Solar energy has become more and more popular these days as a cleaner and cheaper energy source than conventional fossil fuel sources like coal and natural gas. And even though this technology has been around since the 1950s, many are still not familiar with the process of how solar panels can really produce energy. But before we dive into the details, let us first clarify some misconceptions and define our terms.
Misconception #1: Solar panels produce electricity from heat.
This misconception is very easy to have since from our everyday experience, heat and sunlight always go hand in hand. However, we must first understand that the energy that we get from the sun is in the form of light only. This light is only converted into heat when it hits and is absorbed by objects like our atmosphere, the ground, and our roofs. And as the name suggests, solar energy is the energy from the sun, which is light.
Solar panels, therefore, convert sunlight into electricity instead of heat. In fact, similar to electronic devices, they even work better and produce more energy when the ambient temperature is lower, with similar levels of irradiance or sunlight. And since solar panels are not 100% efficient in converting sunlight into electricity, they even produce heat as waste in this process.
Misconception #2: The correct name for solar panels is actually solar modules or solar panels. However, for most purposes, they have been used interchangeably, so we will still use the term solar panel/s throughout the rest of the article.
Solar cells are the smallest power-generating unit of a solar panel. You may have noticed that some solar panels are made up of small blue or black squares inside. These are solar cells, which produce only a very small voltage of less than 1V. This is why the most common solar panels in the market today have 60 or 72 solar cells inside of them, which are all connected in series to raise the voltage to a usable level.
When these solar cells are connected in series, they are placed in a panel, hence, the name solar panel. A solar panel, on the other hand, is a complete, ready to use module that consists of the solar panel, the aluminum frame, the glass, the electric junction box, and the wires.
With these misconceptions out of the way, we can start digging into the details of the science of how solar panels produce energy.
What Solar Panels are Made Up Of
The solar cells are made up of a class of materials called semiconductors. Semiconductors are a class of materials that have an electrical characteristic that is in the middle of that of conductors and insulators. You may be already familiar with conductors and insulators because you encounter them and use objects that are made up of them in your everyday lives. Conductors are basically metals that easily conducts heat and electricity, while insulators are the exact opposite as they do not conduct both heat and electricity very well.
The most common type of semiconductor that is used for solar cells is Silicon, mainly because it is the second most abundant element in the Earth’s crust after Oxygen. 90% of the soil, rock, and dust in the Earth’s crust is made up of silicate materials (rock-forming minerals that are made up of Silicon and Oxygen). Semiconductors have also been used to create transistors, which makes up all of the modern electronic devices that we use today.
Energy Bands in an Atom
To understand why semiconductors have their unique electrical characteristics, we must dive into the physics of it. Remember that an atom is composed of the nucleus, which is composed of protons and neutrons and the electrons which are located in shells that are centered in the nucleus. Depending on the total number of electrons in the atom, it can have multiple larger and larger shells. What’s important here is that the farther an electron is to the nucleus, the higher its energy state.
The outermost shell in an atom is called the valence shell. It is also the farthest shell from the nucleus, which means that the electrons in it(called valence electrons) also have the highest energy levels. The different levels of energy that the electrons can have while in this shell are spread out in an energy band which is called the valence band.
For a material to conduct electricity, a valence electron must receive some amount of energy for its energy level to be higher than the valence band. When this happens, that electron can now freely move away from the atom and flow as electricity. Its current energy level is now in the conduction band and it is said to have “jumped” from the valence band to the conduction band.
For conductors, the conduction band and the valence band overlap with each other, which means that all of their outer electrons already have enough energy to freely move around the whole material. These electrons are called free electrons and this is the reason why this type of material can easily conduct electricity.
For both semiconductors and insulators, there is a certain gap(called the band gap) to their valence and conduction bands which represents the amount of energy that must be added to the valence electron for it to be free. The band gap is very large for insulators and this makes it also very hard for an electron to gain enough energy and jump to the conduction band.
Valence electrons usually get additional energy from heat. For semiconductors, this is sufficient to have a few valence electrons to jump into the conduction band but for insulators, the band gap is so large that it makes this virtually impossible through heat alone. For semiconductors in absolute zero, the valence electrons get no more additional heat from energy, and they also act like insulators.
The great thing about semiconductors is that you can significantly alter and control its characteristics through a process that is called doping. This is done by introducing certain atoms as impurities into a pure semiconductor. When this happens, the semiconductor’s electrical, optical, and even structural properties can be altered. A doped semiconductor is referred to as an extrinsic semiconductor, while a pure semiconductor is called an intrinsic semiconductor.
There are two types of extrinsic semiconductors, n-type and p-type semiconductors. The resulting type from doping depends on the impurity that is added. Phosphorus (P) or Arsenic (As) is used to create n-type semiconductors while Boron is used to create p-type semiconductors.
The valence band of Phosphorus and Arsenic is very close to the semiconductor’s conduction band which makes it very easy for their valence electrons to jump to the conduction band. When this happens, the conductivity of the whole material is increased. On the other hand, the valence band of Boron is very close to the semiconductor’s valence band. This then makes it very easy for the semiconductor’s valence electrons to jump here. An empty space is left behind on the semiconductor’s valence band, which is called a hole and the conductivity of the whole material is also increased.
An amazing thing happens when you put together an n-type and a p-type semiconductor. First, remember that an n-type semiconductor has gotten extra electrons in its conduction band from doping while a p-type semiconductor got empty spaces or holes in its valence band. When these two materials meet, the extra electrons on their point of contact release some energy and move to the holes, creating a portion that is devoid of free electrons where the two materials meet. This portion is called the depletion region because it has been “depleted” of free electrons. Since this is the case, the free electrons from the n-type side will need a certain voltage called the voltage to pass through the depletion region and travel to the p-type side. Because of this voltage barrier, it becomes hard for the electrons to pass through the depletion region and the whole diode.
The resulting material is called a diode. This device is used as an electronic switch that can be turned on and off depending on the polarity and amount of applied voltage. It is very important to discuss diodes here because solar cells are basically just diodes that are designed to use the energy from sunlight to create more free electrons inside of it and generate an electric current. To illustrate this, imagine that a positive voltage is applied to the p-type side of a diode and a negative voltage is applied to the n-type side. The electrons on the p-type side of the depletion region get attracted to the positive terminal, which causes them to move toward it. These electrons then leave behind a hole, which restores the holes on the p-type side of the depletion region. On the other hand, the free electrons on the n-type side get repelled by the negative terminal, pushing them on the n-type side of the depletion region and restoring the free electrons in that area.
The result is that the depletion region gets smaller and along with this, the voltage barrier also decreases. Electric current can then flow freely through the diode. In this case, the diode is said to be forward-biased, which is its on-state.
Let us now examine what will happen if we will reverse the voltage on the diode. This time, the positive voltage is now connected to the n-type side of the diode and the negative voltage is applied to the p-type side.
This time, the free electrons on the n-type side of the diode are attracted to the positive voltage. These electrons then flow through the negative side of the voltage to the p-type side of the diode, filling up the holes near the depletion region. The result is that the depletion region gets bigger, increasing the voltage barrier even more and making it even harder for free electrons to flow through the diode. The diode is now said to be reverse-biased. This is the off-state of the diode.
The Diode’s IV Curve
These characteristics can easily be summarized and memorized through the use of the diode’s IV curve. The x-axis here corresponds to the voltage across the diode, with reference to its forward bias. This means that the positive voltages refer to a forward bias on the diode while the negative voltages refer to a negative bias. The y-axis then refers to the resulting electric current.
As we increase the voltage from 0 as the diode is forward-biased, we can see that the current grows very slowly. But when we hit the voltage at the “knee” of the curve, the current then starts growing exponentially. For silicon diodes, this voltage is called the knee voltage. To “turn on” a diode, you must apply a voltage that is greater than this knee voltage first before a significant amount of current flows in your circuit. Take note here that if a diode is forward biased, it consumes an amount of energy that is equal to the electric current through it times the voltage across the diode, which is just approximately equal to the knee voltage.
When we instead apply an increasing voltage while the diode is reverse-biased, the reverse happens. The current quickly increases(although to a much smaller value) to a value that is called the reverse saturation current and stays on that level. You may be wondering how can there still be current in the reverse-biased diode when we have just discussed that in this case, the depletion region widens, which makes it very hard for electrons to pass through. This is a very valid question and one that is rarely addressed in electronics textbooks and online resources.
This is due to a phenomenon in quantum mechanics that is called quantum tunneling. In quantum mechanics, particles such as electrons are described to behave very differently in the sense that we are always not certain about their position. Quantum mechanics instead describes their position in terms of probability waves, which are spread around the “apparent” position of the electron and decreases with distance.
To understand quantum tunneling, we should analyze an electron in a potential well. Take note that the rectangles on the left and right of the electron in this diagram refer to an energy level that the electron must first have before it can get out of the potential well. We can make the analogy on a rock that is stuck on a real well but we should analyze it in terms of energy. Since the rock is at the bottom of the well, it has a low gravitational potential energy. For the rock to get out of the well, someone must pick it up and carry it to the top, giving it potential energy in the process. If the rock is carried only up to halfway up the well or you give it an insufficient amount of gravitational potential energy, it would just lose it all and fall back down again. This is the exact same case with the electron in a potential well; the only difference is that in the potential well, we are now talking about the electrical potential energy of the electron.
According to quantum mechanics, the electron still has a small chance of getting out of the potential well, even if it does not have the required energy level because of quantum tunneling. This is because, as we have discussed, we are uncertain of the position of the electron. For example, if you put an electron in a small box, it may still suddenly appear out of the box if its probability wave reaches out of the box. In the same way, the electron’s probability wave in the potential well may extend outside, which can cause it to “tunnel” through the potential well and get outside of it.
Electrons in the depletion layer of a diode can be considered as also being in a potential well because they must also have a sufficient amount of energy to be able to escape. However, because of quantum tunneling, some electrons do escape and flow outside as current. This is what constitutes the reverse saturation current of the diode.
Reverse-biased Region and its Similarities to the Solar Cell
The reverse-biased region of the IV curve is one that is most important to us because solar cells also operate in this region. To prove this, we must look at how the IV curve changes with increases in the temperature of the diode. When the diode becomes hotter, the valence electrons in the semiconductor material gain additional energy, which allows some of them to jump to the conduction band and become free electrons. This means that the current must increase.
However, in the forward-biased region, we don’t see such an increase because the number of additional free electrons that the heat generates is very low compared to the total number of free electrons that are already in the material. Meanwhile, in the reverse-biased region, there are almost no free electrons, so the number of additional free electrons that are generated by heat is very large in comparison to the already existing ones. It is important to notice that the reverse saturation current is proportional to temperature, which means that, for example, if you double the diode’s temperature, the reverse saturation current will also double its value. The same thing happens when the additional free electrons get their energy from sunlight instead of heat. And because there are now many more free electrons in the depletion region, more electrons can now tunnel through it and be channeled and used as an electric current.
Therefore, a solar cell is just a diode that is operating in the reverse-biased region, with the only difference being that the solar cell itself is now the one that is producing energy instead of an outside voltage source. With this knowledge, we will also be able to learn the characteristics of solar cells and thus, solar panels:
Similar to a diode, the solar cell can only produce certain pairs of values for its voltage and current and these values are graphed in an IV curve.
In the same way that the reverse saturation current is the maximum reverse current that we can have on the diode, the solar cell also has a maximum amount of current that it can produce, which is called the short-circuit current.
· If the diode is forward-biased, it consumes energy, but when it is reverse-biased, it instead generates a small amount of energy from heat. On the other hand, a solar cell also generates energy while operating in the reverse biased region. It then consumes energy instead when the current flows in the opposite direction. When this happens, the solar cell may be damaged since they are not designed to consume energy themselves. This is the reason why solar panels still need string fuses even though its wires are already capable of handling even its short-circuit current. String fuses prevent an excess amount of current from the other strings in the PV array to go to one faulty string, causing the solar panels in this string to consume all the energy produced and damage them.
· In the same way that the reverse saturation current is proportional to heat, the current that is produced by a solar cell is also proportional to the received irradiance or energy from sunlight.
The Photoelectric Effect and the First Solar Cell
The whole phenomenon of objects generating electricity when hit by light is called the photoelectric effect, which was first discovered in 1887 by the German physicist named Heinrich Rudolf Hertz. Hertz was more famous for his work on radio waves and his discovery of the photoelectric effect actually happened while experimenting on different electromagnetic waves. He noticed that when he shined ultraviolet light(which is also a form of electromagnetic wave, along with all frequencies of light) on the two metal electrodes of a voltage source, the voltage level can be increased to a point that it can cause sparking. It was only until 1902, however, when an explanation of why this happens was proposed by another German physicist named Philipp Lenard. Lenard proposed that electrically charged particles must be dislodged from the metal surface when it is illuminated.
Although Lenard was right, still, nobody understood how this happens. The theory of electromagnetism by James Maxwell was already available at that time, but it was still not enough to explain it. In fact, it even contradicts the predictions of the electromagnetic theory.
When scientists tried to measure the kinetic energy of the electrons that were released due to the photoelectric effect, they find out that it does not vary with the intensity of the light, which is what is predicted by electromagnetic theory. What they found out was that kinetic energies were instead proportional to the frequency of the light. The only effect that light intensity had was to increase the number of electrons that were released.
In 1905, Albert Einstein concluded that light should be made up of particles that were called photons, which contain a fixed amount of energy that is proportional to the light’s frequency. At that time, this is a very bold conclusion to make since the electromagnetic theory has seemed to end the debate on whether light is a particle or a wave by explaining that light is a form of electromagnetic wave.
With the assumption of light being made up of photons, Einstein was able to lay out the math that explains how the photoelectric effect works. But even if this is the case, it was already universally-accepted in the scientific community that light is a form of a wave. Because of this, Einstein’s work on the photoelectric effect was still doubted by many scientists at that time. However, in 1916, the American physicist named Robert Millikan was able to verify Einstein’s mathematical model of the photoelectric effect, which caused Einstein to be awarded a Nobel Prize in Physics in 1921.
More than three decades from then, Einstein’s mathematical model of the photoelectric effect was first put to practical use with the invention of the first solar cell. Gerald Pearson, Calvin Fuller, and Darly Chaplin worked together at Bell Labs and was able to develop the first silicon solar cell in 1954.
Modularity of Solar Energy
Now we know the basic building blocks of solar energy, which is the solar cell and how it works, we can immediately see and understand from this one of the advantages of solar energy compared to all the other energy sources, and that is modularity. Modularity refers to the use of individually distinct functional units and that is exactly how solar energy can easily be scaled up to meet any requirement. For example, individual solar cells can power calculators that require only a little amount of power. Garden lights that require a little more power than that can be powered by a small solar panel. Going larger, street lights can also be powered by solar panels that are just a little bit smaller than the solar panels that are most commonly used. To power a residential home or even a large commercial building, you will just need to install the right number of solar panels on their roofs. Lastly, in the extreme case, if you want to have a specific size of a solar power plant, you will then just use more solar panels to reach your requirement.
This characteristic of solar energy is very unique to it and can never be found on any other energy source. All of the other energy sources require them to be produced in large power plants only, without an option to go smaller and meet smaller energy demands. This makes solar energy very versatile, allowing it to be used in almost any application, from solar-powered calculators to satellites.
Building Solar Panels from Solar Cells
As we have mentioned, solar cells produce only a very low amount of voltage (less than 1V) although they can already produce a decent amount of current. This is a problem because you will not be able to use it to power anything. For example, no matter how large your 12V battery is, you will still not be able to use it to power a machine that needs 24V. To overcome this problem, solar cells are connected in series in a solar panel to increase the total voltage to a useable level. The most commonly used number of cells in series in solar panels are 60 and 72.
If you think about it, 60 or 72 cells in one solar panel can also be arranged in two strings of 30 and 36 solar cells, respectively. But this is never done in actual solar panel designs. The reason for this, however, is not just to reach a higher voltage.
The more number of strings that you have, the higher the total current of the solar panel will be. This is why having more strings of solar cells in a solar panel is not advisable as this will incur more losses in the form of heat. In fact, the amount of energy that is lost due to heat is proportional to the square of the current. This means that if you double the number of strings(and thus, the current), the total energy lost to heat will be quadrupled. Conversely, if you reduce the number of strings by half, the total energy lost will be decreased by one-fourth.
Efficiency in a solar panel is very important because more efficiency will mean more produced energy for the same amount of area. And in the very competitive solar panel market today, even just a 1% difference in efficiency can already translate to many more sales and projects.
The Solar Panel’s IV Curve
We have already discussed the solar panel’s IV curve, but it is very important to look at it more deeply as this will also give us an idea of how solar panels work in actual solar installations. As we have already discussed, this is what a solar panel’s IV curve looks like:
There are four important points on the IV curve that are always shown on any solar panel’s datasheet:
Open-Circuit Voltage – this is the maximum voltage that a solar panel can produce and is also the point where it produces no current. Since power is just the product of the voltage and current, the power produced at this point is 0. This point of the solar panel’s IV curve is the safest and is usually considered as the point where the solar panel is “off”. However, take note that the best way to turn off a solar panel is to cover it. In fact, solar panel manufacturers don’t even recommend disconnecting a solar panel while under the sun as this can cause arcing and fires. For solar PV engineers, the open-circuit voltage is used to calculate the maximum number of solar panels that can be put on one string.
Short-Circuit Voltage – this is the maximum current that a solar panel can produce and is also the point where it produces no voltage. Similar to the open-circuit voltage, the solar panel also produces no power at this point. However, short-circuiting a solar panel to turn it off is not advisable because it will still produce some amount of heat on its wires. But since the wires built-in the solar panels are already designed to withstand its short-circuit current, this is still a relatively safe way to turn it off.
Voltage and Current at MPP – if we study all the points in the IV curve closely, we will notice that the solar panel will produce the maximum amount of power if it operates on one specific point, which is called the maximum power point. The voltage and current at this point are called maximum power voltage and maximum power current, respectively. During operation, we would want the solar panel to always operate on this point to produce the most amount of power. To achieve this, inverters are designed to have a Maximum Power Point Tracking(MPPT) Function that forces the solar panel to operate on its maximum power point(MPP). Since the solar panel produces the most amount of power at its MPP, it also reaches its maximum efficiency when operating at this point.
Changes in the IV Curve
In its operation, a solar panel’s IV curve will change, depending on the available irradiance and the temperature of its solar cells. As we have discussed, a solar panel’s current is proportional to the irradiance, as can also be seen on the graph of the different IV curves for different values of irradiance.
The IV curve also changes with respect to the solar cell’s temperature. As we have discussed, its effect on the total current is negligible, but for the voltage, its effect is huge and must be taken into consideration in designing solar PV arrays. The changes in the solar panel’s voltage are inversely proportional to temperature, which means that lower solar cell temperatures will lead to higher voltages. In the design of solar PV arrays, it is important to note the lowest expected temperature in the area to determine the lowest expected solar cell temperature and thus, the highest voltage that it can produce. This is done so that we are sure that the voltage produced by the solar PV array will never exceed the inverter’s input voltage at any time and day throughout the year.
We can predict the effect of changes in temperature to the IV curve through the use of three temperature coefficients which can be seen on a solar panel’s datasheet. These are the temperature coefficients of current, voltage, and power. If you will take a look at these values on a solar panel’s datasheet, you will see that the temperature coefficient for current is a positive number, while the temperature coefficients for voltage and power are both negative. This matches what we have discussed earlier that increases in solar cell temperature increase the total current by a very small amount and that the voltage increases with lower solar cell temperatures.
What we haven’t discussed yet, however, is how the solar panel’s output power is affected by changes in the solar cell’s temperature. As we can see on the datasheet, the temperature coefficient of power is negative, which means that it actually increases as the solar cell’s temperature decreases. This means that solar panels would actually produce more power during cool, sunny days than during sunny but scorching hot days.
STC and NOCT
Another thing that needs to be discussed that is also related to the changes in the IV curve is the difference between STC and NOCT. STC stands for standard test conditions, which corresponds to the ideal input of 1,000W/m2 and a solar cell temperature of 25 degrees Celcius. The solar panel’s rated maximum power is defined as the power that the solar panel will produce during these conditions. However, take note that these are ideal conditions and are rarely met in actual operation. Actual operating conditions are more accurately represented by NOCT, which corresponds to an irradiance of 800W/m2 and a solar cell temperature of 45 degrees Celsius.
Effect of Shading
Even though solar panels can be installed virtually anywhere, they are still limited by shading from trees, nearby buildings, electrical posts, and other tall objects. This is a problem because shading can disproportionately affect the power production of a string of solar panels. This is because the solar panels in a string are forced to produce the same amount of current and therefore, any amount of shading that will decrease the output current of one solar panel will also decrease the output current of all solar panels in the string. A good way to think about this is by thinking that all solar panels in a string will only perform as well as the worst-performing solar panel.
This problem is even much more worse in the case of severe shading. To understand this, we must first remember that the inverter is actively looking for total voltage and current value for the whole array where the total power will be at its maximum. Now, going back to our case of severe shading, imagine one solar panel in a string that is severely shaded. Also, for this analysis, we will also only consider one string in the solar PV array to make it simple.
The IV curve of the severely shaded solar panel will be significantly lower because of the lower input irradiance. The other solar panels can follow this and also significantly reduce their output current and power, but the inverter’s MPPT function will still look for the point where the string will produce the most energy as a whole. In these cases, the MPP of the whole string may happen at a point where the severely shaded solar panel will be forced to operate in the negative voltage side of its IV curve. When this happens, the solar panel will then consume power, instead of producing it. Its power production will be negative, but since the power production of all the other solar panels in the string are not significantly decreased, the total power output of the string, in this case, will still be at its maximum.
Since solar panels are composed of several solar cells in series, the same disproportionate effect happens in the level of the solar panel. For example, if one solar panel gets 20% of its area shaded, its power production would drop by much more than 20% for the same reason. Since all the solar cells in a solar panel are connected in series, they would also perform only as good as the worst-performing solar cell.
In solar installations, it is a common practice to avoid shading from 9 AM to 3 PM as these are the peak sun hours and is when the solar panel receives most of the irradiance. If shading is still unavoidable, another good solution is to use microinverters instead. In microinverter systems, solar panels connect directly to the microinverter, which means that the solar panels are not anymore connected in series. In fact, each solar panel will even have its own maximum power point tracker, so any form of shading will only affect the shaded solar panel/s.
To minimize the effects of shading on a solar panel, manufacturers have used what are called bypass diodes to effectively “bypass” a severely shaded solar cell, remove it from the string and not let it affect the production of all the other solar cells and also to protect it from damage.
Remember that a severely shaded solar cell can operate in the negative voltage part of its IV curve. When this happens, it will then consume energy instead of producing it and may be damaged in the process. If we put a bypass diode in parallel to it in a way such that a negative voltage from the solar cell will forward bias the diode, that solar cell will then effectively be in parallel with a short circuit. This way, the current from all the other solar panels in the string will choose to flow through the diode instead of the severely shaded solar panel.
In reality, however, the cost of adding a bypass diode to each solar cell outweighs the benefits of having them, so what solar panel manufacturers do is they only add 2 or 4 bypass diodes with each one being connected in parallel to one half or one-fourth of the total number of solar cells.