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Energy

A Beginners Guide

NATTA’s guide to how to get to grips with energy issues

 

Can’t tell a kWh from a gigajoule? Lost when faced with talk of primary energy and entropy?

Never mind. This short booklet should help you make sense of the mysteries of energy production and the equally mysterious terms used by the specialist to describe it.

Designed as a companion text for use by readers of NATTA’s journal RENEW, it should also be of use to a wider audience. In fact to anyone who wants to understand energy- and who slept through their physics lessons.

 

As with all NATTA reports, the views expressed should not be taken to necessarily reflect those of EERU or the Open University.


Contents

 


Energy – a beginners guide

The generation and use energy probably has the largest environmental impact of all human activities - from fuel extraction right through to the emissions from power stations and cars. To get to grips with the issues and the choices it’s useful to have a grasp of the basic science and engineering principles - what is energy, where does it come from, how do we use it? That can equip you with the tools to analyse some of the problems and some of the solutions.

 


1. Energy Sources

The obvious starting point for our exploration of energy and energy issues is to look at energy sources - where energy comes from.

The energy sources available to mankind fall into two fundamentally differing classes - the renewable sources and the non renewable sources.

Renewable sources are the naturally occurring, and naturally replenished, energy flows such as sunlight, the winds, the waves and the tides. The inds and waves are actually indirect forms of solar energy - the differential heating of the atmosphere, the land and sea produces winds, and winds moving over the sea produced waves. The suns heat also drives the hydrological climate system, creating clouds, rain and rivers, whose energy can be tapped in hydroelectric schemes. So that too is, indirectly, a solar source. Tidal energy by contrast is the result of the gravitational interaction of the moon with the seas. So it could be called 'lunar power', although the suns pull also has an effect.

Sunlight provides energy for plant and animal life, and when this dies and gets buried under geological strata over millennia, it gets converted into fossil materials of various types, coal, oil or gas, depending on the location, duration, temperatures and pressures. These fossil fuel reserves have taken millennia to lay down, as, in effect, stored solar energy, but we have used a large proportion of them in the last hundred years or so: our rate of use far outstrips to rate of regeneration, so in practice they are non renewable resources.

By contrast, when we use biological material like wood, at the same rate as it is produced then that can be thought of as a renewable resource. Like fossil fuels, this is stored solar energy - but it can be continually and relatively quickly replaced.

Finally there is nuclear energy - the energy that can be released when the atomic nucleus of certain materials is disrupted. Reserves of the specific materials needed are limited and are not being renewed: they are part of the planets initial inheritance, so nuclear power is not a renewable resource.

On a strict interpretation of the term, the same is true of geothermal energy from the heat within the planet. This is the result of heat released due to the radioactive decay of materials deep underground - so you could see geothermal energy as a 'natural' form of nuclear energy. Furthermore, since the sun is a giant fusion reactor, bathing this planet with solar energy, you might say all the energy sources we have discussed, except tidal, are nuclear sources.

Of course, only a small fraction of the suns energy actually passes through the atmosphere: most of it bounces off into space. And an even smaller proportion gets converted into renewable flows and into stored solar energy.

What matters to us of course is how much energy we might be able to obtain form these various sources. The problem with fossil and nuclear fuels is that the reserves are underground and it is hard to say exactly how much is there: resource estimates vary, but in general, and depending on the rate of use, global oil reserves could begin to get expensive to access within a few decades, gas should last somewhat longer, while coal and uranium reserves will probably have an availability of the order of a few hundred years. Obviously these are only very rough 'ballpark' figures. Even so, the point should be clear: these are inevitably finite reserves.

By contrast, the renewables are not resource limited, although there are practical limits on how much energy we can recover from these sources. However, before we can go much further with our exploration of energy and the quantitative limits to its availability, we need to be bit clearer about what energy actually is - and about how to measure it.


2. What is Energy?

We have looked briefly at various energy sources. But what do we actually mean by energy? It is not as simple to define as you might first think. For energy is a concept rather than an actual thing, it's a quality or capacity that is manifest in certain situations: we say people have energy when they can work or play hard.

The formal definition is that 'energy is the capacity to do work', but to understand this definition you need to appreciate that 'work' here means any activity involving the physical movement of objects: pushing a broken down car up a hill is an obvious example. You have to work hard to do that, and you have to have the energy to do it. The energy required actually depends on the mass of the car and the vertical height you have to raise it (and also the strength of the gravitational pull - it would be easier on the moon!). Where does that energy come from? In this case it is ultimately from food, which, together with the air you breath, make your muscles work. So in this case we say that food is an energy source - a fuel for the human machine.

There are, as we have seen, many other fuels: e.g. wood and coal. Unfortunately, in common usage, the two terms, energy and fuel, often tend to be used interchangeably. However 'fuels' are only potential sources of energy: you have to do something to release the energy. Usually it can be released in the form of heat energy, for example by burning the fuel. But there are also other types of energy, most obviously electrical energy (from electric currents) and, more fundamentally, the kinetic energy of moving objects. However, what matters for our purposes here is the idea that you can convert energy from one form into another.

A dramatic example of this conversion process is if you convert the potential energy you have when standing on a cliff (by virtue of you being at a height above the ground below), into kinetic energy should you jump off. When you hit the ground and your initial potential energy is suddenly dissipated in structural deformation, sound and maybe some heat. Should you survive and for some reason wish to repeat the exercise, then you will have to provide the potential energy again by using stored chemical energy in your body to power your muscles to climb back up to top against the force of gravity. Unless of course there is a convenient chair lift of some sort to do the job for you - possibly using electrical energy.

Power stations are the largest and most obvious example of an energy conversion device: they convert fuels into electricity. But car engines, domestic cookers, light bulbs and so on are also energy conversion devices, converting one sort of energy into another, whether its heat, light, or power for movement. In some cases it’s a single stage conversion process - for example from electricity to light (and a bit of heat) in a light bulb. In others, a complex chain of conversion processes is involve

Let’s go back to power stations as an example of this multiple staged energy conversion process.. First the heat energy in the fuel, lets say it is coal, is released by burning. This heat is used to boil water to make super hot steam. The steam then passes through a series of turbine blades mounted on a shaft, like in a jet engine, pushing against them and causing the complete turbine unit to rotate. This rotary movement is used to drive an electricity generator - essentially a giant dynamo or alternator type device, consisting of coils of wire turning in a magnetic field. The rotation induces an electric current in the wires - and that is how electricity is generated.

Losses in Energy Conversion

Having established the basic idea of energy conversion, the next key thing to realise is that you will loose some energy in the energy conversion process. The efficiency with which energy in one form can be converted into energy in another form can never be 100%. You will always get less out than you put in - there are always losses, and, as we shall see, for many energy conversion devices they can be quite high, often as much as half the input energy and sometimes more.

Part of the reason for these losses is that you cannot avoid producing other types of energy as an accidental by product of the main conversion process- for example noise or heat from friction with mechanical and electrical energy conversion systems. As we shall see, there are also what are called 'thermodynamic' losses with systems in which the energy in a fuel is used to raise steam to drive machinery, or to create hot gases to drive engines or turbines. To put it simply, you can’t convert all the energy from one form into another, some of it remains unchanged i.e. its not all converted in to the form you want. You are bound to get unwanted incidental energy outputs and conversion losses.

Note however that we never actually loose energy. The total amount of energy produced, when you add up all the energy outputs of the energy conversion process, both desired and undesired, is always equal to the energy fed in at the start of the process. So we have what is called the Law of Conservation of Energy. Put simply it says 'energy is always conserved'. Except in nuclear processes, where things are somewhat different, energy cannot be created or destroyed, only converted from one form to another. So although it is common to talk of 'energy generation' and 'energy consumption', strictly, energy is never 'generated' or 'consumed', it is just 'converted' from one form to another.

Power

The term 'power' is used to formally describe the conversion capacity of any specific device i.e. the rate at which it can convert energy from one form to another, and the unit most commonly used is the watt. So although the concept of power is often used as if it meant the same as energy, in fact power refers to the ability of a system to convert fuels into useful energy. Put formally, power is a measure of the rate of conversion or use of energy.

Specific energy 'generating' or 'consuming' devices, like power stations or electric fires or light bulbs, are given a power rating (or 'rated capacity') in watts. For example electric kettles typically are rated at 1000 watts. Since the watt is a quite small unit, it is usual to use multiples of watts e.g. a kilowatt ('kW') is one thousand watts. Note that the abbreviation is spelt with a small letter ‘k’ and large ‘W’. Moving up scale, a megawatt is 1000 kilowatts (it’s written ‘MW’, and spelt with a big ‘M’ and‘W’), a gigawatt (GW) is 1000 megawatts, and a terawatt (TW) is 1000 gigawatts. To give you an idea of scale, a typical large modern coal or nuclear power station has a rated capacity of around 1.3 gigawatts (GW) or 1,300,000 kW, while the UK has around a total of 65 GW of electricity 'generating' capacity.

Energy

When it comes to calculating your electricity bills, while its useful to know the rated capacity of devices, i.e. how much power they use when running, what you really need to know is the actual amount of energy that they have used - and that depends on how long they have been run.

What you pay for is the amount of energy 'consumed' which will depend on the actual 'work' done by the electricity you have bought in running you lights, TV etc. The energy used is defined by the power of each device multiplied by the time for which they are used.

So energy=power x time (i.e. watts x hours).

Energy is usually measured in kilowatt hours or 'kWh' (note the use of a small letter 'h', as well as a small ‘k’, and a big letter ‘W’). This is the unit by which electricity is sold in many countries, (although, obscurely, the USA still makes use of British Thermal Units, the old measure for the heat content of fuels: 1kWh=3413 BTU's). A typical 1kW rated one bar domestic electric fire 'consumes' 1 kilowatt hour (kWh) each hour, and if you did not have anything else running during the quarter that is how it would eventually appear on your electricity bill - as one 'unit of electricity' or 1 kWh. of energy used

For larger quantities of energy, multiples of kWh's are used, most commonly the terawatt hour (TWh) which is 1000,000,000 kWh. To give an idea of scale, the national figure for total UK electricity 'consumption' was about 300 TWh per annum.

Remember however that this is the figure for the consumption of electricity, not total energy consumption: it does not include all the direct heat supplies (gas etc.) or transport fuels (petrol etc.). We will be coming back to look at the overall consumption picture latter, but very roughly, in the UK, electricity use represents about a third of total energy use. And to give you a feel for the general pattern, most of the coal (and nuclear) energy is used to produce electricity, most of the oil is used for transport, and, until recently, most of the gas was used just for heating - with energy use in these three sectors being very roughly equal.

Exercise

Just to check you are making sense of all this, why not try the following calculation

A large electric kettle is rated at 1kW. It takes 6 minutes to boil. How much energy has it used - in kilowatt hours?

Answer: 6 minutes is one tenth of an hour. So the kettle will have used 0.1kWh

Note: currently, electricity in the UK costs consumers 6-7p/kWh, so boiling that kettle full of water will have cost 0.6 - 0.7 pence.

Try the same calculation for an 8kW instantaneous shower, running say for 6 minutes, compared with a bath full of water, which might need an immersion heater running for one hour, and you’ll realise why even powerful showers are cheaper than baths.


3. Primary Energy

The total amount of energy used is often measured in terms of primary energy consumption, that is the amount of energy in the basic fuels used by energy conversion devices, whether they are used for electricity production, heating or transport. National level energy use is often represented in terms of the primary energy feeding in to the country, aggregating together the input energy used by all the various types of conversion devices and systems - power stations, vehicles and so on.

However it is important to remember that 'primary energy' figures, for the total energy in the fuels used by energy conversion devices, are usually much larger than the finally delivered energy, as utilised by consumers, since, as we have seen, there are losses in the energy conversion and power delivery processes, for example in power plants and in transmission along the grid cable network.

This is particularly true of electricity: conventional coal or nuclear fired power plants only have conversion efficiencies (measured in percentage terms as energy out, divided by energy in, times 100) of around 35%. The rest, around two thirds of the primary energy input, is normally wasted - much of it being pushed out into the environment as heat from the cooling towers at power stations.

This is due to the basic process of using fuels to raise steam so as to drive turbines. This is perhaps not the place to get too far into the intricacies of thermodynamics (although Box 1 might help you grasp some of the key points) but the basic explanation of these losses is fairly straightforward. In a conventional stream raising plant, the superhot steam is used to drive a turbo generator to generate electricity: the steam pushes against the turbine blades making the turbine rotate and giving up its energy. However, the turbine can only extract some of the energy from the steam- although it is cooler, steam still emerges out of the exhaust side, and this energy is then dumped, when the steam is converted back to water. That is what the large cooling towers at power station do: they are essentially vast condensers, fed with steam and transferring the waste heat out from of their exterior surfaces to the air around them.

Fortunately, there are ways to recycle some of this energy, as is explained in Box 2, by developing new types of 'combined heat and power’ plant, but, so far, they are not widely used in the UK. The result is that, in effect, in most coal fired power stations in use in the UK, for every three truck loads of coal fed in to a power plant, two truck loads are wasted, and only one truckloads worth ends up being converted to electricity. It does not matter what the initial heat source is. If nuclear power plants, or conventional plants burning gas or oil are used to raise the steam to drive turbines in the conventional way, the result is no better - they still produce as much waste heat.

Moreover, after it has been produced by a power plant, up to 10% of the electricity may be lost (by heating up the wires) when it is transmitted along power lines to consumers, depending on the distances involved. And then, when they finally get it, consumers will use this 'delivered' energy to power a variety of energy conversion devices with varying degrees of efficiency, with much of it often being wasted, for example in poorly insulated buildings. Primary energy figures therefore only tell part of the story. As we shall see in subsequent sections there is also a need, when comparing technologies and energy systems, to consider the overall efficiency of energy conversion and transmission, and the use to which the energy is put.

Also, do remember that there is a big difference between primary energy (i..e. all energy used) and electricity. For example the UK’s nuclear plants produce around 25% of our electricity, but only about 8% of our total primary energy.

 

Box 1 Energy Conversion Efficiency and Thermodynamics

The efficiency with which energy can be converted from one form to another can be an

alysed in theoretical terms. For energy conversion devices like power stations, the efficiency is equal to the difference between the input temperature of the steam going in to the turbine (T in) and its output temperature at the exhaust (T out) divided by the input temperature (T in).

Or in algebraic form:

Efficiency=T in – T out
                    T in

This may look pretty simple, but it is the basis of much of the science of thermodynamics - which governs the design and performance of many energy conversion devices, and tells us that no device using thermodynamic conversion (i.e. using the heat of gases to drive machinery) can have 100% efficiency.

Let’s take the most extreme case to show why. About the hottest you can run any machine without it melting is 600 degree centigrade- steel melts not far above that temperature. About the lowest temperature you can imagine using for the exhaust side is ground or air temperature - around 15 degrees C. To do the sum we have to convert to a different temperature scale, the Kelvin, on which the freezing point of water is 273 (on the Kelvin Scale the absolute zero temperature of space is zero, or minus 273 centigrade).

So our converted temperatures come out as 273 + 15=288 and 273+ 600=873.

The theoretical maximum conversion efficiency is then:

873-288 or around 2/3 - about 67%
    873

In reality few actual devices can attain anything like this. As already noted, in practice most power stations operate at around 35% efficiency. Perhaps now you can see part of the reason why: you cant get T in – T out large enough in practice. Which means there are inevitably going to be wastage's in the energy conversion process.

Not all energy conversion devices are thermodynamically limited in this way: for example the various so called ‘direct conversion’ devices, some of which work at much higher efficiencies. Many of them are electrical, electronic or chemical, like batteries or electric motors, and have conversion efficiencies of around 80%. So does the electrolysis of water to produce hydrogen gas. By contrast, modern photovoltaic solar cells are only around 20% efficient. And although as mechanical systems, wind turbines are not limited by thermodynamic losses, there are aerodynamic losses, so that the maximum theoretical efficiency is 59.3%. So, even with non-thermodynamically limited conversion, there are still some losses: 100% efficiency is not easy to obtain in the real world.

Entropy

The fact that there are inevitable losses and energy wastage when one form of energy is converted in to another, has some interesting implications. It means that although you may create something more useful, like electricity, overall you will have also have produced some degraded form of energy - unwanted by-products of little value. If you kept repeating this conversion process, feeding the high grade energy back to be converted into some other form of energy, you would end up with less and less useful energy and more and more low grade by product/waste.

Physicists describe this process as one of creating increasing 'disorder' i.e. degrading the original energy, and they use the term 'entropy' to describe the degree of disorder. They add that entropy always increases: try as you may to reorder things, in the end, the result will be that the world will move to a more chaotic, lower grade, state.

It may seem hard to see how a lump of coal is higher grade than electricity, but you have to remember the conversion losses and what we do with electricity - which is to eventually turn it into low grade heat in various ways. Each time we convert some primary form of energy into power for us to use, we end up, sometimes at the end of a long chain of subsidiary processes, indirectly warming the planet slightly. This is not the same thing as ‘global warming’ due to greenhouse gas emissions - it’s a longer term and more fundamental process. The whole universe is going the same way on a cosmic timescale, so perhaps you do not have to feel too responsible for your own contribution. Except that, if we use energy conversion devices to continue to rapidly increase our local entropy, we could eventually bring human activities on this planet to an untimely end - the so called ultimate ‘heat death’.

Box 2: Combined Heat and Power

We have seen that, whatever the fuel used to provide the heat, there are significant losses associated with energy conversion in conventional power plants. Surely there must be some way in which these might be reduced? The obvious answer is to use the exhaust steam for heating rather than wasting it. That is actually done on a wide scale, for example in some European countries, with the waste heat being fed to 'district heating' pipe networks to provide heating for domestic and commercial buildings.

There are still, inevitably, losses in the process of energy conversion (you can't beat the laws of thermodynamics) but, in principle, you can utilise perhaps half of the energy that would normally be wasted by recycling the waste heat in this way - in effect doubling the overall conversion efficiency of the power plant. So if instead of operating at say 35% efficiency just converting the input fuel to electricity, we make use of the waste heat as well as the electricity, the plants overall energy conversion efficiency can be increased to say 70% - and it some cases it can be more.

A simple sum might help to make this point clearer. The efficiency of conversion in a conventional plant is defined as the energy output divided by the energy input. Let’s say that we feed three units of fuel in to a power plant and only obtain one units worth of electricity out: the other two emerge as waste heat. The overall conversion efficiency can be defined in percentage terms as the output divided by the input, times 100. In this case it is:

(1/3) x 100%=33.3%.

If we then 'rescue' half of this waste heat, the overall output is doubled, since we now get two useful units worth of energy out - one as electricity, and the other as heat. The overall efficiency is also doubled, since is now (2/3) x 100 or 66.6% .

Power plants operated in this way are called Combined Heat and Power (CHP) plants - in the USA the term co-generation is also used.

However it is not quite as staightforward as it might first seem. The steam from conventional steam turbines is at too low a temperature to be very useful for district heating networks, and it is therefore necessary to modify the power plant to extract steam at slightly higher temperatures. This however reduces the plants efficiency as an electricity producer. It is a 'swings and roundabouts' exercise: you loose some electrical conversion efficiency but gain some energy conversion efficiency overall.

There are some other options for increasing overall energy conversion efficiency, notably the combined cycle gas turbine. In this, natural gas is the fuel and it is burnt to create hot gases which drive a gas turbine, much as in a jet engine. The waste heat from the gas turbine exhaust is then used to raise steam to drive a conventional steam turbine, so it is a two stage device. But it is only used to generate electricity- the temperature of the output steam is usually too low to use for heating purposes. The end result nevertheless is that the overall energy conversion efficiency can be up to 50% or more: not as much as with CHP, but still worthwhile.


4. Making Sense of Units

Given that there are many ways in which energy is generated and used, it is not surprising that there are many different, often confusing, ways in which it is measured and many devotees of rival systems of measurement.

One of the earliest units was of course horse power (HP) - the power of one horse. Cars are still often rated in HP. One horsepower is actually 746 watts or about three quarters of a kilowatt.

As for energy, we have mentioned kWh, which is the most familiar unit to most people since it is what is usually used on consumers' electricity bills. However you may also be familiar with the calorie - an energy unit used by chemists and more recently by weight watchers. Unfortunately, to confuse things, the food Calorie (which is usually written with a capital 'C'), is 1000 larger than the chemists calorie (which usually written with a small 'c'). The (chemical) calorie is defined as the amount of energy needed to raise the temperature of 1 cubic centimetre of water through one degree centigrade. See Box 3 for a discussion of energy in food.

By contrast physicists, and, more recently, energy analysts, sometimes use the basic physical unit for energy, the joule ('J') or multiples of joules. One watt is one joule per second, so (since there are 3,600 seconds in an hour) a kWh is 3,600,000 Joules, and the joule is thus a very small unit. Hence large multiples are common, e.g. mega joules or 'MJ' (1,000,000 joules), giga joules (1000 MJ), tera joules or 'TJ' (1000GJ), peta joules or 'PJ (1000 Tera joules) and exa joules or 'EJ' (1000 Peta joules

Box 3 Energy in Food

Nowadays most food is labelled with its energy content in terms of joules (or rather kJ) usually per hundred gramme serving. One (food) Calorie is equal to 4.19kJ, and to give you an idea of scale, common granulated sugar has an energy content of about 4 Calories per gram or 19.76 kJ per gram. Fatty foods can have an even higher energy content- up to 9 Calories per gram or more. That's the equivalent of nearly 38 MJ per kg.

You might like to compare this figure with the energy content figures for other fuels: see Table 1 ( which is in both joules and watt-hours). As you can see, food is not a bad fuel in terms of energy content per gram!

For example, lets assume an ice cream label says that it contains 250 Calories, or 1050 kJ per 100g. That’s 1.05MJ if you eat a 100g portion. In theory, it works out, from the basic physics of lifting masses up heights, that a person weighing 60 kilograms would only need to eat 5 grams, to provide enough energy to walk up four flights of steps (say 10 metres) - assuming all the energy could be converted into muscle power. That’s about one lick's worth. In principle, this is a reversible process, but as you can see you would have to climb an awful lot of stairs to burn off the energy from the compete 100 gram tub of ice cream: about 80 flights in fact, even assuming a 100% energy conversion efficiency.

Of course, in the real world, the energy conversion process depends on human metabolic rates (and your fitness) and the energy you burn off depends on how easy it is to mount the steps - and how quickly you try to do that. As anyone who has attended a modern health club will know, working against gravity, or other forms of resistance, can be made into quite tough exercise.

Ice cream too much for you? Here are some other food energy contents in MJ/kg- vegetables 0.1 – 0.2, fruit 1.5 - 4.0, potatoes 3.2 - 4.8, lean meat 5.0 - 10.0

However, to confuse things further, in the UK until recently, for statistical comparison purposes, primary energy use, that is the energy in the fuels used, was often measured in terms of the equivalent amount of coal that would be required to be burnt to provide that energy, regardless of what fuel was actually used in power stations i.e. in 'tonnes of coal equivalent' (or, more, usually 'million tonnes of coal equivalent' or 'mtce'). This no doubt reflected the historical predominance of coal in the UK's economy, although, to confuse matters even more, the gas unit, the 'therm', was also sometimes used ( 1 Therm=100,000 British Thermal Units)

In 1994, to make the situation perhaps a little clearer, the UK governments' statisticians decided to adopt the European standard unit, with the energy content of all fuels being rendered, for statistical comparison purposes, in terms of the equivalent amount of oil that would have the same amount of energy content. The energy content of all fuels is therefore now presented in terms of tonnes of oil equivalent, or more usually, million tonnes of oil equivalent or mtoe's.

Mtoe’s are now widely used by energy analysts, but most engineers still use the more familiar kWh, TWh etc. figures. For reference 1 mtoe=11.63 TWh, and 1 TWh=0.086 mtoe. Some more conversions factors are given in Box 4 below. And, to round things off, we include a comparison of the energy contents of various common fuels

Not everyone will find the details of the units used in energy measurement exciting, but obviously it is necessary to have a common measure, and establishing this may not be easy - or uncontentious. The latter point can be illustrated by exploring some of the details of the new approach mentioned above. In addition to switching to the use of millions tonnes of oil equivalent, the changes introduced in 1994 to the way energy statistics are rendered in the UK also involved a subtle shift in the way the energy content is calculated. It is now based on the energy content of the output power produced by power plants, rather than on the energy content of the input fuel needed to generate the power.

This has some interesting results. Previously, on the so-called 'fuel substitution' basis, the primary energy contribution from non-fossil fuel powered devices like the wind turbines was calculated in terms of the energy content of the fossil fuel that would have to be fed to a conventional power station to provide the same amount of power output. On the new 'energy supply' basis, the figures for contributions from devices like wind turbines drop dramatically, since the output is no longer adjusted (by 65% or so) to reflect the scale of the losses associated with energy conversion in conventional power plants. Or to put it another way, these loses are ignored and only the final outputs are compared. The end result of this change, and some other, less dramatic, changes in the way the sums are done, is that the quoted renewable energy contribution is reduced by more than 70%.

Not surprisingly, the new approach is not particularly popular with renewable energy supporters, especially since, at least in some formulations of this new approach, the figures for nuclear power plants are unchanged. Thus in some reports you can have nuclear power and hydro power quoted as both contributing around 2% to total world energy, while in other reports the nuclear contribution is put at 6 or 7%, but that from hydro is still only 2%. It yet other cases, both are put at 6 or 7%.

To re-iterate, the 2% figure are the actual outputs, calculated according to the new ‘energy supply’ scheme, in terms of the actual amount of energy obtained, whereas the 6-7% figures are the primary energy inputs, that is, the amount of fossil fuel that would be needed for a fossil fuel plant to generate the energy specified- taking into account the 65% or so losses/35% efficiency associated with conventional fossil fuelled. Hence the factor of roughly 3 difference.

Both methods of rendering the figures can be useful - and the UK statisticians have decided to also make the figures available on the original substitution basis, since it was accepted that, while it was useful to be able to compare the actual amount of power being supplied to the grid, it was also useful to be able to assess the degree to which renewable sources were substituting for fossil fuel. But switching back and forward between these schemes and comparing them inconsistently can be confusing.

Note that there is also another possible factor of 3 around that might confuse you even further. If we also shift to presenting the data for, say, hydro and nuclear in terms of the percentage of the world total electricity supplied from these sources, the 6-7% figures increase to around 18-20% in both cases - since, very roughly, electricity make up about one third of total energy use.

Let’s hope you are not now totally disenchanted with the rigour of energy statistics! Hopefully, the above will simply ensure that you take care to check what units are being used and how the figures have been calculated.

Box 4. Summary of unit conversion factors

1 watt=1 joule per second

1kilowatt hour=1 watt for 1 hour=3,600,000 joules ( or 3.6 mega joules)

1 calorie=4.18 joules

1 British Thermal Unit=1055 joules

1 million tonnes of coal equivalent=28 PJ or 7.5 TWh*

1 million tonnes of oil equivalent=42 PJ or 12 TWh*

(* approximately, assuming 100% conversion efficiency)


Table 1 Energy contents of various Fuels

Quantities of Heat Joules Watt-hours
1 cubic foot of natural gas 1.1 MJ 0.31 kWh
1 cubic metre of natural gas 39.0 MJ 10.8 kWh
1 litre of petrol 39.6 MJ 11.0 kWh
1 litre of heating oil 41.1 MJ 11.4 kWh
1 kilogram of coal 30.0 MJ 8.3 kWh
1 therm of gas (=105 BTU) 105.5 MJ 29.3 kWh
1 British Thermal Unit (BTU) 1054.8 J 0.293 Wh
1 kilogram of dry wood 15 MJ 4.2 kWh

5. National and Global Energy Use- and its impacts

Having now established some of the basic sources and types of energy and how they are converted and measured, we can move on to look briefly at how we actually use it at present - and its impacts. Primary energy use figures can be derived at various levels - for countries, or for the world as a whole. Within the national context, primary energy use is often broken down in terms of its final destination i.e. in terms of its eventual 'end use' in each sector of the economy.

In terms of where the basic primary fuels are used, as we’ve noted before, in the UK, very roughly speaking, most of the oil is used for transport, most of the coal is used for electricity production and most of the gas was until recently used for heating - although gas is now being used for electricity production instead of coal.

Inevitably the exact figures change with time, but to give an impression of the balance of energy use amongst the various main end use sectors, in the UK in 1994, transport accounted for 33% of primary energy use, industry 25%, the domestic sector 29%, leaving 13% for other uses. So very roughly speaking, the three main sectors, transport, industry and the domestic sector, each use about the same, although transport is the largest, and in fact has begun to get even larger since 1994.

In terms of the actual form of energy consumed, in the UK in 1994, electricity accounted for 16% of the total, gas 32%, petroleum 44% and solid fuels 8%. However, following the privatisation of the UK energy system in 1990, the way in which electricity supplies were generated has changed, with, as we noted above, gas increasingly taking over from coal as the main fuel for electricity generation. For example, before privatisation up to 80% of the UK’s electricity had come from coal, but by 1995 it fell to 47%, primarily due to the use of natural gas fuelled combined cycle turbines for electricity production. By 1999 coal and gas were almost level - at around 33% each. And by the year 2020, if trends continue, gas is predicted to have a 65-70% share, with coal falling to 10-15%.

The patterns in other countries obviously varies, with electricity use being much higher in the developed countries. For example, Norway and Brazil obtain more than half their electricity from hydro, and France obtains more than 70% of its electricity from nuclear power, whereas in some developing countries there is very

little use of electricity from any source.

In terms of total energy use, the overall trend in the UK, and globally, is upwards - global energy use has in general been rising by between 1-2% each year. See BP’s annual digest of global energy statistics for the latest figures at : http://www.bp.com/bpstats Also see the International Energy Agency data at: http://www.iea.org/stats/files/keystats/

Looking further into the future, given the growing world population and rising material expectations, energy use globally is likely to continue to increase. Energy scenarios have been devised to attempt to map out possible patterns of longer term development in energy supply and demand. Some assume that energy demand is likely to increase up to perhaps three times current levels by the year 2060.

Does that matter? The answer must be yes, if we are concerned about the environment. In recent years we have become aware that burning fossil fuels in power stations and cars not only produces air pollution which can damage health, but it also seems to be changing the Climate.

The main culprit in terms of Climate Change is carbon dioxide gas, which, along with other so called greenhouse gases, such as methane, travel to the upper atmosphere where they act something like the glass in a green house, trapping in the sun heat. This seems to be leading not only to global warming but also to other changes in the Climate system, such as violent storms, floods, and droughts, leading in turn to floods, fires, disease, and possibly death and destruction on a significant scale, as temperatures rises, the ice caps melt and sea level rise. All this just from burning a few fossil fuels? Although some ‘contrarians’ dissent*, the balance of the evidence seems to suggest so - for example see http://www.unfccc.de.

Note that the greenhouse gas ‘climate change’ effect is unrelated to ozone depletion and the creation of so called 'ozone holes' in the stratosphere, notably in the polar regions. Although ozone plays a role in the greenhouse effect, the ozone holes are a separate phenomena - the result of a chemical interaction between CFC (Chloro Fluoro Carbon) molecules and other halogens, which destroy ozone. CFC's are man made chemicals which were developed as a supposedly inert gas for use in refrigerators and foam packaging, amongst other things. The results of stratospheric ozone depletion due to the release of CFC's is that dangerous wavelengths of solar radiation can reach the earth's surface where they can cause skin cancers and damage plant growth. That’s serious enough, but the results of the wider process on Climate Change could be even more serious.

* See for example 'Climate of Fear; why we shouldn't worry about global warming' Thomas Gale More (Cato Institute, Washington DC) or Fred Singers spirited contrarian analysis at http://www.sepp.org For a riposte see Ross Gelbspan ‘The Heat is On’ Longman 1997


6. Reducing Environmental Impacts

Surely some of these pollution problems can be avoided? The answer is yes, some can, but others are much harder. A bit of simple chemistry will hopefully put the situation in context.

Lets deal with the easy one first. One of the main air pollution issues is acid rain - the consequence of acidic emissions from power stations, caused primarily by the sulphur content of coal and oil. When burnt, the small sulphur element in these fossil fuels is converted into sulphur dioxide gas. Here is the equation: S + O2=S O2

This gas can dissolve in water vapour in the atmosphere to produce weak sulphuric acid. This acid, although very weak, falls in the rain and can damage trees, and, collected up in lakes, can injure wildlife and fish. Acid rain also damages buildings and crops. Oxides of nitrogen (NO, NO2, NO3 etc, usually collectively labelled NOx) can also be produced by the combustion of fossil fuels in power stations, and also from the combustion of petrol in cars, and they can play a similar role.

In principle, the acid emissions from the combustion of fossil fuels can be filtered out relatively easily - although at a cost. By contrast there is no easy remedial technology for the emission of greenhouse gases such as carbon dioxide. Carbon dioxide is the fundamental product of combustion: put simply burning means using oxygen from the air to convert the carbon in fossil fuels into carbon dioxide gas plus heat. The chemical equation is: C + O2=CO2 + heat.

Fossil fuels are hydrocarbons: they contain varying amounts of hydrogen and carbon. All fossil fuels create carbon dioxide when burnt, but some produce more than others depending on their chemical make up. Coal has a high pure carbon content, and when burnt it therefore produces mostly just carbon dioxide and heat. By contrast methane (natural gas) is made up of one atom of carbon plus four of hydrogen and when burnt the hydrogen is converted into water, so that the ratio of carbon dioxide to heat produced differs. Here is the chemical equation CH4 + 2O2 =CO2 + 2H 2O+ heat

The result is that the combustion of natural gas produces roughly 40% less carbon dioxide per kWh of heat produced than the combustion of coal. By contrast the combustion of the more complex hydrocarbons like oil, produces intermediate levels of carbon dioxide. See Table 2.

Table 2 Carbon Dioxide productio

Fuel kg of CO2 per GJ of heat
Coal 120
biomass 77
oil 75
natural gas 50

Clearly then, one way to reduce carbon dioxide emissions from power station is to burn gas rather than coal - and as we have seen that is being done in the UK. However, there is only so much methane gas available, it’s a finite resource - eventually we have to tackle the problem at source and stop burning fossil fuels.

That may sound a rather drastic remedy. Aren’t there other solutions. Couldn’t we simply collect and store the carbon dioxide? The problem is that carbon dioxide is a tasteless, colourless and chemically inactive gas which dissolves only very slightly in water- it’s what’s in fizzy drinks. That makes it hard to remove from power station or car emissions. In theory you could freeze it out from power station emissions, producing solid carbon dioxide or 'dry ice' - at vast expense.

Alternatively you could separate it out chemically or by osmotic filters, and then store it as a gas, although, once again, that would be expensive. One estimate suggested that chemically separating out carbon dioxide from power station emissions and then storing it as a gas would add 50-75% to the cost of electricity. There may be situations where this 'sequestration' process might make sense: for example it has been proposed that the CO2 gas could be stored in the underground strata which have been emptied by oil and gas extraction. However, you would have to be sure that it would stay where it was put, and the volumes involved are vast.

Another approach would be to plant more trees to absorb carbon dioxide. Certainly the problem of climate change has been made worse by the fact that large areas of forest around the world have been felled, thus removing an important 'sink' for carbon dioxide, since trees absorb it as they grow. Re-afforestation is obviously vital, if only in terms of maintaining biodiversity. However, it would take many decades just to replace what has been lost, and unless carried out on a vast scale, re-afforestation could only play a small role in absorbing the vast amount of carbon dioxide released every year by power plants and cars. It’s worth doing, but one key problem is that trees absorb most carbon dioxide when they are growing: once mature, the absorption rate reduces, and although the established network of living biomass in mature forests can continue to play a role in carbon dioxide absorption, when trees die and decay, or catch fire, some or all of it can be released back in to the atmosphere.

Basically, trying to put the genie back in the bottle once it’s escaped is not the answer - we really ought to deal with the problem at source and not produce carbon dioxide gas in the first place. Which means basically two things - reducing our use of energy and using non - fossil sources to meet or remaining energy needs.


7. Sustainable Energy Options

The first and most obvious solution is energy conservation - using less primary energy. There are many ways in which energy conversion and energy use can be made more efficient. We have already looked at one option for improving overall efficiency of energy conversion - Combined Heat and Power. We’ve also mentioned fuel switching - that is using gas instead of coal in power plants as a way to reduce the amount of carbon dioxide produced per kWh generated.

Moving over to the energy use side, there are many options in terms of cutting demand in all sectors. The UK Industrial sector has already made big changes in this direction- energy use/ GDP output has fallen by 40% since the 1970’s due to the introduction of more efficient systems. The impetus has mainly been financial - as energy has begun to cost more, interest has grown in using it more effectively.

However, for most people, a more familiar energy saving option involves improving the insulation of houses and installing energy efficient devices in them. Energy use in the domestic sector accounts for nearly 40% of the carbon dioxide emissions in the UK and there are many opportunities for reducing this by designing houses, their heating and other power using systems, to use energy more efficiently.

That means loft insulation, cavity wall filling, the use of low energy compact flourescent light bulbs, low energy fridges, washing machines, cookers and other consumer electrical and electronic equipment. See Box 5.

However, there may be a problem with just focussing on energy conservation. What happens to the cash savings that people make when they save energy? They are likely to spend them on energy intensive activities - a dishwasher, or a foreign holiday by air. Not all expenditure is going to be so energy intensive, but even so there is what economists call ‘rebound’. The cash released by energy conservation can lead to more energy being consumed overall.

Box 5 Energy Efficiency – the key areas

Given that energy has been relatively cheap in recent years (it’s now cheaper in real terms than it was before the oil crisis in the mid 1970s) interest in energy efficiency has been low. But with the environmental costs of generating and using energy rising, and feeding through to economic costs, things are changing.

However, the UK, sadly, is far behind many other countries, especially in the domestic sector. We have some of the worst housing stock in the developed world in terms of energy losses. For example the English House Condition Survey, published in 1996, showed that 50% of owner occupiers, 62% of local authority tenants and 95% of private rented sector tenants failed to achieve adequate levels of warmth - 18c in the living room and 16c elsewhere - when the outside temperature is 4c.

There is no need for this. Well insulated houses have been built which need almost no external sources of energy for heating - the lighting and other incidental sources provide sufficient heat. That should not be too surprising. After all, we can quite happily walk around in the open even in the depth of winter just with personal insulation in the form of well designed clothes. Why, when we come indoors, do we suddenly need huge heating systems?

At the very least, we should be able to design houses with a well insulated building envelopes, low emissivity glass and so on, which need to use 50% less energy - and obtain much of that from renewable sources directly. Solar heat collectors on the roof can meet about half the average annual requirement for hot water. PV solar cells on the roof can meet most of the non-heating electrical load, and, if needed, sustainably sourced wood can provide back up heat. Even better, if your own domestic renewables systems produce excess power at some point in the year, you can now sell it at reasonable ‘net metered’ rates, via the grid - you become a power station!

However, complete domestic energy autonomy is not likely to be viable or needed in most locations in the UK. While, as we have seen, some renewable sources like solar energy, are well suited to domestic use, others are better deployed on a larger scale. For example, small windturbines generate much less power per invested than large commercial machines on remote windfarms. And it’s not just the extra cost that is important- the cost reflects the extra materials used per kWh generated, which in turn has implications in terms of the energy used to manufacture them. One study of the total energy costs of renewable energy devices, including the energy embedded in the materials used, found that the worst case was small independent windturbines feeding into batteries. Of course there are transmission losses associated with sending power from remote power stations to users, so that local generation can make more sense in some locations. However, the existence of the national grid can help balance out variations in local power availability from intermittent renewable sources like. So, in most places, totally off-grid self generation is not the best option.

Certainly, for the moment, most people will want to continue to use gas for heating, which can be efficient if burnt in a condensing boiler - and eventually the natural gas may be augmented or replaced by hydrogen gas, produced from renewable sources (hydrogen gas was one constituent of the old town gas, derived from coke, and used before the advent of North Sea gas). In terms of electricity, you can already contract to have green power from remote renewable sources via the national grid - there are currently a dozen or so schemes on offer in the UK. They allow you to go green simply by switching suppliers. For details of prices see http://www.greeprices.com

The other big area for improvement is transport. Energy use in this sector is growing in most countries, and it is hard to see how this can be limited except via tough constraints on the use of private vehicles coupled with heavy investment in mass transport. But technology can help to some extent. There are already high efficiency conventional car engines and in the longer term electric and hydrogen cars will be common, with the energy coming from renewable sources. You may even charge them up while they are parked overnight at home, or at work or at the supermarket. However, although using these vehicles will not produce any emissions, if we end up just with more and more vehicles - queues of electric or hydrogen cars - things will not be much better. The provision of roads to service more and more vehicles has its own environmental and social impacts. Somehow we need a life style change to reduce the need for so much private travel. Electronic communications can help to some extent. But basically we will have to get used to the fact that the era of cheap personal car transport may be over.

The same may also soon apply to cheap Air transport. Emissions from this sector are rising rapidly, which is perhaps not surprising since aviation fuels are one of the few fuels that are not taxed! That honeymoon can’t last much longer….

The same problem exists for the case where people are suffering from ‘fuel poverty’- i.e. people who are unable to afford to keep their houses warm. Any gains obtained from energy conservation are likely to be taken back in keeping homes warmer- thus using more energy and producing more carbon dioxide.

Is there a way out of this ‘rebound’ problem? Yes, if consumers can move away from using power derived from fossil fuels. However, the bad news is that, even if we can avoid the rebound problem, the savings made form energy conservation may still be wiped out if overall consumer demand for energy increases i.e. even with significant energy conservation, the continuing rise in overall energy use is unlikely to be halted.

Obviously, many ‘green’ minded people feel that it would be good to try to reduce our reliance on consumerism as a way of life. However, many people in the world still do not have access to even the basic requirements for a reasonable life. Clearly the industrialised nations have had more than their share of resources - and still produce more emissions than the rest of the world. However the rest of the world is catching up. Given the large populations and low level of economic prosperity, the per capita level of emissions from the developing world may still be very much less than those from the industialised countries, but the total per annum emissions from the former may soon overtake the latter. This is not the place to debate the politics of global economic redistribution, much less population control issues. While social and cultural changes, and changes in political and economic arrangements, could do much to rebalance the massive inequalities that exist between and within countries, technology and new more sustainable approaches to the use of technology can also help. Energy conservation techniques are part of that, but we will also need new sources of energy, just to replace what we are using now, quite apart from meeting any new needs. The message therefore is that, if we want to avoid increasing environmental problems and if the developing economies of the world are to continue to develop, we need to adopt a sustainable energy strategy which combines energy conservation with a switch over to non-fossil fuels.

The only two non-fossil options currently on offer are nuclear power and renewable energy. Although uranium is a finite resource, there are sufficient reserves to fuel the existing type of nuclear plants for maybe a hundred years or more, depending on the rate of use ( i.e. the number of plants in use) and nuclear power plants do not produce carbon dioxide directly. However, they do produce very dangerous radioactive wastes, and the prospects for nuclear expansion, as a response to Climate Change, look slight these days. No new plants have been ordered in the USA since the Three Mile Island nuclear accident in 1979 and, following the Chernobyl nuclear accident in the Ukraine in 1986, most of Europe is in the process of phasing out nuclear power.

The ex-Soviet countries still have nuclear plants and Japan and China still have nuclear programmes - and some other Asian countries would like to. But overall, nuclear power seems to be on the decline. Its current contribution, of around 8% of total world energy, is unlikely to increase and is more likely to decrease. By contrast renewables are on the ascent. The use of natural 'renewable' energy flows, like the sun, winds, waves and tides, produces no carbon dioxide, and is therefore well suited to the needs of developing countries around the world. Current projections suggest that by 2050 around half the world energy could come from renewable sources.

The use of nuclear fusion (the process which occurs on a vast scale in the sun and, also, dramatically, in H-bombs) remains a long term possibility, but, even if the technology can be perfected, it too has its own safety problems, and unknown costs. Perhaps more importantly, fusion is unlikely to be available as a practical technology, if it ever is, in time to help us deal with the urgent problem of Climate Change. Rather than try to create little artificial suns on the earth, in the form of fusion reactors, many environmentalists therefore believe it is more immediately credible to make use of the natural fusion energy than the sun already produces and which reaches us as solar heat and light.


8. The Renewable Future

As we noted earlier, the term renewable energy is used to indicate that these natural energy sources (for example solar energy, the winds, waves, and tides) cannot be used up - unlike fossil or nuclear fuels they are not based on finite reserves, but are naturally replenished. The use of biological resources such as biomass, can, as was indicated, also be considered renewable if the rate of planting equals the rate of use, and certainly the potential is vast. However, there can be problems with trying to use what are generally more diffuse, geographically disperse and sometimes intermittent renewable energy sources.

Perhaps the most immediate issue is environmental impact. No technology can be entirely benign. Although renewables do not have any global environmental impacts, there may still be some local social or environmental impacts, for example visual intrusion by windfarms. This has become an issue in the UK, although this problem can be overstated. So far, the public opinion surveys have found that, typically, 80% of people asked are in favour of windfarms, and support for them actually increases when people have actual experience of wind projects. There may be a small minority who are implacably imposed, but the level of opposition to windfarms should perhaps be compared with the equivalent figures for nuclear power. Opinion polls have indicated that typically 70-80% of those asked are against further nuclear expansion or want nuclear power phased out. In the end it comes down to a matter of choice, all technologies have some impacts, but the impact per kWh from renewables are usually much less, and much more localised, that the impacts per kWh from the conventional energy technologies. Box 6 may help you get a measure of the problem in relation to wind.

What about the diffuse nature of renewable energy flows? Doesn’t that make the energy conversion process inefficient? To some extent, since we would be using natural, freely available, energy, the efficiency of conversion doesn’t matter too much - the conversion process does not generate dangerous emissions. But efficiency does matter in that inefficient converters will have to be larger to collect the same amount of energy - for example larger wind turbines or larger numbers of wind turbines. That will have implications for local visual and environmental intrusion, and also for costs, since more materials would be used per kWh generated. The energy debt associated with providing these materials also has to be taken into account. So the development of well designed, efficient, devices is important, as is their careful location.

A linked problem facing renewables is that not only are the sources usually diffuse, some of them are also intermittent. Fortunately, in most countries in the Northern hemisphere wind and wave power tend to peak during the winter when more power in needed, but, even so, local seasonal and climatic variations do mean that some renewable energy sources cannot supply power continuously. As it happens this intermittancy does not matter unduly, in terms of providing continuous power to consumers, if the power from the locally variable sources can be fed in to the national power grid, which can, in effect, average it out. If the contribution from variable sources like the winds stays below about 20% the national grid can even out local variations in availability - it is usually windy somewhere around the country.

Of course, if intermittent renewables are to contribute more than that, then we will need some form of energy storage, and that can be expensive. In the longer term it could be that we will convert some of the electricity produced from variable solar wind, wave and tidal sources into hydrogen gas, by means of electrolysis - that is separating out the hydrogen and oxygen in water. Given proper attention to safety, hydrogen, which unlike electricity can be easily stored, could well become a major new fuel. It can be transmitted along gas mains and it can be used for heating, like natural gas, and for local electricity production in electrochemical devices called fuel cells, which work like electrolysis in reverse. Hydrogen can also be used to power vehicles - directly as a combustible fuel, or via an on board fuel cell .

Box 6 The Physics of Wind Power

The energy collected from a wind turbine is proportional to the area in the circle swept by the blade i.e. (pi ) r2 where r is the blade length. The result of this 'square law' is that, for example, doubling the size of the wind turbine quadruples the power output i.e. large turbines are much more effective. But beyond a certain size the blades face stress (and fabrication cost) limits.

The power in the wind is proportional to V3, the cube of V, the wind speed. So even a slightly higher wind speed site pays dramatic dividends. You can derive this relationship yourself if you appreciate that the kinetic energy of the moving air is 1/2 mV2, where 'm' is the mass of the air intercepted by the blade as it turns, and 'V' is the velocity of the air. The mass of the air intercepted is, in turn, proportionate to V, since that defines how much air hits the turbine blades. So, feeding this in 'V' to replace the 'm' the original equation, you end up with an equation for the power of the wind with V2 times a V, or V3 .

Armed with this knowledge you can now begin to explore some of the implications of windpower in terms of land use problems. Wind turbines are often grouped together in 'wind farms', since then the connections to the power grid can be shared, as can control systems and road access for maintenance. How much room will they take up?

Typically you need a separation of between 5 and 15 blade diameters between individual wind turbines, (depending on the machines and the site) to prevent turbulent interactions in wind farm arrays, so wind farms can take up quite a lot of space, even though the machines themselves only take up a small fraction of it. This has led to some objections - and to the argument that there would be insufficient room in countries like to UK to generate significant amounts of power.

So to see if they are right, lets now work out the rough land area requirements for a wind farm - and the amount of power that can be produced. Let's assume we are using medium sized windturbines rated at 300kW- that's typical of the machines already widely in use in the UK and elsewhere, although 600kW machine are entering service and there are also now machines rated at 1MW and larger.

Let’s also assume, to make the arithmetic easy, that we have a 10 by 10 array of one hundred 300 kW machines (30 MW in all) each with say 30 metre diameter blades, and with 10 diameter separations. That's quite a large wind farm, but there are some on this scale in the UK. It would cover an area of 3 km by 3 km. However only about 1% of this area (i.e. 90 m2) would actually be occupied by the base of the turbine towers, the rest could be used for agricultural purposes. One hundred wind farms of this sort would give you 3 Gigawatts of installed capacity - equivalent to 3 conventional 1 GW plants.

Of course these wind turbines will not be able to operate continuously at full power since the wind is intermittent. Typically, given the variability of the wind, wind turbines in the UK can operate on full power load for about 30%-35% of the time: this figure being called the 'load factor'. Obviously the precise figure will depend on the wind turbine design, the site and the wind regime.

To make a fair comparison with conventional plants it is important to realise that, although fossil or nuclear fuelled plants do not have intermittent energy inputs and therefore have much higher load factors, even so, they typically can still only achieve load factors of around 60-70%. On this basis, to generate the same amount of power you would need roughly twice as much windfarm generating capacity as you would conventional capacity. Put the other way around, you would expect wind turbines to generate about half as much continuous power as conventional plants with the same capacity. The UK currently has 60 GW or so of conventional installed capacity, leaving aside existing renewable sources like hydro, so if 10% of this were to be replaced by wind turbines we would require 400 wind farms of the scale outlined above. The tower area covered would be 36 km, while the total area covered by the complete arrays would be 3600 km2 . That is just under1.5% of the UK’s total land area, 250,000km2. That, as it happens, is roughly the area that has been estimated as likely to be suitable and available for wind farm projects in the UK without significant intrusion.

Obviously this is only a very rough estimate, based on some broad assumptions. The actual power output in practice would depend on the machines, and sites - and the windspeeds. If larger machines were used you would need less of them, but equally there may be specific siting constraints which could reduce net power availability: some high wind speed sites may not be acceptable and larger machines might be thought to be visually intrusive. Even so, our calculation does suggest that in principle, even in a crowded country like the UK, we could obtain around 10% of the country's current electricity requirements from windturbines. But of course if that’s a problem,you can always go offshore, where there are far fewer environmental constraints and higher average wind speeds- offsetting the extra cost. And the potential North Sea resource is huge. Isn’t the UK fortunate again

That of course is all in the future. For the moment, the bottom line is economics. The existing energy technologies have the advantage of many decades of often heavily subsidised development, whereas most of the renewable energy options are new, and are trying to break in to a well established market. Not surprisingly some therefore look expensive. Even so, rapid technological and economic progress has been made. For example, in the UK context, in 1991 windpower projects received a subsidised price of 11p/kWh (under the Governments Non Fossil Fuel Obligation), but by 1998 wind projects were going ahead at an average price of 2.88p/kWh, and some Scottish projects were down to 1.89p/kWh. For comparison, in 1998, the average price paid by Regional Electricity Companies for conventional power (that is the so-called "pool ' price) was 2.67p/kWh. Similar patterns of cost reduction seem to be underway for most of the other renewables.

Given the rapidly changing situation, and the continuing debate over the impact of past and present subsidies for fossil and nuclear power, quoting comparative 'prices' for the various conventional energy sources is probably not very helpful, but to give you a rough feel for the pattern, in 1998 New Scientist quoted the price of electricity from gas at 2.5p/kWh, coal at 4.0 p/kWh and nuclear 4.5p/kWh. It’s also worth noting that a range of taxes is being introduced (such as the Climate Change Levy in the UK) to reflect the environmental costs of conventional energy technologies - so that the cost comparisons with renewables should improve even further.

In this context, it’s interesting that, whereas, as we noted earlier, energy strategists have in the past been concerned with energy in terms of ‘kWh’ produced, or ‘million tonnes of coal equivalent’ available, nowadays the units that matter are ‘million tonnes of carbon dioxide’ (mtC) i.e how many million tonnes of carbon dioxide production will be avoided by the various new systems and devices. So we have, as it were, moved from ‘mtce’ to ‘mtC’ in a generation or so - and ‘carbon accounting’ is beginning to become almost as important as conventional economic accounting.

 


9. Conclusion

We’ve looked at a range of energy technologies and at their problems, and we’ve concluded that renewable energy combined with energy conservation offers the best way ahead for a sustainable energy future.

Fossil fuels will of course still be with us for a long while, so there is a need to improve the efficiency of energy conservation and reduce emissions as far as possible- and whatever happens to nuclear power, we will still have a lot of nuclear waste to deal with for millennia. But looking to the positive future, renewables have good prospects, even in relatively crowded and cold countries like the UK. And the prospects for renewables in the sunnier parts of the world are even better. It could for example be that large scale solar cell systems will be installed in some of the desert areas of the world and will be used to generate power for the electrolysis of water, with the resultant hydrogen being tanked or piped back to the colder northern areas.

At the same time, some renewable energy technologies are well suited to deployment on the small scale local basis, meeting local needs from local resources. This could be very important in countries without power grids. Globally, 2 billion people live without access to electricity at present - 70% of people in rural areas in the developing world have no access to grid power and little hope of getting it. But smaller scale local generation can also be important in more developed countries. For example, in some situations it makes more sense to generate and use power locally than to transmit it over long distances via the grid cables: the losses can be too great. We could thus expect some degree of decentralised energy production and use to spread even in countries like the UK - although the dream of total domestic self-sufficiency cherished by some people may be somewhat further off.


Energy Glossary

Acid Rain Mildly acidic rain produced as a result of the release into the atmosphere of acidic gasses such as sulphur dioxide, generated by the combustion of fossil fuel in power stations and cars.

Biomass Biologically derived material than can be used as a fuel - e.g. naturally growing wood, plant or animal residues or specially grown energy crops

CCGT- Combined A power station in which natural gas is burnt to drive a gas Cycle Gasturbine, as in a jet engine, with the exhaust gasses being used to Turbine boil water for a stream turbine as a second stage of electricity generation. Can increase overall energy conversion efficiency from 35% to 50%

CHP - Combined The generation of heat as well as electric power in a power Heat and Power station- by the use of the exhaust heat which would otherwise be wasted . Can double overall energy conversion efficiency e.g. from 35% to 70% or more.

Energy A measure of the amount of 'work' that can be done by, or is needed to operate, an energy conversion system, sometimes measured in 'joules'. It is the power of the device (in kilowatts) multiplied by the time it is in use (hours): hence energy is more commonly measured in 'kilowatt hours' (kWh).

Energy Conversion Converting the energy in a fuel or other energy source to some other form of energy e.g. coal into heat. There are inevitably some energy losses in all conversion processes, usually in the form of wasted heat.

Energy Conservation Avoiding or minimising wasteful methods of fuel use by using more efficient energy conversion devices for energy generation or energy using devices at the point of use. (strictly should be 'fuel conservation' since 'energy' is always conserved). In simple terms, reducing energy losses in houses by installing insulation.

End use energy The energy actually consumed at the point of use.

Global Warming The possible increase in average global temperatures as a result of an enhanced 'greenhouse effect' due to the release of gases such as carbon dioxide and methane into the atmosphere: global warming is one element in the resultant process of 'climate change'.

Nuclear Fission The process of splitting the nucleus of certain atoms (e.g. uranium) with the resultant release of heat and radiation, as in atomic bombs or nuclear reactors.

Nuclear Fusion The process of fusing together certain light element ( e.g. hydrogen) to yield heat and radiation, as in the H-bomb and the yet to be fully developed fusion reactor.

Passive solar The use of glazed areas in houses to capture solar heat, much as with greenhouses. Unlike 'active' solar devices, which have pumps to circulate water through a 'solar collector', passive solar systems have no moving parts-hence the name.

Photovoltaics Photovoltaic 'solar cells' (or 'PV' cells) consist of special (PV) semiconductor type materials such as silicon which absorb light and convert it into electricity.

Power The capacity of a device to convert energy from one form to another, sometimes measured in kilowatts (kW) or its multiples. Devices are usually given a 'rated capacity' (in kW, MW, GW etc ) reflecting the rate at which they can convert energy from one form to another.

Pressurised Water The most common form of nuclear power plant, developed in Reactor- 'PWR' the USA, with cooling provided by pressurised water.

Primary energy The energy in the basic fuels or energy sources used e.g. the energy in the fuel fed into conventional power stations

Renewable Energy Energy sources such as the winds, waves, tides which are naturally replenished and can not be used up.


This material has been abstracted from the NATTA booklet ‘Energy: a beginners guide’ (Aug, 2000) which is available from NATTA for 3

Details from NATTA , c/o EERU, The Open University, Milton Keynes, MK7 6AA Tel: 01908 65 4638 (24 hrs) E-mail: S.J.Dougan@open.ac.uk


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Details from NATTA , c/o EERU,
The Open University,
Milton Keynes, MK7 6AA
Tel: 01908 65 4638 (24 hrs)
E-mail: S.J.Dougan@open.ac.uk

The full 32 (plus) page journal can be obtained on subscription
The extracts here only represent about 25% of it.

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