Ground reflection on front side

What this step does

The ground between the rows reflects light onto the front of the row behind. This gain is added to the plane irradiance after the soiling and incidence-angle rows. pvperf computes it only for a bifacial module on fixed rows, under the Unlimited sheds 2D option of the Model tab's Rear side and front ground reflection step.

The math

1. Running total
2. Gain at a timestep

$$E_{after} = E_{before} + E_{row},\qquad E_{row} = \frac{\Delta t}{1000}\sum_i L_i \tag{1}$$ $$L_i = k_{frt}\,\text{albedo}\,\frac{P}{W}\,\big\langle T_{front}(x)\,G_i(x) \big\rangle \tag{2}$$

where

Symbol	Meaning	Unit
$E_{before}$	the running total of the loss diagram before this row	kWh/m²
$E_{after}$	the running total after this row	kWh/m²
$E_{row}$	the row, a gain	kWh/m²
$\Delta t$	the length of one timestep	hours
$L_i$	the irradiance gained at timestep $i$	W/m²
$k_{frt}$	a fitted scale, 0.9357, on the ray sum of the ground light; the ray sum leaves out the cosine at the receiving face	—
$\text{albedo}$	the ground albedo: the bifacial albedo when it is set, otherwise the site's albedo of the month	—
$P$	the row pitch	m
$W$	the collector width	m
$T_{front}(x)$	the share of the light leaving ground point $x$ that reaches the front face of the next row through its glass	—
$G_i(x)$	the light falling on ground point $x$ at timestep $i$	W/m²
$\langle \cdot \rangle$	the mean over the 100 ground points of one pitch	—

Reading the number

A gain, positive: a fraction of a percent of the plane irradiance on the fixed rows of a utility plant. Rooftop systems have no bifacial model and show zero, as do monofacial modules. A bright ground, modules mounted high and a steep tilt make it larger.

References

1. Marion et al. 2017, A practical irradiance model for bifacial PV modules, NREL/CP-5J00-67847

API

loss.front_reflection

Symbols follow the symbol registry.