The sun gifts us a huge amount of energy.
On the average, the amount of solar radiation reaching the edge of the Earth’s atmosphere perpendicular to the sun ray is about 1300w per square meter. According to the UN’s Development Program, the amount of annual solar energy reaching the earth’s surface is 1,575 – 49,837 x 1018 J which is much higher than the total energy consumption of the world in 2012 which was 559.8 x 1018 J. Indeed, a major portion of the solar radiation is either absorbed or dissipated in the atmosphere while reaching the earth. The solar energy reaching the earth can be transformed into the electric power through different methods such as PV systems or solar dish generators.
How does a PV solar system work?
A solar photovoltaic system consists of PV solar modules, charge controller which control the voltage and the current for charging system batteries, battery bank which consists of batteries storing the solar energy and the inverter which converts batteries’ DC voltage to AC voltage to power AC loads. During this pipeline of energy conversion from solar energy to usable electric energy, depending on the efficiency of the system, some amount of energy is lost or dissipated. For example, the existing PV modules on the market convert only 15 to 25 percent of the incident solar beams into electricity. Also, the existing charge controllers allow only a part of the PV output power to the next stage for charging batteries.
What is a charge controller?
A Solar charge controller is placed between the Solar Panel Array and the Battery Bank to control the amount of electric energy produced by PV panels going into the batteries. The core of its function is to make sure that the batteries are properly charged and protected from overcharging, subsequent gassing, loss of electrolyte and possible plate damage
PWM and MPPT charge controllers
There are currently two type of charge controllers commonly used in PV power systems, Pulse-width Modulation (PWM) and Multiple Power Point Tracking (MPPT).
- A PWM charge controllers are, in essence, a switch that acts between the solar array and the battery and its function is to pull down the voltage of the array to near that of the battery to ensure that the battery is properly charged. In other words, they lock the solar panel voltage to the battery voltage by dragging the Vmp down to the batteries system voltage with no change in the current. Due to the nature of their work mechanism, PWM charge controllers cannot use the maximum power produced by the PV system and so their efficiency is low and around 80%.
- An MPPT controller allows maximum power to be harvested. The basics of an MPPT Solar Charge Controller is to optimize the efficiency of the charge controller employed in a photovoltaic system.
How MPPT works
Compared to a PWM system, an MPPT is a more sophisticated and more expensive system. It has been designed to adjust its input voltage to utilize the maximum power output of the solar array and then transform this power to supply the varying voltage requirement.
Let us see it through an example:
Consider a Solar power system using a 12 volt battery, bound to a 130W PV panel with a current of 7.39 A & voltage of 17.V. When the voltage of the battery is low, at around 11.3 volts, the MPPT takes that 17.6 volts at 7.39 amps and converts it to a new configuration suitable for charging the battery without losing any power. MPPT technology allows the charging voltage to reduce to 12.3 V instead of 17.6 V while increasing the charging current from 7.39 A to 10.16 A @ 12.3 volts. In this case, 130W power produced by PV panels are used to charge the battery. So, mostly all power entered the device is passed to be stored into the battery. Actually, the efficiency of MPPT is 95 to 99 percent.
An MPPT charge controller extracts the maximum power from PV module by forcing the PV module to operate at a voltage close to maximum power point. It causes to draw maximum available power from PV module. Another advantage of MPPT is its flexibility in using PV modules with higher voltage compared to the operating voltage of battery system. Using an MPPT solar charge controller, it is possible to wire the PV array for a higher voltage up to 140V (depending on charge controller and PV modules) and bring the full PV power for charging the battery.
The PWM charge controller is suitable for small systems. It provides a low-cost solution and is normally used when the solar cell temperature is between 45°C and 75°C.
MPPT system is normally used when the PV array voltage is substantially higher than the battery voltage. It is the best solution for higher power systems. The MPPT charge controller also provides substantially more power when the solar cell temperature is below 45°C or above 75°C, or when irradiance is very low.