How does ambient air temperature affect solar panel efficiency?
How does ambient air temperature affect solar panel efficiency?
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2765 words // 25 mins
All photos and content written and created by me unless otherwise stated.
introduction
Photovoltaic cells, known by their household name as solar panels, have frequently been touted as a new way of passive, nonmechanical, localized energy generation. As a result of their design and in turn, their typical placement, solar panels frequently are subject to large amounts of solar radiation. This is a prerequisite to an efficient photovoltaic system, but a byproduct of large volumes of solar radiation is the conversion of some energy to heat. This means that for a solar panel to work at its most efficient, it must maximize solar exposure — however, this frequently correlates to high operating temperatures as well.
Frequent advertisements picture these solar panels on household rooftops, even more so recently with rising energy costs. With global temperatures on the rise, I have frequently wondered how the efficiency of a solar panel is affected by rising temperatures, and consequently falling ones.
Light, which is composed of photons, strikes a semiconductor in photovoltaic cells – amorphous silicon crystals, in this case [5]. When photons are incident upon these semiconductors, they behave in one of three ways: reflecting off the cell, passing through the cell, or being absorbed by the cell. Photons that reflect off the cell or pass through do not interact with the cell further. Only absorbed photons contribute to a cell’s ability to produce energy [2]. When the semiconductor absorbs photons with sufficient energy, electrons in the material are excited from the valence band to the conduction band, leaving behind positively charged "holes" in the valence band – absences of electrons. These electron-hole pairs are created as a result of the photon's energy dislodging the electrons from their original positions (see left). [7]
During the manufacturing process, the upper and lower layers of the semiconductor were “doped,” [3][5] or injected with impurities that affect their characteristics. In this case, the upper layer was n-type doped, and the lower layer p-type doped (doping refers to the injection of impurities in the crystal structure of materials, either adding free electrons (n-type) or creating positive holes (p-type) [6]). This encourages electrons generated in the gradient between the upper and lower layers to flow from lower to upper, generating absences of electrons in the lower regions while driving electrons to the glass surface of the panel. This electron gradient forms a potential difference between the two layers. A load is connected to the individual layers, facilitating the flow of electrons through the load and back to the p-type region, where electrons and the “holes” recombine, and flow to the region in between the n-type (surface) and p-type (lower) layers.
working principle
When extra heat is added to any solid system, its molecules gain kinetic energy, which is how a resultant increase in temperature manifests. This means that these electrons, which are now moving faster, collide with each other far more frequently than slow moving electrons before reaching the ends of their gradients. This increased collision rate can greatly disrupt the electron flow that the formation of the electron gradient in the cell relies on, leading to decreased efficiency when it comes to generating a potential difference. [2][3][5]
Also a result of the heightened kinetic energy is an increase in recombination rate, the rate at which electrons recombine with holes to form hole pairs. If this occurs, the electrons that recombine can no longer contribute to the hole-electron gradient, meaning they cannot contribute to the generation of potential difference that solar panel efficiency relies on. Many external factors, like wind currents cooling cells, extra impurities introduced in doping, and other factors may cause conditions to change from the described situation.
Using the found voltage values, the Temperature Constant for the solar panel will be calculated with the formula [8]:
In this formula, Voc, ambient is the open circuit voltage of the panel at a given ambient temperature. This is the voltage when no current is flowing and the circuit is open — hence the name open circuit voltage.
Tc is the temperature constant in V/॰C — this is the drop in voltage for every increase in degrees celsius.
TSTC is the temperature of standardized testing conditions (25 degrees celsius). [5]
Tambient is the air temperature of the air that the panel is in.
Voc, STC is the open circuit voltage of the panel at standard testing conditions, which is 5.0 Volts [8].
hypothesis
The voltage output of the solar panel decreases with increasing ambient air temperatures due to the temperature dependence of semiconductor materials. Specifically, the added kinetic energy increases the carrier recombination rate, leading to a measurable, almost-linear decrease in the open-circuit voltage (Voc, ambient).
To determine the temperature constant, Voc, ambient will be graphed on the y-axis, and ( TSTC -Tambient ) on the x-axis. This leaves the temperature constant as the gradient of the graph. It also leaves Voc, STC, a known value, the y-intercept of the graph.
variables
variables
Independent Variable // Ambient Air Temperature
Measured in degrees Celsius, using a LabQuest with a temperature probe. Increments were chosen as they are near room temperature and reduce risk of damage to surroundings (25, 30, 35, 40, 45॰C ± 0.03).
Dependent Variable // Output Voltage
Measured in Volts, using a multimeter connected to the solar panel.
constants
constants
In order to keep all conditions the same, it was not viable to perform this experiment outdoors. As a result, an enclosure was created, made of a cardboard box and lined with soft bubble foil insulation. A polycarbonate lid was placed on top. Grommets for all holes were added as well.
Air Pressure // Controlled by operating in enclosed box.
Air currents can inadvertently cool the panel, reducing panel temperature but not affecting ambient temperature. This could lead to systematic error in temperature readings.
Light Incident on Panel // Controlled by using standardized light at measured brightness and distance.
Experiment was run in an otherwise dark room with only one LED light source. A polycarbonate sheet was added on top: polycarbonate allows transmission of light, but traps heat. Extra light incident causes increased voltage output (explained further in the second introduction paragraph).
Uniformity in Air Temperature // Controlled with insulation and sealing.
Holes for probes, wires, and hot air inlet were lined with grommets made of the insulating foil mentioned later. This was done to limit temperature and light exchange. Would cause increases in voltage if hot air escaped or extra light entered the box.
materials
materials
Soldering Rework Station
Used to consistently supply hot air into the testing enclosure.
Multimeter (±0.5%)
Foil Insulation
Amorton Photovoltaic Cell - AM-1816CA-DGK-E [5]
Polycarbonate Sheet
Heat Proof Brick
Cardboard Box
LabQuest with Temperature Probe
Uncertainty:
±0.03°C (0 to 40°C)
±0.1°C (40 to 100°C)
procedure
Use the cardboard box and insulating foil to create an enclosure surrounded by insulating foil.
Place an insulative heat brick in the center (further discussed in Safety).
Carve out holes in the side to insert the Soldering Rework Station air nozzle and LabQuest Temperature Probe.
Carve out two more holes, equally sized, for the Multimeter wires.
Place the polycarbonate on top. Hot glue strips of insulation around the border of the panel of polycarbonate to create a compliant interface with the top edges of the box.
This will allow light to enter the enclosure, but keeps heat from escaping due to polycarbonate’s poor thermal conductivity.
data collection
data collection
Turn on the hot air generator. Ensure that the “air volume” dial is set to the minimum if applicable.
Wait for temperature to reach a desired start temperature (25 degrees Celsius) for which you plan to record voltage values.
Note down output voltage at a specific temperature.
Increase the temperature for the next interval, if applicable.
This should give one voltage output for each temperature input.
To prepare for another recording:
Remove the lid. Keep fingerprints off the polycarbonate.
Let the setup air out naturally for approximately 12 minutes.
This will minimize dust on the solar panels and variances in the metrology equipment due to strong air currents that would otherwise come from fans, etc.
Repeat so that five or more trials are taken at each temperature, for five or more temperatures. This will reduce error and increase precision.
methodological limitations
Not all photons embody sufficient energy to excite electrons, and these less energetic photons are reflected as light or dissipated in heat. To cool the enclosure from the residual heat and heat from the hot air input, the polycarbonate panel will frequently have to be removed so hot air can escape. As a result, finger oils and dust will stick to the polycarbonate panel. To minimize this same effect on the solar panel, it will be cleaned lightly with dish soap and a soft bristle brush. Despite this, dust will accumulate on the glass of the panel in between trials. Both of these would unduly reduce the voltage reading. This is because both factors would disrupt the travel of photons to the solar cell, reducing the amount of photons that make it to the cell.
Another way voltage readings are systematically affected is through resistances in the circuit. Resistance arises when charge-carrying particles in a medium collide, causing electrical power to be dissipated as heat and/or light. As one of the solar panel’s terminals arrived unsoldered, I had to re-solder this joint, as well as soldering wire extensions so that the panel’s wires could reach out of the enclosure. This resulted in a total of 4 solder joints throughout the loop, only one of which was done professionally. These solder joints, as well as the wire extensions, caused increased resistance throughout the circuit. Increased conductor length causes increased resistance, shown by:
Where R is resistance, L is length, A is cross-sectional area, and ρ is the resistivity (electrical conductivity of a conductor of known dimensions). By the below formulae:
It is shown that when Resistance (R) increases, more voltage is dissipated due to heat. This results in an artificial and systematic deflation in voltage readings.
data collection
data presentation
Manufacturer-supplied TC is supplied as a percentage of the panel’s VSTC. For ease of verification of results, TC will be processed into a percentage of the panel’s VSTC before a conclusion on the experiment’s results is made.
Example calculation for line of best fit:
evaluation of results
An investigation was performed to answer the question: How does ambient temperatures affect voltage output by an amorphous silicon crystal solar panel? The voltage output of the solar panel decreases with increasing ambient air temperatures due to the temperature dependence of semiconductor materials. Specifically, the added kinetic energy increases the carrier recombination rate, leading to a near-linear, measurable decrease in the open-circuit voltage (Voc).
As the line of best fit goes through all the error bars of each point, it can be assumed that the linear curve is properly used here. From my slope, the temperature constant when expressed as a percentage of the voltage at standard testing conditions, is -0.36% (with a margin of error of ± 0.05%). The value of the Tc means that for every 1 degree Celsius temperature increase, the voltage output drops by anywhere between 0.30% to 0.40% of the voltage at STC. Given that the manufacturer has supplied a figure of 0.30%, my result aligns with the manufacturer-supplied value. This aligns with my hypothesis, which states that there is a decrease in voltage output. Given the linear relationship between the independent variable and dependent, my hypothesis further aligns with the results.
The intercepts discuss the maximum voltage difference (between output and Vstc) that was output. This has little bearing on the effects, as this would change depending on which temperatures were tested. It is meant to be non-zero, which it is. However, the y-intercept (5.28 ± 0.04 V) does not fall within range of the given value of 5V — it is shown in the formula to be the Open Circuit Voltage at Standard Testing Conditions, which is known to be 5 volts.
Human factors may have caused random error, like placing the polycarbonate shield in an incorrect orientation. This would have let hot air rise and escape, inducing systematic error until the shield would be repositioned (which would cause systematic errors in temperature readings for that entire trial by offsetting the recorded temperature from the actual ambient temperature). Environmental conditions, like the aforementioned dust falling on the solar panel itself, also likely caused error by offsetting voltage readings when they interfered with the panel’s ability to intake light, exacerbated by the air currents in the box encouraging the settling of dust.
extensions and improvements
There are some possible extensions to this experiment. First, a wider range of temperatures could be tested for renewable energy applications in conditions that differ from those of Earth. Using uniform cooling or heating of the space around the solar panel would help simulate these conditions. It should be observed if the linear trend continues, or if deviations occur at extreme temperatures.
A further extension may be to explore thermal management systems. If an optimum operating temperature
for these panels is determined, heating or cooling systems could be explored to keep solar panels at this
optimum temperature. Inspiration could be drawn from current active and passive solutions for delicate hardware, like immersion cooling, passive air cooling, and active fluid loop cooling. As some cooling solutions require energy to run, the efficiency of these solutions should be explored as well.
Furthermore, I would use more secure mechanisms of keeping the enclosure thermally isolated, as I believe that any nonlinearity in my data arose from the high temperatures. Due to the fact that the setup was subjected only to increasing temperatures, readings towards the end of the trial (which were of higher temperatures) were subjected to error because the enclosure was more uniformly warmed than readings taken at lower ends of the temperature gradient. One amendment to make is that students should record data from an equal number of runs increasing the temperature from the minimum to the maximum as they record decreasing the temperature from the maximum to the minimum. This should balance out this undue effect of systematically increasing voltage readings on the low end of the temperature scales due to uneven heating in the enclosure.
improvements and closing
There are some possible extensions to this experiment. First, a wider range of temperatures could be tested for renewable energy applications in conditions that differ from those of Earth. Using uniform cooling or heating of the space around the solar panel would help simulate these conditions. It should be observed if the linear trend continues, or if deviations occur at extreme temperatures.
A further extension may be to explore thermal management systems. If an optimum operating temperature
for these panels is determined, heating or cooling systems could be explored to keep solar panels at this
optimum temperature. Inspiration could be drawn from current active and passive solutions for delicate hardware, like immersion cooling, passive air cooling, and active fluid loop cooling. As some cooling solutions require energy to run, the efficiency of these solutions should be explored as well.
This research has many real world applications. As the world heats up due to global warming, there is more of a push towards green energy – solar panels included. However, due to global conditions and the presence of heatwaves or generally high temperatures in some areas of the world, this data is helpful in addressing the viability of solar panels in high-heat environments.
bibliography
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National Grid. (2023, May 18). How does solar power work? https://www.nationalgrid.com/stories/energy-explained/how-does-solar-power-work#:~:text=Solar%20panels%20are%20usually%20made,and%20produces%20an%20electric%20charge
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Teachengineering.org (n.d.). Photovoltaic Efficiency: The Temperature Effect. https://www.teachengineering.org/content/cub_/lessons/cub_pveff/Attachments/cub_pveff_lesson02_fundamentalsarticle_v6_tedl_dwc.pdf
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LoggerPro was used for data analysis and graphing.
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