chapter 1 pv system sizing

8/17/2019 Chapter 1 PV System Sizing

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What is System Sizing

• System sizing is the process used for determining the

minimum panel and battery size needed to deliver therequired electrical energy under the solar conditions

that exist at the system site.

• It balances the output from the system with the solar

input while taking into consideration losses in thesystem


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We need to know:

• The solar energy in kWh/m2/day at the site for the

lowest solar energy month of the year.• The average Wh/day required by the user to operate

the desired appliances and any special needs for

power that go much beyond the average.

• The losses that occur in the PV system that reducesthe energy available to the user


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Estimating the load

• Determine the Watts required by each of the

appliances• Estimate the hours per day that each appliance will

be used.

• For each appliance multiply the Watts times hours to

get Wh/day• Total the Wh/day for all appliances


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Problem

• 4 lights of 11 watts each are installed.

– 1 light will operate 4 hours per day – 3 lights will operate 2 hours per day

• 1 night light of 1 watt is installed

– Nightlight operates 10 hours/day

• 1 Radio of 10 watts is installed – Radio operates 9 hours per day

How many Wh/day will be needed by the appliances?


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The Solar Resource

• Actual measurements at the site are best but at leastone full year is needed and several years is preferred.

Measurements taken with instruments tilted at the same

angle as the solar panels are best but horizontal

“meteorological” measurements are ok.

• NASA satellite measurements are better than “sunshinehours” recorded for the site

• “Sunshine hour” measurements indicate the solar

variation over the year but are not a good measure of

actual solar energy in kWh/m2/day but are better thannothing.

• Choose the average value of solar for the lowest month

as the design basis


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Wh/day that needs to come from the

panel for systems with batteries

• Wiring and connection losses about 10%

• Losses in the battery about 20%

• Total losses around 30% so the panel will need to

produce enough Wh/day for the load plus enough tocover the losses. So it will have to produce about 130%

of the energy required by the load

• To calculate the Wh/d needed from the panel, multiplythe load Wh/d times 1.3


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Calculating the panel generation

factor (1)

• The lowest month kWh/m2/day value is the

starting point. (Typically between about 5 and 6

kWh/m2/day)

• This is the same total energy as would comefrom the sun shining at 1000 W/m2 each day

for the number of hours equal to the

kWh/m2/day figure.


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factor (3)• Corrections include:

– 15% for temperature above 25 C – 5% for losses due to sunlight not striking the panelstraight on (caused by glass having increasingreflectance at lower angles of incidence)

– 10% for losses due to not receiving energy at the

maximum power point (not present if there is aMPPT controller) – 5% allowance for dirt – 10% allowance for the panel being below

specification and for ageing• Total power = .85 X .95 X .90 X .95X .90 = .62 of the

original Wp rating.


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factor (4)

• To get the panel generation factor (Wh/day per

Wp capacity) multiply the daily sun hours times0.62.

• For the example, that would be 5.2x0.62 = 3.22Wh/Wp/day.

• That is, for every Wp capacity in the panel we

can expect to get an average of 3.22 Wh/dayduring the lowest solar month


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Calculating the panel size needed

• Divide the Wh/day needed from the panel (1.3

times the load Wh/day) by the Generation

Factor in Wh/Wp/day. The result is the

minimum Wp of panel needed to meet the

design load for the lowest solar month after alllosses and corrections have been applied.


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Calculating the battery size (1)

• The load electricity is provided by the battery. So

determining the Ah/day needed by the load willdetermine the battery capacity that has to be availableeach day to operate the appliances.

• For a 12V system, Ah/day = Wh/day/12V

• Solar design methods usually choose a 20% dailydepth of discharge (DOD) for deep discharge batteries.For the modified automotive battery used by AMORE,

longer life will be seen if that percentage is reduced to15% DOD.


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Calculating the battery size (2)

• The rate of discharge is about 5 hours a day for

lights. That represents about a C20 discharge

rate if 15% of the battery capacity is used in 5

hours (the discharge rate in Amperes being the

capacity of the battery divided by the hours todischarge).

• So the total battery capacity needs to be thedaily Ah at C20 divided by 0.15 if 15% is to be

the daily depth of discharge.


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Use of automotive batteries

• Automotive batteries are quite sensitive to deep

discharge so the average percentage of daily

discharge should be reduced to 10% to provide

longer life and even then the life probably will

be less than two years.


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Summary of Sizing calculation

1. Estimate the Wh/day of the load

2. Multiply the load Wh/day times 1.33. Determine the kWh/m2/day of sunlight for the lowest

solar month

4. Multiply the kWh/m2/day times .62 to get the

generation factor Wh/d/Wp5. Divide the result of (2) by the result of (4) to get

minimum panel Wp.

6. Divide (1) by the battery voltage (12V) to get Ah/day

7. Divide (6) by .2 to get the minimum Ah of the batteryat C20.


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Estimating the Wh/day that can be

used for a particular size of panel

• To determine the maximum appliance Wh/day that can

be served by a particular size of panel: – Multiply the kWh/m2/day times .62 to get the local

generation factor

– Multiply the local generation factor times the Wp of

the panel. This will give the estimated Wh/day fromthe panel

– Divide the estimated Wh/day from the panel by 1.3

to get the estimated appliance Wh/day that can be

served by that panel


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Example

• A 36 Wp panel is installed at a site having a low month

solar value of 5.2 kWh/m2/day. What is the maximumWh/day of appliance load that this panel can serve?

– Multiply 5.2 x .62 = 3.22

– Multiply 36 x 3.22 = 116

– Divide 116 by 1.3 = 89 Wh/day of appliance use is

possible


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1. Determine power consumption demands

2. Size the PV modules3. Inverter sizing

4. Battery sizing

5. Solar charge controller sizing

Solar PV system sizing


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1. Determine power consumption demand

find out the total power and energy consumption of all loads that need to besupplied by the solar PV system as follows:

i. Calculate total Watt-hours per day for each appliance used.

Add the Watt-hours needed for all appliances together to get the

total Watt-hours per day which must be delivered to the

appliances.

ii. Calculate total Watt-hours per day needed from the PVmodules.

Multiply the total appliances Watt-hours per day times 1.3 (the

energy lost in the system) to get the total Watt-hours per day

which must be provided by the panels.

Solar PV system sizing…cont


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2. Size the PV modules

Different size of PV modules will produce different amount of power. Tofind out the sizing of PV module, the total peak watt produced needs. The

peak watt (Wp) produced depends on size of the PV module and climate

of site location. We have to consider “panel generation factor” which is

different in each site location. For Malaysia, the panel generation factor is

basically around 3.1. To determine the sizing of PV modules, calculate as

follows:i. Calculate the total Watt-peak rating needed for PV

modules.

Divide the total Watt-hours per day needed from the PV

modules [from step 1(ii)] by 3.1 to get the total Watt-peak

rating needed for the PV panels needed to operate theappliances.

ii. Calculate the number of PV panels for the system

Divide the answer obtained in item (i) by the rated output Watt-

peak of the PV modules available to you. Increase any

fractional part of result to the next highest full number and thatwill be the number of PV modules required.



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2. Size the PV modules

Result of the calculation is the minimum number of PVpanels. If more PV modules are installed, the system

will perform better and battery life will be improved. If

fewer PV modules are used, the system may not work

at all during cloudy periods and battery life will beshortened.



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3. Inverter sizing

An inverter is used in the system where AC power output isneeded. The input rating of the inverter should never be lower

than the total watt of appliances. The inverter must have the same

nominal voltage as your battery.

For stand-alone systems, the inverter must be large enough to

handle the total amount of Watts you will be using at one time. Theinverter size should be 25-30% bigger than total Watts of

appliances. In case of appliance type is motor or compressor then

inverter size should be minimum 3 times the capacity of those

appliances and must be added to the inverter capacity to handle

surge current during starting.

For grid tie systems or grid connected systems, the input rating of

the inverter should be same as PV array rating to allow for safe

and efficient operation.



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4. Battery sizing

The battery type recommended for using in solar PVsystem is deep cycle battery. Deep cycle battery is

specifically designed for to be discharged to low

energy level and rapid recharged or cycle charged and

discharged day after day for years. The battery shouldbe large enough to store sufficient energy to operate

the appliances at night and cloudy days.



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4. Battery sizing

To find out the size of battery, calculate as followsI. Calculate total Watt-hours per day used by

appliances.

II. Divide the total Watt-hours per day used by 0.85 for

battery loss.III. Divide the answer obtained in item (II) by 0.6 for

depth of discharge.

IV. Divide the answer obtained in item (III) by the nominal

battery voltage.V. Multiply the answer obtained in item (IV) with days of

autonomy (the number of days that you need the

system to operate when there is no power produced

by PV panels) to get the required Ampere-hour

capacity of deep-cycle battery.



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S l PV t i i t


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5. Solar charge controller sizing

• To find out the size of battery, calculate as followsThe solar chargecontroller is typically rated against Amperage and Voltage capacities.

Select the solar charge controller to match the voltage of PV array and

batteries and then identify which type of solar charge controller is right for

your application. Make sure that solar charge controller has enough

capacity to handle the current from PV array.• For the series charge controller type, the sizing of controller depends on

the total PV input current which is delivered to the controller and also

depends on PV panel configuration (series or parallel configuration).

• According to standard practice, the sizing of solar charge controller is to

take the short circuit current (Isc) of the PV array, and multiply it by 1.3


S l PV t i i t

http://www.leonics.com/product/renewable/solar_charge_controller/solar_charge_en.phphttp://www.leonics.com/product/renewable/solar_charge_controller/solar_charge_en.php


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Remark: For MPPT charge controller sizing will be different.



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• A house has the following electrical appliance usage:

– One 20 Watt fluorescent lamp with electronic ballast used 5 hours perday.

– One 70 Watt television used for 4 hours per day.

– One 80 Watt refrigerator that runs 24 hours per day with compressor

run 12 hours and off 12 hours.

– The system will be powered by 12 Vdc, 150 Wp PV module.

• Answer the following question

1) Determine the power consumptions demand

2) Size the PV panel

3) Inverter sizing

4) Battery sizing

5) Solar charge controller sizing

Example

chapter 1 pv system sizing

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