Near Shading Effect

The Near Shading Effect quantifies the amount of irradiance lost due to shading on individual modules by other modules, or objects such as trees, terrain and buildings near the PV plant.

The near shading calculation consists of three considerations:

View factors for diffuse shading

The sky diffuse component and the reflected component will be reduced. These effects are calculated by considering the view factors from the modules and from the ground to the unobstructed sky. For fixed tilt arrays the view factors and so these effects are constant with time. The diffuse shading factor is pre-calculated and denoted \(d(j)\). The near-shaded diffuse irradiance for submodule j at time t is then:

$$G_{dif,poa,near}\left( j,t \right) = d\left( j \right) \bullet G_{dif,poa,far}(o_{j},t)$$

Similarly the reflected component is:
$$G_{r,poa,near}\left( o_{j},t \right) = d_r\left( j \right) \bullet G_{r,poa,far}\left( o_{j},t \right)$$

Proportion of direct irradiance lost to near shading

The total direct irradiance arrive on the array is reduced when part of the area is in the shade. The shaded and un-shaded fraction of the array area is calculated and used to report the loss tree for irradiance, and used in the loss tree for some of the conversion effects. However these fractions are not used in the conversion calculation result, and do not influence the system energy yield.

Using \(A_{\text{u}}(j,t)\) to denote the unshaded area of submodule j, we can calculate the average global near-shaded plane of array irradiance on submodule j, denoted \(G_{\text{poa,near}}(j,t)\) as:

$$G_{poa,near}\left( j,t \right) = \left\lbrack G_{dif,poa,near}\left( j,t \right) + G_{r,poa,near}\left( o_{j},t \right) \right\rbrack + \frac{G_{dir,poa,far}\left( o_{j},t \right) \bullet A_{u}(j,t)}{A(j)}$$

And the average over the whole plant is:

$$G_{plant,near}(t) = \frac{\sum_{j = 1}^{N_{\text{submodules}}}{G_{poa,near}(j,t) \bullet A(j)}}{\sum_{j = 1}^{N_{\text{submodules}}}{A(j)}}$$

Shading of specific sub-module leading to electrical mismatch

Once the electrical mismatch is considered, the calculation is based on precisely which cells are shaded. The full direct component of the irradiance is passed into the conversion calculation. The irradiance value used to calculate module performance as part of the electrical mismatch calculation includes the direct component when none of the cells in a submodule are shaded. If one or more cells are shaded then the irradiance value consists of the diffuse and reflected components only.

Shading models

There are two shade models in SolarFarmer which are described in detail in Module Shading.

• The full model, described in section Full Shading Model, back traces from discrete points on the submodule surface to the sun, which is used to create a zenith by azimuth lookup table of where the sun is visible.
• The simple model is described in detail in Simple Model. It is based on an infinite sheds model, so doesn't include effects at row ends, and is best used for arrays with long rows. The simple model uses View Factors to account for adjacent rows, but cannot account for other shading obstacles such as trees or buildings.