What energy is, what it does and how it’s measured

If you know what energy is, what it does and how it’s measured you’ll be in a much better position to deal with advice and make sensible decisions about energy management.

Work and Energy

Energy can be converted from one form to another but can never be created or destroyed.

Energy Conversion

When you burn fossil fuels it releases Carbon Dioxide (which is building up in the atmosphere and causing the climate to change).
How are CO2 emissions calculated?

Emissions

Once you know a little bit about energy you can make some big decisions.

Powerful knowledge

And you can even work things out, like costs and payback times.

Working things out.

The pages above simply deal with the basics. If you want to go into a bit more detail, and look at topics such as heat, electricity and where our energy ultimately comes from then follow this link.

The Science of Energy

Work and Energy

Work

All of the units of energy and power are based on a very simple idea. If you push, or pull, something you’re doing mechanical work.

So, work is something done by forces, but only when they move. The force that stops your elbows doing through the table isn’t doing any work because it isn’t moving, but the force you use to push a supermarket trolley is.

The amount of work a force does depends on two things. How big it is and how far it moves. For example, pushing a supermarket trolley down two aisles takes twice as much work as pushing it down one. Likewise, pushing two trolleys at once, and using twice the force, takes twice as much work as just pushing one.

So, Work = Force x Distance

The basic unit of force is called the newton and represents the force that would be needed to give a mass of 1kg an acceleration of 1m/s/s (i.e. speed it up by 1m/s every second)

The basic unit of distance is the metre

The basic unit of energy, the joule(J), is defined as the work done by a force of 1 newton(N) when it moves something by 1 metre(m)

For example, if you pushed a supermarket trolley 10m using a force of 25N you’d have done 250J of work.

Energy

If something has Energy it means you could get it to do some work. The amount of energy it’s got is just the amount of work it could do.

So, Energy = capacity to do work

Power

Energy can get converted from one form to another. E.g. a light bulb converts electrical energy into heat and light, your body converts food energy into heat and motion. The more powerful the device the more quickly this happens.

So, Power = rate of doing work = rate of converting energy from one form to another.

The basic unit of time is the second(s) and the basic unit of power is the watt(W) and

1 watt = 1joule per second or 1W = 1J/s

For example a 100W light bulb converts 100J of electrical energy to heat and light every second. This means that in a minute it would convert 6000J, and in an hour 360,000J.

The kilowatthour

The joule is a pretty small unit of energy and the numbers could soon get unmanageable. The most practical unit of energy is the kilowatthour(kWh). This represents the work done by something with a power of 1kW (1000W) working for 1 hour (3600s). So 1kWh = 3,600,000J

For example, a 100W bulb working for 1 hour uses 0.1kWh of electricity (where 100W = 0.1kW). A 3kW kettle operating for 5 minutes would use 0.25kWh of electricity (3kW x 5/60hrs)

n.b to help do these calculations just remember that 1000Wh = 1kWh and that there are 60 minutes in an hour and 60 seconds in a minute

Other units

There are all sorts of historic units for heat and power. The first of these was the mechanical horsepower, about 750W and not to be confused with the boiler horsepower. Other definitions, and their conversion factors, can be found here.

Converting energy from one form to another

We use lots of devices to convert energy from one form to another

• A gas boiler converts the chemical energy of the gas into heat energy to warm your home.
• A car engine converts the chemical energy of petrol, or diesel, into mechanical energy of motion.
• A light bulb converts electrical energy into heat and light (mainly heat).
• A loudspeaker converts electrical energy into sound energy.
• the retina of your eey converts light energy into chemical energy and hence into the electrical energy of a nerve impulse to your brain.
• A generator converts mechanical energy into electrical energy.
• A growing plant converts light energy into chemical energy (which we can later make use of by eating the plant).

None of these energy conversions is 100% efficient, with the missing energy usually appearing as heat. Indeed, an ordinary light bulb only converts about 5% of the electrical energy into light and a typical car engine only manages to turn about 30% into movement.

For example, an 800hp (600kW) Formula 1 car produces about twice as much heat as it does useful motion. This means that at full power it produces heat at the rate of about 1200kW. Since a typical central heating boiler is rated at less than 20kW, this is the equivalent of a whole street full of boilers (60) going full blast.

The more times you convert energy from one form to another the more of it is likely to get lost along the way.

Energy Efficiency

The first steam engines were only about 2% efficient but, as the science behind them became better understood, the most efficient boilers and steam turbines (found in big power stations) are now over 40% efficient (i.e only 60% of their input energy is wasted). If you also make use of the waste heat (in a combined heat and power plant) then the overall efficiency can be doubled to about 80%.

Compact Fluorescent Lamps (CFLs) are about 5 x as efficient as old fashilned filament bulbs. Not only that, but they last at least 5 x as long.

A condensing boiler (which recovers heat from the water vapour that’s produced when a fossil fuel burns) has an efficiency of well over 80%, compared to the 60% of a typical non-condensing boiler. (i.e. you’ll get the same amount of heat and save at least 25% on your fuel bills)

A typical laptop computer, running off the mains, uses about 30W of electrical power. A desktop model could easily use over 200W to do the same job. This can be even worse if you end up putting in air conditioning to help get rid of the excess heat.

The Range Rover, sat in the traffic jam behind a Fiat Uno, will be using well over 4 times the amount of fuel for the privilege.

Emissions

Whenever we burn a fuel we produce emissions of gases. Some of these, like Sulphur Dioxide which produces acid rain, are directly harmful. Others, like Carbon Dioxide, are greenhouse gases and their accumulation in the atmosphere is causing Climate Change.

Life Cycle Analysis

Whenever we compare different sources of energy we don’t only have to think about the emissions they produce when they’re being used but also the emissions that were produced when they were manufactured.For example, it takes energy to produce a wind turbine, in particular for its concrete base, and so even a wind turbine doesn’t have zero emissions.

So, to be fair, we have to compare different energy systems over their entire life time. We have to produce a life cycle analysis.

The following table, from the International Atomic Energy Agency, shows typical values for the life cycle analysis of different ways of producing electrical power. If there are tow bars these show the range of values that have been determined. The scale is grammes of CO2 per kWh of electricity produced.

Direct Emissions

Direct emissions are those that simply come from burning fuel. To work these out you just need to know how muc fuel you’ve consumed and of what type. The National Energy Foundation has a simple to use emissions calculator.

Indirect emissions

These are the emissions that arise during the manufacture of an object. This includes the emissions from extracting the materials used, processing them and transporting them about. they are sometimes called the embedded emissions.

Indirect emissions are much harder to calculate, but can be minimised by re-using and recycling materials whenever possible. For example, renovating an old building will produce far less indirect emissions than building a new one from scratch.

Powerful knowledge

It’s often said that knowledge is power. Well, knowledge about power is powerful knowledge.

Whether you’re heating a house, lighting an hotel, driving a car, running a factory or building a shopping centre, you’ll be using energy and producing emissions. But, when you flick the light switch you don’t do it to make the meter go around. You do it so that you can see what you’re doing. What matters, isn’t the energy that’s consumed but the end-use that you put it to.

Heating things up.

• The less stuff you heat up the less energy it will take to do it. So, don’t put any more water in the kettle than you’re going to need.
• When you’re heating a house you’re simply replacing the heat that it loses. The cheapest technology for saving energy is insulation.
• The hotter something is the faster it will lose heat to its surroundings and the faster you’ll have to supply heat to replace the heat that’s lost. Turning down the thermostat by 1 C can reduce your heating bills by 20%.

Cooling things down.

• A fridge or a freezer has to chuck out any heat that leaKS in from outside. The better the insulation the less will leak in.
• More heat will leak into a fridge or freezer if it’s kept somewhere warm. So, try not to put it in aheated space.
• Closing the curtains or blinds on a warm sunny day will stop too much heat getting in and reduce the need for an air conditioning.
• Sweating works better, as a way of losing heat, if the air in a room is moving. A fan can do the job of an air conditioner using a fraction of the energy.

Moving ourselves about.

• The fewer trips you make the less energy you’ll use. So, if your making a trip to do something try and see what else you can get done at the same time.
• Buses and trains use much less energy per passenger mile than all but the most efficient, and fully loaded, cars.
• A car with four people in it doesn’t use anything like 4 times as much energy. So, if you’re going the same place as someone else then share your car.
• A car travelling at 70mph uses 25% less fuel than a car going at 80mph. Obey the speed limits and you’ll save both stress and money. Your place in the next hold up will still be reserved especially for you.
• If the engine isn’t running it’s not using any fuel. Turn it off if you’re going to be stationary for more than about 30 seconds.
• Travel less but walk more. Not only is walking good for you but you also learn to appreciate your own neighbourhood and the people that live in it.
• The most efficient device for transportation over land that has ever been invented, or that nature has ever evolved, is the bicycle. In town, the bike isn’t just efficient it’s also quick, cheap and reliable.

Moving stuff about.

• The less stuff you shift the less energy takes to do it. There’s no point hauling bottled water from one end of the country to the other when perfectly decent stuff comes out of the tap.
• The slower you shift it the less energy it will take. A parsnip taking the slow boat from Australia will use much less energy than one that takes a cheap flight from Malaga.
• The shorter the distance you shift it the less energy it will take. Local produce won’t usually have travelled as far (though it might have taken more energy to produce).

Working things out

To work out how much energy something will use all you need to know is how much power it uses and how long you use it for.

To work out how much it costs, the only other thing you need is the price for each unit of energy.

Example 1

A fridge motor is rated at 80W. On average it runs for 6 hours a day.

If electricity costs 12p per kWh, how much does it cost to run the fridge for a year?

Energy consumed over the year = power x time = 80W x 6hours X 365days = 175200Wh = 175.2kWh

Cost = energy consumed x cost per unit of energy = 175.2kWH x 12p = 2102p = £21.02

Example 2

A traditional 100W light bulb is replaced by a 20W CFL.

If the CFL costs £2.99 and is on for 10 hours a day, how long will it take to recover the CFL’s cost?

(Assume electricty at 12p per kWH.)

The replacement bulb saves 80W (100w – 20W) and in 10 hours it saves 80W x 10hours = 800Wh = 0.8kWh of electricity.

The cost of 0.8kWh of electricity = 0.8 x 12p = 9.6p and so the CFL saves 9.6p per day.

Hence the number of days it takes to recover £2.99 = 299p/(9.6p per day) = 31days

So, it’s paid for itself in about a month and will carry on saving for the rest of its life. (8000 hours at 10hours per day = 800days = just over 2 years)

Example 3

You’ve just moved to Oliver’s Heights and work at the hospital (about 5km away).

If you went to work by bike for 40 weeks of the year, and your car emits 200g of CO2 per km (a typical figure), what CO2 emissions would you avoid?

5km one way = 10km both ways.

10km per day for 5 days a week for 40 weeks a year = 10 x 5 x 40 = 2000km per year.

CO2 saved per year = km per year x emissions per km = 2000km x 200g/km = 400,000g = 400kg = 0.4tonnes