Green Cars

What are the different ways in which the car may develop towards having a reduced impact on the environment? 

What changes are needed to make this form of transport supportable in the long term?  We consider the possibilities.


In industrialised countries people have become largely dependent on cars and see them as essential for their personal freedom.  However it is increasingly agreed that our ecosystem is not able to support the emissions that are produced and the reduction in finite resources caused through driving cars.  This is exacerbated by increasing population and industrialisation and therefore there is a need for concerted attention to be given to technical and sociological solutions that will lessen the impact of this form of transport. 

The relationship between cars and other forms of transportation

The way in which the car currently compares with other forms of transport very much depends on the model of car being considered and the way in which it is used.  Buses are generally acknowledged to be the most efficient form of land transport if they are fully utilised, and it is also widely assumed that trains give lower emissions per passenger kilometre than cars.  However these comparisons are not always clear cut, as for example in the UK there may be less carbon dioxide emissions per passenger kilometre for an efficient diesel car carrying four people than for a train, given the inefficiencies of current rolling stock (note that this does not mean that a switch of four passengers to using the train will result in reduced emissions for an individual journey when the train will run anyway).

Cars generally emit less per passenger mile than planes, but a large car may release more emissions per passenger mile than a plane on a long haul flight.  Whilst it is acknowledged that cars have a significant environmental impact, the high demand for cars has driven great innovation and increase in efficiency, more so than with other forms of transport such as trains.  Now that fuel efficiency is more of a priority for all forms of transport, the question may then be what role cars should play in a future transport system with efficiency improvements across the board.  

Most car journeys are used for short journeys.  In the UK, over 70% of journeys are under 5 miles, and in the US over 70% of journeys are under 40 miles, which is a short distance in the US.  These kinds of distances are within the range of new propulsion systems which have not got the same on-board energy density as internal combustion engine (ICE) cars, and these are discussed further on in the article.  Very short journeys are too short to warm up the car’s engine fully and therefore the engine will run more inefficiently, burning more fuel and polluting more.  It may be argued that these very short journeys should be carried out on bicycle or by walking.

There are those who suggest that the embodied energy in the food required to propel a person riding a bicycle or walking is greater than that in the fuel for required for the equivalent car journey.  It is beyond the scope of this article to either challenge or verify this assertion, but even if it were true there are other benefits to walking or cycling that would need to be taken into account, such as increased fitness and health, although it could be argued that activities leading to prolonged life place a greater burden on the planet’s resources.  It can be seen that the global sustainability arguments are complicated, and it is probably fair to say that more research and consensus is needed before they become conclusive.

The extent to which cars are needed depends on the population density of the places being travelled between.  Although there is increasing urbanisation, there remains a need for rural transport.  As with many rural services that are not cost-effective in their own right, public transport has attracted reduced support when fares from the passengers do not cover the running costs.  Coupled with the fact that public transport that is not fully utilised may give a less efficient use of resources, it can be seen how there may be a continued role for cars in areas of less dense population.

In this case, how can the use of these cars be made more efficient?  Car sharing is advocated as a way of reducing the manufacturing burden of cars and also as having the potential for increased utilisation through encouraging more passengers per car.  There are however significant social challenges to be overcome before car sharing becomes widespread. 

As has been suggested already,  people are attracted towards cars for the freedom they give, and there is resistance to anything that acts to restrict this freedom.  Therefore the design, function, and cost of shared cars will need to be such as to make them a more attractive proposition to the user than having their own car.  A range of designs available for different uses could in fact give people more freedom.  For example a small car may be used for quick visits and a larger estate may be used for carrying goods.  

If we consider the model of buses or trains being used over long distances or between urban centres and cars being used locally on a shared basis in areas of low population density, then it is important that the overall journey is as uninterrupted as possible for people to be attracted to this way of travelling.  Car pick-up depots would be needed at critical nodes, either bus or train terminals.  The transition from one form of transport to the other could be made smooth through the use of smart (data) cards to access vehicles. 

It should also be added that bus travel might need to be made more comfortable than it is currently to attract the majority of the population.  Bus travel could possibly be made more attractive than cars with reduced travel time through using bus lanes and increased onboard facilities such as internet access and refreshments.  

Design and development

There are several different designs towards reducing the environmental impact of cars in operation.  Whilst they may not be environmentally benign in their own right, the common feature is generally that they lend themselves to being fuelled by renewable energy either now in the future as the energy supply changes. 

There are also improvements in efficiency of fossil fuel consumption which are reducing the impact of cars.  It should be noted that over the last 10 years the internal combustion engine has seen efficiency improvements of around 1.6% per year, and whilst fossil fuel is still being burnt, this contribution is worth noting. 

As we make the transition towards greener technology there is much we can do to reduce the impact of our existing cars, such as slower acceleration, reduced speed and reduced weight.  This requires both education and incentives or regulation.  It is not clear yet what new technologies will dominate, but some of those that are showing promise are described below.


Little or no modification is needed for an engine to run off biofuels.  However the main issues that need addressing with biofuels are their energy ratio and land area requirements.  The energy ratio (energy available in fuel divided by the energy required to produce the fuel) is often below 2 for the current generation of biofuels, thereby giving less than a 50% saving in CO2 emissions as compared to mineral fuel, assuming mineral fuel is used to produce the biofuels.  If instead it is assumed that the fuel used to produce biofuels is also biofuel, then the impact can be translated into increased land use. 

With the current generation of biofuels there is a question as to whether there is enough available land to meet all our current transport needs by this means, although there are significant areas currently not fully exploited that could be brought into production.  The next generation of biofuels, using more woody crops, promises both better energy ratios and the ability to use marginal land that cannot easily be turned to agriculture, thereby removing any conflict there might be with food or other land uses.  These second generation biofuels are still under development and have yet to be proved viable. 

If we consider the efficiency of energy conversion of land with energy crops for liquid biofuels and land covered in solar arrays, solar arrays may convert more than 20 times the sun’s energy than the energy crops over the same area, and when this is converted into motive power for transport, the factor may be nearer 50. From a land use perspective therefore, it can be seen that solar power would offer an advantage over biofuels, but there is currently a great difference in cost which makes biofuels more commercially attractive.

Other forms of renewable energy, such as wind power use even less land and are less costly than solar, although are not currently as cheap as liquid biofuels as an energy source for transport.  While internal combustion engines remain the main source of car power it seems likely that liquid biofuels will make a significant contribution.   

Electric cars

Electric cars give a local reduction in emissions as compared to ICE cars, and in the UK they already give a reduction in CO2 emissions when compared to ICE cars, taking into account the grid CO2 burden, transmission losses, pre-combustion emissions and relative conversion efficiencies.  However for electric cars to give the required reduction in overall fossil fuel consumption and associated emissions, the electricity will need to come from renewable resources, and given the current struggle to make a sizeable renewable energy contribution to existing electrical demand, this is not without its challenges.  

Electric cars have not yet reached the performance of ICE cars.  The range of electric cars is less, their power is less, and their energy storage (usually batteries) takes up more space and is very heavy.  Electric cars approaching an equivalent performance may be over 5 times more expensive than their mineral fuel equivalent, and therefore have a long way to go before they become attractive to the average customer.  

There are three main types of electric vehicle: electric vehicles (EV), which use a battery driving an electric motor; hybrid electric vehicles (HEV) in which an internal combustion engine charges a battery; or plug-in hybrid electric vehicles (PHEV). 

Because HEVs have two motive power systems, they are heavy and struggle to match the efficiency of modern diesel engines.  For EVs and PHEVs, which are charged from the electrical network, attention needs to be given to the mechanism for charging. 

With current battery technology it is impractical for most motorists to wait around every time for the time it takes to charge a battery whenever they need a refill of power.  One possibility is to have battery stations, as with petrol stations, where motorists can exchange their depleted battery for a fully charged one.  Clearly this will need increased uniformity in battery use for this to be viable. 

Alternatively flow batteries could be used whereby they could be charged by pumping new electrolytes into the system and pumping out the discharged electrolyte.  There is a concerted focus by manufacturers on the reduction of charging time of batteries. One manufacturer claims it is on track to develop an electric car with 26 minutes' charging time. 

There are serious environmental constraints on the widespread use of batteries in vehicles, with the use of scarce materials in their manufacture and the release of toxins when disposed of.  However advances in battery technology promise longer lifetimes as well as higher efficiencies and deeper discharge.  

Fuel cells

Another form of energy conversion that uses an electric motor is the fuel cell.  A fuel cell differs from a battery in that there is a constant flow of fuel through the battery to supply the electrochemical reaction, so although there are significant maintenance requirements, the electrochemical potential is not depleted as it is in a battery.  

Fuel cell cars (and buses) are already in production, but are disadvantaged by the low energy density of hydrogen storage, thereby limiting their range.  In addition their widespread use would depend on the introduction of a hydrogen infrastructure, which would be extremely expensive. 

If fossil fuels are used to produce the hydrogen, there are no significant energy gains through using fuel cells, and therefore for fuel cells to be green there is a need for production of hydrogen with renewable energy.  However, it should be noted that the overall efficiency of conversion for a fuel cell using renewable electricity via an electrolyser is significantly less than that of a battery electric vehicle. 

Compressed air vehicles

Compressed air cars work by storing air at high pressure in a tank and releasing the air through pistons that drive the wheels.  Developers of compressed air cars claim lifecycle emissions comparable to those of battery vehicles, assuming the air compressor is powered from the same mains electricity that would charge a battery. 

They have the advantages that they do not need expensive batteries to be replaced every few years and they take a fraction of the time to charge.  These are still under development, but could potentially have a significant role to play.  

Impact on the electrical distribution network

One challenge faced by electric vehicles is the dealing with the impact that they will have on the electrical network.  In the UK there is a projected shortfall of capacity based on current electrical demand, and anything that will add to capacity requirements will be difficult to support with our existing infrastructure and planned generation. 

To maximise uptake of electrical vehicles with existing electrical generation capacity, there is the possibility of limiting the charging of electric vehicles to night-time when demand is low, thereby utilising spare capacity rather than adding to the burden on existing capacity.   

Whilst the theory would appear to be sound, to ensure that this happens in practice would require fairly radical new measures such as a draconian tariff structure that  would ensure that day charging would be negligible. 

It would inevitably be the case for battery cars that they would run out of power on occasions through the day, and if this was not to put extra demands on the generation capacity and infrastructure, then local electrical storage may be necessary.  It has been suggested that electrical cars could be used as distributed electricity storage for the electrical network while not in use.

The challenges facing this initiative include the possible reduction in battery life through frequent shallow cycling and also difficulty in ensuring the total storage capacity is in line with requirements at any one time, given the unpredictability of car usage. 

Many proposals for the widespread introduction of electric cars assume charging at the extremities of the network (i.e. at 240V), with the typical model being that people would plug their car in at home. 

Aside from the network capacity issues, at this low voltage the losses would be significant.  A separate high voltage network may address this, and would also support the rapid charging model referred to earlier, but this would require huge Government investment. 

Car manufacture and embodied energy

The question is sometimes asked as to whether it is worth replacing an old inefficient car with a new more efficient one, given the investment of embodied energy in the new car. 

The ratio between the embodied energy in the manufacture of a car and the total energy used over its life including its operation does of course vary considerably with different car models and different usage patterns, but to give an idea of scale a range of 15-25% currently appears to be typical ie 15-25% of the lifecycle energy consumption of a car may be from its manufacture, assuming a normal lifespan.

If the car is scrapped prematurely, the proportion of embodied energy in the lifecycle will increase. Other factors that should be taken into account are whether or not the old car continues to be used by someone else and whether or not there is a rebound effect of the driver covering more miles because the fuel bill is lower. Care should therefore be taken when replacing a car purely on energy-saving grounds.  

Making it happen

Whilst there are people for whom environmental concerns will be the dominant factor when they choose their car, for most people other factors are as important or more important, and these include styling, brand loyalty, reliability, function and cost.   All these factors need to be addressed for the successful uptake of green cars.  There are already noticeable shifts occurring towards more fuel efficient cars because they can give a great reduction in cost to the user. 

If there is to be a move away from ICE cars, then there will need to be significant infrastructure investment, which will need to be centrally funded and coordinated.  Given that non-ICE cars vary in their infrastructure requirements (eg fuel cell cars using hydrogen versus battery electric cars), then it may be supposed that one type of car will dominate when there is an emergence from the current niche markets. 

Whatever technological solutions are chosen, in the same way that we may follow a waste hierarchy or energy hierarchy, we might also follow a transport hierarchy: 1) Use transport less 2) Use fuel efficiently 3) Use renewably powered transport.

Green Tariffs versus Green Wash

 Article by Rupert Blackstone - Energy, Environment and Sustainability Group (EESG), IMechE, 10/12/2008

On the 1st December, EESG hosted a talk given by Juliet Davenport, Chief Executive and founder of Good Energy.  Good Energy is a supplier of green electricity, and Juliet’s presentation was to explain what ‘green tariffs’ actually mean and what difference they can make to the total installed capacity of renewable generation.

Rather than promote the virtues of Good Energy as a company, Juliet’s talk gave an overview of the context of green electricity and the various types of green electricity on the market.  She started off by explaining how far the UK needs to go in meeting its 15% renewable energy target as a share of the total EU target of gaining 20% of energy from renewables by 2020.  She pointed out that in the European Union, only Luxembourg and Malta have less renewable energy supply than the UK (currently less than 3% of primary energy consumption).

Good Energy claims that their customers will receive 100% of their electricity supply from renewable energy.  A common criticism of green electricity in the UK is that green electricity suppliers are just allocating to consumers renewable electricity that is being generated anyway as a response to the Government Renewables Obligation financial mechanism, and which is constrained not so much by the market, but largely by planning.  Following this argument, more is needed than a claim of 100% renewable energy supply to demonstrate that the sale of green electricity is actually resulting in new renewable energy capacity.  Juliet covered in her presentation the main additional measures taken by green electricity suppliers.  These included:

  1. Retiring Renewables Obligation Certificates (ROCs) gained for renewable energy generation from the market, thereby increasing the incentive for renewable energy to be generated elsewhere. Juliet was not in support of this entirely, as she believes that ROCs drive technology solutions, given the differentiation in support to different technologies, whereas green electricity supports units of renewable energy supply (one would presume then that it would favour the cheapest).
  2. Green Funds guaranteeing that a certain amount of money is invested by the energy supplier in environmental projects beyond Government commitments. 
  3. Offsetting carbon dioxide emissions.  Juliet’s concern about offsetting was not only that it may not be diverted into renewable energy, but that it is not generally transparent and auditable. 

Juliet emphasised the need for transparency with green electricity, something that is often not evident for the public when reviewing green electricity websites, and she supported auditing as an inexpensive way of improving customer confidence.  Juliet believes that green electricity has an important role in stimulating the market for renewable energy, and that it can realise potential that regulation and policy interventions will not reach alone.  Although the impact of buying green electricity on new renewable energy generation may be limited at the moment, Juliet explained that the investment in green electricity is not for today, but tomorrow when the market is further developed.