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July 22, 2008

The Leaky Bucket Syndrome

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Residential Energy Leakage
Most people have heard about "vampire power" or the leakage power that continues to flow even when a device is turned off.  Many home entertainment systems can draw a several watts while they are in stand-by mode - the equivalent of "off".  Many set-top cable boxes keep most of their circuitry on even when the unit is not "on".  This is required for the cable infrastructure to maintain communications with the box. 

More curious still is the power that appliances draw when "off".  For instance, if you have a 1000 watt microwave oven with a digital display it may actually use more energy when not cooking then in actual use.  This is due to the electronics drawing power continuously while the oven component itself is used only periodically.  If the oven is used for an average of 4 minutes per day, it will use roughly 24 kW-hrs of energy in a year.  The electronics use about 3 watts and are on 24/7/365 so that component consumes roughly 26 kW-hrs of energy - a bit more than the oven.

If you have 4 incandescent bulb night lights that each use 3 watts of power and you leave them on 24/7 (many people do), in a year that would add up to 105 kW-hrs of energy consumed.  Switching to 1 watt LED based units with daylight sensors would save over 88 kW-hrs of energy in a year.  The LED units will last for 100,000 hours effectively never requiring replacement.  If you consider that incandescent bulbs require replacement periodically, the payback for the LED units with improved efficiency and reliability would be under a year.

OK, so saving a watt here or a watt there is like saving a penny here or a penny there - does it really make a difference?  Here’s a similar analogy: does dropping some loose change into a charity’s bucket during the holidays really make a difference.   If 10 million people dropped an average of 25 cents into buckets across the country, that would add up to 2.5 million dollars!  If every household in America dropped an average of 10 watts from their daily consumption (240 watt-hours), with over 100 million households in the US that would add up to over 24 million kW-hrs of energy per day.  That’s the equivalent output of a 1 gigawatt power plant!

Industrial Size Problems
So if saving a few watts here or there can really add up, imagine the potential savings for industrial users which can be thousands of times higher than a single family home.  It is estimated that 65% of industrial electrical use goes to powering motors.  Motors are most efficient when run at their rated loads; however they quickly lose efficiency when run at lighter loads.  This is a similar phenomenon to the efficiency loss seen in switching power supplies when run at lighter than designed loads.  A Department of Energy (DoE) study of nearly 2000 industrial motors from various applications nationwide showed 44% of them were operating at less than 40% of their recommended loading.

So what can be done to improve motor efficiency?  One method is to replace the direct drive systems with VSD or Variable Speed Drive systems.  It turns out that the speed of an AC synchronous electric motor is proportional to the frequency of the AC line current.  By implementing a variable drive system, a savings of anywhere from 5% to 50% can be realized.  With the cost of electricity increasing, the equipment cost can be quickly amortized and true savings can be realized.

What about lighting?  We talked about saving energy with dimmers and CFLs last week (Living With Less - Are Dimmers Better than CFLs?) in our homes. Now imagine how much could be saved by moving to active systems that lower the light level of florescent lights or HID (High Intensity Discharge) units near windows when it’s sunny.  Natural light coming through glass adds to the total available lighting in buildings.  If the lighting system can monitor that amount and make subtle changes to the light from artificial sources, a tremendous amount of energy can be saved.  Take for instance a parking garage.  Normally during the day, only the interior HID units would need to be working since the exterior units are near the open spaces and receive natural daylight.  Only at night would the units near the periphery need to be turned on.  Additionally, systems that could dim the HID units could gradually increase the brightness the darker it gets.

Parking_garage_hid_exampleFor example, a 4 story above ground parking garage would use approximately 280 HID units (each consuming around 215 watts of power).  Each floor would have 70 units arranged in a matrix of 7 x 10.  So there would be 30 units along the periphery that could be turned off completely and 22 units in the second ring that could be dimmed on each story.  Each story could save 215 watts on the outer 30 units and 107 watts on 22 units on the next ring.  This would save roughly 423 kW-hrs per day or 154,350 kW-hrs a year.  At US$0.10 per kilowatt hour, this would save the garage owner US$15,400 per year!  This does not include the savings in lamp replacement due to reduced wear - just a few thoughts on stopping the leaks in your buckets.

Till next time...


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