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Waste and residues are feedstocks that do not directly compete with food production for land, water or inputs, or with biodiversity and carbon sinks for land use, and can provide significant GHG savings, as long as they are collected and used in a sustainable manner. Due to costly collection/conversion, waste/residues tend to be more suited for conversion into heat, electricity or biogas.

Waste and residues often already have a local market (e.g. non-formal markets), and by redirecting flows, indirect effects may be caused. Further research is needed to determine the proper balance of residues that should remain in the field or in the forest to maintain soil fertility and soil carbon content, and improve soil conservation. In that sense, it is important to note that while utilisation of waste and residues can feature as a win-win to address waste disposal and an energy generation, not all waste or residues are easily accessible or economically viable for energy utilization.

Dedicated energy crops are crops grown for energy purposes. Energy crops can be divided into the four groups of sugar crops, starch crops, oil crops and lignocellulosic crops. The theoretical potential for energy crops in many regions is large, but can be limited by land and water availability as well as competition for other uses (food, feed, fibre). The potential environmental and social impacts from the production of dedicated energy crops differ depending on the choice of crop and management system. These impacts (further elaborated in the full version of the DST), as well as a determination of the resource base (see Box 4), should be taken into consid-eration before promoting a specific crop.

Biomass harvested from natural resources is another form of bioenergy feedstock. These include forest, wood-land, grassland or aquatic resources. Some areas might have the potential to harvest naturally growing biomass for local needs; however, the potential is often low and generally not able to supply large-scale bioenergy systems.

Conversion technologies

The choice of conversion technology should be based on the objectives of the programme/project and the resources

available - biophysical, economic, infrastructure, and human resources. A large number of possible feedstocks, conversion technologies and end uses for bioenergy already exist; and, at the same time, innovation processes are creating further options for advanced and more efficient conversion technologies.

Different conversion routes vary in terms of their complexity, as they are adapted to work on different physical and chemical compositions of feedstocks. Figure 3 provides an overview of possible conversion routes in a bioenergy system.

More advanced technologies tend to have higher conver-sion efficiencies, but tend to have higher capital cost as well. Furthermore, an efficient technology does not work effectively if there is no human capacity to absorb it, run it, and maintain it (See Project Level Consideration box). Trade-offs may have to be made in order to find the best technical solution that is viable in a specific country context. For instance, optimizing a system for a particular end use may call for a change in feedstock (and subse-quent human resources) in order to use the requested technology in the most efficient and cost effective way. The technological absorption capacity in a country should be carefully evaluated in relation to human capital, manufacturing and process inputs (FAO, Bioenergy and Food Security Project, Module II).

Box 4: Brief delineation of variables in estimating a resource base

Estimate areas of potentially available and suitable land. Estimate yields associated with areas of suitable land. Calculate the gross potential resource (by land type and/ or subregion).

Estimate production costs and delivered energy prices. Estimate delivered energy prices (and ranges). Final integration and assessment of energy crop potential.

Source: ESMAP - Bioenergy guidelines

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