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Energy Density

The term energy density is used for the amount of energy that can be 'got out of' a fuel or power source. It is somewhat subjective: for example the effective energy density of petroleum within the atmosphere is very different to what it is in space, because in the former an oxidiser is available from the atmosphere while in the latter the oxidiser must be carried. In this section I have, of course, considered only the mass of the fuel, not the oxidiser.

Energy content of some fuels

Many of the figures in this table were taken from the Australian Bureau of Agricultural and Resource Economics (ABARE), energy definitions (http://www.abareconomics.com/pdf/ENERGYDEFINITIONS.pdf; unfortunately no longer available).
Another source I have used is Xtronics.

1 Tonne of oil equivalent (TOE) is defined as being 41.868GJ
By definition a TOE is the amount of energy produced by burning a tonne (metric ton) of oil. In practice, of course, this depends on the composition of the oil.

Fuel	Wh/kg	MWh/tonne	MJ/L	MJ/kg	Source
Ammonia	6 255	–	–	22.5	Wikipedia
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
Kerosene	12 800	12.8	37	46.1	ABARE
Heating oil	12 800	12.8	37.3	46.2	ABARE
Ethanol	8 200	8.2	23.4	29.6	ABARE and Xtronics
Methanol	5 500	5.5	15.6	19.7	ABARE
Coal	2 800 to 8 300	2.8 to 8.3	-	10    to 30	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
Bagasse	2 700	2.7	-	9.6	ABARE

Notes
Coal varies from 10 for wet lignite (brown coal) to 30 for high quality coking (black) coal.

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.

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.

Comparing flammable fuels, weight-for-weight, with batteries and compressed air as energy sources

Energy densities of batteries are much lower than the amount of energy that can be obtained from burning the same weight of flammable fuel - compare with 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:

1. The weight of the fuel tank containing the fuel (particularly important in the case of hydrogen);
2. The figures given for batteries are for the useful energy that they can provide, while the methods of getting useful energy from flammable fuels are usually only about 30% efficient.

However, even taking these factors into consideration, flammable fuels typically provide around 10MJ/kg while batteries yield less than 0.5MJ/kg.

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CO2 released per kWh

Carbon dioxide released per kWh of electricity generated For fossil fuelled power stations Approximate values
Natural gas	= 0.45 kg
Oil	= 0.5 kg
Black coal	= 0.8 kg
Brown coal	= 1.2 kg

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.

1 kg of carbon becomes 1 x 44/12 = 3.7 kg (approximately) of CO₂.

Burning 1 kg 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 3.7 kg = 3.1 kg of CO₂. A litre of petrol weighs roughly 800 grams, so burning a litre would release about 2.5 kg of CO₂

Burning 1 kg of natural gas

Natural gas is mostly methane. A molecule of methane is composed of one atom of carbon and 4 atoms of hydrogen. In atomic weights, 12 for the carbon, 4 x 1 = 4 for the hydrogen; so the molecular weight of methane is about 16, 75% of which is carbon.

So burning one kilogram of methane (natural gas) would release 75% of 3.7 kg = 2.8  kg of CO₂.

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Miscellaneous

Volume
1 barrel (oil) = 158.987L

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Some multipliers used in the SI metric system

A capital M must be used for 'mega' to distinguish it from the lower-case 'm' for milli (one thousandth). Generally capitals are used for multipliers and lower case for dividers.

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.
Comparative costs of energy in oil, electricity, and firewood are discussed on my Sustainability page.

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