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Performance study of a new type of transmissive concentrating system for solar photovoltaic glass curtain wall
Energy Conversion and Management 201 (2019) 112167
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Performance study of a new type of transmissive concentrating system for solar photovoltaic glass curtain wall
T
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Ming Honga, Chaoqing Fenga, , Zhao Xua, Lizhuang Zhanga, Hongfei Zhengb, Gang Wuc a
College of Energy and Power Engineering, Inner Mongolia University of Technology, Hohhot 010051, China School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China c Institute of Environment and Sustainable Development in Agriculture, Chinese Academy of Agricultural Sciences, Beijing 100081, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Solar energy Transmissive concentrating Glass curtain wall Building integrated photovoltaics Light control
There are many problems in Building Integrated Photovoltaics (BIPV) system, such as contradiction between daylighting and electricity generation, unable to match the demand of light control, need to constantly adjust angle for tracking, and so on. A new type of transmissive concentrating system for glass curtain wall is proposed which can improve the performance of solar photovoltaic glass curtain wall. The concentrating characteristic was studied by a series of comprehensive simulation and experiment. The optical software is used to simulate the influence of concentrating characteristics in different incident angles. Compared with solid and trapezoidal structure compound parabolic collector, the concentrating performance of hollow with water structure is also simulated. Simulation results shown that the receiving rate of hollow with water compound parabolic collector is changing slowly when incidence angle is increasing between 0 and 20°, and it can achieve 70% when the incidence angle less than 20°, and it is better than other concentrator with different cross section shapes. Based on simulation results, the experimental samples were processed and tested in a typical sunny weather. The experimental results were in good agreement with the simulation results. The system had a minimum transmittance of 28.2% at noon, but before 9:40 AM and after 15:40 PM, the transmittance exceeds 55% and can meet lighting requirements of rooms. It can be proved that the new system has passive light control function, which is expected to replace the double-layer vacuum glass curtain wall that is widely used nowadays.
1. Introduction Solar energy is one of the most popular renewable energy on the earth, it has the characteristics of clean, environmental and sustainable development, occupies a prominent position in the transformation of the world's energy structure. With the acceleration of urbanization and the continuous expansion of urban scale, building energy consumption has become a problem that cannot be ignored [1]. Research by the International Energy Agency (IEA) on world energy consumption shows that building energy consumption accounts for 32% of world energy consumption [2], and solar energy will provide about 45% of world energy needs in 2050 [3]. Therefore, effective collection of solar energy as a regular energy supplement is an inevitable choice for mankind. Solar energy has features that it can be easily used in buildings, so using solar energy to replace traditional forms of energy to provide building energy has become a hotspot for research and development at domestic and abroad [4]. The combination of solar photovoltaic systems and buildings is
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called the building integrated photovoltaic (BIPV) system that uses to provide building energy consumption. Its working principle is the same as ordinary solar photovoltaic power generation system. The only difference is that solar modules can be used as building exterior wall materials, further reducing the cost of the whole system [5]. At present, BIPV system has rich experience in design and technology [6]. Some countries have even come up the concept of “zero energy building” [7], Jae BumLee [8] examined the energy consumption of the solar photovoltaic building integrated system building in one year, the total energy consumption of the system is 10,4602.4 kWh, and the total power generation is 10,5266.6 kWh, so the energy surplus is 664.2 kWh, it is evaluated as zero energy building. It is means that buildings can supply enough energy to itself based on the renewable energy utilization system. In addition, many scholars have also carried out optimization design, Ruta Vanaga [9] developed a new type of wall that adapts to the climate, which can adapt to different times and seasons and accumulate solar energy; Daniel [10] proposed a concentrating optical system consisting of a fixed wide-angle Fresnel lens
Corresponding author. E-mail address: [email protected] (C. Feng).
https://doi.org/10.1016/j.enconman.2019.112167 Received 17 June 2019; Received in revised form 27 September 2019; Accepted 10 October 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. The working principle of transmissive concentrating system. 1 Air inlet, 2 Side plate, 3 Antireflection coating, 4 Side wall surface of concentrating unit, 5 Outer glass, 6 Oblique incident light, 7 Direct incident light, 8 PV battery, 9 Cooling water, 10 Air outlet, 11 Fan, 12 Internal glass. Table 1 Geometric concentration ratio with different parameters. Light
3 mm 4 mm 5 mm
Height 20 mm
25 mm
30 mm
3.28 2.81 2.50
3.69 3.16 2.81
4.07 3.49 3.10
researchers mainly used walls and roofs to layout solar cells and generated electricity, but the wall and roof are limited for the building especially for modern building which used lots of glass curtain wall. So many researchers study how to use the windows to generate electricity. Wei Lu [12] designed a new fixed asymmetric composite parabolic concentrator with geometric concentration ratio of 2.0, and the system composed of it is suitable for the application of building windows; Ankita [13] conducted research on building integrated semitransparent photovoltaic/thermal (BiSPVT) system and optimized for various design parameters; Guiqiang Li [14] proposed a new static compound parabolic concentrator for BIPV systems, the concentrator consists of a medial mirror CPC and a transparent outer wall structure, which can full use of internal reflection and specular reflection; Daria [15] developed a concentrator called RADTIRC (Rotating Asymmetric Dielectric Total Internal Reflection Concentrator), which is integrated into a small double glazed window and suitable for use on buildings. All of above researches are pay attention to using energy and how to improve the efficiency, but the more energy supply in the building
Fig. 2. Schematic diagram of micro-concentrating unit.
and a moving compound parabolic concentrator that can satisfy architectural features; Yupeng Wu [11] designed intelligent solar concentrators and used in intelligent centralized photovoltaic (CPV) systems, which can control the amount of light that enters into the building by changing the amount of electricity generated by photovoltaic power generation to automatic response climate. Those works of
Fig 3. Schematic diagram of the solar concentrating photovoltaic/photothermal glass curtain wall system. 2
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while maximizing use of surplus solar radiation at noon and generate electricity, and the contradiction between indoor lighting and PV battery can be solved. Combined with the theoretical calculation, simulation and experimental test methods, it is used to comprehensively analyze the influence of concentrating characteristics under incident angle for different angles or different directions of the system. 2. System structure and working principle The new type of transmissive concentrating system is composed of a plurality of hollow micro-concentrating units, it is made by PMMA (Polymethyl methacrylate), its outer surface is CPC structure (compound parabolic concentrator), photovoltaic cells are attached at the bottom, the hollow portion is axially fed with cooling water also an air sandwich between the double glazings, and the function is to absorbing heat generated by the photovoltaic cell. The structure is shown in Fig. 1, and its working principle is shown as follows: The vertically incident ray 7 passes through the upper surface 5 and is refracted in the micro-concentrating unit, after that, concentrated in the bottom of the photovoltaic cell 8 by antireflection coating 3, then it can generate electricity and heat. As the light through into the bottom of the micro-concentrating unit, it will be partially reflected, not conducive to photovoltaic cell 8 reception, therefore, an antireflection coating 3 is placed between the photovoltaic cell 8 and the glass curtain wall, that can increases the amount of ray in order to make it can be receives by the photovoltaic cell 8, thereby the generation efficiency of photovoltaic cell 8 can be improved. The obliquely incident ray 6 passes through the upper surface 5, shot on the curved inner wall 4 of the CPC, then refracted in the micro-concentrating unit and finally shot out from its lower part, and enters the interior of building through the bottom plate 12 for daylighting. The side plate 2 is provided with venting hole, and the fan 11 is installed on the external air outlet 10. It make the air flow through the opposite side inlet 1 into the concentrating system. The photovoltaic cell 8 is heated when it generating, airflow and water flow while cooling the photovoltaic cell 8, and the efficiency and service life of the photovoltaic cell 8 can be increased. The concentrating process of the micro-concentrating unit is shown in Fig. 2, among them, ray 2 is an effective concentrating ray, which refers to the ray that is vertically incident or incident angle is small. When the ray incident on the junction of the bottom and the curved wall, it will be refracted and reflected by micro-concentrating unit, at this time, ray 2 can still be received by the photovoltaic cell at the bottom of micro-concentrating unit; ray 1 is an effective illumination light, and the effective illumination light refers to the light that is
Fig. 4. Micro-concentrating unit model.
windows must cause the less daylight enter into the room, so resolve the conflict between daylighting and electricity generation is very important in BIPV systems. Chaoqing Feng [16] proposed a PV/T/D (Photovoltaic/Thermal/Day lighting) system composed of solid CPC, and used to make a new type of transparent roof, which can realize light control and adjust the thermal environment of the building. This system considered the daylighting and energy supply. In another aspect, not only pay attention to energy supply for green building, but also it should make sure the indoor environment comfort for person and saving energy for building. However, currently, there are still many problems in BIPV system, for example: using a large number of photovoltaic cells affects the indoor lighting, limited battery area limits its ability to receive the maximum radiation [17], and it is unable to reasonably control the light, existing controllable shutter type layout photovoltaic cell [18] has low reliability and need to constantly manually adjust angle, it is very cumbersome and so on. The new type of transmissive concentrator is proposed in this paper, it is an ideal devices to solve these problems, and the solar photovoltaic glass curtain wall composed of this system has passive light control function, it can ensure the indoor lighting demand in morning and night
Fig. 5. Schematic diagram of two kinds of incident angles. 3
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Fig. 6. Optical path diagram of different incidence angles θ1.
Fig. 7. Changing the angle between the incident ray and the cross section of the concentrator θ2.
finally emitted from the bottom of the glass curtain wall after the refraction and reflection of the micro-concentrating unit when the ray is obliquely incident or the incident angle is large. At this time, ray 1
enters the building through the bottom plate to achieve daylighting. This new type of transmissive concentrating system is particularly suitable for solar photovoltaic curtain wall due to its features of
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Fig. 8. Lights distribution of different incidence angles.
Based on the design method of CPC, the calculation formula of CPC model parameters is as follows [20]:
generating electricity, high receiving for vertically incident ray and high transmittance for obliquely incident ray. Fig. 3 is the schematic diagram of the solar concentrating photovoltaic/photothermal glass curtain wall system that can passively control light composed of this new type of transmission concentrating system. Double-layer vacuum glass are used in ordinary glass curtain wall system, a new type of concentrated solar plants will replace the outside glass of double-layer glass curtain, and make sure the water flow direction is perpendicular to the ground. When the glass curtain wall receives the solar radiation, parts of them enter into the house through the glass curtain wall, and the other parts are converted into electric energy output by the PV cell. The PV cell produced heat while generating electricity, and the heat is taken away by the cooling water and the interlayer air. The thickness of the glass curtain wall can be specifically designed according to the requirements. As long as the waterway and the air layer are connected with each other, a large number of the glass curtain wall can be realized by splicing.
L = a (C + 1) C 2 − 1
where L is the height of the condenser, mm; a is half of the length of the light outlet, mm; C is the geometric concentration ratio. According to the practical application of the glass curtain wall, the geometric concentrating of the concentrator with the length of the light outlet is 3 mm, 4 mm, 5 mm, and the height is 20 mm, 25 mm, 30 mm, as shown in Table 1. Considering the relationship between the thickness of glass curtain wall and the geometric concentration ratio, CPC micro-concentrating unit with a height of 25 mm and light outlet length of 4 mm is finally selected according to the application place. Its section is shown in Fig. 4, geometric condensing ratio is 3.16. 3.2. Simulation result Taking the 0.5 mm wall thickness as an example, the angle between incident ray and axial symmetry plane of the concentrator is defined as θ1; and the angle between the incident ray and the cross section of the concentrator is defined as θ2. Schematic diagram of Fig. 5 are shown the two kinds of incident angle. In the simulation, θ1 and θ2 are changed in the range from 0 to 85°. Let the incident ray set as parallel light, then for ray tracing, the light path diagrams are shown in Fig. 6 and Fig. 7 respectively. For the incident angle θ1, when this angle is 0°, a part of lights reach the surface on receiver directly, another part of lights reach the CPC surface, be refracted and reflected then it is concentrated on the receiver surface. Most of light achieve to the receiver. With the increasing of incidence angle, light begin to through the micro-concentrating unit gradually. The lager the incidence angle is, the higher the transmittance is. When the incident ray angle greater than 55°, the receiver will not receive any ray. For the incident angle θ2, when the θ2 is changed, ray will be refracted and reflected multiple times inside micro-concentrating unit. The number of rays received by the receiver at the bottom of the micro-concentrating unit and the number of rays received by receiving surface of the micro-concentrating unit were analyzed; the receiving, reflectivity and transmittance were obtained, as shown in Fig. 8. It can be known by the simulation results: when the incidence angle
3. Optical simulation Performance analysis of the new transmissive concentrating system should be done and parameters of the concentrating system will be evaluated in the different situations. The main parameters of this study include receiving, reflectivity and transmittance. The optical simulation was performed using Trace Pro software, which is a kind of ray simulation software commonly used in lighting systems, optical analysis, radiance analysis and photometric analysis. It has extended numerical precision and a ray tracing tool specifically designed for optical design, specific to each ray that needs to be traced. It must be noted that the transmittance refers to the effective transmittance, which is defined as the ratio of the number of rays that can be emitted from the lower part of the concentrating system to the number of incident rays. 3.1. Model Selection In the current study, the right and left parabola equation of CPC is expressed by formula (1) and (2).
0.9486y − 0.3165x − 2 = 0.0949[0.9486x + 0.3165y − 1.8972]2
(1)
0.9486y + 0.3165x − 2 = 0.0949[0.9486x − 0.3165y − 1.8972]2
(2)
(3)
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Fig. 9. Comparison between solid CPC and hollow CPC.
is 0°, the receiving rate can reach 74%, when incidence angle θ1 is less than 20°, reflectivity increases slightly, and transmittance declined slightly, but amplitude is not large, the receiving rate can maintained above 70%. When the incident angle is greater than 20°, the receiving drops rapidly, when the incident angle is 25°, receiving is less than 5%, but transmittance at this time rapidly increases to 83%. Angle of incidence continues to rise to more than 55°, the receiving and reflectivity both approach zero, all ray passes through the micro-concentrating unit. When the angle θ2 is changed, as the incident angle increases, the receiving gradually decreases and the reflectivity gradually increases. The receiving ranges from 29% to 74%, the reflectivity ranges from 12% to 67%, the transmittance is always low, only 8% at maximum. When the incident angle is 0°, the transmittance is 2%. As the incident angle increases, the transmittance generally shows a downward trend. After the incident angle is greater than 75°, the transmittance is 0%, and the variation of each parameter is close to a linear trend.
3.3. Compared with solid CPC simulation results In order to know the differences of performance between hollow CPC and solid CPC which is proposed in reference [19], solid CPC has the same physical dimensions with hollow CPC. The comparison has been done and the result is shown in Fig. 9. Comparing the simulation results of the hollow CPC with the same physical dimensions of the solid CPC micro-concentrating unit, it can be seen from the result that when the incident angle less than 20°, hollow CPC receiving rate is about 20% higher than solid CPC, and in the range of small incident angle within 20° the reflectivity of the hollow CPC is much lower than the solid CPC, especially vertically incident ray can even lower by about 40%. Significant differences in receiving over a small range of incident angles indicates that hollow CPC is more advantageous than solid CPC in terms of solar energy using, and the transmittance difference between the hollow CPC and the solid CPC is not obvious, it shows that in the aspect of guaranteeing indoor lighting in practical application, hollow CPC and solid CPC are basically the same. But after the incident angle is greater than 20°, the transmittance 6
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Fig. 10. Comparing between two different cross-sectional shapes.
of the hollow CPC is higher than the solid CPC, about 8%~ 43%. In the range of larger incidence angle, the lighting effect of hollow CPC is better than solid CPC.
between incident angle 20° and 25°.
3.4. Comparison of different cross-sectional shapes
From the analysis of optical paths of different incident angles, it is found that the ray has completely different optical paths when it enter the wall thickness portion or the intermediate medium portion, so it is certainly that the wall thickness has an influence on its optical characteristics. For this purpose, get the receiving, reflectivity and transmittance when the wall thickness varied from 0.5 mm to 3 mm. Fig. 11 is shown the results about receiving, reflectivity and transmission under the wall thickness of 0.5 to 3 mm. The results show that when the incident angle is less than 20°, the thinner wall thickness of the model has the higher receiving and the lower transmittance, such as when the incident angle is 5°, wall thickness of 0.5 mm compared with wall thickness of 3 mm. The receiving of the micro-concentrating unit with thickness of 0.5 mm is 76%, and the transmittance is 3%, but the receiving rate of the microconcentrating unit with wall thickness of 3 mm is only 50%, the transmittance is 13%. But when the incident angle is larger than 20°, taking the incident angle of 35°as an example, the receiving of wall thickness of 3 mm is 26%, and the transmittance is 53%, but the receiving of wall thickness for 0.5 mm is only 1%, and the transmittance
3.5. Comparison of different wall thicknesses
Comparison between two different cross-sectional shapes has been studied by simulation, two models have the same upper and lower end geometry size, the same wall thickness, but section is different which is trapezoidal and another is CPC. In order to analyze the performance of two kinds of condenser, simulation has been done in different incident rays angle θ1 and the results are shown in Fig. 10. In the simulation, the cross-section of the trapezoidal has a limit receiving angle of 75°, receiving and reflectivity are both 0% and the transmittance is 100% after incident angle is greater than 75°. Although it can expand the range of the received ray, but CPC shape can ensure a good receiving even at an incident angle is 20°, as high as 70%; The receiving of the trapezoidal shape decreases sharply from 5°, and when the incident ray angle is 20° receiving is only the 39%, which is lower than CPC shape; compared with the trapezoidal and CPC, the transmittance increases with the increase of the incident angle, the trapezoidal shape changes gently and do not fit the need of solar photovoltaic glass curtain wall. The CPC shape has a significant increase 7
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Fig. 11. performance of different wall thicknesses.
achievability in actual use, micro-concentrating units with a wall thickness of 2 mm was selected to make a sample with the material of high transparency of PMMA (polymethyl methacrylate). It is shown in Fig. 12.
is 92%. The difference in reflectivity change is small; the reflectivity is 0% after the incident angle is greater than 50°. The wall is thicker, the change in reflectivity of incident ray at each angle is more pronounced. 4. Sample processing and experimental testing
4.2. Experimental methods Based on the results of simulation research, the system has a higher receiving when the incident angle is smaller, and a higher transmittance when the incident angle is larger. Experiment research about the new type of transmissive concentrating system is needed to verify the simulation and obtain the performance under real weather conditions.
Outdoor experiment was undertaken under real sky conditions. The system was arranged on the sunlight directly in which the major target is to investigate the light transmittance of the new type of system when the sunlight is in different incident angles. Furthermore, a double-layer vacuum glass which is used commonly in modern building was selected to test in the same condition, the test results of two types glass curtain wall were compared and analyzed. The experimental device is shown in Fig. 13. The integral box
4.1. Size selection According to the simulation results and in combination with the 8
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transmittance =
LX 1 × 100% LX 2
(4)
where the units of LX1 and LX2 are Lux. When the double-layer vacuum glass is tested, the same method can be used. In the experiment, the solar cell was attached to the bottom of the micro-concentrating unit, but the electricity generation was not tested in this experiment. Because the focus point of this study is only about transmittance characteristic. When the integrator box is placed, the light at the maximum solar height of the test day is guaranteed to be incident perpendicularly to the test device. Two location situations are selected in the experiment and the results are compared, one is vertical arrangement which is making the axial line of CPC point to north-south orientation, and another is horizontal arrangement which is making the axial line of CPC point to east-west orientation. At the same time, in order to compare the difference between the new glass curtain wall system developed in this paper and the existing glass curtain wall, the comparison have been done when they are in the vertical direction location. The experiment was conducted on May 31 and it is sunny. By calculation, the maximum solar altitude is about 72° and the apparent solar time is 12:11 on that day. Error analysis has been done about the experiment, in this test, illumination is the only test value, but the values test times are different and it is used to calculate the transmittance. So we must record the illumination values as quickly as possible and reduce the time interval between with tests with and without cover plate. In the experiment, time intervals are less than 1 min, so the results can be considered accurate.
Fig. 12. Sample with wall thickness of 2 mm.
transmittance experimental device is designed by the principle of integrating sphere. At the same time, in order to avoid the local illumination unevenness caused by direct light, the inner wall of integral box is covered with diffuse reflection coating to ensure uniform illumination. The illuminance measurement is performed using a multi-function digital photometer with a measuring range of 0.1–100 Lux. The top of the integrator box has the fix hole with the same size as the photometer for easy measurement. The integrator box is placed outdoors, and the light intensity in the integrator box without any covers is measured by the handheld digital photometer to be LX1, then the new transmissive concentrating system cover is placed, and the light intensity in the integrator box is again measured as LX2, the transmittance of the new transmissive concentrating system is:
4.3. The test results Based on the real sky tests, when the axis of CPC is placed north and south, the experimental results are compared with incident angle θ1 in
Fig. 13. Experimental device for transmittance integrating box. 9
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Fig. 14. Comparison of experimental and simulated transmittance.
well. The test results show that transmittance is small when the sunlight incident vertically at noon, but it becomes lager when the incident angle increases at morning and afternoon. The new transmissive concentrating system has the lowest transmittance at around 12:00 noon, only about 30%. In the simulation results, the transmittance is near about 0% when the light incident vertically, it is lower than the test
simulation results at incidence different angles and is shown in Fig. 14(a), and when the axis of CPC is placed east and west, the experimental results are compared with incident angle θ1 in simulation results and it is shown in Fig. 14(b). It can be seen from Fig. 14 that the changing tendency of experimental results and simulation results about transmittance are matched 10
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Fig. 15. Test results comparison between new system and vacuum double glass in real sky.
is true. The experimental in the real sky have been done, results show that the new transmissive concentrating system has low transmittance at the noon, when the outdoor irradiance is close to 1100 W/m2, the transmittance of the system is only 28.7%, most of lights are keep out, so it can prevent indoor illumination to dazzle effectively. Changing tendency of experimental results and simulation results about transmittance are matched well, but there is a difference in transmittance value of numbers, the reason is that scattered light enter into system desultorily. In the morning and afternoon when the room needs lighting, the maximum transmittance can exceed 60%, which can provide enough indoor lighting during this period. Compared with the doublelayer vacuum glass which is widely used in modern buildings, the transmittance of this new type of transmissive concentrating system can be changed with the outdoor lighting conditions, in the morning and afternoon when indoor lighting is needed, its transmittance is higher, but the transmittance is much lower than that double-layer vacuum glass at noon in direct sunlight. In comparison, this new type of transmissive concentrating system can perfectly replace the double layer vacuum glass, suitable for use as a glass curtain wall in BIPV systems, enabling passive light control. It has a good research prospect and practical application space. In addition, it should be pointed out that a typical sunny weather was selected to do experiment for discussion and analysis. But the weather is not so nice every day, for other types of weather, such as cloudy, fog and haze weather, the proportion of scattered light will increase, and the concentrating characteristics of the device will change differently, so it is necessary to study in the future.
results and mainly because that, in simulation, the incidence light are direct light; but in the real sky, direct light and scattered light are all exist, direct light can be received by solar cell but scattered light will go through the transmissive concentrating system disorganized, so most of them are penetrating into test box and it increase the transmittance. And the transmittance gradually increases as the incident angle increases. In the simulation results, after the incident angle is greater than 55°, the transmittance is close to 100%; but in the test, the maximum transmittance can only reach about 60%. The reason is that: the cover plate is produced by PMMA which is not completely transparent and it will influence the transmittance of transmissive concentrating system. The comparison tests between the new transmissive concentrating system device and the double-glazed curtain wall are done and the results are shown in Fig. 15. As can be seen from the Fig. 15, the double-layer vacuum glass cover can maintain a high transmittance level and change little, it remains at 40%~50% all day, but the transmittance of new transmissive concentrating system cover is lowest at noon, which is about 20% lower than that double-layer vacuum glass cover; before 9:40 and after 16:00, the transmittance is higher than double-glazed vacuum glass cover, and the highest even about 10%. It is shown that new transmissive concentrating system can adjust the light condition of indoor environment and avoid glazing at noon. Furthermore, redundant sunshine can be converted to electric at noon, the comprehensive utilization of solar energy is realized in building by the new transmissive concentrating system.
5. Conclusion Based on the ray tracing simulation results of the new transmissive concentrating system, it shows that a higher receiving rate can be obtained in direct incident light or light with a small incident angle. When the incidence angle is 0°, the receiving rate can reach 74%, when incidence angle θ1 is less than 20°, reflectivity increases slightly, and transmittance declined slightly, but amplitude is not large, the receiving rate can maintained above 70%. Compared with the trapezoidal structure and the solid structure, the new transmissive concentrating system is more advantageous in light control characteristic. By comparing the simulation results of different wall thicknesses, it is concluded that for smaller incident angle, the thicker wall thickness has the higher receiving, but when the angle of incidence is large, the opposite
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement This work was supported by the National Natural Science Foundation of China (No. 51766013, No. 51806244).
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Performance study of a new type of transmissive concentrating system for solar photovoltaic glass curtain wall
T
⁎
Ming Honga, Chaoqing Fenga, , Zhao Xua, Lizhuang Zhanga, Hongfei Zhengb, Gang Wuc a
College of Energy and Power Engineering, Inner Mongolia University of Technology, Hohhot 010051, China School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China c Institute of Environment and Sustainable Development in Agriculture, Chinese Academy of Agricultural Sciences, Beijing 100081, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Solar energy Transmissive concentrating Glass curtain wall Building integrated photovoltaics Light control
There are many problems in Building Integrated Photovoltaics (BIPV) system, such as contradiction between daylighting and electricity generation, unable to match the demand of light control, need to constantly adjust angle for tracking, and so on. A new type of transmissive concentrating system for glass curtain wall is proposed which can improve the performance of solar photovoltaic glass curtain wall. The concentrating characteristic was studied by a series of comprehensive simulation and experiment. The optical software is used to simulate the influence of concentrating characteristics in different incident angles. Compared with solid and trapezoidal structure compound parabolic collector, the concentrating performance of hollow with water structure is also simulated. Simulation results shown that the receiving rate of hollow with water compound parabolic collector is changing slowly when incidence angle is increasing between 0 and 20°, and it can achieve 70% when the incidence angle less than 20°, and it is better than other concentrator with different cross section shapes. Based on simulation results, the experimental samples were processed and tested in a typical sunny weather. The experimental results were in good agreement with the simulation results. The system had a minimum transmittance of 28.2% at noon, but before 9:40 AM and after 15:40 PM, the transmittance exceeds 55% and can meet lighting requirements of rooms. It can be proved that the new system has passive light control function, which is expected to replace the double-layer vacuum glass curtain wall that is widely used nowadays.
1. Introduction Solar energy is one of the most popular renewable energy on the earth, it has the characteristics of clean, environmental and sustainable development, occupies a prominent position in the transformation of the world's energy structure. With the acceleration of urbanization and the continuous expansion of urban scale, building energy consumption has become a problem that cannot be ignored [1]. Research by the International Energy Agency (IEA) on world energy consumption shows that building energy consumption accounts for 32% of world energy consumption [2], and solar energy will provide about 45% of world energy needs in 2050 [3]. Therefore, effective collection of solar energy as a regular energy supplement is an inevitable choice for mankind. Solar energy has features that it can be easily used in buildings, so using solar energy to replace traditional forms of energy to provide building energy has become a hotspot for research and development at domestic and abroad [4]. The combination of solar photovoltaic systems and buildings is
⁎
called the building integrated photovoltaic (BIPV) system that uses to provide building energy consumption. Its working principle is the same as ordinary solar photovoltaic power generation system. The only difference is that solar modules can be used as building exterior wall materials, further reducing the cost of the whole system [5]. At present, BIPV system has rich experience in design and technology [6]. Some countries have even come up the concept of “zero energy building” [7], Jae BumLee [8] examined the energy consumption of the solar photovoltaic building integrated system building in one year, the total energy consumption of the system is 10,4602.4 kWh, and the total power generation is 10,5266.6 kWh, so the energy surplus is 664.2 kWh, it is evaluated as zero energy building. It is means that buildings can supply enough energy to itself based on the renewable energy utilization system. In addition, many scholars have also carried out optimization design, Ruta Vanaga [9] developed a new type of wall that adapts to the climate, which can adapt to different times and seasons and accumulate solar energy; Daniel [10] proposed a concentrating optical system consisting of a fixed wide-angle Fresnel lens
Corresponding author. E-mail address: [email protected] (C. Feng).
https://doi.org/10.1016/j.enconman.2019.112167 Received 17 June 2019; Received in revised form 27 September 2019; Accepted 10 October 2019 0196-8904/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. The working principle of transmissive concentrating system. 1 Air inlet, 2 Side plate, 3 Antireflection coating, 4 Side wall surface of concentrating unit, 5 Outer glass, 6 Oblique incident light, 7 Direct incident light, 8 PV battery, 9 Cooling water, 10 Air outlet, 11 Fan, 12 Internal glass. Table 1 Geometric concentration ratio with different parameters. Light
3 mm 4 mm 5 mm
Height 20 mm
25 mm
30 mm
3.28 2.81 2.50
3.69 3.16 2.81
4.07 3.49 3.10
researchers mainly used walls and roofs to layout solar cells and generated electricity, but the wall and roof are limited for the building especially for modern building which used lots of glass curtain wall. So many researchers study how to use the windows to generate electricity. Wei Lu [12] designed a new fixed asymmetric composite parabolic concentrator with geometric concentration ratio of 2.0, and the system composed of it is suitable for the application of building windows; Ankita [13] conducted research on building integrated semitransparent photovoltaic/thermal (BiSPVT) system and optimized for various design parameters; Guiqiang Li [14] proposed a new static compound parabolic concentrator for BIPV systems, the concentrator consists of a medial mirror CPC and a transparent outer wall structure, which can full use of internal reflection and specular reflection; Daria [15] developed a concentrator called RADTIRC (Rotating Asymmetric Dielectric Total Internal Reflection Concentrator), which is integrated into a small double glazed window and suitable for use on buildings. All of above researches are pay attention to using energy and how to improve the efficiency, but the more energy supply in the building
Fig. 2. Schematic diagram of micro-concentrating unit.
and a moving compound parabolic concentrator that can satisfy architectural features; Yupeng Wu [11] designed intelligent solar concentrators and used in intelligent centralized photovoltaic (CPV) systems, which can control the amount of light that enters into the building by changing the amount of electricity generated by photovoltaic power generation to automatic response climate. Those works of
Fig 3. Schematic diagram of the solar concentrating photovoltaic/photothermal glass curtain wall system. 2
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while maximizing use of surplus solar radiation at noon and generate electricity, and the contradiction between indoor lighting and PV battery can be solved. Combined with the theoretical calculation, simulation and experimental test methods, it is used to comprehensively analyze the influence of concentrating characteristics under incident angle for different angles or different directions of the system. 2. System structure and working principle The new type of transmissive concentrating system is composed of a plurality of hollow micro-concentrating units, it is made by PMMA (Polymethyl methacrylate), its outer surface is CPC structure (compound parabolic concentrator), photovoltaic cells are attached at the bottom, the hollow portion is axially fed with cooling water also an air sandwich between the double glazings, and the function is to absorbing heat generated by the photovoltaic cell. The structure is shown in Fig. 1, and its working principle is shown as follows: The vertically incident ray 7 passes through the upper surface 5 and is refracted in the micro-concentrating unit, after that, concentrated in the bottom of the photovoltaic cell 8 by antireflection coating 3, then it can generate electricity and heat. As the light through into the bottom of the micro-concentrating unit, it will be partially reflected, not conducive to photovoltaic cell 8 reception, therefore, an antireflection coating 3 is placed between the photovoltaic cell 8 and the glass curtain wall, that can increases the amount of ray in order to make it can be receives by the photovoltaic cell 8, thereby the generation efficiency of photovoltaic cell 8 can be improved. The obliquely incident ray 6 passes through the upper surface 5, shot on the curved inner wall 4 of the CPC, then refracted in the micro-concentrating unit and finally shot out from its lower part, and enters the interior of building through the bottom plate 12 for daylighting. The side plate 2 is provided with venting hole, and the fan 11 is installed on the external air outlet 10. It make the air flow through the opposite side inlet 1 into the concentrating system. The photovoltaic cell 8 is heated when it generating, airflow and water flow while cooling the photovoltaic cell 8, and the efficiency and service life of the photovoltaic cell 8 can be increased. The concentrating process of the micro-concentrating unit is shown in Fig. 2, among them, ray 2 is an effective concentrating ray, which refers to the ray that is vertically incident or incident angle is small. When the ray incident on the junction of the bottom and the curved wall, it will be refracted and reflected by micro-concentrating unit, at this time, ray 2 can still be received by the photovoltaic cell at the bottom of micro-concentrating unit; ray 1 is an effective illumination light, and the effective illumination light refers to the light that is
Fig. 4. Micro-concentrating unit model.
windows must cause the less daylight enter into the room, so resolve the conflict between daylighting and electricity generation is very important in BIPV systems. Chaoqing Feng [16] proposed a PV/T/D (Photovoltaic/Thermal/Day lighting) system composed of solid CPC, and used to make a new type of transparent roof, which can realize light control and adjust the thermal environment of the building. This system considered the daylighting and energy supply. In another aspect, not only pay attention to energy supply for green building, but also it should make sure the indoor environment comfort for person and saving energy for building. However, currently, there are still many problems in BIPV system, for example: using a large number of photovoltaic cells affects the indoor lighting, limited battery area limits its ability to receive the maximum radiation [17], and it is unable to reasonably control the light, existing controllable shutter type layout photovoltaic cell [18] has low reliability and need to constantly manually adjust angle, it is very cumbersome and so on. The new type of transmissive concentrator is proposed in this paper, it is an ideal devices to solve these problems, and the solar photovoltaic glass curtain wall composed of this system has passive light control function, it can ensure the indoor lighting demand in morning and night
Fig. 5. Schematic diagram of two kinds of incident angles. 3
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Fig. 6. Optical path diagram of different incidence angles θ1.
Fig. 7. Changing the angle between the incident ray and the cross section of the concentrator θ2.
finally emitted from the bottom of the glass curtain wall after the refraction and reflection of the micro-concentrating unit when the ray is obliquely incident or the incident angle is large. At this time, ray 1
enters the building through the bottom plate to achieve daylighting. This new type of transmissive concentrating system is particularly suitable for solar photovoltaic curtain wall due to its features of
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Fig. 8. Lights distribution of different incidence angles.
Based on the design method of CPC, the calculation formula of CPC model parameters is as follows [20]:
generating electricity, high receiving for vertically incident ray and high transmittance for obliquely incident ray. Fig. 3 is the schematic diagram of the solar concentrating photovoltaic/photothermal glass curtain wall system that can passively control light composed of this new type of transmission concentrating system. Double-layer vacuum glass are used in ordinary glass curtain wall system, a new type of concentrated solar plants will replace the outside glass of double-layer glass curtain, and make sure the water flow direction is perpendicular to the ground. When the glass curtain wall receives the solar radiation, parts of them enter into the house through the glass curtain wall, and the other parts are converted into electric energy output by the PV cell. The PV cell produced heat while generating electricity, and the heat is taken away by the cooling water and the interlayer air. The thickness of the glass curtain wall can be specifically designed according to the requirements. As long as the waterway and the air layer are connected with each other, a large number of the glass curtain wall can be realized by splicing.
L = a (C + 1) C 2 − 1
where L is the height of the condenser, mm; a is half of the length of the light outlet, mm; C is the geometric concentration ratio. According to the practical application of the glass curtain wall, the geometric concentrating of the concentrator with the length of the light outlet is 3 mm, 4 mm, 5 mm, and the height is 20 mm, 25 mm, 30 mm, as shown in Table 1. Considering the relationship between the thickness of glass curtain wall and the geometric concentration ratio, CPC micro-concentrating unit with a height of 25 mm and light outlet length of 4 mm is finally selected according to the application place. Its section is shown in Fig. 4, geometric condensing ratio is 3.16. 3.2. Simulation result Taking the 0.5 mm wall thickness as an example, the angle between incident ray and axial symmetry plane of the concentrator is defined as θ1; and the angle between the incident ray and the cross section of the concentrator is defined as θ2. Schematic diagram of Fig. 5 are shown the two kinds of incident angle. In the simulation, θ1 and θ2 are changed in the range from 0 to 85°. Let the incident ray set as parallel light, then for ray tracing, the light path diagrams are shown in Fig. 6 and Fig. 7 respectively. For the incident angle θ1, when this angle is 0°, a part of lights reach the surface on receiver directly, another part of lights reach the CPC surface, be refracted and reflected then it is concentrated on the receiver surface. Most of light achieve to the receiver. With the increasing of incidence angle, light begin to through the micro-concentrating unit gradually. The lager the incidence angle is, the higher the transmittance is. When the incident ray angle greater than 55°, the receiver will not receive any ray. For the incident angle θ2, when the θ2 is changed, ray will be refracted and reflected multiple times inside micro-concentrating unit. The number of rays received by the receiver at the bottom of the micro-concentrating unit and the number of rays received by receiving surface of the micro-concentrating unit were analyzed; the receiving, reflectivity and transmittance were obtained, as shown in Fig. 8. It can be known by the simulation results: when the incidence angle
3. Optical simulation Performance analysis of the new transmissive concentrating system should be done and parameters of the concentrating system will be evaluated in the different situations. The main parameters of this study include receiving, reflectivity and transmittance. The optical simulation was performed using Trace Pro software, which is a kind of ray simulation software commonly used in lighting systems, optical analysis, radiance analysis and photometric analysis. It has extended numerical precision and a ray tracing tool specifically designed for optical design, specific to each ray that needs to be traced. It must be noted that the transmittance refers to the effective transmittance, which is defined as the ratio of the number of rays that can be emitted from the lower part of the concentrating system to the number of incident rays. 3.1. Model Selection In the current study, the right and left parabola equation of CPC is expressed by formula (1) and (2).
0.9486y − 0.3165x − 2 = 0.0949[0.9486x + 0.3165y − 1.8972]2
(1)
0.9486y + 0.3165x − 2 = 0.0949[0.9486x − 0.3165y − 1.8972]2
(2)
(3)
5
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Fig. 9. Comparison between solid CPC and hollow CPC.
is 0°, the receiving rate can reach 74%, when incidence angle θ1 is less than 20°, reflectivity increases slightly, and transmittance declined slightly, but amplitude is not large, the receiving rate can maintained above 70%. When the incident angle is greater than 20°, the receiving drops rapidly, when the incident angle is 25°, receiving is less than 5%, but transmittance at this time rapidly increases to 83%. Angle of incidence continues to rise to more than 55°, the receiving and reflectivity both approach zero, all ray passes through the micro-concentrating unit. When the angle θ2 is changed, as the incident angle increases, the receiving gradually decreases and the reflectivity gradually increases. The receiving ranges from 29% to 74%, the reflectivity ranges from 12% to 67%, the transmittance is always low, only 8% at maximum. When the incident angle is 0°, the transmittance is 2%. As the incident angle increases, the transmittance generally shows a downward trend. After the incident angle is greater than 75°, the transmittance is 0%, and the variation of each parameter is close to a linear trend.
3.3. Compared with solid CPC simulation results In order to know the differences of performance between hollow CPC and solid CPC which is proposed in reference [19], solid CPC has the same physical dimensions with hollow CPC. The comparison has been done and the result is shown in Fig. 9. Comparing the simulation results of the hollow CPC with the same physical dimensions of the solid CPC micro-concentrating unit, it can be seen from the result that when the incident angle less than 20°, hollow CPC receiving rate is about 20% higher than solid CPC, and in the range of small incident angle within 20° the reflectivity of the hollow CPC is much lower than the solid CPC, especially vertically incident ray can even lower by about 40%. Significant differences in receiving over a small range of incident angles indicates that hollow CPC is more advantageous than solid CPC in terms of solar energy using, and the transmittance difference between the hollow CPC and the solid CPC is not obvious, it shows that in the aspect of guaranteeing indoor lighting in practical application, hollow CPC and solid CPC are basically the same. But after the incident angle is greater than 20°, the transmittance 6
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Fig. 10. Comparing between two different cross-sectional shapes.
of the hollow CPC is higher than the solid CPC, about 8%~ 43%. In the range of larger incidence angle, the lighting effect of hollow CPC is better than solid CPC.
between incident angle 20° and 25°.
3.4. Comparison of different cross-sectional shapes
From the analysis of optical paths of different incident angles, it is found that the ray has completely different optical paths when it enter the wall thickness portion or the intermediate medium portion, so it is certainly that the wall thickness has an influence on its optical characteristics. For this purpose, get the receiving, reflectivity and transmittance when the wall thickness varied from 0.5 mm to 3 mm. Fig. 11 is shown the results about receiving, reflectivity and transmission under the wall thickness of 0.5 to 3 mm. The results show that when the incident angle is less than 20°, the thinner wall thickness of the model has the higher receiving and the lower transmittance, such as when the incident angle is 5°, wall thickness of 0.5 mm compared with wall thickness of 3 mm. The receiving of the micro-concentrating unit with thickness of 0.5 mm is 76%, and the transmittance is 3%, but the receiving rate of the microconcentrating unit with wall thickness of 3 mm is only 50%, the transmittance is 13%. But when the incident angle is larger than 20°, taking the incident angle of 35°as an example, the receiving of wall thickness of 3 mm is 26%, and the transmittance is 53%, but the receiving of wall thickness for 0.5 mm is only 1%, and the transmittance
3.5. Comparison of different wall thicknesses
Comparison between two different cross-sectional shapes has been studied by simulation, two models have the same upper and lower end geometry size, the same wall thickness, but section is different which is trapezoidal and another is CPC. In order to analyze the performance of two kinds of condenser, simulation has been done in different incident rays angle θ1 and the results are shown in Fig. 10. In the simulation, the cross-section of the trapezoidal has a limit receiving angle of 75°, receiving and reflectivity are both 0% and the transmittance is 100% after incident angle is greater than 75°. Although it can expand the range of the received ray, but CPC shape can ensure a good receiving even at an incident angle is 20°, as high as 70%; The receiving of the trapezoidal shape decreases sharply from 5°, and when the incident ray angle is 20° receiving is only the 39%, which is lower than CPC shape; compared with the trapezoidal and CPC, the transmittance increases with the increase of the incident angle, the trapezoidal shape changes gently and do not fit the need of solar photovoltaic glass curtain wall. The CPC shape has a significant increase 7
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Fig. 11. performance of different wall thicknesses.
achievability in actual use, micro-concentrating units with a wall thickness of 2 mm was selected to make a sample with the material of high transparency of PMMA (polymethyl methacrylate). It is shown in Fig. 12.
is 92%. The difference in reflectivity change is small; the reflectivity is 0% after the incident angle is greater than 50°. The wall is thicker, the change in reflectivity of incident ray at each angle is more pronounced. 4. Sample processing and experimental testing
4.2. Experimental methods Based on the results of simulation research, the system has a higher receiving when the incident angle is smaller, and a higher transmittance when the incident angle is larger. Experiment research about the new type of transmissive concentrating system is needed to verify the simulation and obtain the performance under real weather conditions.
Outdoor experiment was undertaken under real sky conditions. The system was arranged on the sunlight directly in which the major target is to investigate the light transmittance of the new type of system when the sunlight is in different incident angles. Furthermore, a double-layer vacuum glass which is used commonly in modern building was selected to test in the same condition, the test results of two types glass curtain wall were compared and analyzed. The experimental device is shown in Fig. 13. The integral box
4.1. Size selection According to the simulation results and in combination with the 8
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transmittance =
LX 1 × 100% LX 2
(4)
where the units of LX1 and LX2 are Lux. When the double-layer vacuum glass is tested, the same method can be used. In the experiment, the solar cell was attached to the bottom of the micro-concentrating unit, but the electricity generation was not tested in this experiment. Because the focus point of this study is only about transmittance characteristic. When the integrator box is placed, the light at the maximum solar height of the test day is guaranteed to be incident perpendicularly to the test device. Two location situations are selected in the experiment and the results are compared, one is vertical arrangement which is making the axial line of CPC point to north-south orientation, and another is horizontal arrangement which is making the axial line of CPC point to east-west orientation. At the same time, in order to compare the difference between the new glass curtain wall system developed in this paper and the existing glass curtain wall, the comparison have been done when they are in the vertical direction location. The experiment was conducted on May 31 and it is sunny. By calculation, the maximum solar altitude is about 72° and the apparent solar time is 12:11 on that day. Error analysis has been done about the experiment, in this test, illumination is the only test value, but the values test times are different and it is used to calculate the transmittance. So we must record the illumination values as quickly as possible and reduce the time interval between with tests with and without cover plate. In the experiment, time intervals are less than 1 min, so the results can be considered accurate.
Fig. 12. Sample with wall thickness of 2 mm.
transmittance experimental device is designed by the principle of integrating sphere. At the same time, in order to avoid the local illumination unevenness caused by direct light, the inner wall of integral box is covered with diffuse reflection coating to ensure uniform illumination. The illuminance measurement is performed using a multi-function digital photometer with a measuring range of 0.1–100 Lux. The top of the integrator box has the fix hole with the same size as the photometer for easy measurement. The integrator box is placed outdoors, and the light intensity in the integrator box without any covers is measured by the handheld digital photometer to be LX1, then the new transmissive concentrating system cover is placed, and the light intensity in the integrator box is again measured as LX2, the transmittance of the new transmissive concentrating system is:
4.3. The test results Based on the real sky tests, when the axis of CPC is placed north and south, the experimental results are compared with incident angle θ1 in
Fig. 13. Experimental device for transmittance integrating box. 9
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Fig. 14. Comparison of experimental and simulated transmittance.
well. The test results show that transmittance is small when the sunlight incident vertically at noon, but it becomes lager when the incident angle increases at morning and afternoon. The new transmissive concentrating system has the lowest transmittance at around 12:00 noon, only about 30%. In the simulation results, the transmittance is near about 0% when the light incident vertically, it is lower than the test
simulation results at incidence different angles and is shown in Fig. 14(a), and when the axis of CPC is placed east and west, the experimental results are compared with incident angle θ1 in simulation results and it is shown in Fig. 14(b). It can be seen from Fig. 14 that the changing tendency of experimental results and simulation results about transmittance are matched 10
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Fig. 15. Test results comparison between new system and vacuum double glass in real sky.
is true. The experimental in the real sky have been done, results show that the new transmissive concentrating system has low transmittance at the noon, when the outdoor irradiance is close to 1100 W/m2, the transmittance of the system is only 28.7%, most of lights are keep out, so it can prevent indoor illumination to dazzle effectively. Changing tendency of experimental results and simulation results about transmittance are matched well, but there is a difference in transmittance value of numbers, the reason is that scattered light enter into system desultorily. In the morning and afternoon when the room needs lighting, the maximum transmittance can exceed 60%, which can provide enough indoor lighting during this period. Compared with the doublelayer vacuum glass which is widely used in modern buildings, the transmittance of this new type of transmissive concentrating system can be changed with the outdoor lighting conditions, in the morning and afternoon when indoor lighting is needed, its transmittance is higher, but the transmittance is much lower than that double-layer vacuum glass at noon in direct sunlight. In comparison, this new type of transmissive concentrating system can perfectly replace the double layer vacuum glass, suitable for use as a glass curtain wall in BIPV systems, enabling passive light control. It has a good research prospect and practical application space. In addition, it should be pointed out that a typical sunny weather was selected to do experiment for discussion and analysis. But the weather is not so nice every day, for other types of weather, such as cloudy, fog and haze weather, the proportion of scattered light will increase, and the concentrating characteristics of the device will change differently, so it is necessary to study in the future.
results and mainly because that, in simulation, the incidence light are direct light; but in the real sky, direct light and scattered light are all exist, direct light can be received by solar cell but scattered light will go through the transmissive concentrating system disorganized, so most of them are penetrating into test box and it increase the transmittance. And the transmittance gradually increases as the incident angle increases. In the simulation results, after the incident angle is greater than 55°, the transmittance is close to 100%; but in the test, the maximum transmittance can only reach about 60%. The reason is that: the cover plate is produced by PMMA which is not completely transparent and it will influence the transmittance of transmissive concentrating system. The comparison tests between the new transmissive concentrating system device and the double-glazed curtain wall are done and the results are shown in Fig. 15. As can be seen from the Fig. 15, the double-layer vacuum glass cover can maintain a high transmittance level and change little, it remains at 40%~50% all day, but the transmittance of new transmissive concentrating system cover is lowest at noon, which is about 20% lower than that double-layer vacuum glass cover; before 9:40 and after 16:00, the transmittance is higher than double-glazed vacuum glass cover, and the highest even about 10%. It is shown that new transmissive concentrating system can adjust the light condition of indoor environment and avoid glazing at noon. Furthermore, redundant sunshine can be converted to electric at noon, the comprehensive utilization of solar energy is realized in building by the new transmissive concentrating system.
5. Conclusion Based on the ray tracing simulation results of the new transmissive concentrating system, it shows that a higher receiving rate can be obtained in direct incident light or light with a small incident angle. When the incidence angle is 0°, the receiving rate can reach 74%, when incidence angle θ1 is less than 20°, reflectivity increases slightly, and transmittance declined slightly, but amplitude is not large, the receiving rate can maintained above 70%. Compared with the trapezoidal structure and the solid structure, the new transmissive concentrating system is more advantageous in light control characteristic. By comparing the simulation results of different wall thicknesses, it is concluded that for smaller incident angle, the thicker wall thickness has the higher receiving, but when the angle of incidence is large, the opposite
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement This work was supported by the National Natural Science Foundation of China (No. 51766013, No. 51806244).
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