Most of the current energy efficiency models focus on the primary-secondary-useful energy conversion chain of energy system, however, ignore the chain of useful energy to final service. In fact, what people need is not energy itself, but the energy service it provides. Therefore, this paper extends the analysis boundary of energy efficiency model to final service, to reveal the evolution and the driving factors of energy service efficiency. Firstly, we divide the energy system into six stages to conduct societal exergy analysis, which are energy source, transformation, end-use conversion device, useful energy, passive system and final service. And the whole scenario of energy flow and conversion is mapped in Sankey diagrams. Then, we use LMDI (Logarithmic mean Divisia Index) decomposition method to comprehensively understand factors driving the change of energy service efficiency. Efficiency and structure factors of each stage are incorporated into a novel LMDI decomposition identity to quantify their relative contributions. A case study of China during 2005-2015 reveals that: a) the energy service efficiency in China, from energy source to final service, has increased from 3.7% in 2005, to 4.1% in 2010, and 4.8% in 2015. It shows an increasing trend, but still at a very low level with huge losses. b) The efficiency improvement of each stage, especially that of end-use conversion device and the power and heat generation sector, makes the greatest contribution to the increase of overall energy service efficiency. c) There are large passive losses in passive systems, especially in the passive system of building. The energy efficiency improvement of passive systems has big potential and deserves more attention in the future.
Keywords energy efficiency, energy service, driving factors, societal exergy analysis, LMDI, Sankey diagram