What happens in Rooftop site survey?

6th Nov 17  

Rooftop market in the year 2016-17 has seen a Y-o-Y growth of around 81%. It is also expected that the rooftop market shall add over 2 GW by 2019 and over 3 GW by 2020. Additionally rooftop solar power plant (as we mentioned in our earlier article "40 GW from rooftop PV by 2022 still a myth!”) have added advantages over utility scale power plants. With such huge numbers of rooftop capacity being added annually, it is important to understand how we install a rooftop solar plant. The rooftop of the each consumer (where the solar PV system is installed) is different and it hardly occurs that the two different roofs have same conditions. There is no standard (system) solution which can be provided to the consumer which makes the need to survey the rooftop to determine various parameters before even proposing a solutionto them. This article aims to educate the readers on the basicsof the rooftop site survey and the parameters which need to be considered and/or recorded during the survey.

Location and Orientation of the roof

The location and orientation of the building plays an important part of rooftop survey. This is because the solar irradiance (and hence the power output of the system) primarily depend on the latitude of the system. Additionally the orientation of the roof(whether east west or north south) needs to be determined. This is because an east west building if needed to generate maximum output from the installed system needs to be installed at an orientation which would not match the orientation of the building. The best orientation (figure 1) is the building 2 which can utilize maximum daylight and the worst is building 1. However all these buildings can have rooftop installed but the aesthetic of the building may be compromised.

Figure 1: Different orientation of building (Source: Autodesk)

Module mounting structure and make of roof

The Module Mounting Structure (MMS) holds the solar module firmly with respect to the ground. Additionally they provide structural rigidity to the entire system and prevent any mechanical damage to the modules during harsh (climatic) conditions.

Figure 2: Various MMS for rooftop solar power plant (Source: Google images)

As evident from the figure above, there are various designs (single/dual axis tracker, Adjustable seasonal tilt, etc. are not shown) of MMS and a particular design of MMS may not be suitable for all the system. This is because primarily the MMS (in addition to the maximum local wind speed) depends on the load bearing capacity (and hence the type) of the roof. Some roofs may use MMS made up of light weight material (say Aluminium) and may just be bolted to the ground whereas some roof may use grouting and/or bolting of MMS. It is hence important to determine the roof type of the consumer.

Rooftop measurement and shadow analysis

At the crux of the site survey lies rooftop measurement and shadow analysis (by mapping the shadow casting objects). The rooftop measurement would enable the developer to understand the maximum capacity of solar power plant that can be installed at their rooftop. A good thumb rule is to consider an area of 10 m2 per kW of solar system. However this area is only the shadow free area and hence it is important to map the shadow casting objects. The length, width and height of such objects in addition to its accurate position are required. Additionally, as the sun changes its position in the horizon (season wise), the length of shadow from an object would vary (in length) with maximum at winter and minimum at summer. It is also important to map surrounding objects such as trees, buildings (existing or new), overhead wire cables, etc. which could all cast shadow on the solar panel reducing its power output.The site along with the shadow casting objectsare modelled in software like Google SketchUp for shadow analysis and the shadow free area on the roof is obtained.

Figure 3: Actual pictures from rooftop site survey

Module and Balance of System (BoS) positioning

In our last article on " Orientation and Positioning of solar module" we explained you the orientation and positioning of solar module. However, there are few more considerations/ design aspects considered and/or applied during site survey. While there may be shadow free area, we would not install module there as there is some space which is chalked out for walking. Additionally we also need space (between modules) (or pitch) to ensure that the shading of modules does not affect other modules. Additionally adequate space for carrying out cleaning and scheduled maintenance is also required. Another consideration is positioning of BoS which include Array Junction Box, String Junction Box, DC combiner box, Inverter positioning, etc. These positioning are critical as increase in distance between these components increases cable length increasing both the cost and money. It is hence important to chalk out these positions.

Figure 4: Actual site image at VVP Eng. College (Source: Waaree project gallery) where a clear pathway is visible

Earthing pit and Lightening Arrestor (LA)

Finally, solar plant as we know harness too much of metal. Hence it is important to ground such metal in case of current leakage from solar panels. Additionally, it is also possible that these plants become the path for lightening. Hence it is important to provide (and hence identify position) Earthing pit and LA. It is also necessary that a separate metal strip and earth pit are provided (and hence identify position) for system grounding and LA.

Waaree is capable of giving turnkey solutions for both Rooftop and Utility scale projects. Its highly reliable module is capable to perform to their maximum at any given conditions. Additionally Waaree also is also capable of giving customised solutions for MMS. We believe in anchoring our consumers’ investments safely. Our standard product offering can be seen in the figure below.

Figure 5: Standard offering of MMS (clockwise from top left) Ground mounted fixed structure, Seasonal tilt structure, Single axis auto tracker and Rooftop mounting structure

Let us all pledge to make solar energy the primary source of energy in the near future.