Successfully reported this slideshow.
We use your LinkedIn profile and activity data to personalize ads and to show you more relevant ads. You can change your ad preferences anytime.

15 wittmer p_vsyst_pvpmc_8_albuquerque

482 views

Published on

8th PVPMC Workshop, May 9-10 2017

Published in: Technology
  • Be the first to comment

  • Be the first to like this

15 wittmer p_vsyst_pvpmc_8_albuquerque

  1. 1. PVSYST SA - Route du Bois-de-Bay 107 - 1242 Satigny - Suisse www.pvsyst.com Any reproduction or copy of the presentation, even partial, is forbidden without a written authorization of the author. Modeling PV power optimizers with PVsyst 8th PVPMC Workshop 9-10.5.2017 Albuquerque, USA André Mermoud, Bruno Wittmer Bruno.Wittmer@pvsyst .com
  2. 2. Page 2Page 2 Overview • Introduction to mismatch losses • Electrical shading losses in row-based installations • Recovery of electrical shading losses with power optimizers
  3. 3. Page 3Page 3 Mismatch losses Operate PV cells at same current. The ‘weakest’ element limits the current. Current mismatch comes from spread in cell characteristics and from irradiance differences. Need to operate strings at same voltage. PV-curve determines best operating point. Voltage mismatch comes from spread in cell characteristics, different irradiance and different temperature. Serial connections (Cells and Modules) suffer from Current mismatch Parallel connections (Strings) suffer from Voltage mismatch Current mismatch Voltage mismatch If serial mismatch becomes large, the bypass diodes get activated Parallel mismatch will not trigger bypass diodes Power optimizers are designed to compensate the mismatch
  4. 4. Page 4Page 4 Mismatch losses in PVsyst Module mismatch losses - Variation of module properties (Vmpp, Impp) - Variation of temperature (Vmpp) Electrical Shading & Orientation losses - IV Mismatch due to shading - Mismatch due to different orientations Constant loss factor in simulation (default 1%). A dedicated tool allows to make a detailed analysis based on IV-curves, to estimate this loss factor Calculated in detail in the simulation. Leads to current and voltage mismatch Current mismatch is small (will not activate bypass diodes) Mainly current mismatch due to shadings When Current mismatch becomes large enough to activate bypass diodes => Voltage Mismatch Fully recoverable with optimizers Partially recoverable with optimizers But additional optimizer efficiency loss! Typically ≈1%
  5. 5. Page 5Page 5 Module and Sub-Modules Sub-module (sub-string) protected by bypass-diode Module in landscape orientation Shadings in a row-based installation will progress upwards. Row-based installations: Modules within a string have the same shadings 1/3 of submodules get shaded at once Consider a module with 72 PV cells and 3 bypass diodes => 24 cells / sub-module
  6. 6. Page 6Page 6 Bypass Diodes Behavior Significant shift of VmppSignificant shift of Vmpp Single Cell shaded All Cells in one sub-module shaded Large current mismatch activates bypass diode Large mismatch is caused by shadings Mismatch of a single cell leads already to bypass of entire sub-module When a bypass diode becomes active, Vmpp decreases by 1/3 (1/2 for second bypass diode) The energy dissipated in the bypass diode, accounts for an additional loss : 1st sub-module (shaded) : 1st + 2nd sub-module : 1st + 2nd + 3rd sub-module : unshaded module : Shaded cells : Unshaded cells Bypass diode active Bypass diode not active Bypass diode active Bypass diode not active
  7. 7. Page 7Page 7 Shading a single module First row of cells shaded Four rows of cells shadedTwo rows of cells shaded : shaded cell(s) : unshaded cells : unshaded module The shading proceeds in steps of sub-modules: Each shaded sub-module shifts the step in the IV-Curve to lower V : 1st sub-module : 1st + 2nd sub-module : 1st + 2nd + 3rd sub-module
  8. 8. Page 8Page 8 Shading losses with one or two rows of shaded cells are the same First row of cells shaded Two rows of cells shaded : 1st sub-module : 1st + 2nd sub-module : 1st + 2nd + 3rd sub-module Linear shading loss: first row of cells Electrical shading loss: second row of cells Linear shading loss: Two row of cells Electrical shading loss: none In a partially shaded sub-module the unshaded cells are also lost and not recoverable! The active bypass diode also dissipates power, which is not recoverable. : unshaded module The losses are of equal magnitude but are accounted for differently
  9. 9. Page 9Page 9 Shading a string of modules A string of modules is equivalent to a string of sub-modules => the behavior is similar to the module behavior : shaded sub-modules : shaded + unshaded sub-modules : resulting IV-curve First row of cells shaded One sub-module shaded Three rows of cells shaded Two sub-modules shaded Two rows of cells shaded One sub-module shaded Optimizers on sub-module level can recover diffuse part of shaded modules Shadow growth (8 sub-modules get shaded at a time) : unshaded string
  10. 10. Page 10Page 10 Shading losses in a string of modules : shaded sub-modules : shaded + unshaded sub-modules : resulting PV-curve First row of cells shaded Four rows of cells shadedTwo rows of cells shaded Optimizers on sub-module level can recover diffuse part of shaded modules : unshaded string (2322W) Shadow growth (8 sub-modules get shaded at a time) Displaced MPP can reach inverter voltage limits!
  11. 11. Page 11Page 11 Parallel connection of shaded strings Identical IV-Curves add up in current => multiply current (power) with number of strings IV curves are not identical MPPs are at different voltages => Mismatch losses Same shading on each string Shaded and unshaded strings in parallel : shaded sub-modules : 1st string : 1st + 2nd string : unshaded strings : shaded + unshaded 1451 W 2990 W 1494 W 3142 W
  12. 12. Page 12Page 12 Parallel connection of shaded and unshaded strings Single shaded string Shaded and unshaded strings in parallel Shaded and two unshaded strings in parallel Pmpp Module: Pmpp Strings: 8 x 187 W = 1494 W 1494 W 1451 W + 1691 W = 3142 W 481 W + 2 x 2319 W = 5119 W 8 x (187 W + 290 W) = 3810 W 8 x (187 W + 2 x 290 W) = 6126 W shaded string shaded string shaded string unshaded strings pull MPP to higher Voltage 1494 W 1451 W 1691 W 481 W 2319 W 2319 W 2322 W 1494 W 1451 W 481 W
  13. 13. Page 13Page 13 Losses in a partially shaded string Single shaded string Unshaded strings Relative loss = The order in which the sub-modules get shaded does not matter! Loss in group of parallel strings Power of single unshaded string Irradiance loss Electrical shading loss 1/3 of sub-modules 3 or more strings in parallel: When 1/3 of the submodules in one string are shaded, the contribution of the shaded string is reduced to diffuse part
  14. 14. Page 14Page 14 Power Optimizer Working Principle Full optimizer (Buck-Boost) Current & Voltage boost ‘Buck-only’ Current boost Optimizers expand MPP to an extended IV-range => Mismatch in serial and parallel connections can be compensated MPP constant power Current boost MPP constant power Current boost ‘Buck-only’ optimizers risk to reach current limit (or inverter lower voltage limit) MPP constant power Current boost Voltage boost constant power Current boost Voltage boost ‘Buck-only’ optimizers can act either on module or on sub-module level : Optimizer Output : Module IV curve
  15. 15. Page 15Page 15 Optimizers and Module Mismatch Full optimizer‘Buck-only’ ‘Buck-only’ per sub-module Optimizers recover the Module mismatch (small mismatch, not triggering bypass diodes) 315 W 295 W 275 W 255 W 105 W 98 W 92 W S = 1140 W S = 295 W In PVsyst, systems with optimizers get as default 0% module mismatch
  16. 16. Page 16Page 16 Optimizers and Shaded Sub-modules Full optimizer‘Buck-only’ ‘Buck-only’ per sub-module Optimizers per sub-module can recover the diffuse part of the shaded sub-modules Extended MPP plateau Extended MPP plateau Recovery of sub-module mismatch Optimizer Imax is reached in this example! MPP Extension to lower voltage risks to hit Inverter limitation!: Optimizer Output : Shaded IV curves
  17. 17. Page 17Page 17 Optimizers and Shaded Strings Full optimizer ‘Buck-only’ ‘Buck-only’ per sub-module Optimizers per sub-module can recover the diffuse part of the shaded sub-modules Sub-modules are boosted independently : Optimizer Output : Shaded IV curves Firs row of sub-modules shaded Firs two rows of sub-modules shaded
  18. 18. Page 18Page 18 Strings in parallel Optimizers effectively recover the Voltage mismatch between strings MPP Extension to lower voltage risks to hit Inverter limitation! Shaded and unshaded strings in parallel No optimizer Pmpp = 3.73 kW ‘Buck-only’ per sub-module Pmpp = 4.67 kW ‘Buck-only’ Pmpp = 3.76 kW Full optimizer Pmpp = 4.51 kW Inverter Vmpp min : Resulting IV/PV curves
  19. 19. Page 19Page 19 Impact on yearly simulation - Situations with optimizer benefits do not happen very often - Typically they happen with shadings => sun is low and incidence angle high (small impact) - The overall benefit in row-based installations is small. - ‘Buck-only per submodule’ can recover the diffuse irradiance on shaded sub-modules - Buck-only per sub-module and ‘Full optimizers’ recover voltage mismatch on shaded/unshaded string combinations Sun Height: 22° : Shadow : Shaded String Pmpp = 3.76 kW Electrical shading lossess
  20. 20. Page 20Page 20 Conclusions Using optimizers in row-based PV installations: • Monitoring at the PV Module level • Possibility to centrally disconnect Modules • More flexibility in string design (some optimizers also allow especially long strings) • The module mismatch is always recovered with optimizers (PVsyst will set it to 0% by default) => useful on the long run, when module degradation increases mismatch • Additional losses due to optimizer efficiency (similar order of magnitude as initial module mismatch) • In row-based installations the recovery of electrical shading losses is rather small Recovering electrical shading losses with power optimizers comes mainly into play with irregular shading situations Buck only optimizers are more likely to reach inverter voltage limitations

×