GENERAL DEFINITIONS
Renewable energy sources (RES) can be defined, in general, as those capturing their energy from ongoing natural processes, like sunshine, wind, flowing water, geothermal heat flows and biological processes; they are considered renewable because their flow of energy is replaced by a constant natural process in a short period of time, which is one of the main differences between RES and fossil energy sources (University of Massachusetts, 1997).
RES can be used in different ways, either directly or indirectly, to generate some more convenient form of energy: for instance, to produce electricity through wind turbines or fuels, such as ethanol, from biomass.
Their use is not new in human history, since wood has been the primary energy source since less than 150 years ago; nevertheless, in the last century, the low price of fossil fuels caused a fall in wood use and, even today, it is one of the main obstacles to a widespread development in RES exploitation (Energy Information Administration, 2004).
In relatively recent years, during the 1970s, the concept of renewable energy began to be debated; since then, RES have gained increasing attention due to the emergence of various problematic issues related to the use of fossil fuels and of nuclear energy: in particular their exhaustibility, their polluting emissions and wastes, and quite recently their rising prices.
As a matter of fact, RES are seen as more sustainable than nuclear and fossil sources of energy: first, because they may be classified as “free energy”, which means (in engineering) an energy source available directly from the environment and which cannot be expected to be depletable by humans; besides, RES are commonly considered cleaner, in terms of their final emissions and environmental impact.
In spite of these potential positive effects, several kinds of criticism have arisen, regarding a more extensive use of RES, which would aim at satisfying part of the increasing energy demand of the last years.
One of the main critique on RES is referred to their habitat hazards. In fact, even if they are not supposed to lead to any new global risk, like nuclear wastes, some renewable energy capture systems entail particular environmental problems: for instance, someone claims that wind turbines can be dangerous for flying birds, while hydroelectric dams can create barriers for migrating fishes. Besides, some people disapprove the aesthetic consequences of wind turbines or large solar-electric installations in the countryside.
Another problematic issue deals with the effective availability of a RES and with its need of proximity to the energy demand. Since RES usually provide a relatively low-intensity energy and are intermittent in nature, exploiting such resources on a large scale is likely to require considerable investment in the technology adopted, as well as in transmission and distribution networks. The costs (not only financial, but also in terms of energy utilized) in infrastructures and for the transport and storage of this energy will make two questions arise: on one side, that of the economic profitability; on the other, that of the net energy produced[1].
Finally, there is a debate on the opportunity cost of the land. Large areas should be used to install wind turbines or photovoltaic cells, or to build a dam, or to cultivate energy crops, in order to produce significant level of bio-energy; those areas could be used to other kinds of production, or could even left wild for conservation purposes (Cliff Bowden, 2005).
The relevance of this issue is particularly evident in the case of biomass production, and especially of biofuels by energy crops, since the large amount of land required could be used to produce food crops: the achievement of food security by a country and its bio-energy production become to be seen, in this way, as they were in a sort of competition, as we will underline later.
In the case of biomass production for energy purposes, all the above mentioned issues assume a peculiar relevance, not just that of land availability and opportunity costs; however, before analyzing the peculiar meaning of those issues, we think it is worth trying to give a definition of the term “biomass” and underline its main characteristics.
The term biomass has different definitions, often depending on the defining entity and its purposes. Nonetheless, it can be broadly identified as all kinds of non-fossil organic material that is available on a renewable basis; we include agricultural crop and wood wastes and residues, animal wastes, municipal wastes, other organic waste materials and, of course, dedicated energy crops and trees[2].
Given the high variety of raw materials, several types of technologies are used to transform biomass into bio-energy: among them, we could list direct combustion, co-firing, pyrolysis and anaerobic digestion. On the other side, final uses of biomass are various and diversified: biomass can be used for household heating, as a liquid fuel, to produce bio-fuels or bio-gas.
Some differences can be identified between biomass and the other kinds of RES. From the point of view of its availability, biomass can be considered, among RES, the most independent one from geography, being available at local level in various forms in almost every period of the year. However, geography becomes relevant again in the phase of transformation of biomass into bio-energy and its transport: collection logistics, available transformation technologies and infrastructures are crucial aspects of the biomass supply chain, as well as the distance existing between the production site and the demand (ITABIA, 2003). In this perspective, biomass can be seen as an important resource at territorial level.
In terms of renewability of the source, there is a wide diversity not only between biomass and other RES, but also among different types of biomass: some kinds of biomass are constantly renewed (e.g. municipal or animal wastes), while some others take time and a new productive process to be renovated (e.g. trees and energy crops). It is worth reminding that this second kind of biomass lies in the definition of RES too, because the time it needs to be renewed never goes beyond a human lifespan (CPATS, 1998).
Furthermore, as opposite for the other RES, the final uses of biomass for bio-energy are usually characterized by some sort of polluting emissions, even if at a lower level than fossil energy sources: in fact, these emissions would be compensated by the amount of CO2 absorbed by biomass during its life, resulting in almost no net CO2 emission.
Biomass shares most of the criticisms claimed against RES in general, as we already mention in the case of land opportunity costs. Nevertheless, in the case of biomass, the critical issue of net energy production assumes a peculiar emphasis and ends up being linked to that of net polluting emissions, due to high incidence of transport in its productive process. It is generally agreed upon that total net polluting emissions of bio-energy from biomass (considering both those of the transport phase and those of the final uses) are lower than those of fossil fuels, especially if biomass is transported and use within a reasonable distance from the production site.
Now, we want to focus our attention on bio-fuels, a specific type of biomass obtained by the oil of dedicated crops, like sugar cane, soy, sunflower, which are called energy crops when used for energy purposes.
Liquid biofuels usually produced are bio-ethanol, bio-diesel, as well as virgin vegetable oils. Bio-ethanol can be used in internal combustion engines and in fuel cells; bio-diesel can be used in modern diesel vehicles with little or no modification to the engine and can be obtained also from waste and virgin vegetable and animal oil and fats (lipids); while modifications in diesel engines are needed to use virgin vegetable oils[3].
While the introduction of energy crops can contribute in the increase in bio-diversity of areas previously dedicated to monocolture, the major benefit of biofuels lies in their lower emissions, compared to fossil fuels. Nevertheless, some drawbacks in their use are linked to the fact that the crops need to be grown, collected, dried and fermented, and the oil obtained needs to be transformed to be used safely by common engines. All these steps in the production chain of biofuels require particular infrastructures and technology, resulting in a higher price of the final product and, consequently, in a barrier against a more widespread use of this kind of bio-energy.
Other two obstacles in the development of bio-fuels seem to be land availability, since they would require large areas to be cultivated with energy crops, and the fact that some more productive energy crops (like soy) may have a negative environmental impact, causing habitat damage in those areas in which they are massively grown.
BIBLIOGRAPHY
Cliff Bowden, “Investing in renewable energy sources”, 2005; available at
“Le biomasse per l’energia e l’ambiente – Rapporto 2003”, ITABIA
Donald L. Klass, “Biomass for renewable energy and fuels”, Biomass Energy Research Association, 2004; available at
(Renewable Energy Research Laboratory (RERL) University of Massachusetts, 1997)
(1998)
(US Department of Energy – Energy Efficiency and Renewable Energy, 2005)
(Energy Information Administration, 2006)
(2005)
(2006)
(Biomass, intermediate technology development group)
(national energy foundation)
D.O. Hall and J.I. House, “Biomass energy development and carbon dioxide mitigation options ”, Division of Life Sciences, King's College London;
1
[1] On the criticism on the use of RES, see
[2] For a detailed definition of biomass, see
[3] See the definition of biofuels available at