We’ve come across another “sleeper” in the quest for understanding energy use and off-grid lighting.

LED replacements for kerosene may not always save as much energy as meets the eye…

In many areas, end-users will prefer products that can be grid-charged, e.g., via cell-phone charging shops or other battery-charging methods. If the local grid relies on fossil fuels and the charger efficiency is low, then a non-trivial amount of energy may be consumed and corresponding greenhouse-gas emissions emitted. This is determined in part by differential between power delivered to the AC adaptor--affectionately known as Vampires, thanks to their "toothy" plugs--and that ultimately released by the battery to the light. Off-grid lighting products are often designed to be compatible with telephone chargers.

Worst case: dirty grid, inefficient AC adaptor = no net energy savings compared to kerosene (but, still much more light).

If the adaptors are highly efficient, the issue is not so important.  But, in practice, the adaptors that find their way into developing countries (and even into industrialized countries) often stink. We’ve tested AC adaptors obtained on the streets in Africa with efficiencies as low as 3% (!!) and none better than 25%.  More systematic work is clearly needed.

California has existing minimum efficiency standards for external power supplies, including those for cell phone chargers. The U.S. Department of Energy has begun standards development for battery chargers and external power supplies, which could provide useful information and rating protocols for the off-grid lighting applications. EPRI also has an activity focused on these end uses. The ENERGY STAR program has a rating protocol for AC adaptors (including mobile phones). The best charger on their list as of 21 February 2010 is 96% efficient, and the worst 24% efficient. To get the total amount of grid electricity required for “off-grid” lighting systems, these losses must be combined with battery efficiencies and other losses in power management. More background information on the subject can be found here. 

Greenhouse-gas emissions associated with grid-charging LED lighting systems depend on the power consumption of the system, conversion efficiencies, and emissions factors. In practice, power supply efficiencies vary from ~3% to ~95%. Minimum efficiency standards in California are 50%. SLA battery efficiencies vary from 50% to 90%, depending on the charging strategy. The assessment shown in the chart assumes a grid-electricity emissions factor of 1000 grams/kilowatt-hour (g/kWh) and 20% transmission and distribution losses. Values in developing countries range from to 600 to 1800 (g/kWh), including transmission and distribution losses. For comparison, a typical kerosene lantern results in emissions of approximately 40 grams/hour. In the example given, losses range from 5% to 100% of baseline lantern emissions, but losses rise steeply at the low-efficiency end of the scale. These values do not include standby power.

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Cross-posted from the Lumina Blog