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What are Solar Street Lights?

Solar street light is an effective outdoor lighting to illuminate the street or any open areas through the use of solar energy.  The history of street lights goes back to Greek and Roman civilizations where light primarily served the purpose of safety and security.  Oil lamps, candle lighting and gas lamp lighting were used for public street lighting before the invention of incandescent lights.  Renewable energy comes from natural resources that are constantly replenished.  In 1839, Alexandre Edmond Becquerel discovered the photovoltaic effect which explains how solar energy is converted into electrical energy.  Solar energy has been harnessed by humans for thousands of years and has developed over the years.  Solar street lights are powered by solar panels and turn on and off automatically by sensing the nightfall and the daylight.  Solar (photovoltaic) cells generate direct current by using sunlight as a source of energy.  The controller in solar lights works as a regulator.  It charges the battery and also controls the reverse power flow.  Batteries store the solar energy during the day to power the lights at night.  With the advancement of technology, there are different types of solar street lights available in the market.  Solar street lights are the perfect cost-effective solution not only for the urban population but also for the people living in rural areas that are deprived of electricity.

Solar street lights have components such as SPV Module, charge controller unit, battery, luminary, interconnecting cables and pole.

  • Solar photovoltaic (SPV) module: is fixed on top of the pole with suitable inclination to receive maximum sunlight throughout the day. The SPV module produces suitable voltage and current, which is used to charge the battery. During the night, the lamp which is housed inside the luminary lights up by using the energy stored in the battery.
  • Charge controller unit (CCU): controls the charging. Charge controller unit increases the battery life by not allowing the battery to deep discharge and overcharge. During deep discharge conditions, the CCU turns off the light, so as to stop further discharge of battery.  During overcharge conditions, CCU disconnects the solar module and prevents overcharging of the battery.
  • Battery:  The energy that is generated by SPV module is stored in the battery.  This energy is used to light up the luminary during night time.  For solar street lights, LiFePO4 battery is normally used, which is more compact than the battery box style fixture of traditional street light which houses Tubular Lead Acid batteries. Lithium ion battery used in solar street lights, is housed inside the luminary itself.
  • Luminary:  LED lamp or fluorescent (CFL) lamp is used to provide illumination.  CCU and the battery are incorporated in the luminary itself if the battery used is lithium-ion battery.  If the battery is lead acid, only the CCU is incorporated in the luminary.  
  • Interconnecting cables:  Cables are used to connect the SPV module, luminary, CCU and battery.
  • Pole:  MS street light pole or GI pole is used to mount the SPV module and the luminary at a height of 4-5 meters above ground.

Types of Solar Street Lights.

In India, there are three types of solar street lights available:

1. All in One Solar Street Light - This light is compact where all the components are housed inside a single unit.

2. Integrated Solar Street Light - In this, battery, fixtures, luminary and controller are molded into a single unit and the solar panel comes as a separate unit.

 3. Semi Integrated Solar Street Light - This is a traditional device where luminary and controller are housed into one unit with external solar panel and battery box.

Features of solar street light

  • Automatic:  By sensing outdoor light, most solar street lights turn on and turn off automatically and are designed to work throughout the night with dusk to dawn sensor feature.
  • Motion sensor:  Solar street lights consist of motion sensor that helps to detect the motion and change the intensity of the light.   This feature helps the solar street lights act as security lights.  This helps in extending battery life by reducing the brightness when no motion is detected and gives 100% brightness when movement is observed.
  • Timer based:  To save battery, controller can be set to dim the lights if required.
  • Energy saving and pollution reducing:  Since solar lighting uses solar energy, there is no consumption of any non-renewable energy, hence making it environment friendly.  The biggest advantage of solar street light is they will continue to light up the area even during a power outage.
  • Money saving:  With no need for paying ongoing electricity charges, solar lighting will save money in the long term.
  • No cable work:  No interdependency weakness in solar street lighting.  No need of foundation work of cables or electric equipment is necessary.  No need of any complicated power facilities; hence, saving both cost and labor.
  • Easy maintenance:  Solar street lighting has built-in intelligence and requires no manual operation after installation, thus making solar street lights safe and economical.
  • Weatherproof:  IP65 rating waterproofing is used for the lights to withstand harsh climate changes.

Types of battery used in solar street light

There are mainly three types of batteries used in solar street lighting namely lead acid batteries, lithium-ion (Li-ion) batteries and lithium ion phosphate (LiFePO4), which is the latest addition in lithium battery technology.

LiFePO4 recharges at a faster rate and offers at least 4 to 5 times the number of charge cycles than lithium-ion batteries, They have a life span 10 times longer than a traditional lead acid battery and are also lighter in weight. LiFePO4 batteries provide upper and lower voltage protection and are priced on the higher side comparatively. Lead acid batteries have large current capability and are inexpensive but demands regular maintenance with a shorter lifespan. Although lithium-ion batteries are considered to be expensive, they offer a longer life span and are almost maintenance free.

    Types of light source used in solar street light

    LED, CFL and sodium vapor lights are used in solar street lights.  LED offers higher luminosity and lower energy consumption; hence they are used most commonly in solar street lights.  CFL uses low pressure mercury vapor gas and the power consumption is more compared to LED lights. Sodium vapor lamps are gas-discharge lights and considered high efficient lights but offers only monochromatic color and also produces more heat than LED Technology.

    Types of lighting fixture used in solar street light

    Cobra head style fixtures are the most commonly used fixture in solar street lighting.  Now with advancing LED technology, there are many decorative, lighter in weight and less bulkier fixtures available.   For durability, the housing used for lighting fixture is ABS plastic or aluminum.

    Types of controller used in solar street light

    PWM or MPPT controller is used in solar street light.  With pulse width monitor (PWM) controller, the panel voltage drops to match the battery voltage, more battery reserve capacity, longer battery life, etc.   With maximum power point tracking (MPPT) controller, the solar panel can operate at its maximum power point and generate more power. MPPT controllers are regarded as expensive compared to PWM controllers because of their ability to produce more power with the same number of solar modules than a PWM controller.

    Types of solar panel used in solar street light

    Solar street lights either have a mono-crystalline solar panel or a polycrystalline solar panel.  Mono-crystalline solar panels are made of pure silicon cells that generate higher efficiency.  Poly-crystalline solar panel is the traditional solar panel which is commonly used and less expensive.

    Working principle

    Solar street lights function based on the simple principle of photovoltaic effect.  During the day, the photovoltaic cells (solar cells) in solar panels convert the solar energy into electrical energy.  This energy is stored in the battery.  The controller helps in charging the battery and regulating the voltage.  At night, the solar lamp draws the current from the battery and starts to illuminate the light source from dusk to dawn.

    MNRE standards in solar street lights

    According to The Ministry of New and Renewable Energy, there are four models of White-LED (W-LED) based solar street lighting systems.

    Model-I

    • Lead acid battery is used.
    • The street light operates from dusk to dawn at full brightness.
    • PV module: Crystalline silicon solar cells, output of 40 Wp under Standard Test Condition, certificate of testing conforming to IEC 61215 Edition II/ BIS 14286 from an NABL or IECQ accredited Laboratory.
    • Battery: Lead acid tubular flooded or tubular GEL/VRLA, 12V – 40 AH @ C/10
    • Light source: White light emitting diode, 7 watt
    • Electronic efficiency: Minimum 85% total.

    Model-II

    • Lead acid battery is used.
    • The street light operates from dusk to dawn, the first four hours at full brightness and then at a lower level; with the help of motion sensor, light glows to full brightness only in case of movement and then dims down automatically after some time.
    • PV module: Crystalline silicon solar cells, output of 30 Wp under Standard Test Condition, certificate of testing conforming to IEC 61215 Edition II / BIS 14286 from an NABL or IECQ accredited Laboratory.
    • Battery:  Lead acid tubular flooded or tubular GEL/VRLA, 12V – 20 AH @ C/20
    • Light source: White light emitting diode, 7 watt
    • Electronic efficiency: Minimum 85% total.

    Model-III

    • Lithium Ferro phosphate battery is used.
    • The street light operates from dusk to dawn at full brightness.
    • PV module: Crystalline silicon solar cells, output of 40 Wp under Standard Test Condition, certificate of testing conforming to IEC 61215 Edition II / BIS 14286 from an NABL or IECQ accredited Laboratory.
    • Battery: Minimum 160 Wh Lithium Ferro phosphate battery
    • Light source: White light emitting diode, 7 watt
    • Electronic efficiency: Minimum 85% total.

    Model-IV

    • Lithium Ferro phosphate battery is used.
    • The street light operates from dusk to dawn, the first four hours at full brightness and rest of the time, at a lower level; in case of movement, the light gives out full brightness and then comes back to lower level automatically.
    • PV module: Crystalline silicon solar cells, output of 30 Wp under Standard Test Condition, certificate of testing conforming to IEC 61215 Edition II / BIS 14286 from an NABL or IECQ accredited Laboratory.
    • Light source:  White light emitting diode, 7 watt
    • Electronic efficiency: Minimum 90% total.

    Also, there are technical specifications provided by MNRE for quality, warranty, operation and maintenance of solar street lights.

    How to install solar street lights

    The solar street lights should be installed by professionals.  Appropriate safety guidelines should be followed during installation.  The ground chosen should be flat without any inclination.  The basic tools used for installing solar street lights are wrench, hoisting mechanism (pulling the pole upright with a crane or using manual assistance depending on the weight of the pole), screwdrivers, spanner and Allen key.  A complete street light kit comes with solar panel, light fixture, lamp pole, battery and controller.  The LED light and the solar panel can be secured to the pole when the pole is still on the ground.  The installation position must create a suitable angle for the solar panel to be faced in the right direction to get maximum sunlight.  The pole used for solar street lights must be strong as there are components mounted on top of the pole and wind resistance is also a factor.  The solar street lights should be secured to the ground as per the instructions given by the manufacturer.  It is important to install the light in a shadow free area where direct sunlight is available throughout the day.

    Maintenance of solar street lights

    Although maintenance of solar street lights is very low, the solar panels should be cleaned off dust particles.  The cleaner the panel the better the efficiency will be.  Clean solar panels guarantee longevity of the panels.  Check the wires on a regular basis to look for chewed wires and conduit.  Also, inspect the fixture for any problems from vandalism, weather or wildlife.  Solar panels are designed with a tilt to avoid water stagnation during rain.  IP65 waterproofing is used to withstand tough climatic conditions.  The solar panels should be out of shade to provide maximum brightness.  Trim down the tree branches, if any, covering the light.  The solar panel can be cleaned using a soft bristled brush or cloth with mild shampoo or detergent. You can also hose off the panels to wash off any dirt or dust.  Ensure not to scratch the glass during cleaning.

    Solar street lights – FAQ

    How does motion sensor light work?

    Motion sensors detect movement of objects in the field of its view and glow up the lights. Motion sensors used in solar street lights help to reduce energy consumption and increase the battery life of the lights.

    How does integrator light work with dimming options?

    Dimmers are used to create comfortable light and also to save energy, which eventually help in extending the life of the lamp used. Solar street lights using motion sensor glow up to full brightness in case of movement and dims down automatically after a while.

    Does solar street light work during monsoons?

    Most of the solar street lights feature longer operating hours, they shine for several days before having to recharge. During monsoons, solar panels produce lesser energy. More the sunlight, better the solar panel works. The lights may still function even during continuous rainy days but the brightness may reduce.

    What is the lifespan of solar street lights?

    In India, the solar panels mostly come with minimum 25 years of warranty; however, the lifespan of a solar panel can go up to 30 years depending upon brand and quality and also on how it is maintained over time. The battery offers a life of 3 to 4 years. In Light source of solar street light LED are commonly used and have a rated life of up to 50,000 hours. Lead Acid battery can last up to 3-5 years with regular maintenance. If battery used is Lithium ion or LiFePO4 it can last up to 5+ years with very low maintenance.

    How much area will solar street light cover?

    This directly depends on wattage and lumens of the light chosen. For example, a 30 watt solar street light may cover approximately 20 to 25 meter area.

    What will be the installation height?

    The height of the light pole should be determined on the locality. Normally, 4 to 5 meter pole is used for installation.

    How do we decide which light to buy?

    There is a variety of solar street lights available that you can choose from depending on your needs, whether it is for decoration or for path lighting. The efficiency of the lights is connected to the amount of sunlight the panels receive during the day. The area of installation has to be decided wisely considering this factor in mind. Some may not require the light throughout the night; they can choose the lights with motion sensor option. It is also important to check the wattage and lumens of each fixture and select according to your requirements. You should check the operating hours of the solar street lights before purchasing. Find out if the battery needs full or partial sunlight to charge and also, consider the type of light bulb used in the solar street light. Choosing a good brand and consulting the professionals will help you decide what is best for you.