# The Expression Energy

The expression 'energy' is often used without a great deal of thought and is applied to very different contexts. In this textbook - which only deals with technically usable types of energy, especially renewables - the physical laws describing the utilization of the energy resources will be investigated. Power is inseparably linked with energy. Since many people mix up energy, work and power, the first part of this chapter will point out differences between these and related quantities.

In general, energy is the ability of a system to cause exterior impacts, for instance a force across a distance. Input or output of work changes the energy content of a body. Energy exists in many different forms such as:

• mechanical energy

• potential energy

• kinetic energy

• thermal energy

• magnetic energy

• electrical energy

• nuclear energy

• chemical energy.

According to the definition above, a litre or gallon of petrol is a potential source of energy. Petrol burned in an internal combustion engine moves a car of a given mass. The motion of the car is a type of work. Heat is another form of energy. This can be seen when observing a mobile turning in the hot air ascending from a burning candle. This motion clearly demonstrates the existing force. Wind contains energy that is able to move the blades of a rotor. Similarly, sunlight can be converted to heat, thus light is another form of energy.

The power:

Table 1.1 Conversion Factors for Energy kJ

kcal kWh kg ce kg oe m3 gas BTU

1 kilojoule

1 kilocalorie

1 kilowatt-

hour (kWh) 3600

1 kg coal equivalent

1 kg oil equivalent

1 m3

natural gas 31,736 1 British Thermal

Unit (BTU) 1.0551

0.2388 0.000278 0.000034 0.000024 0.000032 0.94781 1 0.001163 0.000143 0.0001 0.00013 3.96831

7000

7580

8.14

10,000 11.63

8.816

0.123

1.428

1.083

0.086

0.758

0.113

3412

0.923 27,779

1.319 39,683

30,080

### 0.252 0.000293 0.000036 0.000025 0.000033

is the first derivative of the work, W, with respect to the time, t. Thus, power describes the period of time in which the correlated work is performed. For instance, if a person lifts a sack of cement 1 metre, this is work. The work performed increases the kinetic energy of the sack. Should the person lift the sack twice as fast as before, the period of time is half. Hence the power needed is twice that of before, even if the work is the same.

The units of both energy and work according to the SI unit system are joules (J), watt seconds (Ws) or newton metres (Nm), and the unit of power is the watt (W). Besides SI units a few other units are common in the energy industry. Table 1.1 shows conversion factors for most units of energy in use today. Older literature uses antiquated units such as kilogram force metre kpm (1 kpm = 2.72 • 10-6 kWh) or erg (1 erg = 2.78 • 10-14 kWh). Physics also calculates in electronvolts (1 eV = 4.45 • 10-26 kWh). The imperial unit BTU (British Thermal Unit, 1 BTU = 1055.06 J = 0.000293071 kWh) is almost unknown outside the US and the UK. Common convention is to use SI units exclusively; this book follows this convention apart from using electronvolts when describing semiconductor properties.

Many physical quantities often vary over many orders of magnitudes; prefixes help to represent these and avoid using the unwieldy exponential notation. Table 1.2 summarizes common prefixes.

Errors often occur when working with energy or power. Units and quantities are mixed up frequently. However, wrong usage of quantities can change statements or cause misunderstandings.

For example, a journal article was published in the mid-1990s in Germany describing a private photovoltaic system with a total installed power of 2.2

 Prefix Symbol Value Prefix Symbol Value Kilo k 103 (thousand) Milli m 10-3 (thousandth) Mega M 106 (million) Micro 10-6 (millionth) Giga G 109 (billion) Nano n 10-9 (billionth) Tera T 1012 (trillion) Pico P 10-12 (trillionth) Peta P 1015 (quadrillion) Femto f 10-15 (quadrillionth) Exa E 1018 (quintillion) Atto a 10-18 (quintillionth)

Note: Words in parentheses according to US numbering system

Note: Words in parentheses according to US numbering system kW. It concluded that the compensation of €0.087 to be paid per kW for feeding into the public grid was very low. Indeed, such a subsidy would be very low: it would have been 2.2 kW • €0.087/kW = €0.19 in total because it was stated as a subsidy for installed power (unit of power = kW). Although subsidies to be paid for solar electricity were quite low at that time, no owner of a photovoltaic system in Germany got as little as a total of 20 Eurocents. The author should have quoted that the payment per kilowatt hour (kWh) for electricity fed into the grid was €0.087. Assuming that the system would feed 1650 kWh per year into the grid, the system owner would get €143.55 per year. This is 750 times more than the compensation on the power basis. This example demonstrates clearly that a missing 'h' can cause significant differences.

Physical laws state that energy can neither be produced nor destroyed or lost. Nevertheless, many people talk about energy losses or energy gains, although the law of energy conservation states:

The energy content of an isolated system remains constant. Energy can neither be destroyed nor be created from nothing; energy can transform to other types of energy or can be exchanged between different parts of the system.

Consider petrol used for moving a car: petrol is a type of stored chemical energy that is converted in a combustion engine to thermal energy, which is transformed by the pistons into kinetic energy for the acceleration of the car. Stopping the car will not destroy this energy. It will be converted to potential energy if the car climbed a hill, or to ambient heat in the form of waste heat from the engine or frictional heat from tyres, brakes and air stream. Normally, this ambient heat cannot be used anymore. Thus, driving a car converts the usable chemical energy of petrol into worthless ambient heat energy. This energy is lost as useful energy but is not destroyed. This is often paraphrased as energy loss. Hence, 'energy loss' means converting a high quality usable type of energy to a low quality non-usable type of energy.

An example illustrating the opposite is a photovoltaic system that converts sunlight to electricity. This is often described as producing energy, which, according to the law of energy conservation, is not possible. Strictly speaking,

Gas stove

Electric stove

Microwave

Water boiler

Electric kettle

I I Energy demand in Wh/litre I I Costs for 10,000 litres in €

0 50 100 150 200

Figure 1.1 Prices for Water Heating a part of the energy in the incident solar radiant energy is converted to electrical energy, i.e. the photovoltaic system converts non-usable energy to high quality energy.

Technical systems perform energy conversions with varying efficiencies. The following example should illustrate this.

The thermal energy, Q, which is needed to heat up one litre of water (mass m = 1 kg) from the temperature = 15°C to = 98°C is calculated with the heat capacity, c, of water c Q = c • m • (#2 - #i)

A consumer magazine has compared different systems for boiling water. Figure 1.1 shows the results of different electrical appliances and compares them with those from a gas stove. The graph seems to show that the gas stove has the highest energy consumption while the energy costs are the lowest. The explanation is not the low price of gas, but that the graph compares different energy sources.

The electric stove uses electrical energy for water heating. Normally, this type of energy does not exist in nature, except for lightning or in electric eels. Power stations convert primary energy sources such as coal, gas or uranium into useful electricity. Conventional power stations produce large amounts of waste heat, which is emitted into the environment. They convert only a fraction of the energy stored in coal, gas or uranium into electricity, and the

 Term Definition Type of energy or energy source Primary energy Original energy, not yet e.g. crude oil, coal, uranium, processed solar radiation, wind Final energy Energy in the form that e.g. gas, fuel oil, petrol, electricity, reaches the end user hot water or steam Effective energy Energy in the form used by e.g. light, radiator heat, driving the end user force of machines or vehicles

majority is 'lost'. The efficiency, ^, describes the conversion quality and is given by:

profitable energy efficiency n =

expended energy

The average thermal power station in countries such as Germany has an efficiency of around 34 per cent. Two thirds of the expended energy disappears as waste heat. This means that only one third remains as electricity.

Technical conversion of energy has different conversion stages: primary energy, final energy and effective energy. These stages are explained in Table 1.3.

Going back to the example, it has to be emphasized that the calculated thermal energy (see equation (1.2)) is the effective energy, and the values given in Figure 1.1 are final energy. The comparison of energy efficiency should, instead, be based on primary energy when considering different energy carriers such as gas and electricity. The primary energy source for generating electricity is the coal, gas or uranium used in conventional power plants. Natural gas used for boiling water is also a type of final energy. The transport of natural gas to the consumer causes some losses, but these are much lower than the

Primary energy source Natural gas 100% (311 Wh)

Primary energy source Natural gas 100% (311 Wh)

Figure 1.2 Energy Conversion Chain and Losses for Water Heating with a

Gas Cooker

Figure 1.2 Energy Conversion Chain and Losses for Water Heating with a

Gas Cooker

Primary energy sour< e.g. coal 100% (3515 Wh)

Primary energy sour< e.g. coal 100% (3515 Wh)

Waste heat of cooker

Heating water 19% (97 Wh)

Figure 1.3 Energy Conversion Chain and Losses for Water Heating with an

Electric Cooker

Waste heat of cooker

Heating water 19% (97 Wh)

Figure 1.3 Energy Conversion Chain and Losses for Water Heating with an

Electric Cooker losses of the electrical transmission system (see Figure 1.2). Therefore, the primary energy consumption of the electric stove of 515 Wh = 1980 kJ is 65 per cent higher than that of the gas stove, although the final energy consumption is more than 30 per cent below that of the gas stove. This example is summarized in Figures 1.2 and 1.3, in which the energy conversion chain is compared for the electric and gas stove. The gas stove is the most economical appliance when comparing the primary energy demand, and it is the primary energy demand that determines the environmental impact.

Coal and crude oil were not relevant as energy supplies at the end of the 18 th century. Firewood and techniques for using wind and hydro power provided the entire energy demand. Watermills and windmills were common features of the landscape during that time.

In 1769 James Watt laid the foundations for industrialization by developing the steam engine. The steam engine, and later the internal combustion engine, swiftly replaced mechanical wind and water installations. Coal became the single most important source of energy. In the beginning of the 20th century, crude oil took over as it was needed to support the increasing popularity of motorized road traffic. Firewood lost its importance as an energy supply in the industrial nations, and large hydro-electric power stations replaced the watermills.

The world energy demand rose sharply after the Great Depression of the 1930s. Natural gas entered the scene after World War II. In the 1960s, nuclear power was added to the array of conventional energy sources. These relatively new sources have not yet broken the predominance of coal and crude oil, but gas is the energy carrier with the fastest growth. The share of nuclear electricity of today's primary energy demand is still relatively low. The fossil energy sources - coal, crude oil and natural gas - provide more than 85 per cent of the world primary energy demand.