a beginners guide to energy and power en

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Fuel comparisons

Historically we have largely acquired our energy from the consumption of fossil fuels so consideringand comparing fuel sources in terms of the amount of energy they contain per unit of purchasedsource would be instructive. Some guidance (using average values) is given below.

1 kg of anthracite (4% moisture content) = 36MJ = 10kWh

1 kg coal (5-10% moisture content) = 37MJ = 10.3kWh

1m 3 of natural gas = 39MJ = 10.8kWh

1 litre of gasoline = 34MJ = 9.4kWh

1 litre of diesel fuel = 40MJ = 11.1kWh

1 litre of gas oil = 41MJ = 11.4kWh

1 litre of fuel oil = 44MJ = 12.2kWh

(The above values are based on Gross Calorific Value i.e. assumes water formed and liberated duringcombustion is in the liquid form.)

For comparison a 1kg of a renewable fuel like a woody biomass might typically contain 4.2kWh.

Energy content pricing is an interesting concept. For example a litre of diesel fuel containsapproximately 18% more energy than 1 litre of gasoline and yet its price at the fuel pump is only afew percent in excess of gasoline.

Power

Power is the rate of doing work.

Again, the units of power are named after a researcher whose contribution to a particular field iscentral to its understanding. The internationally (non-US) recognised unit of energy is the Watt .

Scottish engineer and inventor James Watt (1736-1819) was employedby Glasgow University to repair a model of the most advanced steamengine of its time, the Newcomen engine. Watt devised a range of thermal

and mechanical improvements to the device that improved its efficiencyby 300% allowing it to become universally adopted wherever mechanical

power was required.

A Watt is an expenditure of 1 Joule per second. The value of the power rating is commonly statedon energy consuming and energy generating devices and tells us something about how quicklyenergy is being either used or made.

1 Watt is not exactly a large amount of power and so a multiple unit, the kilowatt (abbreviated tokW ), i.e. 1000Watts, is commonly in use.


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For illustration purposes the power rating of some devices is given below:

Personal computers: 50 200W (consumption) Microwave ovens: 650-850W (consumption)

Lawn mowers: 1-1.7kW (consumption) 20m 2 of mono-crystalline photovoltaic: 2.5kW peak (generation) A domestic biomass pellet boiler: 15kW (generation) A 50m diameter horizontal axis wind turbine : 500kW peak (generation) Steam turbines: up to 60MW (generation) Gas turbines: up to 100MW (generation)

Energy power relationship

Energy and power are inextricably intertwined. The application of power for a given time will eitherresult in ( generation ) or require ( consumption ) energy.Mathematically the link is simple if you remember to get the units ( kWh, kW and hrs ) right.

The link is:

Energy (kWh) = Power (kW) x time (hrs)

Examples

1. A electric convector heater rated at 1 kW left on for:

1 hr will consume 1 x 1 = 1kWh

30 minutes will consume 1 x (30/60) = kWh

2. A compact fluorescent lamp rated at 20 Watts left on for8hrs will consume (20/1000) x 8 = 0.16kWh

Final words

Power (measured in kilowatts ) and energy (measured in kilowatt-hours or Joules ) are not identical orinterchangeable. However their conversion is relatively simple with a little practice.

If you would like to learn more, follow the links below.http://www.kayelaby.npl.co.uk/http://www.simetric.co.uk
http://www.kayelaby.npl.co.uk/http://www.kayelaby.npl.co.uk/


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Neil Packer is a Chartered engineer and Senior lecturer at the Faculty of Computing, Engineering andTechnology, Staffordshire University, UK. He has been teaching thermo-fluid and environmental engineeringfor nearly 20 years and acts as a Low Carbon Consultant providing a range of energy services to business,industry and local authorities.

Contact details:Faculty of Computing, Engineering and TechnologyStaffordshire UniversityBeaconside, Stafford, ST18 0ADTel 01785 353243 email [email protected]

This information was presented as part of the Renewable Energies Transfer System Project (RETS) funded byINTERREG IVC through the European Regional Development Fund. The project time line is January 2010 toDecember 2012. For more information and to take part in our online community visit: http://www.rets-community.eu/
mailto:[email protected]:[email protected]:[email protected]:[email protected]

a beginners guide to energy and power en

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