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Photovoltaic background, technology

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The photovoltaic effect

This is the ability to transform solar energy into electricity and is achieved by using photovoltaic (PV) cells.

A PV cell (see Figure P3) is capable of generating voltage of between 0.5 V and 2 V depending on the materials used and a current directly dependent on the surface area (5 or 6 inch cells).

Fig. P3Photovoltaic cell manufactured in a silicon plate (source: Photowatt)

Its characteristics are shown in a current/voltage graph as shown in Figure P4.

Fig. P4Typical characteristic of a photovoltaic cell

The photovoltaic effect is dependent on two physical values (see Fig. P5)– irradiance and temperature:

  • As irradiance E (W/m2) increases, so do the current and power produced by the cell
  • As the temperature (T°) of the cell increases, the output voltage decreases significantly, the current increases only slightly, so overall the output power decreases.

In order to compare the performance of different cells, the standard has set out Standard Test Conditions (STC) for irradiance of 1000 W/m2 at 25°C.

To make it easier to use energy generated by photovoltaic cells, manufacturers offer serial and/or parallel combinations grouped into panels or modules.

Fig. P5Irradiance and temperature influence the photovoltaic effect

Photovoltaic modules

These combinations of cells (see Fig. P6) enable the voltage and current to be increased. To optimise the characteristics of the modules, these are made up of cells with similar electrical characteristics.

Fig. P6PW2300 photovoltaic module dimensions: 1638 x 982 x 40 mm (source: Photowatt)

Each module providing a voltage of several tens of volts is classified by its power level measured in Watt peak (Wp).This relates to the power produced by a surface area of one m2 exposed to irradiation of 1000 W/m2 at 25°C. However, identical modules may produce different levels of power. Currently, the IEC standard specifies a power variation of ±3%. Modules with typical power of 160 Wp include all modules with power of between 155 Wp (160 -3%) and 165 Wp (160 +3%).

It is therefore necessary to compare their efficiency which is calculated by dividing their power (W/m2) by 1000 W/m2.

For example, for a module of 160 Wp with a surface area of 1.338m2[1] the peak power is 160/1.338 which gives 120 Wp/m2.

Therefore the efficiency of this module is: 120/1000 = 12%.

Note: Manufacturers may have different production tolerance limits according to local standards or habits (example: JISC8918 specifies ±10%), so it is recommended to always check product catalogues for actual tolerance values.

(see table in Figure P7 as an example)

Cell size 156 x 156 mm
Number of cells 60
Voltage 24 V
Typical power 250 255 260
Power tolerance 0/+5 0/+5 0/+5
Voltage at typical power 30.1 V 30.2 V 30.4 V
Current at typical power 8.3 A 8.4 A 8.6 A
Short circuit current 8.9 A 9.0 A 9.1 A
Open circuit voltage 37.2 37.4 37.5
Maximum system voltage 1 000 V CC
Temperature coefficient

Isc = +0,065%/°C
Voc = -0,34%/°C
Pmax = -0,43 % /°C

Power specifications Under Standard Test Conditions (STC) : irrandiance of 1000 W/m2, spectrum AM 1,5 and cells temperature of 25°C

Fig. P7Electrical characteristics of a PW2300 module (source: Photowatt)

However when photovoltaic cells are connected in series, a destructive phenomenon known as the “hot spot” may occur if one of the cells is partially shaded. This cell will operate as a receiver and the current passing through it may destroy it. To avoid this risk, manufacturers include bypass diodes which bypass damaged cells. Bypass diodes are usually fitted in the junction box behind the module and enable 18 to 22 cells to be shunted depending on the manufacturer.

These modules are then connected in series to achieve the level of voltage required, forming chains of modules or “strings”. Then the strings are arranged in parallel to achieve the required level of power, thus forming a PV array.

A faulty module within a string must be replaced by an identical module and therefore it is important to choose a supplier which is likely to be in business in the long-term.

Since there are increasing numbers of PV module manufacturers throughout the world, it is important to consider the various options carefully when choosing equipment. Installers should also:

  • Ensure the compatibility of the electrical characteristics with the rest of the installation (inverter input voltage).
  • Ensure that they are compliant with the standards.
  • Select suppliers likely to be in business in the long-term to ensure that faulty modules can be replaced as these must be identical to those already installed.

This final point is important as installers are responsible for the warranty granted to their clients.

Different technologies are currently being used to manufacture photovoltaic generators. These are divided into two categories - crystalline modules and thin film modules.

Crystalline silicone modules

There are two main categories of crystalline silicon modules – mono-crystalline modules and multi-crystalline modules.

Mono-crystalline modules are currently best in terms of performance, with efficiency of 16 – 18%. They are also more expensive.

The efficiency of multi-crystalline modules is between 12 and 14%. They are more commonly used, especially in the residential and service sectors.

These modules have a service life of more than 20 years. They lose some of their power over time (< 1% per year) but continue to produce electricity. Depending on the look required, bi-glass modules are available with two plates of glass which make the module semi-transparent, or Tedlar or Teflon glass modules which are less expensive but completely opaque.

Thin film modules

Extensive research is currently being carried out on thin film modules and current efficiency levels of 6 to 8% should increase in coming years. They are cheap and suitable for large areas provided that the surface is not a valuable part of the facility.

This category of thin film modules includes a number of technologies of which there are 3 main types:

  • a-Si – thin film or amorphous silicon
  • CdTe (cadmium telluride)
  • CIS (copper indium selenide)

It should be noted that at present we do not yet have 20 years’ experience of this type of technology and thus still do not know how these modules will age.

In their technical specifications, reputable manufacturers indicate initial and stabilised values.

The table in Figure P8 provides a comparative overview of all these technologies.

Technologies sc-Si

mono-crystalline

mc-Si

multi-crystalline

a-Si

Thin film

CdTe

Thin film

CIS

Thin film

STC module efficiency
Maximum 19% 15% 8.5% 11% 11%
Maximum 14% 13% 6% 8% 8%
Relative cost ($/Wp) 3 3 2 1 1
Temperature coefficient at the power peak (%/°C) -0.3 / -0.5 0.3 / -0.5 -0.2 -0.2 -0.3

Fig. P8Comparison of technologies used in photovoltaic generators

Inverters

These devices which convert direct current into alternating current are special inverters for photovoltaic power supply (see Fig. P9). Different types of photovoltaic inverters or “PV inverters” are available. They fulfil three main functions:

  • Inverter function: Converts direct current into alternating current in the form required (sinusoidal, square, etc.)
  • MPPT function: Calculates the operating point on the photovoltaic surface or array which produces the most power in terms of voltage and current - also known as the Maximum Power Point Tracker (see Fig. P10).
  • Automatic disconnection from the network function: Automatically commands the inverter to switch off and the system to disconnect from the network in the absence of voltage on the electrical network. This protects the inverter and any maintenance staff who may be working on the network.

Fig. P9Conext CL60-A String inverter for Photovoltaic energy supply

Therefore, in the event of a network failure, the inverter no longer supplies energy to the network and energy produced by the photovoltaic modules is wasted. “Grid interactive” systems are nevertheless available which function in back-up mode. Batteries need to be installed for these systems as well as an additional control panel to ensure that the network is disconnected before supplying their own energy.

  • Different models
Some “multi-MPPT” inverters have a double (or triple, quadruple, etc.) MPPT function. This function enables PV supply to be optimised when the array includes strings facing in different directions. There is however a risk of total loss of supply if one inverter is faulty.
Nevertheless, it is possible to install one less powerful inverter per string, which is a more expensive solution but increases the overall reliability of the system.
“Multi-string inverters” are also available. These inverters are not necessarily multi-MPPT as described above. The name simply indicates that several strings can be connected to the inverter and that they are paralleled inside the inverter.

Fig. P10Operating point of a photovoltaic array which produces the most power, also known as the Maximum Power Point Tracker

European efficiency

In order to compare the various appliances, a level of efficiency has been determined based on different operating points, simulating the average daily performance of an inverter. This “European efficiency” is calculated using the following formula:

0.03 x (η 5%) + 0.06 x (η 10%) + 0.13 x (η 20%) + 0.1 x (η 30%) + 0.48 x (η 50%) + 0.2 x (η 100%)

where:

  • (η 5%) (η 10%) ... represent the MPPT static efficiency at 5%, 10% ... partial MPP power.
  • 0.03, 0.06 ... are the weighting factors used to calculate this overall "European efficiency", and have been calculated according to yearly climate data (north-western Germany climate data)

Note: another similar efficiency calculation has been defined by California Energy Commission, eg similar formula but using different weighting factors and operating points.

IP and operating temperature

We strongly advise against installing an inverter in a place exposed to the sun as this will considerably reduce its service life.

Ingress protection and temperature parameters are important when choosing an inverter.

Almost all manufacturers of inverters offer IP65 inverters which can be installed outdoors. However, this does not mean that they should be installed in full sunlight as most inverters operate in degraded mode in temperatures over 40°C (50°C for Xantrex inverters manufactured by Schneider Electric) and thus output power is reduced.

Installing inverters outdoors in full sunlight also incurs the risk of premature aging of some of the inverter’s components such as the chemical condensers. This considerably reduces the inverter’s service life from 10 years to as few as 5 years!

Connections

Photovoltaic installations require special cables and connectors. Since modules are installed outdoors they are subjected to climatic constraints associated with high voltages caused by the installation of modules in series.

Besides being ingress protected, the equipment used must also be resistant to UV rays and ozone. It must furthermore display a high level of mechanical resistance and a high level of resistance to extreme variations in temperature.

Cables

The voltage drop between the PV array and the inverter must be calculated and this must not exceed 3% for nominal current.

The DC cables used should be double-insulated single wire cables and since these are not standardised, cables indicated by the manufacturer as being specifically for PV should be used.

Connectors

In general, photovoltaic modules are supplied with two cables equipped with one male and one female connector. Using these cables, it is possible to connect two modules installed side by side, thus creating a series without any difficulties. The male connector connects to the female connector of the following module and so on until the required level of direct current is reached.

These special connectors with locking systems (for example the Multi-Contact MC3 or MC4 connectors) offer protection if touched while they are disconnected.

This protection is necessary since as soon as a photovoltaic module is exposed to irradiation, it supplies voltage. If the cables connecting the modules are handled (to alter or extend them) they must either first be disconnected or the DC isolator for the DC circuit must be activated at the input to the connection box.

It is also possible to use different connectors available on the market. These should be chosen carefully for their quality, contact and male-female mating to avoid any poor contact which may lead to overheating and destruction

Notes

  1. ^ The dimensions of these modules (L x W x D) in mm are: 1237 x 1082 x 38.