SOUE News Issue 9

The 36th Maurice Lubbock Memorial Lecture, 13 May 2010: Sustainable Energy - without the hot air

Lecture by Professor David JC MacKay FRS

A video of this lecture can be seen on the Department's website:

David MacKay is Chief Scientific Advisor to the Department of Energy and Climate Change. He saw his mission, in this lecture as in his book of the same title1, to help us visualise how we might live without fossil fuels. These are not "sustainable", on three grounds:

His emphasis was on the need to do calculations (and back-of-the-envelope ones would often do) on the feasibility of various solutions. So he started with the story of a lunch with some Shell people, where the question was raised: "If we wanted to power all the vehicles on a given road with bio-fuels, how wide a road-side strip would be needed to grow the crops?" Some plausible assumptions about the traffic led to the conclusion that a continuous 8 km wide strip alongside each traffic lane would be needed. Which seemed to cast some doubt on this solution!

The amount of CO2 in the atmosphere over past centuries can be measured from ice cores. When plotted, it is seen to have been constant from about 1100 AD to the late 18th century, when it started to rise at an accelerating rate. The start of the rise might be approximated to James Watt's patenting of his condensing steam engine in 1769 and the start of the industrial revolution. Current emissions amount to 5-6 tonnes of CO2 per year per person on the planet. Ultimately we would like to reduce this to zero, but an intermediate target could be 1-2 tonnes/year/person by 2050. But of course it varies from country to country. In the UK the current figure is about 11, in the USA and Canada about 24, so getting down to 1 or 2 is going to be a major challenge. In the Congo and Bangladesh it is a mere 1 tonne/year/person right now.

Too many quantitative comparisons between different forms of energy generation and consumption are framed to score points in arguments, and made incomprehensible to the layman by being expressed in millions, billions or trillions. Most people can't tell the difference! The speaker suggested that energy rates (power) be expressed in terms of kWh/person/day. 1 kWh/day is of course about 40 W, the consumption of a small light bulb. The chemical energy content of the food we eat is about 3 kWh/day, 1 litre of petrol contains about 10 kWh. The manufacture of an aluminium or plastic drink container needs about 0.6 kWh. Someone who flies to Los Angeles and back uses about 10,000 kWh in the process; if you do it once a year that is an average of 26 kWh/day. A typical American or British home uses about 80 kWh/day. An average European car uses about 80 kWh over 100 km. On the other hand, the mobile phone charger left on when not charging the phone, which we are always being urged to unplug, uses about 0.5 W, or 0.01 kWh in a day, about as much as a car uses in one second!

The average rate of energy consumption in Britain is about 125 kWh/person/day, of which about a third goes on transport, a third on electricity, and a third on heating. 1 kWh/day is about the physical work capacity of one human, so think of it as employing 125 servants.

In assessing the merits of various forms of renewable energy, it is useful to ask how many watts we can get per square metre of land utilised. MacKay displayed a graph showing the various countries of the world, with energy consumption per day per person plotted against people/km2. The product of the two scales is power needed per unit area. The world-wide mean is about 0.1 W/m2. The USA needs 0.3 W/m2, but the UK with its much higher population density of 250/km2 needs 1.25 W/m2.

Wind farms can produce about 2.5 W/m2, implying that to get all this country's energy that way means using half of all our land area. Could technical progress improve this? Not much, since although large windmills produce more power, and might do so more cheaply, they have to be spaced further apart, because they shield each other if spaced too closely. Larger ones are taller, and this does help a bit. If every 700 people had one 2 MW wind turbine between them, it would supply their present demand for electricity, and occupy an area about half the size of Wales.

Bio-fuel crops can produce about 0.5 W/m2 (more in the tropics), so planting them over the whole of the Earth's land surface could produce five times the Earth's present energy needs. There would be some land left for growing food!

Photovoltaic solar panels are approaching their theoretical efficiency limit, about 31%, and can produce about 5 W/m2. With a large installation, bigger than a house, one might get 150 kWh/day. By putting them in sunny deserts and using mirrors to concentrate the sun's rays, one ought ideally to be able to get a daily average of 250 W/m2, but practical installations have been getting more like 20 W/m2.

As for tidal power, one long-established French station in La Rance estuary generates 2.7 W/m2 of tidal basin area. The Severn estuary has been suggested frequently as ideal for this purpose. And the North Sea is a vast tidal basin. If "underwater windmills" were deployed there in suitable high-current regions, they might generate 8 W/m2.

Hydroelectricity typically produces (in the UK) 0.24 W/m2 of catchment area, but obviously it depends critically on both rainfall and the vertical fall available.

The conclusion of all this is that, to be effective, renewables need to be country-sized. But before we install any of them, we have to consult neighbours, local authorities etc., and the "not-in-my-backyard" reaction can be very strong. If they insist, for example, that wind farms are not erected:

  1. within 2 km of any dwelling, since it upsets the inhabitants;
  2. in tranquil open spaces, because it spoils their amenity;
  3. in any part of the sea where they might interfere with shipping,recreation, radar reception etc.,

then there is practically nowhere left.

So perhaps we could get our renewable power from other countries. Libya perhaps? Or vast photovoltaic arrays in Spain, with 50 GW transmission lines across Europe to get it to Surrey?

An alternative is to just reduce our energy consumption. This might be done by reducing our population, or radically changing our life-styles, but neither is likely to get popular assent.

So what are the alternatives if we are to drastically cut our CO2 emissions?

  1. clean coal (i.e. burying the CO2);
  2. nuclear power;
  3. renewables elsewhere if we don't want them in our own backyard.

Let us look in turn at the main energy consumers:


The dissipation of energy here comes from:

So to reduce the energy requirement, the vehicle should be of small frontal area, low weight, travel slowly at a constant speed, and have an efficient power source. This seems to lead us to a bicycle, which uses2 around 1 kWh/person per 100 km, as opposed to 80 for a typical car. Public transport can manage about 6 kWh/p/100 km. The Cambridge "Eco-car" can do 1.3 (at 15 mile/hour), similar to a bicycle, but is hardly an acceptable vehicle for the general user.

One way to get a substantial improvement is to replace the internal combustion engine with electric motors, which can be around 85% efficient. This converts the 80 kWh/p/100 km to about 21 (measured at the charging socket). The "Tesla" electric car can manage 15, some others can do 6, and an electric scooter can do 3! Hybrid cars, in which the petrol engine is there just to get you home if the battery goes flat, manage about 25 kWh/100 km. If the electricity to charge the battery comes from a 30-40% efficient power station, then most of the advantage is lost. But it doesn't have to be. Wind turbines?


A modern gas-fired condensing boiler is about 90% efficient. To reduce heat consumption, turn the thermostat down 5° (could save 50%) and insulate the house (25%). To do better still, stick insulating panels on the outside of the house, as is done in Germany.

But the boiler's 90% efficiency is at turning the high-grade chemical energy of the gas into heat, a very low-grade form of energy. If we use a heat pump to extract heat from the ground or atmosphere, and feed it at a higher temperature into the house, our effective efficiency can rise to 300 or 400%. This figure is known as the "coefficient of performance" of the heat pump. Heat pumps are used routinely in other countries.

One way to reduce your energy consumption is to read your meters regularly, and act on your observations. In this way, the speaker managed to reduce his own gas consumption from 40 to 13 kWh/day, and electricity from 4 to 2. To get the latter result he just switched off all the items which previously had been left permanently on "standby".


If we are going to electrify most transport, and get our heating from electrically-driven heat pumps, and maintain something like our present life-style, we are going to have to treble our electricity generation. This probably needs a diversified solution. One possibility, not necessarily the one the speaker would himself recommend, might be:

16 kWh/p/day from nuclear power (perhaps as a stop-gap);

16 kWh/p/day from clean coal (ditto);

16 kWh/p/day from other people's renewables;

8 kWh/p/day from wind power.

This scheme3 would also need greatly expanded pumped storage for the better matching of supply and demand.


Getting off fossil fuels is not going to be easy. But it might be fun, for the engineers at least!

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