Where hot underground steam can be tapped and brought to the surface it may be used to generate electricity. Such geothermal sources have potential in certain parts of the world such as New Zealand, USA, Philippines and Italy. Global installed capacity was about 11.7 GWe at the end of 2012, including 2600 MWe in California, 1900 MWe in Philippines and 1200 MWe in Indonesia, and in 2012 geothermal produced at least 72 billion kWh worldwide. In Japan 500 MWe of capacity produces 0.3% of the country’s electricity. In New Zealand 420 MWe produces over 7% of the electricity, and Iceland gets most of its electricity from 200 MWe of geothermal plant, and also most of its district heating. Mexico has 958 MWe geothermal, Italy 843 MWe and Nevada 470 MWe. In Italy, ENEL got 8769 TWh from geothermal in 2012, in Iceland 4974 TWh. Lihir Gold mine in Papua New Guinea has 56 MWe installed, the last 20 MWe costing US$ 40 million – about the same as annual savings from the expanded plant. Geothermal electric output is expected to triple by 2030. The largest geothermal plant is The Geysers in California, producing about 1000 MWe, but diminishing. See also Geothermal Energy Association website.
There are also prospects in certain other areas for hot fractured rock geothermal, or hot dry rock geothermal – pumping water underground to regions of the Earth’s crust which are very hot or using hot brine from these regions. The heat – up to about 250°C – is due to high levels of radioactivity in the granites and because they are insulated at 4-5 km depth. They typically have 15-40 ppm uranium and/or thorium, but may be ten times this. The heat from radiogenic decay* is used to make steam for electricity generation. South Australia has some very prospective areas. The main problem with this technology is producing and maintaining the artificially-fractured rock as the heat exchanger. Only one such project is operational, the Geox 3 MWe plant at Landau, Germany, using hot water (160ºC) pumped up from 3.3 km down (and maybe should be classed as conventional geothermal). It cost EUR 20 million. A 50 MWe Australian plant is envisaged as having 9 deep wells – 4 down and 5 up.
Ground source heat pump systems or engineered geothermal systems also come into this category, though the temperatures are much lower. Generally the cost of construction and installation is prohibitive for the amount of energy extracted. The 1997 Geoscience Australia building in Canberra is heated and cooled thus, using a system of 210 pumps throughout the building which carry water through loops of pipe buried in 352 boreholes each 100 metres deep in the ground. Here the temperature is a steady 17°C, so that it is used as a heat sink or heat source at different times of the year. See 10-year report (pdf).
Beyond traditional direct uses for cooking and warmth, growing plant crops particularly wood to burn directly or to make fuels such as ethanol and biodiesel has a lot of support in several parts of the world, though mostly focused on transport fuel. More recently, wood pellets for electricity generation have been newsworthy. The main issues here are land and water resources. The land usually must either be removed from agriculture for food or fibre, or it means encroaching upon forests or natural ecosystems. Available fresh water for growing biofuel crops such as maize and sugarcane and for processing them may be another constraint.
Burning biomass for generating electricity has some appeal as a means of utilising solar energy for power. However, the logistics and overall energy balance usually defeat it, in that a lot of energy – mostly oil based – is required to harvest and move the crops to the power station. This means that the energy inputs to growing, fertilising and harvesting the crops then processing them can easily be greater than the energy value in the final fuel, and the greenhouse gas emissions can be similar to those from equivalent fossil fuels. Also other environmental impacts can be considerable. For long-term sustainability, the ash containing mineral nutrients needs to be returned to the land. In southeastern USA, 1.75 million tonnes of wood pellets were exported to Europe in 2012, and the figure is projected to grow to over 5 Mt in 2015. Most of this comes from low-value woodchips which formerly went into papermaking. Three 660 MWe units of Drax, Britain’s largest coal-fired power station, are being converted to burn wood, most of it imported (like the coal of higher heat value that it replaces). No CO2 emissions are attributed to the actual burning, on the basis that growing replacement wood balances out those emissions, albeit in a multi-decade time frame. Unlike coal, the wood needs to be stored under cover.
In Australia and Latin America sugar cane pulp is burned as a valuable energy source, but this (bagasse) is a by-product of the sugar and does not have to be transported. In the EU in 2010 over 11 million tonnes of wood pellets were used. In 2012 Europe imported 4.36 million tonnes of pellets and in 2015, 15 million tonnes import is projected, two thirds from North America (as in 2012). The pellets are made from sawmill residues preferably, but also forest residues and low-value timber. UK demand is expected to reach 11 million tonnes by 2015, equivalent to twice that amount of fresh wood.
In 2011 biomass and waste provided 422 TWh of electricity worldwide. By 2030 biomass-fuelled electricity production was projected to triple and provide 2% of world total, 4% in OECD Europe, as a result of government policies to promote renewables. However, such projections are increasingly challenged as the cost of biofuels in water use and pushing up food prices is increasingly questioned. In particular, the use of ethanol from corn and biodiesel from soybeans reduces food production and arguably increases world poverty. The cost in subsidies is also increasingly questioned: in the OECD US$ 13-15 billion is spent annually on biofules which provide only 3% of liquid transport fuel.
In 2008 about 100 million tonnes of grain (enough feed nearly 450 million people) was expected to be turned into fuel. This includes a legislated 40% of the US corn crop, aided by heavy subsidies. In 2012 the US corn crop amounted to about 360 million tonnes, so about 140 Mt would have been used for ethanol. Meanwhile basic food prices have risen sharply, leading the UN Food & Agriculture Organisation in mid-2012 to call for the USA to halt its biofuel production to prevent a food crisis.