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PV-wirefree: Bringing PV systems back to their essentials

wirefree. PV. PV-wirefree: Bringing PV systems back to their essentials. Henk Oldenkamp OKE-Services, The Netherlands 12 May 2003 Web: www.pv-wirefree.com. 3 rd World Conference on Photovoltaic Energy Conversion, Osaka, Japan. Overview presentation . Why PV-wirefree? What is PV-wirefree?

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PV-wirefree: Bringing PV systems back to their essentials

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  1. wirefree PV PV-wirefree:Bringing PV systemsback to their essentials Henk OldenkampOKE-Services, The Netherlands12 May 2003Web: www.pv-wirefree.com 3rd World Conference on Photovoltaic Energy Conversion, Osaka, Japan

  2. Overview presentation • Why PV-wirefree? • What is PV-wirefree? • Design • Comparison of PV-wirefree and string systems • Status and developments • Conclusions

  3. PV-wirefree objectives • To minimize BOS-costs of PV-systems • PV-wirefree will reduce the PV BOS costs with 50% • To minimize costs electricity generated by PV-systems • PV-wirefree will decrease the kWh costs of PV with 25% • To minimize energy pay-back time and optimize LCA of PV-systems

  4. What is PV-wirefee? How to reach these objectives? Currently many components are used in grid-connected PV-systems. At the DC-side we find the following components:

  5. What is PV-wirefee? How to reach these objectives? In this presentation we will show that all these components can be omitted and/or replaced by the only four essential ones:

  6. What is PV-wirefee? – How to reach these objectives? Back to basics approach What are the essential components of a PV-system? • PV-laminates consisting of PV-cells to generate electricity • Conductor to carry the DC-current from the PV-laminate to an inverter • Inverter to convert DC- to AC-power

  7. What is PV-wirefee? – How to reach these objectives? Ultimate integration of functions

  8. What is PV-wirefee? – How to reach these objectives? The basics of PV-wirefree • 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

  9. Design Design method • To abolish so many components  clear design choices  affects and limits design freedom of the other components. • Leading design choice: optimizing the most expensive component = PV-laminate • Step 1: Optimize PV-laminate dimensions • Step 2: Design mounting bus • Step 3: Design module connector • Step 4: Design inverter

  10. Design Touch safety • In PV-wirefree systems bare conductors are used • International safety standards require: • In dry conditions: voltage < 60 V • In wet conditions: voltage < 30 V • PV-wirefree open circuit voltage will be 21 V

  11. Design Energy hazard • Power of PV-wirefree systems > 240 VA • International safety standards require following measures: • Sufficient separation of bare conductors • European requirements: test finger of about 8 cm shall not be able to make a short circuit • American requirements: tool shall not be able to make a short circuit • PV-wirefree uses spacing between mounting busses > 0.50 m

  12. PV-wirefree laminate Mounting bus Module connector Inverter Step 1 Design – Method – Overview Design step 1: laminate

  13. Design – Method – PV-wirefree laminate Basic assumptions PV-wirefree should be suitable for “nearly all” PV market sectors.  The PV-laminate should be designed for: • Flat and sloped roofs, facades and desert plants (VLS-PV) • Both multi and polycrystalline silicone cells • Cells of the following dimensions: 12,5 x 12,5; 15 x 15 and 20 x 20 cm.

  14. Design – Method – PV-wirefree laminate To meet our goals • Minimize material use and costs  replace frame by 4 points mechanical connection ( no pollution like moss-grow) • Minimize material use and costs  limit wiring to cell-to-cell wiring, omit junction box • Minimize labour costs  easy installation  click and fit system • Minimize costs  use standard 4 mm PV-glass (heat strengthened)

  15. Design – Method – PV-wirefree laminate To meet general standards For heat strengthened glass a.o. the following figures apply: • According to IEC 61215 laminates should resist a test wind load of 2400 Pa • Additional safety factors vary per country. Maximum calculated stress must be in the range of 50-80 N/mm² at the actual design wind load. • At most locations in Europe the maximum wind load is well below 1500 Pa • Actual breaking will occur at 200 – 300 N/mm² • Working conditions allow a maximum weight per PV module of about 10 kg

  16. Design – Method – PV-wirefree laminate Design choices The PV-wirefree laminate should: • Withstand a test wind load of 2400 Pa, while the stress of the glass should not exceed 80 N/mm². This implies 50 N/mm² at a wind load of 1500 Pa. • Use standard 4 mm heat strengthened glass (common for PV). For calculations 3,8 mm must be used to cope with tolerances of glass thickness • Have an area of maximal 1 m² (not to exceed weight of about 10 kg per PV-module)

  17. Design – Method – PV-wirefree laminate In search of the right dimensions • Currently: 4 x 9 cells

  18. Design – Method – PV-wirefree laminate Stress analysis: 4 x 9 cells laminate (1) Laminate: • 4 x 9 cells 150 mm • 0.649 m x 1.414 m = 0.981 m² Fixing: • Stiff in the corners Result: • Maximum stress in laminate: approximately 800 N/mm² (at a test wind load of 2400 Pa)

  19. Design – Method – PV-wirefree laminate Stress analysis: 4 x 9 cells laminate (2) Laminate: • 4 x 9 cells 150 mm • 0.649 m x 1.414 m = 0.981 m² Fixing: • Stiff at optimum position: distance between fixings 0.774 m Result: • Maximum stress in laminate: approximately 200 N/mm² (at a test wind load of 2400 Pa)

  20. Design – Method – PV-wirefree laminate Stress analysis: 4 x 9 cells laminate (3) Laminate: • 4 x 9 cells 150 mm • 0.649 m x 1.414 m = 0.981 m² Fixing: • Flexible at optimum position: distance between fixings 0.774 m Result: • Maximum stress in laminate: approximately 130 N/mm² (at at test wind load of 2400 Pa)

  21. Design – Method – PV-wirefree laminate Stress analysisConclusions 4 x 9 cells laminate • 4 points connection not feasible for standard 4 x 9 cells laminates (150 mm cells) since even for an optimized design maximum stress in glass is 130 N/mm² at a test wind load of 2400 Pa

  22. Design – Method – PV-wirefree laminate In search of the right dimensions • Let’s study a 5 x 7 cells module instead of a 4 x 9 cells module

  23. Design – Method – PV-wirefree laminate Stress analysis: 5 x 7 cells laminate (1) PV-wirefree laminate: • 5 x 7 cells 150 mm • 0.802 m x 1.108 m = 0.889 m² Fixing: • Fixed at optimum position: distance between fixings 0.648 m Result: • Maximum stress in laminate: approximately 160 N/mm² (at a test wind load of 2400 Pa)

  24. Design – Method – PV-wirefree laminate Stress analysis: 5 x 7 cells laminate (2) PV-wirefree laminate: • 5 x 7 cells 150 mm • 0.802 m x 1.108 m = 0.889 m² Fixing: • Flexible at optimum position: distance between fixings 0.648 m Result: • Maximum stress in laminate: approximately 100 N/mm² (at a test wind load of 2400 Pa)

  25. Design – Method – PV-wirefree laminate Stress analysis: 5 x 7 cells laminate (3) PV-wirefree laminate: • 5 x 7 cells 150 mm • 0.802 m x 1.108 m = 0.889 m² Fixing: • Flexible at optimum position: distance between fixings 0.648 m • Optimized connector Result: • Maximum stress in laminate: approximately 80 N/mm² (at a test wind load 2400 Pa)

  26. Design – Method – PV-wirefree laminate Stress analysisConclusions 5 x 7 cells laminate • 4 points connection feasible for PV-wirefree laminate of 5 x 7 cells (150 mm cells). • The maximum stress in glass is 80 N/mm² at a test wind load of 2400 Pa • Provided a properly engineered connector located at optimal bus distance

  27. Design – Method – PV-wirefree laminate In search of the right dimensions • Other possible dimensions: 5 x 7 cells (125 mm) and 4 x 5 cells (200 mm)

  28. Design – Method – PV-wirefree laminate Stress analysis: 4 x 5 cells laminate PV-wirefree laminate: • 4 x 5 cells 200 mm • 0.849 m x 1.052 m = 0.893 m² Fixing: • Flexible at optimum position: distance between fixings 0.612 m • Optimized connector Result: • Maximum stress in laminate: approximately 80 N/mm² (at a test wind load of 2400 Pa)

  29. Design – Method – PV-wirefree laminate Stress analysis: 5 x 7 cells laminate PV-wirefree laminate: • 5 x 7 cells 125 mm • 0.677 m x 0.933 m = 0.632 m² Fixing: • Flexible at optimum position: distance between fixings 0.533 m • Optimized connector Result: • Maximum stress in laminate: approximately 52 N/mm² (at a test wind load of 2400 Pa)

  30. Design – Method – PV-wirefree laminate - Dimensions Stress analysisoverall conclusions • 4 points connection feasible for PV-wirefree laminates of • 5 x 7 cells laminates (150 mm cells)  80 N/mm² • 4 x 5 cells laminates (200 mm cells)  80 N/mm² • 5 x 7 cells laminates (125 mm cells)  52 N/mm² at a test wind load of 2400 Pa Note: Provided a properly engineered connector located at optimal bus distance

  31. Design – Method – PV-wirefree laminate Cell interconnection= Additional advantage • 4 x 9 cells  5 x 7 cells • Wiring limited to cell to cell wiring • Increased safety

  32. Design – Method – PV-wirefree laminate Cell interconnection= Additional advantage • PV-wirefree laminate of 5 x 4 cells. • But wiring should be done properly

  33. Design – Method – PV-wirefree laminate Top 3 PV-wirefree laminates

  34. PV-wirefree laminate Mounting bus Module connector Inverter 5 x 7 cells (12,5 x 12,5 and 15 x 15 cm) and 4 x 5 cells (20 x 20 cm) 4 mm re-enforced glass 4 points connection (no frame) Optimum distance between mounting busses Step 2 Design – Method – Overview Design step 2: mounting bus

  35. Design – Method – PV-wirefree mounting bus Design choices The mounting bus should: • be strong enough to carry the weight of the PV-laminates and the wind load on the PV-laminates • have a cross area sufficient to carry the current of the subsystem of parallel connected PV-laminates • be made of material suitable to provide a reliable electrical contact • be able to be fixed easily to the roof • be rugged • be low cost

  36. Design – Method – PV-wirefree mounting bus Material selection

  37. Design – Method – PV-wirefree mounting bus Material selection Comparison design of a wire with a resistance of 0.001 Ohm/m

  38. Design – Method – PV-wirefree mounting bus Material selection: conclusions • Energy content per amount of conduction of aluminum is lower than steel and comparable with copper. • The costs per amount of conduction of aluminum is lower than steel and copper. • Since aluminum is used as common building material and can easily be extruded in virtually any shape  material choice = aluminum

  39. Design – Method – PV-wirefree mounting bus Dimensions • In order to withstand mechanical loads from the PV-modules – caused by the wind load on the PV-modules (2400 Pa) – and to be rugged enough A strength optimized profile of approx 150 mm² is required. • Based on this profile the maximum PV-wirefree subsystem size is approximately 3-4 kW for the 5 x 7 cells laminates and 1 kW for the 4 x 5 cells laminates • Aluminum used: 0.67 kg per module, resulting in a energy pay back time of 2 months (5 x 7 cells, 150 mm)

  40. PV-wirefree laminate Mounting bus Module connector Inverter 5 x 7 cells (12,5 x 12,5 and 15 x 15 cm) and 4 x 5 cells (20 x 20 cm) 4 mm re-enforced glass 4 points connection (no frame) Optimum distance between mounting busses Cross section of > 150 mm² Material aluminum Step 3 Design – Method – Overview Design step 3: module connector

  41. Design – Method – PV-wirefree module connector Design choices The PV-wirefree module connector should: • Provide mechanical support by a 4-points connection • Strong enough to withstand the wind loads at the PV-laminate • Electrically connect the PV-laminate to the mounting bus • Have a life time > 20 jaar • Provide a visually checking possibility for correct mating • Provide suitable means for unmating

  42. Design – Method – PV-wirefree module connector Material selection • Aluminum selected for the mounting bus • Avoid corrosion caused by the difference in electrochemical potential of different metals.  Aluminum-Aluminum contact: all electrically connected outdoor metals should be aluminum • Including contacts • Including terminals coming laminate • This also avoids costs of finishing.

  43. Design – Method – PV-wirefree module connector Al-Al contact principle A reliable outdoor contact between Al-Al requires: • Stabilized normal contact force of about 100 N to crush the thin aluminium-oxide layer, guaranteeing a reliable connection for > 20 years • After mating the contact surfaces shall not move

  44. Design – Method – PV-wirefree module connector Impressions module connector

  45. PV-wirefree laminate Mounting bus Module connector Inverter 5 x 7 cells (12,5 x 12,5 and 15 x 15 cm) and 4 x 5 cells (20 x 20 cm) 4 mm re-enforced glass 4 points connection (no frame) Optimum distance between mounting busses Cross section of > 150 mm² Material aluminum Aluminum – aluminum connection Stabilized normal force should be > 100 N Step 4 Design – Method – Overview Design step 4: inverter

  46. Design – Method – PV-wirefree inverter Design choices The PV-wirefree inverter should: • Have a working input voltage matching the PV-wirefree laminates of approx 15 V (35 cells laminate) or 8 V (20 cells laminate) • Not load the PV-modules below Vmpp for longer periods • Never short the PV-laminates even in single fault conditions, both inside and outside the inverter for longer periods • Have an efficiency comparable with existing inverters • Have redundant and rugged connections to the mounting bus • Have a price per watt comparable with existing inverters

  47. Design – Method – PV-wirefree inverter No short-circuit • To avoid an external short-circuit between the inputs of the inverter, these terminals should be properly separated. • Therefore only the enclosure is grounded and PV-modules are floating, so a short circuit is not possible at single fault conditions

  48. Design – Method – PV-wirefree inverter Latest results • Topology developed meeting these requirements. Even under multiple fault conditions – like mosfet, diode or driver failure – the inputs will not be short-circuited • Testing 350 W model has started • The topology has acceptable efficiency • Well scalable to the size required for PV-wirefree • First model will be approx 700 Wac.

  49. PV-wirefree laminate Mounting bus Module connector Inverter 5 x 7 cells (12,5 x 12,5 and 15 x 15 cm) and 4 x 5 cells (20 x 20 cm) 4 mm re-enforced glass 4 points connection (no frame) Optimum distance between mounting busses Cross section of > 150 mm² Material aluminum Aluminum – aluminum connection Stabilized normal force should be > 100 N Operating input voltage of 15 V (8 V) Efficiency > 92% (overall) Protecting from short circuits in system Ground fault detection Redundant connection to mounting bus Price: < 0.5 €/watt (large numbers) Design – Method – Overview Overview PV-wirefree components and their properties

  50. Intermezzo:Why we can omit so many components?

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