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Solar cells, part 2

by Dave Elliott

In my last post I looked at various types of photovoltaic and photoelectric cell, which convert light in electricity. In most cases cell efficiency falls with increased temperature,   but in this post I look at devices that operate on heat  and  on the infra -red (IR) part of the light spectrum. There are some hybrid solar thermal/PV systems, with, for example, a semi-transparent PV sheet on top of a heat absorbing solar collector.  This ‘PV/T’ approach can not only keep the PV system cool, but also doubles up on land, roof (or wall) space usage. One such system, a PV array integrated with a SolarWall air-heating unit, was installed on a roof in the 2008 Beijing Olympic Village.

However, in parallel with PV and photoelectric devices, there is a range of thermo-electric devices which convert heat/thermal radiation into electricity directly. Some thermo-electric materials work by exploiting temperature differences on each side of a semiconductor material. In such devices, electrons move from the hot side to the cold and thus transform heat into electricity. That could be used with solar radiation providing the heat. One problem is that, so far, the energy conversion efficiencies achieved are low.  But progress is being made

UK company CIP Technologies (CIP) says it has achieved 12% energy conversion efficiency for what they call thermo-photovoltaic (TPV) cells, working in partnership with the University of Oxford. They use first generation single-junction cells based on indium phosphide materials, which absorb infra red heat radiation. They are looking at second-generation cells with a more complex, multi layer construction to improve infrared capture further. This could raise energy conversion efficiency to over 15%. That would put them on a par with Si PV.

Researchers at MIT have developed a flat panel ‘enhanced nano structured’ solar thermo-electric devices that can heat water directly, while, similarly,  Rice University have developed a solar thermal system using nanoparticle absorbers which heat up in sun light and flash off steam when immersed in water. They claim that the system could have overall energy conversion efficiency of 24%.

Nano particle absorbers and cells that works on infrared radiation, open up potentially remarkable new possibilities- infra red heat is re-emitted by the Earth’s surface after the sunset, so these devices can capture some energy at night and would not be as directionally sensitive as PV cells. This promted  New Scientist  (20/12/10) to run an article entitled’ ‘Is night falling on classic solar panels?’  In particular it looked at a very novel concept that the US Department of Energy’s Idaho National Lab has developed. As it explained, tiny nano-scale antennas resonate when hit by light waves, generating an alternating current at very high frequency, which has to be rectified to be useful (Graham-Rowe 2010). That is tricky at nano-scale, and there is some way to go before the high efficiencies claimed (60-70%) as ultimately possible can be attained, but researchers at the University of Connecticut and Penn State Altoona seem to be making progress

Conventional silicon PV cells can also be made to work on the infra-red part of the spectrum by interstitial lattice modification with sulphur molecules, producing what is called ‘Black Silicon’, with an IR-absorbing surface layer. Lab efficiencies of 18.2% have been reported (Nature Nanotechology, vol. 7, pp. 743-8), but so far they are mainly used for IR detection /measurement, although given the potentially high productivity, commercial power cells may emerge

More exotically, researchers at the University of Utah have a device that turns solar or other heat into sound and then into electricity. Converting heat into sound is the novel first stage. The thermo-acoustic prime mover then drives conventional piezoelectric devices that produce electricity when squeezed in response to the pressure of the sound waves. The efficiency is low, but some small cooling devices have been built

Even more exotic, and moving beyond electricity production, is a new solar-based technology which uses solar radiation to produce syngas via ceria redox reactions in a high-temperature solar reactor, with water and carbon dioxide as input feedstock, although as yet it is not very efficient

There are also various photochemistry ideas, some aiming at hydrogen production (e.g. photo-electrolysis). For example, researchers at the University of Rochester, USA, are trying to develop solar driven systems for generating hydrogen using complex light sensitive natural molecules called chromophores and membranes infused with carbon nanotubes/graphene  to ensure that the freed electrons are not reabsorbed by the chromophore Overall, artificial photosynthesis, using a range of techniques, seems to be an exciting new option

While novel thermo-electric and photo-chemical devices may find applications on both the large and smalls scale, for the moment, conventional PV and PE cells dominate.  Several large PV systems are in operation in areas with high insolation. For example there is a 4MW flat plate tracking array at Springerville in Arizona, and Abu Dhabi’s Masdar 10MW solar plant is the largest grid-linked PV unit in the Middle East. The largest globally so far is the 600 MW array in northern Gujarat, India, but there are many smaller arrays elsewhere around the world, including many so-called solar farms in the EU, including in the UK. In addition, smaller domestic units have proliferated. By 2012 there was 100GW of PV in place globally, with 30GW in Germany, 15GW in Italy, and 2GW in the UK. So even in the often cloudy UK, PV is getting established.  DECCs Pathways analysis has suggested that a PV capacity of up to 22GW might be possible by 2020. Moreover, based on German experience, it has been claimed that, given proper support, UK PV could expand to 37GW by 2020  In more favourable climates, very much more is clearly possible, with growth spurred on by falling prices and the new technologies I have looked.  It may even be possible to make use of the new 3D printing techniques, which some say could revolutionise the field.   Certainly, with module costs  predicted to fall by about 70% by 2020 and a further almost 80% between 2020 and 2040 , and  energy (and therefore)  carbon payback times  now  under 2 years, some see solar booming rapidly.  Shell’s 2013 ‘Oceans’ scenario has solar as the largest single energy contributor globally by 2060:






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