2 Energy definitions and concepts
If someone asked you to define energy, what would you say?
The word energy is derived from the Greek en (in) and ergon (work). Energy is defined as ‘the capacity to do work’ – that is, the capacity to move an object against a resisting force. The scientific unit of energy is the joule.
The concept of energy reveals the common features in processes as diverse as burning fuels, propelling machines and charging batteries. These and other processes can be described in terms of diverse forms of energy, including:
- thermal energy (heat)
- chemical energy (in fuels or batteries)
- kinetic energy (in moving substances)
- electrical energy
- gravitational energy
- nuclear energy
In everyday language, the word 'power' is often used as a synonym for 'energy', but this is not strictly correct.
Power is the rate at which energy is converted from one form to another or transmitted from one place to another. The scientific unit of power is the watt.
Renewable energy can take a variety of these forms, and can be defined as:
energy obtained from the continuous or repetitive currents of energy recurring in the natural environment
(Twidell and Weir, 1986)
or as
energy flows which are replenished at the same rate as they are ‘used’
2.1 Units of energy and power
A host of different units are used to describe energy and power, which can seem a bit confusing. But the following Table 1, where we have listed and described some of the most frequently used units, should clarify the meaning of the various terms.
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