Solar Street Lights

A Solar Panel is basically a module that converts light energy (photons) from the sun to generate electricity in direct current (DC) form.

There are two types of solar panels, mainly crystalline and thin-film types.

There are two types of crystalline solar panels :

Poly-crystalline Solar Panel

Mono-crystalline Solar Panel

As for Thin-film types :

Amorphous Silicon ( a-Si )

Cadmium Teluride ( Cd-Te )

Copper Indium Gallium Selenide ( CIGS )

Dye-Sensitized Solar Cell ( DSC )

A LED lamp is a light-emitting diode (LED) product that is assembled into a lamp (or light bulb) for use in lighting fixtures. LED lamps have a lifespan and electrical efficiency that is several times better than incandescent lamps, and significantly better than most fluorescent lamps, with LED able to emit more than 100 lumens per watt. LED are the perfect combinations with solar power as it operates under low voltage, low heat and low power requirement. Like incandescent lamps and unlike most fluorescent lamps (e.g. tubes and CFL).

LED lights come to full brightness without need for a warm-up time; the life of fluorescent lighting is also reduced by frequent switching on and off. Initial cost of LED is usually higher. LED chips need controlled direct current (DC) electrical power; an appropriate power supply is needed. LEDs are adversely affected by high temperature, so LED lamps typically include heat dissipation elements such as heat sinks and cooling fins.

Batteries are used to store the electricity generated by the solar panel. During the day, electricity generated by the solar panels is supplied to the battery and/or the load. When the load demand is higher than the energy received from the solar panels, these batteries will provide stable energy to the load. Solar power applications typically use deep-cycle batteries because they can persist repeated and deep discharges. There are a few types of rechargeable batteries, which are :

Lead-Acid (LA) Batteries

These batteries are the most commonly used in solar powered systems due to its maturity in technology and low pricing. They can only be used with low Depth of Discharge (DOD) in order to extend its lifespan. Its DOD ranges from 60%-80%. There are two types of Lead-Acid batteries, i.e. flooded and Valve Regulated Lead Acid (VRLA) batteries which are maintenance free batteries.

Nickel-Cadmium (Ni-Cad) Batteries

These batteries are expensive and disposing of Cadmium is hazardous. Even though they have several advantages over Lead-Acid batteries, such as longer life span, and tolerance for higher discharge, Ni-Cd batteries is not commonly used in solar powered systems due to its high cost and limited availability.

Lithium-Ion (LI) or Lithium-Polymer (LP) Batteries

Lithium based batteries are considered the future of batteries used in solar powered systems. This is due to a number of factors such as high specific energy, high DOD, and higher number of charging cycles. However, due to its higher cost compared to LA type of batteries, they are still not very widely used.

Charge controllers are used to control the charging of the batteries. Since the output from solar panels are variable and needs adjustments, charge controllers fetches the variable voltage/current from solar panels, condition it to suit the safety of the batteries. The main functions of charge controllers are to prevent over-charging of batteries from solar panels, over-discharging of batteries to the load and to control the functionalities of the load. Charge controllers are basically DC-DC converters, where PWM or MPPT technique is used to regulate the switches of the controller.

There are three general types of charge controller, mainly :

Simple ON/OFF Controller

Pulse Width Modulated (PWM) Controller

Maximum Power Point Tracking (MPPT) Controller Most charge controllers operate at three stages to complete the charging cycle of the batteries. These stages vary according to different times and battery voltages. PWM can be employed to control the charging at the stages :

BULK stage

ABSORPTION stage

FLOAT stage

For Economical and technical viability, optimized solar powered street lights are obtained using sensors. Sensors such as Ultrasonic sensors, IR Sensors and LDR sensors are used to control the LED lamp functionalities in order to achieve a more energy efficient system. In this case the street light must have auto on and off and must have at least two state of dimming function to save energy, from dusk to dawn