This semester I teach an optional course to first year physics students at the University of Sheffield, with Professor David Lidzey, called The Physics of Sustainable Energy (PHY123). This post explains why I think the course is important and some of what we hope to achieve in it.
The prosperous industrial society we live in depends, above all, on access to cheap and plentiful energy. Our prosperity has grown as our consumption of those concentrated energy sources that fossil fuels provide has multiplied. But this dependency is a problem for us; burning all those fossil fuels has materially altered the atmosphere, this has changed the world’s climate and this climate change is set to continue and intensify. We need to put our energy economy onto a more sustainable basis, but at the moment this transition seems a long way away, and the energy debate doesn’t seem to be progressing very fast. The aim of our course is to give physics students some of the tools needed to understand and contribute to that debate.
So what do you need to know to understand the energy debate? As I’m a physicist teaching students of physics, it’s not surprising that it’s with the physics that I will begin, but this needs to be set into a much wider context, including climate science, biology, economics, history and politics. But the starting point is simpler than any of these things – it’s arithmetic. David McKay, physicist turned government energy advisor, makes the point in his excellent book “Sustainable Energy without the Hot Air” that any sensible discussion of energy needs to start by adding up some simple numbers. But deeper physics knowledge helps, too. Obviously one needs to understand the simple stuff about units and conversions, how to convert a million tonnes of oil equivalent per year into mega Watts. But there are some general laws relating to that shadowy but hugely important quantity, entropy, that set hard limits about energy conversions that have to be central to any discussions of energy economies.
This needs to be set into the context of history. This is a deep history, much more important than any narrative about Kings and Queens, that shows how our industrial civilisation was built on the back of a massive increase in energy consumption. As hunter-gatherers our ancestors would have got through energy at a rate of a couple of hundred Watts; if you are a citizen of the UK you’re responsible for an average of more than 4 kW of continuous energy consumption. In the last two thousand years we’ve developed more concentrated energy sources. For early farmers, animals like horses and oxen provided more power than a human could, while from early medieval times wind and water power were harnessed more and more effectively. But right up to the 19th century, across the world as a whole, human labour was still the dominant power source, and the ultimate source of all energy was the sun’s irradiation.
What changed all that was the discovery of fossil fuels, allowing people to harness energy stored over the millennia in a very concentrated form. As it happens, this transition happened earliest here in Britain. The first half of the seventeenth century in England saw a political revolution with the civil war and the execution of Charles 1st, but it also saw an energy revolution, with the total energy obtained from coal exceeding, for the first time, the energy obtained from firewood. Within a hundred years, the use of coal had increased by a factor of ten, and then by 1850 by another factor of ten again, and the widespread adoption of the steam engine drove further increases.
Most people know that the nineteenth century industrial revolution was powered by fossil fuels; perhaps fewer people appreciate the importance of the twentieth century agricultural revolution that fossil fuels brought about. In the early 20th century, the discovery of the Haber-Bosch process made it possible to use fossil fuel energy to make artificial nitrogen fertiliser. It is estimated that the resulting increases in crop yields are responsible for keeping about 2/3 of the world’s current population alive. Between 1900 and 1990, energy inputs to farming per hectare increased by a factor of more than 80. We don’t just rely on fossil fuels for power and transport, in effect we eat them too.
What has been the result of us burning all that fossil fuel? The carbon dioxide content of the atmosphere has increased from a pre-industrial revolution value below 275 parts per million to a current value approaching 400 ppm, increasing every year. That has changed and is still changing our climate. To understand what we know for sure about human caused climate change, and what remains uncertain, needs a little more physics, in an understanding the origins of the greenhouse effect and the nature of the earth’s energy balance. The basis of the effect is not in doubt, though. So why, even today, do many people, including many who are powerful and influential, find this scientific consensus hard to accept? To understand that, we need to leave science altogether, and look for political and sociological reasons to find out, in the words of the title of an excellent book by Mike Hulme, “Why we disagree about climate change”.
So where are we now? If you are a citizen of the UK, you are responsible for a 1/63 millionth share of a rate of energy consumption that amounts to the equivalent of 200 million tonnes of oil per year. Of that, 3.8% comes from biomass, only 1% comes from other forms of renewable energy, and 7.2% from nuclear energy. The rest comes from fossil fuels, with about 35% each from oil and gas and another 19% from coal. So we are a long, long way from decarbonising our energy economy. And that doesn’t fully account for our energy consumption – as a net importer of goods, a considerable amount of energy is in effect imported in those goods. Everything we use has an embedded energy, even the food we eat; while complex goods such as computers and tablets embody a quantity of energy disproportionate to their mere mass.
So how could we move to a more sustainable energy economy? Established renewable energy technologies like hydroelectricity and wind are well understood, so we can get a good idea of what their total potential contribution to the UK’s energy might be, though there are still economic, political and engineering challenges in the way of widespread adoption of technologies like offshore wind. Waves and tides are less well established now but have considerable promise.
Wind, waves and tides have particular attractions for an island like Britain, but in global terms it is solar energy that has the biggest potential to transform our energy supplies. There’s no doubt in principle that solar energy could power the whole world at western levels of energy consumption, but the fractional contribution now is minuscule, despite strong recent growth. Fundamental physics limits the efficiency of solar cells, but in practise it’s the engineering and economics of producing solar cells in much larger areas that is limiting their uptake now. But this is an area in which new scientific discoveries are being made all the time, with entirely new classes of solar cells being discovered and developed – we’re working on this area here at Sheffield University. The excitement this year, for example, was provided by the perovskite materials being pioneered by Henry Snaith at Oxford – these could prove transformative. My Sheffield colleague, Tony Ryan, together with Steve McKevitt, has described the potential for a new solar powered economy in their recent book “Project Sunshine: how science can use the sun to fuel and feed the world”
Biomass is a form of solar energy too, of course. Plants use the sun’s energy to convert water and carbon dioxide into fuel molecules; the biophysics underlying the way photosynthesis works having some parallels with solar cells. As we’ve seen, the world’s energy economy was always based on biomass right through the middle ages to the early modern period – can biomass and biofuels make a contribution at the much larger scale that we’d need now? It’s possible that new understanding of how plants work and new processes to convert biomass into usable fuels will make a difference, but the total land areas needed and competition with food plants are difficult constraints. Given the degree to which modern agriculture is itself very energy intensive, careful accounting is needed to make sure biofuels really do make a positive net contribution to providing low carbon energy.
Nuclear energy was once thought of as the future of clean energy, and maybe its time will come again. Understanding the chequered history of nuclear power needs an understanding both of the basics of nuclear physics, and of the political factors which made the technology unfold the way it did. Questions of nuclear safety, nuclear proliferation, and the disposal of nuclear waste remain important barriers to the adoption of a new generation of nuclear power stations, but they may not always be insuperable. Meanwhile nuclear fusion is notoriously slow in arriving, with the reputation of always remaining fifty years in the future. But maybe recent progress with fusion gives cause for cautious optimism.
The other side of the balance sheet is how we use energy – how much can we maintain our existing standard of living with a lower energy input by being more efficient? There are big opportunities here in making our homes more efficient, and maybe new automobile technologies can help us maintain our addiction to travel with less energy impact.
And what if we fail? There are some unpalatable choices if we don’t manage to reduce our dependence on fossil fuels. We can try and reduce the impact of burning fossil fuels by capturing and storing the carbon dioxide before it escapes to the atmosphere to add to the intensity of global warming. Or we could try and mitigate the effects of global warming by interfering with the climate on a global scale – geoengineering. The first option looks economically difficult, while the second is fraught with uncertainty and controversy.
The energy debate is an urgent one. I hope this course equips the students who take it to contribute to that debate, helps them understand how the physics they learn at University has some desperately important applications to our current predicament, and helps them understand the wider contexts in which that debate takes place.