PV-wirefree versus conventional PV-systems: detailed analysis of difference in energy yield between...
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PV-wirefree versus conventional PV-systems:detailed analysis of difference in energy yield between series and parallel connected PV-modules
19th European Photovoltaic Solar Energy Conference and Exhibition
Henk OldenkampOKE-Services, The Netherlands8 June 2004
Web: www.pv-wirefree.com
Overview presentation
• Definitions• What is PV-wirefree?• PV-wirefree claims …• Focus of this presentation• Test setup• Explanation of the graphs• Measurement results• Conclusions• Finally
Definitions
• PV-string: series connection of PV-modules of which all cells are connected in series
Duality of a PV-string is:• PV-shunt: parallel connection of PV-modules
of which all cells are connected in parallel
But all cells of a PV-module connected in parallel is not practical due to high currents and its associated losses
Definitions
• PV-string: series connection of PV-modules of which all cells are connected in series
• PV-shunt: parallel connection of PV-modules of which all cells are connected in series
What is PV-wirefree?PV-wirefree = PV-shunt(s)
• Large numbers of PV-laminates connected in parallel using a current carrying mounting frame (= mounting bus)
• Each group of PV-laminates connected to one set of mounting busses has its own inverter, and is called a subsystem
What is PV-wirefree?PV-wirefree = PV-shunt(s)
• Large numbers of PV-laminates connected in parallel using a current carrying mounting frame (= mounting bus)
• Each group of PV-laminates connected to one set of mounting busses has its own inverter, and is called a subsystem
What is PV-wirefree?PV-wirefree = PV-shunt(s)
• Large numbers of PV-laminates connected in parallel using a current carrying mounting frame (= mounting bus)
• Each group of PV-laminates connected to one set of mounting busses has its own inverter, and is called a subsystem
PV-wirefree claims
• Increase of annual energy yield especially in suboptimal conditions
• Considerable decrease of costs
Focus of this presentation
• This presentation proves the first claim: a significant increase of annual energy yield
How?
• By measuring the full P versus V curves of nine PV-modules continuously switched between shunt and string under varying shading conditions, excluding any other influences on the array power
Test setup
Every minute the lower 9 pv-modules are connected in shunt and in string, and a photo is taken. A 1000 points power versus voltage sweep takes 7 seconds
Explanation of the graphs
Red line = average shunt power per module versus shunt voltage
Purple line = irradiation during the 7 seconds sweep time
Explanation of the graphs
Blue line = average string power per module versus average module voltageGreen line = irradiation during the 7 seconds sweep
Explanation of the graphs
Black dot marks maximum power point of the string when loaded at 81.5% of Voc (expected MPP)
Measurement resultsDefinition of three types
• Very lightly shaded: the shade covers roughly the area of one cell
• Lightly shaded: the shade covers several cells
• Moderately shaded: the shade covers several modules
Conclusions: measurement resultsOverview shading effects
Condition Very lightly shaded
Lightly shaded
Moderately shaded
MPP range shunt[% of Voc]
80 – 82 80 – 83 82 - 84
MPP range string[% of Voc]
75 - 87 67 – 85 46 - 76
Shunt power gain minimum [%]
2 - 5 10 - 20 0 - 50
Shunt power gain practical [%]
5 - 25 10 - 40 30 - 400
ConclusionsShunts always perform better
Since:• The MPP voltage nearly constant• And simple and efficient MPP tracking
possible
Additional advantages of shunts • Inverters can have significantly narrower
input voltage windows, which will reduce costs and/or increase efficiency of inverters
• MPP tracking efficiency will always be significantly better