Impact of Industrial Symbiotic Agglomeration on Energy Efficiency: Analysis Using Time-Varying Difference-in-Difference Model

Industrial symbiotic agglomeration (ISA) is a new type of circular spatial agglomeration that is favorable towards energy efficiency (EE) improvement. Based on the notion of ISA, this research explores the influence of ISA on EE: (i) This research explores the mechanism of ISA’s influence on EE from the viewpoints of recycling and green technology innovation, and verifies it via a mechanism test, based on the features of ISA. (ii) To quantify the whole factor EE, including undesirable output, and analyze its characteristics, we use the super efficiency slack-based model based on the undesirable output and the Malmquist–Luenberger index. (iii) By using the multi-time point differences-in-differences method (time-varying DID) for benchmark regression, it is found that ISA significantly improves EE, and the results are still significant after the endogenous test and robustness test. And (iv) from the perspective of urban resource endowment and geographical location, it was discovered that the influence of ISA on EE is stronger in non-resource-based cities than in resource-based cities, and that the impact is greater in western and central China than in eastern China.