Session 17 Grid Tied PV Systems – Part 6 Three-Phase Systems Siting and Mechanical Considerations October 29, 2015

Session 17 content Grid-Connected PV Systems o Wrap-up of Residential PV System Example o Design Considerations for Large Scale Systems 2

Learning Outcomes An examination of the impact of size (scale) on photovoltaic system design 3

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 phase differences: 4

Grid-Tied PV – Larger Scale Systems Design of 21kW 3-phase system -> 21/3 = 7kW per phase Three 7kW inverters V OC (max) = 600V 250V < V mp < 480V (MPPT range) Modules V OC = 64V I SC = 6A V m = 55V I m = 5.5A P m = 305W Temperature range: -30 o C < T amb < +62 o C 5

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 7 Schematic of three-phase PV system

Grid-Tied PV Systems – Space Considerations Evaluation of space availability and solar availability 8 Consideration of shadowing on flat roof arrays

Grid-Tied PV Systems – Space Considerations Evaluation of space availability and solar availability 9 Shading profile added to sun motion diagram

Grid-Tied PV Systems – Space Considerations Evaluation of space availability and solar availability 10 Consideration of shadowing on flat roof arrays

Grid-Tied PV Systems – Space Considerations 11 Consideration of shadowing on flat roof arrays

Grid-Tied PV Systems – Space Considerations Evaluation of space availability and solar availability 12 Four common roof types

Grid-Tied PV Systems – Space Considerations Evaluation of space availability and solar availability 13 Hip roof wind zones

Grid-Tied PV Systems – Space Considerations Evaluation of space availability and solar availability 14 Layout for low wind region

Grid-Tied PV Systems – Space Considerations Evaluation of space availability and solar availability 15 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 16

Mechanical Considerations Mechanical system design Selection, sizing, configuring Design requirements Functional requirements Operational requirements Constraints Trade-offs 17

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. 18

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 19

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 20

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 21

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 22

Grid-Tied PV Systems – Wind Considerations 23 Flow over airfoils

Grid-Tied PV Systems – Wind Considerations 24 Flow around plates

Grid-Tied PV Systems – Wind Considerations 25 Array pressure and force (back wind)

Grid-Tied PV Systems – Wind Considerations 26 Array pressure and force (front wind)

Grid-Tied PV Systems – Wind Considerations 27 Maximum uplift – slanted roof mounted array