PVsysts new framework to simulate bifacial systems PVPMC Workshop 24-26.10.2016 Freiburg, Germany André Mermoud, Bruno Wittmer [email protected] .com PVSYST SA - Route du Bois-de-Bay 107 - 1242 Satigny - Suisse www.pvsyst.com Any reproduction or copy of the course support, even partial, is forbidden without a written authorization of the author. Overview • Introduction – Bifacial PV modules – Modelling the backside irradiance • PVsyst Approach do model bifacial installations – Model describing shed installations – Calculation of backside irradiation – Some first qualitative results • Summary and Outlook Page Page 2 2 Approach for Bifacial Modules in PVsyst Treatment of bifacial modules • • • • • Front side irradiance is added to backside irradiance x bifaciality factor (default is 0.8) From this Effective irradiance follows the IV-curve. Loss factor describing shadings of mounting structure and junction boxes An additional mismatch factor is foreseen to account for inhomogeneous rear side illumination This approach is an approximation Back side irradiance x bifaciality factor Front side irradiance Effective irradiance IV-Curve The main challenge is to calculate the additional backside irradiance including its inhomogeneity Page Page 3 3 Standard Irradiance Calculation Irradiance on PV modules has 3 components • • • Direct Subject to near shadings depending on sun position Diffuse Subject to shading factor that is constant for a given plane orientation Calculation of angles that shade the diffuse Albedo Subject to shading factor that is constant for a given plane orientation Calculation of azimuth angles that are blocked Shadings of the direct irradiance Shadings of the diffuse irradiance Azim 0° Azim -20° Azim -40° He 10° He 20° ... Page Page 4 4 Diffuse shading factor is an integral over all hemisphere directions ... Albedo and Near Ground Scattering Albedo is not the same as Near Ground Scattering • • Albedo (of transposition model) Reflections and scattering from ground that is far away. Obstacles will shade all the albedo for a given azimuth. Near Ground Scattering (for bifacial simulations) Light scattered back from ground that is close to the PV modules. Subject to near shadings with solid angle calculation. Albedo scattering Near ground scattering Albedo shaded Albedo visible Ground scattering is only partially shaded Bifacial PV installations can not be described by a modified albedo only.. Page Page 5 5 Systems with Bifacial Modules in PVsyst Basic approach for bifacial modelling Sketch explaining bifacial model • • • • • Fraction of direct irradiance that reaches the scattering ground (depends on sun position) Fraction of diffuse irradiance that reaches the scattering ground (single factor) Factor describing the scattering off the ground (Ground Albedo) Factor for backside acceptance of scattering ground (form factor) (Constant loss factor describing shadings of mounting structures, cabling and junction boxes) Bifacial Module Direct Irradiance Diffuse Irradiance Ground Scattering Incoming light, and light scattered back to modules depend on the position Assumptions for bifacial calculation • • • • • • Only light scattered back from the ground contributes to backside illumination. Direct and sky diffuse irradiance contribute to ground illumination Sky diffuse is isotropic Only scattering is considered (no specular reflections) The diffuse reflection is isotropic (Lambertian Surface) Non-homogeneous illumination of backside is neglected at this stage Page Page 6 6 Bifacial modules in PV installations Bifacial modules are used in different situations • Sheds Light can reach the ground between the sheds and scatter back to the module backside. • Vertical mounting Diffuse and direct light can reach the PV module on both sides. Sketch showing vertical bifacial systems Sketch showing shed bifacial systems In a first step, PVsyst will model bifacial systems only for shed geometries. Page Page 7 7 Bifacial Modules in Sheds PVsyst Model to determine bifacial parameters for regular shed configurations Simplified 2D calculation Rows without boundary effects (infinitely long) Parameters: • Tilt, Azimuth • Width, Pitch • Height above ground • Ground Albedo The factors for the bifacial calculation can be determined by integrating over the distance between rows Direct and sky diffuse are only computed for front side. Near ground scattering is only computed for backside. Calculation proceeds in three steps 1. Ground Acceptance of direct light 2. Ground acceptance of diffuse light Page Page 8 8 3. Backside acceptance of ground (form factor) Calculation of Direct Light on Ground Ground Acceptance of direct light 1. Profile Angle and Limit Angle determine the amount of directly illuminated ground surface 2. Height over ground and profile angle determine the position of the illuminated strips. aprof Profile angle Profile angle aprof Limit Angle tan 𝛼𝑙𝑖𝑚 = Azimuth a alim q P Page Page 9 9 sin 𝜃 𝑃 𝑊 − cos 𝜃 Examples of Direct Light Ground Acceptance Ground Acceptance of direct light Momentary Ground irradiance Ground irradiance over one day 21. June Geneva, 21. June, 12:00h Geneva, 21. June, 19:00h Illuminated ground is composed of homogeneously illuminated strips Page Page 10 10 Calculation of Diffuse Irradiance on Ground Ground acceptance of diffuse light - Diffuse irradiance from sky is isotropic. Ground acceptance of diffuse light is a function of the position on the ground. Underneath the sheds the irradiance is smaller. Inhomogeneity tends to level out with increasing mounting height. sheds at ground level Page Page 11 11 1m over ground 2m over ground Calculation of Form Factor Backside acceptance of ground (Form Factor) - - Ground scattering is isotropic (Lambertian Scattering) Form Factor is a function of the position on the ground. Underneath the sheds the form factor is large. Inhomogeneity tends to level out with increasing mounting height. sheds at ground level Ground scattering is isotropic (Lambertian Scattering) Page Page 12 12 1m over ground 2m over ground Calculation of Total Irradiance on Backside Putting it all together Absolute irradiance Normalized to horizontal Irradiance on ground Combine Ground acceptance with Form Factor Direct Irradiance on Ground is specific for location and geometry. In this case (Geneva): - Almost no direct in winter - Fraction of diffuse on ground is constant over the year Diffuse + Absolute irradiance Normalized to Front For this location and geometry, the additional bifacial gain is obtained mainly in summer Irradiance on Backside: Page Page 13 13 Example shed installation Basic PV system with sheds: 90 kWp in 6 rows of 3 x 20 modules landscape Location Geneva, Switzerland: 46.3° N, 6.1° E 25° Tilt, 6m Pitch, 3m Width Mounted 1m over ground PV surface: 600 m2 Ground surface: 33m x 33m = 1000 m2 Definition of bifacial shed model 3D shading scene Page Page 14 14 Simulation Results PVsyst Report showing results for bifacial modelling Global incident on ground Ground scattering Backside form factor Shadings on backside Bifaciality factor Mismatch for back irradiance Page Page 15 15 Height over ground Height over ground With higher mounting, the opposite behavior of ground illumination and acceptance gets attenuated. For the shed model with no boundaries (infinitely long sheds), the increase saturates (ground will appear homogeneously illuminated) Diffuse on Ground Form Factor sheds at ground level 1m over ground 2m over ground Page Page 16 16 Diffuse Contribution to rear side Impact on best Tilt Impact on best tilt Best tilt does not change significantly for this specific case. Maximum in bifacial curve is slightly flatter. Bifacial definitions: - 2 m over ground - 80% bifaciality factor - GCR: 50% - Ground albedo factor: 0.3 Bifacial Contribution On Rear Side Page Page 17 17 Impact of pitch on bifacial gain Gain increase as function of pitch Increase of pitch (row spacing) reduces mutual shadings and thus increasing the yield. Ground in between rows also gets more irradiance, leading to an increased yield gain for bifacial systems. 4% 3% Normalized to monofacial, pitch 5m Page Page 18 18 Best tilt as function of Pitch Complete optimization has to consider Tilt together with Pitch Optimization tool to study yield as function of tilt and pitch Monofacial Bifacial Page Page 19 19 Ground reflection (Albedo factor) Bifacial Gain as function of Albedo Factor Ground Type For common Albedo factor ranges and the considered 90 kWp shed installation, the simulation predicts 4-10% bifacial gain. Page Page 20 20 Albedo factor Worn Asphalt 0.12 Bare Soil 0.17 Green Grass 0.25 Desert sand 0.40 New concrete 0.55 Fresh snow 0.8-0.9 Next Steps for Bifacial Framework Add further specific bifacial models Model for vertically mounted modules - Both module sides need to be treated equally - Irradiance of Direct, Diffuse, Albedo and Ground Scattering computed for both faces Bifacial treatment of any PVsyst 3D scene Define a ground surface shape. Compute Global irradiance on ground (direct and Diffuse). Calculate view factors for all PV backsides. Limitations: - No specular reflections - Only the ground surface scatters back Statistical approach with a random distribution of ground points Page Page 21 21 Summary and Outlook – The challenge in bifacial calculations for PV systems is to determine the irradiance on the module back side – The module behavior under a given total effective irradiance is similar to a standard PV module – Simulation of Bifacial PV systems with shed (row) layout will be possible with PVsyst V6.60 – Several approximations are made to handle the calculation • • • • – – – – Only light scattered off the ground reaches the module back side Ground reflection is diffuse and isotropic Shadings of the mounting structure on the backside are accounted with a constant derate factor An additional factor accounts for inhomogeneous illumination Main contributions of backside illumination are captured The approach will be generalized to allow the bifacial calculation for any 3D shading scene The final goal is to treat front and back illumination in the same way. Validation with measured data is still necessary Page Page 22 22