The vast majority of residential and commercial and industrial PV is installed on the roof, which not only helps homeowners save money on their electricity bills, but also optimizes the use of roof space and cools the roof.
This article will discuss a few of the most important things you must know about installing solar panels on a flat roof.
Key 1: Which type of installation to use
Different roofs have different methods of mounting PV modules. Common mounting methods include attachment type, ballast type and pile type.
If the roof belongs to the color steel tile type, generally consider the additional type, directly lay the module flat on the color steel tile, especially the general color steel tile roof are no daughter wall, using the flat type windproof effect is the best, the safest; In addition, the color steel tile roof also has a certain angle of inclination, although it may not be the best angle of inclination corresponding to photovoltaic power generation, but increase the angle of inclination to bring the cost of transformation is also need to be considered.
For concrete roofs, which are usually pure flat, the common installation methods are ballast type and concrete foundation pile type, as shown in the above figure.
Many times, building owners do not allow holes to be drilled in a concrete flat roof, either because they are concerned about the strength of the old building or because they do not want to change the water resistance of the roof. This is the reason why ballast type or concrete foundation installation is chosen.
Ballast type can be considered for windproofing on the side since there is no anchor connection made with the roof, especially for concrete roofs without daughter walls.
The main purpose of the concrete base is to ensure that the bracket system remains intact even during the storm season. It ensures a good installation without causing roof leaks, and also improves the efficiency of the solar modules and reduces sunlight blockage by the daughter wall.
Key 2. Flat roofs do not mean flat installation
Installing PV modules on a flat roof in a flat laying manner seems to be the most ideal way to not only maximize the installation volume/area, but also to potentially use the PV as a roof topper and increase the space area.
But in fact, installing PV on a flat roof does not mean that you should also set its tilt to 0 degrees. We need to consider many factors that should be avoided for flat installation, including
l Flat modules tend to accumulate dust, and dust accumulation may cause a 10% or even 30% loss of power generation.
l More inconvenient to clean, and the accumulated water does not flow out easily.
l the power generation will be reduced compared with tilt installation
l The payback period will be longer.
l It may not be possible to remove ash accumulation by natural rainfall.
There will be a significant difference in power generation between tilt-mounted and purely flat PV arrays, as simulated by a 11.2kWp solar system with tilt angles of 10 degrees, 5 degrees and 0 degrees, respectively.
The simulation results show that with a tilt angle of 0 degrees, the 11.2kWp system generates approximately 13,480.3kWh per year, while with a tilt angle of 5 degrees, the system generates 14,066.9kWh per year. and in fact, with a tilt angle of 10 degrees, the system generates 14,520kWh per year.
From the results, it can be seen that for every 5 degrees increase, the system increases its annual power generation by 500kWh (after 15 degrees, the increase decreases significantly, and beyond the optimal inclination angle, it becomes even lower). Moreover, the above simulation does not take into account the loss of power generation due to ash accumulation when installed at low tilt angles.
According to Global PV's communication with industry experts, it is recommended that if you do not want to use the optimal tilt angle for installation, the installation tilt angle should be as high as possible. If a flat roof is necessary, it is recommended that the tilt angle be designed between 5-10 degrees as well. For flat roofs, an angle of 5-10 degrees is flat enough, while for color tile roofs, an angle of 3 degrees or more is usually left, so it is better to go with the flow and follow the slope than the extra cost of increasing the inclination.
Key 3. Wind speed must be considered for flat roof installations
During the design phase of a solar system, the maximum possible local wind speed must be considered, especially in areas where the wind speed exceeds 180 kilometers per hour.
Not one in five years, one in ten years, the life of a PV system is 25 years, one in 50 years must be taken into account!
The typhoon that occurred in Nantong, Jiangsu Province at the end of April 2021 is enough to explain everything, with the quality of construction, for the construction of the Nantong area also, almost no one will take too much into account the typhoon disaster that often occurred thirty years ago, after all, the typhoon in the Nantong area, it can also bring a little impact on the former square, grass house, shed.
But at the end of April this year, the wind that occurred in Nantong, alive the roof of the solar photovoltaic, solar water heater are blown to the ground, during the typhoon caused more than ten deaths, a 50-year event is not exaggerated.
An ordinary flat roof solar power system can withstand a wind speed of 160 km / h (13 typhoon), but if there is no daughter wall to block the wind and no wind protection is considered between the arrays, the instantaneous wind speed caused by airflow in the local area will far exceed the actual wind speed.
At this time for the selection of bracket, structural design, ballast or concrete foundation strength design must rely on the advice of professional institutions, even for the flat roof of color steel tile, scientific analysis is also required.
Key 4. Leakage risks for flat roof installations
Water accumulation is common on flat roofs. Since the roof is flat, there is nowhere for water to go, or the drainage becomes slower and stays on the roof for a longer time on the edge, so any gap or hole in the roof becomes a stopping place for water to accumulate.
At this point, flat roofs with PV systems installed become the biggest leakage risk for flat roof PV due to the possible damage to the waterproof layer caused by the bracket installation and the concrete pile installation.
If it is a color steel tile roof, it is better to use jig installation than to punch holes in the color steel tiles; if the color steel tiles are not suitable for jig connection, using structural adhesive bonding is also an option, Zhejiang Lingzhi silicone has a relevant solution; if holes must be punched, you need to ensure that all holes are properly sealed to waterproof.
Key 5. Need to consider mechanical units on flat roofs
One of the challenges in designing and installing PV on a flat concrete roof is the various types of mechanical units on the roof of existing buildings, such as outdoor air conditioning units, drains, exhaust fans, ventilation facilities, water tanks, railings, roof structures, HVAC systems and plumbing, etc.
These existing devices not only affect the layout of the rooftop PV, affecting the tether spacing and alignment, but may also create shadow shading on the PV array, or affect future operations and maintenance.
Some designers will consider adding PV above these facilities to shield them from the wind and rain, but this brings another problem of height design requirements and wind protection requirements, as well as difficulties in operation and maintenance.
For rooftop facilities, design can only be done on a site-specific basis, but this is an issue that rooftop PV cannot avoid and must be taken seriously.
Key 6. Rooftop flat modules for incremental development
Generally there is no absolute 0 degree angle of the flat installation, but low tilt is really a problem that can not be avoided, even if the concrete roof will have such a need. The roof of Wuxi Tianpeng Food City is a typical application of adding PV canopies to a cement roof: the
For flat roof PV, optimal design still allows for optimal power generation capacity, which can be accomplished in different ways.
Method 1: In the case of flat laying as far as possible to make a certain inclination angle, not necessarily the pursuit of the best inclination angle, especially when greater than 15 degrees inclination, need to carefully consider the roof of the wind problem (the greater the inclination angle, the greater the impact of the wind). Usually if you can do 5-10 degrees is already enough. Also face the best orientation.
Method 2: For near-flat modules with very low inclination, leave a gap between the top and bottom rows of modules so that water from the module surface can flow out of the module surface instead of accumulating to the next module. This is an additional requirement for PV arrays designed for roofs, which are meant to keep out rain, and can be achieved by considering adding drainage channels between modules (this solution is now common in sunroom PV applications, and commercial and industrial roofs can be used as a reference).
Wuxi Tianpeng Food City picture of the component surface, in the lower edge of the border are obvious traces of mud belt, is the component surface of the mud and water can not be timely discharge component surface results.
Method 3: For low inclination roof PV modules, add a product called automatic drainage deslimer to completely eliminate water and ash accumulation on the surface of PV modules with an inclination of 3-15 degrees. Because it is a roof-top application, later climbing up the operation and maintenance of more inconvenience, the selection of drainage deslimer should take into account the later operation and maintenance, such as the use of high-efficiency, long-life materials, to avoid plastic products may need to be replaced after a short period of aging; also need to consider the material and the component aluminum frame between the corrosion of foreign metals, but also to choose a material that does not affect the surface resistance of the aluminum frame to avoid the risk of component PID.
