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IX. Defining “Renewable Energy”

(originally published December 9, 2007)

I’ve been posting on the Electron Economy/Renewable Electron Economy for the past 9 months but have been relying on the Justice Potter Stewart definition (“knowing it when I see it”) of renewable energy. Most people tend to define renewable energy by listing certain natural resources: “Oh, its wind, solar, geothermal, wave, tides, etc.” Some hopefully more illuminating definitions do exist: a Google Web search reveals the following range:

I think it is possible to come up with a more rigorous, more specific definition that gets closer to the reality of renewable energy, even though there will always be an element of complexity and imprecision in the definition.

Interactive Definitions of Energy

Renewable energy is one of a group of terms that you might call interactive definitions of energy: energy as it matters to people, not the energy concepts that are usually taught in basic physics and chemistry classes.

Pure physicists have no use, for instance, for the concept of “primary energy”, which is useful in power engineering and energy economics. “Primary energy” means the type and amount of energy or fuel that needs to be input to produce a certain finished “secondary” energy product. Coal, natural gas, or uranium can function as the primary energy to produce electricity and crude oil or tar sands can function as the primary energy for finished petroleum fuels. Renewable fuels can function as primary energy too: incoming sun, wind or water falling can be seen as the primary energy for electricity or bio-energy. The efficiency of an energy conversion process is measured by how much of the incoming energy is converted to the useful output form of energy (secondary, tertiary, quaternary).

The “primary”-ness of primary energy has only to do with the fact that this is the form in which humans first find the energy resource. It is an interactive definition of energy; in pure physics the only primary energy is that of the Big Bang 13 billion years ago. In the practical world, humans are interested in energy as a resource not as simply an object of scientific study.

The assessment whether something can function as primary energy, an energy resource and furthermore what type of resource it is, renewable or non-renewable, is an interaction between the human variables and the natural or given variables. People need to have discovered or invented a technology to convert primary energy into a usable secondary form and the energy needs to in some way improve upon human beings natural capabilities.

To make a clear definition of naturally occurring energy types, it helps to differentiate a few concepts in interactive energy, the energy that matters for human use.

Energy Flux, Energy Stores and Energy Events

To arrive at a more precise definition of renewables, I’ve worked out three broad categories that differentiate energy in relationship to human use and timescale: energy flux, energy stores, and energy events.

While these concepts might seem abstract I’ll demonstrate below how they can be used to define renewable and non-renewable energy.

Characterizing Energy

Non-renewable Energy

As non-renewable energy is exhaustible, it makes sense that it functions as an energy store.

Renewable Energy

While the above are fairly easy to characterize the two below are a little more controversial and complicated:

Defining Renewable and Non-Renewable Energy

Using these categories, one can define non-renewable energy more easily than renewable energy.

“Non-renewable energy sources are energy stores with zero or a minute rate of replenishment relative to its depletion by human beings. Most non-renewable energy sources are converted to usable energy by thermal or nuclear reactions. Non-renewable energy sources have stored the natural energy flux of Earth’s biological and geological past or of the formation of elements in the early history of the Universe”

Renewable energy on the other hand, appears both as natural energy flux and as an energy store. It is interesting to note that the types of renewable energy that, in the course of the 20th century were most well integrated into the customary energy mix (hydroelectric, geothermal, and biomass) are both either energy stores or have traditionally been integrated with an energy store.

Here is a hopefully more rigorous if somewhat long definition of renewable energy that only resorts to using a list in the second sentence:

“Renewable energy sources are types of natural energy flux useful for human ends regularly occurring on or near Earth’s surface and, additionally, useful natural energy stores that are replenished by natural flux within the timeframe of conceivable human use. All known renewable energy sources originate in, or are close derivatives of, electromagnetic radiation of our Sun, the Earth’s and Moon’s gravitational fields and heat radiating from earth’s interior. Renewable energy sources are practically inexhaustible though some sources such as geothermal and ocean thermal energy conversion may become locally depleted by human use at a rate that exceeds replenishment by natural flux.”

I believe the first sentence is sufficient but the next sentences add a little more detail.

A Paradigm Shift in Power Engineering

Have we learned anything by drawing the distinction between energy flux and energy stores? One pattern that becomes very clear is that conventional energy system is heavily dependent and focused upon energy stores. The ability to time the release of energy from coal, natural gas, uranium, petroleum, dammed rivers or biomass has been a key support for how we manage our energy system and the electric power grid. We can call this the conventional energy paradigm, where energy is defined as a stockpile of fuel in tandem with the appropriate energy conversion devices.

If we are going to build a renewable electron economy, there needs instead to be a focus on how to capture and monitor the strongest or most readily available renewable energy flux and also how to supplement this with non-polluting energy stores where possible. The management of an electric grid with renewable energy flux means investment and innovation in three areas: energy capture devices, storage devices, and more sophisticated natural flux monitoring. The latter has been used already for demand forecasting (weather effects power and energy demand) but now it will also forecast supply, doubling or tripling its importance in the equation. This means more collaboration than ever between meteorology, geology and power engineering. It also means a paradigm shift in how the managers and planners of the electric system and grid discuss energy: changing from the relatively static world of naturally occurring energy stores to deal with the dynamic world of energy flux will take time, effort, and innovation.

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