Monday, August 24, 2015

Embodied Energy and Recycling

I admit to being ambivalent about recycling.  I understand the benefits of recycling in areas where there is high population density.  Pulling recycled material out of the waste stream reduces tipping costs.  That can be done economically because the amount of incremental travel required to collect the material is small when compared  to those of us who live in the hinterboonies. 

For example, we had been taking our recyclables to Charlotte.   The recycling time window is short.  Consequently, we usually made a dedicated (i.e., single purpose) trip.  The round trip distance is 20 miles.

The "embodied energy" cost of driving a typical American vehicle is very close to 10MJ/mile*.  Two-thirds of that is in fuel costs and one-third is in the depreciation of the "embodied energy" represented by the vehicle itself.

So the embodied energy cost of driving twenty miles is 200MJ.

Wikipedia lists polystyrene as having an embodied energy of 88MJ/kg**.  Beverage bottles are usually made of PET (a form of polyester) but I do not have data for PET, so I will used the value for polystyrene for future calculations.

A typical Gatorade bottle weighs 1.4 ounces.  That calculates out as 3.5MJ/bottle (some sources quote up to 5.4MJ/bottle).  Translated into miles, that is about 2.7 Gatorade bottles per mile.  A round trip of twenty miles cannot be justified for anything less than 55 Gatorade bottles.  In our strange, upside-down world, it is notable that the 130 Calories of Gatorade in the bottle delivers 0.55MJ to the customer, or approximately 15% of the embodied energy represented by the bottle.

A typical water bottle weighs 0.8 ounces.  Consequently it takes about 100 empty bottles to be "embodied energy" neutral for a twenty mile round trip.

Paper, cardboard and boxboard:  Forget about it.  It has an embodied energy of about 10MJ/kg.  Reprocessing incurs a substantial penalty because it involves wet processing and drying.  Drying is extremely energy intensive.  Recycling calculations should comprehend net energy savings but that is not information that is readily found***.

Incidentally, some people recycle based on concern about carbon footprint.  Embodied energy is roughly related to carbon emissions.  More energy nearly always entails more CO2 emission.  Recycling is not always favorable to carbon emissions.

*Extrapolated from Australian data to comprehend a 4000 pound curb weight vehicle like a base Ford F-150, the most popular vehicle in the United States.  100,000 mile average lifespan.  Yeah, I know they can last longer but a bunch of them get knocked off the road due to accidents well before that distance.

**By comparison, gasoline has an embodied energy of approximately 48KJ/kg, so "engineering plastics" have about twice the embodied energy as their base feedstocks.  I have some feelers out on this, so these numbers may change.

***Recycling PET beverage containers incurs an energy penalty of approximately 30MJ/kg so recycling really only nets about 60MJ/kg.  Other sources quote an energy penalty of up to 65MJ/kg which leaves an energy net of 23MJ/kg.  These calculations made no attempt to comprehend "net energy".  No penalties were assessed against the recycled stream and no residual value was assigned to the depreciated vehicle.

****The marketing cachet of products with a high percentage of recycled content has driven the cost of recycled materials above that of virgin materials for several types of plastic resin.

*****A major cost in recycling is in sorting the stream into individual types of plastic.  One solution would be to include a specific fluorescent dyes into each material such that they glow a different color in UV light.  Then, DVT technology could be used to sort the product.  Imagine a conveyor belt, UV lights, air jets and video cameras or photo sensors with color filters over them so they could only "see" certain colors.  The air jet(s) could be programmed to turn on when a bright spot went over them. This technology has been proposed for PFEMA based inspections for robotic applied products where there is not enough natural color contrast to use DVT with natural light.


  1. This ties into the found objects discussion. In the third world 2 litre soda bottles are widely used for gas cans.

    Every time I go to the recycle and dump the bin of plastic bottles, I reflect that for most of human history the containers I am throwing away were one of the most valued items they had. Seems like every dig is littered with two durable goods- weapons and containers.

  2. Raven has a great point... And it actually COSTS us money to recycle, a conundrum we tend to ignore...

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  4. Somewhere I read that one of the few articles it make sense to recycle was aluminum, because of the smelting cost. Whether that is true or not I do not know.

    1. Embodied energy in aluminum is between 155-and-220MJ/kg. That is three-to-four-and-a-half times the energy density of gasoline.

      It takes relatively little energy to remelt aluminum. The huge energy cost is in separating it from the oxygen that always accompanies it in nature.

      That is one reason that powdered aluminum is used in pyrotechnics and rocket fuels. That invested energy is returned as heat energy when recombined with oxygen.

      Great comments! Thanks for reading.

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