Introduction to Solar Cell system Design

Photovoltaic modules consist of PV cell circuits sealed in an environmentally protective laminate, and are
the fundamental building blocks of PV systems. Photovoltaic panels include one or more PV modules
assembled as a pre-wired, field-installable unit. A photovoltaic array is the complete power-generating
unit, consisting of any number of PV modules and panels.

PV String
Individual modules can be connected in series, parallel, or both to increase either output
voltage or current. This also increases the output power. When number of modules is
connected in series, it is called a PV string.
In series connection, the negative terminal of one module is connected to the positive terminal
of the next module. In series connections, voltage adds up and the current remain constant.
V Total = V1 + V2+ ... + Vn
I Total = I1 = I2 = … = In
PV Array
Multiple PV strings are connected in parallel to form a Solar Array. Parallel connection increases
the current, while voltage remains the same.
The power that one module can produce is seldom enough to meet requirements of a home or
a business, so the modules are linked together to form an array. Most PV arrays use an inverter
to convert the DC power produced by the modules into alternating current that can plug into
the existing infrastructure to power lights, motors, and other loads. The modules in a PV array
are usually first connected in series to obtain the desired voltage; the individual strings are then
connected in parallel to allow the system to produce more current. Solar arrays are typically
measured by the electrical power they produce, in watts, kilowatts, or even megawatts.
PV Types
The three general types of photovoltaic cells made from silicon are:
a. Mono-crystalline Silicon – also known as single-crystal silicon
b. Poly-crystalline Silicon – also known as multi-crystal silicon
c. Thin Film Silicon

Figure: Thin-film solar cells

Figure: Schematic representation of a bifacial solar panel

PV Module rating
In the solar industry, the peak power rating of a panel is frequently abbreviated as kWp.
kWp is the peak power of a PV module or system that describes the energy output of a system
achieved under full solar radiation under set Standard Test Conditions (STC). Solar radiation of
1,000 W/m2, module temperature of 25°C and solar spectrum air mass of 1.5 is used to
define standard conditions.
Photovoltaic I-V Characteristics Curves
Manufacturers of the photovoltaic solar cells produce current-voltage (I-V) curves, which gives
the current and voltage at which the photovoltaic cell generates the maximum power output and
are based on the cell being under standard conditions of sunlight and temperature with no
shading.

2.2.2. Short Circuit Current (ISC)

A photovoltaic module will produce its maximum current when there is essentially no
resistance in the circuit. This would be a short circuit between its positive and negative
terminals. This maximum current is called the short circuit current (I sc). This value is higher than
Imax which relates to the normal operating circuit current.
Under this condition the resistance is zero and the voltage in the circuit is zero.
2.2.3. Open Circuit Voltage (VOC)
Open circuit voltage (Voc) means that the PV cell is not connected to any external load and is
therefore not producing any current flow (an open circuit condition). This value depends upon
the number of PV panels connected in series. Under this condition the resistance is infinitely
high and there is no current.
2.2.4. Maximum Power (PMAX or MPP)
This relates to the point where the power supplied by the array that is connected to the load
(batteries, inverters) is at its maximum value, where P max = Imax x Vmax. The maximum power
point of a photovoltaic array is measured in Watts (W) or peak Watts (Wp).
Imax and Vmax value occurs at the “knee” of the I-V curve.
Example:
On a clear and a sunny day, a 1kWp PV array received 6 Peak Sun Hours (hours). Total loss
(derating factor) in the system is estimated at 0.70 (70%)
Expected output can be determined as follows:
Expected Output = Peak Sun Hours x Peak Power Output x Total derating factor
= 1kWp x 6 hour/day x 0.70
= 4.2kWh per day (1st year)
Generally, degradation of a good quality module is about 20% during the module life of 25 years
@ 0.7% to 1% per year.

Now considering degradation of module as per the indicative profile above (example only,
actual degradation of module will be based on module quality and climatic conditions)
Energy generation:
= 3.83kWh per day (on 10th year)
= 3.39kWh per day (on 25th year)

3.0. SYSTEM CONFIGURATIONS

There are two main configurations of Solar PV systems: Grid-connected (or grid-tied) and
Offgrid (or standalone) solar PV systems.

3.1 Grid Connected PV Systems

In a grid-connected PV system, the PV array is directly connected to the grid-connected inverter without a
storage battery. If there is enough electricity flowing in from your PV system, no electricity will flow in
from the utility company. If your system is generating more power than you are using, the excess will be
exported into the energy utility grid, turning your meter backwards. During the times when the PV system
isn’t producing electricity, such as at night, the power grid will supply all the building’s demand. The energy
utility company in lieu will provide energy credit to providers based on the solar production. This is called
“Net Metering”. In this process, energy goes in and out through a single meter.
Benefits of Grid Connected System

a. A grid-connected system can be an effective way to reduce your dependence on utility power,
increase renewable energy production, and improve the environment.
b. System doesn’t always require covering all electrical needs
c. Requires less surface area for panels and no batteries
d. Less expensive

3.1.2. Drawbacks of Grid Connected System

a. Does not prevent grid power failures

b. Can be dealt with by small battery bank

3.2 Standalone PV Systems

Off-grid PV systems have no connection to an electricity grid. A simple standalone PV system
is an automatic solar system that produces electrical power to charge banks of batteries during
the day for use at night when the suns energy is unavailable. Deep cycle lead acid batteries are
 generally used to store the solar power generated by the PV panels, and then discharge the
power when energy is required. Deep cycle batteries are not only rechargeable, but they are
designed to be repeatedly discharged almost all the way down to a very low charge.
A charge controller is connected in between the solar panels and the batteries. The charge
controller operates automatically and ensures that the maximum output of the solar panels is
directed to charge the batteries without over charging or damaging them.
An inverter is needed to convert the DC power generated into AC power for use in appliances.

Standalone PV systems are ideal for the electrification of rural areas or offshore sites that don’t
have utility grid service or where it would be very costly to have power lines run to the isolated
buildings. In these cases, it is more cost effective to install a standalone PV system than pay the
costs of having the local electricity company extend their power lines and cables directly to the
home.
3.2.1. Benefits of Off-Grid Systems
a. System meets all electrical need for building
b. No connection to conventional power grid
c. Works in remote locations
d. Protection against power failures
3.2.2. Drawbacks of Off-Grid Systems
a. Requires much more powerful system. It must produce more power than average consumption.
b. Significantly more expensive
c. Could run out of power
3.3 Grid Tied with Battery Backup System
Including a battery bank into the system allows utilization of energy produced from the PV
system and stored in the batteries during a power out-age. A grid-tied PV system with battery
backup is ideal when living in areas with unreliable power from the grid or that experience
power outages due to natural disasters.
How to Design Solar PV System

What is solar PV system?

Solar photovoltaic system or Solar power system is one of renewable energy system which uses PV
modules to convert sunlight into electricity. The electricity generated can be either stored or used
directly, fed back into grid line or combined with one or more other electricity generators or more
renewable energy source. Solar PV system is very reliable and clean source of electricity that can suit a
wide range of applications such as residence, industry, agriculture, etc.
Major system components
Solar PV system includes different components that should be selected according to your system type,
site location and applications. The major components for solar PV system are solar charge controller,
inverter, battery bank, auxiliary energy sources and loads (appliances).
PV module: converts sunlight into DC electricity.
Solar charge controller: regulates the voltage and current coming from the PV panels going to
battery and prevents battery overcharging and prolongs the battery life.
Inverter: converts DC output of PV panels or wind turbine into a clean AC current for AC
appliances or fed back into grid line.
Battery: stores energy for supplying to electrical appliances when there is a demand.
Load: is electrical appliances that connected to solar PV system such as lights, radio, TV, computer,
refrigerator, etc.
Auxiliary energy sources - is diesel generator or other renewable energy sources.

Solar PV system sizing

1. Determine power consumption demands
The first step in designing a solar PV system is to find out the total power and energy consumption of
all loads that need to be supplied by the solar PV system as follows:
1.1 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.

1.2 Calculate total Watt-hours per day needed from the PV modules.
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.
2. Size the PV modules
Different size of PV modules will produce different amount of power. To find 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 Thailand, the panel generation factor is 3.43. To determine the sizing of PV
modules, calculate as follows:
2.1 Calculate the total Watt-peak rating needed for PV modules
Divide the total Watt-hours per day needed from the PV modules (from item 1.2) by 3.43 to get
the total Watt-peak rating needed for the PV panels needed to operate the appliances.
2.2 Calculate the number of PV panels for the system
Divide the answer obtained in item 2.1 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 that will be
the number of PV modules required.
Result of the calculation is the minimum number of PV panels. 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 be shortened.
3. Inverter sizing
An inverter is used in the system where AC power output is needed. 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. The inverter 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.

4. Battery sizing
The battery type recommended for using in solar PV system 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 should be large enough to store sufficient energy to
operate the appliances at night and cloudy days. To find out the size of battery, calculate as follows:
4.1 Calculate total Watt-hours per day used by appliances.
4.2 Divide the total Watt-hours per day used by 0.85 for battery loss.
4.3 Divide the answer obtained in item 4.2 by 0.6 for depth of discharge.
4.4 Divide the answer obtained in item 4.3 by the nominal battery voltage.
4.5 Multiply the answer obtained in item 4.4 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.

Battery Capacity (Ah) = Total Watt-hours per day used by appliances x Days of autonomy
(0.85 x 0.6 x nominal battery voltage)
5. Solar charge controller sizing
The solar charge controller 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
Solar charge controller rating = Total short circuit current of PV array x 1.3
Remark: For MPPT charge controller sizing will be different. (See Basics of MPPT Charge
Controller)
Example: A house has the following electrical appliance usage:
 One 18 Watt fluorescent lamp with electronic ballast used 4 hours per day.
 One 60 Watt fan used for 2 hours per day.
 One 75 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, 110 Wp PV module.
1. Determine power consumption demands
Total appliance use = (18 W x 4 hours) + (60 W x 2 hours) + (75 W x 24 x 0.5 hours)
= 1,092 Wh/day
Total PV panels energy needed = 1,092 x 1.3
= 1,419.6 Wh/day.

2. Size the PV panel

2.1 Total Wp of PV panel = 1,419.6 / 3.4
capacity
needed
= 413.9 Wp
2.2 Number of PV panels needed = 413.9 / 110
= 3.76 modules
 Actual requirement = 4 modules
So this system should be powered by at least 4 modules of 110 Wp PV module.
3. Inverter sizing
Total Watt of all appliances = 18 + 60 + 75 = 153 W
For safety, the inverter should be considered 25-30% bigger size.
The inverter size should be about 190 W or greater.
4. Battery sizing
Total appliances use = (18 W x 4 hours) + (60 W x 2 hours) + (75 W x 12 hours)
Nominal battery voltage = 12 V
Days of autonomy = 3 days
Battery capacity = [(18 W x 4 hours) + (60 W x 2 hours) + (75 W x 12 hours)] x 3
(0.85 x 0.6 x 12)
Total Ampere-hours required 535.29 Ah
So the battery should be rated 12 V 600 Ah for 3 day autonomy.
5. Solar charge controller sizing
PV module specification
Pm = 110 Wp
Vm = 16.7 Vdc
Im = 6.6 A
Voc = 20.7 A
Isc = 7.5 A
Solar charge controller rating = (4 strings x 7.5 A) x 1.3 = 39 A
So the solar charge controller should be rated 40 A at 12 V or greater.

How MPPT works?

The major principle of MPPT is to extract the maximum available power from PV module by making them
operate at the most efficient voltage (maximum power point). That is to say:
MPPT checks output of PV module, compares it to battery voltage then fixes what is the best power that PV
module can produce to charge the battery and converts it to the best voltage to get maximum current into battery.
It can also supply power to a DC load, which is connected directly to the battery.

MPPT is most effective under these conditions:

� Cold weather, cloudy or hazy days: Normally, PV module works better at cold temperatures and MPPT is
utilized to extract maximum power available from them.
� When battery is deeply discharged: MPPT can extract more current and charge the battery if the state of
charge in the battery is lowers.

MPPT solar charge controller

A MPPT solar charge controller is the charge controller embedded with MPPT algorithm to maximize the
amount of current going into the battery from PV module.

MPPT is DC to DC converter which operates by taking DC input from PV module, changing it to AC and
converting it back to a different DC voltage and current to exactly match the PV module to the battery.