Many a times it has been seen that people try to connect solar panels directly to the load. The load can be anything. It can be a light bulb, radio or even a battery for charging, or a gadget.
This is never really a good idea and here is why – when a solar panel is directly connected to the load, then much of the power that the solar panel has the potential to generate is not generated at all.
Let’s explain this further. Every solar panel has its own internal resistance which varies with the output power. When the solar panel is generating peak power the internal resistance at this point is termed as the characteristic resistance of the panel.
A great way to increase solar panel output is to connect it to the load using an MPPT device. An MPPT device matches the internal resistance of the solar panel at its Maximum Power Point and maximizes its power output.
Can I connect solar panels direct to a load?
Now when you connect the solar panel to the load then the closest the internal resistance of the load is to the characteristic resistance of the panel the more the power can be extracted from the panel.
In reality, the output of a solar panel is not only dependent upon the amount of light it is receiving and the temperature of the panel, but also it is dependent upon the value of resistance of the load. As the internal resistance of any load is not known, it is never a good idea to connect it directly to a solar panel.
To match the resistances is difficult, because normally you may have a solar panel which has a characteristic resistance of 3 to 5 ohms, and you are trying to connect it to a battery that may have an internal resistance of less than 1 ohm.
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How to find solar panel internal resistance
The characteristic resistance of the solar panel can be roughly found out by simply dividing the maximum power voltage value with the maximum power current value. They are denoted by Vmp and Imp respectively. Both of these values can be found on the specification sheet of the panel.
For example in a specification sheet we have the maximum power voltage and we also have the maximum power current given in the classification sheet. We divide them and we get the value of 3.4 ohms. This is the characteristic resistance.
If the maximum power current and voltage is not available, then we can use the open circuit voltage value (Voc) and the closed circuit current (Isc) values to obtain the characteristic resistance roughly.
In this case we have a value of 3.4 ohms. If the load resistance is around 3.4 ohms for this panel only, then we will be able to get the peak power from this panel. This panel is rated at 280 watts and if we don’t match the resistances, then we will not be utilizing the panel fully.
As mentioned earlier, it is very difficult to have a load that has similar internal resistance or impedance value as your solar panels. What is the solution? The solution is that it is always advisable to use an MPPT device between your panel and your load.
How does an MPPT device work?
How does the MPPT device work for solar panels?
The device isolates the load from the panel and shows the panel a resistance or impedance value that maximized the power withdrawal from the panel. This MPPT device is nothing but a DC to DC converter with various internal resistances.
The MPPT device will switch to the internal resistance level that will maximize the power from the panel for a given amount of solar insolation and load resistance. The MPPT device or the maximum power point tracking device can be purchased for as low as $15.
Advantages of MPPT
At present, in most charge controllers and inverters the MPPT device is built-in so whenever you buy solar panels make sure you attach it to an MPPT device, be it in your charge controller or be it in your inverter.
There are also large solar panels available in the market with power rating of 250 watt plus – some of them are called smart panels. These panels come with the MPPT device built-in. They are slightly expensive but can pay back the extra cost in two to three years with their higher output for battery charging.
The other advantage of using an MPPT charge controller is that it will protect your battery from overcharging and under charging.
Can micro-inverters increase solar panel output?
In the world of sustainable technology solar radiation being received by the panel is at best of times not uniform. Meaning, some panels can receive much more light energy than the others and the reasons for this can be multiple.
They include different orientation, different inclination, part shading of a panel to dust and even snow cover. In such cases, the overall capability of the whole string of panels is drastically reduced. It might be that only part of one module might be shaded but it still brings down the output of other panels in the same string that are receiving light.
Micro-inverters are connected at solar panel level and can increase panel power output by up to 20%. Power reduction due to shading issues is greatly reduced by handling the DC/AC power conversion at the panel level rather than the string level.
In other words, the whole collection of panels will operate at the output of the worst-performing solar panel if they are connected to a normal string inverter when solar panels are attached in series with each other.
How many solar panels on a string?
This arrangement is called a string and a collection of strings is called an array. The drop of the whole string performance is kind of like the fairy lights, where the failure of one bulb does not allow the other bulbs to operate, even though the filaments of the other bulbs are undamaged.
Solar panels attached to one string in series behave like a single panel when they are connected to a large central inverter. The output from the panel’s can be reduced. This is despite the fact that the central, or the string inverter as it is called, would have MPPT option built into it.
A number of solar panels connected together in series are collectively called a string. The maximum number of panels connected depends on the input voltage of the string inverter, which converts DC power to AC power for household appliances.
For those who are unaware of the MPPT functionality, it is used for maximizing the output from the panels, particularly when they are partially shaded or there is a cloud cover. A central inverter that may have dual MPPT option would be able to get more power if it is connected to, let’s say, two different string of panels and each string is facing a different direction.
In such situation, even if one string gets shaded, it doesn’t affect the power of the second string. Micro inverters take the MPPT functionality from the string level to the panel level and in doing so they make all the panels function independently.
Shading on one panel has no bearing at all on all the other panels and it is for this reason that it is estimated that micro inverters can increase the output of solar systems by more than 20 percent. Why do we have more solar systems with central or string inverter when micro inverters are so beneficial?
- Firstly, micro inverters are a relatively recent development.
- Secondly, the cost of having micro inverters on each panel is more than having one central inverter, but for a small system it may cost less to have micro inverters.
How are micro inverters connected?
Most micro inverters in the market only connect to a single panel at a time, but there are certain micro inverters that can be connected to up to two and even four panels.
Let’s say for a 1 kilowatt system, 2 micro inverters with dual port, or a single 4-panel micro inverter would suffice.
Make sure though, that the micro inverter is compatible with the panel. The other thing to note is almost all micro inverters sold in the market are grid-tied, so they’re most feasible to use for our off grid system.
They can be used for an off-grid system that has a battery backup but this requires a bit of hacking. A coupling of the micro inverter with the inverter charger has to be made, because there has to be a separate device that communicates and takes care of the battery charge.
What do power optimizers maximize?
A device called the power optimizer is attached individually to every panel and allows MPPT functionality at the panel level. It is, however, different to micro-inverters in that the output of the power optimizer is DC and furthermore power optimizers also require a central or a string inverter.
The biggest advantage of micro inverters is that they allow you to increase the size of your solar system at a later time. This way the cost of your investment can be spread out.