Strategies for reducing environmental impact

Having identified the dominant life phase and component that contributes most to a product's environmental impact (using the Eco Audit tool), the next step is to identify the correct strategy for reducing that impact. As mentioned in the introduction, the appropriate strategies are highly dependent on both the type of product and the dominant life phase. Guidance on what impact reduction strategies, and material indices, to consider is accessed by clicking on the dominant life phase bars in the Summary chart.

The strategies for reducing the environmental impact of each life phase can be viewed by clicking on the links below:

Material phase
Manufacture phase
Transportation phase
Use phase
Disposal and end-of-life phase

Strategies for reducing environmental impact: Material phase

Aim

Minimize embodied energy or CO₂ footprint per unit of function.

Actions

Select material with lowest embodied energy and CO₂ footprint per unit of function.
Use as large a 'recycled content' in the material as possible.
Reuse existing components where possible.
Use as little material as possible while retaining enough redundancy for safety.

Conflicts

Watch out for conflict with the Use phase. The material with the lowest direct eco-impact may not be the lightest or the cheapest. Use trade-off methods to resolve the conflict.

Relevant performance indices to minimize embodied energy (CO₂ footprint):

Mode of loading	Stiffness prescribed: minimize	Strength prescribed:minimize

E = Young's modulus, σ_y = yield strength; ρ = density, H_m = embodied energy of material/kg
(for indices to minimize CO₂ footprint: replace H_m by CO₂ = CO₂ footprint of material/kg)

Charts for selecting materials that minimize impact of Material phase

Bubble chart of Modulus (y-axis) vs. Embodied energy × Density (x-axis)
Bubble chart of Yield strength (y-axis) vs. Embodied energy × Density (x-axis)
Bubble chart of Modulus (y-axis) vs. CO₂ footprint × Density (x-axis)
Bubble chart of Yield strength (y-axis) vs. CO₂ footprint × Density (x-axis)

Alternatively use the chart stage 'Advanced' function to create bar charts of the performance index.

Further reading

Ashby MF, "Materials and the Environment", 2nd Edition, Elsevier Butterworth Heinemann, Oxford, UK, 2012.

MacKay, D. (2009) "Sustainable energy – without the hot air" UIT press, Chapters 15 and H.

Strategies for reducing environmental impact: Manufacture phase

Aim

Minimize process energy, CO₂ footprint and waste.

Actions

Select processes with low energy and CO₂ footprint – deformation processing rather than casting for example.
Avoid processes with large processing waste – net-shape processes rather than machining from solid for example.
Reuse existing components where possible.

Conflicts

Check for quality loss on changing process.

Relevant material indices. Select compatible process with lowest process energy H_p or CO₂ footprint CO_{2, p}.

casting, deformation processing, molding, machining processes

Charts for selecting processes that minimize impact of Manufacture phase

Bar chart of Shaping energy
Bar chart of Machining energy

Further reading

Ashby MF, "Materials and the Environment", 2nd Edition, Elsevier Butterworth Heinemann, Oxford, UK, 2012.

MacKay, D. (2009) "Sustainable energy – without the hot air" UIT press, Chapters 15 and H.

Strategies for reducing environmental impact: Transportation phase

Transportation is an energy-conversion process: primary energy (oil) is converted into mechanical power and this is used to provide motion. As in any energy-conversion process there are losses, here most conveniently expressed as the energy dissipated per tonne per kilometer transported (MJ/tonne.km), carrying with it an associated CO₂ footprint (kg/tonne.km). Sea, ground and air transport systems usually burn hydrocarbon fuel, so the CO₂ emission is approximately proportional to the energy. Transport dissipates energy in two ways: as work against drag exerted by air or water, and as work to accelerate the vehicle, lost on braking:

Energy dissipated per unit distance = α C_d A v² + β m v²

where α and β are constants, C_d is the drag coefficient, A the frontal area of the vehicle, v the velocity and m the mass of the vehicle.

The addition of one more unit of freight does not significantly change the frontal area or the drag coefficient, so it is the kinetic energy term that dominates. So the steps to reduce the energy for transport focus on mass, distance, velocity and mode of transport.

Aim

Design for low-impact transport.

Actions

Reduce the mass transported, m. Material-efficient design helps here.
Rethink the transport mode.
Reduce the distance of transport.
Reduce the speed v.

Conflicts

The motive for off-shore manufacture, incurring the need for long haul transport, is that the lower labor costs more than off-set the greater transport costs. Use the Eco Audit tool to explore the impact of different transport modes and distances.

Charts for selecting processes that minimize impact of Transport phase

Material selection to minimize mass can reduce the mass of both products and the vehicles themselves. See strategies for reducing environmental impact of Use phase.
Explore alternative transport modes by specifying different options in the transport section of the Eco Audit product definition.

Further reading

Ashby MF, "Materials and the Environment", 2nd Edition, Elsevier Butterworth Heinemann, Oxford, UK, 2012.

MacKay, D. (2009) "Sustainable energy – without the hot air" UIT press, Chapters 3, 5, 15, A and C.

Strategies for reducing environmental impact: Use phase

The strategies for reducing the environmental impact of the use phase are highly dependent on the type of product and whether the static or mobile use phase is dominant. These can be categorized into three main groups (click category for guidance on impact reduction):

Static mode - mechanical devices
Static mode - heating and cooling systems
Mobile mode - transportation

Static mode – mechanical devices

Rotating disks, drums and shafts have rotational inertia. Energy is dissipated when they are spun up to speed and down again. The energy loss is minimized by giving the component as small a rotational moment of inertia as possible. The same is true of oscillating components like connecting rods, weaving and printing equipment.

Washing machine: power in, waste heat and hot water out.

Aim

Design for energy use.

Actions

Select material with the lowest value of the appropriate index, listed below.

Conflicts

The material choice that minimizes mass may not minimize embodied energy or cost. Use trade-off methods to resolve the conflict.

Relevant material indices to minimize use energy (CO₂ footprint), choose materials with the lowest values of the indices listed below.

Mode of loading	Stiffness prescribed: minimize	Strength prescribed: minimize

Charts for selecting materials that minimize impact of Use phase (static appliances with moving parts)

Bubble chart of Modulus (y-axis) vs. Density (x-axis)
Bubble chart of Yield strength (y-axis) vs. Density (x-axis)

Alternatively, use the chart stage 'Advanced' function to create bar charts of the material index.

Further reading

Ashby MF, "Materials and the Environment", 2nd Edition, Elsevier Butterworth Heinemann, Oxford, UK, 2012.

MacKay, D. (2009) "Sustainable energy – without the hot air" UIT press, Chapter 11.

Static mode – heating and cooling systems

Energy losses from a typical house: Walls 35%, Roof 25%, Floor 15%, Ventilation 15% and Glazing 10%. Refrigerators and freezers, ovens and kilns, space heating and air conditioning use energy to heat or cool space. Energy use is minimized by maximizing the thermal resistance of the walls of the product or building. These walls are usually multi-layers. The thermal resistance or R-value is a measure of thermal resistance of a window, wall, roof or floor unit. It is the temperature difference required to drive a unit flux of heat through the unit:
R = Temperature difference/heat flux = Sum of (thickness/thermal conductivity) for all layers
where q is the heat flux through the unit, ΔT is the temperature difference across it, t_i are the thicknesses of the layers of the unit and λ_i are the thermal conductivities of those layers. The U-value (the transmittance or conductance) is the reciprocal of the R-value. In the SI system the units of R-value are m²·K/W, but the US still uses ft².F.h/Btu.

Aim

Design for minimum thermal loss.

Actions

Select material with the lowest value of the appropriate index, listed below.

Conflicts

The material choice that minimizes mass may not minimize embodied energy or cost. Use trade-off methods to resolve the conflict.

Relevant material indices. When the temperature difference across the wall is constant over long periods of time, choose the material with the largest R value (where R ∝ 1/λ). When, instead, the temperature difference across the wall fluctuates, choose the material with the lowest value of the index listed below.

Temperature profile	To minimize heat loss: minimize
	Thermal conductivity λ
	Combined thermal inertia and conductivity

λ = thermal conductivity, C_p = specific heat; ρ = density

Charts for selecting materials that minimize impact of Use phase (static heating and cooling systems)

Bubble chart of Thermal conductivity (y-axis) vs. Volume specific heat C_p ρ (x-axis)

Alternatively, use the chart stage 'Advanced' function to create bar charts of the material index.

Further reading

Ashby MF, "Materials and the Environment", 2nd Edition, Elsevier Butterworth Heinemann, Oxford, UK, 2012.

MacKay, D. (2009) "Sustainable energy – without the hot air" UIT press, Chapters 7 and E.

Mobile mode – transportation

The use energy of transport systems or of products that form part of them is largely dependent on their mass. The energy dissipated per unit distance is

E_motion = α C_d A v² + β m v²

where α and β are constants, C_d is the drag coefficient, A the frontal area of the vehicle, v the velocity and m the mass of the vehicle. Material substitution to reduce mass and refinement of shape to reduce frontal area and drag coefficient thus reduces the use energy of the product.

Aim

Design for minimum mass.

Actions

Select material with the largest value of the appropriate index, listed below, to reduce mass.
Lean design: use as little material as possible.

Conflicts

The material choice that minimizes mass may not minimize embodied energy or cost. Use trade-off methods to resolve the conflict.

Relevant material indices to reduce mass:

Mode of loading	Stiffness prescribed: minimize	Strength prescribed: minimize

E = Young's modulus, σ_y = yield strength; ρ = density

Charts for selecting materials that minimize impact of Use phase (transportation products)

Bubble chart of Modulus (y-axis) vs. Density
Bubble chart of Yield strength (y-axis) vs. Density

Alternatively, use the chart stage 'Advanced' function to create bar charts of the material index.

Further reading

Ashby MF, "Materials and the Environment", 2nd Edition, Elsevier Butterworth Heinemann, Oxford, UK, 2012.

MacKay, D. (2009) "Sustainable energy – without the hot air" UIT press, Chapters 3, 20 and A.

Strategies for reducing environmental impact: Disposal and end-of-life phases

There are six options for disposal of products at the end of their first life:

Landfill
Combust for energy recovery
Downcycle
Recycle
Re-condition or re-manufacture
Reuse as is

The first, landfill, is the least attractive. Combustion, properly carried out, recovers some of the embodied energy of the materials of the product, but the recovery-efficiency is low, the economics are unattractive and proposals to build combustion plants are often opposed by local residents. Recycling is the best way to extract value from waste and return materials to the supply-stream, preserving material stock. Re-conditioning or re-manufacturing restores used products or recoverable components to as-new condition, but establishing a market and maintaining a supply chain of recondition products is not easy, and issues of warranty and responsibility for malfunction are deterrents. Reuse sounds the most attractive option; passing products from consumers who no longer want them to those willing to accept them in a used state. This requires a market place where seller and buyer can meet and negotiate and an acceptance of used products rather than new.

Aim

Increase end-of-life potential, design for recycling.

Actions

Select material that have high recycle ratio (the 'Recycle fraction in current supply', listed on the material datasheets).
Minimize the number of different materials in the product.
Avoid combining materials that are incompatible if recycled together.
Identify materials used in components, using recycle marks or color coding, preferably with grades, filler type and content.
Design for ease of disassembly: snap fits, fasteners, releasable adhesives.

Conflicts

The material choice that best suits end-of-life may not minimize the use energy. Use trade-off methods to resolve the conflict.

Charts for selecting materials that maximize end-of-life potential

Bar chart of Recycle fraction in current supply

Further reading

Ashby MF, "Materials and the Environment", 2nd Edition, Elsevier Butterworth Heinemann, Oxford, UK, 2012.

MacKay, D. (2009) "Sustainable energy – without the hot air" UIT press, Chapter 15