Catalogue of technical and operational criteria · thermally activated geostructures can be used as...

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.

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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.

mailto:[email protected]

http://www.alpine-space.eu/projects/greta



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.

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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





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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.





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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





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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.





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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.





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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.





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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





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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





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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.





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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


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


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.





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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




http://www.geophysik-dr-rauen.de/tcscan/


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.




http://groundmed.eu/


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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.





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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





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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.





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.





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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




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