- Evaluation Criteria for BIPV Design
Photovoltaic power generation systems can be integrated on buildings in an elegant and aesthetic way and thus become part of the building’s overall integrity. To measure a high-quality building-integrated project, some metrics are needed. Although most photovoltaic buildings achieve good technical performance, the integration of photovoltaic systems into the building is not very good, and the quality of the building is often not very satisfactory. For a well-integrated project, its construction quality needs to meet high standards, and at the same time, the quality of the building itself and the technical performance of the photovoltaic system also need to meet the requirements.
(1) Natural integration Natural integration means that the photovoltaic system becomes a natural logical part of the building. If there is no photovoltaic power generation system, the building seems to lack something. The photovoltaic power generation system and the building form an inseparable whole.
(2) The building is satisfactory Based on a good design, whether the photovoltaic power generation system adds some eye-catching features to the design. Buildings should look more attractive, and the photovoltaic system obviously improves the design. This is a very intuitive question, and there is no doubt that some buildings are simply more attractive than others.
(3) Perfect composition and structure The color and texture of photovoltaic modules need to be consistent with other finishing materials. In most cases, photovoltaic systems are produced in a certain way. For example, by replacing framed components with frameless components, suitable color, transparency, shape and texture can be obtained using special photovoltaic technology.
(4) Grid, fusion and composition The size of the photovoltaic system should match the size of the building and the grid on the building, which determines the size of the components and the size of the grid bars used on the building.
(5) Overall integration The entire exterior of the building should be integrated with the photovoltaic system and consistent with the building as a whole. On historic buildings, tilted photovoltaic systems look better than larger modules. Likewise, high-tech photovoltaic power generation systems should be more suitable for high-tech buildings.
(6) Good quality of the project This not only involves the waterproofing and reliability of the building, but also the perfection of the details. Have the details been conceived in detail and have the designers noticed the details? Is the material wasted? These all determine the effect of the details of the work
(7) Design innovation Photovoltaic power generation is a kind of technological innovation, so it is necessary to have innovative thinking on buildings. New thinking can enhance the photovoltaic market and increase the value of buildings.
- The core problem of BIPV design
① The core part of the design is to establish the number of components, the size of the components, and the overall size of the power generation system integrated on the roof or front.
②The shadow on the module is also an important issue to consider, especially when the modules are connected in a string, when part of the module is blocked by the shadow, the system loses more efficiency than expected. When a row of batteries in the module is blocked by the shadow, The efficiency of the entire string of components is affected. During the day, shadows move, creating a lot of indirect incident light. Diodes are integrated into many assemblies, and these diodes can cause a short circuit when a row of cells is completely or partially shaded. Converting DC mode to AC mode helps isolate the effects of shadows. Other photovoltaic technologies, such as amorphous silicon thin films, titanium dioxide cells, are less affected by shadows due to their different electrical connection properties and their better performance in low light.
③ The performance of the inverter is also very important, and most inverters have an isolation position. In general, shadows should be avoided as much as possible, and space is also required for efficient and safe installation of the inverter. There is a junction box behind the module that connects to the inverter. Installing the inverter close to the module can improve module efficiency. On the inverter side, the grid connection can be realized in AC mode through the electricity meter.
④When the temperature rises, the efficiency of the crystalline silicon cell will decrease, so ventilation is required behind the module, and the junction box behind the module also needs installation space, which depends on the size of the junction box and the gap between the two junction box components. 20~50mm is required (it depends on the size of the junction box). A safety switch also needs to be installed near the inverter of the photovoltaic system.
(1) Maintenance and cleaning of photovoltaic modules In cities or rural areas with poor environment, horizontally placed photovoltaic modules are prone to stratification of dust, which reduces the efficiency of photovoltaic modules by about 4%. Photovoltaic modules with an inclination angle greater than 20° can be self-cleaned by rainwater, and the special treatment of photovoltaic modules helps to keep the modules clean. According to different applications, photovoltaic modules need to meet certain building standards, so the installation of photovoltaic modules needs to consider national and local regulations.
(2) Issues that need to be paid attention to in the city The photovoltaic power generation system needs to meet other functions besides power generation. Photovoltaic modules can effectively replace traditional building materials, thereby reducing system costs. In order to generate as much power as possible, the photovoltaic building integrated power generation system needs to reduce the shadow area projected on the system. The shadow generated by the photovoltaic power generation system mainly comes from buildings and some strong reflected light.
(3) Installation direction and inclination of photovoltaic modules The amount of solar radiation received on photovoltaic modules depends on the latitude of the building and the local climate, and the maximum radiation amount depends on the inclination and direction of the light-collecting surface of the module. For 52° north latitude, the photovoltaic system can receive more than 90% of the radiation when the inclination angle is between 30° and 50° in the southeast and southwest directions. When the inclination is between 10° and 30°, acceptable radiation can be obtained in the east and southeast and west and southwest positions, and the difference between the obtained radiation and the optimal radiation is only 15%.
(4) Distance problem between photovoltaic buildings In order to avoid shadows between buildings, for low-rise buildings, the distance between them is easy to calculate, and shadows are also a relatively easy problem to solve. For a high-rise apartment built next to a low-rise building, it creates a lot of shadows that we don’t want. The density of buildings in a location is also a more critical factor, such as in high-density areas of cities or downtowns, where the distance between buildings is limited and shadows will exist for long periods of the year. In particular, curtain wall systems are more susceptible to shadows and therefore require more space than roof systems.
(5) The impact of trees on photovoltaic power generation systems While trees are greening the environment and improving climate microcirculation, the shadows they produce also have negative impacts. Especially in summer, the canopy is at its maximum; even in winter, the shade from the trunk cannot be underestimated. Because trees grow slowly, people often underestimate the rate of growth. In order to avoid the above problems after the building is built or the photovoltaic system is installed, it is very important to design well. In order to avoid the impact of trees on photovoltaic modules, there are the following solutions:
① Plant trees only on the north side of the building (northern hemisphere), and for buildings located in the southern hemisphere, only plant trees on the south side of the building;
② Only plant relatively short trees or the limit height of the tree is less than the height of the building roof;
③ The trees should be pruned regularly every year to avoid shadows on the light-collecting surface of the photovoltaic modules.
(6) Specialized photovoltaic power generation system area The photovoltaic power generation system located in the urban area may require a dedicated photovoltaic power generation area, and also requires three-dimensional maps, computer models, etc., to determine the boundary of the building area, so as to avoid possible shadows.
(7) The influence of reflected strong light on photovoltaic modules Although reflected strong light is not a typical problem, under certain conditions, undesired strong light will still be generated. In mixed areas of low-rise and high-rise buildings where curtain wall systems are increasing, they will prevent light from the photovoltaic system from being reflected to other buildings. The degree of this obstruction depends on the surface texture of the PV modules, the installation of the system, the position of the sun, and the intensity of direct radiation at specific times of the year. Achieving a certain distance between buildings also helps avoid shadow problems and most potential glare problems.
(8) Shape and color The color of solar cells is generally blue, dark blue or almost black. Other colors are gray, green, red, orange and yellow are also possible. These colors of solar cells are not produced according to standard processes. Yes, they can be more expensive than standard components. But blue solar cells have the highest conversion efficiency.
Component type also has a greater impact on color. When the module has no frame, the overall surface of the system looks more consistent, so the color of the entire system will only be the color of the battery, and the photovoltaic system on the roof looks like a large surface; the framed module gives a special visual experience , the module frame greatly affects the visual effect of the photovoltaic array and the integration with the building. Sometimes the frame of the module can bring unexpected visual effects, the frame of the module will increase the size of the module and also affect the installation side of the roof. The color of the frame and the encapsulation material on the back of the component can also be different, thereby also providing designers with more choices.
- Issues that need attention in the electrical aspects of BIPV
(1) Maximum sunlight reception Since the electrical energy generated on the PV modules is proportional to the intensity of sunlight, the electrical energy generated by the photovoltaic system can be achieved by maximizing the light intensity on the array, but this is not always feasible. In practical applications, such as the mounting orientation of the array, occlusion by nearby objects, and dust on the surface of individual components, all reduce the amount of sunlight received on the components. The ideal photovoltaic array is one that tracks the sun with the array surface always perpendicular to the sun’s rays. Practically, a pedestal-type array support structure tracks the sun through one or several mechanisms. Typical solar tracker applications are on pumped photovoltaic systems.
However, the array of most BIPV systems is fixed, and in most cases, the installation direction of the PV array is limited by the design and orientation of the building.
(2) Influence of shadows The current generated on the photovoltaic array is proportional to the light intensity received on the surface of the array. However, when only part of the array receives light, the photo-generated current is not proportional to the light-receiving area. Even a small amount of shadow can have a significant effect on the power output of the array. This is because the PV modules are connected in series by wires, so the current flowing through the module is the same as the current on the module that produces the least current, i.e. a single module. Being shaded also limits the current flow through other unshaded components.
The effect of this influence depends on the proportion of direct light and diffused light. 10% shading of the light-receiving surface of the photovoltaic array will reduce the power generation of the array by 50%. The relationship between the shadow blocking area and the reduction of power generation is shown in Figure 7-5. Therefore, any building-integrated photovoltaic array must avoid shading as much as possible.
(3) Influence of snow and ice In the northern climate, the influence of snow or ice is also worthy of attention. Photovoltaic arrays often operate under harsh environmental conditions. For example, the surface of photovoltaic modules powered by radio relay stations on the top of the mountain is often covered with thick ice. BIPV systems operating in more environmentally friendly conditions are occasionally covered in ice and snow, and in most cases, when sunlight penetrates the ice, the water formed will flow away with the slanted array of modules. It is important in the design of the modules to minimise bulges on the bottom of the support structure so that the covered snow layer naturally slides away.
(4) Influence of temperature The amount of light received by the array is the most important factor affecting the power generation of the photovoltaic array. BIPV designers need to consider the temperature on the PV array. High temperature will reduce the operating voltage of crystalline silicon PV modules, which will further affect the performance of PV modules; extremely high temperatures will damage PV modules. Since PV modules are directly exposed to sunlight, heat will inevitably be generated. A good PV system installation design should provide module backside ventilation to remove the heat generated. Ideally, the temperature of photovoltaic cells should be kept below 70°C. According to the relevant regulations in Europe, the system installed on the curtain wall will have the following losses:
①No air gap, loss of 10%;
② 5cm air gap, loss of 5%;
③ 15cm air gap, the loss caused by temperature effect is minimized.