Page 35 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

26 Fundamentals of Ocean Renewable Energy

‘wind energy’ and ‘wind power’ that are often used synonymously by the
press are inaccurate. In contrast to energy, power is the rate at which energy
is transformed from one form to another, or the rate that energy is transferred
from one place to another.
Units of Energy and Power
To pick up a 5-kg block and raise it 2 m is work, and it requires energy. A force
must be exerted, which must be sufficient to overcome gravity, g. The force must
be applied directly upwards, against the pull of g, for 2 m [23]. Because

Work = Force × Distance (1.4)
and
2
Force = 5kg × 9.81 m/s = 49.05 N (1.5)
then

Energy = 49.05 N × 2m = 98.1 J (1.6)
where J is a Joule, the unit of energy. 12 A Joule can therefore be defined as
the work done when a force of 1 N is applied over a distance of 1 m. Referring
to the circular definition of work and energy described earlier, 1 J is also the
energy expended in performing this task. Energy is stored and converted over a
vast range of scales, and some examples are provided in Table 1.4, describing
the energy required to melt 1 g of ice (330 J) and the daily energy output of
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the Sun (3.3 × 10 J). By way of illustration, global electricity production was
23,950 TWh during 2015 (Fig. 1.1). Therefore, energy is
19
23, 950 × 10 12 × 3600 = 8.6 × 10 J (1.7)
Power is the rate at which energy is converted from one form to another.
Power is measured in Watts 13 (W), where 1 W = 1J/s. If we consider what
is becoming a fairly obsolete technology, the incandescent light bulb, a 100 W
light bulb converts 100 J of electrical energy into light (and ‘waste’ heat) every
second. Consider again the case described earlier, where a 5 kg block was raised
2 m. We would need to raise this block 2 m every second (and somehow convert
this energy into electrical energy) if we were to power a 100 W light bulb! As
you will have noticed, electricity generation (or consumption) can either by
reported as instantaneous power (e.g. in Watts) or as the electricity generated
over a period of time (e.g. MWh) (i.e. energy). A simple way of looking at this
is, if we have a 1000 W microwave and run it on full power for 1 h (although I
dread to think what state the food would be in after an hour!), we would have
consumed

12. The unit of energy, the Joule, is named after the English physicist James Joule.
13. The unit of power, the Watt, is named after Scottish inventor James Watt.