Types of renewable energy
Important qualities of any energy source, some particularly relevant to renewables, are:
- Despatchability – can it be turned on (and off) when needed?
- Despatchability matters because supply of electricity has to be exactly matched to demand at every instant, so sources of energy which can be turned on and off – or up and down – at will are essential to avoiding power cuts.
- Capacity Factor (or load factor) – what proportion of its maximum output it produces on average.
- Capacity factor is important because when a generator, such as wind or solar, is producing less than its maximum output, then demand for energy has to be met from other sources — which are generally fossil fuels or biomass, but when the wind/solar/etc generator is producing its maximum then the backup source has to be able to reduce its own output to match.
- Energy Density – how much land is required to produce a given amount of energy.
- Energy density tells us what proportion of our land is needed to meet our energy needs from a particular source. For example Britain simply does not have enough land to grow enough biomass to supply all our energy needs even if we wanted to.
- Deployability – how close can it be built to where its energy is needed?
- Energy Return On Energy Invested (EROEI or EROI) – how much energy it produces over its lifetime compared to the energy required to make and install it.
- The overall EROEI of all our energy sources determines how much energy our civilisation has to spare for activities like producing food, housing, transport, education, industry etc.
Main article on Solar energy
The sun's energy can be exploited through passive solar buildings, for water heating or even cooking. Most usefully it can be used to generate electricity, via various technologies. Photovoltaic (PV) panels are the most familiar and widespread, but there are also concentrating solar power (CSP) plants and solar wind energy which uses convection currents in heated air to drive wind turbines in a large chimney structure.
There is also experimental work on using solar energy to make chemical fuels.
Main article on Wind energy
Wind is a form of indirect solar energy, produced by convection currents in warmed air. Wind has been used for millenia to produce mechanical power for milling corn and pumping water, but nowadays it is widely used to generate electricity, mostly via the familiar wind turbine.
There are other designs of wind generator such as vertical axis machines, and also some work being done in using large tethered kites.
Main article on Hydropower
Hydropower is another form of indirect solar energy, produced by the sun's radiation heating water (on land and in the oceans) making it evaporate and rise into the atmosphere where it condenses into clouds and falls as rain (or snow). Some of it falls in hills and mountains where it has the potential to release energy as it descends to lower altitudes (and eventually back to the sea).
The main ways of exploiting this energy are with water-wheels or turbines in flowing rivers, or by building dams creating artificial lakes or reservoirs which store large amounts of water which can drive turbines as it is released.
Main article on Wave energy
Wave energy is derived from the Sun's energy, via wind which acts on seas and other bodies of water to create waves. Like wind, wave energy is intermittent.
There are a variety of machines which have been designed to harness wave energy.
Main article on Tidal energy
Unlike other energy sources tidal energy harnesses the kinetic energy of the Earth's rotation in the gravitational field of the Moon and the Sun. Tidal forces acting on Earth slow its rotation, very slowly increasing the length of our day. Harnessing tidal energy adds minutely to this slowing.
One way of harnessing tidal energy is to place turbines - like wind turbines - underwater, in the path of strong tidal currents. Another is to create a dam or lagoon to hold a body of water which can be allowed to flow to or from the sea when the tide has created a difference in level between the lagoon and the sea.
Like wind and solar, tidal energy is variable, but its fluctuations follow a precisely known pattern, increasing and decreasing twice a day. There is also a 14 day variation in how big the maximum is because of variation in the height between Spring and Neap tides. With suitable placing of tidal energy installations it should be possible to more-or-less even out the daily fluctuations, but not the Spring/Neap differences.
Main article on Geothermal energy
Geothermal energy comes from the heat of the radioactive decay of isotopes in the Earth's crust, and heat from the Earth's molten core, which itself is partly due to the friction of the Moon's and Sun's tidal effect's on the Earth, but mostly the heat stored in the the core from when the Earth itself was red-hot as it had just formed.
David MacKay discusses the potential for either sustainable or short term geothermal energy in Sustainable Energy Without The Hot Air.
Main article on Biomass
The burning of wood, crop wastes and animal dung are the most widespread and ancient forms uses of this form of renewable energy. Their use causes air pollution which damages the health of billions of people, particularly women and children where the use of indoor stoves for cooking by the world's poorest people is widespread. Particulates from smoke can travel long distances and darken ice increasing the melting of glaciers and even ice caps. Demand for fuel drives deforestation and soil erosion. And burning biomass releases CO2 which could be sequestered in trees, grasses and soils if other fuels were available.
In the developed world the widespread use of biomass in power stations drives the destruction of old growth forests, in Europe and America, driven by European Union rules which classify it as carbon neutral. In Eastern Europe demand for wood for fuel may be driving illegal logging and the murder of forest rangers.
Marine - Tide and Wave
Will Tidal and Wave Energy Ever Live Up to Their Potential? SOPHIA SCHWEITZER; Yale Environment 360; 15 Oct 2015
- As solar and wind power grow, another renewable energy source with vast potential — the power of tides and waves — continues to lag far behind. But progress is now being made as governments and the private sector step up efforts to bring marine energy into the mainstream.