Low carbon energy and land use

 

The low carbon energy sources to be considered are solar, wind, geothermal, hydro, bioenergy and nuclear; with the exception of nuclear these are all normally regarded as renewable energy (RE) sources. Comparisons between these energy sources may be made on issues such as greenhouse gas emissions per unit of energy produced, cost and land use, and it is with land use that we will be primarily concerned.

Merrill (2021) writes on the land needed for a zero-carbon economy in the U.S. and compares the areas needed to supply 100W for a year by various means. Hydro power needs 296m²; wind with spaced turbines, 37 m²; solar 14 m²; coal, 0.8 m²; nuclear 0.3 m²; and natural gas, 0.1 m². He also provides figures for current land use in the U.S. for its energy sources. Of a total of 81 million acres used for energy production, 51.5 are used to produce biofuels, 8.7 for hydropower, 6.7 for wind farms (the total area, of which the direct footprint is 0.07), 4.8 for power lines, 4.4 for natural gas, 3.5 for oil and petroleum products, 0.6 for coal, 0.5 for solar, 0.23 for nuclear, and 0.15 for other power plants.

Merrill notes that “Wind farms, solar installations and other forms of clean power tend to take up more space on a per-watt basis than their fossil-fuel-burning brethren” and cites estimates claiming that “the U.S. would need up to four additional South Dakotas” to supply its needs for clean power by 2050. One of Merrill’s sources is a paper by van Zalk and Behrens (2018): these authors discuss renewable and non-renewable power generation in the U.S. in terms of land use and power density. Their metric for comparison is “the power density of electricity production, that is, the electrical power produced per horizontal m2 of surface area”. They consider natural gas, nuclear, oil and coal, and solar, geothermal, wind, hydro and biomass, and find that non-renewable power densities are “three orders of magnitude larger than renewable densities.” Of the renewable sources considered, solar provided the highest power density, and natural gas gave the highest power density among non-renewables. In making power density comparisons, issues such as the intermittent or variable nature of generation from some renewable sources – the capacity factor – have to be taken into account, and so power density is computed on the basis of “the average electrical power actually transmitted to the grid over some time period (usually a year)”.

Median power density figures in W/m², rounded where appropriate, are given as follows: natural gas, 482; nuclear, 240; oil, 194; coal, 135; solar, 6.63; geothermal, 2.24; wind, 1.84; hydro 0.14 and biomass 0.08. Some caveats apply, particularly to geothermal and hydro, where the median figures are derived from a very wide range of individual values; and the authors also note that solar is the only form of RE in which power density is increasing significantly year on year.

Merrill also cites the study Net Zero America (Princeton, 2021) which “aims to inform and ground political, business, and societal conversations regarding what it would take for the U.S. to achieve an economy-wide target of net-zero emissions of greenhouse gases by 2050.” The Final Report Summary refers to “five different technologically and economically plausible energy-system pathways for the U.S. to reach net-zero emissions by 2050”, each of which “results in a net increase in energy-sector employment and delivers significant reductions in air pollution, leading to public health benefits that begin immediately in the first decade of the transition” and concludes that “a successful net-zero transition could be accomplished with annual spending on energy that is comparable or lower as a percentage of GDP to what the nation spends annually on energy today.” The five pathways or scenarios highlight different combinations and degrees of end-use electrification in transport & buildings, wind and solar electricity generation, and biomass utilization for energy. All the pathways rely on large-scale capture and utilization or storage of CO2; one pathway envisages roughly equal contributions from fossil, nuclear, and renewable sources; in another RE accounts for the majority of primary energy in 2050 (60-68%) while in a third it supplies 100%. Nuclear power is retained at present levels in some scenarios, and is increased or eliminated by 2050 in others.

The land use predicted by the Princeton study in the five scenarios varies between a minimum of 0.25 million km² and a maximum of 1.1 million km². Wind farms predominate in these figures, and so it should be noted that their total area is accounted for, an area far greater than that occupied by equipment, since the wind turbines are widely spaced, allowing much of the ground to be used for other purposes. Taking the contiguous land area of the US as about 7.66 million km², the maximum land use figure represents between 14 and 15% of the total.

In a global perspective the comparative land use figures for solar and for wind will vary widely depending on the location of the renewable resources; a solar installation in a sunny area will need less land for a given output than one in a less sunny place, and similarly a wind farm’s land requirement will depend on the local wind speeds. The way land is used will also affect the comparison, particularly in the case of wind farms. While at present the total land area occupied by RE facilities may not seem significant, this could change in future, and the potential land requirements of solar energy in three widely separated regions are addressed by van de Ven et al. (2021). They also consider the associated emissions due to land use change, and make comparisons with bioenergy: since “the energy density of solar energy is a magnitude higher than that of bioenergy … land requirements for reaching certain levels of electricity penetration with solar energy are about a magnitude lower than land requirements to meet those same levels with bioenergy”. The focus here will be on land area for solar. The authors have chosen three geographical areas for their study: the EU, India, and Japan and South Korea taken together. These areas are similar in that high solar energy use seems likely in future, producing land competition, but they differ in significant ways. For example the difference in irradiation between the zones means that in Europe about three times as much land area is needed for a given solar output as in India, and about twice as much as in Japan/ South Korea.

Predictions are made for the total land use in the three regions, taking into account probable increases in solar PV efficiency, and based on three different proportions of solar power in the total energy mix; 26%, 53%, and 79%. For the highest of these figures, the proportion of total land area occupied by solar facilities in each region by 2050 is estimated as 2.1–2.8% for the EU, 1.0–1.4% for India, and 4–5.2% for Japan and South Korea. These land areas are on the same order as total urban areas, and significant in comparison with areas used for crops.

While solar energy infrastructure “currently occupies a negligible amount of land globally” this changes with a “high share of solar energy in the future electricity mix.” Solar energy expansion is likely to replace land used for crops and commercial forest, which would “incentivise the use of currently unused arable land in other regions … indirectly leading to the loss of natural land cover.” A specific prediction is that “for every 100 hectares of solarland in the EU … depending on the solar penetration level, 31 to 43 hectares of unmanaged forest may be cleared throughout all the world.”

The authors point out that their study does not include additional land use required, for example, by the new transmission power lines required by utility scale solar energy, and they call for siting policies which exclude high yield cropland, maximise the use of urban areas, and encourage land management to minimise adverse impacts. Since they concentrate on solar power, this is the only point of comparison with the Princeton report on Net Zero America, in which the scenario with the greatest use of solar RE estimates that it will use about 0.8% of the contiguous land area, about the size of West Virginia.

References 

 

Merrill, D., 2021, The U.S. Will Need a Lot of Land for a Zero-Carbon Economy, Bloomberg Green, online, accessed 19 Jan 2022,

https://www.bloomberg.com/graphics/2021-energy-land-use-economy/

Princeton, 2021, Net Zero America, Princeton University, online, accessed 17 Jan 2022

https://netzeroamerica.princeton.edu/the-report

van de Ven. DJ., et al., 2021, The potential land requirements and related land use change emissions of solar energy, Scientific reports, 2021, online, accessed 17 Jan 2022

https://www.nature.com/articles/s41598-021-82042-5

van Zalk, J., and Behrens, P., 2018, The spatial extent of renewable and non-renewable power generation: A review and meta-analysis of power densities and their application in the U.S., Energy Policy, online, accessed 19 Jan 2022

https://www.sciencedirect.com/science/article/pii/S0301421518305512