Solar cells are constructed of silicon that has been specifically processed to absorb as much light as possible. Within a panel (module), solar PV cells are electrically coupled in series and parallel connections to achieve the necessary output voltage and/or current values. Solar PV panels are typically made up of 36, 60, or 72 interconnected solar cells.
When there is no external load applied, most silicon solar cells produce roughly 0.5 to 0.6 volts DC, which is the main characteristic of a pn-junction. A solar cell creates its maximum output voltage, also known as its open-circuit voltage, VOC, when there is no load connected or a very low current demand.
To achieve the entire output voltage, stronger sunlight (measured in watts per metre squared, W/m2) is necessary as the load current demand from the cell grows. However, regardless of how bright and strong the light is, the amount of current a solar cell can create has a maximum limit.
While individual solar cells can be connected within a single PV panel, solar photovoltaic panels can be connected in series and/or parallel to form an array, which increases the total potential power output for a given solar application as compared to a single panel.
What is the connection between solar cells?
Solar cells are connected in parallel. Panels can only be joined in one of two ways: in parallel or in series. When solar panels are connected in parallel, the current (amperage) is additive, but the voltage remains constant.
In a solar module, how are the solar cells connected?
A bulk silicon PV module is made up of numerous individual solar cells that are connected in series to improve the power and voltage over a single solar cell. A PV module’s voltage is often chosen to be compatible with a 12V battery. Under 25 C with AM1.5 light, each individual silicon solar cell has a voltage at the maximum power point of roughly 0.5V. Most modules have 36 solar cells in line to account for the projected reduction in PV module voltage due to temperature and the fact that a battery may require voltages of 15V or higher to charge. Under conventional test settings, this results in an open-circuit voltage of roughly 21V and an operating voltage of about 17 or 18V at maximum power and operating temperature. The remaining extra voltage is provided to allow for voltage losses induced by other PV system elements, such as operating below the maximum power point and light intensity reductions.
36 cells are connected in series in a typical module to create a voltage adequate to charge a 12V battery.
The number of solar cells determines the PV module’s voltage, while the module’s current is mostly governed by the size of the solar cells. The current density of a commercial solar cell is around 30 mA/cm2 to 36 mA/cm2 at AM1.5 and under ideal tilt circumstances. Single crystal solar cells are typically 15.6 x 15.6 cm2 in size, resulting in a total current of nearly 9 – 10A from a module.
The output of typical modules at STC is shown in the table below. VMP and VOC scale with the number of cells in the module, although IMP and ISC do not.
If all of the solar cells in a module have the same electrical properties and are exposed to the same amount of insolation and temperature, all of the cells will operate at the same current and voltage. In this situation, the PV module’s IV curve is similar to that of the individual cells, with the exception that the voltage and current are higher. The circuit’s equation is as follows:
The total IV curve of a group of solar cells that are all linked is depicted below. The total current is calculated by multiplying the current of each individual cell by the number of parallel cells. as follows: ISC total is equal to ISC M. The total voltage is calculated by multiplying the voltage of each individual cell by the number of cells in series. as follows:
When the cells are identical, the fill factor remains constant regardless of whether they are in parallel or series. When the cells are merged, however, there is frequently a mismatch in the cells, resulting in a reduced fill factor. The cell mismatch could be due to manufacturing flaws or changes in light levels between the cells, with one cell receiving more light than the other.
In a solar array, how are the solar cells connected?
Solar energy is converted into direct-current (DC) power using photovoltaic cells and panels. Solar panels in a single photovoltaic array are connected in the same way that PV cells are connected in a single panel.
The panels in an array can be linked in series, parallel, or a combination of the two, although in most cases, a series connection is selected to enhance the output voltage. When two solar panels are connected in series, for example, the voltage is doubled while the current remains the same.
A solar array can be as small as a few individual PV modules or panels joined together in an urban area and put on a rooftop, or it can be as large as many hundreds of PV panels interconnected in a field to provide power for a whole town or neighborhood. The modular photovoltaic array (PV systemflexibility )’s allows designers to develop solar power systems that can suit a wide range of electrical needs, large or small.
Even though their power, voltage, or current outputs are ostensibly equivalent, solar panels or modules from various manufacturers should not be used together in a single array. This is because variances in the I-V characteristic curves and spectrum response of solar cells are likely to produce extra mismatch losses within the array, lowering its total efficiency.
Why are solar cells connected in series?
Because electrical power in watts equals “volts times amperes” (P = V x I), connecting PV panels in parallel increases current and thus power production. Photovoltaic cells generate electricity at a voltage of 0.5 to 0.6 volts DC, with current proportional to the cell’s area and irradiance.
What metal is used to join solar cells to solar panels?
“Silver is used for connecting solar cells to solar panels,” we may deduce from the alternatives. Because it is an excellent electrical conductor.
What is a PV string, exactly?
Individual PV modules are connected in series and parallel in a bigger PV array. A “string” is a group of solar cells or modules that are connected in series. In PV arrays, the combination of series and parallel connections can cause a number of issues. An open circuit in one of the series strings is one potential issue. The current from the parallel linked string (commonly referred to as a “block”) will thus be lower than the current from the other blocks in the module. This is electrically equivalent to connecting one shaded solar cell to multiple excellent cells, and the power from the entire solar cell block is lost. This impact is depicted in the diagram below.
Larger PV arrays may have mismatch effects. Despite the fact that all modules are identical and the array is not shaded, mismatch and hot spot effects can still occur.
If the by-pass diodes are not rated to withstand the current of the complete parallel linked array, parallel connections along with mismatch effects can cause issues. The by-pass diodes of series connected modules, for example, get connected in parallel in parallel strings with series connected modules, as shown in the diagram below. Due to a mismatch in the series linked modules, current will travel through a by-pass diode, heating it. The effective resistance is reduced when the by-pass diode is heated. The by-pass diodes that are somewhat hotter will now carry the majority of the current. These by-pass diodes then get even hotter, lowering their resistance and increasing current flow even more. Nearly all of the current may eventually pass through a single set of by-pass diodes. The diodes will burn out if they are not rated to handle the current from the parallel combination of modules, causing damage to the PV modules.
In paralleled modules, bypass diodes are used. Each 36 cell module normally has two bypass diodes.
In addition to using by-pass diodes to eliminate mismatch losses, a blocking diode can also be used to reduce mismatch losses. A blocking diode, as shown in the diagram below, prevents current flow from the battery through the PV array, preventing the module from charging the battery at night. Each string to be linked in parallel should have its own blocking diode when using parallel connected modules. This not only reduces the blocking diode’s necessary current carrying capability, but also stops current from flowing from one parallel string into a lower-current string, reducing mismatch losses in parallel linked arrays.
In a solar panel, what is a module?
A solar module, also known as a solar panel, is a single photovoltaic panel made up of connected solar cells. To generate electricity, solar cells absorb sunlight as a source of energy. To power buildings, a variety of modules are employed.
What is the meaning of MPPT?
The theory and functioning of “Maximum Power Point Tracking” as employed in solar electric charge controllers are covered in this section.
A maximum power point tracker, or MPPT, is an electronic DC to DC converter that optimizes the match between the solar array (PV panels) and the battery bank or utility grid. Simply put, they convert the higher voltage DC output from solar panels (as well as a few wind generators) to the lower voltage required to charge batteries.
(These are sometimes referred to as “power point trackers,” not to be confused with PANEL trackers, which are solar panel mounts that follow or track the sun.)
So what do you mean by “optimize”?
Solar cells are fascinating devices. They are, however, not particularly intelligent. Batteries aren’t either; in fact, they’re downright stupid. The majority of PV panels are designed to produce 12 volts nominally. The catch is the word “nominal.” In reality, nearly all “12-volt” solar panels are designed to produce between 16 and 18 volts. The issue is that a nominal 12-volt battery is near to an actual 12-volt battery – 10.5 to 12.7 volts, depending on charge condition. Most batteries require between 13.2 and 14.4 volts to completely charge, which is very different from what most panels are designed to produce.
So, now we’ve got this cool 130-watt solar panel. The first snag is that it’s only rated for 130 watts at a specific voltage and current. 7.39 amps at 17.6 volts are rated for the Kyocera KC-130. 7.39 amps multiplied by 17.6 volts equals 130 watts.
Now the Catch 22
So, what happens if you use a conventional charge controller to connect this 130-watt panel to your battery?
Your panel is capable of delivering 7.4 amps. Your battery is charged to 12 volts: 7.4 amps multiplied by 12 volts equals 88.8 watts. You saved over 41 watts, but you had to pay for 130. That 41 watts isn’t going anywhere; it’s just not being created because the panel and battery aren’t a good match. It’s even worse if you have a really low battery, like 10.5 volts, because you may be losing up to 35 percent of your power (11 volts x 7.4 amps = 81.4 watts). You lost approximately 48 watts.
One alternative you might consider is to design panels that output 14 volts or less to match the battery.
The panel is rated at 130 watts in full sunshine at a specific temperature, which is catch #22a (STC – or standard test conditions). You won’t receive 17.4 volts if the solar panel’s temperature is too high. You might get under 16 volts at the temperatures encountered in many hot climate places. You’re in danger if you started with a 15-volt panel (like some of the so-called “self-regulating” panels), because there won’t be enough voltage to charge the battery. Solar panels must be designed with enough wiggle room to work in the most adverse conditions. The panel will just sit there looking silly, and your batteries will become even more stupid.
What is Maximum Power Point Tracking?
Panel tracking occurs when the panels are mounted on a mount that moves with the sun. The Zomeworks are the most common. These maximize output by following the sun as it moves across the sky. These normally provide a 15% increase in the winter and up to a 35% increase in the summer.
For MPPT controllers, this is the polar opposite of seasonal variation. Because the temperature of the panels is lower in the winter, they produce more power. Due to the shorter days, winter is usually when you require the most power from your solar panels.
Maximum Power Point Tracking is a type of electronic tracking that is commonly done with a computer. The charge controller compares the output of the panels to the voltage of the battery. It then determines what the best power output from the panel is for charging the battery. It converts this to the best voltage possible in order to get the most AMPS into the battery. (Keep in mind that the number of Amps into the battery is what matters.) The conversion efficiency of most current MPPTs is around 93-97 percent. In the winter, you can expect a 20 to 45 percent increase in power, whereas in the summer, you can expect a 10-15 percent increase. The amount of gain varies greatly based on the weather, temperature, battery state of charge, and other variables.
As the cost of solar reduces and utility prices rise, grid connection solutions are becoming increasingly popular. There are a variety of grid-tie only (no battery) inverter brands available. MPPT is incorporated into each of these. The MPPT conversion efficiency on those is from 94 percent to 97 percent.
How Maximum Power Point Tracking works
This is where optimization, also known as maximum power point tracking, comes into play. Assume your battery is at 12 volts and is low. An MPPT converts 17.6 volts at 7.4 amps to 10.8 amps at 12 volts, which is what the battery now receives. Everyone is thrilled since you still have almost 130 watts.
At 11.5 volts, you should receive roughly 11.3 amps for 100 percent power conversion, but you’ll need to give the battery a higher voltage to force the amps in. And this is a simplified description; in reality, the MPPT charge controller’s output may vary continuously to ensure that the maximum amps are delivered to the battery.
A screenshot from the Maui Solar Software “PV-Design Pro” computer program is seen on the left (click on the picture for full-size image). When you look at the green line, you’ll notice a dramatic peak in the upper right corner, which symbolizes the maximum power point. An MPPT controller “looks” for that precise point, then performs the voltage/current conversion to match the battery’s requirements. In real life, that peak shifts with the changing light and weather.
In almost all cases, an MPPT tracks the maximum power point, which will differ from the STC (Standard Test Conditions) rating. Because the power output increases higher as the panel temperature goes down, a 120-watt panel can actually put out over 130+ watts in very cold conditions – but if you don’t have some way of measuring that power point, you’ll lose it. In extreme heat, on the other hand, the power lowers – you lose power as the temperature rises. As a result, you gain less in the summer.
- Cold weather solar panels perform better in cold weather, but without an MPPT, you’ll lose the majority of the benefits. Cold weather is most likely in the winter, when daylight hours are at their lowest and you need the greatest power to recharge your batteries.
- Low battery charge – the lower the level of charge in your battery, the more current it receives from an MPPT – another time when the extra power is most needed. Both of these situations can exist at the same moment.
- Long wire runs – If your panels are 100 feet apart and you’re charging a 12-volt battery, the voltage drop and power loss can be significant unless you utilize extremely wide wire. This can be quite costly. The power loss is substantially lower if four 12 volt panels are put in series for 48 volts, and the controller will convert the high voltage to 12 volts at the battery. This also means that if the controller is fed by a high-voltage panel, you can use much smaller cable.
How a Maximum Power Point Tracker Works:
The Power Point Tracker is a DC to DC converter with a high frequency. To precisely match the panels to the batteries, they take the DC input from the solar panels, convert it to high-frequency AC, and then convert it back to a different DC voltage and current. MPPTs work at very high audio frequencies, usually between 20 and 80 kHz. High-frequency circuits have the advantage of being able to be created using very high-efficiency transformers and compact components. High-frequency circuit design can be difficult due to issues with components of the circuit “broadcasting” like a radio transmitter, producing radio and television interference. Isolation and suppression of noise become critical.
There are a few non-digital (linear) MPPT charge controls on the market. These are far easier and less expensive to construct and design than computerized ones. They do boost efficiency to some extent, but total efficiency varies greatly – and we’ve seen a few lose their “tracking point” and even deteriorate. If a cloud passes over the panel, the linear circuit will look for the next best location, but it will be too far out in the deep end to find it when the sun returns. Thankfully, there aren’t many of these left.
The power point tracker (and all DC to DC converters) work by taking the DC input current, converting it to AC, passing it through a transformer (typically a toroid, which looks like a doughnut), and then rectifying it back to DC before the output regulator. In most DC to DC converters, this is purely an electrical process with no real intelligence involved save for some output voltage management. Solar charge controllers require a lot more intelligence because light and temperature conditions change throughout the day, as well as battery voltage fluctuations.
Smart power trackers
Microprocessor-controlled digital MPPT controllers are available in all modern versions. They recognize when the output to the battery needs to be adjusted, and they shut down for a few microseconds to “look” at the solar panel and battery and make any necessary modifications. Although not exactly new (the Australian company AERL had some as early as 1985), electronic microprocessors have only lately become affordable enough for use in smaller systems (less than 1 KW of the panel). Several firms currently make MPPT charge controls, including Outback Power, Xantrex XW-SCC, Blue Sky Energy, Apollo Solar, Midnite Solar, Morningstar, and a few more.
What is the process of connecting solar panels to the grid?
Connect the solar panels to a power inverter directly and then to the home power grid, or connect the inverter to the battery and then to the home power grid. This power inverter turns solar energy into usable electricity for the home.
Voltage and Amps in Parallel
Connect all of the positive connections on each panel together, then do the same for the negative terminals, to wire solar panels in parallel. The total current generated by the parallel array will be equal to the sum of all panel amperages. The overall voltage, on the other hand, will be equal to the output voltage of a single panel.
For example, we have three 18-volt, 6-amp panels linked in parallel in the diagram above. Despite the fact that the output current is 18 amps (6A + 6A + 6A = 18A), the output voltage remains at 18 volts.