Efficiency

The concept of efficiency is a vital factor whenever one form of energy is converted to another, as we need to do in areas such as coal liquefaction and hydrogen-powered vehicles. This page looks at this subject in more detail.

Definition

Whenever energy is converted from one form to another, some of that energy is lost as heat – that is a basic law of thermodynamics. Efficiency is defined in "Energy: a Guidebook" as:

The efficiency of any energy conversion system is defined as the useful energy output divided by the total energy input.

This is usually multiplied by 100 to give a percentage.

Examples

In the images below, efficiencies for various conversion engines are shown (values taken from "Energy a Guidebook".

The figures in brackets (shown graphically as orange stripes) are the theoretical output; the other figure (shown as red) is the output in practice. For example, if the coal that is burnt in a steam engine begins with, say, 100 joules of energy, then 95% of that energy is lost as heat and only 5 joules are left to do whatever work is intended.

As well as the inefficiency of the steam engine, we can also see that even the most efficient power fossil fuel power stations lose half of their energy in conversion. The internal combustion engine also shows up as an extremely inefficient method of converting primary energy to movement.

The second image above shows some of the conversions used in powering vehicles.

Using efficiencies and conversion data , we can get some idea of whether alternative sources are really worthwhile. For example, the conversion of coal to oil runs at a rate of three tonnes of coal for one tonne of oil (according to the Chinese). The conversion data shows that 1 tonne of coal contains about 29 GJ of energy and 1 tonne of oil about 42  GJ. So 3 tonnes of coal (87 GJ) converts to 1 tonne of oil (42  GJ) which means that we lose half of the original energy in conversion (not including the energy used in the process itself). Clearly, unless there is a clear benefit from having oil as opposed to coal, it would be better to use the original fuel than convert it.

Saving Energy

Another aspect of efficiency is ensuring that we waste as little energy as possible. This will be a vital area to address if we are to survive the changeover from fossil fuels.

Fairly simple measures can save surprisingly large amounts of energy. In the home, for instance, installing loft insulation can save around 23% of your energy bill each year while adding cavity wall insulation can save 21%. Even turning down the thermostat by one degree can maybe save 6%. But to have a serious effect on a country's energy usage, it is necessary for governments to step in and encourage or even compel such schemes. Offering grants on installing insulation and adding new energy efficient regulations for new homes will help. And there are many tiny steps that can help.

A normal 60W tungsten light bulb lasts for up to 1000 hours; a compact fluorescent bulb lasts ten times as long and uses a quarter of the power for the same amount of light. But the latter bulb still costs over thirteen times the former (from a local catalogue). If the government added a 100% tax to the tungsten light and removed the equivalent money from the fluorescent, the difference would be just six times. People would be more likely to buy the efficient bulbs and everybody would benefit - individuals would save money in the long term and the country would save energy.

Energy saving must extend outside the home. The most fuel-efficient driving speed is in the highest gear at about 50 mph. At the moment, the speed limit on motorways is 70 mph although this is rarely enforced so many drive at 80 mph. The difference in fuel consumption between somebody driving at 50 mph and 70 mph is over a third, and between 50 and 80 is 42%. Reducing the speed limit to even 60 mph could save enormous amounts of petrol, a cost benefit for drivers and energy benefit for the country. Of course this speed limit would have to be enforced (and why are modern cars built capable of speeds of 150 mph or more?)

Creating more efficient motor vehicles would save much energy but the costs of converting millions of cars and replacing those too old to convert may well use more energy than is saved. It is far more efficient to save as much as we can with existing vehicles by actions such as reducing speed limits and increasing public transport usage as we gradually replace cars with more efficient new ones.

Most of these energy saving measures can be taken by individuals but to implement them as fully as possible requires government intervention. It is far easier and cheaper to build solar panels into the roofs of new buildings rather than adding them after the house has been built. Since solar panels should face south (in the Northern Hemisphere), the site layout is a decision that has to be made from the start. We also need to stop building supermarkets out of town where it is necessary for people to use a car to reach. Governments need to encourage the companies either to build in urban areas or place them where there is easy access by trains, trams or buses.

A clear and comprehensive initiative at saving energy could mean the difference between a painful and painless transition to renewable energy but it needs a brave and far- thinking government. Let us hope one comes along before it is too late.

The Jevons Paradox

There is a surprising problem when it comes to saving energy and that is named the Jevons Paradox after William Stanley Jevons who lived in the nineteenth century. He observed that Britain’s consumption of coal had soared after the improvement of Thomas Newcomen’s steam engine by James Watt. Because the use of the fuel had become more efficient, the demand for it grew. He thus came to the conclusion that as technological improvements increase the efficiency with which a resource is used, total consumption of that resource may increase, rather than decrease.

We can note it in our modern society with fuel consumption. If somebody buys a new car which uses less petrol than his old one, he is likely to actually use more petrol because it is now cheaper to drive than before and he will make more journeys.

It is this paradox that can present problems for our attempts to save energy. If we make houses and vehicles more efficient, we must be careful that people do not end up using more energy as a consequence.