Notes on Canvas vs. Plastic Bags

Apologies for not formatting this well. I just wanted it to be present so that the details of my calculations were available. I’ve been putting off posting this for a while, so I see no reason to hold it up longer…

I own 5 canvas bags that in total weigh 900g. They weigh 180g each.

39 plastic bags weighed 250g, for 6.4 g each. This is a slight over-estimate, because
not all were grocery bags. So, the 6 gram number below looks good. That means canvas
bags weigh 30 times more than plastic ones.

http://www.plasticbageconomics.com/index.php?option=com_content&task=view&id=30&Itemid=45

Paper: 2511 BTU to manufacture, 1444 BTU to recycle
Plastic: 591 BTU to manufacture, 17 BTU to recycle
not counting shipping; cotton is 7 times heavier
1 BTU = 1055 J

Paper: 2.65 MJ to make, 1.52 MJ to recycle
Plastic: 0.62 MJ to make, 0.018 MJ to recycle

http://everything2.com/title/Your+organic+hemp+shopping+bag+alone+cannot+save+the+world

Same numbers as above.

http://www.buyecofriendlyproducts.com/2008/10/27/energy-consumption-of-reusable-bags-versus-disposable-bags/

“According to Sustainability-Ed.org, 1 kg of cotton fabric requires 140.1 mega joules (MJ) of energy during a two-year life cycle (which includes 12 washes), while 1 kg of polyester uses 171.5 MJ. A medium-weight cotton or polyester tote weighs about 8-10 oz., or between 0.2 and 0.3 kg, so these figures represent the two-year life cycles of about four reusable bags.

Compared to any type of disposable bag, both cotton and polyester bags consume less energy if used regularly for two years in place of disposable bags. According to a life cycle analysis conducted by the American Chemistry Council, the life cycle of 1,500 polyethylene plastic bags consumes 763 MJ of energy. In a two year period, the average American uses about 660 plastic bags, consuming about 336 MJ.”

That translates to 0.51 MJ/bag for plastic. I used this number, to give plastic bags a better chance.

The number for a 180 g canvas bag is 25 MJ for the bag and 12 washes. This seems too low. I get 19.2 MJ for 12 washes alone, and 10 MJ for the fiber alone. I ignored this claim for canvas.

www.agrilink.co.nz/Portals/Agrilink/Files/LCA_NZ_Merino_Wool.pdf
Textile Energy consumption
(MJ/kg fibre)
Nylon 250
Acrylic 175
Polyester 125
Polypropylene 115
Viscose 100
Cotton 55
Wool 63 †

From this, a 180 g bag should take 10 MJ in fiber. This doesn’t include weaving or manufacture, so it is a strict lower bound. I include it to demonstrate that the value I chose is above the lower bound.

http://cottontoday.cottoninc.com/natural-resources/energy/

“These methods have been applied to cotton production in three different studies for cotton grown in the US resulting in a range of 14 to 35 GJ of total energy per metric ton of fiber produced, and average value of 19 GJ per tonne. [6]”

This translates to 3.4 (2.5-6.3) MJ per 180 g (although there is probably loss in this process). This is lower than the New Zealand study, so my estimate is still safely above the lower bound.

www.ifm.eng.cam.ac.uk/sustainability/projects/mass/uk_textiles.pdf

This is actually a document about t-shirts, but I don’t see why the manufacture of canvas bags should be too different. If anything, the bags should require less energy to produce, because the material is coarser, and presumably less-processed.

These numbers are a bit higher than my above estimates for production, and include 25 times washing, drying, and ironing (60% of the total) to get 112 MJ/250 g T-shirt.

It breaks down to 16 MJ for material, 24 MJ to produce, 7 MJ to transport, and 65 MJ to maintain. In my calculation, I assume that bags do not need to be washed after every use; I certainly don’t wash them that often.

So, let’s say 40 MJ to make, 7 MJ to transport. Scale down to a 180 g bag, and I get 29 MJ to make, 5 MJ to transport. These are the main numbers I used.

As I have claimed above, it should take about 0.5 MJ/bag to make a plastic bag, and about 6/180 * 7 MJ = 0.2 MJ to transport it. Therefore, the cotton sack is 58 times more energy-intensive to produce, which corresponds to its extra mass.

http://www.independent.co.uk/environment/green-living/plastic-fantastic-carrier-bags-not-ecovillains-after-all-2220129.html

Claim is that cotton releases 171 times more greenhouse gasses than plastic.

http://www.biodeg.org/position-papers/paper-bags-and-durable-bags/

Similar numbers as above; canvas bags weigh 80 times more than plastic ones.

These articles don’t include primary sources, and they don’t break out the energy use into manufacture, transport, and washing, so I can’t evaluate why they differ from my calculations. Needless to say, I trust my numbers more.

http://www.nashvillewrapscommunity.com/blog/2011/05/reusable-bag-update-plastic-cotton/

Claim is that cotton bags are 300 times more energy-intensive to produce. My calculations suggest this number is way off.

http://www.savetheplasticbag.com/UploadedFiles/LA%20County%20EIR%20summary.pdf

Claim that one needs to re-use bags 104 times to break even. This refers to a Scottish study that doesn’t include canvas, linked below.

http://www.scotland.gov.uk/Resource/Doc/57346/0016899.pdf

Ecobilan report. Does not include canvas.

rapport_carrefour_post_revue_critique_v4.pdf

A report in French for Carrefour by PriceWaterhouseCooper, referring to an Australian
study in which plastic bags were assumed to weigh 6g, cotton sac 125 g, paper bag 43 g
The cotton bag was only used for 1 year, though. It has a table giving the energy use as
200 MJ. Paper was listed at about 700 MJ?

This estimate would imply that I would wash the bag 104 times in a year, or that they washed weekly and their washing took twice as much energy as I think it should. I think this over-estimates energy use.

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Laundry:

I weighed a couple loads of laundry:
Load 1: 11.5 lbs
Load 2: 9.5 lbs
Use 10 lbs = 4.5 kg

http://michaelbluejay.com/electricity/laundry-sources.html

Hot wash: 106 F/ 41 deg C
Warm wash: 88 F/ 31 deg C
Ground water: 57 F/ 14 deg C

Heat capacity of water: 4185.5 J/(kg K). 3.79 kg/ gallon. C = 0.556 F -32. So I
estimate 9231 J/gal/F, or 2.56 Wh/gallon/F

Bluejay: “0.00263 Wh per gallon per 1 F”. I think he means kWh.

BTU (1055 J) heats one pound of water from 60 to 61 deg F => 2.45 Wh/galF

kWh = 3.6 MJ, Wh = 3.6e3 J

Top-loader:
34-40 gallons. Let’s use 35 kg = 133 kg. Half wash, half rinse.

Front-loader:
uses 40-75% less water. About 15 gallons per load = 57 kg. 85% in wash, 15% in rinse.

Hot cycle will take (41-14)*4185.5 = 0.113 MJ/kg water
Warm cycle will take (31-14)*4185.5 = 0.071 MJ/kg water

Cycles on our washing machine, converting gallons of water to mass:
Top Load Front Load
Wash/Rinse (kg): 66/66 48/9
per pag (4.5 kg load) 2.6/2.6 1.9/0.4
——————————————–
Energy used.
Hot/Cold (MJ): 0.29/0.0 0.21/0.0
Warm/Cold (MJ): 0.18/0.0 0.13/0.0
Warm/Warm (MJ): 0.18/0.18 0.13/0.03

So, worst case is 0.36 MJ/bag for a top loader, or 0.21 MJ/bag for a front loader. At best, 0.14 MJ/bag for a front loader.

Running the washing machine takes about 0.26 kWh = 0.94 MJ. Per pag, 0.94 MJ*0.18 kg/4.5 kg = 0.04 MJ/bag. The spin is energetically negligible.

I need to include an efficiency factor, because I use an electric water heater (booo!). Let me assume 45% for electricity generation and transmission. Then my number is 0.56 (range of 0.31 – 0.88) MJ/bag.

Drying:
Electric: 3.0-3.3 kWh per load = 10.8 – 11.9 MJ/load. Per bag, 0.43-0.48 MJ/bag. Penalty of 45% efficiency for electricity generation and transmission, then the cost is 0.90-0.96 MJ/bag.

Gas: 0.21-0.5 kWh + 0.22 (0.17-0.28) therms gas, where 1 therm = 1e5 BTU = 1.055e8 J = 105.5 MJ, so a load is 1.28 (0.76-1.8) + 23.2 (17.9-29.5) MJ = 24.5 (18.7 – 31.3) MJ, so a bag uses 0.98 (0.75 – 1.25) MJ of energy. This is about the same as electricity, surprisingly. I wonder why…

So my total is 0.6 MJ/bag for washing, 1.0 MJ/bag for drying = 1.6 MJ/bag total, with a range from 1.1 – 2.1 MJ/bag. This is below the range of the t-shirt estimate linked above, which I get 2.6 MJ per wash/dry/iron. So, perhaps I’m off by 50% on the low side.

http://www.civilizationsfuture.com/energyNumbers.html

kWh MJ
Clothes Dryer (1 load) 1.89 6.79
Clothes Washer (1 load/hot wash) 6.42 23.1
Clothes Washer (1 load/cold wash) 0.94 3.40

I get a worst-case for a hot wash, including electricity transmission inefficiencies, of 22 MJ. So my number agrees. My cold wash number is a little bit low.

My estimate for drying is much higher than the estimate above, by a factor of 4. So, I think I have reasonable numbers for the energy use for the laundry.

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Cloth vs paper napkins, as a point of comparison.

http://www.treehugger.com/files/2009/07/are-paper-napkins-more-environmentally-friendly.php

http://www.triplepundit.com/2007/06/askpablo-napkins/

“The paper napkin is assumed to weigh 5 g and the cotton napkin is assumed to weigh 32 g. The paper napkin is used only once, while the cloth napkin will be reused 50 times over its lifetime
Recycled paper fiber has a significantly lower material intensity than virgin paper. 1.75 g of CO2 per gram of bleached virgin paper vs. 0.05 g of CO2 per gram of recycled paper. The difference in water use is also impressive, 303 g vs. 15 g. The cotton is even worse than the bleached paper at 2.74 g of CO2 per g and 6814 g of water used per g. (Source: MIPS data tables, Wuppertal Institute)

“Since the cloth napkin will be reused fifty times we need to consider the impact of washing and drying. Washing and rinsing the cloth napkin probably uses around 2.8 to 5 gallons of water per pound of fabric. If we assume a highly efficient machine this means 250 g of water per napkin, so 12,500 g ( 12.5 kg) of water over its lifetime. Add to that the use of soap and the energy used in drying.

“Assume that we are using a 5000 W machine that can dry fifty napkins in twelve minutes, using 1 kWh. Using the CA average emissions factor ( 0.51 mT CO2 / MWh) we get 510 g of CO2 (0.001 MWh x 0.51 mT CO2 / MWh). If we go back and assume a 500 W washing machine this would add around 50 g of CO2.

“So, here’s the final tally: Fifty paper napkins are responsible for 3,750 g of water use, and 462.5 g of CO2 emissions (although we can’t assume the landfill emissions factor to be quite accurate) and the cloth napkin is responsible for 240,548 g of water use, and 648 g of CO2 emissions.”

From this, I am just interesting in comparing the washing numbers. A 5 g napkin uses
250 g of water, so a 180 g bag would use 9 kg of water. I am using half that, so this
writer might be over-estimating water use by a factor of two.

In terms of energy, he is using 1 kWh for 250 g, or 3.6 MJ * 180g/250g = 2.6 MJ to dry,
which is 2.6 times what I use. However, in his estimate, the washing takes 1/5 of the drying energy, so his total is 3.1 MJ. This is roughly twice what I would use. So, I may be under-estimating energy use by a factor of two, but this does not change my conclusions.

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Wrap-up:
Plastic: 0.5 MJ making, 0.2 MJ transporting, no cost to wash, no cost to dry.
Canvas: 25 MJ to make, 5 MJ to transport, 0.6 MJ each wash, 1.0 MK each dry.

Use: 52 times a year. Replaces 3 bags each time (they double-bag plastic), so I am
saving 156 plastic bags with each canvas one.

Say I wash the bags 12 times. I will use 49.2 MJ in a year. This is the equivalent of 70
plastic bags in the first year. I would have to wash the bags 49 times for plastic to
break even.

In the second year, I would use 19.2 MJ, or the equivalent energy of 27 plastic bags.

  1. If “All” shoppers used cotton bags – You left out the cost of having billions of these bags made which would require land to be devoted to cotton farming and the impact that the tractors and chemicals used on that land have on the environment. This large amount of cotton that would need to be grown to meet the needs of “all” shoppers around the world would be devastating. How many acres of rain forest will be sacrificed for your bags? Organic farming will never reach this proportion of out put! Whenever something has to be done by “everyone” it reaches a critical mass that then has drastic unintended consequences!

  2. I think you are overstating the consequences of everyone using canvas bags. As it stands, my family has been using the same 5 bags for about 10 years. In that time, we have gone through many, many more shirts, pants, skirts, dresses, sheets, and towels. Entire closets have been replaced several times over. This uses much more cotton.

    So, if you are worried about the ecological costs of manufacturing reusable bags, you might be better off first considering whether people should do without clothes.

  3. This is crazy. I have had the same bags for 10 years. I wash them once or twice a year, if that. Many of the bags we used are made from recycled plastic bottles therefore no plants need be raised to create them. We also use boxes to bring home our purchases. No bag necessary.

    Plastic shopping bags are killing much of our marine life. Many creatures mistake them for fish or jellyfish and die a horrible death after ingesting them.