Sunday, May 29, 2011

On the Variable Value of Recycling

A new and very interesting paper by Gutowski et al [1] (also up at ScienceDaily) discusses the variable value of recycling - more specifically, whether there are energy savings benefits is not always clear; and in some cases, the benefits, if any, are small.  From [1]:

Remanufacturing products that can substitute for new products are generally claimed to save energy.  These claims are made from studies that look mainly at the differences in materials production and manufacturing.  However, when the use phase is included, the situation can change radically. ... For most of these products, the use phase energy dominates that for materials materials production and manufacturing combined.  As a result, small changes in use phase can overwhelm the claimed savings from materials production and manufacturing.

Gutowski et al look at 25 case studies in 8 product groups, including furniture, clothing, computers, electric motors, tires, appliances, engines, and toner cartridges.  In these 25 case studies, it was found that there were only 8 cases of clear energy savings, generally grouped into office furniture (office chair, office desk), textiles (clothing), and select electronic components (laptops and monitors).  The first two categories experience an energy savings with reuse because the usage energy / energy of operation is so low [1].  For the electronics, on the other hand, "energy efficiency improvements within the same kind of devices over the time period (2001-2005) are not large enough to overcome the manufacturing phase savings achieved by reusing."  On the other end of the spectrum, products which are best bought new include high-usage appliances such as dishwashers and refrigerators, and other electronic components (again, including laptops and monitors).  In these cases [1], "the use phase energy has changed significantly due to efficiency mandates and/or the introduction of new efficient technologies." 

So what does this mean to materials science and engineering?  New materials are being developed to improve the efficiency of product use, such as through lighter weight which leads to improved fuel economy, or through improved energy maintenance / control / storage leading to reduced energy loss.  As outlined in Ashby [2], the Use phase of many product life cycles prevails as requiring the largest energy fraction, e.g. in civil aircraft (~95%), family cars (~85%), and appliances (~70%).  As such, high-performance materials often target the above products because of the potential energy savings in their respective Use phase.  These applications can also justify the high cost of the new materials because of the anticipated reductions in usage cost.  But as Ashby outlines [2], there are three additional material phases in product life cycles: 1) Production, which includes the acquisition of the raw materials and by-products of refinement; 2) Manufacture, which includes the conversion of raw materials into product shape; and 3) Disposal, which includes the ability to re-use, disassemble, and recycle the product - and is more difficult to quantify than the other three. 

The energy associated with a Disposal phase is a function of many things.  Borrowing from Ashby [2], this can include energies (or costs) associated with, for example, waste / recycled material transportation, sorting, and storage; the usable energy contained in the waste (e.g. which may be retrieved through incineration); the energy difference between embodied energy of new materials versus the recycling energy of waste materials; the energy required to develop a succeeding material/product; and perhaps most important, energy surrounding the environmental consequences.  

Although energies associated with the Disposal phase were not considered by Gutowski et al, the authors do acknowledge that the re-manufacturing route can reduce landfill burdens as well as the toxic material generation, and thus tilt increased importance to re-manufactured rather than new products.  However, I've already written on the toxicity of some materials and the projects underway to develop alternatives, e.g. replacement coatings to Cadmium and Chromium; in these cases, there is significant Disposal-based energy driving the development of new products which can reduce toxic material generation.  In order to more completely and evenly assess the value of recycling and its role in a product's life cycle - as compared to new material manufacturing - the energy of the Disposal phase needs to be better quantified and calculable.  This need is further driven by the fact that some products identified by Gutowski et al - namely, electronic devices - are experiencing diminishing services lives (estimated as 4 years in [1]) and thus their Disposal phase energies are contributing more to their overall energy fraction.

[1] T.G. Gutowski, et al.  Remanufacturing and energy savings.  Environmental Science & Technology, Vol. 45 (2011), pp. 4540-4547.
[2] M.F. Ashby.  Mechanical Selection and Mechanical Design, 4th Ed.  Butterworth-Heinemann (2010), pp. 440-447.

No comments:

Post a Comment