Alternative energy is a term that gets bandied about a lot and is often applied to wind, solar, and other forms of renewable generation. But there are multiple other terms that also are applied and misapplied across forms of generation. Often, alternative is used interchangeably with sustainable, carbon neutral, green and distributed. But each of these terms means something different. Further, they are frequently used pejoratively as opposed to accurately.
This article is intended to provide clarity over the definitions and categorize most forms of energy generation used today by those definitions.
Transportation fuel is not included in this, just electrical generation. Similarly, lake- and ocean-side district heat exchange and building geothermal heat exchange are not included.
Alternative energy means a form of generation that isn’t part of the dominant paradigm at a given point in time. As of 2016, that typically excludes legacy forms of generation such as hydro and nuclear because they are mainstream forms of generation that have been in production for decades around the world.
By this definition, utility-scale wind energy and utility-scale and distributed solar photovoltaics are not alternative forms of energy because they are the dominant form of new generation on grids in many parts of the world. In 2015, 145 GW of renewable capacity became operational globally, most of it wind and solar generation.
This term is often misused by proponents of legacy generation forms such as coal and nuclear to assert that wind and solar energy are not mainstream, but still inferior.
What is interesting is that with so much coal generation being sunset globally and so little new coal generation actually being built, the day when coal is considered alternative energy is coming. Similarly, although on a longer timeframe, the day when nuclear and gas generation are considered alternative is foreseeable.
Clean energy would best be defined as forms of generation which over their full lifecycle emit insignificant amounts of chemical, particulate, harmful radiation, or carbon pollution after mitigation and remediation. Any environmental damage is localized, relatively small in scale from a global perspective, does not causes species-level threats to flora or fauna, and issues are insignificant on a per MWh basis.
By this standard, wind and solar are obviously clean with minimal care of siting. Hydro electricity outside of the forested tropics — where anaerobic decomposition of submerged vegetation creates a significant carbon load — is clean energy. Nuclear power plants when sited carefully and operated properly are clean energy; nuclear power plants sited on earthquake faults and tsunami zones are not clean energy.
Fossil fuel generation is never clean generation no matter how much mitigation and remediation is performed in the fuel chain and effluents and waste chain. There is no economically viable way to reduce the negative externalities of fossil fuel generation to anywhere near the levels of wind or solar generation.
The best definition of distributed generation is that it is energy generated at or very near to the point of consumption.
Home rooftop solar is a good example of distributed generation. Run-of-the-river hydro used by a remote lodge is distributed generation. Industrial co-generation which sees a small thermal generation plant generating electricity and heat for a factory is distributed generation. Diesel generators for remote locations are distributed.
Also, obviously, major hydro dams, nuclear power, coal plants, and the vast majority of gas plants are not distributed generation. They are grid-tied, centralized forms of generation of electricity which will be transmitted and consumed at endpoints substantial distances away.
To be clear, utility-scale wind and utility-scale solar generation are not distributed generation by this definition.
Another and also intended-to-be-disparaging use of the term comes from some legacy, centralized generation advocates who consider anything that is dispersed to be distributed and hence not significant in scale or manageable. This is more of a connotation that they infer to the word distributed, and typically people using the term in this way include utility-scale wind and solar in this as a lead-in to attacking them on the irrelevant metric of energy density or watts per square meter. It’s not accurate or helpful.
Sustainability is defined by the consumption of energy inputs which are either renewed automatically by major geological or atmospheric processes or which consume so few resources that they effectively will last tens of thousands of years or longer, and which do not cause substantial and ongoing ecological harm as they operate, are commissioned, or are decommissioned.
By this definition, wind and solar are definitely sustainable, hydro which does not cause species-level risks due to loss of spawning is sustainable, geothermal is sustainable, and, arguably, well-sited nuclear — once again, not on earthquake or tsunami zones and by major bodies of water not subject to drought or overheating — is sustainable.
Standard proviso: I like nuclear as technology. It’s stable, clean, has low environmental or health threat per TWh, emits little CO2e full lifecycle, and works. The problems for nuclear are those of price, slow speed to build, lack of horizontal scalability, and fiscal risk. When nuclear is poorly sited, poorly managed, and something goes wrong, as is the case in Fukushima, then cleanup costs can approach a trillion dollars (USD) and take dozens or hundreds of square kilometres out of economic, habitation, and other uses.
Fossil fuel generation is inherently unsustainable by every portion of the definition.
Carbon-neutral energy is defined as energy which over its full lifecycle of mining, manufacturing, distribution, operation, and decommissioning has an emissions per kWh in the range of 10 to 100 g of CO2e/kWh after any mitigations or remediations.
By this standard, nuclear, wind, and solar are carbon neutral. Hydro outside of tropical forested areas is carbon neutral.
Fossil fuel generation is not carbon neutral and never will be. Methane gas generation emits on average 480 g CO2e per KWH and coal is worse at around 1.1 kg CO2e per KWH. To achieve carbon neutrality, each fossil generation plant would have to capture all of the carbon, ship the carbon to physically different sites, and sequester it. This would add 16.8 to 19.6 cents per kWh to the cost of these forms of generation per analysis based on numbers from organizations promoting CCS. In addition, enhanced oil recovery is the only form of sequestration with significant economic value and high volume, so sequestration by definition would be used to produce more fossil fuels, which is not a net benefit.