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A lot new under the Sun

By Dave Elliott

Solar energy continues to develop as new ideas emerge. There is over 70 GW of solar PV now installed around the world and new cheaper cell technologies are emerging, such as dye-sensitized cells. (www.idtechex.com/research/reports/dye-sensitized-solar-cells-technologies-markets-and-players-2012-2023-000328.asp)

Longer term, other novel options are also on the horizon. For example, researchers at Stanford University have developed a carbon based thin film cell, with a nanotube cathode and a graphene anode sandwiching an active layer made of nanotubes and buckyballs, all made by printing or evaporating from inks. When fully developed this technology holds out the promise of new robust applications- a tough spray on PV surface for use in extreme conditions. (http://pubs.acs.org/doi/full/10.1021/nn304410w)

However, although important, PV cell development may not be the key issue . Solar power is inevitably limited by the fact that it gets dark every night, so energy storage is all important. Although batteries can be used in some situations, in general, storing electricity is hard, but it can be converted into hydrogen gas by electrolysis- and that can be stored, ready for use for electricity generation when needed, or for other fuel uses. So there is some interest in solar PV arrays linked to electrolysis, possibly using focused solar mirrors, since mirrors are much cheaper than solar cells. Large-scale Concentrating PV arrays (‘CPV’) in deserts are one option.

Another approach entirely is to use focused solar heat to make hydrogen directly by thermal dissociation of water molecules- but you need very high temperatures and the process is not very efficient. A more indirect approach, as in the Solar Gas project in Australia, is to use focused solar heat to convert a mixture of methane (natural gas) water and/or carbon dioxide into new higher value synfuels. CSIRO claim that you can get an extra 25% energy gain-a solar upgrade. (http://csirosolarblog.com/tag/solargas)

Meanwhile, Rice University’s Nanophotonics Lab in the USA have developed a solar thermal system using nanoparticle absorbers which heat up in sun light and flash off steam when immersed in water . They have tested a parabolic focusing device. The system is claimed to have an overall energy conversion efficiency of 24%.

Focused solar is already widely used to boil water and produce hot gases for feeding into a combined cycle gas turbine. (www.solugas.com)

But of course that wont work at night. However, it is relatively easy to store heat. Molten salt heat stores are one option. Typically they use a mixture of 60% sodium nitrate and 40% potassium nitrate to store some of the heat from large focused-solar Concentrating Solar Power (CSP) plants, for use for continued stream raising at night, to make 24 hour power generation possible.

Several CSP plants have been built in desert areas around the world, some with molten salt heat stores, and much has been written about the prospects for desert solar in North Africa and the Middle East generating power for local use (including for desalination) and also for export via HVDC supergrids to the EU, as envisaged by the Desertec. (www.desertec.org)

CSP is still expensive and, as noted above, some interest has been shown in large scale PV using focusing mirrors (‘CPV’) , which may prove to be cheaper. However recently Siemens pulled out of the Desertec project, saying they wanted to concentrate on what they felt were more appropriate technologies for them- wind and hydro. Even so, key players like RWE, E.ON, Deutsche Bank, ABB, and the German reinsurer Munich RE, along with 50 other groups, are still involved, and the idea of using desert solar seems unlikely to go away, with local CSP projects emerging independently. Egypt already has a CSP plant just outside of Cairo , Morocco is planning one, Algeria’s 25MW Hassi R’Mel unit started up last year, backed by a 130MW gas-fired plant, and another solar plant is planned there, while Tunisia’s ‘TuNur’ CSP project should ultimately have 2 GW of electricity generating capacity. In parallel, as I have mentioned before, Desertec and others have been looking at the potential for CSP in the Gobi desert, as part of a pan-Asian green supergrid power network. And the Sahara Forest Group is looking to combine CSP, desalination and biomass production in solar greenhouses- their project in Qatar is now running.
(http://saharaforestproject.com)

There is thus no shortage of big ideas for the future. Moving things on even more,
in a paper entitled ‘Solar-Based Man-Made Carbon Cycle and the Carbon Dioxide Economy’, Detlev Mo¨ller outlines a visionary plan to link solar electricity production such as Desertec CSP, with CO2 utilization via (chemical) air capture (i.e. from the atmosphere) as well as conventional CCS. CO2 would then be reacted with electrolytically produced hydrogen to produce fuels for direct use or for electricity production when needed. The ‘SONNE’ approach, as he calls it , would thus seek to build a man-made carbon (CO2) cycle, like the natural assimilation/respiration carbon cycle by which CO2 is recycled and changed from waste (emissions) to a resource, the process energy being supplied by solar energy. (D Moller 2012 AMBIO 41 413 doi:10.1007/s13280-011-0197-6)

As I have noted before, there have been similar ideas circulating for linking hydrogen production from other variable renewables like wind with air captured CO2, to produce green syngases and fuels, or to upgrade biogas produced from biomass. See for example http://www.iset.uni-kassel.de/abt/FB-I/publication/2010-088_Towards-renewables.pdf

Wind power may be the cheapest major renewable available at present, but the global solar resource is very much larger (wind, after all, is just an indirect form of solar energy), and the technology is developing rapidly, so longer-term solar, in all its varieties, seems likely to become a dominant option. There is already over 245GW of solar thermal capacity in use around the world, rivaling wind power. (See my earlier blog post: http://environmentalresearchweb.org/blog/2012/09/solar-power–245gw-so-far.html)

However, as I have indicated above, that could just be the start. Even in the cloudy UK, where so far nearly 2GW of PV has been installed – with more to follow, including a 32MW solar farm near Leicester

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