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What is photovoltaics?

Photovoltaics (PV) refers to the direct conversion of sunlight into electrical energy. This is done by means of the photovoltaic effect: When light hits an electrically conducting or semi-conducting solid body, the latter releases an electric charge. Solar cells, which are mostly produced from the semiconductor material silicon, assist in this process

How is photovoltaics applied?

Photovoltaics was first used in the aerospace industry starting in the 1950s. Over the years, the efficiency of solar cells increased to such a degree that they could be produced at a considerably lower cost and on a larger scale. The range of applications grew along with this development.

The German government initiated the ‘1000 roofs programme’ in 1990, launching the promotion of installing PV systems on the roofs of detached and semi-detached houses. The Renewable Energies Act (EEG), established in 2000, promotes the feeding of energy into the grid. Since then, PV systems for roofs and open spaces have become the main area of application for photovoltaics.

How is solar energy generated?

The electromagnetic rays of the sun are converted into electricity through solar cells built into the solar panels. These use various semiconductor materials that generate free charge carriers when exposed to light (photovoltaic effect). The energy contained in the light hits electrons (negative charge carriers) in the cell and breaks their bonds, allowing them to move freely.

But in order to generate voltage, positive and negative charges must be separated from each other. To achieve this, foreign atoms are incorporated into two layers of the semiconductor (doping). The back of a cell is p-doped, resulting in a lack of electrons. The front, on the other hand, is n-doped, causing an excess of electrons. A depletion region (p-n junction) is created at the point where the p- and n-doped layers meet, containing an electric field that is created through physical processes. This field exerts a force on charged particles.

When light hits the cell, electrons are released and swept to the front of the cell by the electric field. The spaces in the atomic lattice where electrons were previously located are referred to as holes. These positively charged empty spaces are indirectly moved to the back of the cell by the electric field. This is how the charge separation takes place, and the resulting voltage can be taken from the front and back contacts and connected to a load (e.g., energy-saving light bulb).

What are the different kinds of solar panels?

There are monocrystalline, polycrystalline, and thin-film panels. The three types differ in terms of their appearance and method of production as well as their efficiency.

Monocrystalline panels are made of a single silicon crystal, as their name suggests. These are the traditional solar panels. Their efficiency ranges from 15 to 20 per cent, sometimes even higher. In other words: They can convert between 15 and 20 per cent of the sun’s radiation into electricity.

Polycrystalline panels consist of a combination of many smaller crystals. They are not quite as efficient as monocrystalline panels but are considerably less expensive to produce.

The so-called thin-film panels are the least efficient. Due to their extremely thin layer of silicon, they require a larger surface to achieve a certain output. Their cost advantage is so low compared to the latest generation of crystalline panels that they are hardly worth the investment.

Why do I need an inverter?

The solar panels produce direct current electricity. However, the German electricity grid is designed for 230 V/50 Hz alternating current electricity. This is why an inverter is needed to transform the direct current electricity generated in the solar panels into grid-compatible alternating current electricity. The inverter is chosen according to the voltage and current of the generator. Modern, well-designed inverters can convert almost all of the direct current electricity into alternating current electricity. They are also able to operate differently equipped generators, monitor them continuously and provide the data via an online portal. The inverter is essential to the overall performance of the system and should be fine-tuned to the other components. If not, a significant amount of the generated power can be lost during the conversion into alternating current electricity.

What are island systems?

In addition to solar systems on roofs and open spaces, there are ‘island systems’ (also known as ‘stand-alone systems’) which are used for the self-sufficient provision of electricity in areas where there is no connection to a public electricity grid – particularly in Third World countries. This allows remote settlements that are not connected to the electricity grid to receive electricity. Schools and hospitals are at the top of the list.

In industrialised countries, ‘island systems’ are mostly used for small, self-supporting systems. For example, garden lighting or parking ticket machines can run on solar electricity. All island systems are equipped with a battery for the convenient storage of energy. Sailors and campers are also turning more and more to solar energy for their electricity needs while travelling. 

What is solar thermal energy?

Solar thermal energy is often confused with photovoltaics. Unlike photovoltaics, solar thermal energy uses the sun’s rays to provide heat rather than electricity. In individual homes, solar thermal energy can be used to provide heating or hot water. To do this, devices known as solar thermal collectors are attached to the roof. Their black colour causes the fluid inside them to heat up (usually a mixture of water and propylene glycol). A pump sends the fluid to a heat exchanger, which transfers the heat to the water supply. However, photovoltaic systems like Centrosolar’s innovative Cenpac plus solution can now heat water more cost-effectively than solar thermal energy.

Is there enough sunshine in Germany to run a PV system?

Germany receives an annual average of 1,100 kWh of sunlight per square metre, which is enough to economically operate installations as small as 10 square metres. While the sun is stronger in the south of Germany, the performance of systems in the north is increased by the longer days in summer and higher levels of wind that allow the panels to cool more efficiently.

The sun does not always have to be shining for the systems to work. In overcast conditions, modern solar panels can also convert the hazy light into electricity. It is therefore worthwhile to use a photovoltaic system anywhere in Germany.

Is less electricity produced during winter?

No doubt, the most energy can be produced during summer, due to the longer days and a greater amount of sunlight. However, solar energy is produced throughout the whole year, also in winter. The efficiency of the photovoltaic system even increases in winter because the panels work better at lower temperatures. 

Is my roof suitable for a photovoltaic system?

If you have ten square metres of connected roof area, it is already worthwhile investing in a photovoltaic system. The orientation and pitch of the roof are important factors to consider. As a general rule in Germany: The roof should face between south-east and south-west, and its pitch should be at a 20- to 50-degree angle. 

What is the Renewable Energies Act (EEG)?

The German Renewable Energies Act (EEG) requires power companies to accept the electricity fed into the public electricity grid and to pay for it at a specific rate (status 06/2012: 19.11 €ct/kWh). When a PV system is commissioned, the remuneration rate in force at that time is applied for the current year as well as the next 20 years.

What is feed-in, and what is own use?

Operators of PV systems have been allowed to feed up to 100 per cent of their solar electricity into the public electricity grid since 2009. The remuneration rate is guaranteed for 20 years from initial operation. At the end of this period, the remuneration is determined according to the electricity market prices. But system owners can also use the solar electricity that they produce themselves and significantly lower their electricity costs. This also sets them free from rising electricity prices and lessens the burden on electricity grids.

Is it worthwhile to install a PV system on one’s own roof?

The state offers attractive incentives (for example, the CO₂ reduction programme and the KfW Environmental Protection Programme). These can make solar energy quite profitable. The net investment (excluding VAT) is entirely supported, you can save taxes by deducting the costs, and the Renewable Energies Act (EEG) guarantees you a feed-in remuneration for a period of 20 years.

What makes a PV system even more attractive is the own use of the solar electricity that is produced. This sets you free from rising electricity prices and allows you to save more and more electricity costs every year.

Is it worth it to produce solar electricity for self-consumption?

It definitely is. The more solar electricity you use, the more electricity costs you save since you will need to buy less electricity from your energy provider. And because electricity prices will probably continue to increase in the future, your savings will increase every year.

You can increase your self-consumption by running certain household appliances during specific periods when the sun is shining. There are now many advanced solutions available on the market that allow you to control the consumption of your appliances and operate them in synch with the sun’s activity.

By implementing intelligent storage solutions such as Cenpac plus, self-consumption can easily be increased to over 50%. 

What are the current feed-in tariffs in Germany?

On April 1, 2012, the feed-in tariffs according to the German Renewable Energy Act have changed. The following table shows the current remuneration rates.

Start of operation	Installed PV Output
	up to 10 kW	up to 1,000 kW	up to 10 MW
from 01-03-2013	15,93 ct	13,48 ct	11,03 ct
from 01-04-2013	15,48 ct	13,10 ct	10,72 ct
from 01-05-2013	15,05 ct	12,73 ct	10,42 ct
from 01-06-2013	14,63 ct	12,38 ct	10,13 ct
from 01-07-2013	14,22 ct	12,03 ct	9,84 ct

The Renewable Energies Act (EEG) guarantees the feed-in tariff for 20 years.

How long is the payback time for a PV system?

The production and performance of today’s solar installations have become so efficient that they can already pay for themselves within a one- (thin-film panels) to three-year (crystalline panels) period. After this time, you will have generated more solar electricity than was consumed in the production of the installation. Within an operational period of 25 years, an installation produces a total of around eight times as much energy as it has used. Although the energy yield does diminish slightly over the years, a modern, high-quality solar installation can deliver solid yields for up to 35 years.