01
Project Overview
With the formulation of the national "dual carbon" strategy, energy conservation, emission reduction, and the development of clean energy have become key development strategies in my country. Solar energy is considered the cleanest, safest, and most reliable energy source of the future. Bosch, which prioritizes environmental protection as a fundamental corporate principle and a primary focus of technological innovation, is actively responding to this national strategy by planning to install a distributed photovoltaic power generation system on the roof of its Changsha factory. The roof covers approximately 25,000 square meters and has a designed generating capacity of approximately 1.8 MW.
The Changsha Bosch factory roof features a single-layer waterproof membrane roofing system. From bottom to top, the structural layers are: 0.8 mm thick corrugated steel sheet → 0.3 mm thick PE membrane vapor barrier → 50+50 mm thick rock wool insulation board → 1.5 mm thick PVC waterproof membrane (mechanically secured). Because the roofing membrane has been in use for over a decade, it has deteriorated. Continuing to use the existing waterproofing layer would soon necessitate renovation, resulting in the additional costs of decommissioning, dismantling, and restoring the photovoltaic system. Therefore, to support the implementation of the rooftop distributed photovoltaic power generation project and ensure the lifecycle of the rooftop waterproofing membrane coincides with the photovoltaic power generation, a comprehensive roof waterproofing renovation was required prior to construction of the rooftop distributed photovoltaic power generation system.
02
Project Design
2.1
Waterproofing Renovation Plan Design
As normal production inside the factory could not be interrupted, after many discussions with the owner, it was decided to retain the original PVC waterproof membrane and only remove the part of the original waterproof membrane that affected the sealing treatment of the new PVC waterproof membrane. At the same time, in order to avoid the accelerated migration of plasticizers in the newly laid PVC waterproof membrane, an 80 g/m2 polyester non-woven fabric isolation and protection layer was added to the original waterproof membrane. Finally, a new layer of PVC waterproof membrane was laid and constructed using a mechanical fixing method (Figure 1).
Figure 1 Roof waterproofing renovation structure
To ensure the roofing membrane's lifecycle is consistent with the photovoltaic power generation, the new PVC waterproofing membrane selected was Sika Sarnafil's high-end 1.5 mm thick S327-15L PVC waterproofing membrane. This material offers excellent weather resistance (accelerated aging test for 8,000 hours), superior tensile strength, high solar reflectivity (meeting LEED certification requirements), and a special self-cleaning coating that reduces dust adhesion and leaves the roof looking as clean as new after rain.
2.2
Photovoltaic Mounting Base Design
In general, the reserved load for a single-layer waterproof membrane roof is not large enough to meet the weight of the precast concrete blocks. In addition, the compressive strength of the roof base is not high, and the weight of the precast concrete blocks can easily cause local water accumulation on the roof. Therefore, it is not recommended to use the traditional precast concrete block counterweight fixing method (Figure 2) on a single-layer waterproof membrane roof. If the newly added precast steel component support and fixing method (Figure 3) is used, it will cause great damage to the roof waterproof membrane, make waterproof repairs complicated, and prolong the construction period.
Figure 2 Precast concrete block counterweight fixation Figure 3 Added precast steel support fixation
Installing photovoltaics on a single-layer waterproof membrane roof requires a photovoltaic support base that minimizes damage to the roof's waterproof membrane, simplifies repair of damaged membranes, speeds installation, and meets wind load resistance requirements. To address this, a "flexible membrane photovoltaic support" (Figure 4) has been specially developed. This support is composed of a 304 stainless steel disc, a cylinder, and a flexible membrane, all assembled using patented technology.
Figure 4 Flexible roll photovoltaic support
The flexible roll photovoltaic support is made by pre-drilling holes to penetrate the roof waterproofing roll, insulation board, vapor barrier membrane and corrugated steel plate, then inserting a rivet nut, and then using a special tool to fix it on the crest of the corrugated steel plate; then use an M8 fixing screw to connect the support and the rivet nut to fix the support on the roof waterproofing roll; finally, use hot air welding to connect the flexible roll of the support (the same product and quality as the roof waterproofing roll) with the roof waterproofing roll to form a complete waterproof layer, and at the same time repair the perforation (Figure 5).
Figure 5: Connection and fixation of flexible roll photovoltaic support and roof corrugated steel plate
According to the test data conducted by China Building Materials Inspection and Certification Group Suzhou Co., Ltd., the pull-out force test data of the flexible roll photovoltaic support connected to the 0.8 mm steel plate is 5.7 kN. Considering the safety factor of 2 in the design, the design load value of the flexible roll photovoltaic support is 2.85 kN/piece.
2.3
PV Mounting Type and Fixed Base Layout Design
After deciding to use flexible roll-type photovoltaic supports for the photovoltaic support's fixed base, the photovoltaic support design was designed to ensure a secure connection between the fixed base and the photovoltaic support. The fixed base was also strategically arranged to meet the wind load pullout resistance requirements of individual photovoltaic modules.
This project employed a double-row, single-slope support structure with a 16° slope. Each support consists of two column feet connected to the flexible roll-type photovoltaic support, which is fixed to the single-layer waterproof roll roof, via M8 fixing bolts (Figure 6).
Figure 6 PV bracket column foot connection node
The arrangement of fixed bases is based on the relevant standards of GB 50009-2012 "Code for Loads on Building Structures", GB 50797-2012 "Code for Design of Photovoltaic Power Stations" and NB/T 10115-2018 "Code for Design of Photovoltaic Support Structures". The wind load pull-out resistance required by the photovoltaic module is calculated, and the strength of the support rails, support beams and support columns is verified to determine the number and spacing of fixed bases.
03
Roof waterproofing renovation
3.1
Roof base cleaning
Clean up the garbage and dusty areas on the roof, most of which are concentrated near the roof downspouts.
3.2
Laying the non-woven fabric isolation layer
After the roof base is cleaned, lay a layer of 80 g/m2 polyester non-woven fabric isolation protective layer on the original PVC waterproof membrane. The laying should be flat and straight, with an overlap width of 80 mm.
3.3
Installation of new PVC waterproofing membrane
Before formal laying, pre-laying is carried out, then straightened and fixed with coil fixings, and then another coil is laid to cover the coil fixings. The overlap width is 120 mm (the short side overlap width is 80 mm). Finally, the overlap edges are welded into a whole by an automatic welding machine. The effective welding width of the overlap edges is ≥25 mm.
3.4
Waterproofing of Detailed Joints
The original waterproof membranes at the roof details such as parapets, skylights, roof fans, openings and downspouts that affect the closing of the new waterproof membranes were removed and cleaned, and new PVC waterproof membranes were used to re-process the details.
3.5
Completion Cleaning and Self-Inspection
After the construction is completed, clean the roof and tidy it up, then use a special inspection hook to check the quality of the welds. If any problems are found, repair them immediately.
3.6
Waterproofing Work Acceptance
After passing the self-inspection and rainwater inspection, the roof was found to have no leaks and passed the owner's acceptance. Construction of the rooftop distributed photovoltaic power generation system then began. The roof after waterproofing renovation is shown in Figure 7.
Figure 7 Roof after waterproofing renovation
04
Construction of a rooftop distributed photovoltaic power generation system
4.1
Installing the Flexible PV Mounting Sheet
Install the flexible roll photovoltaic support according to the following steps: 1) Locate the spring line according to the layout diagram of the photovoltaic bracket fixed base, and make corresponding adjustments according to the position of the crest of the roof corrugated steel plate to determine the support point; 2) Pre-drill holes in the roof at the support point position, and fix the rivet nut on the crest of the corrugated steel plate through the rivet nut; 3) Anchor the M8 fixing screw into the rivet nut, and at the same time tighten the flexible roll photovoltaic support and the M8 fixing screw to the roof waterproofing membrane; 4) Use a manual welding gun to hot-air weld the flexible roll of the support to the roof waterproofing membrane; 5) After the support is installed, use a vacuum leak detector to check the quality of the weld to ensure that there is no risk of leakage.
4.2
Installing the Photovoltaic Mounting System
After the photovoltaic bracket mounting base is completed, the bracket can be installed. Since the bracket is a metal structure, its edges and corners can easily puncture the roof waterproofing membrane. Therefore, the finished waterproofing membrane must be protected during the hoisting and installation process. Carpeting and wooden boards are used to protect the hoisting area.
When installing a photovoltaic bracket on a single-layer waterproofing membrane roof, cutting and welding should be avoided as much as possible. Therefore, holes are pre-drilled in the bracket during factory fabrication. M8 bolts are then used to connect and secure the bracket during on-site installation. This minimizes the risk of damage to the roof waterproofing membrane throughout the installation process (Figure 8).
Figure 8 Connection and fixation of photovoltaic bracket
4.3
Installation of photovoltaic panels
The photovoltaic panels selected for this project are 550 Wp monocrystalline silicon panels, measuring 2,279 mm × 1,134 mm × 35 mm, with a total of 3,324 panels. The panels are secured to the brackets using metal clamps and M8 bolts (Figure 9).
Figure 9 Laying and fixing of photovoltaic panels
4.4
Laying Cables
Connect the PV panels in series according to the order of installation. Each string should have external connectors at the same end of the bracket. Use PV-1×4 photovoltaic cables for the cables from the strings to the inverter. Route the cables directly below the PV panels along the PV bracket's built-in cable tray. Install additional cable trays where there are no panels.
4.5
Install a lightning protection grounding grid
The positive and negative poles on the DC side of the photovoltaic modules are left floating and ungrounded. Surge protectors are installed in the DC and AC distribution cabinets to prevent damage from lightning-induced overvoltage. A 25 mm x 4 mm hot-dip galvanized flat steel ring is used around the photovoltaic modules, fixed to the photovoltaic brackets and connected to the original building lightning protection network. The grounding resistance is no greater than 4 Ω.
4.6
Grid connection and commissioning
After the photovoltaic power generation system was fully constructed (Figure 10), it was connected to the grid and commissioned. After two months of intensive and orderly construction, the project was successfully connected to the grid and put into operation, with power generation efficiency meeting the owner's expectations .
Figure 10 The roof after the photovoltaic power generation system is completed
Conclusion
A successful rooftop solar system requires careful attention to waterproofing, connection accessories, and operation and maintenance. Installing distributed photovoltaic power generation on large, unobstructed factory roofs is one of the most effective ways for businesses to save energy and reduce emissions, and an increasing number of companies are planning to install photovoltaic systems on their rooftops. While ensuring effective roof waterproofing and proper functioning of indoor facilities, combining a single-layer waterproof membrane roofing system with a rooftop solar system offers an economical, simple, and reliable solution. The waterproofing performance and service life of the rooftop waterproofing membrane, as well as the wind load pullout resistance of the photovoltaic mounting system, are paramount considerations. This means selecting a reliable waterproofing membrane and a matching photovoltaic mounting base are crucial. This project involved waterproofing the original roof and then installing a distributed photovoltaic system. Professional review and calculations were conducted on the structural load-bearing capacity and wind resistance of the supports to ensure the overall safety and reliability of the project. This not only extended the roof's waterproofing function, but also actively responded to green buildings and the reuse of clean energy by utilizing solar power generation. At the same time, the Sika Sarnafil S327L PVC waterproof membrane and flexible membrane photovoltaic support solution used in the project were highly recognized by the owner and can provide reference for peers.
