1. Key points of engineering design of building photovoltaic
(1) Constant load of building photovoltaic for buildings under construction, the load of photovoltaic system has generally been considered in advance. Here, the load of the built buildings is mainly introduced. For the roof photovoltaic system, when the installed roof is accessible roof, after the installation of photovoltaic modules, due to the lack of space for activities, it can be regarded as non accessible roof. The use attribute of the roof is changed from accessible roof to non accessible roof. The resulting installation load margin of photovoltaic modules is 1.417kN/m2, which can generally meet the installation requirements of photovoltaic modules. When the installed roof is a non accessible roof, the installed photovoltaic modules increase the dead load of the roof. At this time, the bearing capacity of the roof needs to be carefully checked. If the roof does not have enough bearing capacity margin, direct installation will pose a threat to the structural safety of the building. In this case, for the building structure with small span, the photovoltaic support is usually directly rooted in the frame column to avoid its association with the roof, so as to avoid the increase of the roof dead load; For the building structure with large span, the roof reinforcement method is generally adopted, but the cost of this method is relatively high.
(2) The wind load of building photovoltaic is very important for the wind resistance design of building photovoltaic system and photovoltaic array. When installing photovoltaic systems on existing buildings, photovoltaic arrays often do not want to be “rooted and installed” when designing their combination with buildings. For the installation of photovoltaic systems on existing buildings, they are often designed as counterweight wind resistant or centralized connected supports. After adopting the appropriate support form, in order to ensure the safety of building photovoltaic, the wind load of photovoltaic system shall be designed according to the national standard GB50009-2001 Load Code for the Design of Building Structures (2006 edition) when installing the building wall or roof support, and it shall be considered that: the wind load of the separated photovoltaic panel shall be included in the wind load on the windward side and the wind load on the leeward side; The wind load of the support shall be included in the wind load from the panel and the wind load directly borne by the support; The panel system shall be divided.
(3) Requirements for installation convenience in order to combine with buildings and facilitate installation, solar cells are generally made into solar cell tiles, and the module structure and installation method are no different from ordinary building tiles; For components applied to photovoltaic curtain wall, the frame structure shall also be the same as the modulus of building curtain wall, and the installation method shall be the same as that of ordinary curtain wall glass. If ordinary photovoltaic modules are used, special brackets or guides shall be made to facilitate the installation of ordinary solar cells on them; In addition, solar cells are often made into frameless components, and the junction box is generally installed on the side of the component for easy installation. In some special applications, the shape of solar cell modules will be required, which is no longer just a conventional square. For example, circular roofs require solar cells to be in a circular shape, buildings with beveled edges require solar cell modules to have beveled edges, and arched roofs require solar cell modules to have a certain degree of curvature.
(4) As we all know, the design life of building PV is more than 50 years for general concrete buildings, but the life of ordinary PV modules is only 25 years, especially for modules using EVA glue. In contrast, the life of modules packaged with PVB is relatively long. The PV grid connected inverter, the key equipment in the building PV system, usually has a service life of only 8 to 10 years. Therefore, to ensure that the PV system has the same service life as the PV building, it is necessary to consider the issue of regular replacement of the PV grid connected inverter. Most of the connecting lines of the ordinary photovoltaic system are open in the atmosphere, with sufficient air convection and low temperature. Most of the connecting lines in the BIPV building system are in closed structures such as curtain wall columns and beams, so the temperature is not easy to dissipate, and the life of wires and cables is also affected. Therefore, the requirements for wires in the BIPV building system are higher. To sum up, in the design of building photovoltaic system, each component should be considered as a whole to make its service life consistent with the building itself or easy to replace.
2. Photovoltaic characteristics of building photovoltaic
(1) Considering the cost of electrical design, centralized grid connected inverter is still recommended; For BIPV system, due to various installation angles of PV modules and scattered installation locations of battery modules, it is generally recommended to use low-power inverter or string inverter. All cables used in buildings shall be flame retardant. In some occasions with high stress, armored cables shall be used. When the cable used for photovoltaic power generation needs to be laid in the same slot, hole and bridge as the building distribution cable, the cable with the same material as the building distribution cable should be selected. The temperature of the cable sheath shall not be higher than 70 ℃ in the places where people can reach the cable laying. In severe cold environment, cables laid outdoors shall be resistant to low temperature. When the cable wiring needs to go through the wall, it is easy to cause “cold bridge”, so special bridge breaking and thermal insulation structure treatment shall be carried out. For the system with low voltage 380V connected to the mains, the biggest factor affecting the distribution network is the rise of the terminal voltage of the distribution network, especially when reverse power occurs, the rise of the voltage will be more obvious. If the voltage rise exceeds the limit, the electric equipment will be damaged. If the irreversible grid connection mode is adopted, the reverse power protection shall be set.
(2) Monitoring communication design The communication of photovoltaic building integration project usually adopts wireless and wired methods. Wireless mainly includes: Zigbee and GPRS for RS485, and WIFI for Ethernet; Wires mainly include twisted pair for RS485 and Ethernet, coaxial cable for Ethernet and video signal, and optical fiber for Ethernet. For photovoltaic building integration projects, the monitoring positions usually include intelligent photovoltaic combiner box, inverter, multi-function meter and environmental monitor. The projects to be boosted also include microcomputer integrated automation system. Microcomputer integrated automation system is generally used in substations. Each high-voltage switch cabinet in the station is an independent monitoring subsystem. These subsystems integrate the relay protection function and monitoring function, realize local data acquisition and local control, and provide remote control channels.
3. Conclusion
In a word, building PV integration is not a simple “stacking” of buildings and PV modules, but an organic combination of building characteristics and PV characteristics in building PV. Building PV systems give full play to the building characteristics and green power characteristics of PV arrays, providing an innovative way for energy-saving buildings and photovoltaic power development.
