1) Introduction

As climate concerns intensify, circular economy strategies have become essential for reducing the environmental footprint of smartphones [1]. In the ICT sector, buy-back programs are increasingly used to promote refurbishment and recycling, yet they may generate two opposing effects: Higher buy-back values can encourage more users to return their devices but can also shorten the phones lifespans. This work develops a steady-state flow model of the smartphone circular economy, integrating both refurbishment and recycling, in which the buy-back price offered to consumers is the main control variable. In this model, numerical computations allow to identify an optimal buy-back policy minimizing CO2eq. emissions and waste in a reference scenario with realistic parameters. Our approach differs from the existing literature in its ease of interpretation, as well as its particular focus on buy-back policies.

2) Description of the model

The proposed model mathematically represents a simplified smartphone market, where a unique decision maker controls the buy-back value S1 of premium smartphones. Fig. 1 illustrates and summarises the model. The diagram is structured around three main categories of devices: premium phones (new, expensive, refurbishable, and with higher environmental impact), basic phones (new, low-cost, non-refurbishable, and with lower environmental impact), and refurbished premium phones. The arrows show the different material flows between extraction, production, refurbishment and recycling. In the model, these flows are defined as time averages induced by the total quantities of materials circulating over characteristic consumption periods and are used to define the CO2eq. total emission flow that the decision maker is willing to minimize. Consumers are characterised by the type of smartphone they use and whether or not they are sensitive to buy-back, i.e., if they take or not their phone back to the shop for the amount of money offered before buying a new one. Based on [2], we assume that sensitive consumers keep their phones for a shorter time than non-sensitive consumers. Phones from non-sensitive consumers are not collected directly and a fraction of these phones is recovered for recycling through collection points or at the initiative of consumers. The non-recovered devices are considered as waste.

3 Overview of the numerical analysis

We define a reference scenario with realistic values and identify in this case an optimal buy-back value minimizing emissions (see Fig. 2) and waste while maximizing circularity and product longevity. In this scenario, the optimal buy-back reduces emissions by 20% compared to a zero buy-back. This value equalizes supply and demand on the refurbished market, meaning that the optimal buy-back enables to satisfy all consumers willing to buy refurbished phones. We conduct a sensitivity analysis revealing that equalizing supply and demand is actually optimal for many parameter configurations, except for some extreme or unrealistic cases. However, the analysis also highlights several key factors influencing the environmental benefits of circularity, such as the relative environmental impact of the various phones (including refurbished ones). Another key aspect seems to be the low number of spare parts used for refurbishment. This finding emphasizes the importance of eco-design principles that facilitate repair and reuse. Overall, in the smartphone circular economy, our findings confirm the interest of relevant buy-back policies as a lever (balancing the return of smartphones and the rebound effect on consumption) to reduce environmental impacts. However, inappropriate buy-back values or poorly designed circularity strategies can reverse the expected benefits and lead to higher overall CO2eq. emissions levels.