How is a solar cell constructed?

Solar cells usually are thin silicon wafers. By doping the pure silicon with alien atoms we obtain one negative and one positive conductive layer. The transition zone between the two layers is charged with an electric field. The top side of the solar cell is equipped with finger-shaped front contacts, the bottom contact, however, is a continuous metal surface.

How does a solar cell turn sunlight into power?

If light is falling on the solar cell, it releases free electrons and holes (so-called p-holes). Electrons and p-holes are getting separated by the electric field. The electrons are conducted towards the negative pole, the p-holes towards the positive pole. If you connect the contacts to an electric consumer, p.e. a light-bulb, the electrons will return to the positive pole through the outer circuit – an electric direct current is flowing.

How does the conversion of solar power into electric power work?

For the most part, solar cells consist of semiconductors. Semiconductors react on input of power by releasing free carriers. These carriers are called electrons (negative carriers) and p-holes (positive carriers). To generate power from these charges, you need an electric field. This may p. e. develop at a p-n transition. The p-n transition is a region where two different dopings meet each other. The transition zone should be located as close to the surface as possible, and it should reach far down into the material. You create the transition zone by purposefully doping the material. The surface layer is strongly n-doped, the layer below is weakly p-doped. If photons are entering this zone, they will set free pairs of electrons and p-holes. The p-holes are accelerated towards the p-material, the electrons in turn towards the n-material.

What is the meaning of Wpeak/KWp/Wp/peak?

kWp (kilowatt peak) is the unit of the peak capacity (= nominal capacity) that a photovoltaic module may achieve under standard test conditions (STC). The STC being present only in very rare cases, the nominal peak capacity of the photovoltaic module is indeed very rarely achieved.

What is the meaning of STC?

STC = standard test conditions. You need standardised test conditions in order to compare the capacity of various solar modules: irradiation 1000 W/m²; temperature 25 degrees centigrade, and AM 1.5 (AM = air mass, describing the thickness of the atmospheric layer which in-falling light has to pass. At the equator, air mass is AM = 1. For Europe, it is about 1.5). The sensitivity of the solar cell is dependent on the spectral composition of the light.

What is the difference between Watt (W) and Watt hours (Wh)?

Watt is the unit of electric power, defined for a time-span of 1 second. If 1 Watt of electric power is generated or consumed in the course of 1 hour, the result of multiplying power (1 W) and time (1 h) is the energy, expressed in Watt hours (1 Wh). In order to avoid dealing with huge numbers at higher powers, 1,000 Watt are called 1 kilowatt (1 kW). 1,000 Watt hours correspond to 1 kilowatt hour (1 kWh) of energy. Example: If a photovoltaic plant has a constant power of 2 kW for a time of 5 hours, it produces 10 kWh of energy.

How much surface to I need per KWp?

That depends on the effectiveness of the module. On a pitched roof you may require 8 to 12 m2. On a flat roof, twice as much surface may be needed, according to the type of module used.

Where should the inverter be installed?

For highest effectiveness of the plant, the inverter should be installed at a place as cool as possible, between the solar generator and the meters. Heat is hazardous to the inverter.

How is the power supplied into the public grid?

The meter facility is expanded with one additional meter. If there still is an empty space in the meter facility, it may be used in agreement with the power provider. Otherwise another small meter cabinet (H = 950 x W = 300 mm) may be installed.

How many phases are supplied to the grid?

A grid feeder/inverter can supply one phase. Up to 5 KWp (4.6 KVA) of power, there is a one-phase feed.
Above 5 KWp, there is a multi-phase feed. Total imbalance may not exceed 5 KWp.

How does automatic separation from the grid take place if the provider disrupts service?

An automatic device is integrated into the inverter. This so-called ENS system disrupts the feed immediately if the power-grid is disrupted.

The inverter has less power than my solar generator. How that?

The power of the solar generator is dependent on the irradiation. But with the major part of the gain being already achieved at low power, it is useless to try arranging the plant for the rarely achieved peak power. A rule-of-thumb is: The input power of the inverter should correspond to at least 85-90% of the generator's peak power (Wp).

What are mono-crystalline modules?

Mono-crystalline cells currently achieve the highest effectiveness among all photovoltaic modules (up to 18 %, in lab conditions already up to 25 %) and a life expectancy of at least 30 years. As the name may tell, the cell consists of one single crystal. Its high effectiveness is the result of the exactly identical alignment of the crystalline structure. This means, however, that the production is very demanding, and so these cells are by far the most expensive ones.

What are poly-crystalline modules?

Poly-crystalline or multi-crystalline modules offer good price/performance ratio. Meaning that the price per KWp is comfortably low. That is because of the cheaper production mechanisms. Silicon of semiconductive purity is melted under protective glass in a low pressure area and then poured into ingot moulds where under steady heat supply it solidifies into aligned structures. The multi-crystalline blocks are with very fine saws cut into columns of a squared cell base. ID saw-blades cut these columns into multi-crystalline silicon wafers of 0.45 mm thickness. Then they get cleaned.

What are amorphous/micro-amorphous modules (thin-film)?

In the plasma phase, amorphous or micro-amorphous modules are vapour-deposited on float glass. They are also made of silicon. In comparison to standard silicon modules, you need for the production phase only 1 % of raw silicon. Therefore these modules are less expensive, but their effectiveness is much lower in comparison to conventionally produced modules. Their most important advantage is that amorphous/micro-amorphous modules are much more reactive. Meaning that in diffuse lighting (dusk or dawn phase, or cloudy sky) the gain in power is already considerable.

What is Matching?

Modules can only be produced within a tolerance limit of +/- 5%. Matching means the proper sorting of modules, according to their actual power. Making use of the serial numbers and the application to power, these modules may be precisely distributed to the wiring. This increases the overall gain of the plant.

What is power tolerance?

The highest deviation from the nominal power. Producers of modules indicate this as being 2.5 %; 5%, or 10%.

What is guaranteed power?

It indicates the highest tolerable deviation from the nominal power within 10, 20, or 25 years of operation.

What is module effectiveness?

The ratio of emitted power to irradiated power within a solar module, by surface.

What is operating voltage/MPP voltage?

It is that voltage which yield the highest power under STC. A module's maximum power point is called MPP.

What do IEC 61215 and IEC 61646 mean?

If a solar module was certificated as conforming to IEC 61215 (crystalline) or IEC 61646 (thin-film), it has a mark of quality with regard to mechanical resistance and compliance to the electric parameters.

How does a solar cell operate?

Solar cells make use of the photo-electric effect in order to convert solar radiation into electric power. They are made of semiconductive processing materials in which positive and negative carriers are separated under the influence of solar radiation. An exhaustive and popular description of how solar cells operate is available in a voluminous article.

What is the performance ratio?

Performance ratio is the ratio between real gain and nominal gain of a photovoltaic plant. It tells you how much of the generated direct current is actually available. In grid-connected plants, this is about 70 to 75%.

How large does the roof have to be?

The larger the better. The more modules may be implemented on the roof, the higher is the gain and, hence, your profit. In order to get a rough estimate of how many modules may fit on your roof, you can use our online photovoltaic module plan. This tool is currently still in beta state, meaning not all functions are yet implemented or tested. We will optimise this tool as quickly as possible and expand it with more functions.

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