Why Are Solar Panels Inefficient?

Solar energy development continues as the market evolves into more profitable photovoltaic 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 480.3 GW at the end of 2018 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 homeowners to own a solar photovoltaic system. Policy and incentive schemes were also not clearly established or profitable, representing high investment risks that worked against the best environmental desires.

Among other reasons, one of the main limitations for entering the residential market was low efficiency of solar systems. Efficiency of solar panels represents how much of sunlight that hits a solar cell gets transformed into electricity. Some of the first solar panels had efficiencies between 8 to 10 percent. 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 for homeowners in most high-income countries. Solar cell efficiency has increased due to advancements in photovoltaic technology to the range between 15 and 22 percent. This number may not seem so competitive to many who have doubts about fully transitioning to solar energy.

Let’s have a look at reasons why are photovoltaic solar panels still inefficient.

Understanding why is solar cell efficiency low?

To understand efficiency of a solar panel, you must first understand its source of energy – the Sun.

Sun emits energy in a form of light which is composed of photons. Each photon has different energy and wavelength range (from ultraviolet to infrared). Photons are used by photovoltaic cells in solar panels to convert sunlight into electricity.

The cells are made of a semiconductor material (p-n junction), which facilitates generation of electric current when photons get absorbed. But a bigger part of sunlight that hits the cells isn’t converted into electricity. Why is that?

The problem with solar cell efficiency lies in the physical conversion of sunlight.

In 1961, William Shockley and Hans Queisser defined the fundamental principle of the solar photovoltaic industry. Their physical theory proved that there is a maximum possible efficiency of 33.7 percent which a standard photovoltaic cell (based on a p-n junction) can achieve to generate electricity from a light source [2]. In other words, if the sun’s direct light is received by a surface per area of 1,000 watts per one square meter, then the maximum power output that a solar panel could produce is 337 watts per square meter.

This physical limitation is known as the Shockley-Queisser limit and is caused by the physical process of photon absorption by the semiconductor material that knocks loose electrons from their atoms and then passes them to the conduction band within the solar cell [2]. Some photons that hit a solar cell are reflected away from the cell’s surface. Out of those photons that are absorbed, some have their energy turned into heat in a process of internal recombination instead of producing electric current.

These efficiency losses are inevitable. There is no manufacturing process or technology development that can change this limit — at least not with silicon p-n junction solar cells.

The Shockley-Queisser limitation defines the theoretical level of maximum possible efficiency of a solar cell, but in the daily life, efficiency of solar energy conversion is affected also by other factors (discussed below).

Standard Test Conditions for verifying solar panel efficiency

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 solar cell efficiency based on Standard Test Conditions (STC).

STC are set on maximum possible solar irradiance (1kW per m2), at temperature of 25 degrees Celsius or 77 degrees Fahrenheit, and an air mass of 1.5 (related to the thickness of the atmosphere) [4].

Nominal Operating Cell Temperature to simulate real life conditions

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.

What factors affect solar panel efficiency in daily life?

#1 Hail, snow, ice and dust

Hail could in extreme scenarios damage the surface of solar panels and reduce their efficiency by decreasing the amount of sunlight they absorb. However, you should also know that solar panels are durable and meant to withstand hail at a speed of 20 to 30 meter per second (45 to 67 miles per hour). For a better idea, it is estimated that a hailstone with diameter of 1 centimeter or around 0.4 inches falls at a rate of 9 m/s (20 mph). Solar panels should resist even two to three times that. You should be safe during that early spring overcast.  

Thick layers of snow can directly block sunlight accessing solar panels and cut efficiency down to zero [6]. It is important to know that most solar panels can still work with a layer of three to four centimeters of snow (approx. 1.5 inches). If the layer is thicker than that, you should remove the snow.

Just like snow, dust and dirt can influence efficiency values on the same principle. When the layer is too thick to obstruct the light from reaching the solar panel surface, it will affect negatively how much energy the panel produces.

Ice is another element that affects solar panels’ efficiency between 25 to 100 percent, depending on the thickness of the ice layer. To prevent this problem, a silicon coating is applied to a solar panel surface, but even despite that it’s advisable to keep ice off your panels during the winter season [6].

#2 Insulation resistance

Besides the safety concerns, faulty insulation decreases solar module efficiency because current leakages can occur along the edges of solar panels. Be especially careful in areas with high humidity. Humidity speeds up corrosion of materials. Advanced level of corrosion eventually leads to insulation issues and decreases overall solar panel efficiency faster than expected.

Problems with insulation may occur in poor quality solar panels made of cheaper materials, or when solar cells are too close to the frame.

Insulation resistance is especially important for utility-scale projects as higher voltage systems require better insulation properties, which are linked directly to selected materials.

#3 Temperature of solar panels

Solar cells are sensitive to warmer temperature. Higher temperature outside means that the semiconductor material of solar panels gets hot quickly and slightly changes properties, which is linked to electrical losses and voltage drops.

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

If you think about it: temperature increase of around 60 degrees Celsius (140°F) could reduce the power outcome of your solar panel by 17.5 percent. So, the effect of temperature can be significant in countries with hot climate. Long term exposure to hot temperature may also shorten lifetime of the material.

On the other hand, you do not need to worry about colder temperatures, as long as the sun shines and solar panels receive unobstructed sunlight during the day, they will be producing at around their estimated efficiency. The only common problem in winter in regard to solar energy production is that the number of sunny days is much lower when compared to summer, and your system generates less on a monthly basis.  

#4 Selection of the solar panel type

Not all solar panels are the same, neither is their price, which very often depicts not only the quality of materials and durability of the product, but also solar cell efficiency in converting sunlight in electricity. In daily life this usually means that you need less solar panels with higher efficiency rating to produce the same amount of power from their surface than you would need if you selected lower efficiency panels.    

The main difference in efficiency values of different panel types lies in the nature of how they have been manufactured and what semiconductor material they are made of. The most common semiconductor used in solar cells is silicon. In case of silicon, efficiency still differs based on the type of cells. The best quality are solar cells made of monocrystalline silicon, other common forms are polycrystalline or amorphous cells.

Monocrystalline panels composed of a single crystal due to their structure allow for the best efficiency between 15 to 22 percent. Polycrystalline panels reach between 13 to 16 percent of efficiency and thin film panels made from amorphous cells have reached to 20 percent.

What also matters is how much light is reflected from the solar cell surface. The goal is to minimize light reflection and that is why many solar panels have special anti-reflection coating and are dark in color – black in case of monocrystalline and thin film panels and dark blue polycrystalline.

#5 Design configuration of solar panels

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

Your solar panels will have the best power output when the solar panel surface is perpendicular to solar rays. However, as the sun moves across the sky, their angle changes as well. What you can do to ensure direct irradiance is to install a tracking system. Its downside are higher installation costs.

Solar panel efficiency also changes over the time. Every year that passes after your solar system installation, the efficiency value drops by about 0.5 percent per year. Nevertheless, solar panel manufacturers have to guarantee that the performance of your solar system will not drop below 80 percent when in warranty, which usually lasts 25 years.

Finally, shading is also an important part of the design of any solar system. If your system is shaded for the sunniest time of the day, solar panel output will be affected severely.

New solar technologies to improve efficiency

Despite low efficiency rates among current solar panels, there are several innovative proposals and technologies that aim to change how efficient can solar panels get 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 a solar panel. They can be easily identified by any person as silver wires forming a grid inside a solar panel. They transport electricity generated by solar cells.

These wires, even as tiny as they are, reflect the incoming light in sections where they pass by. In 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 in the best case to eliminate their interference with the sunlight by placing them on the back of a panel. This would achieve 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 powerful 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 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 around the manufacturing process to provide high efficiency solar cells.

The solar energy 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.

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

Some components seem to be very unstable because cadmium chloride has to be used during the manufacture. However, a new manufacture approach could deliver positive results, if cadmium chloride gets replaced 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 Perovskite, the 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 of 22 percent at lower manufacturing costs.

The secret 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.

How to easily improve efficiency of solar cells

When you would like to improve efficiency of your solar cells, you should consider the effect of factors discussed above. As you can see, there are some factors that cannot be influenced by you, such as weather and outdoor temperature, but some other factors 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: regular solar panel maintenance helps eliminate snow, dust, dirt and ice – all of which obstruct sunlight from effectively reaching the panel’s surface, and therefore reduce efficiency of solar cells.

By keeping your panels clean, you can nicely maintain their efficiency.

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

Some important details that need to be considered are:

• the solar irradiation value in your area;
• average cloud density;
• levels of air pollution;
• the angle of solar panels depending on their location and a selected mounting system.

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.

Questions you may have about solar panel efficiency

We have discussed the limits of the conversion of sunlight 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 photovoltaic 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 solar cells to produce the maximum amount of electricity from a light energy source.

A single cell with low efficiency will produce less power 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 watts 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 watts 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 bigger in size.

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 space availability is limited, and each inch of the panel needs to produce the maximum possible power to supply the required load.

Utility-scale projects must also take into the 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 sufficient 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, high efficiency solar panels should be considered.

It is important to highlight that you should settle the balance between costs, efficiency, power load and space with your solar installer at the beginning because this decision will affect the overall performance of your solar system and the total cost of your project.