Light Emitting Diodes (LEDs) are miniature bulbs with UV resistant, weather proof, unbreakable plastic or aluminium case, super-lamination. Basically, the mode of building up luminaires of a large size is through clustering of miniature sources into a lamp. An individual LED can give approximately 100 to 150 lumens per Watt.
Solid state, opto semiconductor LEDs in conjunction with solar photovoltaic (PV) have the potential for energisation independent of the grid, especially in areas of weak and unreliable power supply. It is a continuously evolving technology, becoming cheaper each day and it serves as an ideal energy saving solution in the street lighting sector. The efficacy of LEDs (laboratory) which was around 100 lumen/W in 2008 went up to 200 lumens in 2022 and the luminaire output up to 160 Lumens per Watt in 2022.
It is an ideal solution for a demand side management for minimizing the load on grid during peak time created because of lighting loads. LEDs using SPV has the added advantage of bringing down the capital cost of the SPV system as the power requirement of the LEDs are far lower than that of conventional lighting systems. Hence, the SPV-based LED lamps are emerging as the front runners for energy saving, grid Independent Street Lighting Systems. The power input of equivalent LEDs is reduced to almost 25% of their conventional Tubular Fluorescent Lamp (TFL) or Compact Fluorescent Lamp (CFL) equivalent. A typical configuration, as fixed by Ministry of New and Renewable Energy (MNRE) is a 40 W solar PV panel, 40 Ah battery and 7 W LED lamps, to give an output lighting level of over 16 lux at 4 m height and 4 m periphery.
A standalone solar photovoltaic street lighting system is an outdoor lighting unit used for illuminating a street or an open area. The Solar Street Lighting System consists of Solar Photovoltaic (SPV) module, a luminaire, storage battery, control electronics, inter-connecting wires/cables, module mounting pole including hardware and battery box. The luminaire is based on Light Emitting Diode, a solid state device that emits light when electric current passes through it. The luminaire is mounted on the pole at a suitable angle to maximize illumination on the ground. The PV module is placed at the top of the pole at an angle facing south so that it receives solar radiation throughout the day, without any shadow falling on it. A battery is placed in a box attached to the pole. Electricity generated by the PV module charges the battery during the day time, which powers the luminaire from dusk to dawn. The system lights at dusk and switches off at dawn automatically.
Both the systems have been tested and results are analyzed in this article.
PV module: The PV module made up of crystalline silicon solar cells, the power output of the module(s) under STC is measured using Sun Simulator. The measured results with graph are as below.
Battery storage is becoming an increasingly popular addition to solar energy systems. Two of the most common battery chemistry types are lithium-ion and lead acid. As their names imply, lithium-ion batteries are made with the metal lithium, while lead-acid batteries are made with lead.
- System 1 : Sealed maintenance free lead acid battery with a capacity of 40 AH, at voltages of up to 12V @ C/10 discharge rate
- System 2 : 12.80V DC, 192 Wh capacity Lithium Ferro Phosphate Battery
The light source is of white LED type.
Requirement: Minimum 16 Lux when it must be measured at the periphery of 4 metre diameter from a height of 4 metre. At any point within area of 2.5 metre diameter periphery the light level should not be more than three times of the periphery value. The illumination should be uniform without Dark Bands or abrupt variations and soothing to the eyes. Higher output would be preferred.
Average duty cycle: 5 hours a day under average daily insolation of 5.5 kWh/ sq.m. on a horizontal surface.
Autonomy: 3 days or Minimum 15 operating hours per permissible discharge.
Electronics (Solar charge controller)
The driver efficiency for a conventional driver is around >85% while for a well-designed MPPT or microcontroller-based driver it can be as high as 98%. Driver efficiency is highly sensitive to the cost of the power source (in the case of solar PV system where capital cost of the solar module is high). Investment in the driver cost is off set by the reduction in the cost of the PV panel.
Electronic protections: Adequate protection is incorporated in the system under various conditions, e.g., when the lamps are removed and the system is switched ON.
Two 40 W solar street lighting systems were tested with Lead acid battery and Lithium ion battery and the results are presented. The solar powered LED street lighting technology, addressed primarily to meet the requirements of rural areas that are weakly connected to the grid or facing power outages.
The LED street lighting systems provide Illumination level >16 lux at a height of 4 m and periphery of 4 m at an energy efficiency of >90 lm/W. The Solar Charge controller provides the electronic efficiency from 85.56% to 89.73%, indicating scope for improvement. There is further scope for improving the Illumination levels with the use of new age technology in improving total light output and light distribution.
Raghu R. is a B. E. in Electrical and Electronics Engineering. He is presently working as an Engineering Assistant at Energy Efficiency and Renewable Energy Division of Central Power Research Institute, Bangalore.