Héctor Cano Esteban
Ongun Berk Kazanci
MSc, Project Manager in “R&D and Projects
Department”, Geothermal Energy S.L, Spain
PhD, Research and Innovation Specialist,
Uponor AB, Västerås, Sweden/ Division of Fluid and Climate Technology, KTH
Royal Institute of Technology, Stockholm, Sweden
Ing. Project Engineer, Viessmann Belgium bvba,
PhD. Assistant Professor, International Centre
for Indoor Environment and Energy
Department of Civil Engineering, Technical
University of Denmark: email@example.com
hybrid GEOTABS is an EU H2020 funded research project
focusing on the design, development and optimization of the innovative concept
combining thermally active building systems (TABS), geothermal, heat pump and
model predictive control (MPC). As the first article of a serial of project
disseminations, this paper introduces the working principle of hybrid GEOTABS,
as well as their main components/modules. The optimization approach of key
modules in common practice is also discussed. This article aims at providing a
basic knowledge of the concept as a preparation for the up-coming optimization
development and integration of the involved key modules.
geothermal heat pump; TABS; radiant heating and cooling
Model Predictive Control and Innovative System
Integration of GEOTABS in Hybrid Low Grade Thermal Energy Systems
hybridGEOTABS is a
four-year project started in 2016 by an active team of SMEs, manufacturers
and research institutes. The project, led by the University of Gent, is a
Research and Innovation Action funded under the EU’s Horizon 2020 programme.
The goal of hybridGEOTABS
is to optimise the predesign and operation of a hybrid combination of
geo-thermal heat-pumps (GEO-HP) and thermally activate building systems
(TABS), alongside secondary heating & cooling systems, including
automated Model Predictive Control (MPC) solutions.
To know more about the
project visit and
hybridGEOTABS project has
received funding from the European Union’s Horizon 2020 research and
innovation program under grant agreement No 723649.
Introduction of hybrid MPC GEOTABS and its key modules
buildings combine an energy efficient heating and cooling system (Thermally
Active Building Systems, TABS) with renewable resource (ground, GEO) to heat
and cool buildings in a sustainable way. The performance of GEOTABS buildings
has been studied thoroughly in an earlier EU project, GEOTABS - Towards Optimal
Design and Control of Geothermal Heat Pumps Combined with Thermally Activated
Building Systems in Offices .
recent EU project (HORIZON 2020-10 project EE-04-2016, Model Predictive Control
and Innovative System Integration of GEOTABS in Hybrid Low Grade Thermal Energy
Systems - hybrid GEOTABS ) is taking the analysis that were carried out
within the GEOTABS project further. A previous publication has already
summarized the project scope, tasks, and goals .
scope of this project, hybrid GEOTABS buildings are studied in detail in terms
of optimal system design and dimensioning methodology, control, and in other
terms, including but not limited to, energy performance, indoor environmental
quality (IEQ), costs, environmental impacts, and so forth. Model Predictive
Control (MPC) algorithms are being developed and the developed algorithms will
be implemented in chosen demonstration buildings. The three demonstration
buildings are an office building in Luxembourg, an elderly care home in
Belgium, and an elementary school in Czech Republic. All these buildings are
equipped with hybrid GEOTABS systems. In addition to these three demonstration
buildings, there are also two case study buildings: a residential building and
another office building.
to the GEOTABS, the hybridGEOTABS buildings can have “hybrid” heat
emission/removal systems and also “hybrid” energy sources. This enables having
other room conditioning systems in addition to TABS, and gives the possibility
of benefiting from other heat sources and sinks than the ground. MPC ensures
the optimal operation of the systems in such buildings.
studies have identified the system concept, individual modules, and the
interfaces between system components of hybrid GEOTABS buildings , the MPC
concept with a focus on hybridgeotabs buildings , together with the detailed
measurements of thermal indoor environment in the chosen demonstration
first article of this serial, this study focuses on introducing the TABS, heat
pumps (ground source) and the ground heat exchangers, and their optimization
process for the application in hybridGEOTABS buildings.
Hydronic radiant heating and cooling systems
(water-based) radiant heating and cooling system refers to a system where water
is the heat carrier and more than half of the heat exchange with the
conditioned space is by radiation (heat emission to or removal from the space
is by a combination of radiation and convection). There are three types of
radiant heating and cooling systems :
heating and cooling panels;
isolated from the main building structure (radiant surface systems);
embedded in the main building structure (Thermally Active Building Systems,
radiant heating and cooling systems are low temperature heating and high
temperature cooling systems. Therefore, the heat carrier (water) circulating in
the pipes has low temperatures in heating and high temperatures in cooling
operation. In some TABS constructions (hollow core concrete decks), also air
has been used as a heat carrier, and electricity can also be used in some
radiant heating applications .
and ceilings can be used as surfaces that provide heating or cooling to the
space. Hydronic radiant surface systems can address only sensible heating and
cooling loads. Therefore, they require a ventilation system to address the
latent loads and to provide the ventilation rates required for indoor air
quality concerns 
heating and cooling systems enable lower airflow rates than all-air systems, in
which the entire heating and cooling loads are addressed by the ventilation
TABS and its optimization approach
emerged as an innovative solution to improve building energy performance and
indoor climate. As introduced, TABS combine cooling and heating system in the
structural concrete slabs/walls of a multi-storey building, which can operate
hydraulic temperature close to ambient temperature from 22–29°C for heating and
16–22°C for cooling 
primary used for sensible cooling and secondarily for base heating. The whole
system works with radiant heating and cooling, which is not any
air-conditioning or radiators, and does not commonly substitute any ventilation
system. Furthermore, TABS stores heat via building structures themselves and
can commonly provide upgraded global thermal comfort than conventional
convective heating/cooling methods 
Due to the
reduced draught, noise levels and improved mean radiant temperature through
less fluctuated surface temperature, local thermal comfort is commonly high in
TABS buildings. All the above advantages have promoted TABS as a competitive
heating/cooling emission system in the current EU building markets. TABS-served
low-temperature heating (LTH) and high-temperature cooling (HTC) provide wide
opportunities for the integrations and applications of renewable energy, such
as geothermal energy or ground-source heat pumps (hereafter refer as GEOTABS)
1 shows a
typical GEOTABS system that serves heating and cooling in a building with
GEOTABS system. 
TABS as a
mature product has been available on the market. The optimization approach of
TABS mostly lies in the configuration design of TABS to maximize its outputs
based on various construction conditions of the slab/ceiling. In principle,
ceiling configurations without insulation or air gap are ideal for maximizing
the output of TABS. Five typical optimal methods, used based on ceiling design,
have been suggested in Table 1  
Table 1. TABS based on basic ceiling configurations.
Optimal application of TABS
slab with or without bonded screed
Concrete slabs with only a thin floor
or bonded screed deliver heating and cooling
into the room
slab with sound insulation
insulation reduces output via the floor. This design option is acceptable in
applications where mainly the effect of cool ceilings is utilized.
slab with raised floor
For a raised floor, the same considerations
apply as for a floor with sound insulation.
This type of ceiling construction is popular
because power supply and cables can be installed in the void.
slab with hollow floor
variant that is frequently used in office buildings is the hollow floor
of performance, it behaves similarly to the false floor. However, because
screed (instead of floor panels) is used, inspection openings must be used
for the underfloor installations.
Heat pump and secondary system module and its
pumps are a major component in the hybridGEOTABS concept. The geothermal heat
pump serves the upgrade from low temperature geothermal energy to high(er)
temperature TABS heating energy. Traditionally geothermal heat pumps are tested
following existing standard (EN 14511, EN 14825) for low temperature
(35°C) and medium temperature application (55°C). As TABS uses lower supply
temperatures than 35°C, (typically 22–29°C, as introduced), it is more
interesting to investigate the performance at even lower supply temperatures.
be done by a lab test where three parameters will be changed:
supply temperature from 35°C to 25°C
the temperature difference at evaporator side
the temperature difference at condenser side.
variables will give information concerning the trade-off between heat pump
efficiency (COP-value) and circulation pump energy consumption. Furthermore,
the varying primary temperature difference will influence the geothermal bore
Not only is
the performance of the heat pump refrigerant cycle important to improve, but
also other parameters of geothermal heat pump system do have an important role
in the general system performance (e.g. next generation refrigerants, next
advanced circulation pumps, the control of the heat pump). Those parameters
will be evaluated together with important market players.
hybridGEOTABS project, the geothermal heat pumps deliver the base load in
heating and cooling mode. Additional heating and cooling energy are generated
by a secondary heating/cooling system. The hydraulic interaction between the
base load of the heat pump and the peak load of the bivalent system is very
important and will have a big impact on system performance. For all demo
buildings in the project, an energy concept is available. In all cases low
temperature TABS heating and high temperature top heating were separated from
each other. Often fossil boilers were used as the peak load system. Those
boilers are less sensible for varying temperatures, temperature differences.
Via the hydraulic design, the risk of overruling the heat pump working should
hydraulic interaction between base load and peak load, the control between both
generators is important. Based on different optimization criteria (e.g.,
functional cost, energy savings, CO2-reduction, bore field utilization)
both generators will be controlled in a different way.
heat pumps in the market have basic input possibilities to communicate with
existing Rule Based Controllers (RBC, going from potential free
liberation/blocking contact towards 0-10V temperature control). Future Model
Predictive Controllers (MPC) may need other data points to write to the future
heat pump controller. Additionally, more reading signals can be interesting for
future MPC controllers. The wish list of possible reading and writing
parameters will get a reality check for current available heat pump and back-up
system controllers. Possibilities to develop/extend current controllers will be
investigated throughout the project.
Geothermal and renewable supply module optimization
One of the
key issues that needs to be optimized for ground heat exchangers (GHEX) is the
optimization of borehole field. In order to optimally size the geothermal borehole
field, the use of a GRT (Geothermal Response Test) is the most prevalent
practice. It is an on-site test to determine the thermodynamic parameters of
the subsoil. Its execution allows to know the effective thermal conductivity,
which describes the heat transfer through conductivity in the subsoil, and the
thermal resistance of the probe. It indicates what should be the thermal leap
between the collector circuit and the subsoil for the dissipation of the power
applied to the circuit.
hybridGEOTABS project, improvements in this process have been achieved by
executing more developed, upgraded and detailed GRTs than common practice. In a
traditional GRT, only the flow and return temperatures at the top of the
borehole are measured, while an Enhanced Geothermal Response Test (EGRT) enable
us to obtain a temperature profile at all the levels of the borehole, measuring
accurately how the temperature changes with depth as a function of the flow and
the thermal stress of the borehole. It allows engineers to understand the
optimal areas of the sub-soil are and even, to evaluate the influence of the
different materials or groundwater in the case that these exists.
realization of the EGRT (See Figure 2), the temperature
of the ground has been obtained, depending on the depth during a heating controlled process. The red curve validates the
typical theoretical depth-temperature unaltered profile of the earth. The rest
colours represent temperature profile along the borehole length, which have
been represented as a function of time to verify the temperature changes while
a constant heat is injected. With all the information, conductivity along the
borehole can be obtained and the optimal depth of the boreholes can be
calculated taking into account the thermal loads of the building which we want
to provide heating/cooling and DHW. Thermal conductivity differences are
explained by the different hydrogeological conditions.
Temperature profile during EGRT execution.
three different simulations were carried out with EED (Earth Energy Designer)
software to show the importance of knowing the conductivity along the borehole.
In all of them VDI 4640 Guideline has been considered  – :
1. Simulations performed by engineering, when
the conductivity is estimated based on geological and hydrogeological
bibliographic studies as well as on the experience.
2. Simulations performed after performing GRT,
when the average conductivity is known.
3. Simulations performed after performing an
EGRT, when the conductivity is known throughout the depth.
required heat exchanger is longer when conductivity has been obtained from
engineering than when has been determined by the GRT, due to the oversizing
that used to be done for the estimation of conductivity. Similarly, the
required exchanger length is higher when conductivity has been obtained from
GRT than when has been determined by the EGRT because it is possible to
optimize the length of the boreholes. After six different EGRTs executed we can
affirm that pre-engineering sizing has a reduction of 4–10% of investment
performing a GRT and a reduction of 14–16% performing an EGRT. That means, an
EGRT has meant a 7% investment reduction compared to the GRT.
EGRT, areas of high conductivity have been found along the depth, and together
with the study through EED software has allowed the optimization of the global
geothermal system. Performing an EGRT is possible to analyse the complete
information about the subsoil and decide the best solution considering all the
project conditions, always optimizing the number of meters to drill. EGRT
obtains savings in investment costs without penalizing the optimum functioning
of the HVAC installation.
renewable source, geothermal is an efficient and abundant energy globally, in
this content, it is more important to use this resource efficiently in
corrected designed systems. The combination of TABS, heat pumps and GHEX shows
to be an efficient solution that has potentials to maximize the advantages of
each component. Different approaches of how each module are optimized as common
practices have been introduced in this article. However, the challenges lie in
the further integration and interactions of the above modules/components by
means of, e.g., MPC control system. These aspects will be continuing introduced
in the following up articles in the serial.
project has received funding from the European Union’s Horizon 2020 research
and innovation program under grant agreement No. 723649.
6 March 2018. [Online].
 , 6 March 2018. [Online].
 Jorissen, Filip, et al. "hybridGEOTABS
project-MPC for controlling the power of the ground by integration." The
REHVA European HVAC Journal 55.3 (2018): 58-64.
 Khovalyg, D., Kazanci, O. B., Parnis, G.,
Cigler, J., & Olesen, B. W. (2019). Hybrid GEOTABS: System Concept, Individual
Modules, and Interfaces. Atlanta, GA: American Society of Heating,
Refrigerating and Air-Conditioning Engineers. 2019 ASHRAE Winter Conference.
 Cupeiro Figueroa, Iago, Jiři Cigler,
and Lieve Helsen. "Model Predictive Control Formulation: A Review with
Focus on Hybrid GEOTABS Buildings." Proceedings of REHVA Annual Meeting
Conference Low Carbon Technologies in HVAC. 2018.
 Kazanci, O. B., Khovalyg, D., & Olesen,
B. W. (2018). Development of a field measurement methodology for studying the
thermal indoor environment in hybrid GEOTABS buildings. Proceedings of the
REHVA Annual Meeting Conference - Low Carbon Technologies in HVAC. Brussels:
REHVA & ATIC.
 REHVA 007: J. Babiak, B. W. Olesen and D.
Petráš, Low temperature heating and high temperature cooling, Brussels: REHVA -
Federation of European Heating, Ventilation and Air Conditioning Associations,
 O. B. Kazanci, "Low temperature heating
and high temperature cooling in buildings, PhD Thesis," Technical
University of Denmark, Kgs. Lyngby, 2016.
 B. W. Olesen, "Using Building Mass To Heat and Cool," ASHRAE Journal, vol. 54, no. 2, pp.
 Uponor. Uponor Contect compendium - Thermally
active building systems. 2014.
 Wang, Qian, and Suleyman Dag. "Business
Model Analysis of Geo-TABS Buildings with Predictive Control Systems."
Cold Climate HVAC Conference. Springer, Cham, 2018.
 Manual de geotermia. Instituto Geológico y
Minero de España (Ministerio de Ciencia e Innovación) e Instituto para la
Diversificación y Ahorro de la Energía (IDAE).
 Cambio climático 2014. Impactos, adaptación y
vulnerabilidad (2014). Contribución del grupo de trabajo II al Quinto Informe
de Evaluación del Grupo Intergubernamental de Expertos sobre el Cambio
Climático. Recuperado de: IPCC
 Evaluación Preliminar de los Impactos en
España por Efecto del Cambio Climático (2005).
 International Ground Source Heat Pump
Association (IGSHPA). Manual “Design and Installation Guide”.
 Convenio de colaboración entre el Ministerio
de Medio Ambiente y la Universidad de Castilla-La Mancha en materia de
investigación sobre una “Evaluación preliminar general sobre los impactos en
España por efecto del cambio climático”. Recuperado de: Proyecto ECCE.