The selected ZCBs have a great diversity of boundary dimensions, which are illustrated in a set of five radar maps (Fig. 10). Several key observations emerge from the comparative analysis:
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On the “when” related boundaries, the Pixel carbon neutral building claims net savings in its annual operational emissions that counterbalance its annual embodied emissions over 50 years. The other cases focus on the operation of the building only, which clearly indicates the principle of net ZCB over a period of one year.
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On the “who” related boundaries, the cases indicate varied driving forces, such as the government, developers and institutions. Despite the specific driving forces, multiple stakeholders are clearly engaged in the development of the ZCBs.
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On the “where” related boundaries, the ZCB cases are largely three story buildings, with a maximum height of 10 stories, suggesting a serious paucity of knowledge and practices for achieving zero carbon in high-rise buildings. The dimensions of the ZCB cases against the geographic, density, climatic and sector boundaries show great diversity. This diversity is partly attributable to how the cases were selected for the study, but also illustrates the increasing awareness and practice of ZCB globally.
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On the “how” related boundaries, despite the complexity of the institutional (PESTEL) ZCB contexts, all of the cases of ZCBs are politically supported and promoted with public and social engagement and use on-site renewable energy systems, typically photovoltaic panels, and energy efficiency measures