1. Grid Tied PV Systems – C&I scale Siting and Mechanical Considerations
Session 15
Grid Tied PV Systems – C&I scale
Three-Phase Systems
Siting and Mechanical Considerations
October 24, 2018

2. Session 15 content Grid-Connected PV Systems
Design Considerations for Large Scale Systems
Mechanical Issues

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

4. Grid-Tied PV – Larger Scale Systems
At most power plants, AC electricity is generated in 3-phase format
Many industrial customers expect 3-phase AC electricity
Three voltage waveforms are produced (transmitted) with the same amplitude and frequency, but 1200 phase differences:

5. 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 < +62oC

6. 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 circuits, each with 8 modules -> 72 modules!

7. Grid-Tied PV Systems – Larger Scale Systems
Schematic of three-phase PV system

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

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

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

11. Grid-Tied PV Systems – Space Considerations

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

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

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

15. Grid-Tied PV Systems – Space Considerations
Evaluation of space availability and solar availability
Layout for high wind region

16. 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

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

18. 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.

19. 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

20. 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

21. 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

22. 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
Professional Societies
ASCE
ASME
ASTM
Trade Organizations
AISC
ASM

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

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

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

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

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