Serial 1.5D models
6.1 Construction types
6 Thermo active components
The thermo active system has so far mainly been used in buildings where the deck has been in-situ cast, which is the typical building tradition in the Central European countries where the system was first introduced. In the Danish building tradition buildings are most often based on fabricated building elements. Therefore, one aim of this work is to investigate pre-fabricated decks where the thermo active components are integrated already in the production.
In section 6.1 the use of pre-fabricated decks designed as thermo active components is described.
In section 6.2 a simple way of integrating thermo active capabilities in an existing pre-fabricated deck will be investigated using a simple test setup. The activation of the deck is achieved through placing a pipe directly in the air cavities in the deck. However, the heat transfer in the cavities is not as effective as pipes integrated in the concrete and at the same time difficult to predict by simulations. Nevertheless if such a setup should prove to have a cooling capacity, which is large enough to cool the building, it would be a very simple way to make a thermo active component.
Thermo active components based on pre-fabricated hollow core decks is tested in section 6.3 in a large test mock-up. The measurements will be used to find the maximum cooling capacity of the decks and to validate the simulation model used by TASim.
Finally, in section 6.4 a simulation study using TASim will be presented.
To make the deck thermo active, the height of the standard version of the deck has been increased by 50mm to make room for the integrated pipe. A few of the practical
considerations concerning hollow floor decks with integrated pipes based on a regular deck will be described.
Assessment of heat transfer
This section the heat transfer from a thermo active deck with integrated pipes will be assessed to establish the basic properties of this type of deck construction. The purpose is to find the thermal properties for different layout of the deck using steady-state calculations.
Generally, Heat2 (Blomberg, 2000; Blomberg, 1996) is used for the simulations.
Unless otherwise noted, the boundary conditions have been chosen to be 26°C and thermal surface resistance of 0.17m²K/W for the floor and 0.1m²K/W for the ceiling. A pipe
temperature of 15°C is used. Notice that while the calculations of the heat transfer in the deck itself are detailed, the boundary conditions are simplified to constant values.
The geometry of the deck is shown in Figure 6.1. The size of the deck is assumed constant;
however, the pipe diameter, cavity dimension and vertical position of the pipe are changed.
Pipe diameter
Table 6.1 shows the total heat flow from the room to the deck by changing the pipe dimension. As it can be seen, changing the pipe diameter has little impact. Therefore it is more important to consider that a smaller pipe diameter gives a larger pressure drop in the pipe, which consequently requires larger electricity consumption for running the pumps.
Table 6.1 Total heat flow from room to pipe for different values of the pipe diameter. The pipe dimension is given as the aa x bb, where aa is the external diameter and bb is the thickness of the plastic pipe.
Therefore a 25 x 2.5mm pipe has an internal diameter of 20mm Pipe dimension
[mm]
Total heat flux [W]
25 x 2,5 89.4 20 x 2 87.0 16 x 2 83.9
Cavity dimension
The effect the cavity dimension is investigated in Table 6.2. The smaller of the two has the same cross sectional area as the standard deck, which is assumed to be the largest possible value to ensure the mechanical properties of the deck. In the calculations, the cavity has been simplified to a rectangular area. It can be seen that by decreasing the cross sectional area of the cavity by 20% the heat flow is only changed by around 2%. Therefore the cavity only has very little influence on the cooling capacity – a natural consequence of the fact that the heat transfer between room and pipe is mainly through the lower surface of the deck, which is not influenced by the conditions above the pipe.
Table 6.2 Total heat flow from room to pipe for different cavity dimensions Cavity dimension
[mm] Total heat flux [W]
100x160 103.0 80x160 105.0
Vertical position
The vertical position of the pipe is dominated by two conditions, which have opposite importance. Firstly, the higher in the deck the pipe is placed the smaller the heat flow, but secondly, the better the use of the thermal mass in the concrete deck and consequent
possibility to even the load during the day, since a larger part of the deck is activated. Another advantage of a position as far from the surface as possible is that the risk of accidentally drilling into the pipe from the ceiling is reduced.
The vertical position of the pipe is changed with the centerline 30mm to 60mm above the ceiling surface. Two situations are calculated; one with no floor covering and one with a floor covering in the form of a wooden floor with a thermal resistance of 0.32m²K/W. The results are shown in Figure 6.2.
25 30 35 40 45 50 55 60 65
0 10 20 30 40 50 60 70 80
Height of pipe (centerline) above ceiling [mm]
Heat flows [W/m²]
Heat flows as function of vertical pipe position
Ceiling surface − no floor covering Floor surface − no floor covering Total heat flow − no floor covering Ceiling surface − wooden floor Floor surface − no wooden floor Total heat flow − wooden floor
Figure 6.2 Steady-state heat flows in the deck construction for different vertical position of the pipe. The heat flows in the deck with floor covering is shown with the thicker line. The room temperature is 26°C and the fluid temperature is 15°C.
The heat flux through the floor surface is almost constant, while the heat flow through the ceiling surface and the total heat flow decrease with higher vertical position. For both the total heat flow and through the floor surface, the model without floor covering has the highest heat flow. This is not the case for the heat flow through the ceiling, which is actually higher with the floor covering, since the deck will be colder when the deck is insulated on the top side, resulting in lower surface temperature on the ceiling surface.
The heat flow is larger when the pipe is placed closer to the deck. The decrease in the total heat flow is around 12% in both situations when the pipe is moved from 30mm to 60mm above the ceiling surface.
The dynamical behavior is tested by finding the time constant for the different vertical
positions of the pipe. The time constant has been defined in section 3.41. Figure 6.3 shows the heat flow through the surfaces. In the figure, the room temperature is changed from 21°C to 26°C with a fluid temperature in the pipe of 15°C. The deck is in a steady state condition before changing the room air temperature. It can be seen that the reaction time in the heat flux across the surfaces of the deck results in the same thermal behavior.
0 6 12 18 24 0
10 20 30 40 50 60 70 80 90 100
Time [hours]
Heat flow rate [W/m²]
Heat flows through surfaces for three different pipe positions
Floor: 30 mm Floor: 50 mm Floor: 60 mm Ceiling: 30 mm Ceiling: 50 mm Ceiling: 60 mm Through ceiling surface
Through floor surface
Figure 6.3 Heat flow through surfaces for different vertical position of the pipe
Conversely for Figure 6.4, where the heat flow from the deck to the pipe is shown. Here the model with the pipe placed closest to the surface has a steeper curve for the increase in the heat flow, and therefore also a shorter time constant.
0 6 12 18 24
0 10 20 30 40 50 60 70 80
Time [hours]
Heat flow rate [W/m² floor surface]
Heat flow from deck to fluid in pipe
30 mm 50 mm 60 mm
Figure 6.4 Heat flow from deck to pipe for different vertical position of the pipe, shown in W/m² floor surface
The time constants for the heat flows in the deck as found in Figure 6.3 and Figure 6.4 are shown in Table 6.3.
Table 6.3 Time constant for the heat flow in the deck construction as found by the calculation shown in Figure 6.3 and Figure 6.4
Time constant for
vertical position of pipe Through floor
[hours] Through ceiling
[hours] Through pipe surface [hours]
30mm 9.0 1.7 2.3
50mm 8.5 1.7 2.7
60mm 8.5 1.8 3.0
As expected, the results show that the time constant is largest for the floor surface, because of the large thermal mass of the concrete above the pipe and lowest for the ceiling surface. Also
the time constant is largest through the floor surface for the pipe position furthest away from the surface. This is also the case for the ceiling surface. However, here the time constant is much smaller due to the proximity to the ceiling surface compared to the floor surface. For the heat flow from deck to pipe, the values are larger than for the ceiling surface, but smaller than for the floor surface.
Preliminary conclusion
Based on these investigations a few preliminary conclusions can be made for the hollow deck with integrated pipes:
- The pipe diameter does not have any significant influence on the cooling capacity. Hence, the pipe diameter can be chosen based on considerations of pressure drop and installation costs
- The cavity size has little influence on the cooling capacity. Hence, the size of the cavity can be chosen based on considerations of static properties and load bearing capabilities and low weight of the deck.
- For the vertical distance of the pipe from the ceiling surface it is more difficult to find a simple conclusion on the best position. Two opposite properties exist. The first is the maximum heat transfer (cooling capacity), which is larger the closer the pipe is to the ceiling surface. However, the time lag between the time when the heat is absorbed by the deck until it is removed by the pipe is shorter. This means that one of the main advantages of the thermo active component, namely the ability to store heat in the deck is decreased by placing the pipe close to the surface.
6.1.2 Hollow deck with pipe in cavity (PIC)
The use of pre-fabricated hollow decks is typical in the Danish building industry. A simple and cheap way to include thermo active capabilities in the deck is to place the pipe directly in the cavities of the standard deck construction. The solution has the advantage that it can use a standard hollow deck.
This solution is called PIC for Pipe In Cavity and is shown in Figure 6.5. Obviously the heat transfer between the pipe and the concrete will be poorer since the pipe is placed in the air cavity and must rely on radiation and convection. This will limit the cooling capacity of the system. However, compared to the type with integrated pipes, the position of the pipe closer to the middle of the deck may give better use of the thermal capacity.
A simple test has been performed to find the cooling capacity. This is described in section 6.2.
Another reason is that simulations of the cooling capacity have been very inconclusive, which means that it is necessary to use a test setup to find the cooling capacity.
220mm
1200mm 160mm
Figure 6.5 Hollow deck with pipe in cavity setup
The placement of the pipe in the floor cavities can be either with one pipe in each cavity or with two pipes placed as a U in each cavity. The U layout means that the pipe needs only to be handled in one end of the deck.
If one pipe is placed in the cavity, it will be placed at the bottom of the cavity, since it should be placed near the ceiling surface, where the largest heat transfer to the room can be achieved.
At the same time it is the easiest position, as a position away from the bottom would require some type of suspension system. With two pipes in the cavity, these will be placed in the bottom half of the deck close to each side of the cavity wall. A position at the top and bottom is also possible though again it will require a suspension system.