1. Introduction
The world population increased from 1 billion in 1800 to 7.8 billion in 2020 [
1]. Although the world population growth rate is decreasing, it is predicted that the world population will reach 9.7 billion by 2064, subsequently declining to 8.8 billion in 2100 [
2]. The population growth has led to global issues, such as overconsumption of resources and energy and pollution. Sustainability is a fundamental attribute playing an increasingly significant role in design nowadays, especially in product design, where it is considered a requirement for solving those global challenges [
3].
In recent years, there has been an increasing focus on sustainable product developments due to environmental regulations and expectations of consumers [
4]. It is imperative for modern firms and enterprises to consider sustainability in product design and development [
5]. Sustainable development is often defined as ‘development that meets the needs of the present without compromising the ability of future generations to meet their own needs’ [
6]. Sustainability commonly involves three interconnected pillars: social, economic, and environmental [
7]. However, sustainability in design is primarily focused on environmental aspects [
4]. Therefore, sustainable design is described as a design approach for reducing the environmental impacts throughout a product’s entire life cycle [
8,
9,
10]. Material extraction, manufacturing, use, and end of life of a product produced all have impacts on the environment [
11]. It is therefore critical for designers, while designing new sustainable products, to select raw materials with the least environmental impact, influence manufacturers to minimise environmental damage, and consider an environmentally friendly manner of using and disposing of these products [
12].
Conceptual design, which involves activities such as concept generation and assessment, is arguably the most significant stage in the design process. As an early stage in design, concept assessment has a powerful impact on the downstream activities, such as product production, use, and end of life [
13,
14,
15,
16]. It is a complex task involving decision making based on multiple criteria [
13,
14], which significantly saves product development cost and time, as well as raises awareness of design concept improvement opportunities. The decisions made have a critical influence on the environmental impact of a product, as well as performance, cost, reliability, and safety [
17]. The decisions often have high impacts at the conceptual design stage, but decrease significantly as the design process progresses [
16].
Environmental impacts occur throughout a product’s entire life cycle, from raw materials, production, and use, to end of life, while the environmental impacts at different stages vary significantly among different products. For example, although both kettles and cameras are consumer electronics, kettles embody most of the environmental impacts at the use stage, while cameras, ignoring digital storage, embody most of the impacts at the production and raw material stages [
18,
19]. However, most of the environmental impacts are ‘locked’ into the product during early design stages, where the product concept is formed, functions and performance are determined, and materials and manufacturing processes are selected [
20]. The ‘lock-in’ of environmental impacts is determined and cumulated throughout the product’s life cycle [
20]. Nevertheless, it is reported that 80% of sustainability impacts are decided at the design stage, involving both conceptual design and detailed design [
11].
According to studies,
Figure 1 indicates that the impact of decisions decreases, while the cumulative ‘lock-in’ environmental impact increases, as the product life cycle matures [
16,
20,
21]. The figure shows the significance and advantages of addressing the sustainability issues of a product as early as possible in the design process, as it is challenging and costly to address sustainability issues at later stages [
21,
22]. For instance, it is easier to design an energy-efficient product rather than educating consumers to use the product in an energy-saving manner with the aim of reducing environmental impacts. Therefore, the opportunity to minimise a product’s environmental impacts mainly exists in the preliminary design stage, especially in conceptual design where decisions have high impacts [
23,
24].
However, product design engineers generally focus on producing products that meet the required technical performance, aesthetics, durability, and cost demands [
25], but lack awareness of the wider environmental impact of the design [
12] (
Knowledge Gap 1). For instance, Brundage et al. [
26] indicated that there is a lack of communication between designers and manufacturers, which limits designers in reducing environmental impacts from a manufacturing perspective. It is also challenging to improve a product’s sustainability once the product is designed [
21,
22] (
Knowledge Gap 2). Therefore, there is a need to project later sustainability-related activities, such as manufacturing, use, and end of life, to early design stages to inform better decision making [
27,
28,
29]. For example, a product designed for easy manufacturing and assembly could increase the chances of it being reused or recycled, leading to a reduction in environmental impacts [
30]. Nevertheless, the majority of existing concept assessment methods or tools are used to evaluate the feasibility and creativity of the concepts generated at early design stages [
31,
32,
33,
34,
35,
36,
37] (
Knowledge Gap 3). Therefore, current concept assessment methods could not guarantee the generation of sustainable product design concepts, which embody minimum environmental impacts. These three knowledge gaps imply that there is a need to come up with an approach to measure the sustainability of design concepts, considering aspects of later activities in the product life cycle and therefore promoting a more sustainable design manner.
The paper aims to offer support to designers in generating sustainable design concepts by considering a wide range of aspects to minimise negative environmental impacts, ultimately leading to sustainable products. The primary objective of the paper is to answer the following research questions: (1) What are the critical factors related to sustainable design, particularly environmental impacts? (2). Is there a set of metrics that can measure sustainable product design concepts?
The remainder of the paper is organised as follows: the next section reviews the related work on sustainability and concept assessment. In
Section 3, four metrics (
material,
production,
use, and
end of life) for measuring sustainable product design concepts are proposed with rationales and measurements. A case study demonstrating the application of the metrics is provided in
Section 4, followed by discussion in
Section 5 and conclusion in
Section 6.
3. Metrics for Measuring Sustainable Product Design Concepts
Four metrics, material, production, use, and end of life, for measuring sustainable product design concepts are proposed based on existing studies on sustainable design and conceptual design. The four metrics are described in the following subsections with the underpinning rationales and measurements, respectively. However, it is challenging to determine the actual value of the negative environmental impacts caused at the conceptual design stage. As a result, to reflect the level of sustainability in a simple but effective manner, measurement scales of low (0), medium (1), and high (2) are employed to indicate sustainability attributes.
3.1. Material
The assessment of materials in sustainable product design concepts is of fundamental importance. Materials have direct impacts on products with regard to the origin, property, and use of materials. Origin of materials refers to where the materials, used in the components and parts of a product, are originally sourced, involving nonrenewable and renewable resources. Nonrenewable resources are limited in supply, which cannot be replenished or replaced, such as fossil fuels, minerals, and metal ores. Popular materials produced from nonrenewable resources involve fossil-based plastics, metals, and glasses, which are often used in product design. Renewable resources refer to those that can be easily regenerated. Materials that are renewable, such as bamboo, mushroom, natural rubber, wood, and cotton, are increasingly used in product design. An increasing number of sustainable materials are being developed by applying renewable resources. For example, bioplastics, which are less or minimally reliant on fossil fuel, are produced by using renewable plants, such as sugarcanes, corns, and potatoes. In addition, there is currently an emerging trend towards utilising waste materials for design. For instance, by-product waste materials (such as chicken feathers and bran), which are secondary products generated from production, are often used as raw materials.
Toxicity, recyclability, and biodegradability are the main indicators of material sustainability properties. A material that is recyclable or biodegradable and nontoxic is often preferred, while materials that are toxic and neither recyclable nor biodegradable should be avoided in product conceptual design.
The use of material in a product refers to the volume/weight of materials and the number of types of materials involved. Using less volume/weight of materials contributes to a positive environmental impact, as it consumes less amount of resources and energies from material sourcing and production, to product end of life. The more types of materials used will increase a product’s complexity, which will lead to more negative environmental impacts throughout the product’s life cycle, as it increases the difficulties in product production and end of life. Therefore, determining which material(s) to use and identifying how the material(s) are used in a product design concept are strongly associated with the product’s sustainability performance.
Measurement of Material
In the conceptual design stage, information such as material origins, material properties, and the use of materials needs to be determined to assist designers with evaluation. As suggested previously, low (0), medium (1), and high (2) are used to indicate sustainability attributes. For example, if the origin of one type of material used is a promising renewable source, then a rating of high, a score of 2, will be given. Similar principles apply to material properties. For example, if the material is toxic, cannot be recycled, or is biodegradable, then a rating of low, a score of 0, will be given. The use of materials includes the weight/volume of materials and the number of types of materials used. Regardless of material origins, lesser volume/weight and fewer types of materials used will lead to less negative environmental impacts. However, it is difficult to judge the absolute quantity (weight/volume) of materials needed for a concept; therefore, this attribute refers to the potential for material quantity reduction at the time when the concept is evaluated. For instance, if the volume/weight of materials of a product concept could be easily reduced without affecting the structure and performance of the product, a high (2) score will be given.
Table 2 summarises the attributes to consider for material sustainability and provides a brief explanation of each level to inform rational decision making. An equation is then developed to quantify the
material sustainability, as shown in Equation (1).
In Equation (1), i refers to the ith type of material used in a concept. A multiplier is used to correlate attributes that have aggregated effect. For example, material original (M1) and use of material—quantity (M3) have a clear aggregated effect; hence, they are multiplied together. These aggregated effects are added together and then divided by the number of material types N to indicate the overall material sustainability, which will vary between 0 and 8. In order to yield an accessible result, a scaling process is performed to ensure that the final score is within the range of 1 to 10, in which 1 means poor and 10 means excellent with regard to sustainability.
3.2. Production (Manufacturing and Assembly)
Producing products in a sustainable manner, such as conserving resources, consuming less energy, and generating less pollution and waste, leads to minimum negative environmental impacts. However, production is a complex process where many design details are determined at the detailed design stage rather than the conceptual design stage. Therefore, only aspects related to manufacturing and assembly are discussed in this paper. Design for manufacturing and assembly (DFMA) is an effective approach to achieve sustainable production. This study extracted core DFMA considerations, for ease of assembly and manufacturing, to measure sustainable production aspects of design concepts, as shown in
Table 3. Minimising the number of parts in a practical manner, as well as using more standardised parts/components and fewer unique parts/components, could reduce inventory cost, process time, and so on. Designing parts for ease of assembly involves better presentation (such as avoiding too large or too small items and employing symmetric features), easy handling (such as avoiding oversize, sharp, slippery, heavy, and fragile items), mistake proofing (such as using symmetric or asymmetric features to prevent parts from being assembled in wrong orientations), and efficient insertion (such as employing self-aligning/locating features). Suitable fabrication methods refer to the identification of the most appropriate technology/process based on the material selected to minimise excessive operations, such as polishing and fine machining.
Measurement of Production
Production details, such as manufacturing methods, manufacturing parameters, and assembly procedures, can be difficult to determine at the conceptual design stage. However, designers are encouraged to consider these attributes with respect to sustainability to steer towards a more sustainable outcome. A similar approach explained in 3.1.1 is adopted here, for example, a
low rating, a score of
0, will be given if a concept requires a considerable number of customised parts/components, implying that more negative impacts are created during production. An explanation of levels for each production attribute is provided in
Table 4. It is worth noticing that these production attributes are concept dependent, and this therefore requires subjective judgement, for instance, whether adequate standardisation has been achieved. In addition, unlike the types of materials, it is also challenging to evaluate each individual part/component with respect to standardisation, fabrication, and assembly; therefore, here they are considered holistically at the concept level. An equation is then developed to quantify the sustainability with respect to production, as shown in Equation (2).
Similar to material, the aggregated effect for production attributes is considered here. This is again reflected on the multiplier. For example, balance between number of parts and their complexity (P1) is closely related to part standardisation (P2), while suitable fabrication method (P4) will amplify their effect, hence, P1 × P2 × P4. The equation leads to an overall score for production sustainability, which will vary between 0 and 12. A similar scaling process is performed to ensure that the final rating will yield a value between 1 (poor) and 10 (excellent).
3.3. Use
The use of a product pertains mainly to the amount of time the product is owned and operated by its user. A product’s lifetime starts from when it is acquired to when the product is discarded, which is primarily determined at the conceptual design stage. Functional obsolescence, maintenance prevention, and aesthetic obsolescence are the main reasons that lead to the end of life of a product. In early studies, product lifetime extension was employed to reduce resource consumptions and waste productions by means such as ease of repair and upgrade. However, a longer life span of a product does not necessarily indicate that the product is more resource and waste efficient. For instance, longer lifetime products usually consume more resources in material and production. These extra resources are wasted if a product’s lifetime is longer than the time of the product being needed by the user. Therefore, product lifetime optimisation, where a balance between extending and shortening the lifetime and use time is achieved, should be used as an effective strategy to minimise the negative environmental impacts of products. For example, less durable materials should be used for short-life or temporary products and parts. Another strategy to decrease environmental impacts at the use stage is to reduce the product’s resource or energy consumption. For example, LED lights consume much less electrical energy in comparison with incandescent lights, but produce the same illumination. Therefore, LED lights should be used rather than incandescent lights while designing products with illumination features.
Measurement of Use
As described previously, the balance between product use time and lifetime needs to be considered during the conceptual design stage. An ideal scenario would be when the product use time is identical to its lifetime, implying that the product enters its end-of-life stage immediately after the use stage. Therefore, the ratio between product lifetime and use time is an attribute to consider, as shown in
Table 5. Despite various product categories being evaluated, the product use time and lifetime balance can be determined in a unified way, meaning that the difference between them should always be minimised. For example, the perfect balance for a disposable coffee cup is that it can be recycled right after people finish their drinks. For a mobile phone, the ideal case would be that it can be recycled right after it breaks or when people get a new one rather than sitting in a drawer. It is possible to use objective values to determine the thresholds (
low,
medium, and
High) of
product use time/lifetime (U1), but they would be largely dependent on the products themselves. Therefore, subjective descriptors, such as ‘significantly shorter/longer’, are employed.
Energy consumption during use directly indicates the energy efficiency; hence, it is used as the second attribute. Different products can vary significantly; hence, it would be difficult to judge without considering the product category. As a result, it would be beneficial to develop a lookup table by collecting data of day-to-day products and come up with a range of specific values for energy consumption for different product categories. By this, the designer could make judgements by referring to the table more easily. However, it is time-consuming to construct such a lookup table, and therefore, subjective descriptors are used for
energy consumption during use (U2) in this study. Robustness, reliability, and maintenance are treated as the third attribute to indicate sustainability. For example, if a design is more robust, reliable, and easy to maintain, it is then unlikely to cause a significant negative environmental impact due to malfunctioning and servicing. Equation (3) is then developed to quantify the
use sustainability. Again, the aggregated effect of attributes is considered here and denoted by multipliers. For example, the effect of
energy consumption during use (U2) will be amplified by the
product use time/lifetime (U1), hence
U1 × U2 in the equation. The same scaling process is applied to ensure that the final sustainability score falls between
1 and
10.
3.4. End of Life
End of life refers to a product that is at the end of its life cycle, where the product needs to be discarded. End-of-life approaches, such as recycling, reuse, repair, and remanufacturing, are considered more sustainable than conventional disposal methods involving incineration and landfill. Employing biodegradable materials and using waste-to-energy technologies are often used to decrease the negative impacts caused by product disposals, such as landfill and incineration. However, product disposal still leads to issues, such as pollutions and contaminations, and therefore is considered unsustainable. Recycling is a process of converting a disposed product into new materials or objects; reuse involves the action of using the product or parts of the product, without changing the structures, for original and new purposes; repair refers to the replacement of nonfunctional or damaged parts of the product; and remanufacturing means returning the product to a ‘like-new’ condition. Product disassembly is often needed and considered a significant process in product end of life, even for landfill and incineration. Ease-of-disassembly tactics, such as employing detachable joints, using standardised fasteners, minimising the number of fasteners, and avoiding glues, should be considered at the conceptual design stage to contribute to sustainable product end of life. In addition to ease of disassembly, strategies such as using compatible materials, employing modular parts, ease of identification and inspection, and ease of sorting could also support product end-of-life processing for better environmental performance.
Measurement of End of Life
Compared with recycling, remanufacturing, and repair, reuse requires the least resource and, therefore, is listed as an individual attribute. Recycling, remanufacturing, and repair all require further handling and processing, which consumes more energy and materials; hence, they are categorised together. Some parts of a product are inevitably not reusable, recyclable, remanufacturable, or repairable and need to be disposed of. As a result, the environmental impact caused by disposal needs to be considered. A product at its end of life often requires disassembling to obtain the parts to be reused, recycled, remanufactured, repaired, or even disposed of. Therefore, the degree to whether the concept is easy to disassemble at its end of life is another important attribute.
Table 6 presents a summary of the explanations for the attributes discussed. Similar to other metrics, the potential aggregated effect is represented by multipliers of attributes; for example, in order to
reuse (E1),
recycle,
remanufacture,
and repair (E2) and
dispose (E3) the components of a product, the
ease of disassembly (E4) of the product is critical. Equation (4) with a scaling operation was developed for the
end-of-life sustainability.
3.5. The Four Metrics
Material,
production,
use, and
end of life are the four metrics, involving 15 attributes, proposed for measuring sustainable product design concepts. A summary of the metrics is depicted in
Figure 2. The four metrics proposed could be used individually to measure specific aspects of a product design concept’s sustainability, and integrated to provide insights into the concept’s overall sustainability. Equations (1)–(4) are developed to indicate the degree of sustainability, from
poor (1) to
excellent (10). A demonstration of utilising the four metrics in a systematic manner for measuring sustainable product design concepts is presented in
Section 4.
5. Discussion
Four metrics, material, production, use, and end of life, are proposed in this study for measuring sustainable product design concepts. The corresponding attributes with associated measurement equations could be used to identify the sustainability level of a concept, with regard to the four metrics, in a quantitative manner. The attribute scores, low (0) to high (2), applied are in the simplest form possible to provide the most straightforward indication of attribute sustainability. The equations developed aim to indicate the sustainability of each metric by using multipliers to link attributes that have aggregated effects, for instance, material origin and use of material—quantity, and using summation to indicate the cumulative effects of the attributes. The equations developed and used are not necessarily the final forms, and modifications can be envisaged. For example, different weights can be assigned to attributes within a metric, referring to different product design applications, to better indicate concept-specific sustainability. The major advantage of using equations is making quantitative comparisons between different concepts. The designer is able to obtain instant results of concept sustainability based on the scores. More significantly, scores for different metrics allow an indication of sustainability improvement directions. For instance, if a product design concept received a poor sustainability evaluation rating for a metric, the designer could explore which attribute(s) of the metric has low ratings and then modify the design concept accordingly for sustainability improvements.
This study has three implications. First, the four metrics identified for measuring product design concepts could effectively improve a product’s sustainability level at a lower cost in comparison with addressing sustainability issues at later stages. Solving sustainability issues once a product is designed or at late design stages is challenging and expensive [
21,
22], whereas decisions made at the conceptual design stage have high impacts on minimising the negative environmental impacts on downstream activities, such as material, production, use, and end of life [
13,
14,
15,
16]. The second implication is that the study has raised the significance of addressing sustainability issues at the conceptual design stage. Many design features related to product sustainability or environmental impacts are not often considered by design engineers, as they generally focus on cost, performance, and durability [
12,
25,
26]. This could result in products’ lack of sustainability considerations, while the introduction of the four metrics for assessing the sustainability of product concepts has the potential to foster designers in considering sustainability design features during conceptual design. The third implication is the need for more and better sustainability concept assessment tools. The review conducted in this study reveals that the majority of existing concept assessment methods or tools are aimed at assessing the feasibility or creativity aspects of product design concepts. Although a few methods exist for evaluating the sustainability of product design concepts, these methods are limited in use [
63,
64,
65]. The metrics for measuring sustainable product design concepts proposed in this study have an extensive application scope in practice, which could also be utilised as a theoretical foundation for developing advanced sustainability concept evaluation tools.
However, the metrics proposed are aimed at recommending design changes at the conceptual design stage, and it might be challenging to suggest final determinations. It therefore requires the designer or evaluator who uses the metrics for evaluation to possess sufficient knowledge and experience in sustainable design and decision-making skills to yield final design decisions. Further explorations, such as conducting more practical case studies, are needed to examine how well the four metrics represent sustainability to increase the metrics’ suitability for conceptual design and to improve the measurement equations.
6. Conclusions
Sustainability plays an increasingly significant role in modern product design and development, while it is indicated that most of the negative environmental impacts are determined at early design stages, such as conceptual design. However, the review of prior literature showed that most of the existing concept evaluation methods are geared to measure the feasibility or creativity of concepts generated rather than the sustainability. The lack of measurements of sustainability at the conceptual design stage often leads to nonsustainable products, which result in negative environmental impacts. Therefore, this paper explored the key sustainable design elements and propose a set of metrics for measuring sustainable product design concepts. The four metrics identified,
material,
production,
use, and
end of life, associated with corresponding attributes and measurement equations, can support designers in producing sustainable design concepts, ultimately leading to sustainable products with minimal negative environmental impacts. Although this paper aims at assessing the environmental aspects of sustainable product design concepts, products produced also impact both the social and economic dimensions of sustainability through manufacturing, use, and end of life, which contribute to both employability and value creation [
66].
The paper is the first study to explore metrics for evaluating product design sustainability at the conceptual design stage. It delivers three significant contributions to engineering design, sustainability, and energy research communities. First, it serves as a guideline to measure the level of sustainability of design concepts for supporting sustainable product design in a quantitative manner. Second, it urges design practitioners and researchers to look into the importance of considering sustainable design aspects at early design stages. Finally, the study offers new research insights into exploring sustainable concept evaluation and can be used as an infrastructure to develop future concept evaluation tools.