Session 17

Grid Tied PV Systems – Part 6

Three-Phase SystemsSiting and Mechanical Considerations

October 29, 2015

Session 17 content

Grid-Connected PV Systemso Wrap-up of Residential PV System

Exampleo Design Considerations for Large Scale

Systems

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Learning Outcomes

An examination of the impact of size (scale) on photovoltaic system design

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Grid-Tied PV – Larger Scale Systems

o At most power plants, AC electricity is generated in 3-phase format

o Many industrial customers expect 3-phase AC electricity

o Three voltage waveforms are produced (transmitted) with the same amplitude and frequency, but 1200 phase differences:

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Grid-Tied PV – Larger Scale Systems

• Design of 21kW 3-phase system -> 21/3 = 7kW per phase

• Three 7kW inverters• VOC(max) = 600V

• 250V < Vmp < 480V (MPPT range)

• Modules• VOC = 64V ISC = 6A

• Vm = 55V Im = 5.5A

• Pm = 305W

• Temperature range: -30oC < Tamb < +62oC5

Grid-Tied PV – Larger Scale Systems

• Range in module number to meet voltage and temperature conditions:

• 4 source circuits * 6 modules/circuit * 305W = 7320W

• 3 source circuits * 8 modules/circuit * 305W = 7320W

• The second option is preferred• Higher voltage, lower current -> higher gauge wire

• System then has 3 inverters, each with 3 source circuits -> 9 source cicuits, each with 8 modules -> 72 modules! 6

Grid-Tied PV Systems – PV system engineering and design

7Schematic of three-phase PV system

Grid-Tied PV Systems – Space Considerations

Evaluation of space availability and solar availability

8Consideration of shadowing on flat roof arrays

Grid-Tied PV Systems – Space Considerations

Evaluation of space availability and solar availability

9Shading profile added to sun motion diagram

Grid-Tied PV Systems – Space Considerations

Evaluation of space availability and solar availability

10Consideration of shadowing on flat roof arrays

Grid-Tied PV Systems – Space Considerations

11Consideration of shadowing on flat roof arrays

Grid-Tied PV Systems – Space Considerations

Evaluation of space availability and solar availability

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Four common roof types

Grid-Tied PV Systems – Space Considerations

Evaluation of space availability and solar availability

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Hip roof wind zones

Grid-Tied PV Systems – Space Considerations

Evaluation of space availability and solar availability

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Layout for low wind region

Grid-Tied PV Systems – Space Considerations

Evaluation of space availability and solar availability

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Layout for low wind region

Mechanical Considerations

• Mechanical issues – another set of disciplines needed to carry out proper PV system design

• Determination of mechanical forces on system• Selection of components, their sizing and

configuration, to support (or resist) the forces with safety margins

• Selection and construction of components that will not deteriorate or degrade unacceptably over life of system

• Location, orientation, and mounting of PV arrays to be exposed adequately to solar radiation

• Design of array support structures that are aesthetically appropriate, installable, and maintainable

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Mechanical Considerations

• Mechanical system design

• Selection, sizing, configuring

• Design requirements• Functional requirements• Operational requirements• Constraints• Trade-offs

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Mechanical Considerations

• Functional Requirements• System is capable of handling mechanical forces,

pressures, loads• Specification of safety factors and margins• Specification of maximum allowable stresses• Limits on performance

• Estimation of durability (lifetimes, coatings or protective measures)

• For tracking systems, determination of motion/hysteresis, operation of drive mechanisms, etc.

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Mechanical Considerations

• Operational Requirements

• Determination of installation procedures, times, etc.

• Specification of location of BOS components• Specifications for accessibility• Development of security measures, protection

against theft, vandalism, etc.• Specifications for maintenance

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Mechanical Considerations

• Forces & Loads

• Dead Loads – Weight of materials transmitted to roof or other supporting structure

• Live Loads – Workers, First Responders, gear

• Soil, Water, Flood Loads• Wind Loads• Snow Loads• Rain Loads• Ice Loads• Earthquake Loads

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Mechanical Considerations

• Steps in Wind Load Design

• Establish the basic wind speed• Determine the velocity pressure• Determine gust effects• Determine pressures• Determine wind loads• Determine forces on critical members and

attachment points• Establish safety factors• Select appropriate components

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Mechanical Considerations

• Standards and Codes• Standards – Specifications for parts, materials,

processes; Limitation of number of items, reduction in custom parts

• Codes – Specifications for analysis, design, manufacturing, installation; Specs for safety, efficiency, performance

o Professional Societies• ASCE• ASME• ASTM

o Trade Organizations• AISC• ASM

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Grid-Tied PV Systems – Wind Considerations

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Flow over airfoils

Grid-Tied PV Systems – Wind Considerations

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Flow around plates

Grid-Tied PV Systems – Wind Considerations

25Array pressure and force (back wind)

Grid-Tied PV Systems – Wind Considerations

26Array pressure and force (front wind)

Grid-Tied PV Systems – Wind Considerations

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Maximum uplift – slanted roof mounted array