Тексты по специальности ТЭС / Dispatchable energy sources

Dispatchable energy sources - hydropower and biomass

Hydropower provided about 2200 TWh, or 18 per cent, of the world's electrical energy in 1990. It is therefore the largest renewable source of electricity. It has been estimated that only about 15 per cent of the world's technically exploitable hydropower potential is now used. The largest remaining resources are found in Asia, South America and Africa. However, the applications are limited by scarcity of capital, environmental effects and often by the cost of long-distance transmission.

Increased attention should be given to the social and environmental impacts associated with the

exploitation of additional hydro resources. The cost of producing hydroelectricity is very sitespecific, but nearly all of it is capital costs since hydropower plants are relatively simple and inexpensive to operate. The external costs are also site-specific. Hydro dams affect flora and fauna and may also require population ressetlement. They can have seismic and climatic effects and change water quality, affecting sedimentation

and aquatic ecosystems. In many areas, the spread of diseases, such as malaria and schistosomiatosis is also an important concern. In general, small-scale and mini-hydro plants (up to a few MW) pose fewer problems than large-scale projects.

Biomass is also an important energy source, especially in developing countries. Here it is often

used inefficiently, mainly for cooking and heating in rural areas, and in small industries. In industrialized countries, some wood fuel is used for domestic heating, but most of the biomass energy is derived from waste in industry or used as an integral part of industrial processes such as chemical pulping for paper production. The major potential sources of biomass for energy are residues from agricultural and forest industries and from dedicated energy plantations.

The energy from the sun is stored in different forms in different type of plants. Sugar- and starch- rich plants can be used for ethanol production as in the Brazilian fuel-alcohol programme. The energy input for production and conversion of the sugar-cane is only 10 to 15 per cent of the output in the form of

ethanol and bagasse (the cane residue) surplus. For the conversion of corn or wheat, for example, the energy balance is much less favourable and may even be negative. The viability of producing vegetable oils is limited for the same reason. More promising uses of biomass for energy include cogeneration of heat and electricity, methanol production through gasification and ethanol production through a chemical process called hydrolysis. In these applications the feedstock would be woody biomass.

The main environmental impacts from energy plantations are associated with land-use

requirements, use of fertilizers and recycling of ashes to maintain soil fertility, use of pesticides and herbicides and atmospheric emissions during conversion. In the case of deforested or otherwise degraded lands, biomass plantations can, if properly managed, improve these lands ecologically. Research and development is needed to ensure the realization and sustainability of high biomass yields under a wide range of growing conditions in addition to local economic, social and cultural conditions.