TABS radiant systems (Thermally Active Building Systems)

As already mentioned in the "Types of radiant systems" article, the "Tabs" systems fall within the family of embedded systems and more precisely in the "E" type. The acronym derives from the expression "Thermally Active Building Systems", but in Italy they are better known as "Mass activation systems". Not many application examples are known in our country and the only realizations concern buildings for tertiary use.

 Differences with other systems

They differ from all other types of radiant systems for the position of the pipes: these are not embedded in the surface layers of the structures (floors, walls and ceilings), but in the internal layers, in correspondence with the central axis.

The goal is in fact to transform the entire building structure into a large energy supply terminal. The main structures used are the horizontal partitions (floors, foundations and roofs), but nothing prevents the vertical ones from being also involved (walls or partitions with an important surface).

Design criteria

TABS systems are the only ones for which standards do not require an insulation layer under the system. The reason is easy to understand: if the goal is to "activate" the entire structure of the building, it is clear that the energy must be able to propagate in both directions. The structure of the building practically becomes a sort of energy reserve, "thermal" in heating and "refrigerant" in cooling, to be released slowly and gradually over the course of the day without compromising the internal conditions of environmental comfort.

The energy spreads from the mass surrounding the pipes to involve the surfaces, and is then transmitted to the environment. It is evident that it is unthinkable to be able to manage such a system intermittently. This means that, considering the great thermal inertia of the structures involved and necessarily having a continuous operation, the operating temperature of the Tabs systems cannot deviate too much from the environmental comfort parameters (normally 20° in winter and 26° in summer).

In fact, in a radiant system with thermal activation of the mass, it is good to let water circulate at a temperature very close to the one you want to guarantee inside the rooms to be air-conditioned. Generally, a single flow temperature of 22° is used in order to guarantee a minimum contribution in heating and cooling, to maintain the comfort temperature respectively of 20° in winter and 26° in summer. For this reason Tabs radiant systems are also known as "low temperature difference systems". For their construction, pipes with a diameter between 17 and 20 mm are generally used, laid with a distance between centers of 150-200 mm.

Limits of use

Operating with a reduced temperature difference between the heat transfer fluid and the environment to be air-conditioned, these systems, as well as the surfaces in PCM (Phase Change Materials), are not able to cover the entire energy needs of the building. Tabs systems must therefore be placed side by side with the actual air conditioning system, which, however, can be undersized thanks to the attenuation of load peaks. Thanks to the energy accumulated in the structures, the daily thermal excursions inside the air-conditioned rooms are attenuated; therefore the heating / cooling needs are reduced and the main system can be reduced in size.

It should be emphasized that these systems have been developed giving priority above all to cooling. Having to supply the Tabs systems with water at a relatively high temperature (22-24°) compared to the rooms to be air-conditioned (26°), it becomes very simple to use the "free-cooling" operation (freecooling, that is, without the use of a generator but using a natural source), for example using fresh groundwater.

Cooling operation is useful for better understanding the concept of night-time energy storage or night-time activation of the mass. During the night, generally from 20.00 to 8.00, the system is activated with the project temperature (18-22°) in order to "charge" the structural mass of the building with energy. During the day, especially in the hottest hours, refrigerated structures are able to absorb the heat that develops inside the rooms (lamps, electrical equipment, people's caloric intake) or that penetrates from the outside through the structures (solar radiation on the perimeter walls and glass surfaces). Therefore, the room temperature is kept within acceptable limits with the help of the actual air conditioning system, albeit underpowered, and any load peaks are delayed (phase shift) towards the evening and night hours, when it is possible to open the windows, if necessary, and when the system is put back on "charge".

Can only Tabs systems be used for heating and cooling?

We have seen what are the conditions for the design and implementation of Tabs systems. We also stressed that these systems cannot fully meet the energy needs of a building. However, the combination of Tabs systems and a Controlled Mechanical Ventilation system with air humidity management may represent an optimal solution in the following conditions:

• Winter thermal loads not exceeding 10-30 W/m2 and summer thermal loads not exceeding 30-60 W/m2.
• Structures affected by pipes with high inertial mass and absence of insulating layers (slabs or internal partitions without insulation).
• Water temperature with a range of 22-26° in winter and 18-22° in summer.

Benefits

• Reduction of the air conditioning needs of the rooms, especially in cooling, attenuation of fluctuations in the ambient temperature and phase shift of the thermal wave.
• Under-dimensioning of the actual air conditioning system by adopting a smaller generator.
• Operation with reduced temperature difference between the system water and the rooms, using renewable sources for generation (heat pumps).
• Activation of the thermal mass at night, ie in the time slot in which it is possible to take advantage of advantageous electricity tariffs (for the operation of the heat pump).
• Consequent energy saving and reduction of polluting emissions into the atmosphere.

Disadvantages

• Any countertops or other thermally resistant layers are not acceptable.
• The regulation cannot be of the intermittent type and not even for a single environment; greater efficiency is achieved with continuous operation and modular regulation of the zones.
• The design is quite complex and expensive. It requires specific simulations and calculations (UNI EN 15377) as it is necessary to evaluate the "building-plant" behavior in relation to the inertial mass, the accumulation of energy in the structures concerned, the phase shift of the thermal wave, the variations in well-being parameters (air temperature and average radiant temperature) within acceptable comfort limits.

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