Elements of Renewable Energy
Introduction and what is renewable Energy
Introduction and what is renewable Energy
We begin by looking at the Sun, that enormous energy source that powers most of the renewables, either directly in the form of heat, light or electricity or indirectly in the form of bioenergy, hydropower, wind or wave power. We also look at two non-solar renewables, geothermal energy and tidal power. We then move on to look briefly at energy supply and demand, first on a world scale and then on the much smaller scale of the UK. Then we look at the problem of global climate change largely caused by emissions of planet warming greenhouse gases, mainly carbon dioxide from burning fossil fuels.
It's now widely accepted that the world needs to phase out fossil fuels and phase in low and zero carbon energy sources such as renewables. We follow this with an overview of the main solar and non-solar based renewals. We then look at current European and UK targets for increasing the share of renewables in the energy mix by 2020 to 2030. We discuss the UK government's Electricity Market Reform measures and we look at the UK Climate Change Committee's estimates of the contributions renewables could make to UK energy needs by 2030. We conclude this week with a case study in the form of three short video segments of Scotland's ambitious aim to produce 100% of its electricity from renewable sources by 2020.
What is 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.
In practice, few energy sources come close to these ideals, but renewable energy sources are generally more sustainable than fossil or nuclear fuels: they are essentially inexhaustible, and their use usually involves fewer health hazards and much lower emissions of greenhouse gases and other pollutants.
Solar power, both in the form of direct solar radiation and in indirect forms such as bioenergy, water or wind power, was the energy source upon which early human societies were based. When our ancestors first used fire fuelled by burning wood, they were harnessing the power of solar driven photosynthesis. Societies went on to develop ways of harnessing the movements of water and wind, both caused by solar heating of the oceans and atmosphere, to grind corn, irrigate crops and propel ships. As civilizations became more sophisticated, architects began to design buildings to take advantage of the Sun’s energy by enhancing their natural use of its heat and light, so reducing the need for artificial sources of warmth and illumination.
Technologies for harnessing the power of Sun, firewood, water and wind continued to improve right up to the early years of the industrial revolution, but by then the advantages of coal, the first of the fossil fuels to be exploited on a large scale, had become apparent. These highly concentrated energy sources soon displaced wood, wind and water in the homes, industries and transport systems of the industrial nations. Today the fossil fuel trio of coal, oil and natural gas provides around 80% of the world’s energy. Concerns about the adverse environmental and social consequences of these fuels have been voiced intermittently for several centuries, but it was not until the 1970s that humanity began to take more seriously the possibility that their continued use could be adversely affecting the planet’s natural ecosystems and global climate.
The development of nuclear energy following the Second World War raised hopes of a cheap, plentiful and clean alternative to fossil fuels. But nuclear power’s contribution to electricity supply has in some countries stalled in recent years, due to concerns about safety, cost, waste disposal and weapons proliferation. In other countries nuclear power supplies continue to expand.
These concerns have been a major catalyst of renewed interest in the renewable energy sources in recent decades.
Direct solar energy Solar radiation can be converted into useful energy directly, using various technologies. Absorbed in solar ‘collectors’, it can provide hot water for washing or space heating. Buildings can also be designed with ‘passive solar’ features that enhance the Sun’s contribution to their space heating and lighting requirements. The Sun’s rays can also be concentrated by mirrors to provide high-temperature steam for generating electricity. 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. Bioenergy, powered by solar-powered photosynthesis in plants, is an indirect manifestation of solar energy. Solar radiation warms the oceans, adding water vapour to the air. This condenses as rain to feed rivers, into which dams and turbines can be located to extract hydropower from the flowing water.
Sunlight heats the tropics to a greater degree than the polar regions, resulting in massive heat flows towards the poles, carried by currents in the oceans and the atmosphere. The energy in such currents can be harnessed by wind turbines. Where winds blow over long stretches of ocean they create waves, and a variety of wave power devices are being developed to extract their energy.
Energy definitions , Efficiency and Capacity factor
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. The concept of energy reveals the common features in processes as diverse as burning fuels, propelling machines and charging batteries. These and other processes can be described in terms of diverse forms of energy, including:
thermal energy (heat)
chemical energy (in fuels or batteries)
kinetic energy (in moving substances)
and various other forms.
In everyday language, the word ‘power’ is often used as a synonym for ‘energy’, but this is not strictly correct, as power is the rate at which energy is converted from one form to another or transmitted from one place to another. The scientific unit of power is the watt.
The host of different units used to describe energy and power can seem a bit confusing, so if you’re not familiar with them, or need a reminder, then have a look at the related document in this section where we have listed some of the most frequently used units.
Efficiency and capacity factor
When energy is converted from one to another, what comes out is never as much as what goes in.
The ratio (usually expressed as a percentage) is called the efficiency of the process:
percentage efficiency = (energy output/energy input) x100
Some types of energy conversion can be really efficient, for example up to 90% in a water turbine; others very low – around 10–20% in a typical car engine.
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
The period of time can be a year, a month, a week or an hour. The units for the output quantities can be kWh, MWh, GWh, etc., and the capacity factor can be expressed as either a fraction or a percentage. The time period to which the capacity factor relates (a year, a month, a week etc.) is usually stated.
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: principal source of renewable energies
The sun is the ultimate source of virtually all the Earth’s renewable energies. It releases huge amounts of energy as solar radiation, just a small part of which is intercepted by the Earth.
Looking at the figure below which summarises the origins and magnitudes of the Earth’s renewable energy sources, it is clear that the principal source is solar radiation which totals some 5.4 million EJ per year.
The sun radiates huge quantities of energy into the surrounding space, and the tiny fraction intercepted by the earth’s atmosphere 150 million kilometers (km) away is nonetheless around 5.4 million EJ per year. About one third of this is radiated back to space, but this still leaves around 3.8 million EJ per year available for use on Earth. This is about 8,000 times humanity’s present rate of use of fossil and nuclear fuels.
The Sun’s radiation is due to its surface temperature of 6,000 degrees Celsius (°C), maintained by continuous nuclear fusion reactions between hydrogen atoms within its interior. This is an enormous but relatively slow process, and 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 isgeothermal energy from the Earth’s interior, which manifests itself in heat emerging from volcanoes and hot springs, and in heat from hot rocks.