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Renewable electricity sources

Renewable electricity


Photovoltaic systems

In years to come, there will not be a single, unique source of electricity, but many renewable electricity sources that will be combined to answer the planet’s electricity needs. Photovoltaic energy and wind energy have in the past 10 years become the dominant form of electricity generation.

With solar cells, it is possible to convert sunlight directly into electricity by the photovoltaic process. When light particles called photons hit the surfaces of a solar cell material (semi-conductor), they transfer their energy to the material’s electrons displacing them from their orbit. If the semi-conductor is doped with suitable impurities then the electrons are attracted to one surface so setting up an electric charge which forms the basis of an electric current.

The principle of a photovoltaic cell

photovoltaic cell

As sunlight is uniformly distributed, this enables every building to have the potential to generate solar electricity locally. The roof is the most convenient place to mount the solar cells if it is correctly orientated to receive solar irradiation.

Typical roof mounted PV panels either side of a set of solar thermal water panels

pv panels

The potential for generating electricity directly from sunlight is very large and is becoming increasingly cost effective as the technology has improved and the price of electricity generated by conventional sources like fossil fuels is continually increasing.


Advantages

  • The technology can be used almost anywhere since sunlight is available everywhere.
  • The PV modules can almost always be installed close to the place where are consumed thereby avoiding electricity losses associated with transmission and distribution.
  • The installation size can be easily adjusted according to needs and available space.
  • There is no operational pollution, no gaseous discharge, no waste, no risk of physical accidents and no safety issues.
  • There is very little maintenance or repairs as there are no moving parts.

Disadvantages

  • The roof of the building has to be correctly orientated i.e. south facing
  • The daily and seasonal variation which can be overcome by either on site storage or connection to the electricity grid

PV modules

The sun’s radiant energy is transformed into electrical energy without any moving parts or noise. Each cell generates a very small amount of electricity. To obtain a stronger electrical current and to increase the power output, the cells are connected in series to form large photovoltaic panels or modules.

As the cells are extremely thin and fragile, they are protected by a weatherproof enclosure and sheet of transparent, solid glass. Modules are generally rectangular in shape and a few centimetres thick. They can be integrated into construction materials (tiles, slate or transparent frames).

PV systems
A photovoltaic array is simply a small electrical power plant installed as close as possible to where the electricity is needed. There is no need to store the electricity since any excess can be exported to the grid to be used in other dwellings. Electricity can therefore be sold and bought by signing a contract with a grid supplier. A kWh meter records the amount of electrical current exported to the grid so that this electricity can be invoiced to the utility.

PV systems

The other option is to add a battery pack to the solar cell array so that when the supply exceeds demand this electricity can be stored for reuse when the demand exceeds the supply. Which option is chosen will depend upon the unit rate for importing and exporting electricity offered by the utility.

If a dwelling is not connected to the national grid, then electricity needs to be stored when supply exceeds demand as mentioned above. Batteries are the common form of storage as these can store electricity for long periods of time.

KITH # Activity Age range
3.7   Potential for electricity production at your home   
  Science
 9 – 15 
  Potential for electricity production at your school   
  Science
 9 – 15  

Wind energy

Wind results from the movement of air due to the differential heating of the earth by the sun. For example, during the day by the seaside the air above the land is warmed faster than the water in the sea and so air flows from the sea to the land creating sea breezes. At night however the air above the land cools faster than the water in the sea so the air flow is reversed creating land breezes.

The windmill in its simplest form comprises a set of sails which converts the momentum of the air it captures into rotational (mechanical) energy of a shaft. This rotating shaft can in turn be used to grind corn or pump water out of the ground. They are still in common usage in dry parts of the world so when ever the wind blows some water is pumped and stored in an adjacent dam for subsequent use. In Crete, for example, windmills have been in continuous use for almost 5000 years (Figure)

windmills

A more recent application of wind energy conversion is that for producing electricity in which the rotation of a set of blades is then converted into electrical energy by mounting an electric generator at the end of the shaft.

small turbines

The current production of wind turbines range from very small turbines of one metre diameter or less which can be used at home to very large machines with blade lengths up to 70 metres.

Since the 1990’s, communities and electricity companies have been using this technology to build arrays of wind turbines whose output can be fed into the grid for supplying electricity.

The conversion process

Starting in the 1950’s, the sails were replaced by rigid aerofoil shapes (like those of the wing of an aeroplane) that are much more efficient in capturing the energy of the wind by having a higher lift to drag ratio. The generation process is the same for all size wind turbines.

wind turbine

A wind turbine works the opposite of a fan. Instead of using electricity to make wind, a turbine uses wind to make electricity. The moving air (wind) turns the blades which are shaped so that their lift is greater than its drag thus inducing rotation in the shaft to which it is attached. This spins the shaft, which is connected to a generator in order to produce electricity. The electricity is sent through transmission and distribution lines to a substation, then on to homes, business and schools.


Tall towers are used to position the turbine at a sufficient height to have unobstructed access to the wind, so that the rotor faces the strongest and steadiest possible winds. A gear box and a switch called a ‘contactor’ is used in-order to ensure that regardless of fluctuations in the wind speed the turbine feeds its electricity into the grid at the correct frequency (50Hz) and voltage (230 volts). The contactor switch remains on until the wind drops to such a level that the turbine cannot capture sufficient wind energy, at which time it ‘drops out’ in-order to allow the rotor to rotate freely.

As the wind direction changes, the wind turbine needs to face into the wind. With large turbines, this ‘yaw’ rotation is carried out using electric motors whilst for small turbines this is done passively by using a vane at the back of the turbine.

Advantages

  • Turbines can be erected wherever the wind blows
  • They can function either as a stand alone or grid connected generating system
  • Wide range of sizes to fit every application
  • Their footprint (i.e land on which they are mounted) is very small
  • There is no operational pollution, no gaseous discharge and no waste
  • Very little maintenance required

Disadvantages

  • Output is not constant as moving air is always more or less varying in speed
  • When there is not sufficient movement in the air, no electricity is generated
  • Some audible noise is generated by the air passing the blade so for large grid connected turbines an exclusion distance of up to 400m is specified for housing (but not for agriculture)

Wind energy systems

grid turbines

Like photovoltaic systems, wind turbines can be operated either be linked to a grid or operated in a stand alone mode. Grid linked is the most common mode and this is especially important for the very large arrays now being erected off the UK coast.


The best known example of stand alone wind generating systems is on Fair Isle, the most isolated island community off the British coast.

Isle turbine

Wind turbines have being supplying electricity for almost 40 years to a cluster of 20 dwellings and the load is matched to the turbine output by varying the frequency from 48 to 55 Hz and having a set of frequency switches which switch loads on or off in each dwelling.

On Foula, another isolated island adjacent to Shetland, use has been made of the availability of an upper and lower loch. When there is an excess of power, water can be pumped from the lower to the upper loch. When demand exceeds supply, water can be released from the upper loch to drive an axial flow generator which produces electricity. The concept of pumped storage is not new and has been widely used to match the demand and supply of the national grid such as in the UK.

For smaller turbines, battery storage would be the most common medium.

Combining different types of renewable electricity sources is common and wind and PV are generally a good combination as PV peaks in the summer and wind in the winter in much of Europe.

KITH # Activity Age range
9.4   Wind potential of your school   
  Science
 9 – 13  
 

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