Complementary investigations (not included in Paper IV)

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)

Due to the limitations of the simulation software, the following simplifications were made:

a) The room air temperature was constant for the whole period of calculation. The air temperature in the room did not change dramatically over the course of the measurements, as can be seen in the previous section. This simplification was therefore assumed to make little difference to the temperature distribution in the wall element.

b) The boundary conditions were kept constant on the external side of the wall element. Changes here make even less difference to the temperature development on the internal side of the wall element.

The thick piping made of cross-linked polyethylene that was used had an outer diameter of 16 mm with a wall thickness of 1.5 mm. The spacing between the pipes was 150 mm, which was assumed to be the average value used in building practice. The layer of concrete in which the pipes were cast was 100 mm thick and the pipes were situated in the middle of the concrete layer. A period from 8 a.m. to 5 p.m. was investigated, and the cooling system was activated for all of this period except from 12 a.m. to 1 p.m. when it was assumed that all the occupants would leave for lunch.

The results of the temperature distribution for both scenarios are shown in Figure 70. It can be seen that the system with the thick piping did not reach a quasi-steady-state, which means that the full cooling potential of the element was not reached. The system with capillary tubes was close to quasi-steady-state after 4 hours of running the cooling system, and the surface temperatures were 20.1 °C, 22.0 °C, and 23.6 °C for scenarios 1, 2, and 3 respectively. The corresponding temperatures in the solution with thick piping were 23.5 °C, 24.3 °C, and 24.9 °C after 4 hours. The surface temperature difference between two systems increased when the cooling water temperature was decreased. Higher temperature differences were also experienced in the early stages of the experiment, because the reaction of the system using thick piping was much slower than the system using capillary tubes. The advantage of the system with capillary tubes in fast reaction to changes in a control system is obvious.

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Figure 70: Comparison of surface temperatures for the scenarios with capillary tubes and with thick tubing

The difference is even more profound when the heat fluxes of the two systems are compared, as can be seen in Figure 71. The surface heat flux in the system with capillary tubes was 6 times, 4 times, 3 times, and 2 times higher than in the system with the thick piping 1 hour, 2 hours, 3 hours, and 4 hours after the start of the experiment respectively. In other words, the system with capillary tubes was able to deliver from 2 to 6 times as much cooling energy as the system with the thick piping. It should be stressed that these differences were obtained for the same temperature of cooling water in both cases.

To discuss and evaluate the comparative dynamic performance of the two systems, the term “4 hours cooling capacity” is introduced. The “4 hours cooling capacity” is defined as the cooling capacity available after 4 hours from the start of the experiment. The reason for this is that after 4 hours the cooling was stopped for 1 hour. The aim is to use the value of the continuous development of cooling capacity for dynamic evaluation. The system with capillary tubes reached 50% of its “4 hours cooling capacity” already half an hour after the start of the experiment, while the system with the thick piping reached only 8.5% of its “4 hours cooling capacity” at the same time.

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Figure 71: Comparison of surface heat fluxes for scenarios with capillary tubes and thick tubing

Dew point temperature investigation

This part of the investigation focused on the dew point temperature of the room air, which limits the surface temperature of a radiant cooling system. The performance of the system with the plastic capillary tubes was compared with the system with the thick tubes on the basis of the results of transient calculations done in program HEAT 2. The need for transient calculations was due to the slow reaction of the system with the thick tubes, as found in the previous section. The occupancy hours in the test room were taken into account and were set to between 8 a.m. and 12 a.m. and 1 p.m. and 5 p.m. (the same settings as in the previous section). There is one hour between those two periods where the cooling system was turned off (lunch time). The temperature of the test room was 26 °C before the experiment started, which is the same as the situation used during the measurements in Paper IV, and the reasoning behind this choice is described in section 9.4.2.

The temperature of the cooling water was constant for all of the following experiments. No measured data were available for the evolution of cooling water temperature for the investigated supply temperature values of the cooling water. Three scenarios were calculated with cooling water temperatures of 10 °C, 15 °C, and 18 °C.

The standard recommends that the humidity is maintained between 30% and 70% [29]. The acceptable indoor operative temperature according to the standard is 26 °C. From the measurements, we can see that the operative temperature is about 1 – 1.5 K below the room air temperature, depending on the position of the measurement in the test room. For further analysis it was therefore assumed that the maximum acceptable room air temperature in the space equipped with the designed solution for radiant cooling was 27 °C. The dew point for air with a temperature of 27 °C and a humidity of 50% (a middle value in the recommended range) is 16 °C. The surface temperature of the cooled surfaces should not fall below the dew point temperature. In the event that the required cooling load is greater and a dehumidification is

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available as part of the air handling unit, the surface temperature could go down to 8 °C, which is the dew point for humidity of 30% with an air temperature of 27 °C. Figure 72 shows that the surface temperature of cooled walls of 15.4 °C is already too low for cooling water with a temperature of 10 °C.

Figure 72: Surface temperatures for different temperatures of cooling water

The maximum cooling load limited by the dew point temperature of the room air (16 °C) was 140 W/m² with the use of cooling water with a temperature of 12 °C. The resulting lowest surface temperature was 16.6 °C. The heat flux development is shown in Figure 73. The results are relevant for the situation when both cooling walls (the south-eastern wall and the south-western wall) were activated.

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Figure 73: Surface heat fluxes for different temperatures of cooling water