Alternative Fuels

 

Beyond battery operated "electric" vehicles (if you really must)

Alternative fuels cover a broad range of propulsion methods. Basically everything that is not based on crude oil. This includes: electricity (in the form of battery electric, hybrid electric, and fuel cell powered vehicles), biofuels (mainly biodiesel and ethanol based variants), natural gas and - for the adventurous - solar powered vehicles.

 

Overview of Alternative Fuels

 

Alternative Fuels in Detail

Hybrid Electric Vehicles

Hybrid Electric Vehicles (HEVs) have become very popular in recent years, in particular in the US and there again in California. The widely known Toyota Prius is a prime example. These vehicles combine an electric motor and a petrol powered engine. The energy saving is achieved by using an electric motor for short inner city trips and the combustion engine for long range trips at higher speeds. Thus, each method of propulsion is used in the context where it is most efficient.

There is a constantly growing range of HEVs available, from compact cars to luxury sedans and even 4x4's. In the case of hybrid SUVs, it is questionable whether a significant improvement in terms of fuel consumption can be achieved, simply because the weight of the vehicle and the aerodynamic drag are so high.

 

Plug-in Hybrid Electric Vehicles

Traditional HEVs have a very limited range in electric-only mode of a few miles only due to their limited battery capacity and high vehicle weight. Plug-in Hybrid Electric Vehicles (PHEVs) are a more recent development. They are based on Hybrid Electric Vehicles (HEVs, see below) which are equipped with more batteries and a power plug. This way PHEVs can be charged directly at a power outlet, and driven in electric only mode for an extended range. Still, the range of PHEVs in electric-only mode doesn't match those of purpose built electric vehicles.

To date there are no production versions available, the existing PHEVs being the result of individual conversion efforts of their owners, and in some cases on a corporate basis (such as Google's PHEV program dubbed RechargeIT). At the Detroit Motor Show in January 2007, several major vehicle manufacturers have announced plans to mass produce PHEVs. Commercial versions are expected to be launched in 2009.

 

Fuel Cell Vehicles

Strictly speaking, fuel cell powered vehicles are also electric vehicles, since they are driven by an electric motor powered by electric energy produced by the fuel cell. A fuel cell produces energy through a controlled reaction of hydrogen and oxygen. While the oxygen can be taken from the air, hydrogen has to be made available to the fuel cell in a pure state. There are no greenhouse gas emissions at tailpipe level, and only water is produced as a byproduct.

The main issue with fuel cells however is around the provisioning of hydrogen. Namely, storage in the car tank (hydrogen has to be cooled to about minus 250 deg Celsius in order to be in liquid form) and distribution (currently there are only very few hydrogen stations available). Then again, hydrogen is not available naturally in a pure form, so it has to be industrially produced, in most cases from methanol. However, this process is very energy intensive, and the question arises Why not use the energy directly to power a car?, which brings us back to battery electric vehicles.

For practical purposes a fuel cell car could use e.g. methanol as "fuel", which can is more readily available in nature and can be more easily stored. Hydrogen is then extracted "on the go" through a process called reforming. It has to be noted however that this process is rather energy intensive and thus lowers the overall energy efficiency, while creating a certain amount of emissions. Also, while storage in the car is less of an issue here, methanol is not widely available at gas stations either.

Last but not least, the cost of fuel cells is currently prohibitively high (well beyond that of even the most powerful battery packs), due to the cost of platinum, the catalyst used to enhance the reaction of oxygen and hydrogen in one of the most common types of fuel cells for mobile applications (PEM - Proton Exchange Membrane or Polymer Electrolyte Membrane fuel cell).

In summary, fuel cells have a potential to play a major role in the transportation of the future, but it can easily take another 10-20 years until they become really feasible for widespread use for personal transport. Even then, the issues around the production of hydrogen in an ecological way remain to be solved.

 

Pure Vegetable Oil and Biodiesel powered vehicles

Fuels derived from organic sources have been around for close to a century (Rudolf Diesel's first motor ran on peanut oil in 1912). The use of pure vegetable oil is limited by the fact that it only works in robust diesel motors, i.e. older ones without sophisticated injection mechanisms. Biodiesel on the other hand is based on vegetable oil that is refined and mixed with about 10% methanol. This way it can be used in high concentrations in many modern (and more efficient) diesel engines. In countries like Germany, diesel fuel contains around 5% of biodiesel since 2004. In addition, scientific studies seem to indicate that exhaust gases from biodiesel pose much less of a carcinogenic risk when compared with vegetable oil.

The main advantage of vegetable oil and biodiesel is that it is made from renewable sources. When it is burnt, it only emits the amount of carbon that was consumed during the growing of the plants. It is thus a carbon neutral type of fuel. Most modern diesel motors can easily "digest" a certain amount of biodiesel mixed with traditional diesel.

On the downside, a lot of land is needed to grow the plants required to produce the fuel. As an example, in order to power all of Great Britain's and Northern Ireland's cars with biodiesel made from rapeseed oil, an area the size of England would be required to grow the plants. Thus, while a small scale use of this kind of fuel makes environmental sense, a large scale application would not have the same positive impact.

As an example, soybean biodiesel reduces greenhouse-gas emissions by 40% with respect to regular diesel. This takes into account the total amount of energy required and all emissions incurred in the production and consumption of the different fuels [MIT Technology Review, Sept/Oct 2006].

 

Ethanol powered vehicles

Ethanol is gained from crop such as corn grain through a process called fermentation and distillation. It is basically pure alcohol that can be used in combustion engines. It is normally blended with regular fuel (gasoline) in different amounts. In fact, in many European states already today gasoline available at gas stations contains 5% of ethanol, so-called E5, which is the level of Ethanol most combustion engines can work with without problems.

High-percentage Ethanol fuel has recently become very popular in the US and in Latin America (in Brazil, around 60% of vehicles are powered by ethanol gained from sugar cane plants). It is commonly offered in the form of E85, i.e. 85% Ethanol mixed with 15% regular gasoline to enhance cold weather starting performance. The advantages of E85 include a significantly lower price, mostly thanks to lower tax levy, and reduced emissions with respect to gasoline on a per mile basis. Also, Ethanol is usually produced from domestically grown renewable sources. On the downside, it requires a refitting of the engines - only so-called Flex Fuel vehicles are capable of running on any degree of mixture between ethanol and gasoline. In addition, the energy density is about 5-10% below pure gasoline, leading to lower mpg levels.

As an example, ethanol from corn grain reduces greenhouse-gas emissions by 12% with respect to regular gasoline. This takes into account the total amount of energy required and all emissions incurred in the production and consumption of the different fuels [MIT Technology Review, Sept/Oct 2006].

Scientists are currently working on processes to turn the whole plant into ethanol (rather than only the seeds) in a process called biomass to fuel (BTL). Several prototype plants are in operation that turn animal manure and remnants into ethanol. Some of these plants are already cost competitive with oil, given today's high oil price. Work is also being carried out on the front of cellulosic material, which will eventually allow to produce ethanol from wood or special fast growing grass types for example. This will yield a significantly improved efficiency considering the energy required to grow the plants and produce the ethanol.

 

Compressed Natural Gas (CNG) or LPG - powered vehicles

While biodiesel and ethanol are a more recent trend, CNG (or alternatively LPG for Liquefied Petroleum Gas) powered vehicles have been around for a long time, at least in parts of Europe. Natural gas can be used relatively easily as a fuel in combustion engines. It burns in a cleaner way than gasoline, with CO2 emissions reduced by 20% per mile with respect to gasoline (source: Quattroruote Jan. 2007). In many cases CNG powered vehicles emit less than 130 grams of CO2 per kilometer, a level only achieved by the most recent diesel engines (which, on the downside, emit a large amount of particles, unless fitted with a particle filter).

On the other hand, vehicles have to be refitted in order to work with CNG; in particular, large tanks have to be installed, since the energy ratio of CNG is lower than that of petrol on a per litre basis. Even with large under floor tanks, in most cases the range is slightly lower than for petrol powered vehicles. In addition, only a small (but growing) share of gas stations offer natural gas. Luckily, most natural gas vehicles are actually bi-fuel and can run on traditional gasoline as well.

The additional cost for CNG capability can reach several thousand Euros, but incentives are available in several European countries (for instance up to 2,000 Euros in Italy). The major advantage of CNG vehicles, apart from the environmental aspect, is the lower operating cost. The cost per mile is roughly one third of the cost for a gasoline powered vehicle, thanks mostly to the very low tax levy on natural gas. Hence, the premium paid to purchase or refit a CNG vehicle is amortized within 2-3 years. Some European Vehicle Manufacturers, most notably Italian and French companies, also offer bi-fuel (natural gas + traditional combustion engine) versions of some of their cars.

 

Solar electric vehicles

There exists a range of mostly experimental electric vehicles which are directly powered by energy from solar panels fitted to the vehicle. While this is a very interesting and environmentally friendly concept (no or only small batteries required, virtually zero negative environmental impact), these vehicles are characterized by a number of limitations. So far, most solar powered vehicles have been built for experimental or demonstration purposes and are often not viable for everyday practical transport.

The main issue with using solar panels for energy creation - apart from the fact that they only work when there is light - is the limited power output of these panels with respect to the surface available on a typical vehicle. This makes light weight and low aerodynamic drag the key design drivers of these vehicle. The result are vehicle shapes and configurations which are not always very practical for everyday use in e.g. urban environments.

While some improvement can be expected in the future from increased efficiency of solar panels, it is more likely that they will be used to top up batteries in battery-electric vehicles. For instance thin film solar panels could be integrated at a relatively low cost into the roofs of EVs. This would help extend the range of the vehicle on sunny days.

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