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Do Solar Panels Need Direct Sunlight?

Exploring solar panel efficiency depending on the radiation type

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Carlos Huerta
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This is a question that many users of photovoltaic energy often ask installers, especially if they do not have the necessary conditions for the installation of solar panels to receive direct sunlight.

The truth is that solar panels do not need direct sunlight to work, because solar radiation reaches the module’s surface in three different ways including: direct, diffused, and albedo. Thus, even when generation would substantially be lower without the direct sunlight component, a solar panel is still capable of generating electricity using diffused solar irradiance or albedo.

Direct solar radiation is associated with the maximum production efficiency. However, the panels will continue producing electricity in cloudy conditions and even in winter, thanks to the action of diffuse radiation and albedo, logically with a lower proportion of electricity generation.

In this article, we will explain how solar radiation influences solar panel’s operation, how photovoltaic performance is affected in different weather conditions, and how to improve the efficiency of solar panels when they are not in direct sunlight.

Solar Radiation and Operation of PV Panels

As already mentioned, there are three ways that solar radiation (see figure 1) reach the earth’s surface, which is capable of making photovoltaic panels generate electricity.

Direct Radiation

It is the one that comes in a straight line from the sun, so it hits the panel directly. It reaches the earth’s surface when the days are totally clear, which is usually during the summer. A panel is considered to reach its maximum power point when it is exposed to the greatest amount of light, which is usually on a clear day between 11 am and 2 pm.

Therefore, it is ideal to install the panels where they can receive direct sunlight so that the production of photovoltaic energy is the most optimal. So, the more radiation that falls on the semiconductor material of the cell, the more photon absorption there will be. Having more photons will produce more movement of the electrons in the cells and thus more current will be generated.

Diffuse Radiation

It is received through the atmosphere as a consequence of the dispersion sun’s radiation, in other words, diffuse sunlight comes from any direction. On sunny days, this energy can account for approximately 15% of global radiation, but on cloudy days, it reaches percentages greater than 60%.

When the modules are exposed to diffuse radiation, either because it was a cloudy day or because of the presence of a shadow, energy production can be reduced by more than half. In addition, it must be kept in mind that although production is affected, it is not null as people tend to think.

Therefore, it is important to consider the meteorological conditions of the area where the solar panels are to be installed when designing a photovoltaic system. This allows to assess how much energy production will be affected.

Albedo

It is the solar radiation received after it is reflected by a surface. The amount of radiation will depend on the coefficient of reflection from the soil, also called the coefficient of albedo.

The albedo that the Earth reflects is around 35% of solar radiation. However, the characteristics of the surfaces are what will determine the amount of albedo that reaches a module. Therefore, light-colored, bright, and smooth-textured surfaces have much higher albedos than dark-colored, matte, and rough surfaces.

In turn, the reflected radiation from the ground is the most used by bifacial panels, because they are designed to produce energy through their front and rear faces. So, the higher the reflected radiation, the more energy will be generated by the rear side of the module.

Solar radiation and its incidence on a solar panel.

Photovoltaic Performance Against Different Weather Conditions

Photovoltaic performance is directly proportional to energy production, which has to do with the solar panel’s ability to transform solar radiation into electrical energy.

Therefore, to obtain the maximum potential of a solar panel, it is necessary to consider the climatic conditions, which vary the performance of the modules. Now, we explain the main factors that affect solar electricity generation.

Temperature and Climate

The performance of the panels decreases at high temperatures (above 25ºC), it can be said that power production decreases by 1% for every 2ºC that the temperature increases after 25 ºC.

As for adverse climates such as rain or snow, it can be said that they have their pros and cons. The rain allows the cleaning of the panels but their performance is affected by the dense layer of clouds that accompanies this meteorological phenomena.

Meanwhile, as for snowfalls, it can be said that the accumulation of snow would block sunlight, which would reduce energy production. However, a very thin layer of snow in the ground can aid in system performance by allowing sunlight to be reflected effectively through the snow and increasing the amount of solar radiation by diffuse and albedo, especially for bifacial solar panels. This applies if the panels are not covered by snow.

Seasonality

The performance of solar panels varies depending on the season of the year. While in summer the solar panels receive a greater amount of sunlight and generate more energy, in winter the performance is reduced, due to the fewer hours of sunlight.

Although, the opposite occurs regarding the thermal losses in summer and winter. In the first case, summer higher temperatures will lead to higher thermal losses while during winter, thermal losses will be considerably lower due to cool temperatures. Even though, thermal losses are less favorable during summer, there is no doubt that the highest solar production will be achieved during this season. Therefore, it is recommended to size the system to maximize production during the summer in general terms. 

As for spring and autumn, energy production is average, due to the warm climate and variable temperatures typical of the seasons.

Improve the Efficiency of Solar Panels When Not Exposed to Direct Light

We know that solar panels work without direct sunlight, although their performance can be affected. Therefore, if we are facing this scenario, we must look for other options that allow us to increase the efficiency of the photovoltaic system. Some of the options we have are the following:

  • Keep the modules clean, free of dust particles, leaves, or other elements.
  • Use bifacial panels to take advantage of reflected radiation especially for ground mounts and flat roofs.
  • Install the modules in the most optimum orientation and tilt angle, in which the radiation reaching the surface is maximized.
  • If there are obstructions that generate shadows in the optimum orientation and cannot be removed, the panels can be placed in areas that are less efficient due to their orientation but where more direct light can be captured.
  • Avoid shadows since they greatly affect the performance of solar panels. For this, if possible, it is advisable to remove some obstacles such as trees or structures that obstruct direct light.
  • Improve the performance of the system by placing micro-inverters or DC optimizers. Unlike the string inverter, these module level electronics control the panels individually, so if only one fails, the production of that single panel will not affect the rest of the array.
  • Another alternative is to install a reverse tilt system. This applies to those cases in which the solar panels must face a non-optimum roof face like north, and do not receive direct sunlight. By applying a reverse tilt, panels can face south, increasing performance substantially and maybe even receive more direct sunlight.

Conclusion

While the panels perform better when in direct sunlight, they can still produce energy with the other solar radiation components. Bifacial panels are made to take full advantage of albedo radiation.

Likewise, shade, cloud cover, rain, or other atmospheric conditions influence the performance of the system. Therefore, it is important to include elements such as micro-inverters or optimizers so that the efficiency of the system is not affected especially when direct sunlight is not available in a roof face.

Frequently Asking Questions:

How do solar panels transform solar energy into electricity?

Silicon cells absorb photons from the sun and these displace the electrons in the molecular structure of the silicon, which generates the electric field capable of generating the electric current necessary to power a charge.

How much sunlight do solar panels need?

Generally, four hours of peak sunshine (HSP) a day is enough to make a system work. So, in terms of irradiation, 4 kWh/m2/day is needed to get a decent performance. The higher radiation levels, the better performance will be obtained of course.

Does shade affect photovoltaic solar panels?

Yes, solar panels are affected by shade, although its impact will be to a greater or lesser extent depending on how long the shade is available, the panel technology used, and the inverter configuration.

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Author Bio

Electrical Engineer with background in solar PV designs for residential and commercial projects as well as power systems development. Fan of renewable energy topics and projects. Technical writer for papers, articles and research in related topics to sustainability and especially solar power.

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