Efficiency of solar cells

How effectively an entire photovoltaic system works essentially depends on the Efficiency of the solar cellsthe Cabling and the performance of the inverter from. All values are summarized in the so-called Performance Ratio.

This is a key figure used in the photovoltaic industry to evaluate the efficiency of a solar system. It is the ratio between the energy actually generated of the plant and the theoretically possible energythat can be generated under optimum conditions. This target yield is calculated from the irradiated energy on the module surface and the nominal module efficiency.

The annual average performance ratio would therefore be 100 %if the photovoltaic system is under Standard test conditions with Optimum cabling and with a Inverter without losses the amount of energy that the sum of all solar cells could theoretically generate. The energy losses through cables and inverters are currently around 5 %. This puts the focus on the current efficiency of solar cells. A lot has happened here in recent years. Through More efficient technology the performance ratio of photovoltaic systems has increased considerably. Whereas in the 1980s it was 50 or 60 %, today it is on average at 80 or even 85 %.  

How do solar cells work?

A solar cell converts light energy into electrical energy and is usually made of the semiconductor material silicon. When Photons hit the solar cell, they release electrons and thus generate electricity. Solar cells consist of several layerswhich are arranged in such a way that they can capture and effectively utilize the incoming light. The layers are typically called n- and p-layers is designated. The n-layer is a Semiconductor materialwhich contains excess electrons, while the p-layer Electron holes contains. Conventional solar cells absorb sunlight up to a Wavelength of 1200 nanometers and transfer the energy to the electrons, which then flow from the n-layer to the p-layer.

Commercial solar cell types and efficiencies

For commercial photovoltaic systems, the following are usually used today monocrystalline silicon cells that are used in series production Efficiency of 25 % can be achieved. Under laboratory conditions in 2022, research centers have already achieved almost 33 % with so-called tandem solar cells or even over 45 % with quadruple-junction solar cells. This is made possible by using additional layers that are sensitive in a spectral range of 300-1780 nanometers. Thanks to this new technology, the value of 33 %, which has been recognized as the upper limit for many years, has been exceeded for the first time.

Polycrystalline silicon cells are less expensive in industrial production and today have a Efficiency of up to 20 %. They consist of several crystallites that are aligned differently, have an irregular bluish structure and are generally somewhat smaller than monocrystalline solar cells. Polycrystalline silicon cells still generate electricity when there is less sunlight and are therefore also suitable for locations with slightly poorer light conditions.

Amorphous silicon cells or thin-film modules consist of a thin layer of amorphous silicon (a-Si) as the active semiconductor material and today have a Efficiency between 10 and 13 %. In contrast to crystalline silicon, which consists of regularly arranged atoms, amorphous silicon is disordered and has a higher defect density. This means that it is less efficient at transporting electrons through the semiconductor. They have the advantage that they are thinner and lighter than crystalline silicon solar cells and that they are more flexible. This means they can be used in flexible solar modules that can bend and adapt to unusual shapes.

Influences that impair the efficiency of solar cells

First of all, the Degradation which affects the efficiency of solar cells. For modern crystalline solar modules, a degradation of 10 % is specified after around 20 years, meaning that they are guaranteed to achieve a remaining output of 90 %. In contrast, thin-film modules can lose up to 30 % of their output in the same period due to degradation.

Another key factor for the efficiency is the Temperature. In general, the colder it is, the better the yield. Solar modules heat up easily even on cold German winter days when the sun is shining and reach temperatures of 20° or more. Despite the low position of the sun, they are then often more effective than a comparable system near the equator. The efficiency of monocrystalline solar cells, for example, decreases by 0.4 % per degree Celsius. On hot summer days, when a panel can quickly heat up to 70°, losses of up to 20 % can be recorded.

Logically, the Alignment of the solar cells also has a major influence on efficiency. However, the solar modules can usually be perfectly adjusted to the solar radiation in rooftop PV systems by elevating them and in solar parks or solar carports by optimally mounting them. This so-called azimuth calculation takes place in the run-up to planning a PV system, during which the total electricity yield is also determined. Possible shading factors are also taken into account.

However, the influence of Soiling on the efficiency of solar cells. At an angle of inclination of 30-35°, which is usual in Germany, losses of 2-3 % are to be expected, depending on interfering influences such as leaves, needles, dust, soot or bird droppings. For flatter systems that do not clean themselves through rain or snow, the efficiency can quickly fall below 90 % of the nominal output if the solar cells are not cleaned directly.

Area efficiency in relation to efficiencies and solar cell types

The size of a photovoltaic system should always be based on a company's consumption. The more electricity is consumed, the more economical it is. The higher the electricity consumption, the more sensible it is to install more solar modules. To calculate the optimum number, the consumption is set in relation to the available area. The nominal output of a PV system in kWp (Kilowatt peak) or MWp (megawatt peak). If the efficiencies of the solar cell types are taken as a basis, the following area requirements result:

Monocrystalline silicon cells per kWp approx. 5 m²

Polycrystalline silicon cells per kWp approx. 6 m²

Amorphous silicon cells per kWp approx. 16 m²

However, these values only provide a rough guide, as different requirements apply to every PV project. CUBE CONCEPTS, for example, mostly installs monocrystalline modules on roof surfaces measuring 100×180 cm. These currently have a maximum rated output of over 420 W. However, the choice of modules naturally depends on the type of system planned, because with Photovoltaics on roof surfaces the respective Roof load reserves so that other module types can also be used if necessary.

Has the efficiency of solar cells increased so much that repowering makes sense?

There is no clear answer to this question and it should at least be answered for PV systems that are older than ten years are always recalculated once. Repowering of PV systems can be useful when older solar cells are replaced by aging or damage their performance or a roof renovation is due anyway. The National Renewable Energy Laboratory (NREL) in the USA has published an interactive study on the efficiency of solar cells. Graphic which provides an overview of the efficiency development of the various solar cell types from 1975 to the present day. If the performance ratio of your photovoltaic system is between 70 or 75 %, it certainly makes sense to contact the energy experts at CUBE CONCEPTS and think about replacing photovoltaic modules or inverters.