Solar LED lighting solution for rural India

BANGALORE, INDIA: Lighting is taken for granted in industrial countries and in many urban areas of developing countries. It is hard for many people to imagine living at night without being able to obtain light at the flick of a switch.

However, about half of humanity (from China to Latin America, from India to Africa) lives without light after the sunset, since they do not have access to grid-connected electricity.

The unavailability of electricity and, consequently, lighting in such rural areas affects not only the lifestyle of the inhabitants but also the economy, the culture, and the safety:

Without lighting, the majority of working activities must be suspended, and productivity is significantly penalized <1>.

Reading, studying, and interacting with other people become activities more difficult to realize when the sunlight is no longer available.

The darkness of the streets impacts safety in different ways (road accidents, crime, etc.).

Among the various possibilities to fulfill this need, solar LED lighting solutions are gaining more and more consideration. These solutions combine the use of an ecological and renewable energy source with an efficient way to convert energy into light.

Solar LED solutions

Lighting in rural areas of developing countries is generally provided by wax candles or combustion lamps (e.g. kerosene), while flashlights powered by batteries are used as portable sources of light for intermittent use.

These sources are cheaper than any form of electricity. On the other hand, the low efficiency, the poor quality of the light and the intrinsic fire risk of all combustion light sources suggest the use of electric lighting in off-grid locations.

The unavailability of the power grid implies that the electrical energy must be produced locally. Among the methods by which energy can be produced, photovoltaic systems (solar cells) are by far the most universally applicable.

The general principle is to convert the sunlight, in particular, energy carried by photons, into electrical energy.

The use of photovoltaic systems brings some advantages:

Solar energy can be produced locally; hence the solar panels can be installed everywhere (also in areas of difficult access) without the need for infrastructure. This also minimizes the transmission/distribution losses.

Solar energy is a renewable source; it does not impact the environmental pollution.

Solar energy facilities can operate with little maintenance or intervention after initial setup; this contributes to reduce the energy cost.

Solar energy generates low voltages; this simplifies the downstream conversion of power.

However, energy production by photovoltaic systems must take into account the intrinsic periodicity of the solar source (e.g. during the night the energy source is absent).

Storing the electricity in batteries would ensure a continuous availability of energy.

Once the electrical energy has been produced and stored, the next step is generating light in a wise way, in terms of energy saving and in respect to the environment.

The use of white LEDs is becoming of primary importance in this direction:

high luminous efficiency (more than 100 lumen/watt), which implies less  wasted power in comparison with other light sources (e.g. incandescent bulbs).

Hazardous materials free (mercury or toxic gases), that features the LEDs as the cleanest light in ecological terms. Low driving voltages, making LEDs particularly suitable for photovoltaic systems.

In the last years, significant steps forward in LEDs technology has been carried out, especially in terms of power capability.

The availability of LEDs up to 5W, combined with the high luminous efficiency, contributes to their diffusion in the lighting market as replacement of traditional and often less efficient light sources.

The solar LED lighting system above described in outline can be represented as in the Figure 1.

PV systems

The operating principle of photovoltaic (PV) systems is quite simple: the sunlight strikes the solar panel, realized by semiconductor materials; the energy carried by the photons causes in the semiconductor the generation of electron-hole pairs that, in turn, generate a current flowing into a load connected to the panel.

PV panels are realized in different technologies: monocrystalline silicon, polycrystalline silicon, thin film.

However the first two technologies are the most widely employed since they offer the highest efficiency (up to 15-16%), where the efficiency is defined as the ratio between the power produced by the panel and the luminous power that strikes the panel.

Thus the peak power achievable with a PV panel at STC (Standard Test Conditions(1)) is around 150-160 W/m2.

PV panels are available in different sizes, proportionally delivering different power: around 200W for the 1-1.2 m2 modules (typically 72 solar cells) used in home applications, up to 80W for the modules for street-lighting, from 12W to 24W panels for solar lanterns.

In a PV panel all the cells are generally connected in series and the typical voltage of each cell, when delivering the maximum power, is around 0.5V (see Figure 2).

This voltage however depends on the temperature and the illumination level.

In the interest of maximizing the power transfer, a dynamic system called Maximum Power Point Tracker (MPPT) should be implemented between the solar panels and the load (battery).

This circuit samples the voltage and the current at the output of the photovoltaic panel. Then, controlling the downstream DC-DC converter (which can be a step-down or step-up depending on the panel size and the load), the output current of the photovoltaic panel is regulated in order to maximize the power transfer (see Figure 3).

Without the use of a MPPT dynamic system, the power delivered by the PV panel would change a lot not only with the illumination and the temperature, but also with the battery charge level.

This can further reduce the PV panel efficiency (as mentioned not higher than 15%) by 30% and more.

(1) Testing conditions to measure PV cells nominal output power that supposes an irradiance of 1kW/m2, solar spectrum of air mass 1.5 and module temperature 25°C.

ST LED driving solutions

The efficiency of the conversion of the electric energy into light is always a key point, but it gains further importance in this specific application with the target of minimizing size and cost of the system (battery and PV panel).

LEDs, as mentioned, contribute with their efficiency in power to luminance conversion.  However, also the way to drive LEDs should be optimized in order to achieve an overall improvement of the efficiency.

The LEDs are current controlled devices. Therefore the main requirement in LED driving is controlling the current, which, in turn, determines the brightness.

The LED driving solution must consider the application conditions:

input voltage range. According to the use, different battery types can be chosen: from 6V batteries in case of solar lanterns, to 12V batteries for home lighting applications, up to 24V batteries mainly dedicated to streetlight solutions.

Number of LEDs and how they are connected. The LEDs can be connected in series in a single string or can be arranged in multiple parallel strings. The number of LEDs in series defines the output voltage of the conversion (typical voltage drop across a white LED is around 3.5V), whereas the number of parallel strings indicates the total current to provide.

LED current. High brightness LEDs are supplied by currents of hundreds of milliamperes (up to 1A). However in lighting applications, the choice of 350mA LEDs is the most frequent.

Depending on the battery voltage and the number of LEDs connected in series in one string, a buck or a boost conversion can be the most suitable solution.

STMicroelectronics, among a wide range of products for LED driving, offers both a buck converter (L6902D, <2>) and a boost converter (LED7707, <3>) dedicated to LED driving. The possibility to control the current and a high efficiency conversion feature both devices.

Buck LED driver

The L6902D is a step-down switching regulator. It is based on a voltage mode architecture and integrates also a current error amplifier. This allows the L6902D to have both a constant voltage and a constant current control.

The input voltage range from 8V to 36V makes the L6902D suitable for applications where 12V (up to 3 LEDs can be driven) or 24V (up to 6 LEDs) batteries represent the input voltage source. This means that this regulator can be mainly used for home and street lighting.

The output voltage of the L6902D used as LED driver is determined by the number of LEDs connected in series and can go theoretically up to 34V.

A simple sense resistor sets the current flowing through the LEDs (see Figure 4).

The low voltage drop across this sense resistor (100mV) contributes to further improve the efficiency of this solution, already characterized by the intrinsic high efficiency of the step-down conversion.

The Table 1 proves the high efficiency performance that can be achieved with L6902D used as LED driver.

The more LEDs are connected (which means higher output voltage), the higher is the efficiency, as presumable in a buck converter.

A possible example of use of the L6902D as LED driver is in street-lighting. A 40W solar panel can be considered suitable for this application and a 12W lamp, considering the high efficiency of LEDs, could be more than enough for illumination of local roads in rural areas.

Very often 3W LED strings (3 LEDs in series, 1W per LED) represent the basic module for 6W, 9W, and so on, lamps.

In this application, the 12W lamp can be realized by two strings of 6 LEDs (connecting in series two 3W LED modules), each one driven by one L6902D.

Taking into account less than ideal illumination conditions, the actual power delivered by the panel can be halved; thus considering on average ten hours of sunlight, the energy produced by the panel during the day can be estimated as around 200Wh.

The 12W lamp driving requires a power of 12.5W, according to the efficiency data in Table 1. This implies that the energy produced by the panel would be enough to supply the lamp for about 16 hours, fully covering the nightly lighting needs in any season of the year.

Boost LED driver

When the application requires a boost conversion, a solution is provided by the LED7707, a DC-DC converter dedicated to LED driving applications. It integrates the boost converter and six current generators. The input voltage ranges from 4.5V up to 36V. Therefore the LED7707 is an appropriate solution for all the typical input voltages (6V, 12V and 24V batteries), whenever the output voltage (fixed by the number of LEDs in series) is higher than the input voltage.

The six current generators can provide from 20mA up to 85mA each one. The current can be simply adjusted by a resistor (see Figure 5).

Since the six channels can be connected in parallel, a maximum current of 0.51A is achievable. Hence 350mA high brightness LEDs can be simply driven by the six current generators connected together, with a current of 60mA per channel.

In solar LED lanterns a typical battery voltage is 6V (ranging from 5.5V up to 7V).

Considering a 3W LED string module, the voltage drop across the LEDs is typically around 11V.

On the basis of these considerations, the use of a step-up conversion is the only solution, illustrated in Figure 5.

Typically a 15W solar panel is appropriate for solar LED lantern solutions. Under the same hypothesis of sunlight duration and illumination conditions mentioned in the previous section, the energy produced by a 15W panel is 75Wh.

Considering the efficiency results of the LED7707 driving 3 LEDs (1W each one) from 6V input voltage (see Table 2), the power required to drive the 3W LED string shown in Figure 5 is 3.5W.  Therefore the energy produced by the 15W panel is by far sufficient, assuring more than 20 hours of autonomy.

Conclusions

The challenging aim of providing light also in rural areas can be achieved by producing energy locally. Photovoltaic systems offer the possibility of exploiting an energy source available everywhere respecting at the same time the environment.

The storage of the energy into batteries overcomes the intrinsic discontinuity of the solar energy.

The use of LEDs, ever-growing in lighting solutions, seems the most appropriate choice for energy saving thanks to the high luminous efficiency.

Moreover the availability of different LED drivers, with buck or boost configuration, provides flexibility in lighting systems design and high efficient power conversion solutions.

Bibliography:

<1>”Imaging India. Ideas for the new century”, pages 258, 454, 458 – Nandan Nilekani, Penguin Books India.

<2>Low Voltage LED Driver Using L6920D, L4971 and L6902D - Application Note

    AN1941, STMicroelectronics.

<3>6 rows – 85 mA LEDs driver with boost converter for LCD panels backlight -  Application Note AN2810, STMicroelectronics.