22
Mon, Oct

Shading loss: An introduction, its impact on economics & mitigation techniques

Industry Insights
Typography

India has been actively supporting solar energy power plants and with a target of 100GW of solar power by 2022, this support is poised to be continual. The solar prices however, on other hand have been falling continually over years (Figure 1) which could be contributed to various factors.

With such reduction in prices, in order to maintain the profitability and sustainability of the power plant, it is important to maintain the economics of the power plant. A slight change of any parameter of the power plant can lead to significant effect on the economics of the plant. One such parameter is shading which in simple terms means that any object (near or far) obstructing direct sunlight flailing on the solar module. Shading in the power plant is generally a result of bad designing considerations. However sometimes, it may also be a result of poor operation & maintenance of power plant. While we explained you in our previous blog “Common problems in PV plant – Part 1” & “Optimizing solar power output in an overcast weather” as to what are the effects of such shadow on solar power plant, it is also important to understand the economic impact of such shadow on a power plant. This blog hence aims to educate its readers on types of shading along with such their economic impact on the solar power plant.

Figure 1: Rapid falling prices of solar energy (Source: Waaree market research)

The shading which is experienced in solar plants may be of three one of these three types:

  1. Self-shading: With an idea to capture more direct irradiance, the solar modules are generally kept at higher tilt angles. However with such increased tilt, the length of shadow casted to the row behind increases. However, with limitation of spaces many plants are made compact to encompass the required power. This leads to self-shading of modules which happens in power plants mostly around winter solstice.
  2. Shading due to near objects: Near objects are those which may be present within the vicinity or objects around 10 feet (such as vent, parapet walls, air-condition compressor unit, etc.). Few objects within 50 to 60 feet of distance such as trees, buildings, utility poles may also be considered. Few of these obstructions may affect the plant for certain time of the day or for the entire day depending on their location, height, distance from solar module, etc.  
  3. Shading from far horizon objects: Sometimes objects which are far away from the solar plant like newly built high skyscrapers, mountain ranges and/or city skylines caste shadows on the solar plant. These types of objects generally cast shadow on the entire array of modules generally when sun is below the line of horizon. While such shadows could be predicted to a fair accuracy, a designer could hardly do anything about it. However a precise calculation needs to be done about the loss of energy from such shadows.

With the knowledge on types of shading, it is also important how do they preliminary effect the power plant. The object causing shade primarily causes two losses in solar power plant. First the (direct) irradiance loss i.e. the object blocks the direct light falling on solar module which leads to direct reduction in power generation. Secondly the module which experiences such shadowing does not produce adequate power output which it should have produced during normal operation. Such difference in power output between modules causes mismatch loss within modules which causes indirect loss of energy. The Figure 2 below gives the list of irradiance, mismatch and total shading loss of few common objects.

Object (Height)

Irradiance loss

Mismatch loss

Total shading loss

Pole (20 feet)

0.4%

1.4%

1.80%

Pole (40 feet)

0.8%

2.2%

3.00%

Pole (60 feet)

1.1%

2.9%

4.00%

Small tree (25 feet)

1.7%

2.8%

4.50%

Large tree (50 feet)

2.7%

4.2%

6.90%

Tree (40 feet)

3.1%

3.3%

6.40%

Nearby building (70 feet)

2.4%

2.1%

4.50%

Figure 2: Irradiance and mismatch losses associated with common obstruction (Data source: Solar Professional)

While we understood the types of shading and its effect, it is also important to understand how and why they originate. We would have to thus understand the considerations that go along while designing the power plant. They are as follows:

  1. For row to row spacing in solar power plant, the height (h) of the adjacent row (as shown in Figure 3) is considered. It is multiplied with a factor of 1.5 and the obtained result is kept as inter row spacing.

Figure 3: Row to row spacing consideration

  1. Parapet wall consideration: For parapet wall, the installation of solar plant starts from a distance of 1.5 times the height of wall. However considering the ground clearance and the block height of understructure, this distance may be reduced.   
  2. Obstruction from near object: For near object obstruction, a simulation from standard softwares (like PVSyst, Google sketchup, etc.) are used. However many a times a simple site survey with basic assumptions of shadow is made and the marked area is not utilized for power production.  

Let us now move forward and understand the effect of shading on the economics of power plant. As far as general understanding goes, any/all the shading falling on the plant shall reduce the energy output of the power plant and hence its economics. To understand its gauge, let us consider an ideal power plant first. This power plant is considered to be on open space with no shading from near or far horizon objects. The total energy output of the power plant (Figure 4) is 434.4 MWh. Thus considering a generic value of energy sale (INR 3.50/kWh) turns it to around INR 1.52 million/year.

Figure 4: Monthly energy generation of ideal power plant (Source: Waaree Energies)

Now we add two near shading objects along with reducing the row to row spacing. The two added near shading objects drastically reduces the power output of the plant for almost the entire year. The effect of reduction in row to row spacing can be seen in the month of November & December where the energy output of plant further reduces due to self-shading. The total energy output of the power plant (Figure 5) is 368.9 MWh. Thus considering the same value of energy sale (INR 3.50/kWh) the revenue turns out to around INR 1.29 million/year, which is 15% below the expected revenue.

Figure 5: Monthly energy generation of power plant with near shading (Source: Waaree Energies)

Further, we add few obstructions at far horizon (sky scrapers) along with the previous obstruction (near shading). This far horizon objects as we know shadows the entire array of modules when sun is below line of horizon. Thus while we could observe a reduced generation in the entire power plant, a severe reduction in generation could be observed in winters (as sun is much lower in horizon). The total energy output of the power plant (Figure 6) is 331.1 MWh. Thus considering the same value of energy sale (INR 3.50/kWh) the revenue turns out to around INR 1.15 million/year, which is 24% below the expected revenue.

Figure 6: Monthly generation of power plant with near & far horizon shading (Source: Waaree Energies)

Thus as clear from above, shading significantly affects the revenue from a solar power plant. While a 15% & 24% revenue fall was seen from above simulation, practical revenue fall considering (only shadowing) can vary from 10% to whopping 60% or even around 80%. As other than newly built sky scrapers, all the other type of shading could be controlled. This makes project planning and designing a crucial step in order to ensure that the plant lives up to its expectations. A few considerations which could be considered while the designing phase are as follows:

  1. Grouping of modules: An important method to mitigate shading effect (to an extent), in this process, we group solar modules according to the energy they produce while keeping the time of its shading in mind. This gives a leeway to designer to ensure he eliminates only the most underperforming modules while he keeps the better modules to ensure the criticality and sustainability of the power plant. As seen in Figure 7 below, we would keep the modules which are only shaded for say 1 & 3 months while we could completely remove modules which faces shadowing for 7 or more months.  

Figure 7: Specific yield of different groups of module (Image source: Solar Professional)

         2. String and central inverter: String inverters are used to connect smaller arrays whereas a central inverter connects arrays (with larger power output). In central inverter system, shading on any module/array of modules may cause the entire plant power to go down. An alternate solution is to connect the system to smaller string inverters which would increase the power output of module.

         3. Micro inverter: A further detailed solution could be using micro inverters. As we explained in our previous blog “Optimizing solar power output in an overcast weather”, micro inverter could mitigate the effect of shading down to single module level.

Waaree Energies is capable of giving turnkey solutions for both Rooftop and Utility scale projects. We have an experience of executing more than 500+ MW EPC project services. Additionally its highly reliable module is capable to perform to their maximum at any given conditions. Such expertise gives us a cutting edge over our competitors to always serve our customers in the most optimal way.

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

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