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Project “Eco-Building International Club for advanced European sustainable energy technology dissemination in Europe and China ”

is funded by the EC DG TREN within ENERGIE Programme

 

 

 

 


Photovoltaic systems

Introduction


Photovoltaic systems are an attractive alternative to fossil or nuclear fuels for the generation of electricity. Sunlight is free, it does not use up an irreplaceable resource, and its conversion to electricity is non-polluting. The photovoltaic process is completely solid- state and self-contained. There are no moving parts and no materials are consumed or emitted. The photovoltaic device that can convert the light directly into electricity is called “solar or photovoltaic cell” and consists of layers of semiconductor materials with different electronic properties. PV cells are connected together to make a module. Modules are the building blocks connected together to make an array.

Cells Technologies

 
The conversion of light to electricity depends on the electronic structure of solar cells with two or more layers of semiconductor material that can absorb photons, the primary energy packets of light. The photons raise the energy level of the electrons in the semiconductor, exciting some to jump from the lower-energy valence band to the higher-energy conduction band. The electrons in the conduction band and the holes they have left behind in the valence band are both mobile and can be induced to move by a voltage. The electron motion, and the movement of holes in the opposite direction, constitute the electric current. In a typical solar crystalline silicon cell, the bulk of the material is silicon, doped with a small quantity of boron to give it a positive or p-type character. A thin layer on the front of the cell is doped with phosphorus to give it a negative or n-type character. The interface between these two layers contains an electric field and is called a junction.

Measuring the PV efficiency
 

The watt peak

Since solar cell output power depends on multiple factors, such as the sun's incidence angle, for comparison purposes between different cells and panels, the measure of watts peak (Wp) is used.

Watt peak is the DC output in Watts of a solar module as measured under an industry standardized light test before the solar module leaves the manufacturers facility. The light test tests the output watts when illuminated under Standard Test Conditions (STC) of 1000 watts of light intensity per square meter, 25°C ambient temperature and a spectrum similar to sunlight that has passed through the atmosphere (air mass 1.5).

The knowledge of the Wp power let possible to determine the amount of energy that it generates under certain conditions such as the tilt angle, the azimuth angle (N,S,W,E orientation) of the modules and the average climate conditions. For instance a kWp south orientated and positioned with 30° degrees tilt angle in Italy could generate from 1200 to 1600 KWh per year.

The PV system efficiency

A solar cell's energy conversion efficiency (?, "eta"), is the percentage of power converted (from absorbed light to electrical energy) and collected, when a solar cell is connected to an electrical circuit. This term is calculated using the ratio of the maximum power point, Pm, divided by the input light irradiance (E, in W/m²) under Standard Test Conditions and the surface area of the solar cell (Ac in m²).

\eta = \frac{P_{m}}{E \times A_c}

The losses of a solar cell may be broken down into reflectance losses, thermodynamic efficiency, recombination losses and resistive electrical loss. The overall efficiency is the product of each of these individual losses.

Photovoltaic systems, applications and power production

Photovoltaic power systems are exceptionally modular, which not only provides for easy transportation and rapid installation, but also enables easy expansion if power requirements increase. PV systems may comprise some or all of the following basic components:

  • The generator which may be a single module or an array of several modules
  • The support structure
  • The power conditioning equipment which is the inverter
  • The electric panel
  • The utility meter
  • The power storage system (usually provided by batteries)
  • The cables

The photovoltaic effect produces DC (Direct Current) electricity. This DC power is usually converted to AC (Alternating Current) electricity to match the national standards of AC frequency and voltage. The conversion is made by an important system component called the inverter. The array and inverter are engineered for efficiency and compatibility. The AC electricity can then be used to power homes, school or business.

The exceeding amount of energy, generated from a photovoltaic system, that isn't immediately used from the system owner can be stored in a batteries system or may be transferred to the grid. In this case the surplus of energy is credited to users as it passes through its utility meter and into the utility grid. This option is known as net-metering.

Grid Connected systems

Net metering enables photovoltaic systems to use their own generation to offset their consumption over a billing period by allowing their electric meters to turn backwards when they generate electricity in excess of the their demand. This offset means that customers receive retail prices for the excess electricity they generate. Without net metering, a second meter is usually installed to measure the electricity that flows back to the provider, with the provider purchasing the power at a rate much lower than the retail rate. Net metering is a low-cost, easily administered method of encouraging customer investment in renewable energy technologies. It increases the value of the electricity produced by renewable generation and allows customers to "store" their energy and use it a different time than it is produced giving customers more flexibility and allowing them to maximize the value of their production.

Stand Alone Systems (Off grid)

Where the electricity grid is largely confined to the main urban areas, and where a substantial proportion of the rural population does not have access to most basic energy services, PV is widely regarded today as the best - and least expensive - means of providing many of the services that are lacking. The main feature of the off grid PV system is the presence of the power storage system which is represented from batteries.

PV off grid systems can be used for:

  • Pumping systems: to supply water to villages, for land irrigation or livestock watering
  • Refrigeration systems: particularly to preserve vaccines, blood and other consumables vital to healthcare programs.
  • Lighting: for homes are community buildings such as schools and health centres to enable education and income generation activities to continue after dark.
  • Battery charging stations: to recharge batteries, which are used to power appliances ranging from torches and radios to televisions and lights
  • Solar home systems: to provide power for domestic lighting and other DC appliances such as TVs, radios, sewing machines, etc.

Benefits and Costs

Like any technology, photovoltaic technology has its benefits and drawbacks. A PV power system has the lowest environmental impact of any electricity generating technology and its benefits range includes:

  • minimal or no impact because photovoltaic systems can be mounted on roofs and other scattered sites
  • no fuel requirement and no emission production
  • easy to install
  • cheap maintaining cost

Costs

The cost of solar power is currently the primary barrier to its use. Still, costs can appear prohibitive for many electricity users, particularly compared to utility power. Markets for photovoltaics have grown, but will depend on further cost reduction to increase demand for photovoltaic products. In same cases the PV systems are the most convenient alternative to have the electricity: for instance, when extension of utility lines is prohibitive, photovoltaics are actually the most economical option in remote areas because they are cost less to install than a new power line.

Usually the cost of a completely installed photovoltaic system is measured on the base of the peak Kilowatt. Average cost of an installed KWp fluctuates between 4000 and 7000 euro, depending especially from the system size, the quality of the PV modules and installation conditions.

As it is possible to see in the picture, the main part (more than 50%) of the investment is due to the modules cost.

Integration in buildings

The best strategy when considering a PV system is to integrate it into the initial design of the building. This can be done in several ways. For example, the system may be integrated into the roof of a building, to become part of the roof's structure as well as providing electricity. Alternatively, PV cells can be laminated into glass being used in the building, such as in the windows of a building or into a day-lighting concept, to provide electricity in combination with shade and sunlight. PV systems can also be easily retrofitted onto existing buildings, on rooftops, on added awnings, on solariums, or on nearby land, if it is available.

Examples of unique applications that have recently been gaining popularity are included below.



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Whether installing a PV system onto a building or incorporating one into an original design, there are several options on what kind of systems could be utilized: systems that track the sun on a single or two axes, or that are installed in a fixed position. For most buildings, the fixed position option has been preferred, especially for those in which the system has been incorporated into the building design.

( document prepared by ISNOVA )