It is clear that each energy source has its advantages and disadvantages. Fossil fuels have high energy density and good storage characteristics. However, their future availability is uncertain (partly because of finite resources but also because of resource location) and their environmental impact is becoming more and more of a global concern. Nuclear fuel energy density is very high but safety and environmental impact considerations (both real and perceived) may be prohibitive. Renewable energy sources are attractive because of their low environmental impact (at least for humans, if not for wildlife) and continually available supply. However, the restricted availability of suitable locations for hydroelectric, geothermal, and wind imposes a severe limit to their future contribution. Also, low energy density and difficulty with storage make them impractical for certain applications. These characteristics and the inconvenient location of some renewable energy sources (i.e. far away from heavy use areas) may provide a market for Earth to Earth relay satellites to distribute power to areas that need it. Of all the energy sources, solar energy is the most attractive option in terms of general availability since it is accessible anywhere on Earth (depending on variation in cloud cover) has the lowest potential environmental impact, and a virtually inexhaustible supply. The major conclusion of this section is that energy sources have markedly different characteristics both physical and operational. Because of these differences they may not be readily interchangeable. It is not enough to determine the amount of energy needed and then to allocate a certain percentage of the energy requirement to a particular energy source based on its energy content. The best example of non-interchangeability is fuel for transportation. Current types of automobiles use varying grades of oil. It is true that other sources of energy can be used to power cars but significant changes in automobile technology would be required to accommodate them. Another aspect of differences in characteristics is the variable cost of collection, transportation, environmental cleanup and storage requirements. For example, when considering biomass as an energy source, the energy and labor cost of collection and transport to use location must be weighed against its inherent energy content. Section 2.2.2 will discuss the different types of conversion and will help illustrate why certain types of energy sources are more appropriate for specific conversion processes. Section 2.2.3 will provide current energy costs and projected costs (where available), and a discussion of how the characteristics described in this section contribute to the cost of energy. In the final analysis, economics will dictate which types of energy sources, and also how much energy, will be used in the future. 2.2.2 Major Uses and Conversion of Primary Energy The section above described the different energy sources. Their energy has to be transformed into the service needed. Usually there are several possibilities to deliver a certain service (e.g., electric heating versus coal heating). A possible subdivision of the different services (transportation, electricity production and direct heat production) is done in the following three subsections with a special emphasis is on the different transformation processes. This section describes how primary energy is transformed into secondary energy. Secondary energy (in contrast to primary energy) is defined as energy in such a state that it can be transformed easily (without big conversion losses) into the energy form required. The two most important forms of secondary energy dealt with here are electric energy and mechanical energy. Transportation In 1991, transportation accounted for 42% of the fossil fuel consumption in USA (Figure 2.2). For all transportation systems, energy is mainly required to generate movement. This form of energy is called kinetic energy or mechanical energy. Other forms of energy that are employed on board the vehicle (light, cooling, heating ...) are usually small compared to the kinetic energy demand and will be neglected here. A major feature of mobile systems is that all (or most) parts of the energy system are moving with the vehicle. That requires them to have certain limitations in terms of weight, size, security in case of accidents, mechanical stability, etc. For many means of transportation the energy supply is the limiting factor. Therefore this section will be subdivided further into three forms of energy supply: vehicles with external energy supply, vehicles with direct conversion and vehicles with internal energy storage.
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