Towards a bio-energy atlas for Africa – Energy potentials of crop residues, a few case studies

Kurzfassung

Access to and steady supply of energy are an important requirements for socio-economic development. During the last decades global energy resources, mainly fossil fuels, were massively exploited to meet the growing energy demand triggered by the growing world population and the increase of economic welfare. When discussing future perspectives of energy availability usually two concerns come in focus. One is carbon output and its influence on climate change. The other one is the question whether existing resources will meet the projected increase of energy demand, especially in non-OECD countries. When dealing with bioenergy, a third concern needs to be discussed: the dilemma of “food versus fuel”. However, although widely discussed the final answer to this question is still open. This contribution analysis the energy-food nexus with a particular focus on energy availability and sustainable resource exploitation. Recent studies report a range of 27-77 EJ yr-1 of global sustainable bioenergy potentials from residues. When looking at regional scales a realistic assessment of available biomass potentials becomes even more relevant. This is especially true for countries with low average incomes, since aside from access to energy, nutrition security can be limited. To estimate sustainable energy potentials of straw a two-step approach was developed. In a first step the remote sensing data driven Biosphere Energy Hydrology (BETHY/DLR) model was applied to calculate Net Primary Productivity (NPP) for agricultural areas. In a second step this data was transferred to biomass increase of straw and subsequently to its usable energy content. The BETHY/DLR model is a Soil-Vegetation-Atmosphere-Transfer (SVAT) model. It can be used to track the transformation of atmospheric carbon dioxide into energy storing sugars, a process known as photosynthesis. BETHY/DLR has recently been used to assess NPP for parts of Europe, Asia and Africa and has been validated and cross-compared with other process based models for agriculture and eddy covariance data. The scheme of modelling photosynthesis with models like BETHY/DLR is widely accepted and serves as input to global dynamic vegetation models, such as the Jena Scheme of Atmosphere Biosphere Coupling in Hamburg (JSBACH). The model output is transferred to straw content by using conversion factors on yield to straw and above to below ground biomass. In order to avoid the dilemma of “food versus fuel” and to secure nutrition, we assume only the straw content of a crop to be usable for energy generation, but not the grain itself. To respect use competitions (e.g. animal housing, soil fertilization) we assume the share of only 30% to be available for combustion or other energetic use. Lower heating values, giving specific energy potentials per kilogram biomass, are used to transfer the straw share of NPP to sustainable bioenergy potentials. In order to achieve comparable results the approach as described above was applied for the countries Egypt, Kenia and Uganda for the year 2012 using global available – and free of charge – data to drive the BETHY/DLR model. We used meteorological data (2m temperature, wind speed, precipitation and cloud cover) from the European Centre for Medium Range Weather Forecast (ECMF), which are available six times a day. Time series of the Leaf Area Index (LAI) are provided by geoland2 as 10-day composites; monthly averaged CO2 concentrations are derived from the greenhouse gases observing satellite (GOSAT); an elevation model (SRTM), a soil type map (Harmonized World Soil Database) and a land cover / land use classification (Global Land Cover 2000) are used as input. We found sustainable energy potentials of 182 PJ for Kenia; 211 PJ for Uganda and 149 PJ for Egypt. For Egypt this result corresponds with ~4% of the gross primary energy demand. For Uganda and Kenia, however this corresponds with up to 80% of the annual gross primary energy demand. This finding indicates that biomass can play a prominent role when debating how Africa can provide sustainable sources to solve its increasing energy demand. This is especially valid for countries which have access to areas with high vegetation activity. A consistent survey like promoted in the Bioenergy Atlas for Africa (BAfA) initiative might thus be a first step to better exploit existing potentials and deliver information to governmental stakeholders in order to improve policy regarding energy use.