CAMBRIDGE, Mass.--Imagine if your country had an unlimited budget but a limited amount of land: what renewable energy has the most potential?
Rutgers University professor Clinton Andrews and colleagues ran the numbers on this thought experiment and came up with some surprises. They identified clear limits on some technologies, notably biofuels, but concluded that the bigger challenges to renewable energy and land relate to siting energy facilities, particularly transmission lines.
Andrews presented an early version of the paper at the Lincoln Institute of Land Policy conference here on Monday. The goal of this analysis and others like it is to size up the land requirements for different renewable-energy sources which, in many cases, require more land than fossil fuels and nuclear power.
As the U.S. and other countries seek to ramp up renewable-energy production,is becoming a more contentious issue. Already plans to build large-scale solar plants and wind farms in the U.S. have been opposed for aesthetic and environmental reasons. Even for distributed energy sources, such as rooftop panels, permitting and siting issues stand to loom large because upgrades to the electricity grid are needed, the study found.
"It's not so much the land that we need for producing the energy. It's how we move to where we want to use it," Andrews said during his presentation on Monday.
These land use issues are directly linked to public support for renewable energy and government policy. Deciding how and where to put renewable energy will challenge regulators, particularly on the question of federal authority of land use for energy production, Andrews said. The Telecommunications Act of 1996 brought up questions of federal versus state authority on infrastructure but energy promises to be more complicated politically.
"I think the siting challenges, especially for transmission, are even more daunting (than the Telecommunications Act of 1996). Those are hard and nobody wants to see them," he said.
Geothermal and concentrating solar
To better understand land limits, the paper's authors calculated the amount of land that would be required to generate all of the world's current energy demand with one type of energy source and how much land would be required to meet 10 percent. Using those criteria and assuming slow growth in energy demand, the authors placed all energy sources into three categories.
In the small footprint category is nuclear, geothermal, coal, solar thermal technology, and natural gas. Although only suitable for certain places, geothermal is a good candidate for expansion because geothermal plants have about the same footprint as oil and gas drilling, since they all use much of the same technology, Andrews said.
Large-scale concentrating solar power systems also have a relatively small land footprint. Concentrating solar thermal technology, which only works in desert areas, uses mirrors to generate heat and make steam which turns an electricity turbine.
By contrast, rooftop solar thermal systems for domestic hot water, which operate at lower temperatures, require more area per energy than the large-scale solar thermal systems.
A high-profile illustration of the significant transmission needs of large-scale solar thermal plans is the Desertec project to build a string of concentrating solar power plants in North Africa and ship the electricity to Europe and elsewhere. The Desertec Foundation proposes building high-voltage direct-current transmission lines to minimize electricity losses. These cables can be buried underground, although that adds to the cost.
In the second category of the land use study is solar photovoltaics and wind, which have land use requirements in the same range as petroleum and hydropower.
"Land-rich" North America can rely more on wind power than Europe, which has less available land, although conflicts over siting is one reason that offshore wind is being pushed aggressively in Europe, according to the study. One advantage of wind is that the land can be used for multiple purposes, such as farming.
Household solar photovoltaic panels (PV) can make a contribution to energy supply but the authors conclude that, based on the area required, solar, like wind, is more likely to be deployed in remote areas. Solar PV panels are unlikely to make U.S. homes self-sufficient because Americans use much more energy in their homes compared to other countries, notably Japan, the study found.
"It's hard to be self-sufficient at the current levels of consumption we're doing," he said.
Land energy hogs
The most land-intensive of all renewable energy sources is biofuels, particularly biodiesel, according to the analysis that Andrews presented.
Meeting 100 percent of the world's energy needs with ethanol from sugar cane would require nearly three times the amount of land now used for farming.
Ethanol from corn or the cellulose in plants requires more land than ethanol from sugar cane, as is burning biomass to make electricity, according to the study. Biodiesel from soy is the most land intensive by far.
"All of those are just beyond the pale. We would have to double the amount of the Earth's land area that we currently devote to human purposes to get energy from any bio sources so it's just not going to happen," he said. "For 10 percent, maybe a niche market, but don't count on them for broad solutions."
Although the study looks at which renewable source could supply all the world's energy, the authors argue that each one will enter the global energy supply based on each country's available resources and that efficiency should be improved.
Also important are technologies that lower the cost of power generation from renewables and upgrades to the electricity grid so that it can better control and store energy from renewable sources.