Part 3 of the blog series: What does COP21 mean for Engineering and Technology? Read previous parts here.

Renewable energy resources are huge and we need to significantly improve their exploitation in order to meet the objectives of the recent COP21 Paris agreement on climate change. However, these resources are generally distributed far from where the energy is required, and moving the energy is fraught with inefficiencies, losses and challenges; there is not one single “silver bullet” that can solve all the problems and a mix of engineering solutions will be required

Most sources of renewable energy generate electricity, and as discussed in yesterday’s blog, electrical energy can be moved large distances with low losses using High Voltage DC links.

However, this is not a perfect solution, for a number of reasons:

  1. Once generated, electricity needs to be used more or less immediately and supply needs to be matched to demand. Renewable energy sources are notoriously variable and hard to predict, so maintaining the extremely reliable “on tap” electricity supply we are used to becomes extremely difficult and relies on having large energy storage facilities which are costly!
  2. For some applications, electrical power will never be the preferred energy source, especially considering that only 22% of our energy usage is currently from electricity.
  3. A completely electric solution means reliance on large pieces of centralised infrastructure, which leads to inefficient markets and makes governments nervous.

Using electricity to synthesise fuels is an alternative which would make for a much more open and liquid (pardon the pun) energy market. Fuels are an incredibly convenient way of storing and transporting energy and there is a reason why we are so hooked on them. The energy density of most conventional fuels is around 50MJ/kg, compared to around 1MJ/kg for even the best battery technologies.

However, the various technologies for synthesising carbon neutral fuels are all a fair way off being commercially viable on a large scale, and all of them require significant energy input (so have a high net energy loss).


Hydrogen fuel has an energy density of around 100MJ/kg, and can be easily generated via electrolysis. But, being the lightest of the elements, it needs to be kept at a pressure of over 200 bar or at less than -253 °C which uses a lot of energy and is hard to do safely. Also, even in the best storage containers it tends to leak at around 1-2% per day.

New “metal hydride” technologies that store hydrogen in solid form (where hydrogen is reversibly bonded with/into a metal compound) are much safer. However, they are a bit like batteries in that you need more physical material to store more energy, so scaling up is costly. An 18 litre storage vessel can cost over $400 however this looks like a promising area for research and development for some mobile applciations.

Ideas exist for carbon-based synthetic fuels (usually methanol and ethanol) which use CO2 as a feedstock so are therefore carbon neutral if the carbon has been removed from the atmosphere in the first place (which is not trivial!). These technologies are still at an early stage of development. A British company “air fuel synthesis”, was in the news back in 2012 with a proposed process for synthesising captured CO2 with hydrogen, but there have been no new developments to this since.

Nevertheless, it would be a great relief to governments worrying about meeting their COP21 commitments if renewable energy could be moved around in liquid form. Not to mention the enormous economic benefit of cracking this technology, so it can be expected to be an area of significant investment in the coming years.

Oliver Montague

Electrical Engineer in the Industrial, Defence Energy and Consumer Group. Key areas of interest are smart systems and energy technology.