Tech Note 2

Getting to grips with energy

There is a great deal of talk about energy - how we must save energy, how the world is running out of energy, and then we are told the world is heating up because it is capturing too much energy from the sun.   We all have a vague idea of what energy is - we know that we need energy to work.   Turning this idea around we have the definition of energy; it is the capability to do work.   Energy comes in many different forms, chemical, electrical, heat, gravitation, kinetic, elastic, nuclear, and they possess the almost magical property that any one can be transformed into any of the others!

When we boil a kettle of water we are just transforming electrical energy into heat energy.   Our electricity bills tell us how many kilowatt-hours we’ve used, but how many of us understands what this means?   How big is a kWh?   To get some idea we must go back to the basics.   All energy is measured in the same fundamental unit called a Joule. Joule was a French scientist who measured heat, mechanical and electrical energy.   A Joule is a very small unit, and it is very easy to get a feel for what it can do.   Take an average apple, say about 100grams, and drop it from waist height, about 1 meter, onto your foot.   The apple has a gravitational energy of about 1J.   As it falls it loses gravitational energy, acquiring kinetic energy, and when it stops on your foot, all that energy of 1J goes into your foot as heat energy!    Clearly 1J is not going to be much use!   To see how many we need to boil a kettle we have to know the connection between energy and temperature.   In the case of water it is simply that to raise the temperature of 1 litre of water 1⁰C requires 4000J!   So to boil a litre you need 400,000J!   If you are wanting to make a pot of tea you don’t want to wait all day, in fact, not more than a few minutes!   If you are prepared to wait for around 3 minutes, say 200 seconds, the electricity needs to supply 2000 Joules every second!   Now a Joule per second is called a Watt, named after the man who developed the steam engine.   So we have that an electric kettle should be around 2kW.

As the Joule is such a small unit it is convenient to take a large number of Joules and give it a new name.   To see how we do this note that if we multiply the number of watts by the time in seconds we get the number of Joules, and so we can call a Joule a Watt-second.   If we divide the number of Joules by 3600 we will have a number of Watt-hours, and dividing again by 1000, we have the number of kWh.   Returning to the electric kettle, the energy used is 400,000J/3600s/1000=1/9kWh.   At around 18p/kWh we see it costs about 2p to boil our kettle!

In Europe, in mid summer, at noon, the energy falling on the Earth from the Sun is around 1kW per square metre.   The average household in Bridport uses around 12 kWh per day.   If we could capture all of this energy and change it into electrical energy, 12 square metres could provide most of our electrical energy for around half of the year.   Unfortunately, at present these photovoltaic panels are only around 20% efficient.   More detailed calculations show that averaged over the year would require 36 square meters to provide all the electrical energy. Nevertheless photovoltaic panels can provide a useful contribution.   Combined with the current feed-in tariffs, whereby you are paid for the electricity you generate, the payback time can be reduced to an acceptable period.   The details are complex and advice should be obtained from an independent source.

A more efficient way of capturing the Sun’s heat, though not in such a convenient form, is to absorb it in panels filled with water, which is circulated to the hot tank.   This will provide more hot water than you need during the summer months, and if the connections are in series with the boiler, will reduce the demand for gas or oil in the winter.   Just preheating the water in 100-litre tank by 10^oC will save around 1kWh of energy.