Download WEEK 5 sizing 1(2)

SELECTED TOPIC GRID-CONNECTED PV SYSTEM DESIGN AND SIZING
Week 5:
Table of Contents
5.0 Design Principle....................................................................................................................................... 2
5.1 Introduction ......................................................................................................................... 2
5.2 Basic Principle in designing a quality on-grid PV system ................................................ 2
5.3 Selecting the Type of Inverter ............................................................................................ 5
5.4 Power losses in a Grid connect PV System ....................................................................... 6
5.4.1 Temperature of the PV module ...................................................................................... 6
5.4.2 Dirt ................................................................................................................................. 8
5.4.3 Manufacturer Tolerances and module mismatch .......................................................... 8
5.4.4 Voltage drop in cables to inverter. ................................................................................. 9
5.4.5 Inverter efficiency .......................................................................................................... 9
5.4.6 Shadows ......................................................................................................................... 9
5.5 Energy yield of a PV grid-connected system .................................................................... 9
5.5 Design example ............................................................................................................. 10
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5.0 Design Principle
5.1 Introduction
- Two parts:
a. Sizing PV and inverter.
-
The purpose of designing or sizing PV with inverter is to optimise the usage
of inverter and PV modules.
-
Designing involves:- Calculating number of modules required to meet certain amount of energy.
- To select suitable type of inverter.
- To configure array i.e number of parallel string and number of modules per
string.
- To estimate the energy yield from the PV installation.
b. Select BOS component
- The purpose is to select suitable size of BOS components that work safely.
- Discussed in week 3.
5.2 Basic Principle in designing a quality on-grid PV system
1. The following steps are usually undertaken when designing on-grid PV system:a. Budget
b. Roof dimensions
c. Which modules best fits into the available roof space
d. Obstruction that causes shading on the panel
e. Perform site audit to access OSH risks, solar resources, space allocation and
mounting structure (roof areas and conditions). Theoretically the amount of
solar irradiance that received at the surface of earth is 1000 W/m2. However,
the amount of actual solar irradiation received by PV panel could be reduced
due to:
i. Shading
ii. Tilt angle of the modules
iii. Orientation of the modules
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-
It is quite often we get help from Structural Engineer to determine the wind
loading of array and evaluate the strength of the support structures (roof).
Shading on PV panel from nearby tall building (Courtesy PVMC)
2. Decide the suitable system to be installed; retrofit or building integrated. The PV
modules are installed on the top with special bracket as shown below. Building integrated
makes use of PV as a roof.
(a)
(b)
Retrofit installation (a) Panels attached on top of the roof (b) Bracket between panel and
roof. (Courtesy PTM)
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Building integrated installation. PV panels become part of the roof.
3.. Choose the appropriate inverter that match with the PV modules.
i. Select the type of inverter; central, string or multi-string.
ii. Match the array voltage to voltage window of inverter;
iii. Select inverter to meet array rating;
iv. Determine the system losses; and
v. Calculate the energy yield and performance factor.
4. Choose and select cables, fuses, SPD and breakers (BOS components)
i. Sizing the cables
ii. Sizing the protection equipment
5. Determine the best location of components and route of cabling.
The cable runs include:
- Solar array to array junction box
- Junction box to inverter (Array cable)
- Inverter to meter (a.c cable)
- Meter to grid connection (if not same location)
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String cable secure in the flexible conduit.
5.3 Selecting the Type of Inverter
There are few factors need to consider when selecting an inverter:i.
ii.
iii.
iv.
v.
vi.
vii.
viii.
ix.
If the total power is greater than 20 kW then use central inverter. Central
inverter utilises three-phase a.c system
If the total power is between 1-10kW then use string inverter which utilises
single-phase a.c system.
If the total power is less than 150W, usually a small module inverter located
under the panel could be used.
The peak rating of the PV array
Whether the solar modules are all in the same plane, that is the same tilt
angle and direction
The type of shading that occurs on the array
The efficiency
Certified by international standard
The capital costs of the different inverters.
-
There are a few types of grid inverter commercially available such as Fronius,
Solarmax and SMA.
-
Among these inverters, which one to select?. As rules of thumb always select
inverter lower than the total array power and look for inverter with large
voltage window. The size of inverter is depending on the type of solar cell used
in the installation. In general;
a. Crystalline module
Norminal ac power of inverter ~ (0.75 to 0.80) of the total array at stc.
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b. Thin film
Norminal ac power of inverter ~ 0.85 of the total array power at stc.
5.4 Power losses in a Grid connect PV System
-
It is important to note that not all power received by the module from the
sunlight could be transform into usable energy.
-
There are power losses occurs in the PV system. Those losses are:i.
ii.
iii.
iv.
v.
vi.
vii.
viii.
ix.
Efficiency of PV cells/module. Almost commercially available PV cells
have very low efficiency.
Temperature of the PV module. Power generated from the modules
reduces as the temperature increases.
Dirt. Dirt accumulated on the surface could reduce the output power of
module.
Manufacturer’s Tolerances and module mismatch.
Voltage drop in DC cables to inverter
Inverter efficiency
Tilt angle of the module
Orientation of the module
Voltage drop in AC cables to point of connection to grid.
5.4.1 Temperature of the PV module
-
During mid-day, the PV modules receive maximum solar irradiance. However, PV
modules also receive maximum temperature during that period. Hot weather
condition reduces the actual among of power generated from the array. The effect of
temperature on the solar module is stated in datasheet as temperature coefficient.
There are four types of temperature coefficient namely;
a.
b.
c.
d.
-
Temperature coefficient for open circuit voltage Voc
Temperature coefficient for maximum power voltage Vmp
Temperature coefficient for short circuit Isc
Temperature coefficient for power Vpmp
Usually those parameters are provided by manufactured. However, in cases
where no temperature coefficients for Pmp stated by the manufacturer, then use
the following assumptions:i.
For Monocrystalline module, Pmp = Vmp = 0.45%/oC
ii.
For Polycrystalline module, Pmp = Vmp = 0.50%/oC
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For thin Film, Vpmp = Vmp = 0.2%/0C
iii.
-
Effective temperature of the cell is:-
The solar module is fabricated with solar cell under the glass. The actual
temperature of the solar cell is different from the ambient temperature and
estimated by the following formula;;
Tcell _ eff  Tave _ amb  250 C
Where
Tcell_eff = the average daily effective cell temperature in 0C
T_amb = the daytime ambient temperature.
-
In general;
Vmp _ cell _ eff _ temp  Vmp _ stc   Vmp (Tcell _ eff  Tstc )
-
In Malaysia condition, the maximum cell effective temperature (T_max_cell_eff) is
750C and the lowest (T_min_cell_eff) is 200C. You need to use this temperature
values in all your calculation for sizing inverter and PV array.
T_max_cell_eff = 750C
T_min_cell_eff = 200C
-
Temperature gives effect on the module’s voltages:-
Vmax_ mp  Vmp _ stc   Vmp (20  25)
Vmin_ mp  Vmp _ stc   Vmp (75  25)
Vmax_ oc  Voc _ stc   Voc (20  25)
-
Effect of temperature on power (Derating Factor)


f temp  1   Pmp  Tcell _ eff  TSTC 
Where
temp = temperature de-rating factor, dimensionless
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Pmp
= power temperature co-efficient per degree Celsius
Tcell.eff = average daily cell temperature, in degrees Celsius
Tstc = cell temperature at standard test conditions, measured in degrees Celsius.
5.4.2 Dirt
- Dirt accumulates on the surface of PV modules reduce the power generated by the
modules. In dusty environment with less rainfall, the factor is higher than the
places where often receives rainfall.
- fdirt is used as derating factor for dirt and it is dimensionless
Recommended range:
Fdirt = 0.97
Less dusty
With receive often rainfall
~
0.90
dusty environment
with less rainfall
5.4.3 Manufacturer Tolerances and module mismatch
-
f mm is used for the de-rating factor for manufacturing tolerance and mismatch
and is dimensionless . Modules are manufactured with a specified power rating
and a manufacturers tolerance. Sometime this factor is obtained directly from
the module’s datasheet.
-
For example;
fmm = (100 – 5)/100
= 0.95
-
However, some cases this factor needs to be calculated as follow;
fmm = Minimum power /Rated Power
Example; for 180W PV module
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fmm = 162/180 = 0.90
5.4.4 Voltage drop in cables to inverter.
- MS1837:2005 Installation of Grid connected Photovoltaic (PV) Systems allows for
a maximum voltage drop of 5% between the solar array and the inverter. If the
inverter is located far from the point of connection to the grid then the AC voltage
drop is also need to be included in this 5%
5.4.5 Inverter efficiency
- There are losses in the inverter due to transformer losses, power switching and
self consumption eg monitoring etc. These losses are expressed as the inverter
efficiency. Most modern inverters achieve above 90% efficiency when the output
is above 10% of the inverter rating. Typically inverters will be operating in the
range of 92- 96% efficiency. It is important to operate the inverter at high input
power close to its maximum so that it will maintain operating at high efficiency.
5.4.6 Shadows
- Shadow affects the output power of the array due to decrease in solar irradiation.
Another effect which is more difficult to predict is reduction in the maximum
power point voltage of the array due to the shadow and hence a reduction in the
maximum power available from the array.
- Use solar path finder to calculate the percentage of derating power due to shading.
5.5 Energy yield of a PV grid-connected system
- To estimate energy yield from a PV grid-connected system (Esys):Esys =Parray_stc x ftemp x fmm x fdirt x Htilt x ηpv_inv x ηinv
Where
Parray_stc = array power at stc
ftemp = temperature derating factor
fmm = derating factor for manufacture tolerance and mis-match
fdirt = derating factor for dirt
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Htilt = average solar irradiation kWh/m2 for the specified tilt angle
ηpv_inv = efficiency of sub-system from PV to inverter
ηinv = efficiency of inverter
5.6 Performance ratio (PR)
- PR is used to access the installation quality. PR provides a normalised basis so
comparison of different types and sizes of PV systems can be undertaken.
PR = Esys/Eideal
Where
Eideal = Parray_stc x Htilt
5.5
Design example
Example 1:
Your customer wants to install a PV grid-connected system with 4.4 kWp PV array (using
40 Mistubishi 110W solar modules) and it is located where the yearly irradiation for the tilt
and direction of the array is 1,400 kWh/m2. Assuming :
 an inverter efficiency of 97%
 average daily maximum ambient temperature of 37 degrees
 dirt derating of 3%
 total voltage drop of 2%
a) What is the expected annual average energy yield from the system?
b) What is the System specific energy yield?
c) What is the performance ratio?
Solution
Given:
Parray_stc = 4.4 kWp
No of module = 40
Module type = Mitsubishi 110W
Htilt = 1400 kWh/m2 per annum
ηinv = 0.97
ηpv_inv = 0.98
fdirt = 0.97
fmm = 0.95 from datasheet
γpmp = 0.478%
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Tambient = 37
ftemp = 1 – γpmp((Tambient + 25) – 25))
= 1 – ((0.478/100)*(37+25-25)
= 0.823
Esys =Parray_stc x ftemp x fmm x fdirt x Htilt x ηpv_inv x ηinv
= 4.4 x 0.823 x 0.95 x 0.97 x 1400 x 0.97 x 0.98
= 4441 kWh per annum
Specifiec Yield (SY) = Esys/Parray
= 4441/4.4
= 1009.3 kWh per annum/kWp
Performance ratio (PR) = (Esys/Eideal)
= (Esys/(Parray_stc x Htilt))
= (4441/(4.4 x 1400))
= 0.72
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Mitsubishi 110W SOLAR MODULE
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Example 2:
As a PV System Engineer, you have been asked to install a PV grid connected system that
will comprise a Fronius IG40 Inverter and Schott 250W modules (Refer attachments for
datasheet). The maximum and minimum cell effective temperatures recorded at the site
are 650C and 250C respectively. The maximum solar irradiance is 1000 W/m2. To complete
your sizing of inverter and PV modules, please answer the following questions:a. Tabulate the following information on Schott 250W PV module and Fronius IG40
inverter
Schott 250W module
Parameter
Value
Unit
Voc_stc
V
Vmp_stc
V
Isc_stc
A
P_rated_stc
Wp
Maximum
system voltage
V
Vmp
V/oC
VIsc
A/oC
Voc
V/oC
Pmp
W/oC
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Fronius IG40 inverter
a.
Parameter
Value
Unit
Maximum
Voltage (Vmax)
V
Maximum
Window voltage
(V_win_max)
V
Minimum
window votage
(V_win_min)
V
Nominal Power
(P_ac_nominal)
Watt
Calculate;
b. The maximum open circuit voltage, maximum peak power voltage and
minimum peak power voltage of the module
c. The maximum number of modules per string such that the maximum string
voltage shall not damage the inverter
d. The maximum number of modules per string such that the maximum string
operating voltage shall not turn off the inverter
e. From (c ) and (d), which one do you select for maximum number of modules
per string? Why?
f. The minimum number of modules per string
g. What will happen if the number of modules per string is installed less than the
number in (f) ?
h. The estimate total array power at stc and total number of modules that
matches with the inverter.
i. The optimum number of parallel string
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Schott 250W PV module
Page 15 of 20
Fronius IG40
Page 16 of 20
Solution:
(a)
Schott 250W module
Parameter
Value
Unit
Voc_stc
60.3
V
Vmp_stc
48.3
V
Isc_stc
5.8
A
P_rated_stc
250
Wp
Maximum system
voltage
600
V
Vmp
V/oC
VIsc
A/oC
Voc
V/oC
Pmp
W/oC
Fronius IG40 inverter
Parameter
Value
Unit
Maximum Voltage (Vmax)
500
V
Maximum Window voltage
(V_win_max)
400
V
Minimum window votage
(V_win_min)
150
V
3500
Watt
Nominal Power
(P_ac_nominal)
b. The maximum open circuit voltage:
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maximum peak power voltage:
minimum peak power voltage of the module:
c. The maximum number of modules per string such that the maximum string voltage
shall not damage the inverter:
By considering 5% safety margins, the voltage window for the inverter:
So, this will allow a maximum number of modules per string:
d. By considering 5% safety margin, the voltage window for the inverter:
So, the maximum number of modules per string such that the maximum string
operating voltage shall not turn off the inverter.:
e. The number of modules is the same for both (c) and (d). So the maximum number
of modules is 7 modules per string. Either (c) or (d) can be used.
f. The minimum number of modules per string:
minimum peak power voltage of the module as calculated in (b) :
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Must allow voltage drop (assume 5%), the effective minimum voltage at inverter
from the module is = 0.95 X 39.2196 = 37.2586 V.
By considering 10% safety margin, the voltage window for the inverter:
So the minimum number in a string:
g. The inverter will turn off because the input dc voltage from PV array is below than
the minimum voltage window of the inverter.
h. The estimate total array power at stc and total number of modules that matches
with the inverter.
Approximate total array power
Aproximate total number of
modules
and also
.
So the total array power at STC is = 18 X 250 = 4500 Wp.
i.
The optimum number of parallel string
Based on previous calculation, the maximum and minimum number of modules per
string:
Maximum number of modules = 7
Minimum number of modules = 5
And the total number of modules in array is 18 modules. So the posible array
configurations are:
1. 2 X 7
2. 3 X 6
3. 3 X 5
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So the best configuration (optimum number) of the array is 3 parallel strings of 6
modules in series (3 X6). So the total modules are 18 modules which will produce
4500 Wp.
The percentage de-rating factor of array power becomes:
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