Hot panels, lower output

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How hot a solar panel gets really affects the amount of energy they produce and this temperature coefficient can vary considerably. 

Heat translates to a loss of energy when it relates to solar panel output. This presentation explores this area of renewable energy and looks at a variety of different panels and their respective temperature coefficients, how location can affect what panel to purchase. Ideal for sustainability experts and any one interested in solar systems, renewable energy and energy efficiency.

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What happens when a solar panel gets hot?  

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Although temperature doesn't affect the amount of solar energy a solar panel receives, it does affect how much power you will get out of it. 

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• As solar panels get hotter, they produce less power from the same amount of sun.
• Normally, electrons at rest (low energy) are excited by the sun (high energy)
• The difference between their excited and rest energies is the potential difference (voltage) 

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But heat also excites electrons (when we heat something we give it energy)

This raises the energy of the electrons at rest. ("warmer" electrons have more energy at rest than their "cold" counterparts.)

Light is composed of a huge number of "photons", particles which carry the energy associated with light:

When they hit the solar cell they bump into an electron and give it the energy they were carrying.

Electrons go from a low-energy state to a high-energy one. 

The solar cell is then designed to extract this electron in the high energy state.

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The action of light hitting a solar cell 

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The solar cell runs it through an electrical circuit to use up this extra energy that the electron has and:

• Sometimes though this high-energy electron will bump into other atoms in the solar cell
• Before it “gets out”the electron loses this extra energy which turns into heat rather than electricity.
• Heat is the vibration of atoms and molecules

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If the solar cell is at a higher temperature that means that the atoms are vibrating faster and this means that:

• It is harder for the electron to get out without bumping into these atoms
• So when the solar cell heats up the output power will drop 
• Because more of the energy is lost before it manages to escape the solar cell.

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Producing power from the sun 

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Power is produced from the difference in the states (at rest and excited by the sun):

• If the electrons have more energy at rest (your solar panels are hotter)
• The difference between the rest energy and excited energy (from the sun) is smaller
• Our solar panels will produce less energy

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Chemical reactions like cooler temperatures  

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The main effect of temperature on solar panels is that it reduces the efficiency of the solar cells at converting solar energy (sunlight) into electricity:

• Chemical reactions that occur within the solar panels are more efficient at cooler temperatures 
• A solar panel in the hot desert may produce a little less electricity per amount of sunshine than a solar panel in a cold climate area
• This inefficiency is made up by the greater number of sunny days in the desert

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How is temperature accounted for with panels?  

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All solar cells have a temperature coefficient:

• The percentage of power lost at a specific degree measurement above the industry standard is divided by the number of degrees above 25°C to provide the temperature coefficient.
• If you’re looking at a solar panel at 28°C, this is at 3°C higher than the industry standard. 
• If you observe a power loss of 1.08% at that temperature, you divide that power loss by 3 to get 0.36, or a temperature coefficient of -0.36%/°C Pmax

Power loss example  

Let’s say we have a 350W monocrystalline solar panel installed on a roof at 65 deg C.

The solar panel’s power loss can be calculated as follows:

• 65 deg C – 25 deg = 40 deg C
• 40 deg C x -0.36% = 14%
• Therefore panel power loss = 14% x 350W = 49W
• Panel peak output is now is 301 watts

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How to Minimize Solar Panel Efficiency Loss  

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The primary way to mitigate loss in efficiency is through the reduction in the temperature of your solar panels. Here are some of the factors that influence the panel’s temperature:

• The type of solar panel installation has a direct effect on the panel’s temperature. For example, if an angled, roof-mounted system is at 30°C, that same system, but mounted flat on the rooftop, maybe at 35°C.
• Thin film solar panels have a lower temperature coefficient than traditional monocrystalline or polycrystalline panels. Thin film panels can see temperature coefficients closer to -0.2% / °C.
• The material of your roof can have an impact on the solar panel’s temperature. Certain rooftops absorb more heat than others. Make sure to take into account your roof type when making your analysis.

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Temperature coefficient comparisons  

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Power temperature coefficient is measured in % per °C - Lower is more efficient

• Polycrystalline cells -                   0.39 to 0.43 % /°C
• Monocrystalline cells -                 0.36 to 0.40 % /°C
• Monocrystalline IBC* cells -        0.29 to 0.31 % /°C
• Monocrystalline HJT** cells -      0.26 to 0.27 % /°C

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Generally cell temperature is 20-30°C higher than the ambient air temperature which equates to approximately 8-12% reduction in power output. Note, cell temperature can rise as high as 80°C when mounted on a dark coloured rooftop during very hot 40+°C, windless days.

*Interdigitated back contact   **Heterojunction technology

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Conclusion

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• The hotter the temperature the less power is produced by the solar panel
• This is due to the nature and movement of electrons
•  All solar panels have a temperature coefficient,  obtained from the data sheet
•  There are other factors that influence a solar panels ability to achieve optimum output

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If you’d like to see more of what Greenwood Solutions get up to in the real world of renewable energy, solar, battery storage and grid protection check out the following pages:

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https://www.greenwoodsolutions.com.au/industry 

https://www.greenwoodsolutions.com.au/commercial

https://www.greenwoodsolutions.com.au/commercial/customer-stories

https://www.greenwoodsolutions.com.au/news