Energy

Energy is the capacity or capability to do work and energy is used when work are done.
The unit for energy is joule - J, where

1 J = 1 Nm

which is the same unit as for work.

Energy forms

There can be several forms of energy, including

• mechanical energy
• heat or thermal energy
• electrical energy
• chemical energy
• nuclear energy
• light energy

Energy Efficiency

Energy efficiency is the ratio between useful energy output and input energy, and can be expressed as

μ = E_o / E_i         (1)
where
μ = energy efficiency
E_o = useful energy output
E_i = energy input

It is common to state efficiency as a percentage by multiplying (1) with 100.

Example - Energy Efficiency

A lift moves a mass 10 m up with a force of 100 N. The input energy to the lift is 1500 J. The energy efficiency of the lift can be calculated as

μ = (100 N) (10 m) / (1500 J)
    = 0.67 or
    = 67 %

Energy Conversion Factor

• 1 electron volt (eV) = 1.6021 10^-19 J
• 1 erg (erg) = 10^-7 J (exact)
• 1 calorie, thermochemical (cal) = 4.184 J
• 1 British thermal unit (Btu) = 1055.6 J
• 1 Q (Q) = 10¹⁸ Btu (exact)
• 1 quad (q) = 10¹⁵ Btu (exact)
• 1 tons oil equivalent (toe) = 4.19 10¹⁰ J
• 1 barrels oil equivalent (bbl) = 5.74 10⁹ J
• 1 tons coal equivalent (tce) = 2.93 10¹⁰ J
• 1 m³ of natural gas = 3.4 10⁷ J
• 1 litre of gasoline = 3.2 10⁷ J
• 1 kilowatthour (kWh) = 3.6 10⁶ J

Energy Density for Various Storage Form 

Storage		Typical Energy Densities
		(kJ/kg)	(MJ/m3)
Thermal Energy, low temperature	Water, temperature difference 100oC to 40oC	250	250
	Stone or rocks, temperature difference 100oC to 40oC	40 - 50	100 - 150
	Iron, temperature difference 100oC to 40oC	30	230
Thermal Energy, high temperature	Stone or rocks, temperature difference 400oC to 200oC	160	430
	Iron, temperature difference 400oC to 200oC	100	800
Conventional Fuels Energy	Crude Oil	42 000	37 000
	Coal	32 000	42 000
	Dry Wood	12 500 - 20 000	10 000 - 16 000
Synthetic Fuels Energy	Hydrogen, gas	120 000	10
	Hydrogen, liquid	120 000	8 700
	Methanol	21 000	17 000
	Ethanol	28 000	22 000
Electrochemical Energy	Lead-acid batteries	40 - 140	100 - 900
	Nickel-cadmium batteries	350	350
	Lithium ion batteries	700	1400
Mechanical Energy	Hydropower, 100 m head	1	1
	Compressed air		15
	Flywheel, steel	30 - 120	240 - 950
	Flywheel, composite materials	> 200	> 100