Impact of Layer Configuration and Chilled Water Temperature on the Cooling Capacity of Thermally Activated Light Shelf (TALS) System

. Light shelves provide a function to control direct sunlight and introduce daylight into indoor spaces. Also, chilled water flowing in the light shelf can mitigate local cooling demands in perimeter zones. Therefore, in this study, a thermally activated light shelf (TALS) system was proposed, and the TALS’s radiative cooling capacity was evaluated. To do this, a mock-up chamber was developed using prototypes of TALS systems based on panel configurations. It is included: ‘Insulated’ (INS), ‘Air layer’ (ARL), and ‘Air layer with fans’ (ARF), which were designed to increase thermal insulation, natural convection, and forced convection in the TALS panels, respectively. The experiment showed that the ARL and ARF increased cooling capacity by 29% and 57% compared to the INS. Natural convective heat transfer and fan-forced airflow in the TALS's air cavity contributed to improving cooling capacity. With the INS and ARL, the air temperature was recorded at 28.6°C, which requires additional space cooling. The ARF reduced air temperature up to 2.0°C because the TALS cavity fans expedited convective heat transfer and mixing air between the cavity and test chamber. The results of this study could be used to estimate TALS cooling capacity and propose an optimal design in buildings.


Introduction
A light shelf is an environmental design element installed inside and outside the building envelope to prevent direct solar radiation and to reflect natural light and transmit it indoor. The natural light reflected on the light shelf improves the uniformity of illumination in the room without the use of an artificial lighting system. Natural lighting is more advantageous than lighting systems alone due to less heat generation and smaller cooling loads [1]. A previous study [2] evaluated the performance of the light shelf according to dimensions, angles, and orientations. In addition, various studies have been conducted, such as photovoltaic (PV) modules to an upper surface of the light class or perforating method to the panel [3,4]. In the literature [5], when the angle and height of the light shelf were properly adjusted, not only the light environment was improved, but also the thermal environment was improved. The cooling effect of the light shelf was also verified when only using the passive effect without any mechanical considerations to improve the thermal environment [6]. Therefore, a concept of a Thermally Activated Light Shelf (TALS) system [7] was proposed to apply a radiant cooling system with a typical light shelf installed indoors, and the cooling capacity and room temperature reduction effect were analyzed. However, there is a limitation in that the analysis was performed only through heat transfer simulation. Therefore, this study attempted to evaluate the cooling * Corresponding author: knrhee@pknu.ac.kr performance of the TALS system according to the panel section configurations, load conditions and supplied chilled water temperatures through a mock-up test. However, this study focused on the thermal environment of the TALS system and did not evaluate the light environment. The different TALS section designs determine different conditions of the lower and upper surfaces of the TALS panels, respectively, using chilled surfaces and solar radiation. In general, the upper and lower surfaces of the TALS are exposed to the air, and loads of solar radiation on the upper surface must also be considered. So, the cooling load was calculated depending on solar radiation gains on the upper surface of the TALS panel. In addition, the surface temperature, cooling capacity, and air temperature reduction effect of the TALS system were evaluated by varying the supplied chilled water temperatures.

Panel section configuration of TALS
The TALS panels are designed to serve as both a light shelf and a radiant cooling panel. The upper surface of the TALS is covered with high reflection films (reflectance of 0.9 to 0.95) to perform the original function of the light shelf. The lower surface is constructed of a material that has high thermal conductivity and emissivity (emissivity of 0.9) so that radiant heat can be transferred. In+. addition, a water pipe was attached to the lower surface for cooling, and the thermal conductive tape was used to ensure a uniform temperature of the lower surface. Fig 2 shows three types of TALS panels. To increase the cooling capacity in the TALS, (a) Insulated (INS) section was designed to minimize heat transfer through the upper surface by filling the insulation inside of the TALS. Also, on the upper surface, overheating could be a matter due to solar radiation in the summer. Therefore, (b) Air layer (ARL) was proposed using an air layer inside the TALS system. In addition, although the side of the panel is perforated, it would not be easy to fully prevent overheating of the upper part in (b) Air layer (ARL). So, (c) Air layer with fans (ARF) experimented with a small fan that was installed inside the panels to form an artificial airflow to promote heat transfer. Fig.3 shows a mock-up test bed composed of an insulated mock-up chamber, a surface heating film for cooling load reproduction, and a heat source for supplying chilled water. In detail, high insulation (thermal conductivity of 0.034 W/mK or less) was applied in the mock-up chamber, including the ceiling, floor, and walls. Heating films were installed on the ceiling and floor of the mock-up chamber to supply the cooling loads, as the upper and lower surfaces of the TALS panels. To minimize the influence of outdoor air, the experimental settings of the mock-up chamber maintained constant room temperature and humidity at 26 °C and 50% relative humidity, respectively, to minimize the effects of outside air. The cooling load condition was controlled to a total of 100 W/m 2 , which was set based on the general cooling load of an office building [8]. In addition, considering the location where the TALS system is installed, the lower heating film was fixed at 50 W/m 2 , and cooling loads of the upper heating film varied, such as 50, 25, and 0 W/m 2 , in consideration of the inflow level of solar radiation. During the experiment, the supplied chilled water flow rate was fixed at 2 lpm, and the chilled water temperatures were set at 13, 15, and 17 °C. These test scenarios are represented in Table 1.

Cooling capacity calculation method
The demonstrations of Fig.4 show measurement points to evaluate the panel surface temperature, cooling capacity, and effect of the TALS system due to ambient temperature. To calculate cooling capacity, as shown in Fig 4 (a), the flow rate at the vertical part of the chilled water pipe and the temperature at the chilled water pipe inlet and outlet in the TALS panels were measured. In addition, as shown in Fig 4 (b), the surface temperature of the TALS panel and the ambient temperatures at heights of 0.3 m and 0.7 m were monitored in the mockup chamber. The cooling capacity was determined by the EN 14240 calculation for the cooling capacity of the radiant cooling panel [9].
Where Mw is the chilled water flow rate in kg/s, Cp is the specific heat of the water in kj/kgK, Tw,r is the return water temperature in °C, Tw,s is supply water temperature in °C and, ATALS is an area of the TALS panel in m 2 .

Panel surface temperature
To apply the TALS system for cooling, the surface temperature of the lower part of the TALS must be uniform and low, and the surface temperature of the upper part of the TALS must be monitored to prevent overheating on the top surface. The upper and lower surface temperatures of the TALS panel were examined at 9 points. The surface temperature distribution of the bottom surface appeared uniform, which indicates that a uniform surface temperature was maintained. In Fig 5. plots show a) the lower surface temperature and b) the higher surface temperature depending on the sectional configurations of (a) INS to (c) ARF and load conditions when chilled water was fixed at 13 °C. As shown by the lowest surface temperatures under all load conditions, INS showed the lowest surface temperature of 17.7°C-19.6°C, and then, ARL followed the temperature range of 18.3°C -20.4°C and ARF was 18.7°C-7.7°C because the INS shape decreased heat transfer on the upper surface. The lower surface of the ARF appeared higher surface temperatures compared to other shapes. However, the upper surface temperature was at approximately 5.7 °C lower than the INS and ARL under full load conditions. It is because the ARL prevents the overheating of the upper surface in response to heat gains from heating films on the upper surface. In addition, the upper surface temperature of INS and ARL was higher than 26 °C of the thermohygrostat setting temperature, which indicates that replicating the insulation layer to the air layer is not proper.

Cooling capacity
The cooling capacity, or the amount of heat removed by the TALS system, was calculated using the equations [1] and [2] presented in Section 2.3. Fig. 6 depicts the cooling capacity depending on the cross-sectional shape and load conditions when chilled water was provided at 13, 15, and 17 °C, respectively. Depending on load conditions, the range of chilled water temperature, and cross-sectional form, the derived cooling capacity was from 45.4 to 107.1 W/m 2 . ARL and ARF showed an improvement in cooling capacity compared to INS in all conditions, with a maximum increase of 29% and 57%, respectively. Because of the holes in ARL and ARF, air convection on the TALS transferred heat to the air in the mock-up chamber, and the air convection by a fan contributed to the increased cooling capacity of ARF. In addition, ARF had a greater cooling capacity than INS, even when chilled water was provided at a 4 °C higher and the load was supplied from the top (Full, Half). The reason for this is that the ARF responded properly to the load given from the top and removed it most effectively. In comparison to other load conditions, when no load is supplied from the top (Low), the effect of increasing the cooling capacity of ARL and ARF was insignificant. The formation of convection through the fan is unnecessary when solar radiation is low.
Through these data, the results verified that the proposed TALS system accurately represented the variation in the load given by the top portion.

Air temperature reduction
During the experiment, the room air temperature was monitored in order for the TALS system to be applied for cooling. To accomplish this, the air temperature of each point in the mock-up chamber was compared. Fig.  7 shows the vertical temperature distribution based on the cross-sectional shape and load conditions when chilled water was supplied at 13 °C. Under full load conditions, the air temperature at each point of INS and ARL appeared comparable. This is because the internal temperature of the panel was 24.7 ° C, and the cold air was constant, so it did not contribute to the cooling of the room. In the case of the ARF, however, the cold air was constant when the panel was circulated, resulting in a 2°C reduction in room air temperature. This effect was similarly found under a condition of half load, and forming air flow was verified as an appropriate approach to the maximum load. However, under the low load condition (no loads on the top), all shapes formed at 26 °C in indoor environmental conditions where insolation is low, and these results are regarded as

Discussion and conclusion
In this study, the TALS system was evaluated based on chilled water temperature, sectional configuration, and load conditions using a mock-up test. In INS, the lower surface temperature of the TALS panel was lower than in ARL and ARF, and in ARF, the upper surface temperature was significantly lower than in INS and ARL because the upper surface responded to the load supplied to the top. However, when the chilled water temperature was 13°C, the lower surface temperature reached 4.7°C-7.7°C, indicating that the thermal conductivity of the panel must be improved. With enhanced thermal conductivity, an appropriate surface temperature can be obtained when a higher chilled water temperature is supplied. In addition, the effect of increasing cooling capacity and decreasing room temperature was demonstrated when the airflow was formed, and the load was supplied from the upper temperature (Full, Half) as compared to INS and ARL. This is because ARF responded to the load supplied from the upper surface.
However, compared to the actual space, the mock-up chamber is a heat-conductive environment, and the results may differ accordingly. Consequently, the TALS system is necessary to be evaluated in real space in further study.