Low carbon footprint justifies solar investment
Given the controversy over the reduction in support for renewables, particularly photovoltaic-generated electricity, I write on behalf of the Renewable Power Committee of the IMechE to set out some basic facts on this form of power generation.
Photovoltaics (PV) play an increasing role in energy supply. The Department of Energy and Climate Change’s central forecast estimates that the UK is likely to reach 10 to 12GW installed capacity of PV by 2020. Systems installed in the UK are either crystalline silicon (mono and multi-Si) or thin-film (amorphous) silicon (a-Si), cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS), with the vast majority being crystalline silicon.
In 2012 the US National Renewable Energy Laboratory led a literature review on the carbon footprint of PV. The study screened 397 research reports for crystalline PV and 109 reports for thin-film PV.
Harmonised median figures for ground-mounted and roof-top installations for crystalline PV were estimated at 40g CO2/kWh for mono-Si and 47g CO2/kWh for multi-Si with irradiation at 1,700kWh/kWp/yr. Figures varied between 14 and 27g CO2eq/kWh for thin-film under irradiation conditions of 2,400kWh/kWp/yr.
Using the same assumptions transposed to the UK would result in figures of 68 and 80g CO2eq/kWh for moni-Si and multi-Si respectively, and 34 to 65g CO2eq/kWh for thin-film. As a comparison, combined-cycle gas turbine (CCGT) generation has a much larger footprint of 488-600g CO2eq/kWh .
A recent UK Climate Change Committee report, Reducing the UK’s Carbon Footprint, which focused on studies between 2000 and 2009, estimates slightly lower emissions for new installations at around 55g CO2eq/kWh for crystalline technologies and 30g CO2eq/kWh for CdTe. A literature review by the International Energy Agency (IEA) also quotes lower figures.
The IEA report gives payback times of 1.7 and 0.8 years for southern Europe. In the UK this equates to 2.9 and 1.3 years. PV panels have an expected lifetime of 25-30 years and so will produce considerably more energy than was used to make them.
The figures quoted all depend on location of manufacture since the carbon intensity of grid power varies. But even with these increases the carbon footprint of PV is still well below that of CCGT generation and the energy produced is many times that needed for its own production.
There are continuous improvements in PV systems arising from manufacturing efficiencies, wafer thickness and conversion efficiencies, so the figures can be expected to reduce still further. For example, standard 60-cell modules were rated at 235Wp in 2012, 275Wp in 2014 and are predicted to be 320Wp by 2016.
Greater solar PV deployment is not without its challenges. For instance, there are still uncertainties related to large volumes of embedded PV generation and its impact on grid system balancing and harmonic generation. Careful planning is needed to ensure the appropriate use of land and buildings. The advent of economical electricity storage would alleviate the grid system balancing issue, allowing generation to be time-shifted to match demand.
Our full report, The Facts about PV Energy, is at http://bit.ly/1mVqCRS
Ian Burdon, London
Confusion reigns
George Horne rightly corrects the renewables figures, but himself repeats “Scottish energy generation” when he means electrical power generation (Letters, PE February). Pressure groups regularly exploit the confusion of the two.
For instance, a single paragraph in a Greenpeace press release slides effortlessly from the reasonable “almost two thirds of global electricity supply could come from renewable energy” to the ridiculous “CO2 emissions could fall from the current 30 gigatonnes a year to 20 gigatonnes by 2030”.
As engineers we realise that the 30Gt refers to all energy production, not the small proportion that goes to make electricity, but politicians and the public are easily misled. As George Orwell predicted, vocabulary constrains thought.
John Cole, Yelverton, Devon
Nuclear beats biomass
Before we get over-enthused about the carbon savings from biomass, take time to consider where the fuel comes from and the associated pollution-displacement or sustainability issues (“Miracle workers,” PE February).
Taking a lifecycle perspective, credible estimates of product carbon for the Drax project are 120-550g CO2eq/kWh depending on the residue being burned. Once displacement is taken into account then the global figure is significantly higher. The most credible figure as yet for nuclear electricity is around 66g CO2e/kWh and other technologies are much better, although once again the low-carbon claims must be met with some scepticism.
As with all large energy projects, reliable data on lifecycle carbon is hard to come by. Little is said of the impact on the UK coal industry and the alternatives of carbon capture. The economics of the Drax project are also questionable and the government subsidy is under challenge.
This is not to detract from the impressive engineering, but the operation of this biomass power plant could easily work against attempts to reduce global greenhouse gas emissions. Biomass clearly has a role in a sustainable energy mix, but the wisdom of large-scale biomass is debatable.
Ian Roberts, Burton on Trent, Staffordshire
Doubts over Drax
I was taken by the front cover of the February issue of PE. The thought occurred to me that if the growth rate of the trees, so elegantly chopped down, was less than the rate at which the boiler consumed the wood pellets the scheme would be environmentally unacceptable. I am assuming world deforestation is not on the agenda.
On the question of weaning Drax off coal, if this biomass is not of UK supply then we are committing ourselves to yet another imported supply for one of our vital commodities (“Miracle workers,” PE February).
This would not seem desirable from a national security point of view, since it would make us very vulnerable to international influences.
The feature does not mention the energy cost to the consumer, a common omission. Seeing how many customers seem to have problems with their energy bills now, it would be a lot worse if there was a dramatic rise in ‘green’ energy costs.
Manfred O Engel, Stockton-on-Tees, County Durham
Storing up problems
The secondary useful life for electric vehicle batteries suggested in “Powerful recycling” (PE January) could very well solve many energy storage problems in stationary applications, although some questions arise.
The ownership cycle apparently causes the batteries to degrade to “more than 70% of their initial capacity”. What would their remaining useful life be in the energy storage application?
Nissan has apparently sold more than 200,000 electric vehicles. If the battery packs of only Nissan EVs were all recovered, massive premises would have to be built, by which time even more spent batteries would have to be accommodated. The plant would have to be connected to the grid, and would have to include the necessary transformer, switchgear and control/supervisory equipment. So it would not be a simple plant.
The above in no way diminishes the logic of the lateral thinking behind the energy storage suggestion.
Antony Kaye, Ekerö, Sweden
Spy story
A couple of nit-picks regarding your review of the exhibition Bond in Motion (Back page, PE February). Spectre was the 24th James Bond film, not the 23rd, while the Rolls-Royce Wraith was a 1948 model.
Yours in an anorak,
Ken Strachan, Nuneaton
Step in the wrong direction
With reference to the article about Pavegen, attention needs to be paid to the ethical issue of forcing pedestrians to expend additional work to provide the energy to generate electricity (“Energy underfoot,” PE February).
If each step taken causes a tile to drop 5mm, the next step requires the expenditure of muscular energy to raise the pedestrian to the original level. That means that, if the average length of a pace is 500mm, an artificial gradient of 1 in 100 is created. I question the legitimacy of forcing pedestrians to walk continuously uphill on a level public space. It can be argued that the gradient is small, but it could be significant for those with limited mobility.
More importantly, it would establish a principle that energy can be extracted from people’s normal activity without their permission.
That this subject has not already caused greater controversy is probably because the public have not yet realised they will have to work harder to walk across one of these floors. It is a topic that needs to be fully aired and I hope the IMechE will be honest about its implications.
Philip Young, Bourne End, Buckinghamshire
Genius and jealousy
I must take issue with the Griffith versus Whittle comments made by Francis Cowell (Letters, PE February). What Dr A A Griffith started researching in 1926 was to use a gas turbine, with axial compressor, to produce sufficient power in the turbine to drive the compressor and a propeller and nothing more.
Sir Frank Whittle, on the other hand, was quite clear that to enable aircraft to fly higher and faster the only answer was to have a powerful jet exhaust. For simplicity Whittle chose a centrifugal compressor. Imagine Concorde at Mach 2 being dragged along by four props.
Many people knew that when Griffith, a well-respected senior scientist, was asked for his opinion his answer was “impossible”. People in the know were convinced that he was jealous of Whittle.
I write as an 88-year-old who once worked for Power Jets R&D and who regarded Whittle as a genius.
John Rogers, Camberley, Surrey
Flight path
I am pleased that my article about the R101 airship (Archive, PE January) rekindled Bernard Poulten’s childhood memory (Letters, PE February) of seeing an airship flying over his house in Walthamstow – although it was almost certainly not R101 on her final flight.
I say this for two reasons. First, the airship’s route between Cardington and Beauvais on 4-5 October 1930 is well documented. After a farewell circuit of Bedford, R101 headed south via Hatfield, Potters Bar, Barnet, Alexandra Palace, Spitalfields and the Isle of Dogs. This route was to the west of Poulten’s childhood home.
Second, the airship had left Cardington at 18:36 GMT (19:36 BST), when it was already dark, and did not reach London until two hours later.
Nevil Shute’s account of his time working on R100 should also be treated with caution. His book Slide Rule is now widely regarded as an autobiographical novel and his description of the ill feeling between the two design teams and the failings of those working on R101 does not stand up to examination of the facts.
Even Barnes Wallis admitted at the time that some of the design features of R101 were more creative than his own work on R100.
It is also not true to say that R100 “was the only airship to be broken up and dismantled”. This was the fate of 11 British rigid airships, not to mention America’s Los Angeles and Germany’s two Graf Zeppelins.
Paul Ross, Hemel Hempstead, Hertfordshire
Passion in print
I just wanted to let you know how much I enjoyed the January issue of PE.
I’m an aerospace engineer by trade, but I’m now working in a broader innovation and design role at a health charity. This means that usually many of the PE articles don’t really interest me as they’re about very conventional and industry-specific topics.
However, I was so impressed with January’s issue – there were so many features I wanted to read and enjoyed reading, and made me reconnect my past and passion for engineering with my current role and passion for design with impact.
I loved the article about the chameleon tongue gripper, the feature on augmented reality, and the article about Nissan Leaf cars being designed to be part of the national grid.
Please, please, please keep up the good work. I would love to see more articles like these that showcase innovative, cross-sector ideas and technology. I would also love to see more talks and events along these lines too.
Mel Nurse, London