Chapter 1 - Basic energy concepts* * by W.S. Hulscher University of Twente The Netherlands
1. Introduction 2. Forms of energy 3. Energy conversion 4. Energy and power 5. Energy sources 6. Some notes on energy terminology 7. Energy flow 8. Energy units and dimensions 9. Energy losses and efficiency 10. Equivalence and replacement of energy forms 11. Energy balance 12. Process energy requirements and gross energy requirements 13. Examples of calculations of energy conversions
1. Introduction Energy is involved in all life cycles, and it is essential in agriculture as much as in all other productive activities. An elementary food chain already shows the need for energy: crops need energy From solar radiation to grow, harvesting needs energy from the human body in work, and cooking needs energy from biomass in a fire. The food, in its turn, provides the human body with energy. Intensifying food production for higher output per hectare, and any other advancement in agricultural production, imply additional operations which all require energy. For instance: land preparation and cultivation, fertilising, irrigation, transport, and processing of crops. In order to support these operations, tools and equipment are used, the production of which also requires energy (in sawmills, metallurgical processes, workshops and factories, etc.). Major changes in agriculture, like mechanisation and what is called the "green revolution", imply major changes with respect to energy. Mechanisation means a change of energy sources, and often a net increase of the use of energy. The green revolution has provided us with high yield varieties. But these could also be called low residue varieties (i.e. per unit of crop). And it is exactly the residue which matters as an energy source for large groups of rural populations. Other sectors of rural life require energy as well. The provision of shelter, space heating, water lifting, and the construction of roads, schools and hospitals, are examples. Furthermore, social life needs energy for lighting, entertainment, communication, etc. We observe that development often implies additional energy, and also different forms of energy, like electricity. Energy is a scarce resource, at least for some groups of people in some places and, maybe, for the world as a whole. A rational use of energy is then necessary for economic and environmental reasons. This applies to agriculture as much as to any other sector of the economy. A key to the rational use of energy is the understanding of the role of energy. The following sections aim to help understand energy in agriculture and rural development. It should help communication between agricultural planners and energy specialists. Anyone familiar with energy concepts should skip this chapter and read immediately Chapter 2.
2. Forms of energy Energy can exist in various forms. Examples are: - Radiation energy: the radiation from the sun contains energy, and also the radiation from a light or a fire. More solar energy is available when the radiation is more intense and when it is collected over a larger area. Light is the visible part of radiation; - Chemical energy: wood and oil contain energy in a chemical form. The same is true for all other material that can burn. The content of chemical energy is larger the larger the heating value (calorific value) of the material is and, of course, the more material we have. Also animate energy (delivered by bodies of human beings and animals) is, in essence, chemical energy. Furthermore, batteries contain chemical energy; - Potential energy: this is, for example, the energy of a water reservoir at a certain height. The water has the potential to fall, and therefore contains a certain amount of energy. More potential energy is available when there is more water and when it is at a higher height; - Kinetic energy: this is energy of movement, as in wind or in a water stream. The faster the stream flows and the more water it has, the more energy it can deliver. Similarly, more wind energy is available at higher windspeeds, and more of it can be tapped by bigger windmill rotors; - Thermal energy or heat: this is indicated by temperature. The higher the temperature, the more energy is present in the form of heat. Also, a larger body contains more heat; - Mechanical energy, or rotational energy, also called shaft power: this is the energy of a rotating shaft. The amount of energy available depends on the flywheel of the shaft, i.e.:. on the power which makes the shaft rotate; - Electrical energy: a dynamo or generator and a battery can deliver electrical energy. The higher the voltage and the current, the more electrical energy is made available. Note that sometimes by "energy form" an energy source (cf. section 5), or even a particular fuel (like oil or coal), is meant.
3. Energy conversion "Utilising" energy always means converting energy from one form into another. For instance, in space heating, we utilise energy, that is, we convert chemical energy of wood into heat. Or, in lift irrigation, a diesel engine converts chemical energy of oil into mechanical energy for powering the shaft of a pump which, in its turn, converts shaft power into potential energy of water (i.e. bringing the water to a higher height). "Generating" energy also means converting energy from one form into another. We can say that a diesel engine generates energy, which means that the engine converts chemical energy of oil into mechanical energy. Also, a wind turbine generates energy, which means it converts kinetic energy from wind into mechanical energy. And a solar photovoltaic cell generates energy by converting radiation energy into electricity. The generation of energy, in fact, deals with a source of energy, whereas the utilisation of energy serves an end-use of energy. In between, the energy can flow through a number of conversion steps. The words "generation" and "utilisation" are a little confusing because, in fact, no energy can be created or destroyed. All we can do is transform or convert energy from one form into another. In generating energy, we make energy available from a source, by converting it into another form. In utilising energy, we also convert energy, often from some intermediate form into a useful form. In all conversions, we find that part of the energy is lost. This does not mean that it is destroyed, but rather that it is lost for our purposes, through dissipation in the form of heat or otherwise (cf. figure 1). Figure 1. Energy conversion Energy conversions can take place from any one form of energy into almost any other form of energy. (Some conversions have no practical value.) Which conversion is desired depends on our purposes. For instance, for power generation, we convert potential energy from hydro resources into mechanical energy, whereas, in water pumping for lift irrigation, we do the reverse. And, with photovoltaic cells, we convert radiation energy into electricity, whereas with light bulbs we do the reverse. Table 5 in section 9 gives examples of conversions and some typical efficiencies of energy converters. Section 13 shows some calculations of energy conversions.
4. Energy and power Energy and power are related but totally different concepts. A tank of petrol contains a certain amount of energy. We can combust this petrol in a certain time period, that is, we convert the energy of the petrol into mechanical energy, perhaps to power a car. The power is the energy produced per unit of time. The combustion process can be fast or slow. In the case of faster combustion, more power is produced. Obviously, the tank will be empty sooner in the case of high power production than in the case of low power production. If power is energy per time unit, then energy is power multiplied by time period. For Instance, if an oxen delivers a certain amount of power, then after a certain time period it will have delivered a cerain amount of energy, i.e. the power times the time period.
The same principle applies to all other energy conversions, whether for energy generation or for energy utilisation. This implies that we characterise energy resources in units of energy (the amount of energy they contain), and energy conversion devices in units of power (the amount of power they can produce or consume). A closer look at the list of forms of energy in section 2 reveals that some of them have actually been described in terms of power (radiation, kinetic, mechanical and electrical energy). They become energy quantities when we specify the time period during which the power is delivered, and multiply the power by this time period. Also in section 2, the quantities of chemical, potential and thermal energy become power quantities when we divide them by a time period during which the energy quantity is being converted.
5. Energy sources Energy sources partly correspond to the energy forms of section 2, but not entirely. The following energy sources can be relevant for rural areas. - Biomass. We distinguish between: woody biomass (stems, branches, shrubs, hedges, twigs), non-woody biomass (stalks, leaves, grass, etc.), and crop residues (bagasse, husks, stalks, shells, cobs, etc.). The energy is converted through combustion (burning), gasification (transformation into gas) or anaerobic digestion (biogas production). Combustion and gasification ideally require dry biomass, whereas anaerobic digestion can very well take wet biomass. Fuel preparations can include chopping, mixing, drying, carbonising (i.e. charcoal making) and briqueting (i.e. densification of residues of crops and other biomass). - Dung from animals, and human excreta. The energy is converted through direct combustion or through anaerobic digestion. - Animate energy. This is the energy which can be delivered by human beings and animals by doing work. - Solar radiation, i.e. energy from the sun. We distinguish between direct beam radiation and diffuse (reflected) radiation. Direct radiation is only collected when the collector faces the sun. Diffuse radiation is less intense, but comes from all directions, and is also present on a cloudy day. Solar energy can be converted through thermal solar devices (generating heat) or through photovoltaic cells (generating electricity). Direct beam solar devices (whether thermal or photovoltaic) would need a tracking mechanism to have the device continuously facing the sun. - Hydro resources, i.e. energy from water reservoirs and streams. We distinguish between: lakes with storage dams, natural heads (waterfalls), weirs, and run-of-river systems. Hydro energy can be converted by waterwheels or hydro turbines. - Wind energy, i.e. energy from wind. Wind machines can be designed either for electricity generating or for water lifting (for irrigation and drinking water). - Fossil fuels, like coal, oil and natural gas. Unlike the previous energy sources, the fossil energy sources are non-renewable. - Geothermal energy, that is, the energy contained in the form of heat in the earth. A distinction is made between tectonic plates (in volcanic areas) and geopressed reservoirs (could be anywhere). Geothermal energy is, strictly speaking, non-renewable, but the amount of heat in the earth is so large that for practical reasons geothermal energy is generally ranked with the renewables. Geothermal energy can only be tapped at places where high earth temperatures come close to the earth's surface. This list only contains primary energy sources. These are the energy sources which are present in our natural environment. Secondary energy sources, like batteries, are not included here. We observe that the primary energy sources are not the ultimate sources of energy. For instance, animate energy comes from biomass, whereas biomass energy ultimately comes from the sun. Apart from geothermal and nuclear energy, all our so-called primary energy sources have ultimately got their energy from the sun! Section 10 will discuss methods for comparing the energy content of energy sources.
6. Some notes on energy terminology Energy sources are sometimes classified according to characteristics like: renewable, traditional, commercial, etc. The terminology is rather ambiguous, as the meaning of the words often depends on the context. Some connotations are given below. Renewable is generally contrasted with fossil. Renewable are biomass, animate, solar, water and wind energy, as well as geothermal energy. Fossil energy is contained in coal, oil and natural gas. Traditional energy is often contrasted with non-traditional energy, and also with new energy. However, what is considered as traditional depends on what one is used to. In industrialised societies which are used to fossil fuels, renewable energies like biomass and animate energy are often called traditional. At the same time, engineers working on "new" energies like wind or solar energy often consider fossil fuels as traditional. Apparently, what people call traditional are the forms they are actually not used to. New and renewable energy sources are often put together. They exclude fossil and nuclear energy. Commercial energy is contrasted with non-commercial energy, and sometimes with traditional energy. Commercial energy certainly includes energy from fossil fuels which have been monetarized, but also some forms of new and renewable energies which are part of the cash economy. Biomass and some other sources of renewable energy (thermal solar energy) are sometimes considered non-commercial, because they are thought to be freely available. However, in many areas, biomass fuels have to be paid for!
7. Energy flow As we have seen, generating and utilising energy means converting energy from one form into another. Often, intermediate steps are Implied. The energy flows through a number of forms, as well as conversion steps, between the source and the end-use. The costs increase accordingly. We distinguish between primary, secondary, final and useful energy. An example is an energy flow which is related to charcoal. Here, the primary energy form is wood. The wood is converted into charcoal in a charcoal kiln. Charcoal is the secondary form of energy, and it is transported to the consumer. What the consumer buys at the market place is charcoal, and this is called final energy. The consumer eventually converts the charcoal into heat for cooking. The heat is the useful energy. Another example of an energy flow is: primary energy in the form of a hydro resource, secondary energy in the form of electricity at the hydro power station, final energy in the form of electricity at a saw mill, and useful energy in the form of shaft power for sawing. energy
technology
examples
primary
coal, wood, hydro, dung, oil, etc. conversion
power plant, kiln, refinery, digester
secondary
refined oil, electricity, biogas transport/transmission trucks, pipes, wires
final
diesel oil, charcoal, electricity, biogas conversion
motors, heaters, stoves
useful
shaft power, heat
Energy flow is represented In the diagram in Figure 2. It refers to the following terminology. Primary energy is the energy as it is available in the natural environment, i.e. the primary source of energy. Secondary energy is the energy ready for transport or transmission. Final energy is the energy which the consumer buys or receives. Useful energy is the energy which is an input in an end-use application. Note that useful energy is almost invariably either in the form of heat or in the form of shaft power. For a few end-uses (e.g. communication equipment), electricity is the form of useful energy. Note that in some cases the primary energy is at the same time the secondary, and even the final energy (c.f. wood gathered for cooking purposes, or animate power for pulling). The breakdown of primary to useful energy is relevant, because with each conversion step some energy is lost. In order to reduce costs and avoid unnecessary losses, we will always aim at eliminating unnecessary steps in the flow of energy. Furthermore, the breakdown of energy flows is relevant for surveys and statistics. We may not simply add primary energy with, say, final energy! (cf. section 10.)
8. Energy units and dimensions So far, we have discussed energy in qualitative terms. In order to proceed, we must discuss energy quantitatively. That means, we need units for measuring quantities of energy and related concepts. We use the International system of units (SI units), which is based on the dimensions and basic units in Table 1. Table 1. Basic SI units dimension
basic unit symbol
length
meter
m
mass
kilogram
kg
time
Second
s
electric current ampere
A
temperature
°K
kelvin
The unit of energy in this unit system is joule (J), and the unit of power is watt (W). These and many other units can be derived from the basic SI units. The relationship between some derived SI units and the basic SI units is represented in Table 2. Table 2. Derived SI units dimension
unit
symbol
area
square meter
m²
volume
cubic meter
m³
speed
meter per second
m/s
acceleration
meter per second
m/s²
pressure
pascal
Pa (=N/m)
volume flow
cubic meter per second m ³/s
mass flow
kilogram per second
density
kilogram per cubic meter kg/m³
force
newton (*)
N(=kg.m/s²)
energy
joule (**)
J(=N.m)
power
watt
W (=J/s)
energy flux
watt per square meter
W/m²
calorific value joule per kilogram
kg/s
J/kg
specific heat joule per kilogram kelvin J/kg.K voltage
volt
V (=W/A)
(*) The force exerted by a mass of 1 kg equals ca. 10 N. (**) The energy required to lift 1 kg by 1 meter. Note that = W.s. In some countries, or in a particular context, other units than SI units are also used. They can be converted into SI units, which are more convenient for calculations. The conversion of some non-SI units into SI units is given in Table 3, for energy and for power. Table 3. Conversion of non-SI units Non-SI unit for energy symbol equivalence in SI-units erg
erg
10-7 J
foot pound force
ft.lbf
1.356 J
calorie
cal
4.187 J
kilogramforce meter
kgf.m
9.8 J
British thermal unit
Btu
1.055 x 103 J
horsepower hour (metric) hp.hr
2.646 x 106 J
horsepower hour (GB)
hp.hr
2.686 x 106 J
kilowatt hour
kWh
3.60 x 106 J
barrel oil equivalent
b.o.e.
6.119 x 109 J
ton wood equivalent
-
9.83 x 109 J
ton coal equivalent
tee
29.31 x 109 J
ton oil equivalent
toe
41.87 x 109 J
quad (PBtu)
-
1.055 x 1018 J
tera watt year
TWy
31.5 x 1018 J
Non-SI unit for power
symbol equivalence in SI-Units 0.377 x 10-3 W
foot pound per hour
ft.lb/h
calorie per minute
cal/min 69.8 x 10-3 W
British thermal unit per hour
Btu/h
0.293 W
British thermal unit per second Btu/s
1.06 x 103 W
kilocalorie per hour
kcal/h
1.163 W
foot poundforce per second
ft.lbf/s
1.356 W
calorie per second
cal/s
4.19 W
kilogramforce meter per second kgf.m/s 9.8 W horsepower (metric)
hp
735.49 W
horsepower (GB)
hp
746 W
The powers of ten are often abbreviated by writing prefixes before the unit. For instance, the symbol G stands for giga, which means 10 to the power 9, i.e. a billion. One billion W is then written as 1 GW (one giga Watt). Common prefixes are given in Table 4. Table 4. SI prefixes prefix symbol
multiplier
exa
E
1018
peta
P
1015
tera
T
1012
giga
G
109 (= 1,000,000,000)
mega M
106 (= million)
kilo
103 (= thousand)
k
hecto h
102 (= hundred)
deca da
101 (= ten)
deci
d
10-1 (= a tenth)
centi
c
10 (= a hundredth)
milli
m
10-3 etc....
micro u
10-6
nano n
10-9
pico
10-12
P
femto f
10-15
atto
10-18
a
Magnitudes of energy forms Now we have Introduced units for measuring energy, we can make quantitative comparisons and calculations. The following results give us some feeling of magnitudes of energy, as represented in different energy forms. The examples are all equivalent to about 100 kJ; - radiation from the sun on the roof of a house (of ca. 40 m²) in 2.5 s - energy released in burning 3.5 g coal or 2.9 g petrol; or the energy stored in 1/4 slice of bread - a large object (1,000 kg) at a height of 10 m - energy produced by a windmill of 3 m diameter in a wind speed of 5 m/s (a breeze) during 20 minutes; or the energy stored in the mass of a car (1,000 kg) moving at 50 km/h heat emanated in cooling three cups of coffee (0.4 kg) from 80°C to 20° C; or the energy needed to melt 0.3 kg ice - an iron flywheel of 0.6 m diameter and 70 mm thick, rotating at 1,500 revolutions per second - energy consumed by a 100 W electric light bulb in 17 minutes Section 13 illustrates the use of energy units in some calculations of energy conversions.
9. Energy losses and efficiency As has been stated in Section 3, energy conversions always imply energy losses. This leads us to the concept of efficiency, as follows. A quantity of energy in a certain form is put into a machine or device, for conversion into another form of energy. The output energy in the desired form is only a part of the Input energy. The balance is the energy loss (usually in the form of diffused heat). It means the converter has less than 100% efficiency. The efficiency of an energy converter is now defined as the quantity of energy in the desired form (the output energy) divided by the quantity of energy put in for conversion (the input energy). The efficiency is usually expressed by the Greek letter h .
Hence: Table 5 gives some typical efficiencies of energy converters. Table 5. Some typical efficiencies of energy converters Converter
form of input energy form of output energy efficiency %
petrol engine
chemical
mechanical
20 - 25
diesel engine
chemical
mechanical
30 - 45
electric motor
electrical
mechanical
80 - 95
boiler & turbine
thermal
mechanical
7-40
hydraulic pump
mechanical
potential
40 - 80
hydro turbine
potential
mechanical
70 - 99
hydro turbine
kinetic
mechanical
30 - 70
mechanical
electrical
80 - 95
battery
chemical
electrical
80 - 90
solar cell
radiation
electrical
8-15
solar collector
radiation
thermal
25 - 65
electric lamp
electrical
light
ca. 5
mechanical
potential
ca. 60
water heater
electrical
thermal
90 - 92
gas stove
chemical
thermal
24 - 30
generator
waterpump
In some of these converters, intermediate forms of energy occur between the form of the input energy and the form of the output energy. For instance, with diesel engines, the intermediate form is thermal energy. When thermal energy is Involved either as the input or as an intermediate form, the efficiency is generally low. The energy converter can be a device, or a process, or a whole system. An example of the efficiency of an energy conversion system is given in Table 6. The overall efficiency equals the product of the efficiencies of the various components of the system. We see that it can be very low indeed. Table 6 energy form
energy converter
efficiency
diesel engine
30%
generator
80%
electric motor
80%
waterpump
60%
chemical energy mechanical energy electricity mechanical energy potential energy efficiency of the system = 30% x 80% x 80%x 60% = 12%
Efficiency of an energy conversion system: An example Where energy is a scarce resource, we want the efficiency of conversion to be high, in order to save energy. But higher efficiency often implies higher costs for better equipment. Optimisation with respect to, on the one hand, the costs of energy and, on the other hand, the costs of equipment, is a major task in energy planning. The problem of optimization is different when energy sources are free (like with wind, solar and some hydro sources). Energy efficiency has then a limited meaning, and the choice of technology will be guided by the cost effectiveness of the equipment. A very high system efficiency can be obtained when heat losses from one converter are utilised as energy inputs in another. We call this waste heat utilisation. It is applicable, for instance, in agro-processing where heat from Industrial converters is utilised for drying of products. Cogeneration is another example, i.e. the utilisation of "waste" heat from electricity production, for purposes of process heat in Industry.
10. Equivalence and replacement of energy forms In principle, the energy content of a fuel is known when the fuel Is specified. For chemical energy, the energy content is given as the calorific value, or heating value, of the fuel. The unit can be MJ/kg. And so we can compare different fuels with different energy contents. We can work out how much of one fuel is equivalent to a quantity of another fuel. For quantifying energy resources, we sometimes use coal as a reference, and the unit for comparison is then ton-of-coal-equivalent (tee). A certain amount of an energy resource is then characterised by its tee. That is, the resource has an energy content equivalent to so many tee. Alternatively, we can express the energy equivalent of a resource in units of ton-of-oil-equivalent (toe), or in barrels-of-oil-equivalent (boe). Table 7 gives the equivalent values of some fuels. Table 7. Energy equivalent values of some fuels fuel
unit
tonnes of coal equivalent tonnes of oil equivalent barrels of oil equivalent GJ (*)
coal
tonne
1.00
0.70
5.05
29.3
firewood (**) (airdried)
tonne
0.46
0.32
2.34
13.6
kerosine (jet fuel)
tonne
1.47
1.03
7.43
43.1
1000 m3
1.19
0.83
6.00
34.8
gasoline
barrel
0.18
0.12
0.90
5.2
gasoil/diesel
barrel
0.20
0.14
1.00
5.7
natural gas
(*) Note that GJ/tonne is the same as MJ/kg. (**) Note that the energy equivalent of wood can vary a factor 3 depending on the moisture content of the wood. However, what we can achieve with an amount of energy depends very much on how the energy is utilised, that is, on the efficiencies of the energy converters applied. Efficiencies can vary enormously for different converters, as we have seen in Section 9. The energy equivalent is then of limited use to us. In practice, when comparing sources of energy, we are more interested in the replacement value of the energy form. The latter Indicates how much of that energy form is required to do the same job (i.e. serve the same use) as another energy form or fuel. Again, as a reference, coal is sometimes used. The replacement value of an energy form is, then again, expressed in tee. However, this value will be different from the equivalent value of that energy form. An easy way of comparing replacement values of different energy forms is by indicating how many units of the energy form (or fuel) can replace one kg of coal. We call this the replacement ratio of the fuel. Replacement ratios of some household energy forms compared with coal are given in Table 8, as taken from a particular survey. (Alternatively, a similar table could be made with oil as a reference.) It should be noted that the figures serve as an example only, as they depend on the actual efficiencies of the conversion techniques applied. Table 8. Coal replacement ratio of some forms of energy energy form or fuel unit coal replacement ratio (kg coal per unit) dung cake
kg
0.30
vegetable waste
kg
0.60
firewood
kg
0.70 - 0.95
soft coke
kg
1.50
charcoal
kg
1.80
kerosire (lamp)
1
2.10
kerosine (stove)
1
5.20 - 7.00
kWh
0.70
electricity
(The coal replacement ratio is the number of kg of coal which is required to effectively replace 1 unit of the energy form or fuel, under certain assumptions.) Good examples of coal replacement are a kerosine lamp and a kerosine stove. The coal equivalent of kerosine was 1.47, which means that the heating value of 1 kg kerosine equals that of 1.47 kg coal. However, the coal replacement ratio for a kerosine lamp is 2.10, which means that 2.10 kg coal would be required to get as much light as from 1 kg kerosine. And the coal replacement ratio of a kerosine stove is around 6, which means that 6 kg coal is required to get as much heat in a pot as from 1 kg kerosine. In Section 7, it was mentioned that the breakdown of energy flows is relevant for surveys and statistics. This is illustrated by the previous discussion of energy equivalence and energy replacement. We can add the primary energy resources of a particular region by adding the energy equivalences of all the various primary energy resources available. This will give us a rather theoretical figure, as it does not say what can be done with this amount of energy. We can also add, say, the consumption of final energy for a particular sector in a region, and work this out in a coal replacement value. Or we can consider, say, the amount of useful energy for particular end-uses, and express this in an oil (or coal) replacement value. For working out the replacement values, we should know the conversion methods and their efficiencies which are involved in the energy flow.
11. Energy balance An energy balance of a region (or country) is a set of relationships accounting for all energy which is produced, transformed and consumed in a certain period. This basic equation of an energy balance is: source + import = export + variation of stock + use + loss Consider a primary energy balance. Sources are the local (or national) primary energy sources, like coal, hydro, biomass, animate, etc. Imports are energy sources which come from outside the region (or country). Exports go to other regions (or countries). Variations of stock are reductions of stocks (like of forests, coal, etc.), and storage. Use can be specified sectoral, or by energy form, or by end-use, etc., as required. Losses are technical losses and administrative losses: · technical losses are due to conversions and transport or transmission · administrative losses are due to non-registered consumptions. An energy balance usually refers to a year, and can be made for consecutive years to show time variations. Energy balances can be aggregate, or very detailed, depending on their functions. They can also be elaborate, showing all sorts of structural relationships between energy production and consumption, and specifying various Intermediate forms of energy. An energy balance can also be set up for a village, a household, a farm, or an agricultural unit. It will show the inputs of energy in various forms, the end-use energy, and the losses. Specific for energy balances of agricultural systems is the fact that parts of the outputs of the system are, at the same time, energy Inputs into the system (agricultural residues, dung). Energy balances have to be built up from surveys of what is actually going on. This requires energy resource surveys, and energy consumption surveys, as well as more technical energy audits. Section 12 goes into some aspects of energy auditing. Energy balances provide overviews, which serve as tools for analysing current and projected energy positions. The overviews can he useful for purposes of resource management, or for indicating options in energy saving, or for policies of energy redistribution, etc. However, care must be taken not to single out energy from other economic goods. That means that an energy balance should not be taken as our ultimate guide for action. Energy data are to be translated into economic terms, for a further analysis of options for action. And, of course, socio-cultural and environmental aspects are equally important.
12. Process energy requirements and gross energy requirements Energy use in agriculture, or in any other productive system, can be analysed at different levels. 1. The direct energy input in the production process and related transport requirements is considered. 2. The same as 1., but, in addition, the energy embodied in the materials (e.g. fertiliser) for the production process and related transport is considered. 3. The same as 2., but, in addition, the energy required by the machines to produce these materials is considered'. 4. The same as 3., but, in addition, the energy required by the machine Cools is considered. Etc.... Which level of analysis is relevant for whom? Let us first distinguish: GER = Gross Energy Requirement is the total amount of energy required for a product. e.g. the GER of milk is 5.2 MJ/pint in the U.K. This includes the energy Co produce fertiliser, grow the grass, feed the cows, process the milk in the dairy, and energy for transport. PER = Process Energy Requirement is the energy required for processing the product. e.g. the PER of milk is 0.38 MJ/pint in the U.K. This is the energy required to process the milk in the dairy itself. Generally, when the PER can be lowered, as a result the GER will also be lowered. However, this will not always be the case, and it can also be the reverse. For instance, energy economies of scale can sometimes be achieved at farm level, at the expense of energy requiring investments in Infrastructure or transport facilities. The answer to the question as to which level of analysis is relevant obviously depends oh which policy or management level is involved. For instance, for management at the farm level, it is the PER which matters, and so the first level of analysis is the relevant one. For regional policy makers, however, level 2 is relevant when regional materials and resources are involved. Furthermore, the linkages between the agricultural sector and other sectors will be a concern. For instance, large scale biogas digesters can be an energy efficient option for agroprocessing plants, but they may compete with alternative utilisation of the inputs (e.g. dung for poor peoples' household fuel). For national policy makers, level 2 or 3 may be relevant. For instance, the establishment of plants for energy intensive goods can be attractive when cheap energy is available (e.g. fertilizer production). The analyses of PER and GER provide data for energy balances. However, these data do not give information on the forms of energy, or time variations (seasonality) in the energy flows, etc. Such information has to be added, as required. PER and GER are part of what is often called energy auditing. This is the monitoring of energy use in productive systems. The analogue in consumption systems is energy end-use analysis. In subsistence agriculture, productive and consumption systems are intertwined, and the two approaches have to be combined in energy surveys.
13. Examples of calculations of energy conversions The following examples aim to illustrate methods of calculations, rather than to arrive at accurate numbers. For convenience, the calculations are made in round figures. More exact figures would, anyway, depend on the accuracy of the input data. 13.1 How much heat is produced by a human body? A man doing no or very little physical work needs about 2,000 kcal (or less) of energy in his daily food. The body converts this energy almost entirely into heat. 1 day = 24 x 60 x 60 s = 86,400 s 1 cal = 4.2 J Hence
We see that a human body doing no work is equivalent to a heat source of about 100 W - the equivalent of a good bulb. 13.2 The power of oil It was said that two teaspoons of diesel oil are equivalent to the work done by a man in a day. Can that be correct? Assume that the power which can be delivered by a man in a day's work is 60 W (cf. example 13.3), and that he can do that for 4 hours per day. So, per day, he delivers: 60 W x 4 h = 240 Wh = 240 x 3,600 Ws = 860 kWs = 860 kJ (1) Note: the power of ca. 60 W delivered by doing work is on top of the 100 W produced by the body as heat (cf. example 13.1). The additional power requires additional kcal in the food! We estimate that two teaspoons are equal to 1/50 litre. Diesel oil has an energy content of 42 MJ/kg. For simplicity, we assume that 1 litre of oil weighs 1 kg. Then, 1 litre of oil contains 42 MJ, and 2 teaspoons contain: 1/50 x 42 MJ =840 kJ (2) Note: the power delivered by a man can be compared with the power which can be delivered by an oxen, which is: 0.3 to 1.3 hp = 220 to 960 W. We see that the figures (1) and (2) are approximately the same. So - the comparison was correct! 13.3 How can we check that a human body can deliver 60 W during a few hours per day? The actual value could be measured, and it will vary a lot, depending on many factors. One way of checking the order of magnitude is the following. Mountaineers know that a man can climb about 300 metres per hour. Assume that his weight is 75 kg. The gravitational force he is counteracting is then: 75 x 9.8 Newton = 750 N The energy delivered by the man in an hour is: 300 m x 750 N = 225 kNm = 225 kJ. The power delivered is:
13.4 How can we compare the power from oxen with the energy from wood? We cannot compare power and energy. We can make a comparison only if we specify a time period, so as to relate power to energy. For instance, the time period that oxen work. An oxen can deliver typically 0.8 hp. With Table 3 on the conversion of non-SI units, we see that this equals about 740 x 0.8 = 600 W. The amount of energy delivered in one year by this oxen can be calculated if we know how many hours the oxen works in a year. Assume this is 4 hours a day during 300 days, i.e. 1,200 hours per year. One hour is 3,600 s. Hence, the energy from one oxen in a year is: 600 x 1,200 x 3,600 Ws = 2,600,000,000 = 2.6 GJ Thus, 4 oxen would deliver about 10 J in one year. From Table 3 it is seen that this equals roughly the amount of energy in one ton of (wet) wood. 13.5 Do we really need more energy under the pot than in the pot? We have seen that a person needs in his food ca. 2,000 kcal per day (cf. example 13.1). This is 8.4 MJ/day for one person. We assume that the food mainly consists of crop products, i.e. biomass. Dry biomass, whether edible or not, has an energy content of typically 18 MJ/kg. The daily energy of 8.4 MJ can thus be delivered by:
On a yearly basis, the biomass for food per person is: 365 days x 0.5 kg/day = 180 kg/year We can compare this amount with the amount of biomass required as fuel by a household. From surveys, we know that a typical household fuel need for cooking purposes is 500 kg/year of dry biomass per person. Hence
This means that roughly 3 times more energy is required under the pot than in the pot! 13.6 On the price of rural electricity A consumer in a town centre is charged Rs 0.75 per kWh for his electricity from the national grid. In a rural area, a consumer has a lamp connected to the local micro hydro unit at a cost of Rs 1 per day. Which consumer pays more for his electricity? Assume that the lamp in the village consumes a power of 40 W and Chat It is switched on for an average of 4 hours per day. This implies an energy consumption of: 4 h x 40 W = 160 Wh = 0.16 KWh for 1Rs. The consumer in the town pays for this amount of energy: 0.16 x Rs 0.75 = Rs 0.12 We see that the villager pays about 8 times more for his electricity than the consumer in the town. References The examples and data in the basic energy concepts are from the International Courses on Rural Energy Planning at Twente University.
