PV cells Modules and Array

By

Wasim Nidgundi

Secab I E T, Vijayapura

Characteristics of PV cells

• Each type of PV cell is unique and has its own

individual characteristics

• when designing an array as the characterisrics of

the cells in one type of module can affect the power

output of the other modules connected to it.

Graphic representations of PV cell performance

• PV cell is graphically represented as a current-voltage

(or I-V) curve.

• An I-V curve tracks the PV cell’s performance and

highlights key features such as Voc, Isc and Pmax.

• A PV cell will always operate along this curve

• A power curve is used to find the maximum power • A power curve is used to find the maximum power

point.

• A power curve plots the voltage along the horizontal

axis and the power (current multiplied by voltage)

along the vertical axis.

• When this is superimposed on the I-V curve for the

same cell then the maximum power point is obtained

Voc and Isc are the x and y intercepts respectively. Ideally the PV

cell operates around the knee of the curve, where the maximum

power point is located

Pmp is the highest point on the power curve; by extrapolating

back to the x-axis, Vmp can be found. Vmp is also on the I-V

curve and so the corresponding current can be found for this

particular voltage (Imp)

Connecting PV cells to create a module• PV cells of identical characteristics are wired

together in series to create a module.

• In series, their voltages will add while the currentremains constant, i.e. the current of the module isequal to the current of one cell.

• The characteristics of cells are given on the• The characteristics of cells are given on themanufacturer’s data sheet.

• A data sheet includes important technicalinformation required to design and install a PVarray.

• It gives information about efficiency, rated powerand physical size

Creating a string of modules• A string comprises a number of PV modules connected

in series.

• The electrical characteristics of PV modules connectedin series to form a string are the same as PV cellsconnected in series to form a module.

• That is the output voltage of the string will be the sumof the output voltages of all the modules and theof the output voltages of all the modules and theoutput current of the string will be the lowest outputcurrent of any module.

• Modules can also be connected in parallel.

• the current output of the modules will add instead ofthe voltage. The output voltage is that of a singlemodule.

When three identical modules are connected in series to form a

string their voltages add and the total current is that of one single

module. The power output of the string is calculated using P = I x V

When non-identical modules are connected in series the voltages will

still add; however, the current of the string will be the lowest current of

any single module (in this case 4A). The power output of the string is

then calculated using

P = I x V

Three identical modules are connected in parallel; the

total current is the sum of each individual current,

while the total voltage is the voltage of a single module.

The power is once again calculated using P = I x V

When non-identical modules are connected in parallel

the currents add while the output voltage is equal to

the lowest single module voltage. The power output of

the modules is then calculated using P = I x V

Creating an array

• Array designers will connect PV modules using a

combination of series and parallel to produce the

output current and voltage suitable to a market

• Modules are typically connected in series to form a

string and these strings are connected in parallel to

form an array.form an array.

• An inverter is required to convert the DC solar

power to AC power and the array is wired in such a

way that the maximum power point voltage of the

array lies within the range of the grid interactive

inverter

Photovoltaic array performance

• The performance of a PV array is affected by a

variety of factors

1) Temperature

2) irradiance

3) shading

Irradiance

• The amount of solar radiation (sunlight) hitting the

cell will largely determine its power output.

• The output of a PV array can be estimated using

performance data provided by the manufacturer on

the data sheet.

• All arrays have a rated peak power output, i.e. an • All arrays have a rated peak power output, i.e. an

array can be described as a 1.5kWp array

• Using this information and local solar insolation

data (see Chapter 2), it is possible to estimate the

output of an array.

• Example: On a clear sunny day a 2kWp PV array

received 6 peak sun hours: the 6 peak sun hours

equate to an energy input of 6000W/m2 per day.

Expected output can be determined as follows:

peak power output × peak sun hours = expected

output

2kW × 6PSH = 12kWh2kW × 6PSH = 12kWh

• The PV array produced 12kWh for that day

• There will be varying insolation data available on a

monthly basis, so the whole year’s output from the

PV array can be estimated from the monthly data.

The I-V curves for a cell operating at different irradiance

values show the increase in power output with

irradiance

4.84 peak sun hours/day

A 2kWp PV array would produce an average of 2kW* 4.84PSH/day

=9.68kWh/day.

This results in an average of 9.68Wh/day*365days/year =3533.2kWh/year.

Temperature• Not only does the solar radiation hitting the modules

produce electricity, it also heats up the modules

• As temperature increases, the open-circuit voltage decreases rapidly while the short-circuit current increases slowly.

• Power output is voltage multiplied by current and so it will decrease.will decrease.

• When designing systems, engineers will often use the following approximation

cell temperature = ambient temperature + 25°C

• As hot temperatures adversely affect power output, output from a PV array has to be calculated taking the temperature effects into consideration.

• cold temperatures can increase the power output due to the voltage increase

As power = current x voltage (I x V), as voltage decreases,

power decreases

In areas that experience extremely cold temperatures, the voltage

increases as the ambient temperature decreases

• The installation of a PV array can directly affect the

operating temperature of the array itself.

• when an array is installed flush to a roof surface

there is limited air flow.

• This can have a doubly negative affect because the

roof itself will give off heat

• so forced ventilation across the roof surface and • so forced ventilation across the roof surface and

behind the modules is often necessary

• If the installation cannot include both methods of

ventilation, the output of the array will have to be

derated to reflect negative aspects of the

installation

Shading• If a cell receives no sunlight due to shading it will not

produce any power

• so when one or several cells are shaded, the current output of the module will be reduced.

• If the module is part of an array, then the current output of the array will also be reduced.

• This will also occur if a cell is damaged and unable to • This will also occur if a cell is damaged and unable to produce power.

• Shading of the array can lead to irreversible damage. Hot spot heating occurs when a cell is shaded

• that its power output is reduced and most of the current being produced by the other (unshaded) cells is forced through that one cell causing it to heat up.

• This often leads to cell damage (cracking) and can

also damage the glass encapsulation.

• It is difficult to prevent shading. However, diodes

can be used to mitigate temporary shading

• When a cell is shaded or damaged, a diode can be

used to give current another path to follow

• It will skip the damaged or shaded cell completely • It will skip the damaged or shaded cell completely

and have minimum impact on the power output of

the array

• This kind of diode is referred to as a bypass diode

Even this small shadow can reduce the amount of electricity a module

produces –a small shaded area can, under certain circumstances reduce

module output by 80–90 per cent as well as affecting the rest of the

array

The discoloured cell in this array was caused by hot spot heating