Environmental Engineering Reference
In-Depth Information
ore in a process known as enrichment. Gasified uranium is spun at veryhigh speed in a
centrifuge. Like the spin cycle of a washing machine, which draws water out of fabric, the
centrifuge draws the heavier U-238 atoms to the outside, leaving a higher proportion of
U-235 at the centre. Low-enriched uranium, with around 5 per cent uranium-235, is the
most common fuel in modern reactors. One kilogram of this fuel can produce the same
amount of energy as 2,000 tonnes of coal in a conventional power plant. Nuclear weapons
require a far higher level of enrichment (more than 80 per cent U-235). The difficulty and
expense of producing highly enriched uranium is one of the reasons why so few countries
possess nuclear weapons.
Biofuels
Initially hailed by environmentalists and industrialists alike, the one renewable source that
seemed to please everyone, biofuels have fallen from grace in recent years. Since the
2007-2008 global food crisis, when food prices increased dramatically, partly in response
to a growing demand for ethanol, biofuels have been seen by many as a way of diverting
foodfromtheplatesoftheworld'spooresttothegastanksofitsrichest.Eventhedefinition
of biofuels is in dispute. Strictly speaking, they include all fuel sources derived from
biomass; namely, wood, liquid fuels (ethanol and biodiesel) and methane gas obtained
from biological sources. However, because biofuel production has been subsidised by
many countries as a means of reducing carbon dioxide emissions and dependency on fossil
fuels, the term has become important from legal, political and economic perspectives. In
this sense the term 'biofuel' is understood to mean the liquid fuels derived from plant
sources and used as a replacement for petrol and diesel in transport. The most common of
these are bioethanol, mainly produced by fermenting sugarcane (in Brazil) and maize (in
North America), and biodiesel, mainly produced by refining rapeseed and sunflower oils in
Europe.
Biofuels can be used in much the same ways as fossil fuels: for transport, household
heating, and in power plants. The main difference between biofuels and fossil fuels is
that biofuels are renewable and the carbon dioxide they give off when burned is largely
counterbalanced by that absorbed during photosynthesis as the plants grow.
2.4 Energy Storage
Whenweovereat,wetakeinmorefoodenergythanweexpendthroughourdailyactivities.
Our body automatically stores the surplus energy in the form of fat, a concentrated energy
source (it has double the energy density of carbohydrates) that sustains animals over
longer periods without food. The way we store fat also has a lot to do with our external
