Sunday, 23 December 2012

On Christmas Lights


A friend we visited last weekend had some solar powered outdoor Christmas lights around a tree in their front garden. They weren't working because the weather has been so bad - not enough sun. In any case the size of the solar panel feeding their battery was miniscule - maybe 20cm on a side. LED lights are the most efficient we have now but they don't make light out of nothing. I checked our solar panels - we are currently getting about 1.5 kWh/day from 9 panels so 0.17 kWh/panel. To power a 10W string of lights for 4 hours you would need 0.04 kWh so a quarter size panel. This one was probably about a fifth the size it needed to be. This got me thinking about other ways - not necessarily totally practical - to power outdoor lights. Also I thought I'd do a reality check on carbon emissions from candles versus LED lights.


Human power is more reliable than solar power so let's look at some options there.

Light from Gravity

The GravityLight uses the power of a falling weight (such as a bag of sand) which you lift up with a pulley system. It was developed for use in developing countries. Its big advantage is that it does not use any batteries which makes it cheap to run.

A 20kg weight falling through 2m can give you up to 400 J. The GravityLight is supposed to work for 30 minutes which means the theoretical maximum power it can generate is (400 J/1800 secs = ) 0.2W. That won't give you much light but probably more than the kerosene lamp that it was designed to replace. To power a 10W string of Christmas lights you would need some combination of more height (say a 5m tree) more weight(s) or more frequent recharge. 2 x 20kg weights falling through 5m gives you about 3 minutes worth of Christmas lights.

Light from Play

Empower playgrounds make merry-go-rounds and glider swings which are connected to electricity generators so that as the children play they also charge a battery. We all know that kids are full of energy - this is one way to use it. This is also designed for third world countries.

Of course adults play too and you can wire up an exercise bike to charge a battery. You can do it yourself - here's how from Instructables. According to LiveStrong you should be able to generate 150W for 30 minutes if you are in good shape. That would power the 10W lights for more than 7 hours!

Light from slugs*

About 10 years ago there were reports of a new gardener's friend called SlugBot. This was a robot which could collect slugs from your garden and dump them into a tank. The slugs decompose generating methane (a small anaerobic digestor (AD) system) which is then used to power a fuel cell to generate electricity for the robot.

Sadly Slugbot does not seem to have become a commercial success but the principle of turning organic matter into energy certainly is. Last year AD plants across the country generated 147 GWh of electricity[1], enough to power 45,000 homes. Assuming that slug meat has similar calorific value to bacon a 10g slug[2] would contain 30 kcal[3] which is 35 Wh or 3.5 hours of the 10W lights. However, the conversion from slug to electricity via methane and fuel cell will not be very efficient - at 10% you would need 10 slugs for 4 hours of lights.

You might get a little better efficiency from a microbial fuel cell which cuts out the methane step. An MFC uses bacteria to digest organic matter in the anode of the battery, without oxygen as in the AD system, but the oxidation reaction completes at the cathode. You get a flow of protons from the anode to the cathode and hence an electric current [4]. Theoretically you can get more than 50% efficiency but the bacteria might prefer sugar to slug meat as food. There was a lot of news about MFC around 2008/2009 but there still haven't been any commercial installations. In July this year Scientific American reported a demostration plant being built in Israel for treating waste water.

* Slugs are thin on the ground at this time of year but any similarly nutritious biomass would do.

Candles versus LED lights

Many people use candfles for festive lights around the house, though rarely on the Christmas tree because of the fire hazard. Candles used to be made from animal fat and these days you can buy organic lights made from plant oils such as palm oil, but most are made from paraffin which is a fossil fuel. Vegetable wax is carbon neutral but there are still sustainability issues; palm oil corporations have been notorious for cutting down rainforest in South East Asia[5]. However, paraffin is a poor alternative from the point of view of  greenhouse gas emissions. Let's compare the emissions from a 1W LED bulb (which I use as a bedside light and is easily bright enough for reading) versus a 25g candle. If the candle burns for 3 hours that comes to 8g/hour of paraffin which generates 25g CO2[6]. The LED light consumes 1 Wh which generates about 0.5g CO2.

Yet another lighting technology

If you don't like LEDs, look out for another lighting technology in development - field-induced polymer electroluminescence. The inventor, Dr David Carroll, professor of physics at Wake Forest University in North Carolina, says the lights are brighter than LEDs and can produce any colour you like, flicker free and long lasting. Since they are made from layers of plastic they can also be any shape you like. Prof. Carroll has a corporate partner and expects to start production in 2013 [7].

My Christmas lights

We don't have any outdoor lights, just on the indoor tree and we don't use candles because I once set my hair alight on one. Also we prefer to be able to see what we are eating. Our indoor christmas tree now uses one string of LEDs which is rather old and inefficient - they take 16W compared to my neighbour's 4W, though to be fair it is also a longer string. Buying new ones would not be worthwhile financially as they are not on for very long. 4 hours per evening for 10 nights comes to only 0.64 kWh or 10p for our 16W string.


[1] Digest of UK Energy Statistics ((DECC)
[2] Speed of animals.com says a 1.3" slug would be 12g - it is the wrong time of year to find slugs in the garden so I can't measure one.
[3] Weightlossresources.co.uk says bacon is 78 calories per 25g
[4] General Principles of MFCs from www.microbialfuelcell.org
[5] Palm Oil (Greenpeace)
[6] Paraffin, like other hydrocarbons, has the formula C(n)H(2n+2). For solid wax n > 20 so you have about 2 atoms of hydrogen per carbon. Carbon has mass 12, hydrogen 1 and CO2 is 44. The CO2 emissions per g candle are (12/14) x (44/12) or 3.14g.
[7] Plastic bulb development promises better quality light (BBC)

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