April 27, 2018 Solar Energy Written by Greentumble Editorial Team
why are solar cells inefficient
Solar energy development continues as the

market evolves into more profitable PV system solutions in the long and medium term. The trend shows an exponential growth that started with around 6 GW of installed capacity in 2006 and evolved to almost 300 GW in 2016 worldwide [1]. Such accelerated growth could not even be foreseen back in the old days by the best specialists.

Ten years ago, solar installation costs were high above the clouds, making it very hard for residential homeowners to obtain a solar PV system. Policy and incentive schemes were also not clearly established or profitable at all, representing high investment risks that worked against best environmental desires.

Among other reasons, one of the main limitations for market penetration was the low efficiency of solar systems (defined as the portion of sunlight energy that can be transformed into electricity). Most panels had efficiencies between 8 to 10 percent, while other traditional sources of energy had efficiency of 40 to 55 percent with the combined cycle generators [2]. The competition was just unbalanced.

Nowadays, solar installation costs are competitive with traditional energy sources. Policy and incentive schemes are solid and profitable in most high-income countries and efficiency values of solar systems have increased.

But they are still limited and the question is: why?
 
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Why is solar panel efficiency limited?

To understand the efficiency of a solar panel, you must first understand the source of energy of this technology – the sun. The sun emits energy in the form of light which contains photons that can be used by solar cells to transform sunlight (radiation) into electricity.

The problem with efficiency lies in the physical energy conversion.

In 1961, William Shockley and Hans Queisser developed a study that is considered the fundamental principle of the solar photovoltaic industry. The physical theory proved that there is a maximum possible efficiency of 33.7 percent which a photovoltaic cell (based on a p-n junction) can achieve to obtain electricity from a light source [2].

In other words, if there is a solar irradiance (incident power received by a surface per area) of 1,000 watts per square meter, then the maximum power output that the solar panel can produce is 337 watts per square meter.

This limitation is known as the Shockley-Queisser limit.

This parameter is associated with the physical process of absorption of a photon to generate an electron, and then pass it to the band of conduction [2]. There is no manufacturing process or technology development that can change this fact (at least not with a silicon p-n junction cells).

This limitation is the theoretical boundary of energy conversion. In practical terms, energy conversion can be modified by other factors that will be discussed below.
 

 

Standard Test Conditions

To establish the efficiency value, manufacturers submit solar panels to several controlled factors that can exist in real world applications. They do this testing to make sure that the design endures these conditions, while at the same time, they verify estimated efficiency (as it is visible in the electrical characteristics of the datasheet).

It is important to know that many manufacturers establish their efficiency based on Standard Test Conditions (STC). STC are set on maximum possible irradiance (1kW/m2), at temperature of 25 degrees Celsius (77 degrees Fahrenheit) and an air mass of 1.5 (related to the thickness of the atmosphere) [4].

It is rare to have the same conditions in the real life. Temperature and irradiance particularly can be very different from those values.

 

Nominal Operating Cell Temperature

That is why some manufacturers add the Nominal Operating Cell Temperature (NOCT) among their technical references. The NOCT is a parameter closer to real life situations and is defined as the temperature reached by an open circuit cell in a module under the following conditions [4].

    • Irradiance of cell surface: 800 watts per square meter

    • Wind velocity: 1 m/s

    • Ambient temperature: 20 degrees Celsius or 68 degrees Fahrenheit

    • Temperature on the surface: 45 degrees Celsius or 113 degrees Fahrenheit

    • Mounting System: Open back side (to consider air circulation behind the solar panel for refreshing purposes)

Now that you know how efficiency is specified by manufacturers, we can proceed with the factors that are taken into account for the design of solar panels to maintain acceptable efficiency values.
 
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What factors determine solar panel efficiency?

#1 Hail

The first factor is the presence of hail at a speed of 20-30 m/s, solar panels must remain undamaged.

Solar panels are designed to withstand even hailstorms.

Solar panels are designed to withstand even hailstorms, but their efficiency is affected by the bad weather.

#2 Snow, ice and dust

The second factor that must be taken into account is the snow. Thick layers of snow can directly block the sunlight to the solar panels, and therefore, reducing solar efficiency values to zero [6].

It is important to know that most solar panels can work with the presence of a three to four centimeter layer of snow (approx. 1.5 inches). If it is more than that, maintenance procedures must be put into motion to avoid further losses. Dust and dirt, are also contaminating elements that can influence efficiency values.

Ice is another element that can affect solar panels’ efficiency between 25 to 100 percent depending on the thickness of the layer. To avoid this problem, during the manufacturing process a silicon coating is applied, and it’s advisable to try to keep the ice off your panels during the winter season [6].
 

#3 Insulation resistance

Additionally, insulation can also affect efficiency because current leakages can occur on the edges of a solar panel.

This factor is especially important for utility-scale projects as higher voltage systems require better insulation properties (linked directly to the selected materials).
 

#4 Environmental conditions

Higher temperature means more heat, which is linked with electrical losses and voltage drops.

It is estimated that an increase per one unit of temperature above the standard test temperature of 25 degrees Celsius or 77 degrees Fahrenheit, the energy output decreases by 0.25 to 0.5 percent (depending on the type of module) [7].

If you think about it: an increase in temperature up to 60 degrees Celsius (140°F) could lead to a loss of 17.5 percent out of 100 percent efficiency of the solar panel.

Humidity is also undesirable to the solar panels because of corrosion.

Advanced level of corrosion eventually leads to insulation issues and decreases overall solar panel efficiency.
 

#5 Selection of solar panel type

The efficiency of monocrystalline panels varies between 22 to 27 percent.

Monocrystalline PV module

Monocrystalline PV module

Polycrystalline panels reach between 15 to 22 percent of efficiency and finally thin film panels between 7 to 13 percent.

Polycrystalline PV module

Polycrystalline PV module

The main difference in efficiency values of different panel types is the nature of the fabrication process.


 

#6 Design configuration of the solar panels

Among other factors associated with the operation of the solar panels, the selection of the orientation towards the sun and the presence of a solar tracking system has a great importance on the overall efficiency of your solar system.

Generally, your solar panels will have the best power output when the surface of the panel is perpendicular to solar rays. However, as the sun moves across the sky, the output will be reduced. The best way to ensure the direct irradiance is to install a tracking system. Its downside are higher installation costs.

Efficiency is also linked to the lifetime of the solar panel. It is estimated that every year that passes after the installation, there is a degradation of the efficiency value at 0.5 percent per year.

Nevertheless, manufacturers must ensure in the warranty policy that by the end of lifetime the solar panel output must remain above 80 percent compared to when it was installed.

Finally, shading is also an important part of the design in any solar system. If your system is shaded for the most sunny parts of the day, solar panel output will be affected severely.
 
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New solar technologies to improve efficiency

Despite low efficiency rates among current solar panels, there are several innovative proposals and technologies that aim to achieve higher efficiency values in the close future.
 

#1 Reducing the shading effects of wires

Among available proposals is the reduction of the shading effect on solar cells caused by bus bars.

Bus bars are located vertically and horizontally across the solar panel. They can be easily identified by any person as the silver wires forming a grid inside the solar panel. They transport the electricity generated by the solar cell.

Close up of bus bars

Close up of bus bars on a polycrystaline solar panel

These wires, even as tiny as they are, cause the reflection of light in the section where they pass by. At the end, this translates into less irradiance received by solar panels, and therefore generation of less power.

The idea is to reduce the thickness of these wires. Or even in the best case to eliminate their interference with the sunlight by placing them on the back of the panel. We would achieve then a uniform smooth colored panel [8].
 

#2 Innovative gallium arsenide triple-junction structure

Another interesting feature that has been set in motion is the development of the new generation of solar cells – from gallium arsenide.

Gallium arsenide cells have a triple-junction structure (different from silicon p-n junction) that can be chemically modified to acquire more light radiation than ordinary cells.

Among the remarkable advantages of these cells is:

  • Excellent ultraviolet, radiation and moisture resistance;
  • Great performance in the presence of low light;
  • Flexibility and low weight;
  • Efficiency values over 28.8 percent (!).

 

#3 Cadmium telluride thin film cells

As new compounds are used for the creation of solar cells, new methodologies are also evolving in manufacturing process to provide higher efficiency values in solar cells.

The market is ruled by silicon semiconductors included even in the design of thin film solar panels, but in the short to medium term, new photovoltaic materials like cadmium telluride are starting to gain their share on the market as promising materials for the development of thin film panels.

Close up of thin film solar cells

Close up of a thin film PV module

The obstacle for their large-scale market introduction arises during the manufacturing process.

The components seem to be very unstable because cadmium chloride has to be used in one step of the manufacture. However, a new manufacture approach could deliver feasible alternatives. The key is to replace cadmium chloride with magnesium chloride [9].

Unlike cadmium chloride, magnesium chloride is abundant and low cost resource that can be obtained from the seawater. This material can also boost efficiency of the thin film panels up to 15 percent.
 

#4 The Perovskite wonder material

The ultimate and most promising technology for improvement of efficiency is the perovskite component.

Perovskite, a compound of calcium, titanium and oxygen, offers the possibility to achieve efficiency levels above the current maximum level of 22 percent at lower manufacturing costs.

The reason lies in the low costs of raw materials and fabrication methods (printing techniques) that do not require high temperatures and such a high precision as the silicon cells do.
 
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How to improve the efficiency of solar cells

When you would like to improve efficiency of your solar cells, you must refer to the factors discussed above. As you can see, there are some factors that cannot be influenced (environmental conditions, hail, etc.), but some other can be controlled or selected to obtain the best possible efficiency.

The key to success is trying to tackle the factors that can be controlled.

For example: solar panel maintenance helps eliminate factors like snow, dust, dirt and ice – all which are obstructions for the sunlight to reach the panel’s surface, and therefore reduce efficiency of solar cells.

By keeping your panels free of dirt, you can improve their efficiency.

Other factors such as the type of a panel, its orientation, and the minimum shade configuration are crucial parameters that will determine efficiency.

Orientation of solar panels is crucial for maximum efficiency.

Orientation of solar panels is crucial for maximum efficiency.

These parameters are generally selected by a solar designer (but that does not mean that you cannot intervene in the process, especially in the selection of the solar panel type) who must consider:

  • the irradiation value in your area;
  • average cloud density;
  • air pollution;
  • the best inclination for the solar panel depending on the location and the selected mounting system.

 
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Conclusion

We have discussed the limits for conversion of light into electricity for silicon p-n junction cells, the overall effect of several factors on the efficiency of solar panels, the new promising technologies or proposals to elevate such efficiency values and the options that you can implement to improve the efficiency conversion of your PV system.

Nevertheless, we haven’t discussed what exactly the use of solar panels with higher or lower efficiency values means for you.

Efficiency is associated with the ability of the solar cell to produce the maximum amount of electricity from a light energy source.

A single cell with low efficiency will produce less power output than another cell of the same size but with higher efficiency.

 

Does it mean that if my solar panels have low efficiency they will produce less power?

No, it doesn’t. If you select a solar panel with a power output of 200 W and an efficiency of 14 percent, and then choose another solar panel with the same power output but with an efficiency of 20 percent, both panels will produce the same 200 W output!
 
Where is the difference then?!

The difference lies in the size of the panel–lower efficiency panels require more space to produce the same power output. This means that a 14 percent efficient solar panel will be a little bit bigger.

So, the question that arises is…
 

Does solar panel efficiency matter?

The answer is: it depends. In some applications like solar cars, satellites, lighting and electronic devices size will matter as the availability of space is limited, and each inch of the panel needs to produce the maximum possible power to supply the required load.

The maximum possible efficiency is important for solar farms.

The maximum possible efficiency is important for solar farms.

Utility-scale projects must also take into account optimal selection of the size and efficiency, as bigger panels require higher installation costs due to the longer wiring and stronger mounting structures (along with higher land rental costs) [11].

However, in residential solar systems, the availability of space on a roof or in a courtyard is generally large enough to supply the desired load. Even though, this also depends on your expectations–meaning that if the demanded energy load exceeds the available space for the installation, solar panels with higher efficiency should be considered.

It is important to highlight that the balance between costs, efficiency, power load and space must be settled between the solar installer and the homeowner because the decision will affect the overall performance of the system and the total costs of the project.

 


References

[1] http://www.ren21.net/wp-content/uploads/2017/06/GSR2017_Full-Report.pdf
[2] https://es.wikipedia.org/wiki/L%C3%ADmite_de_Shockley%E2%80%93Queisser
[3] https://goo.gl/C1NkRG
[4] http://www.energetica-pv.com/index.php?id=447
[5] https://www.homerenergy.com/products/pro/docs/3.11/pv_nominal_operating_cell_temperature.html
[6] https://www.greenmatch.co.uk/blog/2014/11/how-efficient-are-solar-panels
[7] https://livingonsolarpower.wordpress.com/2013/03/06/6-factors-that-effect-solar-pv-system-efficiency/
[8] https://news.energysage.com/how-to-make-solar-panels-more-efficient/
[9] http://www.altenergy.org/renewables/solar/latest-solar-technology.html
[10] https://www.quora.com/Why-are-solar-panels-so-inefficient
[11] https://goo.gl/imMPc9