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The vast majority of residential and commercial PV modules are installed on the roof, which not only helps homeowners save money on their electricity bills, but also optimizes the use of roof space to cool the roof.  

This article will discuss some of the most important things you must know about installing solar panels on flat roofs. 

 

01、Which installation mode is adopted

Different rooftops have different installation methods for PV modules, common installation methods include additional type, ballast type and pile type. 

If the roof belongs to the color steel tile type, generally consider additional type, directly tile components on the color steel tile, especially the general color steel tile roof without parapet, tile type wind protection effect is the best, the most safe;  In addition, the color steel tile roof also has a certain Angle. Although it may not be the best Angle corresponding to photovoltaic power generation, the transformation cost brought by increasing the Angle also needs to be considered comprehensively.  For cement roofs, which are usually flat, the common installation methods are ballast type and concrete foundation pile type, as shown in the figure above.  In many cases, building owners do not allow drilling into flat concrete roofs, either because of concerns about the strength of the old building or because they do not want to change the waterproof properties of the roof.  This is why a ballast type or concrete base installation is chosen.  Since the ballast type is not anchored to the roof, windproof treatment can be considered on the side, especially on the cement roof without parapet.  The main purpose of the concrete base is to ensure that the support system remains intact even during storm seasons.  It ensures a good installation without causing leaks in the roof, improves the efficiency of the solar modules, and reduces sun blocking from the parapet. 

 

02、Flat roof does not mean tiled installation

It seems the most obvious way to install pv modules on flat roofs, not only to maximize the installation volume/area, but also to increase the space area by using PV as a roof canopy.  However, the fact that 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, should avoid flat installation, including: L tile components are easy to accumulate ash, ash may cause 10% or even 30% of power generation loss;  L More inconvenient cleaning, water is not easy to flow out;  L Compared with the inclined installation, the power generation will be reduced;  L Longer investment return period;  L It may not be possible to remove ash by natural rainfall;  The power generation of photovoltaic arrays with inclined installation and flat paving will be significantly different. Through the simulation of a 11.2kwp solar system, the inclination angles are 10 degrees, 5 degrees and 0 degrees respectively. 

The simulation results show that if the dip Angle is 0 degrees, the annual power generation of the 11.2kwP system is about 13,480.3kwh, while under the dip Angle of 5 degrees, the unified annual power generation of the system is 14,066.9kwh.  In fact, at an Angle of 10 degrees, the system produces 14,520kWh a year.  It can be seen from the results that every 5 degrees of increase, the system will increase the annual power generation of 500kWh (after 15 degrees, the increase will be significantly reduced, but will be even lower after the optimal inclination).  Moreover, the above simulation does not take into account the loss of power generation due to ash accumulation during low-angle installation.  If you do not want to use the best Angle to install, install the Angle as high as possible.  If tiling is necessary, it is recommended to design the inclination Angle between 5 and 10 degrees.  For flat roof, 5-10 degrees of Angle is already flat enough, and for color steel tile roof, generally can leave the Angle of 3 degrees above, compared with the extra cost of increasing inclination, let nature take its course, along with slope is also a very good choice. 

 

03、Flat roof installation must consider wind speed

In the design phase of a solar system, the maximum wind speed that can occur in the local area must be considered, especially in areas where wind speeds exceed 180 km/h.  Not once in five years, once in ten years, the life of photovoltaic system is 25 years, must consider once in 50 years!  The typhoon that happened in Nantong, Jiangsu province at the end of April 2021 is enough to tell the whole story. With the improvement of construction quality, almost no one will take into account the typhoon disasters that often happened 30 years ago for the buildings in Nantong. After all, the typhoon in Nantong can bring some influence to the former square houses, straw houses and greenhouses.  However, the wind in Nantong blew the solar photovoltaic and solar water heater on the roof to the ground. During the typhoon, more than a dozen people died, the first time in 50 years.  Ordinary flat-roof solar power generation systems can withstand wind speeds of 160 km/h (level 13 typhoon), but if there is no parapet to break the wind and wind protection between arrays is not considered, the instantaneous wind speed caused by airflow in local areas will far exceed the actual wind speed.  At this time for the selection of support, structural design, ballast or concrete foundation strength design must rely on the advice of professional institutions, even the tiling of color steel tile roof, but also have a scientific analysis. 

 

04、Risk of leakage in flat roof installations

Standing water is common on flat roofs.  Because the roof is flat, water has nowhere to go, or drains slowly and stays on the roof longer, so any gap or hole in the roof becomes a place for water to stay.  At this point, flat roofs with photovoltaic systems are the biggest water leakage risk due to the damage of waterproof layer caused by bracket installation and concrete pile installation.  If it is the color steel tile roof, the fixture installation is better than drilling on the color steel tile;  If the color steel tile is not suitable for fixture connection, the use of structural adhesive is also a choice, Zhejiang Lexus silicone has related solutions;  If you have to punch holes, make sure all the holes are properly sealed to make them waterproof. 

 

05、Mechanical units on flat roofs need to be considered

One of the challenges of designing and installing PV on a flat concrete roof is the various mechanical components on the roof of the existing building, such as outdoor air conditioning units, drains, exhaust fans, ventilation facilities, water tanks, railings, roof structures, HVAC systems and water pipes.  These existing devices not only affect the layout of the rooftop pv, affect the spacing and arrangement of the system supports, but also may produce shade on the PV array, or affect the future operation and maintenance.  Some designers may consider installing photovoltaic panels on top of these facilities to protect them from wind and rain, but the other problem is the height design requirements and wind protection requirements, as well as the difficulties in operation and maintenance.  For roof facilities, only according to local conditions to carry out the design, but this is the roof photovoltaic can not be avoided, must pay attention to the problem. 

 

06、Additional issue of roof tiling components

There is no absolutely 0 degree Angle tiling installation in general, but the low Angle is really an unavoidable problem, even if it is cement roof will also have such a demand.  The roof of Wuxi Tianpeng Food City is a typical application of cement roof with photovoltaic greenhouses: 

For rooftop tiling pv, the optimal design can still achieve the best generating capacity, which can be done in different ways.  

Method 1: Make a certain Angle as far as possible under the tiling condition, not necessarily pursue the best Angle, especially when the Angle is greater than 15 degrees, need to carefully consider the problem of roof wind protection (the greater the Angle, the greater the influence of wind).  Usually 5-10 degrees is enough.  And also for the best orientation.  

Method 2: For a low-angle nearly tiled assembly, leave a gap between the upper and lower rows of assembly so that water from the assembly surface can flow out of the assembly surface rather than accumulate to the next assembly.  This is an additional requirement for photovoltaic arrays designed as ceilings, because ceilings are designed to keep out rain and can be achieved by adding gutters between components (this scheme is now common in solar room PV applications, and industrial and commercial roofs can be used as a reference).  The component surface in the picture of Wuxi Tianpeng Food City has obvious mud belt traces along the lower border, which is the result that the muddy water on the surface of the component can not be discharged on the surface of the component in time. 

Method 3: For low-angle rooftop PV modules, a product called automatic drainage mud remover is added to completely eliminate water and ash accumulation on the surface of pv modules with 3-15 degrees of inclination.  Because it is the roof ceiling application, it is inconvenient to climb up in the late operation and maintenance. The selection of drainage and mud remover should be taken into account in the late operation and maintenance, such as the use of efficient and long-life material, to avoid the need to replace plastic products after aging in the short term;  Heterogeneous metal corrosion between the material and the aluminum frame of the component should also be considered, and materials that do not affect the surface resistance of the aluminum frame should be selected to avoid the PID risk of the component.