Thursday, 8 December 2011

Christmas trees and Christmas lights

I love to see the fairy lights on our Christmas tree, and the colourful illuminations down the high street.  With dusk falling in late afternoon we need a little something to brighten up the gloom don't we? It's only once a year after all. However, energy bills are rising and climate change is a persistent and increasing threat, so it might be worth a quick check to see how much these luxuries are costing us. Also the Christmas tree quandary - is a real tree lower in carbon emissions than an artificial one, or not?

First the lights. We have 2 sets of rather ancient fairy lights for our tree. One is about 18 traditional lights and it takes 22W. The other is a lot of little LEDs - maybe 80 or more - and it takes 16W. I like to use both, a total of 38W. For say 6 hours each evening that comes to 0.17 kWh/day. Our daily electricity use at this time of year is about 12kWh so the lights add just 1.4% and they cost 2p/day - a great deal less than a minced pie.

If you are buying a new set of indoor Christmas lights, there are lots of reasons you might choose LED lights over traditional ones. They are pretty, they are more reliable and you can get some very good deals at the moment. However, it probably isn't worth paying a lot more for LEDs merely in the expectation of  saving money on the electricity consumption, because you won't use them enough to outweigh the higher capital cost. Worst case, an indoor traditional tree string might use 100W. At 6 hours a night for 3 weeks over one season that would be 12.6 kWh costing around £1.50. Even if the LED lights use 90% less power (as I have seen claimed) the cost savings comes to £1.35/year. Of course they should last for years: ours are well over 10 years old.

Outdoor lighting, especially in the high street can use a lot more power than this, and then the benefit of LEDs will be that much greater. There is one famous shop in Cambridge that runs a net of lights over one enormous wall - I haven't measured it but at a guess it might be 20m x 8m high. From looking at commercial grade lights on a typical 3x2m curtain takes 18W. At that rate this enormous net would use 480W, around 12kWh for 24 hours. Commercial users pay less than householders - say 8p/kWh so the whole wall of light will cost less than £1/day to run, though it will have cost several thousand pounds to buy the equipment.

Now lets look at the tree question - which is better, real or artificial. The most detailed life cycle analysis (lca) I have found on this topic is from Quebec in Canada [1]. They conclude that real Christmas trees have a lower environmental impact in general, including lower carbon emissions. They were working on a 6-year lifetime for the artificial tree and they argue that you would have to keep it for 20 years to reach break even with the real tree per year.  That does seem a long time, but our tree is 14 years old now and still going strong (we bought it for our first Christmas in this house). However, they do also point out that the carbon emissions in either case are not very large - overall about 3 kg CO2 for the real tree and about 8 kg CO2 for the artificial tree, (48 kg amortised over 6 years). The tree was 7 ft tall in both cases. In the UK our total carbon emissions are about 30kg/day/person so 6kg for the artificial tree is one fifth of a typical person day.

The biggest source of carbon emissions from the real tree was in transport from the shop to the house. The lca put this at just over 2kg CO2 for a 5km each way trip,  close enough to typical UK car emission factors (0.2kg/km). (The lca also included another 200km from the plantation to the shop, but bulk transport is much more efficient.) The second biggest carbon emissions for the real tree was from disposal. They estimated this was about 1.5 kg CO2/tree based on 50% going to landfill and 50% chipped and used for fuel (instead of heavy oil). If you send your tree for recycling here it will probably be chipped and used for mulch which generates almost no emissions [2].  The Quebec team estimated that during the growth phase carbon absorbed into the wood outweighed fuel and chemicals used giving a net balance of more than 2kg CO2 absorbed. If your tree is used for mulch it will break down only very slowly so the sequestered carbon will be retained a long time. The lca also took into account the stand for the tree which was another 6 kg CO2 but reused for 6 years.

The lca did not take into account vacuum cleaning around the real tree to pick up the needles but this doesn't add much. If you did this every day for 3 weeks, 1 minute per day, with a 1kW cleaner, this would use 0.35 kWh and generate another 0.2 kg CO2.

Overall, for your real tree, if you combine the buying trip with another purpose, reuse your stand forever,  and recycle the tree for mulch, you could achieve a net carbon saving every year, in which case there is no contest with the artificial tree.

For the artificial tree, the carbon emissions were almost all from production (even though this tree was shipped from China). The tree weighed 9.5kg of which about half was steel (including the trunk and twisted wire for the branches). My tree which is only a little smaller - 6ft instead of 7ft -  weighs only 6kg so maybe this is an over-estimate, at least for a cheap UK tree. Adjusting for weight and 14 years use (so far) instead of 6, our tree comes to 2.1 kg CO2/year. This is equivalent to about 4kWh of electricity use, or 23 days of our Christmas tree lights.

I have to admit our artificial tree does work out worse in carbon emissions but it isn't enough to get me worried. It saves vacuum cleaning, a job I hate and it's much less hassle overall. Anyway, having got it, the longer I keep using it the better, so I'm not going to throw it out now.

[2] recyclenow - can it be recycled: Christmas trees

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