Recommend | PV curtain wall design points

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Source: Central South Curtain Wall Author: Liang Shuguang Liang Shulong Hu Bo Wu Yuekuo

Abstract: In this paper, according to the photovoltaic panel layout, power generation calculation, structural design three often encountered in the design stage of the key points of analysis, specifically in the form of photovoltaic curtain wall, grid design, tilt optimization, shadow blocking, power generation calculation, structure of the avoidance of position, wiring, adjust the panel angle and other dimensions to carry out detailed analysis. The optimal installation form of different types of building photovoltaic curtain wall, the size deduction process of crystalline silicon and thin film photovoltaic modules, the relationship between installation inclination and power generation, the simple calculation method of power generation, the design principle of photovoltaic curtain wall structure and other design reference basis are obtained, which can provide design reference for the building and curtain wall designers and relevant unit personnel who initially contact the photovoltaic curtain wall.

Key words: photovoltaic curtain wall; inclination; grid; shadow; power generation; structure

0 Introduction

Photovoltaic curtain wall is a curtain wall containing photovoltaic components and has the function of solar power generation. In order to cope with the climate crisis, the number of buildings with photovoltaic curtain wall is on the rise, so the design technology of photovoltaic curtain wall needs to be popularized. The design of photovoltaic curtain wall is usually understood to be based on the design of traditional curtain wall, and the additional consideration of photovoltaic power generation panel layout inclination, grid optimization, keel structure, carbon reduction calculation and other work. The designed photovoltaic curtain wall should achieve beautiful effect, excellent power generation efficiency, and convenient construction, installation, disassembly and maintenance. In this paper, according to the layout of photovoltaic panels, power generation calculation, structural design of the three points often encountered in the design stage of the analysis.

1 Photovoltaic panel layout

1.1 photovoltaic curtain wall form

The form of photovoltaic curtain wall can be divided into large-sided curtain wall, interlayer curtain wall, photovoltaic railing, photovoltaic lighting roof and photovoltaic shading components according to the installation position. According to the installation materials, it can be divided into crystalline silicon plate power generation curtain wall and thin film power generation curtain wall. According to the installation effect, it can be divided into transparent curtain wall and non-transparent curtain wall. Because the choice of photovoltaic installation form directly affects the building facade effect and power generation efficiency, it is determined in the early stage of the building plan. According to the architectural modeling characteristics, the use of functional requirements, photovoltaic curtain wall form selection priority is also different. Considering the light transmission effect, the thin film power generation component is generally better than the crystalline silicon component. In terms of power generation efficiency, crystalline silicon components are generally superior to thin-film power generation components. The feasibility and priority of photovoltaic curtain wall of common building types are analyzed as follows.

1) In residential buildings, the south facade has many windows and small and scattered real walls, giving priority to light-transmitting photovoltaic railings. Mountain wall solid wall area is larger, priority installation of opaque photovoltaic power generation efficiency is larger, the economy is higher large curtain wall.

2) In large commercial buildings, the lighting of buildings mainly depends on internal lighting. The importance of natural lighting is lower than that of residential buildings. The facades are mostly glass curtain walls or large-area solid walls. The glass curtain walls consider large-surface curtain walls with light transmission, while the real walls consider large-surface curtain walls with opaque light transmission. What can not be ignored is that commercial buildings usually set up a large area of lighting roof, can consider the light transmission of photovoltaic lighting roof.

3) In office buildings, the main use space has high requirements for lighting, while the auxiliary space has low lighting requirements. Considering the unified facade effect, the curtain walls corresponding to these spaces can be used to install light-transmitting photovoltaic modules. In addition, office buildings have certain requirements for sunshade, and external photovoltaic sunshade components can be set in combination with the sunshade function.

4) In cultural buildings, the building has a strong integrity, and the facade elements are relatively regular, mostly large-area walls or glass curtain walls; while indoor lighting is mainly solved by electrical lighting, and the requirements for natural lighting are slightly lower, so the facade resources can be fully utilized to install impervious large facade curtain walls.

In summary, the installation forms of photovoltaic curtain walls for different types of buildings are shown in Table 1:

1.2 lattice design

In this paper, the grid design of curtain wall is analyzed by two different materials of silicon and film. Because the size of photovoltaic modules is not mandatory, the size of photovoltaic modules produced in large quantities by each manufacturer is different. Therefore, when designing specifically, the size of the components to be used should be obtained from the manufacturer to avoid that there is no corresponding photovoltaic module product that can be purchased after the design is completed. First of all, it is suggested that the partition size of curtain wall should be designed according to the manufacturer's standard component size. Due to the processing technology and processing methods, photovoltaic modules of different sizes can be customized. Sometimes the desired component size cannot be obtained, which can be produced and may affect the construction period of the project. Therefore, understanding the principle of photovoltaic module processing is of great significance to self-grasp and customizable size.

For crystalline silicon components, solar cells are the smallest unit of photoelectric conversion, and the common sizes are 156 mmx156mm, 166 mmx166mm, 182 mmx182mm, and 210 mmx210mm. Typically, the size of a standard crystalline silicon component is about 2000 mmx1000mm. To customize the size of photovoltaic modules, first determine the specifications of the cells, then determine the size of the gap between the cells and the size of the edges of the cells and the glass. After simple calculation, the size of the modules can be determined. Before using customized modules, be sure to communicate with the manufacturers of photovoltaic modules. Figure 1 is a customized crystalline silicon photovoltaic module.

Fig.1 Customized crystalline silicon photovoltaic module

For a membrane module, the size of a standard membrane module is 1200 mmx600mm. The size of less than 1200mm x 600mm can be obtained by cutting the film, and the size of more than 1200mm x 600mm, the power generation film needs to be spliced, for example, to obtain a 1500mm x 800mm component, three parts of 800mm x 500mm power generation film can be selected to be spliced. Because of the limitation of process requirements, there is often an obvious black patchwork seam at the splicing position. Also be sure to communicate with PV module manufacturers before using custom components. Figure 2 is a customized thin film photovoltaic module.

Figure 2 Customized thin film photovoltaic modules

1.3 inclination optimization

Photovoltaic tilt angle optimization mainly considers two factors. On the one hand, it is to ensure that the photovoltaic modules receive as much solar radiation as possible, on the other hand, the overall effect of the building should be considered to ensure the coordination of the combination of photovoltaic modules and buildings. For each form, it is necessary to fully tap its characteristics to balance its advantages and disadvantages and obtain the best balance point. The relationship between the orientation and inclination of the photovoltaic curtain wall and the power generation can be referred to Figure 3 below.

Fig.3 Relationship between orientation and inclination of photovoltaic curtain wall and power generation

It can be seen from Figure 3 that the maximum amount of solar radiation is obtained when the photovoltaic module is facing south in the northern hemisphere. The difference in the amount of solar radiation obtained by the photovoltaic module is very large, and the solar radiation value when the tilt angle is 90 degrees is only 70% of the optimal tilt angle. Therefore, in the design of photovoltaic curtain wall, for projects that can adjust the inclination angle, such as interlayer curtain wall, photovoltaic lighting roof, photovoltaic shading construction, etc., the best inclination angle is designed as far as possible to make the photovoltaic module obtain the maximum amount of light radiation. For the east and west oriented photovoltaic systems, amorphous silicon components with strong weak light can be selected to ensure the extension of power generation time to increase power generation.

The method of obtaining the best inclination angle, taking Hangzhou as an example, using PVsyst software to calculate, according to the annual average solar radiation of the component under different inclination angles, the angle with the largest annual average solar radiation is obtained as the best inclination angle. The average solar radiation data of the previous year on different angle inclined surfaces are shown in the following table:

It can be seen from the above table that the maximum amount of solar radiation is received throughout the year when the inclination angle is 20-24 °. The maximum radiation of the optimal installation angle of the lighting surface is 1266 kwh/m². The enlarged angle range, as shown in Figure 4, can be analyzed, when the angle tends to the optimal angle from 0 degrees, the total radiation amount of the inclined surface increases slowly, and after the angle is greater than the optimal angle, the loss of solar radiation begins to decrease significantly.

4 Ratio of annual average solar radiation of different angle array surface to horizontal surface in Hangzhou area

The installation angle of the photovoltaic curtain wall needs to be polished repeatedly by the architectural scheme designer and the photovoltaic curtain wall designer, and finally determined.

1.4 to avoid shadows

In the photovoltaic module, the part that is shaded by the shadow will become a load, causing the local temperature of the component to be too high, which will seriously burn the component and even cause a major fire. This is the spot effect of the photovoltaic module. Therefore, any partial shading of the source of the photovoltaic panel will largely result in a reduction of the output power. Deline's research shows that for a series system, a shadow block can cause a power loss equivalent to 30 times its physical area. For crystalline silicon solar cells, small shading can cause high power loss. Therefore, in the design of photovoltaic curtain walls, the relevant amorphous silicon thin film cells should be used for scenes that are greatly affected by shadows, because the current density of thin film components is small, and the influence of shadows is much smaller than that of crystalline silicon cells. Designers need to use professional tools for shadow analysis, taking full account of the shadows generated by environmental obstructions and installation components, and accurately calculate the amount of solar radiation received by photovoltaic modules throughout the year. There are many tools for shadow analysis, most of the software at home and abroad can only analyze the roof, or the shadow of the ground power station, the complex shadow analysis of the building facade can use Solar PV software, see Figure 5.

Fig.5 Irradiation analysis chart of Solar PV software

In addition, it is necessary to fully consider the cleaning plan of the photovoltaic curtain wall, and clean up the bird droppings, leaves, dust, etc. on the surface of the photovoltaic power generation panel in time.

2 Calculation of power generation

The calculation of power generation is a parameter that building development units and design units attach great importance to, because the fundamental purpose of the combination of buildings and photovoltaics is to generate electricity. The more electricity is generated, the more energy can be saved and the more carbon emissions can be reduced. In the early stage of architectural design, the power generation can be roughly estimated and calculated. In the specific design stage of photovoltaic curtain wall, the power generation of the system should be calculated in detail and accurately. The formula for the calculation of the power generation is usually GB50797-2012 in the Code for the Design of Photovoltaic Power Stations, and the formula provided in the section on the calculation of the power generation:

In the formula:

HA is the total solar radiation in the horizontal plane (kW · h/m², peak hours);

EP is the on-grid power generation;

ES is the irradiance under standard conditions (constant = 1kW · h/m²);

PAZ is component installation capacity (kWp);

K is the comprehensive efficiency coefficient.

The comprehensive efficiency coefficient K includes: PV module type correction coefficient, PV square inclination, azimuth correction coefficient, PV power generation system availability, light utilization rate, inverter efficiency, collector line loss, step-up transformer loss, PV module surface pollution correction coefficient, PV module conversion efficiency correction coefficient.

1) The total solar radiation on the horizontal plane can be purchased from the weather station, or it can be simulated by professional weather software, such as NASA,Solargis,Meteonorm, etc. For those who do not have software, a rough query can be made through the collated experience table.

2) Formula for component installation capacity:

where: is the peak power of each component and N is the number of components.

3) The comprehensive efficiency coefficient is related to many parameters, generally calculated by professional software, in the power generation estimation.

When calculating, the coefficient outside the inclination of the photovoltaic array can be taken as 80%-82%. Because the angle of the photovoltaic curtain wall changes greatly, the inclination coefficient of the photovoltaic square array is considered separately. From Figure 4, it can be roughly estimated that the curtain wall loss of the 90 elevation is about 40%.

Software developed by Zhejiang Zhongnan Curtain Wall Design Institute for photovoltaic design process and common problems in design, PVsyst, Solar PV, archelios, Solar edge and other photovoltaic-related software provide more accurate power generation calculation services.

3 Structural design

As the outer envelope of the building, the curtain wall is an important component to realize the architectural function and express the architectural shape. Curtain wall design is a subject involving a very wide range of aspects. When designing a curtain wall, it is often necessary to consider the requirements of materials, structure, structure, heat, light, lightning protection, fire protection, function, beauty, etc. Photovoltaic curtain wall as a kind of curtain wall, the design should not only meet the requirements of ordinary curtain wall design, but also meet the requirements of avoiding position, wiring, adjusting panel angle and so on.

3.1 Avoidance

Photovoltaic modules will be equipped with junction boxes, and the assembly of curtain wall components should avoid junction boxes to avoid interference. The solar cell module junction box is very important in the composition of the solar module, and its main function is to connect the power generated by the solar cell with the external circuit. The junction box is divided into the back junction box and the side junction box according to the different placement positions. The back junction box is located at the end or middle of the back of the photovoltaic curtain wall assembly, and the side junction box is located at the front and rear or left and right end glass cutting surfaces of the photovoltaic curtain wall assembly. The sizes of junction boxes set by different manufacturers are different, and the preferred positions of junction boxes for different curtain wall systems are different, as shown in Table 3.

Table 3 Preferred positions of junction boxes for different curtain wall systems

The distance between the side of the photovoltaic panel with the junction box and the curtain wall component should be greater than the size of the junction box. The cushion block used when installing the photovoltaic curtain wall component on the beam should be higher than the height of the junction box of the photovoltaic curtain wall component.

3.2 Wiring

Curtain wall has decorative properties, photovoltaic cable routing should be hidden processing, the traditional curtain wall structure is difficult to achieve the effect of hidden cable. Moreover, because there is no reserved line warehouse, the wiring needs to be punched in the curtain wall components, which destroys the original structure of the curtain wall and increases the difficulty of component processing and on-site installation. Therefore, in the early stage of photovoltaic curtain wall design, wiring should be taken into account in the structural design.

There are three ways to hide cables. One is that part of the curtain wall itself meets the requirements of hiding cables. For example, the panel attachment frame of the hidden frame curtain wall can be routed, as shown in Figure 6. This routing method is not convenient for cable maintenance. One is that the curtain wall keel itself is provided with a wire warehouse, as shown in Figure 7. After wiring is completed, the wire warehouse is closed by gland, so as to maintain the effect of the original curtain wall and facilitate maintenance. When maintenance is needed, the gland can be disassembled. The other is to set up an additional wire warehouse, as shown in Figure 8, to hide the cable inside the wire warehouse for positions with low requirements on effect, or for non-transparent curtain wall parts, the cable is combed by binding and placed in the wire warehouse.

Fig. 6 Keel Self-structure Hidden Cable

Fig. 7 The keel itself is equipped with line warehouse (Jiaxing Photovoltaic Museum)

Fig. 8 Additional Line Stores

3.3 adjustment panel angle

To meet the public's pursuit of artistic beauty, the construction and utilization rate of special-shaped buildings accounts for an increasing proportion of domestic buildings. The most important requirement for the construction of special-shaped buildings is that the curtain wall panels can be spliced at any angle. Generally, the angle deformation is absorbed by setting a rotatable aluminum alloy attachment frame adapter frame, as shown in Fig. 9. The angle and azimuth angle of each curtain wall panel of the photovoltaic curtain wall are different. In the case of little difference in angle and azimuth angle, the inverter of the electrical part has MPPT (Maximum Power Point Tracking) function, and the parallel connection of circuits caused by the installation position or environmental impact of photovoltaic curtain wall components can be solved by using multiple MPPT series inverters. In the case of large differences in angle and azimuth, each photovoltaic curtain wall component can be set up separately with a micro-inverter, so that the output power of each component can be near the maximum power point, greatly weakening the short board effect and reducing the loss of power generation.

Fig.9 Photovoltaic curtain wall system with angle adjustment

4 Conclusions

In this paper, the detailed analysis is carried out in the dimensions of photovoltaic curtain wall form, grid design, inclination optimization, shadow shielding, power generation calculation, construction avoidance, wiring, adjustment panel angle, etc. It is concluded that the preferred installation form of different types of building photovoltaic curtain wall, the size deduction process of crystalline silicon and thin film photovoltaic modules, the relationship between installation inclination and power generation, the simple calculation method of power generation, the design principle of photovoltaic curtain wall structure and other design reference basis. At the beginning of the scheme design, the scheme designer can judge the installation position and form of the design building, judge the installation angle, adjust and run-in to produce relatively perfect and well-considered scheme drawings according to the design process, and avoid making major changes to the scheme in the later period. In the curtain wall design stage, the curtain wall designer can consider the grid, inclination, structure and economic calculation of power generation in the early stage according to this design process.


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