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.