The sun has provided the Earth with free and reliable energy for over 4 billion years. Approximately a two billionth part of the total energy radiated by the sun reaches the Earth’s surface. This has been sufficient to enable life on Earth for millions of years. In the south, in Bavaria, the solar radiation is approximately 1,100 kWh per m2, while in the far north, on the North Sea coast, it is approximately 1,020 kWh per m2 per year.
This corresponds to energy radiation of approximately 70,000 kWh on the roof surface facing the sun in the case of a normal single‐family home (this corresponds to approximately 6,600 litres of fuel oil). Each household therefore receives a considerable amount of energy that is free and supplied to the home in a completely environmentally‐friendly way. Solar power is unbeatable when the availability of resources is considered.
On a global scale, oil will be available to us for another 40 years, natural gas for another 50 years and coal or lignite for another 170 years. Sunlight, however, is infinite.
Photovoltaics is the direct conversion of solar energy into electrical energy. For technical use solar modules are installed on the house roof or on greenfield sites by means of a substructure and are linked to each other electrically.
To be able to use the direct current then generated in the home, using a converter the current is converted to 'normal' alternating current as it comes out of the socket. Since the coming into force of the Renewable Energy Law (EEG) on 1st April 2000 practically all solar power is fed into the national grid.
The current dominant technology is crystalline technology (crystalline silicon. The ultrapure silicon produced in blocks is cut into razor‐thin discs, so‐called wafers. In further process steps the properties of these discs are then adapted to the requirements of photovoltaics through targeted contaminants (doping). Anti‐reflective coatings are applied to the front in order to increase the absorption of light.
Since the voltages to be tapped from a single disc are too low for technical applications, the discs are serially interconnected with each other. To protect against mechanical and environmental influences, these wafer systems are then added with a transparent front cover and a back sheet to a module. This technology has been tested over decades, is fully developed and dominates the photovoltaic industry with a market share of approximately 80% (as of 2008).
In the more recent thin‐film technology no wafers derived from raw silicon are used. Instead, the functional layers are applied to a substrate – usually glass substrate – using a vacuum process. The processes are applied, as they are known in architectural glass coating or the production of flat screens. The principal design – front contact, absorber, back contact – is the same as that used in crystalline technology, but the material and energy used in production are significantly lower.
While wafers of approximately 250 μm are used in crystalline technology, the absorbers in thin‐ film technology are only a few micrometres thick. Thin‐film technology is on the rise. Its market share is growing rapidly and is currently approximately 20% (as of 2008; 2005: 5.8%; 2007: 14%).
The energy supply is decentralised, i.e. the energy is generated where it is needed. High overland losses are prevented. A photovoltaic system measuing 1 m2 continues to save approximately 100 kg of CO2 emissions per year.
Due to the extraordinarily good compensation for electricity fed into the national grid a photovoltaic system pays itself off depending on the size, position and orientation after about 13 to 16 years. Depending on the type of financing and the position of the photovoltaic system one can earn a maximum return of between 6% and 10% per year (before tax).
The size of a photovoltaic system is not capped at an upper limit. A module area of 6 to 8 m2 corresponds to a system output of 1 kWp (Kilowatt – peak = peak output), which generates between 800 kWh and over 1000 kWh per year depending on the orientation to the south and the incline of the module.
Anyone who would like to install a personal photovoltaic system on the roof of the home and generate his own electricity needs over the long term can calculate this easily. The average annual electricity consumption for a four‐member household is approximately 3,500 kWh. This electricity requirement can be covered using a 4.5 kWp system (approximately 30 to 35 m2). However, it should be noted in this respect that the electricity requirement is forecast as constant throughout the year, but the generation over the winter months is very low compared to that of the summer months.
In the case of crystalline modules one may assume a service life of approximately 30 to 40 years (source: Bavarian Ministry of the Environment) and as a rule the module producers provide a performance warranty of approximately 25 years at a maximum 80% of the peak output of the module. The roof rack is made of aluminium or stainless steel and has a very long service life just like the special cable of UV‐resistant materials used.
The inverter has a service life of over 10 years. Nevertheless, one should assume that the inverter becomes defective during the service life of the entire system. Reliable inverter manufacturers usually offer a five‐year warranty. Should the device become defective after this warranty period, one receives a replacement device at preferential rates.