With the increasing penetration of variable renewable energy, integrated energy systems (IES) have become increasingly complex. Unfortunately, the widely used constant efficiency model is too simplified to reflect actual operating conditions. Therefore, this study investigated the effects of off-design characteristics on the operation and configuration of integrated energy systems, both singly and collectively. To this aim, a two-stage optimization framework was applied to optimize the operation and configuration of IES. The off-design characteristics include the electric-thermal correlation of gas turbines (GTs), the load rate-efficiency correlation of gas boilers (GBs) and absorption chillers (ACs), and the temperature-efficiency correlation of heat pumps (HPs) and electric chillers (ECs).

When off-design characteristics are considered, the optimal capacities of the energy conversion equipment tend to increase and system costs would increase. For all system equipment, the capacity of the EC increases by 515.4% and the system cost increases by 7.3%. Regarding system operation, the effects of off-design characteristics on the operation of the GT and AC are most significant for all energy conversion equipment. The electric energy storage (EES) and the thermal energy storage (TES) help to reduce the capacity of the GT and system cost. When off-design characteristics are considered, their effects are magnified.

China's anthropogenic methane emissions are the largest of any country in the world. A recent study using atmospheric observations suggested that recent policies aimed at reducing emissions of methane due to coal production in China after 2010 had been largely ineffective. Here, based on a longer observational record and an updated modelling approach, we find a statistically significant positive linear trend (0.36 ± 0.04 () Tg CH4 yr−2) in China's methane emissions for 2010–2017. This trend was slowing down at a statistically significant rate of -0.1 ± 0.04 Tg CH4 yr−3. We find that this decrease in growth rate can in part be attributed to a decline in China's coal production. However, coal mine methane emissions have not declined as rapidly as production, implying that there may be substantial fugitive emissions from abandoned coal mines that have previously been overlooked. We also find that emissions over rice-growing and aquaculture-farming regions show a positive trend (0.13 ± 0.05 Tg CH4 yr−2 for 2010–2017) despite reports of shrinking rice paddy areas, implying potentially significant emissions from new aquaculture activities, which are thought to be primarily located on converted rice paddies.

Offshore wind power, with accelerated declining levelized costs, is emerging as a critical building-block to fully decarbonize the world’s largest CO2 emitter, China. However, system integration barriers as well as system balancing costs have not been quantified yet. Here we develop a bottom-up model to test the grid accommodation capabilities and design the optimal investment plans for offshore wind power considering resource distributions, hourly power system simulations, and transmission/storage/hydrogen investments. Results indicate that grid integration barriers exist currently at the provincial level. For 2030, optimized offshore wind investment levels should be doubled compared with current government plans, and provincial allocations should be significantly improved considering both resource quality and grid conditions. For 2050, offshore wind capacity in China could reach as high as 1500 GW, prompting a paradigm shift in national transmission structure, favoring long-term storage in the energy portfolio, enabling green hydrogen production in coastal demand centers, resulting in the world’s largest wind power market.