Global_Environmental_Research_Vol.25No1&2
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■■∙■𝛤■1■1𝛽■■■■ 𝐹■■■𝑦■■1■exp■■■𝑦■∑𝑋■■■■■𝑦■where 𝐹■■■𝑦■ is the accumulated obsoletion rate of age 𝑦 where the product reaches its end of life under ownership generation 𝛼, 𝑦 is the duration of use from the from another country, 𝑋■ is the duration of ownership generation 𝛼 in other countries, i.e., 𝑋■ is used when another country to Cambodia) and 𝑋■ refers to the another country for a second ownership in Cambodia), 𝑦■ is the average lifespan of two-wheelers in Cambodia, 𝛽 percentage of obsolete two-wheelers. 𝛤 is a gamma function. 𝑅■𝑦■ is the survival rate of two-wheelers in Cambodia in year 𝑦. 2.3 Predicting the Number of Obsolete Two-wheelers The population balance model (PBM) designed by Tasaki was used in many similar studies to predict the number of two-wheelers (Tasaki et al., 2004). This model has been widely used to calculate the number of obsolete products, and it is used to explain scenarios for ELVs in many countries (Yano et al., 2016; Tsasaki et al., 2001; Lin et al., 2018; Yano et al., 2014; Xu et al., 2019). This study also applied the 2.2.2 Lifespans of Two-wheelers in Cambodia Table 1 Average duration of each owner’s two-wheeler use in Cambodia, Vietnam, Thailand, India, China Japan It is necessary to consider The lifespan of two-wheelers domestically produced in Cambodia follows the definition of domestic service lifespan. This study applied the domestic service lifespan of two-wheelers in Vietnam, which had been reported to be 20.3 years, to the case of Cambodia (Kurogi et al., 2021). Cambodia and Vietnam are both Southeast Asian countries and they share a border. Although they differ politically, they have experienced a similar process of economic development with stable price movement (Asian Development Bank, 2011). It was therefore considered reasonable to apply the 20.3 years that had been determined as the domestic lifespan of two-wheelers in Vietnam to the case of Cambodia. To determine the lifespan of two-wheelers imported to Cambodia, their extended service lifespan needed to be considered. In this study, we focused on import data from five countries: Vietnam, Thailand, India, China and Japan. These five countries account for the majority of two-wheelers imported to Cambodia, representing more than 90%. Another assumption made in this study was that the duration of use by each owner in Cambodia would be identical to those in Vietnam, China, Thailand and India. The potential applicability of the case in Cambodia to Vietnam was mentioned in Section 2.2.1. It is, however, reasonable that the lifespan of a motorcycle would be different in each country. As such, an assumption was established for each country based on the lifespan reported in the existing literature for that specific country. The study by Kurogi et al. (2021) appears to be the only study which includes a detailed discussion on the lifespan of motorcycles for each generation of owners in Vietnam and Japan. Since the lifespan of a commodity varies depending on GDP per capita (Kosai et al., 2020), this study assumed that Vietnam, China, Thailand and India fell under the same category as developing countries, while Japan is considered to belong to a different category. In practice, as seen in the similar ownership rate of two-wheelers in Thailand and Vietnam (Bach Duong, 2016), the trend of two-wheeler use in developing countries appears to be similar. Meanwhile, the ownership of two-wheelers in Japan differed significantly from that in developing countries (Kurogi et al., 2021). As such, this study employed the domestic service lifespan (or duration Vietnam and Japan based on Kurogi et al. (2021). Country First owner [years] 12.1 4.8 Second owner Third owner [years] [years] 4.28 2.9 Estimating Material Flow of Two-wheelers in Cambodia lifespans of two-wheelers domestically produced and imported from other countries separately. the 2.14 of each owner) of two-wheelers in Vietnam identified in the previous study (Kurogi et al., 2021) to the case of Cambodia, China, Thailand and India. The average durations of ownership by each owner in Cambodia, Vietnam, India, China, Thailand and Japan are presented in Table 1. Despite the uncertainty under the qualitative assumption, this study at least differentiates among categories of associated countries to some extent. 2.2.3 Estimation of Lifespan Distribution To estimate the lifespan distribution, this study employed the Weibull distribution function, which is considered to be the most suitable simulation model for product lifespan (Wang et al., 2013). In particular, this study also estimated the number of obsolete two-wheelers considering second and third owners in Cambodia. The Weibull distribution function for two-wheelers in Cambodia is described in the next chapter. This model differs slightly from existing models, in that our study uses a modified version of the Weibull distribution function based on the extended service lifespan of two-wheelers in Cambodia. The Weibull point when the two-wheeler is imported to Cambodia there is no use of the two wheeler in another country (corresponding to a new two-wheeler imported from duration of the first ownership in another country (corresponding to a used two-wheeler imported from is a shape parameter estimated by confirming how well the accumulated distribution curve matches distribution the the PBM, as for function the two-wheelers domestically produced and imported from other countries is indicated by the following equation: (3)(4)it describes 87 ■1■𝑅■𝑦■

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