When to replace products with which (circular) strategy? An optimization approach and lifespan indicator
2021
Hummen, Torsten | Desing, Harald
A longer product use requires less resources, causing less environmental impacts. Even though this assumption may appear intuitive, it is not necessarily the case for all products. Consider a product that causes significant impacts during its service life and loses performance through wear and degradation. Its extended use may cause higher impacts overall than replacing it before it reaches its end-of-life with an evolved alternative. In this paper, we propose a method for finding the replacement time when impacts become minimal, i.e. the optimal environmental lifetime (OEL), taking into account non-linear dynamics of technological efficiency improvements and efficiency degradation during usage. Different replacement options, including lifetime extension strategies such as re-manufacturing, are considered. Based on this, we define an indicator to measure the environmental performance of an achieved lifetime in comparison to the optimum. This lifespan indicator ϕ measures the relative achievement of the maximum possible impact savings, when replacing the initial product at OEL. It accounts for unsustainable throughput of materials and waste of resources when deviating from it. To illustrate the application of the method and lifespan indicator, the OEL of residential heating systems are calculated. It reveals that gas boilers shall be replaced in short intervals with evolved gas boilers or – more effectively – with a heat pump. This case study nullifies the common hypothesis that “durability is per se environmentally beneficial”, requiring to evaluate OEL on a case by case basis.
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