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Life cycle Carbon Dioxide effects
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Like other renewable energy devices, wave energy and tidal stream energy devices offer the ability to generate power without causing carbon dioxide emissions. However, some emissions may arise during manufacturing and installation of the devices, depending on the source of power for these activities.

Consequently, questions that are often asked are:Over a device’s entire life-cycle (manufacturing to decommissioning), how much carbon dioxide will be emitted?  How long will it take for a device to ‘pay back’ the energy that was consumed, or emissions that were caused, during manufacturing and installation?

The following calculations, prepared by Black & Veatch for an imaginary wave energy device, show how answers to these questions can be estimated on a preliminary basis. They are solely for illustration and not intended to be rigorous. Also, the results should not be regarded as typical – life-cycle emissions and payback periods will vary between devices.

 Basis of estimates

The estimates are based on the observations for marine energy devices that:

Emissions of carbon dioxide are broadly proportional to energy use; and therefore

The most important life-cycle stage is the manufacturing of structural materials.A corollary of these observations is that the estimates can be made by comparing the emissions due to a device’s use of structural materials with total energy production over its service life.Life cycle carbon dioxide emissions

Carbon dioxide emissions per unit mass of steel: 1.75 tCO2/tonne steel  

Total mass of steel in device: 665 tonnes  Carbon dioxide emissions due to steel manufacturing: 1.75 x 665 = 1160 tCO2 Average annual energy production: 2.3 GWh/year  Service life: 20 years  Total energy production over service life: 2.3 GWh/year * 20 year = 46 GWh

Life cycle carbon dioxide emissions:

In practice, allowing for emissions in other life cycle stages, total emissions are likely to be in the range 1-2 times this amount: 25 gCO2/kWh to 50 gCO2/kWh.

Energy payback time 

Assumptions and data:

The majority of primary energy used in steel manufacturing is a fossil fuel, which is used for heating, and there is relatively little direct electricity usage.

The fossil fuel will vary but is assumed to be coal, which has a carbon emissions factor of 0.30 tCO2/MWh, and the efficiency of conversion to heat  90% = 0.33´is assumed to be 90%. This gives emissions of 0.30 tCO2/MWh  tCO2/MWh, the reciprocal of which is 3.0 MWh/tCO2. 0.33 MWh of grid electricity is consumed per tonne of steel , and the associated emissions are 0.43 tCO2/MWh.

This product of these gives 0.14 tCO2/tonne steel due to electricity. Assuming a generation efficiency of 35%, the primary energy input due to electricity is 0.94 MWh/tonne steel, (0.33 MWh/tonne steel / 35%). Given total emissions of 1.75 tCO2/tonne steel (as above), 1.61 tCO2/tonne steel is due to heat input (1.75 – 0.14 tCO2/tonne steel). The primary energy  1.61 tCO2/tonne steel = 4.82´input due to heat is therefore 3.0 MWh/tCO2  MWh/tonne steel. The total energy intensity of steel manufacturing is therefore 0.94 + 4.82 = 5.77 MWh/tonne steel. Energy used to produce the steel for the wave energy device:
 

665 tonnes steel x 5.77 MWh/tonne steel = 3840 MWh = 3.84 GWh

Energy payback period:
Note that one could also consider that the electricity generated by the wave energy device avoids the use of primary energy conversion to electricity at an efficiency of 35%, in which case the primary energy input avoided is 2.3 GWh/year / 35% = 6.57GWh/year, which gives an energy payback period of only 7 months.

Linking the life-cycle emissions and energy payback calculations together, it is possible to estimate the carbon dioxide emissions payback period. This is not the same as the energy payback period because the carbon intensities of energy consumption and displaced electricity production are different.

Data:

Carbon dioxide emissions due to steel manufacturing (as above): 1160 tCO2 Average annual energy production (as above): 2.3 GWh/year = 2300 MWh/year Electricity produced by the wave energy device displaces grid electricity with emissions 0.43 tCO2/MWh.  0.43 =´Avoided emissions due to displaced electricity production = 2300  989 tCO2/year

Carbon payback period:

In practice, allowing for emissions in other early life stages that are not related to manufacturing of structural materials (see above), total emissions in the first 2 years of the life of the device are likely to be in the range 1-1.5 times the steel manufacturing emissions. Consequently, the carbon payback period is likely to be in the range of 14 months to 21 months (i.e. 1 to 2 years). 
 
Gel not for your hair but for the roof
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Low-tech roof tiles made from hair gel could provide one revolutionary solution to climate change.

Scientists at the Massachusetts Institute of Technology (MIT) found that hair gel when mixed with water, changes colour depending on the temperature.

The team of students used the technology to create special panels that reflected the suns rays away from Earth when it was hot but absorbed them in chilly weather.Nick Orf PhD '09 demonstrates a prototype Thermeleon roof tile, using a heat gun to warm part of it.

The heated area turns from black to white.The carbon-busting roof slates work by mixing the polymer with water and sealing the substance within two flexible plastic layers against a dark background.When it is cold the polymer remains dissolved and the black backing shows through.

This absorbs the available warmth from the sun, which could be harnessed to heat buildings below.The prototype tile, developed by recent MIT graduates, is designed to turn dark in cold weather and white in warm weatherAs temperatures climb the gel condenses into cloud of tiny droplets, creating a white surface that reflects the suns rays.

The team from the US college is now working on a paint version of the discovery that can simply be sprayed onto dark roofs.Lead scientist Joe Walish said the idea behind the colour-changing roof tiles came when the group was sitting around a lava lamp.'We originally set out to create a lava-lamp type fluid affected by temperature so that black liquid would rise covering the surface in winter and white liquid would rise covering the surface in the summer,' he said. 

'This turned out to be very complicated. However, we knew we could produce gels that would change from clear to white in response to a change in temperature.'These include hair gels that are produced on very large scales, making them inexpensive.' 

US Energy Secretary Steven Chu recently declared that the world should tackle climate change by painting every roof white.He claimed this simple act would eliminate a whole year's worth of global greenhouse gas emissions within 20 years, but critics pointed out that in northern cities the gains made with white roofs in the summer would be outweighed by energy losses through the winter months.

The students are confident their tiles could one day provide the ideal solution and are planning to develop them into a marketable product.

 
Consumers create greenhouse gases
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Greenhouse gas emissions created by Britons are probably twice as bad as figures suggest, says the government's new chief energy scientist.

Professor David MacKay said that reductions in carbon dioxide emissions since 1990 are "an illusion". "Our energy footprint has decreased over the last few decades and that's largely because we've exported our industry," he said.

Developing countries now made the goods that Britain buys, he added. He was speaking unofficially in a previously recorded interview, but his comments will increase pressure on the UK to improve its offer of emissions cuts at the upcoming climate change talks. "Other countries make stuff for us so we have naughty, naughty China and India out of control with rising emissions but it's because they are making our stuff for us now," he said.

"It's been estimated by Dieter Helm from the University of Oxford that roughly half of our energy footprint actually lives overseas so our true footprint is twice as big as it looks on paper." Prof Helm's paper suggests if the UK counted "embedded" emissions, its total pollution would have gone up not down.

'Prof MacKay's comments apply to all developed countries whose manufacturing industries have relocated to the developing world. He also tackles sceptics of climate policy who argue the UK's 2% share of current global emissions is trivial.

If you take into account historic CO2, the UK is among the top three world polluters, he points out. "The argument that we are only a small country could be used by pretty much every country on the planet and then we'd all do nothing," he said.

 "Back in 1910 we were burning per capita the same amount as Americans do today so that's an argument for saying we really have an ethical duty to take a lead and show the way and show that it is possible for a developed country to seriously decarbonise its economy.

"By historical emissions per capita the top three are America, Germany and Britain so we are right up there on the winners podium for carbon dioxide emissions per person over the last 125 years. "

Prof MacKay started his new job on Thursday, and his new employers at the Department of Energy and Climate Change (DECC) do not challenge his figures. A spokesman said: "While some emission reductions have resulted from the trend for manufacturing to move overseas, it's internationally accepted that emissions from manufacturing are counted by the country of production."

This is convenient for the UK, which also managed to gain very favourable terms under the Kyoto Protocol climate agreement. Setting a baseline of 1990 for emissions cuts allowed the UK to cut emissions without trying because 1990 was a peak of coal burning in Britain.

A study from the Stockholm Environment Institute estimated when embedded emissions are taken into account, the average UK resident pollutes 15 tonnes a year - almost five times more than the average Chinese person at 3.1 tonnes a year.

The failure to calculate embedded emissions has damaged the reputation of countries such as China which are making goods for export to the West but are then blamed for the pollution that results.

Prof Helm's paper says: "If carbon outsourcing is factored back in, the UK's impressive emissions cuts over the past two decades don't look so impressive anymore. "Rather than falling by over 15% since 1990, they actually rose by around 19%.

Even this is flattering, since the UK closed most of its coal industry in the 1990s for reasons unrelated to climate change. "No doubt, recalculating the figures for other European countries and the US would reveal a similar pattern."

It is consumption and not production that matters, according to Prof Helm. "This means that if global warming is to be limited, the US and Europe will have to take much more drastic action to reduce those emissions embedded in their own consumption," he said.

"Their appropriate emissions reduction targets will have to be based on the consumption of goods that cause those emissions in the first place. "This not only means that the true scale of required emissions reductions in the Western world will be much higher but that the impact on economic growth and living standards there will also be more severe than so far believed."

 
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