Not All Renewables Are Created Equal

This guest post is by Jonathan Trinastic, a physics graduate student interested in renewable energy and the pursuit of safe, sustainable energy policy. He maintains the blog Goodnight Earth, which reviews current research on these topics, and can be followed on Twitter @jptrinastic.

"Even

After

All this time

The Sun never says to the Earth,

‘You owe me.'

Look

What happens

With a love like that,

It lights the whole sky."

- Hafez

The world is warming at rates not seen on this planet for 11,000 years¹, and the primary reason is anthropogenic fossil fuel emissions. Even ignoring the impacts of climate change, peak oil production^2,3 is on the horizon, highlighting the need to transition to a sustainable economy focusing on carbon-neutral, renewable energy resources⁴. But what physically makes fossil fuels a nonrenewable resource? What renewable resources are available to us? And are they all created equally?

The above questions are important because their answers will guide how we rearrange our energy portfolio to transition away from fossil fuels. Popular culture often clumps energy sources into two groups - renewable and nonrenewable - but we'll see that this is overly simplistic. Renewability is a matter of degree, not a category. In this post, I'll discuss how almost all our energy sources originate from light rays emanating from the Sun. How this light transforms into various forms of energy dictates 1) the renewability timescale and 2) the accessibility of each resource, with implications for the construction of new energy policy.

Ode to the Sun

Our journey begins 150 million kilometers way with a magnificent sphere of plasma. Without the Sun, the Earth would be a static, listless place. No light, no wind, no life, no motion at all. How does the Sun provide all these dynamic elements on Earth? Nuclear fusion. Deep in the core of the Sun, hydrogen quadruplets carry out their eternal dance, fusing into helium and releasing tremendous amounts of energy in the form of high-energy light known as gamma rays. These rays lose energy as they escape to the Sun's surface, where they have an initial power density of 6x10⁷ Watts per square meter (W/m²) as they radiate into space. If this dense light were captured in an area of less than a square kilometer (km), we could provide enough power for the entire world! However, the Earth receives an infinitesimal portion because the planet occupies only a small fraction of space through which the light rays travel, as seen below.

By the time the outgoing light has traveled the Sun-Earth distance, the initial power density has spread across the area of a sphere with a radius of D=150 million km (yellow dotted line above). This means that the solar irradiance only has a power density of about 1300 W/m² at the top of Earth's atmosphere. Some of this radiation is reflected by the atmosphere and clouds, so the light rays finally absorbed at the Earth's surface have a power density around 1000 W/m².

Renewability Timescale

How light energy transforms into the final form that we use determines a resource's renewability timescale. One of the major benefits of solar energy is that it requires no such transformation, making solar power the ultimate renewable resource limited only by the lifetime of the Sun (which is about 6 billion years! We can worry about that later...).

Energy sources that require conversion from light to chemical energy lead to longer renewable timescales. This is because chemical energy conversion arises from biological processes catalyzed by photosynthesis with intrinsically longer timescales. To form fossil fuels, organic matter must anaerobically decompose and then be highly pressurized under layers of sediment, which occurs on geological timescales of millions of years!

Wind and hydroelectric resources have near-instantaneous timescales because they rely on natural resource cycles that occur due to light energy warming the Earth's surface. Wind results from heat transfer from the ground to the cooler air above it that creates turbulent motion. In the case of hydroelectric energy, light energy evaporates water that later rains at higher elevations in freshwater rivers. As the water falls naturally or through artificial damming, the gravitational potential energy can be converted to electrical energy through turbines. Both heat and water cycles occur continuously as long as light rays are reaching Earth, although climate change could change where winds are generated or precipitation falls. In contrast, climate change should not significantly alter solar energy distribution.

Accessibility

How the energy source finally localizes in our environment significantly differentiates renewable resources. The Earth absorbs light energy continually over its entire surface, giving solar energy great geographical flexibility and scalability. However, this source can only be used for half the day and thus relies on advanced energy storage mechanisms.

Wind power requires average wind speeds of at least 3-4 meters per second because power dramatically drops off as speed decreases (by the speed cubed) ⁵. Though this slightly limits possible construction sites, wind power still has robust geographical flexibility compared to other sources. However, energy storage is also needed to handle intermittent wind during peak usage.

Hydroelectric energy extraction is limited to regions with flowing and falling water, which, in conjunction with its many environmental costs⁶, severely limits scalability such that it will likely always be relegated as a supplementary resource. Biomass has also become a controversial renewable because any land used to produce biofuel could also be used for food supply⁷. This will become an increasingly divisive political issue as the global population strains Earth's carrying capacity.

Finally, fossil fuels depend on access to underground stores. Dramatically increased consumption since the industrial revolution has drained all the easy-to-reach fields, so more money and technology is required to reach unconventional sources or those in extreme climates, such as the Alaskan and Siberian fields. As more money is spent to extract from these sources, the price of oil will increase, making other energy sources more competitive.

Renewable Choices

We can see that not all renewables are created equally. The key properties we need to consider are 1) the rate at which we consume a given source compared to its renewability timescale and 2) how accessibility limits a source's scalability. The former depends on how long it takes for the initial light energy to transform to the final form used, and the latter depends on how this final energy source is localized in the environment.

In general nomenclature, renewable resources are those with timescales short enough that we can replenish their stocks in reasonable amounts of time. This explains the nonrenewability of fossil fuels - they are technically renewable over the course of millions of years, as new organic material anaerobically decomposes and traps carbon far beneath the Earth's surface. But this timescale is only sustainable if we consumed energy at a rate equal to this renewal, which could never happen! Their accessibility was also an extreme advantage in the last two centuries, but this benefit is waning as sources diminish.

Certain renewables have advantages only in timescale or accessibility, but not both. Biomass is often touted as a potential renewable source, but its competition with land use for food production will likely limit its large-scale accessibility. Similarly, hydroelectric production has a short renewability timescale but its accessibility is limited by its use in rivers and streams. Climate change will lead to drought and change in river volumes, factors that need to be considered before using significant capital to construct hydroelectric facilities.

Considering the balance of renewability and accessibility, solar and wind energy sources appear most promising. Their immediate renewability combined with geographic flexibility make them prime candidates to replace coal-fired power plants that supply energy to most of our electrical grid. The current obstacles for implementation are 1) improving battery and supercapacitor performance so power from these sources can be consistently provided to the grid at peak hours, and 2) improving photovoltaic efficiency, which is currently limited to less than 30 percent in single-junction cells⁸. Wind energy is reaching parity with coal prices^9,10 and its share of total energy production should continue to grow.

All renewables will be an important component to a diversified energy portfolio, but understanding the physical origins of each provides information about consumption rates and scalability, limiting factors which we should keep in mind when considering them as replacements for our fossil fuel economy!

References

1) Marcott et al. "A reconstruction of regional and global temperature for the past 11,300 years". Science, 339(6), 2014.
2) U.S. Energy Information Administration. Short-Term Energy Outlook, 2014.
3) Bardi, U. "Peak oil: the four stages of a new idea." Energy, 34(3), 2009
4) United National Environment Programme, A Case for Climate Neutrality, 2011.
5) Saleh, MA. "Wind energy for rural and remote areas in the ESCWA region." UNESCWA Expert Group Meeting Conference.
6) Rosenberg, DM et al. "Environmental and social impacts of large scale hydroelectric development: who is listening?" Global Environmental Change, 5(2), 1995.
7) Ajanovic, A. "Biofuels versus food production: does biofuels production increase food prices?" Energy, 36(4), 2011.
8) NREL. "Best Research Cell Efficiencies", 2014. Accessed Jan 16, 2014.
9) Khare V, et al. "Status of solar wind renewable energy in India". Renewable and Sustainable Enegy Reviews, 27, 2013.
10) Maki, K. "Cost of wind vs. fossil fuels." Accessed Jan 16, 2014.

Photo credits

Wind turbine photo: by Jurgen on Flickr
Sun diagram: by Image Editor on Flickr
Sun-Earth diagram: by Jabberwok at en.wikipedia