Environmental consequences of electricity transmission and distribution - a life cycle perspective

Abstract

The transmission and distribution of electricity is increasingly important in the context of future energy systems and large investments are expected worldwide for this sector in the coming years. While knowledge and research about environmental consequences of systems for power generation is abundant, data and available studies on environmental impacts for power transmission and distribution systems are somehow limited.

This thesis consists of the investigation of environmental consequences for systems used in power transmission and distribution in a life cycle perspective. Three goals were set for the thesis: one was to investigate the impacts for transmission and distribution systems from a component level. Here the aim was to study the main component types in an electricity grid, e.g., a power line, a transformer, etc. to a level of detail that allowed for the investigation of important processes and causes of environmental impacts in each case. The second goal was to understand the impacts from an electricity grid system. To accomplish this, a real grid system was modeled and life cycle impacts were obtained per kWh transmitted. The third goal was to address a question that has recently been brought up to the attention of energy systems researchers: what are the environmental impacts associated with the scaling up of systems based on renewable energy sources? Here, that question is addressed in light of the transmission grid extensions that are necessary for upgrading the European system in order to integrate renewable energy sources.

The studies presented here cover the main components of an electrical grid. These components are: overhead lines, underground cables, subsea cables and equipment used in substations, e.g., transformers and switchgear. Different equipment ratings are considered, although most of the analyses cover the high-voltage range, or power transmission level. The analyses include both AC systems, which represent most of the transmission assets installed worldwide, and DC systems, which are used for specific applications such as the transmission of power over large distances, subsea transmission or interconnection of two regions operating at different frequencies.

The method used is Life Cycle Assessment. The impact assessment method used in all the studies undertaken is ReCiPe Midpoint, hierarchist version, with European normalisation.

The main conclusions from the thesis can be summarized as follows: at a component level, electrical losses in equipment used for transmission and distribution are the largest contributor to life cycle impacts in virtually all impact categories. After power losses, impacts arise mainly from these processes: for overhead lines, conductors and masts dominate; cable production is an important process in land and sea cables. For transformers and substation equipment, the production of raw materials is also important. Manufacturing of components, e.g., shaping the materials to a final product is important for some studied impact categories, e.g., water depletion. Processes such as maintenance and installation have a smaller contribution to life cycle impacts. Important direct emissions for transmission and distribution (T&D) systems are mineral oil, zinc and SF6 that result from leakages in the equipment throughout the lifetime. For substation equipment using SF6, the impacts due to gas leakages can be the main process for climate change scores. Recycling of metal parts brings benefits for overall performance of the equipment studied. However, the impacts arising from other processes in the end-of-life may outweigh the benefits from avoided production.

At a system level, it was found that the transmission of electricity in the Norwegian grid causes an impact of 1.3 to 1.5 g CO2 eq./kWh delivered to the distribution network. A sensitivity analysis of the results to the electricity mix considered for power losses shows that, for "cleaner" mixes, the share of impacts from infrastructure related processes is higher. For a Norwegian mix of power production, impacts due to losses contribute as much to climate change scores as impacts from infrastructure related processes.

Finally, the investigation of impacts for transmission grid upgrading projects in Europe indicates that over the lifecycle, total metal depletion impacts are of 11.2 Mton Fe eq. The projects also correspond to a total score for climate change of 10.7 Mton CO2 eq. The grid upgrading corresponds to the construction of new grid equipment, such as new lines, subsea and underground cables, etc. and renovation of already installed equipment. For the construction of new equipment, inputs from several materials, e.g., sand, steel, limestone, aluminum, iron, copper, lead, transformer oil, zinc and gravel are significant. The new assets require approximately 2 Mton of iron and steel, 400,000 tons of aluminum and 150,000 tons of copper. A sensitivity analysis for recycling rates of metal parts indicates that the results are greatly affected by the assumptions made.