7.2 Land use
At some stage, all forms of biomass require a surface area of land or water for plant growth. Using this land for non-biological forms of renewable energy rather than for biomass might do more to mitigate the impacts of CO2 (Smith et al., 2000). How do the various land areas compare?
Assuming a bio-fuelled power plant with an annual electrical output of 10 GWh (the equivalent of a small 1.5 MW thermal power station running with a 75% capacity factor), with reasonable yields and conversion efficiencies the area of energy crops required to fuel this power plant would be in the range 600–900 ha (6–9 km2).
A small wind farm might need approximately 100 ha, (including necessary separation distances).
However, PV and wind systems are more likely to be complementary than to be competing for the same land, and as you saw in Week 3, PV arrays could be deployed to considerable advantage in semi-arid and desert areas with high solar input, or on rooftops in urban areas.
Nevertheless, there have been widespread concerns about the use of land for biofuels rather than for food, and about possible reductions in biological diversity through conversion of existing vegetation to fuel crops.
Activity 3 Bioenergy systems
Are there any bioenergy systems you can think of that could allay the above concerns?
Some bioenergy systems, such as short rotation forestry or coppice, can increase biodiversity compared to conventional agriculture. Among more experimental technologies, microalgæ don’t take up agricultural land and may be useful in cleaning up both nutrient residues and carbon dioxide, while secondary biofuels, using agricultural residues that would otherwise be burned or ploughed back in, offer another low impact option. More research is needed into these technologies.
Bioenergy’s effects on soil also need to be considered. Soils can contain large amounts of organic matter or humus formed from the remains of crops that are hard to break down, and act as a store of large amounts of carbon over long periods. However, cultivating the soil exposes this organic material to the air, and it then begins to break down more rapidly, releasing carbon dioxide.