Layout Designing for Utility-Scale Solar Project
A utility-scale solar facility is one which generates solar power and feeds it into the grid, supplying a utility with energy. Layout designing is the major factor for installation of cost-effective solar plant.
The general layout of the plant and the distance chosen between rows of mounting structures will be selected according to the specific site conditions. The area available to develop the plant may be constrained by space and may have unfavourable geological or topographical features. The aim of the layout design is to minimise cost while achieving the maximum possible revenue from the plant. In general this will mean:
- Designing the layout to minimise cable runs and associated electrical losses.
- Creating access routes and sufficient space between rows to allow movement for maintenance purposes.
- Designing inter-row spacing to reduce associate shading losses.
- Choosing a tilt angle that optimises the annual energy yield according to the latitude of the site and the annual distribution of solar resource.
- Orientating the modules to face a direction that yields the maximum annual revenue from power production. In the northern hemisphere, this will usually be true South and in southern hemisphere, this will be true North.
It should be designed in a way to minimize cable runs and associated electrical losses may suggest positioning an LV/MV station centrally within the plant. In this approach, it should be kept in mind that there will be shadow on the panels due to this. Hence, adequate space should be allocated to avoid the possibility of the station shading modules behind it. This step involves positioning of components in such a way to optimize the land and the shading effect of those components on the array field.
During plant layout designing, always take adequate offset from the perimeters to prevent the shading due to fencing/boundary walls of the plant. This space can be utilized as a peripheral road to incorporate access routes for maintenance staff and vehicles at appropriate intervals. It is advisable to make central divisionary roads within the plant to divide the plant in blocks of 2 MW – 5 MW so that the maintenance of the plant will be easy.
Optimizing of Tilt Angle
Every location will have an optimal tilt angle that maximises the total annual irradiation (averaged over the whole year) on the plane of the collector. For fixed tilt grid connected power plants, the theoretical optimum tilt angle may be calculated from the latitude of the site. However, adjustments may need to be made to account for:
• Soiling –
To reduce soiling losses, tilt angles are kept higher. At higher angles, the modules will be cleaned effectively with natural flow of rainwater and snow slides off more easily.
• Shading –
More highly tilted modules provide more shading on modules behind them. As shading impacts energy yield much more than may be expected simply by calculating the proportion of the module shaded, a good option (other than spacing the rows more widely apart) is to reduce the tilt angle. It is usually better to use a lesser tilt angle as a trade-off for loss in energy yield due to inter-row shading.
• Seasonal irradiation distribution –
If a particular season dominates the annual distribution of solar resource (monsoon rains, for example), it may be beneficial to adjust the tilt angle to compensate for the loss. The benefits of this can be assess through Simulation software.
Optimizing of Inter-Row Spacing (Pitch)
The choice of row spacing is a compromise chosen to reduce inter-row shading while keeping the area of the PV plant within reasonable limits, reducing cable runs and keeping ohmic losses within acceptable limits. Inter-row shading can never be reduced to zero: at the beginning and end of the day the shadow lengths are extremely long. Figure below illustrates the angles that must be considered in the design process.
The shading limit angle α is the solar elevation angle beyond which there is no inter-row shading on the modules. If the elevation of the sun is lower than α then a proportion of the module will be shaded. Alongside, there will be an associated loss in energy yield.
The shading limit angle may be reduced either by reducing the tilt angle β or increasing the row pitch d. Reducing the tilt angle below the optimal is sometimes a choice as this may give only a minimal reduction in annual yield. The ground cover ratio (GCR), given by l/d, is a measure of the PV module area compared to the area of land required.
For many locations, a design rule of thumb is to space the modules in such a way that there is no shading at solar noon on the winter solstice (December 21st in the northern hemisphere). In general, if there is less than a 1% annual loss due to shading, then the row spacing may be deemed acceptable.
Detailed energy yield simulations can be carried out to assess losses due to shading, and to obtain an economic optimisation that also considers the cost of land if required.
In the northern hemisphere, the orientation that optimises the total annual energy yield is true South and for southern hemisphere, it will be true North. In the tropics, the effect of deviating from true south may not be especially significant. The effect of tilt angle and orientation on energy yield production can be effectively modelled using simulation software.
Now we will discuss about layout design for one of the case studies for a 10 MW Solar power plant.
In this plant, peripheral roads are provided along the project boundary which will be beneficial also during the construction and phase and after that the roads will be used for maintenance purpose.
For the ease of O&M, it is divided in 5 blocks of 2 MW each, so that DC losses will be decreased and access to modules become easy.
As this plant is situated in the northern hemisphere, its orientation is considered as true South to get the maximum yield.
For the analysis of optimize tilt angle and inter-row spacing(pitch), we do some analysis using PVsyst simulation software for different angles and at different pitches.
We calculated the shading losses and GCR to get the optimize solution. The result from the simulation software were tabulated as shown in the table below:
From the above table it can be seen that how we can save the project cost by using pitch tilt analysis through which we can get optimised tilt in which the land as well as project cost is reduced without affecting the generation so much.
In the above table we have change the tilt angle by keeping the pitch constant for each case so that we can reach to a situation where we will get optimised tilt and pitch for our project. After that we have considered one case as a reference based on which we will do the analysis.
In reference case the pitch is 6.5 m and the tilt are 20° and the generation is 19257 MWh along with specific yield of 1647 kWh/kWp/Year.
Based on the above table we can conclude that the maximum loss which will be seen is 0.3 % in form generation but in return we can save the project cost more. So, the reference case found as an optimised solution for this project.