywhere!
OPEN LOOP SYSTEMS
Open loop systems exchange heat directly with groundwater, which is abstracted by one (or more) well(s) and
usually reinjected into the same aquifer through other well(s) or, otherwise, rejected in channels or rivers. The
reinjection ensures the hydrological balancing of the aquifer’s budget. The characteristics of the groundwater
body, like its depth, temperature and groundwater velocity strongly influence the efficiency of the plant; thus,
the installation of an open-loop system requires specific hydrogeological knowledge at the local scale.
If a productive aquifer is available at a reasonable depth, the use of open loop systems brings advantages
compared to closed loop systems. The direct heat exchange with groundwater allows higher performance
compared to closed-loop systems and more economically worthwhile.
Further, an open loop system can work in free cooling mode: performing the heat exchange directly between
the groundwater and the building distribution circuit. This is a very efficient cooling mode, and the only electric
consumptions are due to the well-pump(s) and the distribution circuit circulator(s), without the heat pump
use. Free cooling is diffused where large cooling demands are present. This is often the case for e.g. industrial
production sites with machinery or process cooling or data centres.
Although it is not strictly a geothermal source, thermal surface water use is often referred to as open-loop
hydrothermal system. Hence, this method is also mentioned here for the sake of completeness. Lakes, rivers
or the sea can be used as a heat source or sink.
GRETA is co-financed by the European Regional Development Fund through the Interreg Alpine Space programme. See more about GRETA at www.alpine-space.eu/projects/greta.
6/18
Potential innovations in the use of shallow geothermal systems
Much of the technology used in the sector of shallow geothermal energy is relatively mature. A correct design,
including a preliminary geological and hydrogeological analysis and an installation compliant with technical
standards, allows the installation of efficient, economical, long-lasting conditioning systems with a low
environmental impact.
The potential for major innovation in the principal components of a shallow geothermal system is therefore
quite limited. However, there is still scope for innovation. In two areas further improvements are currently
developed. First, the different ways how heat can be injected or retrieved from the ground using energy
geostructures. Second, the adaptation of existing systems to perform in new ways, notably to address the
increased cooling demand.
ENERGY GEOSTRUCTURES
The use of civil engineering structures that are in contact with the ground as a heat-exchanger are an area of
interest in many countries and are especially relevant in the Alpine Space given the high density of large-scale
transport infrastructure in mountainous regions (e.g. tunnels, bridges, major earthworks).
Examples of different kinds of energy geostructures exist and are increasingly installed, with the exception of
energy piles, which are yet to see widespread development. Thus, geostructures represent a very small
fraction of shallow geothermal systems. The energy recovered from or stored in the geostructures can be used
either directly for the needs of the infrastructure or as an energy source for nearby users. Cases of different
installations in which, during the design phase, geothermal heat exchange systems can be included, are:
Foundation piles: most pile types can be adapted to include pipes for heat exchange and energy piles are
becoming more common (Bourne-Webb and Da Costa Gonçalves1, 2016).
Retaining walls: retaining walls and diaphragm walls can be thermally activated by including heat
exchange loops during their construction.
Platform and road de-icing and preventing excessive heating in summer: heat exchange loops are
integrated directly in the road surface. This technology can also be combined with geothermal heat
storage, storing solar energy that heats the road in summer for use in winter.
Airport runways: the large ground volume beneath runways available for geothermal energy and the high
costs associated with de-icing runways mean that some airports have installed energy piles (Olgun and
Bowers2, 2016).
Bridge deck de-icing also provides an option for controlling stresses, especially in pre-stressed concrete.
Systems exist using energy piles and exchangers in bridge abutments.
Tunnel wall activation: specially designed tunnel linings allow tunnels to be thermally activated, such as
for example the ENERTUN energy segment developed at Politecnico di Torino. Studies are continuing in
this field, to quantify the heat available and assess the impact of such a system on surrounding ground
and the structure itself (Barla and Di Donna3, 2018).
1 Bourne-Webb, P., & da Costa Gonçalves, R. (2016). On the Exploitation of Ground Heat Using Transportation Infrastructure. Procedia engineering, 143, 1333-1340. 2 Olgun, C. G., & Bowers, G. A. (2016). Experimental investigation of energy pile response for bridge deck deicing applications. DFI Journal-The Journal of the Deep Foundations Institute, 10(1), 41-51. 3 Barla, M., & Di Donna, A. (2018). Energy tunnels: Concept and design aspects. Underground Space.
GRETA is co-financed by the European Regional Development Fund through the Interreg Alpine Space programme. See more about GRETA at www.alpine-space.eu/projects/greta.
7/18
Tunnel water outflow: a specific application for Alpine tunnels involves the recovery of heat from the
water that flows naturally from many tunnels in the Alps, at temperatures that can be as high as 30-40 °C.
An advantage of this application is that little extra infrastructure is required, as the water must be drained
from the tunnel anyway. Planning during the design phase can however reduce the heat losses, thereby
improving the overall heat recovery. The main problem in this case is to find thermal needs in the nearby,
nevertheless already existing installations in Switzerland provide heat for space heating and production of
domestic hot water. A selection of 13 tunnels in Switzerland has been calculated to have a total heat
potential of 30MWth (Wilhelm and Rybach4, 2003).
UNDERGROUND THERMAL ENERGY STORAGE
Whilst all shallow geothermal systems use seasonal heat storage effects to some extent, the combination with
an explicit heat storage has the potential to increase the efficiency and economics of shallow geothermal
systems. Three proven technologies are distinguished:
Aquifer thermal energy storage (ATES): A famous example is the German Reichstag. Here two separate
aquifers are used for heat storage and cold storage. Two well groups supply the parliament buildings
together with a cogeneration unit. (Schmidt and Müller-Steinhagen5, 2005)
Underground Thermal Energy Storage (UTES): Here borehole heat exchangers are usually installed in
circular plan and spaced at a distance of 2 – 3 m in order to minimise the heat losses. Examples: UTES built
with borehole heat exchangers (plus high temperature water tank) in Crailsheim, Germany (Bauer6, 2007)
or in Okotoks, Canada (Liebel and Reuß7, 2006)
Thermal energy storage built with ice storage: Whilst the design and construction of large ice storage
systems require a lot of know-how and care, their advantages are the utilisation of the huge phase change
enthalpy of water and the minimum heat source temperature of 0 °C, which keeps the COP of the heat
pump high.
4 Wilhelm, Jules, and Ladislaus Rybach. "The geothermal potential of Swiss Alpine tunnels." Geothermics 32.4-6 (2003): 557-568. 5 T. Schmidt, H. Müller-Steinhagen, (2005), Erdsonden- und Aquifer-Wärmespeicher in Deutschland, OTTI Profiforum Oberflächennahe Geothermie, Regenstauf, 14.-15. April 2005 6 Bauer, Dan, et.al., DER ERDSONDEN-WÄRMESPEICHER IN CRAILSHEIM, OTTI, 17. Symposium Thermische Solarernergie, Kloster Banz, Bad Staffelstein, 09.-11.05.0 7 Liebel, V., & Reuss, M. (2006). PE-X borehole heat exchangers for high temperature UTES applications. In Proceedings of Tenth International Conference on Thermal Energy Storage, Ecostock.
GRETA is co-financed by the European Regional Development Fund through the Interreg Alpine Space programme. See more about GRETA at www.alpine-space.eu/projects/greta.
8/18
Specifics in the Alpine space
This schematic cross-section gives an overview about the climatic conditions in the Alpine Space from Genova
to Munich. Conditions are extremely heterogeneous, which results in specific needs related to the usage of
shallow geothermal energy in different regions. While the hot Mediterranean climate is predominant in the
southern part of the Alpine space (Genova, Nice), the northern parts, like Munich or Vienna are characterized
by temperate/warm, continental climate (Fehler! Verweisquelle konnte nicht gefunden werden.). The
mountainous regions with their cold climate, steep slopes, narrow valleys and very constricted aquifers pose
unique challenges for the application of shallow geothermal systems.
In the perspective of the system design, two main aspects should be taken into account:
the climatic context and the heating/cooling demand dependent on the building thermal insulation;
the geological/hydrogeological context, allowing the choice of the most appropriate geothermal
system.
USAGE PROFILES IN DIFFERENT CLIMATE CONDITIONS WITHIN THE ALPINE REGION
Europe can be classified into different climate zones, based on heating (HDD) and cooling (CDD) degree days8.
To classify the Alpine Space region of the GRETA partner countries, a method by Tsikaloudaki et al (2012) 9
was adopted, identifying five zones (A-E). The methodology was modified adding the zone F to identify the
coldest mountain areasFehler! Verweisquelle konnte nicht gefunden werden..
The calculated HDD range from < 1500 (zone “A”: e.g. Nice, Genova, Marseille) to > 3750 in for zone “F”, for
example Sestriere, Lech or Davos. Most parts of the Alpine space show no significant cooling demand (less
than 200 CDD). The cities with the highest cooling demands (> 500 CDD) can be found in the coastal region,
for example Koper and Genova while the identified climate zone “C” only appears in Marseille.
Contrary to fossil fuels heat-generators, installation costs of geothermal plants strictly depend on the installed
power. Thus, particular attention should be paid to the correct sizing of the geothermal heat exchanger, in
order to avoid the oversizing of the plant, reduce the investment, maximizing at the same time the efficiency.
8 Heating and cooling degree-days are the sum, along the heating or cooling season, of the differences measured with a daily frequence between the daily average outdoor temperature and a reference values. For this classification, we used the ASHRAE 65°F approach, which considers 65°CF (18.3°C) as the reference temperature for the calculation of both heating and cooling degree-days. 9 Tskikaloudaki K., Laskos K. And Bikas D., 2012, On the Establishment of Climatic Zones in Europe with Regard to the Energy Performance of Buildings, Energies 2012 (5), pp. 32-44, doi:10..3390/en5010032
GRETA is co-financed by the European Regional Development Fund through the Interreg Alpine Space programme. See more about GRETA at www.alpine-space.eu/projects/greta.
9/18
For this reason, it is useful to perform a detailed and precise evaluation of the thermal loads of the building.
The energy demand of the construction depends on a large number of variables, the most relevant of which
are:
- Climate condition (external air temperature and solar irradiance)
- Building type (related to the final use, occupancy and internal heat gains)
- Envelope thermal insulation (thermal resistance of windows, walls, ceilings and floors)
For the assessment of the suitability of shallow geothermal for different building types, transient simulations
by means of the modelling software TRNSYS has been carried out. TRNSYS is a transient simulation software
package widely used in building physics due to its modular structure, which takes into account the relations
between different components of energy systems.
The three basic building types (figure above) identified for this task are residential house (A), office (B) and
hotel (C). According to occupation and internal energy gains, these three building types cover a wide range of
real application cases. Further, the three building types were considered with two different thermal insulation
cases, a good and a poor one. The thermal loads were assessed based on the climatic zonation. Five climatic
zones and three building types with two insulation cases each makes a total number of 30 different model
cases. Evaluation of the simulation results gives a good overview of the specifics for the usage of shallow
geothermal energy regarding different climatic zones and buildings.
Schedule of the heating and cooling systems in different building types.
Effect of insulation (poor/good) on heating/cooling demand of residential building (detached house, case “A” in figure above).
Type
Heating
Monday Week day Weekend
House 6–8 am
4–10 pm 6–8 am
4–10 pm 8 am – 10 pm
Office 4 am – 6 pm
6 am – 6 pm
Always off
Hotel Always on Always on Always on
Type
Cooling
Monday Week day Weekend
House 4 – 10 pm 4 – 10 pm 8 am – 10 pm
Office 7 am – 6 pm
8 am – 6 pm
Always off
Hotel Always on Always on Always on
GRETA is co-financed by the European Regional Development Fund through the Interreg Alpine Space programme. See more about GRETA at www.alpine-space.eu/projects/greta.
10/18
Finally, the peak load and the total annual energy demand are derived from the simulation results. Some of
the conclusions are summarized below, for a full comprehensive summary please refer to the GRETA
Deliverable 3.2.1:
A better thermal insulation strongly reduces the heating demand, but it increases the cooling demand ,
since the solar and internal gains are “trapped” inside the building due to the lower wall and window
transmittance. An example for a residential building is shown at the previous page.
Production of domestic hot water (DHW) through shallow geothermal lowers the efficiency of the heat
pump due to the higher temperature levels. Hence, the seasonal performance factor is influenced
negatively, but it practically does not influence the peak thermal load required, since DHW is produced
slowly and stored in tanks (e.g. about 100 litres for an apartment).
the economic feasibility of shallow geothermal systems increase:
o for larger buildings, since the cost per kW of heat pumps significantly diminishes when the power
increases;
o with increasing life of installation: compared to conventional systems the installation costs are
relatively high. This is compensated due to low operating costs.
o for buildings where, due to their occupancy level and/or comfort requirements, the heating
and/or cooling plant operates for a large number of hours per year. From this point of view, it can
be very profitable for hotels open all the year;
o if heating as well as cooling demand exists. By coverage of the cooling demand excess heat is
stored underground during summer. This heat is available for extraction during the heating period.
o in cold climates since, usually, the colder the climate the higher the number of full-load equivalent
hours per year;
In heating-dominated buildings, which are expected in most of the Alpine Space area, the shallow
geothermal system can be combined with another heat source to cover peak loads. Both the heat pump
and the geothermal installation can therefore be sized on a lower power, thus operating more hours per
year, reducing the installation costs. This improves the economic performance of the system.
GRETA is co-financed by the European Regional Development Fund through the Interreg Alpine Space programme. See more about GRETA at www.alpine-space.eu/projects/greta.
11/18
Operational criteria and their constraints
The technical and the economic feasibility of shallow geothermal systems depends on a number of operational
criteria, which are summarized in the table below and are grouped into given natural conditions, the demand
or design side and economic factors. Depending on the technology adopted, the relative influence of each
parameter varies.
MAIN OPERATIONAL CRITERIA FOR SHALLOW GEOTHERMAL SYSTEMS
Natural operational criteria
Description Unit Dependent on parameters
Ground temperature
Mean ground temperature [°C] Outside air temperature, solar radiation, hillside exposure, hillslope, snow levels, vegetation, heat flow, anthropogenic constructions/ /sealing (buildings, roads)
Thermal conductivity
Property of a material to conduct heat. Dependent on the physical properties of the subsurface
[W/m K] Anisotropy, compactness, crevasse formation, grain size, permeability, porosity, rock type, soil type, water content
Specific or volumetric heat capacity
Amount of energy needed to raise the temperature of one kilogram or one cubic metre of a certain material by 1K.
[J/kg K] or [J/m³ K]
Temperature, Density
Aquifer (groundwater) temperature
Together with the maximum possible temperature spreading indicates the thermal technical potential
[°C] Outside air temperature, surface water interaction, rainfall, snow levels, existing groundwater usage,
Groundwater level Depth to exploitable aquifer [m]
Transmissivity Product of aquifer hydraulic conductivity and saturated thickness.
[m²/s] Hydraulic conductivity, aquifer thickness
Hydraulic gradient Expresses how the water level changes with distance. Together with transmissivity, it gives a measure of the flux within the aquifer.
[-] Transmissivity, groundwater recharge
Design operational criteria
Description Unit Dependent on parameters
Peak power Peak power required by the building in heating and cooling
[kW] Thermal insulation
Thermal demand Thermal energy demand required by the building both in heating and cooling
[kW] Building characteristics and use
Economic operational criteria
Description Unit Dependent on parameters
Drilling costs Cost for the drilling and installation of borehole heat exchangers or water wells
€ Journey to drilling site, total amount of drilled meters, bedrock properties, GW chemistry
Installation costs Cost of the heat pump, installation, and auxiliaries € Type of HP, chemical composition of the GW
Running costs Yearly cost for maintenance and electricity (fuel) for the installation
€ Maintenance, energy price
Financing Plan and execution of financing the investment € Incentive programs, Interest rates
NATURAL OPERATIONAL CRITERIA
The most obvious natural criterion for the applicability of shallow geothermal energy is the underground or
ground water temperature. While in urban areas measured groundwater temperatures as well as
underground data are widely available, these values are harder to assess in rural areas.
There are different possibilities for the evaluation of underground temperature. The most common approach
applies a correlation of underground temperature with the air temperature depending on the surface
elevation above sea level (see Ref.10 for Switzerland). A more sophisticated approach was applied in the case
10 S. Signorelli, T. Kohl, Regional ground surface temperature mapping from meteorological data, Global Planet Change 40(3–4) (2004) 267-284.
GRETA is co-financed by the European Regional Development Fund through the Interreg Alpine Space programme. See more about GRETA at www.alpine-space.eu/projects/greta.
12/18
study area Leogang where additionally the dependency of the solar exposure on underground temperature
was taken into account and validated by numerical modelling (References DEL.3.2.1 & DEL.4.2.1).
Thermal conductivity and heat capacity (material properties) are mostly estimated from standard values for
common types of rocks and sediments, using geological maps. Beneficial would be the additional execution of
a measurement campaigns of rock samples and/or data from thermal response tests (TRT). Within the GRETA
project, thermal conductivity and capacity values of a number of
representative rock samples from the Case Study areas of Cerkno (SLO),
Aosta Valley (IT) and Parc des Bauges (FR) were measured: GEOZS (SLO),
ARPA VdA (IT) and BRGM (FR) performed the sampling; measurements
were performed by GEOZS using a laser TCS device11.
Information about the aquifer can be gathered in areas where
installations (water or geothermal wells) are already existing. Depending
on the country, information about existing wells are publicly available or
can at least be requested from national, regional or local authorities.
Important information includes the depth of the groundwater table, the
transmissivity/productivity of the aquifer and the gradient/flow
velocity. In the GRETA project, we collected such information for the
case-study areas of Oberallgäu (Germany), Saalbach-Leogang (Austria),
the Aosta valley (Italy) and for the Isère Valley (France).
Some regions in the Alpine space also provide information systems with
detailed information about the aquifer and the underground (for more
information see GRETA Deliverable D2.2.1 – Comparison of NSGE
installation in the Alpine region selected for reproducibility and
transferability relevance and Deliverable 4.1.1 - Assessment and mapping
of potential interferences to the installation of NSGE systems in the Alpine
Regions).
DESIGN OPERATIONAL CRITERIA
The term “design operational criteria” refers to the demand side of the system. Depending on the building
type and the climatic zonation, a typical load profile (base and peak load and the total thermal demand) can
be calculated. The energy demand of the construction covered by the geothermal installation, depends on a
large number of variables, of which the most relevant are:
o Climatic conditions (external air temperature and solar irradiance)
o Building type (related to the final use, occupancy and internal heat gains)
o Envelope thermal insulation (thermal resistance of windows, walls, ceilings and floors)
o Domestic hot water: The higher temperature level needed for domestic hot water production
must be considered when dimensioning of the plant.
The design operational criteria can also be called sizing parameters as they generally define the size of the
required installation. It is therefore one of the most decisive factors for the installation costs. The results of
11 Official website at http://www.geophysik-dr-rauen.de/tcscan/
The Austrian Province of Salzburg offers a big variety of hydrological and geothermal data free for download in a webgis application: salzburg.gv.at/sagis Regione Lombardia has an open-access database of existing BHEs where the results of TRTs are also published: www.rinnovabililombardia.it/rsg The federal state of Bavaria offers an online portal with plenty of information on geological, hydrogeological and geothermal topics: http://www.umweltatlas.bayern.de/startseite/
GOOD PR AC TICE
INFO RM ATIO N SYSTEMS
GRETA is co-financed by the European Regional Development Fund through the Interreg Alpine Space programme. See more about GRETA at www.alpine-space.eu/projects/greta.
13/18
the heating demand calculations show that only a very small share of the running hours need the full capacity
(or peak load). This holds true, in particular, for buildings with a long operating schedule (e.g. hotels); the
heating/cooling terminals also play an important role, since radiant panels have a much higher thermal inertia
compared to fan coils; large heat tanks also reduce the number of operating cycles for the heat pump.
Fehler! Verweisquelle konnte nicht gefunden werden.The figure above shows the cumulate distribution of
the heat demand covered (expressed as a percentage on the yearly demand) plotted against hourly heat loads
(expressed as load factor, i.e. normalized on the peak load). The “elbow” in such plots identifies the value of
load factors on which a hybrid plant (heat pump + gas boiler) could be optimally sized. In this case, 85 – 95 %
of the total heating demand can be covered with 50 % of the peak load for a well-insulated residential building
in Bolzano (Italy). The simulation results are compared with a real-world installation, the social housing
complex “Casanova” in Bolzano (Italy). These results show clearly that shallow geothermal is best used for the
coverage of the baseload with renewable energy from the ground.
SYNTHESIS ON OPERATIONAL CRITERIA
A common indicator for the performance of heat pump systems is the Seasonal Performance Factor (SPF), i.e.
the ratio between the heating/cooling demand covered in a heating/cooling season and the electricity
consumed by the heat pump. Adopting a convention suggested by the project GROUNDMED
(http://groundmed.eu/), we referred to SPF1 as the SPF for the heat pump alone, and SPF2 as the SPF
calculated considering the electric consumption of the heat pump and of the circulation pump of the BHEs or
the well pump in open-loop systems. The SPF2 is a more suitable indicator for a comparison with conventional
systems.
GRETA is co-financed by the European Regional Development Fund through the Interreg Alpine Space programme. See more about GRETA at www.alpine-space.eu/projects/greta.
14/18
In Deliverable 3.2.1 of the GRETA project, the efficiency of open and closed loop systems has been analysed
for the two case-studies of Genoa and Davos (climatic area A and F respectively).
The efficiency of both technologies is deeply influenced by the ground or aquifer temperature. Furthermore,
in open loop systems, the risk of icing of reinjected water in very cold regions is also strictly related to this
parameter.
The figure to the right highlights how the SPF of open
loop systems is influenced by thermal recycling and the
depth to water table. Thermal recycling occurs when
the reinjection well is too close to the abstraction well
(i.e. when parameter xmax > 3, see page 16, point 4). This
leads to a reduction of the extracted water’s
temperature, and consequently to a reduction of the
SPF and may even lead to plant failure. Thermal
recycling can be prevented by choosing a proper
spacing between production and reinjection well
depending on yield, transmissivity and the hydraulic
gradient. The risk of thermal recycling can be assessed
using an empirical formula (see del 3.2.1 or del 4.2.1).
On the other hand, the depth to water table influences
the pumping costs. As shown in the figure to the right,
the SPF2 value can fall about 0.8 point as the water table depth passes from 10 m (i.e., a shallow aquifer) to
50 m (a deep aquifer, as it may occur on a highplain), because the energy consumption of the well pump
steadily increases.
Concluding, open loop systems are very efficient, but they need sufficient aquifer productivity. Otherwise, or
in case of high depth to water table, BHEs are suggested. Increasing the number of boreholes increases the
performance, at the expense of a higher investment (see next figure). The use of sizing tools allows case-
specific cost optimizations to find the best trade-off between investment and operation costs.
This figure shows the Seasonal performance factor 2 of a borehole heat exchanger field vs the three main
influencing parameters thermal conductivity, ground initial temperature and number of boreholes in two
different climatic zones: Genoa (A) and Davos (F).
SPF of an open-loop system against the normalized thermal
recycling parameter 𝑿𝒎𝒂𝒙 and the depth to water table in the
case of a well isolated hotel in Davos.
GRETA is co-financed by the European Regional Development Fund through the Interreg Alpine Space programme. See more about GRETA at www.alpine-space.eu/projects/greta.
15/18
ECONOMIC OPERATIONAL CRITERIA
As a basic principle, it can be observed that factors influencing the economic suitability of shallow geothermal
systems are broadly diversified and are heavily fluctuating due to market dynamics and funding programs.
Studies on economics of these, and in particular of combined systems, e.g. with solar thermal collectors, have
shown the price competitiveness compared to conventional heating systems (Gemelli et al, 2011 12 ). In
comparison, open-loop systems are more cost-effective than closed-loop systems, as the efficiency is higher
and the installation is cheaper.
Shallow geothermal systems are among the techniques for heating and/or cooling with the highest costs of
installation; however, they are among the cheapest for the production of heat. They can therefore be
considered as an investment and should be recovered by savings on the operating costs.
Investment costs (drillings, well/BHE completion, heat pump, installation works). They represent the largest
portion of the costs. Thus, the behaviour of the consumer in the whole process – from the planning stage
through the accurate formulation of the call for bids can
influence the costs of the installation substantially. For
BHEs, the drilling represents a high share of total costs,
which makes correct sizing a substantial factor.
Financing a geothermal installation can be supported by
public funding, investment grants, interest rates, etc. As the
initial investment costs usually represent the largest part of
the cost, plus they are relatively high compared to many
conventional heating systems like oil burning systems,
public support schemes for this initial investment are crucial
in the decision process of consumers to choose this type of
energy supply.
Operating costs (electrical energy costs, maintenance). They are among the lowest compared to other
installations producing heat. The main cost factor is the electrical energy consumption of the system, thus, the
aim is to operate the system efficient and reduce pumping power. The price per kWh ranges from about 0.16
– 0.3 € for medium size households within the alpine space countries. Energy prices are usually about twice as
high for private households than for industrial customers. Costs for maintenance can generally be considered
as low and are reported to be a maximum of 10 % of the total operating costs.
At this point, we can conclude that the cost for a shallow geothermal system initially depends on the
characteristics (open loop vs. closed loop system) and the location (geological/hydrogeological conditions) of
the installation, but is variable due to market dynamics and funding schemes. Definitive cost reductions will
arise from ideal dimensioning of the system. The ideal dimensioning includes among others the use of the
underground as storage (performing heating & cooling) and the combination of the heat pump with other
heating supply systems to cover peak loads. Also upscaling from single household installations towards
housing estate uses can result in significant cost reductions, especially when looking towards lowered energy
prices for industrial customers and a more efficient exploitation of the whole system.
12 Gemelli A., Mancini A., Longhi S., 2011, GIS-based energy-economic model of low temperature geothermal resources: A case study in the Italian Marche Region, Renewable Energy 36, pp.2474-2483, doi:10.1016/j.renene.2011.02.014
Initial investment depending on public support schemes
Operatingcosts
GRETA is co-financed by the European Regional Development Fund through the Interreg Alpine Space programme. See more about GRETA at www.alpine-space.eu/projects/greta.
16/18
Description of threshold values environmental / legal / technical
The design of a shallow geothermal system is based on a number of constraints imposed by the law and/or
good technical practice.
Closed-loop systems are usually sized based on a minimum and/or maximum fluid temperature criterion.
Indeed, the ratio between the thermal load and the length of BHEs installed, along with other factors (number,
length and reciprocal distance of boreholes) determines the thermal alteration of the ground and hence the
temperature of the heat carrier fluid. One should avoid freezing the heat carrier fluid: for this reason, the
minimum fluid temperature should guarantee this risk to be avoided with a good safety margin. However, this
is the very minimum requirement for BHE sizing. Usually, the minimum temperature is imposed in order to
guarantee a good efficiency of the heat pump (SPF). The temperature constraint and the thermal load
determine the required length of Borehole Heat Exchangers. For mapping of closed-loop geothermal potential,
we have chosen a minimum fluid temperature, (- 3 °C), which provides a good safety margin for freezing if a
typical antifreeze solution is used.
The design of open-loop systems should consider different factors:
1) The depth of wells depends on the depth to water table and the saturated thickness of the aquifer. It is
generally recommended to build fully penetrating wells reaching the bottom of the aquifer to ensure
constant performance.
2) The abstraction well(s) should not induce an excessive drawdown. A maximum drawdown of 1/3 of the
saturated thickness is advised and has been used in our potential mapping;
3) The injection well(s) should not cause flooding, so the elevated water level should not exceed the ground
surface elevation. We proposed a safety threshold depth of 0.5 m from ground surface or underground
structures like basements and foundations respectively.
4) The wells should be spaced as far as possible in order to avoid thermal recycling, i.e. the return of injected
water to the abstraction well(s). If we consider a well doublet perfectly aligned with the groundwater flow
direction, thermal recycling does not occur if 𝐿 >2𝑄𝑚𝑎𝑥
𝜋𝑇𝑖, where 𝑄𝑚𝑎𝑥 (m3/s) is the maximum flow rate, 𝑇
(m2/s) is the aquifer transmissivity, 𝑖 is the hydraulic gradient, and 𝐿 (m) is the well distance. The ratio
𝑋𝑚𝑎𝑥 =2𝑄𝑚𝑎𝑥
𝜋𝑇𝑖𝐿 should therefore not exceed 1. However, such a low threshold is often not respected. Based
on our experience, values of 𝑋𝑚𝑎𝑥 < 3 are usually compatible with the correct operation of an open-loop
system; higher values should be checked with numerical simulations;
5) The propagation of thermally altered water (thermal plume) should be evaluated with a numerical
simulation. The extension of the isotherms of Δ𝑇 = ±1°𝐶 should be evaluated, as they indicate the
“thermal footprint” of the plant in heating (Δ𝑇 = −1°𝐶) and cooling (Δ𝑇 = +1°𝐶) mode. Thermal plumes
of neighbouring plants can overlap, but we suggest that the plume areas could be used as an indicator for
a sustainable density of open-loop systems in densely settled areas.
In the presence of groundwater flooding issues (point 3 of the list above), water can be disposed in a surface
water body (river, lake, sea), if allowed by local regulations. This solution, which avoids also thermal recycling
(point 4) and plume (point 5) issues, has been adopted in a few cities like Milan, where the very shallow aquifer
tends to flood underground structures, like the metro’s tunnels and is compulsory in Aosta Valley since
reinjection into subsurface is forbidden by a regional law.
GRETA is co-financed by the European Regional Development Fund through the Interreg Alpine Space programme. See more about GRETA at www.alpine-space.eu/projects/greta.
17/18
Awareness of challenges coming along with the installation of shallow geothermal energy systems
Challenges described herein relate on the one hand to geological and geographical conditions and on the other
hand to anthropogenic issues, like contaminated soils or mining areas. They both represent potential risks
arising during the drilling phase and affect borehole heat exchangers and, depending on the final depth of
water wells. Challenges concerning other water rights, like temperature influences, are part of the operation
(cf. Fehler! Verweisquelle konnte nicht gefunden werden.).
As supporters of geothermal energy, we want to point out possible interference of shallow geothermal
installations with peculiar geological conditions and anthropogenic activities, both for the sake of a (cost-)
effective use of shallow geothermal resources and the protection of the environment and other ground uses.
First, the presence of a possible source of hazard does not mean that complications will be inevitable. Meeting
a possible source of hazards while drilling therefore does not necessarily mean the cancellation of the drilling
works, if appropriate technical measures are applied. The majority of the issues described can be overcome
with special drilling devices and specific precautions, but these over-costs can influence the payback-period of
the plant. In this context, the advice is to assign certified and/or experienced companies for planning, drilling
and installation works.
(HYDRO-)GEOLOGICAL AND ANTHROPOGENIC CHALLENGES
Challenges and potential risks in this category result from lithological conditions, like the presence of anhydrite
or karst, or manmade, anthropogenic structures, like contaminated sites or mining areas. In general, an
assessment of these potential risks and special conditions at the plant site is essential for the planning and can
result in technical and legislative restrictions. These restrictions depend on the type and size of the geothermal
plant. Reasons could be the safety of people, property and environment or the protection of natural resources
such as deep aquifers with strategic importance for future water supply. Possible impacts can be of variable
timeframe, with short-term, long-term and irreversible consequences. The table below summarizes potential
risks and shows possible impacts, which are separated into the following four categories:
1) Impacts on groundwater: these impacts can be of hydraulic or chemical nature. Hydraulic impacts are
changes in the groundwater table where water can rise or lower. Problems can be water logging or
deficient withdrawal. Mixing of different groundwater bodies can change the water chemistry and
therefore the reaction of water to the installed system (e.g. chemical precipitation, scaling or clogging).
2) Ground stability problems: rocks like anhydrite or gypsum react with water used during drilling. This
generates reactions that entail changes in the ground stability. Swelling and solution processes cause uplift
and settlements. In the worst case, this leads to damages of buildings and infrastructure near the
installation. Also incorrectly drilling works in karst or mining areas may lead to settlements or slumps.
3) Environmental impacts: Environmental impacts in this case are contamination of groundwater through
carryover during the drilling (e.g. drilling in contaminated sites) as well as the pollution of properties by
uncontrolled water outlets.
GRETA is co-financed by the European Regional Development Fund through the Interreg Alpine Space programme. See more about GRETA at www.alpine-space.eu/projects/greta.
18/18
4) Impacts on plant engineering: while problems due to
technical malfunctions or wrong adjustment can be easily
detected and solved, malfunctions by incorrectly drilling
and installation works can entail elaborate measures
combined with additional costs. These malfunctions can
appear after completion of the work. For example, the
detection of ongoing swelling and solution is only possible
when the first damages have already occurred. A safe and
competent planning and installation by experts reduces
the risks of this malfunction to a minimum. Furthermore,
the quality of the groundwater has impacts on the plant
engineering. For example, highly mineralized groundwater
leads to precipitations that may clog the heat exchanger.
Some groundwater compositions cause reactions with
commonly used grouting materials and may cause leaks in
the borehole heat exchangers providing a source for
ground stability problems and ecological damages.
Insufficient grouting may occur especially in karst areas and can lead to inadequate heat extraction rates.
In this context, we advise the execution of groundwater chemistry investigations in unknown areas, which
is anyway required in the approval procedure of some regions, like in Bavaria.
The following table shows challenges and risks for the implementation and installation of geothermal energy
plants. The table also sums up possible impacts caused by incorrect installation in these areas.
CHALLENGES/RISKS (Hydro-)geological
/technical /anthropogenic challenges
POTENTIAL IMPACTS
Impact on groundwater Ground stability
problems
Environmental
impacts
Impacts on plant
engineering
Hydraulic
Chemistry
Evaporites (e.g. gypsum) x x x x
Karstic rocks / caves x x x x
Artesian/confined aquifers x x x
Swellable rocks (e.g. anhydrite, clay)
x
Multiple layer aquifers x x x x
Mining areas x x
Landfill/ contaminated sites x x
Gas occurrences x x
Coastal aquifers x x
High mineralized groundwater
x x
In the GRETA project, a web GIS covering potential risk areas for the Alpine Space was created. Risk areas
indicated on this large-scale map are provided to serve as an information base to support further investigation,
but cannot substitute detailed planning. The location of an installation in one of the identified endangered
zones solely indicates a higher probability of risk. As geology and hydrogeology of the underground is not fully
predictable, a local assessment is needed to clarify the risk potential.
For further information see the GRETA Deliverable 4.1.1. Also, note that the country or region where the
system is installed might provide information material or guidelines for geothermal energy systems (some of
the, provided in the same Deliverable).
Impacts due to (hydro-) geological or anthropogenic hazards can occur during the installation of geothermal systems. Worst-case scenarios are extremely rare and always due to incorrect planning and performance. Awareness of possible hazards and the correct choice of the drilling method and the application of drilling equipment is crucial for a professional construction. A correct planning excludes systems in high-risk areas and provides the knowledge and technical standards for all other areas. This provides a sustainable use and long living and cost efficient plant.
FACT BOX CHA LLE NGES
Top Related