High Efficiency Lighting
Rural electrification schemes using solar power generation are being implemented all over the developing world. Generally, electricity grids are not practical due to remote locations, impassable terrain and the high cost of transmission installation.
Although many stand-alone systems also provide power for refrigeration, radio and television, lighting remains the primary requirement for virtually every photovoltaic system.
A single solar-powered lamp can provide a real enhancement to the life a family in developing countries. However these families have some of the lowest disposable incomes in the world. Both the initial purchase price and the total life cycle cost of the system must be kept as low as possible.
Achieving this outcome is far more complex than simply combining the cheapest available components. Comparative analysis of various lighting technologies reveals some surprising economic realities for system designers.
LIGHTING TECHNOLOGIES Several lamp technologies are used in small solar lighting systems. Purchase price is often the deciding factor but the choice should take into account the efficiency since this will affect the cost of an appropriate panel and battery.
INCANDESCENT Very small automotive incandescent lamps of around one Watt are common. These lamps are very cheap but provide a meagre illumination at very poor efficiency. They make poor use of the expensive system components such as the panel and battery and represent false economy.
HALOGEN This advanced incandescent technology is a substantial efficiency improvement over standard incandescent bulbs but still falls well short of other lighting equipment. Their service life is highly susceptible to small variations in supply voltage both above and below their nominal rating.
LED Light emitting diode (LED) technology has made enormous advances over the past decade to the point where they have become practical for lighting applications particularly where the light is required in a concentrated beam.
LEDs can provide a bright highly focused light with a very high intensity (measured in lux) in a small area making them suitable for lighting a desk. However the total light produced (measured in lumens) is not very high so they are ineffective at lighting up a room.
However, high performance LEDs such as the 5W Luxeon glass encapsulated devices remain very expensive. Moreover, their efficiency is yet to reach that of an ordinary fluorescent lamp. The efficiency of smaller Luxeon LEDs is lower still and none of the published Luxeon efficiencies account for the losses in control circuitry required to convert from the standard 12-volt supply.
Conventional plastic encapsulated LEDs have quite unimpressive efficiencies and degrade rather quickly due to light emissions yellowing the plastic body.
FLUORESCENT This gas discharge device is well established as a high-efficiency lighting technology. Progress in phosphor chemistry has led to a slow improvement in efficiency but it is essentially now a mature technology.
Fluorescent lamp efficiency is generally a function of size. Large lamps are more efficient at least in part because the proportion of energy used to heat the electrodes is smaller in a longer tube. It is problematic to produce a low-powered lamp while maintaining high efficiency.
Generally, small solar lighting systems use compact fluorescent technology in a range of sizes from 3 Watts to 11 Watts. They are made in vast numbers and appear to represent good value, as they are both reasonably efficient and relatively cheap.
In an effort to save power some small compact lamps dispense with heating the electrodes. However the savings are offset by reduced discharge efficiency and accelerated electrode damage. Although the lamps continue to function, their brightness degrades more rapidly as electrode material is deposited onto the phosphors.
Fluorescent lamps require stable working conditions and are rapidly degraded by variations from their optimum input voltage and ambient temperature.
Battery voltages typically reach 15 volts during boost phases and below 11 Volts when discharged. Many compact lamps overheat and fail if operated at 15 volts for more than a few minutes. Low voltages damage the tube electrodes due to insufficient heating resulting in internal blackening of the tube and a fall in efficiency.
Small systems for developing countries often incorporate a substantial fluorescent lamp for the main lighting task and very low Wattage "night-light". The night-light is used to save power when the full illumination is not required. Generally, an LED lamp would be used for this purpose. However LED lamps are relatively expensive, only being justified because they offset the generation and storage costs of running a larger lamp when only low-level lighting is required.
COLD CATHODE Cold cathode is an advanced gas discharge fluorescent technology, which provides the highest possible efficiency along with several important additional benefits.
Cold cathode fluorescent tubes operate at a much higher voltage and lower current than conventional fluorescent lamps. The higher voltage overcomes the need to heat the tube while the lower current greatly extends the life of the discharge electrodes and associated phosphor degradation.
Dispensing with the wasteful heated electrodes allows high efficiency to be achieved in a small lamp, typically ten to 30 percent more efficient than a comparable fluorescent lamp.
Cold cathode lights have a life expectancy more than twice that of typical compact fluorescent lamps and do not suffer accelerated degradation from variations in supply voltage.
Cold cathode lamps can be dimmed to any point without damage. Without the need for precise electrode temperature the cold cathode lamp can be operated at any desired brightness up to their maximum rating. They can even be continuously operated in flashing applications that would destroy a conventional fluorescent lamp in a day.
The only disadvantage of the cold cathode technology is the higher cost of manufacture. This is partly due to the special transformers for the high tube voltage but also due to low production volume compared to the conventional fluorescent lamps.
SYSTEM LIFE CYCLE ASSESSMENT The full lifetime cost of all components should be taken into account when selecting equipment for solar lighting systems. Factors that affect the life expectancy of consumable components such as batteries and lamps can have a large influence on life cycle costs.
Fortunately a good quality solar panel will typically last beyond the 15 to 20 year guarantees now available even on small solar panels. As the most valuable and long lasting component, it also defines the system lifespan.
For the purpose of the exercise, consider a small PV lighting system with a sealed lead-acid battery (SLA). The actual savings will depend on the design details and size of the system but the basic principle of investment in high efficiency holds regardless.
A simple rule of thumb for matching panel to battery is approximately one Watt of solar charge for each Amp-hour in a 12-volt power system. A system will generally support one Watt of lamp power for four hours per night for each Watt of solar panel capacity.
During a 15 year PV system life, the rechargeable battery will need to be replaced several times making it a major operating expense. The battery life depends on the charge and discharge regime but typically the battery would need to be replaced every two years with a daily discharge of about 50 percent of its capacity.
At a retail cost of say US$2 per Amp-hour for small 12-volt SLA batteries and say US$10 per Watt for small solar panels, the battery cost is actually the largest life cycle cost in the system.
Saving just a single Watt in one lamp will offset nearly $12 in purchase price of panel and first battery and a further $12 in battery replacement costs over the 15 year life of the system.
Clearly, improvements in lamp efficiency can substantially reduce overall expenditure both initially and over the long term. Conversely a solar energy system can provide substantially more light for the same cost by using higher efficiency lighting.
FURTHER ADVANTAGES Dimmable cold cathode lamps can fulfil both primary and night-light functions in a single lamp. Moreover, choice is not limited to the bright fluorescent lamp or the feeble LED. The level of illumination can be precisely selected by the consumer to suit their need at the time.
Useful light can be provided at a fraction of the full current yet still exceed the efficiency of an LED lamp. This feature provides the greatest opportunity to save energy by allowing the consumer to precisely tailor their power consumption throughout the night.
Moreover, the cold cathode lamp will typically last for the 15 year life of the system when used four hours a night. The conventional fluorescent will need to be replaced twice during this period, considerably more often if subjected to variation from the specified 12-volt operation.
The operational costs of the system are then limited to the battery replacement alone, making cold cathode lighting extremely attractive in systems set up by aid programs.
Rainbow Power Company's compact cold cathode fluorescent lamps as well as the Sundaya Ulita & Ulitium range represents a valuable investment in any photovoltaic system. They form an essential part of any small lighting system, providing extended service life and minimising the ongoing costs that represent the greatest hurdle to effective use of PV lighting systems by the world's poorest people.