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PV-wirefree: Bringing PV systems back to their essentials 3 rd World Conference on Photovoltaic Energy Conversion, Osaka, Japan Henk Oldenkamp OKE-Services,

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Presentation on theme: "PV-wirefree: Bringing PV systems back to their essentials 3 rd World Conference on Photovoltaic Energy Conversion, Osaka, Japan Henk Oldenkamp OKE-Services,"— Presentation transcript:

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

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

5 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: What is PV-wirefee?

6 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 What is PV-wirefee? – How to reach these objectives?

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

8 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 What is PV-wirefee? – How to reach these objectives?

9 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 Design

10 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 Design

11 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 Design

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

13 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. Design – Method – PV-wirefree laminate

14 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) Design – Method – PV-wirefree laminate

15 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 Design – Method – PV-wirefree laminate

16 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) Design – Method – PV-wirefree laminate

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

18 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) Design – Method – PV-wirefree laminate

19 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) Design – Method – PV-wirefree laminate

20 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) Design – Method – PV-wirefree laminate

21 Stress analysis Conclusions 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 Design – Method – PV-wirefree laminate

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

23 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) Design – Method – PV-wirefree laminate

24 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) Design – Method – PV-wirefree laminate

25 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) Design – Method – PV-wirefree laminate

26 Stress analysis Conclusions 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 Design – Method – PV-wirefree laminate

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

28 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) Design – Method – PV-wirefree laminate

29 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) Design – Method – PV-wirefree laminate

30 Stress analysis overall 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 Design – Method – PV-wirefree laminate - Dimensions

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

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

33 Top 3 PV-wirefree laminates 5 x 7 cells 125 mm 5 x 7 cells 150 mm 4 x 5 cells 200 mm Dimensions [m x m] 0.677 x 0.9330.802 x 1.1080.849 x 1.052 Area [m²]0.6320.8890.893 Bus distance [m] 0.5330.6480.612 Weight [kg]6.99.8 Power [Wp] @ cell efficiency 88 @ 16%118 @ 15%88 @ 11% Voc [V]21 12 Vmpp [V]17 9 Design – Method – PV-wirefree laminate

34 Design step 2: mounting bus 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

35 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 Design – Method – PV-wirefree mounting bus

36 Material selection AluminumSteelCopper Costs (/kg) Bulk Sheet Extruded 1.5 4.14 3 0.81 1.64 5 Physics Specific mass [kg/m3] Resistivity [ m] 2700 0.03 7857 0.1 8900 0.017 Energy content [kWh/kg] New Recycled Finishing 55-69 8 8-14 3.7-8 > 28 1.4 Design – Method – PV-wirefree mounting bus

37 Material selection AluminumSteelCopper Required cross section [mm²] 3010017 Weight [kg/m]0.0810.7860.151 Costs [/m] Bulk Sheet Extruded 0.12 0.33 0.24 0.64 0.25 0.76 Energy content [kWh/m] New Recycled 3 – 5.6 0.3 – 0.6 6.3 – 11> 4.2 0.2 Design – Method – PV-wirefree mounting bus Comparison design of a wire with a resistance of 0.001 Ohm/m

38 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 Design – Method – PV-wirefree mounting bus

39 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) Design – Method – PV-wirefree mounting bus

40 Design step 3: module connector 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

41 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 Design – Method – PV-wirefree module connector

42 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. Design – Method – PV-wirefree module connector

43 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 Design – Method – PV-wirefree module connector

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

45 Design step 4: inverter 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

46 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 Design – Method – PV-wirefree inverter

47 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 Design – Method – PV-wirefree inverter

48 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. Design – Method – PV-wirefree inverter

49 Overview PV-wirefree components and their properties 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

50 Intermezzo: Why we can omit so many components?

51 PV-string systems A string of modules is illuminated while a few cells are shaded The illuminated cells force the shaded cells to operate in reverse direction Excessive power will be dissipated in shaded cell this will cause hot spots and may eventually lead to cell defects To avoid hot spots bypass diodes must be used Bypass diodes are not fail-safe When one fails the diode may become a short circuit Excessive back-feeding current in one string may occur when more strings are connected in parallel. Overheating of PV-modules and over-current in the wiring. Conclusion: bypass diodes introduced a new problem To avoid over-current fuses must be used –As diodes are not fail safe, fuses are mandatory to limit the back-feeding current Intermezzo – Partial shading and hot spots

52 PV-wirefree systems What will happen when one cell of a PV- module is shaded, while the other cells are illuminated? –Will it cause hot spots like in a series connection? –Do we need bypass diodes to avoid these hot spots? Intermezzo – Partial shading and hot spots

53 No bypass diodes Intermezzo – Partial shading and hot spots – PV-wirefree systems

54 No bypass diodes Intermezzo – Partial shading and hot spots – PV-wirefree systems

55 PV-wirefree systems What will happen when one PV-module is shaded, while the other PV-modules are illuminated? –Will it cause too high back-feeding currents like in a series connection? –Are fuses needed to avoid these back-feeding currents? Intermezzo – Partial shading and hot spots – PV-wirefree systems

56 No fuses Intermezzo – Partial shading and hot spots – PV-wirefree systems

57 No fuses Intermezzo – Partial shading and hot spots – PV-wirefree systems

58 No fuses Intermezzo – Partial shading and hot spots – PV-wirefree systems

59 No fuses Intermezzo – Partial shading and hot spots – PV-wirefree systems

60 No fuses Intermezzo – Partial shading and hot spots – PV-wirefree systems

61 Fuses and bypass diodes : conclusions One shaded cell in a PV-module while the other cells are illuminated –No significant reverse current in this cell no hot spots no bypass diodes required One shaded PV-module while the other PV-modules are illuminated –No significant back-feeding current no string diodes and/or fuses required But only provided that the inverter does not load the PV- modules below Vmpp for longer periods (no short- circuits) Intermezzo – Partial shading and hot spots – PV-wirefree systems

62 Comparison PV-string systems with PV-wirefree systems Subjects Performance, which is determined by: –Mismatch losses Optimal conditions: all PV-laminates are equally illuminated, having same temperature Suboptimal conditions: PV-laminates are not equally illuminated or partially shaded or partially polluted Single fault conditions Multiple fault conditions –Conduction losses Reliability Safety –Touch safety –Energy hazard Costs –Installation costs –Material costs –Energy payback time Comparison PV-string systems and PV-wirefree systems

63 PV-laminate definitions PV-string modules 36 cells of 150 x 150 mm Dimensions: 0.649 m x 1.414 m Area = 0.918 m² Power = 122 Wp Cell efficiency = 15% Voc = 21.6 V Isc = 8 A Vmpp = 17.5 V Impp = 7 A Two bypass diodes per PV- module PV-wirefree laminates 35 cells of 150 x 150 mm Dimensions: 0.802 m x 1.108 m Area = 0.889 m² Power = 118 Wp Cell efficiency = 15% Voc = 21 V Isc = 8 A Vmpp = 17 V Impp = 7 A Comparison PV-string systems and PV-wirefree systems

64 System definitions PV-string subsystem 8 modules in series P = 976 Watt Area =7.344 m² Voc = 175 V Isc = 8 A Vmpp = 140 V Impp = 7 A In total 16 bypass diodes are used Transformerless inverter PV-wirefree subsystem 8 PV-wirefree laminates in parallel P = 944 Watt Area =7.112 m² Voc = 21 V Isc = 64 A Vmpp = 17 V Impp = 56 A Mounting bus length = 6.556 m (20 mm spacing between the PV-laminates) Distance between mounting busses = 0.658 m Comparison PV-string systems and PV-wirefree systems

65 Optimal conditions PV-substring 0 PV-wirefree subsystem 0 PV-wirefree subsystem does not suffer from mismatch losses due to initial differences between Impp of the PV- modules. Transformerless inverters may have a slightly higher efficiency than PV-wirefree inverters. These effects are considered to have about the same effect on the output of the PV-substring/PV-wirefree subsystem Comparison PV-string systems and PV-wirefree systems - Performance

66 Suboptimal conditions Comparison PV-string systems and PV-wirefree systems - Performance

67 Suboptimal conditions Comparison PV-string systems and PV-wirefree systems - Performance

68 Suboptimal conditions Comparison PV-string systems and PV-wirefree systems - Performance

69 Suboptimal conditions Comparison PV-string systems and PV-wirefree systems - Performance

70 Suboptimal conditions Comparison PV-string systems and PV-wirefree systems - Performance

71 Suboptimal conditions Comparison PV-string systems and PV-wirefree systems - Performance

72 Suboptimal conditions Comparison PV-string systems and PV-wirefree systems - Performance

73 Suboptimal conditions Comparison PV-string systems and PV-wirefree systems - Performance

74 Suboptimal conditions: conclusions PV-substring PV-wirefree subsystem ++ PV-substring suffers from mismatch losses due to differences in illumination, temperature etc. of the PV-modules in the string. Therefore each string needs bypass diodes to prevent being damaged under these conditions. When one bypass diode conducts 10% of the power of the 8 modules string is lost. When two bypass diodes are conducting the loss is 25%. PV-wirefree does not need bypass diodes. PV-wirefree subsystem does not suffer from mismatch losses. The mismatch losses at system level are nihil, resulting in an increase of the annual yield in suboptimal conditions with 5-15% Comparison PV-string systems and PV-wirefree systems - Performance

75 Single fault conditions ConditionPV-substringPV-wirefree subsystem Open circuit anywhere --The power of the complete string is lost +Loses only the power of one module Bypass diode short circuit -10% loss of string power ++PV wirefree does not use bypass diodes. No losses. Cell short circuit 00Approximately equal output losses in the subsystem of 0.35% Comparison PV-string systems and PV-wirefree systems - Performance

76 Conduction losses The high conduction losses in the dc-wiring has been one of the main reasons to connect PV- modules in series PV-wirefree system uses mounting frame for conduction. Due to the large cross section the losses in a PV-wirefree subsystem are overall comparable with a PV-substring. For small ( 2-3 kW) PV-substrings are slightly better. Comparison PV-string systems and PV-wirefree systems - Performance

77 Conclusions ConditionPV-substringPV-wirefree subsystem Optimal conditions00 Suboptimal conditions0++ Single fault conditions Open circuit anywhere Bypass diode short circuit Cell short circuit -- - 0 + ++ 0 Conductions losses00 Comparison PV-string systems and PV-wirefree systems - Performance

78 Reliability ConditionPV-substringPV-wirefree subsystem All conditionsIn case one of the components fails, the power of the complete string is lost Many connections in series increases chance of failures. No redundancy of connections ++In case one of the components fails, only the power of one module is lost Components that are likely to fail are omitted (diodes and fuses). Very rugged mounting bus replaces DC-wiring. All connections are made redundant. Comparison PV-string systems and PV-wirefree systems - Reliability

79 Touch safety ConditionPV- substring PV-wirefree subsystem Optimal conditions++Comparable Suboptimal conditions++Comparable Single fault conditions DC-wire or connector isolation damaged Laminate isolation damaged Laminate glass broken ++ In PV-substrings there is a serious chance of personal injuries Comparison PV-string systems and PV-wirefree systems - Safety

80 Energy hazard ConditionPV- substring PV-wirefree subsystem Optimal conditions++Comparable Suboptimal conditions++Comparable Single fault conditions DC-wire or connector isolation damaged Laminate isolation damaged Laminate glass broken Bypass diodes failure (open circuit) * Provided that it is detected and switched off by condition monitoring 0* ++++++++In PV-substring protection against partial shading is lost; cannot be detected at system level. Hot spots may occur Comparison PV-string systems and PV-wirefree systems - Safety

81 Costs PV- substring PV-wirefree subsystem Installation costs+Easy installation PV-wirefree requires less time Material BOS costs0Number of components is reduced considerable Energy pay back time+Energy payback time of PV- wirefree is reduced Comparison PV-string systems and PV-wirefree systems - Costs

82 Conclusions comparison PV-string systems with PV-wirefree systems Performance: In suboptimal conditions PV-wirefree outperforms PV-strings Reliability: Chance on failures in PV-wirefree are reduced an order of magnitude because of redundancy in connections. Touch safety: PV-wirefree remains touch safe under all circumstances, even in multiple fault conditions. Energy hazard: Comparable Costs: PV-wirefree reduces costs considerably Comparison PV-string systems and PV-wirefree systems

83 PV-wirefree project Time schedule April 2002: –Start development October 2002: –First prototype presented at Rome conference January 2003: –Start proof-of-principle project December 2003: –Start field test of PV-wirefree system December 2004: –Ready for market introduction (expected) Status and developments

84 PV-wirefree partners Bear Architects Netherlands Energy Research Foundation ECN NKF Electronics OKE-Services Oskomera Solar Power Solutions BV TNO Bouw Status and developments

85 A challenge for you? PV manufacturers to participate … and for other inverter manufacturers But more work needs to be done: –Adapting the current PV-standard –Support of the PV-community. –It will save lots of energy when all parties start pushing in the right direction from today on. Status and developments - The future

86 Overall conclusion PV-wirefree offers an opportunity to reach the minimum BOS costs of PV systems possible in due time And thus to contribute to the target making photovoltaic solar energy a viable option for producing electricity in a sustainable way Status and developments - The future

87 Finally More information available at www.pv-wirefree.com


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