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Thermally Activated Building System (TABS): Efficient cooling and heating
of commercial buildings
Jan Babiak
1
, Georgios Vagiannis
1,*
1
Uponor GmbH, Hamburg, Germany
*
Corresponding email: georgios.vagiannis@uponor.com
SUMMARY
The Thermally Activated Building System (TABS) is a combined heating and cooling system
with pipes embedded in the structural concrete slabs or walls of multi storey buildings. TABS
operates at temperature close to ambient enabling more efficient utilization of renewable and
free cooling sources. Moreover, it provides optimized thermal indoor environment. Although
the TABS is a mature and well proven technology in Central Europe, due to energy savings
and low investment and operating costs compared to other technologies, there is still limited
experience and evidence in France. A study was conducted comparing TABS to traditional
convective air conditioning and fan coil systems in office buildings in two locations in France,
Paris and Lyon. The current study presents the results for Lyon. The life cycle costs (LCC)
analysis provides an assessment of different methods for heating, cooling, and ventilation.
Cost figures and building energy simulations show that the TABS is performing more cost
effective, decreasing the global cost (whole life cost) for the building (investment including
depreciation, running and energy of a mid-price escalation) significantly by 16 to 27 %
compared to other air based HVAC mechanical schemes. The results also showed that
choosing TABS for the HVAC scheme will improve the quality of indoor thermal
environment (PMV index in class B range by 22% to 24% more occupation hours in two
selected rooms). In conclusion, TABS has been proven adaptable and cost effective for
French conditions.
INTRODUCTION
Water based cooling with thermal mass activation represents a substantial energy savings
potential in commercial buildings compared to traditional air-based cooling systems (air
conditioning, fan coil). The Thermally Activated Building System (TABS) is a combined
heating and cooling system with pipes embedded in the structural concrete slabs or walls of
multi storey buildings [1] [2]. TABS operates at temperature close to ambient enabling more
efficient utilization of renewable and free cooling sources. Although the TABS is a mature
and well proven technology in Central Europe, due to energy savings and low investment and
operating costs compared to other technologies, there is still limited experience and evidence
in France. The present study compares a life cycle costing analysis of an office building with
the TABS to traditional convective air conditioning; fan coils and variable air volume (VAV)
system.
METHODS
An evaluation project has been performed by appointing Building Energy Simulation (BES)
consultant to provide data using IDA-ICE 4 software [3]. Thermal modelling of indoor
climate and energy was conducted and heating/cooling load profiles were provided, for
selected systems. The HVAC schemes were selected based on what is commonly specified in
France for similar projects. Furthermore, HVAC and Quantity Surveying consultant was
appointed to provide the necessary expertise on costs for the study. Based on the results of the
thermal modelling that was carried out for each case, the consultant created schemes for each
method of mechanical services. Costs were obtained from a variety of sources, including
manufacturers, construction economists [4] [5] and the consultant’s own expertise.
The building base model that was used for the study is described below:
Table 1: Building size and location
Building size 1.000 m²
Length / Width / Height 29 / 11 / 12 m
Storey height 2.6 m
Nr. of storeys 4
Room partitioning single and open plan offices
Location Lyon, France
Table 2: Building shell and core characteristics
External wall 0.36 W/m²K
Roof 0.21 W/m²K
External floor against ground 0.34 W/m²K
Internal wall construction 0.65 W/m²K
Internal slab (floor) construction 200 mm concrete with pipes in slab centre
U-value: 3.35 W/m²K
External window glazing U-value 1.7 W/m²K,
SHGC value 0.31, including frame
Glazing in atrium U-value 1.1 W/m²K,
SHGC value 0.27, including frame
Table 3: Set points and internal loads
Room temperature in summer during occupant
time
26 °C
Room temperature in winter during occupant
time
20 °C
Required hygienic air volumes during
occupancy in office spaces
7 l/s/person
Equipment loads 15 W/m²
Occupant loads 90 W/person sensible
(occupation 10 m²/person)
Lighting loads 10 W/m²
The HVAC system cases were selected based on what is commonly specified in France:
Figure 1: HVAC schemes selected for the study.
The hygienic mechanical ventilation system was introduced to create an equivalent indoor air
quality (IAQ) level for compared cases. Toilettes in all cases are equipped with an exhaust fan
only (no air supply).
Local and central plant (HVAC system items) were sized by the HVAC consultant based on
cooling/heating loads and ventilation rates from BES modelling, in the same method in the
course of completing a mechanical scheme design [6].
The recast Energy Performance of Buildings Directive ("EPBD" Directive 2010/31/EU)
required the European Commission to establish a comparative framework methodology for
calculating cost-optimal levels of minimum energy performance requirements for buildings
and building elements. The regulation was published in January 2012 and the LCC calculation
is conducted according to this method [7] in terms of global costs (whole life costs) for a
calculation period of 15 years.
Global costs (whole life costs) for building and building elements are calculated by summing
up the different types of costs (initial investment, energy, running, disposal), with the related
discount rate to be referred to the starting year and the added residual value, as one can see
below:
( ) ( ) ( )
( )
( )
−×+=
∑∑
=
jViRjCCC
f
i
dia
j
Ig
τ
τ
τ
,
1
,
(1)
Where:
C
g
(τ) global cost (whole life cost), referred to starting year τ
0
C
I
initial investment costs for measure or set of measures j
C
a,i
(j) annual running cost during year i for measure or set of measures j
R
d
(i) discount rate for year I
V
f, τ
(j) residual value of one or set of measure j at the end of the calculation period
(referred to the starting year τ
0
), to be determined by a straight line depreciation of the
initial investment until the end of the calculation period and referred to the beginning
of the calculation period
The residual value is determined as remaining lifetime of a building or building system or
component divided by the estimated economic lifetime and multiplied with the last
replacement cost.
As mentioned above, the LCC study included the following categories of costs [6]:
Initial investment costs
– Material and labour cost
– Project management and design cost as a percentage of material cost
Running costs (maintenance, capital for renovation/replacement in end of equipment
lifetime), as a percentage of the initial costs
Energy costs
– Utilities and fuel prices
– Annual energy use
Local and central plant (system items) was grouped based on the expected equipment lifetime
of each item (EN 15459 [8] and VDI 2067 Part 1 [9]). All items in the same category, have
the same predicted life expectancy, as shown in Table 4.
Table 4: Expected lifetime of equipment, according to EN 15459 and VDI 2067 Part 1.
Note: very conservative estimation for TABS and boreholes; expected lifetime the same as building lifetime
RESULTS
The calculated initial investment costs (material and installation) of the investigated scenarios
(HVAC schemes) can be seen in Figure 2 below.
Figure 2: Initial investment costs (material + installation) of HVAC schemes as a sum of equipment
lifetime categories.
The acquired annual delivered energy and the cost of used energy for all schemes is shown in
Figure 3 Electricity price was taken as 0.074 €/kWh [10] and natural gas price 0.056 €/kWh
[11]. Cost of installing a new connection for natural gas is not included in the case with the
GSHP.
Figure 3: Annual primary energy (left) and cost of used energy (right) for Lyon.
Maintenance was assumed to be 5.5% of the mechanical investment cost (according to the
knowledge and experience of the HVAC consultant), excluding boreholes which were
assumed as maintenance free throughout their lifetime.
The life cycle costs (LCC) are evaluated in term of a global cost according to the above
described methodology (1) for a period of 15 years and the results are shown on Figure 4. It
should be noted here that medium price escalation of 3 % for gas and electricity was used.
Figure 4: Global cost (whole life cost) per year, calculation a period of 15 years, and medium 3 % price
escalation for gas and electricity. Lyon, France.
Apart from the life cycle costs, thermal comfort was considered a crucial topic that should be
investigated and compared between the different HVAC schemed. Figure 5 shows results for
acquired PMV (Predicted Mean Vote) [12] and [13] index during occupation time for a single
office oriented on the south facade and corner office at the north facade.
Figure 5: Duration curves of the PMV index during annual office occupancy hours. South facade (left),
north facade corner room (right).
DISCUSSION
Calculating life cycle costs for an office building provides a broader perspective on the
summary of costs related to investment, energy and running costs calculated for a substantial
period of time. Apart from the costs themselves, the office working environment should be
also taken into consideration, since creating a comfortable working results into higher
employee’s productivity and more relaxed customers, contributing to the success of a
business.
The indoor environment depends not only on thermal environment, but also on other factors
like air quality, acoustics, lighting etc. Though ventilation is always required to ensure an
adequate indoor air quality, it can be optimized when coupled together with a radiant heating
and cooling system. By saying optimized, it is meant to be sized smaller (Figure 2) to
exclusively provide a healthy level of indoor air quality and thus come with benefits on
investment cost, maintenance, used energy and thermal comfort. The lower investment and
maintenance costs result from smaller plant, fans and duct sizes (Figure 2). Furthermore, with
a more cost effective air conditioning system, the ventilation can be supplied to the designated
areas in higher temperature (for cooling caser, the opposite for heating), closer to room
temperature, resulting in that way in a better indoor environment. The latter means that with
reduced air flow volumes also cold draughts and circulation of dust and allergens can be
avoided, which are typical with traditional air conditioning systems, related often to Sick
Building Syndrome. One should not forget that radiant heating/cooling is a silent technology,
without the use of fans.
When it comes to indoor thermal environment and thermal comfort the current study showed
that the TABS (case 1 and 2) can provide an indoor thermal environment closest to a
thermally neutral compared to the other cases under investigation. As demonstrated in Figure
5, for a south room under the roof (left case) equipped with TABS the PMV index would stay
within the class B during 86% of occupation time, whereas with fan coil and VAV system
between 64-66%. For the case of a north corner office, the room equipped with the TABS
would remain in B-class during 75% of occupation time, while at the same time the other
HVAC schemes within 51-59% of time. The latter means that for the two rooms shown in
Figure 4, for the rooms equipped with TABS the thermal environment according to the PMV
index would be improved by 22% to 24% more hours within the class B compared to the
other HVAC schemes.
The long lifetime of the TABS and borehole heat exchanger provides many benefits, shown
by the residual value of the components. This long lifetime of 50 years for the TABS and
borehole heat exchanger components (the same lifetime as the building itself), related to the
residual value, results into lower global costs for these HVAC schemes. The latter can be seen
and compared clearly in Figure2 and Figure 4, demonstrating the investment and global
(whole life cycle) costs of the different HVAC schemes. For the case of TABS coupled with
borehole heat exchanger (TABS + GSHP), although the initial investment is higher, in the
long term this scheme constitutes a cost effective solution (Figure 4). One should not forget
that energy costs in this case are significantly decreased (Figure 3), due to free cooling when
conditions allow for it.
It should be mentioned that the total material and installation costs of a TABS radiant emitter
(TABS) represent only 5% of the total investment for the HVAC scheme of the building.
Nevertheless, the current study showed that only a small part of the total system can have
such a significant effect on the total function and characteristics of the scheme; decreased
energy use, lower investment and maintenance costs, higher levels of thermal comfort etc.
CONCLUSIONS
The current study provided a comparative investigation of the dominant and most commonly
HVAC schemes used in France with HVAC schemes utilizing the TABS. Although the TABS
represents only a small part of the investment costs related to the total HVAC scheme, this
LCC analysis demonstrated the significant advantages that it brings when it comes to
investment and maintenance costs, energy use and thermal comfort.
TABS constitutes an efficient heating/cooling solution not only for investors, but also for
property owners, tenants and final users/occupants. The lowest investment costs (12% to
26%) compared to the other HVAC schemes (Figure 2), makes it more lucrative for investors.
At the same time for property owners the lower global costs (Figure 4) would be the decisive
factor upon decision for the most appropriate HVAC scheme (16% to 27%). As shown by
the study, the lower energy costs (Figure 3) of the HVCA schemes equipped with TABS
constitute a strong argument for tenants. Finally, the higher levels of thermal comfort (Figure
5) that the TABS can provide to the occupants, would be a big benefit for the final users of the
building.
ACKNOWLEGMENT
This study was supported from an internal research project of the Uponor Corporation and
was conducted in cooperation with Equa Simulation Finland Oy and Mott MacDonald
Limited, UK.
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