Connolly, David

Grandal, Rune Duban

Mathiesen, Brian Vad

Wenzel, Henrik

Designing a 100 % renewable energy system (RES) for Denmark, the availability of a sustainable biomass resource potential is found to be a limiting factor. The biomass demand derives from specific needs in the system, i.e. 1) storable fuel for energy for balancing fluctuating power production, 2) carbon feedstock for materials and chemicals and 3) energy dense fuels for the more demanding branches of the transportation sector such as aviation, ship freight and long distance road transportation.The challenge of balancing electricity over different timeslots comprise a short term balancing of supply and demand in every second, but also a long term balancing between days and even seasons. Two alternative technologies are available on a significant scale, which are able to deliver balancing services for Denmark. The first is storage in Norwegian hydropower, while the second is electrochemical storage, i.e. storing wind power through electrolysis and further reaction of hydrogen to hydrocarbons with carbon feedstock from biomass. This involves biomass gasification and hydrogenation of the syngas or hydrogenation of recycled CO2. The advantage of hydro storage is a superior energy efficiency and storage capacity, while the advantage of electrochemical storage is flexibility and, most importantly, the option of producing hydrogenated fuels for use in the transport sector, thereby displacing biomass with wind power in even the most challenging branches of the transport sector.Therefore, the needed scale of wind power production will largely be determined by the availability of residual biomass. Two different situations are of interest. The first is characterized by having a (hypothetical) biomass potential of 320 PJ/year. In this situation, the ratio between deficit and surplus electricity production is 1:1. If so, the main task is direct balancing between the surplus and the deficit, making hydro storage a potential alternative. However, the biomass demand in this situation is much above the sustainable biomass potential. To reduce the biomass dependency from this level down to a level of 200 PJ/year, the production of wind power has to be increased to a level of surplus electricity of almost 150 PJ/year in order to ensure sufficient quantities of hydrogen for the hydrocarbon demand. At this high electricity surplus, the ratio between deficit and surplus becomes 1:20. In such a situation the use of hydro storage is limited. The electricity surplus is peak-shaved by electrolytic conversion, and the electricity deficit and, thus, the need for regulating power, becomes very limited

One million ton of hydrogen is the key piece in the Danish renewable energy puzzle

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