Basics of photovoltaic solar energy

I- Presentation

Photovoltaic solar energy is a form of renewable energy. It makes it possible to produce electricity by transforming part of the solar radiation thanks to a photovoltaic cell.

II- Definitions

1- Photovoltaic cells, panels, and fields

The photovoltaic cell is the basic unit that converts light energy into electrical energy.

A photovoltaic panel is made up of an assembly of photovoltaic cells. Sometimes the solar panels are also called photovoltaic modules.

When you combine several panels on the same site, you get a photovoltaic field.

2- What Are Photovoltaics Panels?

To put it simply, photovoltaics are the science behind your basic solar panel. Here is a more detailed explanation on what photovoltaics are.

Photovoltaics is a method of generating electricity by converting solar radiation into direct current electricity using semiconductors that exhibit the photovoltaic effect. Photovoltaic power generation uses solar panels composed of a large number of solar cells that contain a photovoltaic material. Here are a few materials that are currently used for photovoltaics, monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium gallium selenide/sulfide.

There is now a growing demand for alternative energy sources, therefore the manufacture of solar cells and photovoltaic arrays has advanced considerably in recent years.

Photovoltaics are best known as a method for generating electrical power by using solar cells to convert energy from the sun into a flow of electrons. The photovoltaic effect refers to photons of light exciting electrons into a higher state of energy, allowing them to act as charge carriers for electric current.

3- Light power and illumination

Illumination characterizes the light power received per unit area. It is expressed in W / m². The quantity associated with the illumination is noted G Sometimes, this quantity is also called irradiance.

III- Principle of a photovoltaic cell

Photovoltaic cells are made from a PN junction with silicon (diode). To obtain N-doped silicon, phosphorus is added. This type of doping allows the material to easily release electrons (charge -). To obtain P-doped silicon, boron is added. In this case, the material easily creates electronic gaps called holes (charge +).

The PN junction is obtained by doping the two faces of a silicon wafer. Under the action of solar radiation, the atoms of the junction release electric charges of opposite signs which accumulate on both sides of the junction to form an electric generator.

IV- The different types of photovoltaic generators

1- Monocrystalline silicon cells

They represent the first generation of photovoltaic generators. To make them, we melt bar-shaped silicon. During slow and controlled cooling, the silicon solidifies, forming a single large crystal. The crystal is then cut into thin slices which will form the cells. These cells are generally uniform blue. Lifespan: 20 to 30 years.

• advantages :

– good yield, from 20 %

– good Wc / m2 ratio (around 150 Wc / m2) which saves space if necessary

– a high number of manufacturers

• disadvantages:

– high cost

– low efficiency under low light.

2- Polycrystalline silicon (multi-crystalline)

During the cooling of the silicon in an ingot mold, several crystals are formed. The photovoltaic cell is bluish in appearance, but not uniform, there are patterns created by the different crystals.

• advantages :

– square cell (with rounded corners in the case of monocrystalline Si) allowing better expansion in a module

– cheaper than a monocrystalline cell

• drawback:

– less efficient than a monocrystalline cell: 22 %

– Wc / m² ratio less good than for monocrystalline (about 100 Wc / m2)

– low efficiency under low light.

These are the cells most used for electrical production (best value for money). Lifespan: 20 to 30 years

3- Amorphous silicon Silicon

During its transformation, produces a gas, which is projected on a sheet of glass. The cell is very dark gray. It is the cell of calculators and so-called “solar” watches.

• advantages :

– works with weak or diffuse lighting (even in overcast weather)

– a little cheaper than other technologies

– integration on flexible or rigid supports.

• disadvantages:

– low yield in full sun, from 12% to 18%

– need to cover larger areas than when using crystalline silicon (lower Wc / m² ratio, around 60 Wp / m2)

– performances which decrease over time (around 7%).

V- Structures of a photovoltaic installation

1- Isolated site

In the isolated site, the photovoltaic field can provide the electrical energy necessary to operate the receivers (lighting and household equipment). A regulation system and a storage battery store electrical energy in the absence of sunlight.

Batteries are used to store electrical energy in a chemical form. They restore electrical energy according to the user’s needs. The main function of the charge regulator is to protect the battery against overcharging and deep discharge. It is an essential element for the life of the battery. The inverter supplies power to the receivers operating in an alternating mode.

2- Site connected to the network

For this type of site, the photovoltaic field is connected to the network using an inverter. Individuals can resell all or part of the electricity they produce. In this case, it is not necessary to install batteries solar stock for storing the energy produced.

VI- Available solar energy and optimization of the orientation of the photovoltaic panels

1- Apparent movement of the sun and available solar energy

The earth turns around in 24 hours and performs a complete revolution around the sun in 365 days. Seen from the earth (taken as a fixed reference), the apparent movement of the sun is rotational. This movement is added to that of the cyclic declination of the sun. The declination is defined as the angle between the sun-earth axis and the plane of the equator. This angle is noted α in the figure below. Over a year, the declination of the sun varies between + 23 ° (June 21) and -23 ° (December 21).

2- Available solar energy

The cyclic variations in the apparent movement of the sun result in variations in the solar energy available during the year. For example, in the southern hemisphere, the illumination is lowest when the declination of the sun is + 23 °.

3- Orientation and inclination of fixed photovoltaic panels.

In the southern hemisphere, you must orient the fixed panels to the north to capture the rays of the sun throughout the year. This general orientation is not sufficient. It is also necessary to specify the optimum inclination of the panels relative to the ground surface. In the United state, to collect the maximum amount of energy accumulated over a year, the optimum angle is 18 °. This case corresponds to a site connected to the network.

For an isolated site, the goal is to collect as much energy as possible in winter. In this case, the optimum inclination is 35 ° in the United State.

In practice, the roofs which support the panels are rarely oriented due north with an optimum inclination. In this case, the solar energy collected is lower.