Hot rocks, carbon capture and tidal turbines: alternate power sources for a greener future
Not just solar and wind, there are other green technologies competing for a slice of the renewable energy pie.
Renewable energy sources accounted for just under 13.5 per cent of Australia's electricity generationi and almost 23 per cent of the world's electricity generation in 2014. Of that global renewable energy pool, wind, solar and the more traditional hydroelectricity power plants contributed more than 93 per centii.
However experts say it is a more diverse group of technologies that will power cities of the futureiii.
By 2030, Bloomberg expects renewables to account for 37 per cent of global electricity generation in a mix of technologies that will allow power companies to balance capacity and "hedge against volatility in gas and coal prices, security threats, natural disasters and changes in environmental regulation"iv.
So what are the new sources of renewable energy to look out for? The current forerunners include a new spin on geothermal power, reducing the carbon output in biofuel use and harnessing the energy of the sea.
Geothermal hot rocks
Geothermal power plants use heat stored beneath the Earth's surface and have been used to generate electricity commercially since 1913v. The US generates the greatest amount of geothermal powerii, while Iceland is best known for using geothermal energy to heat about 90 per cent of all homesvi.
Energy is traditionally extracted from seismically active "hot spots", where temperatures are highest and water naturally flows through the area, but emerging Enhanced Geothermal System (EGS) technology could allow energy to be extracted from a new, more abundant source: hot, dry rocks deeper undergroundvii.
EGS technology has been tested in Koreaviii, Franceix, the UK, US and Australia. In the US, it could allow at least 20 times more geothermal energy to be extracted and provide "100 Gigawatt (GW) of new capacity by 2050"x.
In Australia, Geodynamics began testing EGS technology in South Australia's Cooper Basin in 2010xi, but abandoned an EGS project in the Hunter Valley this year amid tough market conditionsxii.
According to global renewable energy policy body REN21, growth in geothermal energy generation has been "slow but steady", with cost and project development risk constraining investmentii.
For millennia now, people have been burning biomass - grasses, wood, manure, and organic material - for energy, but the process can be environmentally damaging. Burning releases carbon dioxide into the atmosphere and energy crops take time and resources to replace, making them a less sustainable optionxiii.
New bioenergy with carbon capture and sequestration (BECCS) technology could improve the environment by removing carbon from the atmospherexiv.
BECCS does this in two ways: crops naturally consume carbon dioxide until they are harvested for fuel, while power plants capture carbon dioxide from the burning process and store it underground, or in the oceanxv,xiv.
There were 16 BECCS projects worldwide as of June 2012xiv. Researchers say the technology could cut carbon emissions by 145 per cent in western USxv, and remove 65 million tonnes of carbon dioxide from Australia in 2050xvi.
Gasification technology, used by groups like Californian start-up All Power Labs, takes the concept of carbon capture and storage a step further: carbon is "stored" in a nutrient-rich byproduct that can also be used as fertiliser. All Power Labs' $27,000 machines generate cheap, clean energy and have been popular in countries like Ecuador, Haiti, Thailand and Nicaraguaxvii.
Meanwhile, Finnish power company Vaskiluodon Voima Oy has operated a large-scale biomass gasification plant alongside its pre-existing coal-fired power plant since 2013xviii. The project was granted €10 million from the Finnish government and expected to cut Vaskiluodon Voima's coal use by up to 40 per centxix.
Ocean energy refers to harvesting energy from changes in ocean temperatures, salt content, waves and tides. The technology is still in its infancy, but could yield up to 80,000TWh of electricity, meeting more than four times the world's electricity demandxx.
There are only about 530MW of ocean energy generators in operation worldwide. Most are tidal barrages, of which the largest are a 254MW facility in South Korea and a 240MW station in Francexxi.
Tidal barrages operate like dams, and are built across narrow passages with at least a 16-foot (approximately 4.9m) difference between high and low tides. Emerging technologies like tidal fences and turbines and offshore ocean float systems could allow more energy to be harvested elsewherexxii.
The CETO unit in Australia's Carnegie Wave Energy project is a float system that can generate 20MW of electricity - enough to power 1400 households - from three hectares of seabed . A unit was installed on Perth's Garden Island last November and is expected to begin producing power this yearxxiv.
According to the CSIRO, wave energy could supply up to 10 percent of Australia's electricity needs by 2050xxv.