Air - Wind power

The origins of winds and atmospheric pressure

One square metre (1 m2) of the Earth’s surface on or near the equator receives more solar radiation per year than 1 m2 at higher latitudes. Because of the curve of the earth the angles mean that the sound rays have further to travel with increased latitude so areas such as the tropics are hotter than the areas of higher latitude.

With these temperature variances it means that there will be differences in atmospheric pressure which give rise to movements of atmospheric masses which is the principle behind 

Atmospheric pressure - It’s the pressure resulting from the weight of the column of air above a specified surface area. The unit of atmospheric pressure is the bar, measured using a barometer – usually calibrated in millibars (mbar), i.e. thousandths of a bar.

On a weather map the regions marked ‘high’ and ‘low’ relate to the atmospheric pressure and the contours represent lines of equal pressure called isobars. The high-pressure regions tend to indicate fine weather with little wind, whereas the low-pressure regions indicate changeable windy weather and precipitation (rain or snow).

Energy and wind power

The energy contained in the wind is its kinetic energy, and in accordance with basic physical principles the kinetic energy of a moving mass (moving air in this case) is equal to half the mass, m, (of the air) times the square of its velocity, V:

Wind turbines

There are two types of wind turbines:

Horizontal axis wind turbines (HAWTs)

These can be muti bladed and have been used since the 19th century for water pumping. Modern turbines have 2 or 3 blades and work at much higher rotational speeds making them great for electricity generation. They go from small turbines to massive turbines producing 7.5 MW or more.

Vertical axis wind turbines (VAWTs)

These can harness winds from any direction without having to reposition the blades but they have had little commercial success due to the power quality because of the cyclic loading (repeated stresses and strains on particular components).

The shape of wind turbines is important as an object in an air stream experiences a force from the air stream that is equivalent to two component forces at right angles - drag force and lift force.

V = Velocity, L = Lift force, F = force, D = Drag force

An object in an air stream is subject to a F and this is composed of D which is acting in one with the sir flow direction and the L which is acting at 90 degrees to the direction of the air flow. L and D forces are both proportional tot he energy in the wind. In HAWTs the rotational axis is maintained in line with the wind direction by a 'yawning' mechanism which constantly realigns the turbine. In a VAWT, the wind direction constantly changes through its cycle and therefore the 'suction' side reverses during each cycle.

Calculating the power and energy from wind turbines

The power output of a wind turbine varies with wind speed. The energy it produces depends on its wind speed curve (image below) and the wind speed frequency distribution.

As wind is not constant, the energy produced by the wind turbine can be calculated by taking the wind speed for each cut-in and shut-down using the following formula:

The wind speed distribution at a site is measures using equipment to record the number of hours for which the wind speed lies within a 1 metre per second (1 ms-1) band e.g. 0-1 ms-1 or 1-2 ms-1. The total energy produced in a given period is calculated by summing the energy produced at all wind speeds within the operating range of the turbine.

Wind speed maps, atlases and computer models

The Riso Laboratory in Denmark has produced various maps of various areas within the EU to show annual mean wind speed at 50 metres above ground level for five different topographical conditions.

Positive impacts of wind energy

The generation of electricity by wind turbines does not involve:

• the real ease of carbon dioxide
• pollutants that cause acid rain or smog
• radioactivity
• contamination of land, sea or water courses
• the consumption of water

Large scale implementation of wind energy within the UK would probably be one of the most economic and rapid means of reducing carbon dioxide emissions.

Environmental concerns

Noise

Wind turbines are often described as noisy but they are not companies with other machines.

The noise can easily be reduced by using acoustic enclosures for the machinery and also using slower rotational speeds to reduce aerodynamic noise.

Electromagnetic interference

If turbines are located between some types of radio transmitter, sometimes electromagnetic interference can occur because of the replication of some of the waves. This depends mainly on the turbine blade construction and surface shape.

Aviation

The MOD have expressed concerns about interference with military radar from turbines in low flying areas. Turbine blades are now being adapted to include radar absorbing materials.

Wildlife

Natural England suggests there is little evidence that wind farms in Enlgand have a significant impact on birds, but it provides guidance about wind turbines and birds, and post-construction monitoring of bird impacts.

For offshore wind there are concerns about possible impacts on fish, crustaceans, marine mammals, marine birds and migratory birds. Research it ongoing.

Wind turbines may have an impact on bats. Natural England has produced guidance to help planners and operators take account of potential impacts on bats when assessing wind turbine developments.

Public attitudes and planning

Wind turbines are not as big as people believe. Since the 1990s, on average 80-80% of the public support the development of wind farms in the UK but it is important that the planners work closely with local communities. There are a variety of factors which affect the public opinion of the turbines and this ranges from the types of blades and the number of blades to where the turbines are located and how many there are. 

Offshore wind energy

In 2014, the UK had the world's largest offshore wind energy capacity to date - we had 3600 MW of offshore wind energy capacity installed from 1000 turbines.

The cost of offshore wind farms is currently higher due to engineering costs, electrical connection costs and the need to construct the turbines out of more expensive materials which can withstand the corrosive nature of the sea.

Offshore wind speeds are higher and more consistent than on land so as technology is advancing and experience gained, offshore wind farms are becoming popular and the energy costs are competitive in the medium to long term.

Floating wind turbines are being explored such as those below in order to locate turbines further out to sea as opposed to sitting on the shallow continental shelf.

Future prospects for wind energy

Wind energy looks set to be a major generator of electricity throughout the world. In Europe, the capturing of offshore wind energy is fat becoming on e of the most important means of reducing carbon dioxide emissions from the energy sector. 

A European Commission report states:

With additional research efforts, and crucially, significant progress in building the necessary grid structure over the next ten years, wind energy could meet one fifth of the EU’s electricity demand in 2020, one third in 2030 and half by 2050 (Zervos and Kjaer, 2009).

References.

Beurskens, J. and Jensen, P. H. (2001) ‘Economics of wind energy – Prospects and directions’, Renewable Energy World, July–Aug.

Everett, B., Boyle, G. A., Peake S. and Ramage, J. (eds) (2012) Energy Systems and Sustainability: Power for a Sustainable Future (2nd edn), Oxford, Oxford University Press/Milton Keynes, The Open University.

EWEA (1991) Time for Action: Wind Energy in Europe, European Wind Energy Association.

Natural England (2009a) Technical Information Note TIN051: Bats and onshore wind turbines – Interim guidance, First Edition, February.

Natural England (2009b) Technical Information Note TIN059: Bats and single large wind turbines: Joint Agencies interim guidance, First Edition, September.

NOP (2005) Survey of Public Opinion of Wind Farms, NOP for BWEA, UK.

Risø (2009) European wind resources over open sea, http://www.windatlas. dk/europe/oceanmap.html.

Troen, I. and Petersen, E. L. (1989) European Wind Atlas, Risø, Denmark for the Commission of the European Communities.

YouGov (2010) Public Attitudes to Wind Farm YouGov Survey, YouGov plc for Scottish Renewables.

Zervos, A and Kjaer, C. (2009) Pure Power – Wind energy targets for 2020 and 2030. (2009 update). European Wind Energy Association.

Earth – The origins of the Earth’s renewable energy sources, and at the world’s energy supplies and demands.

Renewable energy

A sustainable energy source can be defined as one that:

• is not substantially depleted by continued use
• does not produce significant pollution or other environmental problems
• does not cause health hazards or social injustices.

Renewable energy sources are more sustainable then fossil fuels and nuclear energy

Brief history of the energy sources of humans

First of all there was wood which was used in fires (which is harnessing the power of solar driven photosynthesis). Society developed and they started using the movements of water and wind to help grind corn and irrigate crops etc and then building were created to make use of the suns warmth to heat and cool where necessary. In the industrial era coal was heavily exploited and soon replaced the use for wood, water and wind. It was started to become apparent that the use of fossil fuels had adverse effects on the environment but it wasn't until the 1970's that people started taking the issue seriously.The fossil fuels trio of coal, oil and gas make up 80% of the worlds energy sources.

After the second world war nuclear energy was thought to be the new and clean energy source to replace fossil fuels but there have been concerns over the costs and the disposal of waste. Its use is continuing to expand in some countries.

Overview of the main renewable energy sources.

Direct solar energy

Solar radiation can be converted into useful energy directly, using various technologies. It can be captured and heat water or spaces in buildings. It can be concentrated by using mirrors and solar radiation can also be converted directly into electricity using photovoltaic (PV) panels, normally mounted on the roofs or facades of buildings.

Indirect solar energy

Solar radiation can be converted to useful energy indirectly, via the other energy forms it causes. This includes Bioenergy (plants powered by the sun), hydropower (which is from solar radiation heating the oceans and causing water vapour which then rains and feeds rivers and is used by dams)

There are also two other sources of renewable energy that do not depend on solar radiation: tidal energy and geothermal energy.

The word energy is derived from the Greek en (in) and ergon (work), and is broadly 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. Power is the rate at which energy is converted and this is measures in watts.

Energy
Name	Description
Joule (J)	Main scientific unit of energy
Kilojoule (KJ)	Equal to 1000 (103) joules
Megajoule (MJ)	Equal to 1 million (106) joules
Gigajoule (GJ)	Equal to 1 billion (109) jou
Exajoule (EJ)	Equal to 1 quintillion (1018) joules
Kilowatt-hour (kWh)	The amount of energy produced by a power of 1 Kilowatt (1 kW) in one hour
Megawatt-hour (MWh)	The amount of energy produced by a power of 1 Megawatt (1 MW) in one hour
Gigawatt-hour (GWh)	The amount of energy produced by a power of 1 Gigawatt (1 GW) in one hour

Power
Name	Description
Watt (W)	Main scientific unit of power – defined as 1 joule per second
Milliwatt (mW)	Equal to 1000th of a watt (10-3)
Kilowatt (kW)	Equal to 1000 (103) watts
Megawatt (MW)	Equal to 1 million (106) watts
Gigawatt (GW)	Equal to 1 billion (109) watts

Table 1: Common units of energy and power 

Efficiency and capacity factor

When energy is converted from one thing to another, there is loss and this ratio (usually expressed as a percentage) is called the efficiency of the process:

percentage efficiency = (energy output/energy input) x100

If you’re trying to assess an energy generator’s productivity in practice, one useful measure is its capacity factor (CF):

Capacity factor = Actual energy output over time / maximum possible output

For example, the annual capacity factor of a 1 MW plant running constantly at a full rated capacity for one year would be:

One year = 365 days x 24 hours = 8760 hours in a year

So, the annual capacity factor = 8760 MWh / 8760 MWh = 1 or 100%

The Sun

The sun is the ultimate source of all the earths renewable energies. Solar radiation equates to  5.4 million EJ per year. Some of this bounces back into space but still means that there is about 3.8 million available to use. The sun should continue to supply power for another five billion years.

Two non-solar renewable energy sources are also shown in the above figure. One is the motion of the ocean tides, principally driven by the gravitational pull of the moon, the source of tidal energy. The other is geothermal energy from the Earth’s interior, which manifests itself in heat emerging from volcanoes and hot springs, and in heat from hot rocks.

Supply and demand

In the UK energy demand is categorised into four main sectors:

• domestic
• services (i.e. commercial and institutional)
• industry
• transport.

Above is an example of the number of conversions that energy has to go to to get to the consumer. The heat energy released when the coal is burned is the primary energy required for that use. The amount of electricity reaching the consumer, after conversion losses in the power station and transmission losses in the electricity grid, is the delivered energy (sometimes called ‘final’ energy). After some minor losses in the local wires, a quantity, the useful energy, emerges as light. Almost one third of UK primary energy is lost in the process of conversion and delivery.

Climate change and energy use

EU 2020 targets

The European Union’s ‘20:20:20’ Directive, passed in 2009, set a target for Europe to achieve by 2020 (European Union, 2011):

• a 20% reduction in carbon emissions

• a 20% contribution to gross final energy consumption from renewable sources

• a 20% improvement in the efficiency of energy use.

UK 2020 targets
The UK Government’s Action Plan (DECC, 2010c) concludes that delivering the 15% target is likely to involve renewables supplying approximately:

• 30% of electricity demand, including 2% from small-scale sources

• 12% of heat demand

• 10% of transport demand
.

EU member countries have agreed to produce National Renewable Energy Action Plans showing how they propose to contribute to these 20:20:20 targets. The UK’s 2020 target is to achieve a 15% contribution from renewables.