ASHRAE Journal - February 2015 - 35

TECHNICAL FEATURE

17,1

Cooling Power (Btu/h)

is needed. Several programs exist such
Energy Plus, TRNSYS and IDA-ICE.
One of the main advantages of TABS are
4
reduced building height. For each story, you
2
may save 500 to 600 mm (1.8 to 2 ft) of build1
3
ing height, which for a seven-story building amounts to an entire story and related
building materials. As no suspended ceiling
is needed to cover air ducts, significant saving of building materials is possible. It is also
possible to operate the system at 30% to 50%
lowered peak loads allowing reduced plants'
1 = Heat Gain
2 = Power Needed for Conditioning the Ventilation Air
sizes and possible operation of heating/cool3 = Power Needed on Water Side
4 = Peak Heat Gain Reduction
ing systems with temperatures close to room
FIGURE 8 Example of peak-shaving (reducing the peak load) effect (time vs. cooling power).17
temperature, allowing increased plants' efficiency and use of renewable energy sources
over 24 hours and by an air system normally over 8 to
(ground heat exchanger, evaporative cool10 hours. After the water circulation in the slab may
ing, etc.).
have been stopped during the day and will be started
Thermally active building systems exploit the high
thermal inertia of the slab to perform peak shaving. The again in the evening, there will be a high peak cooling load between the heated slab and the cool water;
peak shaving reduces the peak in the required cooling
7 so that it is possible to cool the structures of the
but this should not be used to size the chiller as it is a
power,
very short peak and the capacity needed will after some
building during a period in which the occupants are
minutes decrease significantly. It can be somewhat comabsent (during nighttime in office premises). This way,
the cooling can be delayed and lower nighttime electric- plicated to calculate the needed capacity on the water
side (chiller, heat pump); therefore, a dynamic building
ity rates can be used. At the same time, a reduction in
simulation is recommended.
the size of heating/cooling system components (including the chiller) is possible.
System Design
During daytime, the heat is extracted from the occuRadiant system design requires determining heatpied space by the ventilation system and stored in the
ing/cooling surface area, type, pipe size, pipe spacing,
concrete slabs. Then, during nighttime, the level of vensupply temperature of the heat transfer medium, and
tilation is reduced and the circulation of cool water in
design medium flow rate. The design steps are as follows
the slabs will remove the stored heat.
8
(ISO 11855-3 ):
For the conventional air system, the space load will
1. Calculate the design heating and sensible cooling
be the instantaneous system load, because all the heat
load in accordance with a standard for heating and cooldelivered to the space is immediately removed by the
ing load calculation based on operative temperature.
air system. For the radiant system and especially for
2. Determine the minimum supply air quantity
a TABS the calculated design space load should not be
needed for ventilation and dehumidification. In cooling
used as system load. For both an air system and a TABS,
application, calculate latent cooling and sensible cooling
it is important that the room load over a 24-hour day
available from supply air. Determine remaining sen(Curve 1 in Figure 8) is removed within the 24 hours, else
sible cooling load to be satisfied by radiant system. Also,
the room will get warmer and warmer day by day if the
designate or calculate the relative humidity and dew
weather stays the same. The difference is that with a
point, because the cooling system should operate within
TABS this load is removed from the space in three ways:
absorption in the concrete slab, removed by the ventila- a surface temperature range above the dew point, which
shall be specified depending on the respective climate
tion system and removed by the water circulating in the
conditions in the country. By limiting supply water
slabs. Therefore, the load is removed by a slab system
FEBRUARY 2015

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