Photovoltaic solar energy

PHOTOVOLTAIC SOLAR ENERGY

As solar power becomes increasingly popular, more and more solar panels can be seen on the roofs of homes and businesses alike. These solar panels employ one of the most environmentally friendly methods for producing electricity: “photovoltaic”. The term photovoltaic, or PV, is used to describe something that creates electricity when exposed to sunlight. Solar panels, or PV modules, are made up of several solar cells. Each cell is comprised of materials, which have photovoltaic properties.
Photovoltaic cells exploit the photoelectric effect to produce DC current by absorption of solar radiation. This effect allows the cells to directly convert light energy photons into electricity by means of a semiconductor material carrying electric charges.
This semiconductor material is typically made of silicon (a silicon atom has four peripheral electrons). A semiconductor cannot be classified as an isolating (more 4 peripheral electrons) or conductive (less 4 peripheral electrons) material and its electrical properties can be influenced by adding foreign substances (doping).
 
The doping of silicon is to add with other atoms in order to improve the conductivity of the material.

• One of the cell layers is doped with phosphorus atoms, which, themselves, have five peripheral electrons (one more than silicon). We use the term of N doped layer, as Negative, because the electrons (negative charge) are surplus.

• The other layer is doped with boron atoms that have three peripheral electrons (one less than silicon). We use the term of P doped layer, as Positive, due to the deficit of electrons created.

When the two layers are contacted, due to the difference in concentration, the excess electrons in the N area diffuse into the P area, thus creating an electrical field, the boundary layer or “space charge zone”, inside the semiconductor structure.

Functional principle of photovoltaic
(Source: Solarpraxis AG)

The Photovoltaic Effect:

The upper N doped layer in a solar cell is so thin that the photons from sunlight can penetrate it and can transfer their energy to an electron once they are in the space charge zone. The electron that is activated in this manner follows the internal electrical field, leave the space charge zone and reaches the metal contacts of the P doped layer. When an electrical load is connected, the power circuit is closed: the electrons flow across the electrical load to the solar cell’s rear contact and then back to the space charge zone.

From the Cells to the Module:

The sun radiates approximately 1000W per square meter, so a 10 x 10 cm solar cell is exposed to nearly 10 watts of radiated power. Depending on the quality of the cell, it can produce an electrical output of 1 - 1.5 watts. To increase the output, several cells are combined and connected to a PV module. The connection of several PV modules is also referred to as a PV array.

Manufacture of photovoltaic cells: traditional technologies:

Unconcentrated solar photovoltaic:
Crystalline silicon today represents about 85% of the market of materials used in the   manufacture of panels.

Mono-crystalline cell

• Mono-crystalline cells: 
• Efficiency 14-16%,
• Lifespan exceeding 25 years.
• The mono-crystalline photovoltaic cells are solar cells of the first generation. They are made from a block of crystalline silicon in one piece. They have a good performance but the production method is laborious and costly. This is the cell to the calculators and says "solar" watches.

 

 

 

 

 

Polycrystalline cell

 

 

•  Polycrystalline cells:
•  Efficiency 11-13%,
•  Lifespan exceeding 25 years.
•  Polycrystalline cells are made from a block of silicon composed of multiple crystals. They have a lower performance than mono-crystalline cells, but their cost of production is lower. Recent technological developments make it possible to produce polycrystalline thin-film cells in order to save silicon. These cells have a thickness of about a few micrometers. The maximum experimental yield obtained with polycrystalline cells is currently 12 to 20% for commercial applications and 24% in a research laboratory.

• Amorphous cells:

  

  Amorphous cell

• Efficiency 7-8%,
• Average lifespan: 10 years.
•  The amorphous silicon (a-Si) has been used as a photovoltaic solar cell material for devices which require very little power, such as pocket calculators, because their lower performance compared to traditional crystalline silicon (c-Si) solar cells is more than offset by their lower cost of deposition onto a substrate.

UNI-SOLAR flexible solar cells

The main advantage of a-Si in large scale production is not efficiency, but cost.
a-Si cells use approximately 1% of the silicon needed for typical c-Si cells, and the cost of the silicon is by far the largest factor in cell cost.
Recently improving amorphous silicon manufacturing techniques have made these a-SI solar cells more attractive in the case of large-area coverage.
Their intrinsic efficiency lower being linked, at least partially, to their thinness, higher efficiencies can be achieved by stacking several thin-film cells on top of each other, each one designed to work at a specific light wavelength (efficiency 13%)
This approach is not applicable to c-Si cells, which are thicker and are largely opaque, preventing light to reach the other layers in a stack.
UNI-SOLAR produces a version of flexible backings, used in roll-on roofing products.

Manufacture of photovoltaic cells: Promising technologies:

Concentrated solar cell

Concentrated solar photovoltaic:
The mirrors focus sunlight onto a small high efficiency photovoltaic cell. With this technology of concentration, the semiconductor materials may be replaced by less expensive optical systems. To equal power, it allows the use of 1000 times less photovoltaic material than in a conventional solar panel.
This technology is expected to enter the market in the near future.
The maximum theoretical yield of the photon-electron conversion is around 85%. The maximum experimental efficiency obtained with this technology is currently 40.7%. 

The organic components (polymers):

Organic solar cell

 Efficiency 8-10%,
Lifespan exceeding 20 years.
The use of polymeric materials is intended to replace inorganic materials by organic semiconductors, in other word plastic, for the manufacture of photovoltaic cells.
They are inexpensive, have good absorption properties and are easy to withdraw. Their very low cost is combined with particularly attractive features: lighter and less fragile, their flexible nature allows even consider flexible materials of organic polymers or silicones or even textile fibers and photovoltaic inks.
With a short life and a yield of below 5% in laboratory, they will need to be improved before being used industrially. This technology should enter the market in the medium term to 2015.

Hybrid cell

The hybrid cells: thermal and photovoltaic:
The efficiency of photovoltaic solar cells
decreases when the temperature of the panels rises.
Some research centres have had the idea of recovering the heat absorbed and released by the solar panel to optimize simultaneously the electrical performance and get a heating source.
They develop hybrid solar modules combining photovoltaic and heat. Early versions of this technology just reach the market.

Photovoltaic systems according to the grid:

How connect the solar module to the power grid, grid tied, off grid or the two possibilities (hybrid)?

Grid-tied Systems:
Grid-tied systems are configured so that the power they generate is fed directly into the utility grid. The electricity produced is not stored; instead it is delivered directly to the local electric company whenever the system is active. The electric company then uses “your” power to meet the general demand and depending on your state’s utility credit policies, you can receive a credit for the electricity that your system produces. Grid-tied systems are commonly found in homes and businesses that wish to offset the cost of their power usage. 

Equipment for Grid-tied Solar Systems:

• PV modules
• DC Disconnector
• Inverter
• Power Meter

  
Off-grid Systems:

As the name implies, off-grid systems are not connected to the public utility grid and are often referred to as “stand-alone” systems. During the day, the electricity generated is used either by powering loads or to charge storage batteries. At night, power is supplied by the energy stored in the batteries. When the sun comes up the next day, the cycle begins again.
Off-grid systems are typically found in remote homes, weather stations or radio sites and parks.
Off-grid systems can also be used to supply electricity to vacation homes that are not connected to the utility grid. In developing countries, off-grid systems frequently represent the only solution to supplying remote villages with electricity. The demand for these types of systems is extremely large. To date, approximately two billion people worldwide still live without electricity in regions far away from public utility grids.

Equipment for Off-Grid Solar Systems:

• PV modules
• Charge Controller
• Battery Bank
• DC Disconnector
• Inverter (Optional)
• Generator (Optional)

Hybrid Solar Systems:

They combine the best from grid-tied and off-grid solar systems. These systems can either be described as off-grid solar with utility backup power, or grid-tied solar with extra battery storage.
 
Advantages of Hybrid Solar Systems:
1. Less expensive than off-grid solar systems
Backup generator is not really necessary and the capacity of the battery bank can be downsized.
Off-peak electricity from the utility company is cheaper than diesel.
2. Smart solar holds a lot of promise
The introduction of hybrid solar systems has opened up for many interesting innovations. New inverters let home-owners take advantage of changes in the electricity rates throughout the day.
Your home and your electrical vehicle, if you have one, can be programmed to consume power during off-peak hours (or from your solar panels).
Consequently, you can temporarily store all excess electricity your solar panels in batteries, and put it on the utility grid when you are paid the most for every kWh.
The concept will become increasingly important as we transition towards the smart grid in the coming years.

Equipment for Hybrid Solar Systems:
Typical hybrid solar systems are based on the following additional components:

• PV modules
• Charge Controller
• Battery Bank
• DC Disconnector
• Inverter
• Power Meter

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