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101 renewable - micro inverters versus power optimizer

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Last Updated
7th of January, 2020

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Voltage_regulator.jpg

During the solar energy harvesting process, the electrical power generated by solar panels is carried by home runs equipment and wires and stored into battery banks or transferred to the utility grid. It is even used to power on conventional consumer loads. Therefore it must be mitigated in order to obtain maximum transfer efficiency considering the electrical power generated is not constant in value for a given time interval. The process of mitigating the electrical power generated by PV solar panels is done using power optimizers, power conditioners, power controllers or power regulators.

What is the difference between a micro inverter and a power optimizer?

A micro inverter is mounted under a solar panel (or solar module) performing the role of a string or an array inverter at the panel level. A micro inverter receives DC power from a PV panel and moves the power over the home runs as alternative electric power straight from underneath the panel where it is mounted. A micro-inverter gets the max possible power out of that PV panel through MPPT optimization performed at the PV panel level.

A power optimizer is a DC to DC power conditioner working by strictly following the IV curve for that specific PV module where it was attached to. In this way, it extracts maximum electric power flow from the PV module(PV panel) during the period of time when the module is operating in conditions less optimal caused by shades and soiling. A power optimizer is able to recover over 50% of the PV panel power lost. A power optimizer works complementary to a string or an array inverter increasing site MPPT performances and allowing an inverter mounted on the DC site, downstream from the power optimizers, to improve the system by adding protections on DC respective on AC side, general communication functions and battery bank charger capability.

What is the common benefit of using either a power optimizer or a micro inverter?

They do have things in common. They are both mounted under a PV module and they leverage the control at the module level. They both eliminate the Christmas Tree Effect allowing modules to become parallel-connected through either optimizers or micro-inverters. In this way when one the PV module has incapacitated the rest of them are working at full capacity offering the best power output efficiency.

Solar power charge controllers:~

Solar power charge controllers can function in either PWM mode or in MPPT mode.

PWM types of charge controllers come with integrated temperature compensation and are very well suited as battery banks charge controllers.

Traditional solar power regulators featuring PWM (pulse width modulation) charging capabilities operate by connecting the solar array directly to the battery bank. When that array is directly connected to the battery bank, the array output voltage is ‘pulled down’ as value towards the battery voltage values. That occurs because the batteries represent a very large load against the limited current source of a solar array. PWM controllers are compensating for the battery bank request for the charging current (charging amps).

The other type of solar power controllers is the MPPT charge controller. In the case of an MPPT controller, the solar photo-energy input together with the operating temperature at the module level which acts as a deterrent(high temperatures are limiting the output), determines the working voltage value for that array.


solarcell-ivgraph3

An MPPT controller will follow the working value in order to generate the maximum amount of power. The controller harvests power from the PV module at the module's Vmp voltage value and will convert this voltage by stepping that value down to a battery voltage value. In this way, the charging current is jolted up (it will increase) during the MPPT process. This kind of controllers is empowered by a proprietary technology designed to quickly and accurately determine the Vmp (maximum power voltage) of the solar array. The main gain of MPPT charge controllers is the boost of current quantity towards the battery bank in accordance with the charging status.


MPPT_Control_How_is_Working

The difference between both types of charge controllers is related to costs and the type of application they address. PWM controllers are historically cheaper compared to MPPT Controllers but they do work better in low power range (specifically low current) type of applications.

Solar power voltage regulators:~

Solar power regulators should perform at least two operations:~
>> the obvious one is protecting the battery from overcharge at a time when the sun it is strong and we have little consumption.
>> the second one is protecting the battery bank from excessive discharge in bad weather conditions.
Note: both overcharge and deep discharge are harmful to a battery bank (battery in general).

There are several possible ways to regulate solar panel output. We can use a linear series regulator, but it has the disadvantage of causing voltage drops and having a significant internal power consumption at the time when the sun is weak and the load is towards 100% from critical value. In most of the cases, we will use a shunt regulator, which is inactive when the sun is week and springs to life only when there is energy in excess.


state-of-charge

The picture above illustrates how a battery bank is dragging down the solar panel voltage during the charging process using a PWM controller. Normal a 12V panel is working with a Vmp of 17V. During the charging process because of the sheer amount of amps juiced by the batteries, the voltage will lower value somewhere between 10V and 15V corresponding to the hashed areas (the left-hand site curve). In this way, the system will have an energy loss marked on the power curve (the right-hand site curve) between the pick Vmp = 17V and somewhere inside the hashed area. Normally we consider the constant energy lost between the pick at 17V and the upper level at 15V as constant system energy loss. Additional to that we will have an add-on percentage of energy lost during the charging period. That explains the way the PWM charge controller is better serving in a low type of currents. PWM charge Controller is better used with a reduced number of batteries. Their low cost makes the application to be affordable.

 

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