Characteristics of PV module at standard test condition (STC)

1. The STC means the modules are tested at solar irradiance 1000 W/m^2, cell effective temperature 25 degree celcius and air mass 1.5.
2. Definition of cell, module, string and array:

• Combinations of cells form a module.
• Module connected in series form a string.
• Series and parallel modules form an array.

                                         

• Every PV module has datasheet that describe its electrical properties.
• Example

             

1. The maximum output or peak or rated power of the module (Pmax) = 170 W.
2. The manufacturer tolerance of the output power is +/-3%. So for STP 170S-24/Ab-1. PV module the output power could be between 1.03 x 170 W to 0.97 x 170 W. However, in practice most of the PV modules are having negative percentage of power tolerance. So, during design stage we use negative percentage for worst case scenario.
3. During peak power condition, the voltage and current of the module are Vmp and Imp respectively.
4. During no load condition or open circuit, the voltage of the module is Voc.
5. Short circuit current of the module is Isc.

Types of PV modules

Two broad categories of PV module:

     - Crystalline

               - Nanocrystalline

               - Polycrystalline

     - Thin film

               - Amorphous

 

Solar radiation

• To obtain maximum energy from the irradiance, the PV module must always be facing the sun.
• In northern hemisphere the modules will be positioned facing due south.
• Theoretically the module should be tilted equal to the latitude of the site location.
• However in practice, in the latitude from 0 to 15 deg, the module will generally be tilted at 10 degree to the horizontal.
• Solar irradiation reaching the top of the atmosphere is called solar constant Gc=1367 W/m^2
• Solar irradiation at sea level on earth is 1000 W/m^2
• Daily irradiation is commonly referred as peak sun hour (PSH). The number of PSH for the day is the number of hours for which energy at the rate of 1000 W/m^2 would give an equivalent amount of energy to the total energy for that day. 
• Unit for PSH is (kWh/m^2)/(kW/m^2) or hour. 
• The energy in SI unit is Joule. To convert M/J^2 to peak sun hour, use the following formula:

                                                        

• Air mass (AM) is the path length the sun's array has to travel through the atmosphere to reach earth surface.

                                       

Metering

Energy meter is a very important tool to record energy in kWh consumed by the load. In grid-connected system energy meter is also used to record the amount of energy produced by PV array. There are three methods of metering used in PV-grid system:

1. Single net meter. This is a bi-directional energy meter. It just required one unit to be installed. The meter will show the net power consumed at the end of every month. However, this technique will not record the total amount of energy being generated by the PV power generator. 
2. One import meter and one export meter. Each meter is a unidirectional energy meter and both are connected in series. The import meter will record the total energy consumed and the export meter will record total energy being generated to the grid separately. Although the export meter enable to record the total export enegy, this method cannot differentiate between energy generated from the PV array and other resources e.g regenerative energy from the machine operation.
3. Dedicated export meter. This is a uni-directional energy meter that record the energy produced by the PV array and feed the grid separately. 

Classification of PV-grid connected system

PV-grid connected system can be classified into two:
1. Central system

• This system involves large PV array up to few Megawatt size.
• It is used to transfer large amount of power to the grid through three phase connection.

Central system (Courtesy SMA Technology)

2. Distributed system.

    Subdivided further into types:

• Commercial system. The power is usually greater than 10kWp and the panels are located on commercial business building. Usually, the power generated from PV arrays is used for their own consumption.
• Domestic system. The power is typically from 1 to 5kWp and the panels are installed on the roof of houses.

Basic principle operation of PV grid connected system

• When the light strikes the PV modules, DC power will be produced. The DC current will flow through string cable to an Array Junction Box (AJB).
• All the current in each string are combined together in the AJB i.e all the strings cables are combined and terminate in a box called Array Junction Box (AJB).
• Each string is protected by a string fuse and circuit breaker.
• From AJB, the DC current flow to grid inverter through array cable. Here, the current is converted to AC current and transfer to grid.
• The array cable is also equipped with DC breaker to break the DC circuit when required. A surge protection device (SPD) is installed to this line to protect from lightning surge. The lightning surge induces from the PV side divert to the ground through SPD thus protect the inverter.
• The main function of gid inverter is to convert DC power to AC power. This is the special kind of onverter which different from an ordinary standalone inverter. This inverter works with grid only. It cannot be used to power AC load directly. The AC power produced by the inverter feed to grid supply through direct or indirect connection. Direct connection means connection is made at the main fuse link of grid whereas indirect connection makes connection at the distribution board.
• To measure the total amount of energy generated from the PV array, an additional meter which is known as export meter is usually installed. This is important for monitoring and billing purposes.

Introduction to Photovoltaic System

In general photovoltaic system is divided into two broad categories:


1. Off-grid PV System or standalone

• Independent of grid supply
• Usually install at remote area
• Classified further into two types which are standalone PV battery and hybrid.
• PV battery is a small size and consist of PV and battery
• Hybrid is a medium size and consist of PV and generator set or other power generators.

2. On-grid PV system

• Tie to the grid supply. Grid supply must be present at all time.
• The PV grid connected system is also called grid-interactive, grid-intertie, utility connected are built onto your building that transfers DC power from the PV array to the utility supply through grid inveter.
• There are five major component involves namely PV array, array junction box (AJB), DC and AC cables, inverter and export/import energy meters.

A typical block diagram of PV-grid connected system

Some of the advantages of photovoltaic system are:

• Photovoltaic technology make use of the abundant energy in the sun.
• It has little impact on our environment.
• Photovoltaic can be used in a wide range of product, from small consumer items to large commercial solar electric system.
• Very low maintaince.

Disadvantages:

• Works during daytime with sunlight
• Works with present of grid voltage
• High cost