08.b Boek sewtha.pdf

“Okay – it’s agreed; we announce – ‘to do nothing is not an option!’ then we wait and see how things pan out. . . ” Figure 1.11. Reproduced by kind permission of PRIVATE EYE / Paul Lowe www.private-eye.co.uk.

Notes and further reading At the end of each chapter I note details of ideas in that chapter, sources of data and quotes, and pointers to further information. page no. 2 “. . . no other possible way of doing that except through renewables”; “anybody who is relying upon renewables to fill the [energy] gap is living in an utter dream world and is, in my view, an enemy of the people.” The quotes are from Any Questions?, 27 January 2006, BBC Radio 4 [ydoobr] . Michael Meacher was UK environment minister from 1997 till 2003. Sir Bernard Ingham was an aide to Margaret Thatcher when she was prime minister, and was Head of the Government Information Service. He is secretary of Supporters of Nuclear Energy. – Jonathan Porritt (March 2006). Is nuclear the answer? Section 3. Advice to Ministers. www.sd-commission.org.uk 3 “Nuclear is a money pit”, “We have a huge amount of wave and wind.” Ann Leslie, journalist. Speaking on Any Questions?, Radio 4, 10 February 2006. – Los Angeles residents drive . . . from Earth to Mars – (The Earthworks Group, 1989, page 34). – targetneutral.

com charges just £4 per ton of CO2 for their “neutralization.” (A significantly lower price than any other “offsetting” company I have come across.) At this price, a typical Brit could have his 11 tons per year “neutralized” for just £44 per year! Evidence that BP’s “neutralization” schemes don’t really add up comes from the fact that its projects have not achieved the Gold Standard www.cdmgoldstandard.org (Michael Schlup, personal communication). Many “carbon offset” projects have been exposed as worthless by Fiona Harvey of the Financial Times [2jhve6]. 4 People who want to promote renewables over nuclear, for example, say “offshore wind power could power all UK homes.” At the end of 2007, the UK government announced that they would allow the building of offshore wind

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1 — Motivations

19

turbines “enough to power all UK homes.” Friends of the Earth’s renewable energy campaigner, Nick Rau, said the group welcomed the government’s announcement. “The potential power that could be generated by this industry is enormous,” he said. [25e59w]. From the Guardian [5o7mxk]: John Sauven, the executive director of Greenpeace, said that the plans amounted to a “wind energy revolution.” “And Labour needs to drop its obsession with nuclear power, which could only ever reduce emissions by about 4% at some time in the distant future.” Nick Rau said: “We are delighted the government is getting serious about the potential for offshore wind, which could generate 25% of the UK’s electricity by 2020.” A few weeks later, the government announced that it would permit new nuclear stations to be built. “Today’s decision to give the go-ahead to a new generation of nuclear power stations . . . will do little to tackle climate change,” Friends of the Earth warned [5c4olc]. In fact, the two proposed expansions – of offshore wind and of nuclear – would both deliver just the same amount of electricity per year. The total permitted offshore wind power of 33 GW would on average deliver 10 GW, which is 4 kWh per day per person; and the replacement of all the retiring nuclear power stations would deliver 10 GW, which is 4 kWh per day per person.

Yet in the same breath, anti-nuclear campaigners say that the nuclear option would “do little,” while the wind option would “power all UK homes.” The fact is, “powering all UK homes” and “only reducing emissions by about 4%” are the same thing. 4 “water-powered car” New Scientist, 29th July 2006, p. 35. This article, headlined “Water-powered car might be available by 2009,” opened thus: “Forget cars fuelled by alcohol and vegetable oil. Before long, you might be able to run your car with nothing more than water in its fuel tank. It would be the ultimate zero-emissions vehicle. “While water is not at first sight an obvious power source, it has a key virtue: it is an abundant source of hydrogen, the element widely touted as the green fuel of the future.” The work New Scientist was describing was not ridiculous – it was actually about a car using boron as a fuel, with a boron/water reaction as one of the first chemical steps. Why did New Scientist feel the urge to turn this into a story suggesting that water was the fuel? Water is not a fuel. It never has been, and it never will be. It is already burned! The first law of thermodynamics says you can’t get energy for nothing; you can only convert energy from one form to another. The energy in any engine must come from somewhere. Fox News peddled an even more absurd story [2fztd3].

– Climate change is a far greater threat to the world than international terrorism. Sir David King, Chief Scientific Advisor to the UK government, January, 2004. [26e8z] – the glorification of travel – an allusion to the offence of “glorification” defined in the UK’s Terrorism Act which came into force on 13 April, 2006. [ykhayj] 5 Figure 1.2. This figure shows production of crude oil including lease condensate, natural gas plant liquids, and other liquids, and refinery processing gain. Sources: EIA, and BP statistical review of world energy. 6 The first practical steam engine was invented in 1698. In fact, Hero of Alexandria described a steam engine, but given that Hero’s engine didn’t catch on in the following 1600 years, I deem Savery’s 1698 invention the first practical steam engine. – Figures 1.4 and 1.7: Graph of carbon dioxide concentration. The data are collated from Keeling and Whorf (2005) (measurements spanning 1958–2004); Neftel et al. (1994) (1734–1983); Etheridge et al. (1998) (1000–1978); Siegenthaler et al. (2005) (950–1888 AD); and Indermuhle et al. (1999) (from 11 000 to 450 years before present). This graph, by the way, should not be confused with the “hockey stick graph”, which shows the history of global temperatures. Attentive readers will have noticed that the climate-change argument I presented makes no mention of historical temperatures.

Figures 1.5–1.7: Coal production numbers are from Jevons (1866), Malanima (2006), Netherlands Environmental Assessment Agency (2006), National Bureau of Economic Research (2001), Hatcher (1993), Flinn and Stoker (1984), Church et al. (1986), Supple (1987), Ashworth and Pegg (1986). Jevons was the first “Peak Oil” author. In 1865, he estimated Britain’s easily-accessible coal reserves, looked at the history of exponential growth in consumption, and predicted the end of the exponential growth and the end of the British dominance of world industry. “We cannot long maintain our

Copyright David JC MacKay 2009. This electronic copy is provided, free, for personal use only. See www.withouthotair.com.

20

Sustainable Energy – without the hot air present rate of increase of consumption. . . . the check to our progress must become perceptible within a century from the present time. . . . the conclusion is inevitable, that our present happy progressive condition is a thing of limited duration.” Jevons was right. Within a century British coal production indeed peaked, and there were two world wars. 8 Dominic Lawson, a columnist from the Independent. My quote is adapted from Dominic Lawson’s column in the Independent, 8 June, 2007. It is not a verbatim quote: I edited his words to make them briefer but took care not to correct any of his errors. All three numbers he mentions are incorrect. Here’s how he screwed up. First, he says “carbon dioxide” but gives numbers for carbon: the burning of fossil fuels sends 26 gigatonnes of CO2 per year into the atmosphere (not 7 gigatonnes). A common mistake. Second, he claims that the oceans send 36 000 gigatonnes of carbon per year into the atmosphere.

This is a far worse error: 36 000 gigatonnes is the total amount of carbon in the ocean! The annual flow is much smaller – about 90 gigatonnes of carbon per year (330 Gt CO2 /y), according to standard diagrams of the carbon cycle [l6y5g] (I believe this 90 Gt C/y is the estimated flow rate, were the atmosphere suddenly to have its CO2 concentration reduced to zero.) Similarly his “1900 gigatonne” flow from biosphere to atmosphere is wrong. The correct figure according to the standard diagrams is about 120 gigatonnes of carbon per year (440 Gt CO2 /y).

O 16

C 12

O 16

The weights of an atom of carbon and a molecule of CO2 are in the ratio 12 to 44, because the carbon atom weighs 12 units and the two oxygen atoms weigh 16 each. 12 + 16 + 16 = 44.

Incidentally, the observed rise in CO2 concentration is nicely in line with what you’d expect, assuming most of the human emissions of carbon remained in the atmosphere. From 1715 to 2004, roughly 1160 Gt CO2 have been released to the atmosphere from the consumption of fossil fuels and cement production (Marland et al., 2007). If all of this CO2 had stayed in the atmosphere, the concentration would have risen by 160 ppm (from 280 to 440 ppm). The actual rise has been about 100 ppm (from 275 to 377 ppm). So roughly 60% of what was emitted is now in the atmosphere. 10 Carbon dioxide has a warming effect. The over-emotional debate about this topic is getting quite tiresome, isn’t it? “The science is now settled.” “No it isn’t!” “Yes it is!” I think the most helpful thing I can do here is direct anyone who wants a break from the shouting to a brief report written by Charney et al. (1979).

This report’s conclusions carry weight because the National Academy of Sciences (the US equivalent of the Royal Society) commissioned the report and selected its authors on the basis of their expertise, “and with regard for appropriate balance.” The study group was convened “under the auspices of the Climate Research Board of the National Research Council to assess the scientific basis for projection of possible future climatic changes resulting from man-made releases of carbon dioxide into the atmosphere.” Specifically, they were asked: “to identify the principal premises on which our current understanding of the question is based, to assess quantitatively the adequacy and uncertainty of our knowledge of these factors and processes, and to summarize in concise and objective terms our best present understanding of the carbon dioxide/climate issue for the benefit of policy-makers.” The report is just 33 pages long, it is free to download [5qfkaw], and I recommend it. It makes clear which bits of the science were already settled in 1979, and which bits still had uncertainty. Here are the main points I picked up from this report. First, doubling the atmospheric CO2 concentration would change the net heating of the troposphere, oceans, and land by an average power per unit area of roughly 4 W/m2 , if all other properties of the atmosphere remained unchanged.

This heating effect can be compared with the average power absorbed by the atmosphere, land, and oceans, which is 238 W/m2 . So doubling CO2 concentrations would have a warming effect equivalent to increasing the intensity of the sun by 4/238 = 1.7%. Second, the consequences of this CO2 -induced heating are hard to predict, on account of the complexity of the atmosphere/ocean system, but the authors predicted a global surface warming of between 2 ◦ C and 3.5 ◦ C, with greater increases at high latitudes. Finally, the authors summarize: “we have tried but have been unable to find any overlooked or underestimated physical effects that could reduce the currently estimated global warmings due to a doubling of atmospheric CO2 to negligible proportions or reverse them altogether.” They warn that, thanks to the ocean, “the great and ponderous flywheel of the global climate system,” it is quite possible that the warming would occur sufficiently sluggishly that it

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1 — Motivations

21

would be difficult to detect in the coming decades. Nevertheless “warming will eventually occur, and the associated regional climatic changes . . . may well be significant.” The foreword by the chairman of the Climate Research Board, Verner E. Suomi, summarizes the conclusions with a famous cascade of double negatives. “If carbon dioxide continues to increase, the study group finds no reason to doubt that climate changes will result and no reason to believe that these changes will be negligible.” 10 The litany of probable drastic effects of climate change – I’m sure you’ve heard it before. See [2z2xg7] if not. 12 Breakdown of world greenhouse gas emissions by region and by country. Data source: Climate Analysis Indicators Tool (CAIT) Version 4.0. (Washington, DC: World Resources Institute, 2007). The first three figures show national totals of all six major greenhouse gases (CO2 , CH4 , N2 O, PFC, HFC, SF6 ), excluding contributions from land-use change and forestry. The figure on p14 shows cumulative emissions of CO2 only. 14 Congratulations, Britain! . . . in the table of historical emissions, per capita, we are second only to the USA.

Sincere apologies here to Luxembourg, whose historical per-capita emissions actually exceed those of America and Britain; but I felt the winners’ podium should really be reserved for countries having both large per-capita and large total emissions. In total terms the biggest historical emitters are, in order, USA (322 Gt CO2 ), Russian Federation (90 Gt CO2 ), China (89 Gt CO2 ), Germany (78 Gt CO2 ), UK (62 Gt CO2 ), Japan (43 Gt CO2 ), France (30 Gt CO2 ), India (25 Gt CO2 ), and Canada (24 Gt CO2 ). The per-capita order is: Luxembourg, USA, United Kingdom, Czech Republic, Belgium, Germany, Estonia, Qatar, and Canada. – Some countries, including Britain, have committed to at least a 60% reduction in greenhouse-gas emissions by 2050. Indeed, as I write, Britain’s commitment is being increased to an 80% reduction relative to 1990 levels. 15 Figure 1.8. In the lower scenario, the chance that the temperature rise will exceed 2 ◦ C is estimated to be 9–26%; the cumulative carbon emissions from 2007 onwards are 309 Gt C; CO2 concentrations reach a peak of 410 ppm, CO2 e concentrations peak at 421 ppm, and in 2100 CO2 concentrations fall back to 355 ppm.

In the upper scenario, the chance of exceeding 2 ◦ C is estimated to be 16–43%; the cumulative carbon emissions from 2007 onwards are 415 Gt C; CO2 concentrations reach a peak of 425 ppm, CO2 e concentrations peak at 435 ppm, and in 2100 CO2 concentrations fall back to 380 ppm. See also hdr.undp.org/en/reports/global/hdr2007-2008/. 16 there are many other helpful sources on the internet. I recommend, for example: BP’s Statistical Review of World Energy [yyxq2m], the Sustainable Development Commission www.sd-commission.org.uk, the Danish Wind Industry Association www.windpower.org, Environmentalists For Nuclear Energy www.ecolo.org, Wind Energy Department, Risø University www.risoe.dk/vea, DEFRA www.defra.gov.uk/environment/statistics, especially the book Avoiding Dangerous Climate Change [dzcqq], the Pembina Institute www.pembina.org/publications.asp, and the DTI (now known as BERR) www.dti.gov.uk/publications/. 17 factual assertions and ethical assertions. . . Ethical assertions are also known as “normative claims” or “value judgments,” and factual assertions are known as “positive claims.” Ethical assertions usually contain verbs like “should” and “must,” or adjectives like “fair,” “right,” and “wrong.” For helpful further reading see Dessler and Parson (2006). 18 Gordon Brown.

On 10th September, 2005, Gordon Brown said the high price of fuel posed a significant risk to the European economy and to global growth, and urged OPEC to raise oil production. Again, six months later, he said “we need . . . more production, more drilling, more investment, more petrochemical investment” (22nd April, 2006) [y98ys5]. Let me temper this criticism of Gordon Brown by praising one of his more recent initiatives, namely the promotion of electric vehicles and plug-in hybrids. As you’ll see later, one of this book’s conclusions is that electrification of most transport is a good part of a plan for getting off fossil fuels.

Copyright David JC MacKay 2009. This electronic copy is provided, free, for personal use only. See www.withouthotair.com.

2

The balance sheet Nature cannot be fooled. Richard Feynman

Let’s talk about energy consumption and energy production. At the moment, most of the energy the developed world consumes is produced from fossil fuels; that’s not sustainable. Exactly how long we could keep living on fossil fuels is an interesting question, but it’s not the question we’ll address in this book. I want to think about living without fossil fuels. We’re going to make two stacks. In the left-hand, red stack we will add up our energy consumption, and in the right-hand, green stack, we’ll add up sustainable energy production. We’ll assemble the two stacks gradually, adding items one at a time as we discuss them. The question addressed in this book is “can we conceivably live sustainably?” So, we will add up all conceivable sustainable energy sources and put them in the right-hand, green stack. In the left-hand, red stack, we’ll estimate the consumption of a “typical moderately-affluent person;” I encourage you to tot up an estimate of your own consumption, creating your own personalized left-hand stack too. Later on we’ll also find out the current average energy consumption of Europeans and Americans.

Some key forms of consumption for the lefthand stack will be:

consumption production

In the right-hand sustainable-production stack, our main categories will be: • wind • solar

• transport

– photovoltaics, thermal, biomass

– cars, planes, freight • heating and cooling

• hydroelectric

• lighting

• wave

• information systems and other gadgets

• tide

• food

• geothermal

• manufacturing

• nuclear? (with a question-mark, because it’s not clear whether nuclear power counts as “sustainable”)

22

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23

2 — The balance sheet As we estimate our consumption of energy for heating, transportation, manufacturing, and so forth, the aim is not only to compute a number for the left-hand stack of our balance sheet, but also to understand what each number depends on, and how susceptible to modification it is. In the right-hand, green stack, we’ll add up the sustainable production estimates for the United Kingdom. This will allow us to answer the question “can the UK conceivably live on its own renewables?” Whether the sustainable energy sources that we put in the right-hand stack are economically feasible is an important question, but let’s leave that question to one side, and just add up the two stacks first. Sometimes people focus too much on economic feasibility and they miss the big picture. For example, people discuss “is wind cheaper than nuclear?” and forget to ask “how much wind is available?” or “how much uranium is left?” The outcome when we add everything up might look like this:

Total conceivable sustainable production Total consumption

If we find consumption is much less than conceivable sustainable production, then we can say “good, maybe we can live sustainably; let’s look into the economic, social, and environmental costs of the sustainable alternatives, and figure out which of them deserve the most research and development; if we do a good job, there might not be an energy crisis.” On the other hand, the outcome of our sums might look like this:

Total consumption Total conceivable sustainable production

– a much bleaker picture. This picture says “it doesn’t matter what the

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24

Sustainable Energy – without the hot air

economics of sustainable power are: there’s simply not enough sustainable power to support our current lifestyle; massive change is coming.”

Energy and power Most discussions of energy consumption and production are confusing because of the proliferation of units in which energy and power are measured, from “tons of oil equivalent” to “terawatt-hours” (TWh) and “exajoules” (EJ). Nobody but a specialist has a feeling for what “a barrel of oil” or “a million BTUs” means in human terms. In this book, we’ll express everything in a single set of personal units that everyone can relate to. The unit of energy I have chosen is the kilowatt-hour (kWh). This quantity is called “one unit” on electricity bills, and it costs a domestic user about 10p in the UK in 2008. As we’ll see, most individual daily choices involve amounts of energy equal to small numbers of kilowatt-hours. When we discuss powers (rates at which we use or produce energy), the main unit will be the kilowatt-hour per day (kWh/d). We’ll also occasionally use the watt (40 W ≃ 1 kWh/d) and the kilowatt (1 kW = 1000 W = 24 kWh/d), as I’ll explain below. The kilowatt-hour per day is a nice human-sized unit: most personal energy-guzzling activities guzzle at a rate of a small number of kilowatt-hours per day. For example, one 40 W lightbulb, kept switched on all the time, uses one kilowatt-hour per day. Some electricity companies include graphs in their electricity bills, showing energy consumption in kilowatt-hours per day.

I’ll use the same unit for all forms of power, not just electricity. Petrol consumption, gas consumption, coal consumption: I’ll measure all these powers in kilowatthours per day. Let me make this clear: for some people, the word “power” means only electrical energy consumption. But this book concerns all forms of energy consumption and production, and I will use the word “power” for all of them. One kilowatt-hour per day is roughly the power you could get from one human servant. The number of kilowatt-hours per day you use is thus the effective number of servants you have working for you. People use the two terms energy and power interchangeably in ordinary speech, but in this book we must stick rigorously to their scientific definitions. Power is the rate at which something uses energy. Maybe a good way to explain energy and power is by an analogy with water and water-flow from taps. If you want a drink of water, you want a volume of water – one litre, perhaps (if you’re thirsty). When you turn on a tap, you create a flow of water – one litre per minute, say, if the tap yields only a trickle; or 10 litres per minute, from a more generous tap. You can get the same volume (one litre) either by running the trickling tap for one minute, or by running the generous tap for one tenth of a minute. The volume delivered in a particular time is equal to the flow multiplied by the

Figure 2.1. Distinguishing energy and power. Each of these 60 W light bulbs has a power of 60 W when switched on; it doesn’t have an “energy” of 60 W. The bulb uses 60 W of electrical power when it’s on; it emits 60 W of power in the form of light and heat (mainly the latter).

volume is measured in litres

flow is measured in litres per minute

energy is measured in kWh

power is measured in kWh per day

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25

2 — The balance sheet time: volume = flow × time. We say that a flow is a rate at which volume is delivered. If you know the volume delivered in a particular time, you get the flow by dividing the volume by the time: volume flow = . time Here’s the connection to energy and power. Energy is like water volume: power is like water flow. For example, whenever a toaster is switched on, it starts to consume power at a rate of one kilowatt. It continues to consume one kilowatt until it is switched off. To put it another way, the toaster (if it’s left on permanently) consumes one kilowatt-hour (kWh) of energy per hour; it also consumes 24 kilowatt-hours per day. The longer the toaster is on, the more energy it uses. You can work out the energy used by a particular activity by multiplying the power by the duration: energy = power × time. The joule is the standard international unit of energy, but sadly it’s far too small to work with. The kilowatt-hour is equal to 3.6 million joules (3.6 megajoules). Powers are so useful and important, they have something that water flows don’t have: they have their own special units. When we talk of a flow, we might measure it in “litres per minute,” “gallons per hour,” or “cubic-metres per second;” these units’ names make clear that the flow is “a volume per unit time.” A power of one joule per second is called one watt.

1000 joules per second is called one kilowatt. Let’s get the terminology straight: the toaster uses one kilowatt. It doesn’t use “one kilowatt per second.” The “per second” is already built in to the definition of the kilowatt: one kilowatt means “one kilojoule per second.” Similarly we say “a nuclear power station generates one gigawatt.” One gigawatt, by the way, is one billion watts, one million kilowatts, or 1000 megawatts. So one gigawatt is a million toasters. And the “g”s in gigawatt are pronounced hard, the same as in “giggle.” And, while I’m tapping the blackboard, we capitalize the “g” and “w” in “gigawatt” only when we write the abbreviation “GW.” Please, never, ever say “one kilowatt per second,” “one kilowatt per hour,” or “one kilowatt per day;” none of these is a valid measure of power. The urge that people have to say “per something” when talking about their toasters is one of the reasons I decided to use the “kilowatt-hour per day” as my unit of power. I’m sorry that it’s a bit cumbersome to say and to write. Here’s one last thing to make clear: if I say “someone used a gigawatthour of energy,” I am simply telling you how much energy they used, not how fast they used it. Talking about a gigawatt-hour doesn’t imply the

energy is measured in kWh or MJ

power is measured in kWh per day or kW or W (watts) or MW (megawatts) or GW (gigawatts) or TW (terawatts)

Copyright David JC MacKay 2009. This electronic copy is provided, free, for personal use only. See www.withouthotair.com.

26 energy was used in one hour. You could use a gigawatt-hour of energy by switching on one million toasters for one hour, or by switching on 1000 toasters for 1000 hours. As I said, I’ll usually quote powers in kWh/d per person. One reason for liking these personal units is that it makes it much easier to move from talking about the UK to talking about other countries or regions. For example, imagine we are discussing waste incineration and we learn that UK waste incineration delivers a power of 7 TWh per year and that Denmark’s waste incineration delivers 10 TWh per year. Does this help us say whether Denmark incinerates “more” waste than the UK? While the total power produced from waste in each country may be interesting, I think that what we usually want to know is the waste incineration per person. (For the record, that is: Denmark, 5 kWh/d per person; UK, 0.3 kWh/d per person. So Danes incinerate about 13 times as much waste as Brits.) To save ink, I’ll sometimes abbreviate “per person” to “/p”. By discussing everything per-person from the outset, we end up with a more transportable book, one that will hopefully be useful for sustainable energy discussions worldwide.

Picky details Isn’t energy conserved? We talk about “using” energy, but doesn’t one of the laws of nature say that energy can’t be created or destroyed? Yes, I’m being imprecise. This is really a book about entropy – a trickier thing to explain. When we “use up” one kilojoule of energy, what we’re really doing is taking one kilojoule of energy in a form that has low entropy (for example, electricity), and converting it into an exactly equal amount of energy in another form, usually one that has much higher entropy (for example, hot air or hot water). When we’ve “used” the energy, it’s still there; but we normally can’t “use” the energy over and over again, because only low entropy energy is “useful” to us. Sometimes these different grades of energy are distinguished by adding a label to the units: one kWh(e) is one kilowatt-hour of electrical energy – the highest grade of energy. One kWh(th) is one kilowatt-hour of thermal energy – for example the energy in ten litres of boiling-hot water. Energy lurking in higher-temperature things is more useful (lower entropy) than energy in tepid things. A third grade of energy is chemical energy. Chemical energy is high-grade energy like electricity. It’s a convenient but sloppy shorthand to talk about the energy rather than the entropy, and that is what we’ll do most of the time in this book.

Occasionally, we’ll have to smarten up this sloppiness; for example, when we discuss refrigeration, power stations, heat pumps, or geothermal power. Are you comparing apples and oranges? Is it valid to compare different

Sustainable Energy – without the hot air

1 TWh (one terawatt-hour) is equal to one billion kWh.

Copyright David JC MacKay 2009. This electronic copy is provided, free, for personal use only. See www.withouthotair.com.

2 — The balance sheet forms of energy such as the chemical energy that is fed into a petrolpowered car and the electricity from a wind turbine? By comparing consumed energy with conceivable produced energy, I do not wish to imply that all forms of energy are equivalent and interchangeable. The electrical energy produced by a wind turbine is of no use to a petrol engine; and petrol is no use if you want to power a television. In principle, energy can be converted from one form to another, though conversion entails losses. Fossil-fuel power stations, for example, guzzle chemical energy and produce electricity (with an efficiency of 40% or so). And aluminium plants guzzle electrical energy to create a product with high chemical energy – aluminium (with an efficiency of 30% or so). In some summaries of energy production and consumption, all the different forms of energy are put into the same units, but multipliers are introduced, rating electrical energy from hydroelectricity for example as being worth 2.5 times more than the chemical energy in oil.

This bumping up of electricity’s effective energy value can be justified by saying, “well, 1 kWh of electricity is equivalent to 2.5 kWh of oil, because if we put that much oil into a standard power station it would deliver 40% of 2.5 kWh, which is 1 kWh of electricity.” In this book, however, I will usually use a one-to-one conversion rate when comparing different forms of energy. It is not the case that 2.5 kWh of oil is inescapably equivalent to 1 kWh of electricity; that just happens to be the perceived exchange rate in a worldview where oil is used to make electricity. Yes, conversion of chemical energy to electrical energy is done with this particular inefficient exchange rate. But electrical energy can also be converted to chemical energy. In an alternative world (perhaps not far-off) with relatively plentiful electricity and little oil, we might use electricity to make liquid fuels; in that world we would surely not use the same exchange rate – each kWh of gasoline would then cost us something like 3 kWh of electricity! I think the timeless and scientific way to summarize and compare energies is to hold 1 kWh of chemical energy equivalent to 1 kWh of electricity. My choice to use this one-to-one conversion rate means that some of my sums will look a bit different from other people’s.

(For example, BP’s Statistical Review of World Energy rates 1 kWh of electricity as equivalent to 100/38 ≃ 2.6 kWh of oil; on the other hand, the government’s Digest of UK Energy Statistics uses the same one-to-one conversion rate as me.) And I emphasize again, this choice does not imply that I’m suggesting you could convert either form of energy directly into the other. Converting chemical energy into electrical energy always wastes energy, and so does converting electrical into chemical energy.

Physics and equations Throughout the book, my aim is not only to work out numbers indicating our current energy consumption and conceivable sustainable production,

27

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28

Sustainable Energy – without the hot air

but also to make clear what these numbers depend on. Understanding what the numbers depend on is essential if we are to choose sensible policies to change any of the numbers. Only if we understand the physics behind energy consumption and energy production can we assess assertions such as “cars waste 99% of the energy they consume; we could redesign cars so that they use 100 times less energy.” Is this assertion true? To explain the answer, I will need to use equations like kinetic energy =

1 2 mv . 2

However, I recognize that to many readers, such formulae are a foreign language. So, here’s my promise: I’ll keep all this foreign-language stuff in technical chapters at the end of the book. Any reader with a high-school/secondaryschool qualification in maths, physics, or chemistry should enjoy these technical chapters. The main thread of the book (from page 2 to page 250) is intended to be accessible to everyone who can add, multiply, and divide. It is especially aimed at our dear elected and unelected representatives, the Members of Parliament. One last point, before we get rolling: I don’t know everything about energy. I don’t have all the answers, and the numbers I offer are open to revision and correction. (Indeed I expect corrections and will publish them on the book’s website.) The one thing I am sure of is that the answers to our sustainable energy questions will involve numbers; any sane discussion of sustainable energy requires numbers. This book’s got ’em, and it shows how to handle them. I hope you enjoy it!

Notes and further reading page no. 25 The “per second” is already built in to the definition of the kilowatt. Other examples of units that, like the watt, already have a “per time” built in are the knot – “our yacht’s speed was ten knots!” (a knot is one nautical mile per hour); the hertz – “I could hear a buzzing at 50 hertz” (one hertz is a frequency of one cycle per second); the ampere – “the fuse blows when the current is higher than 13 amps” (not 13 amps per second); and the horsepower – “that stinking engine delivers 50 horsepower” (not 50 horsepower per second, nor 50 horsepower per hour, nor 50 horsepower per day, just 50 horsepower). – Please, never, ever say “one kilowatt per second.” There are specific, rare exceptions to this rule. If talking about a growth in demand for power, we might say “British demand is growing at one gigawatt per year.” In Chapter 26 when I discuss fluctuations in wind power, I will say “one morning, the power delivered by Irish windmills fell at a rate of 84 MW per hour.” Please take care! Just one accidental syllable can lead to confusion: for example, your electricity meter’s reading is in kilowatt-hours (kWh), not ‘kilowatts-per-hour’. I’ve provided a chart on p368 to help you translate between kWh per day per person and the other major units in which powers are discussed.

Copyright David JC MacKay 2009. This electronic copy is provided, free, for personal use only. See www.withouthotair.com.

3

Cars

For our first chapter on consumption, let’s study that icon of modern civilization: the car with a lone person in it. How much power does a regular car-user consume? Once we know the conversion rates, it’s simple arithmetic: distance travelled per day energy used × energy per unit of fuel. = per day distance per unit of fuel

Figure 3.1. Cars. A red BMW dwarfed by a spaceship from the planet Dorkon.

For the distance travelled per day, let’s use 50 km (30 miles). For the distance per unit of fuel, also known as the economy of the car, let’s use 33 miles per UK gallon (taken from an advertisement for a family car): 33 miles per imperial gallon ≃ 12 km per litre. (The symbol “≃” means “is approximately equal to.”) What about the energy per unit of fuel (also called the calorific value or energy density)? Instead of looking it up, it’s fun to estimate this sort of quantity by a bit of lateral thinking. Automobile fuels (whether diesel or petrol) are all hydrocarbons; and hydrocarbons can also be found on our breakfast table, with the calorific value conveniently written on the side: roughly 8 kWh per kg (figure 3.2). Since we’ve estimated the economy of the car in miles per unit volume of fuel, we need to express the calorific value as an energy per unit volume. To turn our fuel’s “8 kWh per kg” (an energy per unit mass) into an energy per unit volume, we need to know the density of the fuel. What’s the density of butter? Well, butter just floats on water, as do fuel-spills, so its density must be a little less than water’s, which is 1 kg per litre. If we guess a density of 0.8 kg per litre, we obtain a calorific value of:

Figure 3.2. Want to know the energy in car fuel? Look at the label on a pack of butter or margarine. The calorific value is 3000 kJ per 100 g, or about 8 kWh per kg.

Consumption

Production

8 kWh per kg × 0.8 kg per litre ≃ 7 kWh per litre. Rather than willfully perpetuate an inaccurate estimate, let’s switch to the actual value, for petrol, of 10 kWh per litre. distance travelled per day × energy per unit of fuel distance per unit of fuel 50 km/day = × 10 kWh/litre 12 km/litre ≃ 40 kWh/day.

Car: 40 kWh/d

energy per day =

Congratulations! We’ve made our first estimate of consumption. I’ve displayed this estimate in the left-hand stack in figure 3.3. The red box’s height represents 40 kWh per day per person. 29

Figure 3.3. Chapter 3’s conclusion: a typical car-driver uses about 40 kWh per day.

Copyright David JC MacKay 2009. This electronic copy is provided, free, for personal use only. See www.withouthotair.com.

30

Sustainable Energy – without the hot air

This is the estimate for a typical car-driver driving a typical car today. Later chapters will discuss the average consumption of all the people in Britain, taking into account the fact that not everyone drives. We’ll also discuss in Part II what the consumption could be, with the help of other technologies such as electric cars. Why does the car deliver 33 miles per gallon? Where’s that energy going? Could we manufacture cars that do 3300 miles per gallon? If we are interested in trying to reduce cars’ consumption, we need to understand the physics behind cars’ consumption. These questions are answered in the accompanying technical chapter A (p254), which provides a cartoon theory of cars’ consumption. I encourage you to read the technical chapters if formulae like 12 mv2 don’t give you medical problems. Chapter 3’s conclusion: a typical car-driver uses about 40 kWh per day. Next we need to get the sustainable-production stack going, so we have something to compare this estimate with.

Queries What about the energy-cost of producing the car’s fuel? Good point. When I estimate the energy consumed by a particular activity, I tend to choose a fairly tight “boundary” around the activity. This choice makes the estimation easier, but I agree that it’s a good idea to try to estimate the full energy impact of an activity. It’s been estimated that making each unit of petrol requires an input of 1.4 units of oil and other primary fuels (Treloar et al., 2004).

driving a car - 55.2% passenger in a car - 6.3% bus or coach - 7.4%

What about the energy-cost of manufacturing the car? Yes, that cost fell outside the boundary of this calculation too. We’ll talk about car-making in Chapter 15.

train or tram - 7.1% bicycle - 2.8% on foot - 10% working mainly at home - 9.2%

Notes and further reading page no. 29 For the distance travelled per day, let’s use 50 km. This corresponds to 18 000 km (11 000 miles) per year. Roughly half of the British population drive to work. The total amount of car travel in the UK is 686 billion passenger-km per year, which corresponds to an “average distance travelled by car per British person” of 30 km per day. Source: Department for Transport [5647rh]. As I said on p22, I aim to estimate the consumption of a “typical moderately-affluent person” – the consumption that many people aspire to. Some people don’t drive much. In this chapter, I want to estimate the energy consumed by someone who chooses to drive, rather than depersonalize the answer by reporting the UK average, which mixes together the drivers and non-drivers. If I said “the average use of energy for car driving

0

10 20 30 40 50 60 70 80 90 100

Figure 3.4. How British people travel to work, according to the 2001 census.

Copyright David JC MacKay 2009. This electronic copy is provided, free, for personal use only. See www.withouthotair.com.

31

3 — Cars in the UK is 24 kWh/d per person,” I bet some people would misunderstand and say: “I’m a car driver so I guess I use 24 kWh/d.” 29 . . . let’s use 33 miles per UK gallon. In the European language, this is 8.6 litres per 100 km. 33 miles per gallon was the average for UK cars in 2005 [27jdc5]. Petrol cars have an average fuel consumption of 31 mpg; diesel cars, 39 mpg; new petrol cars (less than two years old), 32 mpg (Dept. for Transport, 2007). Honda, “the most fuel-efficient auto company in America,” records that its fleet of new cars sold in 2005 has an average top-level fuel economy of 35 miles per UK gallon [28abpm]. 29 Let’s guess a density of 0.8 kg per litre. Petrol’s density is 0.737. Diesel’s is 0.820–0.950 [nmn4l]. – . . . the actual value of 10 kWh per litre. ORNL [2hcgdh] provide the following calorific values: diesel: 10.7 kWh/l; jet fuel: 10.4 kWh/l; petrol: 9.7 kWh/l. When looking up calorific values, you’ll find “gross calorific value” and “net calorific value” listed (also known as “high heat value” and “low heat value”). These differ by only 6% for motor fuels, so it’s not crucial to distinguish them here, but let me explain anyway. The gross calorific value is the actual chemical energy released when the fuel is burned.

One of the products of combustion is water, and in most engines and power stations, part of the energy goes into vaporizing this water. The net calorific value measures how much energy is left over assuming this energy of vaporization is discarded and wasted. When we ask “how much energy does my lifestyle consume?” the gross calorific value is the right quantity to use. The net calorific value, on the other hand, is of interest to a power station engineer, who needs to decide which fuel to burn in his power station. Throughout this book I’ve tried to use gross calorific values. A final note for party-pooping pedants who say “butter is not a hydrocarbon”: OK, butter is not a pure hydrocarbon; but it’s a good approximation to say that the main component of butter is long hydrocarbon chains, just like petrol. The proof of the pudding is, this approximation got us within 30% of the correct answer. Welcome to guerrilla physics.

calorific values petrol diesel

10 kWh per litre 11 kWh per litre

Copyright David JC MacKay 2009. This electronic copy is provided, free, for personal use only. See www.withouthotair.com.

4

Wind The UK has the best wind resources in Europe. Sustainable Development Commission Wind farms will devastate the countryside pointlessly. James Lovelock

How much wind power could we plausibly generate? We can make an estimate of the potential of on-shore (land-based) wind in the United Kingdom by multiplying the average power per unit landarea of a wind farm by the area per person in the UK: power per person = wind power per unit area × area per person. Chapter B (p263) explains how to estimate the power per unit area of a wind farm in the UK. If the typical windspeed is 6 m/s (13 miles per hour, or 22 km/h), the power per unit area of wind farm is about 2 W/m2 . 16 14 12 10 8 6 4 2 0

Figure 4.1. Cambridge mean wind speed in metres per second, daily (red line), and half-hourly (blue line) during 2006. See also figure 4.6. Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

This figure of 6 m/s is probably an over-estimate for many locations in Britain. For example, figure 4.1 shows daily average windspeeds in Cambridge during 2006. The daily average speed reached 6 m/s on only about 30 days of the year – see figure 4.6 for a histogram. But some spots do have windspeeds above 6 m/s – for example, the summit of Cairngorm in Scotland (figure 4.2). Plugging in the British population density: 250 people per square kilometre, or 4000 square metres per person, we find that wind power could 30

20

10

0 Jan

Feb

Mar

Apr

32

May

Jun

Figure 4.2. Cairngorm mean wind speed in metres per second, during six months of 2006.

Copyright David JC MacKay 2009. This electronic copy is provided, free, for personal use only. See www.withouthotair.com.

33

4 — Wind generate

Consumption 2

Production

2

2 W/m × 4000 m /person = 8000 W per person, if wind turbines were packed across the whole country, and assuming 2 W/m2 is the correct power per unit area. Converting to our favourite power units, that’s 200 kWh/d per person. Let’s be realistic. What fraction of the country can we really imagine covering with windmills? Maybe 10%? Then we conclude: if we covered the windiest 10% of the country with windmills (delivering 2 W/m2 ), we would be able to generate 20 kWh/d per person, which is half of the power used by driving an average fossil-fuel car 50 km per day. Britain’s onshore wind energy resource may be “huge,” but it’s evidently not as huge as our huge consumption. We’ll come to offshore wind later. I should emphasize how generous an assumption I’m making. Let’s compare this estimate of British wind potential with current installed wind power worldwide. The windmills that would be required to provide the UK with 20 kWh/d per person amount to 50 times the entire wind hardware of Denmark; 7 times all the wind farms of Germany; and double the entire fleet of all wind turbines in the world. Please don’t misunderstand me. Am I saying that we shouldn’t bother building wind farms? Not at all. I’m simply trying to convey a helpful fact, namely that if we want wind power to truly make a difference, the wind farms must cover a very large area.

This conclusion – that the maximum contribution of onshore wind, albeit “huge,” is much less than our consumption – is important, so let’s check the key figure, the assumed power per unit area of wind farm (2 W/m2 ), against a real UK wind farm. The Whitelee wind farm being built near Glasgow in Scotland has 140 turbines with a combined peak capacity of 322 MW in an area of 55 km2 . That’s 6 W/m2 , peak. The average power produced is smaller because the turbines don’t run at peak output all the time. The ratio of the average power to the peak power is called the “load factor” or “capacity factor,” and it varies from site to site, and with the choice of hardware plopped on the site; a typical factor for a good site with modern turbines is 30%. If we assume Whitelee has a load factor of 33% then the average power production per unit land area is 2 W/m2 – exactly the same as the power density we assumed above.

Car: 40 kWh/d Wind: 20 kWh/d

Figure 4.3. Chapter 4’s conclusion: the maximum plausible production from on-shore windmills in the United Kingdom is 20 kWh per day per person.

Power per unit area wind farm (speed 6 m/s)

2 W/m2

Table 4.4. Facts worth remembering: wind farms.

Population density of Britain 250 per km2 ↔ 4000 m2 per person Table 4.5. Facts worth remembering: population density. See page 338 for more population densities.

Copyright David JC MacKay 2009. This electronic copy is provided, free, for personal use only. See www.withouthotair.com.

34

Sustainable Energy – without the hot air Figure 4.6. Histogram of Cambridge average wind speed in metres per second: daily averages (left), and half-hourly averages (right).

0

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14

speed (m/s)

16

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Queries Wind turbines are getting bigger all the time. Do bigger wind turbines change this chapter’s answer? Chapter B explains. Bigger wind turbines deliver financial economies of scale, but they don’t greatly increase the total power per unit land area, because bigger windmills have to be spaced further apart. A wind farm that’s twice as tall will deliver roughly 30% more power. Wind power fluctuates all the time. Surely that makes wind less useful? Maybe. We’ll come back to this issue in Chapter 26, where we’ll look at wind’s intermittency and discuss several possible solutions to this problem, including energy storage and demand management.

Notes and further reading page no. 32 Figure 4.1 and figure 4.6. Cambridge wind data are from the Digital Technology Group, Computer Laboratory, Cambridge [vxhhj]. The weather station is on the roof of the Gates building, roughly 10 m high. Wind speeds at a height of 50 m are usually about 25% bigger. Cairngorm data (figure 4.2) are from Heriot–Watt University Physics Department [tdvml]. 33 The windmills required to provide the UK with 20 kWh/d per person are 50 times the entire wind power of Denmark. Assuming a load factor of 33%, an average power of 20 kWh/d per person requires an installed capacity of 150 GW. At the end of 2006, Denmark had an installed capacity of 3.1 GW; Germany had 20.6 GW. The world total was 74 GW (wwindea.org). Incidentally, the load factor of the Danish wind fleet was 22% in 2006, and the average power it delivered was 3 kWh/d per person.

Copyright David JC MacKay 2009. This electronic copy is provided, free, for personal use only. See www.withouthotair.com.

5

Planes

Imagine that you make one intercontinental trip per year by plane. How much energy does that cost? A Boeing 747-400 with 240 000 litres of fuel carries 416 passengers about 8 800 miles (14 200 km). And fuel’s calorific value is 10 kWh per litre. (We learned that in Chapter 3.) So the energy cost of one full-distance roundtrip on such a plane, if divided equally among the passengers, is 2 × 240 000 litre × 10 kWh/litre ≃ 12 000 kWh per passenger. 416 passengers If you make one such trip per year, then your average energy consumption per day is 12 000 kWh ≃ 33 kWh/day. 365 days 14 200 km is a little further than London to Cape Town (10 000 km) and London to Los Angeles (9000 km), so I think we’ve slightly overestimated the distance of a typical long-range intercontinental trip; but we’ve also overestimated the fullness of the plane, and the energy cost per person is more if the plane’s not full. Scaling down by 10 000 km/14 200 km to get an estimate for Cape Town, then up again by 100/80 to allow for the plane’s being 80% full, we arrive at 29 kWh per day. For ease of memorization, I’ll round this up to 30 kWh per day. Let’s make clear what this means. Flying once per year has an energy cost slightly bigger than leaving a 1 kW electric fire on, non-stop, 24 hours a day, all year.

Just as Chapter 3, in which we estimated consumption by cars, was accompanied by Chapter A, offering a model of where the energy goes in cars, this chapter’s technical partner (Chapter C, p269), discusses where the energy goes in planes. Chapter C allows us to answer questions such as “would air travel consume significantly less energy if we travelled in slower planes?” The answer is no: in contrast to wheeled vehicles, which can get more efficient the slower they go, planes are already almost as energy-efficient as they could possibly be. Planes unavoidably have to use energy for two reasons: they have to throw air down in order to stay up, and they need energy to overcome air resistance. No redesign of a plane is going to radically improve its efficiency. A 10% improvement? Yes, possible. A doubling of efficiency? I’d eat my complimentary socks.

Jet flights: 30 kWh/d

Car: 40 kWh/d Wind: 20 kWh/d

Figure 5.1. Taking one intercontinental trip per year uses about 30 kWh per day.

Queries Aren’t turboprop aircraft far more energy-efficient? No. The “comfortably greener” Bombardier Q400 NextGen, “the most technologically advanced turboprop in the world,” according to its manu35

Figure 5.2. Bombardier Q400 NextGen. www.q400.com.

Copyright David JC MacKay 2009. This electronic copy is provided, free, for personal use only. See www.withouthotair.com.

36

Sustainable Energy – without the hot air

facturers [www.q400.com], uses 3.81 litres per 100 passenger-km (at a cruise speed of 667 km/h), which is an energy cost of 38 kWh per 100 p-km. The full 747 has an energy cost of 42 kWh per 100 p-km. So both planes are twice as fuel-efficient as a single-occupancy car. (The car I’m assuming here is the average European car that we discussed in Chapter 3.) Is flying extra-bad for climate change in some way? Yes, that’s the experts’ view, though uncertainty remains about this topic [3fbufz]. Flying creates other greenhouse gases in addition to CO2 , such as water and ozone, and indirect greenhouse gases, such as nitrous oxides. If you want to estimate your carbon footprint in tons of CO2 equivalent, then you should take the actual CO2 emissions of your flights and bump them up two- or three-fold. This book’s diagrams don’t include that multiplier because here we are focusing on our energy balance sheet.

energy per distance (kWh per 100 p-km) Car (4 occupants) Ryanair’s planes, year 2007 Bombardier Q400, full 747, full 747, 80% full Ryanair’s planes, year 2000 Car (1 occupant)

20 37 38 42 53 73 80

Table 5.3. Passenger transport efficiencies, expressed as energy required per 100 passenger-km.

The best thing we can do with environmentalists is shoot them. Michael O’Leary, CEO of Ryanair [3asmgy]

Notes and further reading page no. 35 Boeing 747-400 – data are from [9ehws]. Planes today are not completely full. Airlines are proud if their average fullness is 80%. Easyjet planes are 85% full on average. (Source: thelondonpaper Tuesday 16th January, 2007.) An 80%-full 747 uses about 53 kWh per 100 passenger-km. What about short-haul flights? In 2007, Ryanair, “Europe’s greenest airline,” delivered transportation at a cost of 37 kWh per 100 p-km [3exmgv]. This means that flying across Europe with Ryanair has much the same energy cost as having all the passengers drive to their destination in cars, two to a car. (For an indication of what other airlines might be delivering, Ryanair’s fuel burn rate in 2000, before their environment-friendly investments, was above 73 kWh per 100 p-km.) London to Rome is 1430 km; London to Malaga is 1735 km. So a round-trip to Rome with the greenest airline has an energy cost of 1050 kWh, and a round-trip to Malaga costs 1270 kWh. If you pop over to Rome and to Malaga once per year, your average power consumption is 6.3 kWh/d with the greenest airline, and perhaps 12 kWh/d with a less green one. What about frequent flyers? To get a silver frequent flyer card from an intercontinental airline, it seems one must fly around 25 000 miles per year in economy class.

That’s about 60 kWh per day, if we scale up the opening numbers from this chapter and assume planes are 80% full. Here are some additional figures from the Intergovernmental Panel on Climate Change [yrnmum]: a full 747-400 travelling 10 000 km with low-density seating (262 seats) has an energy consumption of 50 kWh per 100 p-km. In a high-density seating configuration (568 seats) and travelling 4000 km, the

Figure 5.4. Ryanair Boeing 737-800. Photograph by Adrian Pingstone.

Copyright David JC MacKay 2009. This electronic copy is provided, free, for personal use only. See www.withouthotair.com.

37

5 — Planes same plane has an energy consumption of 22 kWh per 100 p-km. A shorthaul Tupolev-154 travelling 2235 km with 70% of its 164 seats occupied consumes 80 kWh per 100 p-km. 35 No redesign of a plane is going to radically improve its efficiency. Actually, the Advisory Council for Aerospace Research in Europe (ACARE) target is for an overall 50% reduction in fuel burned per passenger-km by 2020 (relative to a 2000 baseline), with 15–20% improvement expected in engine efficiency. As of 2006, Rolls Royce is half way to this engine target [36w5gz]. Dennis Bushnell, chief scientist at NASA’s Langley Research Center, seems to agree with my overall assessment of prospects for efficiency improvements in aviation. The aviation industry is mature. “There is not much left to gain except by the glacial accretion of a per cent here and there over long time periods.” (New Scientist, 24 February 2007, page 33.) The radically reshaped “Silent Aircraft” [silentaircraft.org/sax40], if it were built, is predicted to be 16% more efficient than a conventional-shaped plane (Nickol, 2008). If the ACARE target is reached, it’s presumably going to be thanks mostly to having fuller planes and better air-traffic management.

Short hauls: 6 kWh/d

Frequent flyer: 60 kWh/d

Figure 5.5. Two short-haul trips on the greenest short-haul airline: 6.3 kWh/d. Flying enough to qualify for silver frequent flyer status: 60 kWh/d.

Copyright David JC MacKay 2009. This electronic copy is provided, free, for personal use only. See www.withouthotair.com.

Solar

Cambridge

We are estimating how our consumption stacks up against conceivable sustainable production. In the last three chapters we found car-driving and plane-flying to be bigger than the plausible on-shore wind-power potential of the United Kingdom. Could solar power put production back in the lead? The power of raw sunshine at midday on a cloudless day is 1000 W per square metre. That’s 1000 W per m2 of area oriented towards the sun, not per m2 of land area. To get the power per m2 of land area in Britain, we must make several corrections. We need to compensate for the tilt between the sun and the land, which reduces the intensity of midday sun to about 60% of its value at the equator (figure 6.1). We also lose out because it is not midday all the time. On a cloud-free day in March or September, the ratio of the average intensity to the midday intensity is about 32%. Finally, we lose power because of cloud cover. In a typical UK location the sun shines during just 34% of daylight hours. The combined effect of these three factors and the additional complication of the wobble of the seasons is that the average raw power of sunshine per square metre of south-facing roof in Britain is roughly 110 W/m2 , and the average raw power of sunshine per square metre of flat ground is roughly 100 W/m2 . We can turn this raw power into useful power in four ways: 1.

Solar thermal: using the sunshine for direct heating of buildings or water. 2. Solar photovoltaic: generating electricity. 3. Solar biomass: using trees, bacteria, algae, corn, soy beans, or oilseed to make energy fuels, chemicals, or building materials. 4. Food: the same as solar biomass, except we shovel the plants into humans or other animals. (In a later chapter we’ll also visit a couple of other solar power techniques appropriate for use in deserts.) Let’s make quick rough estimates of the maximum plausible powers that each of these routes could deliver. We’ll neglect their economic costs, and the energy costs of manufacturing and maintaining the power facilities.

Solar thermal The simplest solar power technology is a panel making hot water. Let’s imagine we cover all south-facing roofs with solar thermal panels – that 38

N Nairobi 52◦ equator

S Figure 6.1. Sunlight hitting the earth at midday on a spring or autumn day. The density of sunlight per unit land area in Cambridge (latitude 52◦ ) is about 60% of that at the equator. 200 Incident solar flux (W/sq m)

6

180 160 140 120 100 80 60

London Edinburgh

40 20 0 J

F M A M

J

J

A

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Figure 6.2. Average solar intensity in London and Edinburgh as a function of time of year. The average intensity, per unit land area, is 100 W/m2 .

Copyright David JC MacKay 2009. This electronic copy is provided, free, for personal use only. See www.withouthotair.com.

39