This is another guest post by DR. David Mackay is Professor of Physics at Cambridge, a Fellow of the Royal Society and a member of the World Economic Forum Global Agenda Council on Climate Change.
Another talk from Oxford. This is my report of David MacKay’s talk on Sustainable Energy – without the Hot Air in the Department of Engineering Science on 13 May 2010.
David MacKay said that he likes back-of-the-envelope calculations such as: if we were to grow biofuel crops alongside roads, how wide a band of crops would be needed to power the traffic on that road? Answer – about 8 km – which shows that changing away from fossil fuels will not be easy. We (the developed world) have an addiction to fossil fuels which is not sustainable, for three reasons: 1. Fossil fuels are a finite resource; 2. They produce CO2 which goes into the atmosphere and oceans, and which the scientists tell us is a massive geoengineering experiment that we would do well to stop; and 3. we should be concerned with security of supply.
Ice core data tells us that CO2 levels started to increase about 1769, the year that James Watt invented the steam engine and the industrial revolution began. We should reduce CO2 emissions to zero eventually, and aim to reach 1 – 2 tonnes per person per year by 2050. The world average is currently 5–6 t and only Congo and Bangladesh meet the 1–2 t target.
MacKay then noted the units that he uses. He quotes energy in kWh and power consumption in kWh per day per person. They may not be physicists’ units, but they relate to personal experience and allow us to deal in small numbers, not billions and trillions. So a UK house runs on about 80 kWh/day, a 100km car journey is 80 KWh, and a plane trip London to LA and back is 10,000 kWh. A phone charger left on for one day uses the same energy as driving a car for one second – so it’s important to realise what matters and what doesn’t. In the UK we use roughly one third of our energy on transport and one third on heating and the rest can be largely supplied by electricity, so it makes sense to look at the areas of transport, heating and power supply first. Total energy use in Europe is about 125 kWh/day/person – in the US or Australia it’s twice this amount.
Considering whether renewables can meet this energy demand, MacKay considered the power available per unit area of land – since renewables generally require land (or sea) area – what sort of areas are we talking about?
For instance biofuels only generate 0.5 W/m2 (somewhat more in the tropics), while wind can give 2-3 W/m2 and up to 20 W/m2 is available from solar PV. Superimposing these data on a graph of per capita energy consumption against population density (see versions here on his blog) the amount of land needed to meet countries’ energy needs by different renewables can be calculated. For instance, the UK consumes about 1.5 W/m2 so to gain all our energy needs from wind power, half of the country would have to be covered with wind farms. MacKay has checked these figures for wind farms in the UK, and finds that they do generate about 3 W/m2. But could technological advances push these numbers up? He thinks not – larger windmills need more space, so the energy/ area does not go up by much. There are promising designs, such as kite-based windmills, but they are not expected to use less space. But put another way, to generate 17 kWh /person /day (14% of energy needs), one 2 MW windmill would be needed per 700 people. This would need about 7% of the UK’s area, or half the area of Wales.
Solar panels would be worth fitting to south-facing roofs in the UK. They have been installed on a large scale in Bavaria, on fields as well as roofs (though they use land that would otherwise be usable for farming) and generate about 5 W/m2. Solar PV efficiencies are near their theoretical limit but of course depend on the amount of sunshine. Concentrating solar power in deserts is likely to be the most effective and can reach 20 W/m2. But large areas of panels are needed and they are expensive.
He also considered tide power, undersea ‘windfarms’ using North Sea tidal currents, and offshore wind power – regardless of the type of renewable energy they all need ‘country-sized’(or ‘sea-sized’) areas to generate a significant proportion of a country’s needs. And each scheme, whether land or sea based, attracts protest. Even a 15-fold increase in UK’s renewables could only meet about 20% of our energy needs. But we have room to install a range of different renewable technologies where they are practical, that would reach the total we need.
Can we reduce demand? The country is unlikely to wish to reduce population, and lifestyle changes are unpopular, so can we increase efficiency? For instance in transport, electricity is far more efficient way of powering a car than a petrol engine (current electric models consume about 21 kWh/100km compared to 80 kWh/100km for a typical petrol car – though inefficiencies in electricity generation are not taken into account). But prototypes suggest this could be improved to about 6 kWh/100km, and renewables could be used to generate the electricity.
Domestic heating is another huge area – here MacKay suggested improving insulation, and adopting air source heat pumps, which are popular in Europe but have not been widely introduced in the UK – he’s not sure why. Pilot projects in the UK have not shown the coefficients of performance found elsewhere (300-400% in Germany and Japan based on the electricity used) – again he’s not sure why. And for a simple idea – read your meters and see what makes a difference. But, efficiency gains can lead to increased consumption in other areas from the money saved.
So, in summary, his suggestions on the demand side are to electrify transport, insulate houses and improve heating efficiency particularly using heat pumps. And Britain should generate electricity from a wide range of sources: mainly nuclear, wind and imported solar (from north Africa), with the rest from UK solar, clean coal, hydro, tidal, biomass and so on. A combination of these would more than supply the UK’s energy needs.
Answers to audience questions:
Fluctuations in wind energy are real, and need to be compensated, but this does not need fossil fuel. Wind + nuclear + hydroelectric storage such as Dinorwig would meet much of the needs. If integrated with smart meters, the electricity used to heat water and charge cars overnight could be adjusted to match supply (provided consumers were guaranteed that they would have charged cars and hot water by the morning).
Micro CHP is unattractive as it locks the country into using gas or biomass, and compared to efficient large power stations and condensing boilers saves only about 12%. Would prefer to move to heat pumps.
The supply of many rare metals (e.g. Li for batteries) is about 40 years – but this is probably only an apparent limit since mining companies will explore for more when the supply starts to run out.
How well do other countries do? Denmark is often held up as a role model but has higher GHG emissions per person than the UK, as does Germany. See graph of GHG emissions vs energy use – but note how far away we are from 1 tonne per person: http://www.inference.phy.cam.ac.uk/withouthotair/cI/page_337.shtml