If you think of renewable sources of energy you probably think of things like wind turbines, solar panels, biomass plants, and hydroelectric plants. However, these are just examples. What does it mean to be renewable?
Power From The Sun
Most forms of renewable energy have the sun as their original source. All the various forms of direct solar energy are obviously from the sun. Less clear is the connection between the wind and the sun. Wind is caused primarily by the uneven heating of the earth’s surface by the sun. By using a wind turbine, we are actually drawing out a little bit of the solar energy that went into creating that wind.
Additionally, the water cycle is driven by the sun. Water is heated by the sun, and rises in tiny droplets (which we humans might feel as humidity) which eventually finds its way upwards until it is high in the atmosphere. These tiny droplets eventually form larger drops that become rain. So when we run water through our dams to turn turbines, we are actually using energy that had its origin in the sun. Lastly, the sun is the source of energy for plants to grow. If we use plants for our biomass energy, we are actually using stored solar energy. If we use our plants faster than they grow however, or use non-renewable inputs such as fertilizers based on potash or natural gas, our use of them may be distinctly non-renewable.
‘Green’ Energy Might Not Be Renewable
People often refer to things like landfill gas plants as renewable. This is not really accurate. Being a renewable form of power depends on what form of energy is being tapped into. In the case of landfill gas, the energy comes from all the garbage we throw away. The origin of this energy could be solar, if the garbage is based for instance on plants. However, in this era, all major sources of garbage will be heavily based on petroleum products such as plastics. If the original energy source is petroleum, it is distinctly not renewable. Petroleum is known to take a long time to make (millions of years), and we are certainly pumping it out of the ground faster than it is made. Petroleum is one of the proverbial non-renewable resources along with coal.
Landfill gas plants are desirable in some places as they allow us to capture more of the energy from our garbage that would otherwise be waste. Efficiency is certainly a positive goal for the world, which is why technologies such as these are often heaped in with the renewable forms of energy. These may be a good idea in some places where an opportunity exists to exploit the resource created by our garbage. These plants might deserve the label ‘green‘, but not renewable.
Fusion Is Renewable
The sun runs on the energy created by fusion reactions that take place near its core. If we are able to create fusion energy in power plants on the earth, we will be tapping into the same power source that drives the sun. If we can master the techniques necessary to make fusion power work, we can have energy for many millions to billions of years just based on the easy-to-access hydrogen in the earth’s oceans. This is a form of energy that could outlive the sun. Humanity may settle places beyond the solar system, and continue using fusion power after the sun dies billions of years from now. If any form of power deserves the name renewable, it is fusion.
Is Fission Renewable?
This is a dicey question that we have dealt with in more detail elsewhere. Fission seems to lie somewhere between fusion and non-renewable sources such as coal. Currently fission reactors produce nuclear waste. The actual mass and volume of the waste is extremely small when compared with the fly ash and pollutants from coal production, but it is still notable. It is interesting to note that a typical reactor in this era will unlock only a tiny fraction of the energy stored in the fuel it uses.
There are economic and political reasons for why we are not using techniques to unlock vastly more energy from the fuel. These techniques would involve ‘reprocessing’ the fuel and sending it back through the reactor. In order for the fuel to work again, scientists need to do some chemistry to separate out chemicals that stop the reactions from proceeding safely. Once this separation is complete, the fuel can be used again. In theory, this process can be repeated until we have unlocked almost all of the energy that was in the original fuel. The volume of nuclear waste in this case would be much smaller. This is known as ‘closing’ the nuclear fuel cycle.
It seems safe to say that if we close the nuclear fuel cycle, fission will be mostly-renewable. It will be able to turn matter into energy with only a small amount of waste. This small amount of waste could actually be made harmless (not radioactive) by a particle accelerator. This day may be a long way off, but there are currently some interesting developments. Interested readers are encouraged to learn about the Liquid Fluoride Thorium Reactor.
As things exist today, the amount of waste produced by a nuclear reactor is enough of a problem that we may safely say that this technology is non-renewable in its current form. Options exist that would make this power source incredibly clean, safe and cheap in the future. We aren’t there yet though, and today’s fission power should get the label ‘non-renewable’ but we should keep in mind that we can choose to make it ‘mostly-renewable’ if we choose to support the development of these new reactors.
Geothermal is Mostly-Renewable
Geothermal power uses the heat of the earth, deep under the surface, as its source of energy. By using this heat as an energy source, we cool a small volume of the earth’s crust. Once we stop, these spots are eventually warmed back up by the heat around them, but usually in a slow manner on a scale of decades to centuries. So if we use a geothermal loop to take heat out of the crust and use it to make electricity, eventually we cool that area down. If we shut down our plant and wait a really long time, it should be relatively hot again when we check. On the time scales that we normally discuss with regards to power plants and society, geothermal can be considered to be mostly-renewable.
There are a number of places on the earth where the heat is carried much closer to the surface, such as Iceland. In places such as these, geothermal power is incredibly cheap by comparison and is certainly one of the best options for renewable energy. Iceland has a number of energy-intensive industries because they have such an amazing energy resource that they use for instance to refine Aluminium.
Geothermal power looks different when considering energy on a scale of centuries or eons. The energy contained in the heat of the earth is not infinite. It is a very large amount of energy but it is very hard to get to after we have cooled off all of the easy to access places. In theory the mantle of the earth contains a tremendous amount of energy, but it is not feasible with any technology we are aware of to tap even a large percentage of that energy. Geothermal is a nice addition to our bag of renewable tricks for the coming decades and centuries, but it is not a power source as long lived as fusion.