Energy in our way of living

By Nuno Oliveira, 12ºD
Escola Secundária de Loulé

Energy is an important factor on our daily life. In everything that we do, there are always energy exchanges between systems. The simple fact the we breath or even think, is responsible for a large amount of processing in these exchanges.

Rational use of energy in houses and schools is a major concern in our time. Since most electric power is still produced in thermoelectric power plants using fossil fuels, waist of electric energy will be a way of accelerating fossil fuel consumption and also an extra contribution to global warming, since the fossil fuels that are burned to produce electric energy produce carbon dioxide, is the major atmospheric component that we can consider responsible for global warming. Nuclear Power Plants are still not a solution because, besides the security problems, fission depends on uranium and uranium is not renewable.

A nuclear power plant close to Berlin.

In our houses, schools and cars, energy is always present.
We can think of several ways to optimize energy consumptions in our daily life.
For instance in house construction there are several things that have to be thought that can reduce energy consumption.
The house should be prepared in order to achieve the higher thermal insulation that is possible. This can be done by using double walls, and double glass windows. In the first case the low thermal conductivity of air or a polystyrene layer between the two wall layers can help to prevent easy energy transfer in or out of the house. In double glass windows this is guaranteed by the air layer between the two glasses.

A solar collector for water heating

We can also use local energy resources like sunlight to increase energy use efficency.
In our houses, schools and cars, energy is always present.

Solar Energy

By Vasco Batista, 11ºZi
Escola Secundária de Loulé

A solar cell or photovoltaic cell is a device that converts light energy into electrical energy. Sometimes the term solar cell is reserved for devices intended specifically to capture energy from sunlight, while the term photovoltaic cell is used when the light source is unspecified.

Fundamentally, the device needs to fulfil only two functions: photogeneration of charge carriers (electrons and holes) in a light-absorbing material, and separation of the charge carriers to a conductive contact that will transmit the electricity (simply put, carrying electrons off through a metal contact into a wire or other circuit). This conversion is called the photovoltaic effect.

Solar Power Plant in Serpa, Portugal
Image credit: The Sietch Blog

Since they use radiation from the Sun that is a renewable source photovoltaic cells are a good alternative for the production of electrical energy, particularly for areas with a high incidence of solar radiation.

However, the high cost of installation and low yield (which implies a huge surface to produce significant energy) are the biggest obstacle to the widespread use of this type of energy.

Due to the huge daily insulation period Portugal has made large investments in solar power, making it one of the biggest producers in a worldwide scale.

Nuclear fission energy

By Ricardo Caroço, 12ºC
Escola Secundária de Loulé

Nuclear fission energy has been the energy that has generated more controversy, often by being linked to the common citizen to military devices and many other times by ignorance of all the potential that has this kind of energy.

Nuclear fission power not only follows the release of electromagnetic energy-related links between electrons, protons and atoms, but occurs with the mass conversion of matter into energy. As we will see below, this makes it the largest release of energy that can be obtained from the field.

In 1942, Enrico Fermi has set in motion the first nuclear reactor. Since then nuclear power has been look of the most diverse forms. Earlier seemed it is a clean energy and without risks.
However, time has shown us the opposite with nuclear disasters as Windscale (1957), Three Mile Island (1979) and Chernobyl (1986), and serious problems such as waste and nuclear weapons.

Enrico Fermi (1901-1954).

Nuclear power may in theory be achieved by two methods: nuclear fission and fusion. The first is the energy that is obtained from the fission of heavy nuclei with formation of smaller nuclei and the second corresponds to the energy released when two nuclei come together to form a new nucleus of a greater number of mass.

Most nuclear power plants that are currently used for the production of electricity from nuclear fission are using the uranium-235 isotope as fuel through a chain reaction induced by neutron collision. However, the uranium-238 isotope is the most abundant in nature and represents 99.3% of the total uranium. This isotope rarely undergoes fission and mostly captures neutrons producing plutonium and neptunium without significant release of energy. Therefore, the uranium used in the core has to be artificially enriched in uranium-235 increasing its percentage from the 0.7% that occur in nature up to 3% used in the mixtures of nuclear power plants.

Scheme of a nuclear fission chain reaction of uranium-235.
(Credit: Serway&Jewett,Physics for Scientists and Engineers-with Modern Physics, 6th Ed., Thomson-Brooks/Cole, USA)

This percentage and conditions of temperature and pressure control achieved by the nuclear reaction is self-sustaining, or to the fact that at least one of the neutrons released in the fission of being caught by another uranium-235 isotope and not by an uranium-238 isotope.

The following image schematically represents the operation of nuclear power plants that are currently used to produce electricity commercially.

Scheme of a nuclear fission reactor.(Adapted from Serway&Jewett,Physics for Scientists and Engineers-with Modern Physics, 6th Ed., Thomson-Brooks/Cole, USA)

At the core of the reactor the nuclear fissions of uranium-235 rise the temperature of the water in the primary circuit which is a closed circuit. This water is at high pressure in order not to boil and serves as a moderator of the speed of the neutrons that are released in order to prevent their fusion with the uranium-238 isotopes.

A nuclear reactor with tipical blue of the Cerenkov radiation. Credit: Wikipedia

The hot water of the reactor is then pumped through a heat exchanger which transfers heat through the walls of the pipe to the water in the secondary circuit. This water reaches boiling point and the vapour moves the paddle of a turbine connected to a generator of electrical power. The steam then passes a condenser that cools it back to liquid state. Water is then pumped back to the secondary circuit where the cycle restarts. The electricity produced by the turbine and generator per mass of uranium-235 is enormous compared to the energy produced by the same mass of fossil fuel.
Though the safety of the nuclear power plants increases with the new generations of reactors, nuclear waste will always be a problem because some of the radioactive waste will endure for thousands of years. On the other hand uranium, as fossil fuels, is a non-renewable source. Therefore it cannot solve the energy problem of the generations to come.

ENERGY PRODUCTION IN PORTUGAL

by Nicolai Manalachi and Ruben Veríssimo, 11º Zi
Escola Secundária de Loulé

Carbon dioxide emissions are one of our major concerns in actuality since it is the major responsible for global warming.

Human effort is now applied in developing alternatives that do not emit CO2 into the atmosphere, since climatic changes - namely temperature rise - are now being perceived at a global scale.

CO2 Emissions in 2000 (Credit: New Scientist)

This problem is so severe that Kyoto’s protocol, signed in 1997 by nearly all countries around the World – one exception was the - has deliberated the need of a drastic drop on CO2 emissions. The evolutions developed until now by modification of traditional combustion motors (Diesel or gasoline) are insufficient to guarantee the success of the protocol.

Hydroelectric energy. (a) A dam at Chança stream at Guadiana river. (b) Turbines.

On the follow up of what was agreed in Kyoto, Portugal assumed the European compromise to guarantee that in 2010, at least 39% of the national consumption of electricity will be provided by renewable sources. Thermoelectric production using coal and/or fuel-oil is still the major electricity source, but this would represent a huge advance.

This ambitious goal implied that renewable energies are no longer viewed as a simple curiosity, only with marginal contributions within the energy sector.

The reasons that support Portugal ’s decision are the fact that alternative energy sources are the only endogenous sources this country possesses and they have a decentralized distribution that may contribute to general development of the country.

Brinches photovoltaic power plant (Source: http://engenhariacivil.wordpress.com )

Portugal has defined its strategy based on the following vectors:
•Huge increase in eolic energy production which means passing from the 290 MW in 2002 to 3.750 MW in 2010;
•Increase up to 150 MW of photovoltaic power in 2010;
•Build new hydroelectric power plants up to a total of 800 MW, increasing hydroelectric contribution to around 4.950 MW;
•Investment in 550 MW from other renewable energy sources (biomass, biogas, urban solid waste, waves and the Sun).

In 2004, Portugal already possessed a significant amount of hydroelectric power installed (aprox. 4.150 MW), that associated to the contribution of other forms of renewable energy are now responsible for about 35% of the national consumption.
Many investments have been made and our now presenting results. Recently at Brinches (Serpa) the worlds 2nd biggest photovoltaic power plant has started its production using 52.000 photovoltaic panels that guarantee a production of 11 MW. An even bigger photovoltaic power plant is now being ended at Amareleja (Moura) that will be the biggest in the world with a 62 MW production. The total amount of installed photovoltaic power at the end of 2007 will be 97 MW.

Eolic energy production has also received a major increase.

Still more investment is needed. Nonetheless, one may consider that though the established goal for 2010 was very ambitious it seems Portugal might be on the way to fulfill it.