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photovoltaics  technology M


any of the world’s hottest regions are so sparsely populated that they are not


connected to the electrical grid. Those that live there rely on diesel generators to produce the power that they need for running the local schools, hospitals, mines, water works and so on. But this form of electricity generation is far from ideal: It is expensive; noisy; it leaves a high carbon footprint; and day-in, day-out power generation demands regular fuel deliveries.


Turning to renewable sources can address all of these issues. One option is concentrator solar thermal, which uses the sun’s energy to convert water, directly or indirectly, to steam that drives turbines and generates electricity. This obviously works well in these sunny climes, but large volumes of cooling water are required. That’s not good, because these locations often have very little rainfall and water is a cherished commodity that needs to be used wisely.


A far better technology for taking advantage of strong, plentiful sunshine is concentrator photovoltaics (CPV). With this approach electricity is generated by tracking the position of the sun across the sky and focusing its rays by a factor of several hundred onto triple-junction solar cells.


One company that is hoping to tap into this potentially lucrative opportunity for CPV over the next few years - and then concurrently start competing for contracts for solar farms when it has cut the cost-per-Watt of its systems through economies of scale - is a little-known UK start-up Circadian Solar.


This University of Warwick spin-off claims that it will be able to manufacture systems delivering world-class performance at very competitive prices, due to its holistic approach to system design. And it aims to eventually get the upper hand over its rivals by exploiting its stake in the development of new processes for multi-junction solar cells that promise to slash production costs through multiple re-use of substrates.


Circadian is led by Jeroen Haberland, former CEO of the thin-film photovoltaic manufacturer Johanna Solar Technology, which was recently acquired by Robert Bosch. When Haberland left this producer of solar cells based on copper, indium, gallium, sulfur and selenium, he wanted to work with a new, emerging photovoltaic technology, and opted for CPV, due to its relative maturity. He joined the team on 1 March 2010; an appointment aimed at taking Circadian on from a university spin-off to a manufacturer of solar systems.


Haberland says that the skill set needed for this transition is different from that needed to build up a spin-off. “That’s where I come into the game. Johanna Solar, my last company, was the third high-tech company that I built up.”


Haberland’s strengths include building up companies; setting up structures and processes; securing funding; finding the right location for manufacturing; and bringing products to market.


A holistic approach According to Haberland, one of Circadian’s key advantages over its rivals is its holistic system design that leads to very high efficiencies.


“Overall efficiency depends on a lot of things. But it starts with the alignment of components to have an optimum focus of the sunlight on the cell.”


Haberland claims that many CPV system manufacturers buy components “off the shelf”, and this makes it far harder to bring the components into optimum optical alignment. “The accuracy of the tracker movement is very important, and it’s one of the factors that we have spent a lot of engineering on.”


Fluctuations in the strength and direction of the wind can also reap havoc on system performance. To address this, Circadian employs a robust tracker design and a high quality gearbox capable of very accurate tracking – its accuracy is better than 0.4 degrees.


The company’s CPV system uses two optical elements for focusing sunlight by a factor of more than 600 onto triple- junction solar cells: a Fresnel lens made from PMMA, which also provides some protection to the cell from the weather; and a second element that is positioned very close to the photovoltaic device. Combining these two makes tight focusing more tolerant to any deviations from perfect optical alignment.


The choice of which type of optical system to use to focus the sun’s rays is controversial. Some system makers, such as SolFocus and GreenVolts, are using mirrors rather than lenses. Circadian says that this alternative approach is inferior, because less light hits the cell, a weakness that is compounded if a plate is added on top of the mirror to protect the cell from the elements.


A choice of chips Triple-junction cells are available from several manufacturers, and Circadian is comparing the performance of many of these in its CPV systems. Haberland knows which manufacturer is giving the best results, but he is not willing to name names. But he will say that there are differences in the performance of the cells produced by different chip manufacturers, which are magnified at the system level.


To begin with, Circadian will be placing orders with at least one of these chipmakers, but in the longer term it is


July 2010 www.compoundsemiconductor.net 35


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