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Solar cells, part 1

by Dave Elliott

Photovoltaic (PV) solar is moving ahead rapidly, with over 100GW now installed around the world at various scales.  The attractions of PV are that it is a silent operating, relatively robust and an easy to fit technology, with no moving parts or plumbing requirements. But since PV cells use specially fabricated materials they were initially expensive.  However that changed as volume production increased and technology improved. Indeed PV has one of the best unit cost/installed capacity ‘learning curve’ slopes in the renewable energy field.  Progress down this curve seems likely to continue given that there are many new cell technologies emerging.  They will increase energy conversion efficiency and reduce unit cost, since with higher efficiencies less cell material is needed.  Depending on the type, commercial silicon cells can have energy conversion efficiencies of 10-18%. More advanced cells, using more exotic materials, can achieve more, at least under laboratory condition e.g., the US National Renewable Energy Labs say copper indium gallium selenide (‘CIGS’) solar cells can be almost 20% efficient.

It is of course some way from the Lab tests to commercial scale production, but it can be worth the wait since it is not just efficiency that is important in reducing costs. New cell materials can lead to new cheaper production methods. Thin film cells, with the cell material deposited in a very thin layer on a substrate backing, are cheaper to make, but tend to have lower efficiencies. One option for very high cell production rates is ink dye based cell printing of flexible thin films of cell material, producing sheets at up to 100 feet per minute, although, strictly, liquid ink/dye sensitised systems are photo-electro-chemical cells, rather than solid semiconductor based PV cells.

The idea is based on the so-called ‘artificial leaf’ concept originally developed by Swiss Professor Michael Gratzel. He claimed that his photo-electro-chemical cells had an energy conversion efficiency of over 11%, 10 times that of natural systems, and he has continued to develop the idea. Versions have also been developed by, amongst others, G24i in Wales, based on coloured dye, with crystals of titanium oxide, a pigment used in white paint. When hit by sunlight, the dye releases electrons, which are captured by the specks of titanium oxide and can then be used to produce an electric current: A version using rust, i.e. ferric oxide, is also being developed.

At present sensitised ink/dye cells are mainly used for small (e.g. portable) power applications. For larger scale use, there is still a large market for silicon cells, the first type of thin film cell to be developed, but the more advanced CIGS seem to be a major new option for larger power uses, although Cadmium Telluride cell technology is often seen as the most attractive of the new cells. However it is a rapidly developing field, with many new ideas being developed for so-called third generation solar cells, including new types of organic/plastic (polymer) cells, and cells exploiting quantum tunnelling effects, all aiming for higher efficiencies and lower costs.

One of the most advanced, developed in the USA, is an inverted metamorphic triple-junction gallium indium phosphide /gallium indium arsenide cell, which, with light focussing, is claimed has an energy conversion efficiency of around 40%.

Moving well beyond the lab, Spectrolab’s Point Focus metamorphic concentrating cell, which is claimed to offer 40% efficiency, is widely used for satellites.

In parallel, JDSU offer a multiple quantum well enhanced triple-junction GaInP/Ga(In)As/Ge cell, a version of the QuantaSol cell developed at Imperial College London, which is claimed to have 41% efficiency.

In advanced designs like this, the light is split optically, with each junction working on different parts of the light spectrum, thus increasing the overall efficiency of energy conversion. They also concentrate the impinging light internally and are then called Concentrating Photo-voltaic devices (CPV).  While the cell itself may not have 40% or whatever efficiency, the module as a whole can do, and its cheaper- mirrors and lenses cost less than cells.

Focusing systems are only realistic in direct (rather than diffuse) sunlight, so they tend to be used in desert areas.  Some systems use flat sheet fresnel diffraction lenses to concentrate sunlight, along with tracking mechanisms to orient the cells to follow the sun across the sky, like Concentrix’s 2MW triple-junction (GaInP/GaInAs/Ge) cell array in Spain. They claim that its energy conversion efficiency is 23%.

There are many other new ideas emerging. North Carolina State University researchers have developed solar cell with a water-based gel infused with light-sensitive molecules, using plant chlorophyll. They say efficiency is low but they felt that ‘the concept of biologically inspired ’soft’ devices for generating electricity may in the future provide an alternative for the present-day solid-state technologies’.

Stanford University is developing  a totally carbon based thin film cell, with nanotube cathode and graphene anode sandwiching an active layer made of nanotubes and buckyballs, all made by printing or evaporating from inks. When fully developed it could provide a tough spray-on PV surface.

The advent of cheap organic/polymer ink/dye types cells opens up new possibilities. For example one very nice idea for air conditioning is the ‘smart window’, with a coating which not only reduces light and heat throughput, but also absorbs or reflects some of the energy to power cells, which can feed electricity to an air-conditioning unit.  An early version was proposed by Prof. Keith Barnham at Imperial College London in 2006.

Subsequently, a system was developed by Prof. Marc Baldo’s team at Massachusetts Institute of Technology (MIT). Large glass panes are treated with a transparent but reflective material, which, via a luminescent effect, redirects some of the light energy to PV cells round the edge. So there is a large collector area, but only a few cells. The result, as the BBC noted, is a smoked-glass window that still lets 10% of the sunlight through.

More recently, other researchers at MIT developed transparent organic PV cells, which can be used in windows.

More recently still, an Oxford University spin-off company has developed something similar, with the option of having different coloured glass, and attracted media attention.

Meanwhile, a team from Sheffield and Cambridge Universities have developed a low cost ‘spray-on’ PV coating for windows.

Similarly, a US company offers a room temperature spray-on system using nanoparticles, claimed to cut cost/kW by a third.

The efficiencies for some of these systems may be low, but if these ‘solar windows’ are as cheap as some claim, there is an obvious potential for wide-scale application. Some of these solar window systems extract energy from the infra-red part of the spectrum, and cells that can do that open up new possibilities, as I will be exploring in my next post.

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  1. Trackback: Renewables: Solar Cells and CO2 « aviott

  2. Trackback: Solar cells, part 1 | Nonstop Solar Cell

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