Results and discussion

Dynamic behaviour of the test room during pre-cooling period

This specific scenario was investigated in order to find out the ability of the installed wall radiant cooling system in terms of cooling a highly overheated room. The results may allow engineers to properly design a control system to enable a building to work with good energy efficiency and to make sure that rooms are always preconditioned to the required state before occupancy.

The test room was overheated to 32 °C over the course of a few days. This could be understood as simulating the situation at the beginning of the week after a weekend with very hot weather in an unconditioned room with high solar heat gains. It is desirable to know the behaviour of the installed wall radiant cooling system in such a situation. The cooling of the room from 32 °C to 26 °C took approximately 3.5 hours, as can be seen from Figure 63 (the blue field shows the acceptable range). The operative temperature of 26 °C was chosen as a limiting temperature since it represents the upper comfort range for indoor environment in class B, which was chosen for the investigations [29]. The supply temperature of cooling water was maintained at 15 °C (± 0.5 K). Figure 63 shows that the temperature of the cooled surfaces was influenced immediately by the cooling system when it was activated. The operative temperature in the room was affected after a short delay of about 15 minutes. This was a good sign indicating that the solution used for cooling was well-designed with respect to the dynamics of the system.

This can be attributed to the small thermal mass of the rather slim layer of concrete (30 mm) in the wall and the overall design of the installed wall radiant cooling system described in more detail in section 5.2.

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Figure 63: Cooling of the room before the presence of occupants (Figure 8 in Paper IV)

Dynamic behaviour of the test room during the occupied hours

The room air and surface temperatures were maintained at around 26 °C as a starting situation for the investigation of the development of the indoor climate when people entered the room, and the wall radiant cooling system was started. Three scenarios were investigated to examine the dynamic behaviour of the room when different boundary conditions were applied, see Table 9.

Table 9: Investigated scenarios (Table 1 in Paper IV)

Scenario Internal Temperature of Weather heat gains [W] cooling water [°C]

1 1530 15.5 Overcast

2 1530 18.5 Overcast

3 1530 21.5 Overcast

The room air temperature increased at the beginning of the experiment as a result of the internal heat gains from occupancy. However, after approximately 30 minutes, the room air temperature started to decrease. The operative temperatures started to decrease after 20 minutes as a result of the influence of the cooled surfaces, which already had a temperature about 24.5 °C. The temperature of the cooled surfaces was decreased from 26 °C to 20 °C in the first 4 hours. Stabilization of the temperatures was observed after 4 hours when the operative temperatures within the room were between 23.5 °C and 24.5

°C. The rise in temperatures at a later stage of the measurements is attributed to the increased outside air temperature, as can be seen in Figure 64. In Figure 64, Figure 65, and Figure 66, time is shown in logarithmic scale in order to expand the most relevant part of the measurements right after the start of the cooling.

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Figure 64: Temperature development in Scenario 1 (Figure 9 in Paper IV)

In scenario 2, the temperature of the cooled surfaces was decreased from 26 °C to 21.5 °C after 4 hours of experiment when the situation in the room became fairly stable, as can be seen in Figure 65. The room air temperature was around 26 °C for most of the time. The operative temperatures at the critical points were maintained between 24 °C and 25 °C at that time as a result of the use of the radiant cooling system to achieve class B of standard EN15251 with regard to the temperature in the room [29]. The cooling loop was supplied with water at a temperature of 18.2 °C when stabilized. It is an interesting finding that the operative temperatures were very similar to those measured in scenario 1, even though the temperature of the cooling water was about 3 K higher. The small increase in the temperatures within the room after 4 hours of measurements can be connected with the increase of the outside air temperature shown in Figure 65 by the dotted line.

Figure 65: Temperature development in Scenario 2 (Figure 10 in Paper IV)

The results for scenario 3 are shown in Figure 66. It can be seen that the room air temperature was maintained above 26 °C for most of the duration of the experiment, which is in environmental category C

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[51]. However, the operative temperatures at the critical positions were maintained around 25 °C, which is in environmental category B, even though the temperature of the cooling water was about 21.5 °C.

Figure 66: Temperature development in Scenario 3 (Figure 11 in Paper IV)

Air velocities close to the cooled walls were investigated in Paper III since there was a potential for the creation of a draught. The position of the measuring point was at a distance of 0.2 m from the south-eastern wall and at a height 0.1 m, representing the height of occupants’ ankles, and this is shown by the green dot in Figure 47. This part of the body is usually very sensitive, especially in summer when people wear light clothes and shoes and cooling is activated [14]. The distance of 0.2 m was assumed to be appropriate since the occupants were placed in close proximity of the south-eastern wall, see Figure 47.

The results of the velocities are shown in Figure 67 together with the temperatures of the room air and the cooled surface. Higher velocities were experienced for lower cooling water temperatures. Velocities fluctuated around 0.11 m/s in scenario 1, around 0.1 m/s in scenario 2 and around 0.07 m/s in scenario 3.

In none of the investigated scenarios did the velocities reach the threshold for maximum velocity in the room of 0.19 m/s for summer conditions [29].

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Figure 67: Velocity development at a distance of 0.2 m and a height of 0.1 m (Figure 12 in Paper IV)

The draught rating values were highest for scenario 1 because the high room air velocities were combined with low room air temperatures, see Figure 68. The draught rating reached a maximum value of about 9%, which is well below the recommended limit of 15% [29].

Figure 68: Draught rating results for all the scenarios at a distance of 0.2 m and a height of 0.1 m (Figure 13 in Paper IV)

The proper heat distribution over the wall element was checked during the measurements, since there were concerns about possible temperature differences occurring over the surface of the wall. The reason for this was that the plastic capillary tubes integrated in the wall run vertically in parallel to each other, and connections for both supply and return manifold pipes were made in the upper part of the wall element, see Figure 16. This could potentially result in differences in the surface temperature along the height of the wall element, especially when a small velocity of circulated water is used. The results of our measurements with a water flow rate of 0.125 l/s, however, showed that the temperature was evenly distributed over the whole surface area of the wall. This was checked by making temperature measurements on the surface of the cooled walls and by using a thermo-graphic camera.

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The thermo-graphic investigation showed that one vertical strip on the south-western wall was not functioning properly and did not cool the room, see Figure 69. One possible explanation for this is clogging of the small plastic capillary tubes with impurities present in the circulating water because no special filtration system was used during the experiments. Another reason could be blockage with plastic material created during the production of the capillary tubes during manufacture. The proper functioning of the capillary mats was not assessed prior to the casting in the layer of high performance concrete.

Figure 69: The un-cooled part of the south-western wall (Figure 17 in Paper IV)

9.4.3 Complementary investigations (not included in Paper IV)