Solar energy is an important form of alternative energy that is ever increasing in it’s popularity. I hope that it would one day account for almost all of our everyday energy needs.
This project focuses on the harvesting of solar energy for home/industrial purposes using Photovoltaic (PV) systems.
Basic Solar PV setup
The picture below shows a basic Solar PV setup, it consists of a PV array, a charge controller and a battery bank for energy storage.
Charge controllers (CC) are an important link in solar energy conversion as they play the role of regulating the manner in which energy is transferred to a deep cycle battery storage system.
CCs are basically implemented in three ways, these are:
1. Simple on/off
2. Pulse width modulation(PWM)
3. Maximum power point tracking(MPPT)
CCs would continue energy transfer into a battery storage system until a preset voltage level is attained; at this point they would disconnect the battery from the PVs.
There is a lot of information on these implementations online; however I will just do a brief recap.
Battery charging is done in three stages these are; bulk, absorb and float.
Bulk stage: this is the first stage of charging; the aim at this point is to push in as much charge as is possible back into the battery. Conventionally charge current is set to a constant value while battery voltage increases steadily, however for a solar setup; the varying intensity of the sun might make it impossible to achieve a constant current charge. The MPPT algorithm is used here to here to maximize charging. Another important point of note is the output current-voltage (I-V) characteristics of solar PV panels.
An unloaded panel has a maximum voltage known as Voc with a zero current output; on the other hand with a short circuit across the panel’s terminals a maximum current Isc with zero output voltage is reached. Somewhere along the I-V the product of current and voltage is at a maximum value, the panel voltage here is known as Vmp it is approximately 80% of the Voc ,the current at this point is Imp .
Above diagram shows the ideal I-V curve of a PV panel. In reality not all panels in a PV array would be equally illuminated by the sun, hence the Vmp point would occasionally shift and needs to be tracked by the MPPT controller’s algorithm.
Absorb stage: at this point the battery should be closer to its fully charged state typically from 80% upwards (this varies according to different manufacturers). Charging current reduces while voltage slightly increases till a full charge is achieved.
Float stage: this is the final stage, charging is mostly turned off with only a trickle charge allowed through to maintain the 100% SOC.
Some points are important to note in this design:-
- My MPPT algorithm runs till 55v
- Bulk stage till 57.5v
- A sort of “quasi” float stage is used. Battery voltage is maintained at 57.5v since this is for a completely off grid installation and battery discharge is very likely going to be continuous.
- Input voltage: up to 200v
- Output voltage: approx. 57.5v maximum
- Output current: should handle 60Amps comfortably
- Battery type: 48v lead acid
- Switching frequency: 32 kHz generated by Atmega88 Microcontroller Unit (MCU)