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Guest post by Rhod Jenkins
What is the holy grail of renewable energy production? I work with a lot of chemists and engineers who research the area of chemical renewable energy, and this is one debate we tend to have more than any other. After a lot of raised voices, wagged fingers and offences taken, we tend to conclude that there is no one solution to the problem, but rather it’ll be a mixture of all of them. This is partially due to the massive scale of the energy problem, but it also comes down to the applicability of the technology. What might work for one application may be completely inappropriate for another.
Take transport, for instance. We all rely heavily on powered transport, and in the UK it accounts for over a third of our energy consumption. For this specific application the energy source needs to be carried on-vehicle (except in very rare electric trams), and at the moment this comes in the handy form of liquid fossil fuel. But what are our other options? Actually, there’s quite a few and they come generally under two categories. We can either replace the fuel itself, keeping the current engine technology and infrastructure, or we can develop new transport technology which would require new infrastructure.
There are a few different renewable liquid fuel technologies that can act as drop-in fuels, some of which are in use today. Most well-known are biofuels, such as ethanol and biodiesel, which are used in blends in both the EU and the US, but they are not without problems. Other than the fact they’re derived from food resources, they both contain oxygen which reduces the amount of energy you can get out of the same amount of material, known as the energy density (by around a third for ethanol and 10-12% for biodiesel).[1, 2] The presence of oxygen also increases their reactivity. Ethanol can be corrosive to the engine and solubilise water from the atmosphere, whilst biodiesel can oxidatively degrade, increasing its viscosity and ultimately producing corrosive compounds.
To try and tackle these problems, research is being carried out to make pure hydrocarbons from biomass, such as thermochemical conversion to ’bio-oil’ which can be refined in the same way as crude oil, or by catalytically combining carbon dioxide from the atmosphere and hydrogen to make fuel-like compounds. This research is in the very early stages of development, however, and so far hasn’t shown to be economical using the relatively low amounts of carbon dioxide in the atmosphere (300-400 ppm).
So how about changing the vehicle technology? What about electric cars, such as the Nissan leaf? Electricity has the biggest potential for being the cleanest power source but only if we can harness solar, wind and tidal energy as a clean means of generation. The cars themselves are much more efficient than the internal combustion engine (by around a factor of 3), which means the costs per mile are much lower. The significant lack of moving parts when compared to a traditional engine means they generally need less maintenance, and as electric motors have relatively constant torque they exhibit a higher acceleration performance compared to a similarly-powered engine.
But of course, there are disadvantages. The ranges for most electric cars are much lower, the Nissan Leaf’s maximum range is about 120 miles, as opposed to a typical range of 300 miles for a fuel-powered car. Battery charge time is also an issue as anyone who owns a smartphone will attest to. Usual charging times for car batteries is several hours when using at-home chargers, though high-power public charge points do exist which have the ability to reach an 80% battery charge in around 30 minutes.
The weight of the batteries is another disadvantage. Pound for pound, the energy density of a Li-ion battery is much less than that of liquid fuel (0.7 MJ/kg as opposed to approximately 45 MJ/kg). Research is currently taking place to improve on battery technologies, such as lithium-air batteries which could demonstrate up to 15 times the energy density of current Li-ion batteries. There are also logistical problems concerning the materials found in electric vehicles. Rare elements such as neodymium and dysprosium are essential ingredients in some electric motors and there is a worry that, with a shift toward electric vehicles, the oil-dependent culture of today will change to a rare metal dependency, with 97% of rare earth metals being produced by China.
Apart from battery power, there is another possibility, the new kid on the block in terms of vehicle power – hydrogen. Hydrogen can either be combusted directly using a modified internal combustion engine, or used in a fuel cell to generate the electricity required to run a motor. Using hydrogen to power a car has lots of advantages. No carbon dioxide is produced, the efficiency is higher, and if hydrogen is produced from a sustainable method (such as water electrolysis using renewable electricity) no greenhouse gases are emitted in the entire cycle.
However, nothing can be perfect. Though hydrogen could be produced in renewable ways, it would be very expensive. Current predictions are that production from solar-powered water electrolysis would cost $6.50 per kilogram. Currently, hydrogen is produced in a much more cost-effective method, the steam reformation of fossil fuels (around $1-2 per kilogram), but those fuels are exactly what we are trying to replace! Another problem is the physical density of hydrogen. Though the energy per unit mass of hydrogen is much higher than petrol or diesel (123 MJ/kg versus 45 MJ/kg), hydrogen exists as a gas which greatly decreases its volumetric energy density. Even at the high pressures it’s currently being compressed to – a whopping 700 bar – the volumetric energy density it significantly lower than current liquid fuels (5 MJ/l versus 35 MJ/L). Building vessels capable of coping with these high pressures also increases the amount of material needed, and therefore the weight.
So many options for replacement fuels and fuel systems are under investigation, and each has their supporters and their denigrators. But what about changing the transport technology itself?
There are other alternative technologies, those of much more remarkable design, which could be feasible in public, rather than personal, transport. Wireless charging of electric buses in South Korea has recently been developed, made possible due to their pre-determined routes. Magnetic levitation, or ’Maglev’, trains run on electricity and the levitation reduces the need for traction and friction, therefore increasing efficiency. This technology can be further extended to evacuated tubes, or ’vactrains’, which greatly reduce air-resistance, improving efficiency further. One example of this is ’Hyperloop’, a concept proposed by Elon Musk, CEO of Tesla Motors, who claims it could reach speeds of up to 760mph and would enable travel between Los Angeles and San Francisco (a distance of 360 miles) in 35 minutes.
In terms of personal travel, however, there’s a significant argument for keeping the current engine technology and infrastructure. After all, we’ve spent over a century perfecting the extraction of work from the combustion of liquid fuels, and the longevity of vehicles is ever-increasing. Therefore, in the short to medium term we need a replacement renewable liquid fuel. In the long-term, however, I expect to see many different transport technologies being developed and improved upon. Personally, I’m still looking forward to the hover skateboard from “Back to the Future II”, but that’s probably just me.
Rhod Jenkins is a PhD student within the Centre of Sustainable Chemical Technologies as the University of Bath. His PhD is in the area of biofuels from microbes and waste resources.
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