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PV Design Basics
Aaron Liggett
Applications Engineer
Jan. 27, 2017
San Diego, CA
PV Design Basics
The fundamentals of PV design; session will provide a basic understanding of photovoltaic (PV) design.
What’s to come:
• Common practices
• String sizing
• Design considerations
• Safety
Typical Grid Tied Solar System
Top Down Process
• Modules
• DC cabling
• Racking
• Inverter
• Monitoring
• Interconnection
Note: When grid power is lost the PV system will disconnect.
Why Sizing and Design is Important
• Do not want to exceed 100% yearly offset
• Visually appealing
• Compensate for customers needs/requests
• Comply with NEC and HOA requirements
• Safety
System Design Process
Customers usage:
• Yearly energy usage can be found on utility bill.
Things to Consider:
• Inefficiencies in home
‒ Old appliances
‒ Lack of insulation
System Design Process
• Total usage for the year is 10,298kWh
• Project address:
‒ 123 Main St.
Anytown, CA 95661
• Information needed for analysis:
‒ Azimuth
‒ Tilt angle
‒ Shade
‒ Customer requests
‒ System size that will fit on roof • Remember FSB!!
• Ground mount
PVWatts
• Now that we know the household usage proceed to PVWatts
• Input site information
‒ Module type
‒ Array type
‒ System losses
‒ Tilt
‒ Azimuth • http://tools.solmetric.com/Tools/RoofAzimuthTool
PVWatts
• 1kW of solar will produce roughly 1,260kWh per year
• Take this value and divide it by the total household usage
10,298kWh/1,260kWh≈8kW solar array needed
But what can actually fit on the roof?..
Roof Sizing
• ALWAYS account for Fire Setbacks (FSB)
‒ Roof Access—Residential with Single Ridge
‒ (IFC 605.11.3.2.2)
Single ridge needs two 3’ pathways on array faces along edge of load bearing exterior wall.
• After FSBs we can
only fit 18 modules
Note: FSB vary depending on structure;
please reference the IFC and consult your
local AHJ.
Verifying Structure
• Lag screw length required
‒ Plywood decking, skip sheathing
• Life of roof
• Snow load
• No fire damage or broken/damaged rafters
• Confirm structure is adequate
‒ Can reference span tables
‒ Typ. Array will add 3-5 psf to roof
‒ www.solarabcs.org/permitting
Racking Selection
• UL 2703 for Bonding
‒ NEC Article 250 and 690 Part V
• Follow Panel manufacture spec. for mounting
• Type of roofing
‒ Comp, Tile, Standing seam etc.,
Design tools• actcouncil.org Source for wind and
snow loads
• SnapNrack Configurator
Don’t forget to add extra
attachments for conduit routing!
Characteristics of Solar Modules
• PV module voltage has an inverse relationship with temperature, which means that at lower temperatures, PV modules’ voltage raises and at higher temperatures, PV modules’ voltage falls from its nameplate rating.
Temperature Effects
String Sizing Max. Voltage
REC280TP
• Module Voc 39.2V
• Temp Coef. Voc -0.31%
o All found on module data sheet
• Lowest record temp. -3°C
o SolarABC interactive map
Note: Residential system are not to exceed 600 VDC per NEC 690.7 (c)
𝑉𝑜𝑐 + 𝑅𝑒𝑐𝑜𝑟𝑑 𝑙𝑜𝑤 − 25 ∗ 𝑇𝑒𝑚𝑝 𝑐𝑜𝑒𝑓 ∗ 𝑠𝑡𝑟𝑖𝑛𝑔 𝑙𝑒𝑛𝑔ℎ𝑡
Required Information Formula
String Sizing Max. Voltage
𝑉𝑜𝑐 + 𝑅𝑒𝑐𝑜𝑟𝑑 𝑙𝑜𝑤 − 25 ∗ 𝑇𝑒𝑚𝑝 𝑐𝑜𝑒𝑓 ∗ 𝑠𝑡𝑟𝑖𝑛𝑔 𝑙𝑒𝑛𝑔𝑡ℎ
• Convert temp coefficient from %/°C to V/°C
39.2*-0.31%= -0.122 V/°C
• Input all information
39.2 + −3 − 25 ∗ −0.122 ∗ 939.2 + −28 ∗ −0.122 ∗ 9
39.2 + 3.416 ∗ 942.62 ∗ 9
= 383.6𝑉 < 600V
Calculations
String Sizing Min. Voltage
REC280TP
• Module Vmp 31.9V
• Temp Coef. Vmp -0.39%
o All found on module data sheet
• Location high temp. 28°C
o SolarABC
o Use rise adder for cell temp
Note: Be sure that Min string voltage is above inverter Min operating voltage.
𝑉𝑚𝑝 + (𝑀𝑎𝑥 𝑡𝑒𝑚𝑝 + 𝐴𝐷𝐷𝐸𝑅) − 25 ∗ 𝑇𝑒𝑚𝑝 𝑐𝑜𝑒𝑓∗ 𝑠𝑡𝑟𝑖𝑛𝑔 𝑙𝑒𝑛𝑔ℎ𝑡
Required Information Formula
String Sizing Min. Voltage
𝑉𝑚𝑝 + (𝑀𝑎𝑥 𝑡𝑒𝑚𝑝 + 𝐴𝐷𝐷𝐸𝑅) − 25 ∗ 𝑇𝑒𝑚𝑝 𝑐𝑜𝑒𝑓 ∗ 𝑠𝑡𝑟𝑖𝑛𝑔 𝑙𝑒𝑛𝑔𝑡ℎ
• Max temp 28°C+25°C= 53°C
o Use Vmp coefficient -0.39%/°C or -0.124
• Input all information
31.9 + 53 − 25 ∗ −0.124 ∗ 931.9 + 28 ∗ −0.124 ∗ 9
31.9 − 3.472 ∗ 928.43 ∗ 9
= 255.8𝑉 > inverter startup voltage
More Calculations
Selecting an Inverter
Inverter Data
DC input information• 5.04kW DC • 383.6V Max • 255.8V Min• ≈1:1 DC/AC ratio
AC information is provided• 30A OCP for 240V • NEC 690.8(B)(1)
BOS placement
• Don’t place inverters in direct sunlight w/o shade covers
• Confirm with AHJ and utility for special requirements
• Verify if Rapid Shutdown is required NEC 690.12
• Working spaces NEC Table 110.26(A)(1)
Considerations
Interconnection
• Breaker tie-in
o 120% rule
• NEC 2014 705.12(D)(2)
• Install breaker opposite to main breaker.
o 100% rule
• Supply side connection
o Requires service-rated disco
• Min. 60 amp rating
o Equipment must have AIC rating sufficient for the max. available fault current
o IPC, double-lugged panels
Interconnection Methods
System Conclusion
• (2) strings of (9) REC280TP modules o Min 255.8 VDC Max 383.6 VDC
o PV STC DC size 5.04kW
• (1) Fronius Primo 5.0-1o DC/AC ratio ≈1:1
o OCP- 30A breaker
• Offset
o 5.04kW*1260kWh=6,350kWh/yr
o ≈61.7%
Final Design
Module Level Electronics
Microinverters SolarEdge String
Cost $$$ $$ $
Rapid Shutdown o
Shade mitigation -
Module level monitoring -
Variable string lengths oo
Max DC Voltage 60 350 600
Strings with varing tilt and azimuth -
o Requires more equipment
oo Variable string lengths possible with multiple MPPTs
Safety
• Always comply with OSHA regulations
• Only a qualified person should perform electrical work
• Green Job Hazards
o Fall
o Electrical shock
o Heat/cold stress
• Always wear proper PPE
• Hazardous materials on site
o Asbestos
• Vermiculite
• Siding
Green Job Hazards
https://www.osha.gov/dep/greenjobs/solar.html
Resources and References
• NABCEP
o Reference Guide
• Imagine Solar
• Solar Living Institute
• USGBC
• National Electric Code (NEC)
o CEC
• IFC
• IBC
• AEE Catalog
• UL
Learn More References
Office: (916) 559-8823Mobile: (916) 806-4896
Aaron.Liggett@aeesolar.com
Aaron Liggett
Questions?
Lumber Grade Stamp
String Sizing Max. Voltage Using NEC Table 690.7
• Module Voc: 39.2
• Max low temp: -3°C
39.2*1.12=43.9VDC/module
43.9*9(string length)= 395VDC
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