Using geothermal
energy in office buildings
Fig. 1
䊳 Accompanying measurements and
monitoring during operation can help
systems to achieve their planned efficiency
䊳 A balance between the heat input
and output will ensure that seasonal
geothermal heat storage systems operate
successfully in the long term
䊳 Control strategies must be planned in
detail and adapted during operation
Reinforcement cage for a foundation pile: The integration of heat exchanger
pipes means that geothermal energy can be used in the building.
S
hallow geothermal energy is being used increasingly often in
new office buildings. The advantage of this renewable energy
source is that the ground can be used alternately for both
heating and cooling purposes, depending on the season. The use of
geothermal energy has been further promoted by the German
Renewable Energies Heat Act (EEWärmeG), which has been in force
since the start of 2009 and provides for the partial use of renewable
energy sources for heating and cooling new buildings and for water
heating in new buildings. Shallow geothermal energy can be used
very efficiently in combination with heating and cooling systems that
operate at a temperature level close to that of the ground. In heating
operation, the heating energy supplied by ground-source heat pumps
is between three and five times the drive energy. Free cooling in
summer is even more efficient, as electrical energy is only consumed
by the circulation pumps. Annual system coefficients of performance
ranging between 10 and 35 can thus be achieved. Many of those
involved in this comparatively new area are still relatively inexperienced with the technology available. These systems are very sensitive
to errors and faults because of the low temperature differences
between the ground and the heating/cooling equipment in the building.
Faulty operation can reduce the current efficiency of the system, and
can also affect its performance in the years to come. A research
project conducted by the Institute of Building Services and Energy
Design (IGS) at the Technical University of Braunschweig and supported
by the German Federal Ministry of Economics and Technology has
investigated the energy efficiency and economic efficiency of eleven
buildings that use borehole heat exchangers, energy piles or ground
absorbers. The project also involved the optimisation of operation
of five of the systems evaluated. One of the goals of the project was
to develop operating rules based on the experience gathered. A similar
project supported by the German Federal Ministry of Economics and
Technology and concentrating on well systems and borehole heat exchanger systems is also being conducted by the Technical Equipment
of Buildings Section at the Institute of Design and Building Construction at the Leibniz Universität Hannover. This project’s monitoring
programme covers nine buildings where systems operation will be
examined and optimised. This will provide a basis for improving
planning tools for the design of system components located in the
ground and for specifying measurement and control details for shallow geothermal systems.
䊳
Heat and cold from the ground
Fig. 3:
Temperature of
ground storage
Solar radiation
approx. 1,000 W/m²
Precipitation
Fig. 2: Principle of seasonal storage / active regeneration
Winter
20 °C
Heat regime in upper
layers of ground*
Summer
Winter
Heat radiation
Heat
extraction
Heat
influx
HP
Heat
extraction
– 10 m
Neutral zone
(approx. 10 °C)
– 20 m
Thermal
conduction
through rock
4 °C
Heat pump
operation
Storage
Free cooling
The ground is characterised by its capacity
for storing heat and its relatively constant
temperature level over the course of the entire year. There are various possible means
of harnessing shallow geothermal energy
for heating or cooling purposes, depending
on the ground conditions and the type of
building foundations. This can be achieved
using heat exchangers alone, which may also
be integrated into building elements that
are in contact with the ground, through
which a heat transfer fluid circulates or else
directly using ground water.
Geothermal energy can be brought up to
heating temperature using heat pumps in the
winter, and excess heat can be removed from
the building to the ground using free circulation in the summer months. These systems
are subject to various conditions imposed
by the authorities, depending on the German
federal state and the authority involved.
䊳
Heat pump
operation
Time
The most common geothermal energy systems are as follows:
■
Borehole heat exchangers:
Individually or as an array of exchangers,
right beside or below the building at a
depth of between 50 and 150 m.
■ Energy piles: Heat exchanger pipes
integrated into the building’s foundation
piles, which are necessary for structural
purposes. Depth of between 10 and
30 m. Length and quantity determined
by static considerations.
■ Ground absorbers: Horizontal pipe
loops in or underneath the foundation
slab, similar to underfloor heating.
Heat-transfer surface already defined.
■
Direct use of ground water:
Supply well pumps ground water
through a heat exchanger, water that has
been “thermally exploited” is returned
through an injection well to the ground
water layer.
Geothermal heat flow Temperature increase
0.05 – 0.12 W/m² approx. + 3 K per 100 m
*as per VDI 4640, Sheet 1
– 100 m
The long-term functioning of these systems
is dependent on the ground regenerating
itself – i.e. approaching the undisturbed
ground temperature again – sufficiently
quickly. The flow of ground water contributes to this naturally. Active regeneration as provided by heat influx in summer
(building cooling) alternating with heat extraction during the winter (building heating) is particularly necessary for storage systems with no flow of ground water. If this
regeneration is not sufficient, the temperature level of the ground will change to such
an extent that the system will suffer from a
significant drop in efficiency and may also
become problematic from an ecological
viewpoint.
Monitoring results
Two research projects – Storage of heat and
cold in the ground (German abbreviation:
WKSP) and Thermal monitoring of nonresidential buildings (TherMo) – are investigating the use of shallow geothermal energy
under service conditions in non-residential
buildings. The systems being examined
differ in terms of their geo(hydro)logy,
technical complexity, the fraction of demand
covered, and their redundancy.
WKSP research project
The energy yield of the geothermal heat
storage systems varies from facility to facility. The amounts of heat extracted during
heating operation correspond roughly to
the planned values with a few exceptions,
whereas the heat influx in summer deviates
strongly in certain cases.
Initially, hardly any of the systems examined was being operated efficiently. This is
demonstrated by annual system coefficients
of performance of less than 3 on average for
heating and cooling operation; values of up
to 10 should actually be attainable here.
The systems themselves and the operation
of these systems were optimised as part of
2
Storage
±0m
Flow of ground water
BINE projektinfo 07/10
Fig. 4: Comparison of the projects
Research project
WKSP
Project participant
Technical University of Braunschweig
TherMo
Leibniz Universität Hannover
Project duration
07/2004 – 02/2010
09/2007 – 09/2011
Buildings investigated
11, with 5 of these subject
to more detailed measurement
9
Planning
not planned as part of a research
project, good energy efficiency was
a target
not planned as part of a research
project, very good energy efficiency
was generally not a target
Commissioning
2002 – 2007
2001 – 2009
Gross floor area (GFA)
4,500 – 81,000 m2
7,400 – 54,000 m2
Geothermal energy use
4 x borehole heat exchangers,
5 x energy piles,
2 x ground absorbers
3 x borehole heat exchangers,
6 x well systems
Heating
heat pump,
heat pump,
supplemented by district heating,
supplemented by district heating,
gas, CHP plant, exhaust air heat pump gas condensing boiler
Heat transfer
concrete core temperature control,
concrete core temperature control,
ceiling panels, supplemented by static underfloor heating, static radiators,
radiators, air-conditioning systems
supplemented by ventilation
Cooling
free cooling,
supplemented by chillers, cooling
tower or desiccant cooling system
free cooling, reversible heat pump
(direct cooling), supplemented by
chillers (hybrid coolers, dry coolers)
Cold transfer
concrete core temperature control,
ceiling panels,
air-conditioning systems
concrete core temperature control,
cooling ceilings,
air-conditioning
systems, recirculating coolers (servers)
Ventilation
natural, mechanical
natural, mainly mechanical
Average heating and cooling
powers (geothermal)
50 – 350 kW
300 – 1,500 kW
Fig. 5:
Annual system coefficients of performance for geothermal heat storage systems
2005 – 2009 (WKSP)
Annual system coefficient of
performance for heating and cooling
Facility
Borehole heat exchanger
9
Annual system coefficient of performance =
8
Ground absorbers
Energy supplied (heating and cooling)
Electrical energy consumption incl. circ. pump
7
6
5
4
3
2
1
0
2005
2006
monitoring with the result that all systems
have been achieving annual system coefficients of performance of between 3 and 7
since 2007 (Fig. 5).
For many of the systems investigated, free
cooling operation has not yet been used or has
only been used in a limited manner. The reasons for this were incorrect operating policies,
overheated ground, and system components
that were not adapted for the low temperature differences between the heat sink and
the building cooling system. There is still
scope for further optimisation here.
TherMo research project
The overall primary energy consumptions
of the buildings investigated vary quite sig-
䊳
Energy piles
10
2007
2008
2009
nificantly despite similar building functions
and similar use of geothermal energy with
high fractions of demand covered. The
annual system coefficients of performance
determined so far are around 7 for a facility
with borehole heat exchangers in heat and
cooling operation (alternating operation).
Based on initial predictions, the energy efficiency of individual systems can be increased by up to 30%. Final comparative
evaluations are not yet possible as the measurement phase of this research project is
still in progress. However, monitoring has
already helped to identify problems and
optimise operation in a number of cases so far.
The configuration of several systems was
not favourable, which even led to total fail-
ure of the heat pump in one instance. Elsewhere the transfer systems, e.g. for concrete
core activation, did not work properly as
they had been installed incorrectly. There
were also difficulties with the operation of
wells and borehole heat exchangers, such as
leaks in the exchanger array or failure of the
production pump caused by earth faults.
The control systems were also not set up
optimally in most cases.
The relevant authorities rarely checked the
monitoring strategy that they had prescribed for ground water use during the
construction phase and final acceptance for
the buildings investigated. The measurements
required during operation were often incomplete, or else temperature limit values
were not being adhered to. For this reason,
the authorities even demanded that one of
the systems be shut down for a period.
Even small inaccuracies in measurements
can have a strong impact on the measured
energy balance as these systems operate on
the basis of very small temperature differences. The standard temperature transducers
generally used by the control systems in
measuring temperatures have an error tolerance of +/–1 kelvin and can result in energy balances that are incorrect by up to 36%.
As part of the project, the accuracy of temperature measurements was increased for
three of the buildings and the possible
measurement error for the temperature difference was thus reduced to 11%.
Experience gathered in service
Planning
Heating and cooling loads can be minimised
by suitable architectural design, well-insulated facades, sun protection measures and
ventilation strategies. Only then are systems
that operate with a small temperature difference with respect to the ground able to
provide efficient heating and cooling. In order for the heat or cold from the ground to
actually reach the building, system components for geothermal energy use such as
heat exchangers, pumps and hydraulic
switches must be carefully designed. The
heat pump and, if present, the well pump
should be capable of operating with partial
loads in order to be able to adapt flexibly to
demand – for example, by employing level
switching or a frequency converter. A thermal simulation is useful when designing
larger systems.
Measurement equipment
A certain minimum amount of measurement equipment is the basis for successfully controlling and monitoring operation in
the long term. Alongside temperature sensors, this also includes meters to measure
the amount of heat/cold delivered and electricity consumption measurements for heat
pumps, well pumps and borehole heat ex-
changer pumps. In the case of more complex systems, careful prior consideration
should be given to defining the relevant
envelope boundaries for the calculation of
coefficients of performance so that heat
amounts and electricity consumptions can
be recorded correctly.
Given the small temperature differences
present (heat source side 3–5 K), the temperature sensors must be calibrated very
accurately in order to deliver an exact heat
yield balance. If included in planning at an
early stage, the measurement points necessary for the heat yield balance and for the
monitoring required by the authorities can
be integrated into the building control
systems that are generally already present.
As a rule, these systems should then store all
relevant measurement data.
Control system
Once the requirements demanded of the
building and equipment have been fulfilled,
the efficiency and success of the use of
geothermal energy depend on a suitable
control strategy. In general, this control
strategy should include and coordinate all
items of heating and cooling equipment
present. The overall philosophy should be
checked in detail, and operation should be
accompanied by measurements and optimised
until regular operation is achieved.
Other conclusions drawn by the researchers with regard to control:
■ Storage systems should be commissioned
during the heating period in order to
create a larger heat sink for free cooling
in summer.
■ Chillers should be commissioned at the
end of this season if possible, when the
cooling potential for free cooling has
been exhausted – otherwise it is often no
longer possible to cool the ground storage system back down to the temperature
level required for free cooling.
■ Inefficient operation of concrete core
activation in the transitional periods in
spring and autumn should be avoided
by employing a “dead range” (neither
heating nor cooling) depending on the
average outside temperature.
■ Optimal coordination of control
strategies with a combination of slowacting and fast-acting transfer systems –
otherwise heat extraction from the
ground by means of slow-acting building
element activation will be lower than
required and assumed during planning.
BINE projektinfo 07/10
3
Conclusion
PROJECT ORGANISATION
PROJECT ADDRESSES
WKSP accompanying research
• TU Braunschweig
IGS
Franziska Bockelmann, Herdis Kipry
Mühlenpfordtstrasse 23
38106 Braunschweig, Germany
www.igs.bau.tu-bs.de
WKSP cooperation partner
• Meteocontrol GmbH,
Energy & Weather Services
Spicherer Strasse 48
86157 Augsburg, Germany
WKSP cooperation partner
• TU Braunschweig
Institut für Grundbau
und Bodenmechanik
Gaußstrasse 2
38106 Braunschweig, Germany
TherMo accompanying research
• Leibniz Universität Hannover,
Institute of Design and
Building Construction
Gunnar Harhausen,
Matthias Wohlfahrt
Herrenhäuser Strasse 8
30419 Hannover, Germany
www.iek.uni-hannover.de
Kooperationspartner TherMo
• UBeG GbR,
Dr. Burkhard Sanner
Zum Boden 6
35580 Wetzlar, Germany
4
BINE projektinfo 07/10
▼
▼
The experience gathered in these research projects demonstrates that shallow geothermal energy is essentially well-suited for providing temperature control in office and
administration buildings. With a suitable design and the right operating policy, significant energy cost savings and CO2 reductions are possible compared to conventional
heating and cooling systems. This is dependent on the size, efficiency and degree of
utilisation of the system.
However, almost all of the buildings examined that are using this relatively new technology still show up weaknesses in planning, design and, in particular, in terms of control
and operation. Building operators are often not very conscious of geothermal energy
systems in everyday operation when these systems are working properly or else serve
to supplement the performance of other redundant systems. Operation monitoring is
a good idea because it increases efficiency and also helps to avoid downtime. The inlet
and outlet temperatures of the geothermal heat storage system and the amount of heat
extracted and fed-in should be monitored here as a minimum. In this way, deviations
from regular operation can be detected at an early stage and the necessary measures
can be quickly implemented.
It is generally necessary to adapt the building strategy and the equipment and control
strategy to account for geothermal energy use. A high quality standard must be adhered
to during planning and construction in order to ensure the energy efficiency of systems,
thermal comfort in the building and the long-term functioning of systems. A sufficient
number of accurate measurement devices are necessary in order to monitor operation.
After detailed acceptance and commissioning, an initial adjustment phase of around
two years is then necessary. The system should be accompanied by measurements during
this time so that it can be adjusted for the real framework conditions and so that the
interplay between the building, the geothermal energy systems, other temperature control
equipment and the building users can be optimised. If contractors and system operators
have insufficient experience, this phase may turn out to be significantly longer.
Guidelines for the harnessing of geothermal energy in office buildings by using borehole
heat exchangers, energy piles and ground absorbers have been developed based on the
experience gathered as part of the WKSP project. These guidelines will be published
as a BINE technical book.
ADDITIONAL INFORMATION
Internet
• www.enob.info
Literature (in German)
• Erdwärme für Bürogebäude nutzen.
FIZ Karlsruhe. BINE Informationsdienst,
Bonn. Stuttgart: Fraunhofer IRB Verl.,
(erscheint 1. Quartal 2011). 160 S., 1. Aufl.,
ISBN 978-3-8167-8325-1, 29,80 Euro
BINE-Fachbuch
Picture credits
• Background photo p. 1, Fig. 2, Fig. 5 and
background photo p. 4: IGS, Braunschweig
• Fig. 1: Katzenbach, Darmstadt
• Fig. 3: LUH, Hanover
Service
• This Projektinfo brochure is also available
as an online document at www.bine.info
under Publikationen/Projektinfos.
Additional information in German,
such as other project addresses and links,
can be found under “Service”.
■ Project Funding
Federal Ministry of Economics
and Technology (BMWi)
11019 Berlin, Germany
Project Management Organisation Jülich
Research Centre Jülich
Rolf Stricker
52425 Jülich, Germany
■ Project Number
0327364A, B
IMPRINT
■ ISSN
0937 – 8367
■ Publisher
FIZ Karlsruhe
76344 Eggenstein-Leopoldshafen
Germany
■ Copyright
Text and illustrations from this publication
can only be used if permission has been granted
by the BINE editorial team. We would be
delighted to hear from you.
■ Editor
Dorothee Gintars
BINE Information Service
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䊳