Discussion

12 18 24 6 12 18 24 0

50 100 150

Time [hour of day]

Temperature [°C]

Measured and simulated heat flow in pipe

TASim Measured

Figure 6.52 Comparison of the measured and calculated heat flow in the pipe.

In total, Figure 6.50 to Figure 6.52 shows that TASim is fully capable of modelling the dynamical conditions in the thermo active deck.

Figure 6.53 shows the measured results for the room air temperature and operative temperature during the measurement period.

12 18 24 6 12 18 24

20 21 22 23 24 25 26 27 28 29 30

Time [hour of day]

Temperature [°C]

Room air temperature

TASim Measured 0.1m Measured 0.7m Measured 1.1m Measured 1.8m Measured 3.5m

12 18 24 6 12 18 24

20 21 22 23 24 25 26 27 28 29 30

Time [hour of day]

Temperature [°C]

Operative temperature

TASim Measured

Figure 6.53 Room air temperature and operative temperature compared. The data shown for the operative temperature has been corrected by the offset error on the measurements by -0.4K.

As found above, TASim overestimates the room air temperature while underestimating the operative temperature. As discussed above, the room air temperature is typically close to the highest measured temperature in the room when there is a “high” heat load, while it is in close agreement when there is a “low” heat load, which again indicates that without stratification in the room air (during the cooling periods with low heat load), TASim is fully able to predict the temperature in the room.

The measurements in the mock-up are as expected and therefore the experimental setup can be used for finding the thermal conditions and energy flows. This is the case in spite of the fact that the test mock-up has not been finished at the time of the measurements, which among others means that the guard box has not been installed. This especially influences the

temperature in the guard, which should be identical to the temperature in the room. This gives an unwanted heat flow through the walls. However, it has been shown that this does not have a significant influence since this unwanted heat flux is small because the temperature

differences are generally small between room and guard. Unfortunately it has not been possible to achieve the same thermal conditions in both decks due to differences in the flow unit and the fact that the missing guard resulted in different conditions for the floor surfaces where the upper deck was exposed to laboratory conditions and not guard conditions, which gave a much higher radiant temperature, while the lower deck is placed only 0.5m above the laboratory floor resulting in a much lower temperature. This will be improved by the

installation of the guard where a fan will ensure air circulation in the guard.

In the stationary measurement series, the cooling capacity has been shown to be proportional to the temperature difference between the fluid temperature and the environmental

temperature of the room. This result of a constant cooling capacity coefficient is the same as found for the PIC-setup in section 6.2. This could also be expected since the only causes of non-linearity are the convective and radiative heat transfer coefficients between deck surface and room/guard. While the radiative heat transfer coefficient depends on the temperature difference between the surfaces to the fourth power, it can be linearized in the narrow temperature band in which these measurements take place. Therefore the only “truly” non-linear part is the convective heat transfer coefficient, which again is not non-non-linear enough to influence the cooling capacity in the temperature band under which the thermo active deck operates.

The use of the environmental temperature, which is the average temperature of the air and surface radiant temperatures weighed by the convective and radiative temperatures

respectively, has proven useful for defining the cooling capacity coefficient of the deck. The approach is similar to a method used for characterising windows, where there is often a large difference between air and radiant temperatures, especially between outdoor air and sky temperature during the night. This is – to a lesser degree – also the case for the mainly radiant cooling from the thermo active component. Another advantage is that the method actually does not require the convective heat transfer coefficient to be known, as this parameter has proven difficult to accurately define and measure. This will be discussed further below.

The cooling capacity has been calculated for the floor and ceiling surfaces individually to split the contributions of the actual cooling of the room. This is especially relevant since the

measurements should also be applicable to other constructions – for instance other types of floor constructions. In buildings with thermo active components it is expected that the floor construction will have a higher thermal resistance than used in this mock-up, since both cabling and acoustic measures must be handled by the floor surface as the ceiling surface will need to have clear view of the concrete in order to absorb enough heat from the room.

In this investigation the cooling capacity of the ceiling surface has been found to be five times higher than through the floor surface.

The dynamic measurements show that the deck can absorb the heat from the high heat load during the day and remove it through the pipes during the night. This result is as previously shown in the literature based on simulations and field measurements.

The test mock-up has proven a good tool for measuring the thermal conditions of thermo active components and the versatile design means that many different experiments can be investigated. A few can be mentioned:

– Different acoustic measures such as lowered ceilings.

Installing a lowered ceiling could be used to easily find a “shading” coefficient of the cooling capacity of the deck. This could be used to develop different types of lowered ceilings, perhaps perforated metal sheets that would both act as acoustic measures while also ensuring cooling from the ceiling surface.

One very interesting type of lowered ceiling, which could unfortunately not be included in this thesis, is an operable lowered ceilings formed as elements of perhaps 40cmx400cm, which can be placed in either horizontal or vertical position. Such a ceiling would have two functions. Firstly, it can be used for acoustic measures both in the horizontal and vertical position. Secondly, the fact that it is operable means that the deck can be cooled more during the night because of the lowered ceiling when placed in the horizontal position will be a thermal resistance for the cooling from the deck to the room. Therefore the room will not be as cool in the morning as it would otherwise be. During the day, when the room starts heating up, the ceiling can be turned and open up for the cool surface of the deck. Further, the operability could be made individually controllable, such that persons with different thermal preferences could themselves control the temperature.

– Heating experiments.

In this thesis only cooling has been examined. However, the design of the mock-up means that cooling can also be measured, by cooling the air in the guard.

– Ventilation

Different types of ventilation principles – for instance displacement ventilation – can be tested. One possible investigation would be to use a combination of a lowered ceiling and a ventilation system where the air is pre-cooled by the cool air in the cavity between deck and lowered ceiling and the air is supplied to the room through the perforations in the ceiling surface.

– Alternative deck constructions.

The mock-up has been designed in such a way that the upper deck can be replaced by an alternative construction with the same dimensions

– External conditions on one of the walls. One side of the walls has been designed in such a way that the guard wall and inner wall can be removed. This means that dynamic

conditions can be applied to this surface – for instance solar radiation through a fully glazed façade.

– Pipe in Cavity setup.

The upper deck has been equipped with a loose pipe in the cavities of the upper deck. The results from these measurements have not been included in this thesis, but the mock-up has been prepared for this. An early assessment in the mock-up seems to find a cooling capacity of around 2W/m²K – or roughly one third of the capacity for the deck with integrated pipes.

– Thermal comfort.

The mock-up can be used for more detailed measurements of the thermal comfort in the room, for instance through controlled experiments on test persons.

– Convection.

As found above, the convective part of the heat transfer has been found to be poorly defined especially concerning the validation of TASim. Therefore measurements in the

room can be used to measure the convective heat transfer coefficient and the split between radiant and convective heat transfer from the surface to the room (air and surfaces)

The simulation model has been validated based on the measurements series with the following results:

- For the stationary measurements the simulated heat flows have been found to be in close agreement to the measured data, except for the heat flow through the floor surface of the upper deck which is underestimated by the model. This can however be explained by an unexpectedly high radiant temperature on the surface. While the room air temperature is generally overestimated, the ceiling surface temperature is underestimated. This indicates that the convective heat transfer is not modelled accurately enough as the thermal

stratification is not included in TASim. The difference can be explained by the fact that the room air temperature is higher directly under the deck than the modelled value, which causes a higher convection than expected – this also accounts for the underestimated ceiling surface temperature of the upper deck.

- For the dynamic measurements the simulated results of heat flows and temperatures in the thermo active component are shown to be in close agreement with the measurement results. This has shown that the model is fully able to reproduce the thermal conditions in the decks. For the room, the model is fully able to reproduce the dynamic course of the temperature of both air and surface temperatures. However, the absolute value of the room air temperature is overestimated by about 1K when there is a high heat load in the room, where the predictions made by the model agree closely with the highest temperature in the room, namely the temperature immediately below the ceiling surface. On the other hand, during periods with low heat load in the room and therefore less stratification, the

temperature predicted by the model is very close to the room air temperature, which is the same in all heights of the room.

In total, the results from the convective part of the room model brings up an interesting discussion on the general use of correlations used in building energy simulation programs where there is only one room air node. A correlation for finding the convective heat transfer refers to an air temperature, but generally it is not well-defined which air temperature this is – in fact what air temperature would be correct to refer it to? Generally in this context it must be concluded that the convective part of the heat transfer is not completely satisfactorily defined.

However the problem is most likely of a more fundamental nature towards the definition and use of convective heat transfer coefficients in building energy simulation programs. A natural step is consequently to more accurately define the convective heat transfer. This is also being investigated through detailed analyses using CFD modelling of the air volume in the rooms – but also in building energy simulation programs where the room air can be divided into a few vertical nodes to include the stratification or alternatively through a stratification term on the room air which assumes a linear stratification from bottom to top of the room. In this context, investigations on the convective heat transfer from horizontal surfaces that are heated or cooled in an entire enclosure.

At the same time it is noticed that most of the results from the comparisons shows that the simulated values are within the range of the measurement accuracy.

In total, TASim has proven that it can model the conditions in the test mock-up and reproduce measurement results for heat flows and temperatures.

One final and very important point to mention in this discussion on the validation is that since the room model in TASim has been shown to give credible results for the room model, this is of course also the case for FHSim.