Photovoltaic derives from "Photo" meaning light in Greek, and "voltaic" to remember Alessandro Volta, the first man who studied the phenomenon of electricity. Photovoltaic is the technology that transforms solar radiation directly into electric energy using semi-conductors materials such as silicon.

A PV plant is made up of:

• photovoltaic modules or panels that transform light into direct current (DC)
• inverter to transform DC into alternating current
• supporting structures
• electric cabin and cables to connect the inverter to the electric grid
• meter to measure produced energy

Photovoltaic cell is the basic element. It is a very thin silicon layer from 0.3 to 0.5 mm, circular, square or octagonal-shaped. When sunlight hits the cell, DC is produced like a weak power battery.

A square cell with a 156 mm side generates about 3.5W under standard conditions, with 25°C temperature and with solar radiation power of 1.000 W/m2.

• Numerous cells inter-connected make photovoltaic modules, the key component in a photovoltaic plant.
• Several modules inter-connected in series or in parallel form the strings whose power varies from hundreds to millions of Watts.
• Inter-connected strings make a photovoltaic plant.

It is easy to install photovoltaic plants since it is a very flexible technology that can be adapted to different kinds of surfaces.
Photovoltaic modules can be installed on rooftops, façades, terraces, sunshades, cantilever roofs, car-park roofs, barriers against noise or on the ground.
Optimal conditions in Italy are:

• Southern inclination (South-East and South-West lose some power)
• slope between 25° (in Southern latitudes) and 35° (in Northern latitudes)
• lack of shade
• technical performance of the plant components (modules, inverter and other devices).

Photovoltaic plants are very competitive in Italy:

• the plants provide a good return on investment - pay back period within 6-8 years on average
• photovoltaic modules' average life is around 25-30 years with about 20% estimated decrease after that
• high reliability since there is no mechanical movement
• maintenance costs reduced to the minimum
• modularity of the plant (it is very easy to increase the capacity of a power plant simply by adding other modules)
• no greenhouse emissions and savings of fossil fuel

For 1kW nominal installed power, the plant is estimated to occupy from 6 to 12 m2 surface depending on the technology used (mono, poly crystalline or thin film).
In Italy, a family of 4 people consumes about 4000kWh per year.
Dependin on the region where plant is installed, at least 3kWp of installed power is required which will occupy on average a 24 m2 surface.

There are two types of photovoltaic plants:

• Stand-alone plants are not connected to a National electricity grid. They are located in isolated areas (isolated houses, shelters, Alpine huts, campers, boats, external lightning etc). The owner of the plant directly consumes the produced electricity and then the excess production is accumulated and stored in specific batteries that guarantee energy availability at all times. In case of black-outs, these batteries provide electricity to the owner.

What is the energy break even?

• Grid-connected plants are directly connected to an electricity grid which guarantees energy even if the sun is weak. In addition, all excess energy can be:
  • put into the grid and withdrawn later (plants inferior to 20kW - net metering)
  • sold to the grid

Watt (W) - Power

The unit measures the rate of energy conversion per second.
Multiples symbols unit decimal power
Watt - W - 1W
Kilowatt - KW - 1,000W - 10³ W
Megawatt - MW - 1,000,000W - 10⁶ W
Gigawatt - GW - 1,000,000,000W - 10⁹ W
Terawatt - TW - 1,000,000,000,000 - 10¹² W

Watt hour (Wh)

Kilowatt hour, symbol kWh, is a unit of energy equal to 1000 watt per hour. Energy in watt hours is the multiplication of power in watts and time in hours. The kilowatt hour is most commonly known as a billing unit for energy delivered to consumers by electric utilities.

Watt-peak (Wp)

Watt-peak (Wp) is a measurement of the nominal power of installed capacity. Related units such as kilowatts-peak (kWp) and megawatts-peak (MWp) are also used. In the context of domestic installations, kWp is the most common unit encountered.
The nominal power of a photovoltaic module is determined by measuring current and voltage. The conditions are specified in standards such as IEC 61215, IEC 61646 and UL 1703, specifically:

• light intensity is 1000W/m2
• temperature of the cells at 25°C.
• with a spectrum similar to sunlight hitting the earth's surface at latitude 35°N in summer (airmass 1.5)

Photovoltaic plants produce energy not only when modules are hit by direct light but also by diffused or reflected light.

Italy is a country where the sun represents an abundant source of renewable, free and unlimited energy. Italy is one of the European countries with the highest numbers of solar radiation, ranging from 1,150 to 1,500 hours annually on average, depending on the latitude.

Photovoltaic plants annual electricity production depends on several factors:

• solar radiation in the plant's location
• orientation and slope of the modules surface
• presence/absence of shade
• technical performance of the plant components (modules, inverter and other devices).

Turnkey photovoltaic plants cost from 3,000 to 5,000€/kWp.
Costs vary according to the dimension, level of integration of the plant, rooftops or ground installation.
Maintenance cost is quite low. It represents 1% of total plant cost per year.

Photovoltaic plants have an average lifetime of 20-30 years and our manufacturers guarantee 25 years performance. After 25 years, there is an estimated loss of production of about 20%. Inverters have a shorter lifetime compared to modules.

Photovoltaic was originally created to provide energy to satellites in orbit around the earth. Since maintenance operations are almost impossible in space, photovoltaic technology has proved to be highly reliable and not only for its lifetime. Photovoltaic still powers satellites.

"Conto Energia" is the Italian State incentive mechanism designed to promote electricity energy production from photovoltaic plants connected to the national electric grid. It pays feed-in tariffs for electricity produced by a plant for a fixed term of 20 years.

Incentives are allocated by Gestore dei Servizi Energetici - GSE S.p.a. - www.gse.it

Incentives are allocated for 20 years. After this period of time, benefits from photovoltaic are still high:

• net metering for power plants inferior or equal to 20 kW
• all photovoltaic plants can sell produced electricity to the grid except plants inferior or equal to 20kW that choose net metering.

The pay back period on the investment is estimated to be around 6-8 years on average, depending on:

• solar radiation available (depends on the plant latitude and orientation)
• cost per kW (depends on the plant size)
• increase in value of produced energy (value of feed-in tariffs and value of used energy)

Feed-in tariffs are given to photovoltaic plants whose power is superior to 1 kWp and are connected to the grid.

They are allocated to:

• individuals
• legal companies
• public institutions
• joint owners of houses or buildings

From silicon to photovoltaic modules

Example of a production process of polycrystalline modules:

Silicon (Si) is the most common material for photovoltaic cells.
In the Earth's crust, silicon is the second most abundant element after oxygen, making up 25.7% of the crust's mass.
Purified silicon is used to produce ultra-pure silicon wafers, which are used in the semiconductor industry, in electronics and in photovoltaic applications. Ultra-pure silicon can be doped with other elements to adjust its electrical response by controlling the number and charge (positive or negative) of current carriers.
In the photovoltaic industry, silicon has to be 99.999999% pure (solar type silicon).

The cells are made of a semi-conductor material, generally crystalline silicon that has been properly treated. The cell is usually black or blue and its size varies from 4 to 6 inches.
Cells are protected by two layers of material:

• the front surface of the cell is covered by a layer of tempered glass with high optical performance that protects the module from both of the atmosphere and possible impacts. In this way light is captured and reflection is diminished.
• the cell back surface is covered by a plasticized layer so module is water-airproof.
• The module is laminated to make its surface compact. The aluminum frame allows the module connection to be fixed to its structure. The function, which houses the connections, is welded and sealed.

A photovoltaic module is usually made of cells, the basic element that directly transforms light to electricity. In the industry, several different technologies are available to produce photovoltaic modules of different types: Crystalline silicon (mono and polycrystalline) is a material that consists of small silicon crystals and differs from amorphous silicon which is used for thin film (among which is CIGS).

Monocrystalline silicon

In monocrystalline silicon, the crystalline framework is homogenous. It can be recognized by its dark external colour and its octagonal shape.

Monocrystalline silicon is melted to obtain a cylindrical ingot with 20 cm diameter and one meter length.

Once cooled, the silicon ingot is cut and transformed into wafers. Cells are obtained by doping the wafers to create a p-n junction (positive and negative poles). Electrodes are carved to carry current and then the cells are coated with an anti reflection layer.

Polycrystalline silicon

Polycrystalline silicon is a material that consists of multiple small silicon crystals. The polycrystalline cells can be recognized by a visible grain and its squared shape and blue colour.

Polycrystalline silicon is melted over moulds from which blocks are obtained and, once cooled, cut into ingots of the dimension of the cells. Therefore wafers are shaped and cut very thin (160 to 200 µm).

Cells are obtained by doping the wafers to create a p-n junction (positive and negative poles). Electrodes are inlaid to transport the current and the cells are coated with an anti reflection layer.

Thin films

Thin films made of silicon, a semi-conductor material, are sprayed over a substrate (usually glass) with dimensions superior to the one of the cells. One of the technology consists of immerging the layers in an electrolytic bath. Then silicon layers are cut into the modules through lasers.

The Silicon layer is 2 µm high, i.e 200 times less than crystalline silicon cells. This is why they are called thin films.

Today the most common materials are:

• amorphous crystalline (used for watches, calculators etc.)
• crystalline silicon
• amorphous crystalline and crystalline silicon
• CIS (Copper Indium Selenide) and CIGS (Copper Indiun Gallium Selenide)
• CdTe (Cadmium Telluride)

The thin film technology has numerous advantages:

• it requires less semiconductor material, from 1 to 2 µm compared to 160 to 200 µm of crystalline cells.
• the production process is cheaper, i.e. melting temperature is 200° to 500° compared to 1500° for crystalline cells.

CIGS (Copper Indium Gallium Selenide)

CIGS is a semi conductor material, made of copper, indium, gallium and selenium.

It is used for thin film photovoltaic module production. This material is an alternative to silicon and has high performance in capturing solar rays.
This technology is one of the most promising since it has high performance with perfect design and easy installation.

In both cases, the sun is required as a source of energy and the radiation is captured through specific surfaces. Photovoltaic modules transform solar radiation into electricity whereas thermal solar panels use the heat from the sun to heat water for sanitary purposes or energy production.

"Grid parity" is the cost parity between energy produced from traditional sources and energy produced by photovoltaic.
Energy produced by photovoltaic plants is rapidly increasing in Italy. For that reason Italy could be one of the very first countries to reach "grid parity".