A Simulation Study on the Performance of Radiant Ceilings Combined with Free-Hanging Horizontal Sound Absorbers.
Water-based radiant surface heating and cooling systems are defined as systems, where at least half of the heat transfer from the heated or cooled surface is by radiation (Babiak et al. 2009). An example of water-based radiant surface heating and cooling systems is Thermally Active Building Systems (TABS).
TABS have several advantages, such as benefiting from the low temperature heating and high temperature cooling principle (Olesen 2012; Kazanci 2016), coupling with renewable heat sources and sinks (Kazanci et al. 2016), transferring peak loads to off-peak hours and peak load reductions (Babiak et al. 2009; Meierhans 1993).
TABS use large building surfaces to emit or remove heat; therefore, unobstructed surfaces are required for optimal performance, which could have a negative impact on the acoustic conditions in indoor spaces (Olesen 2012). One common solution for mitigating the noise in indoor spaces is using free-hanging sound absorbers. These sound absorbers could be horizontal or vertical (Dominguez et al. 2017). Using sound absorbers in combination with TABS will affect the performance of TABS, and, hence, the thermal indoor environment conditions.
It is important to quantify the effects of free-hanging sound absorbers on the cooling capacity of TABS for accurate system dimensioning, and for predicting the corresponding effects on the thermal indoor environment. Different authors have studied the effects of sound absorbers on cooling performance of TABS by climate chamber measurements (Pittarello 2007; Weitzmann et al. 2008; Peperkamp and Vercammen 2009; Ruud 2008; Peutz 2013), by field measurements (Muet and Lombard 2015), and recently by dynamic building simulations (Lombard 2014; Rage et al. 2016a; 2016b; Langner and Bewersdorff 2015). However, there is still a further need to quantify the effects of using sound absorbers in combination with TABS, especially under different ceiling coverage ratios, and by validating commercially available dynamic building simulation software.
In this study, measurements were carried out in a climate chamber to quantify the effects of horizontal free-hanging sound absorbers on the cooling performance of TABS and on the thermal indoor environment. In addition to the measurements, the climate chamber was simulated using a commercially available software with a special plug-in that allows simulating the effects of horizontal sound absorbers on the cooling performance of TABS and on the thermal indoor environment (Solar Energy Laboratory 2014; Lombard 2014). The simulation results were compared to the measurement results.
Experimental setup with horizontal sound absorbers
Experiments were carried out in a test facility at the Technical University of Denmark that resembles a room with TABS. The facility consists of a 21.6 [m.sup.2] (233 [ft.sup.2]) room with a ceiling height of 3.6 m (12 ft). The floor and ceiling consists of thermally active concrete decks to obtain realistic conditions of a multi-story building with TABS.
Each TABS deck consisted of three prefabricated concrete decks covering the entire area of the ceiling and floor. The slabs had a thickness of 270 mm (10.6 in.). There was a wooden floor covering with a thickness of 15 mm (0.6 in.) and a thermal conductivity of 0.15 W/(mK) (1.04 Btu*in/h*[ft.sup.2]*[degrees]F). There was a 20 mm (0.8 in.) air gap between the floor covering and the slab. PEX pipes of 20 mm (0.8 in.) outer diameter and 2 mm (0.08 in.) thickness were embedded in the concrete slabs with a pipe spacing of 150 mm (5.9 in.). The pipes were embedded at a depth of 50 mm (2 in.) from the lower surface of the deck. Fig. 1 shows the details of the test facility and the TABS deck.
The horizontal sound absorbers were made of high-density glass wool with the dimensions of 1160 mm x 1000 mm (3.8 x 3.3 ft), and a thickness of 40 mm (1.6 in.). The panels were installed at a distance of 300 mm (11.8 in.) from the ceiling surface with different ceiling coverage ratios (Rage et al. 2016b).
Before installing the acoustic panels with different coverage ratios, measurements were carried out without any acoustic panels (bare-ceiling). This case was used as the reference case. Fig. 2 shows the installation and layout of the horizontal sound absorbers in the test facility for different ceiling coverage ratios.
The climate chamber was configured as a two-person office room for the experiments. Table 1 summarizes the experimental conditions, including the heat gains.
Measurements were performed under steady-state conditions. The thermal indoor environment in the room was assessed by means of air and globe temperatures (operative temperature at 0.6 m (2.0 ft) and at 1.1 m (3.0 ft) heights). Fig. 3 shows the measured physical parameters and the measurements locations in the climate chamber.
Further details of the climate chamber can be found in (Weitzmann 2004), and further details regarding the experimental setup and the measurements can be found in (Dominguez et al. 2017). Evaluation of the TABS cooling performance can also be found in these two publications.
Details of the numerical model
In addition to the experiments, a dynamic building simulation model of the climate chamber was created, which allows simulation of a wide range of systems from energy systems to multi-zone buildings and more.
Physical components (pumps, valves, etc.) are represented by Types in the software. The same applies for buildings, and Type 56 is used to represent multi-zone buildings. This Type was used to represent the climate chamber in this study.
The experimental setup and the measurement conditions were recreated as close as possible to the actual conditions. The walls, floor and ceiling were modeled using the same material properties and the same construction as in the climate chamber. On the backside, walls, floor and ceiling surfaces were assumed to be facing another room with the same thermal indoor conditions as the climate chamber, in order to simulate the room-in-a-room construction (thermal guard) of the climate chamber. There was no infiltration. The total heat load, ventilation rate and supply temperature, water supply temperature and water flow rates to the floor and ceiling decks were implemented as defined in Table 1. The simulations were run with 6 min time steps until steady-state conditions have been reached.
The floor and ceiling TABS constructions were modeled according to Fig. 1. The pipes were modeled as an active layer. The concrete used for the floor and ceiling slabs had a thermal conductivity of 1.6 W/(m K) (11 Btu*in/h*ft2*[degrees]F), specific heat capacity of 1 kJ/(kg K) (0.24 Btu/lb*[degrees]F), and a density of 2300 kg/[m.sup.3] (144 lb/[ft.sup.3]).
A specifically developed plug-in (Type EAE--Ecophon Acoustic Elements) for acoustic elements (Lombard 2014) was used in the model to study the effects of a partially covered ceiling (horizontal sound absorbers) on TABS cooling performance and on thermal indoor environment. Having an accurate simulation tool can give the possibility to designers and researchers to study the effects of horizontal sound absorbers during the design phase.
The acoustic element plug-in interacts with the multi-zone building model through inputs and outputs (Ecophon 2015). In the beginning of a time step, the building model calculates room parameters such as the air temperature, ceiling surface temperature, floor surface temperature and the mean radiant temperature (MRT) for the room without the horizontal sound absorbers using the standard algorithms in the software. These parameters are then provided to the plug-in, and the plug-in calculates the upper and lower surface temperatures of the horizontal sound absorbers and it calculates the radiative and convective heat exchange in the room with horizontal sound absorbers. Based on the new calculations, a set of radiative gains from the horizontal sound absorbers to the ceiling, floor, and wall surfaces are defined, together with the convective gain from the horizontal sound absorbers to zone air.
It is only possible to simulate horizontal sound absorbers, and the layout of the sound absorbers are not considered in the plug-in; it is assumed that horizontal panels are clustered in the center of the room. Further documentation and details of the implementation of the plug-in are given in (Ecophon 2015) (including the calculation of the new MRT for the room), and the details of the thermal model of the plug-in (i.e. how conduction, convection and radiation are treated) are given in details by Lombard (2014).
RESULTS AND DISCUSSIONS
Fig. 4 shows the comparison of cooling capacity coefficient and its reduction between the measurements and the simulations.
The measurements show a cooling capacity reduction of 11%, 23% and 36% for 43%, 60% and 80% coverage ratios, respectively. According to the simulations, the cooling capacity decreased by 17%, 26% and 39% for 43%, 60% and 80% coverage ratios, respectively. The measurements and the simulations show close values of cooling capacity coefficient reduction for higher ceiling coverage ratios; however, for the 43% ceiling coverage ratio, there is a remarkable difference between the reduction predicted by the simulations and the measurements. This difference could be due to the higher sensitivity of the simulation model to the actual panel layout for lower ceiling coverage ratios. The higher reduction in the cooling capacity predicted by the simulations could be due to the limitation of the simulation model in treating the actual panel layout, and assuming that all the panels are clustered in the center of the ceiling. Vercammen (2015) showed that with a ceiling coverage ratio of 58%, the cooling capacity reduction was 33% with evenly distributed panels while it was 37% when the panels were clustered.
Although the experimental results and the simulation results show close reductions in the cooling capacity coefficient, the simulation results overestimate the cooling capacity coefficient of the ceiling compared to the measurements within a range of 13% to 22%. This behavior requires further modifications to the simulation model. The difference in the cooling capacity coefficients between the experiments and the simulations are mainly due to the differences in the heat removed by the ceiling surface, and due to the difference in the heat removed by the ventilation. Other sources of the differences could be limitations in modeling the guard surrounding the climate chamber, which affects the heat transfer from the ceiling slab to the upper side of the slab and temperatures of other surfaces in the room, and the convective heat transfer coefficients used in the simulation model (default values in the software have been used in this study).
Table 2 shows the air and mean radiant temperatures from the measurements and simulations (at 0.6 m (2.0 ft) height). Fig. 5 shows the comparison of the operative temperatures between the measurements and simulations.
The results in Table 2 show that the simulation model is able to predict accurately the air temperature. The biggest difference between the simulated and measured air temperatures was observed for a ceiling coverage ratio of 80%. The results also show that the simulation model does not predict the mean radiant temperature (MRT) as accurately as the air temperature and that the difference between the predicted and the measured MRT can vary up to 1.5 K (2.7[degrees]F). This difference could be due to the differences in the convective and radiative portions of the heat loads in the measurements and in the simulations, limitations in modeling the guard (effects on surface temperatures), and due to the limitations in the modeling of the actual layout of the acoustic panels and the measurement location in the climate chamber. The simulation model requires improvements regarding the estimation of the MRT.
Fig. 5 shows that the operative temperature increases due to the reduction in the cooling capacity of TABS (heat removed by the TABS surface). In the climate chamber, this increase was 0.6 (1.1), 1.5 (2.7) and 1.6 K (2.9[degrees]F) for 43%, 60% and 80% coverage ratios, respectively. In the case of the simulations, this increase was 0.9 (1.6), 1.4 (2.5) and 2.4 K (4.3[degrees]F) for 43%, 60% and 80% coverage ratios, respectively. There is a bigger difference between the simulation results and the measurements of operative temperature compared to the air temperatures in general. For the ceiling coverage ratio of 80%, the simulations and the measurements provide the same operative temperature.
Being able to predict accurately the surface temperature of a radiant cooling system is important for a wider applicability of the model. Fig. 6 shows the comparison of the ceiling surface temperature of TABS between the measurements and the simulations.
The results of both measurements and the simulations show that the ceiling surface temperature decreases when the ceiling coverage ratio increases, opposite to the behavior of the operative temperature, which increases with higher coverage ratios. This is due to the presence of the horizontal sound absorbers; a cold air layer forms in the plenum, which hinders the heat exchange between the occupied zone of the room and TABS.
The results in Fig. 6 show that the simulations with Type EAE can accurately predict the surface temperature of the TABS, and therefore could be used for further analyses in other studies.
Fig. 7 shows a summary of cooling performance reduction as a function of the ceiling coverage ratio available in literature. The measurement and simulation results are presented with yellow and green circles, respectively.
The experimental results obtained in this study closely match with the results of the previous studies. The simulation results obtained in this study are very close to the previously obtained simulation results (Lombard 2010, red circles) and are very close to the experimental results.
The results show that while some aspects of the simulation model could be improved, in its current form it could be used in other studies. Even though not every designer might use this specific simulation software and model, the values given in this study can be used as a first estimate of the effects of acoustic sound absorbers on the cooling capacity of TABS and on thermal indoor environment, together with the experimental results presented in this study.
The effects of horizontal free-hanging sound absorbers on the cooling capacity of TABS and on the thermal indoor environment were investigated experimentally in a climate chamber and by dynamic building simulations.
When horizontal sound absorbers were used, the cooling capacity of TABS decreased by 11%, 23% and 36% for ceiling coverage ratios of 43%, 60% and 80%, respectively. Corresponding operative temperature increase was 0.6 (1.1), 1.5 (2.7) and 1.6 K (2.9[degrees]F) for 43%, 60% and 80% coverage, respectively.
The developed simulation model was able to predict closely the cooling performance reduction of TABS, the ceiling surface temperature, and the thermal indoor environment in most cases. While the model can be improved in certain aspects (prediction of mean radiant temperature and cooling capacity coefficient), the accurate prediction of the surface temperature of the TABS makes the model useful for further studies, which may use differently constructed radiant surface cooling systems.
Further studies will address the limitations of the current simulation model. Once this is completed, the current simulations will be extended to dynamic simulations, and possibly, to heating conditions.
This study was financially supported by Saint-Gobain Ecophon AB (project no. 8006 Acoustics and Thermal Comfort) and the International Centre for Indoor Environment and Energy, Technical University of Denmark.
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Ongun B. Kazanci, PhD
Associate Member ASHRAE
Bjarne W. Olesen, PhD
L. Marcos Dominguez
Student Member ASHRAE
Table 1. Summary of the operating conditions during the experiments. Constant condition Value Total heat gain, W/[m.sup.2] (Btu/h*[ft.sup.2]) 35 (11) Supply air temperature, [degrees]C ([degrees]F) 20 (86) Ventilation rate, ACH 1.35 Supply water temperature to decks, [degrees]C ([degrees]F) 15 (59) Water flow rate (floor/ceiling), kg/h (lb/h) 293 (646)/283 (624) Table 2. Air and mean radiant temperatures from the simulations and measurements. Scenario Air temp., full- Air temp., scale, [degrees]C simulations, [degrees]C ([degrees]F) ([degrees]F) 0% 21.7 (71.1) 21.7 (71.1) 43% 22.3 (72.1) 22.4 (72.3) 60% 23.2 (73.8) 22.9 (73.2) 80% 23.3 (73.9) 23.8 (74.8) Scenario Mean radiant temp., Mean radiant temp., full-scale, [degrees]C simulations, [degrees]C ([degrees]F) ([degrees]F) 0% 21.7 (71.1) 20.2 (68.4) 43% 22.4 (72.3) 21.1 (70.0) 60% 23.2 (73.8) 21.8 (71.2) 80% 23.3 (73.9) 22.9 (73.2)
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|Author:||Kazanci, Ongun B.; Rage, Nils; Olesen, Bjarne W.; Dominguez, L. Marcos|
|Publication:||ASHRAE Conference Papers|
|Date:||Jan 1, 2018|
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