## Tuesday, January 20, 2015

Having had a go at the walls, the windows, the roof, and air-tightness, I've decided to get the last physical improvement done to our house - the floor.

(image kindly supplied by devanainsulation.co.uk)

The area of wooden suspended floor is about 35 square metres. According to page 290 of my book, the U-value of the floor is 0.7 W/m2/K, so the floor has a leakiness of about 25 W/K, which assuming an average temperature difference of 6 degrees, implies a conductive heat loss of 147 W (3.5 kWh/day). (The floor may also contribute some of the ventilation-leakiness of the house, but I'll neglect that.) The floor doesn't just lose heat, it also feels cold, and thus (a) affects quality of life, and (b) perhaps causes us to turn up the thermostat sometimes to improve our feeling of warmth. If we manage to keep the whole house (leakiness 240 W/K) on average 1 degree cooler thanks to improved floor insulation, then that would be an extra 240 W of saving (6 kWh/day).
I didn't do an economic calculation before deciding to get the underfloor insulation. It just feels like the right thing to do, and Retrovive was recommended by someone from Max Fordham, whose judgement I respect highly. Anyway, let's work out a pay-back time. The anticipated new U-value is about 0.25 W/m2/K. It looks like the work will cost about £2900 including VAT (including insulating central heating pipes that run under the floor) (for comparison we are perfectly happy to put down new carpets over a smaller area for a cost of £1150). If the insulation eliminates two thirds of the floor's conductive heat loss (i.e., about 2.2 kWh/day) and delivers say one quarter of the notional 6 kWh/day saving guessed above (if we managed to turn the thermostat down a bit), the total saving might be 3.7 kWh/day. With gas costing 5.2p per kWh, that's 20p per day, or £70 per year. So the payback time might be about 41 years.
If I went to the high end of all my estimates, I might imagine a saving of 9.5 kWh/day on average, in which case the payback would be about £180 per year and the payback time would be 16 years.
I am expecting that the main value of this work will be the improved feeling of comfort. People are happy to pay £36,000 per year to rent a family home [That's what we paid to rent a flat in London, at least]. If the home's main room feels really cosy, how much extra would we be willing to pay? I could imagine 5% or 10%. On those grounds, the comfort of cosiness is worth £1800 to £3600 per year. So the payback time, taking into account this benefit, is just one or two years.
I will post again when the work is done! (April 2015)

## Friday, November 14, 2014

### Oxford's 2050 Pathway (created 2014-15)

I'm giving the Charles Simonyi Annual Lecture tonight (14 November 2014) and during my talk about Why climate change action is difficult, and how we can make a difference, I will mention the 2050 Calculator, a tool to help people have constructive conversations about Britain's energy options, and to support consensus-building.

The options in the calculator include lifestyle changes, and all sorts of technologies for saving energy and sourcing low-carbon energy. I would love to help Oxford crowd-source its own 2050 pathway, but there won't be time in this single lecture to do it; so here is the plan: I'll come back to Oxford in March 2015, and, with Mark Lynas and other celebrities at the Oxford Literary Festival, we will find out what pathway the Oxford audience would like to choose, to keep the lights on, have energy security, and meet the UK's legal climate change targets . To ensure that we have a really good deliberative discussion, the Oxford community is welcome to use the comments area of this blog page as a place for discussion.
Commenting rules: Please discuss options in the calculator, and what you would like the UK to do. Please don't have a pub brawl. Please discuss pathways that add up. It's fine to say you dislike an option, but you should feel an obligation to describe how you would propose to get by without that option, taking into account other people's views about the other options.
At the Oxford Literary Festival event we will take all your comments into account and use them as a springboard for a really constructive discussion. Thank you for joining in!

You may be interested to see what pathways other cities chose, when they took on the "British Energy Challenge" - here are those cities' choices, and my reflections on the British Energy Challenge roadshows of 2013 and 2014.

I look forward to listening in and supporting Oxford's energy pathway discussion in 2015!

My Simonyi lecture slides are here, and Sustainable Energy - without the hot air is free online too. The 2050 Calculator also contains lots of well-written documentation [click on the blue "i" icons] to help guide your decisions.

## Wednesday, September 10, 2014

### Solar power from space?

When I published Sustainable Energy - without the hot air, there were quite a few topics I could have included but didn't because they seemed a little too blue-sky. For example I have draft chapters on Osmotic Power and on Kite Power (and if I were to rewrite the book today I think that I would now include Kite Power after all). Another idea I dismissed at the time was "what if we put solar panels in space, in geo-synchronous orbit?" I dismissed that idea on the grounds that "the advantage of space over the deserts of Libya and Nevada as a location for solar panels is only roughly a factor of 4, and surely that's outweighed by the difficulty and cost of getting panels (and associated power-re-transmission systems) into space, compared with just plopping them on the ground in a desert?" However, Keith Henson has for some time been working out the details of a scheme that might prove me wrong.
It involves many clever ideas, and some ambitious ones - such as the idea of powering a Skylon freight-delivery space-craft by space lasers that are powered from the ground with GW-scale microwave transmitters! I encourage people who are interested to read Keith's 'dollar a gallon' post and his follow-up post.
Further reading: Solar energy in the context of energy use, energy transportation, and energy storage - a paper in which I provide data for the power per unit area of real solar farms, and discuss the need for significantly cheaper energy storage if ground-based solar power is ever to contribute a significant fraction of energy consumption.

## Tuesday, August 12, 2014

### How would the footprint of a shale gas operation compare with the footprint of other ways of delivering a similar quantity of energy?

There are many dimensions to a "footprint". In this blog post, I'll look at land area, vertical height, and vehicle movements.

I'll compare a shale gas pad (which might produce 0.9 billion cubic metres of gas over 25 years) with a 174-MW wind farm and a 380-MW solar park, both of which would deliver roughly 9.5 TWh of electricity over 25 years ­– the same amount of energy as the chemical energy in 0.9 billion cubic metres of gas.

In this table I've highlighted in green the "winning" energy source for each of the footprint metrics.

 Shale gas pad Wind farm Solar park (10 wells) 87 turbines, 174 MW capacity 1,520,000 panels, 380 MW capacity Energy delivered over 25 years 9.5 TWh 9.5 TWh 9.5 TWh (chemical) (electric) (electric) Number of tall things 1 drilling rig 87 turbines None Height 26 m 100 m 2.5 m Land area occupied by hardware, foundations, or access roads 2 ha 36 ha 308 ha Land area of the whole facility 2 ha 1450 ha 924 ha Area from which the facility can be seen 77 ha 5200-17,000 ha 924 ha Truck movements 2900-20,000 7800 3800 (or 7600*)

The total land area of the facility is smallest for the shale gas pad, and largest for the wind farm. The land area actually occupied by stuff is smallest for the shale gas pad, and largest for the solar park ­– the wind farm has lots of empty land between the turbines, which can be used for other purposes.

In terms of visual intrusion, the wind turbines are the tallest, and could be seen from a land area of between 52 and 170 square km, depending how they are laid out. (To roughly estimate an area of visual influence, I computed the land area within which the drilling rig or a wind turbine would be higher than 3 degrees above the horizon, assuming a flat landscape.) By this measure, the shale gas pad creates the least visual intrusion. Moreover, the drilling rig might be in place for only the first few years of operations at the shale gas pad. The solar panels are the least tall, but the solar facility occupies 450 times as much land area as the shale gas pad. (I've assumed that the wind farm and solar parks wouldn't require any additional "intrusive" electricity pylons.)

When it comes to truck movements, all three energy facilities require lots! I've assumed that solar panels are delivered at a rate of 800 (originally 400*) panels per truck; for the wind farm, my estimate is dominated by the delivery of materials for foundations and roads at 30 tonnes per truck; the estimates for the shale gas pad are from DECC's recent Strategic Environmental Assessment and from the Institute of Directors' report "Getting Shale Gas Working". The shale gas pad might require the fewest truck movements, if all water is piped to and from the site. But if water for the fracking is trucked to and from the site, then the shale-gas facility would require the most truck movements.

What can we take from these numbers? Well, perhaps unsurprisingly, there is no silver bullet ­– no energy source with all-round small environmental impact. If society wants to use energy, it must get its energy from somewhere, and all sources have their costs and risks. I advocate deliberative conversations in which the public discuss the whole energy system and look at all the options.

Thanks to Jenny Moore, Martin Meadows, and James Davey for helpful discussions.

 Photo: Wytch Farm, on the perimeter of Poole Harbour in Dorset, is the largest onshore oil and gas field in Western Europe. It is located in an Area of Outstanding Natural Beauty. The photograph shows the 34-metre-high extended-reach drilling rig, from which boreholes longer than 10 km have been drilled.

All estimates are for energy production facilities located in the UK. The estimate of energy produced from a shale gas pad is highly uncertain, since there are no data for actual shale gas production in the UK.

The comparison in the table is based on deeming 1 kWh of electrical output from the wind to be 'equivalent' to 1 kWh of chemical energy in the form of gas. This is the conventional equivalence used for example in DUKES and in Sustainable Energy - without the hot air. The following differences between the energy sources should be noted.

1. The three sources of power have different profiles of power generation. On an annual scale, a single shale gas well produces most power when it is newly fractured, whereas a wind-farm produces a relatively constant average power over its life. On an hour-by-hour scale, the gas from the well is dispatchable – its flow can be turned up and down at will – whereas the power from a wind-farm is intermittent.
2. In a world in which the only conceivable use for gas is making electricity in a power station with an efficiency of about 50%, one might prefer to deem each 1 kWh of gas as 'equivalent' to just 0.5 kWh of electricity.
3. On the other hand, in a world that values gas highly relative to electricity that is generated at times when the wind blows, one might imagine planning (as Germany is said to be planning) to use electricity from wind-farms to synthesize methane (with an efficiency of 38-48%); then one might deem each 1 kWh of wind-electricity as being 'equivalent' to 0.38-0.48 kWh of gas.
4. If one wished to make a comparison in which both power sources are constrained to have very low carbon emissions, the shale-gas well must be accompanied by other assets. For example, if the gas is sent to a power station that performs carbon capture and storage, the gas-to-carbon-free-electricity efficiency might be about 42%, and the land area for the power station and the carbon transport and storage infrastructure should be included; assuming that these assets have an area-to-power ratio of 100 ha per GW(e), each 43.4-MW gas well (which would yield 18.2 MW of electricity) would require an extra 1.82 ha of land, which roughly doubles the 2-ha land area mentioned in the table.

My estimate for vehicle movements for large wind-farms is based on Farr wind-farm. I'm sure there is considerable variation from project to project, and I would welcome more data. For the number of truck movements required for a wind farm, I reckoned there would be about 870 movements to bring in the turbines themselves [counting an in-bound and out-bound trip as two movements], and significantly more movements to bring in the materials for roads and concrete for foundations. Some of these materials may be mined from quarries located on the wind-farm, which would then involve no vehicle movements on public roads; based on Farr wind-farm (where three quarters of the road materials were sourced on site) [sorry, I don't have a link for this fact], the road building would require 2774 vehicle movements for a 174-MW windfarm, and the foundations would require another 4140 or so – in total, about 7800 vehicle movements.

Potential greenhouse gas emissions associated with shale gas production and use.

## Thursday, August 7, 2014

### Embodied energy in a car - update under way

John Biggins sent me a helpful email querying a number in my book's chapter on "Stuff".
I have a question about the embedded energy in a car, which you quote at 76000kWh. That seems awfully high to me. To a first approximation a car is a tonne of steel, with a raw material energy of 6000kWh: an order of magnitude less.The (admittedly biased) Society of Motor Manufacturers & Traders report quote an even lower figure of 2000kWh per car (page 17), which I suspect is probably meant to be simply the energy used per car by the car plant, neglecting materials.
They asked a few manufacturers, and arrive at a figures in the ballpark of about 1-4 tonnes of C02 to produce a car, which we might reverse engineer guessing most of the CO2 comes from coal burning in either steel production or electricity generation, to get ballpark figures of probably no more than 10,000kwh per car.
Since these estimates actually differ from your figure by a magnitude, I thought I'd write and ask whether you particularly believe your 76,000kWh figure. Do you have any back-of-the-envelope type way to understand it?

This blog post is where I will record my working on this question. I will aim to justify or adjust my answer within a month, and will add to the book's Errata if necessary. If anyone wants to send me good references on embodied-car-energy to add to my own, please post a comment. Thanks! David

## Monday, June 23, 2014

### 2014 Longitude Prize Water Challenge

Clean water is crucial not only for humans' direct use but also for agriculture. Attention often focuses on drinking water, but agriculture is far bigger.

Let's put it in numbers. How much drinkable water do you require for drinking and for cooking? Perhaps a few litres per day per person. In the UK, urban consumption of water is about 160 litres per day per person. And in developed countries, even if they are being careful with water, agriculture requires about 340 litres per day per person. [Israel uses roughly 1000 million m3 of water per year for agriculture, and it has a population of roughly 8 million. That's 340 litres per day per person.]

Some lucky countries have plenty of rainfall, so this agricultural requirement can be provided at very low cost. But what if the water for agriculture must be produced by desalinating sea-water?

 Part of the reverse-osmosis facility at Jersey Water’s desalination plant. The pump in the foreground, right, has a power of 355 kW and shoves seawater at a pressure of 65 bar into 39 spiral-wound membranes in the banks of blue horizontal tubes, left, delivering 1500 m3 per day of clean water. The clean water from this facility has a total energy cost of 8 kWh per m3. From Sustainable Energy - without the hot air

Standard reverse-osmosis facilities have an energy cost of 8 kWh per m3, so an agricultural water requirement of 340 litres per day per person implies an energy requirement of about 2.7 kWh per day per person, if we had to make it all by desalination with today's technology. To put that in UK terms, 2.7 kWh/d/p is roughly 17% of the average UK electricity supply; delivering 2.7 kWh/d/p of electricity to the UK would require roughly 7 extra nuclear power stations the size of Sizewell B, or 13,500 2-MW wind turbines.

For people in a less-developed country, the cost of desalinating that much water would be significant - 2.7 kWh/d/p might cost about 30 pence per day per person. More than a billion people live on less than a dollar a day.

This is why the Longitude Prize Water Challenge sets the goal of desalinizing water with significantly less energy than today's technologies. We are especially interested in approaches that could be low-enough in cost not only at large scale but also when rolled out in small-scale facilities.

For an example of an inventive approach to the Water Challenge, Stephen Salter has published a wave-powered desalination invention (2007) using vapour-compression desalination in place of reverse osmosis.

Voting for the Longitude Prize Challenge closes at 7.10pm on June 25th 2014.
2014 Longitude prize FLIGHT Challenge

David MacKay FRS is a member of the 2014 Longitude Committee. He is the Chief Scientific Advisor at the Department of Energy and Climate Change, and Regius Professor of Engineering at the University of Cambridge. He is well known as author of the popular science book, Sustainable Energy — without the hot air.

David MacKay FRS

### 2014 Longitude Prize Flight Challenge

 The aviation industry has made amazing progress. In just 111 years since the first controlled, powered, heavier-than-air flying-machine flew in North Carolina, innovators and industry have produced a spiralling succession of biplanes, monoplanes, jet aircraft, helicopters, airliners, and faster-than-sound planes. Thanks to steady advances in engineering and materials science, aviation records are repeatedly broken; aircraft are getting steadily lighter and more energy efficient. The Boeing 747 travels 20 times faster than an ocean liner, but uses less than half as much energy per passenger-kilometre; if all its seats are occupied, a 747 is as energy-efficient as a standard (33 miles-per-gallon) car with two people in it, even though the plane goes ten times as fast as the car. And the latest ATR72 turboprop is said to be 40% more energy-efficient than the 747.

But if the world is serious about tackling climate change, these fantastic engineering achievements are not enough. Whereas, in the year 2000, aviation contributed 2% of global carbon dioxide emissions, it is projected that by 2050, aviation's growth will increase its carbon emissions five-fold, even allowing for continued improvements in efficiency. Moreover, today's planes emit other greenhouse gases whose effect on climate is estimated to be between two and four times greater than their carbon dioxide emissions.

The Longitude Prize 2014 Flight Challenge sets a new goal for aviation: to design and demonstrate a near-zero-carbon aircraft, which travels fast (though not necessarily as fast as a jet), which has a substantial range (at least London to Edinburgh!), and which is significantly more energy-efficient than a 747. There is no simple solution to this demanding set of constraints, but there are promising approaches that fulfill some of these requirements.

• NASA's 2011 Green Flight Challenge was convincingly won by a four-seat electric battery-powered aeroplane, the Pipistrel Taurus G4, which flew nearly 200 miles in less than two hours, with an energy efficiency equivalent to a 120 miles-per-gallon car. This astonishing achievement delivers all the requirements of the Longitude Prize Flight Challenge except for the range requirement — it may take significant breakthroughs in battery technology if such an aircraft is to win the Challenge.

• The Solar Impulse is an electric aircraft in which heavy batteries are replaced by huge solar panels spread over the wings and tail. It can fly for more than 24 hours and could easily make the trip from London to Edinburgh. But with a speed of only 43 miles per hour, the Solar Impulse is too slow.

• Oxfordshire-based Reaction Engines have a completely different approach on their drawing board. They are designing liquid-hydrogen-burning engines that could be used to launch satellites into orbit. The same engines could also power a hypersonic passenger aeroplane, the LAPCAT, which (thanks to the very high calorific value of hydrogen) could fly from London to Brisbane in a single hop. It doesn't seem likely that the LAPCAT will be more energy-efficient than a 747, but perhaps a lower-speed hydrogen-powered approach might work.

• Cranfield-based Hybrid Air Vehicles are developing aircraft that are a cross between a regular plane and a blimp. The helium-filled Airlander uses a combination of buoyancy and aerodynamics to generate lift. Their current prototype has adequate speed and range, but its fossil-fuel-powered engines emit too much carbon and use too much energy. Perhaps a redesigned hybrid aircraft, optimized to work at a lower speed, might be significantly more energy efficient.

We intend the Challenge to be accessible to small creative teams. Here I've described four current activities that indicate the wide range of perspectives from which the Flight Challenge might be approached. I'm confident the Flight Challenge will stimulate brilliant inventors to develop other exciting ideas for the future of aviation.

 criteria zero carbon emissions in flight energy-efficient speed range carries passengers vehicle Pipistrel Electric plane ✔✔✔ ✔✔ ✔✔ ✘? ✔✔ Reaction Engines' LAPCAT Hydrogen plane ✔? ✘ ✔✔✔ ✔✔✔ ✔✔✔? Solar Impulse ✔✔✔ ✔✔✔ ✘✘ ✔✔ ✘ Hybrid Air Vehicles ✘? ✘? ✔? ✔ ✔✔

Voting for the Longitude Prize Challenge closes at 7.10pm on June 25th 2014.

David MacKay FRS is a member of the 2014 Longitude Committee. He is the Chief Scientific Advisor at the Department of Energy and Climate Change, and Regius Professor of Engineering at the University of Cambridge. He is well known as author of the popular science book, Sustainable Energy — without the hot air.

What's the expected progress under business as usual, or if best efforts are made?
The Advisory Council for Aeronautics Research in Europe (ACARE) have published their vision for 2020, in which there is a target of a 50% reduction in CO2 emissions per seat-km by 2020, relative to a base year of 2000. Of this 50%, 40% is attributed to aircraft-level improvements [in new aircraft], while 10% comes from operational improvements. From 1961 to 2000, aircraft engines have become roughly 40% more efficient [Comet 4 to B777-200], and aircraft have become 70% more efficient overall (in fuel per seat-km). However, improving an engine's fuel efficiency tends to make its NOx emissions worse. Future Aircraft Fuel Efficiencies - Final Report Gareth Horton (92 pages) Horton reckons the upper bound of likely efficiency improvements, in 2050, relative to 2000, is 71% for single-aisle aircraft.
ESTIMATION OF POTENTIAL AIRCRAFT FUEL BURN REDUCTION IN CRUISE VIA SPEED AND ALTITUDE OPTIMIZATION STRATEGIES Jonathan A. Lovegren and R. John Hansman - MIT Report No. ICAT-2011-03 February 2011 focuses on the potential efficiency benefits that can be achieved by improving the cruise speed and altitude profiles operated by flights today in the USA. Their results indicate that a maximum fuel burn reduction of 3.5% is possible in cruise given complete altitude and speed optimization, which corresponds to 2.6% fuel reduction system-wide.
Isn't this a solved problem? Can't we just use biofuel, especially drop-in bio-replacements for kerosene?
Biofuels are often assumed to be "the answer" for aviation, and they may play an important role. There are three reasons for looking for other solutions.
1. A biofuel-derived kerosene substitute, even if truly carbon neutral, will still cause the other [non-CO2] greenhouse-gas emissions (for example NOx), which are estimated to have at least as big a climate-change impact as the CO2 from planes.
2. The land requirements for a biofuel solution would be substantial: if we take the IPCC's projection for global aviation's fuel demand in 2050 [equivalent to 2.5 GtCO2/y], and assume that biofuel [equivalent to 240 g/kWh] is produced with a land-productivity of 0.5 W/m2, we find the land required for biofuel production would be 2.4 million km2, which is (1/4) of the USA, 10 times the UK's area, or 100 Waleses. (1 Wales is roughly the same as 1 New Jersey.) These land requirements may be in tension with other desires for environmental sustainablity and food production.
3. Biofuels are not necessarily "zero-carbon". Some biofuels require substantial energy for their production; and some forms of biofuel production may involve changes in carbon stocks in the landscape, compared to alternative uses of the land, such that the use of the biofuel causes substantial net carbon emissions. These land-use-change emissions may be "one-off" emissions [i.e., incurred once only], but that does mean they can be neglected.
How can electric or hydrogen planes be counted as zero-carbon? Where does the electricity or hydrogen come from?
Yes, "decarbonizing" by switching to electricity makes sense only if there is a proportional additional increase in low-carbon electricity generation. For hydrogen to be considered zero-carbon, it would eventually have to be produced from zero-carbon electricity by electrolysis, or at a carbon-capture-and-storage facility.
What if we make artificial kerosene from CO2 and a zero-carbon energy source?
Yes, artifical fuel synthesis is an important technology option to develop. It avoids the land-requirements and sustainability concerns about biofuels, and instead requires substantial additional energy inputs.
1. "Zero-carbon" artifical kerosene will still cause the other [non-CO2] greenhouse-gas emissions (for example NOx), which are estimated to have at least as big a climate-change impact as the CO2 from planes.
2. Also, it is important to be clear whether the artifical kerosene is truly carbon neutral; it may be [especially if the CO2 is captured directly from the air], but it depends on the source of the carbon that goes into making the fuel. Consider for example a fossil-fuel power station with CO2 capture. If this CO2 is fed to a aeroplane-fuel-synthesis plant, then we cannot declare both the power station and the aeroplane "zero carbon"! The right way to think about this set-up is that it would be getting two uses out of each fossil-fuel carbon atom, before it is released as CO2 by the plane.
Some fuel-synthesis proponents will suggest getting all the required CO2 from biological sources instead - for example by capture from the chimney of a sustainable-biomass-powered power-station. Such an arrangement can certainly be imagined at small scale, but would there be enough biomass for it to work at large scale?

David MacKay FRS

## Saturday, May 24, 2014

### Energia sostenibile - senza aria fritta [Italian translation of Sustainable Energy - without the hot air!]

I'm very grateful to volunteers Alessandro Pastore, Javier Oca, Valentina Rossi, Alberto Marcone, Paolo Errani, and Simone Gallarini for completing the Italian translation of Sustainable Energy - without the hot air.

There is an announcement of the translation, and a synopsis, at this link, and the first draft of the translation is available on the book's translation page.

Energia sostenibile - senza aria frittaNel caso ci fossero sfuggite delle imperfezioni o errori, all'indirizzo email: energia.senzaariafritta@gmail.com, saremo ben lieti di riceverne segnalazione. Il libro in italiano può essere scaricato liberamente da internet all'indirizzo http://www.withouthotair.com/translations.html.

Grazie!

## Saturday, February 22, 2014

### Crowd-sourcing the IET's 2050 Pathway

I am giving the Clerk Maxwell Lecture for the IET on 6 March 2014 at the Royal Institution, London, UK. This post and its discussion area are for the IET audience who are coming to the lecture.

I'm aiming to make a highly interactive presentation in which we will try to crowd-source an "IET consensus pathway" in the UK's 2050 Calculator. To help the discussion go well, I'd like to encourage people who are planning to be in the audience, before the lecture, to play with the calculator, and to identify the levers they would most like to discuss during the lecture. Please use the comments area at the foot of this blog-post now as a discussion area. Please feel free also to discuss your preferred pathways or preferred settings of individual levers, and to discuss particular issues or trade-offs that you think should be part of a useful conversation using the calculator.

The outcome - Here is the pathway that we got to after one hour - I will write a few notes and propose possible tweaks in a moment. Top things that needed doing: (a) check which fuel mix for the CCS power stations works best; (b) check which choice of fuel from bioenergy works best; (c) explore space-heating options - the audience asked for a 15:25:60 mix of fuel-in-home (eg gas boilers):district-heating:heat-pumps, and the "CD" heating mix doesn't match this perfectly. Thank you to the audience for a fun evening!

Update - After the lecture I made a few adjustments to the above pathway which I think the audience would have been content with. The resulting final IET London pathway (March 2014) is here. The changes I made were as follows: (a) I checked which choice of CCS power station fuel (solid/gas) was best for emissions, and selected "D". (b) I checked which "type of fuels from biomass" was best for emissions, and selected "B" (mainly solid). (c) I adjusted the commercial heating choice to "D,A" so as to make the overall heating mix for homes and commercial closer to the heating mix that the audience voted for. (d) I double-checked whether choices (a, b) were still optimal. The resulting pathway achieves a 77% reduction in emissions on 1990 levels (pretty close to the 2050 target of at least 80%), and requires no backup generation in mid-winter when the wind doesn't blow.

## Wednesday, December 11, 2013

### Turboprop update, and a Hot Air Oscar nomination

 My attention was recently drawn to an impressively fuel-efficient turboprop aircraft, the ATR72-600, which is claimed to be about one third more energy efficient than Bombardier's Q400 turboprop, which I featured on page 35 of SEWTHA.

I've consequently written an update on turboprops, celebrating this achievement, but in the interests of balance I feel I should also nominate the advertisers of the ATR72-600 for this year's Hot Air Oscar for the most misleading "green" infographic, for this astonishing picture [at left] showing the difference between the fuel consumption of the ATR 72 and the Q400 on a 250-nautical-mile journey. As the numbers in the picture show, the ATR 72's fuel consumption is 70% of the Q400's, but the volume of the three-dimensional blue barrel shown is 30% of the volume of the orange barrel — a 2.3-fold exaggeration!
 blue barrel : orange barrel ratio of diameters 91 : 134 =0.68:1 ratio of heights 118 : 179 =0.66:1 ratio of volumes (as depicted) =0.68×0.68×0.66 =0.30:1 true ratio of volumes(735:1043) =0.70:1

### Do UK wind farms decline "very dramatically" with age?

In December 2013, Christopher Booker in the Telegraph discusses a study by Gordon Hughes, published by the Renewable Energy Foundation in December 2012, which is said to show, due to wear and tear on their mechanisms and blades, the amount of electricity generated by wind turbines "very dramatically falls over the years". Booker asserts that "Hughes showed his research to David MacKay, the chief scientific adviser to the Department of Energy and Climate Change, who could not dispute his findings." This is not true.
In fact, I doubted Hughes's assertions from the moment I first read his study, since they were so grossly at variance with the data.
Figure 1: Actual load factors of UK wind farms at ages 10, 11, and 15.
a) Histogram of average annual load factors of wind farms at age 10 years. For comparison, the blue vertical line indicates the assertion from the Renewable Energy Foundation's study that "the normalised load factor is 15% at age 10."
b) Histogram of average annual load factors of wind farms at age 11 years.
c) Histogram of average annual load factors of wind farms at age 15 years. For comparison, the red vertical line indicates the assertion from the Renewable Energy Foundation's study that "the normalised load factor is 11% at age 15." At all three ages shown above, the histogram of load factors has a mean and standard deviation of 24% ± 7%.

Moreover, by January 2013 I had figured out an explanation of the underlying reason for Hughes's spurious results. I immediately wrote a technical report about this flaw in Hughes's work, and sent it to the Renewable Energy Foundation, recommending that they should retract the study.
I would like to emphasize that I believe the Renewable Energy Foundation and Gordon Hughes have performed a valuable service by collating, visualizing, and making accessible a large database containing the performance of wind farms. This data, when properly analysed in conjunction with detailed wind data, will allow the decline in performance of wind turbines to be better understood. Iain Staffell and Richard Green, of Imperial College, have carried out such an analysis (in press), and it indicates that the performance of windfarms does decline, but at a much smaller rate than the "dramatic" rates claimed by Hughes. The evidence of decline is strongest for the oldest windfarms, for which there is more data. For newer windfarms, the error bars on the decline rates are larger, but Staffell and Green's analysis indicates that the decline rates may be even smaller.
I will finish this post with a graphical explanation of the flaw that I identified (as described in detail in my technical report) and that I believe underlies Hughes's spurious results.
The study by Hughes modelled a large number of energy-production measurements from 3000 onshore turbines, in terms of three parameterized functions: an age-performance function "f(a)", which describes how the performance of a typical wind-farm declines with its age; a wind-farm-dependent parameter "ui" describing how each windfarm compares to its peers; and a time-dependent parameter "vt" that captures national wind conditions as a function of time. The modelling method of Hughes is based on an underlying statistical model that is non-identifiable: the underlying model can fit the data in an infinite number of ways, by adjusting rising or falling trends in two of the three parametric functions to compensate for any choice of rising or falling trend in the third. Thus the underlying model could fit the data with a steeply dropping age-performance function, a steeply rising trend in national wind conditions, and a steep downward trend in the effectiveness of wind farms as a function of their commissioning date (three features seen in Hughes's fits). But all these trends are arbitrary, in the sense that the same underlying model could fit the data exactly as well, for example, by a less steep age-performance function, a flat trend (long-term) in national wind conditions, and a flat trend in the effectiveness of wind farms as a function of their commissioning date.
The animation above illustrates this non-identifiability. The truth, for a cartoon world, is shown on the left. On the bottom-left, the data from three farms (born in 87, 91, and 93) are shown in yellow, magenta, and grey; they are the sum of a age-dependent performance function f(a) [top left] and a wind variable v_t [middle left]. (The true site 'fixed effects 'variables u1, u2, u3 are all identical, for simplicity.) On the right, these identical data can be produced by adding the orange curve f(a) to the site-dependent 'fixed effects' variables u1, u2, u3 (shown in green), thus obtaining the orange curves shown bottom right, then adding the wind variable [middle right] shown in blue (v_t).

## Monday, November 18, 2013

### Enormous solar power stations

Three spectacularly large solar power stations have recently been in the news: Ivanpah, located in California, but within spitting distance of Las Vegas, is a concentrating solar power station in which 300,000 flat mirrors focus sunshine onto three power-towers. Solana, located in Gila Bend, Arizona, has a collecting field of about 3200 parabolic-trough mirrors, each about 25 feet wide, 500 feet long and 10 feet high, and it can generate electricity at night thanks to its ability to store high-temperature heat in vast molten salt stores. Kagoshima, near the Southern tip of Japan, has 290,000 solar photovoltaic panels.
All three are enormous, and must be amazing to visit: Ivanpah occupies about 14 km2; Solana, 12.6 km2, and Kagoshima, 1 km2.

Now, I'm always interested in powers per unit area of energy-generating and energy-converting facilities, so I worked out the average power per unit area of all three of these, using the estimated outputs available on the internet. Interestingly, all three power stations are expected to generate about 8.7 W/m2, on average. This is at the low end of the range of powers per unit area of concentrating solar power stations that I discussed in Chapter 25 of Sustainable Energy - without the hot air; Andasol, the older cousin of Solana in Spain, is expected to produce about 10 W/m2, for example.

I published a paper on Solar energy in the context of energy use, energy transportation, and energy storage in the Phil Trans R Soc A Journal earlier this year, and these three new data points lie firmly in the middle of the other data that I showed in that paper's figure 8 (original figures are available here). .

These data should be useful to people who like to say "to power all of (some region) all we need is a solar farm the size of (so many football fields, or Greater Londons, or Waleses), if they want to get their facts right. For example, Softbank Corporation President Masayoshi Son recently alleged that "turning just 20% of Japan’s unused rice paddies into solar farms would replace all 50 million kilowatts of energy generated by the Tokyo Electric Power Company". Unfortunately, this is wishful thinking, as it is wrong by a factor about 5. The area of unused rice paddies is, according to Softbank, 1.3 million acres (a little more than 1% of the land area of Japan). If 20% of that unused-rice-paddies area were to deliver 8.7 W/m2 on average, the average output would be about 9 GW. To genuinely replace TEPCO, one would need to generate roughly five times as much electricity, and one would have to deliver it when the consumers want it.

Maybe a better way to put it (rather than in terms of TEPCO) is in national terms or in personal terms: to deliver Japan's total average electricity consumption (about 1000 TWh/y) would require 13,000 km2 of solar power stations (3.4% of Japan's land area), and systems to match solar production to customer demand; to deliver a Japanese person's average electricity consumption of 21 kWh per day, each person would need a 100 m2 share of a solar farm (that's the land area, not the panel area or mirror area). And, as always, don't forget that electricity is only about one third or one fifth of all energy consumption (depending how you do the accounting). So if you want to get a country like Japan or the UK off fossil fuels, you need to not only do something about the current electricity demand but also deal with transport, heating, and other industrial energy use.

Sources: NREL; abengoa.com; NREL; solarserver.com; and google planimeter.

## Monday, October 14, 2013

### Chinese translation of Sustainable Energy - without the hot air

The Chinese translation of Sustainable Energy - without the hot air is now available on amazon.cn
I am very grateful to the Chinese Academy of Sciences and President Li Jinhai for arranging both the translation and its publication. Thank you!

## Sunday, June 9, 2013

### David MacKay's "Map of the World" - an update

I've updated my "Map of the World" which shows, country by country, how human power-consumption per unit area compares with the power-production per unit area of renewables. I originally published this graph on my blog in August 2009. I've made quite a few improvements to it since then, including the representation of country size by point size, and colour coding of continents in the style of Gapminder.
One interesting thing I figured out while working on this graph is that, while the average power consumption per unit land area of the world is 0.1 W/m2, 78% of the world's population lives in countries where the average power consumption per unit land area of the world is greater than 0.1 W/m2 — much as, in a town with some crowded buses and many empty buses, the average number of passengers per bus may be small, but the vast majority of passengers find themselves on crowded buses.
My "Map of the World" graphs are published this year in two journal papers, which I will blog about shortly.
 David J C MacKay (2013a) Could energy-intensive industries be powered by carbon-free electricity? Phil Trans R Soc A 371: 20110560. http://dx.doi.org/10.1098/rsta.2011.0560 This paper also contains detailed information about the power per unit area of wind farms in the UK and USA, and of nuclear power facilities David J C MacKay (2013b) Solar energy in the context of energy use, energy transportation and energy storage. Phil Trans R Soc A 371: 20110431. http://dx.doi.org/10.1098/rsta.2011.0431 This paper also contains detailed information about the power per unit area of solar farms

## Monday, April 8, 2013

### I've been unfair on Hydrogen

In Sustainable Energy - without the hot air I spent a couple of pages discussing hydrogen transportation, under the title "Hydrogen cars – blimp your ride". While I still think that some people have been overhyping hydrogen - even Nature magazine, who praised Governor Arnold for filling up a hydrogen-powered Hummer - some of the criticisms I wrote were incorrect and I wish to correct them.

On page 131 I wrote: ... hydrogen gradually leaks out of any practical container. If you park your hydrogen car at the railway station with a full tank and come back a week later, you should expect to find most of the hydrogen has gone. Both of these statements are incorrect.

First, while hydrogen is a very leaky little molecule, it is possible to make practical containers that contain compressed hydrogen gas for long durations. It's just necessary to have sufficient thickness of the right type of material; this material may be somewhat heavy, but practical solutions exist. The technical term used in the hydrogen community for this topic is "permeation", and it's especially discussed when ensuring that hydrogen vehicles will be safe when left in garages. Hydrogen containers are currently classed in four types, and the metallic containers and containers with metallic liners (Types 1, 2, and 3) have negligible permeation rate. However, hydrogen permeation is an issue for containers with non-metallic (polymer) liners (Type 4) which readily allow the permeation of hydrogen. [Source: P. Adams et al]

Second, when discussing the hydrogen vehicle that is left for 7 days, I incorrectly tarred all hydrogen vehicles with a hydrogen-loss brush that applies only to vehicles that store liquified hydrogen at cryogenic temperatures. There are in fact three types of hydrogen storage: Compressed gas (typically at 350 or 700 bar); Cryogenic (typically at less than 10 bar and at extremely low temperature) and Cryo-compressed (at low temperature and at pressures up to about 350 bar). The hydrogen community discuss the "loss-free dormancy time" and the "mean autonomy time" of a system, which are respectively the time after which the system starts to lose hydrogen, and the time after which the car has lost so much hydrogen it really needs refilling. In the US Department of Energy's hydrogen plans, the targets are for a loss-free dormancy time of 5 days and a mean autonomy time of 30 days. Cryogenic liquid-hydrogen systems (such as the one in the BMW Hydrogen 7, which I featured in my book) do not currently achieve either of these targets. (And the reason is not that the hydrogen is permeating out, it's that heat is permeating in, at a rate of 1 watt or so, which gradually boils the hydrogen; the boiled hydrogen is vented to keep the remaining liquid cold.) However, compressed-gas systems at 700 bar can achieve both of these targets, so what I wrote was unfair on hydrogen vehicles. [Source: EERE 2006 Cryo-Compressed Hydrogen Storage for Vehicular Applications]

I apologise to the hydrogen community for these errors.

I will add a correction to the errata imminently.

## Friday, December 14, 2012

### L'energie durable - pas que du vent!

On 13th December 2012, I helped launch the French translation of Sustainable Energy - without the hot air by giving a talk in Paris, hosted by the Ecole Normale Superieure and the kind volunteer translators, AMIDES. The book was featured today by Le Figaro: Le Professeur MacKay réduit «les émissions d'âneries» .
Just like the original English book, the French translation is available free on-line, and it can be bought at a reasonable price from your favourite retailers.

## Thursday, August 2, 2012

### Personal energy calculator online

Christian Gebbe has made a nice Personal Energy Estimator based on Sustainable Energy - without the hot air. All the key numbers are adjustable, so you can override the default assumptions. It's nicely done, and I am sure Christian will welcome your feedback.

## Monday, July 16, 2012

### Sun spots and temperature

I love data!
Here is an animation showing the evolution of global temperature and the number of sunspots over the last 129 years.

And in case it doesn't display right, here's the final frame of the animation:
And in case it isn't obvious what I think these data show, the message I get from them is that there is no obvious or strong association at all between sunspot numbers and global temperatures.

## Thursday, March 29, 2012

### TEDx talk: People, Power, Area

- how the laws of Physics constrain our sustainable energy options
TEDx Warwick 2012 (18 minutes). [Original slides are also available here]

## Sunday, January 8, 2012

### Version 3 of the 2050 Pathways Calculator

In December 2011, DECC published the Carbon Plan and version 3 of the 2050 Pathways Calculator.
As before, this open-source engineering-based tool is intended to support grown-up conversations about our possible energy futures. The user can choose any combination of demand-side and supply-side actions over the period to 2050, and the calculator computes and displays various consequences - energy flows, areas of land use, greenhouse gas emissions, and some security-of-supply indicators. The significant new feature in version 3 is the inclusion of costs, for the first time. Version 3 of the calculator also includes an air-quality calculator, which, like the costs calculator, is under development. Expert feedback is welcome.
Future costs are uncertain, and there are a range of views of the future costs of key technologies such as building insulation, low-carbon vehicles, nuclear power, wind power, carbon capture and storage, heat pumps, and energy storage technologies, and key fuels such as oil, gas, and energy crops. These ranges are reflected in the calculator's cost sensitivity visualizer by allowing the user to change the costs from the default values to higher or lower values, consistent with the ranges that DECC has found in the expert literature. The user can also visualize the consequences of cost uncertainty by selecting the Uncertain choice for any of the costed items. The calculator then shows the range of possible costs for the user's chosen pathway.
All the cost ranges, and the original sources, are explicitly detailed in an open wiki, to which experts are encouraged to contribute updated data. You can click through to the relevant bit of the wiki from any row of the cost-sensitivity page of the calculator. The wiki contains superb interactive visualizations of the cost ranges from the literature. Here's the Offshore Wind Costs Data visualization, for example.
In the 2050 Calculator, you can compare the costs of your chosen pathway with other pathways, for example a handful that DECC has published, or those of experts. In the "Costs compared" view, you can compare all the pathways' costs simultaneously. In the "Cost sensitivity" view, you can compare your pathway in detail with one other comparator, which you can choose. In the web version of the calculator, costs are expressed in pounds per person per year. These are whole-energy-system costs, not people's home energy bills. For example, the costs of vehicles, building retrofit, and industrial infrastructure are included. Don't forget, the cost difference between two pathways depends on the cost assumptions. You can use the default cost assumptions if you want, but you can be sure that those costs won't turn out to be exactly right! So I encourage users to use the cost-sensitivity feature; taking into account the cost uncertainties will give you a more reasonable picture of future possible cost ranges, and ranges of cost differences.
For me, one key message from this tool is the importance of innovation support to bring down the costs of all the technologies that may be important in the future.
Media coverage - The Carbon Plan and the 2050 Calculator have had a little bit of media coverage in the last month, including a nice mention in an editorial in Nature magazine.
Some of the coverage has been so inaccurate, however, that one is forced to conjecture that the authors of two recent pieces in the Telegraph made little effort to check their facts. For example, Christopher Booker perpetuates the twaddle of a blogger who invented the assertion that the 2050 calculator 'had been designed on the assumption that, with wind power, Britain would require much less energy, because we would have become more “energy efficient”, by insulating our homes and so forth'. This is complete twaddle, as anyone who takes the time to actually look at the open-source calculator can confirm. The user is perfectly free to combine any choice of energy-efficiency measures with any choice of energy-supply mix. Yes, the government's published pathways combine "green" energy sources (eg nuclear and wind) with energy-efficiency choices. But the calculator does not 'assume' or 'force' this choice. You can easily make high-fossil-fuel pathways with strong energy-efficiency action, if you want. It's all up to you, as the user. I think it's an awesome piece of "open-source policy development", and I'd like to congratulate the civil servants who did it, and thank all the hundreds of experts and volunteers who have helped them in their work. I really hope this open, factual tool can now be constructively used by politicians and opinion-formers to help public engagement with the issues of long-term energy security and climate-change action.

## Wednesday, September 21, 2011

### Come and work at DECC!

DECC is advertising roughly a dozen posts for engineers and scientists. Here's the advertisement for 7 grade-7 engineers and 1 grade-6 engineer and 1 SEO. The closing date is 30 September 2011. More jobs are advertised here, including science/engineering specialists to work in the Office of Renewable Energy Deployment. Come and join us, it's a great place to work!

## Saturday, March 5, 2011

### Public debate about 2050 Pathways

DECC is running a public debate, using the new 2050 Calculator, at blog.decc.gov.uk. The debate was opened by eight experts (Mike Childs, Friends of the Earth; Dustin Benton, Campaign to Protect Rural England; Prof Nick Jenkins; Mark Brinkley, Housebuilder's Bible; Duncan Rimmer, National Grid; Dr David Clarke, Energy Technologies Institute; Keith Clarke, Atkins; Mark Lynas, author), who presented and discussed their preferred pathways within the calculator. It's now open to the public to join in. In a couple more days, the opening panel will wrap up their conversation; it'll be interesting if they can achieve consensus on one or two pathways.

### The panelists and their pathways

Mike Childs: demand highly curtailed and very high renewables
In Mike’s pathway, 20% of primary energy will be imported and emissions will be 80% below 1990 levels in 2050.
Mike’s pathway in more detail

Dustin Benton: demand highly curtailed and high renewables
In Dustin’s pathway, 33% of primary energy will be imported and emissions will be 81% below 1990 levels in 2050.
Dustin’s pathway in more detail

Professor Nick Jenkins: maximum electrification of homes and industry
In Nick’s pathway, 54% of primary energy will be imported and emissions will be 82% below 1990 levels in 2050.
Nick’s pathway in more detail

Mark Brinkley: lots of bioenergy
In Mark’s pathway, 66% of primary energy will be imported and emissions will be 79% below 1990 levels in 2050.
Mark’s pathway in more detail

Duncan Rimmer: mix of CCS, nuclear, renewables and all cars electrified
In Duncan’s pathway, 60% of primary energy will be imported and emissions will be 81% below 1990 levels in 2050.
Duncan’s pathway in more detail

Dr David Clarke: mix of CCS, nuclear and renewables
In David’s pathway, 56% of primary energy will be imported and emissions will be 81% below 1990 levels in 2050.
David’s pathway in more detail

Keith Clarke: high electrification of transport, homes and industry
In Keith’s pathway, 58% of primary energy will be imported and emissions will be 77% below 1990 levels in 2050.
Keith’s pathway in more detail

Mark Lynas: lots of geosequestration
In Mark’s pathway, 78% of primary energy will be imported and emissions will be 80% below 1990 levels in 2050.
Mark’s pathway in more detail

## Wednesday, March 2, 2011

### Version 2 of the 2050 Calculator

On Thursday 3rd March, DECC is going to be publishing version 2 of the 2050 Pathways Calculator, along with an updated version of the calculator that runs in your browser - now including energy flow diagrams and maps showing land areas and sea areas.
We're also publishing a simplified "My2050 simulator", aimed at engaging a wider audience in this open-source conversation about energy policy.
To celebrate these publications, I'll be on a live Guardian blog on Thursday 3rd March at lunchtime.

## Saturday, December 4, 2010

### Downwind faster than the wind

In July 2009 I wrote a post about wind-powered vehicles that travel directly downwind faster than the wind, giving links to videos explaining why this surprising idea is in fact possible.
I've now noticed that in July 2010 a fantastic team of enthusiasts fasterthanthewind.org indeed demonstrated a single-person wind-powered vehicle that goes more than twice as fast as the wind, directly downwind. Don't you just love engineers?!

## Friday, December 3, 2010

### Science, Engineering and Technology Award for DECC 2050 Pathways Team

At the Civil Service Awards last month, the Science, Engineering and Technology Award was won by the DECC 2050 Pathways Team for their work, which I highlighted in July.
From right to left, the photo shows Gus O'Donnell (Cabinet Secretary) giving the award to Katherine Randall, Tom Counsell, Clare Maltby, and James Geddes at Buckingham Palace. (To their left are two staff from the Government Office of Science.)

## Sunday, November 14, 2010

### Now available in Japanese! - "Sustainable Energy - without the hot air"

 Many thanks to Katsunori Muraoka! 持続可能なエネルギー—「数値」で見るその可能性 [単行本]

## Sunday, October 17, 2010

### Making numbers stick - desalination, melting, and boiling

I'm always looking for new ways to make physical numbers memorable.
• One method is to use a picture (eg nuclear waste, per person, per year) [page 170, SEWTHA]
• Another general rule is to choose units such that the answer to be remembered comes out between "1 unit" and "200 units", because smallish numbers are easier to remember.
• Another idea is to reexpress the quantity in completely different units, which may be more familiar and more memorable, as illustrated in this earlier post where I converted an incomprehensible 20 x 1022 J into a hopefully more human-friendly ocean temperature rise of 0.2 degrees C.

I'd like to give a few more examples of this trick, all converting unmemorable numbers in awkward units into temperature rises.
Example 1: the cost of desalinating sea water. [This method of making it stick came from Jim Gill, Chancellor of Curtin University, via Sam Wylie.] In SEWTHA (p 93), I report that desalination has an energy cost of 8 kWh per m3. A nice way to make this number more meaningful is to work out what temperature rise you would get if the same energy were put directly into heat in the same volume of water. The answer is ((8 kWh) / (1000 litres)) / (4.2 ((kJ / C) / litre)) = 7 degrees C.
This result brings home that if the desalinated water is going to be used for a shower or for cooking, the energy cost of the desalination is fairly tiny compared to the energy that will be used later in the water's lifecycle.

Example 2: melting ice. The latent heat of melting of ice is 6 kJ/mol, or 333 kJ per kg, a quantity I have never been able to memorise... until now! Using the same trick as above, we can convert this into an equivalent temperature rise, by dividing by the heat capacity. The answer is "the latent heat of melting of ice 'is' 80 degrees C".
I don't think I'll forget that number! It really brings home why mountaineers spend so much time melting snow. The energy to melt the snow is roughly the same as the energy to bring the melted snow up to boiling point!
Example 3: vaporizing water. We can apply the same trick to the heat required to vaporize water (2258 kJ/kg). The answer is (2258 kJ/kg) / (4.2 kJ/kg/C) in C = 538 C. This number violates the "should be between 1 and 200" rule, so it is not super-memorable, but it is quite striking, isn't it - whereas near-boiling water is 373 degrees above absolute zero, the energy required to actually boil it is equivalent to another 538 degrees of temperature rise! Maybe the best way to obey the "1-200" rule is to reexpress this heat once more, comparing it to the energy required to bring the water from 0 to 100 C. It is bigger by a factor of 5.4. So "the time for the kettle to boil itself dry is about 5 times the time taken to bring it to the boil".
Here ends the lesson.

## Wednesday, September 8, 2010

### 'smart ways of seeing numbers' - the BBC like the 2050 Calculator!

In The BBC News Magazine, Go Figure, Michael Blastland says:
"If you've somehow missed it elsewhere, the DECC 2050 energy calculator is worth looking up."
Hurray!