What exactly do we mean by “zero emissions?”

Is it different than “clean energy?” What about “low carbon,” or “carbon free,” or “renewable?” Let’s take a closer look.

Life Cycle Analysis

Life Cycle Analysis (LCA) is a comprehensive way to measure the climate impact of an energy technology. It is the CO2-equivalent emissions per unit of power produced over the expected plant lifetime. CO2-equivalent includes the effect of all emissions that act as greenhouse gases. For example, methane is up to 100 times more potent than carbon dioxide (CO2) as a greenhouse gas over a 20-year time span; this is accounted for in the CO2-equivalent metric (also known as CO2-e).

Source: Schlomer S., et.al., 2014: Annex III: Technology-specific cost and performance parameters. In: Climate Change 2014: Mitigation of Climate Change. Contribution of Working Group III to the 5th Assessment Report of the IPCC.


LCA includes both direct and indirect emissions. Direct emissions are those that result from the immediate act of daily power production; for example, when wood or coal is burned in a power plant, carbon dioxide and other greenhouse gases are released. Indirect emissions are those that result from everything else related to the plant: mining, processing and transportation of fuel; mining, manufacturing and construction of the plant and its equipment, including maintenance; daily operations activities, and plant decommissioning.

Note that all energy technologies and sources have indirect emissions, which can vary greatly depending on amounts and types of materials and energy required to build and operate the facility, fuel type, and quantities of power produced over the plant’s lifetime. Some technologies do not have any direct emissions, either because they are fuel-less systems (solar, wind, geothermal, hydro, wave, etc.), or because they are non-carbon fueled systems (nuclear). For simplicity and clarity, we refer to these as “zero emission” technologies. Zero smokestack, zero emissions!

Technologies involving combustion of carbon fuels, whether fossil fuels (coal, oil and gas), or biomass, can have relatively large direct emissions of CO2-e. The chart above shows that carbon fuels are clearly in a different category than the zero emissions sources.

Hydropower is a noteworthy exception. While the hydro turbines produce no emissions in operation, recent studies have shown that the reservoirs can produce large quantities of methane from decomposition of organic matter. The error bar for hydro emissions is very large, as methane production depends greatly on location, extent of population and agricultural activity in the drainage basin, and other operational factors. Improved reservoir management may be able to reduce methane emissions. This is a relatively new field of inquiry; we have much to learn.

Buffering and System Effects

PV=photovoltaic; E-66 is the Enercon 1.5 MW wind turbine model studied; CSP=Concentrated Solar Power (Sahara site assumed); CCGT=Combined Cycle Gas Turbine; PWR=Pressurized Water Reactor.

There is a further twist on greenhouse gas emissions for intermittent and variable generation sources such as wind and solar–they often must be “buffered” to integrate them into the electrical grid. Buffering measures can include: fast-ramping backup capacity, such as gas turbines or hydro; energy storage, such as pumped hydro or batteries (not available at true grid-scale); long-distance transmission lines; power conditioning provided by synchronous condensers; over-capacity and/or curtailment of intermittent generation; reduced operation of baseload generators (inefficiency). As always, there is no such thing as a free lunch; there is an inevitable energy and emissions cost for these buffering measures.

Electrical grids are highly complex, and are continually changing and evolving. As we learn more, as technology develops, and as our energy portfolio progressively decarbonizes, the CO2-e impacts of grid integration should decrease. However, at present these impacts can be very significant. Some have argued that in worst cases, grid integration costs can eliminate most if not all climate benefits from otherwise zero emissions sources. Policymakers and power engineers should be cognizant of system effects, and strive to minimize them with regard to system-wide emissions. Otherwise, we risk investing resources and precious time, with less climate benefit than expected.

How significant are these buffering costs? A helpful metric for this is EROI, Energy Returned on Investment. Taking an example from the above chart, burning coal provides us with 30 times the energy that was required to extract, process and transport it—an EROI of 30. The higher the EROI, the better. Nuclear has a very high EROI, a measure of its extreme energy density, small material footprint, and long plant life. Wind has EROI of 16, but after accounting for buffering and system effects, the EROI drops to 3.9. Although “energy breakeven” is an EROI of 1, to support the basics of modern society, studies estimate that an EROI of 7 higher is required to be economical or “worth it.” Note that results from EROI studies can vary widely, due to differing assumptions and methodologies. That said, EROI is still a valid technique to help us understand energy relationships, and optimize system design and allocation of resources.

Renewable Energy

The term “renewable” unfortunately has little relevance to climate change impacts. Biomass is considered renewable, even though carbon combustion releases greenhouse gases. Solar and wind are considered renewable, even though, per unit of energy produced, they consume vast quantities of energy-intensive steel, concrete, aluminum and toxic rare earth metals. Nuclear is zero emissions, with an extremely small physical and material footprint, with supplies of uranium and thorium that are essentially inexhaustible, yet it is arbitrarily excluded from the “renewable” category. Hydropower is considered renewable, yet it is subject to fluctuations of drought, and can cause considerable methane emissions in some cases.

“Renewable” is essentially a brand. The term is not a reliable indicator of greenhouse gas emissions for any energy technology. Each must be evaluated on its own merits using life cycle analysis of emissions.

Clean Energy

“Clean energy” means different things to different people. The term is imprecise and confusing, and it’s worthwhile to discuss why that may be.

Nuclear power is considered by many to not be clean, despite the fact that it generates zero airborne emissions from daily operations, and all spent fuel is fully accounted for and safely stored. Permanent disposal of high-level nuclear waste in geological repositories is a political football, not a technical problem. Engineers wish to use most of the material as fuel for next-generation reactors, and are confident about interim storage above ground, yet much of the public remains uneasy and concerned about lack of a permanent solution.

Solar cells, wind turbines and batteries similarly have zero airborne direct emissions, but require large quantities of rare earths, cobalt and other materials which are very toxic and often mined in developing nations under hazardous environmental conditions. In the case of solar panels, there is currently scant provision for recycling and/or disposal of what will be vast amounts of electronic waste after their useful life of 25-40 years. Currently, a majority of electronic waste has unknown final disposition; much of it is believed to end up in developing countries, melted down by the poor with little or no protections, subjecting them to very toxic materials and creating an environmental hazard.

So again, what is “clean energy?” It’s apparent that both nuclear and solar/wind have life cycle environmental impacts to resolve. Since the climate impact of greenhouse gas emissions over the next several decades is our first-order, time-sensitive concern, it makes sense to prioritize deployment of zero emission technologies. Our top priority must be rapid elimination of greenhouse gas emissions. “Don’t let perfect be the enemy of good” is appropriate here.

Zero Emissions

To be clear, as we use it here, “zero emission” refers to the immediate process of power generation. All energy technologies have indirect emissions, some more than others. We can exercise some control over indirect emissions, and minimize them over time. Zero emission technologies are the tools we must work with to craft a global economy with abundant, climate-friendly energy to sustainably and ethically power a modern world and restore balance to our atmosphere and oceans.