Technology Overview

I am not counting biofuels as green fuels since they require agricultural land which is needed for food production. While there is scope to produce some fuels from waste gases emanating from rubbish tips, this is not something that can be scaled up sufficiently.

There is no question, renewables energy is competitive on price, at least in countries such as Australia with abundant wind and sun.

The problem is intermittency. Overlaid onto variable demand, which was already in a problem before renewables, we have variable supply due to the weather and time of day affecting sun and wind.

One way to store energy is to pump water from lower reservoirs into higher reservoirs. This approach is limited by the availability of suitable locations.

Another way is batteries. Lithium batteries are big here. Important to know: Lithium batteries have high energy densities by the standards of batteries but they also use rare and expensive materials. So why are they used in huge, stationary, grid scale batteries, where energy density is not important? Because lithium batteries reached mass production before alternatives which would otherwise be cheaper.

The critical application, though, is transport. It is here were we rely on imported oil and imported fuels made from oil. And this is the focus of this page.

There are two categories of renewable energy used in transport, electric batteries and green fuels. The focus of this page is just to introduce the technologies. I have other pages where I link those fuels to applications. For example, commercial aircraft and cargo ships have very different fuel needs. But that is not covered in detail here.

There are two approaches used with batteries:

• Charging the battery while it stays in the vehicle. We are familiar enough with how that works in cars. I describe how this works for trucks on this page.
• Swapping the battery. This approach is used more seriously in China, both for cars and trucks. I describe this more on this page

Batteries work well enough for countries like Australia that have plenty of capacity to generate electricity from sun and wind. In other countries, green fuels are much more seriously considered and trialled.

The most obvious green fuel is hydrogen. Other green fuels are produced with hydrogen as an ingredient. Hydrogen is produced in a process called electrolysis which uses electricity to split water into hydrogen and oxygen. When hydrogen is used to produce energy it is not typically burned. Instead it is fed into a fuel cell which combines it with oxygen from the air to produce electricity. This process is the reverse of electrolysis.

Hydrogen is obviously the cheapest green fuel for mass production, since almost all other green fuel production uses hydrogen and then needs some extra processing to get the desired fuel. Hydrogen is also incredibly energy dense, relative to its weight, more energy dense than petrol and diesel. However hydrogen has some significant drawbacks. It is a light gas. That means it needs to be compressed a lot for storage. Hydrogen can also permeate many materials.

Despite the disadvantages, Hydrogen is used as fuel and I talk about it more on this page.

Synthetic fuels typically combine hydrogen with carbon dioxide to produce liquid fuels with molecules consisting of hydrogen, carbon and sometimes oxygen. That makes them identical or similar to fossil fuels. Such fuels are called hydrocarbons. It is possible to make synthetic fuels for use in existing fossil fuel burning engines.

Right now, the carbon dioxide for this process is most commonly a waste product of existing industrial or biological processes. There is no harm done in using this carbon dioxide one more time before it goes into the atmosphere. However, we want to reduce carbon dioxide output, and in any case, we can't retrofit carbon dioxide capture to many existing sources of carbon dioxide.

Thus the only sustainable and scalable way to get carbon dioxide for synthetic fuel production is from the air. The process for doing this is is called Direct Air Capture (DAC). Then, when the fuel is burned, the carbon dioxide goes back into the air. But it's no problem because it came from air in the first place.

The second most common type of synthetic fuel is ammonia. Ammonia molecules consist of hydrogen and nitrogen. The hydrogen, again, comes from electrolysis. And just as carbon dioxide for green methanol will ultimately mostly come from air, nitrogen for ammonia production also comes from air. But there is one difference: Air is 78% nitrogen and only 0.04% carbon dioxide. That makes it a lot easier and less energy-intensive to extract nitrogen from the air for ammonia production than to extract carbon dioxide from the air for hydrocarbon fuel production.

Ammonia is also a gas. However, it can be compressed to a liquid much more easily than hydrogen.

Coming back to synthetic hydrocarbon fuels, the simplest to make is methanol. Using DAC, it take somewhat more energy to make methanol than ammonia. But methanol has the advantage of having a higher energy density, both by weight and by volume, than liquid ammonia under pressure.

Given that we have pre-existing technology to work with and are not building a brand new economy from scratch, ammonia has the advantage that is already widely used and transported as it is an ingredient in fertilisers and other industrial chemicals. Methanol has the advantage that it behaves more like conventional fuel and thus dual fuel engines are relatively easy to build and are in fact available commercially.

More fancy hydrocarbon fuels such as kerosine, used in jet engines, are more expensive and less energy-efficient to manufacture than methanol.

Below is a table of data for the various technologies. Unless otherwise indicated, technical performance figures are based on production technology, as opposed to lab technology. I did mention an advanced car battery already installed in a prototype car. This type of battery roughly doubles energy of existing batteries in production cars, and it is not far off, so it is definitely worth mentioning.