The important concepts of energy and power, and the difference
between them, are explained briefly in my
Wind power glossary; it also
includes terms particularly relating to wind power.
|Units of energy|
|1 Watt hour (Wh)||= a flow of 1W for 1 hour (or equivalent)|
|1 kilowatt hour (kWh)||= a flow of 1kw for 1 hour (or equivalent)|
|1 megawatt hour (MWh)||= 1000kWh|
|1 gigawatt hour (GWh)||= 1000MWh|
|1 terawatt hour (TWh)||= 1000GWh|
|1 kilojoule (kJ)||= 1000 Joules|
|1 megajoule (MJ)||= 1000kJ|
|1 gigajoule (GJ)||= 1000MJ|
|1 Watt second||= 1 Joule (J)|
|1 Watt hour (Wh)||= 3600 Joules or 3.6 kilojoules|
|1 kilowatt hour (kWh)||= 3.6 megajoules|
|1 megawatt hour (MWh)||= 3.6 gigajoules|
|1 kJ||= 0.278 Wh|
|1 MJ||= 0.278 kWh|
|1 GJ||= 278 kWh|
|1 TJ||= 278 MWh|
|1 PJ||= 278 GWh|
|1 kWh||= 3.60 MJ|
|1 calorie (c)||= 4.19002 Joules|
|Note 1 calorie is (approximately) the amount of heat required to raise the temperature of one ml of water by 1 degree Celsius. This unit is less used than it was. Note that it is one thousandth of the Calorie used in nutrition (capital 'C'). (So it requires 1000 × 80 = 80 000 calories to heat one litre of water from 20° to boiling point, and 80 000c = 335.2kJ or 93.2Wh.)|
|Propane||13 800||13.8||25.4||49.6||ABARE and Xtronics|
|Petrol (automotive gasoline)||12 900||12.9||34.2||46.4||ABARE and Xtronics|
|Heating oil||12 800||12.8||37.3||46.2||ABARE|
|Ethanol||8 200||8.2||23.4||29.6||ABARE and Xtronics|
to 8 300
|ABARE and others|
|Energy in air-seasoned firewood||4 400||4.4||-||approx. 16.0||ABARE and others|
|Hydrogen||39 000||39||-||142||Several, including Hypertextbook|
While hydrogen has a very high energy content per kilogram, it is very light in weight, even when highly compressed or liquefied. It therefore does not have a high energy content per litre of space required to store it. Also, as all the systems used to store hydrogen weigh much more than the hydrogen they store, the useful energy per kilogram of storage system is low. The best current hydrogen storage systems can manage only about 3MJ/L or 4MJ/kg (4GJ/tonne). See The Industrial Physicist.
|Battery type||Energy density||Cycles|
|Wh/kg||kJ/kg||MJ/kg||Cycle life is to 80% initial capacity|
|NiMH||60-120||220-430||0.22-0.43||300 to 500|
|Lead-acid||30-50||110-180||0.11-0.18||200 to 300|
|Lithium-ion||110-160||400-580||0.40-0.58||300 to 500|
|Lithium-ion-polymer||100-130||360-470||0.36-0.47||300 to 500|
|Reusable alkaline||80||290 (initially)||0.29||50 (to 50% capacity)|
|Taken from Battery University, What is the best battery?|
|Energy source||Energy density|
Energy from falling water
The relevant equation is E=mgh; where, using the SI metric system (kg,m,sec);
|Energy from a kilogram of water falling 1m|
or energy needed to lift 1kg of water 1m
|Energy from a kilolitre of water falling 1m|
(1kL of water = 1 tonne approx.)
|Energy from a megalitre of water falling 1m||9800kJ, 9.8MJ, 2.7kWh|
|Energy from a megalitre of water falling 100m||270kWh|
|Energy from 100ML of water falling 100m||27MWh|
|Energy from 1GL of water falling 100m||270MWh|
|Power from a litre of water per second falling 1m||9.8 Watts|
|Power from a kilolitre of water per second falling 1m||9.8kW|
|Power from a 10kL of water per second falling 10m||980kW|
|Power from a 100kL of water per second falling 10m||9.8MW|
|Power from a 100kL of water per second falling 100m||98MW|
It takes about seven or eight times as much energy to convert boiling water to steam as is needed to melt the same mass of ice and about a fifth as much energy to raise the temperature of water from freezing point to boiling point as is needed to boil it all away.
So when an evaporative air cooler evaporates one litre of water it cools a room by about the same amount as running a simple 1kW heater would warm the room in three quarters of an hour.
Energy content of fuels, above.
Compressed air can also be used as a source of energy. How much useful energy you can get from a tank of compressed air depends on the pressure inside the tank, the size of the tank, and the efficiency of the compressed air engine. The effective energy density for compressed air as an energy source depends on these factors in addition to the weight of the tank.
To be fair, there are two more factors that should be considered in this comparison:
For fossil fuelled power stations
|Natural gas||= 0.45kg|
|Black coal||= 0.8kg|
|Brown coal||= 1.2kg|
How do you calculate the amount of CO2 released from burning one kilogram of carbon?The carbon dioxide (CO2) molecule is made up of one atom of carbon and two atoms of oxygen. Carbon has an atomic weight of 12, the atomic weight of oxygen is 16. Therefore, when one kg of carbon combines with oxygen we have 12 mass units of carbon and 32 units of oxygen being converted into 44 units (12 + 16 + 16 = 44) of carbon dioxide.
1kg of carbon becomes 1 x 44/12 = 3.7kg (approximately) of CO2.
Burning 1kg of petrol (gasoline for USians)Petrol is composed of a mix of short-chain hydrocarbons; I will use heptane for my calculations. A molecule of heptane is composed of seven atoms of carbon and 16 atoms of hydrogen. In atomic weights, 7 x 12 = 84 for the carbon, 16 x 1 = 16 for the hydrogen; so the molecular weight of heptane is about 100, 84% of which is carbon.
So burning one kilogram of heptane (or petrol) would release 84% of 3.7kg = 3.1kg of CO2. A litre of petrol weighs roughly 800 grams, so burning a litre would release about 2.5kg of CO2
|1 barrel (oil)||= 158.987L|
Don't ask me why the abbreviation for kilo (one thousand) is lower case; that's just how it is.
The abbreviations for Watt and Joule are
usually capitalised because they are peoples' names.
|1018||Exa||E||1 000 000 000 000 000 000||Billion billion|
|1015||Peta||P||1 000 000 000 000 000||Million billion|
|1012||Tera||T||1 000 000 000 000||Trillion|
|109||Giga||G||1 000 000 000||Billion|
|106||Mega||M||1 000 000||Million|
Carbon into carbon dioxide
CO2 released per kWh
Compressed air and flywheels
Energy and boiling water
Energy and melting ice
Energy content of fuels
Energy density of some batteries
Energy from falling water
Human power output – approximate
Metric system multipliers