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Design optimization and experimental evaluation of photovoltaic double skin facade
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Design optimization and experimental evaluation of photovoltaic double skin facade Chul-sung Lee, Jongho Yoon∗ Department of Architectural Engineering, Hanbat National University, San 16-1 Duckmyung-dong, Yuseong-gu, Daejeon 305-719 Republic of Korea
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Article history: Received 7 May 2019 Revised 11 July 2019 Accepted 16 July 2019 Available online xxx Keywords: Design optimization Photovoltaic double skin façade (PV-DSF) Photovoltaic vent window Optimal operating angle Heating and cooling energy consumption
a b s t r a c t This study proposed the introduction of a photovoltaic double skin façade (PV-DSF) as a way to economically and easily improve the performance of building envelopes. This system has been designed to be installed not only in new buildings but also in remodeling or renovation projects of existing buildings even in a state where occupants are present inside. For the PV-DSF system suggested, this study aimed to perform experimental evaluation of 1) the optimal operation method to maximize electric energy production of the PV vent window and to reduce the heating and cooling energy consumption of buildings and 2) the thermal behavior and energy performance of the building when the proposed PV-DSF is applied. Analysis results using simulation have shown that the monthly optimal operating angles of the PV vent window to minimize heating and cooling energy demand were the same as the operating angles to maximize power generation. It has also been proven through experiments that the proper operation of the PV vent window can significantly reduce the energy demand of buildings. In particular, the difference between the DSF and outdoor temperatures was 12 °C in winter, showing that it is possible to achieve a significant reduction of heating energy demand. Lastly, when the PV-DSF was applied, air and wall temperatures were decreased by 0.5 °C and 2 °C in summer, respectively, and cooling energy consumption was also at least 10% reduced on average. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Studies have been carried out to reduce building energy demand by improving the performance of building envelopes with the aim of reducing carbon dioxide emissions [1–4]. In addition, attempts have been made to optimize already developed technologies through convergence of existing technologies rather than using a single technology [5–7]. In order to maintain relatively constant indoor environments in buildings in an energy-efficient way under the circumstances of changing external environments, it is necessary to 1) receive sufficient external resources that can be utilized in building envelopes and 2) block unnecessary elements from entering the indoor spaces of buildings. In other words, to reduce energy consumption, the performance of building envelopes needs to be changed according to changing external environments. The double skin façade (DSF) is used not only for the reduction of energy consumption by making a buffer space between the outer and inner spaces, and also for purposes such as preheating, night cooling, noise reduction, etc. [8]. Recently, as part of studies on convergence of renewable energies, researches on PV-DSF
∗
Corresponding author. E-mail address: [email protected] (J. Yoon).
convergence have been carried out to reduce building energy consumption though the improvement of the performance of building envelopes by introducing the DSF and to produce electric energy using the PV [9,10]. Since a PV system is incorporated into a DSF, the PV-DSF can be another alternative method for introducing a new and renewable energy system in the case of buildings where installation locations for introducing the BIPV system are limited. Studies have been conducted to investigate the effects of the PV-DSF on the energy performance of a building according to the control method when the PV-DSF is introduced and the efficiency of the PV-DSF has also been proven [11–13]. On the other hand, when a PV module is applied to the DSF system, PV performance is improved and defects such as hot spots can be reduced [14]. Previous research has experimentally demonstrated that backside ventilation can lower the module temperature by 15 K ∼ 20 K if ventilation is sufficient [15], and it has also been previously shown that reducing the temperature of PV modules can increase power generation and decrease the risk of defects such as hot spots [16]. In other words, the integration of a PV module into a DSF allows sufficient ventilation in summer, thereby reducing the risk of defects such as overheating and hot spots. In addition, the air cavity depth of a DSF is an important determinant of the performance of the DSF. According to a previous
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Please cite this article as: C.-s. Lee and J. Yoon, Design optimization and experimental evaluation of photovoltaic double skin facade, Energy & Buildings, https://doi.org/10.1016/j.enbuild.2019.07.031
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Fig. 1. Example of application (left) of prefabricated lightweight PV-DSF to an existing building and example of control (right).
study [17], the air cavity depth for backside ventilation to optimize energy consumption and maintenance of the DSF system in the cool-summer Mediterranean climate zone is 0.4 m to 0.6 m, which was found to reduce electric energy consumption by 15% compared to the DSF system with an air cavity depth of 0.2 m. In addition, the introduction of a natural ventilation system can reduce electric energy consumption by 35% compared to a building without natural ventilation. In order to maximize the advantages of DSFs described above, it is also necessary to optimize the design of glass performance and optical performance of a DSF as well as the air cavity at the design stage. In addition, design optimization for the operating angle of the PV and establishment of the optimal operation method of the vent window are also required. Furthermore, since the design of DSFs and the operation of shading devices such as blinds have a significant impact on the indoor light environment and energy environment according to the operating method, there is also a need to establish appropriate strategies for them [12]. On the other hand, if PV and DSF systems are introduced into a building, construction costs are increased, and introducing PV and DSF systems into an existing building has a disadvantage that the building cannot be used during the construction period. Therefore, to introduce a DSF not only in the construction of new buildings but also in remodeling or renovation projects of existing buildings, we need to make it possible to perform installation work even while occupants are doing their work inside. Therefore, this study proposes a prefabricated lightweight PV-DSF module, which can be easily attached to and removed from existing buildings by complementing the disadvantages of the PV-DSF system related to construction, as shown in Fig. 1. The developed PV-DSF module is a removable system that can be installed while occupants are present indoors, and it improves the performance of building envelopes and produces energy. The PV-DSF module is designed so that the upper and lower parts can be opened and closed, and the PV module is applied to the upper and lower ventilation windows. The application position of the PV
module is a spandrel area between floors, and there is a space between PV-DSF modules to prevent the influence of shades during the course of a year considering the fact that the power generation performance of the PV module may be influenced by the partial shade generated when the PV ventilation window is opened and closed. In addition, the vision part that provides a view is equipped with a shading device to prevent direct solar radiation from entering the indoor space, and this shading device is controlled so as to minimize lighting energy according to internal and external environments. In order to maximize the performance of the proposed PV-DSF module, it is necessary to optimize the operation of the PV vent window and the design of the DSF. In an attempt to optimize the performance of the PV-DSF module, we analyzed the monthly optimal operating angles of the vent window to maximize power generation of the PV system, and we also derived the optimal design method of the PV-DSF to minimize cooling and heating energy demand through sensitivity analysis using simulation. In addition, we experimentally verified the energy performance of the building according to the application of the PV-DSF and the seasonal performance of the PV-DSF according to the operation of the vent window. For this purpose, in Section 2, the optimal operating angles of PV vent windows to be applied to the upper and lower parts of the PV-DSF were derived by analyzing the standard weather data. In Section 3, sensitivity analysis for the optical performance and thermal performance of DSFs to minimize the cooling and heating energy demand was performed through simulation. In addition, through a comparison with the results of Section 2, the study derived the optimal operating angle to maximize power generation and minimize cooling and heating energy demand. In Section 4, the performance of the PV-DSF according to the operation of the vent window was examined through experiments. In Section 5, the thermal performance and energy saving of the building depending on the application of the PV-DSF were experimentally verified.
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Fig. 2. Percentages of monthly irradiation by operating angle and annual irradiation by operating method.
2. Optimal operating angles of PV vent windows The PV-DSF system proposed in this study was based on the application to the south side to maximize power generation, and the maximum operating angle was limited to 30º considering the building code and wind load resistance. Generally, the PV power generation amount depends on the amount of irradiation reaching the PV cell, and the power generation amount of the PV-DSF depends on the operating angle of the PV system according to the changes of the altitude of the sun. In this study, the amount of irradiation was calculated depending on the operating angle with a maximum limit of 30° for the monthly operating angle based on the standard weather data of 30 years of Daejeon (latitude: 36º; longitude: 127º). In addition, the incident angle correction factor depending on the optical characteristics of the PV front glass was considered in the analysis, and the analysis results are shown in Fig. 2. As a result, it was found that when the vertically installed PV is operated at an angle of 0º ∼ 30º, the amount of irradiation is large from January to March and from October to December, whose solar altitude is low. During summer months (June to September), the monthly irradiation amount was 34% ∼ 53% smaller than the maximum value (tilt angle of 10º in December). In order to maximize power generation of the PV-DSF proposed in this study based on analysis results, the monthly optimal operating angle needs to be 10º ∼ 20º in winter and 30 º in summer and during intermediate seasons. It was found that if the PV vent window is optimally operated, the annual irradiation per unit area is estimated to be 1219 kWh/m2 .yr. A comparison with the case where the PV installation angle is fixed throughout the year showed that the amount of irradiation per year is decreased by 1% (30º) ∼ 17% (0º) depending on the fixed angle compared to when the optimal operating method is used. On the other hand, the operating method of the ventilation window proposed in this study allows Horizontal Axis Solar Tracking by time. In general, Horizontal Axis Solar Tracking is advantageous for tropical regions with high solar altitudes and short daytime [18], and is not economically suitable for mid-latitude regions like Daejeon in South Korea and the current system of which the operating angle is limited to 30°. Analysis was performed separately for the case where Horizontal Axis Solar Tracking was applied, and the comparison of the analysis results showed that the difference in irradiation between Horizontal Axis Solar Tracking and the method of monthly optimal operating angles as shown in Fig. 3 is less than 3%. Therefore, in terms of economical considerations, the application of Horizontal Axis Solar Tracking by time is not advantageous for the PV-DSF. In the case of the PV-DSF, the operating method of the vent window influences heating and cooling energy demand of the building as well as PV power generation. In Section 3 below, design optimization of the PV-DSF to optimize cooling and heating energy performance was performed by simulation, and the optimal oper-
Fig. 3. Monthly optimal operating angles of the PV-DSF system.
ating method for maximizing PV power generation and minimizing heating and cooling energy demand was derived. 3. Design optimization of PV-DSF 3.1. Building simulation model and calibration In this section, as a basic study to verify the performance of PV-DSF, design optimization of PV-DSF for a building actually used as an office building was carried out using building energy simulation and optimization technique [19]. For this purpose, a simulation analysis model was established using the construction drawing information of the building to which the PV-DSF would be applied and ESP-r, a building energy simulation analysis tool. In order to calibrate the analytical model, the thermal behavior of the room and the external weather conditions were monitored for 4 days in the absence of occupants and the analytical model was calibrated for days with sufficient irradiation and for days with insufficient irradiation, as shown in Fig. 4. In order to calibrate the analytical model, the outer wall temperature, the outer window temperature, and the indoor temperature of the room studied were measured and model calibration was performed based on the measured temperature data. The statistical methods used for calibration and verification of the simulation model are NMBE and CV-RMSE. These two indicators are statistical techniques which are mainly used to verify the validity of the simulation analysis model and evaluate the errors of measured values and predicted values by simulation. According to the ASHRAE Guideline 14 [20] and the FEMP Criteria [21], the simulation model for hourly measurement data is considered to have been calibrated if NMBE is within ±10% and if CV-RMSE is within ±30%. The calibration results of the energy simulation model are shown in Fig. 5. Simulation errors were the lowest on May 6 with low irradiation and large on May 4 and 5 with high irradiation.
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Fig. 4. Outdoor weather conditions measured for simulation model calibration (May 4–7).
Fig. 5. Simulation model calibration results (May 4 ∼ 7).
The calculated NMBE and CV-RMSE were both within the tolerance range, so the simulation analysis model was considered to have been calibrated. 3.2. Optimization of the design and operating method through sensitivity analysis The PV-DSF proposed in this study was applied to the calibrated simulation model to carry out modeling, as shown in Fig. 6. Modeling of the airflow by natural convection of the PV-DSF was conducted using air-flow network (AFN) [22] and design optimization to optimize cooling and heating performance of the PV-DSF was carried out through sensitivity analysis using simulation. Sensitivity analysis is to analyze the consequences of the change of a parameter by applying all the possible values of the parameter when the influence of the parameter on building energy demand is uncertain. Therefore, sensitivity analysis can be used to examine the impact of each of the PV-DSF design parameters on building en-
ergy performance [23]. This study used the global method which allows us to perform whole-input area search and self-verification in sensitivity analysis. Moreover, in order to reduce calculation time, we used the Latin hyper-sampling (LHS) method, which is most widely used and has been proven to be an efficient sampling technique in building optimization studies [24–28]. To reduce heating and cooling loads and improve the indoor environment by applying the DSF to the building, the control method of the DSF should be changed according to the seasons and it should be designed centered on the method which can reduce the total energy demand. Therefore, in order to optimize the cooling and heating performance of the BIPV DSF, we conducted sensitivity analysis for 1) the applicability of functional glass [29], and 2) the design and operation method of the DSF. The DSF width and the size of the vent window were selected as the factors for the design and operating method of the DSF which can minimize the building load. The transmissivity, reflectivity, and absorptivity of glass for near-infrared (NIR) radiation were selected as the indexes of
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Fig. 6. Sensitivity of DSF design parameters for minimization of monthly heating and cooling energy demand.
Table 1 Design parameters for sensitivity analysis and application ranges.
Vent area [m2 ] DSF width [m] NIR transmissivity [-] NIR reflectivity [-] NIR absorptivity [-]
Range
Step
0 ∼ 1.5 0.5 ∼ 1.5 0.1 ∼ 0.9 0.1 ∼ 0.9 0.1 ∼ 0.9
0.3 0.1 0.1 0.1 0.1
optical performance. In terms of the reduction of lighting energy use, it is advantageous to transmit as much visual radiation (VIS) as possible, and NIR radiation closely related to cooling energy demand needs to be optimized using coating technology [30,31]. Here, the ratios of visual radiation and near infrared radiation were assumed to be 50% of total irradiance, respectively. The range of design parameters considered for the sensitivity analysis is shown in Table 1 and sensitivity analysis was performed assuming the office building defined in ASHRAE Standard 90.1 [32]. The standardized rank regression coefficient was calculated as the sensitivity index for heating and cooling energy demand of design parameters, and the results are shown in Fig. 6. Here, the sensitivity index indicates the magnitude of the influence of a design variable on energy demand, and the +/- sign of sensitivity indexes indicates the correlation between the design variable and energy demand. That is, if the sensitivity index is a positive value (+), it indicates that energy demand increases as the size of the design variable increases, and if the sensitivity index is a negative value (-), it indicates that energy demand decreases as the design variable increases. Analysis results showed that the correlations between design variables and heating and cooling energy demand exhibit clearly opposite patterns in winter and summer. During winter months, in order to reduce heating energy demand, the vent area of the PV vent window needs to be minimized, and smaller values of NIR absorptivity and reflectivity were found to be more advantageous. Since the PV vent window of the PV-DSF serves as a shading device, a narrower width and higher NIR transmissivity of glass are more desirable for the reduction of heating energy demand. In contrast, in summer, in order to reduce cooling energy demand, the vent area of the PV vent window should be maximized, and the larger NIR absorptivity and reflectivity, the more advantageous they are. In addition, a larger DSF width and smaller NIR
transmissivity are more desirable. The design variables that have the greatest influence on the heating and cooling energy demand are NIR transmissivity and DSF width, followed by NIR reflectivity and the vent area. The highest sensitivity of DSF width for cooling and heating energy demand is attributed to the fact that the PV vent window installed in the spandrel part plays a role of a shading device, so if the width of the DSF is large, the cooling energy demand is reduced because of decreased indoor solar heat gain, but if the width is short, irradiation entering indoor spaces is increased and thus the heating load is reduced. The control of optical performance can reduce the heating energy demand by increasing transmissivity in winter and reduce the cooling energy demand by increasing reflectivity in summer. The control of NIR absorptivity is not very advantageous in terms of the reduction of cooling and heating energy demand due to its low sensitivity. On the other hand, the results of monthly sensitivity analysis indicated that the optimal operating method of the PV vent window are: 1) to close the upper and lower ventilation systems in winter, to reduce the heating energy demand by using the air cavity as a thermal buffer, and 2) to open the ventilation system to the maximum extent in summer to reduce the cooling energy demand. These operating methods are in agreement with the methods of operating PV vent windows to maximize PV power generation presented in Fig. 2 in Section 2. In Section 3 below, the thermal behavior of the DSF according to the operating method of the PV vent window was investigated through experiments. 4. Performance of the DSF depending on the operation of the PV vent window In order to perform an experiment to investigate the cooling and heating performance and power generation performance of the PV-DSF according to the operating method, a test PV-DSF was constructed on the south side of the room used as an office as shown in Fig. 7. Based on the results for sensitivity analysis in Section 3.2, a single color glass with a transmittance rate of about 60% was applied taking into consideration cooling and heating energy demand, the exterior view, and the prefabricated lightweight structure, and for the width of the DSF, a width of 650 mm was applied considering the appearance. The PV-DSF was built so that it would span the two identical rooms mentioned in Section 3.1, and it was possible to control each of PV vent windows of the two rooms
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Fig. 7. PV-DSF spanning two rooms built for evaluation of performance depending on the operating method.
Fig. 8. Experiment under the same conditions and the temperature distribution of the DSF depending on the operation of the vent window during an intermediate season.
separately. For the experiment, a barrier was installed in the middle of the DSF installed over two rooms so that air would not be mixed. The glass-to-glass PV module can be applied to a vent window to produce electricity throughout the year, and this system is operated to maximize temperature control and annual power generation performance. Taking into consideration electric energy production by operating method in Section 2 and heating and cooling energy demand by operating method in Section 3 at the same time, it is advantageous to operate PV vent windows with vent windows closed during the heating period (November to March) and with vent windows maximally open during the cooling period (May to September) in terms of reducing the total energy consumption. In this section, the study investigated how the performance of the DSF changes depending on whether or not the vent window is operated. Experiments were carried out to investigate the thermal behavior of the DSF depending on the operation of the vent window during the intermediate season (September), winter month (January) and summer month (June) 4.1. Performance of the DSF with vent window during an intermediate season An experiment to test the thermal performance of the DSF was conducted depending on the operating method of the vent window by using the DSF which spanned two rooms but had a partition
blocking air movement between the two rooms. The experiment was carried out for about 3 days from September 12 to 15, and for the first 1.5 days, both the vent windows of the DSF were closed to examine whether the thermal behavior was the same, but for the remaining 1.5 days, one vent window was left open to compare the air temperatures of the DSF depending on the operating method. As shown in Fig. 8, when the vent windows of both DSFs were closed for 1.5 days, the measurement results of the air temperature of the DSFs were almost the same, and so it was confirmed that the two DSFs have the same performance. When the vent window was closed, the inside temperature of the DSF was 12 °C higher than the outdoor temperature, and so it was found that cooling energy demand can be greatly increased if the vent window of the DSF is not operated. At noon on September 13 after the experiment under the same conditions, the thermal behavior of the left DSF, which was operated with the vent window open at the maximum operating angle of 30º, was compared with that of the DSF with the closed vent window. As a result of comparing the air temperature of the DSF with the outdoor temperature, the maximum temperature difference between the DSF and the outdoor temperatures was found to be 3 °C when the vent window was open, but 11 °C when the vent window was closed. In other words, if natural ventilation is not introduced in September, the temperature of the DSF was estimated to be higher by 8ºC than when natural ventilation is introduced, resulting in a significant increase in cooling energy demand. Therefore, it is possible to prevent the increase
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Fig. 9. Temperature distribution of the DSF depending on the application of the vent window in summer.
Fig 10. Temperature distribution of the DSF depending on the application of the vent window in winter.
of the cooling energy demand due to the introduction of the DSF if the vent windows of office buildings are appropriately used in September, which belongs to an intermediate season but is a cooling period. 4.2. Summer performance of the DSF depending on the operation of the vent window The experiment to examine the summer performance of the DSF depending on the operation of the vent windows was performed on June 22, when the outdoor temperature and irradiance were high, and the results are shown in Fig. 9. Experiment results showed that when the vent window was closed in summer, there was a temperature difference of up to 5 °C between the DSF and outdoor temperatures. However, when the vent window was operated, the temperature of the DSF became similar to the outdoor temperature. This is because the irradiance incident on the wall with an angle of 90° is not large (see Fig. 9) due to the high in-
cidence angle of the sun in summer, and the temperature rise of the DSF due to irradiation can be sufficiently removed by natural ventilation. Therefore, if vent windows are operated during summer months, the cooling energy demand due to the application of the DSF can be reduced and PV power generation can also be maximized. 4.3. Winter performance of the DSF depending on the operation of the vent window The experiment to evaluate the winter performance of the DSF according to the operation of the ventilation window was conducted on January 25, when the outdoor temperature and irradiation are very low, and the results are shown in Fig. 10. According to the experimental results, the maximum temperature difference between the two DSFs was 2 °C when the vent windows was operated at an operating angle of 10°, which is the maximum angle for PV generation in winter. The difference between
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period, and 0º was 1% ∼ 6. Considering the difference in the PV power generation amount and the amount of the increase of heating energy demand, it is favorable to operate the PV-DSF with the closed PV vent window during the winter season. Therefore, if the PV-DSF is applied and operated in a state where the PV vent window is closed, it is possible to achieve a significant reduction of heating energy demand due to the high air temperature of the DSF, and the indoor thermal comfort [34] is expected to be greatly improved by increasing the internal wall and window temperatures. 5. Building energy performance depending on the application of the PV-DSF
Fig. 11. Mock-up for the test of energy performance depending on the application of the PV-DSF.
the DSF and outdoor temperatures was about 12 °C when the vent window was closed and about 10 °C when the vent window was fully opened with an operating angle of 30° During the winter season, PV vent windows should be operated in consideration of the relationship between the increasing amount of heating energy consumption due to the opening and closing of the vent window and the PV power generation amount. The calculation results of irradiance depending on the operating angle of the PV window are presented in Fig. 2, which shows that the difference in irradiance between 10º, the optimum operating angle of the heating
Section 4 and previous studies [33,34] sufficiently demonstrated the possibility of reducing cooling and heating energy demand by the application of a DSF to building [35]. In this section, the possibility of reducing cooling energy consumption and the thermal behavior of a building was experimentally demonstrated when the PV-DSF system is applied. As shown in Fig. 11, a mock-up system was constructed to investigate the building energy performance depending on the application of the PV-DSF system. In the test mock-up, DSFs were installed on the east, south, and west sides to investigate power generation performance depending on the operating method of the PV vent window, the possibility of overheating of the PV-DSF, and building energy performance. Since the thermal performance of the PV-DSF installed in three orientations depends on the position of the sun, PV-DSF was separated by a partition wall for each orientation in order not to affect each other. The mock-up has 6 rooms with an identical size, and its cooling and heating energy performance depends on the operating method of the PV-DSF on the east side. Experiments to measure the thermal behavior of the building and the cooling energy consumption of the case where the PV-DSF was applied (With_PV-DSF) and the case where the PV-DSF was not applied (Without_PV-DSF) were performed with the two rooms located in the middle of the building. 5.1. Thermal behavior depending on the application of the PV-DSF The indoor temperatures with or without the application of the DSF were compared through statistical analysis of hourly temperature data without operating the cooler. The analysis was
Fig. 12. Distribution of indoor temperatures according to the application of the PV-DSF.
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Fig. 13. Distribution of internal wall temperatures according to the application of the PV-DSF.
Fig. 14. Temperature distribution of top/middle/bottom parts of PV-DSF.
performed for August, when irradiance is very high and outdoor temperatures are high. As a result, the mean indoor temperature difference between the two rooms was as small as about 0.2 °C. However, the mean indoor temperature between 8 and 10 a.m. was about 0.5 °C higher in Without_PV-DSF than in With_PV-DSF. The reason for the higher temperature of Without_PV-DS during these two hours was that the rooms are on the east side, and Without_PV-DS was more influenced by irradiance. The maximum temperature during daytime was also higher in Without _PV-DSF (Fig. 12). As for internal wall temperatures, Without_PV-DSF showed a higher temperature with a maximum temperature difference of 2 °C around 10 a.m. and it showed a higher temperature than With_PV-DSF throughout daytime, when irradiation and the outdoor temperature are high. . The reason for the low temperature of With_PV-DSF is due to shading effect from the PV-DSF. At night-
time without sunlight, both rooms showed similar wall temperatures (Fig. 13). When a DSF is applied, the increase in the cooling load and overheating of the upper part of the DSF needs to be considered. To do this, the temperatures of the top, middle, and bottom parts of the DSF of the test mock-up in the sultry month of August were measured. The experiment showed that the temperatures of middle and lower parts of the DSF were about 2 °C higher than the outdoor temperature during daytime. The temperature of the upper part was higher by a maximum of 8 °C than the outdoor temperature during daytime. The maximum temperature of the upper part was 54 °C, which is not high enough to cause an overheating problem, and the temperature distributions in the center and lower parts were similar to the outdoor temperatures. Therefore, if the vent window is appropriately used, the increase in cooling energy demand due to the DSF application can be reduced (Fig. 14).
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Fig. 15. Distribution of electric power depending on the application of PV-DSF.
Fig. 16. Distribution of indoor temperatures depending on the application of PV-DSF.
5.2. Cooling energy consumption depending on the application of the PV-DSF When applying the PV-DSF to a building, the experiments in Section 5.1 revealed that the introduction of natural ventilation along with the shading effect of the PV vent window can be advantageous in terms of cooling energy demand reduction compared to the conventional DSF system. In this section, in order to evaluate performance of cooling energy reduction depending on the application of the PV-DSF, a comparative experiment of cooling energy consumption was carried out for the room where the PV-DSF was installed (With_PV-DSF) and the room where the PV-DSF was not installed (Without_PV-DSF) in a mock-up. The experiment was conducted for two days on September 6 and 7, and the electric energy consumption per minute of the cooler was measured. As shown in Fig. 15 below, experimental results showed that the cooler of With_PV-DSF operated more intermittently and With_PV-DSF consumed 9% less electricity on September 6 and 13% less on September 7. This result is attributed to the fact that as the PV vent window operating at an operating angle of 30° served as a shading device, the amount of irradiation entering an indoor space
decreased, and that the DSF discharged collected heat to the outside through natural ventilation. Therefore, it was shown that the application of the PV-DSF can reduce cooling energy consumption. For the experiment of cooling energy reduction, the temperature of the cooler was set at 24 °C, and the set-point temperature was satisfied in all the experiments (See Fig. 16). During nighttime when the cooler does not operate, the air temperature of Without_PV-DSF was about 1 °C higher. Without_PV-DSF showed a higher rise in the indoor temperature on September 7 because irradiation was higher in the morning since the two rooms were located on the east side.
6. Conclusion In this study, we proposed prefabricated lightweight PV-DSF modules that can be attached to and removed from an existing building, and optimized DSF design and the operating method of the vent window to maximize power generation performance and energy performance of the building. In addition, we investigated the thermal behavior and energy performance of the DSF
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according to the operating method of the PV vent window through experiments. The results of this study are summarized as follows: • The analysis of monthly optimum operating angles of the PV vent window to be applied to the upper and lower parts of the PV-DSF showed that the optimal operating angle of the PV is 10º∼20º in winter and 30º in summer and during intermediate seasons. When the PV installation angle was fixed throughout the year, the amount of irradiation was decreased by 1% (30 º) ∼17% (0 º) per year according to the installation angel compared to when the optimal operation method was used. • Sensitivity analysis for optical performance and thermal performance of the DSF to minimize cooling and heating energy demand was performed using simulation, and the analysis results showed that the design variable that has the greatest influence on heating and cooling energy demand is NIR transmissivity, followed by DSF width, NIR reflectivity, and vent area. In addition, monthly optimal operating angles of the PV vent window to minimize cooling and heating energy demand were found to be in agreement with operating angles to maximize power generation. • With the PV-DSF applied to the south side, the performance of the DSF according to the operation of the PV vent window was investigated through experiments. As a result, when the PV vent window was opened during the intermediate season, the temperature difference between the PV-DSF and the outdoor temperature was found to be 3 °C or less and the temperature of the PV-DSF was the same as the outdoor temperature in summer. Thus, it is possible to prevent an increase in cooling energy demand due to the introduction of the DSF by operating the vent window. On the other hand, during winter months, when the PV ventilation window was closed, the difference between the PV-DSF and the outdoor temperatures was found to be 12 °C, showing that it is possible to achieve a large reduction of heating energy demand by the introduction of the DSF. • The thermal performance and cooling energy demand reduction performance of the building according to the application of the PV-DSF to the east side were investigated through experiments. Experimental results showed that the mean air and wall temperatures of the room to which the PV-DSF was applied were lower by 0.5 °C and 2 °C in August, respectively. In addition, the analysis of cooling energy consumption showed that the room with the PV-DSF consumed at least 10% less electric energy on average. Simulation and experimental results revealed that if the PVDSF and vent window are introduced and appropriately operated in existing buildings and new buildings, the system can be used not only to produce electric energy but also to reduce heating and cooling energy consumption. Especially, the PV-DSF system proposed in this study, which can be installed while occupants are present, is expected to greatly improve the performance of the building envelope if it is applied to remodeling or refurbishment projects of buildings. However, further research is required to address several related questions. • Above all, building energy performance and building environment performance depending on the operating method of the shading device [36,37] need to be investigated through experiments, although they were not dealt with in this study. • The performance of the PV-DSF depends on the design and control. Therefore, the performance of the building environment such as thermal and visual comfort should be investigated by PV-DSF design and operation. • It is necessary to examine how much the performance of the PV power generation, which can reduce temperatures by the introduction of natural ventilation in PV-DSF, is more advantageous
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compared to the conventional BIPV system which does not have backside ventilation. • Different types of buildings require different ways of designing and operating the PV-DSF. Therefore, more research is required to investigate the performance of the PV-DSF depending on the use, orientation, insulation performance, and windowto-wall ratio of the building as in previous studies [34,38]. Simulation models allow to perform parametric studies and performance evaluations of PV-DSF in a short period of time, and optimal designs can be derived through detailed simulations or small experiments. Declaration of Competing Interest None. Acknowledgement This research was supported by 1) the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (No. NRF-2016M1A2A2936758), and 2) the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20153010130320). Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.enbuild.2019.07.031. Reference [1] A. Sandak, J. Sandak, M. Brzezicki, A. Kutnar, State of the art in building façades, Bio-based Building Skin. Environmental Footprints and Eco-design of Products and Processes, Springer, Singapore, 2019. [2] Suresh B. Sadineni, Srikanth Madala, Robert F. Boehm, Passive building energy savings: a review of building envelope components, Renew. Sustain. Energy Rev. 15 (8) (2011) 3617–3631. [3] Im Hatice Sozer, Improving energy efficiency through the design of the building envelope, Build. Environ. 45 (12) (2010) 2581–2593. [4] R. Pacheco, J. Ordóñez, G. Martínez, Energy efficient design of building: a review, Renew. Sustain. Energy Rev. 16 (6) (2012) 3559–3573. [5] Bjørn Petter Jelle, Christer Breivik, Hilde Drolsum Røkenes, Building integrated photovoltaic products: a state-of-the-art review and future research opportunities, Sol. Energy Mater. Sol. Cells 100 (2012) 69–96. [6] H. Jouhara, J. Milko, J. Danielewicz, M.A. Sayegh, M. Szulgowska-Zgrzywa, J.B. Ramos, S.P. Lester, The performance of a novel flat heat pipe based thermal and PV/T (photovoltaic and thermal systems) solar collector that can be used as an energy-active building envelope material, Energy 108 (2016) 148–154. [7] R.C.G.M. Loonen, M. Trcˇ ka, D. Cóstola, J.L.M. Hensen, Climate adaptive building shells: state-of-the-art and future challenges, Renew. Sustain. Energy Rev. 25 (2013) 483–493. [8] Ziyi Su, Xiaofeng Li, Fei Xue, Double-skin façade optimization design for different climate zones in China, Sol. Energy 155 (2017) 281–290. [9] E. Gratia, A. De Herde, Are energy consumptions decreased with the addition of a double-skin? Energy Build. 39 (2007) 605–619. [10] M.A. Shameri, K. Alghoul, M.F.M. Zain, O. Elayeb, Perspectives of double skin façade systems in buildings and energy saving renew, Sustain. Energy Rev. 15 (3) (2011) 1468–1475. [11] Z. Ioannidis, A. Buonomano, A.K. Athienitis, T. Stathopoulos, Modeling of double skin façades integrating photovoltaic panels and automated roller shades: analysis of the thermal and electrical performance, Energy Build. 154 (2017) 618–632. [12] D. Kim, S.J. Cox, H.∗ Cho, J.∗ Yoon, "Comparative investigation on building energy performance of double skin façade (DSF) with interior or exterior slat blinds, J. Build. Eng. 20 (2018) 411–423 2018. [13] J. Cipriano, G. Houzeaux, D. Chemisana, C. Lodi, J. Martí-Herrero, Numerical analysis of the most appropriate heat transfer correlations for free ventilated double skin photovoltaic façades, Appl. Therm. Eng. 57 (2013) 57–68. [14] R. Charron, A.K. Athienitis, Optimization of the performance of double-façades with integrated photovoltaic panels and motorized blinds, Sol. Energy 80 (2006) 482–491. [15] B.J Brinkworth, B.M Cross, R.H Marshall, Hongxing Yang, Thermal regulation of photovoltaic cladding, Sol. Energy 61 (3) (1997) 169–178. [16] Guohui Gan, Effect of air gap on the performance of building-integrated photovoltaics, Energy 34 (7) (2009) 913–921.
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Energy & Buildings journal homepage: www.elsevier.com/locate/enbuild
Design optimization and experimental evaluation of photovoltaic double skin facade Chul-sung Lee, Jongho Yoon∗ Department of Architectural Engineering, Hanbat National University, San 16-1 Duckmyung-dong, Yuseong-gu, Daejeon 305-719 Republic of Korea
a r t i c l e
i n f o
Article history: Received 7 May 2019 Revised 11 July 2019 Accepted 16 July 2019 Available online xxx Keywords: Design optimization Photovoltaic double skin façade (PV-DSF) Photovoltaic vent window Optimal operating angle Heating and cooling energy consumption
a b s t r a c t This study proposed the introduction of a photovoltaic double skin façade (PV-DSF) as a way to economically and easily improve the performance of building envelopes. This system has been designed to be installed not only in new buildings but also in remodeling or renovation projects of existing buildings even in a state where occupants are present inside. For the PV-DSF system suggested, this study aimed to perform experimental evaluation of 1) the optimal operation method to maximize electric energy production of the PV vent window and to reduce the heating and cooling energy consumption of buildings and 2) the thermal behavior and energy performance of the building when the proposed PV-DSF is applied. Analysis results using simulation have shown that the monthly optimal operating angles of the PV vent window to minimize heating and cooling energy demand were the same as the operating angles to maximize power generation. It has also been proven through experiments that the proper operation of the PV vent window can significantly reduce the energy demand of buildings. In particular, the difference between the DSF and outdoor temperatures was 12 °C in winter, showing that it is possible to achieve a significant reduction of heating energy demand. Lastly, when the PV-DSF was applied, air and wall temperatures were decreased by 0.5 °C and 2 °C in summer, respectively, and cooling energy consumption was also at least 10% reduced on average. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Studies have been carried out to reduce building energy demand by improving the performance of building envelopes with the aim of reducing carbon dioxide emissions [1–4]. In addition, attempts have been made to optimize already developed technologies through convergence of existing technologies rather than using a single technology [5–7]. In order to maintain relatively constant indoor environments in buildings in an energy-efficient way under the circumstances of changing external environments, it is necessary to 1) receive sufficient external resources that can be utilized in building envelopes and 2) block unnecessary elements from entering the indoor spaces of buildings. In other words, to reduce energy consumption, the performance of building envelopes needs to be changed according to changing external environments. The double skin façade (DSF) is used not only for the reduction of energy consumption by making a buffer space between the outer and inner spaces, and also for purposes such as preheating, night cooling, noise reduction, etc. [8]. Recently, as part of studies on convergence of renewable energies, researches on PV-DSF
∗
Corresponding author. E-mail address: [email protected] (J. Yoon).
convergence have been carried out to reduce building energy consumption though the improvement of the performance of building envelopes by introducing the DSF and to produce electric energy using the PV [9,10]. Since a PV system is incorporated into a DSF, the PV-DSF can be another alternative method for introducing a new and renewable energy system in the case of buildings where installation locations for introducing the BIPV system are limited. Studies have been conducted to investigate the effects of the PV-DSF on the energy performance of a building according to the control method when the PV-DSF is introduced and the efficiency of the PV-DSF has also been proven [11–13]. On the other hand, when a PV module is applied to the DSF system, PV performance is improved and defects such as hot spots can be reduced [14]. Previous research has experimentally demonstrated that backside ventilation can lower the module temperature by 15 K ∼ 20 K if ventilation is sufficient [15], and it has also been previously shown that reducing the temperature of PV modules can increase power generation and decrease the risk of defects such as hot spots [16]. In other words, the integration of a PV module into a DSF allows sufficient ventilation in summer, thereby reducing the risk of defects such as overheating and hot spots. In addition, the air cavity depth of a DSF is an important determinant of the performance of the DSF. According to a previous
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Fig. 1. Example of application (left) of prefabricated lightweight PV-DSF to an existing building and example of control (right).
study [17], the air cavity depth for backside ventilation to optimize energy consumption and maintenance of the DSF system in the cool-summer Mediterranean climate zone is 0.4 m to 0.6 m, which was found to reduce electric energy consumption by 15% compared to the DSF system with an air cavity depth of 0.2 m. In addition, the introduction of a natural ventilation system can reduce electric energy consumption by 35% compared to a building without natural ventilation. In order to maximize the advantages of DSFs described above, it is also necessary to optimize the design of glass performance and optical performance of a DSF as well as the air cavity at the design stage. In addition, design optimization for the operating angle of the PV and establishment of the optimal operation method of the vent window are also required. Furthermore, since the design of DSFs and the operation of shading devices such as blinds have a significant impact on the indoor light environment and energy environment according to the operating method, there is also a need to establish appropriate strategies for them [12]. On the other hand, if PV and DSF systems are introduced into a building, construction costs are increased, and introducing PV and DSF systems into an existing building has a disadvantage that the building cannot be used during the construction period. Therefore, to introduce a DSF not only in the construction of new buildings but also in remodeling or renovation projects of existing buildings, we need to make it possible to perform installation work even while occupants are doing their work inside. Therefore, this study proposes a prefabricated lightweight PV-DSF module, which can be easily attached to and removed from existing buildings by complementing the disadvantages of the PV-DSF system related to construction, as shown in Fig. 1. The developed PV-DSF module is a removable system that can be installed while occupants are present indoors, and it improves the performance of building envelopes and produces energy. The PV-DSF module is designed so that the upper and lower parts can be opened and closed, and the PV module is applied to the upper and lower ventilation windows. The application position of the PV
module is a spandrel area between floors, and there is a space between PV-DSF modules to prevent the influence of shades during the course of a year considering the fact that the power generation performance of the PV module may be influenced by the partial shade generated when the PV ventilation window is opened and closed. In addition, the vision part that provides a view is equipped with a shading device to prevent direct solar radiation from entering the indoor space, and this shading device is controlled so as to minimize lighting energy according to internal and external environments. In order to maximize the performance of the proposed PV-DSF module, it is necessary to optimize the operation of the PV vent window and the design of the DSF. In an attempt to optimize the performance of the PV-DSF module, we analyzed the monthly optimal operating angles of the vent window to maximize power generation of the PV system, and we also derived the optimal design method of the PV-DSF to minimize cooling and heating energy demand through sensitivity analysis using simulation. In addition, we experimentally verified the energy performance of the building according to the application of the PV-DSF and the seasonal performance of the PV-DSF according to the operation of the vent window. For this purpose, in Section 2, the optimal operating angles of PV vent windows to be applied to the upper and lower parts of the PV-DSF were derived by analyzing the standard weather data. In Section 3, sensitivity analysis for the optical performance and thermal performance of DSFs to minimize the cooling and heating energy demand was performed through simulation. In addition, through a comparison with the results of Section 2, the study derived the optimal operating angle to maximize power generation and minimize cooling and heating energy demand. In Section 4, the performance of the PV-DSF according to the operation of the vent window was examined through experiments. In Section 5, the thermal performance and energy saving of the building depending on the application of the PV-DSF were experimentally verified.
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Fig. 2. Percentages of monthly irradiation by operating angle and annual irradiation by operating method.
2. Optimal operating angles of PV vent windows The PV-DSF system proposed in this study was based on the application to the south side to maximize power generation, and the maximum operating angle was limited to 30º considering the building code and wind load resistance. Generally, the PV power generation amount depends on the amount of irradiation reaching the PV cell, and the power generation amount of the PV-DSF depends on the operating angle of the PV system according to the changes of the altitude of the sun. In this study, the amount of irradiation was calculated depending on the operating angle with a maximum limit of 30° for the monthly operating angle based on the standard weather data of 30 years of Daejeon (latitude: 36º; longitude: 127º). In addition, the incident angle correction factor depending on the optical characteristics of the PV front glass was considered in the analysis, and the analysis results are shown in Fig. 2. As a result, it was found that when the vertically installed PV is operated at an angle of 0º ∼ 30º, the amount of irradiation is large from January to March and from October to December, whose solar altitude is low. During summer months (June to September), the monthly irradiation amount was 34% ∼ 53% smaller than the maximum value (tilt angle of 10º in December). In order to maximize power generation of the PV-DSF proposed in this study based on analysis results, the monthly optimal operating angle needs to be 10º ∼ 20º in winter and 30 º in summer and during intermediate seasons. It was found that if the PV vent window is optimally operated, the annual irradiation per unit area is estimated to be 1219 kWh/m2 .yr. A comparison with the case where the PV installation angle is fixed throughout the year showed that the amount of irradiation per year is decreased by 1% (30º) ∼ 17% (0º) depending on the fixed angle compared to when the optimal operating method is used. On the other hand, the operating method of the ventilation window proposed in this study allows Horizontal Axis Solar Tracking by time. In general, Horizontal Axis Solar Tracking is advantageous for tropical regions with high solar altitudes and short daytime [18], and is not economically suitable for mid-latitude regions like Daejeon in South Korea and the current system of which the operating angle is limited to 30°. Analysis was performed separately for the case where Horizontal Axis Solar Tracking was applied, and the comparison of the analysis results showed that the difference in irradiation between Horizontal Axis Solar Tracking and the method of monthly optimal operating angles as shown in Fig. 3 is less than 3%. Therefore, in terms of economical considerations, the application of Horizontal Axis Solar Tracking by time is not advantageous for the PV-DSF. In the case of the PV-DSF, the operating method of the vent window influences heating and cooling energy demand of the building as well as PV power generation. In Section 3 below, design optimization of the PV-DSF to optimize cooling and heating energy performance was performed by simulation, and the optimal oper-
Fig. 3. Monthly optimal operating angles of the PV-DSF system.
ating method for maximizing PV power generation and minimizing heating and cooling energy demand was derived. 3. Design optimization of PV-DSF 3.1. Building simulation model and calibration In this section, as a basic study to verify the performance of PV-DSF, design optimization of PV-DSF for a building actually used as an office building was carried out using building energy simulation and optimization technique [19]. For this purpose, a simulation analysis model was established using the construction drawing information of the building to which the PV-DSF would be applied and ESP-r, a building energy simulation analysis tool. In order to calibrate the analytical model, the thermal behavior of the room and the external weather conditions were monitored for 4 days in the absence of occupants and the analytical model was calibrated for days with sufficient irradiation and for days with insufficient irradiation, as shown in Fig. 4. In order to calibrate the analytical model, the outer wall temperature, the outer window temperature, and the indoor temperature of the room studied were measured and model calibration was performed based on the measured temperature data. The statistical methods used for calibration and verification of the simulation model are NMBE and CV-RMSE. These two indicators are statistical techniques which are mainly used to verify the validity of the simulation analysis model and evaluate the errors of measured values and predicted values by simulation. According to the ASHRAE Guideline 14 [20] and the FEMP Criteria [21], the simulation model for hourly measurement data is considered to have been calibrated if NMBE is within ±10% and if CV-RMSE is within ±30%. The calibration results of the energy simulation model are shown in Fig. 5. Simulation errors were the lowest on May 6 with low irradiation and large on May 4 and 5 with high irradiation.
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Fig. 4. Outdoor weather conditions measured for simulation model calibration (May 4–7).
Fig. 5. Simulation model calibration results (May 4 ∼ 7).
The calculated NMBE and CV-RMSE were both within the tolerance range, so the simulation analysis model was considered to have been calibrated. 3.2. Optimization of the design and operating method through sensitivity analysis The PV-DSF proposed in this study was applied to the calibrated simulation model to carry out modeling, as shown in Fig. 6. Modeling of the airflow by natural convection of the PV-DSF was conducted using air-flow network (AFN) [22] and design optimization to optimize cooling and heating performance of the PV-DSF was carried out through sensitivity analysis using simulation. Sensitivity analysis is to analyze the consequences of the change of a parameter by applying all the possible values of the parameter when the influence of the parameter on building energy demand is uncertain. Therefore, sensitivity analysis can be used to examine the impact of each of the PV-DSF design parameters on building en-
ergy performance [23]. This study used the global method which allows us to perform whole-input area search and self-verification in sensitivity analysis. Moreover, in order to reduce calculation time, we used the Latin hyper-sampling (LHS) method, which is most widely used and has been proven to be an efficient sampling technique in building optimization studies [24–28]. To reduce heating and cooling loads and improve the indoor environment by applying the DSF to the building, the control method of the DSF should be changed according to the seasons and it should be designed centered on the method which can reduce the total energy demand. Therefore, in order to optimize the cooling and heating performance of the BIPV DSF, we conducted sensitivity analysis for 1) the applicability of functional glass [29], and 2) the design and operation method of the DSF. The DSF width and the size of the vent window were selected as the factors for the design and operating method of the DSF which can minimize the building load. The transmissivity, reflectivity, and absorptivity of glass for near-infrared (NIR) radiation were selected as the indexes of
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Fig. 6. Sensitivity of DSF design parameters for minimization of monthly heating and cooling energy demand.
Table 1 Design parameters for sensitivity analysis and application ranges.
Vent area [m2 ] DSF width [m] NIR transmissivity [-] NIR reflectivity [-] NIR absorptivity [-]
Range
Step
0 ∼ 1.5 0.5 ∼ 1.5 0.1 ∼ 0.9 0.1 ∼ 0.9 0.1 ∼ 0.9
0.3 0.1 0.1 0.1 0.1
optical performance. In terms of the reduction of lighting energy use, it is advantageous to transmit as much visual radiation (VIS) as possible, and NIR radiation closely related to cooling energy demand needs to be optimized using coating technology [30,31]. Here, the ratios of visual radiation and near infrared radiation were assumed to be 50% of total irradiance, respectively. The range of design parameters considered for the sensitivity analysis is shown in Table 1 and sensitivity analysis was performed assuming the office building defined in ASHRAE Standard 90.1 [32]. The standardized rank regression coefficient was calculated as the sensitivity index for heating and cooling energy demand of design parameters, and the results are shown in Fig. 6. Here, the sensitivity index indicates the magnitude of the influence of a design variable on energy demand, and the +/- sign of sensitivity indexes indicates the correlation between the design variable and energy demand. That is, if the sensitivity index is a positive value (+), it indicates that energy demand increases as the size of the design variable increases, and if the sensitivity index is a negative value (-), it indicates that energy demand decreases as the design variable increases. Analysis results showed that the correlations between design variables and heating and cooling energy demand exhibit clearly opposite patterns in winter and summer. During winter months, in order to reduce heating energy demand, the vent area of the PV vent window needs to be minimized, and smaller values of NIR absorptivity and reflectivity were found to be more advantageous. Since the PV vent window of the PV-DSF serves as a shading device, a narrower width and higher NIR transmissivity of glass are more desirable for the reduction of heating energy demand. In contrast, in summer, in order to reduce cooling energy demand, the vent area of the PV vent window should be maximized, and the larger NIR absorptivity and reflectivity, the more advantageous they are. In addition, a larger DSF width and smaller NIR
transmissivity are more desirable. The design variables that have the greatest influence on the heating and cooling energy demand are NIR transmissivity and DSF width, followed by NIR reflectivity and the vent area. The highest sensitivity of DSF width for cooling and heating energy demand is attributed to the fact that the PV vent window installed in the spandrel part plays a role of a shading device, so if the width of the DSF is large, the cooling energy demand is reduced because of decreased indoor solar heat gain, but if the width is short, irradiation entering indoor spaces is increased and thus the heating load is reduced. The control of optical performance can reduce the heating energy demand by increasing transmissivity in winter and reduce the cooling energy demand by increasing reflectivity in summer. The control of NIR absorptivity is not very advantageous in terms of the reduction of cooling and heating energy demand due to its low sensitivity. On the other hand, the results of monthly sensitivity analysis indicated that the optimal operating method of the PV vent window are: 1) to close the upper and lower ventilation systems in winter, to reduce the heating energy demand by using the air cavity as a thermal buffer, and 2) to open the ventilation system to the maximum extent in summer to reduce the cooling energy demand. These operating methods are in agreement with the methods of operating PV vent windows to maximize PV power generation presented in Fig. 2 in Section 2. In Section 3 below, the thermal behavior of the DSF according to the operating method of the PV vent window was investigated through experiments. 4. Performance of the DSF depending on the operation of the PV vent window In order to perform an experiment to investigate the cooling and heating performance and power generation performance of the PV-DSF according to the operating method, a test PV-DSF was constructed on the south side of the room used as an office as shown in Fig. 7. Based on the results for sensitivity analysis in Section 3.2, a single color glass with a transmittance rate of about 60% was applied taking into consideration cooling and heating energy demand, the exterior view, and the prefabricated lightweight structure, and for the width of the DSF, a width of 650 mm was applied considering the appearance. The PV-DSF was built so that it would span the two identical rooms mentioned in Section 3.1, and it was possible to control each of PV vent windows of the two rooms
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Fig. 7. PV-DSF spanning two rooms built for evaluation of performance depending on the operating method.
Fig. 8. Experiment under the same conditions and the temperature distribution of the DSF depending on the operation of the vent window during an intermediate season.
separately. For the experiment, a barrier was installed in the middle of the DSF installed over two rooms so that air would not be mixed. The glass-to-glass PV module can be applied to a vent window to produce electricity throughout the year, and this system is operated to maximize temperature control and annual power generation performance. Taking into consideration electric energy production by operating method in Section 2 and heating and cooling energy demand by operating method in Section 3 at the same time, it is advantageous to operate PV vent windows with vent windows closed during the heating period (November to March) and with vent windows maximally open during the cooling period (May to September) in terms of reducing the total energy consumption. In this section, the study investigated how the performance of the DSF changes depending on whether or not the vent window is operated. Experiments were carried out to investigate the thermal behavior of the DSF depending on the operation of the vent window during the intermediate season (September), winter month (January) and summer month (June) 4.1. Performance of the DSF with vent window during an intermediate season An experiment to test the thermal performance of the DSF was conducted depending on the operating method of the vent window by using the DSF which spanned two rooms but had a partition
blocking air movement between the two rooms. The experiment was carried out for about 3 days from September 12 to 15, and for the first 1.5 days, both the vent windows of the DSF were closed to examine whether the thermal behavior was the same, but for the remaining 1.5 days, one vent window was left open to compare the air temperatures of the DSF depending on the operating method. As shown in Fig. 8, when the vent windows of both DSFs were closed for 1.5 days, the measurement results of the air temperature of the DSFs were almost the same, and so it was confirmed that the two DSFs have the same performance. When the vent window was closed, the inside temperature of the DSF was 12 °C higher than the outdoor temperature, and so it was found that cooling energy demand can be greatly increased if the vent window of the DSF is not operated. At noon on September 13 after the experiment under the same conditions, the thermal behavior of the left DSF, which was operated with the vent window open at the maximum operating angle of 30º, was compared with that of the DSF with the closed vent window. As a result of comparing the air temperature of the DSF with the outdoor temperature, the maximum temperature difference between the DSF and the outdoor temperatures was found to be 3 °C when the vent window was open, but 11 °C when the vent window was closed. In other words, if natural ventilation is not introduced in September, the temperature of the DSF was estimated to be higher by 8ºC than when natural ventilation is introduced, resulting in a significant increase in cooling energy demand. Therefore, it is possible to prevent the increase
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Fig. 9. Temperature distribution of the DSF depending on the application of the vent window in summer.
Fig 10. Temperature distribution of the DSF depending on the application of the vent window in winter.
of the cooling energy demand due to the introduction of the DSF if the vent windows of office buildings are appropriately used in September, which belongs to an intermediate season but is a cooling period. 4.2. Summer performance of the DSF depending on the operation of the vent window The experiment to examine the summer performance of the DSF depending on the operation of the vent windows was performed on June 22, when the outdoor temperature and irradiance were high, and the results are shown in Fig. 9. Experiment results showed that when the vent window was closed in summer, there was a temperature difference of up to 5 °C between the DSF and outdoor temperatures. However, when the vent window was operated, the temperature of the DSF became similar to the outdoor temperature. This is because the irradiance incident on the wall with an angle of 90° is not large (see Fig. 9) due to the high in-
cidence angle of the sun in summer, and the temperature rise of the DSF due to irradiation can be sufficiently removed by natural ventilation. Therefore, if vent windows are operated during summer months, the cooling energy demand due to the application of the DSF can be reduced and PV power generation can also be maximized. 4.3. Winter performance of the DSF depending on the operation of the vent window The experiment to evaluate the winter performance of the DSF according to the operation of the ventilation window was conducted on January 25, when the outdoor temperature and irradiation are very low, and the results are shown in Fig. 10. According to the experimental results, the maximum temperature difference between the two DSFs was 2 °C when the vent windows was operated at an operating angle of 10°, which is the maximum angle for PV generation in winter. The difference between
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period, and 0º was 1% ∼ 6. Considering the difference in the PV power generation amount and the amount of the increase of heating energy demand, it is favorable to operate the PV-DSF with the closed PV vent window during the winter season. Therefore, if the PV-DSF is applied and operated in a state where the PV vent window is closed, it is possible to achieve a significant reduction of heating energy demand due to the high air temperature of the DSF, and the indoor thermal comfort [34] is expected to be greatly improved by increasing the internal wall and window temperatures. 5. Building energy performance depending on the application of the PV-DSF
Fig. 11. Mock-up for the test of energy performance depending on the application of the PV-DSF.
the DSF and outdoor temperatures was about 12 °C when the vent window was closed and about 10 °C when the vent window was fully opened with an operating angle of 30° During the winter season, PV vent windows should be operated in consideration of the relationship between the increasing amount of heating energy consumption due to the opening and closing of the vent window and the PV power generation amount. The calculation results of irradiance depending on the operating angle of the PV window are presented in Fig. 2, which shows that the difference in irradiance between 10º, the optimum operating angle of the heating
Section 4 and previous studies [33,34] sufficiently demonstrated the possibility of reducing cooling and heating energy demand by the application of a DSF to building [35]. In this section, the possibility of reducing cooling energy consumption and the thermal behavior of a building was experimentally demonstrated when the PV-DSF system is applied. As shown in Fig. 11, a mock-up system was constructed to investigate the building energy performance depending on the application of the PV-DSF system. In the test mock-up, DSFs were installed on the east, south, and west sides to investigate power generation performance depending on the operating method of the PV vent window, the possibility of overheating of the PV-DSF, and building energy performance. Since the thermal performance of the PV-DSF installed in three orientations depends on the position of the sun, PV-DSF was separated by a partition wall for each orientation in order not to affect each other. The mock-up has 6 rooms with an identical size, and its cooling and heating energy performance depends on the operating method of the PV-DSF on the east side. Experiments to measure the thermal behavior of the building and the cooling energy consumption of the case where the PV-DSF was applied (With_PV-DSF) and the case where the PV-DSF was not applied (Without_PV-DSF) were performed with the two rooms located in the middle of the building. 5.1. Thermal behavior depending on the application of the PV-DSF The indoor temperatures with or without the application of the DSF were compared through statistical analysis of hourly temperature data without operating the cooler. The analysis was
Fig. 12. Distribution of indoor temperatures according to the application of the PV-DSF.
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Fig. 13. Distribution of internal wall temperatures according to the application of the PV-DSF.
Fig. 14. Temperature distribution of top/middle/bottom parts of PV-DSF.
performed for August, when irradiance is very high and outdoor temperatures are high. As a result, the mean indoor temperature difference between the two rooms was as small as about 0.2 °C. However, the mean indoor temperature between 8 and 10 a.m. was about 0.5 °C higher in Without_PV-DSF than in With_PV-DSF. The reason for the higher temperature of Without_PV-DS during these two hours was that the rooms are on the east side, and Without_PV-DS was more influenced by irradiance. The maximum temperature during daytime was also higher in Without _PV-DSF (Fig. 12). As for internal wall temperatures, Without_PV-DSF showed a higher temperature with a maximum temperature difference of 2 °C around 10 a.m. and it showed a higher temperature than With_PV-DSF throughout daytime, when irradiation and the outdoor temperature are high. . The reason for the low temperature of With_PV-DSF is due to shading effect from the PV-DSF. At night-
time without sunlight, both rooms showed similar wall temperatures (Fig. 13). When a DSF is applied, the increase in the cooling load and overheating of the upper part of the DSF needs to be considered. To do this, the temperatures of the top, middle, and bottom parts of the DSF of the test mock-up in the sultry month of August were measured. The experiment showed that the temperatures of middle and lower parts of the DSF were about 2 °C higher than the outdoor temperature during daytime. The temperature of the upper part was higher by a maximum of 8 °C than the outdoor temperature during daytime. The maximum temperature of the upper part was 54 °C, which is not high enough to cause an overheating problem, and the temperature distributions in the center and lower parts were similar to the outdoor temperatures. Therefore, if the vent window is appropriately used, the increase in cooling energy demand due to the DSF application can be reduced (Fig. 14).
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Fig. 15. Distribution of electric power depending on the application of PV-DSF.
Fig. 16. Distribution of indoor temperatures depending on the application of PV-DSF.
5.2. Cooling energy consumption depending on the application of the PV-DSF When applying the PV-DSF to a building, the experiments in Section 5.1 revealed that the introduction of natural ventilation along with the shading effect of the PV vent window can be advantageous in terms of cooling energy demand reduction compared to the conventional DSF system. In this section, in order to evaluate performance of cooling energy reduction depending on the application of the PV-DSF, a comparative experiment of cooling energy consumption was carried out for the room where the PV-DSF was installed (With_PV-DSF) and the room where the PV-DSF was not installed (Without_PV-DSF) in a mock-up. The experiment was conducted for two days on September 6 and 7, and the electric energy consumption per minute of the cooler was measured. As shown in Fig. 15 below, experimental results showed that the cooler of With_PV-DSF operated more intermittently and With_PV-DSF consumed 9% less electricity on September 6 and 13% less on September 7. This result is attributed to the fact that as the PV vent window operating at an operating angle of 30° served as a shading device, the amount of irradiation entering an indoor space
decreased, and that the DSF discharged collected heat to the outside through natural ventilation. Therefore, it was shown that the application of the PV-DSF can reduce cooling energy consumption. For the experiment of cooling energy reduction, the temperature of the cooler was set at 24 °C, and the set-point temperature was satisfied in all the experiments (See Fig. 16). During nighttime when the cooler does not operate, the air temperature of Without_PV-DSF was about 1 °C higher. Without_PV-DSF showed a higher rise in the indoor temperature on September 7 because irradiation was higher in the morning since the two rooms were located on the east side.
6. Conclusion In this study, we proposed prefabricated lightweight PV-DSF modules that can be attached to and removed from an existing building, and optimized DSF design and the operating method of the vent window to maximize power generation performance and energy performance of the building. In addition, we investigated the thermal behavior and energy performance of the DSF
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according to the operating method of the PV vent window through experiments. The results of this study are summarized as follows: • The analysis of monthly optimum operating angles of the PV vent window to be applied to the upper and lower parts of the PV-DSF showed that the optimal operating angle of the PV is 10º∼20º in winter and 30º in summer and during intermediate seasons. When the PV installation angle was fixed throughout the year, the amount of irradiation was decreased by 1% (30 º) ∼17% (0 º) per year according to the installation angel compared to when the optimal operation method was used. • Sensitivity analysis for optical performance and thermal performance of the DSF to minimize cooling and heating energy demand was performed using simulation, and the analysis results showed that the design variable that has the greatest influence on heating and cooling energy demand is NIR transmissivity, followed by DSF width, NIR reflectivity, and vent area. In addition, monthly optimal operating angles of the PV vent window to minimize cooling and heating energy demand were found to be in agreement with operating angles to maximize power generation. • With the PV-DSF applied to the south side, the performance of the DSF according to the operation of the PV vent window was investigated through experiments. As a result, when the PV vent window was opened during the intermediate season, the temperature difference between the PV-DSF and the outdoor temperature was found to be 3 °C or less and the temperature of the PV-DSF was the same as the outdoor temperature in summer. Thus, it is possible to prevent an increase in cooling energy demand due to the introduction of the DSF by operating the vent window. On the other hand, during winter months, when the PV ventilation window was closed, the difference between the PV-DSF and the outdoor temperatures was found to be 12 °C, showing that it is possible to achieve a large reduction of heating energy demand by the introduction of the DSF. • The thermal performance and cooling energy demand reduction performance of the building according to the application of the PV-DSF to the east side were investigated through experiments. Experimental results showed that the mean air and wall temperatures of the room to which the PV-DSF was applied were lower by 0.5 °C and 2 °C in August, respectively. In addition, the analysis of cooling energy consumption showed that the room with the PV-DSF consumed at least 10% less electric energy on average. Simulation and experimental results revealed that if the PVDSF and vent window are introduced and appropriately operated in existing buildings and new buildings, the system can be used not only to produce electric energy but also to reduce heating and cooling energy consumption. Especially, the PV-DSF system proposed in this study, which can be installed while occupants are present, is expected to greatly improve the performance of the building envelope if it is applied to remodeling or refurbishment projects of buildings. However, further research is required to address several related questions. • Above all, building energy performance and building environment performance depending on the operating method of the shading device [36,37] need to be investigated through experiments, although they were not dealt with in this study. • The performance of the PV-DSF depends on the design and control. Therefore, the performance of the building environment such as thermal and visual comfort should be investigated by PV-DSF design and operation. • It is necessary to examine how much the performance of the PV power generation, which can reduce temperatures by the introduction of natural ventilation in PV-DSF, is more advantageous
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compared to the conventional BIPV system which does not have backside ventilation. • Different types of buildings require different ways of designing and operating the PV-DSF. Therefore, more research is required to investigate the performance of the PV-DSF depending on the use, orientation, insulation performance, and windowto-wall ratio of the building as in previous studies [34,38]. Simulation models allow to perform parametric studies and performance evaluations of PV-DSF in a short period of time, and optimal designs can be derived through detailed simulations or small experiments. Declaration of Competing Interest None. Acknowledgement This research was supported by 1) the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (No. NRF-2016M1A2A2936758), and 2) the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20153010130320). Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.enbuild.2019.07.031. Reference [1] A. Sandak, J. Sandak, M. Brzezicki, A. Kutnar, State of the art in building façades, Bio-based Building Skin. Environmental Footprints and Eco-design of Products and Processes, Springer, Singapore, 2019. [2] Suresh B. Sadineni, Srikanth Madala, Robert F. Boehm, Passive building energy savings: a review of building envelope components, Renew. Sustain. Energy Rev. 15 (8) (2011) 3617–3631. [3] Im Hatice Sozer, Improving energy efficiency through the design of the building envelope, Build. Environ. 45 (12) (2010) 2581–2593. [4] R. Pacheco, J. Ordóñez, G. Martínez, Energy efficient design of building: a review, Renew. Sustain. Energy Rev. 16 (6) (2012) 3559–3573. [5] Bjørn Petter Jelle, Christer Breivik, Hilde Drolsum Røkenes, Building integrated photovoltaic products: a state-of-the-art review and future research opportunities, Sol. Energy Mater. Sol. Cells 100 (2012) 69–96. [6] H. Jouhara, J. Milko, J. Danielewicz, M.A. Sayegh, M. Szulgowska-Zgrzywa, J.B. Ramos, S.P. Lester, The performance of a novel flat heat pipe based thermal and PV/T (photovoltaic and thermal systems) solar collector that can be used as an energy-active building envelope material, Energy 108 (2016) 148–154. [7] R.C.G.M. Loonen, M. Trcˇ ka, D. Cóstola, J.L.M. Hensen, Climate adaptive building shells: state-of-the-art and future challenges, Renew. Sustain. Energy Rev. 25 (2013) 483–493. [8] Ziyi Su, Xiaofeng Li, Fei Xue, Double-skin façade optimization design for different climate zones in China, Sol. Energy 155 (2017) 281–290. [9] E. Gratia, A. De Herde, Are energy consumptions decreased with the addition of a double-skin? Energy Build. 39 (2007) 605–619. [10] M.A. Shameri, K. Alghoul, M.F.M. Zain, O. Elayeb, Perspectives of double skin façade systems in buildings and energy saving renew, Sustain. Energy Rev. 15 (3) (2011) 1468–1475. [11] Z. Ioannidis, A. Buonomano, A.K. Athienitis, T. Stathopoulos, Modeling of double skin façades integrating photovoltaic panels and automated roller shades: analysis of the thermal and electrical performance, Energy Build. 154 (2017) 618–632. [12] D. Kim, S.J. Cox, H.∗ Cho, J.∗ Yoon, "Comparative investigation on building energy performance of double skin façade (DSF) with interior or exterior slat blinds, J. Build. Eng. 20 (2018) 411–423 2018. [13] J. Cipriano, G. Houzeaux, D. Chemisana, C. Lodi, J. Martí-Herrero, Numerical analysis of the most appropriate heat transfer correlations for free ventilated double skin photovoltaic façades, Appl. Therm. Eng. 57 (2013) 57–68. [14] R. Charron, A.K. Athienitis, Optimization of the performance of double-façades with integrated photovoltaic panels and motorized blinds, Sol. Energy 80 (2006) 482–491. [15] B.J Brinkworth, B.M Cross, R.H Marshall, Hongxing Yang, Thermal regulation of photovoltaic cladding, Sol. Energy 61 (3) (1997) 169–178. [16] Guohui Gan, Effect of air gap on the performance of building-integrated photovoltaics, Energy 34 (7) (2009) 913–921.
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