User:Samir Muzaffar

= Energy and units =

1.1 Introduction
Energy is needed for work and energy is needed for life. Yet despite its allencompassing

nature energy is something that, until recently, many took for

granted – giving little thought of where it comes from, or how it is used.

Over the past decade energy has steadily risen up both the political agenda

and public consciousness. There are at least three reasons why energy

attracts so much attention:

- Climate change and fears that consumption of fossil fuels is changing

global weather patterns.

- World energy resources and fears that energy use is unsustainable in

the face of limited energy reserves.

- Energy security and fears that energy supply may become unreliable.

The energy sector as a whole is a complex web of interlinked activities,

markets and technologies. What happens in one sector affects all others:

- For instance if a global dispute decreases gas supplies this will force

up not only the price of gas but also the price of electricity – in many

countries significant electricity generation capacity is fuelled by gas.

- A rise in the price of electricity will make renewable energy

technologies more competitive and stimulate this sector.

- Industrial expansion in China will increase global demand for petroleum

which in turn will stimulate efforts to find substitutes, such as bio-diesel,

ethanol and hydrogen.

Energy is a key driver for socio-economic change, for instance the industrial

revolution was only possible because of coal mining and steel production – the

development of the steam engine was also required:

- Coal was formed from dead plant matter which over millions of years

was transformed successively from peat to lignite, to bituminous coal

and finally into anthracite.

- The energy from the sun was stored by ancient plants (by way of

photosynthesis) and this energy, in turn, became trapped as the plant

material as it was subsequently buried and converted into coal.

Further industrial development, during the late 19th and 20th centuries, was

largely a story of the internal combustion engine replacing the steam engine,

which in turn lead to an increased demand for hydrocarbons:

- Oil was form by the deposition and burial of aquatic plant and animal

remains – generally zooplankton and algae.

- Under deep burial these organisms were gradually transformed into

hydrocarbons – both oil and gas (gas is also associated with coal).

This chemical energy, collectively known as fossil fuels accumulated over

millions of years and has only recently been extracted on an industrial scale.

1.2 Units
Units have been discussed in other courses and students should be familiar

with SI units – where possible these units should be used. However, other

units are often used in the global energy sector as will be discussed..

Energy is a primitive term and has been defined in several ways. Perhaps the

best approach is to say that energy is a consequence of the First Law of

Thermodynamics:

- The First Law of Thermodynamics is commonly known as the principle

of conservation of energy.

- If a system is taken through a cycle, i.e. starts in a state, undergoes a

sequence of processes and then returns to the same state.

- And, if all the interactions between the system and surrounding (heat

and work flows) are measured accurately, then they will sum to zero.

- There must be “something” stored in the system, that has varied in

accordance with these interactions, but has ultimately returned to its

original value – that “something” is called the energy of the system.

Over the centuries the output of machines has increased dramatically:

Note the large increase in power output since the industrial revolution, until

then the main sources of power were animals, windmills and waterwheels.

1.2.1 Unit of Power
In SI the unit of mechanical power is the watt (W) defined as follows:

1 W = 1 J/s.

Power the rate at which energy is being produced, thus power is energy per

unit time. In SI the unit of energy is the joule (J) defined as follows:

1J = 1 N m

Energy in the form of work is the product of force times the distance moved.

However, it was shown by Joule that work can be completely converted into

heat (although completely converting heat entirely into work is not possible).

In the US system, the unit of mechanical power is the horsepower (hp). The

horsepower was originally developed by Watt to compare the mechanical

power output of his steam engines to that of a horse:

1 hp = 550 ft lbf/s

(1 ft Ibf = 1.3558 J)

The horsepower has since been standardised with the watt and is now

approximately

1 hp = 745.7 W

Since heat and work are both forms of energy, it follows that heat is also

measured in joule (J) and the rate at which heat is being transferred is also

measured in watt (W) – this is sometimes known as “thermal power”.

A non- SI unit for heat is the calorie (cal) – this is the amount of heat needed to raise the temperature of 1 gram of water by 1 degree centigrade. The

calorie is now also standardised against the joule and is defined as

1 cal = 4.1868 J

In US units, the unit of heat is the British Thermal Unit (BTU) which is the

amount of heat needed to raise 1 lb of water by 1oF. The BTU can vary

depending on the temperature used – the IT (International Table) conversion is

1 BTU = 1055.06 J

In the US, however, the BTU is defined in terms of the 15oC calorie to be

1 BTU = 1054.804 J

An old measure of gas consumption was the therm (thm) – it is approximately

the amount of energy released by burning 100 cubic feet of gas. However, the

therm was originally defined as an exact number of BTU:

1 Therm = 105 BTU = 1.055 x 108 J

While the therm is useful for domestic consumption, the “quad” (quadrillion) is

used by US Department of energy to discuss global and national energy

usage. Like the therm the quad is defined as an exact number of BTU:

1 Quad = 1015 BTU = 1.055 x 1018 J

In SI the rate of heat transfer is the watt or the kilowatt, while in the US the rate

of heat transfer is measured in terms of (BTU/h)

1 BTU/h = 0.2931 W

1.2.2 Kilowatt-hour

If power is energy per unit time, then it follows that power times time must be

equal to an amount of energy.

In this way a kilowatt-hour (kWh) is an amount of energy transferred over a

one hour period of time:

1 kWh = 1 kW × 1 hr

To obtain the conversion between (kWh) and (J) start with the SI definition of

the watt

1 W = 1 J/s

1 kW = 1 kJ/s

Multiplying across by time leads to

1 kW s = 1 kJ

To change (kWs) into (kWh) divide the LHS by 3,600 (s/h). Finally multiply

across by the factor “3,600” to isolate (kWh) on the LHS

1 kWh = 3,600 kJ = 3.6 x 106 J

This clearly shows that the (kWh) is an amount of energy – across Europe

domestic energy usage, both electric and gas, is expressed in (kWh).

The kilowatt-hour is a good size for domestic and commercial energy usage.

1.2.3 Barrels of Oil Equivalent
Another unit of measure used when comparing different energy sources (in

particular different type of fossil fuel such as coal, oil and gas) is the “barrel of

oil equivalent” (bboe or BOE).

One Barrel of Oil Equivalent (BOE) is the approximate amount of energy

released when 1 barrel of crude oil (42 US gallons) is completely burned.

Due to variations in the calorific value of oil it is fixed as follows:

1 BOE = 6.12 x 109 J

When discussing the energy value of very large quantities of hydrocarbon

reserves this is scaled up to “billons of barrels of oil equivalent” (BBOE):

1 BBOE = 6.12 x 1018 J

1.2.4 Tonnes of Oil Equivalent

Another related measure is “tonnes of oil equivalent” (toe) – again it is the

notional energy released when I tonne of oil is burned and is fixed at

1 toe = 4.187 x 1010 J

When talking about larger quantities of energy this can be scaled up to

“millions of tonnes of oil equivalent” (Mtoe)

1 Mtoe = 4.187 x 1016 J

1.2.5 Solar Power Output
The solar constant, 1377 W/m2, is the average energy flux (energy per unit

surface area) arriving at the earth’s atmosphere from the sun. This gives a

total energy flow arriving at the outer atmosphere of some 5.46 x 1024 J/yr:

• Of this energy input about 30% is reflected by the atmosphere with a

further 23% being used to drive evaporation and precipitation.

• Most of this heat input is then re-radiated back into space as longer

wave radiation and only a small fraction, some 4.1 x 1017 J/yr, is being

laid down currently as future fossil fuels.

Human energy consumption, mainly derived from fossil fuels, is rising fast but

may be estimated at some 5.3 x 1020 J/yr (2010, EIA). Clearly consumption of

fossil fuels outstrips the rate at which new supplies are being laid down.

1.2.6 Conversion Efficiency
Power output of any device is always less than power input. The difference

between the power input and the power output are the energy losses

associated with the item of equipment.

The efficiency of any energy conversion process is defined as power output

divided power input – it may be expressed either as a fraction or as a

percentage.

$$\eta=\left ( \frac{Power Output}{Power Input} \right )\times100\%$$

1.3 Reference

 * Dr Sandy Kerr. MSc Course: Energy in the 21st Century.
 * MSc Course: Petroleum Engineering.