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| 1 | +.. _bifacial: |
| 2 | + |
| 3 | +Bifacial modeling |
| 4 | +================= |
| 5 | + |
| 6 | +This section reviews the bifacial modeling capabilities of |
| 7 | +pvlib-python. |
| 8 | + |
| 9 | +A bifacial module accepts light on both surfaces. Bifacial modules usually have |
| 10 | +a front and back surface, with the back surface intended to face away from |
| 11 | +the primary source of light. The primary challenge in modeling a PV system |
| 12 | +with bifacial modules is estimating the irradiance on the front and back |
| 13 | +surfaces. |
| 14 | + |
| 15 | +pvlib-python provides two groups of functions for estimating front and back |
| 16 | +irradiance: |
| 17 | + |
| 18 | +1. a wrapper for convenient use of the pvfactors package: |
| 19 | +:py:func:`~pvlib.bifacial.pvfactors.pvfactors_timeseries` |
| 20 | + |
| 21 | +2. the infinite sheds bifacial model: |
| 22 | +:py:func:`~pvlib.bifacial.infinite_sheds.get_irradiance` |
| 23 | +:py:func:`~pvlib.bifacial.infinite_sheds.get_irradiance_poa` |
| 24 | + |
| 25 | + |
| 26 | +pvfactors |
| 27 | +========= |
| 28 | + |
| 29 | +The `pvfactors <https://sunpower.github.io/pvfactors/>`_ package calculates |
| 30 | +incident irradiance on the front and back surfaces of an array. pvfactors uses |
| 31 | +a 2D geometry which assumes that the array is made up of long, regular rows. |
| 32 | +Irradiance is calculated in the middle of a row; end-of-row effects are not |
| 33 | +included. pvfactors can model arrays in fixed racking or on single-axis |
| 34 | +trackers. |
| 35 | + |
| 36 | + |
| 37 | +Infinite Sheds |
| 38 | +============== |
| 39 | + |
| 40 | +The "infinite sheds" model [1] is a 2-dimensional model of irradiance on the |
| 41 | +front and rear surfaces of a PV array. The model assumes that the array |
| 42 | +comprises parallel, equally spaced rows (sheds) and calculates irradiance in |
| 43 | +the middle of a shed which is far from the front and back rows of the array. |
| 44 | +Sheds are assumed to be long enough that end-of-row effects can be |
| 45 | +neglected. Rows can be at fixed tilt or on single-axis trackers. The ground |
| 46 | +is assumed to be horizontal and level, and the array is mounted at a fixed |
| 47 | +height above the ground. |
| 48 | + |
| 49 | +The infinite sheds model accounts for the following effects: |
| 50 | + |
| 51 | + - limited view from the row surfaces to the sky due to blocking of the |
| 52 | + sky by nearby rows; |
| 53 | + - reduction of irradiance reaching the ground due to shadows cast by |
| 54 | + rows and due to blocking of the sky by nearby rows. |
| 55 | + |
| 56 | +The model operates in the following steps: |
| 57 | + |
| 58 | +1. Find the fraction of unshaded ground between rows, ``f_gnd_beam`` where |
| 59 | + both direct and diffuse irradiance is received. The model assumes that |
| 60 | + there is no direct irradiance in the shaded fraction ``1 - f_gnd_beam``. |
| 61 | +2. Calculate the view factor, ``fz_sky``, from the ground to the sky accounting |
| 62 | + for the parts of the sky that are blocked from view by the array's rows. |
| 63 | + The view factor is multiplied by the sky diffuse irradiance to calculate |
| 64 | + the diffuse irradiance reaching the ground. Sky diffuse irradiance is thus |
| 65 | + assumed to be isotropic. |
| 66 | +3. Calculate the view factor from the row surface to the ground which |
| 67 | + determines the fraction of ground-reflected irradiance that reaches the row |
| 68 | + surface. |
| 69 | +4. Find the fraction of the row surface that is shaded from direct irradiance. |
| 70 | + Only sky and ground-reflected irradiance reach the the shaded fraction of |
| 71 | + the row surface. |
| 72 | +5. For the front and rear surfaces, apply the incidence angle modifier to |
| 73 | + the direct irradiance and sum the diffuse sky, diffuse ground, and direct |
| 74 | + irradiance to compute the plane-of-array (POA) irradiance on each surface. |
| 75 | +6. Apply the bifaciality factor, shade factor and transmission factor to |
| 76 | + the rear surface POA irradiance and add the result to the front surface |
| 77 | + POA irradiance to calculate the total POA irradiance on the row. |
| 78 | + |
| 79 | +Array geometry is defined by the following: |
| 80 | + |
| 81 | + - ground coverage ratio (GCR), ``gcr``, the ratio of row slant height to |
| 82 | + the spacing between rows (pitch). |
| 83 | + - height of row center above ground, ``height``. |
| 84 | + - tilt of the row from horizontal, ``surface_tilt``. |
| 85 | + - azimuth of the row's normal vector, ``surface_azimuth``. |
| 86 | +View factors from the ground to the sky are calculated at points spaced along |
| 87 | +a one-dimensional axis on the ground, with the origin under the center of a |
| 88 | +row and the positive direction toward the right. The positive direction is |
| 89 | +considered to be towards the "front" of the array. Array height differs in this |
| 90 | +code from the description in [1], where array height is described at the row's |
| 91 | +lower edge. |
| 92 | + |
| 93 | +This model is influenced by the 2D model published by Marion, *et al.* in [2]. |
| 94 | + |
| 95 | +References |
| 96 | +---------- |
| 97 | +.. [1] Mikofski, M., Darawali, R., Hamer, M., Neubert, A., and Newmiller, |
| 98 | + J. "Bifacial Performance Modeling in Large Arrays". 2019 IEEE 46th |
| 99 | + Photovoltaic Specialists Conference (PVSC), 2019, pp. 1282-1287. |
| 100 | + doi: 10.1109/PVSC40753.2019.8980572. |
| 101 | +.. [2] Marion. B., MacAlpine, S., Deline, C., Asgharzadeh, A., Toor, F., |
| 102 | + Riley, D., Stein, J. and Hansen, C. "A Practical Irradiance Model for |
| 103 | + Bifacial PV Modules".2017 IEEE 44th Photovoltaic Specialists Conference |
| 104 | + (PVSC), 2017, pp. 1537-1543. doi: 10.1109/PVSC.2017.8366263 |
| 105 | +
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