Photovoltaic Training Course - Module 1.3 - PV facility calculations

Photovoltaic Systems Training
Design
PV FACILITY CALCULATIONS
www.leonardo-energy.org

Radiation (Wh/m2)
Electric Energy (Wh)
PR = 0,74 - 0.78
System Losses
Considerations
1. The values considered in the following slides are estimated values and should only be used as an approach.
They may vary depending on each location.
2. A detailed Performance Ratio study is fundamental to evaluate the profitability of each solar facility
Defining Performance Ratio (PR)

1. Temperature: (9%) +10ºC 4% received energy
2. Inverter: About 6%. New inverters can reach 4%
3. Cables: AC, DC & other electric devices: < 2%
4. Panel tolerance: It shouldn’t be higher than 3%
5. Pollution, dispersion & reflectance:
1. Fixed panel: approx.3%
2. Suntracking system: 2%.
In urban areas pollution, dispersion and reflectance effects increase system losses by
a further 2%
6. Shadowing: Below 4%. With suntracking systems, a shadowing study might be necessary.
7. Other losses (incidents, etc):
1. Fixed panel: 2%
2. Suntracking system: 4%.
100%
91%
87,4%
85,6%
83%
80,6%
77,3%
75,8%
System Losses evaluation

 Choose cool locations, as elevated areas
 Select inverters with high efficiency and Maximum Power Point Tracking (MPPT)
 Consider extra cable sizing avoiding long traces with voltage drops
 Choose solar panels with tolerances between +/- 2-3%
 Cleaning the modules in long periods without rain
 Balance the separation between panel rows (to avoid shadowing) with the optimization of the surface area
 Minimize the impact of breakdowns, with a preventive maintenance.
Keys to optimize the PR

Depending on the type of installation, the results of the shadowing study, surface optimization and therefore,
project profitability, may vary.
Study aspects:
 Azimuthal deviation from the south (Northern hemisphere) or north (Southern hemisphere)
 Tilt of the solar panel
 Shadows of extern elements
 Shadows of own elements
Shadowing evaluation
FIX - GROUND SUNTRACKING-GROUND FIX - ROOF INTEGRATION

Distance between panel rows
 A basic rule would be to avoid shadows during the 4 central hours of the day, in the day of the year with
less radiation.
 This implies calculating the angle of the sun (height regarding the line of the horizon) to +/-2 hours
regarding the solar midday. This angle will vary depending on the latitude
 The objective is to avoid that the top of the front panel projects a shadow to the lowest part of the panel
that is placed behind.
Fix - Ground
d= h / k
Latitude 29° 37° 39° 41° 43° 45°
k 1,600 2,246 2,475 2,747 3,078 3,487

Tilt angles
 The optimum tilt angle of the solar panel can be expressed by the following simplified formula: Tilt =
Latitude – 10º
 In Spain, tilt angles from 30 to 33º is considered as optimum, but tilt angles between 20 – 40º don’t
mean considerable system losses
 Tilt angles below 15º in urban areas may cause system losses due to pollution and dirt accumulation on
the panels.
 Local land slope will be logically taken into account, which can help reducing distance between the
panel rows to improve the surface profit. (Obviously, the opposite effect can happen)
Fix - Ground

Orientation angle
 The most favorable orientation is 0º South (North hemisphere).
 An orientation deviation below 20º (East or West) cause negligible system losses.
 The following graph (which is valid for a 40º latitude) shows how additional losses may appear
depending on the combination of orientation and tilt angle.
Fix - Ground

A practical example: Solar Plant in Valdecarabanos (Spain)
Placement optimization
Suntracking - ground

Previous tasks:
 Environmental conditions
 Urban conditions
 Topography
 External elements shadowing study (trees, electrical posts, etc)
 Own elements shadowing study: direct & crossed (in suntracking cases)
 Definition of the distance between suntrackers (or panel rows)
Location optimization

Location optimization
Shadowing study

As grid connected solar facilities are considered as an investment, we have to choose between
the following cases:
 To place the solar panels at the optimum tilt and orientation angle.
 To adapt the solar panels to the roof shape
Take into account:
 Impact of angle orientation.
 Impact of tilt angle.
 Impact of shadows
 Comparison between adapted VS optimum
 Roof geometrical limits
Remarks:
Be careful with panels from the same “row” in different planes
Fix - Roofs
OPTIMUM ANGLE & ORIENTATION
ROOF ADDAPTED

Two possibilities:
To avoid visual impact, adapting the solar panels to the roof shape
To integrate the panel as a constructive element with a certain function:
 Electricity generation
 Sunshade effect: special panels which allow some sunlight to go through
 Innovative design: usually special structures are required, and this may increase the installation costs
In architectural integration, the solar facility is not considered as just an profitable investment, but also as
an image and design element
Architectural integration

Radiation, to the south of Madrid (Spain), for a certain year can be around 4.77 kW-h/m2 (Average)
Annual production

Production by kWp (installed)
(4.7 kW-h/ m2 –day x 0.74 x 1.15 x 365 day x 1 kW) / 1 kW/m2
 Expected production for this horizontal radiation, with a PR = 0.74, would be: 1460 kW-h
STC
instincdaymed
annual
I
PyeardaysfPRH
kWpE


 /
/
Annual production
Hmed-day Average solar radiation per day
PR Performance ratio for the solar installation. Dimensionless
F inc Tilt coefficient: a ratio normally obtained from the optimum tilt for a fixed panel (Which
optimizes its performance). In Spain (Latitude = 40º) it is 1.15
Pinst Installed solar power
ISTC Average irradiance in the horizontal plane

 Once the modules and inverters are selected, the configuration of the system allows to maximize the produced
energy
 It is possible that in some cases we should consider the use of a different module or inverter in order to
improve the system performance.
 The configuration of the systems takes into account:
 Maximum input voltage of the inverter
 Maximum input current of the inverter
 Voltage and current at Maximum Power Point
When designing the solar panel configuration in series and parallels, we must take into account that the voltage
and current of the branch will change depending on the temperature. Therefore it will be necessary to choose
extreme values of the region for the calculation.
System configuration

System configuration
Source: PVsyst
A configuration example of a designing software for Solar Plants (PVSYS screen shot )

Take into account:
 Maximum current in the cables
 Maximum allowed voltage drop.
 If there is a long distance the main factor to determine the cable section will be the voltage drop.
 If there is a very short distance the current that flows along the cable will determine the section of the cable
Tramo
Long.
Vnom(V)
Wp inst (kWp)
Inom(A)
Conduct.
∆Vmax(%)
∆Vmax(V)
Seccion (mm2)
Seccióncalc.(mm2)
Seccionestandar(mm2)
Imax_admisible
100% 70% 30% 100% 70% 30%
ZA01 93 541 72 50 22 133 93 40 35 1,0 5,4 131 92 39 97 150 338
ZA02 97 541 72 50 22 133 93 40 35 1,0 5,4 136 95 41 101 150 338
ZA03 115 541 72 50 22 133 93 40 35 1,0 5,4 162 113 48 120 150 338
ZA04 133 541 38 27 12 71 50 21 35 1,0 5,4 100 70 30 74 95 245
Electrical calculation

Electrical design
In order to do a simplified earthing calculation, we can start with the following formulas depending on the soil
resistivity and the electrode characteristics
The average values of the resistivity, depending on the type of soil are:
Electrode Soil resistivity (Ohm)
Buried plate R = 0,8 ρ/P
Vertical peg R = ρ/L
Buried conductor R = 2 ρ/L
ρ, soil resistivity (Ohm x m)
P, Plate perimeter (m)
L, Peg or conductor length (m)
Type of Soil Soil resistivity (Ohm)
Cultivable and fertile soils, compact and wet soils 50
Cultivable non fertile soil, or other soils 500
Naked rock soils, and dried and permeable soils 3.000

Electrical calculations
The cable sizing is based on the following formulas:
Three Phases
One Phase
Where:
• P = Power
• L = Cable length
• γ = Cable conductivity
• E = Allowed voltage drop
• U= Line voltage
• For example, for LV in Europe:
• 400V in Three-phase
• 230V in One-phase
TABLE OF CONDUCTIVITY DEPENDING ON THE TEMPERATURE
Material γ 20 γ 70 γ 90
Copper 56 48 44
Aluminium 35 30 28
Temperature 20 ºC 70 ºC 90ºC

 A lightning may produce a transitory overvoltage of short duration,
with a huge amplitude.
 The overvoltage produced due to network unbalances is a permanent
overvoltage, with a longer duration and a lower amplitude.
 In order to protect our installation against overvoltage, electrical
dischargers can be connected at the input and output of each device to be
protected.
There are three different protection levels:
High Middle Low
Over Voltage
Source: Cirprotect
DEVICE PROTECTION
LEVEL
INVERTER
METER
CC CABINET
TRANSITORY OVERVOLTAGE
PERMANENT OVERVOLTAGE

Transformers connection topology
RING
STAR
CABLE BREAK DOWN
NO PRODUCTION
LOSSES
PRODUCTION
LOSSES
In installations where more than one Medium Voltage transformer is required, it is important to define the correct
topology for the connection between all the MV transformers and the main grid (Power line).
The possible connections options are:

23
Single-line diagram
FUSE
DC
MCB
DIFERENTIAL
PROTECTION
AC
MCB
ELECTRICAL COMPANY
DEVICE

Photovoltaic Training Course - Module 1.3 - PV facility calculations

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