Let’s start with the bad news: According to the current state of the art, all PV systems widely available on the market operate with relatively low efficiency. And now the good news: efficiency does not equal performance. In other words, as long as our solar system performs as it should, i.e., when operating optimally, it lowers our electricity bill and covers our total annual electricity consumption, the efficiency levels at which it does so are negligible.
Efficiency of the PV system: absorbed and emitted energy
Of course, there is a correlation between power and efficiency, but they are by no means synonymous and the low efficiency of a PV system should not be a cause for concern at all.
PV systems convert sunlight into electricity. Solar cell efficiency essentially expresses the ratio of the amount of light energy that arrives at the surface of the solar cells and the amount of electricity generated from it. Let’s take a look at what this all means in practice, using the most common types of PV systems.
Monocrystalline solar cell
The monocrystalline solar cell consists of one silicon block and therefore has a very high efficiency: On average, it can be around 21%. However, due to the larger silicon block, the manufacturing costs and thus the price are relatively high.
Polycrystalline solar cell
The polycrystalline solar cell consists not of one large silicon block, but of several smaller ones. Its efficiency is lower, although only slightly, and is about 20%. The production is cheaper, which is also reflected in the price.
Thin film solar cell
The thin-film solar cell is also based on silicon, but is manufactured by an evaporation process in which only a thin layer of it is applied to the surface of the structure. This makes its production not only cheaper, but also much faster than its monocrystalline and polycrystalline counterparts. Unfortunately, it shows a significant drop in efficiency: only 6-8%.
It can be seen that the differences are sometimes smaller, sometimes larger for some types. In the long term, these can lead to significant yield losses. At the same time, the issue should also be studied from the point of view of cost implications in order to get a realistic picture of total costs and payback.
The problem of measurement
Before photovoltaic manufacturers launch their solar modules on the market, they carry out a so-called “final measurement” in which they illuminate the solar modules in a measurement chamber with artificial light of a specific spectrum and then measure the level of the output current and the value of the DC voltage. Since the spectrum of the artificial light source cannot completely match the spectrum of the solar radiation, the result obtained in this way is not exact, but only approximate values.
Cell efficiency = PV system efficiency?
The cells inside the solar panels do not work with the same efficiency, they are classified according to different aspects during production, and then a kind of mixed set is formed. The average efficiency of the cells cannot fully correspond to the efficiency of the whole panel. This value can not be determined accurately or reliably in this way.
Alignment and inclination
The angle of inclination between 30 and 40 degrees is the most favorable for the system, as it ensures the optimal angle of incidence of 90 degrees. It is also possible to install the panels on a flat roof, in which case the same conditions apply as for installation on the ground, with the difference that the solar panels are distributed at a more favorable height.
The longer the panels are exposed to the sun, the more energy they can produce. A southerly orientation is most optimal; a southeasterly and southwesterly orientation can expect a loss of about 5 percent, while a westerly orientation can expect a loss of 10-15 percent. None of these have a significant impact on the payback of the investment. We pose a real problem only when the roof is oriented to the north. In this case it is not recommended to install the PV system on the roof. At the same time, there is no such orientation that would be a reason for exclusion, at most the installation conditions change accordingly. Finally, a PV system can be placed on a garage or other outbuilding, on the roof of a covered terrace or even on the garden ground.
Ideal or optimal?
In terms of conditions, the ideal state for PV systems is like the perfect happiness of man: It exists almost only for a moment. For example, we might think that the higher the heat, the better the efficiency: on a hot day, we can produce more electricity with our private mini solar power plant. However, this is not really the case. Most solar panels work most efficiently at 25 degrees Celsius. Therefore, manufacturers measure solar cells with 1000 watts of rectangular irradiation at 25 degrees Celsius, which is optimal in all respects, an environment that very rarely occurs in reality or only for short periods of time.
There are types of products that produce relatively high power even in cloudy weather, while in others productivity drops more drastically. So when it comes to efficiency, you can’t think in terms of black or white. However, it is certain that certain environmental factors have at most a slightly different impact on the operation of the overall system. The efficiency of PV systems is constantly changing due to various environmental influences.
Under the given circumstances
The use of solar energy is also affected by humidity, cloud cover, smoke, smog and temperature values, the geography of the area and the angle of incidence of sunlight.
Of course, the efficiency of PV systems is much higher in summer than in winter, but there are also differences depending on the landscape. For example, in the lowlands, there are more hours of sunshine in summer than in the mountains due to cloud cover. In winter, on the other hand, the opposite is true: due to the fog that falls on the flat terrain, the sun’s energy is better utilized in the mountains. Smog can also become a problem from time to time around larger cities. Regardless, it is important to point out that PV systems produce energy everywhere and at all times of the year, just to varying degrees. Of course, it would be possible to create a system that is incapable of doing this, but it loses its raison d’être on the design table even before it is implemented.
Despite the fact that the layer of snow blocking the light path slides off more easily from clean, dust and dirt-free panels, and efficiency is also better, you do not have to worry about the separate cleaning. On the one hand, the rain always solves this problem for us, on the other hand, cleaning is strongly discouraged, because the panels can be easily damaged. We can also improve our electricity production with a rotating support structure. However, careful consideration is absolutely necessary here, as these devices significantly increase investment costs and their maintenance also requires a greater financial outlay compared to almost maintenance-free fixed support structures.
Smart System: smart solution?
One of the weak points of PV systems is that the overall system is tuned to the module with the lowest power. If a shadow falls on a module or leaves remain permanently attached to the surface after a strong storm, the efficiency of the overall system decreases. Smart systems can improve the efficiency of the PV system. They optimize the system and coordinate the operation of solar modules with different tilt angles and locations. In addition, they also indicate possible faults.
An installed smart system can lead to an increase in performance of 5-25%, depending on individual conditions.
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