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Solar radiation is electromagnetic radiation released by the Sun. While every area on Earth receives some sunlight over the year, the amount of solar radiation that reaches any spot on the Earth’s surface differs. Solar cells catch this radiation and transform it into useful forms of energy.

There are two main types of solar power technologies—photovoltaics (PV) and concentrating solar-thermal power (CSP).

  • Photovoltaics

When the Sun shines on the solar panel, energy from the sunlight is absorbed by PV cells in the panel. This energy produces electrical charges that move in response to an internal electric field in the cell, causing electricity to flow.

  • Concentrating solar-thermal power (CSP)

This system uses mirrors to reflect and concentrate sunlight on receivers that convert  solar energy to heat..It is  then used to produce electricity or kept for later use..

How to calculate solar power efficiency?

Efficiency is interpreted as the ratio of energy production from the solar cell to input energy from the Sun. 

In addition to considering the performance of the solar cell, the efficiency depends on the spectrum and intensity of sunlight and the temperature of the solar cell. 

Therefore, conditions under which efficiency is measured need to be precisely controlled to compare the performance of one device to another.

The primary solar panel energy output is measured by examining the panels under typical conditions, identified as Standard Test Conditions (STC). 

Under STC, a 250-watt panel generates 250 watts of electricity when the sunlight on the panel is 1000 watts per square meter, and the panel is at 25°C. 

Solar panel efficiency is another factor that influences how much energy a panel will produce. The efficiency of a panel indicates the ability of the panel to turn sunlight into usable energy. 

In a panel with 20% efficiency, 20% of all the light that hits it will be converted into electricity. A panel with a higher efficiency rating will convert more sunlight into energy. Most solar panels have around 15% to 18% efficiency.

To calculate the efficiency of the panel, multiply the amount of sunlight that reaches the Earth’s surface in the specific area (known as the “incident radiation flux”) by the area of the panel (in square meters). 

For example, if 2,000 watts per square meter of sunlight hits (assumed during STC testing) and the panel is 2 square meters, there will be 4,000 watts. 

If the panel is advertised as producing 400 watts, the efficiency rating is 20% (400÷2000 is 0.2, and 0.2 x 100% = 20%).

However, conditions are usually different from STC in the real world. So a regular panel seldom produces its advertised maximum wattage. 

The temperature of the solar panel, for example, is unusually fixed 25°C (the temperature used to determine STC testings). 

Most solar panels are 20°C hotter than the outside temperature. In other words, if the temperature outside is 20°C, the solar panel’s temperature is probably around 40°C.

The solar power efficiency formula

The efficiency of a solar cell is defined as an incident of power, which is converted to electricity:

 

Where:

Voc is the open-circuit voltage;

Isc is the short-circuit current;

FF is the fill factor and

η is the efficiency.

Factors that affect solar panel efficiency

Few factors affect solar panel efficiency and the ability to convert sunlight into energy for use in homes and businesses.

  • Temperature

Warmer temperatures have an unfavourable impact on the production of solar panels. Some may assume warmer temperatures would be better because there’s more sunlight when it’s warmer, but extended heat exposure can prematurely degrade solar cells.

Solar cells perform better in the cold climate, and as things stand, panel efficiency is estimated at 25˚C, which can be different from the outdoor situation. For each degree rise in temperature above 25˚C, the panel output decays by about 0.25% for amorphous cells and about 0.4-0.5% for crystalline cells

Thus, in hot summer days, panel temperature can easily reach 70˚C or more. What it means is that the panels will put out up to 25% less power compared to what they are rated for at 25˚C. 

For everyday production, high temperatures cause a reduction in voltage and, in turn, a decrease in power. Power is the output of voltage and current (P = V x I). 

Sunlight is made up of charged particles called photons that come from the collision between hydrogen atoms in the Sun’s core. As these photons affect the semiconductive setting of the panels, they expose their energies into the electrons in the cell. 

Once these electrons are stimulated to a higher energy state, they move around and are eventually gathered and channelled into a stream of electrons.

If there is a temperature rise, the electron’s “rest temperature” is dramatically increased. Since it takes less energy to stimulate the electron, less energy is transferred from the photon. As a result, the system produces less electricity.

One way to prevent the impact of higher temperatures is to mount the panels a few inches off the roof. This allows air circulation to cool them naturally.

  • Shading

Shading is the enemy of solar cells because much like clouds, it is blocking the sunlight required to produce electricity.

So, for instance, if there are lots of surrounding trees, there is a need to trim or remove it to go solar. Roof blocking like vents or chimneys can also be a problem. 

It is ideal for solar panels to be placed where there will never be shadows because a shadow even on a small part of the panel can have a massive effect on the output. 

The reason shade causes such a significant problem has to do with how solar panels are wired. The cells in a panel are usually all wired in groups, and the shaded cells affect the current flow of the entire panel. 

But there can be situations where it cannot be avoided, and thus the effects of partial shading should be considered while planning. 

If the affected panel is wired in series with other panels, then the output of all those panels will be affected by the partial shading of one panel. In such a situation, an obvious solution is to avoid wiring panels in series if possible.

Another great solution to this problem is using Power Optimisers. Essentially a Power Optimiser isolates an individual module such that its performance does not affect the other modules in an array.

  • Orientation

To achieve maximum efficiency, solar panels need to be positioned at an angle to point directly at the Sun. 

This means that solar panels in the United States need to face south. Solar trackers can also be installed for automated adjustment of the panels. 

This ensures they receive maximum sunlight throughout the day. If the pitch of the roof needs to be altered to accommodate this, racks can be used.

For the calculation of the energy production of a photovoltaic installation, it is essential to know the solar irradiance in the plane corresponding to the installation and the solar path in the place at different seasons of the year. 

The situation of the sun at any time is determined by the height and azimuth of the sun.

The performance of photovoltaic panels is determined by the orientation and inclination, which influences the amount of producible energy. Depending on the inclination, solar energy will be maximum when the position of the panel is perpendicular to the irradiation.

The orientation of the photovoltaic panel is the angle of difference from the geographical south of a surface or north in the southern hemisphere. Maximum differences of +/- 20 degrees are allowed. 

In the northern hemisphere, the standard rule for the placement of solar panels is that they must be situated towards the true south (and in the south, the true north) to receive direct radiation throughout the day. 

  • Maintenance

Solar panels are very long-lasting and can stay good for 25-30 years. However, cleaning solar panels is essential to maximise the amount of light available to turn into electrical power. Making frequent physical inspections can help solar panels absorbing light effectively.

Maintenance is the key to a strong, efficient panel setup. In general, a solar panel system requires very little mechanical maintenance, even if it is wired in series. 

They have no moving parts, unlike generators which need repairs or replacement, but they may require some electrical maintenance. A loose connection can severely affect energy production.

Solar panels also do not rust or break down, but keeping them clean and in good condition does form part of its necessary maintenance.

Solar panels can crack like glass. This problem is almost always caused by extreme weather conditions like high winds and hail storms. 

When even a small crack occurs, condensation can form within the panel. This fogs up the glass and hampers energy production. Manufacturer or installer need to handle this problem. 

What is the maximum efficiency of solar cells?

The maximum general efficiency calculated is 86.8% for a pile of cells, using the incoming intense sunlight radiation. 

When the incoming radiation comes only from an area of the sky the size of the Sun, the efficiency limit drops to 68.7%. 

What types of solar panels are most efficient?

Most of the solar panel options currently available fit in one of three types: 

  • Monocrystalline
  • Polycrystalline 
  • Thin-film

These solar panels differ in how they’re made, appearance, performance, costs, and the installations.

Each solar panel has its unique advantages and disadvantages: 

 

Solar Panel Type Advantages Disadvantages
Monocrystalline
  • High efficiency/

Performance

 

  • Aesthetics
  • Higher Cost
Polycrystalline
  • Low Cost
  • Lower efficiency/

Performance

Thin-film
  • Portable and flexible
  • Lightweight
  • Aesthetics
  • Lowest efficiency/

Performance

Of all panel types, monocrystalline has the highest efficiency and power. Monocrystalline solar panels can reach efficiencies higher than 20%. While polycrystalline solar panels usually have efficiencies between 15% to 17%.

Monocrystalline solar panels tend to produce higher power than other panels not only because of their efficiency but because they have higher wattage modules. 

Most monocrystalline solar panels come in more than 300 watts (W) of power capacity, some even 400 W. On the other hand, polycrystalline solar panels have lower wattages. 

Both monocrystalline and polycrystalline solar panels come with 60 silicon cells each, with 72 or 96 cell variants. Despite the same number of cells, monocrystalline panels are capable of producing more electricity.

A new kind of solar technology has set a world record for the most efficient generation of energy by a solar cell. By stacking six different photoactive layers, the record-setting multi-junction cell has reached 47% efficiency in the lab and nearly 40 percent in the field.

The six-junction solar cell now holds the world record for the highest solar conversion efficiency at 47.1%, which was measured under concentrated illumination. A variation of the same cell also set the efficiency record under one-sun illumination at 39.2%.

This panel exceeds typical panels by combining six kinds of collectors into one micro-thin surface. Researchers say the same tech could be fine-tuned to reach a full 50 percent efficiency.

Most panels are 15 to 18 percent efficient. By stacking the technologies from six different solar cells, solar researchers can ratchet up that efficiency multiple times over.

In total, there are 140 layers of the six different solar collector materials. Even so, the entire collecting surface is one-third the thickness of a human hair. 

The research team used different semiconductors and carefully arranged them to maximize usable surface area through all 140 layers. Further reduction of the series resistance within this structure could realistically enable efficiencies over 50%.

Efficiency reduction of solar power over time

Panels are typically warrantied for 25 years so that users can presume it to last at least that long. But in reality, studies have shown panels continue to perform at reduced efficiency long after the warranty expires.

Most panels are reported to provide at least 80% of their measured output during the warranty. 

78% of the systems tested held a degradation rate of less than 1% per year. That means that after 25 years of use, about 4 out of 5 solar panels still function at 75% efficiency or better. 

Methods of improving efficiency

Choosing optimum transparent conductor

The lighted side of some types of solar cells has a transparent conducting film to allow light to enter into the active material and to collect the produced charged particles. 

Typically, films with high transmittance and high electrical conductance, are used for the purpose. 

There is a trade-off between high transmittance and electrical conductance. Thus the maximum density of conducting nanowires or conducting network formation should be chosen for increased efficiency of solar panels.

Promoting light scattering in the visible spectrum

Lining the light-receiving surface of the cell with nano-sized metallic studs can mainly increase cell efficiency. 

Light reflects off these studs at a diagonal angle to the cell, improving the length of the light route through the cell. This increases the number of photons absorbed by the cell and the volume of current produced.

The primary materials used for the nano-studs are silver, gold, and aluminium. 

Aluminium absorbs only ultraviolet radiation and reflects both noticeable and infrared light, so energy loss is minimised. Aluminium can increase cell efficiency by 22% (in lab conditions).

Radiative cooling

An increase in solar cell temperature of approximately one degree causes an efficiency decrease of about 0.45%. A transparent silica crystal layer can be applied to solar panels to prevent the reduction. The silica layer acts as a thermal black body which emits heat as infrared radiation into space, cooling the cell up to 13 °C.

Anti-reflective coatings and textures

Antireflective coatings result in harmful interference of light waves from the Sun. Therefore, the sunlight would be transported into the photovoltaic. 

Texturising, in which the surface of a solar cell is altered so that the reflected light strikes the surface again, is another technique used to reduce reflection. These surfaces can be created by etching or using lithography. 

Adding a flat back surface in addition to texturising the front cover helps to trap the light within the cell, thus providing a longer optical path.

Thin film materials

Thin-film materials show a lot of promise for solar cells in terms of low costs and adaptability to existing structures and frameworks in technology.

Since the materials are so thin, they lack the optical absorption of bulk material solar cells. Attempts to correct this have been made. 

Since this is the dominant recombination process of nanoscale thin-film solar cells, it is crucial to their efficiency. Adding a passivating thin layer of silicon dioxide could reduce recombination.

Conclusion

Solar panels are usually able to process 15% to 22% of solar energy into usable energy, depending on factors like placement, orientation, weather conditions, and similar. 

The amount of sunlight that solar panel systems are able to convert into actual electricity is called performance, and the outcome determines the solar panel efficiency.

To determine solar panel efficiency, panels are tested at Standard Test Conditions (STC). STC specifies a temperature of 25°C and an irradiance of 1,000 W/m2. 

This is the equivalent of a sunny day with the incident light hitting a sun-facing 37°-tilted surface. Under these test conditions, a solar panel efficiency of 15% with a 1 m2 surface area would produce 150 Watts.

 

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