Results and discussion

The temperatures were evenly distributed over the surface of the inner layer of the high performance concrete wall element even with increased spacing of the plastic capillary tubes, as can be seen in Figure 44

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and in Table 5. This finding is the main precondition for creating an element which will contribute to a uniform thermal environment in the room and will maximize the potential of natural sources of energy.

Figure 44: Temperature distribution in the inner layer of the high performance concrete element (Figure 4 in Paper II)

Table 5: Differences in Kelvin between the maximum and minimum temperatures over the inner surface of the high performance concrete wall element (Table 2 in Paper II)

Max. Temperature differences over the surface of HPC wall

Distance of CMT Heating Cooling

Mm 21 °C 22 °C 24 °C 18 °C 21 °C 22 °C 30 0.006 0.010 0.018 0.031 0.020 0.016 50 0.024 0.042 0.076 0.136 0.085 0.067 70 0.055 0.060 0.173 0.310 0.216 0.152 100 0.110 0.125 0.347 0.622 0.385 0.306

As the required power output of heating and cooling systems depends on a particular installation, the aim of this study was to investigate how different configurations of plastic capillary tubes behave under different operating conditions.

Figure 45 shows the results of the heat flux on the inner surface of the high performance concrete wall element for each the investigated scenarios. The temperature of heating and cooling water has the biggest

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influence on the experienced power output from the wall element. The heat removed from the room can be increased by 60% when the temperature of cooling water decreases 3 K.

However, the idea in this project was to use water at a similar temperature to the room air temperature.

Therefore further investigations focused on various other ways of increasing the power output from the wall without changes in the temperature of the water. One way to increase the power output is to decrease the spacing between the plastic capillary tubes. The increase in the power output was about 50%

when the spacing was decreased from 100 mm to 30 mm. Another way is to increase the diameter of the plastic capillary tubes. This resulted in a considerable increase in power output, see Figure 45.

The influence of the thickness of the high performance concrete layer of the wall element on the power output was also investigated, because there can be situations where the designed 30 mm will not be sufficient. From Figure 45 it can be seen that an increase in the thickness of the high performance concrete layer from 30 mm to 50 mm has a rather small influence on the power output. This is due to the rather high thermal conductivity of the high performance concrete used, resulting in a mild increase in the thermal resistance of the high performance concrete layer. This is a positive finding, which means that the thickness of the inner layer of the high performance concrete wall element should not be a limiting factor in the design of the solution presented.

Figure 45: Heat flux of the high performance concrete element towards an indoor environment for various scenarios (Figure 6 in Paper II)

The limiting factor for the cooling could be the dew point temperature of the room air, which limits the surface temperature of the cooled walls. The ISO 7730 standard recommends maintaining the humidity level in the room between 30% and 70% [29]. The acceptable indoor operative temperature according to ISO 7730 is 26 °C (for environmental class B) [29]. From the measurements described in section 9.4, it can be seen that the operative temperature is 1 – 1.5 K lower than the room air temperature, depending on the position of the measurement in the room. Therefore the maximum acceptable room air temperature was assumed to be 27 °C for further analysis in this paper. The dew point for air at 27 °C, assuming the middle value in the recommended range for humidity of 50%, is 16 °C. The minimum surface temperature of the

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scenario with spacing of 30 mm and a capillary tube diameter of 3.5 mm was found to be 20.6 °C, which means that condensation would not be present. The resulting values from this scenario for cooling, recalculated to the floor area of the test room used for the investigations in this thesis (described in section 5.1), were 59.6 W/m², 37.6 W/m², and 29.5 W/m² for cooling water temperatures of 18 °C, 21 °C, and 22 °C respectively. It was found during measurements and CFD calculations in Paper III that the indoor air temperature was 24.5 °C – 25 °C when a cooling power of 29 W/m² was used and 22 °C – 23 °C when a cooling power of 59 W/m² was used. The value 29 W/m² can therefore be assumed as a suitable and sufficient amount of cooling power for the scenarios investigated in the test room to maintain a comfortable indoor climate without draughts. Based on these findings, it can be concluded that sub-hypothesis 2 is true for the cooling mode.

The targeted value for heat flux to the indoor environment for the heating scenario was set to 10 W/(m²·K).

This value was chosen as reference so that the results would be comparable with other radiant heating systems, for example with one of the most commonly used radiant heating systems composed of thick plastic (cross-linked polyethylene) pipes cast in the floor construction. The value of 10 W/(m²·K) is the usual design value for the required power of floor heating for passive houses. The required energy output of the system depends on the type of building and the location it is built in. The variables influencing the design are the heat loss of the building (including transmission and ventilation heat loss), comfort issues (temperature of the surface, draft, etc.), the surface area available for installation of the RHCS, and the required reaction time of the system. Different scenarios were therefore investigated to assess the performance of the system under different circumstances. The resulting values of the scenario with spacing of 30 mm and a capillary tube diameter of 3.5 mm for heating, recalculated to the floor area of the test room used for the investigations in this thesis (described in section 5.1), were 28.8 W/m², 14.8 W/m², and 6.9 W/m² for heating water temperatures of 24 °C, 22 °C, and 21 °C respectively. The heating was not needed in the test room because the heat gains (between 51.5 W/m² and 77.8 W/m² of floor area in the test room) were higher than those actually needed for heating. However, the results for heating would be applicable for any other type of building and are therefore relevant as part of the study. It can be concluded that sub-hypothesis 2 is true for the heating mode.

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Increased energy losses to the outside environment were experienced during the investigations as a result of changes in the temperature of the inner layer of high performance concrete: increases in the case of heating mode and decreases in the case of cooling mode. The configurations with integrated plastic capillary tubes were compared with the situation without any tubes in the inner layer of the high performance concrete. Figure 46 shows the increase of the heat flux to the outside environment for various configurations in heating mode. The heat loss increased substantially with increased heating water temperature and the increase is about 20% for a heating water temperature of 24 °C. This finding further supports our initial aim of using heating and cooling water with a temperature very close to the room air temperature. However, this also means that large areas should be available in a building for the application of such a solution. Thorough discussions in the early stages of a project are therefore necessary between all the parties involved in the design of a building. The resulting value for heat loss is rather high and could possibly influence the overall heat balance of the building to a considerable degree. This finding could mean limitations in the use of radiant systems for heating and cooling in buildings in general. The next step in the investigation was to find out how big the influence of increased heat loss from the high performance concrete wall element is on the overall heat losses from the building. The energy performance calculations were carried out using the tool called IDA ICE for dynamic calculations of buildings [75]. The overall heat loss increase was found to be 2.6% for the case under investigation.

Figure 46: Increase of heat flux to the outside environment as result of the use of CPTs in the high performance concrete inner layer (Figure 9 in Paper II)

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