Scientific principles of renewable energy

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The definitions of renewable (green) and finite (brown) energy supplies (Section 1.3.1) indicate the fundamental differences between the two forms of supply. As a consequence the efficient use of renewable energy requires the correct application of certain principles.

1.4.1 Energy currents

It is essential that a sufficient renewable current is already present in the local environment. It is not good practice to try to create this energy current especially for a particular system. Renewable energy was once ridiculed by calculating the number of pigs required to produce dung for sufficient methane generation to power a whole city. It is obvious, however, that biogas (methane) production should only be contemplated as a by-product of an animal industry already established, and not vice versa. Likewise for a biomass energy station, the biomass resource must exist locally to avoid large inefficiencies in transportation. The practical implication of this principle is that the local environment has to be monitored and analysed over a long period to establish precisely what energy flows are present. In Figure 1.1 the energy current ABC must be assessed before the diverted flow through DEF is established.

1.4.2 Dynamic characteristics

End-use requirements for energy vary with time. For example, electricity demand on a power network often peaks in the morning and evening, and reaches a minimum through the night. If power is provided from a finite source, such as oil, the input can be adjusted in response to demand. Unused energy is not wasted, but remains with the source fuel. However, with renewable energy systems, not only does end-use vary uncontrollably with time but so too does the natural supply in the environment. Thus a renewable energy device must be matched dynamically at both D and E of Figure 1.1; the characteristics will probably be quite different at both interfaces. Examples of these dynamic effects will appear in most of the following chapters.

The major periodic variations of renewable sources are listed in Table 1.2, but precise dynamic behaviour may well be greatly affected by irregularities. Systems range from the very variable (e.g. wind power) to the accurately predictable (e.g. tidal power). Solar energy may be very predicable in some regions (e.g. Khartoum) but somewhat random in others (e.g. Glasgow).

1.4.3 Quality of supply

The quality of an energy supply or store is often discussed, but usually remains undefined. We define quality as the proportion of an energy source that can be converted to mechanical work. Thus electricity has high quality because when consumed in an electric motor >95% of the input energy may be converted to mechanical work, say to lift a weight; the heat losses are correspondingly small, <5%. The quality of nuclear, fossil or biomass fuel in a single stage thermal power station is moderately low, because only about 33% of the calorific value of the fuel can be made to appear as mechanical work and about 67% is lost as heat to the environment. If the fuel is used in a combined cycle power station (e.g. methane gas turbine stage followed by steam turbine), then the quality is increased to ~50%. It is possible to analyse such factors in terms of the thermodynamic variable energy, defined here as 'the theoretical maximum amount of work obtainable, at a particular environmental temperature, from an energy source'.

Table 1.2 Intensity and periodical properties of renewable sources

System

Major periods

Major variables

Power relationship

Comment

Text reference (equation)

Direct sunshine

24 h, 1 y

Solar beam irradiance G*( Wm-2);

Angle of beam from vertical 0Z

PaGJ cos 6Z Pmax= IkWm-2

Daytime only

(4.2)

Diffuse sunshine

24 h, 1 y

Cloud cover, perhaps air pollution

P < 300WrrT2

Significant energy over time

(4.3)

Biofuels

iy

Soil condition, insolation,

Stored energy

Very many chemical types and

Table 1 1.1

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  • ermanno endrizzi
    What are the scientific principles of renewable eneegy resources?
    2 years ago
  • Ambessa
    What are the scientific principles on renewable energy?
    2 years ago
  • MALVA GARDNER
    What scientific principle is involved with energy?
    7 months ago

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