Abstract
The concept of net-zero-CO2 power systems has gained increased attention by the EU goal to be a climate neutral continent by 2050. As potential pathways towards a net-zero-power system, this work analyzes future power systems based on intermittent renewable electricity with long-term storage through chemical energy carriers, so called Power-to-Fuel-to-Power systems, and a system based on the combustion of natural gas with 100% carbon capture and storage. The chemical energy carriers selected for electricity storage are hydrogen, methane and ammonia. Using life cycle assessment, we determine and compare the environmental impacts of 1 kWh of dispatchable electricity produced by the two pathways on seven impact categories. There was not one single pathway that had the most environmental benefits on all seven impact categories. Of the Power-to-Fuel-to-Power systems assessed the use of hydrogen for storage has the lowest environmental impact in all categories. Additionally, all the Power-to-Fuel-to-Power systems have a lower environmental impact on climate change, photochemical ozone formation and fossil resource depletion compared with the natural gas with carbon capture and storage system. The natural gas with carbon capture and storage system has a lower environmental impact on particulate matter formation, marine eutrophication and mineral resource scarcity. Our work is complemented by an analysis of pathways from a net-zero-direct-CO2 to a life-cycle net-zero-CO2-equivalent power system which is actually climate neutral, achieved by direct air capture of the residual CO2 from the atmosphere. However, this leads to an increase in all other impact categories of 11% for the Power-to-Fuel-to-Power systems and 21% in the natural gas combustion with carbon capture and storage system. A system sizing study also highlights the very low capacity factors of the capital employed for electricity storage, raising the point of economic feasibility.
Highlights
The European Green Deal states that the European Commission aims to be the first climate neutral continent by 2050 (European Commission, 2019)
Comparing the two Natural Gas Combined Cycle (NGCC) reference systems, the figure shows that fully abating the direct CO2 emissions of the NGCC system (“Carbon Capture and Storage (CCS)” in Figure 8) reduces its life cycle climate change (CC) impact by around 70% compared to the unabated NGCC system, but it leads to an increased impact on all other impact categories
The noticeable increase (∼25%) in fossil resource scarcity is largely due to the reduced efficiency of the NGCC and the increased use of natural gas, which was observed by the aforementioned authors
Summary
The European Green Deal states that the European Commission aims to be the first climate neutral continent by 2050 (European Commission, 2019). Where there are many technologies proposed for short-term electricity storage, longer term (e.g., seasonal) storage of iRES is currently limited to storage in chemical energy carriers (e.g., through power-to-gas systems) and to a lesser extend pumped hydro. Ammonia can be stored as a liquid at only 10 bars of pressure, which may be advantageous from the perspective of storage and energy required for pressurization These chemical energy carriers can subsequently be used for the production of electricity, as described in Sutter et al (2019), whence they are colloquially called Power-to-X-to-Power (P-X-P) systems. This raises the question of what net-zero really means, which relates to the question of stewardship. If a country or region wants to reach net-zero at a given moment in time, should it compensate for indirect emissions, and should it compensate for emissions outside its own region, but attributable to its power supply system? Given the nature of this contribution, we will not speculate on the political or legal aspects of such questions, but we can show how the full value chain could be made climate neutral over its life cycle, and what this implies for other environmental impacts
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