Steps and Key Points for Rooftop Distributed Photovoltaic Installation

Detailed Steps and Key Points for Rooftop Distributed Photovoltaic Installation, Including Specific Calculation Methods

Preliminary Preparation

• Selection of Roof
  • Prioritize new or relatively new buildings to reduce operation and maintenance costs due to building maintenance.
  • For concrete roofs, choose rigid waterproof accessible roofs, as these are convenient for construction, operation, and maintenance, and typically meet the load-bearing requirements of photovoltaic stations.
  • For non-accessible roofs, load verification must be conducted by the original design unit or a qualified third party to ensure safety.
• Estimate of Installation Capacity
  • Calculation of Usable Area: Calculate the effective roof area, deducting shadows from parapets, equipment, and skylights.
    • The utilization coefficient for profiled steel roofs is generally 0.75.
    • The utilization coefficient for concrete roofs is 0.85.
  • Calculation of Photovoltaic Module Area: Based on the effective roof area and the dimensions of the photovoltaic modules, calculate the number of installable photovoltaic modules.
    • For example, if the effective roof area is 1000 square meters and the photovoltaic module size is 1.65m × 1m, the number of installable modules would be:
      Number of installable modules = (Effective Roof Area)/(Photovoltaic Module Area) = (1000)/(1.65 × 1) ≈ 606 modules
  • Calculation of Photovoltaic Module Installation Angle: Consider local solar resources and surrounding building parameters to calculate the optimal installation angle for the photovoltaic modules.
    • For example, for a project located in Shandong, the optimal angle can be calculated using the following formula:
      Optimal Angle = Latitude + 10°
    • If the latitude is 36 degrees, the optimal angle would be 46 degrees.
• Assessment of Grid Absorption Capacity
  • Confirm that the installed capacity of the rooftop distributed photovoltaic power generation system matches the local grid’s absorption capacity.
  • For example, if the local grid absorption capacity is 500 kW and the calculated installed capacity is 600 kW, either the installed capacity needs to be adjusted or alternative absorption methods need to be sought.

Rooftop Distributed Photovoltaic Design

• System Design
  • Selection of Photovoltaic Modules: Choose photovoltaic modules suitable for the local climate and roof conditions.
  • Selection of Inverter: Choose an appropriate inverter based on project capacity and grid voltage level.
    • If project capacity is small (less than 100 kW) and uses low-voltage grid connection, use string inverters.
    • If project capacity is large (more than 100 kW) and uses medium-voltage grid connection, use centralized inverters.
  • Installation Method for Photovoltaic Modules: Choose a suitable installation method based on the type of roof.
    • For profiled steel roofs: generally installed flat with a low angle (3°~5°).
    • For concrete roofs: can set brackets with optimized angles to improve power generation efficiency.
• Bracket Design
  • Bracket Design for Color Steel Tile Roof:
    • Brackets should be fixed at the peaks of the color steel tiles with clamps and self-tapping screws.
    • To improve generation efficiency, photovoltaic modules can be installed at an angle on color steel tile roofs that meet structural review requirements.
  • Bracket Design for Concrete Roof:
    • Use hot-dip galvanized or hot-dip zinc bracket systems.
    • Leave a 15 cm~20 cm air gap between the brackets and the roof for natural cooling.
  • Calculation of Support Reaction Force:
    • For photovoltaic bracket with connection beams:
      N_x = 0.21 kN
    • For photovoltaic bracket without connection beams:
      N_x = 0.15 kN
  • Structural Safety Assessment:
    • New buildings should include the load calculations of the rooftop distributed photovoltaic system in the building’s main structure and enclosure calculation.
    • Existing buildings should consider the building’s service life and functional requirements and perform structure and electrical safety reviews.
    • Calculate the load conditions of the rooftop distributed photovoltaic system, including the weight of photovoltaic modules, brackets, bracket foundations, wind, rain, snow loads, and loads under seismic and temperature effects.
    • For example, based on GB 50009—2012 “Building Structural Load Code,” the wind load calculation formula is:
      F_w = (1/2) ρ v² C_d A
      Where, ρ is air density, v is wind speed, C_d is wind pressure coefficient, A is wind-affected area.
• Electrical Design
  • Location of Junction Box and Inverter: Place as close to the grid connection point as possible avoiding attic areas.
  • Cable Selection:
    • DC cable design should prioritise cost-saving and reasonable wiring paths.
    • Prioritize aluminium alloy cables.
    • Use Class C flame-retardant cables for junction box to inverter DC side.
    • High-voltage cable selection must undergo thermal stability verification, introduced into the switch station by direct burial, with a burial depth of no less than 0.7m, greater than local frost depth.
  • Location of Grid Connection Point: Determine the grid connection point and connection voltage level to ensure smooth grid connection.

Rooftop Distributed Photovoltaic Installation

• Bracket Installation
  • Bracket Installation for Color Steel Tile Roof:
    • Use clamps or self-tapping screws to fix the C-shaped steel rafters of the photovoltaic brackets.
    • Leave a 15 cm~20 cm air gap between brackets and the roof for natural cooling.
  • Bracket Installation for Concrete Roof:
    • Fix the main beam position according to the original factory steel purlin positions to determine the bolt hole positions for the brackets.
    • Flip over the main beam, place brackets under the rafters, and fix them to the original roof steel purlins with self-tapping screws.
    • Seal around the brackets and screw holes with silicone sealant to prevent roof leaks.
  • Main Beam Splicing:
    • Apply two beads of silicone sealant inside the aluminium joint, insert the two ends of the main beams, and rivet each joint end with six rivets, ensuring adjacent rivets are spaced more than 50 mm apart.
    • Clean up iron filings and debris inside the main beams, apply silicone sealant to the splicing seams, smooth it out, and then stick butyl waterproof tape to create a triple waterproof effect.
• Module Installation
  • Layout along the Roof Ridge: Set up drainage channels below to ensure the ridge is installed straight.
  • Sequential Installation of Modules and Channels: Install seal strips between gaps of adjacent modules and fix them with pressure blocks.
  • Installation Method:
    • For profiled steel roofs: modules are installed parallel to the roof, leaving 15 cm~20 cm air gap between the brackets and the roof.
    • For concrete roofs: install modules based on the calculated optimal angle, using either single-row vertical installations or double-row horizontal installations.
• Electrical Connections
  • Inverter Installation: Inverters should be centrally installed in the roof machine room or top floor machine room, ensuring shade and rain protection to keep the operating environment dry and ventilated.
  • Connect AC and DC switches to the inverter ends, ensuring the system has overcurrent protection functionality.
  • Install measuring instruments to monitor system operation status and user power consumption.

Rooftop Distributed Photovoltaic Maintenance

• Regular Inspections
  • Check for cracks or detachment at the battery bracket connections.
  • Check if there are cracks, color changes on the module surface, and if there are large gaps or deformations in the frames.
  • Check if the brackets are properly grounded.
• Temperature and Current Measurements
  • Use infrared thermometers to measure temperatures on photovoltaic modules, ensuring temperature differences on the same module’s surface do not exceed 20℃.
  • During normal system operation, measure currents at the inverter to confirm current deviations among modules do not exceed 5%.
• Cleaning Photovoltaic Modules
  • Regularly clean the module surface from fine dust particles and other contaminants to improve power generation efficiency.
  • Monthly cleaning is recommended or adjusted based on actual conditions.
  • Cleaning should be done in the early morning, evening, night, or on cloudy days to avoid working under intense sunlight.
  • Use a pressure washer to thoroughly rinse the module surface and then wipe with a clean soft cloth.

Economic Evaluation

• Investment Estimation
  • Initial Investment: Includes the purchase costs of photovoltaic modules, inverters, brackets, etc.
  • Operation and Maintenance Costs: Includes daily maintenance, cleaning, troubleshooting, and repair expenses.
  • Building Structural Reinforcement Costs: Based on specific building conditions and the load conditions of rooftop distributed photovoltaic systems.
• Income Estimation
  • Annual Power Generation Estimation: Based on installed capacity and local solar resource situation.
    • For example, assuming an installed capacity of 100 kW, with an average annual solar radiation of 1200 kWh/m² and a system efficiency of 80%, the annual power generation would be:
      Annual Power Generation = Installed Capacity × Average Annual Solar Radiation × System Efficiency = 100 × 1200 × 0.8 = 96000 kWh
• Economic Evaluation
  • Evaluate the economics of the rooftop distributed photovoltaic power generation system based on investment and income estimates.
  • For example, if the initial investment is 500,000 yuan, annual operation and maintenance costs are 50,000 yuan, building structural reinforcement costs are 100,000 yuan, with an annual power generation of 96,000 kWh, and the electricity price is 0.8 yuan/kWh, then the annual income would be:
    Annual Income = Annual Power Generation × Electricity Price = 96000 × 0.8 = 76,800 yuan
  • Investment Recovery Period Calculation:
    Investment Recovery Period = Total Investment / Annual Income = (500000 + 100000 + 50000) / 76800 ≈ 7.17 years

Grid Connection Acceptance

• Grid Connection Acceptance Process
  • The grid company’s acceptance process includes application, review, testing, and approval.
  • Application: Submit grid connection application along with relevant project documents.
  • Review: The grid company reviews the project to ensure compliance with grid connection standards.
  • Testing: Conduct grid connection testing, including electrical performance testing and safety performance testing.
  • Approval: After passing the tests, the grid company approves the grid connection.

Through the detailed steps and key points outlined above, the smooth installation and long-term stable operation of rooftop distributed photovoltaic systems can be ensured, maximizing their economic benefits and environmental benefits.

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